Base-Free Selective Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid over Cu–Mn Mixed Oxide Catalysts under Mild Conditions

The selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) under base-free conditions represents a pivotal step toward the sustainable synthesis of biomass-derived monomers. In this work, we report a cost-effective, non-noble metal catalytic system based on Cu–Mn mixed oxides, synthesized through a hydrothermal method followed by controlled thermal treatment. A series of catalysts with varying Cu/Mn ratios and calcination temperatures were systematically evaluated for their activity in HMF oxidation using tert -butyl hydroperoxide ( t -BuOOH) as a green oxidant. Among them, the Cu 3 Mn 1 -500 catalyst─obtained by calcining the precursor at 500 °C─exhibited optimal performance, achieving 100% HMF conversion and a 77.5% FDCA yield at 90 °C within 4 h. Notably, the catalyst maintained high activity (>99% HMF conversion and >75% FDCA yield) over five consecutive cycles, demonstrating excellent stability and reusability. Structural characterization revealed that the catalyst predominantly comprises CuO and Mn 2 O 3 phases with uniform elemental dispersion. Mechanistic insights suggest that high-valent species (Cu 2+, Mn 4+ ) facilitate reactive oxygen generation, while low-valent species (Cu +, Mn 3+ ) and oxygen vacancies cooperatively enhance electron transfer and redox cycling. This study provides fundamental understanding of structure–performance relationships in Cu–Mn binary oxides and offers a practical, scalable pathway for FDCA production under environmentally benign, base-free conditions.