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A new class of materials invented and developed by
Omar Yaghi at the University of Michigan can store vast amounts of
carbon dioxide. And one member of the class has the highest carbon
dioxide capacity of any porous material, Yaghi and co-worker Andrew
Millward report in a paper published online Dec. 1 in the Journal of
the American Chemical Society.
The materials, called metal-organic frameworks (MOFs)
and sometimes referred to as crystal sponges, previously have been
shown to have great potential for storing hydrogen and methane. On the
molecular level, MOFs are scaffolds made up of metal hubs linked
together with struts of organic compounds, a structure designed to
maximize surface area.
Just one gram of a MOF, in fact, has the surface
area of a football field. By modifying the rods in various ways, Yaghi
and his team have been able to increase the material's storage
capacity, making it possible to stuff more gas molecules into a small
area without resorting to high pressure or low temperature.
Yaghi compares the principle by which MOFs store CO2
to placing a honeycomb in a room full of bees. "All the bees will come
to the honeycomb, so you're able to contain a large number of bees in
a small volume. What we've created is a material that acts like a
honeycomb for adsorbing carbon dioxide."
The star performer in Yaghi's cast of MOFs is one
dubbed MOF-177, which sops up 140 percent of its weight in CO2 at room
temperature and reasonable pressure (32 bar).
Put another way, "if you have a tank filled with
MOFs, you can store in that tank as much carbon dioxide as would be
stored in nine tanks that do not contain MOFs," Yaghi said. By
comparison, a tank filled with porous carbon - one of the current
state-of-the-art materials for capturing CO2 in power plant
flues - would hold only four tanks worth of CO2.
MOFs can be made in large quantities from low-cost
ingredients, such as zinc oxide - a common component of sunblock - and
terephthalate, which is used in plastic soda bottles. And finding
effective, low-cost ways of reducing CO2 emissions is
crucial, said Yaghi, who is the Robert W. Parry Collegiate Professor
of Chemistry.
"Almost every region of the world is using more
energy than ever before, and the prediction is that this will continue
to increase, not just for petroleum, but also for coal and natural
gas. Whenever you're burning fossil fuels, you're releasing CO2
into the atmosphere, with devastating environmental effects that
include melting the polar ice caps and changing the ocean's acidity.
In the United States alone, each person is responsible for generating
more than 15 tons of carbon dioxide a year, largely from automobile
and power plant emissions," Yaghi said.
"I'm not exaggerating when I say that we are
digging a big, black hole for ourselves by not addressing the problem
of carbon dioxide emissions." |
