Base-free selective conversion of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid over a CoOx-CeO2 catalyst

Ce oxide-based catalysts with different molar ratio of Co/Ce (denoted as CoCe-x, x represents the Co to Ce ratio) were examined as the catalyst of 5-hydroxymethylfurfural (5-HMF) oxidation to 2,5-furandicarboxylic acid (FDCA). The Co to Ce ratio, catalyst amount and reaction temperature all have remarkable effects on yield and selectivity. Complete conversion of 5-HMF with 86.3% selectivity of FDCA has been achieved over the CoCe-0.15 catalyst at 130 °C with 0.6 MPa O2 pressure (gauge) for 4 h. The good performance is attributed to the high specific surface area and the high oxygen vacancy concentration to promote the mobility and adsorption of oxygen species. In addition, the CoCe-0.15 catalyst is reused for five times without significant activity loss. Finally, the reaction mechanism over the CoCe-0.15 catalyst under base-free condition is discussed. 5-HMF is oxidized by the lattice oxygen via Mars-van Krevelen mechanism with the redox of Ce4+/Ce3+ and Co3+/Co2+, and the lattice oxygen is replenished by adsorbing and activating O2 molecules.