Room-Temperature Oxidation of Methane by α-Oxygen and Extraction of Products from the FeZSM-5 Surface

Room-temperature oxidation of methane to methanol by alpha-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 O-alpha concentration of 100 mu mol/g. This value exceeds 3-15 times the O-alpha 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 + O-alpha surface reaction proceeds by the hydrogen abstraction mechanism. This mechanism leads to hydroxy and methoxy groups residing on alpha-sites. The methanol formation takes place by hydrolysis of (Fe-OCH3)(alpha) 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 center dot radicals formed in the H-abstraction step is suggested to explain the reaction stoichiometry CH4:O-alpha = 1:1.75 and a deficit of the carbon balance at extraction.