Methanol is a key feedstock for the production of chemicals, some of which are used to make products such as plastics, plywood, and paints. Methanol also can fuel vehicles or be reformed to produce high-grade hydrogen for fuel cells. The current method for producing methanol from methane- or coal-derived synthesis gas involves a multi-step process that is neither efficient nor economical in small-scale applications. As a result, methane emissions from oil wells — accounting for 210 billion cubic feet of natural gas annually — are being vented and flared, according to the U.S. Energy Information Administration.
Meanwhile, the growth of hydraulic fracturing, or fracking, and the subsequent use of shale gas — the chief component of which is methane — have dramatically increased the natural gas supply in the United States, and accelerated the desire to upgrade methane into more valuable chemicals, such as through oxidation to methanol, or carbonylation to acetic acid.
As a result, scientists have been seeking more efficient and less expensive ways to convert methane with a process that uses inexpensive molecular oxygen in mild conditions in which relatively low temperatures and pressures are used. However, the direct oxidation of methane — found in natural gas — into methanol at low temperatures has not yet been accomplished.
A way to accomplish this using a heterogeneous catalyst and cheap molecular oxygen was developed using molecular oxygen and carbon monoxide for the direct conversion of methane to methanol, catalyzed by supported mononuclear rhodium dicarbonyl species, anchored on the internal pore walls of zeolites, or on the surface of titanium dioxide supports that were suspended in water under mild pressure (20 to 30 bar) and temperature (110 to 150 °C).
The same catalyst also produces acetic acid through a different reaction scheme that does not involve methanol as an intermediate. Carbon monoxide is essential to the catalytic reaction, which is heterogeneous. Tuning the reaction to either methanol or acetic acid is possible by properly controlling the operating conditions, especially the acidity of the support. Even after many hours of reaction, there is no leaching of the catalyst in the water.
Supported Rh catalysts were prepared through relatively simple synthesis procedures. The main focus was to atomically disperse the rhodium species, which was achieved by a special heat treatment protocol on the zeolite support and by anchoring rhodium precursor species on reduced titania assisted by UV irradiation. The atomic rhodium state is necessary for the reaction to occur.