Methanol is one of the most important chemical feedstocks. It is more desirable than methane as it is a liquid that can be easily transported and stored, and has been considered as a suitable on-board hydrogen carrier for low-temperature fuel cells. Therefore the conversion of methane to methanol has attracted much attention both in academia and industry. In remote natural gas wells, a simple oxidation of methane to methanol would be most attractive. However, no such method is currently available. The commercial process is the two-step steam reforming of methane to syngas followed by methanol synthesis. This is a high energy- demanding overall process, and is not economic at small scales. While many metal catalysts have been considered for the direct oxidation of methane to methanol over the last several decades, the main barriers have not been overcome. These are the activation of the C−H bond of CH41, and the prevention of the over-oxidation of methanol or other oxygenates to CO2, while water remains quite challenging in the conversion process as well. It is also desirable to use gaseous oxygen for the conversion, rather than expensive oxidants such as hydrogen peroxide. Therefore a good catalyst that can catalyze the direct conversion of methane to methanol with gaseous oxygen has to overcome these difficulties.
Activation of the alkane C-H bond by single-site metal catalysts has received a lot of attention.1 Under investigation in our lab are single-site metal catalysts, including Fe/ZSM-5 and Ni/ZSM-5, that can activate methane in the presence of oxygen at moderate temperatures (< 200 oC) and form M-CH3 species. These are also potential candidates for the conversion of methane to methanol and acetic acid using O2 as the oxidant and CO as a key component in aqueous phase at mild conditions. The single-site catalysts were prepared by an impregnation method and a following reduction step. EXAFS analysis verified the lack of metal-metal coordination. In the presence of CO, the functionalization of the M-CH3 species can proceed through oxygen insertion reactions, forming methanol as the product.2 Alternatively, the carbonyl species can also undergo migratory insertion to the methyl group, forming acetic acid as the product.2 This observation opens up a new route for the low temperature catalytic methane oxidation to oxygenates.
1. P.J. Perez, Alkane C−H Activation by Single-site Metal Catalysts; Springer: Berlin, 2012
2. R.A. Periana, O. Mironov, D. Taube, G. Bhalla, and C.J. Jones; Science 301, (2003) 814