Porous Polymer Captures CO₂

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.