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Room-temperature oxidation of methane to methanol by α-oxygen is of great mechanistic interest for both conventional and biomimetic oxidation catalysis. This work was carried out using new-generation FeZSM-5 samples that have the Oα concentration of 100 μmol/g. This value exceeds 3−15 times the Oα concentration on the earlier studied samples, thus providing more precise quantitative measurements related to the reaction mechanism. Fourier transform infrared spectroscopy data confirmed an earlier conclusion that CH4 + Oα surface reaction proceeds by the hydrogen abstraction mechanism. This mechanism leads to hydroxy and methoxy groups residing on α-sites. The methanol formation takes place by hydrolysis of (Fe-OCH3)α groups at the step of extraction. For the first time dimethyl ether (DME) was identified in the reaction products, its amount comprising 6−7% of the methane reacted. In distinction to methanol, DME is readily extracted both by dry solvents (acetonitrile, tetrahydrofuran, ethanol) and their mixtures with water. A reliable extraction procedure was developed, which provides a 75% recovery of the methane oxidation products (methanol + DME). The missing products are shown to remain on the catalyst surface and can be quantitatively recovered in the form of COx at heating the sample. A mechanism involving CH3• radicals formed in the H-abstraction step is suggested to explain the reaction stoichiometry CH4:Oα = 1:1.75 and a deficit of the carbon balance at extraction.
Starokon et al. (Wed,) studied this question.