Structure and catalytic properties of the Zn-modified ZSM-5 supported platinum catalyst for propane dehydrogenation

Zinc containing ZSM-5 zeolite was hydrothermally synthesized and then was used as support for platinum catalyst in propane dehydrogenation. To investigate the location and the influence of Zn concentration on the catalyst structure and the reaction performance, the prepared samples were studied by several techniques, including XRD, nitrogen adsorption, SEM, NH₃-TPD, TEM, hydrogen chemisorption and H₂-TPR. It was found that some parts of zinc species could be incorporated into the framework of ZSM-5 zeolite and the introduction of zinc at the synthesis resulted in the formation of the agglomerated crystal rods, which in consequence increased the specific surface area. Additionally, the presence of zinc poisoned the strong acidity of the zeolite evidently. Compared with the Zn-free support, the substitution of Zn in the support strengthened the interaction of platinum with support and increased the platinum dispersion effectively. In this case, relatively homogeneous distribution of metallic particles was found due to the "geometric effect" of Zn. Unlike the impregnation method, the substitution of Zn was more beneficial to reflect the modification effect of the promoter to the metal phase and support acidity. In our experiments, the PtNa/Zn(1.0%)-ZSM-5 catalyst exhibited the highest reaction activity and stability. Nevertheless, with the continuous increase of Zn amount, the metal character had been changed. The formation of PtZn alloy resulted in the loss of catalytic activity and stability, while had a promoting effect for the reaction selectivity.