Aromatization of methane over supported and unsupported Mo-based catalysts

The interaction of methane with unsupported and supported molybdenum compounds (Mo, MoO2, MoO3, Mo2C, and MoC(1-x)) has been investigated at 973 K. ZSM-5 was used as a support. Reaction products were analyzed using gas chromatography. Changes in the composition of catalyst samples were followed by X-ray photoelectron spectroscopy. Molybdenum metal and oxides interacted strongly with methane at 973 K to give H-2 (Mo) and H2O and CO2 (oxides), but only a trace amount of ethane. When these compounds were contacted with ZSM-5, the reaction pathway of methane initially was the same. Afterward, however, a dramatic change occurred in the product distribution: the formation of ethane, ethylene, and benzene came into prominence. This was particularly the case when these compounds were highly dispersed on ZSM-5. The selectivity to benzene was 80-85%. XPS analysis of Mo-containing catalysts demonstrated the formation of Mo carbides during the catalytic reaction. Unsupported Mo carbides behaved like metallic Mo; the dominant process was the decomposition of methane to hydrogen and carbon. The deposition of Mo2C on ZSM-5 in a well-dispersed state, however, produced a very active and selective catalyst for the conversion of methane into benzene. The results suggest that Mo2C is the active surface species in the Mo-containing catalysts, which converts methane into ethylene, the primary compound for the production of benzene on the zeolite surface. (C) 1997 Academic Press