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New compound provides a better cage for carbon dioxide


mg-mof-74Capturing carbon dioxide is simple chemistry. In fact, you may have seen it in your high school chem lab. Remember that tightly sealed bottle of sodium hydroxide, aka lye? Simply popping the top off that strong base and exposing it to air resulted in a chemical reaction in which the ambient CO2 was absorbed and the lye became sodium carbonate.

So it would seem like carbon capture and storage might be relatively simple—thus neatly solving global climate change.

Unfortunately, it's not quite that simple, largely because getting the CO2 back out of any of a number of different materials put forward over the years has proven energy-intensive. For example, getting CO2 out of carbonate requires heating to more than 900 degrees Celsius in an industrial kiln.

However, in the November 30 Proceedings of the National Academy of Sciences, chemists at the University of California, Los Angeles, (U.C.L.A.) report that a new compound—affectionately known as Mg-MOF-74—readily traps CO2 and then releases 87 percent of it back again at room temperature*. "If we continue to blow, we blow the CO2 back off again," says U.C.L.A. chemist David Britt, lead author on the paper presenting the results.

The molecular lattice can absorb roughly 9 percent of its weight in CO2, and heating to a relatively mild 80 degrees Celsius releases the rest for your preferred storage option. "Mg-MOF-74 strikes an excellent balance between strong adsorption and ease of regeneration that makes it highly promising as a carbon dioxide capture material," Britt says.

The U.C.L.A. chemists began exploring Mg-MOF-74—a metal-organic framework employing magnesium ions—because they knew it released relatively little heat when adsorbing CO2, suggesting that it might require relatively little heat to get the CO2 back out again. Plus, magnesium is a key component of the plant enzyme RuBisCO that allows photosynthesizers to begin the process of turning CO2 into food. Essentially, the new metal-organic framework works as a crystal cage, allowing mixed gas to flow through it and selectively trapping the CO2.

The latter trick is important since power plants and other sources of carbon dioxide, rarely deliver a pure stream of the greenhouse gas. In tests using a mix of 20 percent CO2 and 80 percent methane, the Mg-MOF-74 captured only the CO2 while allowing the methane to pass through (also of vital interest to the natural gas purification industry).

But the chemical filter will probably still need some tweaking, particularly if the compound will be exposed to CO2 and water vapor, according to Britt. "Given the nearly endless possibility of MOF structures, we expect that it will be possible to tailor the environment of the [magnesium] ion to address this issue," he says.

Plus, it'll be relatively inexpensive. "Magnesium is a cheap and highly abundant metal and the organic linker in Mg-MOF-74 is commercially available," Britt says. If that proves to be the case, and engineering kinks can be worked out, then the compound could even be used in devices to capture CO2 directly from the air. Not only would that make its discoverers eligible for the $25-million Virgin Earth Challenge, it also just might help combat climate change.

Image: Courtesy of David Britt

* Correction (1-11-10): This sentence was changed to reflect the fact that the compound in question has been known since 2008. Thanks to chemist Antek Wong-Foy of the University of Michigan for pointing this out.


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