Theoretical Investigation into the Oxidation of 5-Hydroxymethylfurfural to 2,5-Diformylfuran over Metal–Nitrogen-Doped Carbon Catalysts

Selective electrochemical oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) represents a sustainable pathway for producing value-added bio-based chemicals. However, the rational design of efficient electrocatalysts remains hindered by an incomplete understanding of structure–activity relationships. Herein, a series of transition-metal (TM) single-atom–doped M–N4/C catalysts (M = Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au) were systematically constructed, and density functional theory (DFT) calculations were employed to investigate the activity and selectivity of HMF oxidation to DFF. Our results demonstrate that the electronegativity and charge of the metal center interact with the alcohol group, which governs the charge distribution, thereby influencing the reactivity and selectivity. TM centers with lower electronegativity and higher positive charges preferentially form M–O bonds, stabilizing an O-type adsorption configuration of HMF. Correlation analysis reveals that this adsorption mode significantly facilitates C–H bond cleavage at the rate-determining step of alcohol oxidation, thereby enhancing overall catalytic activity. In contrast, TM centers with higher electronegativity and lower positive charges favor M–H bond formation and stabilize an H-type adsorption configuration, which is associated with substantially higher reaction barriers. Among the 12 catalysts examined, Fe–N4/C, Ni–N4/C, and Ru–N4/C emerge as the most promising candidates, due to their low energy barriers for the rate-determining step and strong selectivity toward alcohol oxidation during HMF conversion. Orbital-resolved analyses further reveal that effective catalysis originates from directional hybridization between the TM d orbitals (dz2/dxz/dyz) and the oxygen atom of the key intermediate R–CH2O*, forming σ-type interactions that promote C–H bond activation. Overall, by connecting electronic structure–adsorption configuration–reaction pathway–activity/selectivity, this work establishes a coherent structure–activity–mechanism relationship, which provides transferable guidance for the design of single-atom catalysts in biomass oxidation.