Molecular Car Park: Material packs in carbon dioxide

A crystalline material composed of metal and organic building blocks holds more carbon dioxide than other porous substances do, chemists report. The discovery could lead to a device that reduces power plant emissions of this greenhouse gas.

About 40 percent of the carbon dioxide released in the United States in 2003 came from electric power plants, according to the Department of Energy. A potential strategy for reducing emissions is to fit plant flues with materials that capture the gas from exhaust.

Metal-organic frameworks had previously stored hydrogen (SN: 6/14/03, p. 382: Available to subscribers at Convenient hydrogen storage?). In the new study, Omar M. Yaghi and Andrew R. Millward of the University of Michigan in Ann Arbor measured the adsorption of carbon dioxide by nine different frameworks, each composed of organic compounds and either zinc or copper.

The researchers exposed each framework to increasing amounts of carbon dioxide gas in a closed system at room temperature. A framework containing zinc swallowed the largest amount of carbon dioxide—33.5 millimoles of gas per gram of material, or 1.4 times its own weight. The structure has a surface area of 4.5 square kilometers per gram.

The frameworks excel at containing carbon dioxide, says Yaghi, because they can “bring the gas molecules close to each other, like cars in a car park.” The gas molecules repel one another, but the attraction between the gas molecules and the metal-organic framework is stronger than the repulsive force. So, in the pores of the material, the gas occupies a smaller volume then it would alone, Yaghi says.

A container filled with the winning framework can take up nine times as much carbon dioxide as the empty container would and twice as much as a container filled with a carbon-based material previously tested for carbon dioxide storage. The researchers report these results in the Dec. 28, 2005 Journal of the American Chemical Society.

The researchers are now working with engineers to scale up the technology and study its effectiveness in flues. They envision a power plant column housing tons of the framework.

The material is reusable because in the absence of the high pressures found in the flue exhaust, the carbon dioxide freely leaves the material. Gas captured from the exhaust might be used to manufacture polymers or incorporated into other industrial materials, says Yaghi.

“It’s an interesting piece of work,” says Mark Thomas of the University of Newcastle upon Tyne in England. However, he’s not yet convinced that the technology is economically feasible.