(461e) 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 that their conversion to value-added products by catalytic routes be conducted in the liquid-phase which often necessitates the use of a solvent. 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 compare the behavior of the same catalyst in vapor-phase (where we ensure ideality) or the liquid phase. Catalytic cycles are comprised of multiple steps, all of which may be influenced by the presence of solvent. If individual steps are influenced to a greater extent than other steps, barriers associated with the reaction coordinate can differ from the gas-phase.

In this work, we demonstrate the impact of solvent on the adsorption of pyridine and the impact of pH on the adsorption of amines in solid acids (zeolites) using isothermal titration calorimetry (ITC). In a single experiment, ITC can yield the various thermodynamic parameters associated with titrant-Brønsted acid interactions, such as enthalpy, entropy, free energy and binding stoichiometry. ITC experiments demonstrate that the choice of the solvent strongly affects adsorption thermodynamics of pyridine on ZSM-5. The apparent adsorption enthalpy of pyridine on HZSM-5 is -35 kJ/mol in water and -65 kJ/mol in acetonitrile. The basicity of amine also influences binding energetics with the adsorption enthalpy of isopropylamine on HZSM-5 in water is -58 kJ/mol. We also examined the impact of solvent addition on the gas-phase adsorption thermodynamics of amines on Brønsted acid sites using calorimetry. A comparison of the calorimetry results in the two phases demonstrate the influence of non-idealities on acid-base reactions on zeolites. We further demonstrate the impact of zeolite topology on the thermodynamics of adsorption in condensed phase media.