Aromatization of propane over Ga-modified ZSM-5 catalysts

H-ZSM-5 was modified with gallium by the ion-exchange method and characterized with XRD, SEM, BET, NH₃-TPD, Py-IR, ICP, and XPS techniques. The influences of the Si/Al mol ratios and the pretreatment conditions of the Ga-modified ZSM-5 ((Ga-H)-ZSM-5) on their acidity, gallium species state, and aromatization performance of propane were investigated. It was found that the Si/Al mol ratio had significant effects on the catalyst acidity and the interaction of non-framework Ga species with zeolite, and hence, the catalytic activity and selectivity for propane aromatization. At 550℃ and WHSV of 1.0h^-1, (Ga,H)-ZSM-5 with a Si/Al mol ratio of 30 showed the highest catalytic activity. It was shown that Ga species improved the aromatics selectivity and inhibited cracking reactions. After being reduced in hydrogen, Ga₂O₃ was most likely transformed into Ga⁺ and GaH⁺₂ species, and migrated into the zeolite channels. Subsequent air-oxidation treatment of the reduced (Ga,H)-ZSM-5 led to formation of GaO⁺ species, thus greatly increasing the aromatization activity.