Auδ−–Ov–Ti3+: Active site of MOx-Au/TiO2 catalysts for the aerobic oxidation of 5-hydroxymethylfurfural

Abstract

Despite wide applications of noble metal-based catalysts in 5-hydroxymethylfurfural (HMF) oxidation, promoting the catalytic performance at low loading amounts still remains a significant challenge. Herein, a series of metal oxide modified MOx-Au/TiO2 (M = Fe, Co, Ni) catalysts with low Au loading amount of 0.5 wt% were synthesized. Addition of transition metal oxides promotes electron transfer and generation of the Auδ−–Ov–Ti3+ interface. A combination study reveals that the dual-active site (Auδ−-Ov-Ti3+) governs the catalytic performance of the rate-determining step, namely hydroxyl group oxidation. Auδ− site facilitates chemisorption and activation of O2 molecules. At the same time, Ov-Ti3+ site acts as the role of “killing two birds with one stone”: enhancing adsorption of both reactants, accelerating the activation and dissociation of H2O, and facilitating activation of the adsorbed O2. Besides, superoxide radicals instead of base is the active oxygen species during the rate-determining step. On this basis, a FDCA yield of 71.2% was achieved under base-free conditions, complying with the “green chemistry” principle. This work provides a new strategy for the transition metal oxides modification of Au-based catalysts, which would be constructive for the rational design of other heterogeneous catalysts.

Keywords

• 5-Hydroxymethylfurfural
• Base-free oxidation
• Interfacial catalysis
• Active oxygen species