Effects of Co-fed O₂ and CO₂ on the deactivation of Mo/HZSM-5 for methane aromatization

The aromatization of methane in the presence of H₂, CO and CO₂ over a 2wt.% Mo/HZSM-5 catalyst was studied. The results were compared with those for the aromatization reaction in pure methane and with added O₂. The addition of O₂ up to 5.3% or CO₂ up to 12.8% reduces deactivation so that, at the reaction temperature of 770°C, an aromatic yield of ca. 4% can be maintained for 6h; in the absence of the gaseous additive, however, the catalyst would have been completely deactivated for aromatic formation within 4h. XPS analysis revealed that, in the presence of O₂ or CO₂, the molybdenum oxide supported on the HZSM-5 located at the reactor inlet was not converted to molybdenum carbide, whereas in the zone away from the reactor inlet, Mo₂C was found. Investigation by temperature-programmed surface reaction showed that the production of aromatic compounds was always preceded by the reaction of molybdenum oxide with methane to form Mo₂C and CO. The beneficial effect of adding CO₂ and O₂ in low concentration was mainly attributed to the formation of CO and H₂ by oxidation and the reforming of methane in the zone closed to the reactor inlet. H₂ enhances the stability of the catalyst by suppressing the excessive dehydrogenation of the reaction intermediates into inactive entities. When the concentration of CO₂ and O₂ was too high, the entire catalyst bed remained oxidized and the methane aromatization reaction could not occur.