Intensified Biphasic Electrochemical Oxidation of Benzylic Alcohols in a Flow System with Continuous Electrolyte Recycling

Electrochemical methods for the oxidation of alcohols to aldehydes and ketones have been extensively studied over the past few years as a safer and more sustainable alternative to stoichiometric oxidants. However, electrolysis often requires significant amounts of supporting electrolyte to provide sufficient conductivity to the organic solvent, which is detrimental to process green chemistry metrics. Herein, we report an electrochemical method for the selective oxidation of alcohols based on a biphasic mixture consisting of ethyl acetate and 0.1 M aqueous sulfuric acid. The electrochemical method was successfully transferred to a single-pass continuous-flow protocol by using an oscillatory flow electrochemical reactor. Combination of the oscillatory flow with static mixing elements provided excellent mixing of the biphasic mixture, even at the relatively low flow rates required for single-pass processing. A liquid-liquid separator after the reactor output enabled continuous recycling of the aqueous electrolyte, allowing for the processing of gram amounts of material with a small amount of electrolyte. The electrochemical process with continuous electrolyte recycling could be operated for 8 h without any decrease in the reaction performance, resulting in an improvement of the process mass intensity (PMI) from 51 in batch mode to 11 in flow mode.