(731f) Consequences of Solvation and Acid Strength for Catalysis by Faujasite Zeolites
The effects of extraframework Al (Alex) 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 Alex 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 (Alf) 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 Alf 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 Alex 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.