Selective Production of C3 Polyols from Cellulose over Hydrogen Spillover Promoted Pd–Mo/TiO2 Catalyst with Adjacent Lewis Acid Pairs

Abstract

The use of cellulose as a substitute for fossil resources has gained significant attention due to the depletion of nonrenewable energy sources and the increasing potential environmental crisis. The selective hydrogenolysis of cellulose to C₃ polyols, namely, 1,2-propanediol and glycerol, is one of the attractive biomass depolymerization and utilization pathways. However, coordinating sugar isomerization and retro-aldol condensation remains a challenge. In this paper, we manipulate the hydrogen spillover behavior of the Pd–Mo/TiO₂ catalyst by tuning the density and aggregation states of Mo/Pd species to promote the yield of C₃ polyols in the one-pot cellulose hydrogenolysis reaction. The optimal Pd–Mo/TiO₂ catalyst realizes a ∼50% C₃ polyol yield with the total polyol yield approaching 70%, surpassing the performances of known heterogeneous catalysts. We demonstrate that the strong hydrogen spillover on the Pd/TiO₂ facilitates the reduction of MoO_X and enhances the formation of continuous oxygen vacancies on Ti–O–Mo sites as adjacent Lewis acid pairs that serve as the adsorption sites for the tridentate complex of hexose and efficiently catalyze the isomerization and subsequent retro-aldol condensation steps. The acceleration of glucose → fructose isomerization and inhibition of undesirable condensation account for the high product yield.