Identification of diverse silanols on protonated ZSM-5 zeolites by means of FTIR spectroscopy

The infrared spectral region corresponding to the vibrations of the silanols of ZSM-5 zeolites has been carefully examined. Four different kinds of silanols were identified: two inner species, located in the channels, and two species corresponding to external sites, at the boundaries of the crystallites. The behavior of these species with thermal treatment and pyridine adsorption was investigated. On the zeolites as received, the outer species are exclusively geminal silanols, a new identified OH group, which convert to free silanols with thermal treatment. One of the species shows also Brønsted acid properties.