Critical Practices in Improving the Electrochemical Oxidation of 5‑Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid in 0.1 M KOH

The electrochemical oxidation of 5-hydroxymethylfurfural (HMF) is an efficient method to produce 2,5-furandicarboxylic acid (FDCA). The moderate alkaline condition (pH ≈ 13) is a compromise between stability concern and catalytic efficiency. Herein, several critical issues are studied to improve the electrochemical oxidation of HMF to FDCA in 0.1 M KOH using NiO as the catalyst. The direct oxidation process of HMF is observed before the formation of Ni³⁺ species, which follows an electrochemical reaction mechanism. By increasing the electrode area/electrolyte volume ratio and NiO loading, the reaction potential can be extended to a lower value (1.30 V_RHE) with high FDCA yield (>95%). Moreover, the FDCA yield of 97% is realized at the feeding HMF concentration of 40 mM in 0.1 M KOH by using an anion exchange membrane (AEM) as the separator of H-type electrolyzer in a three-electrode system. In addition, the adsorption of products by the electrolysis system and the compound penetration through the AEM from anolyte to catholyte cannot be ignored in a two-electrode flow cell. The slight decrease of alkali concentration in the catholyte is conducive to decreasing the penetration of products from anolyte to catholyte due to the inhibition of water transfer in the opposite direction in the AEM.