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Crystal sponges capture carbon emissions

From New Scientist Print Edition.
Andy Coghlan

Crystalline sponges pocked with pores that are just the right size to trap carbon dioxide molecules could filter the fumes from power stations and cars. What's more, the trapped CO₂ can then be sucked from the crystals and piped into containers and buried underground, allowing the crystals to be reused.

Carbon capture and storage has been touted as a powerful weapon against global warming. Until now, the only way to strip CO₂ from car exhaust, flue gases or power-plant emissions was to bubble them through a solvent that reacts with CO₂. The trouble is that subsequently removing the gas from the solvent requires heat, limiting the efficiency of the process. "Anything that has the potential to reduce this 'energy penalty' is extremely valuable," says Stuart Haszeldine, an expert on carbon capture at the University of Edinburgh, UK.

So Omar Yaghi and colleagues at the University of California, Los Angeles, created powders made of porous crystals that soak up CO₂. When the crystals are fully loaded, the gas can be released by a drop in pressure, which sucks the CO₂ right out.

Their crystals, called "zeolitic imidazolate frameworks" (ZIFs), have pores that are big enough to allow CO₂ molecules inside, but small enough to retain them. The crystals are created by blending cobalt or zinc with organic molecules called imidazolates - the pore size depends on the imidazolate used.

The researchers created 25 different ZIFs but only three had pores of about the right size to trap CO₂. The team bubbled a mixture of CO₂ and carbon monoxide (CO) gas over each of them. All three trapped CO₂, with the most efficient soaking up 83 times its own volume, but little CO (Science, DOI: 10.1126/science.1152516). To bury CO₂ the gas must not be contaminated by other molecules - which might be toxic and leach out - so this selectivity is key.

"We're optimistic that within a year or two these materials will be ready for testing in power stations," says Rahul Banerjee, a member of the UCLA team. ZIFs should be tough enough to survive harsh conditions inside power stations, he says. In tests, they withstood temperatures of 400 °C.

From issue 2644 of New Scientist magazine, 23 February 2008, page 26

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