Impact of the presence of Mo carbide species prepared ex situ in Mo/HZSM-5 on the catalytic properties in methane aromatization

The catalytic activity of HZSM-5 supported Mo-oxide (MoO_x) and Mo-carbide (MoC_y) for methane aromatization was studied using a packed-bed microreactor. MoO_x/HZSM-5 catalysts with 3, 6, 10, and 12 wt.% Mo loading were prepared by incipient wetness impregnation method followed by calcination at 500 °C. The MoC_y/HZSM-5 catalysts were prepared ex situ by treating the oxide catalysts by temperature-programmed reduction and carburization. The MoC_y/HZSM-5 catalysts show significantly higher activities and stability compared to those of the MoO_x/HZSM-5 catalysts. Unlike the oxide catalysts, not only methane conversion and benzene yield improve with higher Mo loading but also the deactivation rate becomes much slower for the carbide catalysts. The optimum carbide catalyst has a Mo loading of 10 wt% The catalysts were characterized by XRD, N₂ adsorption, TPR, NH₃-TPD, TPO,TGA and ²⁷Al-NMR. The results show that the oxide catalysts at a high loading face pore blockage after few hours of reaction, which makes the active sites inaccessible to CH₄. Carbide catalysts, on the other hand face no pore blockage even after a long period of reaction, making them much better catalysts than the oxides for nonoxidative methane aromatization reaction.