Taking the direct approach

19 Jul 2011

It is possible to chemically fix waste carbon dioxide gas onto organic molecules without the need for transition metal catalysts


© iStockphoto.com/Vesivus

Recycling carbon dioxide (CO2) into a chemical feedstock helps reduce greenhouse gas emissions and has a range of potential benefits for the chemical industry. Yugen Zhang and Dingyi Yu at the A*STAR Institute of Bioengineering and Nanotechnology have now discovered a way to add CO2 directly to organic molecules without the need to use expensive transition metal catalysts—an innovative protocol that could significantly lower costs for this kind of carbon capture.

Chemists have a tough time reacting CO2 with other substances because its carbon–oxygen double bonds are extremely stable. Cracking these connections requires an additional reagent with a steady supply of electrons, a role traditionally filled by high-priced metal catalysts. Recently, however, Zhang and Yu discovered a more economical technique. By mixing cheap organic catalysts called N-heterocyclic carbenes (NHC) with copper (Cu) metals, the team successfully fixed CO2 onto acetylene-bearing compounds at room temperature.

During further investigation of the mechanism underlying the catalytic activity of Cu–NHC, the researchers realized that the fixation process encompassed several different chemical equilibria. One of these steps involved the use of a base—another driving force for splitting CO2 bonds. “From there, we proposed that under the right conditions, the reaction could take place without metal catalysts,” says Zhang.

Because this process involves a gas reactant, Zhang and Yu predicted that CO2 pressure alone could drive a metal-free fixation reaction forward. Experiments revealed that their hunch was correct. After optimizing the temperature conditions, the researchers found that CO2 pressures two and a half times greater than normal could convert a phenyl–acetylene molecule into a carboxylic acid with over 95% efficiency in the presence of a carbonate base. Higher CO2 pressures enabled the reaction to proceed at even faster rates.

Choosing the correct base proved critical to the team’s success. They found that cesium carbonate—an inorganic base widely used in organic synthesis—allows this mode of efficient CO2 fixation. With this reagent, the researchers were able to convert a variety of aromatic- and alkyl-acetylene substrates containing electron-donating and electron-withdrawing functional groups into carboxylic acids with good to excellent yields and no discernable byproducts.  

Zhang notes that this new carbon capture process is a good complement with their previous efforts. Whereas metal-free CO2 fixation requires elevated temperatures and pressures, the Cu–NHC system can proceed under ambient conditions if water is rigorously excluded from the reaction—two options that provide increased flexibility for manufacturers turning to ‘green’ chemical practices.

The A*STAR-affiliated researchers contributing to this research are from the Institute of Bioengineering and Nanotechnology.

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Yu, D. & Zhang, Y. The direct carboxylation of terminal alkynes with carbon dioxide. Green Chemistry 13, 1275–1279 (2011). | article

This article was made for A*STAR Research by Nature Research Custom Media, part of Springer Nature