Stabilization of 5-HMF in highly alkaline electrolytes through acetalization for the selective electrooxidation to FFCA

Abstract

The oxidation products of 5-hydroxymethylfurfural (HMF) and its derivatives are promising monomers for the production of renewable polymers. However, the stability of HMF during electrooxidation in alkaline electrolytes remains challenging due to its degradation into humins, which reduces the carbon yield. To increase HMF stability in alkaline media, a protection strategy based on acetalization of the formyl group is plausible, but it has not yet been evaluated for electrochemical processes. In this study, we successfully transferred this protection strategy to the electrochemical oxidation of HMF in alkaline media. We demonstrate that acetal-protected HMF is highly stable in alkaline electrolytes, even at elevated concentrations and temperatures. Furthermore, we show that the overall selectivity of the electrooxidation shifts from 2,5-furandicarboxylic acid (FDCA) to 5-formyl-2-furancarboxylic acid (FFCA), which is typically not obtained during the alkaline electrooxidation of HMF. High yields (95%) and Faradaic efficiencies (87%) of FFCA were achieved, even at elevated substrate concentrations (250 mM) in 5 M KOH. The carbon balance remained closed throughout the electrooxidation, demonstrating that acetalization of HMF effectively suppresses degradation into humins.