(401al) Relationship between pKa of Amines in Microgel Particle Membranes and CO2 Permeance

Technology development of CO₂ capture technology from mass CO₂ emission sources such as thermal power plants is needed to decrease CO₂ concentration in the atmosphere. One of the most realistic strategies to reduce the CO₂ emission is capturing (separating) CO₂ from exhaust gases and storing them under ground. Membrane separation method is theoretically the most energy-saving separation method driven only by transmembrane pressure difference. However, there is a trade-off between selectivity and permeability.⁽¹⁾ Development of materials to overcome the trade-off and to realize reduction of CO₂emission is highly required.

We have developed CO₂ selective membranes using amine-containing gel particles (GPs) as the separation function layer on a support. The GPs consist of N-Isopropylacrylamide (NIPAm), N,N’-Methylene-bis-acrylamide (BIS) and amine-containing monomer. Properties of GPs such as particle size, amount of amine, swelling behavior, and pK_a of amines can be controlled by tuning polymerization procedures.⁽²⁾ In this paper, we report the effects of pK_a on CO₂separation efficiency.

GPs with variety of pK_a were prepared by free radical polymerization of monomers. Concentration and pK_a of amines were quantified by acid base titration. CO₂ separation membranes were prepared by spray coating of GPs solution to the porous support membranes. Membranes whose thickness of prepared GPs layer ranged from 2 µm to 130 nm determined by scanning electron microscope are prepared. Humidified gas mixture of CO₂/N₂(10/90 by vol.) was fed to the membranes, and the composition of the permeate was quantified by gas chromatography equipped with a thermal conductivity detector.

CO₂ permeance of amine-containing GPs membranes is in reverse proportion to thickness of the GPs layer when the thickness of GPs layer was more than 500 nm. Meanwhile, CO₂ permeance didn’t change dramatically when the thickness of GPs layer was less than 500 nm. It is suggested that surface reaction at the membrane surfaces was a rate-limiting step of CO₂ permeation when the thickness of GPs layer was less than 500 nm. What’s more, CO₂ permeance depended on pK_a of amines in GPs when the thickness of GPs layer was 250 nm. The GP with pK_a of 6.6 showed the CO₂ highest permeance of 195 GPU, while 61 GPU for the GP with pK_a of 8.3. It has been reported that CO₂ absorption rate depends on pK_a.⁽³⁾ Thus the CO₂ absorption and desorption rates were equal at pK_aof 6.6, and permeation resistance was the smallest, which resulted in giving the highest permeance.

In conclusion, we developed amine-containing GPs membranes with tunable pK_a and found that CO₂ permeance was the fastest at pK_a of 6.6. It can be for the first time to clarify that pK_a value determines the CO₂ permeance. This finding would provide an insight for further development of CO₂ separation membranes.

References

1. Robeson, L. M., J. Membr. Sci, 320, 390-400 (2008).

2. Yue, M., Hoshino, Y., Ohshiro, Y., Imamura, K., and Miura, Y., Angew. Chem. Int. Ed. 126, 2692-2695 (2014).

3. Littel, R. J., Swaaij, W. V., and Versteeg, G. F., Aiche. J. 36, 1633-1640 (1999).