Abstract:

Cu/SiO₂ catalyst was prepared by ammonia evaporation method using nano-silica vapor as carrier and the contents and types of hydroxyl groups on the carrier surface were changed by step heating roasting. The structure, acidity and alkalinity of the Cu/SiO₂ catalysts were characterized by BET, FTIR, DRIFT, XRD, TEM, H₂-TPR, NH₃/CO₂-TPD, XPS and AES. Hydrogenation of dimethyl oxalate to ethylene glycol over the Cu/SiO₂ was carried out in a continuous flow fixed-bed reactor to evaluate the catalytic activity at low temperature (448K) and low pressure (1.5MPa). The results showed that calcination of SiO₂ at high temperature could significantly change the structure of the Cu/SiO₂ catalyst and reduce its acidity and alkalinity, and greatly reduce the selectivity of alcohols or ethers in the hydrogenation of dimethyl oxalate. The selectivity of ethylene glycol over the Cu/SiO₂-4 catalyst prepared by calcination of silica at 873K increased from 92% over the Cu/SiO₂ prepared under low temperature roasted silica to more than 97% under the optimum reaction conditions. However, the calcination of silica lowered the performance of Cu/SiO₂ catalyst, which should be improved by increasing the ratio of hydrogen to ester to obtain the best reaction result.

Key words: catalyst, silicon hydroxyl groups, ion exchange, dimethyl oxalate, hydrogenation, ethylene glycol

摘要：

通过程序升温焙烧改变气相纳米二氧化硅表面的羟基含量及种类，并以其为载体，采用蒸氨法制备了Cu/SiO₂催化剂，使用比表面积测试（BET）、傅里叶变换红外光谱（FTIR）、漫反射傅里叶变换红外光谱（DRIFT）、X射线衍射（XRD）、透射电子显微镜（TEM）、H₂程序升温还原（H₂-TPR）、NH₃/CO₂程序升温脱附（NH₃/CO₂-TPD）、X射线光电子能谱（XPS）、俄歇电子能谱（AES）等方法研究了Cu/SiO₂催化剂的结构和酸碱性，采用固定床反应器在低温（448K）、低压（1.5MPa）的反应条件下进行草酸二甲酯加氢制备乙二醇的反应，评价其催化活性。结果表明，高温焙烧二氧化硅载体可显著改变后续合成Cu/SiO₂催化剂的结构并降低其酸碱性，对提高乙二醇选择性和降低草酸二甲酯加氢过程中醇类或醚类副产物的选择性具有明显的促进作用。但同时该过程会导致催化剂的活性降低，载体焙烧（473K）后合成的催化剂均需要提高氢酯比方能获得最佳反应结果。其中经873K焙烧的二氧化硅制备的Cu/SiO₂-4催化剂，在最佳反应条件下乙二醇的选择性由低温焙烧后的92%左右提升到97%以上，草酸二甲酯转化率保持在100%。

关键词: 催化剂, 硅羟基, 离子交换, 草酸二甲酯, 加氢, 乙二醇