Zinc promoted H-ZSM-5 catalysts for conversion of propane to aromatics II. Nature of the active sites and their activation Original Research Article

ZnH-ZSM-5 catalysts, prepared by ion exchange (Zne) and impregnation (Zni) of a parent H-ZSM-5 as well as by physical mixing of ZnO with H-ZSM-5 (Znm) were characterized by TPRH2, TPSRCO and TPDA. Their catalytic activity after thermal pretreatment in air or hydrogen was determined in the conversion of propane. Zinc species in Zni and Zne were hardly reducible. TPSRCO revealed a generation of CO2 and H2 explained as a reaction of [Zn(OH)]+ ions with CO. The corresponding TPSRCO profiles indicated two peaks denominated as α- and β-peak and attributed to [Zn(OH)]+ ions located in the channels (α-species) or on the external surface of the zeolite crystals (β-species). Residual ZnO, not reacting with CO was denominated as γ-species, presenting ZnO outside the crystals. Zni and Znm samples could be activated by pretreatment in hydrogen. These samples also exhibited a self activation if they were mildly calcined before the testing. A correlation between the content of α-species and the activity has been demonstrated. TPSRCO has proved to be a suitable new method for the determination of the active portion of zinc in ZnH-ZSM-5 catalysts.