Until recently, factory smokestacks that produced nothing but carbon dioxide
and water vapor were considered exemplary. Now CO2 has become notorious as a
greenhouse gas, and the danger of climate change has become one of the most
pressing environmental problems of our time. How can we slow the increasing
release of CO2? Efficient methods for the separation of this greenhouse gas
from industrial exhaust are being sought. Korean researchers have now developed
a porous material that can bind and store CO2 efficiently and highly selectively.
As Myunghyun Paik Suh and Hye-Sun Choi report in the journal Angewandte
Chemie, the lattice-like network contains flexible "columns" that
can open the pores of the three-dimensional lattice for CO2.
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Many porous materials are able to absorb CO2 and other gas molecules. However,
the selective, room-temperature extraction of CO2 at atmospheric pressure from
industrial exhaust containing other gases such as nitrogen, methane, and water
remains a major technical challenge.
The research team has now developed porous, three-dimensional networks of coordination
polymers. Various nickel complexes and organic molecules are used as building
blocks that assemble into two-dimensional lattice-like planes, which in turn
grow into stacks held together by “columns”. The special trick in
this case is that the columns are not rigid, but very flexible. The corresponding
cavities in the structure are thus of variable size and can adjust to the guest
molecules that enter.
The symmetric molecule carbon dioxide has a permanent electrical quadrupole
moment that can be described as two electrical dipoles sitting back-to-back
and pointing in opposite directions. This quadrupole interacts with the three-dimensional
lattice, and this effect causes the columns to open the “gates”,
allowing the gas to enter the cavities. In contrast, nitrogen, hydrogen, and
methane have much smaller quadrupole moments. The pores thus remain closed to
them. The exclusion of nitrogen, which makes up a large proportion of air, is
essential for any potential CO2 capture. In addition, the new nickel-containing
materials are stable at temperatures up to 300 °C and are air- and water-
stable—also an important requirement for potential industrial application.
If the surrounding pressure is reduced, the stored CO2 is released. This type
of material is thus suited for processes in which carbon dioxide must be cyclically
stored and then released through a change in pressure.
Posted August 24th, 2009
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