Kolbe already discovered in 1849 that electrochemical oxidative decarboxylation of carboxylic acids is feasible and leads to formation of alkanes and CO2, via alkyl radical intermediates. We now show for Pt electrodes that Kolbe electrolysis of acetic acid is favored in electrolytes with a pH similar to, or larger than the pKa of acetic acid, suppressing the formation of O2. However extended duration of electrolysis of acetate at basic pH results in loss of Faradaic efficiency to ethane, compensated by the formation of methanol. This change in selectivity is likely caused by the dissolution of CO2 near the electrode-electrolyte interface, resulting in enlarged concentration of bicarbonate/carbonate. On the positively polarized, and oxidized Pt surface, these anions seem to inhibit homocoupling of methyl radicals to ethane. These results demonstrate that reaction selectivity in acetic acid (acetate) oxidation using oxidized Pt electrodes is determined by the pH and the anionic composition of the electrolyte.
- acetic acid
- bio-oil upgrading
- electrochemical Kolbe oxidation
- pH-dependent product selectivity