Density Functional Theory Study On The Structure Of The Active Centre And The Catalytic Mechanism For Methane Activation Over Mo,W/HZSM-5

The direct conversion of methane into value-added chemicals such as benzene under non-oxidative conditions opens a novel approach to the effective utilization of natural gas. The Mo/HZSM-5 catalyst has been proved to be the best one among the tested catalysts. In this paper,the DFT method is employed to investigate the structures and catalytic properties of Mo carbide. The geometry optimizations are performed and electronic properties are examined,and the relationships between structures and catalytic activities are explored. The activation mechanism of methane C–H bond dissociation on the designed Mo carbide models is investigated. In summary, the obtained results are as follows:1. Four active center models of the monomer and dimer models are proposed, which are Mo(CH₂)₂/ZSM-5, Mo(CH₂)₂CH₃/ZSM-5, Mo₂(CH₂)₄/ZSM-5 and Mo₂(CH₂)₅/ZSM-5. Mo=CH₂ is formed in both the monomer and dimer models. The activation the C–H bond of methane is heterogeneously dissociated with the H⁺ and the H₃C^– moiety attack the C and Mo atom of the Mo=CH₂ bond, respectively, and theÏ€bond is broken simultaneously. The calculated activation energies of methane C–H bond on the four designed models are between 106 and 196 kJ/mol. The Mo₂(CH₂)₅/ZSM-5 model shows the best activity for methane C–H bond dissociation.2. Correspondingly, four active center models of the tungsten carbide, monomer and dimer models are proposed, which are W(CH₂)₂/ZSM-5, W(CH₂)₂CH₃/ZSM-5, W₂(CH₂)₄/ZSM-5 and W₂(CH₂)₅/ZSM-5. The activation mechanism of methane C–H bond dissociation on the designed W carbide models is the same as on the Mo active centers. The calculated activation energies of methane C–H bond on the four designed models are between 86 and 198 kJ/mol, lower than on the Mo active centers.3. Considering the influence of the neighbor acid site to the structures of the active center and the dissociation of the methane C–H bond, the monomer model T6T9-Mo(CH₂)₂/ZSM-5 located at the T6T9 NNN-site as well as the monomer model T6T6-Mo(CH₂)₂/ZSM-5 and T6T6-Mo(CH₂)₂CH₃/ZSM-5 located at the T6T6 NNNN-site are designed. The results shows that the effect of the steric hindrance of the neighbor acid site for the dissociation of methane C–H bond is very small, and the T6T9-Mo(CH₂)₂/ZSM-5 mode shows lower activity.