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Electro-organic reactions on organic electrodes
321
J. Electroanal. Chem., 318 (1991) 321-326 Elsevier Sequoia S.A., Lausanne
JEC 01748
Electra-organic
reactions on organic electrodes
Part 16. Electrolysis using composite-plated electrodes: V Influence of hydrophobicities of anode, substrate and electrolytic solution on electro-oxidation of alcohols * Yoshihito Kunugi and Tsutomu Nonaka Department of Electronic Chemistry, Tokyo Institute of Technology, 4259 Nagatsura, Midori-Ku, Yokohama 227 (Japan)
Yong-Bo Chong and Nobuatsu Watanabe Applied Science Research Institute, 49 Tanaka Ohi-Cho, Sakyo-Ku, Kyoto 606 (Japan)
(Received 24 April 1991; in revised form 23 July 1991)
Abstract In the electro-oxidation of alcohols at nickel anodes in aqueous alkaline solutions, the current efficiency for the corresponding products increased with an increase in the hydrophobicity of the anodes. An increase in the hydrophobicity of alcohol molecules also caused an increase in the efficiency at a hydrophobic anode, while, in contrast, at a hydrophilic anode the efficiency decreased with the increase in the hydrophobicity of the alcohol. However, an increase in the hydrophobicity of electrolytic solutions resulted in a decrease in the efficiency at the hydrophobic anode but not at the hydrophilic anode.
INTRODUCI’ION
In our previous work [2-41, it has been found that a hydrophobic Ni + PTFE (polytetrafluoroethylene; Teflon@) composite-plated nickel electrode gave high current efficiencies for electrolytic oxidation and reduction of various organic compounds by suppressing oxygen and hydrogen evolution in aqueous solutions. This hydrophobic plated electrode promotes organic electrode processes but has
l
For Part 15, IV, see ref. 1.
0022-0728/92/$05.00
0 1992 - Elsevier Sequoia S.A. All rights reserved
322
rather low hydrogen and oxygen overvoltages compared with an unplated electrode [l]. Therefore the promotion of the organic electrode processes can be rationalized as being due to an unusually strong interaction of organic substrates with the hydrophobic electrode surface. From this point of view, in the present work the influence of the hydrophobicities of the electrode, the substrate molecule and the electrolytic solution on the oxidation of alcohols as a test reaction was examined. EXPERIMENTAL Ni + PTFE electrode
An Ni + PTFE composite-plated nickel electrode was prepared by compositeplating in a nickel sulfamate + PTFE particle (diameter ca. 0.2 p,m> dispersion bath (Metaflon FS Bath@, C. Uyemura&Co. Ltd.) [3,5]. The nickel cathode (3 cm x 3 cm> was located between two nickel plates as anodes (5 cm x 6 cm) in the plating bath (250 ml) which was stirred with a magnetic bar; 360 mA of current was passed for 20 min at 45°C. An Ni + PTFE composite-plated film prepared in this way appeared to have a hydrophobic smooth surface, and the volume content of PTFE particles in the film was estimated by surface and cross-section scanning electron microscopy (SEM) observation. The PTFE content in the film depended on the PTFE content (30-100 g drnm3) in the bath. Surfaces of films containing 5-20 vol.% PTFE particles had contact angles with water of 95”-115”, whereas the contact angle of an unplated nickel surface was ca. lo”. Ni + SiO, electrode
An Ni + SiO, (silica gel) composite-plated nickel electrode was prepared by composite-plating a nickel plate in a nickel sulfamate + SiO,, particle dispersion bath (Ni(NH,SO,), - 4H,O, 350 g dmm3; NiCI,, 30 g dmp3; H,BO,, 40 g dmp3; SiO,(diameter ca. 0.02 km), 60 g dme3). The procedures for plating were similar to those used for Ni + PTFE plating except for the plating current (900 mA) and the time (9 min). The SiO, content in the plated film obtained could not be estimated by SEM observation because the particle size of SiO, was too small, but the presence of SiO, in the film was confirmed by secondary ion mass spectrometry (SIMS) analyses (primary beam, Cs+, - 10 kV; secondary polarity, negative). The film surface was smooth and very hydrophilic (contact angle with water, ca. o”>. Electrolysis and analysis
Electrolysis and product analysis were performed as described below, unless otherwise stated. Alcohols such as ethanol, 2,2,2-trifluoroethanol, propan-2-01, cyclohexanol and 4-carboxyl-butan-2-01 (0.1-0.5 M) were oxidized in aqueous KOH solutions (50
323
TABLE 1 Anodic oxidation of propan-2-01 (0.5 M) at various nickel anodes in 1.0 M aqueous potassium hydroxide solution Entry no.
Anode Plated material
PTFE content/ vol.%
Contact angle with water/deg
1 2 3 4 5
Ni+SiO, None b Ni + PTFE Ni + PTFE Ni + PTFE
_= 0 5 10 20
0 10 90 110 115
Current efficiency for acetone/% 7 I 29 42 84
a The SiOz content was not estimated b An unplated nickel anode was used.
cm3> in a divided H-shaped cell equipped with an anode (3 cm x 3 cm) and a platinum cathode (3 cm X 4 cm). A constant current (10 mA cme2> was supplied until the charge passed reached 241 C (0.1-0.5 electrons/molecule). The anolyte was neutralized and then the oxidation products were analyzed by gas chromatography (PEG 6000/5o”C for propan-2-01 and acetone; UNISOLE 30T/170”C for levulinic acid) and high pressure liquid chromatography (ULTRON S-C18/0.1% H,PO, at an analytical wavelength of 210 nm for acetic and trifluoroacetic acids). Some of the experimental data for propan-2-01 and cyclohexanol are quoted from a previous paper [31.
RESULTS AND DISCUSSION
Influence of anode hydrophobic&
The influence of anode hydrophobicity on propan-2-01 oxidation was examined using a variety of nickel anodes with different contact angles to water. As shown in Table 1 (entries 1 and 2), propan-2-01 was oxidized to acetone at very low current efficiency (7%) at Ni + SiO,-plated and unplated nickel anodes with low hydrophobicity (contact angles with water, 0” and 10” respectively). In contrast, hydrophobic Ni + PTFE-plated anodes (PTFE content, 5-20 vol.%; contact angle with water, 90”-115”) gave much higher efficiencies (29%-84%, entries 3-5). These results clearly indicate that the efficiency increases with an increase in the anode hydrophobicity. Propan-2-01 should be more hydrophobic/ less hydrophilic than water itself. Therefore the observed influence of hydrophobicity of anodes on the current efficiency can be rationalized as due to the fact that a more hydrophobic surface interacts more strongly with propan-2-01 than with water.
324
100
50
0
Water content/ % Fig. 1. Contact angle of aqueous solutions of (a) CF&H,OH cosolvent (water) on the Ni + 20 vol.% PTFE electrode.
and (b) CH,CH,OH
containing a
Influence of alcohol hydrophobicity
The influence of alcohol hydrophobicity on their oxidation at plated and unplated anodes was examined. The hydrophobicity of the alcohols used is presumed, from their molecular structures, to increase in the following order: -0OCCH ,CH,CH(CH,)OH (in alkaline solution) < CH ,CH,OH < CFsCH,OH < cycle-C,H,,OH. In fact, it was confirmed experimentally that CF,CH,OH is more hydrophobic than CH,CH,OH because of the presence of the hydrophobic CF, group, though both ethanols have very similar molecular skeletons and volumes. As shown in Fig. 1, the contact angles of aqueous solutions of CF,CH ,OH with a hydrophobic Ni + 20 vol.% PTFE-plated electrode surface are smaller than those of CH,CH,OH. This result clearly suggests that the hydrophobicity of CF,CH,OH is greater than that of CH,CH,OH, though the angles of both the neat alcohols cannot be measured directly because they are too small (almost zero degrees). Table 2 shows the electrolytic results for alcohols at the Ni + 20 vol.% PTFE composite-plated nickel anode (contact angle with water, 115’) and the unplated nickel anode in aqueous potassium hydroxide solutions. The plated anode oxidized
325 TABLE 2 Anodic oxidation of alcohols at the Ni + 20 vol.% FTFE composite-plated in aqueous potassium hydroxide solutions
and unplated nickel anodes
Current efficiency for oxidation/%
Entry no.
Alcohol
6b
HOOCCH ,CH ,yHOH
Ratio ’ of current efficiency
At Ni + PTFEplated anode
At unplated anode
35
24
1.5
3.1 5.3
CH3
;‘,
CF,CH,OH CH,CH,OH
49 63
12 16
9e
0
16
1
a b ’ d ’
OH
76
Ratio of current efficiency at plated anode to current efficiency at unplated anode. 0.5 M alcohol+0.7 M KOH solution. 0.1 M alcohol+ 1.0 M KOH solution. 0.5 M alcohol + 1.0 M KOH solution. 0.25 M alcohol+ 1.0 M KOH+20 vol.% CH,CN solution.
primary and secondary alcohols to the corresponding carboxylic acids and ketones respectively, at higher current efficiencies than was the case with the unplated anode. This result is rationalized as due to the fact that any alcohol is more hydrophobic than water itself. More interestingly, the current efficiency at the hydrophobic plated anode increases in the order of the hydrophobicity of alcohol, but this behaviour is reversed at the unplated anode which is regarded to be rather hydrophilic. Influence of the hydrophobicity of the electrolytic solution
The influence of the hydrophobicity of the electrolytic solution was examined in the oxidation of propan-2-01 to acetone at the Ni + 20 vol.% PTFE-plated and unplated anodes in aqueous KOH solutions containing t-BuOH as an organic cosolvent at different contents (O-50 vol.%). As shown in Fig. 2, an increase in the t-BuOH content, i.e. the hydrophobic&y, caused a decrease in the current efficiency for acetone at the plated anode, while it did not at the unplated anode. It *is likely that propan-2-01 and t-BuOH interact competitively with the hydrophobic plated anode surface, though their relative hydrophobicities are unknown. Therefore the interaction of propan-2-01 is weakened at a high t-BuOH content and consequently the current efficiency is lowered. However, at the hydrophilic unplated anode the interaction of water with the surface is exclusive and consequently the current efficiency apparently does not depend on t-BuOH contents less than 50 vol.%. According to this hypothesis, an increase in current efficiency may be observed at a much higher t-BuOH content (above 50 vol.%) if a preference for the interaction of propan-2-01 to that of water is not compensated
326
0 t-BuOH
I
I
25
50
content/ %
Fig. 2. Anodic oxidation of propan-2-01 (0.5 MI at (a) the Ni + PTFE composite-plated and (b) the unplated nickel anode in 0.2 M aqueous potassium hydroxide solutions containing t-BuOH.
by the competitive interaction of t-BuOH. This has not been verified experimentally at present because of a technical problem. ACKNOWLEDGEMENTS
This study was financially supported in part by a Grant-in-Aid for Scientific Research on Priority Areas (New Functionality Materials) from the Japanese Ministry of Education, Science and Culture. The authors are also grateful to Dr. T. Fuchigami, Tokyo Institute of Technology, for helpful discussions. REFERENCES 1 2 3 4
Y. Kunugi, Y.-B. Chong, N. Watanabe and T. Nonaka, Electrochim. Acta. in press. Y. Kunugi, T. Fuchigami and T. Nonaka, Chem. Lett. (1989) 1467. Y. Kunugi, T. Fuchigami, S. Matsumura and T. Nonaka, J. Electroanal. Chem., 287 (1990) 385. Y. Kunugi, R. Kumada, Y.-B. Chong, N. Watanabe and T. Nonaka, J. Electroanal. Chem., 314 (1991) 215. 5 S. Matsumura, Shikizai, 56 (1983) 328 (in Japanese); Chem. Abstr. (1983) 99, 77201h.
J. Electroanal. Chem., 318 (1991) 321-326 Elsevier Sequoia S.A., Lausanne
JEC 01748
Electra-organic
reactions on organic electrodes
Part 16. Electrolysis using composite-plated electrodes: V Influence of hydrophobicities of anode, substrate and electrolytic solution on electro-oxidation of alcohols * Yoshihito Kunugi and Tsutomu Nonaka Department of Electronic Chemistry, Tokyo Institute of Technology, 4259 Nagatsura, Midori-Ku, Yokohama 227 (Japan)
Yong-Bo Chong and Nobuatsu Watanabe Applied Science Research Institute, 49 Tanaka Ohi-Cho, Sakyo-Ku, Kyoto 606 (Japan)
(Received 24 April 1991; in revised form 23 July 1991)
Abstract In the electro-oxidation of alcohols at nickel anodes in aqueous alkaline solutions, the current efficiency for the corresponding products increased with an increase in the hydrophobicity of the anodes. An increase in the hydrophobicity of alcohol molecules also caused an increase in the efficiency at a hydrophobic anode, while, in contrast, at a hydrophilic anode the efficiency decreased with the increase in the hydrophobicity of the alcohol. However, an increase in the hydrophobicity of electrolytic solutions resulted in a decrease in the efficiency at the hydrophobic anode but not at the hydrophilic anode.
INTRODUCI’ION
In our previous work [2-41, it has been found that a hydrophobic Ni + PTFE (polytetrafluoroethylene; Teflon@) composite-plated nickel electrode gave high current efficiencies for electrolytic oxidation and reduction of various organic compounds by suppressing oxygen and hydrogen evolution in aqueous solutions. This hydrophobic plated electrode promotes organic electrode processes but has
l
For Part 15, IV, see ref. 1.
0022-0728/92/$05.00
0 1992 - Elsevier Sequoia S.A. All rights reserved
322
rather low hydrogen and oxygen overvoltages compared with an unplated electrode [l]. Therefore the promotion of the organic electrode processes can be rationalized as being due to an unusually strong interaction of organic substrates with the hydrophobic electrode surface. From this point of view, in the present work the influence of the hydrophobicities of the electrode, the substrate molecule and the electrolytic solution on the oxidation of alcohols as a test reaction was examined. EXPERIMENTAL Ni + PTFE electrode
An Ni + PTFE composite-plated nickel electrode was prepared by compositeplating in a nickel sulfamate + PTFE particle (diameter ca. 0.2 p,m> dispersion bath (Metaflon FS Bath@, C. Uyemura&Co. Ltd.) [3,5]. The nickel cathode (3 cm x 3 cm> was located between two nickel plates as anodes (5 cm x 6 cm) in the plating bath (250 ml) which was stirred with a magnetic bar; 360 mA of current was passed for 20 min at 45°C. An Ni + PTFE composite-plated film prepared in this way appeared to have a hydrophobic smooth surface, and the volume content of PTFE particles in the film was estimated by surface and cross-section scanning electron microscopy (SEM) observation. The PTFE content in the film depended on the PTFE content (30-100 g drnm3) in the bath. Surfaces of films containing 5-20 vol.% PTFE particles had contact angles with water of 95”-115”, whereas the contact angle of an unplated nickel surface was ca. lo”. Ni + SiO, electrode
An Ni + SiO, (silica gel) composite-plated nickel electrode was prepared by composite-plating a nickel plate in a nickel sulfamate + SiO,, particle dispersion bath (Ni(NH,SO,), - 4H,O, 350 g dmm3; NiCI,, 30 g dmp3; H,BO,, 40 g dmp3; SiO,(diameter ca. 0.02 km), 60 g dme3). The procedures for plating were similar to those used for Ni + PTFE plating except for the plating current (900 mA) and the time (9 min). The SiO, content in the plated film obtained could not be estimated by SEM observation because the particle size of SiO, was too small, but the presence of SiO, in the film was confirmed by secondary ion mass spectrometry (SIMS) analyses (primary beam, Cs+, - 10 kV; secondary polarity, negative). The film surface was smooth and very hydrophilic (contact angle with water, ca. o”>. Electrolysis and analysis
Electrolysis and product analysis were performed as described below, unless otherwise stated. Alcohols such as ethanol, 2,2,2-trifluoroethanol, propan-2-01, cyclohexanol and 4-carboxyl-butan-2-01 (0.1-0.5 M) were oxidized in aqueous KOH solutions (50
323
TABLE 1 Anodic oxidation of propan-2-01 (0.5 M) at various nickel anodes in 1.0 M aqueous potassium hydroxide solution Entry no.
Anode Plated material
PTFE content/ vol.%
Contact angle with water/deg
1 2 3 4 5
Ni+SiO, None b Ni + PTFE Ni + PTFE Ni + PTFE
_= 0 5 10 20
0 10 90 110 115
Current efficiency for acetone/% 7 I 29 42 84
a The SiOz content was not estimated b An unplated nickel anode was used.
cm3> in a divided H-shaped cell equipped with an anode (3 cm x 3 cm) and a platinum cathode (3 cm X 4 cm). A constant current (10 mA cme2> was supplied until the charge passed reached 241 C (0.1-0.5 electrons/molecule). The anolyte was neutralized and then the oxidation products were analyzed by gas chromatography (PEG 6000/5o”C for propan-2-01 and acetone; UNISOLE 30T/170”C for levulinic acid) and high pressure liquid chromatography (ULTRON S-C18/0.1% H,PO, at an analytical wavelength of 210 nm for acetic and trifluoroacetic acids). Some of the experimental data for propan-2-01 and cyclohexanol are quoted from a previous paper [31.
RESULTS AND DISCUSSION
Influence of anode hydrophobic&
The influence of anode hydrophobicity on propan-2-01 oxidation was examined using a variety of nickel anodes with different contact angles to water. As shown in Table 1 (entries 1 and 2), propan-2-01 was oxidized to acetone at very low current efficiency (7%) at Ni + SiO,-plated and unplated nickel anodes with low hydrophobicity (contact angles with water, 0” and 10” respectively). In contrast, hydrophobic Ni + PTFE-plated anodes (PTFE content, 5-20 vol.%; contact angle with water, 90”-115”) gave much higher efficiencies (29%-84%, entries 3-5). These results clearly indicate that the efficiency increases with an increase in the anode hydrophobicity. Propan-2-01 should be more hydrophobic/ less hydrophilic than water itself. Therefore the observed influence of hydrophobicity of anodes on the current efficiency can be rationalized as due to the fact that a more hydrophobic surface interacts more strongly with propan-2-01 than with water.
324
100
50
0
Water content/ % Fig. 1. Contact angle of aqueous solutions of (a) CF&H,OH cosolvent (water) on the Ni + 20 vol.% PTFE electrode.
and (b) CH,CH,OH
containing a
Influence of alcohol hydrophobicity
The influence of alcohol hydrophobicity on their oxidation at plated and unplated anodes was examined. The hydrophobicity of the alcohols used is presumed, from their molecular structures, to increase in the following order: -0OCCH ,CH,CH(CH,)OH (in alkaline solution) < CH ,CH,OH < CFsCH,OH < cycle-C,H,,OH. In fact, it was confirmed experimentally that CF,CH,OH is more hydrophobic than CH,CH,OH because of the presence of the hydrophobic CF, group, though both ethanols have very similar molecular skeletons and volumes. As shown in Fig. 1, the contact angles of aqueous solutions of CF,CH ,OH with a hydrophobic Ni + 20 vol.% PTFE-plated electrode surface are smaller than those of CH,CH,OH. This result clearly suggests that the hydrophobicity of CF,CH,OH is greater than that of CH,CH,OH, though the angles of both the neat alcohols cannot be measured directly because they are too small (almost zero degrees). Table 2 shows the electrolytic results for alcohols at the Ni + 20 vol.% PTFE composite-plated nickel anode (contact angle with water, 115’) and the unplated nickel anode in aqueous potassium hydroxide solutions. The plated anode oxidized
325 TABLE 2 Anodic oxidation of alcohols at the Ni + 20 vol.% FTFE composite-plated in aqueous potassium hydroxide solutions
and unplated nickel anodes
Current efficiency for oxidation/%
Entry no.
Alcohol
6b
HOOCCH ,CH ,yHOH
Ratio ’ of current efficiency
At Ni + PTFEplated anode
At unplated anode
35
24
1.5
3.1 5.3
CH3
;‘,
CF,CH,OH CH,CH,OH
49 63
12 16
9e
0
16
1
a b ’ d ’
OH
76
Ratio of current efficiency at plated anode to current efficiency at unplated anode. 0.5 M alcohol+0.7 M KOH solution. 0.1 M alcohol+ 1.0 M KOH solution. 0.5 M alcohol + 1.0 M KOH solution. 0.25 M alcohol+ 1.0 M KOH+20 vol.% CH,CN solution.
primary and secondary alcohols to the corresponding carboxylic acids and ketones respectively, at higher current efficiencies than was the case with the unplated anode. This result is rationalized as due to the fact that any alcohol is more hydrophobic than water itself. More interestingly, the current efficiency at the hydrophobic plated anode increases in the order of the hydrophobicity of alcohol, but this behaviour is reversed at the unplated anode which is regarded to be rather hydrophilic. Influence of the hydrophobicity of the electrolytic solution
The influence of the hydrophobicity of the electrolytic solution was examined in the oxidation of propan-2-01 to acetone at the Ni + 20 vol.% PTFE-plated and unplated anodes in aqueous KOH solutions containing t-BuOH as an organic cosolvent at different contents (O-50 vol.%). As shown in Fig. 2, an increase in the t-BuOH content, i.e. the hydrophobic&y, caused a decrease in the current efficiency for acetone at the plated anode, while it did not at the unplated anode. It *is likely that propan-2-01 and t-BuOH interact competitively with the hydrophobic plated anode surface, though their relative hydrophobicities are unknown. Therefore the interaction of propan-2-01 is weakened at a high t-BuOH content and consequently the current efficiency is lowered. However, at the hydrophilic unplated anode the interaction of water with the surface is exclusive and consequently the current efficiency apparently does not depend on t-BuOH contents less than 50 vol.%. According to this hypothesis, an increase in current efficiency may be observed at a much higher t-BuOH content (above 50 vol.%) if a preference for the interaction of propan-2-01 to that of water is not compensated
326
0 t-BuOH
I
I
25
50
content/ %
Fig. 2. Anodic oxidation of propan-2-01 (0.5 MI at (a) the Ni + PTFE composite-plated and (b) the unplated nickel anode in 0.2 M aqueous potassium hydroxide solutions containing t-BuOH.
by the competitive interaction of t-BuOH. This has not been verified experimentally at present because of a technical problem. ACKNOWLEDGEMENTS
This study was financially supported in part by a Grant-in-Aid for Scientific Research on Priority Areas (New Functionality Materials) from the Japanese Ministry of Education, Science and Culture. The authors are also grateful to Dr. T. Fuchigami, Tokyo Institute of Technology, for helpful discussions. REFERENCES 1 2 3 4
Y. Kunugi, Y.-B. Chong, N. Watanabe and T. Nonaka, Electrochim. Acta. in press. Y. Kunugi, T. Fuchigami and T. Nonaka, Chem. Lett. (1989) 1467. Y. Kunugi, T. Fuchigami, S. Matsumura and T. Nonaka, J. Electroanal. Chem., 287 (1990) 385. Y. Kunugi, R. Kumada, Y.-B. Chong, N. Watanabe and T. Nonaka, J. Electroanal. Chem., 314 (1991) 215. 5 S. Matsumura, Shikizai, 56 (1983) 328 (in Japanese); Chem. Abstr. (1983) 99, 77201h.