Effect of Na addition on catalytic performance of PtSn/ZSM-5 catalyst for propane dehydrogenation

PtSnNa/ZSM-5 catalysts with different amounts of Na were prepared by the sequential impregnation method on HZSM-5 zeolites, and their catalytic activities and stabilities in propane dehydrogenation were investigated. The effect of the addition of sodium on the physicochemical properties was studied by means of XRD(X-ray diffration), IR spectrum of adsorbed pyridine, NH3-TPD, H-2 chemisorption, TPR(temperature-programmed reduction), etc. The results indicated that the addition of Na had an obvious impact on the catalytic performance of the PtSn/ZSM-5 catalyst. A suitable concentration of Na could not only reduce the Bronsted acid sites and moderate/strong Lewis acid sites, thus preventing the catalyst from coking and improving the catalytic stability, but also increase the fraction of bare metallic Pt on the surface of the catalyst, thus increase the catalytic activity. It was found that the propene selectivity and yield could reach the maximum, when the Na content was 1.0%(w). After reaction for 30 h, the propane conversion was still high (36.4%). However, the continuous addition of Na increased the weak Lewis acid sites and promoted the fin species to be reduced to metallic tin. In this instance, the cracking and hydrogenolysis of propane could be carried out easily, which was disadvantageous to the dehydrogenation.