Effect of transition metals (Cr, Mn, Fe, Co, Ni, Cu and Zn) on the hydrogenation properties of benzene over Ru-based catalyst

•The transition metals were present in the corresponding metallic oxides or hydroxides.•These metallic oxides or hydroxides could not enhance the selectivity to cyclohexene.•These metallic oxides influenced the formation of the (Zn(OH)2)3(ZnSO4)(H2O)x salt.•The (Zn(OH)2)3(ZnSO4)(H2O)x salt directly improved the selectivity to cyclohexene.•Ru–Mn(0.23), Ru–Fe(0.47) and Ru–Zn(0.27) catalysts gave high cyclohexene yields. 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 Cu4(SO4)(OH)6(H2O) salt. The presence of the chemisorbed Cu4(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.