基于量子化学理论的苯丙氨酸热解过程中NO_(x)前驱体生成机理研究

STUDY ON FORMATION MECHANISM OF NO_(x) PRECURSOR IN PYROLYSIS PROCESS OF PHENYLALANINE BASED ON QUANTUM CHEMISTRY THEORY

选取苯丙氨酸为含氮模型化合物,针对其结构特点设计2种反应路径。采用密度泛函理论的B3LYP方法和6-31G(d,p)基组,对热解过程中所有的反应物、中间体、过渡态及产物进行几何构型的全优化及频率计算,并采用热重-红外联用(TGFTIR)的实验方法对苯丙氨酸热解产生的NO_(x)前驱体进行在线监测。结合理论计算和实验结果对苯丙氨酸的整个热解反应路径进行分析,并据此研究不同结构氨基酸的热解过程和热解产物分布的差异,结果表明苯丙氨酸热解有2种反应路径:第1种为反应物苯丙氨酸结合化合物8并脱去氨形成化合物2,再经过脱羧反应,最终形成含N芳香化合物异喹啉5;第2种为化合物8经过脱水缩合,形成2,5-哌嗪二酮,再通过2种不同的开环方式,最终形成HNCO、HCN等小分子气体。NO_(x)前驱体的释放分为2个阶段——第1阶段NH_(3)、HCN和HNCO均大量析出,而第2阶段HCN为主要析出的气体。从整个热解阶段来看,HCN的析出量最大,NH_(3)次之,HNCO析出量最小;链式结构的简单氨基有利于NH_(3)的形成,含杂环的氨基酸则更有利于HCN的产生,而含苯环结构的氨基酸会使得更多的N进入焦油和焦炭中。

Phenylalanine was selected as a nitrogen-containing model compound,and two reaction paths were designed according to its structural characteristics.The energy gradients of reactants,products,intermediates and transition states for each reaction pathway were optimized,and vibration frequencies of which were calculated using B3 LYP/6-31 G(d,p)based on density functional theory.The NO_(x) precursor produced during phenylalanine pyrolysis process was monitored using thermogravimetric-infrared spectroscopy(TG-FTIR).Based on the theoretical calculation and experimental results,the whole pyrolysis path of phenylalanine was analyzed,and the differences of pyrolysis process and pyrolysis product distribution due to amino acids with different structures were studied.The results show that there are two reaction pathways in the pyrolysis of phenylalanine.The first reaction path is the reaction of phenylalanine and compound 8 where the compound 2 is formed via a condensation reaction,and the isoquinoline 5 with an aromatic compound containing N is finally formed through decarboxylation.The second veaction path is that compound 8 is dehydrated and condensed to form 2,5-piperazinone,and small molecular gases such as HNCO and HCN are formed by two different ring-opening methods.The release of NO_(x) precursor can be divided into two stages.In the first stage,a large amount of NH_(3),HCN and HNCO are released,while HCN is the main gas released in the second stage.In the whole pyrolysis process,the amount of HCN production is the most,which is followed by NH_(3),and the HCNO production is the least.The simple amino group with a chain structure is beneficial to the formation of NH_(3).The amino acid containing heterocycle is more beneficial to the production of HCN,and the amino acid with a benzene ring structure will make more N into tar and char.