应用密度泛函理论(DFT)研究了Mo/HZSM-5分子筛上碳化钼活性中心的几何结构和电子结构,以及甲烷C-H键在该活性中心上的活化机理.设计了两种碳化钼单体模型Mo(CH_2)_2/ZSM-5和Mo(CH_2)_2CH_3/ZSM-5,两种碳化钼双体模型Mo_2(CH_2)_4/ZSM-5和Mo_2(CH_2)_5/ZSM-5.其中单钼模型构建在ZSM-5分子筛孔道交叉点T6位的Br(o)nsted酸位上,双钼模型构建在T6-T6相邻双酸位上.这些模型中都有Mo=CH_2键,结构优化后得到的Mo-C键长与实验值吻合.所有模型的前线分子轨道都在Mo=CH_2的π键上.甲烷活化过程是发生C-H键异裂,H~+和H_3C~-残基分别进攻Mo=CH_2键的c和Mo,使π键同时断裂.在以上4种碳化铝模型上,甲烷C-H键活化能都在106～196kJ/mol,且Mo_2(CH_2)_5/ZSM-5在甲烷活化过程中显示出最高的催化活性.
Density functional theory (DFT) calculation was employed to investigate the geometric and electronic structure of molybdenum carbide loaded on ZSM-5 zeolite and the catalytic mechanism for methane C-H bond dissociation. Four active center models of the monomer and dimer models were proposed, which were Mo(CH_2)_2/ZSM-5, Mo(CH_2)_2CH_3/ZSM-5, Mo_2(CH_2)_4/ZSM-5, and Mo_2(CH_2)_5/ZSM-5. The monomer model was located at the Bronsted acid site of the T6 site positioned at the intersection of the channels of ZSM-5 zeolite. The dimer model was constructed at the T6---6 Br(o)nsted acid sites. Mo-carbene, in the form of Mo=CH_2, was formed in both the monomer and dimer models, and the optimized bond length of Mo-C was in reasonably good agreement with the corresponding experimental value. The frontier molecular orbitals in the active center were assigned to the p orbitals of the Mo=CH_2 bonds in all four models. The catalytic activity of the Mo carbide active centers was investigated. It was found that the C-H bond of methane was heterogeneously dissociated with the H~+ and the H_3C~- moiety bonded on the C and Mo atoms of the Mo=CH_2 bond, respectively, and the p bond was broken simultaneously. The calculated activation energy of the methane C-H bond in the four models was between 106 and 196 kJ/mol. The Mo_2(CH_2)_5/ZSM-5 model showed the highest activity for methane C-H bond dissociation.
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