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

用沉淀法制备了单金属纳米Ru（0）催化剂，考察了ZnSO₄和La₂O₃作共修饰剂对该催化剂催化苯选择加氢制环己烯性能的影响，并用X射线衍射（XRD）、X射线荧光（XRF）光谱、X射线光电子能谱（XPS）、俄歇电子能谱（AES）、透射电镜（TEM）和N₂物理吸附等手段对加氢前后催化剂进行了表征. 结果表明，在ZnSO₄存在下，随着添加碱性La₂O₃量的增加，ZnSO₄水解生成的（Zn（OH）₂）₃（ZnSO₄）（H₂O）_x（x=1，3）盐量增加，催化剂活性单调降低，环己烯选择性单调升高. 当La₂O₃/Ru 物质的量比为0.075 时，Ru催化剂上苯转化率为77.6%，环己烯选择性和收率分别为75.2%和58.4%. 且该催化体系具有良好的重复使用性能. 传质计算结果表明，苯、环己烯和氢气的液-固扩散限制和孔内扩散限制都可忽略. 因此，高环己烯选择性和收率的获得不能简单归结为物理效应，而与催化剂的结构和催化体系密切相关. 根据实验结果，我们推测在化学吸附有（Zn（OH）₂）₃（ZnSO₄）（H₂O）_x（x=1，3）盐的Ru（0）催化剂有两种活化苯的活性位：Ru⁰和Zn²⁺. 因为Zn²⁺将部分电子转移给了Ru，Zn²⁺活化苯的能力比Ru⁰弱. 同时由于Ru和Zn²⁺的原子半径接近，Zn²⁺可以覆盖一部分Ru⁰活性位，导致解离H₂的Ru⁰活性位减少. 这导致了Zn²⁺上活化的苯只能加氢生成环己烯和Ru（0）催化剂活性的降低. 本文利用双活性位模型来解释Ru基催化剂上的苯加氢反应，并用Hückel分子轨道理论说明了该模型的合理性.

ZnSO4 and La2O3 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 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: Ru⁰ and Zn²⁺. The ability of Zn²⁺ to activate benzene was much weaker than that of Ru⁰ owing to some electron transfer from Zn²⁺ to Ru⁰, which was confirmed by the XPS and AES results. Furthermore, Zn²⁺ could cover some of the Ru active sites because Ru and Zn²⁺ have similar atomic radii, which decreased the number of Ru⁰ active sites for activating H₂ molecules. As a result, the benzene activated on Zn²⁺ 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.