ZSM-5分子筛催化1-己烯生成cis-2-己烯的理论研究  

Theoretical Study of the Double-Bond Isomerization of 1-Hexene to cis-2-Hexene over ZSM-5 Zeolite

作　　者：李延锋[1] 朱吉钦[1] 刘辉[1] 贺鹏[1] 王鹏[2] 田辉平[2] 

机构地区：[1]北京化工大学化工资源有效利用国家重点实验室,北京100029 [2]中国石油化工股份有限公司石油化工科学研究院,北京100083

出　　处：《物理化学学报》2011年第5期1081-1088,共8页Acta Physico-Chimica Sinica

基　　金：国家重点基础研究发展计划项目(973)(2010CB732301)资助~~

摘　　要：基于54T团簇模型,采用ONIOM分层计算方法,研究了1-己烯在ZSM-5分子筛上进行顺式双键异构的反应机理.计算结果表明,1-己烯的顺式双键异构反应通过只有分子筛Brφnsted酸部分起作用的机理进行.首先,1-己烯与分子筛的Brφnsted酸性位形成π配位复合物.接着,酸质子发生迁移使1-己烯的双键端基碳原子被质子化,同时双键的另一碳原子与失去质子的Brφnsted酸羟基的氧原子成键,形成稳定的烷氧基中间体.然后,烷氧基中间体中的C―O共价键被打断,同时Brφnsted酸羟基的氧原子从C6H13基团提取一个氢原子还原分子筛的酸性位,并且生成cis-2-己烯.这一反应路径与借助于分子筛活性位的酸-碱双功能性质的反应路径是相互竞争的.计算得到的表观活化能是59.37kJ·mol-1,该值与实验值非常接近.这一结果合理解释了双键异构过程中的能量特征,并且扩展了对分子筛活性位本质的理解.We investigated the double-bond isomerization reaction of 1-hexene to cis-2-hexene on the surface of ZSM-5 zeolite using density functional theory with a 54T cluster model simulating the local structures of zeolite materials. We found that the double-bond isomerization proceeded by a mechanism that did not involve the bifunctional （acid-base） nature of the zeolite active sites but exclusively involved the Brφnsted acid sites. According to this mechanism, 1-hexene is the first physically adsorbed onto the zeolite acid site resulting in the formation of a π-complex, and then the acidic proton of the zeolite transfers to a carbon atom of the double bond of the physisorbed 1-hexene. The other carbon atom of the double bond of the physisorbed 1-hexene bonds with the Brφnsted host oxygen and yields a stable alkoxy intermediate. Thereafter, the Brφnsted host oxygen abstracts a hydrogen atom from the C6H13 fragment and the C―O bond of the alkoxy intermediate is broken, which restores the zeolite active site and yields physisorbed cis-2-hexene. The proposed reaction pathway competes with the bifunctional pathway. The rate-determining step is the decomposition of the alkoxy intermediate with an activation energy of 134. 64 kJ·mol-1. The calculated apparent activation energy for the isomerization reaction is 59. 37 kJ·mol-1, which is in good agreement with the reported experimental value. These results well explain the energetic aspects during the double-bond isomerization and extend the understanding of the nature of zeolite active sites.

关 键 词：ZSM-5 活性位 己烯 密度泛函理论 双键异构 

分 类 号：O643.32[理学—物理化学]