Electrochemical decarboxylative oxidation of carboxylic acid mixtures on boron doped diamond electrodes

Abstract

Electrochemical decarboxylation of carboxylic acids is of emerging interest for upgrading of pyrolysis oil. In this study, electrooxidation of propionic acid using BDD electrodes in batch mode is 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 relative ethanol over ethylene Faradaic Efficiency (FE_{ethanol/ethylene}) can be achieved at low current density (25 mA/cm²) and an electrolyte pH of 5, while the FE_{ethanol/ethylene} is affected by 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, but that surface mediated reactions are also relevant. Finally, we propose mitigation strategies for consecutive oxidation of ethanol at high current densities (100 mA/cm²), using flow cells and pulsed electrolysis to control the near-surface concentration of hydroxyl radicals. In conclusion, this study provides a comprehensive analysis of decarboxylation reactions using BDD anodes and additionally emphasizes the significance of engineering approaches to achieve substrate specificity, a field that has hitherto remained largely unexplored.