Deactivation of NaCl/B₂O₃/Fe₂O₃ catalysts and their improvement for the oxidative coupling of methane

Abstract

In the present work, the deactivation of the NaCl/B₂O₃/Fe₂O₃ catalysts was studied for the oxidative coupling of methane. Several techniques, such as XPS, XRD, SEM, H₂-TPR, and flow-reaction, were employed to examine the function of each catalyst component, and its change during the catalytic reaction. NaCl and B₂O₃ show a synergistic effect on the Fe₂O₃ surface. B₂O₃ modifies the oxidative activity of Fe₂O₃ and makes the first reduction peak of Fe₂O₃ shift from 490 to 750°C. The NaCl modified B₂O₃/Fe₂O₃ catalyst has high reduction rate, high activity and selectivity at about 750°C. It is demonstrated that the deactivation of the NaCl/B₂O₃/Fe₂O₃ catalysts is a complicated process, consisting of chloride loss, sodium change, B₂O₃ loss, silica deposition and catalyst sintering. The chloride loss enhances the surface basicity, which causes the silica deposition and sodium change, and aggravates the catalyst sintering. The silica deposition and catalyst sintering cause permanent deactivation. The B₂O₃ loss is not a direct reason for catalyst deactivation. NaCl crystal diluted NaCl/B₂O₃/Fe₂O₃ catalysts have a better stability. The deactivated catalyst has a more stable structure. When it is regenerated by impregnating with NaCl again, a more stable catalyst can be obtained.