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Ultrasound in organic electrosynthesis
Ultrasonics Sonochemistry 7 (2000) 163–167 www.elsevier.nl/locate/ultsonch
Ultrasound in organic electrosynthesis P. Cognet a, *, A.-M. Wilhelm a, H. Delmas a, H. Aı¨t Lyazidi a, P.-L. Fabre b a Laboratoire de Ge´nie 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 continuous cleaning of the electrodes by ultrasound irradiation of the electrochemical cell or the enhancement of mass transfer at the electrodes are examples of such activation. Finally, ultrasonication can play an important part for the orientation of reactions whose selectivities are very sensitive to stirring. Two very different examples have been chosen to illustrate these phenomena: the indirect electrooxidation of di-ketone-L-sorbose into the corresponding ketogulonic acid and the direct electroreduction of acetophenone into 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
Irradiation of liquids by power ultrasound leads to cavitation phenomena: microbubbles present in the solution are submitted to growing, vibration and finally implosion. Effects of cavitation in a variety of homogeneous chemical systems (outgassing, stirring) have been studied widely, but beneficial processes can be obtained in heterogeneous media at a solid–liquid interface, such as particle size modification, cleaning of surfaces or the formation of fresh surfaces [1]. Such a heterogeneous interface exists between an electrode surface and an electrolyte. In this way, some attempts have been made to apply ultrasonic effects to electroorganic processes in order to increase product yields, modify electropolymerised polymer properties or promote sacrificial electrode reactions. We report below some results obtained for electrolyses assisted by ultrasound in two different applications. In the first case, ultrasonication was used during electrolysis for the activation of a suspended electrode used in the electrooxidation of diacetone-L-sorbose into diacetone-2-keto-L-gulonic acid. The second example deals with the electroreduction of acetophenone into pinacol under ultrasound.
2.1. Reaction scheme
* Corresponding author. Tel.: +33-62-252329; fax: +33-62-252329. E-mail address: [email protected] (P. Cognet)
The oxidation of diacetone-L-sorbose into diacetone2-keto-L-gulonic acid is a well-known step of the synthesis of the C vitamin. It can be carried out electrochemically, using a nickel working electrode in an alkaline medium [2,3]. Nickel hydroxide is first anodically oxidised into peroxide. Then it reacts with the alcohol (DAS) to give the acid (DAG) and regenerate the hydroxide. Hydrogen evolution is the main cathodic reaction (Scheme 1).
Scheme 1. Mechanism of the indirect oxidation of DAS in alkaline aqueous medium.
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P. Cognet et al. / Ultrasonics Sonochemistry 7 (2000) 163–167
of the DAG yield, for both concentrations. For this application, the ultrasound irradiation acts in different ways. Firstly, an analysis of the granulometry has shown that the size of the nickel hydroxide particles was reduced: the mean particle diameter changes from 0.88×10−3 m without sonication to 0.12×10−3 m after sonication. Having in mind that the DAS oxidation is a heterogeneous chemical process, a decrease in particle size leads to an increase in catalyst surface, resulting in a higher reaction rate. Secondly, it can improve the transfer of the DAS from the bulk solution to the peroxide particle, where it reacts, and also at the surface of the nickel foam electrode, which is also activated.
Fig. 1. Electrochemical Grignard reactor.
2.2. Sonoelectrochemical reactor Experiments were carried out in a 1 l Grignard electrochemical reactor ( Fig. 1), fitted with two cylindrical electrodes, isolated by a polyethylene film. Nickel hydroxide was introduced as a suspension in a 1 M KOH aqueous solution [4]. The outside electrode (anode) was made of nickel foam (96 cm2 surface area), whereas the internal electrode (cathode) was made of expanded platinated titanium (56 cm2 surface area). Ni(OH ) (4 g) was used as the suspended electrode. It 2 was irradiated by a 500 kHz ultrasonic horn, connected to a 100 W ultrasound generator. A mechanical stirrer (speed 25 s−1) was added to ensure sufficient stirring. The electrochemical cell was undivided and electrolyses were performed in an intensiostatic way (I=1 A). Temperature was maintained at 60°C by means of a cooling jacket. 2.3. Sonoelectrooxidation
2.3.2. DAS concentration Electrolyses were carried out for different initial DAS concentrations. Results are shown in Fig. 2. It can be observed that an increase of the DAS concentration results in a lower chemical yield. This can be explained by the increase in viscosity of the solution with the DAS concentration, which is detrimental to ultrasound efficiency. Nevertheless, the same yield (70%) was obtained for a DAS concentration of 50 and 100 kg/m3.
3. Acetophenone electroreduction 3.1. Reaction scheme Direct reduction of acetophenone in a protic medium is a well-known reaction [5–8]. One product is 2,3-diphenyl-2,3-butanediol (two diastereoisomers d,l and meso). It is obtained by one-electron reduction followed by duplication. The other is 1-phenylethanol which is obtained by a two-electron reduction (electrochemical dihydrogenation). For pinacol, both diastereoisomers d,l and meso are formed. The choice of experimental conditions determines the chemoselectivity but also the stereoselectivity of the reaction. The most determining parameter is pH. In neutral and weakly
2.3.1. Ultrasound irradiation The impact of ultrasonication on the process was studied first. Electric power of 100 W (22 W in the solution) was applied. As can be seen in Table 1, ultrasonic irradiation results in an important increase (30%) Table 1 Effect of ultrasound irradiation on DAG chemical yield Ultrasound power ( W )
m (g) DAS
Chemical yield (%)
0 22 0 22
11 11 22 22
68 86 54 72
Fig. 2. Influence of the DAS concentration on the DAG yield after passing the theoretical electricity amount (4 F/mol ).
P. Cognet et al. / Ultrasonics Sonochemistry 7 (2000) 163–167
165
alkaline media, acetophenone exhibits a single twoelectron wave. In this pH range, the proton availability near the electrode surface is lower and so the neutral ketone is the electroactive species. It is reduced to a radical anion (I ), which immediately abstracts a proton from the solvent. The resulting radical (II ) is then quickly reduced, resulting in the observed two-electron wave ( ECE mechanism). But, if concentrated solutions are used for electrolysis, dimerisation of I or II can compete with the reduction of II ( EC mechanism) (Scheme 2).
Before use, the electrodes were washed with dilute nitric acid and then rinsed with distilled water. Electrolyses were carried out under dinitrogen atmosphere after outgassing the electrolysis solution by dinitrogen bubbling for 25 min. The electrolysis solution composition was as follows: acetophenone (3.8×10−2 M ); supporting electrolyte, Na SO (0.1 M ); solvent, water; co-solvent, methanol 2 4 (from 0 to 50% of the total volume of 200 ml ).
3.2. Experimental part
3.3.1. Sonoelectrolysis without co-solvent The impact of ultrasonication on electrode passivation was studied first. This study was made without methanol and at a constant current of 150 mA. It can be observed in Fig. 3 that the cathode potential decreases dramatically with electrolysis time from −1.65 to −2.80 V/SCE, because of the pinacol film deposition. Ultrasonication allows us to remove the pinacol film on the cathode, resulting in the stabilisation of the cathodic potential which varies between −1.6 and −1.7 V/SCE. Figs. 4 and 5 show the molar profiles of acetophenone and products obtained during electrolyses with and without ultrasound. The cathodic potential drop which
Experiments were carried out in a conventional nondivided Pyrex cell with a cooling jacket [9]. The internal volume was 300 ml. It was fitted with a PVC cover which permitted the introduction of the electrodes. For conventional electrolyses, the electrolytic solution was stirred magnetically. For sonoelectrochemical electrolyses, the cell was placed in a sonicated bath ( ELMA, 505×137×100 mm3, 35 kHz). The electrode configuration was axial. The working electrode (cathode) consisted of an expanded zinc (geometrical surface area 57×10−4 m2). The space between the two electrodes was equal to 4 mm. The counterelectrode consisted of an expanded Ti/Pt (surface area 46×10−4 m2) grid. Temperature was maintained at 25°C by circulating water in the cooling jacket in case of overheating the solution by Joule effect. Electrolyses were monitored by a Tacussel PJT 35 V-2 A potentiostat connected to a current integrator, using the three-electrode configuration. To measure or set the potential to the working electrode, a double junction reference saturated calomel electrode (SCE ) was used. It was immersed in the solution through a Luggins capillary.
3.3. Galvanostatic electrolysis under ultrasound
Fig. 3. Evolution of cathodic potential during electrolysis, with and without ultrasonication.
Scheme 2. Mechanism of the reduction of acetophenone in neutral pH aqueous medium.
Fig. 4. Molar profiles of acetophenone, alcohol and pinacol during electrolysis without ultrasound, I=150 mA.
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Fig. 5. Molar profiles of acetophenone, alcohol and pinacol during electrolysis under ultrasound, I=150 mA.
is observed without ultrasonication leads to a decrease in the acetophenone conversion and to dihydrogen production. After passing 1.65 F/mol, only 40% of acetophenone was consumed. As a result, pinacol selectivity falls and alcohol becomes the main product (Fig. 4). On the contrary, ultrasonication permits us to obtain better conversions (from 38% without ultrasound to 47% with ultrasound), and a better selectivity in pinacol ( Fig. 5). 3.3.2. Sonoelectrolysis with co-solvent Using methanol as the co-solvent results in a better conversion of acetophenone (Fig. 6). This can be attributed to ultrasound mechanical effects on mass transfer and surface cleaning. Pitts et al. [10] showed that the presence of dioxygen in the medium had an inhibiting effect on the benzophenone reduction reaction. Radicals electrochemically generated react with dioxygen to give the starting reagent, benzophenone. He proposed the following mechanism: $
(C H )COH+O HO$+(C H ) CO 6 5 2 2 6 52
Fig. 7. Alcohol/pinacol molar ratio as a function of consumed electricity, I=150 mA.
The benzophenone reduction is then slowed down but not stopped. The same effect was observed by Stocker and Sidisuntharne [11] for the reduction of acetophenone in solutions saturated with dioxygen. No effect was observed on the stereoselectivity. The better conversion obtained under sonication can then be partially attributed to outgassing. Indeed, dioxygen anodically formed by water electrolysis is eliminated by sonication. As a consequence, conversion of radicals into the starting reagent is stopped. Fig. 7 shows the evolution of the alcohol/pinacol ratio during electrolysis with and without ultrasound. Without sonication, the ratio increases with electrolysis time. Under ultrasound, it remains almost constant and inferior to 1 in the case of 10% methanol. A slow methanol concentration favours the pinacol formation, but in all cases ultrasonication allows us to dramatically reduce the alcohol production. It permits us to minimise the co-solvent concentration while avoiding electrode passivation.
$
HO$+(C H )COH+O H O +(C H ) CO. 2 6 5 2 2 2 6 52 4. Conclusions
Fig. 6. Acetophenone conversion as a function of electrolysis time, I=150 mA.
The activation of electrosynthesis processes has been illustrated by two different reactions: one indirect electrooxidation using a suspended electrode and one direct electroreduction. In the first case, the electrooxidation of DAS, the application of an ultrasonic field changes the granulometry of the suspended electrode, thus increasing the electrode surface. It results in better faradaic and chemical yields. In the second case, the electroreduction of an aromatic ketone, ultrasound technology proved to be of great interest for electrode depassivation and mass transfer rate enhancement. Selectivity is deeply affected by ultrasonication, which is probably the most remarkable result. These two examples show all the interest of applying ultrasound
P. Cognet et al. / Ultrasonics Sonochemistry 7 (2000) 163–167
technology to electrochemical processes. Further developments have to take the energetic aspect into account.
References [1] D.J. Walton, S.P. Sukhvinder, Adv. Sonochem. 4 (1996) 205. [2] G. Vertes, G. Horanyi, Acta Chim. Acad. Sci. Hung. 67 (1971) 145.
167
[3] P.M. Robertson, P. Berg, H. Reimann, J. Electrochem. Soc. 130 (1983) 591. [4] H. Aı¨t-Lyazidi, The`se de doctorat de la Faculte´ des Sciences de Meknes, 1996. [5] D. Brown, L. Horner, J. Lieb. Ann. Chem. (1977) 77. [6 ] S.G. Mairanovski, Russ. J. Phys. Chem. 38 (1964) 980. [7] E. Laviron, J.C. Lucy, Bull. Soc. Chim. Fr. (1966) 2202. [8] A. Honnorat, P. Martinet, Electrochim. Acta 28 (1983) 1703. [9] A. Ghanem-Lakhal, The`se de doctorat I.N.P.T., Toulouse, 1997. [10] J.N. Pitts, R.L. Letsinger, R.P. Taylor, J. Am. Chem. Soc. 81 (1959) 1068. [11] J.H. Stocker, P. Sidisuntharne, J. Am. Chem. Soc. 82 (1960) 3913.
Ultrasound in organic electrosynthesis P. Cognet a, *, A.-M. Wilhelm a, H. Delmas a, H. Aı¨t Lyazidi a, P.-L. Fabre b a Laboratoire de Ge´nie 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 continuous cleaning of the electrodes by ultrasound irradiation of the electrochemical cell or the enhancement of mass transfer at the electrodes are examples of such activation. Finally, ultrasonication can play an important part for the orientation of reactions whose selectivities are very sensitive to stirring. Two very different examples have been chosen to illustrate these phenomena: the indirect electrooxidation of di-ketone-L-sorbose into the corresponding ketogulonic acid and the direct electroreduction of acetophenone into 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
Irradiation of liquids by power ultrasound leads to cavitation phenomena: microbubbles present in the solution are submitted to growing, vibration and finally implosion. Effects of cavitation in a variety of homogeneous chemical systems (outgassing, stirring) have been studied widely, but beneficial processes can be obtained in heterogeneous media at a solid–liquid interface, such as particle size modification, cleaning of surfaces or the formation of fresh surfaces [1]. Such a heterogeneous interface exists between an electrode surface and an electrolyte. In this way, some attempts have been made to apply ultrasonic effects to electroorganic processes in order to increase product yields, modify electropolymerised polymer properties or promote sacrificial electrode reactions. We report below some results obtained for electrolyses assisted by ultrasound in two different applications. In the first case, ultrasonication was used during electrolysis for the activation of a suspended electrode used in the electrooxidation of diacetone-L-sorbose into diacetone-2-keto-L-gulonic acid. The second example deals with the electroreduction of acetophenone into pinacol under ultrasound.
2.1. Reaction scheme
* Corresponding author. Tel.: +33-62-252329; fax: +33-62-252329. E-mail address: [email protected] (P. Cognet)
The oxidation of diacetone-L-sorbose into diacetone2-keto-L-gulonic acid is a well-known step of the synthesis of the C vitamin. It can be carried out electrochemically, using a nickel working electrode in an alkaline medium [2,3]. Nickel hydroxide is first anodically oxidised into peroxide. Then it reacts with the alcohol (DAS) to give the acid (DAG) and regenerate the hydroxide. Hydrogen evolution is the main cathodic reaction (Scheme 1).
Scheme 1. Mechanism of the indirect oxidation of DAS in alkaline aqueous medium.
1350-4177/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S1 3 5 0 -4 1 7 7 ( 0 0 ) 0 0 03 6 - 5
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P. Cognet et al. / Ultrasonics Sonochemistry 7 (2000) 163–167
of the DAG yield, for both concentrations. For this application, the ultrasound irradiation acts in different ways. Firstly, an analysis of the granulometry has shown that the size of the nickel hydroxide particles was reduced: the mean particle diameter changes from 0.88×10−3 m without sonication to 0.12×10−3 m after sonication. Having in mind that the DAS oxidation is a heterogeneous chemical process, a decrease in particle size leads to an increase in catalyst surface, resulting in a higher reaction rate. Secondly, it can improve the transfer of the DAS from the bulk solution to the peroxide particle, where it reacts, and also at the surface of the nickel foam electrode, which is also activated.
Fig. 1. Electrochemical Grignard reactor.
2.2. Sonoelectrochemical reactor Experiments were carried out in a 1 l Grignard electrochemical reactor ( Fig. 1), fitted with two cylindrical electrodes, isolated by a polyethylene film. Nickel hydroxide was introduced as a suspension in a 1 M KOH aqueous solution [4]. The outside electrode (anode) was made of nickel foam (96 cm2 surface area), whereas the internal electrode (cathode) was made of expanded platinated titanium (56 cm2 surface area). Ni(OH ) (4 g) was used as the suspended electrode. It 2 was irradiated by a 500 kHz ultrasonic horn, connected to a 100 W ultrasound generator. A mechanical stirrer (speed 25 s−1) was added to ensure sufficient stirring. The electrochemical cell was undivided and electrolyses were performed in an intensiostatic way (I=1 A). Temperature was maintained at 60°C by means of a cooling jacket. 2.3. Sonoelectrooxidation
2.3.2. DAS concentration Electrolyses were carried out for different initial DAS concentrations. Results are shown in Fig. 2. It can be observed that an increase of the DAS concentration results in a lower chemical yield. This can be explained by the increase in viscosity of the solution with the DAS concentration, which is detrimental to ultrasound efficiency. Nevertheless, the same yield (70%) was obtained for a DAS concentration of 50 and 100 kg/m3.
3. Acetophenone electroreduction 3.1. Reaction scheme Direct reduction of acetophenone in a protic medium is a well-known reaction [5–8]. One product is 2,3-diphenyl-2,3-butanediol (two diastereoisomers d,l and meso). It is obtained by one-electron reduction followed by duplication. The other is 1-phenylethanol which is obtained by a two-electron reduction (electrochemical dihydrogenation). For pinacol, both diastereoisomers d,l and meso are formed. The choice of experimental conditions determines the chemoselectivity but also the stereoselectivity of the reaction. The most determining parameter is pH. In neutral and weakly
2.3.1. Ultrasound irradiation The impact of ultrasonication on the process was studied first. Electric power of 100 W (22 W in the solution) was applied. As can be seen in Table 1, ultrasonic irradiation results in an important increase (30%) Table 1 Effect of ultrasound irradiation on DAG chemical yield Ultrasound power ( W )
m (g) DAS
Chemical yield (%)
0 22 0 22
11 11 22 22
68 86 54 72
Fig. 2. Influence of the DAS concentration on the DAG yield after passing the theoretical electricity amount (4 F/mol ).
P. Cognet et al. / Ultrasonics Sonochemistry 7 (2000) 163–167
165
alkaline media, acetophenone exhibits a single twoelectron wave. In this pH range, the proton availability near the electrode surface is lower and so the neutral ketone is the electroactive species. It is reduced to a radical anion (I ), which immediately abstracts a proton from the solvent. The resulting radical (II ) is then quickly reduced, resulting in the observed two-electron wave ( ECE mechanism). But, if concentrated solutions are used for electrolysis, dimerisation of I or II can compete with the reduction of II ( EC mechanism) (Scheme 2).
Before use, the electrodes were washed with dilute nitric acid and then rinsed with distilled water. Electrolyses were carried out under dinitrogen atmosphere after outgassing the electrolysis solution by dinitrogen bubbling for 25 min. The electrolysis solution composition was as follows: acetophenone (3.8×10−2 M ); supporting electrolyte, Na SO (0.1 M ); solvent, water; co-solvent, methanol 2 4 (from 0 to 50% of the total volume of 200 ml ).
3.2. Experimental part
3.3.1. Sonoelectrolysis without co-solvent The impact of ultrasonication on electrode passivation was studied first. This study was made without methanol and at a constant current of 150 mA. It can be observed in Fig. 3 that the cathode potential decreases dramatically with electrolysis time from −1.65 to −2.80 V/SCE, because of the pinacol film deposition. Ultrasonication allows us to remove the pinacol film on the cathode, resulting in the stabilisation of the cathodic potential which varies between −1.6 and −1.7 V/SCE. Figs. 4 and 5 show the molar profiles of acetophenone and products obtained during electrolyses with and without ultrasound. The cathodic potential drop which
Experiments were carried out in a conventional nondivided Pyrex cell with a cooling jacket [9]. The internal volume was 300 ml. It was fitted with a PVC cover which permitted the introduction of the electrodes. For conventional electrolyses, the electrolytic solution was stirred magnetically. For sonoelectrochemical electrolyses, the cell was placed in a sonicated bath ( ELMA, 505×137×100 mm3, 35 kHz). The electrode configuration was axial. The working electrode (cathode) consisted of an expanded zinc (geometrical surface area 57×10−4 m2). The space between the two electrodes was equal to 4 mm. The counterelectrode consisted of an expanded Ti/Pt (surface area 46×10−4 m2) grid. Temperature was maintained at 25°C by circulating water in the cooling jacket in case of overheating the solution by Joule effect. Electrolyses were monitored by a Tacussel PJT 35 V-2 A potentiostat connected to a current integrator, using the three-electrode configuration. To measure or set the potential to the working electrode, a double junction reference saturated calomel electrode (SCE ) was used. It was immersed in the solution through a Luggins capillary.
3.3. Galvanostatic electrolysis under ultrasound
Fig. 3. Evolution of cathodic potential during electrolysis, with and without ultrasonication.
Scheme 2. Mechanism of the reduction of acetophenone in neutral pH aqueous medium.
Fig. 4. Molar profiles of acetophenone, alcohol and pinacol during electrolysis without ultrasound, I=150 mA.
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P. Cognet et al. / Ultrasonics Sonochemistry 7 (2000) 163–167
Fig. 5. Molar profiles of acetophenone, alcohol and pinacol during electrolysis under ultrasound, I=150 mA.
is observed without ultrasonication leads to a decrease in the acetophenone conversion and to dihydrogen production. After passing 1.65 F/mol, only 40% of acetophenone was consumed. As a result, pinacol selectivity falls and alcohol becomes the main product (Fig. 4). On the contrary, ultrasonication permits us to obtain better conversions (from 38% without ultrasound to 47% with ultrasound), and a better selectivity in pinacol ( Fig. 5). 3.3.2. Sonoelectrolysis with co-solvent Using methanol as the co-solvent results in a better conversion of acetophenone (Fig. 6). This can be attributed to ultrasound mechanical effects on mass transfer and surface cleaning. Pitts et al. [10] showed that the presence of dioxygen in the medium had an inhibiting effect on the benzophenone reduction reaction. Radicals electrochemically generated react with dioxygen to give the starting reagent, benzophenone. He proposed the following mechanism: $
(C H )COH+O HO$+(C H ) CO 6 5 2 2 6 52
Fig. 7. Alcohol/pinacol molar ratio as a function of consumed electricity, I=150 mA.
The benzophenone reduction is then slowed down but not stopped. The same effect was observed by Stocker and Sidisuntharne [11] for the reduction of acetophenone in solutions saturated with dioxygen. No effect was observed on the stereoselectivity. The better conversion obtained under sonication can then be partially attributed to outgassing. Indeed, dioxygen anodically formed by water electrolysis is eliminated by sonication. As a consequence, conversion of radicals into the starting reagent is stopped. Fig. 7 shows the evolution of the alcohol/pinacol ratio during electrolysis with and without ultrasound. Without sonication, the ratio increases with electrolysis time. Under ultrasound, it remains almost constant and inferior to 1 in the case of 10% methanol. A slow methanol concentration favours the pinacol formation, but in all cases ultrasonication allows us to dramatically reduce the alcohol production. It permits us to minimise the co-solvent concentration while avoiding electrode passivation.
$
HO$+(C H )COH+O H O +(C H ) CO. 2 6 5 2 2 2 6 52 4. Conclusions
Fig. 6. Acetophenone conversion as a function of electrolysis time, I=150 mA.
The activation of electrosynthesis processes has been illustrated by two different reactions: one indirect electrooxidation using a suspended electrode and one direct electroreduction. In the first case, the electrooxidation of DAS, the application of an ultrasonic field changes the granulometry of the suspended electrode, thus increasing the electrode surface. It results in better faradaic and chemical yields. In the second case, the electroreduction of an aromatic ketone, ultrasound technology proved to be of great interest for electrode depassivation and mass transfer rate enhancement. Selectivity is deeply affected by ultrasonication, which is probably the most remarkable result. These two examples show all the interest of applying ultrasound
P. Cognet et al. / Ultrasonics Sonochemistry 7 (2000) 163–167
technology to electrochemical processes. Further developments have to take the energetic aspect into account.
References [1] D.J. Walton, S.P. Sukhvinder, Adv. Sonochem. 4 (1996) 205. [2] G. Vertes, G. Horanyi, Acta Chim. Acad. Sci. Hung. 67 (1971) 145.
167
[3] P.M. Robertson, P. Berg, H. Reimann, J. Electrochem. Soc. 130 (1983) 591. [4] H. Aı¨t-Lyazidi, The`se de doctorat de la Faculte´ des Sciences de Meknes, 1996. [5] D. Brown, L. Horner, J. Lieb. Ann. Chem. (1977) 77. [6 ] S.G. Mairanovski, Russ. J. Phys. Chem. 38 (1964) 980. [7] E. Laviron, J.C. Lucy, Bull. Soc. Chim. Fr. (1966) 2202. [8] A. Honnorat, P. Martinet, Electrochim. Acta 28 (1983) 1703. [9] A. Ghanem-Lakhal, The`se de doctorat I.N.P.T., Toulouse, 1997. [10] J.N. Pitts, R.L. Letsinger, R.P. Taylor, J. Am. Chem. Soc. 81 (1959) 1068. [11] J.H. Stocker, P. Sidisuntharne, J. Am. Chem. Soc. 82 (1960) 3913.