Methane activation by iron

28 May 2014 - Chemical engineering

methane_conversion.PNG Natural gas (methane) is an important chemical feedstock. It can be converted into syngas via CH4 + H2O -> CO + 3 H2 and then into methanol via CO + 2H2 -> MeOH. Translated, this route implies easy methane hydrolysis with one hydrogen equivalent as a bonus. Methanol then enters chemical industry as the starting material for a host of secondary chemicals. Do not fix something that is not broken? Guo et al. in a recent report try a different approach to methane activation eventually arriving at ethylene.

They note that the big hurdle for methane is overcome is the high carbon-hydrogen bond strength and low polarizability. Oxidative coupling of methane to ethylene (2CH4 + O2 -> CH2CH2 + 2H2O) suffers from over-oxidation leading to carbon dioxide formation and coke formation.

The Guo team has come up with a catalyst they call Fe@SiO2, made by mixing fayalite (the iron mineral Fe2SiO4) and silicon dioxide in a ball mill, fusing it at 1973 K for 6 hours and leaching it with nitric acid. At 1363 K this catalyst system was able to convert methane to ethylene and hydrogen gas and some benzene and naphtalene with 48% efficiency. Without the catalyst or with SiO2 not containing any iron a similar setup yielded only 95% coke. Several known catalysts based on iron/silicon did not work either. The main product from conventional methane pyrolysis is acetylene and again coke.

The rationale behind the apparent success of just this catalyst system according to Guo is the isolation of the iron sites in the silicon matrix (HAADF data included). This prevents C-C coupling reactions taking place at the surface and hence coke formation. During the catalyst lifetime the iron atoms also get better embedded in the matrix. External carbon also dissolves in the matrix forming Fe-C bonds that further help stabilize the iron center. The role of the iron center is to eject methyl radicals from the surface. These radicals have been detected by (take a deep breath) "vacuum ultraviolet soft photoionization molecular-beam mass spectrometry". The radicals then combine to form ethane and then split of hydrogen to form ethylene.
The source of the external carbon is not fully explained. Does it limit the final 48% yield? By adding ethane to the gas input the yield also increases again no explanation is given.