Porous Inorganic Membranes for CO 2 Capture: Present and Prospects

Porous Inorganic Membranes for CO2 Capture: Present andProspectsMarc Pera-Titus*

Institut de Recherches sur la Catalyse et l’Environnement de Lyon (IRCELYON), Universite de Lyon, UMR 5256CNRSUniversite Lyon 1, 2 Av. A. Einstein, 69626 Villeurbanne Cedex, France

Eco-Efficient Products and Processes Laboratory (E2P2L), UMR 3464 CNRSSolvay, 3966 Jin Du Road, Xin Zhuang IndustrialZone, 200012 Shanghai, China

*S Supporting Information

CONTENTS

1. Introduction B1.1. Precombustion, Oxy-Combustion, and Post-

combustion CO2 Capture: Which Separa-tions Are To Be Addressed? D

1.2. Pre- and Postcombustion CO2 CaptureTechnologies: Adsorption vs Membranes E

1.3. Polymer/Mixed Matrix Membranes, DenseMembranes, and PIMs for Pre- and Post-combustion CO2 Capture F

1.4. R&D Concepts from Academia and Industryfor CO2 Capture Based on PIMs H

1.5. PIMs for CO2 Capture: The Triple Challenge J2. Membrane Separation: Adsorption vs Diffusion

Selectivities K2.1. Adsorption Mechanisms K

2.1.1. Amine-Functionalized Silicas K2.1.2. Zeolites N2.1.3. Metal−Organic Frameworks Q2.1.4. Taxonomy of Materials Based on Ad-

sorption Selectivity W2.2. Diffusion Mechanisms: Molecular Sieving

and Correlation Effects for CO2 Separation X2.2.1. Diffusion Mechanisms in Mesoporous

Solids: Selectivity beyond the KnudsenThreshold? Z

2.2.2. Diffusion Mechanisms in Zeolites andMOFs Z

2.2.3. High-Temperature Mechanisms: Appli-cation to H2 Separation AC

2.2.4. Taxonomy of Materials Based on Dif-fusion Selectivity AC

3. CO2 Permeation and Separation Properties AE3.1. Silica Membranes AE

3.1.1. Microporous Silica Membranes AE3.1.2. Amine-Functionalized Mesoporous

Membranes AG3.1.3. Ionic Liquid Membranes (RTILMs) Based

on Mesoporous Alumina and Silica AH3.2. Zeolite Membranes AI

3.2.1. Effect of the Operation Variables on theCO2 Permeation and Separation Proper-ties AL

3.2.2. Influence of the Membrane Structure onthe CO2 Permeation and SeparationProperties AR

3.3. MOF Membranes AT3.3.1. H2 Separation: Molecular Sieving Mech-

anism AT3.3.2. CO2 Separation via Preferential Adsorp-

tion AV3.4. Taxonomy of Materials Based on Perme-

ation Properties under Ideal/Real Conditions AX3.4.1. CO2/N2 Separation AX3.4.2. CO2/CH4 Separation AY3.4.3. CO2/H2 and H2/CO2 Separation AY3.4.4. Insights into in Silico Studies AZ

4. Membrane Synthesis and Microstructure AZ4.1. Membrane Supports: From Tubes to Hollow

Fibers AZ4.2. Silica Membranes BA

4.2.1. Sol−Gel Routes BA4.2.2. Polymer Route in the Presence of a

Surfactant BB4.2.3. Chemical Vapor Deposition/Infiltration BC4.2.4. Modification of Silica Membranes BC

4.3. Zeolite Membranes BD4.3.1. Direct in Situ Crystallization BE4.3.2. Secondary Growth Method BF

4.4. MOF Membranes BG4.4.1. Solvothermal Synthesis BG4.4.2. Stepwise Layer-by-Layer Synthesis BH

4.5. Membrane Microstructure BH4.5.1. Macrodefects BH4.5.2. Grain Boundaries BJ

Received: May 2, 2013

Review

pubs.acs.org/CR

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4.6. Taxonomy of Materials Based on LayerIntergrowth and Microstructure BK

5. Final Remarks and Outlook BKAssociated Content BM

Supporting Information BMAuthor Information BM

Corresponding Author BMNotes BMBiography BM

Acknowledgments BNDedication BNGlossary BN

Greek Symbols BNSubscripts BNSuperscripts BNAcronyms (for more acronyms see the Support-ing Information) BN

References BP

1. INTRODUCTION

There is growing scientific consensus that the risingatmospheric levels of CO2 as a result of man-made activities(e.g., fossil fuel burning, cement and lime production, ammoniaand ethylene oxide synthesis, aluminum and glass industries,fermentation, deforestation, use of fertilizers) is responsible forthe warming effect on the climate. Indeed, CO2 emissionsaccount for ca. 70% of the gaseous irradiative force causing thegreenhouse effect, followed by CH4 and N2O.1,2 Sincepreindustrial times, the CO2 concentration in the tropospherehas increased from 280 ppmv in 1750 to >380 ppmv at present,with an annual increase of about 1 ppm.1 This increase hasbeen escorted by a rise of about 0.6−0.7 °C in the earth’ssurface temperature during the past century, with a prominentincrease during the past 20 years from an annual growth rate of1.1% in the period 1990−1999 to >3% during the period2000−2004.3 Worldwide CO2 emissions from fuel combustionreached a level of 30 Gtonnes in 2010, 41% of which beingrelated to energy production.4 On current trends, theIntergovernmental Panel on Climate Change (IPCC) projectsan average increase of the global temperature by 1.8−4.0 °Cduring this century, triggering irreversible consequences formankind and ecosystems (e.g., increase of ocean levels).5

Climate models also suggest that the negative effects of a globaltemperature increase could even be amplified due to cumulativemechanisms (e.g., massive liberation of CO2 or CH4 stored incarbon hydrates in the oceans, fusion of perpetual ice, or even achange in the pattern of the Gulf Stream), driving the climateto hardly predictable evolution scenarios.Since the beginning of the 1990s, climate change has moved

up high in the international political agenda. The KyotoProtocol, approved by more than 140 nations in 1997 underthe auspices of the UN Framework Convention on ClimateChange (UNFCCC) and extended during the last climatesummit held in Doha (Qatar) in October 2012, constitutes aninternational milestone to counteract global warming.6,7 Thegoal is to use market forces to restrict emissions by creating“flexible mechanisms”, such as a trading allowance greenhousegas (GHG) emission market, a clean development mechanism(CDM), and a joint implementation (JI) strategy. Industrializedcountries were entitled to cut emissions of GHGs (includingCFCs and HFCs, responsible for stratospheric ozonedepletion) by 5.2% from 1990 levels in the period 2008−

2012.6,7 New stringent mandatory post-Kyoto targets (post-poned until 2015) will be negotiated for the horizon 2015−2020 after endorsement of Adaptation Funds.In the past 15 years, the European Union (EU) has passed

some pioneering legislation to meet Kyoto targets.8−12 TheEU-15 committed to cutting GHG emissions by 8% below1990 levels before 2012, by 20% onward before 2020, and by80−95% before 2050 by setting an ambitious EuropeanClimate Change Program (ECCP).13−15 The proposed strategyrelies on the premise that reducing the energy demands byincreasing energy efficiency and productivity and promoting thetransition to a low-carbon sustainable economy (e.g., by usingrenewable energies, natural gas cogeneration and integratedgasification combined cycle (IGCC) power plants, andbiofuels) is the best way to cut CO2 emissions and boosteconomic growth. Moreover, despite the serious concernsamong the scientific community and the negative publicopinion about the future of “green” nuclear energy after theFukushima disaster (March 11, 2011),16 it appears that the roleof nuclear energy will still be manifest in the forthcoming yearsto promote energy autonomy and to meet the headline CO2emission targets. In this regard, the midterm implementation ofthird-generation EPR reactors combined with fuel cells andbatteries seems a realistic scenario. Nevertheless, according tothe present state-of-the-art, many efforts are still necessary toreduce fuel cell costs (€6000−8000 per kilowatt vs €30−50 perkilowatt for thermal systems17), improve hydrogen storagetechnologies, and boost the power storage capacity ofaccumulators. Note that in any case neither fuel cells norbatteries involve per se a net zero-carbon balance, since CO2 isunavoidably generated from steam reforming (hydrogenproduction) or combustion (power generation) of carbon,natural gas, or biomass.In this context, CO2 capture, transport, and long-term

storage or sequestration (CCS) is visualized as a promisingstrategy for mitigating CO2 emissions at short- and midterms,especially in stationary sources, as put forward in a number ofrecent technical reports and papers.18−23 Three strategies havebeen invoked for long-term large-scale CO2 storage: (1)geological storage in depleted oil and gas fields, deep salineaquifers, or unminable coal seams (formation of CO2 pools orsolid gas hydrates), (2) deep ocean storage in “CO2 lakes” andmarine sediments, and (3) industrial fixation in inorganiccarbonates. Injection of CO2 into suitable depleted oilreservoirs has been used for a long time in Texas for enhancedoil recovery (EOR). The maximum available CO2 geologicaland deep ocean storage capacity has been estimated at 2 and 1Ttonnes, respectively.24 CO2 transport to the storage site couldbe carried out either at high pressure by gas duct or byliquefying CO2 at moderate pressure (up to 7 bar) and −20 °C(offshore transport by ship).Among the three steps of the CCS chain, CO2 capture is by

far the most expensive one, accounting for 50−90% of theoverall chain cost depending on the CO2 emission source.5

Chemical absorption with alkanolamines (or variants) con-stitutes today the benchmark technology for CO2 capture fromflue gas steams in large emission sources.25 In this technology,the sorbed CO2 in the form of stable carbamate and/orbicarbonate species must be further liberated in a separatevessel by raising the temperature and/or lowering the pressureabove the solution, the regenerated solvent being recycled tothe absorption unit. Despite the significant improvements interms of liquid stability in the presence of O2 and SO2,

26

Chemical Reviews Review

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toxicity, and regeneration energy demands, this process isintrinsically noncontinuous (i.e., semicontinuous) and stillhighly energy intensive (>2.5 GJ/tonne of CO2 avoided27 or€12.5−62.5 per tonne of CO2 avoided for coal powerstations24).This review focuses on membranes and more specifically on

porous inorganic membranes (PIMs) as alternative candidatesto chemical absorption and other mature and prospectivetechnologies for CO2 capture in different pre- and post-combustion scenarios. In general terms, a membrane can bedefined as a selective barrier allowing the separation of one ormore species from a mixture driven by their preferential affinityfor the membrane. Unlike solid adsorbents, the term “affinity”is regarded here in a broad sense, since it comprises not onlythe favorable interaction of one or more molecules from amixture with the solid, but also its/their promoted/inhibiteddiffusion within the active film or top layer.Although some studies point out the advantages of chemical

absorption over membrane technologies for CO2 capture,28,29

most of these statements focus on nonoptimized membranematerials (e.g., resistant to water and acids, manufacture of thinlayers, reproducibility of synthesis protocols), offering a biasedview of the field. Joining the view of Favre,28 three mainchallenges are addressed for boosting membrane technologiesto promote industrial implementation (section 1.3): (1) theselectivity challenge, (2) the energy challenge, and (3) theproductivity challenge. These challenges impose not only theoptimization of existing membrane materials, but also thedevelopment of new materials with balanced adsorption anddiffusion properties for a given CO2 capture application.Although PIMs for gas separation are certainly still in an early

technological stage, they show potential for pre- andpostcombustion CO2 capture for different reasons. First,PIMs show higher thermal, chemical, and mechanical stability

than polymers, offering potentially longer lifetimes in serviceand more reduced maintenance costs. Second, PIMs offerhigher permeabilities (usually at the expense of lowerseparation factors), allowing smaller layouts and investmentcosts than polymer membranes. Finally, unlike adsorbents, thepossibility of operating in continuous mode and treating higherflow rates offers PIMs an added value to their industrialimplementation.In addition to these general advantages, a new generation of

hybrid materials based on functionalized silicas, novel zeolites,and metal−organic frameworks (MOFs) offer a plethora ofsynthetic possibilities to generate new membranes with suitablefunctional groups for CO2 capture via selective adsorption and/or diffusion.30,31 Recent reviews compiling CO2 adsorption dataon such materials for pressure-swing and thermal-swingadsorption column design,32−36 and covering thin filmsynthesis,37−40 have recently appeared, but with only briefinsights into membrane design for CO2 and H2 separations. Weintend here to fill this gap with a special focus on the assets andlimits of such materials in light of current industrial challengesin gas separation technologies for different pre- andpostcombustion CO2 capture scenarios (e.g., high- and low-temperature applications). To this aim, a compilation of themost relevant and updated recent permeation and separationdata, structural stability, and optimization perspectives of theabove-stated materials for membrane design is presented. Thereader will find along the review a multidisciplinary vision of themembrane field, including solid-state chemistry, materialsscience, separation, and process design (R&D). The lack ofexchange between these different disciplines is usuallyrecognized as an important barrier to breakthrough discoveries.This paper intends to provide some keys to the differentcommunities, including industrial researchers, to facilitatereciprocal exchange.

Figure 1. CO2 capture strategies as a function of the combustion type, implementation in the overall strategy for energy production and CO2capture/storage, strategic target separations, and relevant FP6 and FP7 European projects recently accomplished.


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This review is organized in five sections. The analysis startsfirst with the enumeration of the three main challenges inmembrane design with respect to absorption and adsorptiontechnologies for CO2 capture in terms of cost and materialdevelopment (section 1). Threshold “academic” selectivity andpermeability values for different CO2 capture scenarios areoutlined here combined with recent R&D developments fromindustry. Section 2 provides a systematic study of CO2, N2,CH4, and H2 adsorption and diffusion properties on a broadseries of silicas, zeolites and MOFs to boost membranepermeances and selectivities for low- and high-temperatureCO2 capture applications. The materials are ranked at the endof each chapter on the basis of their adsorption and diffusionselectivities, making use when necessary of recent in silicostudies. Section 3 includes a comprehensive body of separationand permeation data of the different materials emerging fromsection 2 with an insight into the influence of water andpollutants on the separation and permeation properties. Acritical analysis is provided at this level on the capacity of thedifferent materials to accomplish the three main challengespointed out in section 1 and to conceive and benchmark

realistic membrane-based solutions for real CO2 capturescenarios. Section 4 provides the reader with a description ofthe synthetic analogies and differences of silica, zeolite, andMOF membrane films and nanocomposites for CO2 and H2separation with a critical discussion on the extrapolation ofcontrasted synthesis protocols for powders to membrane layers.This section also lists the main factors affecting membranereproducibility in the preparation protocols and the role of themembrane microstructure on the permeation and separationproperties. Finally, section 5 provides a critical appraisal of thedifferent membrane materials and technological solutions fordifferent CO2 capture scenarios in terms of CO2 separation andpermeation properties, as well as performance under realisticconditions and present technico-economical manufactureconstraints.

1.1. Precombustion, Oxy-Combustion, andPostcombustion CO2 Capture: Which Separations Are ToBe Addressed?

Schematically, CO2 capture in a fossil fuel combustion processcan be achieved following three different strategies (Figure

Table 1. Typical Flue and Exhaust Gas Compositions for Stationary and Mobile CO2 Emission Sourcesa

moleculecoal combustion

flue gascoal-fired IGCC

flue gasgas-fired flue

gaswaste incineration

flue gascoal gasification

fuel gasmethane steam reforming

flue gasdiesel vehicleexhaust gasb

CO2(%, v/v)

∼11 ∼7 ∼3 6−12 4−13 27−34 ∼11

O2 (%, v/v) ∼6 ∼12 ∼14 7−14 ∼2N2 (%, v/v) ∼76 ∼66 ∼76 balance balance 0.3−2.2 ∼75CO(%, v/v)

0.001−0.060 40−58 <0.65

H2 (%, v/v) ∼30 34−46H2O(%, v/v)

∼6 ∼14 ∼6 10−18 1−4 18−38 ∼12

SO2 (ppmv) 10−200 200−1500H2S (ppmv) 1000−4000 0.14−0.17NOx(ppmv)

500−800 10−100 10−300 200−500 2500−3000

HCl (ppmv) 400−3000 500−600dioxine(ppbv)

≪1 1−10

aData adapted from refs 42 and 43. bData kindly provided by J-P. Roumegoux, Institut Francais des Sciences et Technologies des Transports, del’Amenagement et des Reseaux (IFSTTAR; Bron, France).

Figure 2. Summary of technologies for CO2 capture stressing the intimate synergy between absorption and adsorption concepts for membranedesign (violet, precombustion; blue, oxy-combustion; orange, postcombustion). A detailed comparison of the different technologies can be found inthe Supporting Information (Table S1).


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1):41 (1) oxy-combustion, where the combustion is performedwith pure oxygen instead of air, so that a CO2/H2O mixture isproduced; (2) precombustion, where the carbon from the fuelis removed prior to combustion (decarbonization) as CO2, as asolid, or in other forms and whereby the primary fuel heatingvalue is transformed into hydrogen through partial oxidation,methane-steam reforming, or autothermal reforming withsubsequent water−gas shift (WGS) reaction; (3) postcombus-tion, where CO2 recovery is performed at the end of a pipefrom a wet exhaust flue gas, usually at 10−30 vol % CO2concentration. Standard flue gas compositions from differentprocesses are summarized in Table 1.The target separations to achieve in these processes to make

them feasible are: O2/N2 for oxy-combustion, CO2/H2 forprecombustion, and CO2/N2 for postcombustion CO2 capture.Different technological solutions (and indeed membranematerials and operational conditions) are necessary to performsuch separations. Bredesen and co-workers41 carried out adetailed analysis of the economical impacts of CO2 capture inthe energy costs of these three technological solutions,transferring in each case the CO2 capture problem to thehydrogen and energy production plants (see more detailsbelow). Among the three CO2 capture strategies, postcombus-tion CO2 capture is by far the most challenging option, since adiluted, low-pressure, wet, and hot CO2/N2 mixture has to betreated. Nevertheless, it corresponds to the most widelyextended industrial application, showing the essential advantageof being compatible with a retrofit strategy (i.e., pre-existinginstallations can be a priori subjected to this adaptation).

1.2. Pre- and Postcombustion CO2 Capture Technologies:Adsorption vs Membranes

A panel of technologies have been proposed in the past for CO2capture with different degrees of maturity. Figure 2 provides avisual comparison of the different technologies, whereas Figure3 shows a general scheme of their time scale and benefits fordifferent CO2 capture scenarios. Furthermore, Table S1

(Supporting Information) provides detailed information ontheir main advantages and drawbacks for industrial implemen-tation. On one hand, in the case of oxy-combustionapplications, three main technologies have been proposed,including: (1) N2/O2 separation by cryogenic distillation, (2)chemical looping using metals with redox properties (e.g., Fe,Ni, Cu, and Mn), and (3) oxygen transfer membranes(OTMs). The latter two technologies rely on the selectiveretention of oxygen by the formation of metal oxides (chemicallooping) or by selective oxygen transfer within redox switchablematerials (e.g., OTMs based on perovskites). Despite thepotentials of these technologies, their short-term applicationsare discouraged due to the high energy demands for metalreduction and oxygen transfer. On the other hand, three maintechnologies emerge for pre- and postcombustion CO2capture:41,44,45 (1) chemical and physical absorption (wetscrubbing) using alkanolamines, ethylene glycol, solutions ofalkali-metal carbonates, or “chilled” ammonia, (2) pressure andthermal swing adsorption (PSA and TSA, respectively) usingeither physical adsorbents (e.g., activated carbon, alumina, silicagel, or molecular sieves) or chemical adsorbents such aslimestone and hydrotalcites (dry scrubbing), and (3)membrane-based separations relying on the selective passageof CO2 (polymers and ceramics) or hydrogen (ceramics anddense or composite Pd/Ag).Separation processes based on PSA (VSA) or TSA using

well-engineered (physi)sorbents at the molecular level (mostoften activated carbons and zeolites, but also amine-function-alized silicas) are visualized as future strategies for CO2separa t ion at low gas flow streams (Figure 2 ,right).33−35,46−48 Adsorption technologies rely on the differentphysical attraction/repulsion forces between the sorbent andsorbate species, allowing the separation of different species in agas mixture based on different affinities. Table 2 lists the main

physicochemical properties of the usual components present influe gases emitted from pre- and postcombustion sources.Adsorption-based technologies involve two basic steps:(1) Adsorption step. N components of a gas mixture are

selectively adsorbed on the solid in a packed column atrelatively high pressure, leaving the gas stream enriched in Mcomponents (M < N) in the raffinate stream corresponding tothe weakly sorbing components.(2) Regeneration or desorption step. The N−M sorbed

components are desorbed from the solid by lowering thepressure (PSA) or the temperature (TSA) inside the column.After this operation, the adsorbent is ready for a further cycle.

Figure 3. Cost reduction and estimated commercialization time for theindustrial implementation of CO2 capture technologies. Nomencla-ture: IL, ionic liquid; OTM, oxygen transfer membrane. Image adaptedfrom the original scheme presented by the DOE National EnergyTechnology Laboratory during the MEGA Symposium in Baltimore,MD, 2008.

Table 2. Physicochemical Properties of Molecular SpeciesPresent in Pre- and Postcombustion Flue Gasesa

moleculekinetic

diameter (Å)polarizability

(Å3)dipole

moment (D)quadrupole

moment (D Å)

CO2 3.30 2.507 0.000 4.30CH4 3.80 2.448 0.000 0.02H2 2.89 0.787 0.000 0.66N2 3.64 1.710 0.000 1.54O2 3.46 1.580 0.000 0.039CO 3.76 1.950 0.110 2.50NO 3.49 1.700 0.159H2O 2.65 1.501 1.850 2.30H2S 3.60 3.780 0.978

aData obtained from refs 49 and 50.


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The gas mixture obtained from the regeneration (the extract) isenriched in the more strongly adsorbed components.In common practice, several columns are operated in swing

mode to make the process continuous (or more properlysemicontinuous) and additional steps are added to the basic cycleto maximize productivity and energy savings. As a consequence,industrial PSA/TSA systems include 4−16 vessels connected bya complex network of pipes and valves switching the gas flow.Recent process developments include the rapid cycle PSAapproach (RCPSA) patented by QuestAir, relying on a systemof proprietary multistream rotatory valves that results incompact PSA units especially tailored for low-capacityapplications (up to 0.1 MSm3/d; MSm3 = mega standardcubic meters).51,52 The adsorption step usually ends before themore strongly adsorbent component breaks through the bed,while the regeneration step is terminated before the bed is fullyregenerated. A low-pressure purge step is usually applied in thecycle to promote the adsorbent regeneration by recycling partof the raffinate (purge or sweep gas) produced during theadsorption step. The term “vacuum swing adsorption” (VSA)identifies a particular PSA process where adsorption is run atnear-ambient pressure and desorption under vacuum. Thisapproach is restricted to the situation where either a highly pureextract stream is required or strongly adsorbed components arepresent.Despite the success of adsorption-based technologies for gas

treatment and conditioning in terms of energy economy (by

$12−35 per tonne of CO2 avoided for a six-step VSA cyclewithout purge53), these processes are usually restricted to lowflow rates (<4 MSm3/d; Supporting Information, Table S1)and limited by their semicontinuous operation mode as inphysical/chemical absorption (Figure 2, left). Membranes cancircumvent this shortcoming by reformulating the adsorbent ina filmlike configuration, also offering additional selectivitybenefits via differences in surface diffusion patterns. Further-more, membranes can also benefit from general concepts ofphysical/chemical absorption via liquid confinement inmembrane pores (e.g., impregnation or grafting), offering apiece of creativity for design (see more details in section 2).Figure 4 illustrates graphically the main differences betweenadsorption and membrane technologies for a postcombustionCO2 capture application. Figure 4 also illustrates the alternativepotential uses of membranes for either the flue gasconcentration (in CO2), or drying and conditioning from O2,NOx, and SOx species. Among the different technologies,significant progress in PIMs for pre- and postcombustion CO2capture applications is expected at midterm (Figure 3).1.3. Polymer/Mixed Matrix Membranes, DenseMembranes, and PIMs for Pre- and Postcombustion CO2Capture

In general terms, the membrane materials for pre- andpostcombustion CO2 capture can be classified into threemain families: (1) polymers, (2) dense or metal membranes,and (3) porous inorganic membranes (PIMs). This classi-

Figure 4. Postcombustion CO2 capture: top, adsorption column; bottom, membrane separation with feed pressurization and principle of membraneseparation.


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fication should not be regarded as absolute, since examples ofhybrid membranes including combinations of these threefamilies are frequent and even desired for merging the generaladvantages of each individual family. This is the case, forinstance, of mixed matrix membranes (MMMs) consisting of apolymer matrix doped with solid particles, or hybrid silicasconsisting of a silica matrix, usually micro- or mesostructured,impregnated or doped with a polymer. Such examples and themain consequences on membrane design are described in detail

in the following sections. The main characteristics of the threedifferent families are addressed below, as well as advantages anddrawbacks for CO2 capture applications. A relative comparisonof research efforts among these three families in terms of openpublications and patents is provided in Figure 5. Furthermore,Table 3 summarizes the main characteristics of the threefamilies for pre- and postcombustion applications.The first family of materials is based on polymer membranes

and related materials. Polymer membranes and hollow fibers

Figure 5. Total number of open publications and patents retrieved from the Web of Science and European Patent Office (July 2, 2013), respectively,for the period 2000−2013 using the keywords “membrane” + “separation” + “H2” (or “CO2”) + “X”, where “X” = “polymer” or “organic”, “mixedmatrix” or “hybrid”, “silica”, “zeolite”, “mof” or “zif”, and “dense” or “metal”.

Table 3. Classification and Characteristics of Membranes for Pre- and Postcombustion CO2 Capture


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(e.g., cellulose, polyimide, polysulfone, polycarbonate, poly-(ethylene oxide), and poly(ether imide)) have been widelyinvestigated for gas separation applications. This importantactivity is testified by the number of papers covering H2 andCO2 separations in the period 2000−2013 (about 1000 and900, respectively), as well as by the important R&D activities inthe field, with about 70 patents claimed on polymer materialsfor H2 separation (Figure 5). Nevertheless, the application ofpolymer membranes to CO2 separation is usually argued aslimited due to their insufficient thermal, mechanical, andchemical stabilities and their intrinsic low permeances. Indeed,a CO2/N2 permselectivity of 50 associated with a CO2permeance of 3 nmol·m−2·s−1·Pa−1 (1 μm thick) can betaken as a classical upper limit for polymeric membranes. Acomprehensive survey of glassy and rubbery membraneimplementation in pre- and postcombustion processes can befound in some excellent reports.54−58 A similar conclusion canbe drawn a priori for emerging hybrid membranes or MMMsrelying on mesostructured silicas, zeolites, and carbons (e.g.,poysulfone/MCM-41,59 polysulfone/ITQ-29,60 PES/4A,61

poly(ether imide)/HSSZ-13,62 and polyimide/carbon,63) dueto a low symbiosis between the solid nanoparticles and thepolymer matrix. Even if the field of MMMs has experienced asustained development in the past years, this is still less activethan the field of polymer membranes, with about 90 and 175published papers covering, respectively, H2 and CO2 separa-tions. In the meantime, 8 and 10 patents have been claimed,respectively, on R&D developments for H2 and CO2separations, UOP LLC being the main actor in the field.Notwithstanding the general shortcomings of polymer and

hybrid membranes, some outstanding exceptions deserve to bementioned. This is the case for instance of poly-(benzenimidazole) (PBI) membranes,64,65 recently commer-cialized by MTR, showing H2/CO2 separation factors as high as50 and H2 permeances of about 0.1 μmol·m−2·s−1·Pa−1 withpotentials for precombustion CO2 capture applications. Otherrecent relevant improvements involve the development ofMMMs based on MOFs (e.g., polymer/MOF-5,66 polysulfone/MIL-53(Al)-NH2,

67polyimide/MIL-53(Al)-NH2,68 Matrimid/

CuBTC/MIL-53(Al),69 Matrimid/ZIF-8,69,70and Ultem/ZIF-871) and dual zeolite/MOF combinations,72 where the organiclinkers can couple with the polymer matrix, allowing MOFloadings up to 50 wt %. MOF/polymer membranes usuallyshow improved fluxes compared to bare polymers and zeolite-based MMMs. Erucar and Keskin73 have underlined thepotentials of MOF/polymer MMMs for CO2/CH4 separationsin a recent in silico study.The second family of materials is constituted by metal

membranes, most often based on dense Pd alloys eithersupported or not on ceramic supports, for H2 separation inprecombustion CO2 capture applications. This field has beenvery active in the past years, with about 250 open publicationsappearing and ca. 300 patents claimed covering not only thepreparation of novel formulations for specific applications, butalso the conception of intensified processes for H2 or energyproduction (e.g., WGS, fuel cells). Unlike other cheaper metals,Pd films and Pd/ceramic nanocomposites offer comparativelyhigher H2 permeabilities and a higher stability against oxidationand carburation. Since Pd films have the tendency to becomebrittle on H2 dissolution and can be reactive in the presence ofdefects, Pd is usually combined with other metals (e.g., Ag) toincrease their stability under operation. Moreover, Pd films onrefractory metal supports are usually thick (40 μm) to avoid

interdiffusion between Pd and the refractory metals. Takinginto account the maturity of Pd-doped membranes forprecombustion CO2 capture applications (>900 papers havebeen published and about 300 patents have been claimed in theperiod 2000−2013, with Toyota Motor Corp., Nissan Motor,and Tokyo Gas Co. Ltd. being the industrial main players in thefield), we exclude this family of materials from the scope of thisreview, for which the reader will find authoritative surveys inrefs 41 and 74−80. However, when necessary, some examplesof high-temperature membrane concepts for postcombustionCO2 capture applications will be tackled. The application ofdense films (e.g., molten carbonates) has been less studied withonly a few publications (<20) and patents (<15) appearing inthe literature in the past years (Figure 5).Finally, the third and last family, what we call here “porous

inorganic membranes” or PIMs, encompasses purely inorganicor mineral membrane materials for H2 and CO2 separation.These materials can be classified into three differentsubfamilies: (1) silicas, (2) zeolites, and (3) MOFs. Carbonmolecular sieve membranes, although they can be classified as“inorganic”, are not considered here and accordingly have notbeen included in Table 3. As can be deduced from Figure 5,unlike polymer or dense metal membranes, PIMs cover areduced field of published papers and patents, but withsignificant progress in the past years. Among the three differentsubfamilies, silica membranes have been mostly applied to H2separations, with more than ca. 400 papers published and 20patents claimed with a remarkable activity of Noritake Co. Ltd.in the period 2000−2013. In the case of CO2 separation, ca.160 papers and only 4 patents appeared, attributed mainly toKyocera Corp. A similar trend has been observed for zeolitemembranes, with ca. 650 papers published and 11 patentsclaimed mainly by ExxonMobil on H2 separation applicationsand ca. 250 papers and more than 20 patents claimed on CO2separations with an important activity of Hitachi Shipbuilding& Engineering Co. Finally, in the field of MOF membranes,more than 160 and 110 papers have been published,respectively, on H2 and CO2 separations, with only 3 patentsand 1 patent, respectively, covering technological developmentsfor each separation, ExxonMobil being again the most dynamicactor. These three families of PIMs are at the core of thisreview. The main properties and potentials of the differentmaterials for H2 and CO2 industrial applications are describedin detail in the following sections.

1.4. R&D Concepts from Academia and Industry for CO2Capture Based on PIMs

To increase the efficiency and reduce the economic impact ofmembrane-based CO2 capture processes in both pre- andpostcombustion applications, Hagg and co-workers havepublished a series of technico-economical studies for biogasupgrading81 and postcombustion CO2 capture

82,83 focusing onpolymer materials. These studies underline the benefits ofmembrane cascade units including at least two modules, wherethe retentate stream of the second module can leave themembrane system or feed either the inlet or outlet of theformer one. Competitive CO2 capture costs in the range of$15−40 per ton of CO2 avoided are proposed by the authorsfor a two-module cascade system.83,84

In the case of PIMs, two recent developments have beenpublished and patented on the implementation of MFI zeolitemembranes and hollow fibers to postcombustion CO2 captureapplications in incineration plants (stationary CO2 capture)

85,86


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and in a 40 tonne heavy vehicle consuming 25 L/h diesel fuel(on-board CO2 capture).87−89 Figures 6 and 7 reproduce,respectively, the flowsheet of each process. On one hand, theCO2 capture cost in the former application was estimated to be$150−180 per tonne of CO2 avoided (cascade of threemultitubular modules operating under vacuum permeate,Figure 6), which is much larger that the cost range proposedby Hagg and co-workers for polymer membranes (vide supra).This difference is mainly attributed to the cost of aluminasupports, a limitation that has been traditionally argued as adeterrent for porous inorganic (or molecular sieve) membraneapplications. The quest for affordable supports for inorganicmembrane design is therefore imperative to allow membrane

implementation. This point is tackled in section 5. On the otherhand, the on-board CO2 capture solution depicted in Figure 7consists of a cascade of two modules based on MFI−aluminahollow fibers also operating under vacuum permeate. Thesystem includes a first step of water removal and cooling at 303K to optimize the permeation performance of the MFI hollowfibers. The resulting exhaust gas is then submitted to thehollow-fiber unit to capture at least 75% (molar basis) of theCO2 present in the stream and further evacuated to theatmosphere. The concentrated CO2 permeate stream leavingthe second module (CO2 purity ca. 97%) is compressed tosupercritical conditions at a pressure up to 100 bar and roomtemperature to be in situ stored in high-pressure reservoirs in

Figure 6. Flowsheet of a membrane cascade system based on three modules for postcombustion CO2 capture from a standard incineration planttogether with the operation conditions and main characteristics of each unit. Adapted with permission from ref 85. Copyright 2012 Wiley.

Figure 7. Flowsheet of a hollow-fiber unit for in situ CO2 capture in a heavy vehicle (the heat exchangers in each compression are not included forsimplicity). Adapted from ref 87. Copyright 2009 American Chemical Society.


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the vehicles and further removed (for instance, during fuelrefilling). The energy consumption of this solution wasestimated at <6% for an autonomy of about 10 h.Besides these applications for postcombustion CO2 capture,

some interesting developments have also been disclosed inrecent patents for precombustion CO2 capture in reforming/WGS units. Okada et al.90 at Renaissance Energy ResearchCorp. have recently patented a process involving a membranereactor for WGS reaction of syngas to produce H2 including afirst CO2 and steam separation from H2 using an inorganicmembrane (molten carbonate) followed by a second unit forCO2/steam separation and further steam recycling to a formerreforming unit. Niitsuma et al.91 at Nippon Oil Corp. havedisclosed a process for producing H2 and recovering CO2 aftercarbon gasification followed by the WGS process combining afirst PSA unit for H2 separation and purification with twosubsequent membrane-based units for CO2 and H2 separation,respectively. The CO2 separated in the first membrane unit(based on zeolite or amine-containing silica membranes) isliquefied and stored, whereas the H2 separated in the secondmembrane unit (based on microporous silica or zeolitemembranes) is recycled to the PSA unit to increase the H2partial pressure and facilitate its purification. Ziaka andVasileiadis92 have patented a similar concept for methane andhydrocarbon reforming units, but the membrane material (e.g.,microporous silica) was chosen to be simultaneously selectivefor CO2 and H2 for further methanol production. Finally,Doong et al.93 have developed a membrane reactor system forWGS reaction of syngas combining simultaneous removal ofhydrogen sulfide prior to WGS reaction and separation of H2and CO2 after the reaction. The invention includes threedifferent membranes in the same unit with a dense metal

carbonate or metal oxide membrane for CO2 separation at hightemperature (300−600 °C).1.5. PIMs for CO2 Capture: The Triple Challenge

Favre28,29 has recently published two incisive technico-economical studies on the potentials of membrane-basedtechnologies for CO2 capture. The main conclusion drawnfrom these studies is that membranes might potentiallycompete with chemical absorption in terms of energy demands(<0.75 GJ/tonne of CO2 avoided) in postcombustion CO2capture applications for flue gases with a CO2 composition >20vol % (e.g., cement production). At lower compositions,postcombustion CO2 capture might only be feasible at a level of1−2 GJ/tonne of CO2 avoided given the present state-of-theart of polymer membranes and PIMs. Membranes can also becombined with other technologies (e.g., absorption) formitigating the energy costs.To make membranes more competitive in terms of both

OPEX and CAPEX for postcombustion CO2 captureapplications (this point is certainly extensible to precombustionapplications), Favre28 has recently evoked three strategic andsynergistic challenges (the Holy Trinity of membranes!):(1) Selectivity challenge. A minimal CO2/N2 permselectivity

of 100−200, corresponding to a mixture separation factor ofabout 10−20, appears to be essential to reduce the energy costs(OPEX) down to the level of 2 GJ/tonne of CO2 avoided.(2) Energy challenge. Even with a membrane showing a CO2/

N2 permselectivity >200, a flue gas with a 10% molar fractioncan hardly be subjected to a simultaneous 90% CO2 recoverywith 90% CO2 purity in the permeate in a single step.Multistage operation should be applied if these targets are to befulfilled, involving an energy requirement estimated to a value>4 GJ/tonne of CO2 (OPEX) for a membrane permselectivity

Table 4. Summary of the Structural Features of Silicas, Zeolites, and MOFs with Potential for Membrane Design for CO2Capture


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>120.28 Increasing the membrane selectivity might onlymoderately decrease the energy demand. The use of vacuumoperation in the permeate combined with moderate compres-sion in the feed might reduce the energy requirements down tothe threshold level of 2 GJ/tonne of CO2.

28,94 Overall, thispoint is consistent with the general conclusions drawn byMerkel et al.64 on a recent technico-economical study.However, Koros and co-workers95 have pointed out thathowever the energy consumption can be dramatically reduceddown to a level of 0.50 GJ/tonne of CO2 avoided with acomplete heat integration plant design.(3) Productivity challenge. The required membrane area for a

given separation can be reduced by narrowing the effective layerthickness and promoting the intrinsic gas permeance (>0.1μmol·m−2·s−1·Pa−1), mitigating the capital costs (CAPEX).Increasing the gas permeance is usually considered easier toachieve than boosting the membrane selectivity. Moreover, themembranes have to keep a high permeability under realoperation conditions, especially in the presence of high waterpartial pressures, and in some cases resist harsh environmentsdepending on the combustion/syngas generation conditions,respectively, for post- and precombustion applications (e.g.,presence of NOx and SOx, Table 2).Table 4 lists the main textural and structural properties of the

different silica, zeolite, and MOF/zeolitic imidazolate frame-work (ZIF) materials considered in this review offeringpotential perspectives for membrane design in different pre-and postcombustion CO2 capture that scenarios. Whenrequired, additional materials will be portrayed to provide asystematic view of the field.To properly compare the results reported by different

authors, the following classical definitions are hereinafter usedfor PIMs (see additional definitions for polymer membranesand MMMs in refs 96 and 97):(1) Gas permeance (Πi), flux within the membrane (Ni)

divided by the log difference of partial pressure between theretentate and permeate (ΔPi)

Π · · · =Δ

− − − NP

(mol m s Pa )ii

i

2 1 1

(1)

(2) Gas permeability (Pi), gas permeance multiplied by thetop-layer effective thickness

· · · = Π− − −P (mol m s Pa )i i1 1 1

(2)

(3) Permselectivity (Pij), quotient between the pure gaspermeances of species i and j

=ΠΠ

Piji

j (3)

(4) Separation factor (SFij), feed-to-permeate ratio betweenthe molar compositions of species i and j

=y x

y xSF

/

/iji i

j j (4)

2. MEMBRANE SEPARATION: ADSORPTION VSDIFFUSION SELECTIVITIES

The first idea to keep in mind when developing a porousmembrane for a target separation is to choose a material with aconvenient affinity for the desired species to separate. As statedabove, the term “affinity” has to be regarded in a broad sense,

comprising not only the favorable interaction of one or moremolecules from a mixture with the solid, but also its/theirpromoted/inhibited motion within the porous framework. Twokey selectivities then emerge (i.e., adsorption and diffusion)which should be maximized at the first step of membranedesign.Unfortunately, systematic studies providing concomitant

adsorption and diffusion properties of membrane materialsfor CO2/N2 and CO2/H2 separations are dramatically missing.In silico studies relying on molecular simulations appearpossible and feasible to fill this gap,111,112 but only if accurateinformation about sorbate arrangement (e.g., presence ofclustering effects, description of “pockets”) and correlationeffects between the diffusing sorbate species, as well as detailedinformation on the membrane microstructure (by far the mostcomplex aspect!), is well-known and conveniently described.These questions will be tackled in more detail in section 4. Ouraim in this section is to address the main adsorption anddiffusion mechanisms allowing CO2 discrimination in micro-and mesoporous frameworks that can be exploited formembrane design with a devoted connection to the frameworkstructure and surface chemistry.

2.1. Adsorption Mechanisms

The adsorption properties of a solid sorbent are governed bythe nature and strength of force fields and their distributionalong the active surface and pores. These interactions dependon the structure, framework composition, crystal size, andpurity of the sorbent. At first glance, one can make use of twomain forces for sorbate discrimination: (1) electrostatic forces(e.g., polarization forces, surface field−molecular dipole andsurface field gradient−molecular quadrupole interactions), and(2) van der Waals or nonspecific forces, directly correlated withthe sorbate molecular polarizability. Electrostatic forces arestrongly linked to Lewis acid−base interactions and segregationand clustering effects at the molecular level, while van derWaals forces increase with the degree of sorbate confinementfollowing the classical analysis of Derouane.113−115 Thedifferent energetic contributions to adsorption of a givensorbate (in particular CO2) can be discriminated experimentallyby calorimetric techniques. This point is addressed below foreach family of materials.CO2 and N2 adsorption on polar surfaces is mainly promoted

by surface field gradient−molecular quadrupole interactions.Conversely, adsorption of large nonpolar molecules (e.g.,hydrocarbons) is essentially ascribed to molecular polarizabilityand in some cases configuration entropy effects.116 Keepingthese ideas in mind, some general trends on the adsorptionselectivity can be established that will be exposed at the end ofthis section. A list of the most relevant CO2, CH4, N2, and H2adsorption data on amine-functionalized silica, zeolite, andMOF materials, including a broad compilation of Henry’sconstants and heats of adsorption, can be found in theSupporting Information (Tables S2−S5). Nonspecific forcesplay an important role in N2 and H2 adsorption, leading insome cases to compensation effects between the enthalpy andentropy of adsorption.117,118

2.1.1. Amine-Functionalized Silicas. 2.1.1.1. Chemicalvs Physical Adsorption. Micro- and mesoporous silicamaterials show an inherent CO2 adsorption capacity due tothe presence of surface silanol groups (usually in the range of1−5 SiOH groups/nm2) with weak alkaline behavior, providingonly modest CO2/N2 and CO2/CH4 adsorption selectiv-


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ities.116,117 Indeed, the zero-loading isosteric heats of CO2, N2,and CH4 adsorption on TPABr-templated amorphous silicasare as low as 24, 21, and 22 kJ/mol, respectively. Theadsorption strength becomes even more moderate in thepresence of water vapor, blocking partially the accessibility ofCO2 to the silica micropores.The intrinsic CO2 adsorption strength of silicas can be tuned

either by impregnating Lewis acids such as imidazolium-basedionic liquids (ILs),119 or by incorporating alkaline surfacegroups such as amines (e.g., physically adsorbed, covalentlygrafted, or by formation of SiNH2 surface groups viaammonolysis), taking advantage of the well-known aminechemistry gained from chemical adsorption in wet scrubbers.The final adsorption properties depend not only on the natureof the amine (i.e., primary, secondary, or tertiary), but also onthe loading of accessible amine surface groups and theirinteraction with the silica walls. Jones and co-workers120 haverecently published a devoted review on the potentials ofamine−oxide hybrid materials for acid gas separation (includingCO2).In addition to electrostatic forces, the interaction between

CO2 and alkaline amines can be governed by two additionalmechanisms involving, respectively, the formation of surfacecarbamates (C−N bonds) and ammonium (bi)carbonates(monodentate and bidentate). In the former mechanism,amine moieties behave as both Lewis and Brønsted bases,promoting the formation of C−N bonds as acid−base adductsthrough a mechanism involving zwitterion intermediates with amaximum amine efficiency of 0.5 mol of CO2/mol of N. Theamine efficiency increases with its alkaline character andloading, promoting deprotonation of the zwitterion intermedi-ate.121,122 Characteristic IR bands for carbamates have beenreported at 1595−1680 cm−1 (ν(N−H), ν(CO)), 1441−1563 cm−1 (νas(COO

−), δ(N−H)), and 1330−1430 cm−1

(νs(COO−)),122−125 while characteristic 13C NMR resonances

have been reported at 160 and 164 ppm, respectively, forcarbamates and carbamic acid.126

In contrast, under humid conditions, amines behave solely asBrønsted bases, promoting a maximum amine efficiency of 1.0mol of CO2/mol of N. Water can also stabilize the aminesurface groups upon CO2 adsorption/desorption cycles.127 Theformation of (bi)carbonate surface species during humid CO2adsorption yields IR bands centered at 1470−1493 and 1422−1432 cm−1, respectively, for monodentate and bidenatebicarbonates and at 1337−1363 and 1541−1575 cm−1,respectively, for monodentate and bidentate carbonates.128−130

Note that the presence of (bi)carbonates does not exclude theformation of carbamates at the early stage of adsorption,transforming further into more stable (bi)carbonates. Thenature and number of (bi)carbonate species can also changeduring CO2 adsorption due to partial displacement of CO2 bywater, evolving from mono- to bidentate with stronger bindingenergies. The relative importance of each mechanism dependson the nature of the amine moieties and on the presence ofhumidity in the gas phase.118,131 In general terms, primary andsecondary amines (sterically hindered or not) tend to reactdirectly with CO2, forming carbamates, whereas (bi)carbonatesare favored for tertiary amines with a more pronounced alkalinenature.2.1.1.2. Synthetic Approaches for Amine Loading. Within

this general framework, three approaches can be in principleconsidered for amine loading: (1) incorporation of aminegroups on silica walls either by direct co-condensation of

aminoalkosilanes during the synthesis, (2) incorporation ofamino groups by impregnation or grafting, and (3)ammonolysis, involving the direct generation of SiNH2 moietieswith no need for an intermediate carbon chain.The simplest method for incorporating amines in silicas is by

wet impregnation of an amine polymer, most often poly-(ethylenimine) (PEI), tetraethylenepentamine (TEPA), oreven dendrimers, from a solution and further evaporation ofthe solvent. The amine polymer is stabilized by the formationof hydrogen bonds with surface silanol groups, which facilitatesits distribution throughout the pore volume. This interactionresults in a decrease of the maximum decomposition temper-ature with the polymer loading.132 This approach, generatingthe so-called “molecular baskets” in the case of mesoporoussilicas,133−140 is discouraged for membrane functionalization,since a significant amount of amine groups can be stericallyhindered within the pore volume and also form agglomeratesbetween particles, both aspects affecting negatively the CO2adsorption/diffusion properties and being at the origin ofanomalous adsorption patterns with temperature (i.e., increaseof CO2 loading with increasing temperature). A similarconclusion can be in principle drawn for recently reporteddual impregnated tethered amine−silicas.141 However, CO2uptakes higher than 1.2 mmol/g can be achieved at lowpressures (<10 kPa) for PEI-impregnated SBA-15, even atatmospheric concentrations (i.e., ca. 400 ppm), either under Zrdoping at the optimum value of 0.07,142 or in the presence ofother metal heteroatoms (e.g., Al, Ti, and Ce143), by finelytuning the acid/base surface properties of the silica backbone.Furthermore, although amine-impregnated mesoporous alumi-nas might show better stability than silica counterparts,144 theiruse is also discouraged for membrane design due to thecomplex manufacture of mesoporous alumina films.The general limitations of amine-impregnated silicas can be

overcome by grafting the silica surface with convenient amineprecursors following the pioneering study of Leal et al.145 onsilica gel, providing in some cases a partial control of the localdistribution of the amine moieties (see section 4.2.4.2 for moredetails). Figure 8, top, shows examples of CO2 isotherms onamine-tethered MCM-41 and SBA-16 silicas. This strategy isattractive for membrane design, since amine grafting mightavoid pore blockage and amine leaching. Given the potentials ofsuch materials, the Supporting Information collects a list of themost outstanding and recent CO2 adsorption properties ofmono-, di-, and triamine-tethered mesoporous silicas comparedto parent silicas (Table S2). Furthermore, Figures S1−S4compare the CO2 adsorption patterns of some representativeamine-functionalized silica materials as a function of the porearchitecture, the nature and loading of amine groups, thetemperature, and the presence or absence of humidity in thegas stream.A first possibility for grafting amines on silica is via a two-step

process involving a first functionalization step with alkyl halidegroups followed by SN2 reaction with the amine precursor.122

Amine groups can also be directly grafted via condensation ofprimary and secondary (aminoalkyl)trialkoxysilanes with sur-face silanol groups. The final amine loading is limited by thesurface area, the number of accessible silanol groups, the alkylchain (most often methyl), and the synthesis conditions.Although the amine loading increases in the sense mono-aminosilane < diaminosilane < triaminosilane, the number ofgrafted silane molecules follows the inverse order due to stericconstraints, leaving some silanol groups inaccessible to the


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silane precursor. The maximum amine loadings reported so farusing primary mono-, di-, and triaminosilanes fall into theranges 1.3−5.1, 1.6−5.5, and 4.6−8.0 mmol of N/g ofadsorbent, respectively, depending on the silica material.121

The amine grafting efficiency can be improved by includingwater in nonanhydrous media during postsynthesis, reducingthe density of residual silanol groups, as well as by increasingthe specific surface of the silica material.146 The incorporationof amine groups in mesoporous silicas usually encompasses apore size contraction ascribed to pore occupation. However,the pore size can be increased via postsynthetic incorporationof pore-expanding amine agents (e.g., dimethyldecylamine,DMDA), tuning the pore size in MCM-41 materials in therange of 3.5−20 nm.147

The grafting of hyperbranched amines has also beendemonstrated by Jones and co-workers on HMS silica,providing CO2 loadings as high as 5.5 mmol/g at 298 K anda CO2 pressure of 10 kPa.32,148,149 These loadings are usuallylower than the values that can be commonly reached withimpregnated silicas (up to 8.0 mmol/g under simulated flue gasconditions for mesoporous silica capsules150), but at theexpense of a higher stability of the amine−silica hybrids andsilica mesophases even after exposure to moisture.151 Hyper-branched amines might be relevant for membrane design if theamines can be preferentially localized near pore mouths.Finally, amines can also be incorporated onto silica supports

by ammonolysis to generate alkaline SiNH2 and −Si−NH−Si−

surface groups on KCC, SBA-15, and MCM-41 silicas byreaction of surface silanol and −Si−O−Si− moieties withammonia at temperatures between 1073 and 1373 K. Such aconcept, recently reported by Polshettiwar and co-workers152

for CO2 adsorption, allows the generation of highly stablesilicas compared to aminoalkane hybrid counterparts withbetter accessibility to N sites. However, to our knowledge, thisapproach has not yet been demonstrated for membrane design.

2.1.1.3. Amine Stability. Detailed experimental studies onthe stability of amine-functionalized silicas are scarce. Sayariand co-workers153 studied the stability of PEI-impregnated andamine-grafted MCM-41 and SBA-15 silicas with mono-, di-, andtriamines after several thermal and chemical treatments. Exceptfor secondary monoamines, these authors observed anextensive degradation of the different amine-impregnated andamine-grafted silicas in the presence of dry CO2, particularly athigh temperature. However, in the presence of moisture, theCO2 uptake on PEI-modified SBA-15 remained essentiallyunchanged. Interestingly, the authors attributed this differentstability to the formation of stable urea linkages at the expenseof amine groups under dry CO2, preventing the formation ofsurface carbamates. Tanthana and Chuang154 also reported apartial degradation of TEPA-impregnated silica after treatmentwith dry CO2 by the formation of carboxylate species. Thestability of the samples could be remarkably improved by thesimultaneous impregnation of polyethylene glycol (PEG) withTEPA by slowing the formation of carboxylate species uponCO2 adsorption via the formation of hydrogen bonds betweenTEPA (NH2/NH) and PEG (OH). The introduction ofnonionic surfactants together with the amine promoted thedispersion of the latter within the silica pores.155

Amine-functionalized silicas can also suffer from degradationin the presence of air. Two devoted studies have recently beenreported tackling this issue. Sayari and co-workers156 reported astrong degradation of PEI-impregnated SBA-15 upon exposureto air even at moderate temperatures, whereas it remainedstable in the presence of humidified gases containing both CO2and O2. In the case of amine-grafted silicas, Jones and co-workers157 observed a remarkably higher stability of primaryand tertiary monoamines on propyl linkers supported on silicamesocellular foams (MCFs) than in the case of secondarymonoamines. The latter authors also found that oxidation ofsecondary amines led to concomitant degradation of terminalprimary amines in diamines. Similar conclusions were reportedby Sayari and co-workers158 on PE-MCM-41 silicas grafted withprimary, secondary, and tertiary monoamines, secondarymonoamines being extensively deactivated upon direct contactwith a stream containing moisture and oxygen. Under similarconditions, triamine-grafted silicas also suffered from strongdeactivation compared to monoamine-grafted counterparts.

2.1.1.4. Amine Efficiency for CO2 Adsorption. Triamine-grafted mesoporous silicas usually provide higher CO2adsorption capacities than mono- and diamine counterpartsdue to their higher amine contents at comparable load-ings.121,146,159−163 Interesting comparative studies at low CO2pressures (<15 kPa) under anhydrous dry conditions have beenreported by Hiyoshi et al.159 on SBA-15 functionalized withmonosilane, [3-(methylamino)propyl]trimethoxysilane(MAPS), and [3-(dimethylamino)propyl]trimethoxysilane(DMAPS), by Sayari and co-workers146,160−163 on PE-MCM-41 grafted with mono- and triamine groups, and by Llewellynand co-workers121 on SBA-12 grafted with (3-aminopropyl)-trimethoxysilane (APS), MAPS, and [3-(phenylamino)propyl]-

Figure 8. Isosteric heats of CO2 adsorption under dry conditions forPE-MCM-41 and SBA-16 silicas and for the corresponding amino-grafted counterparts. The amino moieties were 2-[2-(3-trimethoxysilylpropylamino)ethylamino]ethylamine for PE-MCM-41and TEDA = N-[3-(trimethoxysilyl)propyl]ethylenediamine for SBA-16. Data adapted, respectively, from Serna-Guerrero et al.165 andKnofel et al.167


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trimethoxysilane (PAPS). The latter authors reported amineefficiencies as high as 0.49, 0.45, and 0.43 mol of CO2/mol ofN, respectively, for monosilane, MAPS, and PAPS groups. Kimet al.164 also reported high amine efficiencies for silicas preparedvia anionic surfactant mediated templating (up to 0.428 mol ofCO2/mol of N). The presence of a partially occluded nonionicsurfactant (e.g., poly(propylene oxide)) in amine-impregnatedmesoporous silicas can promote the CO2 adsorption capacityby dispersing the guest species, as well as by stabilizingcarbamate intermediates via specific interactions between CO2,the amines and hydroxyl groups.133

CO2 isotherms in amine-functionalized silicas are charac-terized by a sharp increase of the CO2 loading at low partialpressures, most often about 20−30 kPa, involving Henry’sconstants up to 2.8 mmol·g−1·kPa−1 driven by moderately highheats of CO2 adsorption in the range of 48−150 kJ/mol (zero-loading value) due to the formation of carbamate and(bi)carbonate species127,129,165−169 compared to 20−35 kJ/mol found for unmodified silicas.124,170 The isosteric heats ofadsorption in amine-loaded silicas usually show a drasticdecrease with the CO2 loading (Figure 8, bottom), suggesting ahigh degree of heterogeneity of the samples. The largerenthalpies of adsorption at low coverage correlate with thestrong reactivity between CO2 molecules and the amine sites,while the lower enthalpies at higher loadings reflect a majorcontribution of electrostatic interactions. Grand canonicalMonte Carlo (GCMC) simulations can be used to quantifythe relative proportion of chemically/physically adsorbed CO2in a silica surface in the presence of amine moieties.171

The distribution of amine moieties on amine-grafted silicasurfaces can play a key role in CO2 adsorption under dryconditions, since carbamates involve the participation of twoamine moieties for adsorbed CO2. To tackle this issue, Stranoand co-workers172 proposed a semiempirical isotherm equationallowing a straightforward computation of the adsorptionisotherm of an arbitrary surface configuration of grafted aminesfor different lattices with a given surface distribution (i.e., z-histogram). This modeling approach can be useful foradsorbent and membrane design, since it addresses the optimaldistribution of amine moieties on a silica surface for a givenamine function to promote CO2 adsorption. From a practicalviewpoint, attempts have been reported to promote the localvicinity of tethered amine groups using a carbamate-protectedamine tether such as [3-(triethoxysilyl)propyl]-tert-butylcarba-mate (TESPtBC) followed by carbamate removal under mildheating to “deprotect” the amine, but with a modest success.173

2.1.2. Zeolites. Zeolites are metastable (alumino)silicateswith defined topologies generating an intracrystalline networkof cavities and channels with a rich surface chemistry dependingmainly on the Si/Al ratio and the amount and nature ofexchangeable cations. Indeed, cations in zeolites can behave asLewis acid sites, while framework oxide anions near cationsmight act as Brønsted/Lewis basic sites and silanol groupsmight behave as Brønsted acid sites. Figure 9 plots somecomparative CO2 adsorption trends in different zeolitesbelonging to the MFI, FAU, CHA, and DDR families. Detailedadsorption data arranged by zeolite families are reported in theSupporting Information (Figures S5−S8). Also in theSupporting Information, Tables S3 and S4 compile a broadseries of CO2, N2, and CH4 adsorption data on different zeoliteframeworks (i.e., LTA, MFI, FAU, CHA, DDR, MOR, BEA,natural erionite, and clinoptilolite) in terms of maximum CO2

adsorption capacity, Henry’s constants, and (isosteric) heats ofadsorption.We describe below the main implications and consequences

of the surface chemistry and zeolite topology for CO2adsorption and discrimination from other gases in view ofmembrane design. We also illustrate the main differences withamine-functionalized silicas (section 2.1.1) and some commonand divergent points with MOFs (section 2.1.3).

2.1.2.1. Effect of the Zeolite Topology. The CO2 adsorptionproperties of zeolites are strongly influenced by their topologyby locally tuning the energy interactions, promoting in somecases size and shape selectivity, clustering, and segregationeffects. The effect of the zeolite topology is not usuallystraightforward, since most zeolites can hardly be synthesized inpure silica form or at similar compositions (i.e., similar Si/Alratios and cation amounts) allowing a proper comparison.Remarkable exceptions to this rule are pure siliceous MFI(silicalite-1),179 DDR (clathrasil DD3R),180,181 CHA,182,183 andLTA (ITQ-29).184 To avoid this shortcoming, GCMC andmolecular dynamics (MD) simulation tools have beendeveloped in recent years affording the study of all-silicazeolite topologies, underlying the main characteristics andtextural properties of zeolites favoring CO2 adsorption. Acrucial point in these simulations is the use of accurate sets offorce field parameters to properly describe fluid−fluid andespecially solid−fluid interactions, the latter parameters mostoften being validated by fitting to experimental adsorption data,although some recent efforts have been made to deriveparameters from ab initio calculations.185,186

Fischer and Bell187 reported in a recent study acomprehensive analysis of 18 different pure silica zeolitetopologies for postcombustion CO2 capture. This study showsintrinsic isosteric heats of CO2 adsorption in the range of 16−34 kJ/mol for conventional zeolite topologies. For mostsystems, the isosteric heat of adsorption showed a slightincrease with the CO2 loading, reflecting the presence of lateralinteractions between neighboring guest CO2 molecules due to ahomogeneous distribution of interaction energies. Lateralinteractions could promote clustering effects between guestmolecules below the critical point, triggering unexpectedsorption patterns with temperature deviating from the classicalArrhenius behavior.188

Figure 9. Comparison of the CO2 adsorption isotherms at 303 K for aseries of zeolite frameworks with different Si/Al ratios. Data adaptedfrom Walton et al. (NaZSM-5),174 Barrer and Gibbons175 (NaX,NaY), Li et al. (SAPO-34),176 Cui et al. (T),177 and Himeno et al.(DDR).178


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The study of Fischer and Bell187 also points out a smallnumber of pure siliceous zeolite topologies (i.e., GME, MOR,and RHO) with a heterogeneous distribution of interactionenergies encompassing strong sites localized in specificpositions, whereas the interaction in other areas is compara-tively weaker. In this situation, fluid−fluid interactions betweenneighboring guest molecules tend to be weakened and theisosteric heat of CO2 adsorption shows a decreasing trend withthe loading, as is commonly observed in aluminosilicatezeolites. This site configuration is regarded as more attractivefor CO2 separation applications, especially when the zeolitetopology maximizes regions with nearest-neighbor frameworkatom distances in the range of 3−4.5 Å.189 The presence of abroad distribution of adsorption sites with different strengthscan promote segregation effects in cage-type zeolite topologies(e.g., LTA, FAU, and CHA) due to competition between siteslocated in the cavities or near the windows.Size and shape selectivity effects in zeolites can play a

relevant role for CO2 separation applications in narrow-porezeolites on the basis of the smaller kinetic diameter of CO2compared to those of CH4 and N2 (3.30, 3.64, and 3.80 Å,respectively, Table 2). Separation by molecular sieving canprovide a priori size and shape selectivity, such as CO2/N2separation in ETS-4 (pore size 3−4 Å).190 However, pure sizeand selectivity effects for separation in zeolites are rare, theseusually being linked to the zeolite topology and surfacechemistry. In contrast, molecular sieving effects can bepromoted by preferential diffusion, tuning the selectivity of agiven zeolite to CO2 (see section 2.2.1).2.1.2.2. Effect of the Surface Chemistry: Si/Al Ratio and

Exchangeable Cations. The Si/Al ratio leads to significantdifferences in the ability of zeolites to adsorb CO2 by tuning theamount and position of exchangeable cations, as well as theacid−base properties of the zeolite framework. Among thecommon zeolitic structures (in Na form for LTA, MFI, andFAU), faujasites offer the highest CO2 adsorption capacities(up to 5−10 mmol of CO2/g at 101 kPa and 303 K; FiguresS5−S8, Supporting Information). The CO2 affinity increases inthe order NaY < NaX < NaA (Supporting Information, TableS4). This trend is explained by the presence of a higher amountof type III cations at the entrance of supercage cavities, showingpreferential interaction with CO2,

191 and by a decrease of thepore size. Corma and co-workers184 showed for a series ofisomorphous LTA zeolites (i.e., ITQ-29) that the CO2adsorption capacity increases dramatically with the Si/Al ratio(Figure 10, top). This property is outstanding for membranedesign in view of preferential CO2 separation by finely tuningthe polarity and surface chemistry of the LTA framework forspecific separations.Aluminosilicate zeolites show Henry’s coefficients for CO2

adsorption that are systematically 1−2 orders of magnitudelarger than those of N2 and CH4 for the different zeolitesconsidered in Table S3 (Supporting Information). Thesedifferences can be attributed to the higher heat of CO2adsorption compared to the heats of N2 and CH4 adsorption(compare, for instance, the value of 34.3 kJ/mol given in TableS4, Supporting Information, for the mean adsorption enthalpyof CO2 in zeolite 13X and the values of 13.8 and 16.7 kJ/molfor N2 and CH4). As an example, Figure 11 plots the evolutionof the CO2 and N2 Henry’s constants on X-type zeolites fordifferent framework cations, showing a trade-off betweenelectrostatic cation−quadrupole and acid−base interactionsfor CO2 adsorption. This trade-off is at the origin of a

maximum of the CO2 Henry’s constant for Rb-exchanged X-type counterparts, making this cation a priori more suitable interms of CO2 adsorption strength. Figure 11 also shows adecrease of the maximum CO2 adsorption capacity with thecation size, this observation being attributed to the reduction ofthe number of supercages (1 g of LiX possesses moresupercages than 1 g of RbX!).The framework basicity of zeolites increases with the

framework Al content and with the cation electropositiv-ity.192,193 Kiselev and Du194 estimated values up to 55% and23%, respectively, for the contribution of ion−quadrupole

Figure 10. Effect of the Si/Al ratio on the isosteric heat of CO2adsorption for a series of ITQ-29 zeolites displaying the LTA. Imagesadapted from ref 184.

Figure 11. Evolution of the CO2 and N2 Henry’s constants formonovalent−divalent X-faujasites. Data obtained from Walton et al.196


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interactions to the total gas−solid interaction energy of CO2adsorption in zeolite NaX. The hypothesis of a truly chemicalinteraction between adsorbed CO2 and alkaline cations issupported by the classical paper of Ward and Habgood,195

reporting characteristic IR bands assigned to carbonate speciesfor CO2 adsorption in LiX, NaX, and KX zeolites. Largercations such as Rb or Cs providing stronger framework basicitymight compensate electrostatic cation−quadrupole interactions,promoting the affinity for CO2.

196 The framework basicity ofzeolites can also be tuned by incorporating amine groups to thezeolite framework, resulting in some cases in a phase change ofthe zeolite and in the formation of Si−N−Si moieties (e.g.,methylamine adsorption in high-silica MFI197). Some examplesof MEA- and TEPA-impregnated zeolites reveal an increase ofthe CO2 adsorption properties via chemical interactions as inamine-modified silicas,198−200 but at the expense of a drasticreduction of the specific surface of the modified zeolite.Although some examples of amine-grafted large-pore zeolites(e.g., APDMES in NaX) have been reported,201,202 thesematerials also show a dramatic reduction of the specific surfaceeven under optimized conditions (e.g., choice of a solvent witha convenient dielectric constant), making them not suitable apriori for membrane design.The cationic form of a given zeolite plays a relevant role in its

adsorption properties and energy heterogeneity. The nature ofcations and their location within the zeolite framework mightinfluence the molecular sieving properties of zeolites. Aparadigmatic example can be found in K-and Cs-exchangedchabazites, showing striking molecular trapdoor properties by atemporarily reversible cation displacement from the center ofcage windows induced by guest accommodation.203 Anotherinteresting example can be found in Na-rho zeolite, displaying asorbate-induced structural modification upon CO2 adsorp-tion.204 Moreover, cations can also influence the electric fieldand the acid−base properties in zeolite micropores. Bajusz etal.205 showed that low degrees of Ca2+ exchange in NaX zeolites(<20%) reduce the amount of type I Na+ cations, nonaccessibleto CO2 during adsorption, promoting the location of Ca2+

cations in sites II and III (Figure 12, top right) and in its turncation−quadrupole interactions. This behavior is inversed withthe introduction of Mg2+ and La3+ cations in NaX zeolites, mostof the ions in Mg−Ca and La−Ca zeolites not interactingdirectly with sorbate molecules.206

Theoretical studies on CO2 adsorption in cage-type zeolitesreveal that sorbed CO2 molecules locate preferentially near typeII and III Na+ cations (Figure 12, top right, for FAUzeolites).185,208−210 Neutron powder diffraction and MDsimulations provide evidence of cation rearrangement in FAUzeolites (dehydrated) at low and intermediate CO2 loadings,Na+ cations either migrating from site II to the supercage center(NaY) or being displaced from site III′ to a nearby vacant site(NaX).211,212 At 4 K, CO2 molecules locate preferentially intwo crystallographically independent sites bonding to Nacations (Na10) in site II, showing, respectively, a linearconfiguration (interaction with Na10 via one terminal oxygen)and a bent O−C−O configuration (148.3°) with both oxygenatoms coordinating to two symmetry-related Na10 atoms (i.e.,carbonate precursor). Dehydration at high temperatures wasreported to affect in some cases the cation distribution withinthe zeolite framework (e.g., zeolite L), promoting an “intrinsicion exchange”.213 The CO2 adsorption capacity and strength ofa zeolite can also be promoted by “artificially” modifying therelative position of Na+ cations in the framework by subjectingthe samples to, for instance, post-treatment with a NaOHsolution in the presence of a kaolin binder, the latter being inturn partially converted into zeolite 13X.214

MFI and FAU zeolites show increasing CO2 adsorptioncapacities in the sense silicalite < HZSM-5 < NaZSM-5 <KZSM-5 < BaZSM-5, LiX > NaX > KX and CaX < SrX < BaX(Supporting Information, Figures S6 and S7). The higher CO2adsorption capacity of MFI zeolites with larger cations isattributed to strong electrostatic interactions between CO2 andcations, in turn being ascribed to a stronger electrical field inthe zeolite channels (6.8 V/nm in NaZSM-5 using CO as theprobe molecule, ion−quadrupole interactions contribu-ting about 85% to the overall electrostatic effect of Na+

cations215). Moreover, larger cations show lower acid character,contributing to a higher basicity of the MFI framework. Na-and Ba-exchanged ZSM-5 zeolites are considered to adsorbCO2 more strongly on cation sites than on pore walls.Zero-loading isosteric heats of CO2 and N2 adsorption in

most zeolite frameworks including cations increase with the ioncharge density in the order Cs < Rb < K < Na < Li and Ba < Sr< Ca < Mg on the basis of the stronger polarizing power ofsmaller cations, contributing to stronger cation−quadrupoleelectrostatic interactions and shorter OCO distances.216−222 Aninteresting exception is CO2 adsorption in FAU-type zeolites,Rb and Cs samples showing stronger adsorption strengths thanLi, Na, and K counterparts. Pirngruber et al.223 revealedthrough DFT simulations that this peculiar trend in FAUzeolites can be explained by a stronger interaction of CO2 withcations located in hexagonal windows between supercages andsodalite cages and framework oxygen atoms in 12-MRconnecting two supercages. In the case of LTA zeolites,although the CO2 adsorption capacity is lower than for FAUzeolites due to their lower pore volumes, the CO2 affinity ismuch higher. Indeed, the isosteric heats of CO2 adsorption canreach a level of 70 kJ/mol in zeolite CaNaA (Table S4,Supporting Information) due to the formation of carbonateschemisorbed on framework cations, as reported by Masuda etal.224 from a series of IR studies on A-type zeolites (presence ofa characteristic peak centered at 1450−1460 cm−1). In the caseof Na- and K-FER zeolites (Si/Al = 8.6), bridged CO2complexes can be formed between two neighboring cationscharacterized by a characteristic IR band centered at 2370 and2357 cm−1 for Na- and K-FER, respectively.225

Figure 12. Isosteric heats of CO2 and N2 adsorption for silicalite(squares), NaZSM-5 (circles), and NaX (triangles) zeolites. Dataadapted from refs 196 and 207. Top right: different cations present inthe FAU framework (represented as circles).


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As a general trend, in the presence of cations, the isosteric/differential heat of CO2 adsorption for a given zeolite shows amarked decreasing trend with the CO2 loading, though lesspronounced than in the case of CO2 adsorption in amine-functionalized silicas, due to the participation of sites ofdifferent nature. Figure 12 provides comparative trends for CO2adsorption on pure silicalite-1, NaZSM-5, and NaX zeolites,while Figure 10, bottom, compares the isosteric heat pattern onITQ-29 zeolites displaying the LTA structure with different Si/Al ratios.2.1.2.3. Role of Water in CO2 Adsorption in Zeolites. Water

is known to influence the CO2 adsorption properties of zeolites.The CO2 capacity can be strongly inhibited in the presence ofhigh water partial pressures due to CO2 and water competitionfor the sorbent sites.226 Water adsorbs preferentially onexchangeable cations, reducing the electrical field in zeolitecavities and channels. For example, the CO2 adsorptioncapacity on CaX zeolite measured at 0.06 bar of CO2 and323 K shows a drastic decrease from 2.5 to 0.1 mmol/g for awater concentration increasing from 0.8 to 16.1 wt % (Figure13).227 A similar qualitative observation was reported by Liu etal.228 on NaX and NaA zeolites.In contrast, at low CO2 concentrations (<300 ppm), water

preadsorption might promote CO2 uptake due to the formationof surface bicarbonate species. This enhancement decreasesgradually as the CO2 concentration increases beyond >1000ppm, the CO2 loadings being lower than for dry CO2adsorption. Moreover, water adsorption on FAU-type zeolites

can affect the cation distribution and involve the formation ofcyclic hexamers located in 12-MR windows, altering the CO2adsorption pattern.229−232 Using elution chromatography,Choudary et al.233 found that the N2 adsorption strength inNaA and CaNaA zeolites can be strongly inhibited in thepresence of preadsorbed water or CO2 due to a reduction ofelectrostatic interactions between N2 quadrupoles and cations.In the case of CaNaA zeolite, water partially hydrolyzes theCa2+ ions to give Ca(OH)+ sites. A similar qualitativeobservation was reported by Coe and Kuznicki234 on X-typezeolites.In the case of zeolites with higher Si/Al ratios (e.g., MFI), the

water isotherm shows a type II behavior differing from thecharacteristic type I (Langmuir-type) trend observed in LTAand FAU zeolites according to the Emmett−Teller classi-fication. Water adsorption proceeds via the formation of waterclusters/islands in strong sites in MFI channels (e.g., surfacedefects or silanol groups in silicalite-1),235−237 involving amoderate reduction of the CO2 adsorption capacities uponwater preadsorption. This type II adsorption behavior canevolve into a formally type V pattern in silicalite-1 with neitherAl nor silanol groups in the framework (i.e., waterintrusion).238,239 The incorporation of germanium into theMFI framework also promotes the hydrophobic properties ofMFI zeolites compared to the incorporation of boron (Figure14), but with only a moderate effect on the CO2 adsorption

properties. A type V adsorption pattern was also reported forwater intrusion in pure-silica CHA, the isotherm showing aspring behavior and the presence of two kinds of watermolecules trapped in the Si-CHA cage: water moleculesstrongly bonded to silanol groups and a number of liquidlikewater molecules interacting with the former molecules.240

2.1.3. Metal−Organic Frameworks. Metal−organicframeworks (MOFs) constitute a new class of hybrid materialsconsisting of coordinated metal clusters by organic linkers.Some “robust” MOFs possess pore volumes and windowdiameters largely exceeding the values commonly found inzeolites. For the IRMOF series based on the MOF-5 structure,the pore size can be tuned from 0.38 to 2.88 nm by selectinglinkers of various sizes.241 Exceptionally high internal surfaceareas have been reported for Cu(bdc)teda,242,243 MOF-177,244,245 and MIL-101109 with values of 4000, 4500, and5300 m2/g, respectively. These properties open promising

Figure 13. CO2 adsorption isotherms (top) and Henry’s constants(bottom) on CaX at 323 K for different preadsorbed water loadings.Data obtained from Brandani et al.227

Figure 14. Water adsorption at 303 K on crushed Al-MFI (Si/Al =30), B-MFI-100 (Si/B = 100), B-MFI-50 (Si/B = 50), and Ge-MFI-10(Si/Ge = 10). Image adapted from ref 88.


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perspectives for both adsorbent and membrane design.Authoritative reviews on metal open framework structuresand functionalities and on adsorption and catalytic propertiescan be found in refs 246−258.CO2 adsorption in MOFs is driven by the framework

topology as in zeolites, as well as by the surface chemistry andvan der Waals and dipole−quadrupole interactions in pores andcavities. Unlike zeolites, the surface chemistry of MOFs offers apriori more opportunities for generating adsorption sites bytuning the nature of pending functional groups on the ligands,the nature of metal clusters, and the relative flexibility of theframework to promote cooperative intermolecular interactions.To this aim, simulation studies have been reported on thedesign of MOFs with suitable textural properties and activesites by promoting Lewis acid and base and electrostaticinteractions, as well as confining effects.259−262

Some relevant CO2 adsorption properties for promisingMOFs in view of membrane design are collected in Tables S5and S6 (Supporting Information), whereas some representativeCO2 isotherms on these materials are collected in Figures15−17 and in Figures S9−S11. CO2 adsorption in rigid MOFsusually follows a type I pattern as in zeolites and amine-graftedsilicas. Deviation from this behavior has been observed in theIRMOF series at low temperatures (<273 K), the isothermsfollowing a formally type V pattern due to preferentialadsorption near the corners of the MOF cavities at lowpressures followed by pore filling at higher pressures.263

Some examples of MOF materials able to selectively adsorbCO2 over CH4 by molecular sieving/size exclusion have beenreported in the literature, including manganese formate,264

showing a 3D framework with 1D channels, the coordinativeinterpenetrated PCN-17,265 with large cavities linked by smallapertures, MIL-96(Al),266 and Zn2(cnc)2(dpt).

267 Zeolite-likeMOFs (ZMOFs), and more particularly ZIFs, based ontransition metals (Zn, Co, Cu, In) and imidazolates as linkersgenerating zeo-type frameworks containing large cavitiesinterconnected by small apertures, also show promising CO2adsorption properties and potentialities for membrane syn-thesis. In the latter case, the ZIF topology plays a fundamentalrole in the adsorption properties. Laird and co-workers268

recently showed that topologies with smaller pores (e.g., ZIF-7and ZIF-94) show higher CO2 loadings at low pressure (<1bar), whereas the opposite situation is observed for larger poreZIFs (e.g., ZIF-11) at higher pressures. Using DFT simulationsand heat of adsorption measurements, the authors found that,despite the higher adsorption strength of smaller pore size ZIFs,their narrower pores circumvent multiple CO2 accommodation,making larger pore ZIFs more competitive for CO2 adsorptionat higher pressures.The most promising CO2 adsorption capacities have been

reported so far on MOFs with open metal sites, favoring Lewisacid−base interactions. For instance, equimolar CO2/CH4 andCO2/H2 selectivities in the range of 3−9 and 80−150,respectively, were reported for HKUST-1 at room temperatureand 1−3 bar,271,272 the electrostatic interactions between CO2and the Cu(II) sites driving the sorbent selectivity.273 A similaradsorption mechanism was proposed for CO2 separation on aseries of isostructural MOFs with the general formula[M2(dhtp)] or [M2(dobdc)] (CPO-27 or MOF-74; M = Ni,Co, Zn, or Mg),274−276 showing a potential remarkable impacton CO2 adsorption at low pressures.277 Indeed, from MCsimulations coupled with the ideal adsorbed solution theory(IAST) theory, Snurr and co-workers278 predicted CO2/CH4

selectivities as high as 30 for the pillared-layer Zn2(ndc)(dpni)material.In general terms, MOFs exhibit comparatively lower heats of

CO2 adsorption than zeolites (most often <25 kJ/mol for meanvalues, Table S6 in Supporting Information). This fact involveslower CO2 adsorption capacities and Henry’s constants, theisotherms usually approaching the linear adsorption pattern ofhigh-silica MFI zeolites at near-ambient pressures. In contrast,in some cases, the zero-loading isosteric or differential heat ofCO2 adsorption can approach the values reported on zeolites.Indeed, the largest zero-loading isosteric heats have beenreported on MIL-100 and MgMOF-74 materials, with values,

Figure 15. CO2 adsorption isotherms on rigid MIL-type frameworksdeveloped by Ferey and co-workers at near room temperature at high(top) and intermediate (middle) pressures and evolution of theisosteric heats of adsorption with the CO2 loading (bottom). Imagesadapted from refs 110, 265, and 280.


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respectively, of 63 kJ/mol280 and 47 or 72 kJ/mol dependingon the team.228,281 These values are in good keeping with thecorresponding heats of adsorption reported on CaNaA (about70 kJ/mol, Supporting Information). For such a system, DFTsimulations reflect the formation of an angular complexbetween CO2 and the Mg(II) sites, with a MgOCO angle ofabout 129°.280

2.1.3.1. Structural vs Postfunctionalization Routes for CO2Adsorption. The intrinsic CO2 adsorption capacity of MOFscan be increased through two main strategies, i.e., structural and

functional routes, being easily transposable to syntheticmembrane protocols. The structural route can involve areduction of the micropore size (usually >1.5 nm) down tothe common size of medium- and large-pore zeolites (0.5−1.2nm), either by interpenetration or by interweaving to promotewall−molecule interactions and attain molecular sievingmechanisms.282 IRMOF-9, -11, -13, and -15 consist ofinterpenetrated MOF-5 frameworks, but with lower pore sizethan that of the parent structure.241 Another example is thetriply interpenetrated Co2(ndc)2(bipyen) with pore crosssections of 4.4 × 3.5 Å.283,284

Another approach involves the generation of structures withcoordinatively unsaturated metal sites serving as charge-bindingsites for CO2 adsorption, which can be obtained afterdesolvation of the material under vacuum and/or high-temperature treatment.285,286 A classical example of a MOFmaterial with unsaturated metal sites is Cu3(btc)2 (HKUST-1),287 affording open Cu(II) sites providing a zero-loadingisosteric heat of CO2 adsorption of 35 kJ/mol (Figure 16,bottom).269 Other typical materials with unsaturated metal sitesinclude Cr3O(H2O)3F(btc)2 (MIL-100) and Cr3O(H2O)2F-(bdc)3 (MIL-101)280 and the M2(dobdc) (MOF-74 or CPO-27) family.108,288 In the former MIL-type materials, exposedCr(III) metal sites generated after water removal afford zero-loading heats of CO2 adsorption of 62 and 44 kJ/mol,respectively, for MIL-100 and MIL-101 (Figure 15, bottom). Inthe case of MOF-74 materials, unsatured Mg(II) and Zn(II)sites can be generated inside the hexagonal 1D pores of theframework, allowing a zero-loading isosteric heat of CO2adsorption of 42 and 26 kJ/mol, respectively, for MgMOF-74and ZnMOF-74.The ligand can also be subjected to functionalization prior to

synthesis by incorporation of functional groups with N atoms(most often amines) to promote CO2 adsorption. PromisingCO2 adsorption enhancements were obtained for[Ni2(NH2dbc)2(dabco)] (dabco = 1,4-diazabicyclo[2.2.2]-octane) and [In(OH)(NH2dbc)] relative to nonfunctionalizedcounterparts.289 Promising results were also reported foramine-functionalized MIL-101(Al)290 and MIL-53(Al).291

Other interesting MOF structures with amino-decoratedpores offering high and selective CO2 uptake includeCo2(ad)2(CO2CH3)2 (bio-MOF-11), Zn2(C2O4)(C2N4H3)2·0.5H2O, N-heterocycles,

292,293 [Cu(Me-4py-trz-ia)] (Me-4py-trz-ia = 5-(3-methyl-5-pyridin-4-yl-4H-1,2,4-triazol-4-yl)-isophthalate),294 {[Zn(btz)]·DMF·0.5H2O}n (H2btz = 1,5-bis(5-tetrazolo)-3-oxapentane),295 and Zn2(atz)2(ox).

296 Fi-nally, Bai and co-workers297 recently reported improved CO2adsorption properties of the series [Cu(L1)·2H2O·1.5DMF]∞(L1 = 5-pyridin-4-ylisophthalic acid) (SYSU), [Cu(L2)·DMF]∞ (L2 = 5-pyridin-3-ylisophthalic acid) (NJU-Bai7),and [Cu(L3)·DMF·H2O]∞ (L3 = 5-pyrimidin-5-ylisophthalicacid) (NJU-Bai8) by shifting the coordination sites of ligands tofinely tune the pore size and by polarizing the inner surfacewith N groups.In addition to the aforementioned structural approach, MOF

materials can also be subjected to postsynthetic modifica-tion,298−301 whereby both the metal unit and the ligand canundergo heterogeneous chemical transformations while keepingthe overall crystalline topology of the material. A panel ofMOFs have been successfully subjected to postsyntheticfunctionalization with alkylamine groups such as HCu-[(Cu4Cl)3(BTTri)8] (H3BTTri = 1,3,5-tris(1H-1,2,3-triazol-4-yl)benzene), which can be further reacted with diamines such

Figure 16. Comparison of CO2 and N2 adsorption capacities on Cu-BTC (top) and evolution of the isosteric heats (blue) and entropy(red) of adsorption with the CO2 loading (bottom). Images adaptedfrom Wang et al.269 and Liang et al.270

Figure 17. CO2 adsorption isotherms on ZIF-8 and related materialsobtained by MC simulations, as well as the evolution of thecorresponding isosteric heats of adsorption at zero loading as afunction of the dipole moment of the imidazolate linker group. Imageadapted from ref 279.


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as N,N′-dimethylethylenediamine (mmen).302 Amine impreg-nation/adsorption has also been demonstrated in some MOFs,encompassing a significant increase of the CO2 adsorptionstrength compared to the parent MOFs. Recent examples havebeen reported by Jones and co-workers303 on Mg2(dobdc)modified with ethylenediamine (ED) coordinated to unsatu-rated Mg(II) sites (one ED molecule per unit cell). Thefunctionalization with ED increases not only the CO2 capacityat ultradilute concentrations (400 ppm CO2), but alsopromotes the regenerability of the material. Long and co-workers304 also reported a positive increase of the CO2 loadingon Mg2(dobpdc) and Zn2(dobpdc) with ED coordinated tounsaturated Mg(II) cations, reaching a value of 3.14 mmol/g(12.1 wt %) at 0.15 bar and 313 K. However, these materialscan show partial structural deformation upon N2 adsorption at77.4 K and a pronounced apparent “gate-opening” behavior(see below) at low CO2 pressures in the presence of amines.ZIFs modified with alkaline or polar functions and N-

functionalized with 4- and 5-azabenzimidazolate and purinateligands305 also show enhanced CO2 adsorption capacities andstrengths. In particular, ZIF-68, ZIF-69, and ZIF-70306−308

display excellent CO2/CO separation capacities, while a relatedmaterial, Zn(bdc)(4,4′-bipy)0.5, shows a good CO2 separationperformance from binary CO2/N2 and ternary CO2/CH4/N2mixtures.305 ZIF-95 and ZIF-100 show high affinity for CO2 atroom temperature over N2, CH4, and CO, with CO2/N2selectivities up to 25 (see comparative CO2 and N2 isothermsin Figure S9, Supporting Information).105 Finally, the GCMCsimulation studies reported by Babarao et al.309 predicted highCO2/H2 selectivities on rht-ZMOF and rho-ZMOF (Na-type).The high CO2 separation performance in ZIFs is ascribed to acombined effect of strong quadrupolar interactions betweenCO2 and N atoms present in the pore surface and pore sizeapertures being similar to the kinetic diameter of CO2 (3.65and 3.35 Å, respectively, for ZIF-95 and ZIF-100 vs 3.30 Å forCO2). Amrouche et al.279,310 showed through MC simulationsthat the CO2 adsorption capacity and CO2/CH4 selectivity ofZIF-8 frameworks can be improved by substitution of themethyl group in the imidazolate linker in ZIF-8 for CHO (ZIF-90) and NO2 (ZIF-NO2) groups. ZIF-7, with an SOD-typeframework based on a hexagonal arrangement of large cavitiesinterconnected by narrow windows of 3.0 Å, just between thekinetic diameters of H2 and CO2, offers a remarkable CO2adsorption capacity relying on a marked gate-openingphenomenon.311

Unsaturated metal sites can also be inserted into some MOFsby postsynthetic treatment. A typical example is Al(OH)-(bpydc) (MOF-253), exhibiting a topology similar to that ofAl(OH)(bdc) (MIL-53), but possessing open bipyridine sitesalong 1D pores that generate vacancies where metal cations(e.g., PdCl2 and Cu(BF4)2 in MOF-253) can be inserted afterpostsynthesis treatment.312 Another recent example is theimpregnation of [Zn3(tcpt)2(HCOO)][NH2(CH3)2] (SNU-100′) with Li+, Mg2+, Ca2+, Co2+ and Ni2+ cations, resulting inan increase of the zero-loading isosteric heat of CO2 adsorptionfrom 29.3 kJ/mol in the parent SNU-100′ material to 34.5−37.4 kJ/mol in the cation-impregnated counterparts.313 An andRosi314 showed that the pore size of Zn8(ad)4(bpd)6O·2Me2NH2 (bio-MOF-1) can be postsynthetically modified bycation exchange and that such a modification can be used tosystematically tune the CO2 adsorption capacity of the material.Ahn and co-workers315 also demonstrated the ion-exchangepotentials of sod-ZMOFs for tuning the CO2 adsorption

properties as in the case of zeolites. Finally, the reduction of theframework through Li incorporation was also demonstrated as apromising option for promoting electrostatic interactions inMOF cavities for CO2 separation,316,317 showing segregationeffects for HKUST-1.318 The hydration of Li+ can in some caseslead to a reduction of the free volume and consequently to alower CO2 adsorption loading.319

2.1.3.2. Gate-Opening and Breathing Phenomena inFlexible MOFs. Unlike zeolites and rigid coordination polymers,some MOF materials can show an inherent structural flexibilitydue to weaker bonds (e.g., π−π stacking, hydrogen bonds, andvan der Waals interaction), promoting either a gradual (elastic)swelling driven by guest accommodation as for the MIL-88/pyridine system,320 or a rich variety of abrupt but reversiblephase transitions between metastable phases involving eitheramorphous-to-crystal or crystal-to-crystal transformations. Theabove-stated structural changes in SPCs translate intoanomalous adsorption isotherm patterns (in the presence ofeither polar or nonpolar sorbates) such as the paradigmatic“gate-opening” and “breathing” phenomena.321,322 Thesephenomena are characterized by large hysteresis loops betweenthe adsorption/desorption branches. A classical example ofbreathing phenomena can be found in the well-known MIL-53(Cr,Al), exhibiting an abrupt phase transition upon CO2adsorption.323,324 Water promotes CO2 adsorption in thepressure range of 12−18 bar as the hydrated structure opens toaccommodate gas molecules while blocking the entry of CH4molecules. Breathing phenomena are also observed for ZIF-7upon CO2 and C2 and C3 alkane adsorption,

311,325,326 showingin this case only one narrow-to-large-pore (np → lp) phasetransition upon guest accommodation. An example of MOFdisplaying gate-opening effects upon CO2 adsorption can befound in the MAMS material with a framework consisting ofhydrophilic channels with tert-butyl groups interacting via weakvan der Waals forces.327 The accessibility to the hydrophilicchannels increases with the temperature by reducing theinteraction between tert-butyl groups, promoting a gate-opening phenomenon favoring the passage of CO2 molecules.Striking gate-opening phenomena between nonporous andporous phases have also been reported in the ELM and CIDseries,328,329 showing, respectively, narrow and broad ranges ofintegral free energies of sorbate adsorption /desorption nearthe phase transition zone.330 Finally, Chabal and co-workers331

recently reported an unusual CO2 uptake behavior inZn2(bpdc)2(bpee) (bpdc = 4,4′-biphenyldicarboxylate; bpee =1,2-bis(4-pyridyl)ethylene) by preferential twisting of the bpeepillars.CO2 adsorption/desorption hysteresis is not a priori desired

for membrane design, since the material has to adsorb at veryhigh pressures and desorb at very low values. Moreover, the factthat one species triggers pore opening might reduce the abilityof the material to discriminate between different molecules,involving a loss of selectivity. Moreover, expansion orcontraction of the unit cell upon adsorption could involve thegeneration of cracks or preferential intercrystalline pathways inMOF films, affecting negatively the membrane selectivity.However, some flexible organic frameworks could be foreseenfor membrane applications if one of the phases shows a broaddomain of stability during operation. This is the case, forinstance, for the partially or totally amine-modified MIL-53(Cr,Al), where the presence of the amine favors the stabilityof a flexed and narrow-pore phase after desolvation (Figure 18)through the formation of hydrogen bonds between the amine


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moieties and the Al clusters in MIL-53(Al). DFT simulationson pure MIL-53(Al)-NH2 have revealed that the interactionsbetween CO2 and hydroxyl groups are more intense comparedto those in the parent MIL-53(Al) due to the increased acidityof the hydroxyl moieties.332 Interestingly, increasing breathingpressures have been reported for the narrow-pore → large-porephase transition in MIL-53(Al) with the amine loading,333,334

this property being attractive for membrane design viapreferential CO2 separation.2.1.3.3. Hydrothermal Stability and Influence of Water on

CO2 Adsorption Properties. Unlike zeolites and silicas, theadsorption properties of MOFs can be strongly affected bywater during exposure to pre- and postcombustion flue gasesdue to the intrinsically weaker metal−ligand bonds comparedto Si−O bonds, resulting in some cases in a framework collapse.Low et al.335 provided steam stability maps for different MOFsover a broad range of steam levels and temperatures. A typicalexample of a MOF material showing low hydrothermal stabilityis MOF-5, extensible to the IRMOF series constituted by zinccarboxylate clusters, due to the fast hydrolysis of zinccarboxylate bonds.336−338 The materials showing the highesthydrothermal stability belong to the MIL family (especially therobust MIL-100 and M101 based on trinuclear Cr(III)c l u s t e r s ) , 1 0 9 , 3 3 9 , 3 4 0 t h e Z r ( I V ) - b a s e d MOFZr6O4(OH)4(CO2)12 (UiO-66),

341 Ni3(btp)2 based on azolatelinkers,342 and ZIFs, being especially indicated for membranedesign.In addition to hydrothermal stability, water can play a

relevant role in the CO2 adsorption properties of MOFs and intheir capacity to discriminate CO2 from other gases. Partialpore blockage by water upon CO2 adsorption has beenreported on rigid MOFs, though at a more moderate level thanin the case of hydrophilic zeolites, due to the competition ofwater molecules for active sites. For instance, Mg2(dobdc) canshow a strong deactivation upon water adsorption through areduction of the CO2 binding energy,343 whereas Ni2(dobdc)and Co2(dobdc) show better recoveries after water exposure,keeping steady-state CO2 loadings.344,345 HKUST-1, withunsaturated Cu(II) sites, shows a decrease of the CO2adsorption capacity after the coordination of two watermolecules to the Cu sites.228 However, upon the adsorptionof one water molecule, the CO2 adsorption capacity shows adramatic enhancement, which has been attributed to

preferential electrostatic interactions between the coordinatedwater molecule and CO2.

346 In contrast, in the case of hydratedMIL-101, terminal water molecules can act as additionalbinding sites for CO2 adsorption, promoting CO2/CH4separation at low pressures.347

A particular behavior upon water exposure is observed inflexible MOFs such as the MIL-53 family, where water caninduce reversible gate opening and breathing effects, improvingin some cases the CO2 adsorption properties.324 Flexiblematerials subjected to several adsorption/desorption cycles canhowever experience partial loss of their stability.348 Water canalso promote the stability of the narrow-pore phase in pureMIL-53(Al)-NH2,

332 this property being outstanding formembrane design.

2.1.3.4. Isotherm Models for Describing CO2 Adsorption.On the basis of their regular microporous system, zeolites andMOFs usually show type I (or Langmuir-type) adsorptionisotherm patterns. In the most general situation, the generalizedLangmuir (LG) isotherm is used to account for adsorptionequilibrium in energetically heterogeneous surfaces, where themolar loading, q(P), can be obtained by solving the generalintegral equation of adsorption349 using the Langmuir equationfor local molar loading, qL:

∫ χ δ=q P q P E E E( ) ( , ) ( )E

E

Lmin

max

(5)

where P is the pressure and χ(E) is the distribution function ofadsorption energies, taking values from a minimum to amaximum energy, Emin ≤ E ≤ Emax. The generalized Langmuirequation is expressed as follows:

θ = =+

β⎛⎝⎜

⎞⎠⎟

qq

yy1M (6)

with

δ= = − Δ °δ δ∞ ⎜ ⎟⎛

⎝⎞⎠y KP K P

HRT

( ) ( ) exp(7)

where qM is the molar saturation loading, K is the adsorptionconstant, K∞ is a pre-exponential factor collecting theadsorption entropy, ΔH° is the adsorption enthalpy, and θ =q/qM is the fractional occupancy.The resolution of the generalized Langmuir isotherm

provides a number of adsorption isotherms accounting fordifferent degrees of energy heterogeneity (ascribed to physicaldefects in the lattice, to the chemical nature of the solid, or toadmixing of solid crystals with different ions) depending on therelative values of the parameters β and δ. As shown in Table 5,the generalized Langmuir isotherm tends to the single-site

Figure 18. CO2 adsorption/desorption isotherms at 303 K on MIL-53(Al) (A), MIL-53(Al)-11.1%NH2 (B), MIL-53(Al)-20%NH2 (C),MIL-53(Al)-50%NH2 (D), MIL-53(Al)-66.7%NH2 (E), and MIL-53(Al)-NH2 (F). Empty and filled symbols refer, respectively, toadsorption and desorption data. Data adapted from ref 334.

Table 5. Isotherms Derived from the Generalized LangmuirIsotherm for Modeling Gas Adsorption

adsorptionisotherm β δ equation

single-siteLangmuir

1 1 θi = KiPi/(1 + KiPi) (9)

Freundlicha 0 ≤ β ≤ 1 δ = 1 θi = (KiPi)βδ (10)

β = 1 0 ≤ δ ≤ 1Tothb β > 1 0 ≤δ ≤ 1 θi = KiPi/[1 + (KiPi)

δ]1/δ

(11)

aβδ ≤ 1. bβδ = 1.


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Langmuir (SL) isotherm for β = δ = 1, the generalizedFreundlich (GF) isotherm for 0 ≤ β ≤ 1 and δ = 1, theLangmuir−Freundlich (LF) isotherm for β = 1 and δ < 1, andthe Toth (T) isotherm for β > 1 and 0 < δ ≤ 1.CO2 adsorption in zeolites and robust MOFs is usually

modeled using the single-site Langmuir, SIPS, Langmuir−Freundlich, and Toth isotherms depending on the degree ofheterogeneity of the samples.196,228,350−355 In the case of silicas,CO2 isotherms can be most often modeled using the Dubinin−Radushkevich and Freundlich isotherms for microporous silicamaterials,356 while amine-functionalized silicas usually showCO2 sorption patterns evolving from linear or Henry’s type toLangmuir type or even displaying a hybrid behavior dependingon the affinity between CO2 and the amine moieties (Figure19).165 In the case where CO2 shows preference for different

sorption sites (e.g., Ca- and Ba-exchanged FAU zeolites), CO2adsorption is better represented by a dual-site Langmuirisotherm:351

=Κ+ Κ

+Κ+ Κ

q qP

Pq

PP1 1M,A

A

AM,B

B

B (8)

where subscripts A and B indicate independent adsorption sites,for instance, channel interiors and intersections in the MFIframework.An alternative modeling approach has been recently reported

accounting for gas adsorption in a broad variety of porousmaterials.357−359 Such an approach, inspired from the solutionthermodynamics formalism proposed by Myers,360 consists ofthe representation of an isotherm by a thermodynamicequivalent that can be fitted to a universal isotherm equationwith a reduced set of energy heterogeneity, m, and affinity, k,parameters in its original formulation. A valuable equation foran isotherm can be obtained by representing the integral freeenergy of adsorption relative to saturation, expressed as aKiselev integral, Ψ, against the inverse of the chemical potentialof the sorbate, Z. The equation reads

∫ δθ− Ψ = − Π = °+θ λ λ

λ λ+RT

G[ ln( )]

1

1

1 2

1 2 (12)

where G° is the total dimensionless free energy dissipatedduring the adsorption process, Z = 1/−ln(Π), λ1 = k1Z

m1, andλ2 = k2Z

m2, with m1> m2 and k1 > k2 for most host/guestsystems. Equation 12 relates −Ψ/RT with Π in such a way that

−Ψ/RT → G° at Π → 0 and −Ψ/RT ∝ Zm2,α at higher Πvalues, showing the presence of two linear trends in doublelogarithmic axes with slopes m1 and m2, respectively.Finally, in the case of mixture adsorption (e.g., CO2/N2 and

CO2/CH4), the extended Langmuir (EL) isotherm has beenwidely employed, taking into account the similarity of themolecular sizes of CO2, N2, and CH4:

θ =+ ∑ =

K P

K P1i

i i

jC

j j1 (13)

The EL isotherm does not properly represent mixtureadsorption in the presence of species with different molecularsizes (for instance, in the presence of water vapor). In thissituation, the IAST modeling approach developed by Myersand Prausnitz361 using the concept of spreading pressure (andreformulated in terms of surface potentials for solutionthermodynamics362) provides a thermodynamically consistentframework for modeling mixture adsorption in zeolites withoutthe need for detailed physical models for the sorbate. The IASTmodel has been applied with success for modeling simultaneousadsorption of CO2 and other gases such as N2, ethylene, andpropane in zeolites353,363 and in rigid MOFs and SPCs.364,365

Modified versions of the original IAST formulation such as thereal adsorbed solution theory (RAST)366 and the predictiveadsorbed solution theory (PRAST) of Sakuth et al.367 havebeen proposed to account for nonideality of the sorbatemixture.Briefly, the IAST model relies on the definition of the surface

potential of each species in the adsorbed phase that can bederived using a modified Gibbs−Duhem equation at isothermalconditions:349

∫ δ− Φ =°

= + ° ≥

=

° Φ

RT

q P

PP q K P

i C

( )ln(1 ) 0

1, ...,

Pi i

ii i i i

0

( )

M,

(14)

The IAST model allows the derivation of mixture adsorptionisotherms from only pure adsorption data by assuming thatmixture adsorption occurs at constant surface potential (i.e., Φw= ΦE = Φ). In analogy with Raoult’s law, the molar fractions ofthe gas and adsorbed phases are related by

γ= Φ ° Φ =p x T P T i C( , ) ( , ) 1, ...,i i i i (15)

where γi(T,Φ) is the activity coefficient of species i in thesorbate, which tends to 1 in the IAST.Unlike Raoult’s law, there is a subtle difference in the

meaning of Pi°(T,Φ) in eq 15. In the case of vapor−liquidequilibrium (VLE), Pi° is the saturation vapor pressure ofspecies i corresponding to the solution and only depends onthe temperature, while in the case of an adsorbed phase itcorresponds to the adsorptive saturation pressure at a giventemperature and surface potential.The IAST approach requires the resolution of eq 14 for all

the n species in the mixture. This allows the representation of ay−x equilibrium diagram. In addition to this representation, aloading diagram is also necessary, which links the total adsorbedamount, qi, with the vapor-phase mole fraction of each species.Following the guidelines of Sakuth et al.,367 the followingexpressions are proposed:

Figure 19. Schematic representation of CO2 adsorption on amine-functionalized mesoporous adsorbents. Image adapted from Serna-Guerrero et al.165


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∑ ∑ γ=

° Φ+

∂ Φ∂Φ= =q

xq

RT1

( )

ln ( )

i

Ci

i i

Ci

T1 1 (16)

=q x qi iT

(17)

where qT is the total loading, qi°(Φ) is the loading of purespecies i at surface potential Φ, and qi is the loading of species ifor the given mixture.2.1.4. Taxonomy of Materials Based on Adsorption

Selectivity. Given the CO2 adsorption properties of thedifferent amine-modified silica, zeolite and MOF materialsmentioned in the previous sections, a taxonomy of the differentmaterials can be established relying on the relative affinity ofthe different materials for CO2 adsorption. Krishna and vanBaten,99,111 Scholl and Keskin,368−371 and Smit372,373 reportedcomprehensive series of CBMC simulation studies on theability of different zeolites, MOFs, and ZIFs to discriminateCO2 from mixtures in view of adsorbent and membrane design.In these studies, the selectivities are computed using anexpression analogous to eq 4:

=Sq

q

x

xi ji

j

j

i,ads

(18)

A complete and critical summary of the main trends obtainedin these series is presented below, which has been extended toamine-functionalized silicas. Figures 20 and 21 compile somesimulated trends for the CO2/H2, CO2/N2, and CO2/CH4selectivities as a function of the total fugacity of the feed streamfor a series of zeolites, MOFs and ZIFs. For the sake ofcomparison, Figure 20 also includes some experimental trendsfor the CO2/N2 selectivity on amine-functionalized silicas.Among the different zeolite materials considered, thesesimulations point out the supremacy of NaX zeolites forseparating CO2 at moderate pressures. Likewise, in the case ofMOF materials, rho-ZMOF shows values higher than 50 forCO2/H2, CO2/N2, and CO2/CH4 selectivities. MgMOF-74,with exposed metal cation sites, exhibits high selectivities forCO2 separation that are comparable to those obtained for rho-ZMOF. The adsorption selectivity decreases most often withthe CO2 loading in the feed.369 The use of covalent organicframeworks (COFs) might provide adsorption selectivites up to120 at 5 bar and room temperature for the separation ofequimolar CO2/H2 mixtures and up to 4 at 20 bar and roomtemperature for CO2/CH4 equimolar mixture separation.374

Figure 22 shows the evolution of the mixture CO2/N2adsorption selectivities in cation-exchanged Y zeolites, reflectingthe crucial role of the cation size in the CO2/N2 selectivity bytuning the electrostatic interactions in the FAU windows andcavities (see section 2.1.2.2 for more mechanistic insights).Other cage-type zeolite materials not represented in Figure 22might even show higher adsorption selectivities due topreferential CO2 adsorption in cage windows and pockets(e.g., all-silica AFX zeolite99). As has been suggested byKrishna, these “pocket” centers are hardly foreseeable to beactive in real CO2 capture applications due to their small size,limiting CO2 diffusion in and out the pockets. Likewise, thepreferential location of CO2 molecules in window regionsexplains the high selectivities in all-silica CHA.208,375

In general terms, ZIF materials show lower CO2 separationselectivities than MOF-177, MgMOF-74, and rho-ZMOF(Figure 21). Among the different ZIF materials, those basedon the original ZIF-8 framework with imidazolate frameworks

modified with HCO and NO2 groups (denoted as ZIF-90 andZIF-NO2, respectively) show the most promising CO2/N2 andCO2/CH4 selectivities. ZIF-3 and ZIF-90 appear to be moreindicated a priori for CO2/H2 separations, with selectivities>100. However, experimental validation is still required onthese materials.For the different separations, the selectivity remains almost

constant up to 0.1 MPa for ZIF-2, ZIF-4, ZIF-8 and ZIF-9, andup to 1 MPa for ZIF-5. In contrast, at high pressures, thequalitative behavior of the different selectivities dependsspecifically on the particular ZIF: the selectivities tend toincrease for ZIF-2 and ZIF-8, while they show a decreasingtrend for ZIF-9 and ZIF-10. ZIF-3 also shows an increasingtrend in the separation of CO2/CH4 equimolar mixtures,

Figure 20. Evolution of the CO2/H2 (top), CO2/N2 (middle), andCO2/CH4 (bottom) adsorption selectivity as a function of the total gasfugacity computed from CBMC simulations for a series ofrepresentative zeolite frameworks (all-silica FAU, LTA, DDR, CHA,and MFI and NaY, NaX, NaZSM-5, and NaA). Images adapted fromref 111. More trends in adsorption selectivities can be found in thesame reference and its corresponding Supporting Information.


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whereas the behavior is more complex (presence of aminimum) in the separation of CO2/H2 (15−25% CO2)mixtures. This observation has been attributed to the differentcage sizes in the different ZIFs and to the relative interplaybetween enthalpic and entropic factors. In the case of ZIF-68and ZIF-70, Sholl and co-workers370 reported a prominentdecline of the CO2/CH4 adsorption selectivity until 1 MPatotal pressure in the separation of 10:90 CO2/CH4 mixtures.Beyond this pressure, the selectivity remained essentiallyunchanged (ZIF-68), or showed a slight increase with thepressure (ZIF-70).

2.2. Diffusion Mechanisms: Molecular Sieving andCorrelation Effects for CO2 Separation

In addition to adsorption selectivity, the separation capacity of amembrane can be strongly affected by the selective diffusion ofone or more species of a mixture. The diffusion selectivity of amembrane depends not only on the relative kinetic diameter ofthe molecules, but also on the diffusion mechanisms involved,

Figure 21. Evolution of the CO2/H2 (top), CO2/N2 (middle), and CO2/CH4 (bottom) adsorption selectivity as a function of the total gas fugacitycomputed from CBMC simulations for different MOFs (left) and ZIFs (right). Images adapted from refs 99, 279, 371, 375, and 376.

Figure 22. Adsorption selectivity of ion-exchanged FAU-type zeolitesfor a pure and an equimolar CO2 and N2 mixture at 308 K and 101kPa total pressure. Data obtained from ref 377.


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which depend to a great extent on how sorbate moleculesinteract with the pore walls. In general terms, molecules can betransferred on the basis of elastic collisions with the pore walls

(Knudsen mechanism) on mesoporous solids, by skippingalong the pore walls for functionalized mesoporous solids, andby jumping between adjacent sites in channels and cavities in

Table 6. Summary of the Structural Features of Silicas, Zeolites, and MOFs with Potential for Membrane Design for CO2Capture

Table 7. Transport Diffusivities of CO2, N2, and CH4 on Zeolite and MOFs at Low Coverage (<1 molecule/uc)a

sorbate zeolite D at 300 K (m2/s) technique ref

CO2 amine−silica 4.9 × 10−11 ZLC 380MFI 7.0 × 10−9 MBR 384MFI 5.9 × 10−10 MBR 385MFI 4.5 × 10−9 MD 386MFI 1.3 × 10−8 MD 387MFI 5.0 × 10−9 MD 388MFI 7.0 × 10−9 QENS 389b

MFI 1.5 × 10−10 MBR 390MFI 1.8 × 10−7 MBR 179CHA 2.3 × 10−9 MD 386DDR 1.2 × 10−9 MD 386DDR 5.0 × 10−9 MD 391c

LTA 3.5 × 10−9 MD 3865A 2.3 × 10−11 13C-MRI 389

FAU 1.7 × 10−8 MD 386FAU 1.1 × 10−10 MBR 392e

NiMOF-74 1.0 × 10−11 VOL 228MgMOF-74 1.3 × 10−8 MD 99MOF-5 9.6 × 10−13 TGA 393MOF-5 1.2 × 10−9 VOL 394MOF-5 9.3 × 10−10 MBR 395MOF-177 2.3 × 10−9 VOL 394Zn(tbip)f 2.9 × 10−9 IR 396ZIF-8 6.1 × 10−11 MC 376ZIF-8 2.5 × 10−10 IR-GCMC 397ZIF-9 3.9 × 10−9 MC 376

N2 MFI 1.3 × 10−8 MBR 384MFI 1.8 × 10−8 MD 386MFI 5.0 × 10−9 QENS 389FAU 1.8 × 10−10 MBR 392e

MFI 7.5 × 10−10 MBR 398CHA 4.0 × 10−9 MD 386DDR 2.2 × 10−9 MD 386

sorbate zeolite D at 300 K (m2/s) technique ref

LTA 8.0 × 10−9 MD 386LTA 1.2 × 10−14 FR 398d

LTA 9.0 × 10−15 TGA 399d

DDR 8.4 × 10−9 MD 391c

FAU 3.0 × 10−8 MD 386MgMOF-74 8.0 × 10−8 MD 99ZIF-8 2.4 × 10−9 MC 376ZIF-9 2.2 × 10−11 MC 376

CH4 MFI 2.0 × 10−8 MD 386MFI 1.5 × 10−8 MD 387MFI 5.0 × 10−10 MBR 386MFI 1.1 × 10−7 MBR 179CHA 3.0 × 10−11 MD 386DDR 1.5 × 10−10 MD 412LTA 3.0 × 10−10 MD 386LTA 3.6 × 10−15 FR 398LTA 5.0 × 10−15 TGA 399FAU 3.5 × 10−8 MD 386DDR 5.8 × 10−9 MD 391c

MOF-177 1.0 × 0−12 VOL 394MgMOF-74 5.5 × 0−8 MD 99MOF-5 1.8 × 10−9 VOL 394MOF-177 1.5 × 10−9 VOL 394ZIF-8 1.8 × 10−9 MC 376ZIF-9 1.5 × 10−12 MC 376

H2 MgMOF-74 5.5 × 10−7 MD 99ZIF-8 5.5 × 10−8 MC 376ZIF-9 4.3 × 10−9 MC 376

aNomenclature: ZLC, zero-length column; TGA, thermogravimetricanalysis; VOL, microvolumetry; MBR, membrane. bAt 2 molecules/uc.cSimulations considering a flexible structure. dAt 273 K. eAt 308 K.fH2tbip = 5-tert-butylisophthalic acid.


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zeolites, MOFs and microporous silica. Table 6 illustrates themain mechanisms that can play a role in diffusion selectivity.The main characteristics of these mechanisms are describedbelow, including relevant diffusion data, either measured orsimulated, on powders and films. The main structuralcharacteristics of solids that help maximizing diffusionselectivities for pre- and postcombustion membrane separationsare also pointed out.2.2.1. Diffusion Mechanisms in Mesoporous Solids:

Selectivity beyond the Knudsen Threshold? Mass transferwithin mesoporous membranes can occur via Knudsen andsurface diffusion pathways. Knudsen diffusion can efficientlydiscriminate molecules with significant molecular weightdifferences. This is the case, for instance, of CO2/H2 separationin precombustion CO2 capture applications, mesoporous silicamembranes showing preferential hydrogen transfer and amaximum theoretical H2/CO2 permselectivity of 5. Withoutprior functionalization, raw mesoporous silica membranesdisplay low CO2/N2 Knudsen selectivities (up to 0.8) ascribedto the very similar molecular weights of CO2 and N2.In the case of amine-functionalized silicas, CO2 can be

hindered due to preferential adsorption on amine groupshanging on pore walls, while the N2 and H2 diffusion behavioris expected to be only slightly affected due to their weakeradsorption strengths.378 To our knowledge, the sole papersreporting so far CO2 diffusion coefficients and activationenergies on amine-functionalized silicas have been published bySayari and co-workers379 on triamine-grafted PE-MCM-41 andby Ostwal et al.380 on an amine-functionalized Vycormembrane with (3-aminopropyl)triethoxysilane (APTES).On the one hand, the former authors reported a D/r2

diffusion constant for CO2 computed by direct fitting ofFick’s law to experimental breakthrough curve data showingvalues of about 2.4 × 10−3 s−1 at 298 K with an activationenergy of <3 kJ/mol. On the other hand, Ostwal et al.380

reported CO2 diffusivities of 4.92 × 10−11 and 2.86 × 10−10 m2/s, respectively, at 298 and 373 K with an activation energy ofabout 32 kJ/mol (Table 7). These values are much lower thanthe standard CO2 diffusivities commonly measured in aqueousAMP (0.78 × 10−9 m2/s381) and DEA (1.04 × 10−9 m2/s382)solutions at 298 K. Using DFT calculations, Ostwal et al.380

proposed a hopping mechanism for CO2 diffusion involving aCO2 transfer between adjacent aminopropyl moieties.From a practical viewpoint, Hiyoshi et al.170 measured time

lags for CO2 of 4, 11, and 13 min, respectively, on mono-, di-,and triamine-functionalized SBA-15 under a 30 cm3 (STP)/minflow rate and a composition of 15% CO2, 12% H2O andbalance N2, corresponding to ca. 82%, 97%, and 97% of theirequilibrium capacities.159 Huang et al.383 determined that about80% of the equilibrium capacity is reached after 30 min underdry conditions, with an adsorption halftime of about 5 min.Sayari and co-workers146 measured an adsorption halftime ofca. 1.5 min on triamine-grafted PE-MCM-41 under dryconditions. These authors also found a maximum adsorptionuptake rate of 1.8 mmol of CO2·g

−1·min−1 for a TRI-PE-MCM-41 sample prepared at 358 K with addition of 0.3 cm3 of H2O/gof SiO2 during the synthesis, the sorbent reaching 80% of fullcapacity after 1.5 min. A slower adsorption uptake rate of 0.24mmol of CO2·g

−1·min−1 was measured by Zelenak et al.121 onmonoamine-grafted SBA-12 under dry conditions.2.2.2. Diffusion Mechanisms in Zeolites and MOFs.

Two comprehensive reviews on gas diffusion in zeolite crystalsand membranes have recently been published by Gavalas400

and Krishna401 with special insight into CO2 diffusionproperties. In general terms, sorbate molecules tend to transferwithin microporous materials via surface or conf igurationaldif fusion pathways due to their interaction with the potentialfield created in the micropores (channels and cavities in thecase of zeolites/MOFs). It is generally acknowledged thatmolecules with a size larger than 60% of the micropore sizemigrate by this mechanism.402,403 Two different types ofdiffusivities can be measured:404,405 Fick or transportdiffusivities, DT, and self-diffusivities, D*. The fundamentaldifference between both diffusivities relies on the presence orabsence of finite gradients. Transport diffusivities are measuredunder nonequilibrium conditions in which finite gradients ofloading exist (∇qT ≠ 0), while self-diffusivities are measuredunder equilibrium conditions (∇qT = 0) and involve masstransfer of identical but labeled molecules. Transportdiffusivities can be measured using macroscopic techniques(zero-length column, gravimetry, frequency response, transientpermeation across a film), while self-diffusivities are measuredusing microscopic techniques (pulsed-field gradient NMR,quasi-elastic neutron scattering, neutron spin echo). Self-diffusivities and transport diffusivities have also been estimated,respectively, using equilibrium and nonequilibrium MDsimulations.Multicomponent mass transfer of sorbate species in micro-

porous materials can be conveniently modeled through thegeneralized Maxwell−Stefan (GMS) equations using theformalism developed earlier by Krishna.406−408 The GMStheory assumes that the translation of a species is caused by adriving force that is balanced by the friction experienced withthe other sorbates and the surroundings. Taking the isothermalgradient of the chemical potential of the ith species, −∇Tμi, asthe driving force and treating vacancy sites as active species, thegeneral form of the GMS equations applied to surface diffusionreads

∑ρ μ− ∇ =−

+

=

=

≠

q

RT

q N q N

q ĐNĐ

i j C

( )

, 1, ...,

ii

j

j i

Cj i i j

j ij

i

ip T

1

S S

M,S

S

VS

(19)

where qi and NiS are, respectively, the molar loading and the

surface flux of the ith species.The first term on the right-hand side in eq 19 reflects the

friction exerted between two sorbate molecules, while thesecond term represents the friction between a sorbate moleculeand the pore wall. Both interactions can be modeled,respectively, by means of MS counterexchange dif fusivities, Đij

S,and MS surface or “jump” dif fusivities, Đi

S. The MS counter-exchange diffusivities can be estimated in many cases, forinstance, as a function of MS jump diffusivities using Vignesinterpolation formulas in the absence of strong hydrogenbonds.The surface chemical potential gradients may be expressed in

terms of the molar loading gradients by introduction of thematrix of thermodynamic factors, Γij:

∑μ∇ = Γ ∇=

q

RT

q

qqi

ij

C

iji

jj

1

,M

,M (20)


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Γ ≡∂∂

=⎛⎝⎜⎜

⎞⎠⎟⎟

q

q

q

PPq

i j C, 1, 2, ...,ijj

i

i

i

i

j

,M

,M (21)

The form of the thermodynamic factors is determined by theform of the mixture adsorption isotherm. In the case of usingthe extended Langmuir isotherm, the classical Darken equationis obtained relating fickian diffusivities with MS surfacediffusivities (Di

S vs ĐiS):

θθ

=−

D qĐ

( )( )

1iiS TS T

T (22)

Krishna and co-workers408,409 developed a modifiedexpression for eqs 20 and 21 accounting for systems followinga dual-site Langmuir (or multisite) adsorption pattern.Moreover, Kapteijn and Krishna410 derived expressions usingthe IAST formalism for describing mixture adsorption forspecies with different saturation loadings (see section 2.1.3.4 forfurther details).In general terms, the temperature exerts a double effect on

the gas diffusion properties in zeolites and MOFs. On the onehand, increasing the temperature reduces the adsorptionstrength, contributing to a reduction of the sorbate loadingand in turn increasing the MS diffusivity. On the other hand, ahigher temperature promotes the mobility of the sorbatemolecules to overcome the activation barrier. This contributioncan be well accounted for by an Arrhenius-type dependence ofthe MS surface diffusivities at zero coverage, Đi

S(0), on thetemperature:411

= −⎛⎝⎜

⎞⎠⎟Đ q A

ERT

( ) expi iiS T SS

(23)

where AiS is the pre-exponential factor and Ei

S is the activationenergy.Most often, the activation energy is found to be loading

dependent.412 Accordingly, when determining activationenergies for diffusion, the MS diffusivities should be measuredat a given loading, preferably at the limit of low loading. Theactivation energy tends to increase as the kinetic diameter ofthe guest molecules approaches the pore size of the zeolite/MOF material due to an increase of the number of molecule−wall interactions and as the adsorption strength increases, forinstance, in the presence of cations; this is exemplified by thecomparison of diffusion data for all-silica LTA (or ITQ-29) andCaA.413 As a general trend, the activation energies at lowloadings (<1 molecule/uc) for CO2, CH4, and N2 diffusion inMFI zeolites fall into the range of 6−15 kJ/mol,179,406,407 whilevalues of 24 and 18 kJ/mol for CH4 have been reported,respectively, on DDR and CHA zeolites.386 However, thesevalues can show important variations as a function of thedensity and nature of intercrystalline defects in the zeolitelayers (see section 4.5.2).412

Table 7 collects relevant transport MS surface diffusivities ofCO2, N2, and CH4 at low coverage (<1 molec/uc) on amine-functionalized silicas, zeolites and MOFs, where the fickian andMS diffusivities should converge. Noteworthy, these diffusivityvalues should be considered as only indicative for comparisonpurposes between different sorbates, since the measured valuesdepend strongly on the analytical technique used and on thepretreatment protocol of the zeolite samples prior to thediffusion measurements. Despite the similarity of CO2, N2, andCH4 kinetic diameters (Table 2) acting as a deterrent for

molecular sieving adsorption mechanisms, CO2 and N2 canshow detectable variable diffusivities by tuning the size of FAUcavities through cation exchange. The transport diffusivitiesshow a monotonous decreasing trend with the cation size forN2 and beyond a threshold value for CO2 (Figure 23).

At this point, as inferred from Krishna, the diffusionselectivity toward CO2 can be promoted through two majoreffects: (1) promotion of CO2 loading due to larger CO2adsorption, and (2) speeding up adsorption of CO2 andslowing down adsorption of N2/CH4 due to correlation effectsascribed to the counterexchange diffusion properties of thesolids. These aspects are tackled below.

2.2.2.1. Loading Dependence of MS Diffusivities. Mecha-nistically, the MS surface diffusivity, Đi

S, can be related to thedisplacement of the sorbate molecules, λ, and the jumpfrequency, ν(qT), which in general can be expected to dependon the total molar loading, qT,408:

λ ν=Đ qz

q( )1

( )i iS T 2 T

(24)

where λ is the average jump distance, z is the number ofnearest-neighbor sites, and νi(q

T) is the jump frequency ofspecies i (s−1).Figure 24 plots some characteristic loading dependences

reported by Krishna using MD for MS surface diffusivity ofCO2, N2, and CH4 on a series of zeolites and MOF materials.Following the own terminology of Krishna, light hydrocarbondiffusion in MFI zeolites usually proceeds under a weak

Figure 23. Relationship between the transport diffusivity and theexchanged cation size in FAU-type zeolites for CO2 (top) and N2(bottom) mass transfer at 308 K and 101 kPa total pressure. Theempty and full symbols refer, respectively, to pure and binarydiffusivities. Images adapted from Kusakabe and co-workers.377,392


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confinement scenario in zeolite channels. In this situation, thejump frequency remains constant, i.e., νi(q

T) = νi(0), regardlessof the molar loading. In contrast, CO2 shows a quasi-lineardecreasing trend with the sorbate occupancy, which can beattributed to the interaction of the sorbate with the intersectingchannel structure.In the case of cage-type zeolites (e.g., LTA, FAU, CHA,

DDR, and SAPO-34) and MOFs (e.g., ZIF-8), MS surfacediffusivities can be strong functions of the sorbate occupancydue to strong confinement, especially at high pressures, due tosegregation effects.386 CO2 is a more slender molecule than N2and CH4, resulting in lower energy barriers for hoppingbetween cages. Consequently, the MS diffusivity for CO2 inCHA and DDR zeolites does not show the sharp initial increaseobserved for N2 and CH4.

414 The presence of a maximum inthe loading dependence of the MS diffusivity (formally type IVaccording to the classification of Karger and Pfeifer415) isattributed to the presence of molecular clustering at temper-atures lower than the critical temperature.416

In the presence of a strong confinement scenario, frameworkflexibility can play an important role in the intercage hoppingacross narrow windows in cage-type frameworks. The effect offramework flexibility is expected to be larger in MOFs, althoughthe effect of flexibility in zeolites is not expected to be negligiblefor very strongly confined sorbates as in the case of N2 or CH4in CHA.417 For example, MD simulations have shown thatframework flexibility can promote by 1 order of magnitude theethane diffusivity within the channels of Zn(tbip) (0.45 nm,1D).418

To quantify the loading dependence in zeolites, Krishna etal.410,419 proposed the use of the classical Reed and Ehrlichmodel for describing surface diffusion of sorbate mole-cules.401,420 In this model, the intermolecular interactionswithin a cage are assumed to influence the jump frequency ofmolecules between cages by a factor ϕi = exp(δEi/RT), whereδEi represents the reduction in the energy barrier for diffusion.The values of δEi can be calculated from the free energy profilesobtained by molecular simulations. The Reed−Ehrlich modelleads to the following expression for the MS diffusivity as afunction of the fractional loading, θi:

εε ϕ

=++

−Đ q Đ( ) (0)

(1 )(1 / )i i

iz

i iz

S T S1

(25)

where z is the coordination number, representing the maximumnumber of nearest-neighbor sorbate molecules within a cage.The other parameters are defined as follows:

εβ θ ϕ

θ=

− +−

( 1 2 )

2(1 )ii i i

i (26)

β θ θ ϕ= − − −1 4 (1 )(1 1/ )i i i i (27)

The MS surface diffusivity converges into a constant value (i.e.,weak confinement) for ϕi → 1.An alternative modeling approach based on the so-called

relevant site model (RSM) has recently been reported byKapteijn and co-workers422,423 for systems displaying segre-gated adsorption. The model improves the classical Reed−Ehrlich formulation by reducing in theory one fitting parameterin its formulation. The model consists of a modified version ofthe MS equations, but only relying on the diffusion propertiesof the relevant site (*, most often near the cage windows) andincluding a proper calculation of the void volume in the loadingdependence of the MS surface diffusion coefficients. The twofundamental equations of the model read as follows assumingthat all of the void volume (i.e., 1 − θT) participates indiffusion:

θ= −*

Đ q Đq

q( ) (0)(1 )i i

i

i

S T S T

(28)

* =*

+ **q q

K f

K f1i iM i i

i (29)

The model was applied with success to the description of bothpure and mixture diffusion in cage-type zeolites under a strongconfinement scenario (e.g., DDR181). The model successfullypredicts the characteristic diffusion maxima with the temper-ature for type IV diffusing systems (e.g., N2 in all-silica DDR)and provides a proper description of side “pockets”.Thonhauser and co-workers424 have recently published acombined experimental and DFT study describing themacroscopic implication of side pockets on H2 and CO2diffusion within MgMOF-74.

2.2.2.2. Role of Correlation Effects in CO2 Separation. Inaddition to the loading, the CO2, N2, CH4, and H2 diffusionpattern within zeolites and MOFs can be strongly affected bycorrelation effects. Correlation effects compute the influence ofthe friction exerted between two sorbate molecules on theoverall diffusion of a target species. The weight of correlationeffects on the diffusion pathways can be estimated using theratio between the MS diffusivity and the MS counterexchange

Figure 24. MD simulation results on the loading dependence of theMS diffusivity Đi of CO2 and N2 (top) and CH4 (bottom) in differentzeolite frameworks and MOFs. Images adapted from Krishna and vanBaten419,421 and Jee and Sholl.369


dx.doi.org/10.1021/cr400237k | Chem. Rev. XXXX, XXX, XXX−XXXAB

self-diffusivity, i.e., ĐiS/Đii

S.425,426 Higher values of this parameterinvolve higher correlation effects.Figure 25 plots the evolution of the Đi

S/ĐiiS ratio for CH4 with

the sorbate occupancy for a variety of zeolite and MOF

topologies. The ĐiS/Đii

S ratio increases with the sorbate loading,involving a higher degree of correlation due to a higheradsorption on recently abandoned sites for unsuccessful jumps.Correlation effects are stronger for 1D and in intersectingchannel structures than in larger pore 3D open structures withhigh connectivity and are more stressed for strong adsorbingspecies (see the example in Figure 25, bottom, for the MFIframework). In the latter case, the mobility of the sorbatespecies i is not expected to contribute to the mobility of thesorbate species j (i.e., Đij

S → ∞).406,408 In this particularsituation, the first term on the right-hand side in eq 19 vanishes,providing the simplified Habgood expression427

ρ μ= − ∇N Đq

RTi ii

iS

pS

T (30)

In the case of binary mixtures, correlation effects are capturedby counterexchange MS diffusivites, Đij

S (eq 19). According toKrishna, this parameter can be estimated on the basis of theself-exchange coefficients of the corresponding pure speciesusing Vignes interpolation formulas. Correlation effects are atthe origin of speeding up and slowing down effects in cage-typezeolites and MOFs. For these systems, CO2 can be lodgedpreferentially at window regions, hindering the transport ofother guest species (e.g., N2 and CH4). As a consequence, theMS diffusivity can be slightly promoted, whereas that of the

partner species can be dramatically reduced.208,427 Thisproperty is outstanding and particular for cage-type zeoliteand MOF topologies and can be used as a tool to promote CO2selectivity in membrane design.

2.2.3. High-Temperature Mechanisms: Application toH2 Separation. Gas permeation in microporous silicamembranes occurs between nearby micropores and usuallyproceeds at higher temperature (>500 K) than in the case ofamine-grafted mesoporous silicas, zeolites and MOFs. Twomodel approaches have been considered to describe gasdiffusion within microporous silicas: (1) molecular sievingand (2) “activated” or gas translational (GT) diffusion withinmicropores if the micropore walls exert an effect on gaspermeation. The corresponding expressions for the gaspermeance for the ith species read as follows

Π = Π −⎛⎝⎜⎜

⎞⎠⎟⎟

E

RTmolecular sieving: expi i

i,0

,effS

(31)

αΠ = −⎛⎝⎜⎜

⎞⎠⎟⎟T

E

RTactivated diffusion: expi

i ,effS

(32)

where Π0 and α are constants that depend on the system andEi,effS is the effective or activation energy, which corresponds to

the difference between the true activation energy and theadsorption enthalpy, i.e., Ei,eff

S = EiS + ΔHi

S.Equation 31 can be deduced from an adsorption/diffusion

model relying on the MS formalism at low loadings and can beregarded as a pseudo-Arrhenius law. In contrast, eq 32,originally formulated by Shelekhin et al.426 and further adaptedby Yoshioka et al.402 to account for the permeation of hardlyadsorbable species in silica and zeolite membranes at relativelyhigh temperatures (Henry’s adsorption regime), assumes thatthe sorbed phase shows a “gaslike” behavior and is stronglyaffected by the potential field exerted by the pore walls. Gasdiffusion is then regarded to follow a Knudsen mechanismcorrected by an Arrhenius exponential term. A consequence ofactivated diffusion at high temperatures is that amorphous silicausually exhibits pure He permeances larger than those of H2, asexpected from a mechanism driven by molecular sieving.Takaba et al.428 found using GCMD modeling that thetransition from the sieving to the activated diffusionmechanisms occurs when the pore size of the membranebecomes smaller than 1.2 times that of the molecular diameterof the permeating species.Table 8 collects some reported values on the apparent

activation energies for H2, CO2, and N2 permeation within silicamembranes computed from eq 31. Figure 26 plots somerepresentative Arrhenius trends for these gases. The activationenergies on microporous silica membranes depend strongly ontheir synthesis protocols, increasing inversely with the micro-pore size. In general terms, silica membranes prepared bychemical vapor deposition (CVD) show higher activationenergies for H2 permeation than those prepared by the sol−gelmethod, this observation being attributed to closer interstitialstructures with lower sized micropores in the former case. Inany case, the activation energies of silica membranes preparedby both methods are lower than the common values obtainedon vitreous (Vycor) silica glass, the latter showing a value ofabout 37 kJ/mol for H2 permeation.

429,430

2.2.4. Taxonomy of Materials Based on DiffusionSelectivity. A taxonomy based on diffusion properties has

Figure 25. Ratio of the MS diffusivity, Đi, with respect to the self-exchange coefficient, Đii, for CH4 on different zeolites and MOFs(top) and for CO2, N2, CH4, and H2 on silicalite-1 (bottom). Imagesadapted from Krishna.208,425


dx.doi.org/10.1021/cr400237k | Chem. Rev. XXXX, XXX, XXX−XXXAC

recently been established by Krishna and other authors for afast discrimination of materials in view of CO2 captureapplications. In principle, materials displaying higher diffusionselectivities should be desirable for membrane developments.Krishna111 has demonstrated that a reasonable estimate of thediffusion selectivity can be computed as a ratio of the mixtureself-diffusivities, that is

=+

+≈S

ĐĐ

Đ Đ

Đ Đ

D

D

1 ( / )

1 ( / )i ji

j

j ij

i ij

i

j,diff

S

S

S S

S S,selfS

,selfS

(33)

Equation 33 should be regarded with care when estimatingdiffusion selectivities. On the one hand, this expression doesnot consider a potential influence of surface barriers on crystalsurfaces on the gas diffusion properties, most often related tothe presence of moisture.396 On the other hand, self-diffusivitiescan show strong deviations compared to MS and transport

surface diffusivities in the presence of type V adsorptionpatterns. This is the case for adsorption of polar molecules inhydrophobic frameworks. For instance, using IR microscopy(IRM) imaging, Chmelik et al.448 recently reported enhancedself-diffusivities for methanol diffusion on ZIF-8 by an opposingflux of labeled molecules. The authors rationalized theirobservation on the basis of a dominating role of intercagehopping during molecular propagation and due to strongsorbate−sorbate interaction, involving unexpectedly highthermodynamic factors (i.e., Γ ≫ 1). Such effects might playa remarkable role in CO2 diffusion within hydrophobicframeworks in the presence of water or other polar pollutantsand are not explicitly considered in eq 33.Figures 27−29 compile a broad series of trends for diffusion

selectivities with the total sorbate loading for CO2/H2, CO2/

Table 8. Summary of Reported Values of ApparentActivation Energies (in kJ/mol) of H2, CO2, and N2Computed by Fitting Eq 31 to Experimental PurePermeation Data

method T (K) H2 CO2 N2 ref

sol−gel 298−573 9.1 0.1 16.8 431295−473 11 10 432301−473 12.3, 21.7 3, 6.8 433323−473 −3.4 434373−673 7.6 −2 6 435

8.0 −4308−588 −3.6 1.2 436473−673 3.7 6.8 9.6 437

5.4 11.0 9.8298−500 −29 22 438b

298−573 6.0 3.3 439250−435 −3.5 1.0 440a

250−435 13.0 1.0 441373−473 5.0 4.5 4.3 442b

353−462 15.3 −14.2 443c

310−350 −1.8 0.0 44411−20.4 −10 to +1.1 445

CVD 313−473 0 0 3, 19 446300−873 13.6 447

aData obtained from MD simulations. bMembrane templated usingMTES. cSiO2−NbO2 (Si/Nb = 1.5).

Figure 26. Evolution of the surface diffusivity as a function of thekinetic diameter for an MTES-templated silica/alumina compositemembrane. Image adapted from Moon et al.445

Figure 27. Evolution of the CO2/H2 (top), CO2/N2 (middle), andCO2/CH4 (bottom) diffusion selectivities as a function of the totalsorbate loading computed from MD simulations for different zeoliteframeworks. Images adapted from Krishna.99,111


dx.doi.org/10.1021/cr400237k | Chem. Rev. XXXX, XXX, XXX−XXXAD

N2, and CO2/H2 separations obtained from MD simulations.These trends should be regarded as indicative, since noinformation about the framework flexibility, as well as thematerial microstructure (i.e., presence of defects), is usuallytaken into account in the simulations.In the case of CO2/H2 mixture separation, microporous

materials favor the transfer of hydrogen within micropores dueto its much smaller molecular size. In most cases, this trendopposes that observed for the adsorption selectivity, especiallyfor NaY and NaX zeolites. Note however that the presence ofcations in the zeolite framework tends to reduce the CO2mobility by increasing the residence time of the sorbatemolecules near the cations. The preferential diffusion ofhydrogen compared to CO2 in zeolites and microporous silicacan be exploited in high-temperature membrane separations forprecombustion CO2 capture, where the adsorption selectivity infavor of CO2 is discouraged. At lower temperatures, cage-typematerials with narrow pores such as ZIF-8 and ZIF-90, offeringa low degree of correlation between CO2 and H2, also showpotentials for selective H2 separation. In contrast, as pointedout by Krishna, the design of membranes that are selective forCO2 requires micropore topologies involving a strong degree ofcorrelation between CO2 and H2, compensating the intrinsicdifferences in mobility between both molecules. To this end,

Krishna points out the preference for 1D channel structures likethose found for TON or MTW zeolites.In the case of CO2/N2 separations, the diffusion selectivity

favoring either CO2 or N2 separation is modest due to thesimilar molecular size of both species. The maximum diffusionselectivites are reached for NaX, favoring CO2 separation. Thisproperty combined with the higher CO2/N2 adsorptionselectivity for this zeolite at low temperature renders thismaterial a priori desirable for membrane design forpostcombustion CO2 capture separations.Finally, in the case of CO2/CH4 separations, the diffusion

selectivity is promoted for all-silica CHA (e.g., SAPO-34), ITQ-29, DDR, and LTA frameworks (with or without ex-frameworkcations), showing narrow-sized windows. This observation ismainly attributed to the preferential location of CO2 moleculesat window regions, hindering the intercage hopping of CH4. Asimilar conclusion has to be made for ZIF-8.449

3. CO2 PERMEATION AND SEPARATION PROPERTIES

In section 2, we discussed the crucial role of pore architecturefor providing the necessary adsorption fields, confinementeffects, and degree of correlation between the diffusing speciesfor different CO2 capture scenarios. On the guidance of theseconcepts and data, this chapter presents a collection of the mostrelevant permeation and separation properties of silica, zeoliteand MOF/ZIF membranes with special insight into theirperformance under real operation conditions. According to themain trends that will be unveiled, a taxonomy of the differentmembrane materials is proposed at the end of the chapter forpre- and postcombustion CO2 capture applications.

3.1. Silica Membranes

3.1.1. Microporous Silica Membranes. Microporoussilica membranes show promising potential in two separations:(1) H2 separation from H2/CO2 mixtures at high temperatures(>473 K) driven by preferential H2 diffusion and a molecularsieving effect, and (2) near-room temperature CO2 separationfrom CO2/N2 and CO2/H2 mixtures based on preferential CO2adsorption (amine-functionalized silicas). In the former case,microporous membranes compete with dense membranes (e.g.,Pd(Ag)), offering higher H2 permeances and higher resistanceto poisoning. For this application, the CVD method usuallyyields microporous membranes with lower H2 permeances thansol−gel membranes, but offering higher selectivities (seesection 4.2.2 and section 4.2.3 for further details). Themicropore size can be tuned in CVD by changing the size ofthe silica alkoxide precursor (e.g., by introducing differentnumbers of phenyl groups450,451). In contrast, at sufficientlylow temperature, some authors reported partial CO2 selectivitydriven by selective CO2 adsorption in silica micropores,especially when incorporating amine groups by including(aminoalkyl)silanes in the sol formulation (see Xomeritakis etal.455).Tables 9 and 10 list the available body of data on the

separation properties of sol−gel and CVD microporous silicaand related membranes for post- and precombustion CO2capture. Moreover, Figures 30 and 31 plot some representativetransient and steady-state permeation trends for silicamembranes, illustrating the predominant role of differentseparation mechanisms as a function of temperature. In generalterms, gas permeation at low temperatures (<500 K) can bewell described by the MS formalism for a weakly confinedscenario (see section 2.2.2). Moreover, Figures 32 and 33 plot

Figure 28. Evolution of the CO2/H2 (open symbols) and CO2/CH4(filled symbols) diffusion selectivities as a function of the total sorbateloading computed from MD simulations for different MOFs. Imageadapted from Krishna.99,111

Figure 29. Evolution of the CO2/H2 adsorption (filled symbols) anddiffusion (open symbols) selectivities as a function of the feed fugacitycomputed from MD simulations for ZIF-8 (500 K) and ZIF-90 (300K). Image adapted from Krishna111and Keskin.371


dx.doi.org/10.1021/cr400237k | Chem. Rev. XXXX, XXX, XXX−XXXAE

Table 9. Summary of Reported Data on CO2 Separation at Low-to-Moderate Temperature Using Microporous and MesoporousSilica Membranes

Table 10. Summary of Reported Data on H2 Separation at High Temperature Using Microporous Silica Membranes


dx.doi.org/10.1021/cr400237k | Chem. Rev. XXXX, XXX, XXX−XXXAF

some trends of H2/CO2 and CO2/N2 permeances onsupported microporous silica membranes, respectively, as afunction of the water partial pressure and operation time. Thegas permeance is usually strongly inhibited in the presence of

water vapor in the gas stream despite the potential increase ofthe CO2 adsorption strength, this reduction being lesspronounced when incorporating hydrophobic groups into thematerial. In the case of acid-resistant microporous membranes(e.g., zirconia), a postsynthesis treatment using an aqueousH2SO4 solution (50 M) can promote the H2 permeance andH2/CO2 selectivity. An acid treatment also allows theincorporation of superacid sulfate groups on zirconia.465

3.1.2. Amine-Functionalized Mesoporous Mem-branes. A few studies have recently appeared in the literatureon the functionalization of mesostructured silica membranes forCO2 separation. Although some examples of PEI-impregnatedMCM-48 membranes were already reported by Kumar et al.,468

the membranes showed modest N2/CO2 separation factors andextremely low gas permeances (<0.1 nmol·m−2·s−1·Pa−1),approaching the values commonly observed for polymermembranes. Another example of this concept has beenreported in a recent patent focusing on the infiltration ofsodium glycinate on a porous Ni support with a mesoporoussilica layer on one side.469 The membrane showed a muchimproved CO2 permance in the separation of a CO2/O2/N2mixture with a feed composition of 15.3:5.3:79.4 at 353 K,reaching a value of 0.1 μmol·m−2·s−1·Pa−1 and a CO2/N2separation factor of ca. 10. The sole studies on amine-graftedmesoporous silica membranes for CO2 separation have beenpublished by Sakamoto et al.,458 Ostwal et al.,380 and Guliantsand co-workers.470 The former authors reported equimolarCO2/N2 separation factors of up to 800 at 373 K on MCM-48membranes functionalized with APTES, as expected for anadsorption/diffusion mechanism activated via the formation ofsurface carbamate species. However, the membranes stillshowed extremely low CO2 mixture permeances compared tothose of raw silica membranes (2 vs 34 nmol·m−2·s−1·Pa−1),suggesting partial pore blockage. A maximum of the CO2/N2separation factor with the temperature was observed at ca. 373K (Figure 34). Evidence of the formation of carbamate speciesin APS-grafted silica membranes was provided by Ostwal etal.380 using DFT calculations, CO2 diffusion proceeding via ahopping mechanism between adjacent APS moieties.Ostwal et al.380 also reported the preparation of well-

intergrown membranes based on APTES-grafted Vycor

Figure 30. Transient permeation fluxes of a microporous silicamembrane for an equimolar CO2/N2 mixture at 448 K under a 500kPa feed step. Image adapted from Moon et al.438

Figure 31. Time dependence of the H2 permeance and the H2/CO2binary separation factor on a TEOS/TFPTES-derived hydrophobicsilica membrane under hydrothermal conditions. Image adapted fromWei et al.439

Figure 32. Representative Arrhenius plots of pure H2 (circles), N2(squares), and CO2 (triangles) permeance on amorphous silicamembranes.

Figure 33. Effect of the RH on the CO2 permeance (open symbols)and CO2/N2 separation factor (filled symbols) of a microporous silica(circles, 338 K, 10% CO2), an APTES−TEOS-derived aluminosilicatemembrane (triangles, −NH2/Si = 0.2, 295 K, 1% CO2), and a Ni-doped silica membrane (squares, 335 K, 10% CO2) after exposure for60−160 h to a humid atmosphere. Adapted with permission from ref456. Copyright 2009 Elsevier.


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membranes with larger mean pore sizes (4.7 vs 2.0 nm),showing a 1 order of magnitude lower CO2 permeance (0.22nmol·m−2·s−1·Pa−1) and a comparatively lower CO2/N2separation factor (up to 10) at 373 K for a 20% (v/v) CO2feed composition, involving the formation of surface carbamatespecies. The membranes displayed an improved permeationand separation performance at higher temperatures and lowerCO2 compositions in the feed stream (Figure 35). The authorsattributed these observations to a lower stability of carbamatespecies and increased CO2 diffusion at higher temperatures andto a higher saturation of carbamate species at higher pressures,hampering the hopping of CO2 molecules within themembrane by closing down the pores.In contrast, the latter authors reported preferential N2/CO2

separation overcoming the Knudsen threshold on MCM-48membranes being surface-modified by silylation and (3-aminopropyl)silyl groups. Such behavior could be explainedon the basis of the preferential CO2 retention on silica wallsdriven by electrostatic interactions, as pointed out by Krishna ina recent study.471

3.1.3. Ionic Liquid Membranes (RTILMs) Based onMesoporous Alumina and Silica. A prospective applicationof mesoporous silica and alumina membranes is based onroom-temperature ionic liquid membranes (RTILMs) relyingon the intrinsic adsorption capacity and selectivity of ionicliquids (ILs) toward CO2.

473 Here, instead of infiltrating themembrane with an amine, the membrane is used as a contactorfor supporting an IL, the latter acting as the true separationagent. The separation properties can be described by a classicalsolution/diffusion model as that in which gas molecules adsorbin the IL immobilized in the membrane at the feed side, diffusein the liquid phase across the membrane, and desorb at thepermeate side. Unlike impregnated amine silicas, RTILMs donot experience a significant IL loss during operation due to theextremely low volatility of ILs.Two different concepts emerge for the development of

RTILMs: (1) immobilization into a porous matrix (supported

IL membranes), and (2) IL copolymerization with polymericmembrane precursors (composite IL membranes). Table 11collects the most significant CO2 permeation and separationdata on these materials. The membrane is not intended topromote the gas/liquid contact as in the case of catalyticmembrane contactors, but only to confine the ionic liquid inthe pores by capillary forces.The first study on RTILMs for gas separation purposes dates

back to 1995 and was based on mixtures of tetraalkylammo-nium salt hydrates supported on porous polypropylene (PP)membranes.474 Since then, a number of teams have studiedpure gas permeation and gas mixture separation properties ofRTILMs based most often on alkyl-substituted imidazolium-based ILs supported on polymer membranes. The influence ofthe alkyl substituents on the imidazolium ring,475−478 thenature of the counterion,479−481 the temperature,482 and thehydrophilic/hydrophobic nature of the support483 have beeninvestigated and optimized. The highest permeabilities reportedon this type of IL membrane are on the order of 1000 barrers.Less common ILs based on phosphonium cations have beenstudied,484 but usually exhibit lower permeabilities thanimidazolium-based RTILs. Despite the relatively high CO2/N2 and CO2/CH4 selectivities that can be achieved using thesematerials (>50) and their stability in the presence of humidgases,483,485 these usually show relatively low gas permeances(on the order of 10−10−10−9 mol·m−2·s−1·Pa−1 or 100−1000barrers), comparable to those obtained with dense polymermembranes.Only very few studies have used porous ceramic supports for

confining ionic liquids. Porous α-Al2O3 disks were spin-coatedor soaked with ILs.486,487 Single-gas permeation experimentsrevealed low CO2 permeabilities (10−9−10−10 mol·m−2·s−1·

Figure 34. Dependence of the CO2 (blue) and N2 (red) permeancesand CO2/N2 selectivity (green) on temperature for a monoamine-grafted mesostructured silica membrane. Bottom image adapted fromSakamoto et al.458 SEM micrograph reprinted with permission fromref 458. Copyright 2007 Elsevier.

Figure 35. Evolution of the CO2 permance and the CO2/N2separation factor with the CO2 partial pressure in the feed andtemperature. Images adapted from Ostwal et al.380


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Pa−1), but high CO2/N2 ideal selectivities (30−127).Asymmetric porous tubes with a top layer with pores of 5nm impregnated with ILs were found to withstand a differentialpressure of up to 55 bar without any liquid loss.488 Similarly,Oyama and co-workers489impregnated α-alumina tubes andhollow fibers with pore sizes ranging from 5 to 150 nm withvarious ILs. The SRTILMs displayed CO2 permeance in therange of 5 × 10−10 to 5 × 10−9 mol·m−2·s−1·Pa−1, with idealCO2/CH4 selectivities of 5−30. In an original approach,Vangeli et al.490 grafted silylated ILs to surface hydroxyl groupsin alumina or mesoporous silica membranes with top-layer poresizes between 1 and 5 nm. The pores appeared to beincompletely filled with the ILs, resulting in high fluxes andnegligible separation factors, except for a sole IL.

3.2. Zeolite Membranes

Tables 12 and 13 compile, respectively, the main data availablein the literature on CO2 and H2 separation using zeolitemembranes in view of post- and precombustion CO2 captureapplications. In most cases, the membranes were prepared ondisks or tubular α-alumina or SS supports by in situ or seededhydrothermal synthesis (see section 4 for more details). Beforethe permeation tests, the membranes are commonly subjectedto a pretreatment at high temperature (usually in the range of473−673 K) under flow or primary vacuum to removemoisture and adsorbed vapors (see a comprehensive appraisalof activation methods in ref 491). The best CO2/N2 separationfactors reported to date correspond to FAU and T membranes,showing values, respectively, up to 70 and 200 for equimolarCO2/N2 mixtures at room temperature (Table 12). Theselectivity of these membranes is governed by the selective CO2adsorption in the zeolite material (Figure 21).The best trade-off between selectivity and permeation

corresponds to the MFI membranes prepared by Guo et

al.542 on SS wafers. These materials show room-temperatureCO2/N2 separation factors and mixture CO2 permeances ofabout 65 and 0.7 μmol·m−2·s−1·Pa−1, respectively, for equimolarCO2/N2 mixtures under Wicke−Kallenbach (WK) conditions(i.e., sweep under He flow and zero transmembrane pressure).These CO2/N2 separation factors overcome the traditionalvalues attainable on MFI zeolites (silicalite-1, HZSM-5,NaZSM-5, BaZSM-5, B-HZSM-5, and B-NaZSM-5), the latteroffering room-temperature CO2/N2 separation factors up to 20and 10, respectively, at equimolar and 10:90 CO2/N2 feedcompositions. The isomorphous substitution of Si(IV) forB(III) in the MFI framework provides materials withcomparable CO2 permeation and separation properties, butwith a better stability than unmodified counterparts.In the case of CO2/CH4 mixture separations, the highest

separation factors have been reported on H-SAPO-34, T, andDDR membranes, showing room-temperature separationfactors, respectively, up to 136−250,522,523,525 400,177 and1000526 (equimolar mixtures). The maximum CO2 permeanceson these materials are about 1.8, 0.04, and 0.055 μmol·m−2·s−1·Pa−1, respectively. Dealing with DDR membranes, molecularsieving (diffusion selectivity) governs the separation behavior ofthe material, while both adsorption and diffusion selectivitiesplay an active role in the preferential CO2 selectivity of SAPO-34 membranes. The weight of each contribution depends onthe operation conditions, most especially on the feedcomposition.MFI-type zeolite membranes have also been explored for

CO2/CH4 separations, but with modest results. Poshusta etal.503 reported CO2/CH4 separation factors of 5.5 and amixture CO2 permeance of 0.64 μmol·m−2·s−1·Pa−1 on filmlikeHZSM-5 membranes (Table 12). Sandstrom et al.495 measuredcomparable separation factors, but with much improved CO2

Table 11. Selected Data (Best Performances Reported) on CO2 Permeability Using Ionic Liquids Supported or Embedded inPolymer or PIMs


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Table 12. Summary of Reported Data on CO2 Separation Using Zeolite Membranes


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permeances (>6 μmol·m−2·s−1·Pa−1), on ultrathin silicalite-1membranes prepared using a masking technique. The sameteam showed that the CO2 separation ability of MFImembranes can be improved by introducing amino groupsinto the channels. Indeed, Lindmark and Hedlund506 reportedenhanced CO2/CH4 separation factors (up to 12) atcompetitive CO2 permeances (up to 0.9 μmol·m−2·s−1·Pa−1)on high-silica MFI membranes modified with methylamine.Finally, dealing with CO2/H2 separations, only a small

number of examples have been reported so far in the literature.

In the case of FAU-type membranes, Ginnakopoulos andNikolakis543studied the permeation of CO2 and H2 with amaximum CO2/H2 separation factor of 19.1 at 60 °C, but theCO2 permeance was as low as 0.1 μmol·m−2·s−1·Pa−1. Yucel etal.392 reported a CO2/H2 separation factor of 28 and a CO2

permeance of 0.7 μmol·m−2·s−1·Pa−1 (NaY). Much higherCO2/H2 separation factors could be afforded by Hong et al.523

on SAPO-34 membranes (about 140), but at the expense ofreduced CO2 permeances (about 0.04 μmol·m−2·s−1·Pa−1).

Table 12. continued

Table 13. Summary of Reported Data on H2 Separation Using Zeolite Membranes


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MFI-type membranes have also been studied for CO2/H2separations, but usually with modest success. Bakker et al.492

prepared silicalite-1 membranes on SS supports with a CO2/H2separation factor of 12 and a CO2 permeance of 0.38 μmol·m−2·s−1·Pa−1 at 22 °C. Guo et al.542 reported similar results onSS-embedded silicalite-1 membranes (CO2/H2 separationfactor of 15 and CO2 permeance of around 0.7 μmol·m−2·s−1·Pa−1 at 20 °C). Noteworthy, it is likely that in both studiesFe(III) cations could be incorporated into the MFI frameworkby leaching from the SS support, resulting in a more polar MFIframework compared to standard silicalite-1 films grown onalumina or carbon supports (see, for instance, the study ofKanezashi et al.501). A remarkable exception to these studies isa recent work by Sandstrom et al.495 on ultrathin silicalite-1membranes prepared using a masking technique (vide supra).These authors measured exceptionally high CO2/H2 separationfactors (>16) at very high CO2 permeances (>9 μmol·m−2·s−1·Pa−1) in the separation of CO2/H2 equimolar mixtures at 296 Kand 1010 kPa. The CO2/H2 separation factor could even beimproved below room temperature (277 K), reaching a value>50.The lack of optimal CO2/H2 separation properties for

microporous membranes relies on the opposing separationmechanism between CO2 and H2: while selective adsorptionacts as the driving force for CO2 passage, H2 is transferredmainly by selective diffusion due to its small molecular size. Incontrast, preferential H2/CO2 separation for precombustionCO2 capture applications has been reported by several authors.Hong et al.533 reported a H2/CO2 separation factor of 5 and aH2 permeance of 0.01 μmol·m−2·s−1·Pa−1 at 295 K on a boron-substituted MFI membrane (Si/B = 12.5). An ALPO-4membrane has also shown potential for H2 separation, with aH2/CO2 separation factor as high as 10 and a H2 permeance of0.2 μmol·m−2·s−1·Pa−1.Other studies have reported preferential H2 separation at

high temperature by promoting H2 diffusion pathways andblocking CO2 adsorption. Two topical examples have beenreported on MFI501,544 and DDR540,545 membranes. Kanezashiet al.501 reported H2/CO2 separation factors and H2permeances of about 3 and 1.3 μmol·m−2·s−1·Pa−1, respectively,at 673 K from equimolar H2/CO/CO2 mixtures on ZSM-5membranes blocking nonzeolite pores. Van den Berg et al.540

reported preferential H2 separation from H2/CO2 mixtures onDDR with H2/CO2 separation factors up to 4 and H2permeances of about 0.04 μmol·m−2·s−1·Pa−1 at 953 K. LTA/carbon composite membranes have also shown promisingperspectives for H2 permeation at high temperature (423 K)despite a modest H2/CO2 separation capacity, most likely dueto only partially intergrown layers.546

3.2.1. Effect of the Operation Variables on the CO2Permeation and Separation Properties. 3.2.1.1. Effect oftemperature. Figures 36−40 plot some examples of theinfluence of temperature on the CO2 permeation andseparation properties toward separation of CO2/CH4 andCO2/N2 mixtures on a series of representative zeolitemembranes (i.e., MFI, T, SAPO-34, and DDR). The CO2permeance shows in most cases a decreasing trend withtemperature, which is accompanied by a decreasing trend of theCO2/N2 and CO2/CH4 separation factors as the CO2 loading isreduced in both zeolites. A maximum of the CO2 permeance/separation factor can be obtained, even below room temper-ature, as in the case of SAPO-34 membranes.

The gas permeation behavior within zeolite membranes canshow in certain cases a dramatic increase with the temperaturebeyond a threshold value (about 400−500 K for lighthydrocarbons in silicalite-1 membranes494,547−549). Generally,the membrane permeance does not follow an adsorption-only-driven mechanism after going through a maximum in keepingwith adsorption, increasing again at higher temperatures.548,550

To face the complication of an increasing flux at hightemperatures, some authors have introduced a “gas-activatedtransport” 549,551 on top of a MS mechanism on the guidance of

Figure 36. Permeance (red, blue) and separation factor (green) of anequimolar CO2/N2 mixture as a function of temperature for a ZSM-5membrane subjected to postsynthesis modification with SiO2. Imageadapted from Shin et al.498

Figure 37. Gas permeation and separation factor of a CO2/N2equimolar mixture as a function of temperature for a NaY zeolitemembrane under dry (open symbols) and moist (closed symbols)conditions. Image adapted from Gu et al.515


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the feedback gained from silica membranes (see section 2.2.3).This complex permeation behavior and its intimate link to themembrane microstructure is addressed in more detail in section4.5.2.1. The latter mechanism does not prevent however amoderate H2 separation from CO2/H2 mixtures at hightemperature.3.2.1.2. Effect of the Feed/Permeate Composition. Figures

41−43 plot some representative data on the effect of the feedcomposition for a series of zeolite membranes (i.e., MFI, FAU,and zeolite T) in the presence of a sweep gas in the permeatestream. Irrespective of the zeolite material, the CO2/N2 andCO2/CH4 separation factors and the CO2 transmembrane flux

usually increase with the CO2 partial pressure. Competitiveadsorption occurs, the strongest adsorbed component, i.e.,CO2, occupying most of the sorption sites. Consequently,CO2/N2 and CO2/CH4 separation factors often show highervalues than the corresponding permselectivities, the effect ofpore blocking increasing with the CO2 feed composition.CO2 fluxes are only slightly affected by weakly adsorbed

species, except at very low CO2 feed concentrations, where N2can compete with CO2 for the sorption sites. This posesobvious limitations for achieving high selectivities in CO2separation at low CO2 molar fractions, which is the caseoften encountered in practical postcombustion CO2 captureapplications. Some zeolite membranes, however, still offer highseparation factors at relatively low CO2 concentrations. Aparadigmatic example is zeolite T membranes. Cui et al.177

showed that these membranes, when duly intergrown, can showCO2/CH4 and CO2/N2 separation factors up to 70 and 250,respectively, at CO2 feed compositions as low as 10% (Figure42). However, the CO2 mixture permeances remain at relativelylow values (<0.1 μmol·m−2·s−1·Pa−1).Although industrial applications discourage the use of a

sweep gas in the permeate side of the membrane, mostlaboratory-scale applications make use of it for carrying out theCO2 separation tests. In this case, CO2/N2 separation factorsare usually promoted by increasing the sweep gas flow rate.This result is traditionally attributed to a reduction of the CO2partial pressure in the permeate side, increasing the drivingforce across the membrane. The use of a sweep gas poses therisk, however, of sweep gas counterdiffusion within themembrane in the absence of strongly adsorbing sorbates or at

Figure 38. Permeance and separation factor of CO2/N2 mixtures as afunction of temperature for a zeolite T membrane in dead-end mode.Image adapted from Cui et al.177

Figure 39. Permeance and separation factor as a function oftemperature for a SAPO-34 membrane in the separation of (top)equimolar CO2/CH4 mixtures and (bottom) nearly equimolar CO2/H2 mixtures. Images adapted from Li et al.176 and Hong et al.523

Figure 40. Permeance (red, blue) and separation factor (green) ofequimolar CO2/CH4 (top) and CO2/N2 (bottom) mixtures as afunction of temperature at different feed pressures for a DDRmembrane. Image adapted from van den Berg et al.181


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sufficiently high temperatures. Figure 44 shows an example ofthe effect of temperature on the He counterdiffusion within asilicalite-1 membrane for a series of alkane sorbates. Themaximum temperature avoiding He counterdiffusion increaseswith the chain length due to a higher adsorption capacity of thealkane in the MFI framework.3.2.1.3. Effect of the Feed and Transmembrane Pressures.

Another key variable in the performance of zeolite membranesis the feed pressure. Figures 45−48 plot the influence of thetotal feed pressure on the CO2 separation performance for a

series of representative well-intergrown zeolite membranes(silicalite-1, T, and DDR). Furthermore, Figure 49 comparesthe CO2 separation selectivity of different zeolite materials onthe basis of published data based on standard binary mixturesusing GCMC + MD simulations. As a general trend, thepressure dependence of the CO2/X separation factor (X = H2,N2, CH4) for a given membrane depends strongly on the feedcomposition: the higher the CO2 feed composition, the morepositive the slope of the CO2/X curve with the feed pressure(Figure 45 for CO2/N2 separation within a silicalite-1membrane).The CO2 separation factors remain essentially unchanged

until 10 bar for different standard CO2 separation scenarios.Beyond this threshold pressure, different trends are observed:(1) the CO2/N2 separation factor still remains invariable withthe feed pressure, which can be explained by the essentially

Figure 41. Binary flux, separation factor, and ideal separation factor ofa CO2/N2 mixture as a function of the CO2 molar fraction for asilicalite-1 membrane. Image adapted from van den Broeke et al.492

Figure 42. Permeance and separation factor of a CO2/N2 mixture(closed symbols) and a CO2/CH4 mixture (open symbols) as afunction of the CO2 molar fraction for a zeolite T membrane. Imageadapted from Cui et al.177

Figure 43. Permeance and separation factor of CO2/CH4 (top), CO2/N2 (middle), and CO2/H2 (bottom) mixtures as a function of the CO2feed molar composition for a SAPO-34 membrane. Images adaptedfrom Li et al.520,560 and Hong et al.523


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constant adsorption and diffusion selectivities with pressure(Figures 20 and 27); (2) the CO2/H2 separation factor shows amaximum with the feed pressure for narrow-pore zeolitemembranes and a decreasing trend for NaY and NaX zeolitemembranes, which can be well accounted for by the trends ofCO2/H2 adsorption selectivities; (3) the CO2/CH4 separation

factors most often show a decreasing trend with the feedpressure, which can be explained by the general behavior of theCO2/CH4 diffusion selectivity with the feed pressure for thedifferent membranes.

3.2.1.4. Effect of Water and Impurities on CO2 Permeationand Separation Properties. Systematic CO2 separationexperiments at cycle times approaching those required inindustry (>3000 h) are needed to conceive realisticapplications, focusing specifically on the effect of water andimpurities such as acidic or alkaline vapors, promoting in somecases dealumination or desilication under current operationconditions. However, most of the reported permeation and

Figure 44. Permeance of the sweep gas (He) from the permeate to thefeed side of a silicalite-1 membrane as a function of temperature forCH4, C2H6, and n-C4H10 in the feed side. Image adapted from van denBroeke et al.492

Figure 45. Separation factor of a CO2/N2 mixture as a function of thefeed pressure and different CO2/N2 feed compositions for a silicalite-1membrane. Image adapted from van den Broeke et al.492

Figure 46. Permeance and separation factor of a CO2/N2 mixture as afunction of the total pressure for a zeolite T membrane. Image adaptedfrom Cui et al.177

Figure 47. Permeance (red, blue) and permselectivity (green) as afunction of the total feed pressure for CO2 and CH4 permeation for aDDR membrane. Image adapted from van den Berg et al.181

Figure 48. Permeance (H2, red; CO2, blue) and separation factor(green) as a function of the total feed pressure for CO2 and H2permeation within an ultrathin MFI membrane. Image adapted fromSandstrom et al.495


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separation data have been measured for dry flue gases, omittingthe effect of water on pore blockage. In the case of membranesoperating via selective CO2 adsorption, care should be takenwhen dealing with strongly hydrophilic zeolites (e.g., FAU)provided that the relative humidity is kept below a thresholdvalue determined by the form of the water isotherm.Following the study of Gu et al.,515 this threshold humidity

can be extended to higher values at higher temperatures drivenby a reduction of the water loading. Figure 50 plots theevolution of the CO2 permeance and CO2/N2 separation factoras a function of temperature for an FAU membrane subjectedto dry and humid atmospheres (2.64 kPa water pressure). TheCO2 and N2 permeances showed a drastic decrease in the

presence of water due to pore blockage (about 2 orders ofmagnitude for CO2 at 300 K), the initial values only beingpartially recovered at temperatures of >425 K.Gu et al.515 also reported a drastic reduction of steady-state

CO2 and N2 gas permeances with the water partial pressure at473 K (more than 1 order of magnitude) for partial pressures>20 kPa. Beyond this level, the CO2/N2 separation factorsshowed a progressive decrease from a maximum value of about4.5. Keeping this picture in mind, hydrophobic or lowhydrophilic materials such as MFI membranes, relying onselective CO2 adsorption, appear to be more suitable forrepelling water and preventing pore blockage during operation.Indeed, Nicolas and Pera-Titus88 reported a net reduction ofthe gas permeance with water incorporation to the feed,irrespective of the degree of B, Ge or Cu substitution in MFI−alumina hollow fibers. Beyond 2% humidity, the CO2 and N2permeances showed a steady-state decrease of the CO2permeance to ca. 40% of the initial value when incorporatingwater into the feed stream. However, at lower waterconcentrations, the permeance reduction appeared to bemore sudden, probably due to favored water adsorption inmicropores (sitting near cations). The CO2 and N2 permeationpatterns were accompanied by CO2/N2 separation factorsshowing a slight increase with the water concentration in thefeed stream. Such a positive effect might be attributed to partialwater condensation in intercrystalline mesopores, blockingnonselective defective pathways. Moreover, an effect of partialintercrystalline pore shrinkage due to crystal swelling uponwater adsorption cannot be excluded (section 4.5.2.2).

Figure 49. Evolution of the CO2/H2 (top), CO2/N2 (middle), andCO2/CH4 (bottom) permeation selectivity (separation factor) as afunction of the total gas fugacity computed from CBMC + MDsimulations on different zeolite frameworks (all-silica FAU, LTA,DDR, CHA, MFI, ERI, ITQ-29 + NaY, NaX, SAPO-34). Data adaptedfrom ref 111.

Figure 50. Gas permeation and separation factor of CO2/N2equimolar mixtures as a function of the water partial pressure forGe-MFI (top) and NaY (bottom) zeolite membranes. Images adaptedfrom Nicolas and Pera-Titus88 and Gu et al.515


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Systematic studies on the permeation properties of high-quality SAPO-34 and DDR membranes have been reported,respectively, by Noble and Falconer552 and Himeno et al.180 forthe separation of CO2/CH4 mixtures. Figure 51 reproduces

some representative results obtained by both groups, showingthe effect of water introduction in simulated flue gases on themembrane permeation properties. In both cases, the mem-branes showed gas permeances and separation factors that werefairly stable with time after water introduction. Water onlycontributed to a significant decrease of CO2 permeance in thecase of DDR membranes. As all-silica DDR membranes have anintrinsic hydrophobic nature, the reduction of CO2 permeancein these materials might be ascribed to single-file diffusioneffects due to preferential passage of water compared to CO2and CH4 on the basis of the smaller kinetic diameter of theformer (Table 2).Noble and Falconer552 reported an interesting study on the

long-term stability of SAPO-34 membranes in the presence ofwater. A typical permeation pattern observed for low- andintermediate-quality SAPO-34 membranes is reproduced inFigure 52. Such materials might suffer from aging effects afterlong exposure to humid atmospheres, and this phenomenonmight be accelerated in the presence of nonzeolite pores byincreasing the accessibility of water to SAPO-34 crystals. As aconsequence, CO2 permeation is reduced, while the CH4

permeance might be promoted due to preferential passagewithin defective domains. The presence of large amounts ofwater and linear hydrocarbons in the gas stream can promotecrystal expansion during the permeation experiments, blockingintercrystalline domains and increasing accordingly themembrane selectivity to CO2.

553 Such observation providesadditional evidence of the crucial and subtle role of themicrostructure on the membrane permeation and separationproperties (see section 4.5.2.2 for more details).Hedlund and co-workers506 reported a dramatic increase of

the CO2/CH4 and CO2/H2 selectivity for equimolar mixturesin the presence of water vapor for HZSM-5 membranes (Si/Al> 100) modified with methylamine. Figure 53 plots an exampleof the permeation and separation trends for ternary CO2 (49.6kPa)/H2 (49.6 kPa)/H2O (2.1 kPa) mixtures during acomplete adsorption/desorption cycle. An interesting hystereticphenomenon was observed, showing higher hydrogen (ormethane) permeances when cooling. The authors attributedthis phenomenon to partial water desorption when cooling,reducing channel blockage by water. The CO2/H2 separationfactor was promoted by water adsorption, showing a maximumwith the temperature at a value of 6.5 with a hydrogenpermeance as high as 1.3 μmol·m−2·s−1·Pa−1 at about 325 K.This behavior contrasts with the much lower CO2/H2separation factor (about 3) near room temperature for anunmodified MFI membrane.Water vapor can also exert a positive effect on H2 separation

from CO2/H2 mixtures in high-temperature applications. Wangand Lin544,561 reported interesting examples on ZSM-5membranes (Figure 54). The H2 and CO2 permeances bothshowed a sustained decrease with the water pressure, beingmore dramatic for the H2 permeance. The H2/CO2 separationfactor showed a value of >3 irrespective of the water pressure.Moreover, the introduction of H2 either in the feed or in thepermeate streams has been reported to provide a stabilizingeffect on the permeation of hydrocarbons and xylenes withinZSM-5 membranes. Such an effect could play an important rolein the separation of CO2/H2 or H2/CO2 mixtures.

555

Finally, Nicolas and Pera-Titus88 studied the effect of NOand propane addition on the CO2/N2 separation factors ofMFI−alumina hollow fibers. The presence of NO in the feed(5000 ppm) did not significantly modify the CO2/N2separation properties of the material. Although NO permeatedfaster than N2, the CO2/N2 separation factor was not affected

Figure 51. Permeance and selectivity as a function of time of anequimolar CO2/CH4 mixture containing (top) 0.02% water at 297 Kfor a SAPO-34 membrane and (bottom) 3% water at 298 K for a DDRmembrane. In both cases, the experiments were performed in WKmode at a feed and permeate pressure of about 100 kPa. Imagesadapted from Tomita et al.527and Poshusta et al.552

Figure 52. Transient permeation of CO2 and CH4 within SAPO-34membranes with different initial qualities in the presence of a humidatmosphere. The humidifier was turned off at time 0 h. Image adaptedfrom Poshusta et al.552


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by the presence of NO for the entire range of temperaturetested. Figures 55 and 56 summarize the results obtained on theseparation of a feed mixture with molar composition10:1:2:0.5:86.5 CO2/propane/O2/NO/N2. Introducing pro-pane to the feed stream led to a significant reduction of theCO2, N2, and NO permeances, the former by a factor of 3, butstill maintaining a value of >0.4 μmol·m−2·s−1·Pa−1 at 303 K forthe Al-MFI, B-MFI-50, and Cu-MFI samples (>0.7 μmol·m−2·s−1·Pa−1 at 373 K for Al-MFI and Cu-MFI samples). Thesamples preferentially permeated propane at a level of 1.5μmol·m−2·s−1·Pa−1 at 303 K.In contrast, the room-temperature CO2/N2 separation factor

increased to a value of about 6 in the presence of propane. Theauthors attributed such an observation to the strong adsorption

of propane in the MFI framework and to correlation effects inthe surface diffusive patterns between the sorbate species,slowing the permeation of lower adsorbing species, i.e., NO andN2. The presence of a maximum in the curve plotting theevolution of the CO2 permeance with the temperature reflecteda competitive adsorption between CO2 and propane. Sucheffects were predicted and modeled by Krishna and vanBaten.425 The beneficial effect of propane adsorption in theCO2/N2 separation factor was hindered beyond 373 K.

3.2.2. Influence of the Membrane Structure on theCO2 Permeation and Separation Properties. 3.2.2.1. In-fluence of Exchangeable Cations. Following the commentsstated in section 2.1.2.2 on the role of exchangeable cations onthe CO2 adsorption properties of high-Al zeolites, the CO2permeance and separation capacity of zeolite membranes canbe affected by the presence of cations. A series ofcomprehensive studies on this issue were reported by Kusakabeand co-workers377,392,556,557 for FAU zeolite membranes. TheCO2 permeance within these membranes decreases in the orderLi > K > Na and Ba > Ca > Mg due to a combined effect ofvariable CO2/N2 sorption capacity and CO2/N2 surfacediffusivities. Figure 57 plots some pure and mixture CO2permeation data on ion-exchanged FAU membranes. In generalterms, NaY and KY membranes show the highest CO2/N2separation factors. Partial impregnation of Rb+ and Cs+ cationsin the zeolites might increase the CO2/N2 separation factor to150 depending on the cation exchange level.556 FAU zeolites,incorporating higher Al amounts, provide improved CO2/N2and CO2/CH4 separation factors.557

In the case of MFI zeolite membranes, the CO2 permeationperformance can only be tuned for low Si/Al ratios. For a Si/Alratio of 25, Aoki et al.558 reported increasing pure CO2permeances in the order K+ < Ba2+ ≈ Ca2+ < Cs+ < Na+ ≈H+, approximately matching a decrease in the cation size.Lindmark and Hedlund505 reported a comprehensive study onthe CO2/H2 permeation and separation properties of silicalite-1and ion-exchanged MFI membranes with Li, Na, and Ba(Figure 58). Ba-MFI membranes show the highest CO2/H2separation factors, but at the expense of a lower CO2permeance due to a reduction of the channel size upon Baincorporation into the MFI framework. Li et al.559 also showeda variation of the H2 and CO2 permeation properties with the

Figure 53. Ternary CO2/H2/H2O permeances and separation factorsas a function of temperature for an H-ZSM-5 membrane in theseparation of H2 (49.6 kPa)/CO2 (49.6 kPa)/H2O (2.1 kPa) mixturesat atmospheric pressure under WK conditions. Images adapted fromLindmark and Hedlund.506

Figure 54. High-temperature permeance (red, blue) and separationfactor (green) of a CO2/H2 equimolar mixture for a MFI membrane asa function of the water partial pressure. Image adapted from Zhu etal.561


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degree of Ge substitution into the MFI framework (Figure 59).However, no separation properties were reported.In the case of SAPO-34 membranes, the CO2/CH4

separation properties are strongly affected by the crystal sizeand the Si/Al ratio in the top layer.540,560 An optimal crystalsize was found at about 0.8 μm, whereas an optimal Si/Al ratiowas found at 0.13, providing the highest selectivities. Higher Si/Al ratios might enhance the presence of the SAPO-5 phase andnonzeolite pores in the zeolite layers, reducing the CO2/CH4

separation factors to a value of ca. 40 and increasing the CO2

permeance to a value of >2 μmol·m−2·s−1·Pa−1. Concentration

polarization effects at the feed/membrane interface can alsoexert a positive effect on the CO2 separation in SAPO-34membranes.554

3.2.2.2. Influence of Silica Deposits. The separationperformance of zeolite membranes can be improved bypreferential sealing of nonzeolite pores (see section 4.5.1.2).Shin et al.498 used this approach to improve the separationperformance of MFI membranes toward the separation ofCO2/N2 mixtures. Furthermore, the separation performance ofzeolite membranes can be tuned to make them selective for theless adsorbing species at low temperatures. Hong et al.533 found

Figure 55. Evolution of the CO2 (open circles), N2 (open squares), C3H8 (inverse open triangles), and NO (open tilted squares) permeances andCO2/N2 separation factor (filled triangles) as a function of temperature for the Al-MFI, B-MFI-50 (Si/B = 50), Ge-MFI-10 (Si/Ge = 10), and Cu-ZSM-5 hollow fibers. Experimental conditions: feed composition 10:1:2:0.5:86.5 CO2/C3H8/O2/NO/N2; feed flow rate, 500 cm3 (STP)/min;sweep flow rate, 200 cm3 (STP)/min; feed pressure, 200 kPa; transfiber pressure, 50 kPa. Images adapted from Nicolas and Pera-Titus.88

Figure 56. Comparison of the CO2/N2 permeation and separation performance of Al-MFI, B-MFI-50, Ge-MFI-10, and Cu-MFI hollow fiberswithout and with the presence of propane in the feed stream. Experimental conditions: feed flow rate, 500 cm3 (STP)/min; sweep flow rate, 80 cm3

(STP)/min; feed composition 10:89:0.5 CO2/N2/NO; feed pressure, 200 kPa; transfiber pressure, 50 kPa. Images adapted from Nicolas and Pera-Titus.88


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that the intrinsic CO2 separation capacity of B-ZSM-5membranes in the separation of CO2/H2 mixtures could betuned to H2 by silylation. Figure 60 plots the evolution of theH2 permeance and H2/CO2 separation factor as a function oftemperature for this membrane after modification. Zhu et al.561

monitored a progressive increase of the H2/CO2 separationfactor at 723 K upon CVD of silica on a ZSM-5 membrane.The same group reported an improvement of the H2/CO2separation factor for a DDR membrane. Contrary to the trendsusually observed in zeolite membranes, the H2 permeance andH2/CO2 separation factors can experience a dramatic enhance-ment at high temperature in these materials. This observation isattributed to a strong promotion of H2 mass transfer by surfacediffusion within silica domains at higher temperatures.3.3. MOF Membranes

A few studies have appeared recently providing CO2 and H2permeation and separation data on continuous and well-intergrown MOF films. Tables 14 and 15 collect the availabledata reported in the literature. Most of the successfulmembrane materials show preferential H2 separation overCO2 at moderate to high temperatures beyond the Knudsenselectivity threshold, promoting in some cases molecular sievingeffects. Preferential CO2 separation also appears possible, buton duly functionalized materials providing optimal electrostaticpotentials for selective adsorption. The main separationcharacteristics of the different families of MOF materials withproven separation properties and reproducibility are addressed

below. Section 4 compiles the main details on the manufacture,layer intergrowth, and microstructure of the different materials.

3.3.1. H2 Separation: Molecular Sieving Mechanism.3.3.1.1. MOFs Based on Cu(II) Clusters. The first andchronologically older family of MOF membranes displayingH2 separation is based on materials including Cu(II) clusters.An example of this material is HKUST-1 with a mean pore sizeof 9 Å. Guo et al.585 prepared HKUST-1 films (60 μm) onpartially oxidized Cu supports with a H2/CO2 (equimolar)mixture separation factor of 6.8 at room temperature. The H2permeance increased while the permselectivity decreased withthe temperature in the range of 273−343 K. Guerrero et al.586

Figure 57. Effect of ion exchange on the gas permeance in theseparation of CO2/N2 mixtures for ion-exchanged Y-type membranes.Image adapted from Kusakabe and co-workers.377,392

Figure 58. Ternary CO2/H2/H2O permeances (top and middle) andseparation factors (bottom) as a function of temperature for silicalite-1and ion-exchanged ZSM-5 membranes (Li, Na, Ba) in the separationof H2 (49.6 kPa)/CO2 (49.6 kPa)/H2O (2.1 kPa) mixtures atatmospheric pressure under WK conditions. Images adapted fromLindmark and Hedlund.505


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also prepared HKUST-1 membranes on porous aluminasupports by seeded solvothermal synthesis, but with lowerH2/CO2 permselectivities (3.5) at room temperature. Ranjanand Tsapatsis587 prepared oriented [Cu(hfipbb)(H2hfipbb)0.5](MMOF) films on porous α-alumina substrates by seededsolvothermal synthesis, the material showing extremely low

pure CO2 permeances (0.8 nmol·m−2·s−1·Pa−1) and a H2/CO2permselectivity of about 4, approaching the Knudsen threshold.

3.3.1.2. MOFs Based on Zn(II) Clusters. A second class ofMOF membranes is constituted by materials with Zn(II)clusters. An example of a Zn-based membrane with partial H2/CO2 permselectivity is IRMOF-1 (MOF-5)395grown on ahollow-fiber support. The materials showed competitive room-temperature H2/CO2 permselectivity compared to Knudsenselectivity.In addition to this material, a comprehensive series of studies

have been devoted to the preparation of Zn-based ZIF-typemembranes, showing promising hydrothermal and chemicalstability. The most systematic body of results on ZIFmembranes with H2 selectivity has been reported by Caro’sgroup. In a pioneering study, this team reported the preparationof well-intergrown c-oriented ZIF-8 films on titania supports bya microwave (MW)-assisted solvothermal process in methanolafter impregnating the support with PEI.573 After activation atroom temperature, the membrane showed an equimolar H2/CO2 separation factor of 7.0 overcoming the Knudsenselectivity threshold, but with a reduced H2 permeance (15nmol·m−2·s−1·Pa−1). In addition, some H2/CH4 separationexperiments revealed partial CH4 passage through the porenetwork of the membrane, even if the window size of ZIF-8(3.4 Å) is smaller than the kinetic diameter of CH4 (3.8 Å).The authors attributed this observation to the frameworkflexibility. As a matter of fact, the ZIF-8 framework is flexibledue to the flip-over of the imidazole ring, involving anexpansion of the window size from 3.4 to 4.0 Å.Xu et al.574 also reported high H2/CO2 permselectivities (up

to 55) and H2 permeances as high as 1 μmol·m−2·s−1·Pa−1 onZIF-8−alumina hollow fibers. Figure 61 plots the characteristicpermeation trends with time for H2, N2, CH4, and CO2 on thismaterial. Among the different gases tested, CO2 showed thelongest stabilization times, while H2 showed a practicallyinstantaneous steady-state permeation. Membrane pretreat-ment protocols for ZIF-8 activation before gas permeation/separation tests are usually stricter than for zeolite membranes,especially in the presence of open metal sites, involving heatingat 473−573 K under inert gas flow for 24−72 h. Related to thisstudy, Lai and co-workers577 and Tao et al.578 recently reportedthe preparation of ZIF-8 films supported on the outer surface ofYSZ and ceramic hollow fibers with very high H2 permeances(>10 μmol·m−2·s−1·Pa−1), promising room-temperature H2/

Figure 59. Pure gas permeances at 473 K as a function of the kineticdiameter for a series of Ge-ZSM-5 membranes grown on SS supports.Image adapted from Li et al.559

Figure 60. H2 permeance and equimolar H2/CO2 selectivity as afunction of temperature for a B-ZSM-5 membranes ubjected topostsynthetic silylation. Image adapted from Hong et al.533

Table 14. Summary of Reported Data on CO2 Separation Using MOF and ZIF Membranes


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CO2 permselectivities (3.8 and 5.5, respectively), and highequimolar H2/propane separation factors (474). Unfortunately,no separation results were provided to assess the relative role ofpotential mesoporous intercrystalline defects on the membraneselectivity.Caro and co-workers579 manufactured ZIF-7 membranes on

porous α-alumina supports by seeded MW-assisted solvother-mal synthesis with a high equimolar H2/CO2 separation factor(6.5) after activation at 473 K driven by a molecular sievingmechanism. In a further development, a ZIF-7 membrane withan equimolar H2/CO2 separation factor of 14 at 493 K wasreported with an increasing trend of the H2 permeance with thetemperature and a remarkable hydrothermal stability.580 Figure62 plots the time behavior of the membrane, showing a stablepermeation and separation performance. Finally, the same teamreported the preparation of c-oriented ZIF-7 membranes with apromising equimolar H2/CO2 separation factor at 473 K (8.4),but at the expense of a low H2 permeance, most likely due tothe anisotropy of the pore structure.588

Relying on these results, Caro and co-workers582 developedZIF-22 membranes on titania supports by solvothermalsynthesis using APTES as a covalent linker. The equimolarH2/CO2 separation factor was 7.2 at 323 K driven by amolecular sieving effect. Likewise, ZIF-90 membranes wereprepared on α-alumina supports by chemical bonding betweenZIF-90 crystals and the support via the APTES linker. Theequimolar H2/CO2 separation factor was 7.3 at 473 K with astable permeation and separation performance even when watervapor was added into the feed stream.583 The covalentpostsynthetic functionalization of the former ZIF-90 membrane

by ethanolamine enhanced the equimolar H2/CO2 separationfactor to 16 at 498 K, involving the formation of imine surfacegroups.584 The authors attributed such improvement to a slightreduction of the pore size and to an increase of the layerintergrowth (see section 4.4.1.1 for more details).

3.3.1.3. MOFs Based on Al(III) Clusters. The third family ofMOF membranes displaying selectivity to H2 relying onpreferential diffusion is based on MIL-53(Al)-NH2. Two veryrecent studies have been published showing H2/CO2

permselectivities overcoming the Knudsen threshold.570,571

Interestingly, the former study also addressed H2 separationfrom equimolar H2/CO2 mixtures at near room temperature,achieving a separation factor of ca. 31 and a H2 permeance >1μmol·m−2·s−1·Pa−1. The membrane was operated at near-ambient pressure with a transmembrane pressure of 101 kPa,far enough from the np → lp phase transition. The H2/CO2

separation factor showed a slight decrease with the temperaturein the range of 290−350 K, whereas the H2 permeanceincreased slightly, as expected for a membrane governed bypreferential diffusion (Figure 63).

3.3.2. CO2 Separation via Preferential Adsorption.MOF materials with partial CO2 separation from CO2/N2 andCO2/CH4 mixtures have been developed in the past 5 yearsfocusing on the enhanced CO2 strength of the material. Thebest trade-off between CO2 permeance and selectivity in ZIF-type membranes was reported by Venna and Carreon566 (ZIF-8on α-alumina) in the separation of equimolar CO2/CH4

mixtures. These authors reported room-temperature CO2

permeances and CO2/CH4 separation factors as high as 240

Table 15. Summary of Reported Data on H2 Separation using MOF and ZIF Membranes


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μmol·m−2·s−1·Pa−1 and 5.1, respectively, under a finitetransmembrane pressure difference.Aguado et al.589,590 recently reported the synthesis and CO2

separation properties of SIM-1 (a variety of ZIF-8) membranes.The membranes showed mixture CO2 permeances and CO2/N2 separation factors up to 40 nmol·m−2·s−1·Pa−1 and 4.5,respectively, in the separation of a ternary 10:87:3 CO2/N2/H2O mixture at 324 K, a 4 bar feed pressure, and a 4 kPatransmembrane pressure. With the exception of the studyreported by Venna and Carreon,566 this material matches the

permeation properties of ZIF-8 and ZIF-7 membranes reportedso far by Caro and co-workers (Figure 64).

Besides ZIF membranes, Betard et al.562 recently reportedthe preparation of membranes based on [Cu2(BME-bdc)2(dabco)] with BME-bdc = 2,5-bis(2-methoxyethoxy)-1,4-benzenedicarboxylate. The membrane displayed a room-temperature equimolar CO2/CH4 separation factor of 4.5,which is larger than the Knudsen selectivity (0.6). Moreover,Lin and co-workers395 developed an MOF-5 membranedisplaying preferential CO2 separation from CO2/H2 andCO2/N2 mixtures at near room temperature for a CO2 molarcomposition in the feed stream >0.8. At equimolar concen-trations, the membrane only displayed a moderate CO2/H2separation factor of 2.5 and a CO2 permeation of 0.2 μmol·m

−2·s−1·Pa−1.In addition to the above-stated MOF films with a

polycrystalline structure, Takamizawa et al.591 prepared aCu2(bza)4(pyz) (bza = benzoate; pyz = pyrazine) single-crystalmembrane with a 1D chain structure constituted of orientednarrow channels (<4 Å) in two crystallographic directions. Aseries of pure permeation tests on this membrane reflected afaster permeation pathway along the channel direction thanalong the direction perpendicular to the channels (up to 60-

Figure 61. Time evolution of the pure H2, CO2, N2, and CH4permeances, as well as the H2/CO2, N2/CO2, and CH4/CO2permselectivites, for a ZIF-8 film grown on an α-alumina hollowfiber. Images adapted from Xu et al.574

Figure 62. Stability test of a ZIF-7 membrane in the separation of anequimolar H2/CO2 mixture with addition of a 3 vol % steam at 493 K.Image adapted from Li et al.580

Figure 63. Permeation and separation performance as a function oftemperature of a H2/CO2 equimolar mixture for an MIL-53(Al)-NH2membrane. Image adapted from Zhang et al.570

Figure 64. Comparison of pure gas permeation behavior of ZIF-7(493 K) and SIM-1 (303 K) membranes as a function of the molecularkinetic diameters. Image adapted from Li et al.579and Aguado et al.590


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fold, Figure 65). The membrane displayed a CO2/CH4

permselectivity of 25.

Comprehensive studies on the effect of the feed pressure onMOF and ZIF membranes are very scarce. In silico studies ofsuch materials reveal as a powerful tool to unveil the maintrends of the membrane separation capacity with the feedpressure. Figure 66 plots some representative trends of CO2/X(X = H2, N2, CH4) separation factors as a function of the totalfeed pressure (fugacity) for a series of MOF membranes usingGCMC + DM simulations as inferred from the studiespublished by Krishna99 and Keskin.592−595 The separationfactors appear to be invariable with the feed pressure until 7MPa for the different materials, except for CO2/H2 and CO2/CH4 separation on CuBTC, CO2/H2 separation on bio-MOF-11, and CO2/CH4 separation on IRMOF-1 and ZIF-10membranes. In such materials, the separation factors increasewith the feed pressure driven by preferential CO2 adsorption.3.4. Taxonomy of Materials Based on PermeationProperties under Ideal/Real Conditions

We have summarized in section 3.1−section 3.3 the mostoutstanding permeation and separation properties of the mainfamilies of porous inorganic materials with promisingapplications as membranes for pre- and postcombustion CO2capture. Now let us compare these different materials to drawcorrelations between the membrane selectivity and thepermeability of the fastest diffusing species (CO2 and H2,respectively, for pre- and postcombustion applications) instandard Robeson plots. Figure 67(left) shows the Robesonplot for the near-room-temperature separation of equimolarCO2/N2, CO2/CH4, and CO2/H2 mixtures. For the sake ofcomparison, the same figure also plots some data reported byKrishna and van Baten using CBMC + MD simulations onpurely silica zeolites and some MOFs for which the membranesynthesis and performance have not been achieved ordemonstrated to date (i.e., MOF-177 and MgMOF-74).Furthermore, to take into account the effect of the top-layerthickness, Figure 67(right) plots the variation of the CO2/Xseparation factors (X = N2, CH4, H2) with the permeance of thefastest diffusing species.3.4.1. CO2/N2 Separation. Among the different materials,

NaY membranes offer the best trade-off between CO2/N2separation capacity and CO2 permeability, exceeding the

Robeson upper bound. Even if the standard thickness of NaYmembranes is considered in the computation of the CO2permeance, this material still shows the best performancedriven by preferential adsorption selectivity of CO2 over N2.Although MFI-type membranes do not appear to compete withNaY membranes for CO2/N2 separation, the two MFImembranes recently reported by Guo et al.542 (HZSM-5)and Shin et al.498 (NaZSM-5) provide an opposite perspective.Indeed, these materials show CO2/N2 separation factors, CO2permeabilities, and CO2 permeances comparable to the bestvalues reported on NaY membranes. Given the higher Si/Alratio of ZSM-5 membranes compared to NaY counterparts, theabove-stated MFI membranes appear to be more indicated forCO2/N2 separation in the presence of moisture. Althoughamine-functionalized membranes show competitive separation

Figure 65. Comparison of the pure gas permeance for different gasesacross a single-crystal [Cu2(bza)4(pyz)]n membrane: (red) along thechannels (channel membrane); (light-blue) perpendicular to thechannels (nonchannel membrane). Image adapted from Takamizawaet al.591

Figure 66. Evolution of the CO2/H2 (top), CO2/N2 (middle), andCO2/CH4 (bottom) adsorption selectivity as a function of the total gasfugacity computed from CBMC simulations for a series of MOFs andZIFs. Images adapted from refs 99 and 592−595.


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factors, their modest CO2 permeances, comparable to thoseavailable on polymer membranes, make these materials notcompetitive at their present stage of development.3.4.2. CO2/CH4 Separation. In the case of CO2/CH4

separations, DDR and SAPO-34 membranes provide apparentlythe best performance, as well as all-silica ERI, CHA, and ITQ-29, although no experimental data on the latter have beenreported to date. In all cases, separation is driven by preferentialCO2 diffusion with reduced correlation effects. The highestCO2 permeance of these materials is about 1 × 10−6 mol·m−2·s−1·Pa−1 given the present preparation protocols. MOF and ZIFmembranes usually show a modest trade-off between CO2/CH4

separation and CO2 permeability, approaching the upperbound. One exception is the ZIF-8 membrane reported byVenna and Carreon,566 offering an unexpectedly high CO2

permeance at a modest CO2/CH4 separation factor <10, butovercoming the upper bound.

3.4.3. CO2/H2 and H2/CO2 Separation. Finally, twodifferent scenarios are possible for CO2/H2 separation: (1)preferential CO2 separation, and (2) preferential H2 separation.In the former case, NaY and BaZSM-5 membranes appear toperform the best, showing preferential CO2 adsorption and ahigh degree of correlation effects compensating the preferentialH2 mobility within the microporous framework. In contrast, inthe case of H2/CO2 separation, separation must proceed byselective H2 diffusion with a low degree of correlation effects. Inthis case, the ZIF-8 membranes prepared by Xu et al.574 appearto offer the best trade-off between H2/CO2 selectivity and H2

permeability. However, care should be taken with this result,since it refers to permselectivity measurements. In addition tothis membrane, CuBTC, ZIF-22 and ZIF-90 offer comparable

Figure 67. Taxonomy of silica, zeolite, and MOF/ZIF materials for equimolar CO2/N2 (top), CO2/CH4 (middle), and CO2/H2 (bottom)separation at near room temperature and ambient pressure for post- and precombustion CO2 capture applications and comparison with establishedupper bounds for polymeric membranes.596 The left-handed graphs represent the Robeson plots (separation factor or permeation selectivity(separation factor) vs permeability of the fastest species in barrers, 1 barrer = 3.348 × 10−16 mol·m−1·s−1·Pa−1), while the right-handed graphsrepresent the evolution of the given separation factor with the permeance of the fastest species. For comparison, the figures also show CBMC + MDsimulated data (black squares) at near room temperature according to Krishna and van Baten99 (T = 300 K, P = 1 MPa, 15:85 CO2/N2, 50:50 CO2/CH4, 15:85 CO2/H2).


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or even higher H2 permeabilities, but with H2/CO2 separationfactors <10. Transposing these data into H2/CO2 separationfactors and CO2 permeances, sol−gel microporous silicamembranes can compete in certain cases with the above-statedMOF materials on the basis of the comparably narrowerthickness of silica layers, usually <1 μm.3.4.4. Insights into in Silico Studies. The taxonomy of

materials proposed in this section joins grosso modo theconclusions drawn by Krishna and van Baten99 in their in silicodiscrimination of zeolite and MOF materials for CO2 captureapplications. It should be stressed from this study the potentialsof MgMOF-74 membranes for CO2/H2 and even CO2/CH4separations. However, to our knowledge, no experimental studyhas been reported to date on the synthesis and permeationproperties of such membranes. In the latter case, only veryrecently the synthesis of the NiMOF-74 was achieved on α-alumina disks with a H2/CO2 permselectivity of 9.1.Furthermore, COFs also show promising potentials for CO2/H2 separations as can be inferred from the simulation studyreported by Keskin on CH4/H2 separation.

597 However, to ourknowledge, no proof of concept is available in neither thepatent nor the open literature on the extrapolation of suchresults to CO2 separation.

4. MEMBRANE SYNTHESIS AND MICROSTRUCTURE

In the former chapter, we have illustrated the most relevantCO2 separation and permeation trends for silica, zeolite andMOF/ZIF membranes for different CO2 capture scenarios. Forpractical applications, this information has to be necessarilycompleted with detailed data on the membrane microstructure,offering a detailed picture of the intercrystalline structure of theactive layers. This chapter provides the reader with a criticaldescription of the main characteristics of intercrystallinedomains in PIMs as a function of the synthesis protocols andof their potential role as defect sources and low-energy barriersfor gas diffusion. Moreover, we also address the reproducibilityof the synthesis protocols and upscaling potentials of thedifferent formulations as key issues for industrial success.

4.1. Membrane Supports: From Tubes to Hollow Fibers

The first and a key element for developing well-intergrownmembrane layers is the right choice of supports with optimalquality and cost. As a matter of example, the cost of a zeolitemembrane without housing has been estimated to be $1000−2500 per square meter, the support accounting for more than75% of the overall cost.87,598

Most often, gas separation membranes are manufacturedonto a symmetric or asymmetric planar tubular support (usually

α,γ-alumina, titania, and stainless steel), providing the requiredmechanical resistance. From the standpoint of the industrialapplication, tubular membranes are more suitable than flatconfigurations, because tubes are easier to upscale inmutichannel modules and monoliths. Extrusion or isostaticpressing followed by sintering are conventional techniques fortubular support manufacture. Centrifugal casting is a noveltechnique for the cost-effective one-step formation ofasymmetric supports with improved microstructural homoge-neity and surface roughness.598

In the special case of silica membranes, these materialsusually require an intermediate mesoporous γ-alumina or Vycorglass (7930 glass, Corning Inc.) layer between the silica top filmand the macroporous support. The mean pore size of thisintermediate layer usually lies in the narrow range of 3−5 nmand is manufactured most often via a colloidal sol−gel route toachieve higher permeability and resistance at high temperature.The most recent developments on silica, zeolite and MOF

membranes have been performed on ceramic capillaries andhollow fibers (most often based on α-alumina), offering highersurface-to-volume ratios compared to conventional tubes(>1000 vs <500 m2/m3 depending on the inner diameter).Capillary and hollow-fiber units have been commercialized byTNO599,600 and HyFlux.601 Ceramic hollow fibers (o.d. < 1.5mm, thickness <200 μm) are most often prepared by a wetspinning process via phase inversion of a polymer.602−605 Inthis process, the solid particles are first mixed with a solution ofthe polymer (usually polysulfone in N-methylpyrrolidone). Theslurry is then spun through a spinneret into a water bath,promoting the precipitation of the polymer incorporating theceramic particles. The resulting particles (usually showing acharacteristic green color) are cut into pieces and sintered tofully porous ceramic hollow fibers.Interesting developments on capillary and hollow-fiber

supports for gas separation have been reported by Benes andco-workers.599 These authors synthesized silica layers on top ofalumina hollow fibers with high He permeances and He/N2separation factors lying in the range, respectively, of 1.1−2.9μmol·m−2·s−1·Pa−1 and 100−1000. Xu et al.606 reported thesynthesis of zeolite NaA membranes on 0.4 mm diameterceramic hollow fibers, showing a continuous 5 μm film offeringtypical single permeances of ∼30 nmol·m−2·s−1·Pa−1. Richter etal.607 reported the synthesis of ZSM-5 membranes oncapillaries (4 mm i.d.) with single-gas permeances of around0.5 μmol·m−2·s−1·Pa−1. MFI membranes have also beenprepared on alumina hollow fibers with selectivity for CO2and higher fluxes (about 30%) unlike tubular supports.502 Aqualitative comparison of alumina hollow fibers and tubes used

Figure 68. From left to right, different supports used for zeolite, cross section image of an alumina hollow fiber, and MFI crystals plugging the fiberpores after synthesis. Adapted from ref 87. Copyright 2009 American Chemical Society.


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for MFI membrane synthesis is provided in Figure 68. Finally,Xu et al.574 and Lai and co-workers577 reported the recentdevelopment of ZIF-8 membranes on α-alumina and YSZhollow fibers, respectively.The support quality plays a crucial role in the final active

layer microstructure. The presence of large-type defects orsurface impurities can act as a serious deterrent for a properlayer intergrowth. The structure of the solid then needs propercharacterization before the synthesis of the active layer. Theeasiest way for support characterization is via gas−liquiddisplacement. In this method,608 after careful sealing (this stepis especially critical for hollow fibers; see ref 609), themembrane support is soaked in a solvent with low surfacetension (e.g., ethanol) for at least 24 h to ensure proper porefilling by capillarity. Subsequently, the wetted support issubjected to an increasing transmembrane pressure under N2,and the corresponding flux through the membrane ismonitored. Figure 69 shows the typical N2 flux curves vs the

transfiber differential pressure obtained in these tests. The“wet” curve represents the N2 flux for the wetted samplestarting at the first bubble point (FBP) and showing a rapidincrease with the transfiber pressure as the solvent is expelledfrom the fiber pores. After complete removal of the solvent, thepressure is reduced to the starting value and then increasedagain, thereby obtaining the “dry” curve. At sufficiently hightransfiber pressures, the wet and dry curves should converge,the trend becoming linear as expected from the Hagen−Poiseuille equation for a system accomplishing the heuristiccondition Pmd > 0.1 Pa·m (omission of Knudsen contribu-tion),610 where Pm is the mean pressure between the retentateand permeate sides of the fiber, being computed as Pm = Pret −ΔP/2.The pore size distribution was obtained from the evolution of

the N2 flux with the transfiber pressure by solving the Fredholmequation of the first kind defined by

∫ δ= Δ∞

J K d P P f d d( , , ) ( )N 0m2 (34)

where JN2is the N2 flux, K is the Kernel, f(d) is the pore size

distribution, and d is the pore size, the latter being related tothe transfiber pressure by the Laplace law:

γ θ=Δ

dP

4 cos(35)

where d is the pore size, γ is the surface tension of the solvent atroom temperature (taken as 23.0 mN/m), θ is the contactangle (taken as 0), and ΔP is the transfiber pressure.The Kernel in eq 34 can be expressed as a function of the

pore size, d, the mean pressure, Pm, and the transfiber pressureby the Hagen−Poiseuille equation omitting the Knudsencontribution, that is

μ τ= ΔK d P

d PRT

P( , )32m

2

G

m

(36)

4.2. Silica Membranes

4.2.1. Sol−Gel Routes. 4.2.1.1. Colloidal Route. The firstand oldest family of PIMs for gas separations are based on sol−gel microporous materials coated on porous supports. Thefundamentals of sol−gel chemistry for membrane synthesis canbe found in classical reports and recent reviews.611−615

Nanoparticulate metal oxide sols are commonly prepared bythe acid-catalyzed hydrolysis and condensation of thecorresponding alkoxide precursors. The hydrolysis behaviorand final properties of the sols depend to a great extent on thepartial charge and the R substituents of metal alkoxides, theirdegree of oligomerization, the coordination capacity of themetal during hydrolysis, and the pH. After coating, the sol issubjected to a drying step (353−623 K) to promotecondensation followed by a thermal treatment above 623 Kto achieve cross-linking of the coated particles and removeresidual carbon.The classical “colloidal” sol−gel route provides coarse-

grained mesoporous membranes suitable for ultra- andnanofiltration applications, but lacking CO2 separation proper-ties beyond the Knudsen selectivity threshold. Notwithstandingthis fact, some recent patents describe the manufacture ofalumina-supported mesoporous functional films based onmolten BaTiO3

616 and MgO617 with a high chemical affinityto CO2, providing CO2/N2 separation factors of >120 at 623 Kin the case of MgO films, but with modest permeances. Lackneret al.618 recently patented a molten carbonate carrier phase(e.g., K2CO3) formulated as a film combined with a poroussolid oxide that reacts with CO2 at the feed side, diffusesthrough the molten carbonate channels within the solid oxidestructure, and desorbs at the permeate side. The correspondingoxide anion is transferred back through the oxide support, as isschematically shown in Figure 70. The system operates abovethe melting point of the carrier (in the range of 873−1173 K),offering CO2/H2 selectivities of up to 500. Huang et al.619 havedisclosed a similar concept in a recent patent, but employing alithium zirconate (Li3ZrO3) as the carbonate carrier, achievingCO2/N2 separation factors up to 20 at 873 K.Likewise, Lin and co-workers620 developed a dense Li2CO3/

K2CO3 membrane at the eutectic composition as an anionicconductor to promote CO3

2− mass transfer from a Ni anode toa NiO cathode. A scheme of this concept is illustrated in Figure71. The CO2 permeances are usually as low as 1 nmol·m−2·s−1·Pa−1 at about 900 K with a CO2/N2 permselectivity of about16. The process might suffer from strong deactivation at hightemperatures due to the oxidation of the metal support ordissolution of metal to the molten carbonate, implying a strongreduction of the permeation performance. A patent on thisconcept has recently been disclosed.621

Figure 69. Typical curves of N2 flux versus transmembrane differentialpressure for an α-alumina hollow fiber obtained from gas−liquiddisplacement and corresponding pore size distribution obtained afterdata processing according to eqs 32−34. Adapted with permissionfrom ref 609. Copyright 2010 Elsevier.


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4.2.1.2. Polymer Route. To achieve microporous architec-tures with smaller intercrystalline voids down to the microporelevel, the so-called “polymer” sol−gel route is recommended. Inthis approach, the polymeric gel layers are produced throughsequential polymer growth in the sol under acidic conditions,providing a 3D network at the gel stage.622 This networkgradually collapses and cross-links during solvent removal,generating an intercrystalline microporosity due to theinterpenetration of low-branched polymeric clusters. Followingthis route, ultra- and supermicroporous “almost dense”amorphous silica thin films431−433,623−631 and to a lesser extentzirconia, titania, and zirconia/titania films632−640 can bemanufactured, as well as binary silica/alumina, silica/titania,s i l i c a / z i r con i a , s i l i c a /n iob i a , and s i l i c a /bo ronfilms,443,631,641−644 most often onto or between mesoporousγ-alumina or silica supports. The incorporation of anintermediate mesoporous layer (most often made of γ-alumina)between the support and the microporous silica layer helps toimprove the membrane reproducibility and gas permeancewhile maintaining the separation properties.645

An organic template can be used as a structure-directingagent (SDA) during the sol−gel transition to promote thegeneration of residual microporosity after calcination. Twoapproaches are in principle possible: (1) incorporation ofnonionic surfactants (e.g., octaarylpoly(ether alcohol)s) intothe gelation medium, being inert to the chemical process,646

and (2) use of modified alkoxides (e.g., methylated silane orMTES and tetrapropylammonium bromide or (TPA)Br),where a molecular group acting as a template is covalentlybonded to the Si atoms434,454,455,645,647−651 and subjected to aco-condensation mechanism with the silica precursor. Figure 72

illustrates the generation of microporosity using bothapproaches. The use of (TPA)Br increases the pore size from3 Å for the raw silica to 5−6 Å.434 Some additives such asformamide, dimethylformamide, and oxalic acid can also beadded during the drying step to promote layer inter-growth.652,653

More recent studies have also reported the synthesis ofmembranes via sol−gel techniques using partially decomposedzeolites as precursors. For instance, Nishiyama et al.654 reportedthe synthesis of silica/alumina membranes starting fromdealuminated LTA zeolite using HCl. The final sol was thenspin-coated on a Vycor glass plate to obtain microporousmembranes based on zeolite nanoblocks, keeping the localstructure of the zeolite but lacking eight-membered oxygenrings as permeable pores.

4.2.2. Polymer Route in the Presence of a Surfactant.In this route, a surfactant is used as an SDA to obtain orderedmesoporous films driven either by solvent-induced self-assembly (EISA) of the silica precursor or by conventionalhydrothermal synthesis.655−657 Depending on the templatesurfactant nature, its concentration, and the synthesisconditions, the final silica structure might exhibit hexagonal,cubic, or lamellar symmetry. The addition of swelling agents(e.g., mesilylene) might also increase the hydrophobic volumeof self-assembled aggregates, leading to larger pore sizes.658

Unlike nontemplated mesoporous membranes, mesostruc-tured membranes exhibit improved structural stability (e.g., nograin growth or phase transformations), well-controlled poresizes and pore order (dictated by the choice of the surfactantand the synthesis conditions), high surface areas, andchemically tunable pore walls. Most of the reported studiesfocus on the synthesis of 3D cubic silica membranes (e.g.,MCM-48), either by hydrothermal treatment659−671 or theEISA method,671,672 without spatial restrictions againstpermeation. Further insights into the synthesis and structuralproperties of mesostructured silica membranes can be found inthe review paper of Kumar and Guliants.673 The support can bemodified with an aminosilane prior to synthesis (e.g., APTES)

Figure 70. Schematic of a molten carbonate facilitated transportmembrane for postcombustion CO2 capture.

618 Image redrawn fromthe original appearing in the patent.

Figure 71. Scheme of dual-phase molten carbonate fuel cellpostcombustion CO2 capture. Adapted from ref 620. Copyright2005 American Chemical Society.

Figure 72. Schematic illustration of organic/inorganic templatestrategies for generation of microporosity in silica membranes.Image adapted from Roger et al.646 Image redrawn from the originalpublished by the Royal Society of Chemistry.


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to render the support surface hydrophilic and positivelycharged.674 Other methods for mesostructured silica filmsynthesis include flow-field-induced growth and aerosol-assisteddeposition.675

The main drawback traditionally argued against 2Dhexagonal silica membranes is that the porous network tendsto arrange randomly and even parallel to the support surfaceduring the synthesis, hindering permeation. Preferentialchannel orientation perpendicular to the support in MCM-41thin film structures has only been achieved by Brossiere et al.676

(MSU-X) and Tolbert et al.677 (MCM-41), the latter authorsusing magnetic field alignment.Surfactant removal after the synthesis can be achieved via

calcination or solvent extraction. Calcination of as-synthesizedmesoporous silicas is generally conducted at about 723 K forseveral hours, while extraction is carried out using ethanol ormethanol in the presence of an acid. The method used forsurfactant removal plays a relevant role in the final properties ofthe mesoporous silica membranes. Although calcinationprovides higher surfactant removal efficiency, extractiondecreases the risk of film shrinkage and collapse of mesoporouschannels due to pore contraction after surfactant removal, aswell as thermal shock (see an illustrative scheme in Figure 73).

To prevent silica membranes from cracking, Yang et al.678

proposed the use of a liquid paraffin for solvent removal in theEISA method. The density of surface silanol groups can be

tuned after calcination by hydrothermal treatment of thecalcined samples.

4.2.3. Chemical Vapor Deposition/Infiltration. Inparallel to sol−gel routes, CVD has been intensivelyinvestigated for preparing microporous silica membranes,eliminating the drying and calcination steps as in sol−gelprotocols. This method consists of the thermal decomposition(>773 K) of a silica precursor (e.g., tetraethyl orthosilicate(TEOS), tetramethyl orthosilicate (TMOS), SiCl4, SiH4, or analkoxysilane) followed by a chemical reaction with an oxidantagent (e.g., air, O2, N2O, or H2O) either on the surface or inthe pores of a hot substrate, promoting film growth. CVDmethods can be classified into two categories depending onhow precursors are supplied: (1) concurrent CVD, and (2)countercurrent CVD or chemical vapor infiltration (CVI). Incocurrent CVD, the precursors are provided from one side ofthe substrate, keeping the other side under dynamic vacuum,while in countercurrent CVD both reactants are supplied fromthe opposite sides of the support. In the latter case, silicadeposition occurs inside the pores, encompassing the formationof nanocomposite architectures (Figure 74). The macrodefectdensity and the layer thickness can be controlled by changingthe flow of reactants (pure or diluted in a carrier gas), thedecomposition temperature, and the synthesis time. Highreproducibility of the synthesis protocols is in principle possibleusing this technique.Gavalas et al.679 were among the first to prepare H2-

permselective silica membranes (over N2) at high temperatures(723−1023 K) on mesoporous Vycor glass supports by theCVD method using SiH4 as silica precursor and O2 as oxidant.Other examples of silica membranes prepared by CVD and CVItechniques can be found, respectively, in refs 680−684 and refs444, 461, and 685−688 The temperature can be reduced tovalues <773 K (one refers then to low-temperature CVD) usingstrong oxidizing agents (e.g., O3

446,689), by heating with aplasma (plasma-enhanced CVD),690−693 or by incorporating acatalyst (catalytic atomic layer deposition, ALD).694

4.2.4. Modification of Silica Membranes. 4.2.4.1. En-hancement of Hydrothermal Stability. Microporous silicamembranes might exhibit some drawbacks ascribed to aging inthe presence of steam, resulting in a reduction of microporosity,as well as a lack of stability at high temperatures with a risk ofsintering and cracking.695 This lack of hydrothermal stabilitycan be attributed to the condensation of silanol groups in thesilica layer catalyzed by water, being accelerated at hightemperatures and in the presence of humid environments. Thehydrothermal stability of silica membranes can be improvedthrough hydrophobization by incorporating organoalkoxysi-lanes (e.g., MTES or MOTMS) into the synthesis sol-

Figure 73. Effect of the surfactant removal method on the quality ofmesoporous silica membranes.

Figure 74. Schematic illustrations of SiO2 layers: (A) dip-coating, (B) cocurrent CVD, and (C) countercurrent CVD or CVI.


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ution431,439,460,461,696−699 or inorganic additives into the solsuch as alumina, zirconia, or titania,700 as well as Ni701 andCo.702,703

The incorporation of organic moieties into the silicaframework can be clearly assessed by 29Si NMR-MAS (presenceof Q3 and Q4 peaks for silica and T3 peaks for methyl andphenyl groups). Noteworthy, this approach allows the removalof surface silanol groups by generating organic/inorganic hybridmaterials through the incorporation of methyl moieties in thebare silica. The hydrothermal stability of silica membranes canalso be improved by modifying the surface with carbon (i.e.,carbonized-template molecular sieving silica membranes,CTMSS),704 acting as molecular barriers blocking silanolmigration and condensation (Figure 75). The latter methodoffers the advantage of improving the hydrothermal stability ofthe microporous structure of silica without remarkable loss ofits intrinsic hydrophilic properties.4.2.4.2. Amine-Functionalized Silica Membranes by Co-

Condensation of Aminosilanes. Direct co-condensation of anaminosilane allows the one-pot synthesis of silica membraneswith amine moieties. In the first approach, first proposed byStein et al,705 an aminosilane (usually APTES) and the silicaprecursor are mixed together and subjected to aging to allowhydrolysis and further condensation of the silica precursor. Acationic or anionic surfactant can be added when preparingmesophases.455,706,707

Care has to be taken during the co-condensation reaction toavoid phase separation of the precursors to obtain uniformdistributions of functional groups and to avoid Si−C bondcleavage during sol−gel synthesis and surfactant removal. Thismethod offers the important advantage of preventing poreblockage and often provides homogeneous surface aminedistributions (see, for instance, refs 164, 708, and 709),resulting in higher CO2 loadings (see, for instance, thecomparative studies reported by Yokoi et al.710 and Wang etal.711). Nevertheless, the degree of mesoscopic order decreases

with the aminosilane concentration in the reaction mixture,leading ultimately to totally disordered amorphous solids. Themethod also requires the use of extraction techniques forsurfactant removal to preserve the organic moieties. In thisregard, Hao et al.712 showed a positive effect of periodicfluoride ion introduction into the synthesis solution on thelong-range ordering of SBA-15 upon introduction of APTMS.

4.3. Zeolite Membranes

Several strategies have been developed for the synthesis ofzeolite membranes. Among them, dry-gel conversion andliquid-phase hydrothermal synthesis are the most popular. Theformer method consists of the deposition of a layer containingthe Si and Al precursors as a dry amorphous aluminosilicate gelonto the support using sol−gel techniques followed byzeolitization in the presence of vapors for promoting nucleationon the support. With this general approach, two routes can bedistinguished, namely, the vapor-phase transport (VPT)method, where the organic SDA is not included in the dryparent gel, and steam-assisted crystallization (SAC), where thedry gel contains the SDA (generally a quaternary amine) andonly a saturated vapor is supplied. Classical examples of zeolitemembranes (e.g., MFI, MOR, FER, and FAU) prepared onalumina and SS supports using both approaches can be foundin refs 507 and 713−720. Furthermore, zeolite membraneshave been prepared by vapor-phase regrowth of colloidal MFI-type zeolite particles deposited on disks721 and by liquid-phasehydrothermal treatment of a dry gel barrier previouslyincorporated onto an alumina support.722

In hydrothermal synthesis, the porous support is immersedinto an alkaline synthesis solution that contains the required Siand Al precursors together with one or more optional SDAs,and the membrane is synthesized under atmospheric orautogenous pressure in one or more synthesis cycles.723

Hydrothermal synthesis can be carried out either in situ (director in situ crystallization) or with a preliminary seeding step(seeded hydrothermal synthesis or secondary growth method).

Figure 75. Proposed mechanism of pore structure stabilization by CTMSS as opposed to hydrothermal instability. Image adapted from ref 704. Imageredrawn from the original published by Elsevier.


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4.3.1. Direct in Situ Crystallization. The so-called directin situ crystallization constitutes the most widely extendedmethod for zeolite membrane synthesis. In this process, bothzeolite nucleation and growth take place in the presence of asupport. The bare support is put into contact with the synthesissolution under hydrothermal conditions similar to those usedfor zeolite powder synthesis. This simple and widespreadstrategy presents however some drawbacks affecting themembrane microstructure. As a matter of fact, the zeolitelayers should be formed from nuclei that appear during thehydrothermal treatment. Their number and distribution on thesupport depend on the surface properties (e.g., smoothness),which are difficult to control. Moreover, heterogeneous nucleiformation competes with the crystal growth process. Thisimplies in practice the need of growing thick zeolite films toobtain continuous and well-intergrown layers. The reproduci-bility of the synthesis can also be affected by the transfer ofnuclei synthesized in the homogeneous phase to the supportsurface and by partial dissolution of the support.One of the challenges during in situ hydrothermal synthesis

is to ensure a high nucleation site density on the support.Different pretreatment methods prior to hydrothermal syn-thesis have been proposed to promote the formation of nucleimoieties by increasing the surface hydrophilic behavior. Thesemethods include the treatment of the support with NaOH,724

spin-coating of the appropriate template or a metal oxide todirect the formation of crystallites,725 and UV radiation oftitania surfaces to increase the number of defect sites and/orclean the support.726,727 Nucleation can also be promoted bycoating the support with a mesoporous silica layer, acting as anintermediate precursor for zeolite synthesis.728−730

Randomly oriented MFI zeolite membranes are usuallysynthesized in an autoclave at 443−473 K in alkaline conditionsunder autogenous pressure using (TPA)OH or (TPA)Br as thetemplate. Alternative methods involving the use of fluorideanions as a mineralization agent at moderate pH and/or underionothermal conditions have also been attempted, but withmodest control of layer intergrowth. Alkaline conditions arealso used for the synthesis of SAPO-34 and DDR membranes,but with devoted templates ((TEA)OH for SAPO-34 and 1-

adamantanamine for DDR) and addition of a P source in theformer case. The support type and surface characteristics canfavor the growth of the zeolite material on top of the support asa film and partially within the pores.731 To remove thetemplate, the membranes are usually calcined at 673−823 Kafter the synthesis, which might result in the formation ofcracks or defects in the zeolite layers caused by compressionstresses during cooling (vide infra). In an attempt to avoid acalcination step, Heng et al.732 proposed template removal bylow-temperature oxidation with ozone. Alternative methods forzeolite detemplation include treatment with a combination ofammonia, water, and hydrogen peroxide at high temperature, aswell as a combination of choline, hydrogen peroxide, and asurfactant.733 Other authors have attempted the synthesis oftemplate-free MFI membranes as well, but with moderateproperties for gas separation.734−739 In the special case of DDRzeolite membranes, template removal can be critical, providinglow-intergrown materials.740

In a series of studies, Miachon and Dalmon741−745 amongother researchers731 reported the synthesis of highly reprodu-cible and thermally resistant “nanocomposite” MFI−aluminamembranes (i.e., the material is grown in a porous matrix withno film formation) via “pore-plugging” in situ hydrothermalsynthesis. Figure 76 provides a general scheme of the concept.In this method, at least one interruption during the synthesis isrequired to allow nutrients to diffuse into the support pores andnucleate. The composition of the precursor solution and thesupport pore size play a relevant role in the final membranequality. This membrane architecture offers three generaladvantages over the conventional zeolite films: (i) reducedeffect of thermal expansion mismatch between the support andthe zeolite; (ii) ease of upscaling synthesis protocols at thecommercial level; (iii) ease of membrane handling and moduleassembly, mitigating abrasion and shocks. This approachimplies a subtle interplay between nutrient diffusion andnucleation in the support, being in principle only applicable tosystems with low induction times (i.e., MFI). This method-ology has proven to be successful for the synthesis ofnanocomposite MFI layers in hollow fibers502 and multitubularbundles.746

Figure 76. (Left two panels) Schematic comparison between film (left) and nanocomposite (right) membrane structures. (Right two panels) TEMmicrograph of an MFI−alumina composite membrane with the area shown located inside the support top layer (0.2 μm pore size). Adapted withpermission from refs 731 and 745. Copyright 2002 American Chemical Society and 2008 Elsevier, respectively.


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The above-stated templated membrane materials exhibit anintrinsic hydrophilic behavior ascribed to the presence of silanolgroups (or nests) generated during template removal, affectingthe surface properties of the membranes during separation. Inthe particular case of silicalite-1 (and TS-1), the degree ofhydrophobicity of the membranes, and accordingly theircapacity to reject water and permeate CO2, can be tunedusing ammonium carbonate or fluoride crystallization agents,747

as well as via postsynthetic capping using small amounts ofalkali and quaternary ammonium salts.748,749

LTA and FAU membranes are usually prepared at 343−373K at atmospheric pressure with or without an SDA (i.e.,TMA).750 To reduce the synthesis times, several authorsstudied the hydrothermal synthesis of LTA zeolite membranesunder MW heating to reduce induction times.751 Someattempts of synthesis of nanocomposite architectures for LTAand FAU membrane synthesis were reported using SDAs withpartial layer intergrowth.752 In such a situation, a comprehen-sive understanding of the nucleation mechanisms in LTA andFAU zeolites and the subtle role of cations as SDA agents isnecessary to rationalize the formation of the membranes frominitial amorphous gel particles. In this context, the genesis oftemplate-free nanosheets at very soft reaction conditions (i.e.,near room temperature and clear solutions) might help togovern the nucleation of zeolite moieties on the support.753 Inthis way, Valtchev and co-workers754 recently reported anoriginal synthetic approach for the preparation of well-intergrown FAU membranes with a nanocomposite architec-ture via the infiltration of the synthesis solution at lowtemperature (about 273 K) to slow its gelation.In addition to these studies, Caro and co-workers755 have

shown that better intergrown LTA membranes can be preparedusing small positively charged intergrowth supporting sub-stances (ISSs, usually APTES) during the synthesis. The use ofISS might shift the strong negative charge of zeolite crystals tothe isoelectric point, promoting the ability of negatively chargedmoieties to reach the growth interface and therefore the layerintergrowth. Using such a concept, the same team succeeded inthe synthesis of LTA,535 FAU,537 and sandwich LTA/FAU536

films presenting an optimal layer intergowth. Likewise, Huangand Yang756 and Seike et al.757 reported, respectively, thesynthesis of LTA and FAU membranes using an electrophoretictechnique, where an electrical field helps to drive nuclei to thegrowth interface. Moreover, Huang and Caro758 recentlydemonstrated the benefits of bidentate linkers (e.g., 1,4-diisocyanate) as molecular binders covalently bonded on thesupport surface to anchor LTA nutrients onto the supportsurface for the development of thin (ca. 3.5 μm) and well-intergrown LTA films. Such an approach might help thereaction of surface silanol groups of the support with the LTAprecursor, as well as the linkage of neighboring crystallites inthe growing film. Evidence of the fine structure of an LTAmembrane with a composite layer was recently reported by Liuet al.759 using TEM microscopy and a focused ion beam (FIB)for sample preparation. Figure 77 shows representativemicrographs of the nanocomposite layer, pointing out theoptimal layer intergrowth.Finally, Rey and co-workers760 succeeded in the synthesis of

silica-rich LTA (ITQ-29) films in a fluoride medium in thepresence of Ge using 4-methyl-2,3,6,7-tetrahydro-1H,5H-pyrido[3.2.1-ij]quinolinium and tetramethylammonium hy-droxide ((TMA)OH) as SDAs with a priori tunable Si/Alratios. In a similar approach, Caro and co-workers538 modified

the synthesis solution by incorporating Kryptofix or K222template, reaching the genesis of well-intergrown ITQ-29 filmswith partial H2 selectivity.

4.3.2. Secondary Growth Method. The quality andreproducibility of zeolite membranes can be improved byseeding the support prior to synthesis by decoupling thenucleation and growth processes.725,761−763 This step is usuallyfollowed by calcination to promote the attachment of the seedsonto the support, ideally via condensation reactions affordingcovalent bonding. Since the nutrient concentration needed forsecondary growth is lower than for in situ hydrothermalsynthesis, further nucleation is strongly reduced and almost allcrystal growth takes place over the existing seeds. By carefullycontrolling the seed layer coating the support using aconvenient technique (e.g., rubbing,764−766 dip-coating,767−770

spin-coating,770 slip-casting,721,771 cross-flow filtration,772,773

vacuum seeding,774 ,775 or pulsed-laser deposit ion(PLD),776,777 crystallization of undesired zeolite phases canbe discouraged and the rate and directionality of crystal growthcan be controlled to a certain extent.510,778−780 Detailed studieson the effect of seed loading and size on membraneperformance can be found in refs 109, 517, and 781. Seededhydrothermal synthesis by dip-coating of nanoparticles hasbeen applied with success in the synthesis of SAPO-34782 andDDR527 membranes.

Figure 77. TEM images (a, c) and HAADF-STEM images (b, d) takenfrom a composite LTA layer. Arrows in images a and b indicate voids.Adapted from ref 759. Copyright 2006 American Chemical Society.


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The electrostatic attachment of seeds onto the support canbe promoted by surface-charge control via pH, by adsorption ofcationic polymers777 or covalent linkers783 prior to seeding, orby electrophoretic deposition (EPD).778,784 Clet et al.779,785

proposed the use of seeded nuclei moieties generated from anadditional alkaline gel and further incorporated into thesolution. Hedlund et al.778 showed that seed infiltration intothe support during seeding can be circumvented using amasking technique. This might help in the development ofultrathin membranes with high permeation performance.Composite zeolite/carbon materials have recently been

reported via the deposition of zeolite nanoparticles on aphenolic resin followed by seeding on the support by dip-coating and further carbonization. Well-intergrown LTA/carbon508 and SAPO-34/carbon522 membranes were manufac-tured using this approach with gas separation properties.

4.4. MOF Membranes

MOF membranes constitute the most novel and exciting fieldof study in PIMs. The Fischer,786,787 Zhao,788 and Jeong789

teams have published comprehensive reviews describing therecent progress and specific challenges in the manufacture ofMOF films on substrates. Relying on the knowledge gained inthe past on the genesis of silica and zeolite layers, most MOFfilms reported to date have been manufactured by direct growthor deposition from hydro/solvothermal mother solutions andby stepwise layer-by-layer growth onto a great variety ofsubstrates.The surface chemistry of the support (presence or absence of

functionalities) plays a key role in the nucleation and growth/attachment of MOF crystals, which is perhaps more relevantthan in classical zeolite chemistry. Unlike zeolite membranes,the attachment/growth of MOF films on raw alumina, silica ortitania can be discouraged without prior functionalization of thesupport,790,791 requiring most often the seeding of the supportbefore synthesis. Both approaches are described herebyunderlying the main impacts on layer intergrowth and possiblecontrol of the crystal orientation.4.4.1. Solvothermal Synthesis. 4.4.1.1. Direct Solvother-

mal Synthesis. The functionalization of the support before orduring direct solvothermal synthesis is often crucial for the(inter)growth of MOF films via the promotion of secondarybonding (e.g., H-bonding or van der Waals interactions) orcovalent attachment. Guo et al.585 were among the first toreport that partial oxidation of Cu supports helps to anchorHKUST-1 crystals, promoting layer intergrowth.Self-assembled organic monolayers (SAMs) have been used

as SDAs to promote the generation of supported MOF nucleimoieties and in some cases to direct the crystal orientation.Some scholar examples were reported by Fischer and co-workers790 on the growth of MOF-5 crystals at room

temperature on COOH- and COOH/CF3-terminated goldsubstrates and oriented HKUST-1 crystals under conventionalsolvothermal growth conditions (110−120 °C) on COOH-and CF3-terminated SiO2/Si substrates. Caro and co-workersalso impregnated alumina supports with APTES prior tosolvothermal synthesis to promote the growth of ZIF-22582

(ZIF-22 = Zn(5-azabenzimidazolate)2 on and ZIF-90583 filmsvia the covalent attachment of the imidazolate linker with thesilane tether, thereby promoting the heterogeneous nucleationand growth of ZIF crystals (see the general scheme of theconcept in Figure 78). Kang et al.792 reported the synthesis ofhigh-quality ZIF-8 membranes with the preliminary depositionof APTES-functionalized silica supports. Similarly, Xie et al.575

prepared ZIF-8 membranes with the preliminary deposition ofAPTES-functionalized alumina particles onto a macroporoussupport for reducing the pore size and simultaneously boostingthe density of heterogeneous nucleation sites. Most of thesyntheses were carried out under MW heating to promote layerintergrowth and reduce induction times for the nucleation ofZIF moieties.The covalent attachment of the ligand can also occur during

solvothermal synthesis without the need for an externalbinder.793,794 Jeong and co-workers306 reported the preparationof ZIF-8 and ZIF-7 films by the high-temperature covalentlinkage of the imidazole linker to an alumina support via theformation of Al−N Lewis adducts. The same group reportedthe benefits of basic sodium formate addition duringsolvothermal synthesis for promoting linker deprotonation inthe genesis of well-intergrown ZIF-8 films.306

4.4.1.2. Secondary Growth Method. As in zeolitemembrane synthesis, the secondary growth method can alsobe applied to the synthesis of MOF films after a convenientdeposition of seed crystals on the support surface. Unlikezeolite membranes, MOF membranes usually require a polymerbinder to attach seed crystals to the support. Ranjan andTsapatsis587 showed that partial impregnation of the supportswith PEI binder before seeded solvothermal synthesis promotesthe growth and crystal orientation of [Cu(hfipbb)(H2h®pbb)0.5](MMOF) thin films. Lin and co-workers563 manufacturedcontinuous MOF-5 films on alumina disks by dip-coating thesupport with MOF-5 seeds before solvothermal synthesis. Caroand co-workers579,580 demonstrated the benefits of seedingalumina supports with ZIF-7 nanocrystals together with PEIbinder to promote the growth and continuity of ZIF-7 films.Tao et al.578 prepared continuous ZIF-8 films on ceramichollow fibers (the material was not specified) by rubbing thesupport with ZIF-8 crystals before solvothermal synthesis.Finally, Zhang et al.570 and Fan et al.571 manufacturedcontinuous MIL-53(Al)-NH2 films on glass frit disks previouslyseeded with colloidally assembled MIL-53(Al)-NH2 crystals.

Figure 78. Scheme of the preparation of ZIF-90 membranes using APTES as a covalent linker between the ZIF-90 layer and the alumina support viaan imine condensation reaction. Adapted from ref 583. Copyright 2010 American Chemical Society.


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Li and co-workers795 recently reported an innovativetechnique for the synthesis of free-standing CuBTC and ZIF-8 films using electrospun nanofibrous mats as skeletons. In thisapproach, CuBTC and ZIF-8 crystals are first blended in asolution of polystyrene and subsequently poured into a plasticsyringe that is pressurized using an injection device. A voltage(12 kV) is then applied between the spinneret and the collectorto generate electrospun nanofibers that can be further used asseeds during solvothermal synthesis for the genesis ofhierarchically nanostructured free-standing films.In addition to the studies involving an external binder, Hu et

al.796 reported the preparation of high-quality MIL-53(Al)membranes on a porous alumina support by the “reactiveseeding” of MIL-53(Al) crystals on the support. In thisapproach, a seed layer is first prepared on a support includingthe relevant cation of the MOF material (Al3+ in that case)together with the ligand at the concentration and temperatureconditions for promoting the nucleation of MOF moieties onthe support. Subsequently, the seeded support is subjected tostandard solvothermal synthesis to generate the continuouslayer. In a further study, the same group extended the methodto the manufacture of MIL-96 films.797 The authors proposed aseeding mechanism involving the formation of γ-AlO(OH)moieties by reaction of α-alumina with water that might furtherreact with the linker (H3btC) to generate MIL-96 seed crystals.Lin and co-workers581 recently extended this method to thesynthesis of continuous ZIF-78 films on ZnO supports.Furthermore, Jeong and co-workers586 developed a “thermal

seeding” method for the synthesis of HKUST-1 films withoutan external polymer binder. This method involves first thecontact of a suspension of HKUST-1 seed crystals with the hotalumina support (473 K) followed by rinsing under sonication.Later on, the seeded support is subjected to solvothermalsynthesis to grow continuous HKUST films.The use of MW heating during solvothermal synthesis of

seeded supports can help in the development of well-intergrown and preferentially oriented MOF films with atleast one fast growing facet. Examples of oriented ZIF-7 andZIF-8 films have been reported by Caro and co-workers.573,588

Figure 79 shows an illustrative SEM image of a c-oriented ZIF-7film.

4.4.2. Stepwise Layer-by-Layer Synthesis. In addition tosolvothermal synthesis, stepwise layer-by-layer (LBL) synthesishas recently been proposed as a possible synthetic approach forthe manufacture of continuous and preferentially orientedMOF films. This method involves the consecutive and alternateimmersion of the support into metal ionic and linkersolutions.798 This approach has been applied with success tothe synthesis of well-intergrown membranes based on[Cu2(BME-bdc)2(dabco)] (BME-bdc = 2,5-bis(2-methoxye-thoxy)-1,4-benzenedicarboxylate and dabco = 1,4-diazabicyclo[2.2.2]octane) with nanocomposite architectureand Ni-MOF-74 membranes displaying H2/CO2 permselectiv-ity.569 The method has also been applied to the seeding ofsupports for the further solvothermal synthesis of continuousHKUST-1 films.799

4.5. Membrane Microstructure

4.5.1. Macrodefects. 4.5.1.1. Crack Generation. The maindrawback when preparing supported membranes is thepresence of large intercrystalline defects (or simply macro-defects or cracks) in the nearby micro- or mesoporous crystalsthat can be generated either during membrane preparation orupon template removal by thermal treatment.In addition to such effects, macrodefects might also appear

during cooling, drying, or operation, in all cases beingdetrimental to selectivity. Concerning the latter point, thinfilms should be cooled slowly and if possible when immersed inthe synthesis solution to avoid crack formation upon cooling.The use of solvents with low boiling points in the case ofMOFs, such as methanol or dichloromethane, is beneficialbecause the film can be activated at moderate temperature.Micro- and mesoporous silica membranes can also shrink in

the presence of water vapor unless the framework is stabilizedeither via hydrophobization (e.g., introduction of Si−C bondsvia MTES or silylation), or by the introduction of stabilizingatoms (e.g., Zr800). Hydrothermal stability can also be an issuefor some MOF films. An example is IRMOF-1, which can sufferfrom crack generation in contact with ambient humidity due toexchange reactions of carboxylates with water.801 Yoo et al.802

have recently shown that the resistance of IRMOF-3 films tomoisture can be improved by postsynthetic functionalization ofthe layers just after activation with an amphiphilic surfactant(Span 80) providing hydrophobization via alkyl groups. Theincorporation of hydrophobic functional groups near bipyridinelinkers increases the stability of carboxylate-based MOFs andincreases the hydrothermal stability of the films.803

Finally, crack formation can be an issue during activation,especially in the case of MOF films, due to capillary stressesupon drying. A standard method to reduce stresses is todecrease the drying rate at nearly saturated conditions, usuallytaking a few days.586 Another method to increase the stability ofMOF films upon drying is by coating the films with a surfactant(Span 80) to mitigate capillary stresses. The benefit of thisapproach was demonstrated by Yoo et al.802 for IRMOF-3films.In addition to this general behavior, some materials might

exhibit an intrinsic lack of stability at high temperature. Crackformation can be especially critical in high-Al zeolitemembranes (e.g., LTA and FAU). As suggested by Caro andco-workers,804 LTA and FAU films are likely to show negativeexpansion coefficients until 373 K and positive coefficientsfurther on, increasing drastically with temperature. This mightpromote a mismatch between the thermal expansion

Figure 79. Cross section (right) SEM image of a c-oriented ZIF-7 filmon a porous α-alumina support. Adapted with permission from ref 588.Copyright 2010 Wiley.


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coefficients of the zeolite layer and the support, creatingthermal stresses at the origin of membrane shrinkage. A similarmechanism was advocated to explain crack formation insupported HKUST-1 films upon thermal treatment.586

4.5.1.2. Defect Plugging. Defect plugging is in principlepossible by deposition of microporous silica by reaction withsilicon alkoxides or silylation agents,561,805−809 microporousalumina,810 Pd,811,812 or coke,813 also opening routes formembrane functionalization and hydrophobization. For in-stance, Noble and Falconer810 showed that the selectivedeposition of alumina in the narrow-sized intercrystallinedefects of MFI and SAPO-34 membranes provides membraneswith H2/CO2 selectivity, opposing the expected trend of theraw materials. Some details on the permeation and separationperformance of “repaired” membranes have been summarizedin section 3.2.Besides these approaches, Zhang et al.814 developed a defect

patching technique for counterdiffusion chemical liquiddeposition of silicalite membranes. Co-condensation of TEOSand (3-chloropropyl)triethoxysilane (3CP-TES) at the organic/aqueous interface generates a silsesquioxane/silicate hybrid filmat the pore mouths of meso- and macrodefects, decreasing themean defect size to 1.3 nm.4.5.1.3. Membrane Characterization. A number of

techniques are available for the characterization of polycrystal-line membranes. Briefly, these techniques include microscopy(e.g., SEM, HRTEM, AFM, confocal microscopy),815,816 Hgporosimetry,817 permporometry,778,815,818−821 and temperature-programmed desorption (TPD).822 Kyotani et al.823 alsoreported interesting correlations between FTIR-ATR spectraand TEM images for characterizing the fine structure of LTAmembranes. The main shortcoming of these techniques is thatactive pores to permeation cannot always be easilydistinguished from dead-end pores, under- or overestimatingintercrystalline pore sizes.

Single-gas permeance measurements constitute the simplestand by far more extended method for a rapid assessment of thepresence of macrodefects in PIMs. For instance, Sanchez etal.824 used the time-lag method to characterize large pores incomposite MFI membranes from experimental He and SF6permeance data. Hanebuth et al.825 developed an algorithm forestimating the contribution of intracrystalline diffusion ofsorbate molecules and Knudsen diffusion in large pores andgrain boundaries in H2 and SF6 permeation. Kumakiri et al.

826

conceived an extremely sensitive method to detect leaks anddefects in zeolite layers using pure H2 permeation data. Finally,Noble and Falconer827 developed a spatially resolved gaspermeation technique at the membrane surface for tubularconfigurations relying on a movable mass spectroscopy probe.The technique provides an accurate cartography of the gaspermeance, affording the location and characterization ofintercrystalline domains of SAPO-34 membranes.The most straightforward and reliable method for character-

izing intercrystalline pores is, however, by gas separation. Threedifferent kinds of separations have been traditionally consideredfor zeolite membrane testing (most often for MFI membranes),relying on differences in (1) molecular size (e.g., N2/SF6 or N2/DMB), (2) surface diffusion rate (e.g., n-butane/isobutane),and (3) adsorption strength (e.g., n-butane/H2). The low-temperature separation of n-butane/H2 mixtures within MFImembranes is so sensitive that grain boundaries andintercrystalline domains can be identified depending on themembrane pretreatment protocol before separation (e.g.,heating under the presence of an inert gas flow, heatingunder vacuum, or simply storing in an oven491). As a matter offact, any mesoporous defect in the membrane would locallyinverse the selectivity (turning to a Knudsen mechanism) andreduce the separation factor. Other tests such as molecular-sieving-based separations (e.g., N2/SF6) or diffusion-basedseparations (e.g., n-butane/isobutane) might not be so

Figure 80.Microstructure of zeolite membranes: (top) fluorescence confocal microscopy (FCOM) images of grain boundaries in an MFI membraneat high magnification at different positions from the membrane surface, (bottom left) HRTEM micrograph of the microstructure of an LTAmembrane showing the presence of grain boundaries, (bottom right) schematic representation of the microstructure indicating intracrystalline andintercrystalline (grain boundaries and large defects) regions. Microscopy images adapted from ref 816. Copyright 2001 Elsevier.


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discriminative, considering that Knudsen-type defects caneither show some separation efficiency (N2/SF6) or be neutral(n-butane/isobutane). Moreover, mesopores can also displaysome moderate selectivity driven by adsorption differences.While these tests have been widely considered for zeolite

membrane characterization, to our knowledge no extrapolationor convenient discussion on their applicability for thecharacterization of large intercrystalline defects in MOF/ZIFmembranes has been reported so far. As has been argued byNoble and Falconer in a series of recent studies, suchtechniques might provide an incomplete picture of themembrane microstructure in the presence of small pores (i.e.,narrow mesopores or supermicropores). In such a situation, apotential crystal swelling induced by the adsorption of gases orvapors can affect the intercrystalline pore sizes and accordinglythe corresponding pore volumes. Such behavior is expected tobe promoted for MOF/ZIF membranes, showing more flexiblestructures than zeolite counterparts. However, no dedicatedexperiments to survey such effects are available in the openliterature. More details on such effects are described below.4.5.2. Grain Boundaries. Mesoporous and microporous

membranes for gas separation usually consist of polycrystallinefilms and nanocomposites, generating an assembly of grainboundaries between nearby grains. Figure 80 shows an exampleof a microstructure of an A-type film visualized by HRTEM andfluorescence confocal microscopy (FCOM), clearly showingthe location of grain boundaries.The number and nature of grain boundaries can promote the

membrane anisotropy, affecting the permeation fluxes.828

Furthermore, molecules in grain boundaries might showadsorption and diffusion properties radically different fromthose in standard micropores of zeolites and MOFs/ZIFs. Thedifferences are usually difficult to quantify because of theirregularities in the shape and size of grain boundaries.Notwithstanding this fact, Nomura et al.829 assessed differentpathways for mass transfer through zeolite pores and grainboundaries in MFI membranes after CVD of silica in grainboundaries. Using MD simulations, Takaba et al.830 computeddecreasing activation energies for methane diffusion within MFImembranes with small mesoporous intercrystalline domains(from 21.4 to 6.4 kJ/mol for decreasing intercrystalline poresizes from 1.0 to 2.0 nm). A possible role of grain boundaries as“fast diffusion paths” has also been suggested to explain whywater diffusivities are up to 3−4 orders of magnitude higherwhen obtained from experimental PV fluxes within zeolite NaAmembranes than from adsorption kinetics data.831

4.5.2.1. High-Temperature Effects. Grain boundaries canalso alter the membrane microstructure depending on theoperation conditions. An illustrative example can be found forsilicalite-1 membranes, showing an increase of pure gaspermeances beyond 400−500 K (see section 3.2.1.1). Thisobservation, originally ascribed to an “activated diffusion” masstransfer pathway, was reinterpreted by Miachon and Dalmon interms of intercrystalline pore openings due to crystalcontraction at high temperatures.832 Figure 81 plots somesimulated pore openings on filmlike HZSM-5 membranes dueto thermal expansion mismatch between the zeolite crystals andthe support, underlying a higher mismatch for larger zeolitecrystals. This phenomenon can be overcome in nanocompositearchitectures, where the active phase is embedded into the hostporous network, conferring to the material higher resistance tolong-range thermal stresses and avoiding pore opening.

4.5.2.2. Sorbate-Induced Pore Contraction. Related to thelatter observations, Noble and Falconer384 have also shown thatadsorption of appropriate molecules in defective MFImembranes can induce expansion of zeolite crystals,unexpectedly shrinking grain boundaries and in turn promotinggas separation. A detailed review paper on such phenomena hasrecently been published by the same group.833 Figure 82provides a qualitative illustration of such phenomena on afilmlike B-ZSM-5 membrane together with some indicativetrends of the He flux (He adsorption is very weak in mostzeolites) after preadsorption of different gases and vapors.Crystal expansion and defect shrinkage are outstanding when

linear C3−C6 hydrocarbons are adsorbed in MFI membranesand might exert a remarkable influence on the permeation andseparation properties. For instance, the intercrystalline volumecan be reduced up to 50% in MFI membranes after n-hexaneadsorption at an activity as low as 0.01, as inferred from 2,2-dimethylbutane (DMB) adsorption (not adsorbing in MFIchannels).819 The mean pore size of good-quality MFImembranes estimated from DMB adsorption falls into therange of 1.5−4.0 nm.834,835 The expansion behavior of zeolitecrystals upon gas/vapor adsorption can be inhibited by selectivedeposition of large molecules (i.e., cyclodextrin) in grainboundaries. Examples have been provided for supportedMFI836 and SAPO-34837 membranes.Furthermore, a He flux reduction down to 2% of the initial

value was reported for a filmlike B-ZSM-5 membrane uponpropane preadsorption at a relative pressure of 0.002.384 Thisexample is relevant for membrane-based CO2 captureapplications, since light hydrocarbons can be present duringCO2 separation and accordingly affect the membrane behavior.Water shows a practically negligible expansion effect on He fluxreduction in MFI membranes for relative pressures <0.1,remaining comparable to that observed for CO2 and N2 (Figure82, bottom).553 However, the effect becomes more pronouncedat higher relative pressures and can be intensified at highertransmembrane pressures. In the case of branched alkaneadsorption (e.g., isobutane), MFI crystals might show either anexpansion or a contraction depending on the sorbate loading.As a consequence, the He permeance might show a dualbehavior with the sorbate loading, showing an increase at lowerloadings and a decrease at higher loadings.838 A similar type of

Figure 81. Intercrystalline pore opening due to the thermal expansionmismatch between the support and zeolite crystals in filmlikemembranes. Calculations were based on nonoriented crystals as afunction of the crystal size.744


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effect was reported for water adsorption on NaA zeolitemembranes.839

In addition to these studies, Weh et al.841 have shown thatFAU and MFI membranes with adsorbed azobenzene (AZB)can display photoswitchable permeation properties due to thetrans−cis photoisomerization of AZB. The pure gas perme-ances, as well as the equimolar CO2/N2 and CO2/CH4separation factors, are higher for the trans form of thezeolite-encapsulated AZB than for the cis form. These resultscan be explained by selective plugging of mesoporousintercrystalline defects due to crystal expansion induced bycis−trans photoisomerization.4.6. Taxonomy of Materials Based on Layer Intergrowthand Microstructure

Given the different available methodologies for the synthesis ofsilica, zeolite, and MOF/ZIF membranes, two issues have to beaddressed for a proper transposition at the industrial level: (1)the reproducibility of the synthesis protocols, and (2) thepossibility of upscaling such protocols. These issues, thoughessential for industrial implementation, have only been tackledin a very small number of studies. Indeed, most of thepermeation and separation data compiled in Tables 9−15 dealwith the synthesis of membrane materials with permeationsurfaces smaller than 10 cm2 and without a strict assessment ofreproducibility. Furthermore, the need for optimal layer

intergrowth via classical crystallization techniques discouragesthe synthesis of thin films, penalizing the membranepermeation performance.Comprehensive studies on membrane reproducibility are

dramatically scarce. Some exceptions can be found in the caseof microporous silica and MFI membranes. In the latter case,rigorous studies on the reproducibility of hydrothermalprotocols were conducted by Gora et al.,842 Hedlund etal.,738and Miachon et al.741,742 Specific studies on membranereproducibility have also been reported by Noble and Falconeron SAPO-34 membranes, pointing out the major role of theparticle size on the layer intergrowth.The reproducibility of the preparation protocols is usually

critical for high-Al zeolites (e.g., ERI- or FAU-type) due to thefast nucleation and crystallization kinetics even in the presenceof organic templates, reducing the layer intergrowth. Althoughthe proposed synthetic approaches to circumvent this intrinsiclimitation (e.g., gelation at low temperature, incorporation of atemplate, tuning the support charge) seem promising, theirimpact on the reproducibility has not been addressed yet indetail. The studies of Kusakabe and co-workers on FAU-typemembranes constitute a milestone in the genesis of Al-richzeolite membranes for CO2 separation. The membranereproducibility can also be an issue in the case of DDRmembranes, for which crack formation during template removalprevents the genesis of well-intergrown materials and affects theeconomy of the fabrication process. Despite the promisingresults reported by Tomita and Himeno on their manufacture,reproducibility studies are still missing in the open literature.Finally, MOF-type and more extensively ZIF-type mem-

branes constitute one of the emerging fields of study with muchroom for improving the reproducibility of the syntheticprotocols and for progressively decreasing the layer thicknesswhile maintaining the intergrowth level. The studies reportedby Caro’s team on MW-assisted seeded solvothermal synthesisfor the genesis of reproducible ZIF-7 and ZIF-8 thin filmsconstitute a milestone in the field.

5. FINAL REMARKS AND OUTLOOKI have described in the previous sections the recent progress inthe synthesis of porous inorganic membranes and theirpotentials for CO2 capture in pre- and postcombustionscenarios with a critical regard on adsorption and diffusionmechanisms for boosting permeation and selectivity. We haveshown that a rational design of adsorption and diffusionproperties of the materials is compulsory for a convenientmembrane design, with a critical analysis on the impact ofmoisture and hydrocarbons on the stability of the materials andthe potential synergistic blocking effects during separation.Given the four different taxonomies outlined above, namely,(1) adsorption, (2) diffusion, (3) permeability/permeance, and(4) layer intergrowth/microstructure, the final question thatemerges is how to establish a final ranking of membranematerials for pre- and postcombustion CO2 capture applica-tions. To help in the decision, Table 16 compiles the mainconclusions addressed from such taxonomies for the differentmaterials.In the case of precombustion CO2 capture, carbon-templated

microporous silica and MFI membranes constitute the mostmature materials for membrane design on the basis not only ofthe reproducibility of their synthesis protocols, but also of theirnarrow thickness down to the micrometer level. This latteraspect is especially relevant for silica membranes, compensating

Figure 82. (Top) Schematic description of sorbate-induced crystalexpansion and contraction of MFI crystals and correspondingshrinkage/expansion of nanosized grain boundaries. (Bottom)Normalized He flux as a function of the gas/vapor activity for afilmlike B-ZSM-5 membrane. Images adapted from Noble andFalconer.384,840


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the relatively modest permeabilities driven by preferential H2diffusion selectivity for precombustion H2/CO2 separations atmoderate to high temperatures. These properties make carbon-templated silica membranes promising candidates for CO2capture in WGS units and even for the design of membranereactor applications in line with recent patents (section 1.4),competing with Pd(Ag) membranes and more conventional gasabsorption techniques. The ZIF-8 and ZIF-90 membranesmanufactured by Caro’s group constitute promising compet-itors to silica materials for H2/CO2 separation. Despite theirmodest H2/CO2 separation factors, these materials show a highhydrothermal stability and potential for improving thereproducibility of the synthesis protocols, making themcandidates of choice for precombustion CO2 captureseparations. Other materials such as MIL-53(Al)-NH2, thoughin a preliminary stage of development, might find potential forfuture developments for H2/CO2 separations. In the case ofmaterials displaying preferential CO2/H2 separation, BaZSM-5membranes appear to be the most attractive, preferential CO2adsorption and a high degree of correlation effects compensat-ing the preferential H2 mobility within the microporousframework.In the case of postcombustion CO2 capture, MFI membranes

can find suitable applications for CO2/N2 separations driven bypreferential CO2 adsorption. The presence of moderate Si/Alratios in these materials provides a trade-off for preferentialCO2 adsorption and moderate poisoning by moisture below athreshold value (usually <2 vol % at 323 K). Although theCO2/N2 separation factors remain modest al low CO2concentrations (usually <10 vol %), the membranes can showhigh CO2 permeances exceeding 1 μmol·m−2·s−1·Pa−1 either

when dealing with nanocomposite architectures or when usingmasking techniques during hydrothermal synthesis. However,the studies reported by Guo et al.542 (HZSM-5) and Shin etal.498 (NaZSM-5) have shown that exceptionally high CO2/N2separation factors can be attained with MFI materials, matchingthe selectivity and permeance challenges of Favre forpostcombustion CO2 capture (see section 1.5). Moreover, thepresence of VOCs in the gas stream might be beneficial interms of intercrystalline pore sealing.Despite the general advantages of hydrophilic membranes

including alkaline cations (e.g., NaY) for CO2/N2 separation onthe guidance of Favre’s selectivity and permeation challenges,the permeation properties of these materials might be stronglyinhibited by the presence of moisture due to microporeblockage. Moreover, the presence of cations templating zeolitenucleation and growth during synthesis acts as a deterrent forthe reproducibility of the synthesis protocols, impelling themanufacture of well-intergrown thin layers down to themicrometer level. The studies of Kusakabe and co-workershave demonstrated that well-intergrown thick NaY and NaXfilms (>10 μm) are technically feasible, but at the expense of areduction of the CO2 permeance. Potential improvements ofFAU-type membrane reproducibility could be achieved bypromoting gelation at low temperature, by using cheaptemplates, or via novel synthesis concepts involving zeolitenanosheets.Finally, in the case of CO2/CH4 separations, the SAPO-34

membranes prepared by Noble and Falconer can showpotentials with a proven reproducibility of synthesis protocols.However, their application can be discouraged in the presenceof high water concentrations due to a lack of hydrothermal

Table 16. Final Taxonomy of Porous Inorganic Materials for Different CO2 Capture Scenarios


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stability. In such a situation, DDR membranes might behavemore robustly if convenient preparation protocols becomeaffordable. For such separation, all-silica ERI, CHA, and ITQ-29 might provide promising potentials according to molecularsimulations, although only a few experimental data have beenreported to date. Separation is expected to be mainly driven bypreferential CO2 diffusion with reduced correlation effects. TheZIF-8 membranes reported by Venna and Carreon566 also offerpromising perspectives for CO2/CH4 separation at a reasonablereproducibility of synthesis protocols.Despite the potentials of the above-stated materials for CO2

separation in different CO2 capture scenarios, long-term studiesfocusing on the stability of membrane materials under realisticoperation conditions are still dramatically missing with only afew exceptions for MFI and SAPO-34 membranes. Practicaldetails on membrane activation, thermal cycling, and modulesealing are missing for many membrane materials, beingespecially crucial in high-temperature applications.Moreover, there is still room for improving the synthesis

protocols in view of industrial R&D developments to boost themembrane reproducibility, reduce the layer thickness, andmitigate possible cracking during template removal in the caseof high-silica zeolite membranes. Regarding the first aspect,nanocomposite architectures might be potentially useful, alsopreventing crack formation during operation. This architecturehas been demonstrated for micro/mesoporous silica and MFImembranes, but to my knowledge no report is available onMOF and ZIF-type materials beyond the preparation of hybridmembranes supported on ceramic matrices. Also, the use ofmasking techniques or the synthesis of nanocompositearchitectures could be useful concepts inherited from zeolitemembrane praxis to improve the quality of MOF membranes.Such concepts are also interesting for the design ofmesostructured silica membranes, for which surfactant removalby calcination is usually a critical step. The use of specificdetemplation techniques (vide supra) avoiding calcination isdesired for preventing crack formation in zeolite membranes.The support itself poses serious drawbacks for the

industrialization of gas separation membranes. To avoid thisshortcoming, the use of cheaper supports such as silica ormullite involving partial zeolitization if necessary in thepresence of an organic template could be advantageous. Thisrequires in turn the manufacture of supports with good surfaceand porous properties allowing the genesis of continuous filmsand nanocomposites, which might not be straightforwardbeyond traditional α-alumina and titania supports. Further-more, unlike zeolite or amorphous silica membranes, the choiceof the right support and its possible functionalization is crucialin the preparation of MOF/ZIF membranes to promote crystalnucleation and growth. The use of supports with high surface/volume ratios (e.g., capillaries and hollow fibers) is highlydesirable, but still challenging. The extension of membranesynthesis protocols to this kind of supports could open upnovel applications where volume is a prerequisite, especially inminiaturized systems (e.g., on-board CO2 capture applications).Despite the general feeling among the scientific community

that porous inorganic membranes might not find applicationsfor CO2 capture, this vision is in my opinion elusive. I havetried to show in this review that a suitable and smart design ofmembrane pores with convenient adsorption and diffusionproperties might provide materials with competitive separationand permeation properties. These properties, combined with anoptimal reproducibility of the synthesis properties on affordable

supports, cheap organic templates and ligands when necessary,and mild operation conditions (e.g., hydrothermal/solvother-mal synthesis under MW heating), might allow the develop-ment of membrane solutions that can compete with moremature gas absorption and adsorption technologies. In thisperspective, the recent field of MOF membranes is open, andfresh new contributions are expected to appear in the followingyears offering new and competitive solutions for CO2 capture.

ASSOCIATED CONTENT

*S Supporting Information

Adsorption data and heats, Henry’s constants, and representa-tive isotherms on functionalized silicas, zeolites, and MOFs,detailed information on the main advantages and drawbacks forindustrial implementation of CO2 capture technologies,additional MOF structures, and additional acronyms. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author

*Phone: +86 2124089654. Fax: +86 2154424351. E-mail: [email protected].

Notes

The authors declare no competing financial interest.

Biography

Marc Pera-Titus received a double M.Sc. degree in ChemicalEngineering (2001) and Physical Chemistry (2002), and a Ph.D.(cum laude) in Membrane Technology (2006) at University ofBarcelona (Spain). In 2007, he joined as postdoc the group of Jean-Alain Dalmon at IRCELYON/CNRS (France) with a Marie Curie EIFgrant. In 2008, he was appointed CNRS researcher at the sameinstitution. He was Visiting Professor at HKUST University in 2008and 2009, in the group of Prof. K.L. Yeung. In 2011, he received hisHabilitation degree (HDR) from the University Claude Bernard Lyon1. Marc is author of 60 papers, 8 book chapters, and inventor of 7patents in the fields of zeolite membranes, membrane reactors,adsorption and heterogeneous catalysis with more than 1300 citations.In 2009, he received the Elsevier Award for highly cited author inCatalysis. Since November 2011, Marc is project leader and expert atthe Eco-Efficient Products and Processes Laboratory (E2P2L),CNRS/Solvay international joint lab in Shanghai (China) carryingout industrial and academic research.


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http://pubs.acs.org

http://pubs.acs.org

mailto:[email protected]

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ACKNOWLEDGMENTSThis research was funded through the CO2 program of theFrench Agence Nationale de la Recherche (ANR) under AwardANR-07-PCO2-003. I thank S. Durecu (TREDI), B. Siret(LAB), P. Chanaud (Pall Corp.), and L. Schrive (CEA-Marcoule; CEA = Commissariat a l’Energie Atomique et auxEnergies Alternatives) for rich discussions and advice duringthe preparation of this manuscript. Although some of the viewsexpressed in this paper are mine alone, I am indebted tonumerous colleagues and students for discussions during thepast years. I thank Dr. J. Sublet for helpful discussions onpolymer and hybrid membranes, as well as Dr. N. Guilhaume atIRCELYON and Dr. Z. Yan at E2P2L for valuable suggestionsand for carefully reading some parts of the paper draft.

DEDICATIONI dedicate this paper to Prof. J.-A. Dalmon, recently retired, andDr. S. Miachon, who passed away 3 years ago, for theirinsightful advice and friendship since my appointment to theCNRS staff.

GLOSSARYAS pre-exponential factor in eq 23 (m2/s)d diameter (m)D diffusion coefficient (m2/s)ĐS(0) Maxwell−Stefan diffusivity at zero loading (m2/s)E adsorption energy (J/mol)ES activation energy (J/mol)f(d) pore size distribution function in eq 34G° total free energy of adsorption dissipated during the

adsorption process in eq 12H Henry’s constant (mol·g−1·Pa−1)JN2

N2 flux (mol·m−2·s−1)

k affinity parameter in eq 12K kernel in eq 34

thickness (m)m energy heterogeneity parameter in eq 12Ni flux of ith species (mol·m−2·s−1)P pressure (Pa)P° saturation vapor pressure in the sorbate in IAST (Pa)Pi permeability of species i (mol·m·m−2·s−1)Pij permselectivity between species i and j (dimensionless)q adsorbed loading (mol/kg)qM saturation loading (mol/g)Qst isosteric heat of adsorption (J/mol)Sij selectivity between species i and j (dimensionless)SF separation factorT temperature (K)TS titanosilicalitexi molar fraction of ith species in the feed/retentate, molar

fraction in the sorbate phaseyi molar fraction of ith species in the permeatez number of nearest-neighbor sites in eq 24Z inverse of the chemical potential of the sorbate (mol/

kJ)Greek Symbols

α constant in eq 32β, δ characteristic parameters in eqs 6 and 7 (dimensionless)χ distribution function of adsorption energies in eq 5

(dimensionless)ΔP transmembrane or transfiber pressure (Pa)ε, ϕ parameters in eqs 26 and 27

Φ surface potential (J·kg−1)

Γthermodynamic factor in the MS formalism (dimension-less)

γ activity coefficient in the sorbate phase or surface tension(mN/m)

Κ adsorption constant (Pa−1)λ displacement of sorbate molecules (m) or variable in eq

12μ chemical potential (J·mol−1) or viscosity (kg·m−1·s−1)ν jump frequency in eq 24 (s−1)Π reduced pressure (dimensionless)Πi gas permeance of ith species (mol·m−2·s−1·Pa−1)Πo parameter in eq 31 (mol·m−2·s−1·Pa−1)θ fractional occupancy (dimensionless) or contact angle

(deg)λ pressure-related variable in eq 12 or average jump

distance in eq 24 (m)τ tortuosity (dimensionless)Ψ integral free energy of adsorption relative to saturation

(kJ/mol)

Subscripts

* self-diffusion coefficienteff effectivem mean valueret retentateT transport diffusion coefficientV void

Superscripts

ads adsorptiondiff diffusionT total* relevant site

Acronyms (for more acronyms see the SupportingInformation)

abim azabenzimidazolead adeninateADM 1-adamantanamineAFM atomic force microscopyALD catalytic atomic layer depositionAMP 2-amino-2-methyl-1-propanolAPDMS (3-aminopropyl)methyldiethoxysilaneAPS (3-aminopropyl)trimethoxysilaneAPTES (3-aminopropyl)triethoxysilaneatz 3-amino-1,2,4-triazoleAZB azobenzenebdc 1,4-benzenedicarboxylateBEA β zeolitebim benzimidazolebipy bipyridinebipyen trans-1,2-bis(4-pyridyl)ethyleneBME-bdc 2,5-bis(2-methoxyethoxy)-1,4-benzenedicar-

boxylatebmim 1-butyl-3-methylimidazolebpdc 4,4′-biphenyldicarboxylatebpee 1,2-bis(4-pyridyl)ethylenebpydc 2,2′-bipyridine-5,5′-dicarboxylatebtc 1,3,5-benzenetriscarboxylatebza benzoateCAPEX capital expenditurecbim 5-chlorobenzimidazoleCBMC configurational bias Monte Carlo


dx.doi.org/10.1021/cr400237k | Chem. Rev. XXXX, XXX, XXX−XXXBN

CCS carbon capture and storageCDM clean development mechanismCFC chlorofluorocarbonCHA chabaziteCLC chemical looping combustionC2mim 1-ethyl-3-methyl-1H-imidazoliumC4mim 1-n-butyl-3-methylimidazoliumC3NH2mim N-(aminopropyl)-3-methylimidazolium bis-

[(trifluoromethyl)sulfonyl]imidecnc 4-carboxycinnamateCOF covalent organic framework3CP-TES (3-chloropropyl)triethoxysilaneCTMA cetyltrimethylammoniumCTMSS carbonized-template molecular sieving silica

membranesCVD chemical vapor depositionCVI chemical vapor infiltrationdabco 1,4-diazabicyclo[2.2.2]octaneDDR decadodecasil 3RDEA diethanolamineDF dimethylformamideDFT density functional theoryDIPA diisopropanolamineDMAPS [3-(dimethylamino)propyl]trimethoxysilaneDMB 2,2-dimethylbutaneDMDA dimethyldecylamineDME dimethyl etherdobdc 2,5-dioxido-1,4-benzenedicarboxylate (=dhtp)dobpdc 4,4′-dioxido-3,3′-biphenyldicarboxylateDOE U.S. Department of EnergyDPA dipropylaminedpni N,N′-bis(4-pyridyl)-1,4,5,8-naphthalenetetra-

carboxydiimidedpt 3,6-bis(4-pyridyl)-1,2,4,5-tetrazineEC European CommunityECCP European Climate Change ProgramED ethylenediamineedia N-ethyldiisopropylamineEISA solvent-induced self-assemblyEL extended Langmuiremim 1-ethyl-3-methylimidazoliumEOR enhanced oil recoveryEPD electrophoretic depositionEPR European Pressurized ReactorETS Engelhard titanium silicateEU European UnionFAU faujasiteFCOM fluorescence confocal microscopyFER ferrieriteFIB focused ion beamfma fumarateFT Fischer−TropschGCMC grand canonical Monte CarloGCMD grand canonical molecular dynamicsGHG greenhouse gasGlyNa sodium glycinateGMS generalized Maxwell−StefanGT gas transationalH3BTP 1,3,5-tris(1H-pyrazol-4-yl)benzeneH3BTTri 1,3,5-tris(1H-1,2,3-triazol-4-yl)benzeneH2btz 1,5-bis(5-tetrazolo)-3-oxapentaneHF hollow fiberHFC hydrofluorocarbon

H2hfipbb 4,4-(hexafluoroisopropylidene)bis(benzoicacid)

Hmim 1-hexyl-3-methylimidazoliumHMS hexagonal mesoporous silicaH2NC3H6mim 1-(3-aminopropyl)-3-methylimidazolium bis-

[(trifluoromethyl)sulfonyl]imideH2tbip 5-tert-butylisophthalic acidH3tcpt 2,4,6-tris(4-carboxyphenoxy)-1,3,5-triazineHRTEM high-resolution transmission electron micros-

copyIAST ideal adsorbed solution theoryica imidazolate-2-carboxyaldehydeIGCC integrated gasification combined cycleIL ionic liquidim 4-methyl-5-imidazolecarboxaldehydeIPCC International Panel on Climate ChangeIRM infrared microscopyISS intergrowth supporting substanceIZA International Zeolite AssociationJI joint implementationL1 5-(pyridin-4-yl)isophthalic acidL2 5-(pyridin-3-yl)isophthalic acidL3 5-(pyrimidin-5-yl)isophthalic acidLBL layer-by-layerlp large poreLTA Linde type A zeoliteLUS Laval University silicaMAMS mesh-adjustable molecular sievesMAPS [3-(methylamino)propyl]trimethoxysilaneMBR membraneMCF mesocellular foamMCM mobile corporate materialMD molecular dynamicsMDEA N-methyldiethanolamineMEA monoethanolamineME-4py-trz-ia 5-[3-methyl-5-(pyridine-4-yl)-4H-1,2,4-triazol-

4-yl]isophthalateMFI mobile fiveMIL Materiau Institut Lavoisiermim 2-methylimidazoleMMM mixed matrix membraneMOF metal organic frameworkMOR mordeniteMOTMS [3-(methacryloxy)propyl]trimethoxysilane2-MP 2-methyl-2-pyrrolidoneMS Maxwell−StefanMSU Michigan State UniversityMTES methyltriethoxysilaneMW microwavenbIm 5-nitrobenzimidazolendc naphthalene-2,6-dicarboxylic acidnIm 2-nitroimidazolenp narrow poreOPEX operating expenditureOTM oxygen transfer membraneox oxalatePAPS [3-(phenylamino)propyl]trimethoxysilanePBI poly(benzeneimidazole)PCN porous coordination networkPE pore-expanded, polyethylenePEG polyethylene glycolPEI poly(ethylenimine)PES poly(ether sulfone)


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PF-NMR pulsed-field nuclear magnetic resonancePIM porous inorganic membranePLD pulsed layer depositionPP polypropylenePPC polypropylene carbonatePRAST predictive adsorbed solution theoryPS polysulfone, polystyrenePSA pressure swing adsorptionPSZ polysilazanePTFE poly(tetrafluoroethylene)PVA poly(vinyl alcohol)PVDF poly(vinylidene fluoride)pyz pyrazineQENS quasi-elastic neutron scatteringRAST real adsorbed solution theoryRCPSA rapid cycle pressure swing adsorptionRSM relevant site modelRTILM room-temperature ionic liquid membraneSAC steam-assisted crystallizationSAM self-assembled organic monolayerSAPO silicoaluminophosphateSBA Santa Barbara amorphousSBU secondary building unitSDA structure-directing agentSEM scanning electron microscopySPC soft porous crystalSNU Seoul National UniversitySRTILM supported room-temperature ionic liquid

membraneSS stainless steelTEA triethanolamine(TEA)OH tetraethylammonium hydroxideTEDA N-[3-(trimethoxysilyl)propyl]ethylenediamine(TEHA)Br triethylhexylammonium bromideTEM transmission electron microscopyTEOS tetraethyl orthosilicateTEPA tetraethylenepentamineTESPtBC [3-(triethoxysilyl)propyl]-tert-butylcarbamateTFPTES (3,3,3-trifluoropropyl)triethoxysilaneTHF tetrahydrofuranTMA tetramethylamineTMOS tetramethyl orthosilicateTPA tetrapropylamineTPD temperature-programmed desorption(TPO)Zr tripropyl orthozirconateTS titanosilicaliteTSA temperature swing adsorptionTVSA temperature vacuum swing adsorptionUiO Universitetet I OsloUNFCCC UN Framework Convention on Climate

ChangeVLE vapor−liquid equilibriumVOL microvolumetryVPT vapor-phase transportVSA vacuum swing adsorptionYSZ yttria-stabilized zirconiaWGS water−gas shiftZIF zeolitic imidazolate frameworkZLC zero-length columnZMOF zeolite-like MOFZPO zirconium propoxide

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