Engineering octahedral redox sites in NiMnxCo2-xO4 spinel oxides for selective base-free aqueous phase 5-hydroxymethylfurfural oxidation to FDCA

Biomass-derived 5-hydroxymethylfurfural (HMF) is a key platform molecule for the sustainable production of 2,5-furandicarboxylic acid (FDCA), a promising renewable alternative to petroleum-derived terephthalic acid. However, efficient FDCA production commonly relies on noble-metal catalysts and alkaline additives, limiting industrial sustainability. In this study, a series of crystal-engineered spinel oxides, NiMnxCo2−xO4 (x = 0–2.0), were synthesized through controlled octahedral-site substitution to elucidate the structural and electronic descriptors governing HMF oxidation activity. Comprehensive characterization using XRD, Raman spectroscopy, FTIR, BET, FESEM, TEM, ICP-MS, and XPS revealed that Mn substitution significantly alters the redox properties and oxygen activation capability of the spinel lattice. Among the investigated catalysts, NiMn1.0Co1.0O4 exhibited the optimum catalytic performance, achieving 96.4% HMF conversion and 72.1% FDCA yield under base-free aqueous conditions using molecular oxygen as the sole oxidant. The enhanced activity was attributed to a balanced Mn³⁺/Mn⁴⁺ ratio (~0.91), which promotes efficient lattice oxygen participation and oxygen vacancy replenishment through synergistic Mn–Co redox cycling. Kinetic studies revealed that HMF oxidation proceeds predominantly through the HMF → DFF → FFCA → FDCA pathway, with FFCA oxidation representing the rate-determining step. Furthermore, ESI-MS analysis identified HMFCA-derived oligomers as major humin precursors responsible for carbon loss under prolonged reaction conditions. The catalyst demonstrated excellent structural stability and reusability over six consecutive cycles. This work establishes octahedral-site engineering as an effective strategy for designing earth-abundant spinel catalysts for sustainable biomass valorization and green production of FDCA.