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

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(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 Brönsted 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 Brönsted acid sites. Mo-carbene in the form of Mo=CH2 was formed in both the monomer and dimer models, and the optimized bond lengths of Mo-C were in reasonably good agreement with the corresponding experimental values. The frontier molecular orbitals in the active center were assigned to the p orbitals of the Mo=CH2 bonds in all four models. The catalytic activity of the Mo carbide active centers were investigated. It was found that the C-H bond of methane was heterogeneously dissociated with the H+ and the H3C− moieties bonded on the C and Mo atoms of the Mo=CH2 bond, respectively, and the p bond was broken simultaneously. The calculated activation energies of the methane C-H bond in the four models were between 106 and 196 kJ/mol. The Mo2(CH2)5/ZSM-5 model showed the best activity for methane C-H bond dissociation.

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 Brönsted 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∼196 kJ/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.

Graphical abstractThe Mo2(CH2)5/ZSM-5 was proved as the effective active center for methane activation by using DFT calculations. The C-H bond dissociation occurred on the p orbital of Mo=CH2 with the activation energy of 106 kJ/mol.Figure optionsDownload full-size imageDownload as PowerPoint slide