Magnetic Catalysts And Its Selective Hydrogenation Performance For Benzene On Magnetically Stabilized Bed

Cyclohexene is an important raw material, which can be widely used in production of medicine, pesticide, dye, washes, dynamite, feedstuff, polyester, and other fine chemicals. Selective hydrogenation of benzene to cyclohexene is becoming an important chemical process for production of cyclohexene. A magnetically stabilized bed (MSB) has many advantages of high mass-transfer efficiency, low pressure drop, high conversion, and easy to avoid channeling flow, interphase backmixing and residence time over conventional fixed bed and fluidized bed, since the magnetic catalyst particles are dispersed in a uniform and stable arrangement by the magnetic field. The preparation process of alumina nanospheres and the magnetic core-shell alumina nanospheres were explored in this paper. Selective hydrogenation of benzene on the Ru and Ru-Zn-B catalysts supported on Al₂O₃-ZrO₂ composite was investigated. Three kinds of magnetic core-shell alumina microspheres were prepared, which have both suitable magnetism and high activity for hydrogenation, therefore it is capable of applying to the MSB. The final goal of this paer is to develop novel magnetic catalysts and MSB technology for selective hydrogenation of benzene.Alumina nanospheres and magnetic core-shell alumina nanospheres were synthesized by sol-gel method. It presents a simple and efficient approach to synthesize alumina nanospheres, which can be produced using lauric acid as template in 1-propanol system. A novel and uniform alumina nanospheres with a size of 80–120 nm has been successfully prepared by lauric acid via combination of the sol-gel process with the surfactant self-assembly approach. The alumina nanospheres possess average pore diameter 4.9 nm, a large pore volume 0.8 cm³/g, and a high specific surface area 550 m²/g. On the basis of above method, well-structured nanospheres with a magnetic core/alumina shell (MFeCA) structure were obtained by self-assembly of stearic acid/alumina species complex in 1-propanol system. The thickness of alumina shell is very uniform (30nm). The surface area, pore volume and average pore size of the sample is 140m²/g, 0.22 cm³/g and 5.2 nm, respectively. The saturation magnetization values of the sample is as high as 50.2 emu/g.Magnetic alumina composite microspheres withγ-Fe₂O₃ core/Al₂O₃ shell structure were prepared by the oil column method. The results show that the specific surface area and pore volume of theγ-Fe₂O₃/SiO₂/Al₂O₃ composite microspheres calcined at 500℃were 200 m²/g and 0.77cm³/g, respectively. The saturation magnetization is 15.1emu/g. Magnetic alumina composite microspheres consisting of Fe₃O₄ core, an intermediate layer of SiO₂ and Al₂O₃ shell structure were also prepared by the oil column method. The sample is of uniform particle size and a strong superparamagnetic. The specific surface area, pore volume and average pore diameter of the microsphere were 201m²/g, 0.76cm³/g, 17nm, respectivly. The particle size distribution of the sample is 80³00μm. In addition,α-Fe₂O₃/SiO₂/γ-Al₂O₃ microspheres composed of the sphericalα-Fe₂O₃ or the needle type ofα-Fe₂O₃ were prepared by oil column method. Magnetic Fe₃O₄/SiO₂/γ-Al₂O₃ and Fe/SiO₂/γ-Al₂O₃ microspheres were obtained by hydrogen reduction. The former represents paramagnetic magnetization 14.0emu/g, while the latter shows the ferromagnetic saturation magnetization 26.0emu/g, coercivity 430 Oe. Therefore, it is easy to get different magnetic alumina carrier by controlling the reduction temperature and time.Ru-Zn-B/Al₂O₃-ZrO₂ and Ru-Zn-B/Al₂O₃ catalyst were prapared by chemical reduction method using sodium borohydride as chemical reductant. The performance of two catalysts was characterized using selective hydrogenation of benzene in autoclave reactor. The results show that ZrO₂ is favorable to increase the selectivity of cyclohexene on Ru-Zn-B/Al₂O₃ catalyst obviously, the highest selectivity of cyclohexene occurs when the Zr/Al atomic ratio is 0.1. Ru catalyst containing a small amount of zinc is in favor of improving selectivity of cyclohexene. The reason may be that ZnSO₄ solution enhances the hydrophily of the surface of the catalyst obviously, and Zn²⁺ dilutes Ru to increase the dispersion of Ru nanoparticles, therefore improves the selectivity for cyclohexene.A model MSB unit was designed and constructed, and was used to investigate the flow state with Fe fine particles as model catalyst, nitrogen as gas and water as liquid. The results indicated that the high mass-transfer efficiency is obtained when the magnetically stabilized bed is in the state of chain operation. On the basis, Ru-based magnetic microspheres (Ru/γ-Fe₂O₃-SiO₂-γ-Al₂O₃) catalysts with uniform and egg-type distribution of Ru on catalyst pellets were prepared by impregnation method, respectively. A MSB unit for hydrogenation was designed and constructed, and used to characterize the selective hydrogenation of benzene. The effects of catalyst preparation parameters and reaction conditions on the selective hydrogenation of benzene were investigated in detail. The results show that the state of chain operation in MSB and the egg-type distribution of Ru on catalyst pellets increase the selectivity for cyclohexene obviously, this verify that the magnetic ruthenium-based catalysts and magnetically stabilized bed have a good prospect of industrial applications.