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XIAOXING WANG/PENN STATE
KAREN UFFALUSSY/U OF PITTSBURGH
BY DESIGN Custom synthesis methods convert polyethylenimine and silica into
CO2 sorbents with tailored textures and structures, such as hollow nanobubbles (TEM
image, left) and micrometer-sized particles (SEM image).
ers of CO2 to avoid the cost of transporting
CO2 by permitting them to capture it and
use it on-site.
In a study published earlier this year in
Environmental Science & Technology, Jones
and coworkers compared CO2 uptake
and regeneration of their hyperbranched
aminosilica materials in tests with two
types of gas mixtures, one containing 10%,
the other 400 ppm CO2. They report that
the sorbent’s performance is “only marginally influenced” by the large difference in
CO2 concentration (Environ. Sci. Technol.,
DOI: 10.1021/es102797w).
MOF COMPOUNDS are another class
of solid sorbents widely investigated for
carbon-capture applications. These compounds are crystalline materials composed
of metal ions or clusters that are connected
by organic linkers. Because MOFs can
be tuned so broadly by tweaking various
chemical handles—for example pore sizes
can be adjusted by modifying the organic
units—research groups such as Omar M.
Yaghi’s at the University of California, Los
Angeles, have been able to prepare dozens
of MOFs, some of which claim record-setting CO2-uptake capacities.
Meanwhile, Zoey R. Herm, Kenji Sumida,
and other members of Long’s UC Berkeley
group have been designing MOFs that are
tailor-made to tackle various aspects of the
carbon-capture challenge. Long’s group
just described a strategy for tuning MOFs
to adsorb CO2 with high selectivity in the
presence of hydrogen, a key requirement for
precombustion CO2 capture (J. Am. Chem.
Soc., DOI: 10.1021/ja111411q).
The trick, which they demonstrated
by comparing the performance of MOFs
known as Cu-BT Tri and Mg-DOBDC with
that of other high-surface-area MOFs, is
to incorporate a high concentration of
exposed metal cation sites. That property,
Long explains, favors uptake of CO2 over
hydrogen because of CO2’s enhanced
po larizability.
Much the same strategy led Long’s
group to prepare the first Cr(II)-based
MOF, Cr3(BTC) 2, which selectively adsorbs
oxygen in high capacity in the presence of
nitrogen (J. Am. Chem. Soc., DOI: 10.1021/
ja1027925). That feature could help spur
development of technology for oxyfuel
combustion.
Regardless of chemical composition,
sorbents will need to be affordable to be
adopted widely. That requirement is driving researchers to examine low-cost starting materials, including various forms
of carbon. Song’s group for example,
recently showed that molecular basket
compounds prepared from commercial
carbon blacks and PEI have a CO2 capacity that nearly matches the group’s more
expensive SBA- 15 samples (Energy Fuels,
DOI: 10.1021/ef101364c). Similarly, Marta
Sevilla and Antonio B. Fuertes of Spain’s
National Institute of Coal, in Oviedo, just
reported that cellulose and sawdust can
be used to make activated carbon with
record-setting CO2 uptake (211 mg per
gram) (Energy Environ. Sci., DOI: 10.1039/
c0ee00784f).
There’s no shortage of ideas for capturing carbon. Even a weeklong session such
as the one in Anaheim is hardly broad
enough to touch on all of the relevant
proposals. But as Edinburgh’s Haszeldine
and other like-minded scientists point out,
government action mandating limits to
CO2 emissions has barely started. If carbon
capture and storage is to play a large role in
limiting climate change, he says, urgent action is required no w. ;
