物理化学学报 >> 2011, Vol. 27 >> Issue (05): 1081-1088.doi: 10.3866/PKU.WHXB20110516

理论与计算化学 上一篇    下一篇

ZSM-5分子筛催化1-己烯生成cis-2-己烯的理论研究

李延锋1, 朱吉钦1, 刘辉1, 贺鹏1, 王鹏2, 田辉平2   

  1. 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
  • 收稿日期:2011-01-06 修回日期:2011-02-08 发布日期:2011-04-28
  • 通讯作者: 刘辉 E-mail:hliu@mail.buct.edu.cn
  • 基金资助:

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

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

LI Yan-Feng1, ZHU Ji-Qin1, LIU Hui1, HE Peng1, WANG Peng2, TIAN Hui-Ping2   

  1. 1. State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China;
    2. Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, P. R. China
  • Received:2011-01-06 Revised:2011-02-08 Published:2011-04-28
  • Contact: LIU Hui E-mail:hliu@mail.buct.edu.cn
  • Supported by:

    The project was supported by the National Key Basic Research Program of China (973) (2010CB732301).

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Brønsted酸部分起作用的机理进行. 首先, 1-己烯与分子筛的Brønsted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Brønsted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Brønsted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59.37 kJ·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.

关键词: ZSM-5, 活性位, 己烯, 密度泛函理论, 双键异构

Abstract:

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 Brønsted 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 Brønsted host oxygen and yields a stable alkoxy intermediate. Thereafter, the Brønsted 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.

Key words: ZSM-5, Active site, Hexene, Density functional theory, Double-bond isomerization

MSC2000: 

  • O641