Just add water: Simple step boosts polymer’s ability to filter CO2 from mixed gases

An international team of researchers has found it can significantly boost an existing polymer’s ability to selectively remove carbon dioxide (CO₂) out of gas mixtures by first submerging the material in liquid water.

“Normally, improving the permeability of a gas through a material impairs the material’s selectivity,” says Rich Spontak, co-corresponding author of a paper on the work and Distinguished Professor of Chemical and Biomolecular Engineering and Professor of Materials Science and Engineering at North Carolina State University. “To explain this using CO₂ as an example, the more easily gases can pass through a material, the less able the material usually is to remove CO₂ from a gas mixture. It lets through the CO₂, but it lets through other gases as well. There’s a real tradeoff when engineering polymers for use as gas-separation membranes.

“What’s remarkable about our finding is that we were able to drastically improve the polymer’s CO₂ permeability while also slightly improving its CO₂ selectivity. And the process that led to this substantial improvement was related to transforming the microstructure of the membrane in low-cost and nontoxic fashion – we submerged the material in water.”

Polymer membranes that can filter out CO₂ are desirable for use in a variety of applications, such as removing CO₂ from natural gas and sequestering CO₂ in order to limit emissions from industrial facilities.

The polymer at issue here is a thermoplastic elastomer that is recyclable, relatively tough, and has been shown to have desirable properties for a wide range of contemporary technologies. For this work, the researchers set out to see how the morphology of the material – how the molecular sequences comprising the polymer molecules are arranged relative to each other – affects its performance as a CO₂-selective membrane.

The permeability of gases through polymers is frequently measured in Barrer units. When dry, the permeability of CO₂ through the polymer examined in the paper is less than 30 Barrer. Previous work reported by members of the team had shown that inclusion of water vapor in the feed could improve CO₂ permeability, boosting it to as high as 100-190 Barrer at relative humidity levels above 85%.

“With these new results, we’ve shown we can reach a permeability of almost 500 Barrer at 90% humidity,” says Liyuan Deng, Professor of Chemical Engineering at the Norwegian University of Science and Technology and co-corresponding author of the paper. “At the same time, the selectivity of CO₂ relative to nitrogen (N₂) increases to as high as ~60. For comparison, the best commercial polymer membranes that could be used for CO₂ capture possess a permeability up to ~200 Barrer and a CO₂/N₂ selectivity up to ~50. It’s very important that both of these performance metrics are considered simultaneously to achieve competitive membranes.

“This work demonstrates the polymer’s potential for use in industrial gas separation and carbon capture technologies, with benefits for both manufacturing efficiency and efforts to mitigate global climate change. It also provides a previously unexplored and facile route by which to transform the morphology of a polymer membrane and achieve tremendous improvement in gas transport properties.”

The paper, “Highly CO₂-permeable membranes derived from a midblock-sulfonated multiblock polymer after submersion in water,” is published in the journal NPG Asia Materials. First author of the paper is Zhongde Dai of the Norwegian University of Science and Technology (NTNU). The paper was co-authored by Jing Deng, Hesham Aboukeila and Luca Ansaloni of NTNU; Marco Giacinti Baschetti of Università di Bologna; Jiaqi Yan, a Ph.D. student at NC State; and Kenneth Mineart, a former student in Spontak’s lab who is now on the Chemical Engineering faculty at Bucknell University.

The work was done with support from the European Commission within the NanoMEMC2 project in the Horizon 2020 research and innovation program and the NC State University Nonwovens Institute. The research also used resources at the Advanced Photon Source, a U.S. Department of Energy Office of Science User Facility operated by Argonne National Laboratory.

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Note to Editors: The study abstract follows.

“Highly CO₂-permeable membranes derived from a midblock-sulfonated multiblock polymer after submersion in water”

Authors: Zhongde Dai, Jing Deng, Hesham Aboukeila, Luca Ansaloni, and Liyuan Deng, Norwegian University of Science and Technology; Marco Giacinti Baschetti, Università di Bologna; Jiaqi Yan and Richard J. Spontak, North Carolina State University; and Kenneth P. Mineart, Bucknell University

Published: Oct. 1, NPG Asia Materials

DOI: 10.1038/s41427-019-0155-5

Abstract: To mitigate the effect of atmospheric CO₂ on global climate change, gas separation materials that simultaneously exhibit high-CO₂ permeability and selectivity in gas mixtures must be developed. In this study, CO₂ transport through midblock-sulfonated block polymer membranes prepared from four different solvents is investigated. The results presented here establish that membrane morphology and accompanying gas transport properties are sensitive to casting solvent and relative humidity. We likewise report an intriguing observation: submersion of these thermoplastic elastomeric membranes in liquid water, followed by drying prior to analysis, promotes not only a substantial change in membrane morphology, but also a significant improvement in both CO₂ permeability and CO₂ /N₂ selectivity. Measured CO₂ permeability and CO₂/N₂ selectivity values of 482 Barrer and 57, respectively, surpass the Robeson upper bound, indicating that these nanostructured membranes constitute promising candidates for gas separation technologies aimed at CO₂ capture.

This post was originally published in NC State News.