Membranes made from a porous organic polymer featuring a nitrogen heterocycle, tetrazole, exhibit exceptional gas permeability and carbon dioxide separation and uptake properties, according to a team of researchers (Nat. Mater., DOI: 10.1038/nmat2989). The study may advance efforts to develop low-cost and energy-efficient methods to capture CO2 and other greenhouse gases (C&EN, May 2, page 30).
So far, the most developed method to capture CO2 from power plant flue gas relies on aqueous solutions of amines, which react readily with CO2. But the high-temperature regeneration step saps a large fraction of the plant’s energy output.
Researchers are therefore studying other types of nitrogen-containing CO2-capture materials including microporous polymers, which can be processed conveniently via standard solution-phase methods and have unusually high surface areas.
A key challenge is designing a material that captures CO2 with high selectivity and yet allows it to permeate the membrane easily. Most membranes are hampered by a trade-off between those properties. The top-performing porous polymers reported until now are characterized by a CO2/N2 selectivity of roughly 20 and a CO2 permeability of roughly 1,000 Barrer.
Now, Naiying Du and Michael D. Guiver at the Canadian National Research Council in Ottawa, Ontario; Ho Bum Park of Hanyang University, Seoul; and coworkers have come up with a class of novel membrane materials with a CO2/N2 selectivity of roughly 30 and a CO2 permeability greater than 2,000 Barrer.
To produce the new polymer, which features pendant tetrazole groups with high affinity for CO2, the team reacted a polymer containing aromatic nitrile groups with an azide compound via cycloaddition chemistry.
The group’s data show that CO2 molecules adsorb strongly to the polymer’s micropores, thereby blocking adsorption and transport of nitrogen, says membrane specialist Neil B. McKeown, a chemistry professor at Cardiff University in Wales. Nitrogen can then be vented through another flue channel.