Theoretical study on the cracking reaction catalyzed by a solid acid with zeolitic structure: The catalytic cracking of 1-hexene on the surface of H-ZSM-5

The cracking mechanism of 1-hexene over H-ZSM-5 zeolite has been studied by using the ONIOM(B3LYP/6-31G(d,p):UFF) method with an 88T cluster model and the B3LYP/6-31G(d,p) method with a 5T cluster model. The computation results show that the cracking process follows a two-step mechanism involving a carbenium ion intermediate. When the 88T cluster model is used, a stable carbenium ion can be found. When the 5T cluster model is used, no stable carbenium ion is obtained, however, the intrinsic reaction coordinate prompts that there exists a carbenium ion intermediate during the reaction process. The theoretical researches indicate that the H-ZSM-5 zeolite environment plays a significant role in stabilizing the hexyl carbenium ion. The adsorbed carbenium ion on the surface of zeolite is an active high energetic species, and the cleavage of the CC bond on β position occurs easily. The extended zeolite framework has a large effect on the energetics of the adsorbed complexes. The apparent energy barrier of the cracking reaction with 88T cluster model is 104.73 kJ/mol and that with 5T cluster model is 149.50 kJ/mol.

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► The H-ZSM-5 zeolite is found to have high activity and selectivity in light olefin cracking reactions.
► The cracking mechanism of hexene over H-ZSM-5 zeolite is investigated by using ONIOM method.
► The olefin cracking process follows a two-step mechanism involving a carbenium ion intermediate.
► The H-ZSM-5 zeolite environment plays a significant role in stabilizing the hexyl carbenium ion.
► The extended zeolite framework also has profound effects on the energetics of the system.