(681f) Mechanistic Studies of Roles of Extra-Framework Species in MFI Zeolite on Catalytic Cracking of Propane

Catalytic cracking is a key reaction in refining industry and is often catalyzed using microporous acid catalysts, specifically, silicon/aluminum catalysts known as zeolites. Within zeolites the framework aluminum atoms are charge balanced by Bronsted acid sites, while rare earth species are introduced to increase the hydrothermal stability of the structure. These extra framework species also have an impact on the reaction’s turnover frequency and selectivity. However, the exact role of these extra framework species (e.g., Al, Si, La,...) remains unclear and under debate. In this study, we report density functional theory calculations to reveal the specific roles of extra framework aluminum, lanthanum, and cerium (Ce³⁺) within MFI zeolite using propane cracking as a probe reaction. The enthalpy and entropy of each step is calculated. We find that the presence of extra framework species stabilizes the transition states by redistributing the charges of the protonated transition states and that such an electronic effect can be tuned by changing the nature of the extra framework structures. This work thus provides fundamental insights for designing the active sites and their local environment for promoting the cracking activity and selectivity (cracking with respect to dehydrogenation).