Carbon capture drives metal purification

Capturing the carbon dioxide emitted by fossil fuel power stations or industrial plants offers a vital way to reduce global greenhouse gas emissions. But carbon capture technologies are still considered too costly for widespread deployment.

Now chemists at the University of Lyon have developed a method to capture CO₂ while simultaneously purifying mixtures of metals. By coupling these two processes together, they argue, CO₂ capture can generate value in the form of purified metals, potentially making carbon capture more economically viable (Nat. Chem. 2019, DOI: 10.1038/s41557-019-0388-5).

Current carbon capture systems often use amines that react with CO₂ in flue gas to form carbamates. These compounds can be separated and heated to release a stream of pure CO₂, regenerating the amine so that it can be cooled and reused. But the huge temperature swings involved make this a costly approach.

Meanwhile, the CO₂ released from the amines is sometimes used to help displace oil from underground reserves. “The oil extraction process is effectively paying for the price of capturing CO₂,” says Julien Leclaire from the University of Lyon. But this process ultimately leads to further carbon emissions when the oil is burned.

So Leclaire’s team looked for an alternative way to add value to carbon capture. They settled on a polyamine called diethylenetriamine (DETA), which reacts with CO₂ to produce monocarbamate or dicarbamate anions, along with inorganic bicarbonate anions. Mixed with metal chlorides, the amine, carbamates, and bicarbonate can act as ligands or counterions to form a range of metal complexes.

Most of these complexes are in reversible equilibria with each other, and interconvert by ligand exchange. But some complexes are insoluble, and under the right conditions they form solid precipitates that contain a single metal compound. “The idea is that each metal can find its ideal combination, in terms of ligand and counterion, that allows each of them to be separated,” Leclaire explains.

As a proof of principle, the researchers prepared a mixture of lanthanum chloride and nickel chloride in water, added DETA, and then bubbled CO₂ directly from a car’s exhaust through the solution. This triggered a series of ligand exchange reactions that created insoluble lanthanum carbonate, La₂(CO₃)₃, which the team removed by centrifugation.

“If you can get a process that selectively picks out individual metals, you have the start of a very exciting industrial symbiosis,” says Peter Styring, who works on carbon capture, utilization, and storage technologies at the University of Sheffield. “This makes absolute sense.”

Leclaire’s team applied a similar approach to separating the metals in a lanthanum-cobalt-nickel alloy (La₂CoNi₉) that is found in the anodes of nickel metal hydride batteries, used in some older electric vehicles. After forming the metal chlorides and adding DETA, they used a stream of pure CO₂ to recover more than 99.4% of the original lanthanum at 99.8% purity. In subsequent steps, ethanol and isopropanol helped precipitate 97% pure cobalt chloride monocarbamate, leaving the nickel-amine complex in solution.

The isolated solids can be heated to drive off a pure stream of CO₂ for subsequent use or storage—just like a conventional carbon capture system, but with the added benefit of having purified metals along the way.

Leclaire and his colleagues are now using their method to purify the metals found in rare earth magnets, which are widely used in wind turbines, and they hope to collaborate with metal recycling companies to develop the technology further.