The aim of a molecular modeling study is often to calculate accurate reaction kinetics of elementary steps. In this respect, one often calculates free energy barriers, which are related with rate coefficients via the Arrhenius law. Understanding trends in free energy barriers can be guided by a deconvolution into individual enthalpic and entropic contributions. When considering catalysis in confined space such as in a zeolite, entropic contributions are of paramount importance, especially since many zeolite-catalyzed reactions occur at high temperature. Nonetheless, entropic effects are not well understood as they are difficult to measure or calculate. When using microkinetic models, accurate enthalpy and entropy barriers for each reaction step are required. These arguments underline the necessity of an accurate determination of entropic contributions when modeling catalytic reactions at operating conditions.<br/><br/>Our goal is to study zeolite-catalyzed reactions at operating conditions based on a molecular dynamics (MD) approach as this allows to go beyond the often applied Harmonic Oscillator (HO) approximation. The latter is based on results at 0 K and does not properly account for finite temperature effects and the anharmonic character of the potential energy surface. As such, the HO approximation might no longer be valid at operating conditions. Moreover, this approach fails in accounting correctly for the contribution of collective motions characterized by low vibrational modes. In our study we benchmark several advanced entropy sampling methods, encompassing metadynamics with the potential energy as reaction coordinate and methods relying of the vibrational density of states (VDOS) computed from MD simulations. We apply these methods to elementary reaction steps in the framework of the conversion of methanol into hydrocarbons.

Entropy sampling for zeolite-catalyzed reactions at operating conditions

253rd National Meeting (2017)

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