A giant polyoxomolybdate molecular catalyst with unusual Mo6+/Mo5+ synergistic mechanism for oxidation of hydroxyfurfural under atmospheric pressure

Selective oxidation of 5-hydroxymethylfurfural (HMF) under atmospheric conditions is challenging due to the slow reaction rate and multiple parallel reaction routes. This work reports a green approach for selective oxidation of HMF to 2,5-diformylfuran (DFF) by using a giant Keplerate-type polyoxometalate of [(NH4)42[MoVI72MoV60O372(CH3COO)30(H2O)72] (Mo132). Mo132 features extraordinary catalytic activity under atmospheric and even anaerobic conditions, achieving the conversion of 99.2% with DFF selectivity of 100%. Mo132 also shows outstanding recyclability with the activity decreasing by only 3.4% after five recycle tests. The intermediate valence state content of Mo5+ in Mo132 is about 46%, which displays superior catalytic activities during the reactions. Mechanism exploration firstly indicates that the proposed proton coupled electron transfer occurs on special terminal oxygen atoms in Mo6+ sites being surrounded by Mo5+ species. This unveils a new Mo6+/Mo5+ synergistic effect to enhance the efficiency for catalyzing the oxidation of HMF. This work expands the application of high-nuclear molybdenum clusters in catalysis, and supplies a new strategy for the justified design and synthesis of POMs-based catalysts in biorefinery.