Theoretical study of 1-hexene activation over La/ZSM-5 zeolite

Theoretical investigations of the interaction of 1-hexene with an La-modified ZSM-5 zeolite, and the mechanism of the activation reaction of 1-hexene on the surface of the La-modified ZSM-5 zeolite have been carried out using density functional theory and the ONIOM method with 54T cluster models simulating the local structures of the zeolite materials. The study includes the geometry optimization and characterization of the reactant, product, and transition state, and the calculation of the activation energies for the process. It was found that there was no π-complex formed, but instead a weaker interaction of the alkyl group and the zeolite framework or acidic center present in the adsorption complexes. The activation of 1-hexene proceeds via heterolytic O-H bond cleavage in the La species, in which case, the acidic proton of the La species transfers to one carbon atom of the double bond of the 1-hexene, while the other carbon atom of the double bond of the 1-hexene bonds with the oxygen of the La species, yielding a stable alkoxy activated product. The calculated activation barrier for the activation of 1-hexene is 160.64 kJ/mol. The results can explain the experimental phenomena observed during the course of the catalytic cracking of olefins over the La-modified ZSM-5 zeolite.