Electrooxidation of Anhydrous Glycerol: Kinetics and Selectivity

Abstract

As a by-product of biofuel production, glycerol needs to find its use in various applications, for example, as a substrate for electrosynthesis of more valuable chemicals. The glycerol oxidation reaction (GOR) in aqueous media is technologically feasible but produces various products with hard-to-control selectivity. Less explored is the electrooxidation of anhydrous glycerol, which theoretically limits possible products to aldehydes/ketones, including high-cost glyceraldehyde. Herein, the GOR with gold electrodes is investigated using glycerol and acetonitrile as solvents without and with base, TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl) redox mediator, and a copper(I)-bipyridyl catalyst added into the lithium bis(trifluoromethanesulfonyl)imide electrolyte solutions. Both redox-mediated and heterogeneous oxidation are slow when glycerol is used as a solvent even at 90 °C, as in particular probed by Fourier-transformed alternating current voltammetry. Redox-mediated glycerol oxidation to glyceraldehyde is achieved in acetonitrile at a yield rate of 6 ± 3 nmol s⁻¹ cm⁻², but the reaction essentially stops after ≈1 h of electrolysis. Heterogeneous catalytic GOR in acetonitrile is more stable but requires significantly more positive potentials and produces a mix of products. While demonstrating the possibility of selective anhydrous glycerol electrooxidation, our results highlight the need for improvements in the mediator and catalyst designs.