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The carbon dioxide is captured using a new class of materials designed
by Yaghi and his group called zeolitic imidazolate frameworks, or ZIFs.
These are porous and chemically robust structures, with large surface
areas, that can be heated to high temperatures without decomposition
and boiled in water or organic solvents for a week and still remain
stable.
Rahul Banerjee, a UCLA postdoctoral research scholar in chemistry and
Anh Phan, a UCLA graduate student in chemistry, both of whom work in
Yaghi's laboratory, synthesized 25 ZIF crystal structures and
demonstrated that three of them have high selectivity for capturing
carbon dioxide (ZIF-68, ZIF-69, ZIF-70).
"The selectivity of ZIFs to carbon dioxide is unparalleled by any
other material," said Yaghi, who directs of UCLA's Center for
Reticular Chemistry and is a member of the California NanoSystems
Institute at UCLA. "Rahul and Anh were so successful at making new
ZIFs that, for the purposes of reporting the results, I had to ask
them to stop."
The inside of a ZIF can store gas molecules. Flaps that behave like
the chemical equivalent of a revolving door allow certain molecules
- in this case, carbon dioxide
- to pass through and enter the reservoir while blocking larger
molecules or molecules of different shapes.
"We can screen and select the one type of molecule we want to capture,"
Phan said. "The beauty of the chemistry is that we have the freedom to
choose what kind of door we want and to control what goes through the
door."
"The capture of carbon dioxide creates cleaner energy," Yaghi said. "ZIFs
in a smokestack would trap carbon dioxide in the pores prior to its
delivery to its geologic storage space."
In ZIFs 68, 69 and 70, Banerjee and Phan emptied the pores, creating
an open framework. They then subjected the material to streams of
gases - carbon dioxide and carbon monoxide, for example, 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.
Carbon dioxide is killing corral reefs and marine life, damage that
will be irreversible, at least for many centuries, Yaghi noted.
Currently, the process of capturing carbon dioxide emissions from
power plants involves the use of toxic materials and requires 20 to 30
percent 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.
"For each liter of ZIF, you can hold 83 liters of carbon dioxide,"
Banerjee said.
The word zif, Yaghi noted, is used in the Bible to describe a region
of splendor. It also means comeliness and brightness. This name is
fitting for this new class of materials, he said, because its members
are many and of quite beautiful constructions.
On a fundamental level, the invention of ZIFs has also addressed two
major challenges in zeolite science. 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.
"We keep producing new crystals of ZIFs every day," Banerjee said.
"These reactions produce crystals that look as beautiful as diamonds."
Co-authors 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. Yaghi can
change the components of MOFs nearly at will. Like ZIFs, MOFs have
pores � openings on the nanoscale in which Yaghi and his colleagues
can store gases that are usually difficult to store and transport.
Yaghi's laboratory has made several hundred MOFs, with a variety of
properties and structures. Molecules can pass in and out of them
unobstructed.
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