反应修饰剂ZnSO_4和预处理对苯选择加氢制环己烯Ru-Zn催化剂性能的影响

Effect of Reaction Modifier ZnSO_4 and Pretreatment on Performance of Ru-Zn Catalyst for Selective Hydrogenation of Benzene to Cyclohexene

共沉淀法制备了Ru-Zn催化剂,考察了反应修饰剂ZnSO_4和预处理对苯选择加氢制环己烯Ru-Zn催化剂性能的影响。结果表明,反应修饰剂ZnSO_4可以与Ru-Zn催化剂中助剂Zn O反应生成(Zn(OH)2)3(ZnSO_4)(H_2O)盐。随反应修饰剂ZnSO_4浓度增加,(Zn(OH)2)3(ZnSO_4)(H_2O)盐量逐渐增加,Ru-Zn催化剂活性逐渐降低,环己烯选择性逐渐升高。因为(Zn(OH)2)3(ZnSO_4)(H_2O)盐中的Zn2+可以使Ru变为有利环己烯生成的缺电子的Ruδ+物种,而且还可以占据不适宜环己烯生成的强Ru活性位。但当反应修饰剂ZnSO_4浓度高于0.41 mol·L-1后,继续增加ZnSO_4浓度,由于Zn2+水解浆液酸性太强,可以溶解部分(Zn(OH)2)3(ZnSO_4)(H_2O)盐,RuZn催化剂活性升高,环己烯选择性降低。环己烯选择性略微降低,是由于ZnSO_4溶液中大量的Zn2+可以与生成的环己烯形成配合物,稳定生成的环己烯,抑制环己烯再吸附到催化剂表面并加氢生成环己烷。在ZnSO_4最佳浓度0.61 mol·L-1下对Ru-Zn催化剂预处理15 h,Ru-Zn催化剂中助剂Zn O可以与ZnSO_4完全反应生成(Zn(OH)2)3(ZnSO_4)(H_2O)盐,在该催化剂上25 min苯转化68.2%时环己烯选择性和收率分别为80.2%和54.7%。而且该催化剂具有良好的稳定性和重复使用性能。

A Ru-Zn catalyst was prepared by a co-precipitation method, and the effects of the modifier ZnS04 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（ZnSOa）（H20） 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 modifierZnS04 was higher than 0.41 m01·L-1, the further increased concentration of （Zn（OH）2）3（ZnSO4）（H20） salt due to the strong acidity for the hydrolysis of Zn2＋ ZnS04 could dissolve part of the of ZnS04, thus the activity of Ru- Zn 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 ZnS04 concentration of 0.61 mol. L-1, the promoter ZnO of Ru-Zn catalyst could completely react with ZnS04 to form the （Zn（OH）2）3（ZnSO4）（H20） 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.