Comparative Investigation of Electrocatalytic Oxidation By Proton-Exchange-Membrane and Anion-Exchange-Membrane Electrolyses

Abstract

Oxidation is one of the most important reactions in chemical industries. However, the oxidation reaction generally requires a large input energy and problematic oxidants. On the other hand, the electrochemical oxidation is expected to be a key technology for the reduction of greenhouse gas emissions because this reaction can generally be performed under mild conditions. In addition, it does not require any hazardous redox agents and produce less waste than other conventional oxidation processes [1]. However, electrochemical oxidation also has some disadvantages. In order to provide sufficient conductivity, a large amount of supporting electrolyte is usually required. The presence of the supporting electrolyte might cause separation problems in the reaction workup. Moreover, in an ordinary electrolysis system, solution resistance is generated between the working and counter electrodes, resulting in a decrease in energy efficiency [2]. In addition, it is difficult to realize direct oxidation of hydrocarbons because their redox potentials are higher than those of solvents and electrolytes [3].To overcome these problems, we focused on a solid polymer electrolyte (SPE) electrolysis technologies. The SPE electrolysis cell was originally developed for fuel cell technologies. As shown in Figure, a membrane electrode assembly (MEA) is integrated into the central part of the reactor. The MEA consists of an ion exchange membrane sandwiched between a pair of catalyst layers on the anode and cathode sides, having the dual roles of electrode. For this reason, the substrate solution does not require a supporting electrolyte and the cell resistance can be minimized. Furthermore, the oxidative conversion can be carried out at a potential lower than that required to effective its direct electrooxidation because it uses reactive oxygen species generated on the anode catalyst from H2O.The main types of SPE electrolysis technologies are proton exchange membrane (PEM) electrolysis and anion exchange membrane (AEM) electrolysis. The former PEM has been used widely as a fuel cell and a water electrolysis equipment. However, because of acidic environment at the electrochemical reaction fields, it may cause undesired side reactions. Moreover, electrocatalysts are limited to some noble metals. On the other hand, in the later AEM, many earth-abundant metal catalysts can be used because of its alkaline environment [4,5]. However, the durability of AEM is not yet suitable for commercialization. With these in mind, in this work, we have demonstrated electrocatalytic oxidation of cyclohexene by using both PEM and AEM electrolyzers. We have also optimized the applied voltage and selected the suitable anode catalyst to realize highly selective and efficient oxidation process.The authors gratefully acknowledge support by JST CREST Grant No. JPMJCR18R1, Japan.