Effect of La addition on catalytic performance of PtSnNa/ZSM-5 catalyst for propane dehydrogenation

Abstract

The addition of La can have obvious influence on catalytic performance of PtSnNa/ZSM-5 catalyst for propane dehydrogenation. It is found that suitable content of La can improve the catalytic stability and selectivity to propene effectively. However, the opposite effect is observed when the loading of La is excessive. In order to discuss the possible reasons, the catalysts are characterized by several techniques and particular emphasis is focused on the changes of catalytic acidity and character of Pt active sites by the modification of La. PtSnNaLa/ZSM-5 catalysts with different amounts of lanthanum were prepared by sequential impregnation method, and characterized by XRD, nitrogen adsorption, NH 3 -TPD, FT-IR, TPR, H 2 -TPD, XPS and TPO techniques. It was found that the La species were well dispersed on the internal and external surfaces of ZSM-5 zeolite. With a little addition of La, the catalyst acidity decreased slightly and the active sites of the catalyst were stabilized, which resulted in the increased catalytic selectivity and stability. In PtSnNaLa/ZSM-5 catalyst containing 1.4% La, the stability of catalyst reached the best state and the catalytic selectivity was at the maximum. The average yield of propene was about 35.2% over 80 h for the reaction of propane dehydrogenation at 590 °C. However, as the La content increased, the character of Pt active sites changed obviously and the amount of Lewis acid sites increased, which in consequence decreased the catalytic activity and selectivity. Results from XPS analysis indicated that the addition of La could inhibit the reduction of tin species. Temperature-programmed oxidation (TPO) experiments showed that suitable loading of La could prevent the catalyst from coking effectively, whereas the opposite effect was observed when the content of La was excess.