It's the reason why chemists envy green plants: by using photosynthesis, plants can easily fix the carbon dioxide that is so plentiful in air to make biomass, or organic compounds. Chemists would also like to be able to simply produce carbon compounds out of CO2 from air. In contrast to the usual sources of carbon used today—fossil fuels and natural gas—carbon dioxide is a renewable resource and an environmentally friendly chemical reagent. Unfortunately, its carbon–oxygen bonds are too strong to be broken easily. Researchers working with Yugen Zhang and Jackie Y. Ying at the Institute of Bioengineering and Nanotechnology in Singapore have now developed a novel reaction scheme by which CO2 can be efficiently converted into methanol under very mild conditions. As reported in the journal Angewandte Chemie, it is based on an N-heterocyclic carbene catalyst and a silane as the reducing agent.
The basic framework of an N-heterocyclic carbene is a five-membered ring made of two nitrogen and three carbon atoms. Instead of having the usual four bonds, one of these carbon atoms only has two. The two electrons left over in the form of a lone pair, which makes this species highly reactive—reactive enough to attack CO2.
The researchers in Singapore produced the carbene catalyst used in the reaction in situ from a precursor. The carbene activates the CO2, but is then split off again to end the reaction cycle in its original state. The formal reaction partner is a hydrosilane, an organosilicon compound that acts as a reducing agent. The reaction product into which the CO2 is converted can easily be collected in the form of methanol in the last step of the reaction series. Methanol is an important starting material for many chemical syntheses and serves as an alternative fuel and as a raw material for the production of energy in methanol fuel cells.
The big advantage: unlike prior reaction mechanisms using metal-containing catalysts, air can be used as the source of the CO2 because the carbene catalyst is not sensitive to oxygen. The carbene is more efficient than the metal-containing catalysts as well, and the reaction can be carried out under very mild conditions.