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

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 MoOX 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.