Electrochemical Decarboxylative Oxidation of Carboxylic Acid Mixtures on Boron Doped Diamond Electrodes

Electrochemical decarboxylation of carboxylic acids is of emerging interest for pyrolysis upgrading. In this study, electrooxidation of propionic acid using BDD electrodes in batch mode will be shown to form ethanol and ethylene. The influence of process conditions is elaborated on and we generally demonstrate that the oxidation of propionic acid on BDD is largely induced by solution based hydroxyl radical chemistry. A high FEethanol/ethylene can be achieved at low current density (25 mA/cm2) and an electrolyte pH of 5, while the relative ethanol over ethylene selectivity is effected due to consecutive oxidation of ethanol to acetaldehyde and/or CO2. This is further corroborated by investigating the specificity of BDD in electrooxidation of short chain carboxylic acid mixtures. It is shown that the oxidation specificity is not solely dependent on the rate constant of Formic, Acetic, and Propionic acid with hydroxy radicals. Finally, we propose mitigation strategies for consecutive oxidation of ethanol at high current densities (100 mA/cm2), using flow cells and pulsed electrolysis as viable means of controlling the near-surface concentration of hydroxyl radicals. In conclusion, this study provides a comprehensive analysis of Kolbe electrolysis of carboxylic acid mixtures using BDD anodes and additionally emphasizes the significance of engineering approaches to achieve substrate specificity in carboxylic acid oxidation, a field that has hitherto remained largely unexplored.