ZnO催化草酸二甲酯和乙醇酯交换合成草酸二乙酯研究*1

Synthesis of diethyl oxalate by transesterification of dimethyl oxalate with ethanol over ZnO catalyst

摘要:

通过控制制备方法和工艺合成了一系列不同形貌的ZnO,作为催化剂用于催化草酸二甲酯(DMO)和乙醇经酯交换法合成草酸二乙酯(DEO)反应,并采用XRD、N₂低温物理吸附、XPS、SEM和CO₂-TPD等技术对催化剂结构性能进行表征,系统讨论了ZnO在DEO酯交换法合成过程的构效关系。结果发现,DMO与乙醇的酯交换反应活性与ZnO形貌密切相关,其中花状微球ZnO(ZnO-f)的催化活性最高,在最优条件下DMO转化率和DEO选择性分别可以达到99.8 %和75.8 %；重复使用4次后,DMO转化率和DEO选择性分别降低至93.9 %和24.7 %,Zn(OH)₂的生成是催化剂失活的主要原因。动力学研究显示DMO和乙醇在ZnO-f表面的酯交换反应为一级反应,活化能为35.7 kJ/mol,其表面存在的丰富的中强和强碱性位是DMO和乙醇酯交换反应的活性位。

Abstract:

A series of ZnO samples with different morphologies were synthesized by controlled preparation methods and processes,and employed as catalysts for the synthesis of diethyl oxalate (DEO) from dimethyl oxalate (DMO) and ethanol by transesterification route. The structure-activitySrelationships of this transesterification reaction over ZnO were systematically discussed with the help of XRD, N₂Sphysisorption, XPS, SEM and CO₂-TPD techniques. It was found that the catalytic activity of ZnO catalysts in the transesterification of DMO with ethanol is closely related to the ZnO morphologies. Among them, the flower-like ZnO-f is found to be the most active catalyst, and the DMO conversion over ZnO-f can reach 99.8 % with the selectivity to DEO of 75.8 % under the optimal conditions. The DMO conversion and DEO selectivity decrease from 99.1% and 68.2% to 93.9 % and 24.7 % after four cycles, respectively. This catalystSdeactivation can be ascribed to the presence of Zn(OH)₂ phase. Kinetic studies showed that the transesterification of DMO with ethanol over ZnO-f catalystSfollowsSfirst-orderSreaction with activation energy of 35.7 kJ/mol, and the abundant medium and strong basic sites over ZnO-f is responsible for its excellent catalytic activity in the transesterification reaction of the DMO reaction with ethanol.