Ab initio study of methanol and ethanol adsorption on Brønsted sites in zeolite H-MFI.

Abstract

We examine the interaction of methanol and ethanol with a bridging OH group of H-MFI (Al12-O20(H)-Si3 site). The computational standard approach for molecule-surface interaction uses density functional theory with inclusion of dispersion for energies and harmonic vibrational frequencies for entropies and finite temperature effects for enthalpies. At 300 K, this yields -117 and -135 kJ mol-1 for adsorption enthalpies of methanol and ethanol, respectively, and 59 and 61 kJ mol-1, respectively for their entropy terms -T·ΔS. To reach chemical accuracy (±4 kJ mol-1) we go beyond this approach. The energies are calculated using a hybrid QM:QM scheme (QM - quantum mechanics) which combines plane-wave density functional theory accounting for the periodicity of the system with wave function-based methods (Møller-Plesset perturbation and Coupled Cluster theories). Finite temperature and entropy contributions are calculated from anharmonic vibrational partition functions. This yields as final predictions for methanol and ethanol -84 and -104 kJ mol-1, respectively, for the enthalpies of adsorption, 56 and 48 kJ mol-1, respectively, for the -T·ΔS term, and -28 and -56 kJ mol-1, respectively, for the Gibbs free energies at 300 K.