Density Functional Theory Calculations of the Activation of Methanol by a Broensted Zeolitic Proton

Abstract

Density functional theory is used to determine transition states, adsorption, and dissociative complexes of Bronsted- acid-activated methanol. The respective activation barriers and adsorption and desorption energies for the reactions of hydrogen exchange and dehydration of methanol are presented. The activation barriers were found to be 11 and 212 kJ/mol for hydrogen exchange and dehydration, respectively. The methoxonium ion intermediate of the hydrogen exchange reaction was found to be a transition state corresponding to a maximum in the potential energy surface, rather than a chemisorbed species. The dehydration reaction forms a methoxy group that is a methyl group surface-bonded to the basic oxygen lattice. An analysis of the equilibrium constants shows that for both reactions methanol will adsorb initially with the hydroxyl group directed to the basic oxygen of the zeolite cluster model, perpendicular to the zeolitic surface (end-on). The dehydration reaction proceeds via a fast equilibration between this first mode of adsorption (end-on) and an adsorption mode where now the methyl group is directed to the basic oxygen of the zeolite cluster, parallel to the zeolite surface (side-on). From the calculated activation barrier and vibrational, rotational, and translational partition functions, reaction rate constants have been evaluated using transition state reaction rate theory