QUANTUM CHEMICAL MOLECULAR-MODELS OF OXIDES .2. METHANOL ADSORPTION ON SILICA AND ZEOLITES

Quantum chemical molecular models are used to interpret the IR spectra of methoxy species formed in methanol adsorption on silica and zeolites. The calculations are carried out by the ab initio SCF MO method with 3-21G basis set, using a scaling procedure to correct the ab initio C-H stretching frequencies. Comparison of the calculated and observed frequencies shows that at low coverage of silica the [GRAPHICS] structure is the most relevant among the physically adsorbed methanol species, while at high coverages the physically adsorbed methanol tends to form cyclic hydrogen-bonded polymers with the O3SiOH group participation. Assignment of the C-H stretching bands at 2980 and at 2870 cm-1 to the methoxy group -> SiO(CH3) ... Al <- in zeolites, suggested in the literature, is in large disagreement with the predicted vibrational frequencies for this structure. We suggest that these bands belong to strongly physically adsorbed methanol, stabilized on the bridging OH groups by two hydrogen bonds.