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(Bronsted 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 δsym(NH+3)(RNH2 adsorbed on a Bronsted 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 δasym(NH+3). Bronsted 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.