Zeolites are one of the most promising solid acid catalysts for the conversion of renewable biomass-derived alcohols into fuels and chemicals. Dehydration of alcohols to alkenes is a well-known prototypical acid catalyzed reaction, where confinement and entropic effects impact the rates of these reactions. For such conversions, H-ZSM-5 zeolite is commonly used as a platform for acid catalyzed reactions due to its strong acidity and enhancement of reaction rates due to confinement in pores. However, the reactions are strongly affected in aqueous environment. In this talk, we present the structure and thermochemistry of ethanol adsorption on the Brønsted acid site of H-ZMS-5 in the presence of water by means of ab initio molecular dynamics (AIMD) simulations directly. Simulations were performed using two different ethanol loadings with different H2O concentrations at different temperatures (300 K ≤ T ≤ 500 K). This enables us to take into account enthalpic and entropic effects caused by the dynamics of the motion of the reaction intermediates and water presence. AIMD simulations show that properties of ethanol adsorption and protonation are significantly modified compared to those without water in the system. We also observe that hydrogen transfer from the zeolite scaffold to water/ethanol occurs as temperature and water content increases. The excess proton becomes soluble in the form of a mobile H3O+. Estimates of entropy and enthalpy of adsorption were determined from the computed VDOS along with a quasi-harmonic approximation. This study also exemplifies how anharmonic effects, as captured by AIMD, are critical for the quantitative modeling of the free energetics of zeolite-catalyzed processes.

The role of anharmonicity in the confinement effect in zeolites: Structure, spectroscopy and adsorption free energy of ethanol on H-ZSM-5 in aqueous phase

255th National Meeting (2018)

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