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 ZSM5-like silicalite, namely, free and hydrogen-bonded -->SiOCH3 groups, which are in thermodynamic equilibrium. Upon decreasing the temperature, the relative concentration of the free -->SiOH groups increases due to the -->SiOH transition from the hydrogen-bonded -->SiOCH3 groups into hydrogen-bonded -->SiOH aggregates. Physically adsorbed methanol forms large hydrogen-bonded polymers, even at high concentration of unoccupied -->SiOH 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 -->SiOH 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.