Effect of (Zn(OH)2)3(ZnSO4)(H2O)5 on the performance of Ru-Zn catalyst for benzene selective hydrogenation to cyclohexene

A series of Ru-Zn catalysts with different Zn contents were prepared by co-precipitation. The catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) - Ar+ sputter, transmission electron micrographs (TEM)-energy dispersion scanning (EDS) and temperature-programmed reduction (TPR). The performances of the catalysts for benzene selective hydrogenation to cyclohexene were investigated in the presence of 0.6 mol/L of ZnSO4. The results showed that the Ru and Zn in Ru-Zn catalyst were in metallic Ru and ZnO respectively and the ZnO was rich on the surface. The ZnO alone could not improve the selectivity to cyclohexene of Ru-Zn catalyst. However, the ZnO on the surface could react with ZnSO4 to form a (Zn(OH)(2))(3)(ZnSO4)(H2O)(5) salt. The (Zn(OH)(2))(3)(ZnSO4)(H2O)(5) salt chemisorbed played a key role in improving the selectivity to cyclohexene of Ru-Zn catalysts. After Ru-Zn(8.6%) catalyst was pretreated 22 h in ZnSO4 solution at 140 degrees C and 5 MPa H-2, a cyclohexene selectivity of 81.4% at a benzene conversion of 54.0% was achieved at 10 min and a maximum cyclohexene yield of 58.9% was reached. Moreover, the activity was stable above 50% and the cyclohexene selectivity and yield were steadily above 76% and 40% on this catalyst in the first six recycles, respectively. (C) 2012 Elsevier B.V. All rights reserved.