Developing Dawson-Type Polyoxometalates Used as Highly Efficient Catalysts for Lignocellulose Transformation

Abstract

Selective breakage of the β-O-4 bond in lignin is the key procedure for full conversion of lignocellulose; however, non-noble metal-based catalysts usually require harsh reaction conditions in the cleavage of the β-O-4 bond and show low selectivity in heterogeneous catalysis. Despite the tremendous development in recent years, it still remains a great challenge to develop versatile catalysts with high efficiency, convenient regeneration, and multifunctionality to achieve full lignocellulose valorization. Herein, a strategy of "atom-by-atom" replacement of the central atom (P5+ by V5+) was employed to obtain the polyoxometalate (POM) catalyst, H6V2Mo18O62 (H6V2Mo18), which exhibited a significantly enhanced activity on the cleavage of β-O-4 lignin models (compared to the original H6P2Mo18O62). The optimized electronegativity of Mo and O atoms induced by the inserted vanadium at the central site and the modified acidic/redox ability of H6V2Mo18 had been extensively analyzed by density functional theory (DFT) and experiment. Deep eutectic solvent cation betaine (Bet+) was further used to solidify H6V2Mo18 to obtain the BetH5V2Mo18, which acted as a trinitarian catalyst with controlled acidic/redox ability and thermosensitive ability for mass-transferring confirmed by molecular dynamics simulations, DFT, and experiments. Using BetH5V2Mo18 as a highly efficient catalyst, full utilization of lignocellulose can be easily achieved with the one-pot method via temperature-programmed treatment. This work is opening new research frontiers in the design of multifunctional-site POMs with a specialized micro-environment in biomass valorization, and this new trinitarian catalyst could lead to a new trend in catalyst design.