Structure and acidity of Mo/ZSM-5 synthesized by solid state reaction for methane dehydrogenation and aromatization

The Mo/ZSM-5 catalysts were synthesized by solid-state reaction with the mixture of MoO3 and HZSM-5 under N2 atmosphere at 500 °C, and characterized by XRD, MAS NMR, BET, NH3-TPD, FT-IR and powder XRD structural analysis. The results showed that the Mo species were well dispersed on the internal and external surface of the zeolite ZSM-5, while the framework of ZSM-5 was retained. One part of Mo species inside the channels of ZSM-5 can be described as [Mo5O12]6+ units in the Mo/ZSM-5 with 8 wt% MoO3 loading. The [Mo5O12]6+ units are situated at the cross-point of the zigzag channel and the straight channel. They interacted with the framework of ZSM-5, and decreased the Brönsted acid sites remarkably. There was about 0.47 [Mo5O12]6+ unit in every cell of the ZSM-5 on an average. The other part of Mo species is distributed on the external surface of the ZSM-5 crystals, and can be described as crystallites of molybdenum oxides. The distribution of Mo species on HZSM-5 increased the Lewis acid sites in the catalysts. The [Mo5OxCy]n+ units inside the channels of ZSM-5 partially reduced by methane maintained the catalytic activity of the catalysts and were beneficial for the formation of benzene during methane dehydrogenation and aromatization at 700 °C.