摘要
在催化裂化过程中,负氢离子转移反应是C—H键的主要反应。针对催化裂化反应,从反应化学观点阐述了一系列烷烃C—H键异裂反应以及与其相关的正碳离子反应,讨论了C—H键异裂和正碳离子稳定性对负氢离子转移反应的影响。基于烷烃的C—H键异裂能,计算了一些反应的负氢离子转移反应热,给出了裂化过程中正碳离子碳数分布对负氢离子转移反应的影响规律。从L-H和Rideal机理观点,论述了双分子负氢离子转移反应途径。从分子轨道对称规则考虑,按Rideal机理进行的负氢离子转移反应途径是对称允许。基于正碳离子化学、量子化学和动力学的部分研究结果,论述了有关负氢离子转移反应的中间体和过渡态。分子筛催化剂上,负氢离子转移反应的活性中间体非常相似于吸附的非经典的五配位正碳离子,它同催化剂之间的作用力是库仑力。笔者力求在所论述的几个方面展示出新的视野,对负氢离子转移反应的研究提出一些见解。
The hydrogen is transferred as a negative species in hydride transfer, which plays a wellrecognized important role in catalytic cracking. Hydride transfer as well as heterolytic cleavage of C--H bond were described from a viewpoint of reaction chemistry. The carbonium ion is key active intermediate related to hydride transfer reactions. The heat of hydride transfer reaction （e. g. nC16H34＋R^＋ →nC16 H33^＋＋RH） was calculated according to the heterolytic cleavage energies of C--H bonds of nC16 H34 and RH. The calculation gave a series of regular results ： the distribution of carbon numbers of carbonium ion in catalytic cracking was an important factor influencing hydride transfer reactions, H^＋ as hydride acceptor was more effective than carbonium ion for hydride transfer reactions. Langmuir-Hinshelwood mechanism and Eley-Rideal mechanism were discussed for understanding the mechanism of hydride transfer reaction. The interesting application of orbital symmetry rules is to predict the course of reactions. The course of hydride transfer reactions based on Eley Rideal model is seen to be orbital symmetry allowed. The quantum-chemical calculations provided a reasonable description of hydride transfer reactions in catalytic transformations of paraffins on solid acid catalyzed hydrocarbon reactions. Hydride transfer reactions between alkanes and carbonium ions were found to proceed through a mechanism consisting of two steps： the formation of tight carbonium ion intermediate complexes and then the proceeding of hydride transfer reactions. The activated complexes of hydride transfer reaction resembled very much the adsorbed nonclassical carbonium ions, and these transient species were strongly held at the surface active sites by the Coulomb interaction. The optimized geometry of the intermediate complexes was linear and with an open three-center two-electron C--H--C bond, that is more or less symmetrical depending on the nature of the involved carbon atoms. Up till now, as the＂ true＂ mechanism of hydride transfer reaction has poorly been understood, it remains to be further studied. Some suggestions for study on reaction mechanism of hydride transfer were presented.
出处
《石油学报（石油加工）》
EI
CAS
CSCD
北大核心
2008年第4期365-369,共5页
Acta Petrolei Sinica(Petroleum Processing Section)
关键词
氢转移
负氢离子
C-H键
异裂
正碳离子
催化裂化
hydrogen transfer
hydride
C--H bond
heterolytic cleavage
carbonium ion
catalytic cracking
