5-Hydroxymethylfurfural oxidation reaction (HMFOR) is regarded as a promising approach to attain biomass-derived high-value chemical products. As the HMFOR process is complicated, and the two-step oxidation of the aldehyde group and hydroxyl group in 5-hydroxymethylfurfural (HMF) is typically involved, it is fundamentally significant to understand the different catalytic processes for HMFOR. In this work, we identify direct and synergistic oxidation types for HMFOR on cobalt oxide catalysts. For the direct HMFOR process, Co3O4 was found to have a higher activity for the aldehyde group than for the hydroxyl group due to the higher reaction barrier of hydration in the hydroxyl oxidation. By studying the hydroxyl oxidation behaviors in transition metal oxides, NiO exhibited optimal hydroxyl activity owing to the appropriate OH adsorption energy for alcohol dehydrogenation. Therefore, the optimal HMFOR performance was achieved by accurately introducing Ni into the tetrahedral catalytic sites of cobalt spinel oxides to improve the hydroxyl activity. The integrated catalytic sites enhanced the overall activity of HMFOR with 92.42% FDCA yield and 90.35% faradaic efficiency. This work provides a promising perspective for designing efficient electrocatalysts for HMFOR.
