Copper-Based Catalytic Anodes To Produce 2,5-Furandicarboxylic Acid, a Biomass-Derived Alternative to Terephthalic Acid

2,5-Furandicarboxylic acid (FDCA) is a key near-market platform chemical that can potentially replace terephthalic acid in various polyesters such as polyethylene terephthalate (PET). FDCA can be obtained from oxidation of 5-hydroxymethylfurfural (HMF), which can be derived from cellulosic biomass through isomerization and dehydration of hexoses. In this study, electrochemical oxidation of HMF to FDCA is demonstrated using Cu, one of the cheapest transition metals, as the catalytic anode. The oxidized Cu surface is not catalytic for water oxidation, which is the major reaction competing with HMF oxidation in aqueous media. Therefore, a wide potential window to oxidize HMF without inducing water oxidation was available, enabling high Faradaic efficiencies for FDCA production. Cu was prepared as nanocrystalline and bulk electrodes by electrodeposition, and key differences in their surface oxidation and electrochemical HMF oxidation were investigated. The oxide and hydroxide layers formed on the nanocrystalline electrode appeared to have an intrinsically different catalytic ability for HMF oxidation from those formed on the bulk electrode. Both the HMF conversion and FDCA production by the nanocrystalline electrode were nearly perfectly proportional to the amount of charge passed with no significant accumulation of any intermediate oxidation product during the course of HMF oxidation. After the stoichiometric amount of charge was passed, the nanocrystalline electrode achieved a FDCA yield of 96.4%. In contrast, the bulk electrode accumulated a significant amount of 5-formyl-2-furancarboxylic acid (FFCA) during HMF oxidation and achieved an FDCA yield of 80.8%. The morphology and composition of the oxide and hydroxide layers formed on the nanocrystalline and bulk electrodes were investigated systematically before and after HMF oxidation.