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

[Display omitted] •A new approach to generating zeolite-supported Mo carbide active species with improved performance for methane aromatization is presented.•Mo carbides formed ex situ constitute more stable catalysts than Mo oxides where Mo carbides are formed in the reaction induction period.•10 wt% Mo carbide catalyst exhibits a benzene yield of 8% and little deactivation; in contrast the best Mo oxide catalyst deactivates completely.•The amount of carbon deposits formed after reaction does not play a role in the catalyst deactivation.•Careful preparation of supported Mo carbide species overcomes the rapid deactivation characteristic of Mo oxide supported systems. The catalytic activity of HZSM-5 supported Mo-oxide (MoOx) and Mo-carbide (MoCy) for methane aromatization was studied using a packed-bed microreactor. MoOx/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 MoCy/HZSM-5 catalysts were prepared ex situ by treating the oxide catalysts by temperature-programmed reduction and carburization. The MoCy/HZSM-5 catalysts show significantly higher activities and stability compared to those of the MoOx/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.