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

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 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 host oxygen and yields a stable alkoxy intermediate. Thereafter, the 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 ratedetermining step is the decomposition of the alkoxy intermediate with an activation energy of 134. 64 kJ· mol. 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. 1081 Acta Phys. ⁃Chim. Sin. 2011 Vol.27