February 15, 2024
Journal Article
Promotion of adsorptive and catalytic properties of zeolitic Brønsted acid sites by proximal extra-framework Si(OH)x groups
Abstract
Steric confinement in zeolites influences the catalytic conversion of alkanes. For zeolitic Brønsted acid sites, proximate extra-framework species introduce additional confinement to the pore constraints, enhancing the catalytic activity of alkane cracking. Although extra-framework alumina has been the most studied, here we report the element-specific impact of silica species. By grafting extra-framework silica species close to Brønsted acid sites in H-ZSM-5 zeolite, the binding of bases like pyridine and amines is strengthened via van der Waals interactions with their aryl or alkyl chains. Brønsted acid sites close to extra-framework silica achieve a higher reaction rate of protolytic cracking of n-pentane via enthalpic (unlike entropic, as with extra-framework alumina) stabilization of the transition state by 24–51?kJ?mol-1. The lower activation energy points to an earlier transition state than in the presence of extra-framework alumina, with a better stabilization of the carbonium ions in the transition state compared to the parent zeolite. Conceptual work was supported by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES), Division of Chemical Sciences, Geosciences and Biosciences (Impact of catalytically active centers and their environment on rates and thermodynamic states along reaction paths, FWP 47319). We gratefully acknowledge the Leibniz Supercomputing Center for funding this project by providing computing time on their Linux-Cluster. R.Z. is grateful to the Chinese Scholarship Council for financial support. We also acknowledge M. Iqbal and M. Neukamm for technical support concerning the nitrogen physisorption and elemental analysis, respectively.Published: February 15, 2024