Theory Study Of The Structure And Activity Of Mo/HZSM-5 Catalyst

Catalytic dehydrogenation and aromatization of methane in the absence of gas-phase oxygen into highly value-added chemicals such as benzene and naphthalene turn out to be a promising route for the direct conversion of methane. It is generally accepted that the molybdenum-modified zeolite catalysts which show good catalytic performance in the reation of MDA are bi-functional catalysts. However, the real structure of Mo active center and the mechanism of methane C–H bond dissociation are still unclear. In this paper, we address the geometry and the electronic properties of active Mo species using ab initio density functional theory (DFT) method. According to the experimental studies, we designed the geometry structures of Mo active centers. By studying the electronic structures and the energies of molecular orbitals, the relationships between structures and activities are revealed. Moreover, based on the optimized models for active centers, we further studied methane C–H bond splitting mechanisms and procedures, calculated the activation energy. The main findings are summarized as follows:1. (MoO₂)⁺,(MoO₂)²⁺ monomer and (Mo₂O₅)²⁺ dimmer anchored on ZSM-5 are the reasonable models of Mo-oxo active species. The natural bond orbital (NBO) analysis revealed that there are conjugatedÏ€orbital O≡Mo≡O systems existing in each of the models. The Mo atoms are coordinated with the framework oxygens throughσbond orσ-donation bond. By calculating the formation energies, we concluded that (Mo₂O₅)²⁺ dimer is more preferred to form at NNNN-acidic sites than at NNN-acidic sites; the (MoO₂)²⁺ monomer can only be formed at NNN-acidic sites and the formation energy is higher than that of dimer.2. Mo(CH₂)₂,Mo(CH₂)₂CH3,Mo₂(CH₂)5 and Mo₂(CH₂)4/ZSM-5 models are the reasonable Mo carbide active centers. There areÏ€orbital conjugations in C=Mo=C system, which extends to the framework O–Al–O bridge. The Mo atoms are coordinated with the framework oxygens throughσbond orσ-donation bond.3. According to the electronic structures and the energies of the molecular orbitals, we predict that the reaction between methane and the Mo(CH2)2 and Mo(CH2)2CH3 species would happen between the HOMO of CH4 and the LUMO of Mo active centers, whereas the reaction between methane and the Mo₂(CH₂)5 specie would happen between the LUMO of CH4 and the HOMO of Mo active center. The possible reaction mechanism of methane activation is proposed that the C–H bond of methane splits into CH3 moiety and H, which bond to the Mo atom and the carbon atom of Mo carbide species, respectively, forming stationary intermediate product. The activation barriers for the methane dehydrogenation over Mo(CH2)2/ZSM-5, Mo(CH2)2CH3/ZSM-5 and Mo₂(CH₂)5/ZSM-5 were 112.03,196.17 and 106.28kJ/mol, respectively.