The Structure, Stability, and Reactivity of Mo-oxo Species in H-ZSM5 Zeolites:  Density Functional Theory Study

The geometry and binding characteristic of Mo-oxo species anchored in the channels of H-ZSM5 zeolites were investigated by the density functional theory (DFT) method. The structures of the (MoO<sub>2</sub>)<sup>2+</sup> monomer and the (Mo<sub>2</sub>O<sub>5</sub>)<sup>2+</sup> dimer were optimized based on the 6T cluster model. The calculations revealed that the (Mo<sub>2</sub>O<sub>5</sub>)<sup>2+</sup> dimer preferred to form at the next-next-near-neighbor-positioned Brönsted acid sites. The calculated Raman vibrational frequencies are in good agreement with the experimental result. The binding characteristics and electronic configurations of the (MoO<sub>2</sub>)<sup>2+</sup> monomer and the (Mo<sub>2</sub>O<sub>5</sub>)<sup>2+</sup> dimer were examined by using natural bond orbital (NBO) analysis. The HOMO (highest occupied molecular orbital) in (Mo<sub>2</sub>O<sub>5</sub>)<sup>2+</sup>/ZSM5 is related to the p orbital of the framework oxygen, whereas the LUMO (lowest unoccupied molecular orbital) is assigned to the antibonded π orbital of the Mo⋮O triple bond. The reactivity of the (Mo<sub>2</sub>O<sub>5</sub>)<sup>2+</sup> dimer toward methane C−H bond dissociation was examined, and the transition state was determined with an activation energy of 63.5 kcal/mol.