|A ZIF structure. Click to enlarge.|
Researchers led by Omar Yaghi at UCLA have developed a new class of materials—zeolitic imidazolate frameworks (ZIFs)—that exhibit “unusual” selectivity for capturing carbon dioxide from gas mixtures and “extraordinary” capacity for storing CO2. The work is reported in the 15 February issue of the journal Science.
The researchers synthesized 25 ZIF crystal structures and found that three of them (ZIF-68, ZIF-69, ZIF-70) exhibited selectivity for capturing carbon dioxide from gas mixtures. One liter of ZIF-69 can hold approximately 83 liters of CO2 at 273 kelvin (-0.15°C) under ambient pressure.
|ORTEP drawing of zinc atom surrounded by four linkers of ZIF-69. Click to enlarge.|
The ZIFs are highly porous (with surface areas up to 1,970 square meters per gram) and chemically robust structures that can be heated to high temperatures without decomposition and boiled in water or organic solvents for a week and still remain stable.
The technical challenge of selectively removing carbon dioxide has been overcome. Now we have structures that can be tailored precisely to capture carbon dioxide and store it like a reservoir, as we have demonstrated. No carbon dioxide escapes. Nothing escapes—unless you want it to do so. We believe this to be a turning point in capturing carbon dioxide before it reaches the atmosphere.—Omar Yaghi
Flaps in the ZIF structure behave like the chemical equivalent of a revolving door, allowing certain molecules—in this case, carbon dioxide—to pass through and enter the pores while blocking larger molecules or molecules of different shapes.
In ZIFs 68, 69 and 70, researchers Rahul Banerjee and Anh Phan emptied the pores, creating an open framework. They then subjected the material to streams of gases—carbon dioxide and carbon monoxide, and another stream of carbon dioxide and nitrogen—and were able to capture only the carbon dioxide. They are testing other ZIFs for various applications.
Currently, the process of capturing carbon dioxide emissions from power plants involves the use of toxic materials and requires 20 to 30% of the plant’s energy output, Yaghi said. By contrast, ZIFs can pluck carbon dioxide from other gases that are emitted and can store five times more carbon dioxide than the porous carbon materials that represent the current state-of-art.
Zeolites are stable, porous minerals made of aluminum, silicon and oxygen that are employed in petroleum refining and are used in detergents and other products. Yaghi’s group has succeeded in replacing what would have been aluminum or silicon with metal ions like zinc and cobalt, and the bridging oxygen with imidazolate to yield ZIF materials, whose structures can now be designed in functionality and metrics.
Banerjee and Anh automated the process of synthesis. Instead of mixing the chemicals one reaction at a time and achieving perhaps several reactions per day, they were able to perform 200 reactions in less than an hour. The pair ran 9,600 microreactions and from those reactions uncovered 25 new structures.
Co-authors of the paper are Bo Wang, a UCLA graduate student in chemistry in Yaghi’s laboratory; Carolyn Knobler and Hiroyasu Furukawa of the Center for Reticular Chemistry at the UCLA’s California NanoSystems Institute; and Michael O’Keeffe of Arizona State University’s department of chemistry and biochemistry.
In the early 1990s, Yaghi invented another class of materials called metal-organic frameworks (MOFs), which have been described as crystal sponges and which also have implications for cleaner energy. Like ZIFs, MOFs have pores in which Yaghi and his colleagues can store gases that are usually difficult to store and transport. (Earlier post.)
Yaghi’s laboratory has made several hundred MOFs, with a variety of properties and structures. Molecules can pass in and out of them unobstructed.
BASF funded the synthesis of the materials, and the US Department of Energy funded the absorption and separation studies of carbon dioxide.
(A hat-tip to Curtis!)
Rahul Banerjee, et. al., High-Throughput Synthesis of Zeolitic Imidazolate Frameworks and Application to CO2 Capture. Science 15 February 2008: Vol. 319. no. 5865, pp. 939 - 943 DOI: 10.1126/science.1152516