Theoretical Study On The Reaction Mechanisms Of Cyclohexene Aromatization Over HZSM-5Zeolite

Using QM/MM hybrid ONIOM2method with two layer76T cluster model, we studied the dehydro-aromatization reaction mechanisms of cyclohexene over HZSM-5zeolite and the dehydrogenation reaction mechanism of cyclohexene on GaH2/ZSM-5zeolite. The main contents of the paper are listed as follow:1. The dehydro-aromatization of cyclohexene over HZSM-5zeolite was performed through four steps. Firstly cyclohexene was adsorbed on the Br(?)nsted acid site of zeolite. The dehydrogenation reaction included cleavage of a C-H bond by acidic proton. After release of hydrogen molecule, the cabonium moiety was bonded with the basic framework oxygen, resulting in surface cyclohexene alkoxide. Then the alkoxide released the proton to the framework oxygen which led to generation of B-acid site and the produced cyclodihexene adsorbed on the B-acid site. After further dehydrogenation and deprotonation steps, the final product of benzene moleclule was produced. The activation barriers for dehydrogenation reactions were279.64and260.21kJ/mol, respectively, and for deprotonation steps were74.64and59.14kJ/mol, respectively, The results indicated that the dehydrogenation processes is the rate-limiting step. With the unsaturation state going up, the deprotonation and dehydrogenation reactions become much easier. All the products of dehydrogenation steps are surface alkoxide intermediates and the reaction are endothermic, and the subsequent deprotonation are exothermic and have lower energy barriers. In addition, the three competitive reactions about cyclohexene were also been compared, which included dehydrogenation, protonation and hydrogen exchange reaction mechanism. It was found that cyclohexene preferred to dehydrogenate with zeolite proton.2. The new catalytic active center was obtained by exchange (GaH2)+with proton in HZSM-5zeolites. On this active center, dehydrogenation reaction of cyclohexene was performed through the stepwise mechanism or concerted mechanism. The stepwise mechanism includes deprotonation, dehydrogenation and further deprotonation steps. The concerted mechanism was started by adsorption of cyclohexene on GaH2/ZSM-5zeolite, then C-H bond rupture, dehydrogenation and π bond formation proceeded synchronously. The results indicated that the active barrier for dehydrogenation was quite small, but the deprotonation was not; the reactions occurring through the stepwise mechanism showed smaller activation barriers than the concerted mechanism. However, the energy barrier for the stepwise mechanism seemed to be strongly influenced by the size of the reactants.