Effect of Water on Elementary Reactions in Solid-Acid Catalyzed Reactions: Thermodynamics of Acid–Base Interactions in H-ZSM-5

The influence of water on acid–base interactions in zeolites was quantified by measuring the enthalpy of pyridine adsorption on H-ZSM-5 with three Si/Al ratios (11.5, 40, and 140) in water using isothermal titration calorimetry (ITC). This calorimetric study reconciles the liquid- and gas-phase energetics of acid–base interactions in a solvated, pore-filled zeolite through a Born–Haber cycle analysis. In water, the apparent adsorption enthalpy is about 150 kJ/mol less exothermic than the gas-phase value and becomes more exothermic as the Si/Al ratio increases, whereas the gas-phase enthalpy is near −200 kJ/mol and independent of the Si/Al ratio. Since the intrinsic acid strength of the framework does not vary with Si/Al ratio, this increasing apparent adsorption energy trend with increasing Si/Al ratio reflects the influence of solvation rather than acidity. Water stabilizes both the acidic proton and the pyridinium ion, and a fraction of this stabilization is lost when adsorbed pyridine displaces water from the acid site. By combining ITC data with vapor-phase water adsorption isotherms and differential scanning calorimetry to construct a Born–Haber cycle for each zeolite, the aqueous-phase enthalpies are quantitatively reconciled with their gas-phase counterparts, and their dependence on Si/Al ratio is traced to the different extents to which the initial and final states are solvated.