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.