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New way to lock up carbon dioxide

Sun, 6th Dec 2009

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Scientists may have found a much more cost-effective way to extract the CO2 from exhaust gases.

Smokestacks from a wartime production plant, World War II.With Copenhagen just around the corner the attention of the world is firmly fixed on the question of cleaning up our emissions.  But efficient ways to selectively scrub CO2 from the waste gas streams leaving power-stations or other heavy industries have proved hard to find.

The best contenders to date have involved dissolving the gas in solutions of amines, but this is problematic because to re-release the gas from the amine - to store or sequester it - consumes large amounts of energy, making the process inviable in energy terms.

But now a new molecular structure may have come to the rescue, the MOF or metal organic framework.  These are large molecular structures resembling chemical cages linked together in a repetitive sequence.  But what's special about these cages is that the 'bars' consist of organic molecules that plug into metal atoms at the vertices and this combination gives these structures very exciting chemical properties including the ability to selectively lock away certain gases inside the cages but simultaneously reject others.  This means that they can behave like molecular sieves and California Nanosystems Institute scientist David Britt and his colleagues have developed a MOF that is very good at selectively grabbing CO2.

Writing in PNAS, the team present a molecule called Mg-MOF-74, which contains magnesium atoms linked to the organic material DOT, short for 2,5-dioidoterephthalate.  Exposed to a mixture of gases that includes a low concentration of CO2, this MOF selectively sequesters the carbon dioxide by linking it to the magnesium atoms inside the molecular cages; the other gas molecules meanwhile, slip through unimpeded.  Fully charged the MOF can soak up close to 9% of its weight in CO2; better still is can re-release the CO2 just with gentle warming to only 80 degrees C, liberating the gas and regenerating the MOF.

This say the scientists meakes these molecules excellent candidates as CO2 capture media; they also offer the advantage over existing methods of being non-toxic and non-corrosive, which are unpleasant features of the amine solutions currently being trialled.

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