Liquid Phase Hydrogenation of Benzene to Cyclohexene over Ru-based Catalysts
Cyclohexene, which has a highly active double bond, is widely used as an intermediate for producing medicine, pesticide, agrochemical, feed additive, polyester fine chemicals etc, especially for the synthesis of polyamide fibre through its intermediates, cyclohexanone and hexanedioic acid. Cyclohexene can be made by several methods such as dehydration of cyclohexanol, hydrodehalogenation of halogenated cyclohexane, or dehydrogenation of cyclohexane. These processes for producing cyclohexene have disvantages of complicated multiple steps, low yield, and high production cost. A route via partial hydrogenation of benzene to cyclohexene possesses the low price of feedstock, the simplicity of the process, along with the atomically economical character of the reaction. The Asahi
Strong magnets Chemical Industry of Japan has commissioned the first plant in the world for manufacturing cyclohexene from the partial hydrogenation of benzene route since Oct. 1988. In China, Shenma Group Company introduced this technology in 1996, but had to pay high costs for the patent and catalyst. Thus, the development of efficient catalyst system is of great significance academically and industrially.However, the hydrogenation of benzene to cyclohexane is thermodynamically much more favorable. Notwithstanding this difficulty, a search for the appropriate catalyst, additives and reaction conditions is on for maximizing the yield of cyclohexene. Based on numerous works, it has been acknowledged that ruthenium is the most suitable catalyst, and the liquid phase reaction is the most promising for industrialization.
Therefore, we prepared a series of novel Ru catalysts and tested their catalytic performance in the liquid phase hydrogenation of benzene to cyclohexene.In order to promote the Ru active center, by using SBA-15 as support, we prepared the catalyst by the "two solvents method", and modified the catalyst by La or Ce; the yield of cyclohexene could be improved greatly. As to the reaction modifier, it is generally needed to add ZnSO4 to lower the catalytic activity and improve the selectivity to cyclohexene. For the first time we reported the synergistic effect of ZnSO4 and CdSO4 modifiers, which can efficiently improve the selectivity to cyclohexene. As to the catalyst support, in order to improve the yield of cyclohexene, we choose the support with a hydrophilic character. We prepared the MgAl2O4 spinel material by a hydrothermal method, after being impregrated with Ru, the stability of the catalyst was improved greatly. ZrO2, which has abundant surface defects, weak acid site, weak base site and re-dox character, is an excellent catalyst support and dispersant in the partial hydrogenation of benzene. On one hand, by using commercial ZrO2 as support, we studied the bimetallic effect (RuPdB/ZrO2, RuPtB/ZrO2) on the partial hydrogenation of benzene. On the other hand, considering the low surface area of commercial ZrO2, and different methods available to prepare ZrO2 with different crystal structure, surface area and surface acid-base character, we synthesized two kinds of ZrO2 materials (monoclinic and tetragonal) as supports for RuB catalysts, and studied the effect of crystal structure and surface acid-base character of the support on the liquid phase hydrogenation of benzene. The main results are summaried as follows:Since the
http://www.chinamagnets.biz/ discovery of M41S mesoporous materials by Mobil’s Company in 1992, mesoporous materials show wide applications in catalytic hydrogenation, oxidation fields due to its high surface area, narrow pore size distribution and uniform pore structure. But there are few reports about the preparation of Ru catalysts by using mesoporous silica as support for the liquid phase hydrogenation of benzene. Meanwhile, for the Ru catalyst, it is generally needed to add promoter to lower its catalytic activity and improve the selectivity to cyclohexene. China has abundant rare earth resources, which show excellent promoting effect in the catalytic hydrogenation reaction. So, we first studied the effect of La or Ce promoter on the Ru/SBA-15 catalyst in the liquid phase hydrogenation of benzene to cyclohexene. Combing with the XRD, TEM, TPR, XPS characterizations and reaction results, the following conclusions could be drawn:1)
The RuCe/SBA-15 and RuLa/SBA-15 catalysts prepared by the "two solvents method" showed much higher selectivity to cyclohexene compared with un-modified Ru/SBA-15 catalyst, with La promoter showing the best result. The maximum yield of cyclohexene could reach 57.4% at 413 K of reaction temperature,4.0 MPa of hydrogen pressure,0.42 M of ZnSO4 and 1.56×10-3 M of CdSO4.2) The modification of Ce or La promoter reduced the surface Ru active sites; improved the electron density of Ru and the hydrophilicity of the catalyst, all favorable for the formation of cyclohexene.3) The effect of reaction temperature and hydrogen pressure was studied over the RuLa/SBA-15-0.8 catalyst. The reaction temperature influences the desorption of cyclohexene from the catalyst surface and the solubility of cyclohexene in the water thin film, while the effect of hydrogen pressure was due to the competitive adsorption of hydrogen and benzene molecule on the catalyst surface.In order to improve the yield of cyclohexene, the addition of a modifier to the reaction system is indispensable during the liquid phase hydrogenation of benzene. Generally, there are two kinds of reaction modifiers:organic modifier and inorganic modifier. ZnSO4 has been regarded as the most efficient modifier and all published documents (patents) have focused on the promoting effect of a single modifier. There are no reports about the synergistic effect of two modifiers. In the present work over the RuLa/SBA-15-0.8 catalyst, we found that by controlling the concentration of CdSO4, a much better yield of cyclohexene could be obtained, and a combination of CdSO4 and ZnSO4 performed much better than either of them alone. In order to elucidate the individual role of CdSO4 and ZnSO4 in the reaction, we fell to theoretical calculation to obtain the information about the interactions of Cd2+ and Zn2+ ions with benzene and cyclohexene. Based on experimental data and theoretical calculations, we concluded that CdSO4 acted as surface modification, suppressing more the adsorption of cyclohexene than that of benzene, while the function of ZnSO4 was mainly the stabilization of cyclohexene in the liquid phase, accelerating the desorption as well as hindering the re-adsorption of cyclohexene, thus improving the yield of cyclohexene.Although a high yield of cyclohexene could be obtained by using Ru/SBA-15-series catalysts, but due to the low hydrothermal stability of SBA-15 which results in its poor catalyst stability. Spinel is a kind of material with good chemical and thermal stability, which shows wide applications in ammonia synthesis, ethanol reforming and WGSR reactions etc. We prepared the MgAl2O4 material by a hydrothermal method and calcined at different temperatures to obtain better crystal structure of spinel.
The catalyst prepared by using MgAl2O4 calcined at 1023 K as support showed the best result in the liquid phase hydrogenation of benzene, with the maximum yield of cyclohexene reaching 38.5% at 413 K of reaction temperature,4.0 MPa of H2 pressure,0.28 M of ZnSO4 and 0.39×10-3 M of CdSO4, much higher than the corresponding values on Ru/Al2O3 and Ru/MgO catalysts. Under the same reaction conditions, the maximum yield of cyclohexene
http://www.chinamagnets.biz/ on the Ru/Al2O3 catalyst was higher than that on the Ru/MgO catalyst. The Ru/MgAl2O4-1023 catalyst could be re-used for 3 times without significant lowering the yield of cyclohexene. Compared with the Ru/SBA-15-series catalysts, the catalyst stability was improved greatly, but the maximum yield of cyclohexene was lower. The different yield of cyclohexene is due to the different hydrophilic character of the support. Support with a hydrophilic character is more favorbale for the formation of cyclohexene, and the acid-base character of the support also has great influence on the formation of cyclohexene.The maximum yield of cyclohexene over Ru/MgAl2O4-series catalysts is lower than the expected in industry, meanwhile, numerous literatures and patents have showed that ZrO2 is an excellent catalyst support and dispersant in the liquid phase hydrogenation of benzene. In order to obtain higher yield of cyclohexene, the promoter is needed to modify the Ru catalyst, including Zn, Fe, Co, Ni, and rare earth elements etc. There are few reports about the promoting effect of noble metal element in the liquid phase hydrogenation of benzene. By choosing a suitable noble metal element (Pd, Pt), it is possible to form alloy with Ru, while the formation of alloy (RuPd) had shown better promoting effect in the hydrogenation of dimethylbenzene, nitrobenzene aromatic hydrocarbons.Thus, in this chapter, we use commercial ZrO2 as support and modify the Ru catalyst by Pd or Pt element to study the bimetallic effect in the liquid phase hydrogenation of benzene. The experimental results showed that the introduction of Pd or Pt could improve the catalytic activity and the yield of cyclohexene greatly. The maximum yield of cyclohexene increased from 38.5%(RuB/ZrO2) to 43.8%(RuPdB/ZrO2-0.2) and 44.2% (RuPtB/ZrO2-0.15), while the reaction time for the maximum yield of cyclohexene was shortened from 80 min (RuB/ZrO2) to 60 min (RuPdB/ZrO2-0.2) and 45 min (RuPtB/ZrO2-0.15). The improvement of the catalytic performance was due to the improvement of the dispersing degree, the thermal stability of the catalyst and the formation of surface alloy. Due to the low surface area of commercial ZrO2 (<10 m2·g-1), which restricts its application.
There are many methods available to prepare ZrO2, resulting in different surface area, crystal structure and surface acid-base character of ZrO2. Herein, we prepared two kinds of ZrO2 (monoclinic and tetragonal) as supports for the RuB catalysts to study the effect of crystal structure and surface acid-base character on the partial hydrogenation of benzene. Py-IR characterization showed that monoclinic ZrO2 (ZrO2-M) had both Lewis acid site and Br(?)nsted acid site while tetragonal ZrO2 (ZrO2-T) had only Lewis acid sites. Under the same reaction conditions, by calcining the ZrO2-T and ZrO2-M materials at 873 K as supports to prepare supported catalysts and test their catalytic performance in the liquid phase hydrogenation of benzene, the maximum yield of cyclohexene was 46.6% (RuB/ZrO2-T-873) and 37.8%(RuB/ZrO2-M-873). The catalysts prepared by using ZrO2-T as support were more favorable for the formation of cyclohexene. By combining the experimental results and literature reports, benzene adsorbed on the Br(?)nsted acid site is more favorable for the formation of cyclohexane with the spill-over hydrogen from the metal active site, resulting in lower yield of cyclohexene. For the first time, we reported the influence of different acid site of the ZrO2 support in the liquid phase hydrogenation of benzene. The RuB/ZrO2-T-series catalysts may have excellent application in industry.
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