Ultrasonics Sonochemistry 7 (2000) 163–167www.elsevier.nl/locate/ultsonch

Ultrasound in organic electrosynthesis

P. Cognet a,*, A.-M. Wilhelm a, H. Delmas a, H. Aıt Lyazidi a, P.-L. Fabre ba Laboratoire de Genie Chimique, UMR CNRS 5503, INPT-ENSIGC, 18 chemin de la Loge, 31078 Toulouse Cedex 04, France

b Laboratoire de Chimie Inorganique, UPS, IUT Chimie, avenue G. Pompidou, 81100 Castres, France

Abstract

Mechanical effects induced by ultrasonication can be very helpful for the activation of electrochemical reactions. The continuouscleaning of the electrodes by ultrasound irradiation of the electrochemical cell or the enhancement of mass transfer at the electrodesare examples of such activation. Finally, ultrasonication can play an important part for the orientation of reactions whoseselectivities are very sensitive to stirring. Two very different examples have been chosen to illustrate these phenomena: the indirectelectrooxidation of di-ketone-L-sorbose into the corresponding ketogulonic acid and the direct electroreduction of acetophenoneinto pinacol. © 2000 Elsevier Science B.V. All rights reserved.

Keywords: Acetophenone electroreduction; Diacetone-L-sorbose electrooxidation; Electrosynthesis; Sonoelectrochemical reactor; Ultrasound

1. Introduction 2. Di-ketone-L-sorbose electrooxidation

2.1. Reaction schemeIrradiation of liquids by power ultrasound leads tocavitation phenomena: microbubbles present in the solu-tion are submitted to growing, vibration and finally The oxidation of diacetone-L-sorbose into diacetone-

2-keto-L-gulonic acid is a well-known step of the synthe-implosion. Effects of cavitation in a variety of homogen-eous chemical systems (outgassing, stirring) have been sis of the C vitamin. It can be carried out electrochemi-

cally, using a nickel working electrode in an alkalinestudied widely, but beneficial processes can be obtainedin heterogeneous media at a solid–liquid interface, such medium [2,3].

Nickel hydroxide is first anodically oxidised intoas particle size modification, cleaning of surfaces or theformation of fresh surfaces [1]. Such a heterogeneous peroxide. Then it reacts with the alcohol (DAS) to give

the acid (DAG) and regenerate the hydroxide.interface exists between an electrode surface and anelectrolyte. In this way, some attempts have been made Hydrogen evolution is the main cathodic reaction

(Scheme 1).to apply ultrasonic effects to electroorganic processes inorder to increase product yields, modify electropolymer-ised polymer properties or promote sacrificial electrodereactions.

We report below some results obtained for electro-lyses assisted by ultrasound in two different applications.In the first case, ultrasonication was used during electrol-ysis for the activation of a suspended electrode used inthe electrooxidation of diacetone-L-sorbose into diacet-one-2-keto-L-gulonic acid. The second example dealswith the electroreduction of acetophenone into pinacolunder ultrasound.

* Corresponding author. Tel.: +33-62-252329;Scheme 1. Mechanism of the indirect oxidation of DAS in alkalinefax: +33-62-252329.

E-mail address: patrick.cognet@ensigct.fr (P. Cognet) aqueous medium.

1350-4177/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.PII: S1350-4177 ( 00 ) 00036-5

164 P. Cognet et al. / Ultrasonics Sonochemistry 7 (2000) 163–167

of the DAG yield, for both concentrations. For thisapplication, the ultrasound irradiation acts in differentways. Firstly, an analysis of the granulometry has shownthat the size of the nickel hydroxide particles wasreduced: the mean particle diameter changes from0.88×10−3 m without sonication to 0.12×10−3 m aftersonication.

Having in mind that the DAS oxidation is a hetero-geneous chemical process, a decrease in particle sizeleads to an increase in catalyst surface, resulting in ahigher reaction rate. Secondly, it can improve thetransfer of the DAS from the bulk solution to theperoxide particle, where it reacts, and also at the surfaceof the nickel foam electrode, which is also activated.

2.3.2. DAS concentrationElectrolyses were carried out for different initial DAS

concentrations. Results are shown in Fig. 2. It can beobserved that an increase of the DAS concentration

Fig. 1. Electrochemical Grignard reactor.results in a lower chemical yield. This can be explainedby the increase in viscosity of the solution with the DAS

2.2. Sonoelectrochemical reactor concentration, which is detrimental to ultrasound effi-ciency. Nevertheless, the same yield (70%) was obtained

Experiments were carried out in a 1 l Grignard elec- for a DAS concentration of 50 and 100 kg/m3.trochemical reactor (Fig. 1), fitted with two cylindricalelectrodes, isolated by a polyethylene film. Nickelhydroxide was introduced as a suspension in a 1 M 3. Acetophenone electroreductionKOH aqueous solution [4]. The outside electrode(anode) was made of nickel foam (96 cm2 surface area), 3.1. Reaction schemewhereas the internal electrode (cathode) was made ofexpanded platinated titanium (56 cm2 surface area). Direct reduction of acetophenone in a protic mediumNi(OH)2 (4 g) was used as the suspended electrode. It is a well-known reaction [5–8]. One product iswas irradiated by a 500 kHz ultrasonic horn, connected 2,3-diphenyl-2,3-butanediol (two diastereoisomers d,lto a 100 W ultrasound generator. A mechanical stirrer and meso). It is obtained by one-electron reduction(speed 25 s−1) was added to ensure sufficient stirring. followed by duplication. The other is 1-phenylethanolThe electrochemical cell was undivided and electrolyses which is obtained by a two-electron reduction (electro-were performed in an intensiostatic way (I=1 A). chemical dihydrogenation). For pinacol, both diastereoi-Temperature was maintained at 60°C by means of a somers d,l and meso are formed. The choice ofcooling jacket. experimental conditions determines the chemoselectivity

but also the stereoselectivity of the reaction. The most2.3. Sonoelectrooxidation determining parameter is pH. In neutral and weakly

2.3.1. Ultrasound irradiationThe impact of ultrasonication on the process was

studied first. Electric power of 100 W (22 W in thesolution) was applied. As can be seen in Table 1, ultra-sonic irradiation results in an important increase (30%)

Table 1Effect of ultrasound irradiation on DAG chemical yield

Ultrasound power (W ) mDAS (g) Chemical yield (%)

0 11 6822 11 860 22 54

Fig. 2. Influence of the DAS concentration on the DAG yield after22 22 72passing the theoretical electricity amount (4 F/mol ).

165P. Cognet et al. / Ultrasonics Sonochemistry 7 (2000) 163–167

alkaline media, acetophenone exhibits a single two- Before use, the electrodes were washed with dilutenitric acid and then rinsed with distilled water.electron wave. In this pH range, the proton availability

near the electrode surface is lower and so the neutral Electrolyses were carried out under dinitrogen atmo-sphere after outgassing the electrolysis solution by dinit-ketone is the electroactive species. It is reduced to a

radical anion (I ), which immediately abstracts a proton rogen bubbling for 25 min.The electrolysis solution composition was as follows:from the solvent. The resulting radical (II ) is then

quickly reduced, resulting in the observed two-electron acetophenone (3.8×10−2 M); supporting electrolyte,Na2SO4 (0.1 M); solvent, water; co-solvent, methanolwave (ECE mechanism). But, if concentrated solutions

are used for electrolysis, dimerisation of I or II can (from 0 to 50% of the total volume of 200 ml ).compete with the reduction of II (EC mechanism)(Scheme 2). 3.3. Galvanostatic electrolysis under ultrasound

3.3.1. Sonoelectrolysis without co-solvent3.2. Experimental partThe impact of ultrasonication on electrode passiv-

ation was studied first. This study was made withoutExperiments were carried out in a conventional non-divided Pyrex cell with a cooling jacket [9]. The internal methanol and at a constant current of 150 mA. It can

be observed in Fig. 3 that the cathode potential decreasesvolume was 300 ml. It was fitted with a PVC coverwhich permitted the introduction of the electrodes. For dramatically with electrolysis time from −1.65 to

−2.80 V/SCE, because of the pinacol film deposition.conventional electrolyses, the electrolytic solution wasstirred magnetically. For sonoelectrochemical electro- Ultrasonication allows us to remove the pinacol film on

the cathode, resulting in the stabilisation of the cathodiclyses, the cell was placed in a sonicated bath (ELMA,505×137×100 mm3, 35 kHz). potential which varies between −1.6 and −1.7 V/SCE.

Figs. 4 and 5 show the molar profiles of acetophenoneThe electrode configuration was axial. The workingelectrode (cathode) consisted of an expanded zinc (geo- and products obtained during electrolyses with and

without ultrasound. The cathodic potential drop whichmetrical surface area 57×10−4 m2). The space betweenthe two electrodes was equal to 4 mm. The countere-lectrode consisted of an expanded Ti/Pt (surface area46×10−4 m2) grid.

Temperature was maintained at 25°C by circulatingwater in the cooling jacket in case of overheating thesolution by Joule effect.

Electrolyses were monitored by a Tacussel PJT 35V-2 A potentiostat connected to a current integrator,using the three-electrode configuration. To measure orset the potential to the working electrode, a doublejunction reference saturated calomel electrode (SCE)was used. It was immersed in the solution through aLuggins capillary.

Fig. 3. Evolution of cathodic potential during electrolysis, with andwithout ultrasonication.

Scheme 2. Mechanism of the reduction of acetophenone in neutral pH Fig. 4. Molar profiles of acetophenone, alcohol and pinacol duringelectrolysis without ultrasound, I=150 mA.aqueous medium.

166 P. Cognet et al. / Ultrasonics Sonochemistry 7 (2000) 163–167

Fig. 7. Alcohol/pinacol molar ratio as a function of consumed electric-Fig. 5. Molar profiles of acetophenone, alcohol and pinacol duringity, I=150 mA.electrolysis under ultrasound, I=150 mA.

is observed without ultrasonication leads to a decreasein the acetophenone conversion and to dihydrogen The benzophenone reduction is then slowed downproduction. but not stopped. The same effect was observed by

After passing 1.65 F/mol, only 40% of acetophenone Stocker and Sidisuntharne [11] for the reduction ofwas consumed. As a result, pinacol selectivity falls and acetophenone in solutions saturated with dioxygen. Noalcohol becomes the main product (Fig. 4). On the effect was observed on the stereoselectivity. The bettercontrary, ultrasonication permits us to obtain better conversion obtained under sonication can then be par-conversions (from 38% without ultrasound to 47% with tially attributed to outgassing. Indeed, dioxygen anodi-ultrasound), and a better selectivity in pinacol (Fig. 5). cally formed by water electrolysis is eliminated by

sonication. As a consequence, conversion of radicals3.3.2. Sonoelectrolysis with co-solvent into the starting reagent is stopped.

Using methanol as the co-solvent results in a better Fig. 7 shows the evolution of the alcohol/pinacolconversion of acetophenone (Fig. 6). ratio during electrolysis with and without ultrasound.

This can be attributed to ultrasound mechanical Without sonication, the ratio increases with electrolysiseffects on mass transfer and surface cleaning. Pitts et al. time. Under ultrasound, it remains almost constant and[10] showed that the presence of dioxygen in the medium inferior to 1 in the case of 10% methanol. A slowhad an inhibiting effect on the benzophenone reduction methanol concentration favours the pinacol formation,reaction. Radicals electrochemically generated react with but in all cases ultrasonication allows us to dramaticallydioxygen to give the starting reagent, benzophenone. reduce the alcohol production. It permits us to minimiseHe proposed the following mechanism: the co-solvent concentration while avoiding electrode

passivation.(C6H5)C$

OH+O2�HO$2+(C6H5)2CO

HO$2+(C6H5)C$

OH+O2�H2O2+(C6H5)2CO.

4. Conclusions

The activation of electrosynthesis processes has beenillustrated by two different reactions: one indirectelectrooxidation using a suspended electrode and onedirect electroreduction. In the first case, the electrooxida-tion of DAS, the application of an ultrasonic fieldchanges the granulometry of the suspended electrode,thus increasing the electrode surface. It results in betterfaradaic and chemical yields. In the second case, theelectroreduction of an aromatic ketone, ultrasound tech-nology proved to be of great interest for electrodedepassivation and mass transfer rate enhancement.Selectivity is deeply affected by ultrasonication, whichis probably the most remarkable result. These twoFig. 6. Acetophenone conversion as a function of electrolysis time,

I=150 mA. examples show all the interest of applying ultrasound

167P. Cognet et al. / Ultrasonics Sonochemistry 7 (2000) 163–167

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[1] D.J. Walton, S.P. Sukhvinder, Adv. Sonochem. 4 (1996) 205. [10] J.N. Pitts, R.L. Letsinger, R.P. Taylor, J. Am. Chem. Soc. 81[2] G. Vertes, G. Horanyi, Acta Chim. Acad. Sci. Hung. 67 (1971) (1959) 1068.

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