Ru-Zn/ZrO_2催化剂在苯部分加氢反应中的孔径效应

Pore Size Effect of Ru-Zn/ZrO_2 Catalyst on Partial Hydrogenation of Benzene to Cyclohexene

采用沉淀法和溶剂热法合成了三种具有相同晶型但不同孔径的四方ZrO_2(t-ZrO_2),以此为载体,采用沉积沉淀-硫酸锌溶液中还原的方法制备了Ru-Zn/ZrO_2催化剂,考察了Ru-Zn/ZrO_2催化剂的孔径对苯部分加氢性能的影响.采用粉末X射线衍射(XRD)、N_2物理吸附、电感耦合等离子体原子发射光谱(ICP-AES)、CO化学吸附、X射线光电子能谱(XPS)、X射线吸收近边结构(XANES)、X射线激发俄歇电子能谱(XAES)、H_2程序升温还原(H_2-TPR)和透射电子显微镜(TEM)等手段对载体和催化剂进行了系统的表征.研究表明,在苯部分加氢反应中,Ru-Zn/ZrO_2催化剂的孔径对环己烯的选择性有显著影响.随催化剂孔径的增大,苯的转换频率(TOF)基本不变,环己烯初始选择性(S0)则逐渐升高,孔径为11.7 nm的ZrO_2(ZrO_2(11.7))负载的Ru-Zn/ZrO_2(11.7)催化剂的S0及得率最高,分别可达88%和54%.结合催化剂的表征和加氢结果,讨论了孔径影响苯部分加氢活性和选择性的原因.

Partial hydrogenation of benzene to cyclohexene is an important industrial process and features exceptional superiority to processes such as dehydration of cyclohexanol, dehydrogenation of cyclohexane, and the Birch reduction in terms of inexpensive feedstock, succinct reaction route and consequently, improved operational simplicity. In this work, the pore size effect on the partial hydrogenation of benzene to cyclohexene over the Ru-Zn/ZrO2 catalysts was studied for the first time. Three ZrO2 supports with the same tetragonal crystallographic form（t-ZrO2） but different pore sizes were synthesized by the precipitation and the solvothermal methods. Using these ZrO2 samples, the Ru-Zn/ZrO2 catalysts were prepared by the deposition-precipitation method followed by reduction in Zn SO4·7H2O aqueous solution. The supports and catalysts were characterized by powder X-ray diffraction（XRD）, N2 physisorption, inductively coupled plasma-atomic emission spectroscopy（ICP-AES）, CO chemisorption, X-ray photoelectron spectroscopy（XPS）, X-ray absorption near-edge structure（XANES）, temperature-programmed reduction of H2（H2-TPR）, and transmission electron microscopy（TEM）. It is identified that the Ru nanoparticles（NPs） on these catalysts had similar size and chemical state. In the partial hydrogenation of benzene to cyclohexene, a pronounced pore size effect of the catalyst was identified. With the increase in the pore size, while the turnover frequency（TOF） of benzene was essentially unchanged, the initial selectivity（S0） to cyclohexene increased steadily. The Ru-Zn/ZrO2（11.7） catalyst with the ZrO2 support having the pore size of 11.7 nm exhibited the highest S0（88%） and yield（54%） of cyclohexene. On the basis of the characterization results, the similarity in the TOFs of benzene on the Ru-Zn/ZrO2 catalysts with different pore sizes is associated with the identical sizes of the Ru NPs. On the other hand, we tentatively propose that the ZrO2 support with large pore size is beneficial for the out-diffusion of the cyclohexene nano-droplets formed in the pore channels, thus avoiding consecutive hydrogenation to cyclohexane and improving the S0.