Liquid-Phase Oxidation of Methane into Oxygenates with O2

Methane has been used as a clean energy source because it emits the smallest amount of CO₂ per heat produced among hydrocarbons. Recently increasing attention has been paid to its chemical conversion owing to the upsurge of shale gas production. However, due to the lack of the economic process for the utilization of methane as a chemical feedstock in a small scale, about 35% of methane is just being flared away in the United States. In the convention chemical process, methane is first converted into syngas (a mixture of CO and H₂), which can be further transformed into various chemicals. Since the typical syngas process is energy-intensive, only large-scale methane-based process can be cost-effective compared with petrochemical process to produce the same chemical. To utilize the small gas well efficiently, the efficient direct methane conversion technology into liquids needs to be developed.

Since methane has a strong C-H bond, its functionalization is quite demanding under mild conditions. Therefore, strong oxidants including H₂O₂ have been used to oxidize methane in the liquid phase. Hutchings and his colleagues reported the direct conversion of methane into methanol and formic acid using heterogeneous catalysts such as Cu-promoted Fe/ZSM-5 in the aqueous phase. Because of the high price of H₂O₂, there have been many efforts to use O₂ or air as an oxidant. However, high yields of methane oxygenates could not be accomplished in a single-step process. To compromise these two processes, in situ generation of H₂O₂ from H₂ and O₂ can be proposed in order to oxidize methane selectively under mild conditions.

In this work, Pd/C and ferrous ion was utilized to generate H₂O₂ from a mixture of H₂ and O₂ and to oxidize methane, respectively. The ferrous ion is essential to form two oxygen-radical species (hydroxyl and hydroperoxyl radical) from H₂O₂ to oxidize methane in the aqueous phase. The effect of catalyst compositions on the catalytic activity was examined systematically to find out the role of each component in this catalytic reaction. It can be concluded that methane can be oxidized into formic acid in the co-presence of H₂ and O₂ using the catalyst system composed of Pd/C and ferrous ion.