Study on the Effect of Electrolyte pH during Kolbe Electrolysis of Acetic Acid on Pt Anodes

Abstract

Kolbe already discovered in 1849 that electrochemical oxidative decarboxylation of carboxylic acids is feasible and leads to formation of alkanes and CO 2 , 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 O 2 . 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 CO 2 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.

Closing the Gap: Towards a Fully Continuous and Self‐Regulated Kolbe Electrosynthesis

“…These changes might be due to increasing carbonate concentrations during the Kolbe process as recently described as a negative factor for the acetic acid coupling. [27] Carbonate formation is based on the cleavage of one molecule of CO 2 per molecule carboxylic acid in combination with an alkaline solution pH. The resulting carbonate/bicarbonate equilibrium stabilizes the solution pH at a value around 10 (see Figure 1A and B and Figure 2A) and side reactions like the butanol formation prevail.…”

Electrochemical decarboxylation of acetic acid on boron-doped diamond and platinum-functionalised electrodes for pyrolysis-oil treatment

“…Moreover, the carbocation can react with deprotonated acetate to form the respective ester. Several parameters influence the fate and selectivity of (non)Kolbe electrolysis such as pH, applied current density, solvent, and electrolyte composition 5,10 . The electrode material is also crucial, and platinum electrodes have been reported to be ideal for Kolbe electrolysis at negligible water discharge rates.…”

“…Several parameters influence the fate and selectivity of (non-)Kolbe electrolysis, such as pH, applied current density, solvent, and electrolyte composition. 5,10…”

“…Several parameters inuence the fate and selectivity of (non-)Kolbe electrolysis, such as pH, applied current density, solvent, and electrolyte composition. 5,10 The electrode material is also crucial, and platinum electrodes have been reported to be ideal for Kolbe electrolysis at negligible water discharge rates. For Pt electrodes, the formation of a barrier layer consisting of carboxylates is suggested to be essential to prevent water oxidation.…”

“…5,27 Still, only a few reports are available related to acetic acid oxidation on BDD, 28 and usually electrooxidation is performed in methanol-, sulphate- or perchlorate-containing electrolytes, while the product distribution is not well described. The decarboxylation of acetic acid on BDD is considered to occur via the formation of weakly adsorbed hydroxyl radicals or other radicals (such as peroxysulphates), 10 which oxidise the acetate, omitting a direct PCET. A direct electron-transfer reaction is only observed for compounds with thermodynamic potentials well below those of water oxidation and formation of OH radicals.…”