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

Abstract

The methanol-to-hydrocarbon process is known to proceed autocatalytically in H-ZSM-5 after an induction period where framework methoxy species are formed. In this work, we provide mechanistic insight into the framework methylation within H-ZSM-5 at high methanol loadings and varying acid site densities by means of first-principles molecular dynamics simulations. The molecular dynamics simulations show that stable methanol clusters form in the zeolite pores, and these clusters commonly deprotonate the active site; however, the cluster size is dependent on the temperature and acid site density. Enhanced sampling molecular dynamics simulations give evidence that the barrier for methanol conversion is significantly affected by the neighborhood of an additional acid site, suggesting that cooperative effects influence methanol clustering and reactivity. The insights obtained are important steps in optimizing the catalyst and engineering the induction period of the methanol-to-hydrocarbon process.

Methanol loading dependent methoxylation in zeolite H-ZSM-5

“…In line with methanol loading dependent room temperature methoxylation experiments, recent simulations of methoxylation show that the energy barriers decrease from 160 to 119 kJ mol –1 when the loading is increased from 1 to 4 methanol molecules per Brønsted acidic site, respectively. 66 The energy barriers calculated are in line with the previous literature. 20 , 33 – 36 However, the energy barrier of 119 kJ mol –1 for 4 methanol molecules per acidic site remains significant in the context of a process observed at room temperature.…”

Layered double hydroxide membrane with high hydroxide conductivity and ion selectivity for energy storage device

“…However, it remains a challenge for the direct observation of the transport behavior of hydroxide in restricted interlayer gallery of LDHs under operating conditions, and thus confirming the conduction mechanism convincingly. In order to investigate the transport mechanism of OH − in LDHs layer, ab initio molecular dynamics (AIMD) simulation was explored, which is a powerful tool for providing the dynamic behavior of various species under operating conditions [40][41][42] . The optimized LDHs model was shown in Fig.…”

Concluding remarks: Reaction mechanisms in catalysis: perspectives and prospects

“…Metadynamics is a powerful tool, which allows the investigation of activated processes which would be inaccessible to conventional molecular dynamics, enabling free energy barriers to be calculated. A review of its application to catalytic processes is available from Van Speybroeck et al 6 A recent example is provided by the work of Nastase et al, 7 who applied metadynamical simulation techniques to the study of the reactivity of methanol in ZSM-5. Ab initio dynamics rst established that methanol clusters can deprotonate framework acid sites.…”