Breaking the C[sbnd]C bond of glucose on tungsten oxide-based catalysts in aqueous phase

Abstract

In chemistry, selective activation of CC bonds enables the direct production of valuable chemicals from widely available and inexpensive natural materials but remains a fundamental challenge due to their kinetic inertness. The selective cleavage of CC bond in glucose by tungsten oxide-based catalysts in aqueous phase pioneers a path for converting cellulose biomass into valuable ethylene glycol. However, debates regarding the active phase and how it selectively breaks the C6 into C2 fragments have persisted for over a decade. In this study, we present a comprehensive mechanistic investigation by modeling three potential active phases, i.e. the reduced WO3-x surface, the dissolved tungstic acid, and tungsten bronze, in explicit solvent waters. By constrained molecular dynamics simulations, we have demonstrated that the low-coordinated W center can chelate with glucose, forming a metallacyclic complex with a 5-membered ring after the protonation of carbonyl group. The formation of 5-membered ring serves as the premise for the selectivity to C2 fragments via homolytic cleavage of CC bond. Furthermore, the reduced W5+ center is suggested to be crucial in facilitating the cleavage process by stabilizing the dissociated C4 intermediates via a redox process. In conclusion, we propose that the surface decoration of reduced and low-coordinated W sites can act as active heterogeneous catalysts for the selective conversion of cellulose in aqueous phase. These recent findings have the potential to provide valuable insights and strategies for CC bond activation in both biomass conversion and organic synthesis.