Investigation of Pt/Zr-(MoOx or WOx)@HMS catalysts in n-heptane isomerization reaction Nastaran Parsafard* Kosar University of Bojnord, Department of Applied Chemistry, North Khorasan, 9415615458, Iran *corresponding author:
[email protected]
ABSTRACT Pt/ZrO2-HMS catalysts with Si/Zr molar ratio=5, promoted by MoOx and WOx, were studied and compared with their undoped equivalents for n-heptane isomerization reaction at 200–350 oC. Catalysts characterization shows that the addition of Zr/(Mo or W) has a remarkable influence on the surface and textual properties depending on the modification method. Reaction results demonstrate that the modification with MoOx exhibits a significant advantage over ZrO2 and WOx that the higher conversion of n-heptane and selectivity to isoheptane are obtained. The results show that the addition of MoOx improves the selectivity to isoheptane. Keywords: Pt/ZrO2-HMS, Isomerization, MoOx, WOx, Isoheptane. . 1.
INTRODUCTION With the growing awareness of environmental protection, the amount of aromatics and olefins in gasoline has been severely restricted [1]. Although these materials have a strong influence on the environment and human health, they are considered a factor in improving fuel quality. Therefore, in order to prevent the loss of fuel quality by eliminating these compounds, catalytic isomerization is suggested as an important and green channel to upgrade the quality of liquid fuels in the modern petrochemical industry that produces compounds with the same quality to the aromatics and olefins [2]. The isomerization reaction of linear saturated hydrocarbons usually involves a multipath reaction process to obtain the branched isomers, and simultaneously accompanying the occurrence of other side reactions such as cracking, aromatization and hydrogenolysis reactions. Among these side reactions, cracking is the most common undesired reaction and must be avoided [1]. According to this purpose, many different catalytic systems have been studied to obtain high selectivity to isomerization products [1-4]. Among these researches, solid acid catalysts based on zirconia appear to be the best candidate for isomerization reaction. These catalysts have favorable advantages such as high catalytic activity at low temperature because of their stronger acidity, no corrosion in the reactor and no environmental problems in disposing of the used catalysts [3, 4]. This class of catalysts has been used with different metal oxides such as WO 3 and MoO3. These mixed oxides catalysts have shown very good activity. Among these super acids, WO3/ZrO2 [3] supported catalysts have more stable conversion and no leaching of catalyst component into the products; however, these catalysts are less active compared to SO42−/ZrO2 supported materials [3]. These catalysts have no problems of previous catalysts but their non-uniform pore size and their low surface area limit their catalytic performance [3, 4]. Literatures in this field demonstrate that mesoporous silica supports with high surface area and facilitate diffusion of bulky organic molecules in and out of the porous structures can eliminate the defects of such oxide catalysts and so can generate appropriate activity and selectivity for isomerization reaction [5]. Herein, four types of hexagonal mesoporous silica supported Pt catalysts with ZrO2, WO3 and MoO3 and Si/Zr molar ratio equal to 5 were prepared and compared with Pt/HMS. The catalytic performances of n-heptane isomerization over these catalysts were studied as catalyst activity, selectivity, research octane number measurements and stability to coke deposition. Finally, the obtained results with the aim of selecting the best catalyst for n-heptane isomerization were judged. 2. Experimental 2.1. Catalysts preparation The preparation method of HMS, Zr-HMS, WOx-HMS and MoOx-HMS supports is similar to the reported recipe [3, 4, 6]. Briefly, these materials were prepared by sol-gel method. In this preparation method, the appropriate amounts of zirconyl (IV) nitrate hydrate (Aldrich) as zirconium source, dodecyl amine (Merck) as surfactant sources and tetraethyl orthosilicate (Merck) as silica source were used. After depositing zirconyl (IV) nitrate hydrate on HMS, tungstate and molybdite were introduced in Zr-HMS by impregnation method of ammonium meta tungstate hydrate
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(Aldrich) and ammonium heptamolybdate (Aldrich) solution. The solid products, with W (or Mo)=12 wt% and Si/Zr=5 molar ratios were separated by filtration and then dried at 110 °C overnight and calcined at 600 °C for 6 h in air. The Pt(0.6 wt%)-MoO3 (WO3)/ZrO2-HMS catalysts were also prepared by a certain concentration of hexachloroplatinic acid (Aldrich) solution. The platinated materials were filtered and dried at 110 °C overnight. Subsequently, these solid samples were calcined in air at 300 °C for 4 h. 2.2. Activity evaluation The isomerization of n-heptane (n-C7) was performed in a fixed-bed flow reactor. A quantity of 1 g catalyst was loaded into the isothermal region of the reactor tube for all reaction tests. Prior to the reaction, the catalyst was activated in hydrogen stream at 450 oC for 2 h, and then cooled down to a reaction temperature. A dose of n-C7 was passed over the activated catalyst under the following reaction conditions: reaction pressure of 1 atm, molar H 2/n-C7 ratio of 7:1, and 200-350 oC temperature range. The products were analyzed by an online Agilent Technologies 7890A gas chromatograph equipped with a flame ionization detector. Since weight percentage of isoheptane, aromatics, hydrogenolysis and the cracking products could be obtained by area normalization method in GC peaks, the conversion and the selectivity of each produst were calculated and reported. 2.3. Stability study The major problem in the catalytic processes is the coke deposition on the catalysts' surfaces and their deactivation. So the stability of prepared catalysts for further study in the catalytic performances was also examined under the operating conditions similar to activity performance at a selected constant temperature (300 oC) for 10 h on stream. The products were also identified with an on-line gas chromatograph. 3.
Results and discussions To investigate the effect of metal addition on the catalytic activity of Pt/HMS catalyst in n-C7 isomerization, the reactions were firstly performed over unmodified and then Zr/(Mo or W)- modified Pt/HMS catalysts at 200-350 oC. As presented in Fig. 1, the conversion of n-C7 over modified catalysts especially Pt/Zr-Mo-HMS catalyst is higher than that over the unmodified catalyst.
Fig. 1. Catalytic activities over various prepared catalysts. The results show that the selectivity to isoheptanes (Fig. 2) is enhanced after Zr/(Mo or W) modification and all the Zr/(Mo or W)-modified catalysts almost behave the same selectivity to isoheptanes, except for Pt/Zr-Mo-HMS, which exhibits a notably higher isoheptanes selectivity with a higher n-C7 conversion. To promote the octane number for improving fuel quality, in accordance with the environmental limitations, it is needed to increase isomer products while aromatic compounds, especially benzene, and also light compounds were reduced. The results (Fig. 2) show that our prepared catalysts have high selectivity to isomerization products. The aromatic selectivity for all mentioned catalysts was zero that this result is exactly favorable for environmental legislation.
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Fig. 2. Influences of the reaction conditions on (a) monobranched isomers, (b) multi branched isomers and (c) total produced isomers selectivitiy. In order to compare the catalysts activity in this process, we further carried out the n-C7 isomerization reaction over prepared catalysts at different time on stream (10 h). All four catalysts demonstrated the similar trends of conversion and selectivity with increasing reaction time. The catalytic activities are reduced over time. One reason for this reduction is the formation of coke on the catalytic surfaces. This is an important problem in continuing processes of the industrial catalytic practices. For this purpose, catalysts' stability was evaluated for n-C7 isomerization at 300 oC. It was observed that the catalytic activities and also the selectivity to isomerized products for all catalysts diminish with time until reaching stable values. The lowest decrease in conversion and selectivity were observed for Pt/HMS. Although other prepared catalysts do not differ significantly in the amount of coke produced. Another important calculation to judge the catalytic performance for isomerization reaction is research octane number (RON) that is presented in Fig. 3.
Fig. 3. RON vs. temperature for various catalysts. This parameter shows anti-knocking capacity of fuels. To calculate RON, the following equation was used: (1) In this equation, RON is the measured octane number for total products includes isomers or other side products after the isomerization reaction of n-C7, RONi is the pure-component octane number for each molecule that denoted as i and yi is the volume fractions of molecule i. Results show that Pt/Zr-Mo-HMS at 350 oC compared to other catalysts provides higher RON (~56), because of producing molecules with higher RONi and as expected from the isomers and other products.
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Conclusion This study successfully introduced the Zr/(Mo or W) into Pt/HMS. The results show that the n-C7 conversions at all temperatures for Pt/Zr-Mo-HMS are more than MoO3-free catalysts. It seems the interaction of MoO3 and ZrO2 oxides and the impact of this interaction on the catalytic acidity achieves better conversion results than other catalysts. Also the catalytic activity increases with reaction temperature and the highest conversion (63.2%) obtains for Pt/Zr-MoHMS at 350 oC. This observation can be explained that the production of some compounds such as cracking increases with raising temperature and since the conversion is defined as the converting of the feedstock into various products; therefore it is not unexpected that the maximum value of the conversion is observed at highest temperature. Furthermore, the addition of Zr/Mo improves the selectivity to isoheptane and also RON. Acknowledgment I would like to thank Dr. Mohammad Hasan Peyrovi from the Department of Petroleum Chemistry and Catalysis of Shahid Beheshti University for his help in providing the laboratory. REFERENCES [1] Shi, L., Song, X., Liu, G., Guo, H. (2017). Effect of catalyst preparation on hydroisomerization of n -heptane over Pt/silicalite-1. Catalysis Letters, vol. 147, no. 10, p. 2549-2557. [2] Parsafard, N., Peyrovi, M. H., Jarayedi, M. (2017). Catalytic study and kinetic modelling of the n -heptane isomerization over Pt/Al-HMS/HZSM-5 hybrid catalysts. Energy & Fuels, vol. 31, no. 6, p. 6389–6396. [3] Parsafard, N., Peyrovi, M. H., Parsafard, N. (2017). Effect of WO x promoter on Pt/ZrO 2-HMS catalysts for nheptane isomerization: Catalytic performance and kinetics study. Chinese Chemical Letters, vol. 28, no. 3, p. 546-552. [4] Parsafard, N., Peyrovi, M. H., Parsafard, N. (2017). Pt-HMS catalyst promoted with MoO x/ZrO2 mixed oxides for n-heptane isomerization: catalytic performance and kinetics. Reaction Kinetics, Mechanisms and Catalysis, vol. 120, no. 1, p. 231-246. [5] Kaucký, D., Wichterlová, B., Dedecek, J., Sobalik, Z., Jakubec, I. (2011). Effect of the particle size and surface area of tungstated zirconia on the WO x nuclearity and n-heptane isomerization over Pt/WO 3–ZrO2. Applied Catalysis A: General, vol. 397, no. 1, p. 82-93. [6] Peyrovi, M. H., Parsafard, N., Peyrovi, P. (2014). Influence of Zirconium Addition in Platinum–Hexagonal Mesoporous Silica (Pt-HMS) Catalysts for Reforming of n-Heptane. Industrial & Engineering Chemistry Research, vol. 53, no. 37, p. 14253-14262.
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