Correlations between Dispersion, Acidity, Reducibility, and Propane Aromatization Activity of Gallium Species Supported on HZSM5 Zeolites

Abstract

Gallium-modified ZSM5 zeolites, containing 2, 3, and 4 wt % Ga, were prepared by incipient wetness impregnation with gallium(III) nitrate solutions of HZSM5 samples with SAR values of 24 and 35. The modified zeolites were subjected to two consecutive treatment cycles of reduction in hydrogen and reoxidation in air. The catalysts were characterized by nitrogen physical adsorption, 29Si and 27Al MAS NMR, temperature-programmed reduction, temperature-programmed desorption of ammonia, X-ray absorption spectroscopy, and infrared spectroscopy in the OH stretching region and in the region of skeletal vibrations of adsorbed pyridine. Catalytic activity and selectivity were measured in propane conversion reactions at 703 K. Results were rationalized in terms of correlations among the framework Ga/Al ratio, density, strength and type of acidity, gallium cluster sizes, reducibility of gallium, and propane aromatization activity. It was found that the dispersion of the gallium species was a function only of the framework Ga/Al ratio. Lewis acidity and reducibility of the gallium species were positively correlated with each other and with their degree of dispersion. A correlation was found between propane aromatization activity and the product of the concentrations of strong Lewis and Brønsted acid sites in the oxide form of the catalysts.