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Electrooxidation of benzyl alcohol on Pt-Nafion composite electrode
Il. pp.
Ekrwochimica Acta. Vol. 40, No. 1785-1787. 1995 Copyright 0 1995 Ekvicr Science Ltd. Printed in Great Brimin. All rightsreserved
) Pergamon
CO&4686195 b.50 + 0.00
SHORT COMMUNICATION ELECTROOXIDATION OF BENZYL ALCOHOL Pt-Nafion COMPOSITE ELECTRODE MIKITO
YASUZAWA,~
KYOUHIRO
KAN,~
AKIRA
ZEN-ICHIRO
KIJNUGI,*~
ZEMPACHI
ON
OGUMI$
and
TAKEHARA~
t Department $ Division
of Chemical Science and Technology, Faculty of Engineering, The University of Tokushima, Minamijosanjima-cho, Tokushima 770, Japan of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-01, Japan (Received 25 July 1994; in revisedform
18 January
1995)
Abstract-The electrooxidation of benzyl alcohol was investigated employing a Pt-Nation composite electrode in both polar and non-polar solvents. The use of hexane as a solvent caused the current effciency for benzaldehyde formation to increase greatly compared with that in acetonitrile. Furthermore, the stability of the Pt-Nafion composite electrode for the repeated electrolyses was largely improved by the use of hexane. Key words: electrooxidation non-polar solvent.
of benzyl alcohol,
SPE electrolysis
INTRODUCTION
The solid polymer electrolyte (SPE) electrolysis method employing a Pt-Nafion@ composite electrode is of potential use for electro-organic syntheses because no supporting electrolyte is required. Therefore, the choice of an usable solvent is extended, and also product separation and the product work-up are simplified. Some of the present authors have reported applications of this method to several organic syntheses and showed their usefulness[l-71. The electrooxidation of benzyl alcohol has been extensively investigated since benzaldehyde is an important intermediate for the production of a variety of medicines[8-151. The present paper concerns the oxidation of benzyl alcohol on the PtNafion composite electrode. Attention is also focused on the utility of non-polar solvents such as hexane and benzene, which are hardly employed in conventional electroorganic syntheses. EXPERIMENTAL
The Pt-Nafion composite electrode was prepared by the deposition of platinum onto one side of the Nafion@ 324 (E.I. Du Pont de Nemours, Inc.), employing an electroless plating method described in an earlier journal[16]. The surface resistance of the * Author
to whom correspondence
should be addressed.
method,
Pt-Nation
composite
electrode,
Pt layer deposited on the Nafion membrane was lower than 2ohm at 1cm distance. The electrolytic cell was composed of two compartments. Each compartment was separated by the Pt-Nafion composite electrode. Its geometric surface area was 3.1 cm’. The membrane surface coated with Pt faced the working electrode compartment, which was filled with a solution containing benzyl alcohol. The non-metallized side faced the counter electrode compartment, which was charged with a 0.05moldmW3 H$O, aqueous solution. A platinum wire electrode was inserted into the counter electrode compartment. Electrolysis was carried out galvanostatically using a Hokuto-Denko Model HA-301 potentiostat/galvanostat. The total charged passed during each run was 2 F mol- I. Ohmic contact with the Pt in the Pt-Nafion composite electrode was achieved with a gold ring inserted between the Pt-Nafion composite electrode and the body of the working electrode compartment. Product mixtures were analyzed by a Shimadzu Model CC-14A gas chromatograph fitted with a capillary column OV-101 (25 m). Propiophenone was used as an internal standard for quantitative analysis. The amount of benzoic acid produced was determined with hplc, using Japan Spectroscopic Model UVIDEC-100-W fitted with Finepak SIL C8-5 column. The organic products detected under the conditions used in this work were benzaldehyde and benzoic acid. The amount of the gaseous product oxygen produced, was determined by a gas absorption method using a pyrogallol solution.
1785
1786
Short communication RESULTS AND DISCUSSION
Table 1 summarizes the oxidation of benzyl alcohol on the Pt-Nafion composite electrode in different solvents at a constant current density of 2OOAm-‘. The conversion of benzyl alcohol was calculated using the difference between the benzyl alcohol content of the anolyte before and after electrolysis. The yields of benzaldehyde and benzoic acid were calculated by dividing the amounts of benzaldehyde and benzoic acid produced by the amount of consumed benzyl alcohol. In every solvent, benzaldehyde was produced together with benzoic acid. It is noteworthy that the use of hexane gave a higher current efficiency and yield for benzaldehyde formation than the use of acetonitrile, which is often used for electroorganic synthesis. Moreover, the use of hexane permitted triple repetitions of electrolyses in nearly the same current efticiency and yield; ie, the degradation of the Pt-Nafion composite electrode, such as the removal of platinum from the Nafion membrane, was sparingly caused by the repeated electrolysis. This can be inferred from Fig. 1 which shows the microphotographs of the platinum metal surfaces of Pt-Nafion composite electrodes before and after electrolysis in hexane and acetonitrile,
respectively. Before electrolysis, the surface of the PtNafion electrode is uniformly covered with platinum (Fig. la) except for dark parts, which correspond to the intersection of ptfe net embedded in the Nafion 324 film. After electrolysis in hexane, the covered platinum parts remained virtually intact (Fig. lb); whereas, in acetonitrile, partial peeling-off of deposited platinum was observed (Fig. lc). This degradation could also be recognized from a significant increase in electric resistance of the Pt-Nafion electrode after electrolysis in acetonitrile. In hexane, however, the electric resistance was almost unchanged. Such a difference in the stability of the electrode for hexane and acetonitrile might be attributable to the difference in degree of swelling of the Nafion. In hexane, the observed swelling of the Nafion membrane was slight. However, in acetonitrile, benzene or thf, significant swelling of Nafion was recognized within a few minutes of dipping it in these solvents. Such swelling leads to the disconnection and the peeling-off of deposited platinum, which lowers the current efficiency for benzaldehyde formation. Figure 2 shows the dependence of the current efliciencies for the formation of benzaldehyde, benzoic acid, and oxygen on the current density in hexane.
Table 1. The influence of solvent in the direct electrooxidation comnosite electrode* Benzaldehyde
of benzyl alcohol on Pt-Nafion
Benzoic acid
Solvent
C.E.t (%)
Yield (%)
C.E.I (%)
Yield (%)
Conversion (%)
Acetonitrile Hexane Benzene Tetrahydrofuran
21 49 19 6
62 74 51 34
19 9 7 trace
28 6 9 trace
33 66 37 17
* Benzyl alcohol: 0.14moldm-3. 2Fmol-‘. t Current effkiency
Fig. 1. Micrographs
Current density: 2OOAm-‘. The amount of charge passed:
of Pt-Nafion composite electrode: (a) before electrolysis; (b) after electrolysis in hexane, and (c) after electrolysis in acetonitrile.
Short communication
: n >
do not need to be added to the working electrode compartment. Therefore, observation of somewhat lower current efficiency and yield can be tolerated
60 -->.
and
40
0
\
*3&, f
20 j 0
I787
100 Current
200 density
300
/ A rn-’
Fig. 2. The variation in the current efficiencies of benzaldehyde (a), benzoic acid (A), and oxygen (m), with current density in hexane solution.
The current efliciency for benzaldehyde formation decreased with increasing current density, while that for benzoic acid formation was constant. At high current densities, the current efficiency for oxygen evolution was considerably increased, and this increase resulted in the decrease in the current eficiency for benzaldehyde formation. The current efficiency observed for the formation of benzaldehyde in hexane was around 50% and the yield was around 70%, while in other solvents both current efficiency and yield were much lower. These values were not so high compared with other results observed from recent studies[lO-14). In these other electrolyses, benzyl alcohol was indirectly oxidized by the introduction of mediators such as triarylaminecl l] and tempo[ 121, and by using both phase transfer catalysts and redox mediators[lO, 13, 141. The isolating process of the electrolysis mixture is significantly simplified by the use of this Pt-Nafion composite electrode in that supporting electrolytes
accepted.
Acknowledgements-Financial assistance Ministry of Education, Science and Grant-in-Aid for Scientific Research on 05235235), is gratefully acknowledged by
from the Japanese Culture, through Priority Area (No. the authors.
REFERENCES 1. Z. Ogumi,
T. Mizoe, Z. Chen
and Z. Takehara,
Bull.
Chem. Sot. Jpn. 63, 3365 (1990).
2. Z. Chen, T. Mizoe, Z. Ogumi and Z. Takehara, Bull. Gem. Sot. Jpn. 64,537 (1991). 3. Z. Chen, Z. Ogumi and Z. Takehara, Bull. Chem. Sot. Jpn. 64, 1261 (1991). 4. Z. Ogumi, K. Inatomi, J. T. Hinatsu and Z. Takehara, Electrochim. Acta 37, 1295 (1992). 5. M. Inaba, Z. Ogumi and Z. Takehara, J. electrochem. Sot. 140, I9 (1993). 6. M. Inaba, J. T. Hinatsu, Z. Ogumi and Z. Takehara, J. electrochem. Sot. 140, 706 (1993). 7. M. Yasuzawa, A. Kunugi, M. Inaba and Z. Ogumi, Denki Kagaku 62, 1183 (1994). 8. H. Lunds, Acta Chem. Stand. 11, 1323 (1957). 9. E. A. Mayeda, L. L. Miller and J. F. Wolf, J. Am. Chem. Sot. 11, 6812 (1972). IO. R. E. W. Jansson and N. R. Tomov, J. app[. Electrothem. 10, 583 (1980). I I. K-H. G. Brinkhaus, E. Steckhan and W. Schmidt, Acta Chem. Stand. 37,499 (1983). 12. A. Deronzier. D. Limosin and J.-C. Moutet, Elecrrochim. Acta 32, 1643 (1987).
13. J. S. Do and T. C. Chou, J. appl. Electrochem. (1989). 14. J. S. Do and T. C. Chou. J. appI. Electrochem.
19, 922 22, 966
(1992).
15. J. Burzyk and I. Zjawiony. Pal. J. appl. Chem. 37, 99 (1993). 16. H. Takenaka and E. Torikai. Kokai Tokkyo Koho (Japan Patent) 55. 38 934 (1980).
Ekrwochimica Acta. Vol. 40, No. 1785-1787. 1995 Copyright 0 1995 Ekvicr Science Ltd. Printed in Great Brimin. All rightsreserved
) Pergamon
CO&4686195 b.50 + 0.00
SHORT COMMUNICATION ELECTROOXIDATION OF BENZYL ALCOHOL Pt-Nafion COMPOSITE ELECTRODE MIKITO
YASUZAWA,~
KYOUHIRO
KAN,~
AKIRA
ZEN-ICHIRO
KIJNUGI,*~
ZEMPACHI
ON
OGUMI$
and
TAKEHARA~
t Department $ Division
of Chemical Science and Technology, Faculty of Engineering, The University of Tokushima, Minamijosanjima-cho, Tokushima 770, Japan of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-01, Japan (Received 25 July 1994; in revisedform
18 January
1995)
Abstract-The electrooxidation of benzyl alcohol was investigated employing a Pt-Nation composite electrode in both polar and non-polar solvents. The use of hexane as a solvent caused the current effciency for benzaldehyde formation to increase greatly compared with that in acetonitrile. Furthermore, the stability of the Pt-Nafion composite electrode for the repeated electrolyses was largely improved by the use of hexane. Key words: electrooxidation non-polar solvent.
of benzyl alcohol,
SPE electrolysis
INTRODUCTION
The solid polymer electrolyte (SPE) electrolysis method employing a Pt-Nafion@ composite electrode is of potential use for electro-organic syntheses because no supporting electrolyte is required. Therefore, the choice of an usable solvent is extended, and also product separation and the product work-up are simplified. Some of the present authors have reported applications of this method to several organic syntheses and showed their usefulness[l-71. The electrooxidation of benzyl alcohol has been extensively investigated since benzaldehyde is an important intermediate for the production of a variety of medicines[8-151. The present paper concerns the oxidation of benzyl alcohol on the PtNafion composite electrode. Attention is also focused on the utility of non-polar solvents such as hexane and benzene, which are hardly employed in conventional electroorganic syntheses. EXPERIMENTAL
The Pt-Nafion composite electrode was prepared by the deposition of platinum onto one side of the Nafion@ 324 (E.I. Du Pont de Nemours, Inc.), employing an electroless plating method described in an earlier journal[16]. The surface resistance of the * Author
to whom correspondence
should be addressed.
method,
Pt-Nation
composite
electrode,
Pt layer deposited on the Nafion membrane was lower than 2ohm at 1cm distance. The electrolytic cell was composed of two compartments. Each compartment was separated by the Pt-Nafion composite electrode. Its geometric surface area was 3.1 cm’. The membrane surface coated with Pt faced the working electrode compartment, which was filled with a solution containing benzyl alcohol. The non-metallized side faced the counter electrode compartment, which was charged with a 0.05moldmW3 H$O, aqueous solution. A platinum wire electrode was inserted into the counter electrode compartment. Electrolysis was carried out galvanostatically using a Hokuto-Denko Model HA-301 potentiostat/galvanostat. The total charged passed during each run was 2 F mol- I. Ohmic contact with the Pt in the Pt-Nafion composite electrode was achieved with a gold ring inserted between the Pt-Nafion composite electrode and the body of the working electrode compartment. Product mixtures were analyzed by a Shimadzu Model CC-14A gas chromatograph fitted with a capillary column OV-101 (25 m). Propiophenone was used as an internal standard for quantitative analysis. The amount of benzoic acid produced was determined with hplc, using Japan Spectroscopic Model UVIDEC-100-W fitted with Finepak SIL C8-5 column. The organic products detected under the conditions used in this work were benzaldehyde and benzoic acid. The amount of the gaseous product oxygen produced, was determined by a gas absorption method using a pyrogallol solution.
1785
1786
Short communication RESULTS AND DISCUSSION
Table 1 summarizes the oxidation of benzyl alcohol on the Pt-Nafion composite electrode in different solvents at a constant current density of 2OOAm-‘. The conversion of benzyl alcohol was calculated using the difference between the benzyl alcohol content of the anolyte before and after electrolysis. The yields of benzaldehyde and benzoic acid were calculated by dividing the amounts of benzaldehyde and benzoic acid produced by the amount of consumed benzyl alcohol. In every solvent, benzaldehyde was produced together with benzoic acid. It is noteworthy that the use of hexane gave a higher current efficiency and yield for benzaldehyde formation than the use of acetonitrile, which is often used for electroorganic synthesis. Moreover, the use of hexane permitted triple repetitions of electrolyses in nearly the same current efticiency and yield; ie, the degradation of the Pt-Nafion composite electrode, such as the removal of platinum from the Nafion membrane, was sparingly caused by the repeated electrolysis. This can be inferred from Fig. 1 which shows the microphotographs of the platinum metal surfaces of Pt-Nafion composite electrodes before and after electrolysis in hexane and acetonitrile,
respectively. Before electrolysis, the surface of the PtNafion electrode is uniformly covered with platinum (Fig. la) except for dark parts, which correspond to the intersection of ptfe net embedded in the Nafion 324 film. After electrolysis in hexane, the covered platinum parts remained virtually intact (Fig. lb); whereas, in acetonitrile, partial peeling-off of deposited platinum was observed (Fig. lc). This degradation could also be recognized from a significant increase in electric resistance of the Pt-Nafion electrode after electrolysis in acetonitrile. In hexane, however, the electric resistance was almost unchanged. Such a difference in the stability of the electrode for hexane and acetonitrile might be attributable to the difference in degree of swelling of the Nafion. In hexane, the observed swelling of the Nafion membrane was slight. However, in acetonitrile, benzene or thf, significant swelling of Nafion was recognized within a few minutes of dipping it in these solvents. Such swelling leads to the disconnection and the peeling-off of deposited platinum, which lowers the current efficiency for benzaldehyde formation. Figure 2 shows the dependence of the current efliciencies for the formation of benzaldehyde, benzoic acid, and oxygen on the current density in hexane.
Table 1. The influence of solvent in the direct electrooxidation comnosite electrode* Benzaldehyde
of benzyl alcohol on Pt-Nafion
Benzoic acid
Solvent
C.E.t (%)
Yield (%)
C.E.I (%)
Yield (%)
Conversion (%)
Acetonitrile Hexane Benzene Tetrahydrofuran
21 49 19 6
62 74 51 34
19 9 7 trace
28 6 9 trace
33 66 37 17
* Benzyl alcohol: 0.14moldm-3. 2Fmol-‘. t Current effkiency
Fig. 1. Micrographs
Current density: 2OOAm-‘. The amount of charge passed:
of Pt-Nafion composite electrode: (a) before electrolysis; (b) after electrolysis in hexane, and (c) after electrolysis in acetonitrile.
Short communication
: n >
do not need to be added to the working electrode compartment. Therefore, observation of somewhat lower current efficiency and yield can be tolerated
60 -->.
and
40
0
\
*3&, f
20 j 0
I787
100 Current
200 density
300
/ A rn-’
Fig. 2. The variation in the current efficiencies of benzaldehyde (a), benzoic acid (A), and oxygen (m), with current density in hexane solution.
The current efliciency for benzaldehyde formation decreased with increasing current density, while that for benzoic acid formation was constant. At high current densities, the current efficiency for oxygen evolution was considerably increased, and this increase resulted in the decrease in the current eficiency for benzaldehyde formation. The current efficiency observed for the formation of benzaldehyde in hexane was around 50% and the yield was around 70%, while in other solvents both current efficiency and yield were much lower. These values were not so high compared with other results observed from recent studies[lO-14). In these other electrolyses, benzyl alcohol was indirectly oxidized by the introduction of mediators such as triarylaminecl l] and tempo[ 121, and by using both phase transfer catalysts and redox mediators[lO, 13, 141. The isolating process of the electrolysis mixture is significantly simplified by the use of this Pt-Nafion composite electrode in that supporting electrolytes
accepted.
Acknowledgements-Financial assistance Ministry of Education, Science and Grant-in-Aid for Scientific Research on 05235235), is gratefully acknowledged by
from the Japanese Culture, through Priority Area (No. the authors.
REFERENCES 1. Z. Ogumi,
T. Mizoe, Z. Chen
and Z. Takehara,
Bull.
Chem. Sot. Jpn. 63, 3365 (1990).
2. Z. Chen, T. Mizoe, Z. Ogumi and Z. Takehara, Bull. Gem. Sot. Jpn. 64,537 (1991). 3. Z. Chen, Z. Ogumi and Z. Takehara, Bull. Chem. Sot. Jpn. 64, 1261 (1991). 4. Z. Ogumi, K. Inatomi, J. T. Hinatsu and Z. Takehara, Electrochim. Acta 37, 1295 (1992). 5. M. Inaba, Z. Ogumi and Z. Takehara, J. electrochem. Sot. 140, I9 (1993). 6. M. Inaba, J. T. Hinatsu, Z. Ogumi and Z. Takehara, J. electrochem. Sot. 140, 706 (1993). 7. M. Yasuzawa, A. Kunugi, M. Inaba and Z. Ogumi, Denki Kagaku 62, 1183 (1994). 8. H. Lunds, Acta Chem. Stand. 11, 1323 (1957). 9. E. A. Mayeda, L. L. Miller and J. F. Wolf, J. Am. Chem. Sot. 11, 6812 (1972). IO. R. E. W. Jansson and N. R. Tomov, J. app[. Electrothem. 10, 583 (1980). I I. K-H. G. Brinkhaus, E. Steckhan and W. Schmidt, Acta Chem. Stand. 37,499 (1983). 12. A. Deronzier. D. Limosin and J.-C. Moutet, Elecrrochim. Acta 32, 1643 (1987).
13. J. S. Do and T. C. Chou, J. appl. Electrochem. (1989). 14. J. S. Do and T. C. Chou. J. appI. Electrochem.
19, 922 22, 966
(1992).
15. J. Burzyk and I. Zjawiony. Pal. J. appl. Chem. 37, 99 (1993). 16. H. Takenaka and E. Torikai. Kokai Tokkyo Koho (Japan Patent) 55. 38 934 (1980).