Structure and Acidity Changes in Ultra-Stable Y Zeolites During Hydrothermal Aging: A Solid-State NMR Spectroscopy Study

doi:10.11938/cjmr20192745

Abstract

Abstract: Structure and acidity changes of the acid sites in ultra-stable Y zeolites of different cell sizes during the hydrothermal aging process were studied by solid-state nuclear magnetic resonance (NMR) combined with molecular probes. The experimental results demonstrated that, during the initial stage of hydrothermal aging, parts of the framework aluminum species were removed from the framework and formed five-coordinate extra-framework aluminum species. In the meanwhile, the five-coordinate extra-framework aluminum species migrated to the surface and formed poly-aluminum species, resulting in decreases of both Brønsted acid sites and Lewis acid sites. The acidity of ultra-stable Y zeolites changed considerably during the first 3 h of the aging process, and the change of Brønsted acid content was proportional to the unit cell dimension of the fresh zeolites agent. With the increase of aging time, the aluminum distribution tended to become stable, with little further changes in the amounts of Brønsted acid and Lewis acid sites. The synergy between Brønsted/Lewis acids in the ultra-stable Y zeolites existed during the entire aging process, resulting in a stably enhanced acidity for the Brønsted acid sites.

Key words: solid-state NMR, Brønsted acid and Lewis acid, probe molecules, ultra-stable Y zeolites, hydrothermal aging

CLC Number:

O482.53

GAO Xiu-zhi, ZHANG Yi, WANG Xiu-mei, ZHANG Zhi-hua, XU Guang-tong. Structure and Acidity Changes in Ultra-Stable Y Zeolites During Hydrothermal Aging: A Solid-State NMR Spectroscopy Study[J]. Chinese Journal of Magnetic Resonance, 2020, 37(1): 95-103.

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