Highlights

The carbon dioxide (CO₂) that streams from car exhausts and factory smokestacks takes much of the heat for global warming events because of its resiliency; the carbon–oxygen double bond that holds together a CO₂ molecule is extremely stable. Consequently, chemists have been seeking an economical way to split this long-lasting, atmospheric pollutant into molecules that they can use in reactions to produce alternative fuels.

Now, postdoctoral fellow Liuqun Gu and team leader and principal research scientist Yugen Zhang from the A*STAR Institute of Bioengineering and Nanotechnology in Singapore have developed a catalytic pathway that breaks apart CO₂ and transforms it into carbon monoxide (CO) under very mild reaction conditions. Because researchers can use CO molecules to convert water into hydrogen fuel, this new process may play an essential role in future ‘green’ energy technologies.

At the heart of Gu and Zhang’s system are organocatalysts, small organic compounds that can mimic the catalytic behavior of complex biomolecules and rare transition metals. Compared with other methods for splitting CO₂, “organocatalysis systems have higher efficiency because the catalysts are much cheaper and the reaction conditions are milder,” explains Zhang.

Previously, Zhang was part of a team that used organocatalysts called N-heterocyclic carbenes (NHCs) to convert CO₂ into methanol via a silicon-based intermediate*. The central component of NHC catalysts, an unsaturated hydrocarbon–nitrogen ring known as imidazole, has an unshared pair of electrons that can trap and activate CO₂ molecules for further chemical reactions.

Gu and Zhang studied how NHC organocatalysts react with aromatic aldehydes, benzene-based compounds that can act as oxygen acceptors. After initiating the reaction with a base, they found that without CO₂, the aldehyde self-reacted; but with CO₂, the NHC complex caused a catalyzed reaction that generated an aromatic carboxylic acid and CO.

They propose that after NHC binds to CO₂ and activates it, the complex attacks the aldehyde to form a meta-stable intermediate. Then, the base initiates a shift of atoms that splits CO₂ into the new products. Theoretical results revealed that this mechanism would be a net exothermic process. These results were backed by experiments that showed that a wide range of aromatic aldehydes could react with NHC to catalytically split CO₂ at room temperature.

The researchers note that investigations of this sustainable catalytic system have only just begun. “Our future work will focus on combining NHC catalysts with other systems for more challenging CO₂ transformations,” says Zhang.

The A*STAR-affiliated researchers mentioned in this highlight are from the Institute of Bioengineering and Nanotechnology.