Title : Confinement effect of alcohols and water in MFI Zeolite from Ab Initio molecular dynamics simulations
Zeolites are one of the most promising solid acid catalysts for chemical conversion of renewable biomass-derived alcohols into fuels and chemicals through alcohol dehydration condensation and alkene oligomerizaiton. 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. One grand challenge is to understand how confinement and solvent influence entropic effects and ultimately impact the rates of these reactions. In this talk, I will present the structure and thermochemistry of ethanol and water adsorption on the Brønsted acid site of H-ZMS-5 by means of ab initio molecular dynamics (AIMD) simulations. Structural and spectroscopic properties obtained from simulations will be discussed and directly compared with in situ infrared (IR) and NMR spectroscopy. Entropic component of free energy is strongly influenced by the confinement and solvent effect. We will discuss entropy and enthalpy of adsorption estimated from the computed vibrational density of states (VDOS) using a quasi-harmonic approximation. Our results show a good agreement with available experimental measurement. This enables us to take into account enthalpic and entropic effects caused by the dynamics of the motion of the reaction intermediates. AIMD simulations show that hydrogen transfer from the zeolite scaffold to water/ethanol occurs as temperature and water/alcohol content increases. In the simulations with only ethanol g, proton transfer occurs via relay between H-bonded ethanol molecules. When water is present the excess proton becomes soluble at a mobile H3O+[H2O] cluster. Overall, this study exemplifies how anharmonic effects, as captured by AIMD, are critical for the quantitative modeling of the free energetics of zeolite-catalyzed processes.