Pt-Cr/ZSM-5 catalysts for propane and cyclohexane conversions

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Abstract

The effect of chromium on the structural and catalytic properties was investigated for Pt-Cr/ZSM-5 system. From TPR measurements it is seen that the reduction profiles were modified by the addition of Cr to the Pt/ZSM-5. Chemissorption data showed that Cr promoted the Pt dispersion. The propane and cyclohexane reactions showed higher activity and aromatic selectivity for the bimetallic samples when compared to the single Pt/ZSM-5 system. Unlike the Pt/ZSM-5 catalyst at higher cyclohexane conversions the hydrogenolysis did not occur on the bimetallic system, and this behavior increases the efficiency by eliminating undesirable parallel reactions.

The Pt–Cr interaction seems to be favored and not only prevents the migration of Pt particles to the external zeolite surface but also weakens the Pt interaction with reactant intermediates, inhibiting hydrogenolysis

Introduction

Pt/ZSM-5 catalysts have been used in processes of light alkane conversions that involve bifunctional sites [1], [2]. However, the interaction between the noble metal and the zeolitic structure can lead to the generation of electronic deficient sites, which have less dehydrogenation character. The presence of a second element that possesses available d electrons can modify the Pt electronic density and, therefore, the catalytic selectivity. In the case of metals on zeolites, it has also been reported that cations from transition metals can interact both with the Pt atoms and the zeolitic structure, which helps to diminish the migration of metallic particles, as in the case of Fe2+ and Cr3+ [3].

In a previous work [4], we investigated the influence of the calcination procedure on the catalytic activity of Pt/HZSM-5 and the effect on propane aromatization. TPR, CO chemisorption and XPS measurements showed that there are significant differences due to the existence of different platinum species after calcination procedures and reduction step, which could explain the differences on catalytic behavior. High calcination temperature promoted a great reduction of Pt2+ to Pt0 and a surprisingly high dispersion, consistent with the increasing catalytic activity.

However, literature is scarce concerning the effect of the addition of a promoter, which has a beneficial effect on the metallic dispersion. Chromium is one of the elements that should promote the Pt dispersion in this system. Therefore, the aim of this work is to study if chromium affects the electronic structure of the Pt/ZSM-5 catalyst by using simple model reactions, like propane and cyclohexane conversions. These reactions help to discriminate the effects based on the product selectivity and besides other characterizations have also been made to look for possible Pt–Cr interaction.

Section snippets

Preparations

The monometallic Pt/ZSM-5 catalyst was prepared by ion exchange of HZSM-5 zeolite (silica to aluminium rate, SAR=28 and specific surface area, Sg=325 m2/g) with a Pt(NH3)4Cl2 solution. For bimetallic catalysts Cr was introduced by impregnation of a Cr(NO3)3·9H2O solution, followed by drying and calcination and then Pt was exchanged. A monometallic Cr/ZSM-5 sample was also prepared using impregnation. The drying step was carried out overnight at 120 °C, followed by calcination at 550 °C for 2 h with

TPR

Fig. 1 shows the TPR profiles of the monometallic and bimetallic samples. Pt(08) catalyst presented two reduction peaks: one with maximum temperature at 220 °C and the other at 420 °C, which are in good agreement with literature data for Pt/zeolite systems [5]. The broad peak area corresponding to a higher reduction temperature indicates a higher ratio of Pt species strongly interacting with the zeolite.

Both monometallic Cr/ZSM-5 catalysts presented identical reduction profiles, which suggests

Discussion

Introduction of Pt in the zeolite enhanced aromatic production, leading to dehydrogenation, however also a significant amount of ethane from hydrogenolysis was observed. The bimetallic catalysts exhibited better activity and aromatic selectivity than Pt alone, which can be attributed to a preferential dehydrogenation process, in spite of hydrogenolysis. Probably, there is a combination of a promotional dispersion effect caused by the presence of Cr and the effect of Cr activity.

The literature

Conclusions

The bimetallic Pt-Cr catalysts exhibited better activity and aromatic selectivity than Pt alone, which can be attributed to a preferential dehydrogenation process, in spite of hydrogenolysis. Probably, there is a combination of a promotional dispersion effect caused by the presence of Cr and the effect of Cr activity.

The easier interaction between metallic Pt and chromium cations, which are difficult to reduced, as in the case of Cr3+, seem to enhance the Pt dispersion and formation of small

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