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Designing Carbon-Hungry Sponges to Capture and Store CO2

Monday, August 30, 2010

Coal-fired power plants release a whopping 10 billion tons of carbon dioxide every year.  The search is on for a way to scrub out this notorious greenhouse gas from smokestack exhaust – and scientists are betting on ZIFs.

To curb carbon emissions from power plants, one idea is to fit smokestacks with a reusable trap that temporarily stores carbon dioxide.  Scientists hope a new crystalline material, nicknamed ZIFs, can clean the exhaust while withstanding the heat.

ZIFs, or zeolitic imidazolate frameworks, are highly porous yet extremely stable crystals with extraordinary amounts of internal space.  How much space?  If you unfolded and flattened a sugar-cube sized ZIF, it would cover an entire football field.

To spur discovery of the best ZIFs for industrial use, the Department of Energy recently awarded a five-year, $11.5 million grant to create an Energy Frontier Research Center.  The center involves researchers at the University of California Los Angeles (lead institution), University of California Berkeley, the University of Kansas and Eastern Washington University.

One of the collaborators, Prof. Omar Yaghi at UCLA, pioneered a technique for making similar types of frameworks.  Now, robots are speeding up production and simultaneously creating hundreds of variations of these cage-like structures.  

Brian Laird, professor of chemistry at KU and the Center for Environmentally Beneficial Catalysis (CEBC), Yao Houndonougbo, assistant professor of chemistry at EWU and former CEBC postdoctoral fellow, and Mark Asta, professor of materials science at UC-Berkley, are working to understand how ZIFs actually soak up carbon dioxide and keep it trapped.

They are using computer simulations to do this.  Laird said the goal is to design ZIFs with even better carbon-capturing capability. 

In a recent journal article, Laird and his colleagues compared five ZIFs, each with the same underlying basic shape but with different functional groups attached to the “i” part of the ZIF, or the imidazolate.  These groups of atoms are aimed into the empty space inside the ZIF where they can interact with trapped gas molecules. 

By combining their computer simulations with Yaghi’s experimental studies, they found that these five ZIFs can store between 3 and 10% of their weight in carbon dioxide at basically room temperature and pressure.  The ZIF with the highest CO2 storing capacity had two different functional groups bound to the imidazolate (–CN and –NH2). 

“This scenario promotes electrostatic and van der Waals interactions between the framework and carbon dioxide,” said Ning He, postdoctoral researcher in Laird’s lab.      

But in order for ZIFs to be used in smokestacks, many questions still need answers.  For example, what will happen to the captured CO2?  A change in pressure would release the gas, but what can be done with it then? Store it underground? Or is there a way to economically convert it into useful chemicals or fuels?

Clearly ZIFs are promising carbon-hungry sponges.  Yet more research is needed to see if they can help reduce the world’s growing carbon footprint.

--Story by Claudia Bode

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