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 RuxCoOy(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 Ru4CoOy(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 molFDCA molRu–1 h–1) compared to those of Ru-based benchmark catalysts in the literature. Moreover, this catalyst was considerably stable during seven reuses.