On the Role of Gallium for the Aromatization of Lower Paraffins with Ga-Promoted ZSM-5 Catalysts

H–ZSM-5 showed a high selectivity of propane and low selectivity of aromatic hydrocarbons in butane conversion at 450°C, whereas Ga-supportd ZSM-5 (Ga–ZSM-5) showed higher selectivities of aromatic hydrocarbons and hydrogen. A small amount of oxygen in the feed promoted butane conversion to aromatic hydrocarbons on H–ZSM-5. Hybrid catalysts composed of the physical mixture of Ga/Al2O3 and H–ZSM-5 showed comparable activities and aromatic selectivities to those of the Ga-supported ZSM-5 zeolite (Ga–ZSM-5). Hydrogen was adsorbed quickly on the Ga–ZSM-5 and on the hybrid catalyst at 400°C and the adsorbed hydrogen was desorbed at around 450°C during the temperature programmed desorption experiment from both catalysts. However, little hydrogen was adsorbed on H–ZSM-5 and was desorbed from it. It was pointed out that the supported Ga promoted the zeolite-catalyzed aromatization of lower paraffins by promoting hydrogen desorption via the “reverse spillover” effecct.