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

Abstract

PtSnNaLa/ZSM-5 catalysts with different amounts of lanthanum were prepared by sequential impregnation method, and characterized by XRD, nitrogen adsorption, NH₃-TPD, FT-IR, TPR, H₂-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.

Introduction

Bimetallic platinum–tin catalysts are widely used in the process of propane dehydrogenation. It is known that the catalytic dehydrogenation of propane is of increasing importance because of the growing demand for propene [1], [2]. However, the deactivation of the catalyst due to coke formation is inevitable because of the rigorous reaction conditions [3]. Therefore, many efforts have been made to improve the catalytic selectivity to propene and the resistance to coke deactivation. It has reported that introducing a third metal is a feasible way to solve the problems [4], whereas the stability of this catalyst is still relatively poor. Recent studies have investigated the support effect on catalytic behavior of Pt–Sn catalysts for propane dehydrogenation, supports such as silicon nitrides, clinoptilolite zeolites and CIT-6 have been studied [5], [6], [7].

ZSM-5 zeolite is a member of crystalline microporous aluminosilicates with low aluminium content. Due to the specific framework structure and the low aluminium content ZSM-5 zeolite has special properties and is used widely in many important industrial processes like catalytic aromatization [8], hydroisomerization [9] and disproportionation [10]. On the other hand, there is growing interest in developing platinum-impregnated HZSM-5 zeolite for the catalytic dehydrogenation of propane. Grasselli et al. [11], [12] used 0.7 wt% Pt-Sn-ZSM-5 in catalytic dehydrogenation (DH) of light paraffins combined with selective hydrogen combustion (SHC) process and achieved a near-equilibrium propene yield of 25% at 550 °C. Our previous work [13], [14], [15], [16], [17] discussed the effect of promoter and reaction conditions on catalytic performance of PtSn/ZSM-5 catalyst for propane dehydrogenation and found that PtSnNa/ZSM-5 catalyst with suitable amount of Na showed relatively high propane conversion and propene selectivity.

In order to further improve the catalytic property of PtSnNa/ZSM-5 catalyst, changing the catalyst acidity and the character of Pt active sites is very important. As is well known, the role of lanthanides on alumina support can improve its thermal stability [18]. Moreover, the existences of lanthanides can strongly modify the TPR profiles of supported noble metals [19] and improve the Lewis acidity of alumina [20], thus affecting the catalytic properties of PtSn/γ-Al₂O₃ catalyst obviously. Angel et al. [21] systematically studied the effect of lanthanum on catalytic properties of bimetallic PtSn/γ-Al₂O₃ catalysts and pointed out that the catalytic performances of PtSn/γ-Al₂O₃–La catalysts were mainly depended on the preparation methods used. Li et al. [22] investigated PtSnLa/γ-Al₂O₃ catalysts prepared by Pt–Sn bimetal cluster and conventional impregnation for propane dehydrogenation and concluded that the catalytic stabilities were improved by introducing lanthanum into the above-mentioned catalysts. However, the dispersive status of La species on Pt–Sn catalysts supported on different carrier may result in the variant behaviors, which may affect on the catalytic performance significantly.

In the present study, PtSnNaLa/ZSM-5 catalysts with different amounts of lanthanum were prepared and used for propane dehydrogenation. The catalysts were studied by several techniques, including XRD, nitrogen adsorption, NH₃-TPD, IR spectrum of adsorbed pyridine, TPR, H₂-TPD, XPS and TPO. Particular emphasis was focused on the changes of acidity and character of Pt active sites in PtSnNa/ZSM-5 catalyst by the modification of lanthanum. All the results were correlated with a corresponding catalytic reaction of propane dehydrogenation. This can provide us with important information to understand the effect of La addition on supported metallic catalyst.