Liquid phase isomerisation of dichlorobenzenes over H-zeolites

Applied Catalysis A: General 243 (2003) 301–307

Liquid phase isomerisation of dichlorobenzenes over H-zeolites

Dalibor Kaucky1, François Fajula∗, Patrice Moreau, Annie FinielsLaboratoire de Matériaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 ENSCM-CNRS,

Ecole Nationale Supérieure de Chimie, 8 rue de l’Ecole Normale, Montpellier Cedex 5, F-34296 France

Received 9 September 2002; received in revised form 16 September 2002; accepted 16 October 2002

Abstract

Liquid phase isomerisation of dichlorobenzenes (dCBs), i.e.ortho-dichlorobenzene (o-dCB) and/orpara-dichlorobenzene(p-dCB) to meta-dichlorobenzene (m-dCB) catalysed by solid acid catalysts could provide an alternative to the traditionalprocess based on AlCl3-type catalysts. The reaction was studied over a series of H-zeolites in a batch autoclave reactor at atemperature of 340◦C and a pressure of 40 bar, with pure reactants, i.e. without solvent. Under these conditions, H-ZSM-5and mordenite exhibited the highest activity, while�-zeolite, faujasite and ferrierite showed only low activity. These resultsare directly related to the pore architecture of the different zeolites, which governs deactivation by coke formation. The natureof the coke formed in the two most active zeolites H-ZSM-5 and mordenite has been analysed by different techniques.© 2002 Elsevier Science B.V. All rights reserved.

Keywords: Dichlorobenzenes; Isomerisation; Deactivation; Coke formation; H-ZSM-5; H-Mordenite; Liquid phase; Heterogeneous catalysis

1. Introduction

Isomerisation of dichlorobenzenes (Scheme 1) rep-resents the most challenging problem in present in-dustrial chemistry of chloroaromatics[1].

This is because of disadvantageous product com-position resulting from classical Friedel–Crafts chlo-rination of benzene or chlorobenzene. These reactionsproduce mainlyo-dCB and p-dCB, while the for-mation of m-dCB is minimal. Having on mind thatm-dCB begins to be an important raw material inagrochemical, pharmaceutical and dye industry[1],the isomerisation ofo-dCB and/orp-dCB represents

∗ Corresponding author. Tel.:+33-4-67-14-43-91;fax: +33-4-67-14-43-49.E-mail address: [email protected] (F. Fajula).

1 On leave from J. Heyrovsky Institute of Physical Chemistry,Academy of Sciences of the Czech Republic, Dolejskova 3, Prague8, CZ-182 23 Czech Republic.

an important route form-dCB manufacture, alter-native to complicated many-stage processes. Envi-ronmentally unfriendly AlCl3/HCl/H2O, BF3·HF orSbF5·HF catalysts were employed so far[2–5], forthe isomerisation ofo-dCB. These catalysts are un-stable, very difficult to recover and highly corrosive.Consequently, the effort of several groups has beenfocused on searching effective solid catalysts for theisomerisation of dCBs. Gas phase isomerisation ofo-dCB was investigated over a series of acidic zeo-lites, including faujasite (Y, USY), mordenite (MOR),ferrierite (FER) and�-zeolite (BEA) [6–13]. Them-dCB was produced in significant yield in most ofthese catalysts, with the maximum yield of 44% re-ported over H-MOR at 370◦C [6,7]. However, fastdeactivation by formation of coke, has been observedin all cases. Coq et al.[7] have especially reportedthat, under gas phase conditions at 370◦C, the spe-cific activity of the sites in the isomerisation reactionwas H-ZSM-5> H-MOR > H-BEA > H-OFF, and

0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0926-860X(02)00552-5

302 D. Kaucky et al. / Applied Catalysis A: General 243 (2003) 301–307

Scheme 1.

was related to the acidity strength of the differentzeolites, while the deactivation rate was in the orderH-MOR > H-BEA, H-OFF> H-ZSM-5.

Only very limited information on the isomerisationreaction of dCBs in liquid phase catalysed by solidcatalysts in general, and zeolites in particular, is avail-able. Toray[14] and more recently Bayer[15] havedisclosed such a process, using the same type of zeo-lites as catalysts, namely ZSM-5, MOR and FAU, infixed-bed reactors. These patents claim high yields inm-dCB but with a severe catalyst deactivation. In thepresence of 5% molar hydrogen in a feed of dCBscontaining 60%para isomer, H-ZSM-5 zeolite provedthe best catalyst in terms of stability andm-dCB yields(42% at 360◦C and 30 atm[15]). The productivity ofthe catalyst was however low since such a conversionwas obtained at a space velocity of 0.1 h−1.

In view of the limited information available regard-ing the behaviour of zeolite catalysts in the liquidphase isomerisation of dCBs, we have undertaken thepresent study where the activity and stability of a seriesof five zeolites have been compared using a batch re-actor and their performances analysed in relation withtheir acidic and textural characteristics.

2. Experimental

2.1. Catalysts and products

The zeolites H-MOR (10), H-USY (15) and H-BEA(12.5) were obtained from PQ Corporation, respec-tively CBV 20A, CBV 720 and CP 814E (the figures inparentheses refer to the Si/Al ratio of the samples). TheH-ZSM-5 (25) was from CONTEKA, CBV 5524G.The H-FER (9) was a gift from J. Heyrovsky Instituteof Physical Chemistry of Prague (A6106a). Calcina-tion of zeolites was performed in a flow of dry air at500◦C for 6 h with a heating rate of 60◦C h−1. The

o-dCB andp-dCB, provided by Tessenderlo Chemie,were used as such (99.5% purity).

2.2. Catalytic experiments

The catalytic experiments were performed in a300 ml high-pressure, titanium-wall covered reactor(PARR Instruments Co.), operating under batch con-ditions. The maximum temperature limit was 350◦C.A sampling pipe immersed into the reaction mixtureenabled withdrawing of liquid through a filter. Exceptotherwise mentioned, the general procedure, definedas “standard conditions”, was as follows: 37.5 mlpureo-dCB, 3 g freshly calcined catalyst, temperature340◦C, pressure 40 bar, stirring rate 800 rpm.

2.3. Analysis

The liquid samples of reaction mixture were with-drawn as a function of time and analysed by gaschromatography (HP 5890 Series II gas chromato-graph equipped with FID, capillary column Carbowax20 M-type, 25 m× 0.53 mm, 1.0�m film thickness,carrier gas H2, 60–150◦C, 10◦C min−1). The detec-tion efficiency for all three isomers was practically thesame.

2.4. Catalysts characterisation

Chemical analyses of the solid samples were per-formed at the Service Central d’Analyse du CNRS(Solaize, France) and at the CIRAD (Montpellier,France). Powder X-ray diffraction (XRD) patternswere recorded on a Philips instrument (40 kV, 20 mA)using Cu K� radiation (λ = 0.154 nm). The 2θ angleranged from 20 to 30◦. Thermogravimetric experi-ments (TG) were carried out on a Setaram TG 851000◦C microbalance in air (80 ml min−1) from 25to 750◦C (ramp: 10◦C min−1, dwell: 150◦C for30 min). Microporous volumes were determined byN2 adsorption with a Micromeritics ASAP 2000 in-strument. The samples were outgassed in vacuum(2 × 10−4 Pa) at 250◦C overnight before N2 ad-sorption. Temperature Programmed Desorption ofammonia (NH3-TPD) was carried out in a quartzflow-type microreactor; the catalyst calcination in airflow at 600◦C and subsequent sorption of ammonia at150◦C preceded each TPD scan (ramp:10◦C min−1).

D. Kaucky et al. / Applied Catalysis A: General 243 (2003) 301–307 303

Acidity was characterised by the total amount of am-monia adsorbed and the temperature (Tmax) at whichthe rate of desorption was the highest. UV-Vis wereobtained on a Perkin-Elmer Lambda 14 spectrome-ter. 13C NMR spectra were recorded on a BruckerAvance DSX 200 spectrometer operating at a fre-quency of 200 MHz, using cross-polarisation withmagic angle spinning (CP/MAS). The nature of thecoke was also analysed according to the proceduredescribed by Guisnet and Magnoux[16,17]: afterdissolution of the used zeolite by HF, the organicresidue was extracted by a solvent (CH2Cl2). The sol-uble fraction was analysed by GC/MS and the solidone, corresponding to condensed aromatic rings, wasweighted.

3. Results and discussion

3.1. Comparison of the various catalysts

The activity of the five zeolite catalysts has beenevaluated in the isomerisation of pureortho-dichloro-benzene (o-dCB) in the batch reactor under the ex-perimental conditions reported above. As shown inTable 1, the activity of these zeolites, expressed by theyield of m-dCB reached after 24 h reaction, stands inthe following sequence:

ZSM-5 > MOR � BEA, USY > FER.

A yield of 28% of m-dCB is obtained with ZSM-5(25), while a yield of 20% is typically obtained withH-MOR (10) under identical conditions. Much loweryields are associated with BEA, USY and FER zeo-lites. All the catalysts, except H-FER, produced sig-nificant amounts of coke.

Table 1Zeolite properties and catalytic behaviour in the liquid phase isomerisation ofo-dCB

Catalyst (Si/Al) Pore structure (Å) Acidity Product compositiona (%) Coke (wt.%)

(meq. g−1) Tmax (◦C) ortho meta para

H-ZSM-5 (25) 5.5× 5.1, 5.6× 5.3 0.65 440 65 28 7 6.4H-MOR (10) 7.0× 6.5, 5.7× 2.6 1.19 580 73 20 7 10H-USY (15) 7.4× 7.4 0.49 490 93 5 2 12.3H-BEA (12) 7.6× 6.4, 5.5× 5.5 1.05 520 92 6 2 13.5H-FER (9) 5.4× 4.2, 4.8× 3.5 2.07 510 99 1 0 <1

a After 24 h reaction at 340◦C.

Fig. 1. Isomerisation ofo-dichlorobenzene (o-dCB) over H-ZSM-5(25) at 340◦C and 40 bar. Product distribution as a function oftime: m-dCB (�), p-dCB (�).

A typical time evolution curve of the yieldsof m-dCB and p-dCB during a standard experi-ment, i.e. using pureortho-dichlorobenzene as re-actant and H-ZSM-5 (25) as catalyst, is shownin Fig. 1. As said above, the yield ofm-dCBwas found to be 28% (reproducibility±2%) after24 h reaction. Longer runs did not allow to im-prove m-dCB formation. Thepara isomer was alsoformed in some extent (7%); no other productswere detected, except traces of mono-chlorobenzene(CB).

From the data ofTable 1, it is clear that the abilityof the catalysts in producingm-dCB and coke fromo-dCB cannot be explained by their acid characteris-tics, taking into consideration either the number or thestrength of the sites measured by ammonia desorption.The catalytic results are better discussed in relationwith the reaction mechanism and the topology of thezeolite frameworks.


Scheme 2.

The overall reactional process can be representedby the following general scheme (Scheme 2). Thereaction involves two sets of competitive reactions:consecutive reversible steps in the acid-catalysedisomerisation reaction (k1, k−1, k2 andk−2), and suc-cessive rearrangements (ka, kb, kc), leading to theformation of coke from the three dichlorobenzeneisomers. These two series of reactions occur on theporosity of the solid catalyst. The desorption of theproducts—and the composition of the liquid phase—is governed by the partition coefficients of reactantsand products between the liquid phase and the zeolite(K1, K2, K3).

The isomerisation reactions being reversible, therate of isomerisation tom-dCB will depend on thedistribution of o-dCB, m-dCB and p-dCB isomersin the mixture. Thus, when this distribution will ap-proach the equilibrium value, the rate of themetaisomer formation will strongly decrease. On the con-trary, with regards to the formation of coke, the threeisomers can be considered as a single substrate, allcontributing to coking. Therefore, the rate of cokeformation will be less dependent on the compositionsince [o] + [m] + [p] = constant. Consequently,in such a process the isomerisation phenomenonis overtaken by the formation of coke and, as the

two reactions compete for the same type of acidsites, the formation of coke will be always kineti-cally favoured over the formation of the targetmetaisomer.

Because confinement effects[18,19] play a majorrole in zeolite catalysis, the various pathways leadingto either isomerisation or deactivation by coking willbe strongly dependent on the topology of the zeoliteframeworks. Actually, diffusion of the reactants andproducts as well as coke formation in pores withmolecular dimensions are shape-sensitive processes.In our case, the inactivity of H-FER can be well ex-plained by the small dimension of its pores whichcan hardly accomodate the more hinderedortho andmeta isomers of the dCB molecule. Such a restrictiondoes not exist in the case of H-USY and H-BEAwhich feature three-dimensional networks of poresaccessible through 12 member-ring (MR) pore open-ings. In those catalysts, fast formation of coke leadsto rapid fouling by accumulation of carbonaceousresidue which consumes—or impedes access to—theacid sites before reaching a significant isomerisa-tion level. Under our conditions, therefore, H-ZSM-5(two-dimensional network of 10 MR, channels) andH-MOR (uni-dimensional 12 MR network of pores)represent the best compromise between activity


in the isomerisation reaction and stability towardsdeactivation.

3.2. Nature of coke and catalyst reactivation

The nature of the carbonaceous deposits on usedH-ZSM-5 (25) and H-MOR (10) catalysts has beenanalysed by different techniques.

The coke content, determined by thermogravime-try from the weight loss between 350 and 700◦Camounted to 10 and 6.4 wt.% for H-MOR andH-ZSM-5, respectively. These deposits are responsi-ble for a significant decrease of the free microporevolume of the zeolites, 25% for H-ZSM-5 and 70%for H-MOR, suggesting a higher toxicity of coke forthe latter. Elemental analysis for carbon shows that thedeposits contain 84 wt.% carbon and 16 wt.% chlo-ride for ZSM-5, while the composition of the cokeon mordenite corresponds to 65% carbon and 35%chloride. These figures indicate, on the one hand, thata significant amount of chlorine is lost (probably asHCl which was indeed detected, but not measured, inthe system) during coke formation (the starting reac-tant contains equal amounts of the two elements) and,on the other hand, that the nature of the coke whichis deposited on the two zeolites is different. This lastassumption is confirmed by UV-Vis and13C NMRexamination of the used catalysts and by analysis ofthe coke after extraction.

Thus, a difference in the aromaticity of the twodeposits is first indicated by the difference of coloursof the two samples, respectively greyish for usedH-ZSM-5 and dark-blue for used H-MOR; the lat-ter colour is directly related to an important bandat 580 nm in the UV-Vis spectrum, characteristic ofcondensed aromatic rings. The13C CP/MAS NMRspectra reveal a well-resolved peak at 42.5 ppm inaliphatic region in the case of H-ZSM-5 which isabsent form the spectrum of coked H-MOR, whilethere are no significant differences in the aromaticregion (116–154 ppm). The signal at 42.5 ppm can beassigned to CH2 groups between two aromatic rings,and might indicate a certain degree of branching ofthe coke formed on H-ZMS-5 sample.

Coke analysis by the method of Guisnet and Mag-noux [16] allows to differentiate CH2Cl2 “soluble”and “insoluble” coke. Insoluble coke consists of pol-yaromatic molecules resulting from the transformation

of lighter molecules (soluble coke) trapped inside thezeolite micropores. For the H-ZSM-5 sample, the sol-uble extracted coke molecules (about 70–90%), iden-tified by GC/MS, consist mainly in diphenylmethanederivatives, chlorinated or not. On the contrary, forthe H-MOR zeolite, the soluble coke content is only10–20%; thus, with H-MOR, the quantity of insolublecoke is very important; such a feature is in good agree-ment with a highly polyaromatic character of coke inH-MOR.

The formation of aromatic coke is known to be pro-moted in open structures with a high density of strongacid sites[20,21]. As the H-MOR features larger poresand is more acidic than H-ZSM-5, both in terms ofstrength and density of acidic sites (Table 1), the dif-ference observed in the nature of the coke on the twosolids is consistent.

Deactivated catalysts re-used after Soxhlet extrac-tion in dichloromethane led to only traces ofm-dCB.On the other hand, when the used catalysts were re-activated in a flow of air at 550◦C, the original whiteaspect of the catalyst was recovered together with asimilar activity as that of the fresh sample. Additionalcharacterisations by XRD and NMR allow to excludeany irreversible change in the catalyst structure afterseveral reaction, reactivation cycles.

3.3. Maximum yield achievable with thebatch reactor

The maximum yields inm-dCB achievable underour batch conditions have been determined by per-forming a series of experiments using pureo-dCB,p-dCB, and m-dCB as well as mixtures of themas starting reactants. The results are presented in aconventional ternary triangular diagram inFig. 2.

Starting from pureortho (standard experiment (�)),the final composition (point A) after 24 h correspondsto 65%ortho, 28%meta and 7%para. Starting frompurepara, the corresponding composition (�) is 34%of meta, with only 3% ortho. The experiment start-ing from a 50/50 mixture ofortho-dCB andpara-dCBleads to a final composition of 30%meta, 38% or-tho and 32%para (). Experiment starting from puremeta (�) demonstrates a minimum (5%) conversionof this isomer.

Furthermore, an experiment was carried out withthe reactional mixture obtained from a previous


Fig. 2. Isomerisation of dichlorobenzenes over H-ZSM-5 (25)at 340◦C, 40 bar. Equilibrium compositions achieved after 24 hreaction.

experiment starting from pureortho and correspond-ing to the composition attained at point A, with 3 gof fresh catalyst. The final composition was 51% ofo-dCB, 37% ofm-dCB and 12% ofp-dCB (point B).The same experiment was repeated, but with an initialmixture prepared from fresho,m,p-DCB chemicals(clear colour, fresh mixture of the same compositionas the mixture represented by point A). The result waspractically the same in both cases. Moreover, if a newexperiment is carried out over a fresh catalyst fromthe mixture represented by point B, only a minimumchange is detected (1% of overall conversion).

These results are in line with the overall reactionscheme (Scheme 2) discussed above and indicate that ayield of m-dCB of ca. 38% would therefore correspondto the maximum achievable at a reasonable rate underbatch conditions.

Though integrated and instantaneous conversionscannot be compared directly, our data point to aslightly lower activity of H-ZSM-5 when operatedin batch with respect to fixed-bed reactor[14,15].Such a difference has been already pointed out, forinstance in the case of the acetylation of toluene using�-zeolites as catalysts[22]. Work under continuousoperation does not totally prevent for catalyst deacti-vation, but allows for a better control of the contacttime and, overall, a better desorption of the productsand coke precursors, therefore improving conversion.

4. Conclusion

Mordenites and ZSM-5 zeolites are effective cata-lysts in the isomerisation of dichlorobenzenes in liquidphase under batch conditions. H-ZSM-5 exhibited thehighest activity, leading to 28 and 34% ofmeta isomerat 340◦C after 24 h reaction starting fromo-dCB andp-dCB, respectively. Mordenite shows moderate activ-ity. Deactivation, which occurred in all cases as a resultof coke formation, restricts the activity of the catalysts.The content and the nature of the coke are dominatedby structural characteristics and the acidity of the zeo-lite. The lower amount and lower aromatic character ofthe coke formed in H-ZSM-5, in comparison to mor-denite, are attributed to the dimensions of the poreswhich sterically hinder the formation of condensedpolyaromatics and to a lower density and strengthof acid sites. Under the reported batch conditions,over H-ZSM-5 zeolite, the maximum yield ofm-dCBachievable from a mixture of dCBs would amount to38%. Despite severe catalyst deactivation, liquid phaseisomerisation of dichlorobenzenes catalysed by acidzeolites in batch reactor could therefore constitute apromising alternative owing to the high selectivity ofthe reaction and the environmental advantages versusestablished process based on AlCl3-type catalysts.

Acknowledgements

The authors thank Tessenderlo Chimie, Belgium,for financial support and Drs. Marc Belmans and FrankBoers from R&D, Fine Chemicals Division of thisfirm for stimulating discussions. Dr. Patrick Magnoux(University of Poitiers, France) is acknowledged forhis contribution and discussions for the determina-tion of coke nature. The authors thank Dr. VladimirBosacek (J. Heyrovsky Institute of Physical Chem-istry, Prague, Czech Republic) for recording13C NMRspectra.

References

[1] Ullmanns Encyclopedia of Industrial Chemistry, fifth ed., vol.A6, 1996, p. 330.

[2] L.A. Mattano, US Patent 2,727,075 (1955), to Standard OilCompany.

[3] J.W. Angelkorte, US Patent 2,920,09 (1960), to Union CarbideCorporation.


[4] Y.G. Erykalov, G. Bekker, A.P. Belokurova, Zh. Org. Khim.4 (12) (1968) 2126.

[5] G.A. Olah, W.S. Tolgyesi, R.E.A. Dear, J. Org. Chem. 27(1962) 3441, 3449.

[6] J. Pardillos, B. Coq, F. Figueras, Appl. Catal. 51 (1989) 285–293.

[7] B. Coq, J. Pardillos, F. Figueras, Appl. Catal. 62 (1990) 281–294.

[8] B. Coq, J. Pardillos, F. Figueras, Stud. Surf. Sci. Catal. 59(1991) 581.

[9] B. Coq, R. Durand, F. Fajula, C. Moreau, A. Finiels, B.Chiche, F. Figueras, P. Geneste, Stud. Surf. Sci. Catal. 41(1988) 241.

[10] T. Takahashi, T. Kai, Stud. Surf. Sci. Catal. 84 (1994) 1989.[11] I.I. Lishchiner, V.A. Plakhotnik, A.N. Kuzmicheva, E.R.

Mortikov, Russ. Chem. Bull. 43 (11) (1994) 1809.

[12] H. Litterer, European Patent 0,140,123 (1985), to HoechstA.G.

[13] J. Pardillos, D. Brunel, B. Coq, P. Massiani, L.C. de Ménorval,F. Figueras, J. Am. Chem. Soc. 112 (1990) 1313.

[14] K. Tada, H. Imada, T. Inoue, Japan Patent Showa 58–144,330(1984), to Toray Co. Ltd.

[15] M. Pies, H. Fiege, L. Puppe, J. Kaesbauer, US Patent5,466,881 (1995), to Bayer A.G.

[16] M. Guisnet, P. Magnoux, Appl. Catal. 54 (1989) 1.[17] M. Guisnet, P. Magnoux, Appl. Catal. A: Gen. 212 (2001) 83.[18] L.D. Rollman, D. E Walsh, J. Catal. 56 (1979) 139.[19] E.G. Derouane, J. Catal. 100 (1986) 541.[20] E.G. Derouane, Stud. Surf. Sci. Catal. 20 (1985) 221.[21] S.K. Sahoo, N. Viswanadham, N. Ray, J.K. Gupta, I.D. Singh,

Appl. Catal. A: Gen. 205 (2001) 1.[22] E.G. Derouane, J. Mol. Catal. A: Chem. 137 (1998) 29.

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Liquid phase isomerisation of dichlorobenzenes over H-zeolites