DENSITY-FUNCTIONAL THEORY CALCULATIONS OF ADSORPTION AND REACTIVITY OF METHANOL AT ALUMINOSILICATE BRONSTED ACID CENTERS

First-principles density functional theory calculations have been carr ied out in order to elucidate the reaction path for formation of surfa ce methoxy species from methanol at a zeolitic Bronsted acid site; a p ossible 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 exten sive 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 b ase oxygen into the C-O bond of methanol, was shown to proceed via a h igher-energy activation barrier (230-240 kJ mol(-1)) than mechanisms p reviously studied although inclusion of long-range electrostatic effec ts (neglected in the current cluster calculations) is expected to redu ce this barrier. The nature of the methanol-acid site adsorption compl ex is discussed in detail. It is concluded that the experimental IR an d NMR spectra are dominated by responses due to physisorbed methanol i n which there is one strong hydrogen bond between the Bronsted hydroxy group and the methanol oxygen, and one weak hydrogen bond between the methanol hydroxy group and a lattice oxygen.