Korean researchers have developed a new, highly porous material that can soak up carbon dioxide efficiently and highly selectively. The material, characterised by X-ray diffraction and other techniques, might one day be used to "scrub" out the greenhouse gas from power stations and factories that burn fossil fuels for energy.

Chemists Myunghyun Paik Suh and Hye-Sun Choi of the Seoul National University, in the Republic of Korea, have developed highly selective carbon dioxide capture materials from flexible three-dimensional coordination polymer networks. They report details in the journal Angewandte Chemie.

The ominous towers of the industrial revolution have belched out sulfurous smoke and black soot for almost two centuries. In some parts of the world that is still the case, but the advent of pollution-control technology that can scrub out sulfur compounds and other pollutants, has remedied the problem to some degree. Indeed, factory smokestacks that produce nothing but carbon dioxide and water vapour have become exemplars of the greening of industry.

However, colourless, odourless, non-toxic carbon dioxide has become the most notorious pollutant of all against which noxious nitrogen oxides and acid-rain forming sulfur compounds almost pale into irrelevance as CO₂, took on the mantle of greenhouse gas, and is the harbinger of climatic doom on which almost all attention is now focused.

The pressing risks of irreversible climate change mean that finding ways to eradicate carbon dioxide from industrial smokestacks and the exhaust fumes of fossil-fuel powered electricity plants is now high on the technological agenda.

Attempts to avoid burning fossil fuels to power industry and drive society are not yet addressing the runaway demands of climate control. As such, researchers across the globe are trying to find methods for separating the carbon dioxide from the inevitable industrial exhausts that are energy efficient and cost effective, and moreover have a sufficiently small carbon footprint of their own.

There are numerous porous materials that can adsorb gases. Some of these can soak up carbon dioxide. However, finding materials that are selective for this gas alone and can extract it from a complex exhaust mixture at room-temperature and atmospheric pressure without being affected by the presence of nitrogen, methane, or water vapour remains a major technical challenge.

Suh and Choi think they may have taken the first steps towards such a material. They have developed a porous, three-dimensional network from a coordination polymer based on a nickel complex and an organic molecule as its building blocks. The starting materials assemble into two-dimensional lattice-like planes, which grow into stacks held together by flexible columns, to produce a novel three-dimensional porous structure with an interesting feature.

The flexibility of the columns allows the cavities in the structure to vary in size and so accommodate different gaseous guest molecules that might enter their pores. However, the porous material does not flex its molecular muscles for any gas molecule, as Suh and Choi explain.

The researchers point out that the symmetric carbon dioxide molecule has an electrical quadrupole moment that can be described as two electrical dipoles sitting back-to-back and pointing in opposite directions. This quadrupole interacts with the three-dimensional lattice, and this effect causes the columns to open the "gates", allowing the gas to enter the cavities.

Nitrogen, hydrogen, and methane are not so well endowed as carbon dioxide when it comes to their quadrupole moments and cannot open those porous gates.

The exclusion of nitrogen, which makes up a large proportion of air, is essential for any putative carbon dioxide capture system. In addition, the new nickel-containing materials are stable at temperatures up to 300 Celsius and are stable in air and in the presence of water, which is essential for an industrial carbon dioxide scrubber.

The trapped carbon dioxide can be released by reducing the pressure on the porous material. Given that carbon dioxide is a pollutant in some contexts but a useful industrial gas in others, this potentially cyclical approach to carbon dioxide capture and released could be made commercially and environmentally viable.

Related links:

• Angew Chem Int Edn, 2009, 48, 6865-6869
• Suh's Page

Article by David Bradley