Abstract:

A promising chemical looping-oxidative coupling of methane (CL-OCM) oxygen-carrier catalyst, Na₂WO₄/Mn₇SiO₁₂-SiO₂, was obtained by adding extra Mn₂O₃ to Mn₂O₃-Na₂WO₄/SiO₂ and in-situ activating in the reaction stream. After experiencing an induction period, the oxygen-carrier phase transformed from Mn₂O₃ to Mn₇SiO₁₂ in association with an improvement in C2-C3 selectivity but decreased CH₄ conversion. The Na₂WO₄/Mn₇SiO₁₂-SiO₂ oxygen-carrier catalysts could also be obtained by directly calcining the Na₂WO₄/Mn₂O₃-SiO₂ precursor at 800 ℃ in air. At 750 ℃ and a CH₄ residence time of 12 s, the catalyst achieved 12% (or 7%) CH₄ conversion and 81.5% (or 90.0%) C2-C3 selectivity using a m_Cat/m_CH4 weight ratio of 27 (or 13.5). Notably, only C₃H₆ was detected as C3 products, whose selectivity was about 5%. The CL-OCM reaction proceeded selectively through the redox cycle mode of Mn₇SiO₁₂ $ \leftrightarrow $ [MnSiO₃ + MnWO₄]. The lattice-oxygen mobility in Mn₇SiO₁₂ was much weaker than that in Mn₂O₃, which improved C2-C3 selectivity but decreased CH₄ conversion. Our findings provide guidance for the exploration of more advanced catalytic oxygen-carrier catalysts toward efficient CL-OCM process.

Key words: oxidative coupling of methane to ethylene, chemical looping, oxygen-carrier catalyst, lattice-oxygen oxidation, manganese