Heterogeneity of OH groups in HZSM-5 zeolites: splitting of OH and OD bands in low-temperature IR spectra

Abstract

The IR band of hydroxyl groups in HZSM-5 zeolites is split into five sub-maxima. The splitting was observed in low-temperature (170 K) spectra of HZSM-5 with Si/Al > 50. The OH/OD exchange distinctly improves the quality of the hydroxyl spectrum and makes the splitting more easily seen. We attribute the splitting of the hydroxyl band to the heterogeneity of the hydroxyl groups in HZSM-5 and the presence of hydroxyls of various acid strength. The heterogeneity of OH groups is explained by the presence of (SiO) 3Si–OH–Al(OSi) 3 groups of various bridge geometries.