Effect of Reaction Modifier ZnSO4 and Pretreatment on Performance of Ru-Zn Catalyst for Selective Hydrogenation of Benzene to Cyclohexene

A Ru-Zn catalyst was prepared by a co-precipitation method, and the effects of the modifier ZnSO4 and the pretreatment on the performance of the Ru-Zn catalyst for selective hydrogenation of benzene to cyclohexene were investigated. The results showed that the modifier ZnSO4 could react with the promoter ZnO of the Ru-Zn catalyst to form a indissoluble (Zn(OH)(2))(3)(ZnSO4)(H2O) salt. Moreover, the amount of this salt increased with the increase of the concentration of the modifier ZnSO4, which made the activity of this catalyst increase and the selectivity to cyclohexene decrease. This is because the Zn2+ of the salt not only could transform the active sites Ru into the electron-deficient species in favor of the formation of cyclohexene, but also could occupy the strong active sites Ru unfavorable for the formation of cyclohexene. However, when the concentration of the modifier ZnSO4 was higher than 0.41 mol.L-1, the further increased concentration of ZnSO4 could dissolve part of the (Zn(OH)(2))(3)(ZnSO4)(H2O) salt due to the strong acidity for the hydrolysis of Zn2+ of ZnSO4, thus the activity of RuZn catalyst increased and the selectivity to cyclohexene decreased. But the selectivity was slightly decreased, which was because Zn2+ of ZnSO4 in the solution could form a complex with cyclohexene stabilizing cyclohexene formed and preventing cyclohexene from being adsorbed on surface of the catalyst again and being hydrogenated into cyclohexane. When the Ru-Zn catalyst was pretreated for 15 h under the condition of the optium ZnSO4 concentration of 0.61 mol.L-1, the promoter ZnO of Ru-Zn catalyst could completely react with ZnSO4 to form the (Zn(OH)(2))(3)(ZnSO4)(H2O) salt, and this catalyst gave a selectivity of 80.2% and a cyclohexene yield of 54.7% at the benzene conversion of 68.2% at 25 min. Moreover, this catalyst had a good stability and a excellent reusability.