Silylation of H-ZSM-5: an X-ray photoelectron and infrared spectroscopy study

H-ZSM-5 zeolite has been silylated using hexamethyldisilazane, trimethylchlorosilane and triphenylchlorosilane as the silylating agents. The acidity of silylated samples was further detected by both FTIR spectroscopy and XPS of chemisorbed pyridine. The results suggest that the removal of a Brønsted acid site produces a framework trigonal Al atom, which is the Lewis acid site in H-ZSM-5 zeolites. Compared with the infrared spectra, the XP spectra of chemisorbed pyridine are more sensitive in distinguishing Brønsted acid sites of different strengths: i.e., the strong and weak Brønsted acid sites. The removal of external silanol groups does influence the distribution of surface acid sites by increasing the proportion of strong surface Brønsted acid sites.