Effect of hydrothermal treatment on catalytic properties of PtSnNa/ZSM-5 catalyst for propane dehydrogenation

Abstract

The effect of hydrothermal treatment on PtSnNa/ZSM-5 catalyst was analyzed during the dehydrogenation of propane with the presence of hydrogen. Fresh and hydrotreated catalysts were characterized by XRD, 27Al MAS NMR, nitrogen adsorption, NH 3-TPD, IR spectrum of adsorbed pyridine, TEM, TPR and hydrogen chemisorption, to determine the effect of hydrothermal treatment on catalytic acidity and metallic character. It was found that under the mild treatment, the pore volume and the average pore diameter of the catalyst increased. However, the opposite effect was observed with the increase of hydrothermal temperature and time. Acidity characterization showed that the hydrothermal treatment decreased the acid amount and weakened the acid intensity. Moreover, the intensity of Lewis acid sites decreased slightly from 400 °C to 550 °C, and important loss of acidity took place at 650 °C. Transmission electron micrographs and results of hydrogen chemisorption experiment indicated that the hydrothermal treatment had obvious impact on the dispersive status of Pt particles on the external surface of the catalyst. Sample hydrotreated at 650 °C experienced Sn species loss and Pt sintering strongly, which caused the activity decrease and selectivity modifications during the reaction. It was suggested that the dealumination of ZSM-5 was the original reason of the catalytic deactivation and the subsequent loss of Sn species was just the main reason of the crystallitic Pt sintering. Finally, a model was proposed for the influence of hydrothermal treatment on catalytic properties of PtSnNa/ZSM-5 catalyst for propane dehydrogenation.