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Environmental and Carbon Dioxide Issues

Efficient absorption of CO2 by introduction of intramolecular hydrogen bonding in chiral amino acid ionic liquids Mingguang Pan, Yongsheng Zhao, Xiaoqin Zeng, and Jianxin Zou Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00879 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on April 28, 2018

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Efficient absorption of CO2 by introduction of intramolecular hydrogen bonding in chiral amino acid ionic liquids Mingguang Pan,*,† Yongsheng Zhao,‡ Xiaoqin Zeng,†,§ Jianxin Zou*,†,§ †

National Engineering Research Center of Light Alloy Net Forming and State Key Laboratory of Metal Matrix Composite, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China ‡

Department of Micro/Nano-electronics, Shanghai Jiao Tong University, Shanghai 200240, China

§

Shanghai Innovation Institute for Materials, Shanghai, 200444, China Ionic Liquids; chiral amino acid; intramolecular hydrogen bonding; multi-site cooperative interactions; CO2 storage. Supporting Information Placeholder ABSTRACT: Developing advanced materials such as ionic liquids is highly desired for post-combustion capture of CO2. In this communication, we first develop a series of chiral amino acid ionic liquids for efficient, fast and reversible capture of CO2. Our data reveal that the dianionic form of IL is beneficial to the formation of intramolecular hydrogen bonding, which can remarkably mitigate the viscosity increase during CO2 absorption. The enhanced absorption capacity of CO2 in the IL is mainly due to the multi-site absorption from both the amino group and the negative oxygen group. And the dominating absorption pathway is that the amino group reacts with CO2 via an intramolecular proton transfer from the amino group to the negative oxygen group, leading to the formation of the intramolecular hydrogen bonding between the carbamate and the protonated oxygen group. As a result, the enhanced absorption performance of aminofunctionalized ILs is achieved, especially a controllable viscosity change during the uptake, providing an important foundation for building smart absorption systems based on amino-functionalized ILs.

1. INTRODUCTION Accumulation of carbon dioxide (CO2) in the atmosphere has reached 400 ppm, a level unprecedented in recent history. Such an increase is largely attributed to the excessive consumption of fossil fuels.1 To mitigate the CO2 crisis, the conventional method is aqueous amine solutions for chemical absorption of CO2, however, exhibiting severe inherent drawbacks including solvent loss, corrosion, and energy-extensive consumption.2 Thus, it is a promising approach to develop advanced functional materials or technologies for post-combustion capture of CO2. For example, various solid materials including zeolites, metal-organic frameworks (MOFs), porous polymers and activated carbons are devoted to this significant field–CO2 capture and storage (CCS).3 These adsorbents offer high surface areas to accelerate the uptake of CO2 by virtue of their porous nature. However, in most cases they suffer from problems such as low CO2/N2 selectivity, significant degradation in CO2 uptake capacity especially at moisture condition, and high energy demand for regeneration.

Ionic liquids (ILs), which are entirely composed of ions, are a class of low-temperature molten salts with melting points usually below 100 ˚C.4 Since the first report about the physical solubility of CO2 in the 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF6] was a breakthrough, considerable efforts then focused on understanding and increasing the physical absorption of CO2 in ILs.5 However, purely physical dissolution of CO2 in ILs is too low, only up to a molar fraction of 0.035 (mass capacity is around 0.4 %) at ambient condition, which is inaccessible to practical application.6 Therefore, chemisorption of CO2 by functionalized ILs may be the key to improve the CO2 absorption capacity to a large extent. The first chemisorption of CO2 by an aminofunctionalized IL was designed in 2002, reaching up to a molar fraction of 0.5 under ambient condition, far beyond the previously reported results in ILs.7 Recent decades have witnessed that functionalized ILs are greatly developed as state-of-the-art solvents for CO2 absorption on account of their unique properties including negligible vapour pressure, high thermal stability, non-flammability, wide liquid temperature range, virtually unlimited tunability and excellent CO2 affinity.8 Among them, amino-functionalized ionic liquids (AFILs) are the most widely used for CO2 absorption as the amino group has sufficiently strong CO2 binding energy.9 In comparison to aqueous amine solutions, AFILs may be able to reduce the solvent degradation and energy demand during the CO2 uptake process because of no use of water. However, the viscosities of traditional AFILs, in most case, increased by up to two orders of magnitude because of the formation of a strong and pervasive hydrogen-bonded network during the uptake process of CO2.10 As a result, the absorption kinetics is extremely slow and absorption capacity remains poor. Therefore, a carefully structural design of AFILs is highly desired to achieve considerable absorption performance of CO2. As revealed in previous literatures, the presence of intermolecular hydrogen bonding is principally responsible for the dramatical increase of viscosity in AFILs during the absorption of CO2. There are two main strategies to solve this issue: i) Developing non-amino-functionalized ILs such as phenolate or azolide ILs due to the lack of proton donor and acceptor in such systems;11 ii) Avoiding the formation of extensive intermolecular hydrogen bonded network by a series of means such as

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introducing an carboxyl group in the close vicinity of the amine,9b or creating intramolecular hydrogen bonding or intramolecular proton transfer by adjusting the loaction of the proton donor such as NH2 group and the proton acceptor such as O- in close proximity.9a,12 A recent work revealed that the viscosity in aminofunctionalized pyridine-based ILs decreased around 40% by intramolecular proton transfer reactions.12 In this regard, the route ii is preferred for the promotion of CO2 uptake in the present work. Chiral molecules possess the feature that they cannot be superimposed due to their mirror images and have been employed as building blocks into more complicated architectures, such as specifically designed peptides, DNAs, sol-gel systems, etc.13 Here, the chiral effect of amino acid anions was firstly investigated in ionic liquids for modulating CO2 absorption. Furthermore, the establishment of dianion mode in the ILs is to create intramolecular hydrogen bonding during the CO2 uptake, thus resulting in an improved absorption capacity and fast absorption rate. In the current work, the chiral amino acids with two different proton donors, hydroxyl group (OH) and carboxylic acid group (COOH) respectively, were selected to act as the anion parts of ILs. Among these amino acids, the amino group (NH2) was set on the adjacent position of the COOH group. Once the proton on COOH took away, the mono-anionic type of ILs formed. And the formed carboxylate ion (COO-) play a significant role in tuning the CO2 reaction mode of neighbouring amino group. Specially, the COO- group can prevent the intermolecular proton transfer process from one amino group to another upon the uptake of CO2 due to the electrostatic instability of the zwitterions (Scheme S1).9b Further deprotonation of another proton donor group OH can produce the di-anionic type of ILs. Mono-anionic type of these amino acid ILs is prone to form dense hydrogen bonded network during the CO2 capture, resulting in extremely slow absorption kinetics. The creation of the di-anionic type of ILs can give rise to a much faster, higher molar absorption of CO2 due to the formation of intramolecular hydrogen bonding.

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Tributyl(ethyl)phosphonium bromide [P4442]Br was synthesized by reacting tributylphosphine and bromoethane using the following procedures.14 Tributylphosphine (20.2 g, 0.998 mol) was transferred to a roundbottom flask containing dry acetonitrile. Approximate 11.5 g (0.1055 mol, in excess) of bromoethane was added and the mixture was refluxed under nitrogen for 24 h. After completion of the reaction, acetonitrile and excess bromethane were removed under reduced pressure. The resulting white solid [P4442]Br was washed three times with hexane and dried under vacuum while heating. Then, [P4442][OH] was prepared from [P4442]Br by anion-exchange method. Detailed procedures are as follows. [P4442]Br was dissolved in ethanol and then transformed into [P4442][OH] ethanol solution by passing it through the Amberlite IRA-402(OH) resin column. If no precipitates appear when adding [P4442][OH] ethanol solution with AgNO3/HNO3 solution (0.5 mol·L-1), it indicates the completion of anion exchange. The concentration of OH- in [P4442][OH] ethanol solution was determined by titration with potassium hydrogen phthalate (KHP) aqueous solution. Charaterization. All ionic liquid (IL) samples were dried under vacuum at 80 oC for 24 h to reduce possible trace of water or solvent. CO2 gas in ultra high purity grade was passed through a drying column to avoid moisture contamination before use. 1H NMR and 13C NMR spectra were recorded on a Bruker Avance DMX-400 spectometer in DMSO-d6 with tetramethylsilane as the standard. FT-IR spectra were obtained using a Nicolet 6700 FT-IR spectrometer. UV-vis spectra and circular dichroism spectra were recorded on Chiral IR-2X. Optical activity was determined on PMS / JASCO P-2000. The viscosities of the ILs were measured by Brookfield DV2T Viscometer coupled with a CPA-40Z or CPA-51Z spindle. The water contents in the ILs before and after CO2 capture were determined with a Karl Fisher titration (JK-101, China). CO2 Absorption Measurements. In a typical CO2 absorption by basic ILs, CO2 gas in ultra high purity grade after passing through a drying column was bubbled through circa 1.0g ILs in a glass container at a flow rate of 60 mL min-1 at atmospheric pressure (Figure 1). The glass container with an inner diameter of 10 mm was partly immersed in a metal bath controlled at desired temperature. The amount of CO2 absorbed was determined at regular intervals by an electronic balance with an accuracy of ± 0.1mg. CO2 captured was released by heating under N2 bubbling.

2. EXPERIMENTAL SECTION Chemicals and Synthesis. All reagents including D-serine, Lserine, tributylphosphine and bromoethane were obtained in the highest purity grade possible, and were used as received unless otherwise stated. The ion exchange resin, Amberlite IRA-402(OH) was purchased from Alfa. Both CO2 gas and N2 gas (purity: 99.999%) were obtained from Shanghai Li Kang Gas Co., Ltd., China. Tributyl(ethyl)phosphonium amino acid ILs were prepared by neutralizing tributyl(ethyl)phosphonium hydroxide [P4442][OH] and the amino acid precursor (e.g., D-serine) at a molar ratio of 2:1 or 1:1 and stirring at room temperature for 12 h.9a Subsequently, the enthanol and water were primarily removed by distillation at 45 ˚C on the rotavap under reduced pressure. The products thus obtained were dried in high vacuum for 24 h at 80 ˚C to reduce possible trace of solvent and water. The P4442 cation and five anions (including mono-anionic and di-anionic amino acid forms) are shown in Scheme 1. Detailed NMR data are given in the supporting information.

Figure 1. Schematic diagram of the CO2 absorption apparatus: (1) glass container with a stirring bar, (2) metal bath, (3) constant temperature heating device, (4) control valve, (5) gas rotameter, (6) drying column, (7) CO2 cylinder.

3. RESULTS AND DISCUSSION

Scheme 1. Chemical structures of the anion and the cation in these amino acid ILs for CO2 capture.

Synthesis and Charaterization. The di-anion and mono-anion types of amino acid ionic liquids were prepared by mixing the amino acid such as D- or L-serine and an ethanol solution of phosphonium hydroxide [P4442][OH] at the molar ratios of 1:2 and 1:1 respectively (Scheme 1). The P4442 cation was selected due to its small molecular weight and relatively high stability, benefiting for a high gravimetric capacity of CO2. The structures of the ILs were characterized by 1H and 13C NMR spectra and FT-IR method (see the supporting information). Moreover, as revealed in the IR spectra, compared with [P4442][L-Ser-H], the peak at 3184 cm-1

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disappeared and the strong peak at 1158 cm-1 emerged in [P4442]2[L-Ser] (Figure S1), indicating that the R-OH group should almost convert to R-O- completely. Chirality. The chirality of the ILs was studied by circular dichroism (CD) spectroscopy. For example, a mirror-image CD spectrum for [P4442]2[D-Ser] and [P4442]2[L-Ser] was observed (Figure 2). There was a negative cotton effect centered at around 215 nm for [P4442]2[D-Ser], and on the contrary, a positive CD signal for [P4442]2[L-Ser]. It indicates that [P4442]2[D-Ser] and [P4442]2[L-Ser] are right-handed and left-handed, respectively, corresponding to the data obtained from automatic polarimeter. Our data revealed that the CD signal of D-serine or L-serine was much stronger than that of corresponding IL [P4442]2[D-Ser] or [P4442]2[L-Ser] (Figure S2), possibly losing partial chirality due to the addition of basic [P4442][OH].15 However, the CD signal of [P4442]2[D-Ser] or [P4442]2[L-Ser] did not have an obvious change when varying the heating tempeature (ranging to 100˚C) or heating time, indicating that heating may have no effect on the chirality of IL in a certain temperature range.

Figure 2. CD and UV-vis spectra of [P4442]2[D-Ser] (black) and [P4442]2[LSer] (red). Solvent: H2O; Concentration: around 2.0 mM.

CO2 Absorption. The CO2 absorption experiment was conducted in these di-anionic chiral amino-functionalized ILs (Figure 3). As revealed, the CO2 capacities for [P4442]2[L-Ser] and [P4442]2[D-Ser] are similar with the values of 1.10 and 1.06 mole CO2 per mole IL, respectively; the absorption kinetics for both ILs is almost equally competitive. The absorption time for reaching to CO2 uptake equilibrium is no more than 80 min. The viscosities of these two ILs were 102.7 cP and 105.4 cP at 25 ˚C, respectively, and after absorption of CO2, increased to 1329 cP and 1174 cP, respectively (Table 1). Therefore, it is concluded that the chiral effect played an insignificant role in tuning the CO2 absorption. Notably, the absorption capacites are more than equimolar CO2 per mole IL. Moreover, the absorption capacities of CO2 in ILs, [P4442]2[L-Ser] and [P4442]2[D-Ser] under 15 vol.% CO2 at 25 ˚C are 1.04 and 0.91 mole CO2 per mole IL respectively (Figure S3). We speculate that the multi-site interactions are present in these absorption systems and the chemical binding between amino group and CO2 is the dominating interaction. Viscosity Change. It is generally accepted that intermolecular hydrogen bonding dominated absorption systems always show extremely slow absorption rate of CO2 as well as decreased absorption capacity. Meanwhile, the viscosity would generally increase above 100-fold during the uptake process. However, as aforementioned, the viscosity of [P4442]2[D-ser] increased from 105.4 cP to 1174 cP (only around 11-fold increase). This

moderate viscosity change should originate from the formation of intramolecular hydrogen bonding. In contrast, the mono-anion type of ILs exhibited a dramatical increase of viscosity during the CO2 absorption and became gel-like liquids. For example, the viscosity of [P4442][L-Ser-H] exhibited a remarkable change from 684.5 cP to 154892 cP. And in the condition of almost same weight (~1.0 g), the CO2 uptake time for [P4442][L-Ser-H] is above 3 hours, almost three times as long as that for [P4442]2[L-Ser]. The viscosity in [P4442][L-Ser-H] increased significantly by above 200fold, indicating that the dense and strong intermolecular hydrogen bonded network is primarily responsible for the slow absorption kinetics of CO2 (Table 1). Thus, the mode of di-anion in these amino acid ILs is essential to prevent the formation of intermolecular hydrogen bonds during the absorption of CO2. Variation of the relative position of the amino group and negative oxygen atom through insertion of the CH2 group on the alkyl chain, i.e. replacement of the dianion [L-Ser] to [L-HSer], results in a slight increase of viscosity both before and after the CO2 uptake (Table 1).

Figure 3. Absorption of CO2 in the dianionic chiral amino acid ILs at 25 ˚C and 1 bar. Table 1. Absorption capacity of CO2 under 1 bar at 25 ˚C. Solubility

Solubility

Viscosity η [cP][a]

[mol/mol]

[%g/g]

Before/after CO2 uptake

[P4442]2[L-Ser]

1.10

8.55

102.7/1329

[P4442]2[D-Ser]

1.06

8.24

105.4/1174

[P4442]2[L-HSer]

1.07

8.12

108.8/1671

[P4442][L-Ser-H]

0.86

11.28

684.5/154892[b]

[P4442][D-Ser-H]

0.82

10.76

745.6/139518[b]

Ionic liquid

[a] The viscosity η was determined at 25 ˚C. [b] The value was calculated by the equation, lg(1/η) = a * T + b, where a and b are the constants, because the viscosity exceeds detection limit at 25 ˚C.

Spectroscopic Analyses. FT-IR and 13C NMR spectra are the favored means to gain insight into the absorption mechanism of CO2 in these chiral amino acid ILs. For mono-anionic ILs such as [P4442][L-Ser-H], as revealed in IR spectrum, a broad peak at 1682

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cm-1 produced after CO2 absorption and no ammonium bands in the ranges of 1900–2100 cm-1 and 2700–2800 cm-1 formed.16 A broad peak appeared at around 1682 cm-1 attributable to the carbamate group. Combined with the dramatical viscosity change in [P4442][L-Ser-H] after CO2 capture (above 200-fold), it indicated that strong and dense hydrogen bonded network formed after CO2 uptake and no zwitterion products generated. In other words, the intermolecular proton transfer from one amino group to another in [P4442][L-Ser-H] did not occur during the uptake of CO2, but a mass of intermolecular hydrogen bonds should be present in [P4442][L-Ser-H]-CO2 adducts. More importantly, for di-anionic ILs such as [P4442]2[L-Ser], in the IR spectrum, the NH2 group in the fresh IL exhibited symmetric and asymmetric stretches centered at 3374 cm-1 with a shoulder at 3300 cm-1 (Figure 4a). This pair transforms into a single peak centered at 3337 cm-1 with the shoulder at 3399 cm-1 after CO2 uptake. It indicated that the NH2 group in [P4442]2[L-Ser] chemically reacted with CO2. The signal at 2337 cm-1 is assigned to the physically absorbed CO2. And as expected, the ammonium bands in the range of 1900–2100 cm-1 and shoulder peak at 2700–2800 cm-1 didn’t emerge.16 Moreover, two new characteristic peaks were observed at 1699 cm-1 and 1500 cm-1, which were attributed to the COOasymmetric and symmetric stretches of the carbamate anion, respectively. Notably, the COO- asymmetric stretch at 1699 cm-1 in [P4442]2[L-Ser]-CO2 adduct was much sharper than that at 1682 cm-1 in [P4442][L-Ser-H]-CO2 complex for a blueshift of 16 cm-1 (Figure S1). These data indicated that the amino group in [P4442]2[L-Ser] reacted with the CO2 molecule to produce the intermediate carbamic acid, which then primarily formed the intramolecular hydrogen bonding with the negative oxgen group instead of the carboxyl group. And the proton should be located closer to the negative oxygen group than the carbamate group, seemingly resulting in the formation of the carbamate anion as revealed in the IR spectra. Furthermore, it is seen in Figure S4 that one new broad signal in the 13C NMR spectra at around 157.8 ppm apppears after the absorption of CO2. The present NMR data could not tell the reaction product clearly, possibly carbamate or carbonate or both. Specially, one would expected there shoud be at least 7 peaks for [L-Ser] di-anion after CO2 absorption if ammonium ion is present.7 But here there are no more than 5 peaks. Thus, the NMR data confirmed that no ammonium ions generated upon the CO2 uptake, that is to say, the intermolecular proton transfer reaction from one amino group to another did not occur. Recently, a new NMR method, i.e. No-deuterium 13C NMR spectrocopy, was developed to distinguish the species of CO2 adducts.12 This approach was also applied to this absorption system. As revealed in Figure 4b, a new and strong peak at 159.9 ppm attributable to the carbamate carbon was clearly observed, a small peak at 158.2 ppm attributable to the carbonate carbon also produced and no additional peak was found. The carbonate peak was small in the NMR spectrum and wasn’t observed in the IR spectrum, indicating that a smart proportion of negative oxygen group can chemically react with CO2. Therefore, the aboved spectroscopic data indicate that the major absorption route (Path A) is that the amino group reacts with CO2, maybe via an intramolecular proton transfer from the amino group to the negative oxygen group, thus forming intramolecular hydrogen bonding between the carbamate and the pronated oxygen group; the minor absorption route (Path B) is that the immediate carbamic acid form intramolecular hydrogen bonding with the carboxyl group; and then, the negative oxygen group can receive another CO2 molecule instead of the proton, thus assisting the total CO2 capacity towards an excess of 1 mol/mol (Scheme 2).

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Scheme 2. Plausible pathways of CO2 absorption in [P4442]2[L-Ser].

Figure 4. a) FT-IR and b) No-deuterium 13C NMR spectra (C6D6 capillary, inset: enlarged view of partial 13C NMR spectra at 164–156 ppm) of [P4442]2[L-Ser] before and after CO2 absorption.

Theoretical Calculation. The interaction energy (∆E), which is the difference in energy between the conformer and the corresponding isolated ions, is a direct approach to judge the reaction pathway of CO2 (Scheme S2).17 Hence, the density functional theory caculations at the B3LYP/6-311 + + G(d, p) level with a Gaussian 09 program was empolyed to investigate the interaction of [L-Ser] anion with CO2 (Scheme 2). As seen in Scheme 2, ∆Ea for Path A is -165.3 kJ/mol. This value is much higher than the ∆E values (∆Eb1= -78.8 kJ/mol and ∆Eb2= -116.4 kJ/mol) for Path B. Thus, Path A is the dominating reaction route between [L-Ser] and CO2. RDG analysis is another useful method to further study non-covalent interaction such as hydrogen bonding in the present work. The spike corresponding to O···H-N for [L-Ser] is located at -0.0210 a.u., on the contrary, the spike corresponding to O···H-O for [L-Ser]-CO2 complex is located at -

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0.0516 a.u. (Figure 5), indicating the presence of strong intramolecular hydrogen bonding between chemically attached CO2 on NH2 group and the pronated oxygen group (OH). The ADCH charges (atomic charges caculated by atomic dipole moment corrected Hirshfeld population method)18 on the CO2 molecule also have significant changes (C: 0.412 to 0.470; O: 0.206 to -0.693 and -0.602 respectively) upon the absorption (Path A). These caculation results are in good agreement with the absorption data and spectroscopic analysis, indicating that Path A is the dominating route for CO2 absorption in [P4442]2[L-Ser] and the formation of intramolecular proton transfer/hydrogen bonding gives rise to enhanced absorption performance.

enhanced absorption performance of amino-functionalized ILs is achieved, especially a controllable viscosity change during the uptake, providing an important foundation for building smart CO2 absorption systems based on amino-functionalized ILs. Further attempts will be put onto the promotion of mass uptake of CO2 by both suitable structural design of functionalized anion and the decrease of molar mass of IL. ASSOCIATED CONTENT Supporting Information NMR spectra, FT-IR results, and other supporting data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author [email protected]; [email protected]. Notes There are no conflicts to declare. ACKNOWLEDGMENT Prof. Zou acknowledges the support of National Natural Science Founation of China (No. 51771112) and “Shuguang” Scholar Project (16SG08) from Shanghai Education Commission. Dr. Pan and Dr. Zhao are supported by the Projects funded by China Postdoctoral Science Foundation (2017M621476 and 2017M621477, respectively).

Figure 5. RDG scatter plots (isovalue = 0.5 a.u.) and surface plots (s = 0.7 a.u.) of (a, b) [L-Ser] anion, and (c, d) [L-Ser]-CO2 complex. The isosurfaces are colored on a blue-green-red scale according to values of sign(λ2)ρ, ranging from -0.03 to 0.020 a.u. Blue indicates strong attractive interactions and green indicates weak Van der Waals interactions.

CO2 Cycling. CO2 absorption and desorption cycles were performed to observe the reversibility of IL (Figure S5). After 5 cycles, the absorption capacity of [P4442]2[L-Ser] remained around 1.0 mol/mol, and the desorption was almost complete. The complete desorption was also verified by FT-IR spectra for being almost identical as the fresh IL after cycling (Figure S6). Compared with [P4442]2[D-Ser], [P4442]2[L-Ser] exhibited slightly better reversibility during the CO2 cycling as seen in Figure S5. 4. CONCLUSION In summary, the di-anionic chiral amino acid ILs were prepared through an acid-base neutralization reation by the addition of [P4442][OH] into the amino acid (D-serine, L-serine, or LHomoserine) at the molar ratio of 2:1. In comparison with [P4442]2[L-Ser], [P4442]2[D-Ser] exhibited a comparable absorption capacity except for a slightly inferior CO2 reversibility, indicating the chirality of IL played an insignificant role in tuning the CO2 uptake. Our data reveal that the dianionic form of IL is beneficial to the formation of intramolecular hydrogen bonding, which can remarkably mitigate the viscosity increase during CO2 absorption. The absorption capacities of CO2 towards more than 1 mol/mol in these di-anionic chiral amino acid ILs are mainly due to the multisite absorption from both the amino group and the negative oxygen group. And the dominating absorption pathway is that the amino group reacts with CO2 via an intramolecular proton transfer from the amino group to the negative oxygen group, leading to the formation of the intramolecular hydrogen bonding between the carbamate and the protonated oxygen group (OH). As a result, the

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