Alcohols are the major derivatives from the conversion of agriculture-based biomass feedstocks with billions of gallons production annually. While direct alcohol oxidation is limited by slow reaction kinetics and irreversible substrate adsorption, indirect oxidation carried out by redox mediators can circumvent these problems. Due to its vast abundance in sea water, chloride anion has been considered as a promising mediator for this purpose thanks to its fast kinetics as well as the generation of active species that can readily oxidize substrates. With spin-coated HxWO3 semiconductor photoelectrodes, selective chloride oxidation to hypochlorous acid has been achieved, which subsequently oxidizes ethanol to acetaldehyde and acetic acid. Despite the high integrated yield, acetaldehyde is found to be a long-lived intermediate with the acid as the thermodynamic product. Stopping the reaction at the 2-electron aldehyde products remains as a key challenge that requires alternation in mechanism. By shifting into neat ethanol (solvent-free condition) and using an alkylammonium chloride electrolyte on a glassy carbon electrode, 1,1-diethoxyethane (DEE), which could serve as a protective platform for acetaldehyde from overoxidation, was prepared electrochemically with a high and stable current density &gt;15 mA/cm2 and a high faradaic efficiency &gt; 95%. UV-vis spectroscopy reveals the generation of an ethyl hypochlorite intermediate as the only chloride oxidation product, which decomposes unimolecularly to release hydrochloric acid and acetaldehyde. In the presence of acid, acetaldehyde undergoes nucleophilic attack from the ethanol solvent and forms DEE. Tafel measurement reveals faster kinetics for the Cl-mediated process compared to direct ethanol oxidation. This indirect electrochemical oxidation mechanism enables a much lower potential input and provides selectivity towards the value-added DEE product that can then be hydrolyzed to release acetaldehyde. Such a scheme provides a novel mediated electrochemical approach with high atom economy for the selective oxidation of other alcohols with a series of abundant and sustainable mediator.

Photoelectrocatalytic and electrocatalytic chloride mediated selective ethanol oxidation in solvent-free and aqueous conditions

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ACS Fall 2022

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