Shape-Selectivity of Cyclopentenyl Cation Isomerization: Investigating Kinetic Control in Medium-Pore Zeolites

Abstract

We performed periodic density functional theory (DFT) calculations to study the dynamics of alkyl substituents on cyclopentenyl cations in medium-pore acid zeolites, H-ZSM-5 and H-ZSM-22. Our study seeks to shed light on how zeolite shape selectivity can influence key reaction intermediates in the methanol-to-hydrocarbon process, which can lead to both value-added products and coke species. We considered the isomerization of 4-ethyl-4,5,5-trimethylcyclopentenyl cation (A), which has been shown by Hernandez et al. [ACS Catal.2021, 11, 12893–12914] to lead to IR spectra and alkyl substitution patterns that vary with zeolite pore structure, in contrast to DFT-predicted thermodynamics. Here, we investigate the role of kinetic control on zeolite shape-selectivity by computing exhaustive DFT dynamics of substituent rearrangement in medium-pore zeolites starting with cation A. We used the Rule Input Network Generator (RING) code to enumerate isomerization pathways from A to five product isomers that differ mainly in the presence or absence of an alkyl substituent on the central allylic carbon of the cyclopentenyl ring, yielding a reaction network with a total of 24 distinct species. We combined metadynamics and climbing-image nudged elastic band (CI-NEB) methods to compute the free-energy landscapes, including barriers for all species in H-ZSM-5 and H-ZSM-22 zeolites. Integrating kinetic equations for the reaction network in the two zeolites predicts that equilibrium product distributions are obtained after 103 s in H-ZSM-5, while 108–109 s is required for equilibration in H-ZSM-22, suggesting the clear possibility of kinetic control depending on zeolite structure.