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, O-2 as the oxidant, and no soluble base additives. Various physicochemical techniques, including ICP-MS, PXRD, DRIFT, TEM, H-2-TPR, O-2-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, O-2 pressure, reaction temperature, and time) and the reaction pathway was investigated. HMF oxidation was initiated mainly from the OH group on metallic Ru-0 active sites, and the cooperation of the Co(II)/Co(III) redox pair allowed the effective transformation of O-2 to reactive lattice oxygen and the regeneration of Ru-0 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 degrees C and 10 bar of O-2 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.