The properties of boralites studied by infrared spectroscopy

Abstract

Because of its small dimensions boron is three-coordinated in boralites and is situated in one face of a tetrahedral hole. The i.r. band of the B—O asymmetric stretch in BO₃ units is split into two maxima (at 1380 and 1405 cm^–1), thus suggesting that two kinds of boron sites exist in boralites. Boron becomes four-coordinated following adsorption of the electron-donor molecules NH₃, H₂O and pyridine, but not of benzene. Three kinds of OH groups (at 3680, 3720 and 3460 cm^–1) are formed following the subsitution of Na⁺ ions by protons. The 3720 cm^–1 OH groups act as Brønsted-acid sites in the reaction with pyridine. The concentration of Brønsted sites determined by pyridine sorption is comparable with theoretical values. The low strength of the Brønsted-acid sites (illustrated by pyridine thermodesorption experiments) can be explained by the large separation and weak interaction between boron and the OH groups in the O₃B⋯OH–SiO₃ units. The electrostatic field of the Lewis-acid sites in boralites is stronger than that in zeolites.