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Identifying Active Sites, Quantifying Site Heterogeneity, and Revealing Site-Site Synergies in Zeolite and Zeotype Catalysts
November 21, 2019 @ 10:00 am - 11:00 am
Abstract
Zeolite HZSM-5, a member of the MFI family of structures, is an important catalyst from both practical application and fundamental structure-function perspectives. With known uses ranging from cracking, isomerization, and methanol-to-hydrocarbon chemistries, the traditional view of only one type of acidic Brønsted proton site in HZSM-5 has been called into question by recent experimental data on zeolites with relatively high framework Al content. Motivated by surprising increases in reaction rates observed for some hydrocarbon/HZSM-5 probe reactions in the presence of sub-stoichiometric amounts of water, a series of experiments involving catalyst synthesis, selective post-synthetic modifications, cation titrations, and advanced solid-state NMR techniques reveal that multiple Brønsted acid protons can exist in zeolites. The number and type of acid sites can be varied, with significant impacts on reactivity. In addition, crystalline framework Brønsted protons proximate to extra-framework or non-crystalline framework sites are the most reactive species, and when such species are selectively removed, catalyst activity decreases by an order of magnitude or more. Recently acquired data at ultra-high magnetic field strengths of 35.2 Tesla (1.5 GHz) suggest that the highly active sites form at partially-coordinated framework Al atoms, and that positive synergistic effects from water co-addition require their presence. Implications on catalyst synthesis, post-synthetic chemical modifications, thermal history, and responses to phosphorous addition will be discussed.