Roger Rousseau
Pacific Northwest National Laboratory, USA
Title: The role of anharmonicity in the confinement effect in zeolites: structure, spectroscopy and adsorption free energy of ethanol in h-zsm-5
Biography
Biography: Roger Rousseau
Abstract
Zeolite is one of 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, HZSM-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. In this talk, we present the structure and thermochemistry of ethanol adsorption on the Brønsted acid site of the HZMS-5 by means of ab inito molecular dynamics (AIMD) simulations directly compared with in-situ IR spectroscopy and thermochemical measurements on the same material. Simulations were performed using two different ethanol loadings (with/without deuterium substitution) at different temperatures (100 ≤ T ≤ 700). 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 ethanol occurs as temperature increases. In the simulations with higher ethanol loading, proton transfer occurs via relay between H-bonded ethanol molecules. Calculated projected vibrational density of states (VDOS) obtained from velocity autocorrelation function show a broad peak around 1600 cm-1 related to H-O-H bending mode which is also observed experimentally. We estimated entropy and enthalpy of adsorption using the computed VDSO along with a quasi-harmonic approximation, which shows good agreement with experimental measurement Conversely, the more commonly employed harmonic vibrations lead to free energy estimates that deviate from experiment substantially. Overall, this study exemplifies how enharmonic effects, as capture by AIMD, are critical for the quantitative modeling of the free energetics of zeolite-catalyzed processes.
