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CO2-selective absorbents in air: Reverse lipid bilayer structure forming neutral carbamic acid in water without hydration Fuyuhiko Inagaki, Chiaki Matsumoto, Takashi Iwata, and Chisato Mukai J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b01049 • Publication Date (Web): 17 Mar 2017 Downloaded from http://pubs.acs.org on March 17, 2017
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CO2-selective absorbents in air: Reverse lipid bilayer structure forming neutral carbamic acid in water without hydration Fuyuhiko Inagaki*, Chiaki Matsumoto, Takashi Iwata and Chisato Mukai Division of Pharmaceutical Sciences, Graduate School of Medical Sciences, Kanazawa University, Kakuma-machi, Kanazawa 920-1192 (Japan)
Supporting Information Placeholder ABSTRACT: Emission gas and air contain not only CO2 but also plentiful moisture, making it difficult to achieve selective CO2 absorption without hydration. To generate absorbed CO2 (wet CO2) under heating, the need for external energy to release the absorbed water has been among the most serious problems in the fields of carbon dioxide capture and storage (CCS) and direct air capture (DAC). We found that the introduction of the hydrophobic phenyl group into alkylamines of CO2 absorbents improved the absorption selectivity between CO2 and water. Furthermore, ortho-, meta-, and paraxylylenediamines (OXDA, MXDA, PXDA, respectively) absorbed only CO2 in air without any hydration. Notably, MXDA·CO2 was formed as an anhydrous carbamic acid even in water, presumably because it was covered with hydrophobic phenyl groups, which induces a reverse lipid bilayer structure. Dry CO2 was obtained from heating MXDA·CO2 at 103~120 °C, which was revealed to chemically involve the Grignard reaction to form the resulting carboxylic acids in high yields.
Carbon dioxide capture and storage (CCS)1-3 is currently one of the most effective methods for reducing CO24-7 in emission gas. Direct air capture (DAC)8-15 is another technology for reducing CO2 gas. From the perspective of the installation location, DAC systems could be more valuable than CCS because a DAC system captures CO2 (ca. 400 ppm = 0.04 vol%) from the air, which is abundant everywhere on earth. The adsorbents/absorbents for CCS and DAC are mainly classified into three types: amines (e.g., monoethanolamine [MEA], polymeric amine), inorganic salts, and metalorganic frameworks (MOFs)16-21. Emission gas and air contain not only CO2 but also plentiful moisture, and water contamination requires additional external energy at the step of CO2 generation under heating condition. However, as far as we know, no CO2 absorbent without water contamination has been reported. Recent efforts in our laboratory have focused on investigating DAC rea-
gents using low molecular weight compounds, and we found that the absorption of CO2 in air using MEA formed (CO2)·3(MEA)·3(H2O) (1) in 85~95% yields, suggesting the generation of carbonate ions (CO32- or HCO3-) (Figure 1c).15 The absorption ratio of (CO2)·3(MEA)·3(H2O) (1) revealed that MEA functions as both a hygroscopic material and a CO2 sorbent. Similar contamination must be caused in the CCS systems using MEA because exhaust gas also contains water derived from burned fossil fuels. On these grounds, our current aim is to explore CO2 absorbents with no hygroscopic properties. Plausible mechanisms for the absorption of CO2 and water using amines are shown in Figure 1a. Acidic CO2 is captured by basic amines to form the corresponding carbamic acid A (RR’NCO2H). Then, the hydration of A yields ammonium hydrogencarbonate B (RR’NH2+HCO3-). In parallel, carbamic acid A would also be neutralized with another amine to provide the neutral carbamic acid C (RR’NH2+RR’NCO2-). Both the neutralization of B with another amine and the hydration of C yield the common bisammonium carbonate D ([RR’NH2+]2CO32-). We hypothesize that if the hydrophobic moiety is introduced at the substitution group of the amine (R), the hydration of the neutral carbamic acid 2 might be inhibited owing to the hydrophobicity (Figure 1b). To pursue the above goal, we selected amines bearing benzene moieties, such as benzylamine (BZA, 4a), phenethylamine (PEA, 4b), N-methylbenzylamine (NMBZA, 4c), p-methoxybenzylamine (PMBZA, 4d), m-xylylenediamine (MXDA, 4e), p-xylylenediamine (PXDA, 4f), and o-xylylenediamine (OXDA, 4g) (Figure 1c). Firstly, CO2 absorption from the ambient air was examined in an enclosed space (35.7 L). The results are shown in Figure 2a. The initial CO2 concentration (502582 ppm, 0.84-0.97 mmol CO2) was highly dependent on uncontrollable variables in the experimental room.
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R
O N R' A OH
R NH + CO2 R'
H 2O
O
R NH 2 O R'
O
R NH 2 O C
b R NH 3 O 2
c NH 2
NH 2
H 2O
O
R NH 2 R'
R'
O
N R H
H 2N R'
O R NH 3
? N H
R O
D
X H 2O
O hydrophobic group
R NH R'
N R
R'
OH
B
R NH R'
O
O
H 3N R
3 Me
NH 2
MeO Benzylamine Phenetylamine N-Methylbenzylamine p-Methoxybenzylamine (BZA, 4a) (PEA, 4b) (NMBZA, 4c) (PMBZA, 4d) NH 2
NH 2
NH 2 NH 2 m-xylylenediamine p-xylylenediamine (MXDA, 4e) (PXDA, 4f)
NH 2 NH 2 o-xylylenediamine (OXDA, 4g)
Figure 1. CO2 absorption/generation systems side-by-side with hydration/dehydration. a. Plausible mechanism for the reaction of amines, CO2, and water. b. Our hypothesis regarding hydration blocking for the hydrophobic groups on the amines. c. Candidates for the amines bearing hydrophobic moieties.
When 0.33 mL (5.0 mmol) of BZA 4a was incubated in the sealed box, the CO2 concentration immediately decreased from 551 ppm to less than 50 ppm within 14 h, which is quite similar to the previously reported result for MEA. The absorption ability of PEA 4b, PMBZA 4d, MXDA 4e, and OXDA 4g were also similar to that of BZA 4a. Furthermore, the CO2 absorption rate of PXDA 4f was slightly slower than that of MEA 1. The absorption ability of NMBZA 4c was obviously inferior to those of the other amines. Next, we investigated the ratio of CO2 absorption in air using several amines (4b, 4d-f). We omitted the highly volatile BZA 4a, NMBZA 4c, which exhibits low absorbency, and OXDA 4g, which is too expensive (ca. 1850 times the price of MXDA 4e). For this experiment, 75 mmol of the amines (4b, 4d-f) was placed on a scale exposed to air (i.e., not enclosed, CO2 ca. 300~600 ppm), and it was weighed over time. The average mass increases over time from three replicates are shown in Figure 2b. Although PEA 4b acted as a CO2 absorbent in the enclosed space, the total amount decreased owing to its volatility. PXDA 4f gradually increased in weight. The initial rate of increase using PMBZA 4d was faster than that of PXDA 4f. Both the increment and rate of MXDA 4e represented the best results. To determine the composition of the mixture between amines (4b-g) and air, elemental analyses were then conducted (Figure 2c). Samples collected after 2 weeks were compared and analysed, revealing that the mixture x(CO2)·y(PEA 4b)·z(H2O) contained CO2, PEA 4b, and H2O at a ratio of approximately 9 : 4 : 1. The ratio among CO2, PMBZA 4d, and H2O was the same as the ratio for 4b. Based on the results of elemental analy-
sis of 4b,d containing half the amount of CO2, amine, and a small amount of water seemed an equilibrium between ammonium carbamate C (RR’NH2+RR’NCO2-) and bisammonium carbonate D ([RR’NH2+]2CO32-). Furthermore, similar examinations using several alkyl amines (N-methylaminoethanol, ethylenediamine, N,N’dimethylethylenediamine, diethylenetriamine) were also conducted for comparison (details are shown in supporting information). The alkylamines absorbed approximately 5.5~16 times more H2O than CO2. Nevertheless, while PEA 4b and PMBZA 4d allowed some hydration, we could trust that the hydrophobicity from introducing the phenyl moiety was effective for selective absorption. Furthermore, we found that the xylylenediamines 4e-g selectively absorbed CO2 without any water contamination. As the resulting MXDA·CO2, PXDA·CO222, and OXDA·CO2 were insoluble or have very low solubility in most solvents, solid 13C-NMR was used for the analysis (Figure 2d). All complexes showed one peak near 163~165 ppm (carbonyl moiety).23,24 Judging from the results of elemental analysis and solid 13C-NMR, MXDA·CO2, PXDA·CO222, and OXDA·CO2 would indicate the intermediate of neutral carbamic acid C (RR’NH2+RR’NCO2-: Figure 1a). Based on these investigations, we finally judged that MXDA 4e was an optimum CO2 absorbent without hydration. a
b 3.5 3 2.5 Increment/g →
a
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2 1.5 PEA 4b PMBZA 4d MXDA 4e PXDA 4f
1 0.5 0 -0.5
0
1
-1
c amines
air ca. CO2 300~600 ppm rt, 2 weeks
2
3
4
5
6
7
Absorp; on ; me/day →
d
MXDA·CO2 DA
x(CO2)y(amine)z(H 2O)
CO 2 amine H 2O x y z amine 1 3 3 MEA 4 9 1 PEA 4b 9 1 PMBZA 4d 4 1 1 0 MXDA 4e 1 1 0 PXDA 4f 1 1 0 OXDA 4g
OXDA·CO2 CO 2 : H 2O 1: 1: 1: 1: 1: 1:
3 0.25 0.25 0 0 0
DA
PXDA·CO2 DA 200
150
100 Chemical Shift (ppm)
50
0
Figure 2. Alkylamines bearing phenyl moieties selectively absorb CO2 rather than water. a. The ability of alkylamine derivatives 4 (5 mmol) to absorb atmospheric CO2 in sealed box (35.7 L). b. The average increment of the reaction with alkylamines 4b,d-f (75 mmol) and open air. c. The results of elemental analysis (within ±0.4% error) using products derived from alkylamines 4b,d-g and air for 2 weeks at room temperature. d. The spectra of solid 13C-NMR for the insoluble CO2-absorbed complexes MXDA·CO2, OXDA·CO2, and PXDA·CO2.
Our next experiments focused on clarifying water resistance using MXDA 4e. The reaction of MXDA 4e under abundant air yielded the MXDA·CO2 complex 5e in quantitative yields (Figure 3a). The generating solid
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MXDA·CO2 stayed in solution as precipitations or suspension under mixing condition. Although the liquid MXDA 4e was highly soluble in water, the resulting solid MXDA·CO2 5e showed very low solubility in water (ca. 0.463 × 10-3 mol/L). Exposing the solution of MXDA 4e to CO2 gas in water also provided the same solid. More recently, Zhu and co-workers25 reported that the reaction of MXDA and 1 atm of dry CO2 formed MXDA·1.1(CO2). However, the product was speculatively identified based on the amount of released CO2, and no structural evidence of MXDA·1.1(CO2) was reported. In our examination, MXDA fully reacted with CO2 at a 1:1 ratio. To elucidate the composition and structure of the product, X-ray crystallography was performed. Despite the very low solubility of MXDA·CO2 5e, a crystal was obtained from a water solution for the X-ray analysis. The ORTEP result (Figure 3b) clearly proved that 1) the structure contains no water, 2) the ratio of MXDA:CO2 was 1 : 1, and 3) MXDA·CO2 5e contains a carbamic acid, which is neutralized with another amine. Thus, the formed MXDA·CO2 5e must be self-assembled without hydration, even in water.
a
b air NH 2 rt NH 2 >99% yield
NH 2
NH C O OH MXDACO 2 5e solid very low water solublity (
