Mo/ZSM-5分子筛碳化钼活性中心结构及甲烷活化机理的密度泛函理论研究  

Density Functional Theory Study on Structure of Molybdenum Carbide and Catalytic Mechanism for Methane Activation over ZSM-5 Zeolite

作　　者：邢双英[1] 周丹红[1] 曹亮[1] 李新[2] 

机构地区：[1]辽宁师范大学化学化工学院功能材料化学研究所,辽宁大连116029 [2]中国科学院大连化学物理研究所催化基础国家重点实验室,辽宁大连116023

出　　处：《催化学报》2010年第4期415-422,共8页

基　　金：国家自然科学基金(20073058);辽宁省教育厅高等学校科学研究项目(A类05L211)

摘　　要：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(CH2)2/ZSM-5和Mo(CH2)2CH3/ZSM-5,两种碳化钼双体模型Mo2(CH2)4/ZSM-5和Mo2(CH2)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Brnsted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=CH2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=CH2的π键上.甲烷活化过程是发生C–H键异裂,H+和H3C–残基分别进攻Mo=CH2键的C和Mo,使π键同时断裂.在以上4种碳化钼模型上,甲烷C–H键活化能都在106～196kJ/mol,且Mo2(CH2)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 mono-mer and dimer models were proposed, which were Mo（CH2）2/ZSM-5, Mo（CH2）2CH3/ZSM-5, Mo2（CH2）4/ZSM-5, and Mo2（CH2）5/ZSM-5. The monomer model was located at the BrΦnsted 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---T6 Brnsted acid sites. Mo-carbene, in the form of Mo=CH2, 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 π orbitals of the Mo=CH2 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 H3C– moiety bonded on the C and Mo atoms of the Mo=CH2 bond, respectively, and the π 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 Mo2（CH2）5/ZSM-5 model showed the highest activity for methane C–H bond dissociation.

关 键 词：密度泛函理论 碳化钼 ZSM-5分子筛 甲烷 活化能 

分 类 号：O643.36[理学—物理化学]