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Filtering carbon dioxide, a greenhouse gas, from factory
smokestacks is a necessary, but expensive part of many manufacturing processes.
However, a collaborative research team from NIST and the University of Delaware
has gathered new insight into the performance of a material called a zeolite
that may stop carbon dioxide in its tracks far more efficiently than current
scrubbers do.
Zeolites are highly porous rocks—think of a sponge made of
stone—and while they occur in nature, they can be manufactured as well. Their
toughness, high surface area (a gram of zeolite can have hundreds of square
meters of surface in its myriad internal chambers) and ability to be reused
hundreds of times makes them ideal candidates for filtering gas mixtures. If an
unwanted molecule in the gas mixture is found to stick to a zeolite, passing
the mixture through it can scrub the gas of many impurities, so zeolites are
widely used in industrial chemistry as catalysts and filters.
The team explored a zeolite created decades ago in an
industrial laboratory and known by its technical name, SSZ-13. This zeolite,
which has octagonal “windows” between its interior pore spaces, is special
because it seems highly capable of filtering out carbon dioxide from a gas
mixture. “That makes SSZ-13 a promising candidate for scrubbing this greenhouse
gas out of such things as factory smokestacks,” says Craig Brown, a researcher
at the NIST Center for Neutron Research (NCNR). “So
we explored, on an atomic level, how it does this so well.”
Using neutron diffraction, the team determined that SSZ-13’s
eight-sided pore windows are particularly good at attracting the long, skinny carbon
dioxide molecules and holding onto their “positively-charged” central carbon
atoms, all the while allowing other molecules with different shapes and
electronic properties to pass by unaffected. Like a stop sign, each pore halts
one carbon dioxide molecule—and each cubic centimeter of the zeolite has enough
pores to stop 0.31 g of carbon dioxide, a quantity that makes SSZ-13 highly
competitive when compared to other adsorbent materials.
Brown says a zeolite like SSZ-13 probably will become a
prime candidate for carbon scrubbing because it also could prove more
economical than other scrubbers currently used in industry. SSZ-13’s ability to
attract only carbon dioxide could mean its use would reduce the energy demands
of scrubbing, which can require up to 25% of the power generated in a coal or
natural gas power plant.
“Many industrial zeolites attract water and carbon dioxide,
which are both present in flue exhaust—meaning both molecules are, in a sense,
competing for space inside the zeolite,” Brown explains. “We suspect that this
novel carbon dioxide adsorption mechanism means that water is no longer
competing for the same site. A zeolite that adsorbs carbon dioxide and little
else could create significant cost savings, and that’s what this one appears to
do.”
Brown says his team is still collecting data to confirm this
theory, and that their future efforts will concentrate on exploring whether
SSZ-13 is equally good at separating carbon dioxide from methane—the primary
component of natural gas. Carbon dioxide is also released in significant
quantities during gas extraction, and the team is hopeful SSZ-13 can address
this problem as well.
