Materials Science Materials Science
In this model of the tetrazole-functionalized polymer of intrinsic microporosity (TZPIM), the dotted
line indicates possible hydrogen bonding between the tetrazole ring (blue) and the oxygen atoms on the ether
linkages of the polymer chains. This bonding acts to tighten the molecular structure, increasing the
selectivity of CO2 over N2.
By Tyler Irving
Posted July 2011
The field of carbon capture and storage (CCS) has taken a big step forward. Researchers at the National Research Council (NRC) have created a polymer membrane that shows unprecedented performance in separating CO2 from various gas mixtures.
Microporous polymer membranes for gas separation have been around for decades but have always been subject to a trade-off between permeability and selectivity. As the pore size is increased to let gases through at a higher rate, the ability to select which molecules go through and which stay behind is decreased. In order to do CCS from real industrial flue gas, membranes must be able to maintain their selectivity at extremely high throughput rates — up to 500 cubic metres per second. Another problem is that many polymer molecules change their shape on exposure to CO2, which degrades their performance.
At the NRC, Michael Guiver, Naiying Du, Mauro Dal-Cin and their team have been working on a substance known as a polymer of intrinsic microporosity (PIM). Unlike most polymers that have flexible chains, PIMs are made from molecules that are rigid, planar and at right angles, which prevents them from contorting. The team further modified this polymer by substituting tetrazole groups on to the monomer. These nitrogen-based ring structures attract CO2 and allow it to diffuse through the polymer faster than other gases by acting like molecular channels. Importantly, this works even better when CO2 is mixed with other gases; it appears that the same process that speeds up CO2 can slow down other gases like N2 when both are present.
“The unexpected surprise was seeing the mixed gas selectivity higher than the pure gas,” says Dal-Cin. Guiver agrees. “It’s got a very high novelty, and I think the results are really groundbreaking,” he says. The researchers believe that if these films can be made thin enough, they could reduce the cost of carbon capture to below $20 per ton, which the United States Department of Energy has targeted as critical for the future of CCS technology. The research was published in the April 2011 issue of Nature Materials.
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