Periodic density functional theory analysis of direct methane conversion into ethylene and aromatic hydrocarbons...

Mo/ZSM-5-catalyzed methane conversion into aromatic hydrocarbons is an important reaction to produce ethylene and benzene, but the detailed reaction mechanism has not been investigated due to its high complexity. In the present study, density functional theory combined with a periodic model was used to investigate the reaction mechanism of direct methane conversion into aromatic hydrocarbons catalyzed by Mo/ZSM-5. The calculation results show that the active phase for Mo is Mo₄C₂ instead of MoO_x. The whole reaction processes processed via the following steps: the C-H bond in methane was first activated by Mo₄C₂ with an energy barrier of 1.01 eV and then converted into ethylene species via the coupling of two CH₃ species as well as two successive dehydrogenation steps (2CH₃ → C₂H₆ → C₂H₄ + 2H). The rate-controlling step for the processes to form ethylene is the coupling of two methyl species with a barrier of 1.22 eV. The produced ethylene species then react with each other to produce C₆H₈via the reaction of 3C₂H₄ → C₃H₈ + 2H₂, and molecular benzene is formed by successive dehydrogenation of C₆H₈. The rate-limiting step for benzene formation from ethylene is the cyclization step of chain C₆H₈ with an energy barrier of 1.21 eV. Additionally, molecular propane (C₃H₈) is formed by the reaction of C₂H₄ + CH₄ → C₃H₈, and the controlling step C₃H₇ + H → C₃H₈ has a barrier of 1.46 eV. Molecular C₁₀H₁₂ is produced via coupling of C₆H₈ and C₂H₄, where the limiting step is the dehydrogenation step of C₈H₁₂ with an energy barrier of 1.44 eV. Our present calculation results indicate that the selectivity of benzene was the largest among the possible products, that is, C₂H₄, C₃H₆, C₆H₆ and C₁₀H₁₂, based on the corresponding controlling step barrier. Importantly, the rate-controlling step for the whole reaction process from methane to benzene is the dissociative adsorption of methane (CH₄ → CH₃ + H) with an energy barrier of 1.83 eV when considering entropy contribution. The present study may help people design a good catalyst for the formation of benzene from methane; in other words, the catalyst should have a good ability to activate the C-H bond of molecular methane.