Towards molecular control of elementary reactions in zeolite catalysis by advanced molecular simulations mimicking operating conditions

Abstract

Title: Towards molecular control of elementary reactions in zeolite catalysis by advanced molecular simulations mimicking operating conditions Advanced ab initio molecular dynamics techniques enable study of the infl uence of catalyst topology and acidity, reaction temperature and the presence of additional guest molecules on methylation reactions of benzene and propene, which are crucial reaction steps in the conversion of methanol into hydrocarbons. Our advanced simulations provide detailed insight into how operating conditions impact the competition of the concerted and stepwise methylation mechanism and thus the reaction kinetics. As featured in:See and mechanism of individual reaction steps at operating conditions. Herein we use state-of-the-art advanced ab initio molecular dynamics techniques to study the influence of catalyst topology and acidity, reaction temperature and the presence of additional guest molecules on elementary reactions. Such advanced modeling techniques provide complementary insight to experimental knowledge as the impact of individual factors on the reaction mechanism and kinetics of zeolite-catalyzed reactions may be unraveled. We study key reaction steps in the conversion of methanol to hydrocarbons, namely benzene and propene methylation. These reactions may occur either in a concerted or stepwise fashion, i.e. methanol directly transfers its methyl group to a hydrocarbon or the reaction goes through a framework-bound methoxide intermediate. The DFT-based dynamical approach enables mimicking reaction conditions as close as possible and studying the competition between two methylation mechanisms in an integrated fashion. The reactions are studied in the unidirectional AFI-structured H-SSZ-24, H-SAPO-5 and TON-structured H-ZSM-22 materials. We show that varying the temperature, topology, acidity and number of protic molecules surrounding the active site may tune the reaction mechanism at the molecular level. Obtaining molecular control is crucial in optimizing current zeolite processes and designing emerging new technologies bearing alternative feedstocks.

On the stability and nature of adsorbed pentene in Brønsted acid zeolite H-ZSM-5 at 323 K

“…This method has recently been applied successfully in various zeolite catalysis studies [63,70]. During an NVT MTD run with similar settings as for the MD simulations, Gaussian hills are added every 25 fs along two collective variables (CVs), described by coordination numbers (CN), which are able to describe the reaction coordinate for transformations between the various adsorbed species.…”

Insight into the Role of Water on the Methylation of Hexamethylbenzene in H‐SAPO‐34 from First Principle Molecular Dynamics Simulations

“…This reaction may occur either via a concerted mechanism in which methanol transfers its methyl group directly towards the aromatic or via a stepwise mechanism, both represented in Figure b. In this stepwise mechanism, methanol first reacts with the catalyst framework, forming a methoxide species, which then transfers its methyl group to the HP species . Literature suggests that both mechanisms occur within the zeolite and the prevailing mechanism largely depends on the topology and the operating conditions .…”

“…In this stepwise mechanism, methanol first reacts with the catalyst framework, forming a methoxide species, which then transfers its methyl group to the HP species . Literature suggests that both mechanisms occur within the zeolite and the prevailing mechanism largely depends on the topology and the operating conditions . The studies show that with increasing temperature and decreasing pressure the prevailing mechanism shifts from concerted to stepwise .…”

“…The studies show that with increasing temperature and decreasing pressure the prevailing mechanism shifts from concerted to stepwise . This effect is attributed to the entropic gain of the intermediate release of water during the stepwise mechanism …”

“…During the simulations the system adopts more realistic configurations in the pores of the material ( vide infra ). Thanks to these adaptations, the conclusions of Case 1 can be generalized to experimentally more realistic pore environments and the influence of the assisting effect of additional protic molecules on both methylation mechanisms can be assessed . For the system under study, molecular dynamics methods are of utmost importance to properly account for the dynamic interplay between the methylating agent, hexamethylbenzene and water.…”