Liquid Phase Hydrogenation Of Benzene To Cyclohexene Over A Series Of Novel Ruthenium Catalyst

Cyclohexene, which has a highly reactive double bond, could be used as an intermediate material for producing adipic acid, nylon-6, nylon-66, and fine chemicals. Cyclohexene can be made by several methods such as dehydration of cyclohexanol, hydrodehalogenation of halogenated cyclohexane, or dehydrogenation of cyclohexane. In any of the methods mentioned above, the starting materials are the compounds derived from aromatic hydrocarbons. These processes for producing cyclohexene require complicated multiple steps, the efficiencies are poor, thus leading to high production cost. A route running via selective hydrogenation of benzene to cyclohexene possesses the low price of the raw material, the simplicity of the process, along with the atomically economical character of the reaction. The Asahi chemical industry of Japan has commissioned a plant for manufacturing cyclohexanol using benzene to cyclohexene route. In china, Shenma group company also introduced this technology to build a plant, but had to pay high costs for Japan's patent. Thus, the preparation of cyclohexene through such route is of great value academically and industrially.The hydrogenation of benzene to cyclohexane is thermodynamically much more favorable. Under normal conditions, there is a strong tendency of the reaction to proceed to cyclohexane in one step. Notwithstanding this difficulty, a search for the appropriate catalyst, additives and reaction conditions is on for maximizing the yield of cyclohexene. The hydrogenation capability of the catalyst should not be so strong that over-hydrogenation prevails, or so weak that reaction rate is low. Based on numerous works, it has been acknowledged that ruthenium is the most suitable metal for this reaction, and liquid phase reaction is the most promising in industrialization. Therefore, the present studies deal with the liquid phase hydrogenation of benzene to cyclohexene over a series of Ru novel catalysts.For the ruthenium catalyst, two aspects are selected to investigate benzene selective hydrogenation. On the one hand, we studied metal hydroxide supported Ru catalysts besides the generally traditional support. On the other hand, we studied some catalysts such as RuZn, PVP-RuB and the effect of strong metal support interaction on the catalysisperformance from the point view of Ru active center. Some new conclusions are drawn as the following,1. Metal hydroxide supported Ru catalyst and liquid phase hydrogenation of benzeneMetal hydroxide is seldom used as catalyst supports due to its thermal stability. Recently, some researchers found that Au catalyst supported on metal hydroxides displayed a good dispersity and catalysis performance in low temperature oxidation of CO to CO2. Additionally, reaction system of benzene selective hydrogenation contains a special phase, water phase. This prompts us to have an idea to investigate a series of metal hydroxide supported catalysts and make a preliminary study in benzene selective hydrogenation.A series of metal hydroxides supported Ru catalyst were prepared by coprecipitation and then followed by hydrogen reduction in liquid phase. The XRD, XPS characterizations suggested the catalyst was Ru/AIOOH, Ru/TiO2-nH2O, Ru/La(OH)3 and Ru/Mg(OH)2, respectively. The catalytic behavior in liquid phase selective hydrogenation of benzene to cyclohexene was studied and compared with that of the corresponding oxide supported catalyst reported in the literature. The metal hydroxide catalysts are found more reactive than the corresponding oxide supported catalyst, and the maximum yield of cyclohexene is also more than that over the latter.2. Aluminum hydroxide supported Ru catalyst and alumina supported catalyst(1) Aluminum hydroxide preparation and its hydrothermal stability1) Bayerite preparation and characterizationBayerite with irregular shapes was obtained by aging the precipitate from the reaction of aluminum nitrate and ammonia, but it is contaminated with a minor component of gibbsite, as evidenced by XRD, TG-DTA, and TEM results.2) Effect of hydrothermal conditions on boehmite formationThe transformation of bayerite to boehmite is attempted to carry out under different hydrothermal conditions. Unstirred condition resulted in uncompleted transformation, where about mass ratio of 1:1 of bayerite to fibrillous boehmite reached. The effect of the atmosphere in the autoclave on the boehmite properties has been studied. The air atmosphere leads to monodisperse boehmite nanocrystals, while the hydrogen causes the agglomeration of boehmite nanocrystals. Thesystem pressure acts on the crystal growth in this research. Boehmite plates commence to aggregate as increasing the system pressure to 4 MPa. On the other hand, a larger pore on the exterior surface of the boehmite under relative high pressure was created in comparison with that under autogenous pressure. The effect of atmosphere kind on the surface pore is not distinct in the same condition. 3) Transformation mechanism from bayerite to boehmiteTemperature-programmed technique was applied for the first time to detect the transformation process and revealed the crystal-transformation mechanism at different atmospheres. The fissuring of bayerite occurs parallel to the {001} crystal faces for both atmosphere at same temperature of 150°C. A monodisperse boehmite nanocrystal comes into being under air atmosphere, while the aggregates of boehmite nanocrystals were formed under hydrogen atmosphere, which arises from the positive role of hydrogen in bayerite dissolution (or decomposition).(2) Ru/AIOOH Catalyst: Its Preparation, Characterization and Benzene Partial Hydrogenation Behavior1) A novel 4 wt.% Ru/AIOOH catalyst was prepared by the coprecipitation method and characterized by XRD, TG/DTA, TEM and nitrogen physisorption.2) In liquid phase benzene partial hydrogenation, Ru/AIOOH catalyst exhibited the highest cyclohexene selectivity and moderate activity as compared to Ru/y-AhO-} catalyst prepared by calcining the titled catalyst or by the wetness impregnation method. It is suggested that the surface hydroxyl groups and the large pores in boemite are essential for a high yield of cyclohexene.(3) Colloidal RuB/AhC^-JcH^O catalyst for liquid phase hydrogenation of benzene to cyclohexene1) A colloidal RuB/A^Os-jcE^O catalyst has been synthesized through a combined coprecipitation-crystallization-reduction strategy and characterized in detail with techniques including ICP-AES, N2 physisorption, XRD, TG/DTA, PSD and TEM.2) The catalytic behavior in liquid phase selective hydrogenation of benzene to cyclohexene was studied and compared with that of the RuB/y-A^Os catalyst prepared by the wetness impregnation method. The RUB/AI2O3XH2O catalyst is found more reactive than the RuB/y-AkOs catalyst, and the maximum yield of cyclohexene is about fourfold of that over the latter. The better activity of thecolloidal catalyst is assigned to the higher dispersion of the smaller RuB particles, whereas its superior selectivity is attributed to the improved hydrophilicity due to higher content of structural water and surface hydroxyl groups.3. Preparation of alumina nanofiber and Ru/5-alumina nanofiber catalyst(1) Attempts have been made to prepare alumina nanofibers by hydrolyzing aluminumnitrate in the presence of hexamethylenetetramine (HMTA) followed by the supercritical fluid drying (SCFD) process. The samples were characterized by XRD, TEM and nitrogen physisorption. The results show that 8-AI2O3 nanofibers with diameter of 2 nm, length of 50 run and with BET surface areas of 412.6 m2-g"' were successfully synthesized. The thermal evolution of the fibers and the role of hexamethylenetetramine were also briefly discussed.(2) Introducing the Chemical mixing method and in combination with the sol-gel-SCFD process, we prepared a Ru/5-alumina nanofiber catalyst. The results of characterization suggest Ru particles were very large, approximately to 25 nm, which may be derived from the crystal growth in high temperature calcination and the following reduction. The benzene hydrogenation over such catalyst produced a low cyclohexene yield.4. Zirconia supported Ru catalysts and its application in selective hydrogenation of benzene to cyclohexene(1) Partial hydrogenation of benzene to cyclohexene on a RuZn//n-ZrO2 nanocomposite catalystA RuZn//ra-ZrO2 nanocomposite catalyst for benzene partial hydrogenation to cyclohexene was prepared by coprecipitation of ruthenium trichloride and zirconium oxychloride with ammonia, followed by hydrogen reduction in an aqueous zinc sulfate solution. By use of TEM, XRD and X-ray photoelectron spectroscopy (XPS), the reduction of zinc cation to metallic zinc and the transformation of zirconium hydroxide to monoclinic zirconia were proposed and explained. The hydrogenation parameters, such as the amount of zinc sulfate in the pretreatment process, the hydrogenation temperature, the stirring rate, and the hydrogen pressure, influenced the yield of cyclohexene. The pronounced...