Green Solvent-Mediated Catalysis for Efficient Biorefinery: Unravelling the Kinetic and Thermodynamic Impacts

Understanding solvent effects on catalysis is pivotal to a more energy-efficient process design. This research elucidated the effects of binary solvent systems (mixtures of H2O and γ-valerolactone (GVL), isopropanol (IPA), or acetone) on the kinetics of Brønsted acid-catalyzed cellobiose hydrolysis and Lewis acid-catalyzed and Brønsted base-catalyzed glucose isomerization. While solvents influenced the reaction rate to different extents in diverse catalytic reaction systems, the trend of activation energy, i.e., GVL/H2O ≤ H2O < IPA/H2O ≤ acetone/H2O, seemingly arises as a generality in this study. In the case of hydrolysis, the evaluation of H2SO4 second acid dissociation pointed to the unlikelihood of higher proton concentrations in mixed solvents, whereas molecular dynamics simulation suggested no correlation between proton stability (relevant to reactivity) and the reaction rate. Isothermal titration calorimetry revealed a subtle solvent impact on the enthalpy change (<1.5 kJ/mol) of cellobiose protonation. The significance of solvents is instead highlighted from the perspective of system energetics. We observed that the enthalpy of activation and entropy of activation in different solvents formed a linear correlation regardless of the catalytic systems, whereas the free energy of activation was invariable. Solvents might promote stabilization, lessening the enthalpic penalty (ΔH‡) while increasing the entropic loss component (TΔS‡) to the same extent, as the reaction proceeds from the initial state to the transition state. This study evidenced the generic effects of solvent-induced energetic trade-off. Our finding inspires a new perspective in the search for and development of sustainable, promising alternatives to conventional hazardous solvents.