ACID PROPERTIES OF MAZZITE ZEOLITES STUDIED BY IR SPECTROSCOPY

The adsorption of ammonia, pyridine and benzene on non-dealuminated (S i/Al=4.4) and dealuminated (Si/Al= 10 and 30) mazzite samples was foll owed by FT-IR spectroscopy. The dealumination of mazzites (by steaming and acid treatment) made all their narrow channels accessible to pyri dine. Pyridine and ammonia adsorption and desorption showed that the b road OH band at approximately 3600 cm(-1) was composed of several subm axima corresponding to hydroxyl groups of various localizations and va rious acid strengths. With all the mazzite samples, the concentrations of Bronsted acid sites determined by ammonia adsorption were comparab le with the values calculated from the chemical composition of zeolite s (amounts of Al minus amount of Na) whereas those determined from pyr idine adsorption were lower than these values. The greatest difference was found for the non-dealuminated MAZ-4.4, in which pyridine could n ot reach the sites of the narrow channels. The values of the extinctio n coefficients corresponding to the OH groups located in the large cha nnels were higher than those corresponding to the OH groups of the nar row channels, thus showing the stronger acidity of the former OH group s. The acid strength of OH groups increased with the dealumination, as seen from the increase in the corresponding extinction coefficient an d frequency shift Delta v due to the hydrogen bonding of hydroxyl grou ps 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. (C) 1998 Elsevier Science B.V.