| The preparation and characterization of Ru-Zn catalysts for benzene selective hydrogenation to cyclohexene were studied in this paper. Some conclusions were drawn :1. The preparation and characterization of Ru-Zn catalysts prepared by precipitation method: Preparation conditions: Ru-Zn catalysts were prepared by general precipitation and alkali-washed precipitation method respectively. General precipitation method: Zn content of the catalyst should be about 6%. ZrCh can improve the activity but decrease the selectivity; the precipitation temperature sh6uld be near 60, where both the activity and the selectivity of the catalysts are better. The concentration of active component has an effect on the Ru crystallite size, it should be 3.5-4mol/L (calculated by Ru). Alkali-washed precipitation method: When the Zn contents is about 5wt%, the catalyst showed more activity and selectivity. Proper NaOH concentration for dissolving the excess Zn is 2-4%. Compared the two methods, the Ru-Zn catalysts prepared from alkali-washed precipitation method are more efficient using for the benzene partial hydrogenation to cyclohexene because of the pore structure from the alkali-wash. With Ru-Zn catalysts prepared by alkali-wash precipitation method, the yield of cyclohexene reaches to 30% at benzene conversion 53 %.Characterization: There are only peaks corresponding to Ru were determined in XRD patterns, indicating Ru and Zn come into being solid solution with Zn atoms dispersed incidentally and disorderly into Ru crystal grating; the reaction of benzene selective hydrogenation to cyclohexene is sensitive to structure, so the Ru crystallite size should be controlled within 10nm; With the aid of thiophene adsorption experiment, it can be found that the selectivity is influenced evidently by sulphur even ppm amount, suggesting that the active sites which covers only a small part of the surface has an import effect on the selectivity to cyclohexene; Ru-Zn catalysts has a suitable BET surface area ( 62.5m2/g ) and pore structure(open capillary columns), and distribution of the utilizable pores(10-100nm) is homogeneous, average pore diameter of which is 16.05 nm; the surface of Ru-Zn catalyst can be seen from SEM pictures, the catalyst particals is composed of small crystal. In this paper, the activity, nature of active central and the relationship between surface area, pore nature and activity and selectivity was discussed.2. The preparation and characterization of Ru-Zn catalysts prepared by chemical reduction method:Preparation of catalysts: the high activity and selectivity Ru-Zn-B/ZrO2 catalysts were prepared by chemical reduction method, the selectivity reached about 82% at benzene conversion 40%, The activity and selectivity index y wand S40are superior to which of the imported catalyst and the the performance of the catalyst can be repeated well, which suggests that catalysts are of industrial interest. The preparation and operation conditions are studied. The suitable Zn/Ru ratio is 2wt%; the promoter Zn can not only improve the characteristion of the catalyst, but also longer the life of the catalyst; the proper Ru/Zr02 ratio is 13wt%; ZrO2 can not only make the active agent high dispersion but also improve the activity and selectivity, and it should be exist in the form of supports; the concentration of KBH4 should be around 0.25mol/L when the concentration of active agnent (calculated by Ru) is 2mol/L; the precursor of the promoter Zn should choose ZnSO4 7H2O; The pH of the washing liquid should be neutrality or alkalescency; the effect of element B on the activity and selectivity was studied, evidently, the element boron attributes to the high selectivity to cyclohexene. The addition of the forth promoter such as Fe, Co is unfavorable to the selectivity of catalysts. By comparison of this method with the precipitation method, high selectivity and yield of cyclohexene can be obtained with the catalyst prepared by chemical reduction method, and the procedure is so easy to handle that the preparation and th... |
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