A FTIR study of silanol nests over mesoporous silicate materials

IR spectroscopy and DFT calculation were used to investigate the adsorption of water and toluene on aerosil calcined at a temperature of 250 degrees C as well as on silica produced by high-power electron-beam evaporation. It was shown that isolated SiOH groups (3746-3750 cm(-1)) on the surface of silicate materials weakly retain water and are easily displaced by toluene. An amount of adsorbed water raises slightly with vapor pressure. On the surface of aerosil there exist silanol nests that are characterized by two pairs of absorption bands: 3540, 3730 cm(-1) and 3657, 3740 cm(-1), which correspond to stretching vibrations of isolated and H-bonded hydroxyl groups, respectively. Vicinal OH groups in the nests adsorb molecular water according to the stoichiometry 2H(2)O: 1SiOH even at low water pressures. Owing to a higher acidity of the silanol nests, water is adsorbed much stronger, so that toluene cannot displace them. As a result, it was explained why MFI zeolites with a low silicate modulus are more hydrophobic than similar zeolites with a high modulus. Finally, a reliable spectral criterion of silanol nests existence in silicate materials was established; it consists in the presence of a pair of bands in IR spectra: a broad band in the region below 3600 cm(-1) and a narrow band in the range of 3700-3740 cm(-1).