Spectra of hydroxyl groups in zeolites in the near-infrared region

Abstract

Diffuse reflectance ir spectra of hydroxyl groups in decationized and cationic forms of zeolites, X, Y, and mordenite have been studied for the first time in a wide spectral range, including the fundamental stretching vibrations of OH bonds, their overtones and combination bands. Surface OH groups of amorphous aluminosilicate and silica gel have also been investigated. The combination bands of stretching and bending vibrations of OH groups are shown to be more sensitive to the environment of the hydroxyls than are the stretching vibrations (fundamental and overtones). The observation has been made that in the supercages of X-zeolite there are two different types of structural hydroxyls, which are not resolved in the fundamental region of the spectra, but differ both in their bending and combined frequencies. Cations introduced into zeolites by ion exchange influence the structural OH groups, resulting in a shift of their combination bands, in comparison with decationized samples, to lower or higher frequencies, depending on the nature of the cation and on the zeolite structure. The change in the bending frequencies of different OH groups is considered to be connected with the strength of their coordination to the neighboring aluminum atoms. The Morse function parameters of the potential curves x e and v e and the isotopic effects of OH and OD groups in zeolites are calculated. The applicability of these data to characterize the acidic properties of OH groups is discussed.