(569dg) Deconvoluting the Role of Extra-Framework Cations in MFI Zeolite

Zeolites are a commonly used catalyst for the petroleum refining industry due to their acidity and high density of active sites. High temperature and pressure treatment of zeolites can cause dealumination, removing acid sites from the catalyst while also generating extra framework aluminum (EFAl) species. EFAl located at a synergistic site has been shown to influence the product selectivity in catalytic cracking and dehydrogenation; however, the mechanism behind the influence of extra framework species remains to be explored. It has been proposed that these species provide a confining environment with increased van der Waals (vdW) and/or entropic stabilization, potentially altering the acid strength of the Brønsted acid sites (BAS) or stabilizing the charged transition states through electrostatic interactions. In this study, density functional theory (DFT) calculations are used to separate several energetic contributions, namely vdW stabilization, structural deformation, and electronic interactions in intrinsic activation barrier for acid-catalyzed dehydrogenation and catalytic cracking of propane. Calculations are performed for extra-framework cations hosted at a synergistic site with varied distances between the reacting BAS and the synergistic site, the results of which are compared to the calculations for an isolated BAS. We find that EFAl, in certain configurations and at an ideal distance from the active site, can stabilize transition states through electrostatic interactions. The enhancing effect is highly sensitive to their location in the zeolite framework; transition states may be destabilized when the external species is either too close or too far from the BAS active site. This work provides fundamental insights into the role of extra-framework cations in MFI zeolite and deconvolutes the energetic contributions involved.