Liquid-crystalline nanostructured membranes for CO₂ separation

Abstract

We report herein that self-organized subnanoporous membranes prepared from ionic liquid-crystalline (LC) compounds exhibit CO₂ separation properties (αCO₂/N₂ ≈ 60) in humid conditions. A bicontinuous cubic (Cub_bi) LC film shows N₂ barrier properties, whereas the CO₂ permeability is kept as permeable.

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The development of CO₂ separation membranes^1–3 that allow rapid and selective CO₂ permeation is critical for rebalancing the global carbon cycle.⁴ Membranes fabricated from polymers,^5–8 graphene oxide,⁹ metal–organic frameworks,¹⁰ zeolites,¹¹ organic/inorganic interpenetrating networks,¹² and liquid water¹³ have been prepared for CO₂ gas separation. To achieve optimal performance, defect-free ultrathin separation membranes enabling fast sorption, diffusion, and desorption of CO₂ need to be designed, while minimizing the sorption of competing gases.

Thin films derived from polyamine-based hydrogel particles^14–16 and gel membranes of ionic liquids^17–19 were also examined for CO₂ separation. Since the humidity of gas strongly affects its permeability through polymer membranes,^7,14–20 gas permeation behaviors under various humid conditions were examined. Poly(ionic liquid) membranes have been shown to be useful for CO₂ separation.¹⁷ These materials contain ionic groups such as ammonium, imidazolium, and phosphonium moieties. However, these membranes form basically amorphous states.^17–19 We expected that the self-organized structures of ionic subnanoporous membranes^21–25 in humid conditions may enhance the gas separation properties. These structures are fixed by in situ polymerization of ionic liquid-crystalline (LC) compounds in the ordered state.

Our intention was to develop self-organized ionic LC membranes^21–29 for CO₂ gas separation. Several examples were reported as LC gas separation membranes.^30–32 Bara et al. reported CO₂ permeation through the membrane prepared from a lyotropic columnar LC monomer having a carboxylic acid moiety.³⁰ The membrane with an ordered cylinder structure showed higher CO₂ separation properties (αCO₂/N₂ ≈ 27) compared to the non-ordered membrane prepared from the isotropic phase (αCO₂/N₂ ≈ 21). It was assumed that interactions between CO₂ and the carboxylate groups of the ordered membrane with the aquatic environment of the pores caused higher CO₂ solubility and gas separation. Kloos et al. described gas separation using a smectic polymer membrane prepared from rod-shaped monomers having a crown ether moiety.³¹ The membranes preserving more ordered smectic structures showed CO₂/N₂ separation properties whereas membranes having disordered structures did not exhibit the gas separation.³¹ Jones et al. reported lyotropic liquid crystals showing light-driven columnar-to-cubic phase switching and the phase transition changed their CO₂ permeability.³² Development of membranes with ordered pores and the examination of the relationship between the organized structures and performance of the membranes are of interest.

Here, we describe the CO₂ gas separation properties of subnanoporous LC membranes in high humidity conditions. A wedge-shaped LC compound (1) and a rod-shaped LC compound (2) (Fig. 1) were used for the evaluation of gas permeation. These polymer films were originally developed as ion-conductors^26–29,33 and water-treatment membranes.^21–25 Compound 1 exhibits a bicontinuous cubic (Cub_bi) phase and its nanostructured membrane shows salt rejection comparable to those of nano-filtration membranes.^21,22 Compound 2 forms smectic (Sm) phases exhibiting two-dimensional channels showing enhanced water permeability and lower salt rejection ability since the Sm phase shows larger ratio of the hydrophilic nanopores.²³

Fig. 1 Molecular structures of the LC monomers.

The ionic LC membranes show unique ion permeation/rejection selectivity due to interactions among the ionic moieties, solutes, and water molecules.^{21,22,34–37} It is of interest to examine the performance of those organized ionic sites for CO₂ separation.

Compounds 1 and 2 (Fig. 1) were synthesized according to the procedures previously reported.^21–24 Polymer membranes (P1 and P2) for the gas separation tests were prepared from the LC monomers 1 and 2, respectively, with the transcription method using a poly(vinyl alcohol) (PVA) layer as a sacrifice layer (Fig. S1, ESI†).²¹ On a PVA substrate, compound 2 forms vertically aligned nanochannels in the Sm phase.²⁵ Thin films of 1 or 2 were obtained by in situ polymerization of the spin-coated mixture of the monomer and a photoinitiator on the PVA substrate. The nanostructured LC polymer layer was supported by porous substrates composed of polysulfone and poly(ethylene terephthalate) (PET) (Fig. 2). These supporting layers impart mechanical toughness to the membranes.

Fig. 2 Schematic illustrations of the nanostructured LC polymer membranes in controlled humid conditions.

The gas-permeation properties of the LC membranes (diameter = 25 mm) were evaluated by flowing simulated post-combustion gases containing 10 vol% CO₂ and 90 vol% N₂ (40 °C, 100 mL min⁻¹) on the top side of the membranes of the LC polymer film (Fig. S2, ESI†). The gas mixture was humidified by passing through a temperature-controlled water bath. The backside of the membranes was swept with helium gas to quantify the amount of gaseous species in the permeate by gas chromatography. The humidity of the backside gas was also controlled so that the gas humidity on both sides was the same.

Fig. 3 shows the permeance of CO₂ and N₂ permeation through P1 and P2 membranes when the relative humidity (RH) of the supplied gas was set to 50%, 80%, and 90%. P1 and P2 membranes showed high CO₂ and N₂ permeance below 80% RH. The permeances of CO₂ and N₂ were calculated to be of over 1000 gas permeation units (GPU, where 1 GPU = 1 × 10⁻⁶ cm³(STP) (s cm² cmHg)⁻¹). The CO₂ selectivity, αCO₂/N₂ (permeance of CO₂/permeance of N₂) was almost 1, indicating that these membranes did not work as CO₂ separation membranes below 80% RH.

Fig. 3 Effects of humidity on CO₂ (red bars), N₂ (black bars) permeance and selectivity (blue dots) of (a) P1 and (b) P2 LC membranes at 40 °C.

The N₂ permeance of the Cub_bi membrane P1 decreased dramatically from 1500 GPU to 3 GPU when the humidity was increased from 80% RH to 90% RH (Fig. 3a). The N₂ permeance through the P2 membrane also decreased under high humidity, but it was about 80 GPU (Fig. 3b). The N₂ barrier property of the smectic P2 membrane was approximately 1/30th that of Cub_biP1 membrane. The CO₂ permeance also decreased when increasing the RH from 80% to 90%. However, the CO₂ permeances through the sub-nanoporous membranes were greater than those of N₂ for each membrane and the CO₂ permeance was almost independent from the structures of P1 and P2 in contrast to the N₂ permeance. The CO₂ permeance of the P1 membrane was about 180 GPU and that of P2 was about 360 GPU at 90% RH, respectively. Consequently, these LC membranes exhibited CO₂ selectivity under humid conditions. In particular, the Cub_biP1 membrane at 90% RH showed the highest CO₂ selectivity (αCO₂/N₂ ≈ 60). This reflects the high N₂ barrier performance of the Cub_bi membranes under high humidity conditions.

Since post-combustion exhaust gas has a high water vapor partial pressure with a humidity of over 90% RH, the development of CO₂ separation membranes that function at high humidity is important, and thus various membranes for such purpose have been reported.^{7,9,14–20,38–40} Typically, membranes such as amine-added membranes and hydrogel membranes absorb water at high humidity, and the absorbed water facilitates CO₂ to dissolve and diffuse in the membrane.^{7,14–16,19,20,40} On the other hand, since N₂ has low polarity, its solubility in membranes decreases due to moisture absorption by the membrane. The high CO₂ selectivity that appeared in the LC membrane under high humidity is due to the low solubility of N₂ caused by moisture absorption by the ionic liquid crystal, while the solubility of highly polar CO₂ was maintained. The differences in N₂ barrier properties and CO₂ selectivity under high humidity between liquid crystal membranes with different phases may be due to the efficiency of their ionic moieties. It is assumed that the effects of ionic moieties are further critical in the Cub_bi structure with smaller nanochannels compared to the Sm structure shown as their ion removal properties.^21,23

To clarify the cause of the difference in gas permeance of P1 and P2 membranes at high humidity, we quantified the amount of water absorbed by the P1 and P2 membranes under highly humidified conditions with a thermogravimetry differential thermal analysis system equipped with a humidity controller (TG-DTA/HUM, Fig. S3, ESI†). For the TG-DTA/HUM measurements, nanostructured polymer membranes without the polysulfone substrate were used. Fig. 4 shows the mass changes of each membrane. Both the membranes P1 (Cub_bi) and P2 (Sm) absorbed water when the humidity was gradually increased at 40 °C. They absorbed water by weight of approximately 10% of the dry weight of the membrane at 90% RH. The amount of absorbed water was not significantly different between the Cub_bi and Sm films. This indicates that under high humidity, the polarity of the membrane increases as the membrane absorbs water, and as a result the solubility of N₂, which is non-polar, decreases, resulting in an increase in barrier properties against N₂.

Fig. 4 Effect of humidity on the amount of water absorbed by P1 (Cub_bi) (triangle) and P2 (Sm) (circle) membranes at 40 °C.

In conclusion, we have found that an ionic self-organized membrane that preserves a bicontinuous LC cubic structure exhibits the CO₂/N₂ selectivity of 60 at 90% RH, whereas no selectivity was observed below 80% RH. This selectivity at higher humidity is caused by water molecules adsorbed in the ionic subnanoporous membranes, which disturb the permeation of non-polar N₂ molecules.

T. K. and Y. H. conceived and designed the project. T. K., K. I., T. S. and Y. H. wrote the manuscript. T. S. synthesized the LC molecules and prepared the LC membranes. K. I. performed the gas separation experiments and analyzed the data. All of the authors read the paper.

This work was supported by a JSPS KAKENHI (JP19H05715; Grant-in-Aid for Scientific Research on Innovative Areas of Aquatic Functional Materials). This research was also supported by the MEXT Program, Data Creation and Utilization-Type Material Research and Development Project (Grant Number JPMXP1122714694), and JST (Grant Number JPMJPF2114).

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cc06751g

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