Characterization of zeolite acidity. An infrared study using ammonia, methylamine and n-butylamine

Abstract

Characterization of the acidity of an HY and a number of mordenite zeolites (SiO₂/Al₂O₃ ratios 8–26) has been studied by infrared spectroscopy using ammonia, methylamine and n-butylamine. The characteristic bands of adsorbed ammonia appeared at 1445 cm^–1(Brønsted type) and 1623 cm^–1(Lewis type), with the disappearance of the acidic OH band at 3609 cm^–1 for mordenites and of the acidic OH bands at 3643 and 3568 cm^–1 for HY. When methylamine and n-butylamine were adsorbed on the catalysts the acidic hydroxyls were found to be accessible to the amines and a band of δ_sym(NH⁺₃)(RNH₂ adsorbed on a Brønsted site) appeared in the 1500–1540 cm^–1 region with a band at ca. 1610 cm^–1, which is a combination of RNH₂ on Lewis sites and δ_asym(NH⁺₃). Brønsted acidity of the catalysts as determined from the band intensities of the adsorbed ammonia, methylamine and n-butylamine, although not in numerical agreement, showed the same trend. The alkylamines were decomposed into alkenes at elevated temperatures and the decomposition was prominent in the case of n-butylamine.