Abstract: 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[KG10x](Model A) and Mo(CH)CH2[KG10x](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[KG-45x]-[KG-20x]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 σ 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[KG-45x]-[KG-20x]H bond to the π*-orbital of Mo[KG-45x]-[KG-20x]C bond. After heterogenous splitting of C[KG-45x]-[KG-20x]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 [WTHZ]A[WTBZ] was 119.97 kJ/mol. On Model [WTHZ]B[WTBZ], 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.

Key words: MCM-22 zeolite, molybdenum carbide, density functional theory, methane, activation energy