Influence of Impurities on the Electrochemical Upcycling of Biomass

The electrochemical oxidation of sugar-derived compounds such as furfural and 5-hydroxymethyl furfural (HMF) to the corresponding carboxylic acids is a crucial step in unlocking biomass as a renewable carbon feedstock. For instance, 2,5-furandicarboxylic acid, the oxidation product of HMF, can replace crude-oil derived terephthalic acid in the ubiquitous polymer polyethylene terephthalate. Hence, establishing an electrochemical process for the refinery of biomass requires oxidation of furfural and HMF with high current densities. It is therefore noteworthy that we show in this work that trace impurities undetectable by NMR spectroscopy and HPLC analysis impair the kinetics of electrochemical HMF and furfural oxidation. We therefore evaluate different methods for the removal of impurities from HMF and furfural that form during synthesis and storage of both chemicals. We find that purification by distillation of furfural and by recrystallization of HMF improves the kinetics of their electrochemical conversion best. Since both procedures can be adopted readily by other labs, the present work provides practical guidelines for the pretreatment of chemicals, which may also prove relevant for the development of processes at scale.