Characterization of the acidity of ultrastable Y, mordenite, and ZSM-12 via NH 3-stepwise temperature programmed desorption and Fourier transform infrared spectroscopy

Abstract

The acidity of ultrastable Y (USY), mordenite, and ZSM-12 of variable Si/Al ratios has been characterized by coupling NH 3-stepwise temperature programmed desorption (STPD) and FT-IR. The former technique allows us to quantify accurately the ammonia chemisorbed on acid sites of different strengths. In contrast to other acidic oxide catalysts, it was found that protonated zeolites possess distinct limits of acid strength and that chemisorbed ammonia desorbs from each type of site only within particular temperature ranges. It was observed that for zeolites with Si/Al ratios smaller than about 20 the number of ammonia molecules chemisorbed on both Lewis and Brønsted sites is smaller than the total number of Al atoms present in the zeolite. However, an increase in the Si/Al ratio above this threshold results in a 1:1 relation between the total number of acid sites and the amount of ammonia adsorbed. It was found that for mordenite and ZSM-12 an increase of the dealumination severity results in a decrease of the acid site strength. For USY the acid strength decreased with steam treatment, but then increased with acid leaching. The FT-IR studies revealed that for the protonated USY, Lewis-bound ammonia is evolved within each temperature stage. For mordenite, the Lewis sites are associated with desorption up to 250°C. The non-dealuminated ZSM-12 (Si/Al=35) sample possesses a relatively small number of Lewis sites, whereas the ZSM-12 samples with higher Si/Al ratios had only Brønsted acid sites. A peak deconvolution procedure was performed in order to decompose accurately the complex hydroxyl region of USY and mordenite and was used to separate the Lewis peak from structural vibrations for ZSM-12. As a result, important information concerning the nature of the aluminum species present in these zeolites (extraframework species, framework species present within the side pockets of mordenite, framework species present within the β cages of USY, etc.) was obtained.