Acta Phys. -Chim. Sin. ›› 2014, Vol. 30 ›› Issue (7): 1332-1340.doi: 10.3866/PKU.WHXB201405072

• CATALYSIS AND SURFACE SCIENCE • Previous Articles     Next Articles

ZnSO4 and La2O3 as Co-Modifier of the Monoclinic Ru Catalyst for Selective Hydrogenation of Benzene to Cyclohexene

SUN Hai-Jie1,2, LI Yong-Yu2, LI Shuai-Hui1, ZHANG Yuan-Xin1, LIU Shou-Chang1, LIU Zhong-Yi1, REN Bao-Zeng3   

  1. 1. College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, P. R. China;
    2. Institute of Environmental and Catalytic Engineering, Department of Chemistry, Zhengzhou Normal University, Zhengzhou 450044, P. R. China;
    3. School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, P. R. China
  • Received:2014-02-28 Revised:2014-05-04 Published:2014-06-30
  • Contact: LIU Zhong-Yi E-mail:liuzhongyi@zzu.edu.cn
  • Supported by:

    The project was supported by the National Natural Science Foundation of China (21273205, U1304204), Innovation Found for Technology Based Firms of China (10C26214104505), and Postdoctoral Science Foundation of Henan Province, China (2013006).

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Abstract:

A nano-scale monometallic Ru(0) catalyst was prepared by the precipitation method, and the effect of using ZnSO4 and La2O3 as co-modifiers on the performance of the catalyst for selective hydrogenation of benzene to cyclohexene was investigated. The catalysts before and after hydrogenation were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), transmission electron microscopy (TEM), and N2-physisorption. It was found that increasing the amount of alkaline La2O3 increased the amount of the ((Zn(OH)2)3(ZnSO4)(H2O)x (x=1, 3) salt formed by the hydrolysis of ZnSO4, which resulted in a gradual decrease of the activity of the Ru(0) catalyst and a gradual increase of the selectivity for cyclohexene. When the molar ratio of La2O3/Ru was 0.075, cyclohexene selectivity of 75.2% and cyclohexene yield of 58.4% at a benzene conversion of 77.6% were achieved in 25 min over the Ru(0) catalyst in the presence of ZnSO4. Moreover, this catalytic system had good reusability. The mass transfer calculation results indicated that the liquid-solid diffusion constraints and pore diffusion limitations could all be ignored. This suggested that the high cyclohexene selectivity and cyclohexene yield could not be simply ascribed to physical effects, and were closely related to the catalyst structure and the catalytic system. Based on the experimental results, we suggest that the surface of the Ru(0) catalyst on which the (Zn(OH)2)3(ZnSO4)(H2O)x (x= 1, 3) salt chemisorbed had two types of active sites for activating the benzene molecules: Ru0 and Zn2+. The ability of Zn2+ to activate benzene was much weaker than that of Ru0 owing to some electron transfer from Zn2+ to Ru0, which was confirmed by the XPS and AES results. Furthermore, Zn2+ could cover some of the Ru active sites because Ru and Zn2+ have similar atomic radii, which decreased the number of Ru0 active sites for activating H2 molecules. As a result, the benzene activated on Zn2+ could only be hydrogenated to cyclohexene, and the activity of the Ru(0) catalyst decreased. A dual active site model is proposed, for the first time, to explain the reaction of benzene hydrogenation over the Ru-based catalyst, and Hückel molecular orbital theory was used to show the reasonableness of the model.

Key words: Benzene, Selective hydrogenation, Cyclohexene, Ruthenium, Znic, Lanthanum