CO2 sponge

Macromolecule that was accidentally discovered seems to trap the greenhouse gas from the air

A macromolecule that was accidentally discovered when scientists left stuff sitting on a lab bench seems to soak up atmospheric carbon dioxide, a study now suggests.

CARBON SPONGE Solid crystals made up of a newly discovered macromolecule (the large ring-shaped structure in this image) can trap carbonate ions (the four-atom group shown at center). Those ions form spontaneously in an alkaline solution and incorporate carbon dioxide pulled from the atmosphere, new analyses suggest. J. Tossell

The original find was made by a research team led by chemists at the University of Southampton in England. They were trying to design and create molecules that could capture negatively charged ions, such as chlorides and phosphates, on the surfaces of bioengineered cells. In one experiment, the researchers set aside an alkaline solution of various organic substances to evaporate, says geochemist John A. Tossell, author of the new study. When analyzing the crystals that formed, the team found that the organic macromolecule that made up the crystal unexpectedly contained carbonates, which form in solutions containing carbon dioxide.

The carbon dioxide in those carbonates probably came from the air in the lab and was converted to carbonate in the solution, says Tossell, of George Washington University in Washington, D.C. He describes, in the Aug. 3 Inorganic Chemistry, the macromolecule’s ability to absorb carbonate.

Though the carbonate isn’t chemically bonded to the macromolecule, spontaneous absorption of carbon dioxide from the air suggests that the combination is very stable, Tossell says. Theoretical analyses show the macromolecule is a giant ring composed mostly of ring-shaped subcomponents and that the negatively-charged carbonate ion nestles within the positively-charged center of that macromolecule, he notes.

Scientists are evaluating many chemical means to absorb and store carbon dioxide to help diminish its atmospheric buildup and slow climate change (SN: 5/10/08, p. 18). To use this macromolecule to soak carbon dioxide from the air or from industrial emissions on a large scale, engineers would need to separate the carbonate from the macromolecule. This could be done one of several ways, Tossell suggests. Heating the combination would drive out the carbonate, but might require large amounts of energy — and producing that energy using fossil fuels would create carbon dioxide. Alternatively, making the alkaline solution more acidic would cause the carbonate ion to latch on to protons, which would make the combination less stable and therefore more easily broken apart.  

Though the macromolecule may be too expensive to be used in large-scale CO2 scrubbers, Tossell says, studying the macromolecule and how it’s made may help future efforts to make similar materials more cheaply.

“This work is careful and thorough”, says Omar Yaghi, a chemist at UCLA. However, he notes, the new material may not have the capacity to absorb large amounts of carbon dioxide and might absorb other gases as well, which could reduce the material’s effectiveness.

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