(735c) Solvent Effects on Elementary Reactions in Solid-Acid Catalyzed Reactions: Acid-Base Interactions in Zeolites

Biomass-derived molecules are characterized by high oxygen content and boiling points necessitating their conversion to value-added products by catalytic routes in the liquid-phase. Solvents introduce thermodynamic non-idealities that if accounted for can be used to rigorously compare the behavior of different catalysts in the condensed phase or in the vapor-phase (where ideality is generally assumed). In this work, we demonstrate the impact of solvent on the adsorption of pyridine in solid acids (zeolites) using isothermal titration calorimetry (ITC). ITC experiments demonstrate the choice of solvent strongly affects the adsorption thermodynamics of pyridine on HZSM-5. We examined the impact of solvent addition on the gas-phase adsorption thermodynamics of pyridine on Brønsted acid sites using ITC. A comparison of liquid- and gas-phase calorimetry results for pyridine adsorption on H-ZSM5 will be shown for two solvents – water and heptane. The comparison of the calorimetry results in the two phases demonstrates the impact of non-idealities on acid-base reactions within the voids of zeolites depends on their Si/Al ratio. Compared to the gas-phase adsorption enthalpy of pyridine on zeolites which is independent of Si/Al ratio, the adsorption enthalpy of pyridine on zeolites increases with increasing the Si/Al ratio in water, decreases with increasing the Si/Al ratio in acetonitrile and is independent of Si/Al ratio in heptane. Differences in measured (apparent) enthalpies are caused by the different extent to which the initial state and the final state are solvated, and not differences in intrinsic acidity of intrapore protons. Differences in the measured enthalpy from ITC in different solvents can be explained by developing Born-Haber cycles for pyridine adsorption on zeolites with different Si/Al ratios. We will further discuss the relationship between enthalpies of adsorption of different amines on HZSM-5 in the presence of water and the basicity of amines in gas- and liquid phase, respectively.