Tailored Octahedral Site Engineering in Spinel NiMn _<i>x</i> Co _{2– <i>x</i>} O ₄ Oxides for Base-Free Aerobic Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid in an Aqueous Medium

Transition metal oxides have emerged as viable, earth-abundant alternatives to noble metals for biomass valorization. Among them, cobalt- and manganese-based spinel oxides stand out for 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) oxidation. It has been found that the octahedral sites in spinels are responsible for their catalytic activities. However, little is known about the contributing parameters of these spinels that influence HMF oxidation activity. Herein, a series of NiMnxCo2–xO4 (x = 0–2.0) spinel oxides are synthesized to elucidate the structural–electronic descriptors governing HMF oxidation activity. We reveal that both Mn and Co cations contribute with Mn acting as the primary active species in synergy with Co. The catalytic performance strongly depends on the Mn/Co ratio, with NiMn1.0Co1.0O4 delivering the highest activity and enabling 96.4, 14.6, and 72.1% HMF conversion, 5-formylfuran-2-carboxylic acid (FFCA), and FDCA yield, respectively, at 120 °C temperature and 24 h time. The kinetic study and activity trend revealed that the base-free aerobic oxidation of HMF occurs via a primary series–parallel reaction through HMF → 2,5-diformylfuran (DFF) → FFCA → FDCA, where FFCA → FDCA is the rate-determining step. Structural insights from ESI-MS reveal 5-hydroxymethyl-2-furancarboxylic (HMFCA)-derived oligomers as the dominant humin precursors formed under prolonged exposure or elevated temperature under a base-free reaction medium. A characterization study reveals that the coexistence of Mn3+/Mn4+ in a balanced ratio close to 1 (∼0.91) is the key to achieving the highest HMF to FDCA oxidation activity. A plausible mechanism was developed, supported by an experimental study and the Mars-van Krevelen pathway, where lattice oxygen and synergistic Mn–Co redox cycling drive stepwise oxidation. These findings unify compositional and electronic parameters as descriptors of spinel activity, offering design principles for earth-abundant oxide catalysts in selective biomass-based upgradation reactions.