摘要
为了研究催化裂化过程中,烷烃分子中的C-H键和C-C键异裂生成三配位正碳离子时可能的异裂位置。利用密度泛函理论的量子化学从头算法,计算2-甲基戊烷分子中的伯C-H键、仲C-H键、叔C-H键、仲C-C键和叔C-C键的异裂能。结果表明,在C-H键和C-C的2种不对称的异裂中,总是较大的基团带正电荷,较小的基团带负电荷,即异裂成较大基团为正碳离子载体,较小基团为负碳离子/负氢离子载体时,有利于分散电子云,异裂所需能量较低。C-H键和C-C键的异裂能由大到小的顺序为:伯C-H>仲C-C>仲C-H>叔C-C>叔C-H。异裂能越小,C-H键和C-C键异裂的反应性能越高,其C-H键和C-C键异裂的反应性能由大到小的顺序为:叔C-H>叔C-C>仲C-H>仲C-C>伯C-H。C-H键的异裂能最低,低于异裂能最低的C-C键的异裂能。由于催化裂化过程中,烷烃要参与催化裂化生成正碳离子,如经由质子化裂化和负氢离子转移反应,生成三配位正碳离子,必须按照异裂的方式进行。从异裂能的角度分析,烷烃分子经由催化裂化反应生成正碳离子时,可能是优先从烷烃分子中的C-H键处得以活化。
Carbenium ions would be generated in the heterolysis of C-H bonds and C-C bonds in alkanes in catalytic cracking process, in order to determine the possible bond cleavage in the alkanes. DFT-based （density functional theory） ab initio quantum mechanical methods had been applied to calculate the heterolytic energy of C-H bonds and C-C bonds in 2-methyl pentane. The results suggested that positive charge would be located in the bigger groups and the negative charge in the smaller groups in the heterolysis, the electron atmosphere would be decentralized uniformly when the positive charge located in the bigger groups to generate carbenium ions. The heterolytic energy of C-H bonds and C-C bonds in 2-methyl pentane was primary C-H 〉 secondary C-C 〉 secondary C-H 〉 tertiary C-C 〉 tertiary C-H. The reactivity of C-H bonds and C-C bonds was correlated with the heterolytic energy. The lower heterolytic energy of C-H bonds and C-C bonds, the higher reactivity they were. The reactivity of C-H bonds and C-C bonds： tertiary C-H 〉 tertiary C-C 〉 secondary C-H 〉 secondary C-C 〉 primary C-H. The lowest heterolytic energy of C-H bonds was below the lowest heterolytic energy of C-C bonds. During the protolytic cracking and hydride transfer reaction in catalytic cracking process, the alkanes would be transfer into carbenium ions by hetrolysis of C H bonds or C-C bonds in the alkanes. It suggested that in the catalytic cracking process, the heterolysis reaction took place easily from the C-H bond in the alkanes in order to generate carbenium ions in terms of heterolytic energy.
出处
《计算机与应用化学》
CAS
CSCD
北大核心
2010年第7期871-874,共4页
Computers and Applied Chemistry
基金
国家重点基础研究发展规划"973"项(2006CB202505)资助
