Solidification of Aerial CO2‐Captured Aralkylamines in Water Triggered by Self‐Assembly Against Affinity for Formation of Reverse Lipid Bilayer

It is found that aralkylamines solidified even in water by absorption of aerial CO2. Despite being amphiphilic, the aralkylamines absorbed CO2 and self‐assembled against the amphiphilicity to form a reverse lipid bilayer structure and solidified. Once the solid‐liquid separation of aerial CO2‐captured amines in water is achieved, it becomes possible to select the optimum conditions for both absorption and desorption.

It is generally well known that the small amine molecule, which is a chemical absorbent, increases its CO 2 absorption efficiency in aqueous solution presumably due to the reduction of viscosity.However, increasing the water content means that extra energy for heating the water is required to release CO 2 .For example, it has been reported that 80% of the total heating energy is equivalent to water heating when CO 2 is released from a general condition of 30% MEA aqueous solution. [9]Therefore, this DOI: 10.1002/admi.202300881conventional method has the paradox that optimum conditions for both absorption and release cannot be selected.
In water, amphiphilic compounds aggregate according to their affinity to form lipid bilayer-and micelle-like structures (Figure 1).Recently, we [21,22] have found that aralkylamines, [22] which have hydrophobic phenyl groups and amines in close proximity, selectively absorb CO 2 from the air.It has also been confirmed that most amines change their state from liquid to solid after CO 2 absorption.For example, MXDA (m-xylylenediamine) absorbed only CO 2 from the air, with the resulting MXDA•CO 2 forming a reverse lipid bilayer structure as shown by X-ray crystallography.Thus, we considered that, even in the coexistence of water, aralkylamines absorbing low concentrations of CO 2 might self-assemble against the affinity to form a reverse lipid bilayer and solidify.Herein, we report that aerial CO 2 -absorbed aralkylamines solidified in an aqueous condition to form a reverse lipid bilayer.

Results and Discuss
Initially, we focused on the relationship between the concentration of aralkylamine aqueous solution and CO 2 absorption efficiency.MXDA was selected for this examination (Figure 2a).A mixed gas containing 1% CO 2 and 99% N 2 was flowed into the MXDA (7.6 mmol) aqueous solution (0.075≈3.1 m) at a rate of 25 mL per min by a mass flow controller (MFC), and the resulting CO 2 concentration on the outlet side was measured over time (Figure 2b).The results are shown in Figure 2c,d.With the MXDA stock solution (100 wt.%), the global minimum CO 2 concentration was ≈3 000 ppm at 2 h.In the aqueous solutions (1, 3, 6, 13, 23 and 30 wt.%), the minimal values were obtained at similar times, all below 1000 ppm. [23]The 6 wt.% solution gave the lowest value of 118 ppm at 2 h.When aqueous solutions of 6% or higher concentration were used, the resulting solids precipitated after CO 2 absorption.Figure 2e shows the amount of CO 2 absorbed per amine depending on the concentration of MXDA solution.At concentrations higher than 6wt.%, the amine efficiency values were lower than 0.5.This suggests that the carbamic acid formed by the amine and CO 2 is neutralized by another amine (R-NHCO 2 − •R-NH 3 + ).On the other hand, at concentrations lower than 6 wt.%, the amine efficiency values were higher than 0.5, which implies the inclusion of ammonium hydrogen carbonate (R-NH 3 + •HCO 3 − ) generated from amine.To clarify the further potential of CO 2 absorption with MXDA, an investigation under the atmospheric concentration of CO 2 was conducted.In this study, air containing about 400-600 ppm CO 2 directly flowed through the MFC (250 mL min −1 ) into 6 wt.% MXDA (94 mmol) aqueous solution under stirring (200 rpm) (Figure 3a).As shown in Figure 3b, CO 2 concentrations immediately dropped to 160 ppm, and this reaction condition continued to capture the low concentration (<300 ppm) of CO 2 for more than 300 h.In this examination, the precipitated solid was detected in the solution.After filtration and elemental analysis of the solid, it was found that the resulting powder was anhydrous MXDA•CO 2 recovered in 89% yield.Subsequent particle size analysis showed that the particle size (Dv(50) 73.488, Dv(10) 27.780, Dv(90) 169.753 μm) of the precipitated MXDA•CO 2 was much larger than 1 μm and all the precipitates could be filtered out using standard filter paper (Figure 3c).It can be seen in the scanning electron microscope (SEM) image that the plate-shaped crystals were laminated to form this solid (Figure 3d).
With the required results regarding MXDA in hand, our next endeavor focused on solidification with other aralkylamines and their mechanisms.To investigate the structuresolidification relationship, benzylmonoamines, such as pmethoxybenzylamine (PMBZA) bearing an electron-donating group and p-chlorobenzylamine (PCBZA) bearing an electronwithdrawing group, and one-carbon homologated phenethylamine (PEA) were selected (Figure 4a).Using the same device of Figure 2b, 6 wt.% of corresponding aralkylamines (7.5 mmol) in water was reacted under air-flow conditions (250 mL min −1 , 200 rpm). [23]For comparison, 6 wt.% of monoethanolamine (MEA) and NaOH (7.5 mmol) in water were also examined.As shown in Figure 4b, aralkylamines (MXDA 103 ppm, PEA 68 ppm, PMBZA 50 ppm) absorbed lower minimum CO 2 concentrations than the standard MEA (114 ppm) and NaOH (144 ppm) except for PCBZA (218 ppm).In addition, it was confirmed that MXDA and PCBZA maintained low concentrations of CO 2 for a longer time.
The desired solids were obtained after absorbing atmospheric CO 2 from the aqueous solution of aralkylamines.For PEA, a solid was formed only under a high concentration of CO 2 .Xray crystal structure analyses of the single crystals obtained in the presence of water were performed. [24]The results are shown in Figure 4c.MXDA bearing two ─CH 2 NH 2 groups absorbed CO 2 to form MXDA•CO 2 . [25,26]In PMBZA and PCBZA possessing one ─CH 2 NH 2 group, two molecules of amine reacted with one molecule of CO 2 to afford (PMBZA) 2 •CO 2 and (PCBZA) 2 •CO 2 , respectively. [27]In one-carbon homologated PEA ). [28]ext, the intramolecular interactions of the resulting complexes were analyzed.In all cases, phenyl groups are arranged on both outer surfaces to form a hydrophobic layer, and neutralized carbamic acid (or bicarbonate) composing hydrophilic layers are ar- ranged inside.In other words, the CO 2 -absorbed aralkylamine self-assembled against its amphiphilicity even in the presence of water, forming a reverse lipid bilayer structure. [29]

Conclusion
In summary, it was found that in the coexistence of aralkylamine and water, not only was the absorption performance of CO 2 in the atmosphere improved, but the absorbed CO 2 could be solidified with the amine.Therefore, no energy is required for heating water to release CO 2 .By this method, it is possible to choose the optimal conditions for both CO 2 absorption and desorption, which is not possible with conventional aqueous amine solutions.It was also clarified that the related aralkylamines selfassembled "against" amphiphilicity along with CO 2 absorption even under water conditions to [30 form a waterproof reverse lipid bilayer structure.Further study for CO 2 selective absorption is ongoing.

Figure 1 .
Figure 1.Relationship between amphiphilic molecule and affinity in water.

Figure 3 .
Figure 3. CO 2 absorption of 6 wt.% MXDA (94 mmol) aqueous solution under air.a) Reaction formula and photograph showing the state after the reaction, b) relationship between concentration of outlet side CO 2 densitometer and time, c) particle size analysis of the precipitated and filtered MXDA•CO 2 , and d) SEM image of the precipitated and filtered MXDA•CO 2 .