Anharmonicity and Confinement in Zeolites: Structure, Spectroscopy, and Adsorption Free Energy of Ethanol in H-ZSM-5

To account for thermal and entropic effects caused by the dynamics of the motion of the reaction intermediates, ethanol adsorption on the Bronsted acid site of the H-ZSM-5 catalyst has been studied at different temperatures and ethanol loadings using ab initio molecular dynamics (AIMD) simulations, infrared (IR) spectroscopy, and calorimetric measurements. At low temperatures (T <= 400 K) and ethanol loading, a single ethanol molecule adsorbed in H-ZSM-5 forms a Zundel-like structure where the proton is equally shared between the oxygen of the zeolite and the oxygen of the alcohol. At higher ethanol loading, a second ethanol molecule helps to stabilize the protonated ethanol at all temperatures by acting as a solvating agent. The vibrational density of states (VDOS), as calculated from the AIMD simulations, are in excellent agreement with measured IR spectra for C2H5OH, C2H5OD, and C2D5OH isotopomers and support the existence of both monomers and dimers. A quasi-harmonic approximation (QHA), applied to the VDOS obtained from the AIMD simulations, provides estimates of adsorption free energy within similar to 10 kJ/mol of the experimentally determined quantities, whereas the traditional approach, employing harmonic frequencies from a single ground state minimum, strongly overestimates the adsorption free energy by at least 20 similar to 50 kJ/mol. This discrepancy is traced back to the inability of the harmonic approximation to represent the contributions to the vibrational motions of the ethanol molecule upon confinement in the zeolite.