连续流动条件下含氮Beta分子筛催化甲缩醛-异丁烯Prins反应

Prins Reaction of Methylal and Isobutene Catalyzed by Nitridated Beta Zeolites under Continuous-flow Conditions

采用高温氨气氮化法,制备了一系列含氮Beta分子筛样品,并在连续流动条件下,考察了其催化甲缩醛与异丁烯经Prins缩合制异戊二烯的反应性能.利用XRD、N2吸附/脱附、元素分析、29SiMAS NMR、NH_(3)-TPD、CO_(2)-TPD和TG等方法对氮化分子筛的结构、酸碱性和反应后积碳进行了表征,研究了反应条件及样品的酸碱性对反应性能的影响.结果表明,以35%甲缩醛水溶液为反应物,在320℃、2 MPa、异丁烯与甲缩醛摩尔比为7的适宜条件下,铵型Beta为前驱体制备的催化剂对甲缩醛的转化率达99%,异戊二烯的产率达85%.氮化后少量的酸性位和较多的碱性位有利于维持甲缩醛较高的转化率、较高的异戊二烯产率及较低的反应积碳量.催化剂再生6次后仍保持75%异戊二烯产率,样品中Si―N键削弱导致酸碱性变化是再生后反应性能下降的主要原因.

Isoprene is a very important C5 commodity,with wide utilization in the synthesis of rubber,thermoplastic polymers,and fine chemicals.The production of isoprene directly via Prins reaction of formaldehyde and isobutene in a fixed-bed reactor has received increasing attention from academia and industry owing to its simple process,low energy consumption.The balance of basicity and acidity drives for the high selectivity of isoprene.Nitrogen-containing zeolites have both basic sites and acidic sites,and their amounts and strength can be tuned feasibly.In this article,methylal was used to produce formaldehyde in-situ,followed by Prins condensation between formaldehyde and isobutene to isoprenol,and final conversion to isoprene on nitridated Beta zeolites under continuous flow conditions.XRD,N2 adsorption/desorption,elemental analysis,29Si MAS NMR,NH_(3)-TPD,CO_(2)-TPD and TG techniques were used to characterize the structure,acidity,basicity and the coke content of the nitridated Beta zeolites.Nitridated Beta zeolite obtained from the ammonia-form precursor shows a high isoprene yield of 85%at a methylal conversion of 99%under the optimized conditions of 2 MPa,320℃,isobutene/methylal molar ratio of 7 and methylal aqueous solution concentration of 35%.NH_(3)-TPD and CO_(2)-TPD characterizations indicate that a small amount of acid sites and a sufficient amount of middle-strong and strong strength basic sites are beneficial for the methylal conversion and isoprene selectivity.In addition,the coke amount over the spent nitridated Beta zeolites decreases significantly due to their less acid sites.In addition,it also shows good regeneration capability,with isoprene yield remaining as high as 75%after six successive regeneration cycles.The variation of basicity and acidity caused by the weakening of Si―N species in regenerated catalyst accounts for the decline of reaction performances after regeneration.