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 2 wt.% 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 6 h; in the absence of the gaseous additive, however, the catalyst would have been completely deactivated for aromatic formation within 4 h. 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