Effects of Reducing and Oxidising Pre-Treatments on the State of Gallium in Ga/H-ZSM-5 Propane Aromatisation Catalysts

摘要

Various techniques have been used to identify the state and local environment of Ga species in Ga/H-ZSM-5 zeolite prepared by ion exchange. Small GaO(OH) particles (O ～ 4-5 nm) are present after synthesis at the outer surface of the zeolite, together with few isolated large particles (needles 0.5-2 μm long and 0.1-0.2 μm wide) of gallium oxide. The former are transformed upon calcination at 823 K into small Ga_2O_3 particles (O ～ 4-5 nm). Reducing/oxidising cycles at 823 K enable the migration of these Ga species into the zeolite, most likely as ion-exchanged GaO~+ species. Bifunctional sites, combining zeolite acid Bronsted sites and redox GaO~+ sites, enhance the propane dehydrogenation and aromatisation performances of the Ga/H-ZSM-5 catalyst.