Zinc promoted H-ZSM-5 catalysts for conversion of propane to aromatics .2. Nature of the active sites and their activation

Zn/H-ZSM-5 catalysts, prepared by ion exchange (Zn-e) and impregnation (Zn-i) of a parent H-ZSM-5 as well as by physical mixing of ZnO with H-ZSM-5 (Zn-m) were characterized by TPR/H-2, TPSR/CO and TPDA. Their catalytic activity after thermal pretreatment in air or hydrogen was determined in the conversion of propane. Zinc species in Zn-i and Zn-e were hardly reducible. TPSR/CO revealed a generation of CO2 and H-2 explained as a reaction of [Zn(OH)](+) ions with CO. The corresponding TPSR/CO profiles indicated two peaks denominated as alpha- and beta-peak and attributed to [Zn(OH](+) ions located in the channels (alpha-species) or on the external surface of the zeolite crystals (beta-species). Residual ZnO, not reacting with CO was denominated as gamma-species, presenting ZnO outside the crystals. Zn-i and Zn-m 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 alpha-species and the activity has been demonstrated. TPSR/CO has proved to be a suitable new method for the determination of the active portion of zinc in Zn/H-ZSM-5 catalysts.