Effect of clay binder on sorption and catalytic properties of zeolite pellets

Abstract

Sorption of N2, O2, Ar, and CH4 in zeolites X, Y, and mordenite in powder as well as pellets was studied using gas chromatography. Heats of sorption in pellets formed using clay as a binder increased by 34-78% for N2 and 7-18% for CH4 relative to the respective zeolite powder. Surface acidity of zeolites upon pelletization as measured by ammonia temperature-programmed desorption showed that high-strength acid sites completely disappear for both zeolite Y and mordenite when bound with bentonite and attapulgite. Decreases in weaker acid sites upon pelletization for mordenite and HY are 47-52% and 34%, respectively. Fourier transform infrared spectroscopy also showed a decrease in acidity upon pelletization of zeolite powders as observed from the reduction in the intensity of the peaks corresponding to acidic hydroxyl groups. Influence of the decrease in surface acidity upon pelletization was also observed from conversion values for alkylation of toluene with methanol in zeolite pellets. For example, reduction in total xylene formed was by 50%, 87%, and 100% for HY520, HY510, and HM510, respectively. These observations have been interpreted in terms of solid-state migration of cations present in clay interlayer space to zeolite extraframework sites during pelletization. Reexchange of cations in zeolites HY and HM with an aqueous solution of Na+/Ca2+ salts shows sorption isotherms closer to the respective pelletized samples. X-ray diffraction studies of zeolites in powders as well as pellets confirm migration of clay cations to zeolite extraframework sites.