Acid properties of mazzite zeolites studied by IR spectroscopy

Abstract

The adsorption of ammonia, pyridine and benzene on non-dealuminated (Si/Al = 4.4) and dealuminated (Si/Al = 10 and 30) mazzite samples was followed by FT-IR spectroscopy. The dealumination of mazzites (by steaming and acid treatment) made all their narrow channels accessible to pyridine. Pyridine and ammonia adsorption and desorption showed that the broad OH band at approximately 3600 cm −1 was composed of several submaxima corresponding to hydroxyl groups of various localizations and various acid strengths. With all the mazzite samples, the concentrations of Brönsted acid sites determined by ammonia adsorption were comparable with the values calculated from the chemical composition of zeolites (amounts of Al minus amount of Na) whereas those determined from pyridine adsorption were lower than these values. The greatest difference was found for the non-dealuminated MAZ-4.4, in which pyridine could not reach the sites of the narrow channels. The values of the extinction coefficients corresponding to the OH groups located in the large channels were higher than those corresponding to the OH groups of the narrow channels, thus showing the stronger acidity of the former OH groups. The acid strength of OH groups increased with the dealumination, as seen from the increase in the corresponding extinction coefficient and frequency shift Δν due to the hydrogen bonding of hydroxyl groups with benzene. Furthermore, the protonic acidity of the dealuminated mazzite of Si/Al = 30 was found to be stronger than that of mordenite and beta zeolite with comparable Si/Al.