Mechanisms of Methanol Adsorption on Silicalite and Silica - Ir-Spectra and Ab-Initio Calculations

Combined IR and ab-initio studies suggest the presence of two chemisorbed species of methanol on ZSMS-like silicalite, namely, free and hydrogen-bonded 9SiOCH3 groups, which are in thermodynamic equilibrium. Upon decreasing the temperature, the relative concentration of the free 9SiOH groups increases due to the >SOH transition from the hydrogen-bonded 3SiOCH3 groups into hydrogen-bonded 3SiOH aggregates. Physically adsorbed methanol forms large hydrogen-bonded polymers, even at high concentration of unoccupied S i O H groups. The CH frequencies of the chemisorbed and physically adsorbed species on silicalite are equal to those on silica, indicating the identity of these forms on both solids. Owing to the extremely high local concentration of 3SiOH groups in atomic-scale defects in silicalite, the effect of the reversible transformation of two chemisorbed forms to each other, on changing the temperature, is more pronounced for this solid than for silica.