Interaction of Methanol with Alkali Metal Exchanged Molecular Sieves. 2. Density Functional Study

Abstract

The structure and stability of various methanol adsorption complexes on alkali metal exchanged zeolites was studied by a density functional method using cluster models of six-rings. In particular, O-bound species coordinated only by the oxygen atom to the metal cation were compared to O,H-bound species coordinated in addition with a hydrogen bond of the methanol proton to a zeolite oxygen center. The coordination bond between the methanol oxygen atom and the metal cation furnishes the main contribution to the interaction of the adsorbate with sodium- or potassium-exchanged zeolites (40−55 kJ/mol), while the hydrogen bond contributes in addition about 15 kJ/mol. The hydrogen bond between the methanol hydroxyl group and a zeolite oxygen center significantly influences the OH stretching frequency in the IR spectra of adsorbed methanol. The calculated red shift of the OH frequency depends strongly on the proton affinity value of the zeolite oxygen center; for a change of the proton affinity by 54 kJ/mol it in...