Acta Phys. -Chim. Sin. ›› 2011, Vol. 27 ›› Issue (05): 1081-1088.doi: 10.3866/PKU.WHXB20110516

• THEORETICAL AND COMPUTATIONAL CHEMISTRY • Previous Articles     Next Articles

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).

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