(392f) Molecular Dynamics Study of Ethanol Adsorption and Protonation in H-ZSM-5
Zeolitic materials, such as H-ZSM-5, offer promising perspectives for the catalytic conversion of renewable biomass-derived alcohols into fuels and chemicals. The first step in such acid catalyzed reactions is the protonation of the alcohol molecule. However, considering the very flat potential energy surface for the proton transfer between the alcohol molecule and the zeolite, finite temperature and dynamical effects must be considered to better understand the nature of adsorbed alcohols in zeolites. Moreover, for loosely bonded complexes in zeolites where there are many soft degrees of freedom, entropy losses calculated based on the harmonic oscillator approximation can be largely overestimated. In order to take into account thermal and entropic effects caused by the dynamics of the motion of the reaction intermediates, ethanol adsorption on the Brønsted acid site of the H-ZSM-5 catalyst has been studied at different temperatures and ethanol loadings using ab initio molecular dynamics (AIMD) simulations. At low temperatures (T ≤ 300 K), a single ethanol molecule adsorbed in H-ZSM-5 forms a Zundel-like structure where the proton is shared between the oxygen of the zeolite and the oxygen of the alcohol. In contrast, when the temperature is elevated to 500 K, the adsorbed ethanol starts presenting its protonated form. This transition occurs around 400 K. Interestingly enough, a second ethanol molecule helps to stabilize the protonated form of adsorbed ethanol at all temperatures by acting as a solvating agent. The projected vibrational density of states (VDOS), as calculated from the velocity autocorrelation function, shows a broad peak around 1600 cm-1 related to the H-O-H bending mode of the protonated ethanol, while the quasi-harmonic entropy estimates are in a good agreement with the experimental data. Overall, this study exemplifies how AIMD simulations can capture anharmonic effects and provide invaluable insights in zeolite-catalyzed processes.