Researchers from the Universities of Valencia, Huelva and Toulouse, led by Professors Gregorio Asensio, Pedro J. Pérez and Michel Etienne, have developed a methodology for transforming methane into more complex organic molecules. A paper on their work is published in the journal Science.
The use of methane, the simplest hydrocarbon and main component of natural gas, as a source for the production of more complex organic compounds is of great interest from both economic and environmental points of view. However, methane has the strongest C-H links in the whole series of alkanes.
Another challenge for chemically transforming methane derives from of its gaseous nature and its low solubility in common solvents. These features make it difficult for methane to come in contact with the catalysts and reagents that perform the chemical reaction and, therefore, this does not occur or it does but with great difficulty. For these reasons, very few processes are known to be effective for the functionalization of this hydrocarbon.
The reaction involves a silver catalyst specifically designed to activate the C-H methane bonds, a process that had already proved effective with heavier hydrocarbons. The challenge of attaining effective contact between the catalyst and the reagents needed for the transformation and methane was achieved by using carbon dioxide in supercritical state as the reaction medium.
Carbon dioxide is a gas under normal conditions, but at temperatures and pressures above their critical values (32 °C and 74 atmospheres) it is a fluid similar to a liquid and capable of solubilizing the molecules involved in the reaction. These properties of supercritical carbon dioxide have found wide industrial applications such as, for instance, the decaffeination of coffee. In addition, the chemical inertness of carbon dioxide prevents it from reacting with the catalyst or the reactants involved in the conversion of methane, and therefore is an ideal solvent for these reactions.
The transformation involves a carbene insertion into a C-H methane bond catalyzed by silver complexes with halogenated scorpionate ligands in supercritical carbon dioxide. The described process establishes the feasibility of the insertion of carbenes into C-H methane bonds catalyzed by transition metals. The reaction leads to the creation of a C-C bond over the methane to give ethyl propanoate with a yield of 19% and opens new perspectives to the process of functionalization of methane and of hydrocarbons in general.
The research was funded by the Spanish Ministry of Science and Innovation, the Regional Governments of Valencia and Andalusia, and the European Union through its ERA Chemistry program.
Ana Caballero, Emmanuelle Despagnet-Ayoub, M. Mar Díaz-Requejo, Alba Díaz-Rodríguez, María Elena González-Núñez, Rossella Mello, Bianca K. Muñoz, Wilfried-Solo Ojo, Gregorio Asensio, Michel Etienne, Pedro J. Pérez (2011) Silver-Catalyzed C-C Bond Formation Between Methane and Ethyl Diazoacetate in Supercritical CO2. Science Vol. 332 no. 6031 pp. 835-838 doi: 10.1126/science.1204131