Net-zero reaction. To deal with carbon dioxide, befriend it

Countless philosophers have pointed out that you can disarm an adversary by treating him as a friend. Now is the time to apply that principle to mankind’s worst enemy — carbon dioxide.

In theory, this global warming greenhouse gas can also be an industrial product — you can use it in aerated drinks, for example — but industry needs less than one per cent of the carbon dioxide that humans produce.

Can this be increased? Yes, if you can make high-value products out of the gas. As such, researchers have been preoccupied with the valorising of carbon dioxide. Making value-added products means getting carbon dioxide to react with something. But the gas with one carbon atom bonded to two oxygen atoms does not readily react with other materials.

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There are two ways to break this bond. The first is to remove one oxygen atom (reduction) to produce carbon monoxide and oxygen; but this requires a lot of energy. To break the bond between carbon and oxygen calls for 805 kilo-joules of energy per molecule.

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The other way, non-reduction, is to ‘activate’ the carbon to react with other material. This requires a lot less energy.

We know that a molecule of anything is incredibly small, too small for us to imagine, yet each molecule has its own geography. A carbon dioxide molecule, too, has different ‘sites’, one of which is called ‘lowest unoccupied molecular orbital’. This region loves electrons; if you supply electrons, it will accept them and this, in turn, will ‘activate’ the carbon dioxide molecule. The trick is to find a suitable catalyst that will provide electrons to the carbon dioxide molecule at the desired site.

Researchers working in this area have found out that you can make ‘cyclic carbonates’ with carbon dioxide. Cyclic carbonates — there are eleven of them — are versatile compounds and have wide applications in Li-ion batteries, pharmaceutical manufacturing, and in the manufacture of many fine chemicals. “Depending on the nature of the reaction and catalyst used, different products, such as dimethyl carbonate, heterocycles, formates, formic acid, methanol, alpha- and beta-unsaturated carbonyl compounds, polycarbonates, urea, urethanes, carbon monoxide, etc can be obtained by carbon dioxide conversion,” says a scientific paper produced by researchers led by Prof Venkata Krishnan at the School of Basic Sciences and Advanced Materials Research Centre, Indian Institute of Technology, Mandi, Himachal Pradesh.

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Prof Krishnan’s team has developed a catalyst, ‘metal-free boron doped graphitic carbon nitride’, for the job. Indeed, that catalyst has been tried earlier — Prof Krishnan acknowledges the pioneering work done by Prof Zhen Zhao of Shenyang Normal University, China — but Krishnan has tweaked the catalyst further for better results by turning it into nanosheets. It is also pertinent to note that there are several ways of activating carbon dioxide, and using a catalyst is one of them.

The catalyst provides electrons to activate carbon dioxide, but to make the high-value products — cyclic carbonates — you need to bend the carbon dioxide molecule, which is a straight line (oxygen-carbon-oxygen), into a triangular ring. At this point, chemicals called ‘epoxides’ enter the scene. An epoxide molecule is triangular with one vertex occupied by an oxygen atom and the other two vertices by carbon atoms, which in turn could be part of any other molecule. So, activated carbon dioxide is made to react with a suitable epoxide. By adding (polymerising) carbon dioxide to a suitable epoxide and manipulating them, you get cyclic carbonates.

This is a promising conversion method, and many types of catalysts have been reported to show good results, says a February 2023 paper by Ting Yan, et al of Shanghai University.

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Prof Krishnan told Quantum that the pathway to produce cyclic carbonates discovered at IIT-Mandi has several advantages. It requires much less power — about 100 degrees Celsius. It needs no pressure, so does not require additional energy. Importantly, it is solvent-free — solvents can be costly and toxic. Furthermore, you can make the entire range of cyclic carbonates through this pathway.

The most relevant of these products is polycarbonate, a versatile material with a wide range of industrial applications — from automobiles and aerospace to electrical insulation, lens making, medical equipment, packaging, and toys. It can be made with carbon dioxide and epoxides. Who said carbon dioxide is bad?

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