Ethane-to-aromatics conversion over gallium-modified FAU zeolite: a two-layer ONIOM theoretical study

Abstract

The catalytic performance of gallium-modified FAU (Ga-FAU) zeolite on the ethane-to-aromatics (ETA) process was studied by a two-layer ONIOM (our Own N-layered Integrated molecular Orbital and molecular Mechanics) method implemented in Gaussian software. The whole ETA mechanism includes two pathways: ethane dehydrogenation to ethylene and ethylene aromatization. Four different Ga models ([GaH₂]⁺, [GaH]²⁺, [GaO]⁺, and Ga⁺) have been used over the Ga-FAU zeolite. For the ethane dehydrogenation, the order of reactivity is [GaH]²⁺ > [GaH₂]⁺ > [GaO]⁺ > Ga⁺. We selected the [GaH₂]⁺ site to study the ethylene aromatization. On the [GaH₂]⁺ model, the ethane dehydrogenation could take place through both the stepwise pathway (three steps) and the concerted pathway. The three-step pathway is more favorable than the concerted pathway. The rate-determining step of ethylene aromatization is the dehydrogenation of cyclohexene cation. The ethylene aromatization proceeds more slowly than the ethane dehydrogenation due to the higher energy barrier, and thus, the ethylene molecule should be the major product in the whole ETA process on Ga-FAU. The differential charge density (DCD), reduced density gradient (RDG), and local orbital locator (LOL) were employed to analyze the direction of electron migration and the nature of interactions in different TS fragments. The RDG analysis suggests that except for attractive force, there is strong spatial repulsive interaction between fragments that are close in distance, especially in organic carbon ring. The LOL maps indicate that partial covalent interactions often exist in the region where the chemical bonds are forming or breaking. The DCD plots reveal the variation of electron densities of different TS fragments. The electrons always migrate from the fragments with the negative DCD values to the fragments with the positive DCD values.

Graphical abstract