Pt-Cr/ZSM-5 catalysts for propane and cyclohexane conversions
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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2013, Applied Catalysis B: EnvironmentalCitation Excerpt :At T1, the reduction of Cr(VI) species weakly interacted with the zeolite support takes place. However, the formation of Cr(IV) and Cr(V) species like reduction intermediates might occur at the same temperature [60]. The reduction of chromate species anchored to Cr2O3 occurs at T2 [39,56,59], while the reduction of Cr(III) to Cr(II) takes place at T3 according to Ilieva et al. [61] and based on our experiment performed with pure Cr2O3 oxide [39,56,59].
Dehydrogenation of C<inf>3</inf>-C<inf>4</inf> paraffin's to corresponding olefins over slit-SAPO-34 supported Pt-Sn-based novel catalyst
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Preparation and characterization of Pt-promoted NiY and CoY catalysts employed for 4-nitrophenol reduction
2013, Applied Catalysis A: GeneralCitation Excerpt :Specifically, it is of interest to investigate the activity of platinum supported on NaY zeolite for the development of these catalysts for the industrial applications of organic pollutants removal by hydrogenation reactions. Bimetallic platinum with a second metal such as PtFe/Al2O3 [1], PtCo/Al2O3 [2], supported PtNi [3,4], and Pt-Cr/ZSM-5 [5] have been prepared using different preparation methods such as successive impregnation, incipient wetness and competitive ion exchange methods. They have been used in some reactions such as CO oxidation, isomerization reactions and alkane conversion.
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