Theoretical Exploration of the Origin of Alkaline Dependence in the Oxidation of 5-Hydroxymethylfurfural Catalyzed by NiO2Hx

Alkaline dependence is a common phenomenon in the electrochemical oxidation of biomass, and investigating the influence of alkalinity on the oxidation mechanisms is crucial for enhancing both the activity and the selectivity of biomass oxidation. Herein, we constructed five NiO2Hx catalysts under different alkaline environments (pH ≈ 9–13) and employed density functional theory methods to investigate the reaction mechanisms of the selective oxidation of the biomass platform molecule 5-hydroxymethylfurfural (HMF). The origin of alkaline dependence in HMF electrochemical oxidation was analyzed, and the results indicate that the catalyst alkalinity determines the oxidation pathway of HMF to 2,5-furandicarboxylic acid: under weak alkalinity (pH ≈ 9–11), the adsorption and activation of the alcohol group in HMF are more favorable, resulting in preferential oxidation of the alcohol group; under strong alkalinity (pH ≈ 11–13), the aldehyde group of HMF is more prone to adsorb and activate on the catalyst surface, resulting in a predominant aldehyde oxidation. With the increase in the alkalinity of NiO2Hx, there is a decrease of H atom coverage and an increase in the valence state of Ni, resulting in a transformation in the preferential oxidation pathway of HMF from the alcohol group to the aldehyde group as well as a transition in the oxidation mechanism from direct oxidation to indirect oxidation. NiOOH under moderately alkaline environment effectively reduces the Gibbs free energy change for C–H/O–H bonds cleavage, lowering the reaction energy. It significantly enhances the oxidation ability of alcohol and aldehyde groups, exhibiting the highest catalytic performance in the conversion of HMF to FDCA. The study not only provide a comprehensive explanation for the alkaline dependence of HMF oxidation but also offer guidance on how to rationally design catalysts for improving oxidative performance through alkalinity adjustments.