Solvent Effect in Catalytic Upgrading of Biomass-Derived Furanics

In liquid-phase heterogeneous catalysis, solvent provides an additional degree of freedom for tuning the catalytic activity and selectivity. Aqueous-phase upgrading of small oxygenates is particularly desirable for biomass conversion since pyrolysis of biomass produces large amounts of water and water-soluble small oxygenates. Fundamental understanding of the solvent effect will be valuable for further promoting the liquid-phase heterogeneous catalytic upgrading strategy. Here we will focus on furfural and discuss the solvent effect on its catalytic reactions through a synergistic combination of experiment, theory and computation. We find rate enhancement of furfural conversion and improved selectivity towards hydrogenation of the carbonyl group to form furfuryl alcohol in an aqueous phase as compared to reactions in organic solutions. We show that the water solvent has multiple, distinct effects. The aqueous phase stabilizes the intermediate and final products by forming hydrogen bonds and changes the rate-determining step. Moreover, hydrogen can also be transferred by a water molecule, which further reduces the activation barrier. We will also discuss the effect of the dielectric screening of water on stabilization of the charged transition state.