(691e) Computational Modeling of Bimolecular Hydrocarbon Transformations in Brønsted-Acid Zeolites

Hydride transfers and β-scission are key elementary reactions prevalent in solid-acid-catalyzed hydrocarbon transformations. Bimolecular in nature, they feature relatively bulky transition states (TS) and different zeolites are known to display significantly different reactivities towards these reactions due to the intricate interactions between the transition-state complexes and the unique active-site environment in each zeolite. These features, however, make it a challenging task to model them in a consistent and predictive manner. To address this challenge, we have recently developed a method that uses force field-based sampling to generate quasi-TS complexes, followed by semi-automated TS refinement based on ab-initio density-functional theory (DFT) calculations.¹ The method was demonstrated on the protolytic cracking of n-butane inside the TON, MFI, LTA, and FAU zeolites. Here, we present a comprehensive study of hydride transfer and β-scission reactions across different active sites in these four zeolites, for linear and branched reactants containing 4–6 carbon atoms. Compared to monomolecular TS in protolytic cracking, substantial variations were observed for the bimolecular mechanisms, with values ranging from 0–95 kJ/mol for β-scission and 0–37 kJ/mol for hydride transfers. Further considering the reactant-state energies, ensemble-averaged intrinsic barriers can be computed, which are found to be dominated by a few sites with the lowest TS energies.

1. Malviya, S. & Bai, P. Computational Investigation of Site-Dependent Activation Barriers of Zeolite-Catalyzed Protolytic Cracking Reactions. ACS Catal. 13, 179–190 (2022).