Catalytic properties and nature of active centers of ferrospheres in oxidative coupling of methane

• Oxidative coupling of methane was investigated over ferrospheres possessing more than 76 wt% Fe2O3 isolated from fly ashes.
• The quantity of Fe-containing phases, the state, and distribution of iron within the crystallographic sites were determined.
• The catalytic active site of the OCM reaction was established.

Ferrospheres with the Fe2O3 content in the range from 76 to 97 wt% were applied as catalysts for the oxidative coupling of methane (OCM). To identify their phase composition and distribution of iron sites, the ferrospheres were characterized by X-ray powder diffraction and Mössbauer spectroscopy before and after the OCM reaction. Magnetite-based ferrite spinel, hematite and aluminosilicate glasses were established to be the main phases. The ferrospinel of all ferrospheres partially oxidized to hematite after the OCM reaction. It was established that the yield of C2-hydrocarbons sharply increased at the ferrospheres with Fe2O3 content higher than 89 wt% The spinel phase of these ferrospheres includes Fevv3+(B) sites with a Ca2+ tetrahedral cation and an octahedral cation vacancy among the nearest neighbors. A linear correlation between the yield of C2-hydrocarbons (ethane and ethylene) and the content of such sites was established, thus indicating that their electrophilic oxygen species participate in selective CH4 conversion to C2H6.

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