Ca-Mg-Al复合氧化物催化尿素与1,2-丙二醇制碳酸丙烯酯

Synthesis of propylene carbonate from urea and 1,2-propanediol catalyzed by Ca-Mg-Al composite oxide

采用共沉淀法制备了系列Ca-Mg-Al复合氧化物催化剂,通过对沉淀剂配比、沉淀液pH值及焙烧温度等制备条件的考察,得到以Na2CO3为沉淀剂、pH=9.5、850℃下焙烧4h制备的Ca-Mg-Al催化活性最高。在n(PG)∶n(urea)=1.5∶1、反应温度为145℃、绝压20kPa、反应时间4h、催化剂用量为尿素质量的5%时,碳酸丙烯酯收率达到84.6%。采用XRF、XRD、NH3-TPD、SEM及BET对催化剂的组成、晶型及酸性进行了表征,发现随着沉淀剂中Na2CO3的含量增加,催化剂中CaO∶MgO的比例增大,碳酸丙烯酯的收率亦升高;经850℃焙烧后,催化剂中存在CaO和MgO两种活性中心,起协同催化作用;随着焙烧温度由700℃升高到850℃,NH3-TPD脱附曲线向低温方向偏移,且强酸中心NH3脱附峰面积比由81.14%明显下降为0,中强酸中心NH3脱附峰面积比由0增加到78.07%,而碳酸丙烯酯收率由68%增加到84.6%,这表明催化剂强酸性位的减少是催化活性增加的主要原因。

A series of Ca-Mg-Al mixed oxide catalysts were prepared by co-precipitation method.Through the investigation of the preparation conditions such as the ratio of precipitant,pH of precipitation solution and calcination temperature,it was found that the highest activity of Ca-Mg-Al catalyst was obtained when Na2CO3 was used as precipitant,pH=9.5 and calcined at 850℃for 4h.When the ratio of n(PG)∶n(urea)is 1.5∶1,the reaction temperature is 145℃,20kPa(absolute pressure),the reaction time is 4h and the amount of catalyst is 5%,the yield of propylene carbonate is 84.6%.The composition,crystal form and acidity of the catalyst were characterized by XRF,XRD,NH3-TPD,SEM and BET.It was found that with the increase of the content of Na2CO3 in the precipitant,the ratio of CaO to MgO in the catalyst increased,and the yield of propylene carbonate also increased.After calcination at 850℃,there were two active centers of CaO and MgO in the catalyst,which played a synergistic catalytic role.With the increase of calcination temperature from 700℃to 850℃,the NH3-TPD desorption shifted to low temperature,and the area ratio of NH3 desorption peaks in strong acid center deceased significantly from 81.14%to 0.The area ratio of NH3 desorption peaks in middle acid center increased from 0 to 78.07%,and the yield of propylene carbonate(PC)increased from 68%to 84.6%,which indicates that the decrease in the strong acid sites of the catalyst is the main reason for the increase in catalytic activity.