DFT Studies On The Structure Of Mo/HMCM-22 Catalyst And The Reaction Mechanism For Methane Activation

MCM-22 possesses a unique crystal structure containing two independent pore systems. The Mo loaded MCM-22 catalyst has been shown to be an excellent catalyst for methane dehydroaromatization with higher benzene selectivity. It has been proposed that the acid sites in supercage of HMCM-22 zeolite play an important role for its shape-selective catalysis.In the present work, DFT calculations were used to study the structures of molybdenum oxo species and molybdenum carbide at the T4 position in HMCM-22 zeolite. We used the natural bond orbital (NBO) calculations to analyze the bonding characteristics of the active centers. The reactions of methane on the active centers were investigated and two possible reaction pathways were proposed. The transition states and the activation energies of these reactions were obtained. The formation process of molybdenum oxo species were investigated through calculating the thermodynamic properties. The main conclusions are as follows1) ConjugatedÏ€orbital system exists in the active center, and Mo is bonded to framework oxygen throughσbond. The highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO) are concentrated in the Mo=C bond. In terms of the composition and the energy of the frontier orbital, it was suggested that the activation of methane on Mo carbide active center would happen between the HOMO of methane molecule and the LUMO of Mo carbide. Namely, the electrons preferred to transfer from theσ-orbital of C–H bond to theÏ€*orbital of Mo–C bond. This is crucial for the C-H bond activation of methane. A reaction mechanism is as follows: After heterogeneous splitting of C–H bond, the H₃C^- group was bonded to Mo and the H+ was bonded to C in Mo carbide species, which led to the relative stable intermediates. 2) The reactions between MoO₂(OH)₂ and B acid sites may produce two kinds of Mo oxo species: MoO₂ and Mo₂O₅. The MoO₂(OH)₂ reacts with the protons on two adjacent B-acid sites, resulting in the grafted MoO₂ by releasing two water molecules. Whereas, if two MoO₂(OH)₂ molecules react with the protons on two adjacent B-acid sites, the anchored two MoO₂(OH) groups will be dehydrated to form Mo₂O₅ species at the B acid sites.3) From the point of energy, the loading of the first Mo(OH)₂O₂ molecule on the B acid is a spontaneous process. The adjacent B acid site is preferred to react with another MoO₂(OH)₂ molecule rather than to react with MoO₂(OH) group to form MoO₂ species. The loaded two MoO₂(OH) groups can dehydrate to form Mo₂O₅ species. Through comparing the stability of products and the trend of reactions, we can draw the conclusion that the Mo₂O₅ group is most likely to load on acid sites. The precise locations are T1-T1(3N),T1-T4(4N) sites of the 12-MR in the entrance of supercage.