Mechanistic Insights into the Dealumination of an H-ZSM-5 Zeolite Using Reactive Molecular Dynamics Simulations

Abstract

Zeolites are among the most successful catalysts used in the petrochemical industry, as well as in more environment-friendly processes such as biomass conversion to fuels and chemicals. A major challenge in bioresource utilization is the presence of water, so addressing fundamental aspects of zeolite-water interactions is imperative. Among the different zeolite water-driven phenomena, dealumination is ubiquitous and affects several stages of the catalyst lifetime. To better understand this phenomena, this contribution investigates the dealumination mechanism of an H-ZSM-5 catalyst using reactive molecular dynamics simulations and implementing the ReaxFF force field. The strength of the approach used here relies on its capability to consider entropic and dynamic effects, which are usually ignored by the widespread static density functional theory (DFT) calculations that are often conducted on this kind of materials. Simulations were run at 1000, 1300, and 1600 K, and for H₂O:Al loads of 4:1, 6:1, and 8:1 during 1 ns using a constant number of molecules, pressure, and temperature (NPT ensemble). The results show a percentage of framework-associated aluminum (FAAl) between 40 and 50%, in good agreement with the experimental range reported in the literature of 30–40%. The mechanism of zeolite dealumination displayed three relevant dynamic features not reported in previous theoretical studies but that are supported by experiments: water-assisted proton transport, framework flexibility, and silicon-assisted aluminum dislodging. The dynamic chemistry of the dealumination also showed the most relevant step demonstrated by previous DFT studies, namely, the water adsorption and its subsequent dissociation to form silanol and adsorbed hydroxyl groups. This reaction is favored in the last stages of the dealumination and at high water loads. Static DFT calculations performed in this work and others reported in the literature indicate that water adsorption on the aluminum atom is spontaneous at high temperatures, including those above 1000 K, when the ideal gas phase reference state of water is modified to include effects of the zeolite environment, that is, enthalpy and entropy loses. Kinetic factors in heating protocols similar to calcination also appear to promote water adsorption and dealumination over transport to the gas phase, which should still be significant at temperatures as high as 1000 K. Besides, the structure of the identified extra-framework aluminum and FAAl matches the documented descriptions drawn from nuclear magnetic resonance spectroscopy analysis. The results of this work complement the body of research about zeolite dealumination aimed at the rational design of biomass conversion processes.