The influence of transition metals on the hydrogenation properties of benzene over Ru-based catalysts had been investigated in the presence of ZnSO4 acting as the reaction modifier. It was found that the (Zn(OH)(2))(3)(ZnSO4)(H2O)(x) salt (x=1 or 3) chemisorbed on Ru surface played a key role in improving the selectivity of Ru catalysts to cyclohexene. The promoter Cr in Ru-Cr(x) catalysts existed as Cr2O3. The amount of the (Zn(OH)(2))(3)(ZnSO4)(H2O)(x) salt (x=1 or 3) chemisorbed on Ru surface slightly decreased with the increase of the amount of Cr2O3, resulting in a slight decrease of the selectivity to cyclohexene for the Ru-Cr(0.42) catalyst. Co and Ni promoters were present as Co3O4 and Ni(OH)(2) in the Ru-Co(x) and Ru-Ni catalysts, respectively. Increasing amounts of Co3O4 and Ni(OH)(2) increased the chemisorption of (Zn(OH)(2))(3)(ZnSO4)(H2O)(x) salt (x=1 or 3), leading to a slight decrease in activity and a slight increase in selectivity of the Ru-Co(x) and Ru-Ni(x) catalysts to cyclohexene. The promoters Mn, Fe and Zn existed as Mn3O4, Fe3O4 and ZnO, respectively, in the Ru-Mn(x), Ru-Fe(x) and Ru-Zn(x) catalysts. These could react with ZnSO4 to form the (Zn(OH)(2))(3)(ZnSO4)(H2O)(x) salt (x=1 or 3). The amount of chemisorbed (Zn(OH)(2))(3)(ZnSO4)(H2O)(x) salt (x=1 or 3) was significantly increased with the content of Mn3O4, Fe3O4 and ZnO, resulting in a prominent decrease in activity and a significant increase in cyclohexene selectivity of the Ru-Mn(x), Ru-Fe(x) and Ru-Zn(x) catalysts. The Ru-Mn(0.23), the Ru-Fe(0.47) and the Ru-Zn(0.27) catalysts provided maximum cyclohexene yields of 55.3%, 56.7% and 53.4%, respectively. The Cu promoter in the Ru-Cu(0.49) catalyst was present as CuO. Part of the CuO could react with the ZnSO4 in the slurry to form a Cu-4(SO4)(OH)(6)(H2O) salt. The presence of the chemisorbed Cu-4(SO4)(OH)(6)(H2O) and (Zn(OH)(2))(3)(ZnSO4)(H2O)(x) (x=1 or 3) salts improved the selectivity of the Ru-Cu(0.49) catalyst to cyclohexene. (C) 2013 Elsevier B.V. All rights reserved.