Aromatization of methane over zeolite supported molybdenum: active sites and reaction mechanism

The conversion of methane to aromatics, mainly benzene, has been studied. Zeolite type catalysts, ZSM-5, MCM-22, containing molybdenum were used. The reaction was carried out at 973-1073 K and atmospheric pressure. Aromatics were formed after an induction period during which molybdenum carbide was formed. Induction period was followed by analyzing infrared (IR) band of the acidic OH. Zeolite plays an important role for increasing Mo dispersion. Comparison between ZSM-5 and MCM-22 showed that Mo/MCM-22 was twice more active than Mo/ZSM-5. This was attributed to more open topoly of MCM-22 which facilitated the migration of Mo ions towards exchangeable positions thus favoring Mo dispersion, and also reduced diffusion limitation of the aromatic products. Analysis of the products at high space velocity showed that acetylene was formed as primary product along with possibly ethylene. Acetylene was converted into aromatics (benzene, naphthalene) as the space velocity decreased. IR analysis in the OH stretching vibrations showed that H+ ions, in H-ZSM-5, are progressively exchanged during the calcination of Mo/HZSM-5, and almost completely removed after the induction period following CH4 reaction at 823 K. At high temperature in O-2 molybdenum species, possibly MoO2+ (Mo2O5)(2+), migrate in the zeolite framework and replace W. These species were further converted into Mo2C by reaction with methane, residual carbon species (coke) poisoning the residual zeolite protons. The reaction of CH4, C2H2 and C2H4; over acidic H-ZSM-5, H+ poisoned Mo2C/H-ZSM-5 and non-acidic Mo2C/SiO2 was studied. It resulted that H+ sites are not prerequisite for aromatization of C2H2 or C2H4. It was concluded that the principal route for the aromatization of CH4 is the formation of C2H2 over Mo2C which again over the molybdenum carbide forms benzene. The bifunctional mechanism apparently was less important. (C) 2002 Elsevier Science B.V. All rights reserved.