(731f) Consequences of Solvation and Acid Strength for Catalysis by Faujasite Zeolites

The effects of extraframework Al (Al_ex) and of Na⁺ titrants in FAU zeolites on turnover rates and acid strength have been contradictorily interpreted in previous studies. Here, we examine partially-Na⁺-exchanged samples of steam-dealuminated FAU (USY) and USY dealuminated chemically (CD-USY) to remove Al_ex species. Protons were measured by titration with 2,6-di-tert-butylpyridine during methanol dehydration catalysis, with Na⁺ and with dimethyl ether. On both samples, the number of H⁺ sites was smaller than the number of framework Al atoms (Al_f) given by ²⁷Al MAS nuclear magnetic resonance spectroscopy and X-ray diffraction. Monomolecular isobutane cracking and dehydrogenation rate constants (per H⁺; 763 K) and their ratios were independent of Na⁺ content on USY and CD-USY. These kinetic data, together with infrared spectra, indicate that Na⁺ stoichiometrically replaces H⁺ sites of uniform reactivity. Rate constants inappropriately normalized by Al_f atoms, however, decreased monotonically with Na⁺ titration, giving the appearance that Na⁺ selectively titrates more reactive protons. Methanol dehydration rate constants confirmed that acid sites on USY and CD-USY are equivalent in strength but that solvation effects are slightly stronger within USY supercages, because of the larger Al_ex content and smaller void spaces than in CD-USY. These findings demonstrate that thermal and chemical treatments do not change acid strength but can subtly influence the solvation effects of confinement within voids of molecular dimensions.