摘要
应用密度泛函理论研究了Mo/MCM-22分子筛上碳化钼活性中心的几何结构和电子结构,以及甲烷在该活性中心上的活化机理.设计了两种结构的活性中心模型:Mo(CH2)2(模型A)和Mo(CH)CH2(模型B);它们都嫁接在MCM-22分子筛超笼边缘的T4位的Brφnsted-酸性位上,用3T簇模型代替分子筛的骨架,对所设计的模型进行了几何结构优化和电子结构分析.结构优化结果显示,Mo与CH2端基以双键结合,键长为0.18～0.19nm,而Mo与CH端基以叁键结合,键长为0.17nm.通过自然键轨道分析,证明中心钼原子以配位键与骨架氧原子结合.根据前线分子轨道的分析,预测了甲烷活化反应将发生在甲烷分子的HOMO和钼活性中心的LUMO之间,即C-H键的电子流向Mo-C键的π*轨道.甲烷C-H键发生异裂,H+和H3C-基团分别与Mo-C键上的Mo和C成键.在模型A上,甲烷活化反应的活化能为119.97kJ/mol;在模型B上,甲烷的H原子可以分别结合到CH2端基和CH端基上,对应的活化能分别为91.37和79.07kJ/mol.
Density functional theory was employed to study the geometric and electronic structure of molybdenum carbide loaded on MCM-22 zeolite and predict the mechanism for CH4 activation. Two models of active center, Mo（CH2）2 （Model A） and Mo（CH）CH2 （Model B）, were designed; they were located on the T4 site of the supercage in MCM-22 zeolite. The geometry optimization and electronic structure analysis of the models were performed based on 3T cluster model. The optimized geometry showed that the Mo connected with CH2 terminal group by double bond （bond length 0.18-0.19 nm） and with CH terminal group by triple bond （bond length 0.17 nm）. The natural bond orbital calculation revealed that the Mo was bonded to framework oxygen through a bond. In terms of the composition and the energy of the frontier orbitals, it was suggested that the CH4 activation on Mo carbide active center would happen between the HOMO of CH4 molecule and the LUMO of Mo carbide. Namely, electrons preferred to transfer from the σ-orbital of C-H bond to the π ＊ -orbital of Mo-C bond. After heterogenous splitting of C-H bond, the H3C group was bonded to Mo and the H^＋ was bonded to C in Mo carbide species. The calculated activation energy on Model A was 119.97 kJ/mol. On Model B, there were two possible pathways, in which the hydrogen in CH4 could attach to CH2 and CH terminal groups. The corresponding activation energy was 91.37 and 79.07 kJ/mol, respectively.
出处
《催化学报》
SCIE
EI
CAS
CSCD
北大核心
2007年第2期180-186,共7页
基金
辽宁省教育厅科研基金(A205L211).
关键词
MCM-22分子筛
碳化钼
密度泛函理论
甲烷
活化能
MCM-22 zeolite
molybdenum carbide
density functional theory
methane
activation energy
