Methane dehydroaromatization over Mo and W catalysts supported on ZSM-5

Abstract

Methane aromatization reaction to produce benzene using tungsten and molybdenum catalysts supported on ZSM-5 was investigated at 800 oC. Catalysts were prepared by impregnation of tungsten and molybdenum salts on ZSM-5 zeolite with various metal loadings in the range of 2-10 wt. %. In order to obtain the catalytic structures before and after the reaction, catalysts were characterized by XRD and FTIR analysis. It was indicated from reactor tests that increase of metal loading on the catalyst surface leads to increase 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 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 time of stream equals 100 min), and increase of Mo loading leads to enhancement of catalytic activity and methane conversion, which indicate initial activation of methane is 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 include 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 show increase of support channel diameter leads to reduce the possibility of ring formation.