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

The methylation of hexamethylbenzene with methanol is one of the key reactions in the methanol‐to‐olefins hydrocarbon pool reaction cycle taking place over the industrially relevant H‐SAPO‐34 zeolite. This methylation reaction can occur either via a concerted or via a stepwise mechanism, the latter...

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Vydáno v:ChemCatChem Ročník 11; číslo 16; s. 3993 - 4010
Hlavní autoři: Bailleul, Simon, Rogge, Sven M. J., Vanduyfhuys, Louis, Van Speybroeck, Veronique
Médium: Journal Article
Jazyk:angličtina
Vydáno: Weinheim Wiley Subscription Services, Inc 21.08.2019
Témata:
Alkenes
density functional theory
enhanced sampling
First principles
Free energy
H-SAPO-34
Hydrogen bonding
Methanol
Methylation
Molecular dynamics
Reaction mechanisms
Sampling methods
water
Water chemistry
Zeolites
ISSN:1867-3880, 1867-3899
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Popis
Shrnutí:The methylation of hexamethylbenzene with methanol is one of the key reactions in the methanol‐to‐olefins hydrocarbon pool reaction cycle taking place over the industrially relevant H‐SAPO‐34 zeolite. This methylation reaction can occur either via a concerted or via a stepwise mechanism, the latter being the preferred pathway at higher temperatures. Herein, we systematically investigate how a complex reaction environment with additional water molecules and higher concentrations of Brønsted acid sites in the zeolite impacts the reaction mechanism. To this end, first principle molecular dynamics simulations are performed using enhanced sampling methods to characterize the reactants and products in the catalyst pores and to construct the free energy profiles. The most prominent effect of the dynamic sampling of the reaction path is the stabilization of the product region where water is formed, which can either move freely in the pores of the zeolite or be stabilized through hydrogen bonding with the other protic molecules. These protic molecules also stabilize the deprotonated Brønsted acid site, created due to the formation of the heptamethylbenzenium cation, via a Grotthuss‐type mechanism. Our results provide fundamental insight in the experimental parameters that impact the methylation of hexamethylbenzene in H‐SAPO‐34, especially highlighting and rationalizing the crucial role of water in one of the main reactions of the aromatics‐based reaction cycle. Role play: The methylation of hexamethylbenzene with methanol in the industrially relevant H‐SAPO‐34 zeolite is systematically investigated here, focusing on the influence of the complex reaction environment via realistic water loadings and concentrations of Brønsted acid sites. First principle molecular dynamics simulations are performed using enhanced sampling methods to characterize the reactants and products in the catalyst pores. The constructed free energy profiles reveal that dynamic sampling is prerequisite to take the stabilization of the product state through hydrogen bonding and mobility of water into account.
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ISSN:1867-3880
1867-3899
DOI:10.1002/cctc.201900618