Selective Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid over CoMn ₂ O ₄ –NiO _<i>x</i> Spinel Catalysts

The CoMn2O4–Ni/NiOx tetragonal spinel catalyst, synthesized via low-temperature precursor-ordering modulation to tailor intrinsic oxygen vacancy (OV) density and Mn4+/Mn3+ redox pairs, achieves exceptional efficiency in the selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) under ambient conditions. Mechanistic analysis reveals a Mars–van Krevelen pathway, where lattice oxygen (OL) from Mn4+–O–Mn4+ motifs drives sequential dehydrogenation via 2,5-diformylfuran (DFF) and 5-formyl-2-furancarboxylic acid (FFCA) intermediates, while OV rich Ni–Mn interfaces enhance HMF adsorption and stabilize reactive •OCl– species from NaClO. The catalyst’s optimized defect architecture, with 72% OV and Mn4+/Mn3+ redox cycling, enables 93% FDCA yield with a record productivity of 0.921 h–1 at room temperature, surpassing conventional systems. The role of Mn4+–OV–Ni2+ junctions moderate interfacial charge transfer and suppress overoxidation by stabilizing reactive intermediates. The adsorption of the aldehyde group of HMF on Mn4+ sites polarizes the C═O bond, facilitating hydration to a gem-diol intermediate, followed by β-H abstraction and oxidative dehydrogenation to FDCA. This work establishes a blueprint for defect-engineered transition-metal oxide catalysts, emphasizing redox synergy and adsorption kinetics to advance sustainable FDCA production for industrial biomass valorization.