The quantitative prediction of rates is a grand challenge towards a better understanding of how confinement influences entropic effects and ultimately its impact on the rates of these reactions. Zeolites are one of the most promising solid acid catalytic media for chemical conversion such as alcohol dehydration alcohols to alkenes. 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. In this talk, we present the structure and thermochemistry of ethanol adsorption on the Brønsted acid site of H-ZMS-5 by means of ab initio molecular dynamics (AIMD) simulations in direct comparison with complementary in situ infrared (IR) spectroscopy and thermochemical measurements on the same material. A series of simulations were performed representing the different ethanol and water loadings used for the experimental studies (with/without deuterium substitution) at different temperatures (100 K ≤ T ≤ 700 K). Our approach enabled 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 content increases. In the simulations with higher ethanol loading, proton transfer occurs via a relay between H-bonded ethanol molecules. When water is present, 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, and showed a good agreement with available experimental measurements. Conversely, the more commonly employed harmonic approximation led to free energy estimates that deviate substantially from experiment. 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.

Role of anharmonicity in the confinement effect in zeolites: Structure, spectroscopy and adsorption free energy

254th National Meeting (2017)

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