Consequences of Solvation and Acid Strength for Catalysis by Faujasite Zeolites

Thursday, October 20, 2011: 5:15 PM
200 I (Minneapolis Convention Center)
Rajamani Gounder, Andrew J. Jones, Robert T. Carr and Enrique Iglesia, Department of Chemical and Biomolecular Engineering, University of California at Berkeley, Berkeley, CA

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.

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