Base-Free Aerobic Oxidation of 5‑Hydroxymethylfurfural on a Ru(0) Center in Cooperation with a Co(II)/Co(III) Redox Pair over the One-Pot Synthesized Ru–Co Composites

A novel Ru_xCoO_y(OH) composite catalyst was prepared by the simple one-pot hydrolysis of Ru and Co salts at room temperature in the presence of NaBH₄. This catalyst showed promise for aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) under environmentally benign conditions, i.e., using water as the solvent, O₂ as the oxidant, and no soluble base additives. Various physicochemical techniques, including ICP-MS, PXRD, DRIFT, TEM, H₂-TPR, O₂-TPD, XPS, and CO₂-TPD, were used to study the catalytic properties that depend on the Ru load. The influence of reaction parameters (i.e., HMF/Ru ratio, O₂ pressure, reaction temperature, and time) and the reaction pathway was investigated. HMF oxidation was initiated mainly from the OH group on metallic Ru⁰ active sites, and the cooperation of the Co­(II)/Co­(III) redox pair allowed the effective transformation of O₂ to reactive lattice oxygen and the regeneration of Ru⁰ sites. It was shown that the Ru₄CoO_y(OH) catalyst with an enhanced Ru–Co metal interaction was the most active in giving a 99.9% yield of FDCA at 140 °C and 10 bar of O₂ after 18 h, corresponding to a high productivity (3.3 mol_FDCA mol_Ru^–1 h^–1) compared to those of Ru-based benchmark catalysts in the literature. Moreover, this catalyst was considerably stable during seven reuses.