Theoretical Study of Reaction Mechanism of Cyclohexene Aromatization over H-ZSM-5 Zeolite

By using the QM/MM hybrid ONIOM2 (B3LYP/6-31G(d,p):UFF) method, the dehydro-aromatization reaction of cyclohexene over H-ZSM-5 zeolite was studied. The calculation was based on the 76T cluster model with Al substituted at the 112 site. The results indicated that cyclohexene was adsorbed on the Bronsted acid (B-acid) site of H-ZSM-5 zeolite. The dehydrogenation reaction consisted of cleavage of a C H bond by acidic proton. After release of dihydrogen, the cabonium moiety was bonded with the basic framework oxygen, resulting in surface cyclohexene alkoxide intermediate. Then the alkoxide released the proton to the framework oxygen which led to generation of B-acid site, and the produced cyclodihexene was adsorbed on the B-acid site. After subsequent dehydrogenation and deprotonation steps, a benzene molecule was produced as final product. The activation energy for dehydrogenation steps was 279.64 and 260.21 kJ/mol, respectively, and the activation energy for deprotonation process was 74.64 and 59.14 kJ/mol, respectively. All dehydrogenation steps are endothermic to lead to surface alkoxide intermediates, and the subsequent deprotonation have lower energy barriers and are exothermic. In addition, the three competitive reactions of cyclohexene, i.e. protonation, hydrogen exchange, and dehydrogenation, were also compared. It was found that cyclohexene preferred to dehydrogenate with zeolite proton.