ZIFs: New Framework Materials for the Capture and Storage of CO2

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. 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. 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. 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, 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.

A ZIF structure.

ORTEP drawing of zinc atom surrounded by four linkers of ZIF-69.

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. 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.

Researchers 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. 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.