Infrared spectroscopic investigation of hydroxy-group siting in H faujasites

Abstract

The total hydroxy-group and the separate high-frequency (h.f.) and low-frequency (l.f.) hydroxy-group concentrations in H faujasites, observed by i.r. spectroscopy and corroborated by other methods, show a dependence of the maximum occupation of h.f. OH groups on the Si/Al ratio of between 15 and 32 OH groups per unit cell for zeolites HY and HX, respectively. In the case of HX zeolites above 70% exchange, decomposition is unavoiable. As a result of band separation the separate amounts of seven kinds of h.f. OH subspecies have been obtained from measurements of their i.r. absorbance and the integrated extinction coefficients of the h.f. OH stretching band for HX and HY zeolites. These subgroups show the same i.r. sub-bands in both HX and HY zeolites, but of different intensities. This may be explained on the basis of different Si/Al ordering schemes by O₁H groups bonded to silicon atoms, surrounded by 0–4 Al atoms, and bonded to Al atoms, which are influenced by electrostatic interactions with neighbouring aluminium atoms.