有机胺改性对ZIF-8催化Knoevenagel缩合反应活性的影响

Effect of Amine Modification on Catalytic Activity of ZIF-8 in Knoevenagel Condensation Reaction

采用不同结构的有机胺改性溶剂热法合成ZIF-8催化剂,探讨胺结构特性对ZIF-8催化Knoevenagel缩合反应活性的影响.结果表明,有机胺改性后ZIF-8保持菱形十二面体结构,形貌规则与未改性材料无明显差别,1,2丙二胺、二乙烯三胺和三乙烯四胺改性ZIF-8后的BET比表面积分别为1893 m2·g^-1、1885 m2·g^-1和1861 m2·g^-1,较改性前下降约6.5%,这主要是由于接枝在ZIF-8表面的有机胺堵塞了其孔道;采用乙醇作溶剂,催化剂添加量(摩尔分数)为0.6%(相对于苯甲醛用量),反应温度80℃,210 min时,1,2丙二胺改性ZIF-8对Knoevenagel反应的催化活性最高,α-氰基肉桂酸乙酯的产率达97.8%,循环10次后,产率依旧保持90%以上,较未改性催化剂产率提高35.3%.对胺改性ZIF-8的催化机理研究表明:有机胺改性ZIF-8可增加其催化活性位点,而1,2丙二胺因链短,空间位阻小,其N活性位点更易与反应物接触,与ZIF-8上原有的咪唑N位点一起通过孤对电子与反应物苯甲醛的亚甲基上的-H配位,从而显著提高催化剂的活性.

In this paper,different kinds of organic amines were used to modify ZIF-8 catalysts prepared via solvothermal method,and the effects of organic amine structural characteristics on the catalytic activity of ZIF-8 for Knoevenagel condensation reaction were discussed.The results show that ZIF-8 modified by organic amine still maintains the rhombohedral dodecahedron structure and possesses regular morphology,without obvious difference from that of unmodified materials.The BET specific surface areas of ZIF-8 modified by 1,2-propylenediamine,diethylenetriamine and triethylenetetramine are 1893 m2·g^-1,1885 m2·g^-1 and 1861 m2·g^-1,respectively,decreased by about 6.5%compared with that of the unmodified materials,mainly due to the blockage of pores for ZIF-8 by organic amines.When ethanol works as solvent,the amount of catalyst is 0.6 mol%(molar ratio to benzaldehyde),the reaction temperature is 80℃.When the reaction time keeps for 210 min,1,2-propanediamine modified ZIF-8 exhibits the highest catalyst activity,and the yield of ethylα-cyanocinnamate reaches 97.8%.After 10 cycles,the yield of this reaction is still up to 90%,an increase of 35.3%higher than that of unmodified ZIF-8.The studies on the catalytic mechanism of amine-modified ZIF-8 indicate that amines modified ZIF-8 can improve the amount of activity site,while 1,2-propanediamine has a shorter chain and less steric hindrance,making its N sites easier contact with reactants.It combines with the original imidazole N site on ZIF-8 to coordinate withα-H on the methylene of benzaldehyde through lone pair electrons,therefore significantly improving the catalytic activity of the catalyst clearly.