Solvation and acid strength effects on catalysis by faujasite zeolites

Demystifying faujasite: Isobutane cracking (C) and dehydrogenation (D) rate constants (per framework Al; Alf), but not their ratio, decreased monotonically with Na+ content in ultrastable Y (USY) zeolite. Similar Na+ and 2,6-di-tert-butylpyridine titrant uptakes completely suppressed catalytic rates, indicating that there are fewer H+ sites than Alf atoms and that Na+ stoichiometrically replaces H+ sites that are uniform in reactivity and acid strength. [Display omitted] ► Protons are counted directly by titration with Na+, CH3 groups via dimethyl ether, and 2,6-di-tert-butylpyridine. ► There are fewer protons than framework aluminum atoms on FAU zeolites. ► Na+ stoichiometrically replaces H+ sites that are similar in reactivity and acid strength. ► Thermal treatments do not create Brønsted acid sites of increased strength. ► Thermal and chemical treatments influence supercage void size and solvation properties. Kinetic, spectroscopic, and chemical titration data indicate that differences in monomolecular isobutane cracking and dehydrogenation and methanol dehydration turnover rates (per H+) among FAU zeolites treated thermally with steam (H-USY) and then chemically with ammonium hexafluorosilicate (CD-HUSY) predominantly reflect differences in the size and solvating properties of their supercage voids rather than differences in acid strength. The number of protons on a given sample was measured consistently by titrations with Na+, with CH3 groups via reactions of dimethyl ether, and with 2,6-di-tert-butylpyridine during methanol dehydration catalysis; these titration values were also supported by commensurate changes in acidic OH infrared band areas upon exposure to titrant molecules. The number of protons, taken as the average of the three titration methods, was significantly smaller than the number of framework Al atoms (Alf) obtained from X-ray diffraction and 27Al magic angle spinning nuclear magnetic resonance spectroscopy on H-USY (0.35 H+/Alf) and CD-HUSY (0.69 H+/Alf). These data demonstrate that the ubiquitous use of Alf sites as structural proxies for active H+ sites in zeolites can be imprecise, apparently because distorted Al structures that are not associated with acidic protons are sometimes detected as Alf sites. Monomolecular isobutane cracking and dehydrogenation rate constants, normalized non-rigorously by the number of Alf species, decreased with increasing Na+ content on both H-USY and CD-HUSY samples and became undetectable at sub-sto