ZnSO 4 and La 2 O 3 as Co-Modifier of the Monoclinic Ru Catalyst for Selective Hydrogenation of Benzene to Cyclohexene

A nano-scale monometallic Ru(0) catalyst was prepared by the precipitation method, and the effect of using ZnSO4 and La2O3 as co-modifiers on the performance of the catalyst for selective hydrogenation of benzene to cyclohexene was investigated. The catalysts before and after hydrogenation were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), transmission electron microscopy (TEM), and N2-physisorption. It was found that increasing the amount of alkaline La2O3 increased the amount of the ((Zn(OH)2)3(ZnSO4)(H2O)x (x=1, 3) salt formed by the hydrolysis of ZnSO4, which resulted in a gradual decrease of the activity of the Ru(0) catalyst and a gradual increase of the selectivity for cyclohexene. When the molar ratio of La2O3/Ru was 0.075, cyclohexene selectivity of 75.2% and cyclohexene yield of 58.4% at a benzene conversion of 77.6% were achieved in 25 min over the 1332 孙海杰等: ZnSO4和La2O3作共修饰剂单金属Ru催化剂上苯选择加氢制环己烯 No.7 1 引 言 苯选择加氢制环己烯具有重要的工业价值, 因 为它提供了一条环境友好的生产环己酮和环己醇 的路线. 目前, 全球生产环己酮和环己醇(KA油)的 85%采用催化空气氧化环己烷的方法获得, 该路线 成本高、能耗大、工业三废多、且危险性高. 而苯-环 己烯路线避免了空气氧化步骤. 唯一的副产物环己 烷也是重要的化工原料. 然而热力学上, 苯加氢更 倾向于生成环己烷. 因此, 高选择性苯加氢制环己 烯催化剂的研发是该技术的核心. 反应修饰剂是提高Ru基催化剂环己烯选择性 最简单的方法. ZnSO4和NaOH是研究最多的无机 反应修饰剂. 旭化成公司利用ZnSO4作Ru-Zn催化 剂的反应修饰剂实现了苯选择加氢制环己烯的工 业化, 苯转化 40%时环己烯选择性和收率分别为 80%和 32%. Ning等 利用ZnSO4作反应修饰剂在 SiO2稳定的Ru胶态催化剂上获得了 42%的环己烯 收率. Sun等 利用ZnSO4作反应修饰剂分别在RuZn、Ru-Mn、Ru-Fe、Ru-Ce、Ru-La和Ru-Co-B/ZrO2催 化剂上获得了 58.9%、61.3%、56.7%、57.4%、58.5% 和 62.8%的环己烯收率. Zhang等 利用NaOH作反 应修饰剂在羟基磷灰石负载的Ru-Zn催化剂上获得 了 33%的环己烯收率. 谭晓荷等 利用乙醇胺作反 应修饰剂在Ru-B/MOF催化剂上获得了24%的环己 烯收率. 双反应修饰剂(包括无机-无机和无机-有机) 比单一的效果更佳. Liu等 用ZnSO4和CdSO4作双 反应修饰剂在Ru-La/SBA-15催化剂上获得了 57% 的环己烯收率. Fan等 用ZnSO4和乙二胺作双反应 修饰剂在Ru-Co-B/γ-Al2O3催化剂上获得了34. 8%的 环己烯收率. Sun等 用ZnSO4和醇类作双反应修饰 剂在 Ru-Zn 催化剂上获得了 64.5%的环己烯收率. 双反应修饰剂为苯选择加氢环己烯催化剂和催化 体系的研发开辟了新的思路. 然而, 提高Ru催化剂环己烯选择性的关键是 什么, 至今仍然是一个存在较大争议的问题. Struijk 等 根据传质理论计算的结果将高环己烯收率的获 得归结于H2的传质限制. Milone等 认为在高转化 率下环己烯的液-固扩散对环己烯收率有重要影 响. 本课题组发现化学吸附在Ru催化剂表面的(Zn (OH)2)3(ZnSO4)(H2O)x (x=1, 3, 5, 7)盐起着提高Ru催 化剂环己烯选择性的关键作用. 那么究竟是苯、 环己烯或H2的传质作用, 还是催化剂组成和结构在 提高Ru催化剂环己烯选择性起着关键作用呢? 为 解决这个问题, 本文考察了ZnSO4和La2O3作共修饰 剂对苯选择加氢制环己烯单金属纳米Ru(0)催化剂 性能的影响, 并计算了苯、环己烯和H2的Carberrry 准数和 Wheeler-Weisz 模数, 研究了传质对Ru(0)催 化剂性能的影响. 2 实验部分 2.1 实验试剂 实验所需的试剂: RuCl3∙3H2O、NaOH和ZnSO4∙ 7H2O为分析纯, La2O3(国药集团化学试剂有限公司, 优级纯), 实验用水均为蒸馏水. 2.2 纳米单金属Ru(0)催化剂的制备 将20. 0 g RuCl3∙3H2O溶于200 mL蒸馏水中, 搅 拌下快速加入30% (w) NaOH溶液200 mL, 于80 °C 下搅拌 4 h. 静置后弃去上层清液, 将得到的黑色固 Ru(0) catalyst in the presence of ZnSO4. Moreover, this catalytic system had good reusability. The mass transfer calculation results indicated that the liquid-solid diffusion constraints and pore diffusion limitations could all be ignored. This suggested that the high cyclohexene selectivity and cyclohexene yield could not be simply ascribed to physical effects, and were closely related to the catalyst structure and the catalytic system. Based on the experimental results, we suggest that the surface of the Ru(0) catalyst on which the (Zn(OH)2)3(ZnSO4)(H2O)x (x= 1, 3) salt chemisorbed had two types of active sites for activating the benzene molecules: Ru and Zn. The ability of Zn to activate benzene was much weaker than that of Ru owing to some electron transfer from Zn to Ru, which was confirmed by the XPS and AES results. Furthermore, Zn could cover some of the Ru active sites because Ru and Zn + have similar atomic radii, which decreased the number of Ru active sites for activating H2 molecules. As a result, the benzene activated on Zn could only be hydrogenated to cyclohexene, and the activity of the Ru(0) catalyst decreased. A dual active site model is proposed, for the first time, to explain the reaction of benzene hydrogenation over the Ru-based catalyst, and Hückel molecular orbital theory was used to show the reasonableness of the model.