摘要/Abstract

碳酸酯催化加氢制甲醇作为二氧化碳定向转化的间接路径具有重要意义.采用蒸氨法合成了一系列助剂铬修饰的Cr_x-Cu/SiO₂催化剂，系统考察了其对碳酸二乙酯催化加氢性能的影响.研究表明，3 wt%铬修饰量的催化剂活性最优.在反应温度503 K、氢气压力2.5 MPa及液时空速1.0 h^-1条件下，碳酸二乙酯的转化率可达99%，目标产物甲醇的收率和时空得率分别为86.2%和5.6 mmol_MeOH·g_cat^-1·h^-1.采用X射线粉末衍射（XRD）、N₂吸脱附、透射电镜（TEM）、氢气程序升温还原（H₂-TPR）、X射线光电子能谱（XPS）和原位漫反射傅里叶变换红外光谱（In-situ DRIFTS）等手段表征了铬的修饰对催化剂物化性质的影响.结果表明，相较于未修饰的Cu/SiO₂催化剂，少量铬修饰所得Cr_x-Cu/SiO₂催化剂表面活性铜物种的分散度显著提高，且由部分铜和铬相互作用形成的亚铬酸铜物相优化了不同价态铜物种的表面分布状况及催化剂对底物的吸附构型，有效提升了其对碳酸二乙酯催化加氢制甲醇的反应性能和稳定性.
关键词: Cr_x-Cu/SiO₂, 碳酸酯, 加氢, 分散性, 促进效应
Recently, it has been widely reported that CO₂ was utilized to produce valuable chemical feedstock with copper/zinc and metal oxide based catalysts, yet harsh conditions (high pressure and high temperature, etc.) are still essential for the activity and selectivity. Compared with the harsh conditions required in the direct conversion of CO₂ to achieve high selectivity and activity, mild conditions in the indirect conversion of CO₂ through the carbonate intermediate provides an alternative. Since CO₂ can be easily transferred to carbonate under mild and even atmospheric pressure of CO₂ in many reports, hydrogenation of carbonates to methanol at ambient condition presents an attractive strategy for the indirect conversion of CO₂ with higher catalytic activity. In our previous work, we have reported that Cu/SiO₂ catalyst achieved satisfying performance for the hydrogenation of diethyl carbonate with poor stability at long term running due to the agglomeration of active metal. Herein, we present that the catalytic activity and stability of the catalysts in the hydrogenation of carbonates could be efficiently improved by the addition of Cr. In this research, various Cr-promoted Cr_x-Cu/SiO₂ catalysts were synthesized through an ammonia evaporation method. The effect of added Cr on the catalytic performance was investigated by the hydrogenation of diethyl carbonate (DEC) as a probe reaction system. The results showed that the Cr_x-Cu/SiO₂ catalyst with 3 wt% Cr performed the preferable activity. Under the reaction conditions of temperature of 503 K, hydrogen pressure of 2.5 MPa and liquid hourly space velocity (LHSV) of 1.0 h^-1, the conversion of DEC could be 99%, while the selectivity of product methanol (86.2%) and space-time yields (STY) of methanol (5.6 mmol_MeOH·g_cat^-1·h^-1) were enhanced significantly. The physicochemical properties of Cr_x-Cu/SiO₂ catalysts were characterized by X-ray diffraction (XRD), N₂ physical adsorption and desorption, transmission electron microscopy (TEM), H₂ temperature-programmed reduction (H₂-TPR) and in-situ diffuse reflection infrared Fourier transform spectroscopy (In-situ DRIFTS). The results revealed that the dispersion of active copper species was significantly improved. The copper chromite species formed by the interaction of copper and chromium could optimize the distribution of Cu(0) and Cu(I) and regulate adsorption construction of reactant, efficiently improving the catalytic performance and stability for the hydrogenation of diethyl carbonate to methanol.
Key words: Cr_x-Cu/SiO₂, carbonate, hydrogenation, dispersion, promotional effect


PDF全文下载地址:

点我下载PDF