Methane Dehydroaromatization over Mo and W Catalysts Supported on ZSM-5

Methane aromatization reaction to produce benzene using tungsten and molybdenum catalysts supported on ZSM-5 was investigated at 800 ºC. Catalysts were prepared by impregnating tungsten and molybdenum salts on ZSM-5 zeolite with various metal loadings in the range of 2-10 wt. %. To obtain the catalytic structures before and after the reaction, catalysts were characterized by XRD and FT-IR analysis. It was indicated from reactor tests that an increase in metal loading
on the catalyst surface leads to an increase in methane conversion (1.1% and 3.2% for 2W/ZSM-5 and 6 W/ZSM-5, and 2.4% and 4.8% for 2W/ZSM-5 and 6 W/ZSM-5, respectively, at the time of stream, equals 120 min). It was also concluded that Mo catalysts show higher activity and stability than W (methane conversion of 3.2 and 9 % using 10Mo/ ZSM-5 and 10 W/ZSM-5 catalysts respectively, at the time of stream equals 100 min). An increase in Mo loading leads to the enhancement of catalytic activity and methane conversion, indicating that methane's initial activation has occurred on metal sites of catalysts. This activation leads to occur the later reactions and production of final benzene. These conditions confirm the two-factor mechanism which includes two stages: i) hemolysis break of C-H bond and CH3 radical formation and then ethylene formation, and ii) cyclization of ethylene species in the presence of acidic sites within the zeolite channels. Investigations on mesoporous HMS support showed no aromatic production, which shows an increase in support channel diameter leads to reduce the possibility of ring formation.