A Theoretical Study on the Catalytic Mechanisms of Ethane to Aromatics over Zn-FAU Zeolite

The ethane-to-aromatics (ETA) mechanisms on Zn-modified FAU have been studied by a two-layer ONIOM approach. The ETA process contains two stages: ethane to ethylene (ethane dehydrogenation) and ethylene to aromatics (ethylene aromatization). Two models, (Zn-O-Zn)²⁺ and (O-Zn-O)^2-, are used as catalytic centers in the Zn-FAU. Overall, the (Zn-O-Zn)²⁺ site is more favorable for the ETA process than the (O-Zn-O)^2- site. The rate-limiting step of ethane dehydrogenation is ethyl dehydrogenation to produce ethylene on the (Zn-O-Zn)²⁺ and (O-Zn-O)^2- models. For ethylene aromatization, the cyclization step of the -CH₂-CH₂-CH₂-CH₂-CH═CH₂ group is rate-limiting on the (Zn-O-Zn)²⁺ site. On the (O-Zn-O)^2- model, the dehydrogenation of the cyclohexene carbenium ion is the rate-limiting step. Compared with the ethane dehydrogenation, the ethylene aromatization will take place more slowly because of a higher activation barrier. Ethylene will be the main product for the ETA mechanism over Zn-FAU, which is in accord with the experimental findings. The nature of transition states (TSs) and electron flow between different TS fragments are analyzed by quantum wave functions. The complicated attractive force, repulsive force, and partial covalent interactions between different TS fragments are found. The direction of electron migration has been revealed in detail.