Simple method improves membrane CO2 separation performance

AN international team of researchers has, for the first time, shown that the carbon dioxide (CO₂) separation performance of an existing polymer can be “significantly” improved by first submerging the material in water.

Due to their relatively low-cost, easy fabrication, and simple scale up, polymer membranes have emerged as a practical alternative to traditional gas separation processes for large-scale industrial applications. Polymer membranes capable of separating CO₂ from gas mixtures have potential use in a variety of applications, such as removing CO₂ from natural gas and sequestering CO₂ to reduce emissions from industrial facilities.

In their research, the international team used a midblock-sulfonated polymer as the separation membrane. The material behaves as a thermoplastic elastomer, a class of copolymers that consists of materials with both thermoplastic and elastomeric properties. It is recyclable, tough, and self-sterilising.

They discovered that submerging the material in deionised water for 24 hours and then drying it at 22.5°C improved the polymer’s permeability and selectivity. This is a “remarkable” finding according to Richard Spontak, as typically “improving the permeability of a gas through a material impairs the material’s selectivity”. Spontak is a Distinguished Professor of Chemical and Biomolecular Engineering and Professor of Materials Science and Engineering at North Carolina State University (NCSU), US, which was involved in the research.

Using the submersion method when the gas stream was at 89% relative humidity (RH), the researchers were able to achieve a CO₂ permeability and carbon dioxide/nitrogen (CO₂/N₂) selectivity of 482 Barrer and 57, respectively, for polymer cast in chloroform (CF-cast). Spontak explained that the Barrer is a unit of measurement for gas permeability that reveals the rate of throughput, and selectivity provides a measure of the quality of separation between two gases. Submersion improved permeability by about 650% and CO₂/N₂ selectivity by about 17% on average, over the entire RH range.

For comparison, the best commercial polymer membranes that could be used for CO₂ capture have a permeability of about 200 Barrer and a CO₂/N₂ selectivity of about 50. This is according to Liyuan Deng, Professor of Chemical Engineering at the Norwegian University of Science and Technology (NTNU), Norway, which was also involved in the research.

The researchers attributed the change in polymer membrane performance to a transformation in the microstructure of the membrane via the developed low-cost, non-toxic method, according to Spontak. Submersion of the polymer containing spherical ionic microdomains (a highly nonequilibrium morphology for the polymer examined) caused swelling and created interconnected pathways that served to enhance molecular mobility through the membrane, he added.

The use of a humidified gas stream further swells the diffusion pathways and further enhances CO₂ permeability.

Spontak said: “Most studies of membranes designed around block copolymers seek to achieve highly ordered morphologies with continuous diffusive pathways. This can be an onerous task on commercial production scales.

“Our study demonstrates that nonequilibrium morphologies can provide competitive gas-separation performance. In this case, a nonequilibrium morphology can be transformed into the desired continuous pathway through a simple and eco-friendly post-process: swelling in water.”

The choice of casting solvent also affects the polymer membrane performance. To achieve the highly nonequilibrium morphology capable of swelling and irreversibly transforming following submersion in water, non-polar or moderately polar (co)solvents need to be used, said Spontak. Cosolvents are substances added to a primary solvent to aid compound solubility or miscibility. In the study the researchers used low-polarity (co)solvents to generate these structures.

To cause swelling and achieve an irreversible transformation of the polymer morphology, the submersion solvent must be a highly polar medium, such as water, glycerol, or ethylene glycol, that doesn’t dissolve the polymers, said Spontak.

According to Spontak, as far as the researchers are aware this is the first time this submersion approach has been introduced and if it is generally applicable to nanostructured amphiphilic polymers, such as the midblock-sulfonated polymer employed in this study, it could provide a new membrane design paradigm. Amphiphilic molecules are those that have both hydrophilic and hydrophobic components.

Currently, researchers from NTNU and NSCU are investigating chemically dissimilar nanostructured amphiphilic polymers and their preliminary findings suggest similar results can be achieved. This provides evidence that the process-based approach is general, Spontak added.

Deng said: “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.”

This research was performed with support from the NANOMEMC2 project a research and innovation action of the EU’s Horizon 2020 research and innovation programme. NANOMEMC2 aims to reduce the cost, energy, and process limitation of CO₂ capture processes that are non-viable in many industries. The NCSU Nonwovens Institute, the world’s first accredited academic programme for the interdisciplinary field of engineered fabrics, also supported the research.

The research also used resources at the Advanced Photon Source, a US Department of Energy Office Science User Facility located at the Argonne National Laboratory.

NPG Asia Materials: http://doi.org/ddqx