Caging carbon

by Tyler Shendruk

Published: 11/17/10

The problem
INDUSTRIAL NATIONS EMIT countless millions of tons of carbon dioxide (CO2) into the atmosphere every year. Coal combustion produces approximately a third of all that pollution and there is an immediate need to reduce emissions. One controversial idea is to bury the emissions deep in the ground before the CO2 can escape into the atmosphere and contribute to the greenhouse effect.
But you can’t just bury gas. You have to capture it first. Unfortunately, current methods of scrubbing CO2 out of a coal plant’s exhaust would require at least a quarter of all the energy produced by the power plant. It’s a prohibitively expensive procedure.

The researcher
Tom Woo is a researcher in the the Department of Chemistry and Centre for Catalysis Research and Innovation at the University of Ottawa. Woo specializes in molecular simulations and uses computer algorithms to model chemical systems at the molecular level. His simulations give fellow chemists insight into their experimental results and point them toward potential new designs for engineering materials.

The project
Compounds called metal-organic frameworks are special crystals of metal ions linked together by organic molecules. They are special because they can form very porous structures. In fact, these nanoporous materials can selectively capture CO2 molecules in their pores and hold the greenhouse gas trapped there. The rest of the combustion exhaust would float by and the CO2 would be left, filtered out of the gas.
But there’s one problem: the energy binding the CO2 to the pore is a little too weak. The material currently captures water vapour better than CO2. If the interaction trapping gas can be increased and the material made to not bind water, then nanoporous materials could be the short term solution to reducing carbon emissions.

The key
In order to design nanoporous material that better imprisons CO2, chemists must first understand the forces that hold the pollutant gas in the pore cavity. Woo’s simulations show that the forces responsible for keeping the CO2 captured are almost entirely made up of dispersion forces—a type of force that is weaker than most chemical bonds.
Woo believes that future materials can be designed to replace dispersion forces with stronger electrostatic forces. Using a stronger force ensures that the CO2 stays securely imprisoned while discouraging the seizure of water. Nanoporous materials engineered to use electrostatic interactions to selectively bind CO2 to their cavities would be an important step forward in carbon capture technology.