Density functional theory calculations of adsorption and reactivity of methanol at alumino-silicate Brønsted acid centres

Abstract

First-principles density functional theory calculations have been carried out in order to elucidate the reaction path for formation of surface methoxy species from methanol at a zeolitic Brønsted acid site; a possible key step in methanol to gasoline conversion. The reaction was studied using both valence double zeta (DZVP) and valence triple zeta (TZVP) basis sets within the BLYP approximation. In addition, an extensive study of the methanol and water dimers was also made in order to gain an insight into the quality of the method and the basis sets for calculations of binding energies, dipole moments, vibrational and NMR properties. The reaction mechanism, that of insertion of a conjugate base oxygen into the C–O bond of methanol, was shown to proceed via a higher-energy activation barrier (230–240 kJ mol ^-1 ) than mechanisms previously studied although inclusion of long-range electrostatic effects (neglected in the current cluster calculations) is expected to reduce this barrier. The nature of the methanol–acid site adsorption complex is discussed in detail. It is concluded that the experimental IR and NMR spectra are dominated by responses due to physisorbed methanol in which there is one strong hydrogen bond between the Brønsted hydroxy group and the methanol oxygen, and one weak hydrogen bond between the methanol hydroxy group and a lattice oxygen.