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

Abstract

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.

Mechanistic Insight into the Framework Methylation of H-ZSM-5 for Varying Methanol Loadings and Si/Al Ratios Using First-Principles Molecular Dynamics Simulations

“…The obtained 2D free energy surface is then projected on to the difference between the two CVs. The phenomenological free-energy barrier is computed by the procedure proposed by Bučko et al ( 57 ) Further details on the methodology and case studies are provided in the work of Bailleul et al ( 58 )…”

Combined Ex and In Situ Measurements Elucidate the Dynamics of Retained Species in ZSM‐5 and SAPO‐18 Catalysts Used in the Methanol‐to‐Olefins Reaction

“…In the small‐pore materials, all the aromatics are retained in the cavities and their maximum size is limited to the cavity size (with tetracyclic aromatics being the largest species found in SAPO‐34 and SAPO‐18 catalysts) [41, 42] . The active species are normally monocyclic aromatics, with hexamethylbenzene being a key active species for SAPO‐34 catalysts, [43] whereas tetramethylbenzenes and pentamethylbenzenes are inferred to be the key active species in SAPO‐18 catalysts [44] . Other studies have revealed that bicyclic aromatics can be active species in SAPO‐34 catalysts at high reaction temperatures [39, 45] .…”

Meet the Women of Catalysis

“…Very recently, she is further developing research activities to simulate more realistic systems including spatial heterogeneities. She is also an elected member of the Royal (Flemish) Academy for Science and the Arts of Belgium …”