Protic Ionic Liquid‐Based Deep Eutectic Solvents with Multiple Hydrogen Bonding Sites for Efficient Absorption of NH3

Protic Ionic Liquid-Based Deep Eutectic Solvents with Multiple Hydrogen Bonding Sites for Efficient Absorption of NH

Yongkang Cao

, Xiangping Zhang

, Shaojuan Zeng

, Yanrong Liu

, Haifeng Dong

, Chun Deng

a Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex

Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China

b College of Chemical Engineering and Environment, China University of Petroleum, Beijing, Beijing

102200, China

c College of Chemical and Engineering, University of Chinese Academy of Sciences, Beijing 100049,

China

d Energy Engineering, Division of Energy Science, Luleå University of Technology, 97187 Luleå,

Sweden

*Corresponding author: S. J. Zeng

Email address: sjzeng@ipe.ac.cn

Accepted Article

This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as

doi: 10.1002/aic.16253

© 2020 American Institute of Chemical Engineers (AIChE) Received: Nov 11, 2019; Accepted: Apr 04, 2020

Abstract

The emerging of ionic liquids (ILs) provides an efficient and sustainable way to separate and recover NH

due to their unique properties. However, the solid or highly viscous ILs are not suitable for traditional scrubbing. Therefore, an effective strategy was proposed by combining the protic ILs (PILs) with acidic H and low viscous ethylene glycol (EG) to form IL-based deep eutectic solvents (DESs) for NH

absorption. The results indicated that these PIL-based DESs not only have fast absorption rate, but also exhibit exceptional NH

capacity and excellent recyclability.

The highest mass capacity of 211 mg NH

/g DES was achieved by [Im][NO

]/EG with molar ratio of 1:3, and was higher than all the reported ILs and IL-based DESs, which was originated from multiple hydrogen bonding between acidic H and hydroxyl groups of the DESs and NH

. This work will provide useful idea for designing IL-based solvents for NH

separation applications.

Keywords: protic ionic liquids, deep eutectic solvents, NH

absorption, hydrogen bonding, multiple

Accepted Article

Introduction

Ammonia (NH

) is an important chemical raw material, especially in nitrogenous fertilizers and nitric acid

. However, NH

-containing gases released to air by fertilizer industries, fossil fuel combustion, refrigeration processes and so on

, also cause serious environmental issues by combining with NO

or SO

in atmosphere to form ammonium salts, a main component of PM2.5 (Particulate Matter 2.5)

. The traditional methods to remove NH

are water scrubbing and acid scrubbing, however, water scrubbing needs higher energy consumption for NH

recovery due to the high volatility of water, and acid absorption produces irreversible ammonium salts that does not satisfy the sustainable requirement

.

In order to develop the efficient process for NH

separation and recovery, more and more attention has been paid to ionic liquids (ILs) due to their unique characterizes, such as tunable properties and low volatility

. Up to date, conventional ILs

, hydroxyl-functionalized ILs

, protic ILs (PILs)

and metal ILs

were designed for NH

separation, however, most of the functionalized ILs possess relatively high viscosity

or present solid state under ambient temperature, which are not favorable for gas transfer and diffusion in absorption processes. In order to overcome the transfer limitation of gas in such ILs, several molecular solvents were mixed with ILs to form IL-based absorbents with relatively low viscosity at room temperature for gas absorption and separation. For example, Zhang et al

studied the hybrid systems of the IL 1,4-Bis(3-methylimidazolium-1-yl)butane prolinate ([Bis(mim)C

][Pro]

) and water for CO

capture. It was found that the pure [Bis(mim)C

][Pro]

has very low

Accepted Article

CO

capacity of 0.03 mol CO

/mol IL in 10 min at 40 ℃, which don’t reach absorption equilibrium due to the high viscosity up to 2094 mPa

s at 40 ℃. However, for the IL-based solvent with 20 wt% [Bis(mim)C

][Pro]

, the viscosity dramatically decreased to 8.8 mPa

s at 25 ℃, and the CO

capacity is improved to 1.72 mol CO

/mol IL within 5 min due to the addition of water. Huang et al

synthesized two acid salt ILs (ASILs), solid tetraethylammonium diglutarate ([N

][diglutarate]) and dimethylethanolammonium diglutarate ([DMEA][diglutarate]) with high viscosity under room temperature, and mixed the ASILs with sulfolane to form binary mixtures containing 40 wt% ASILs for SO

absorption. The SO

absorption rate was found to be quite fast with an equilibrium time within 2 min, which arises from the relatively low viscosities of the mixtures of 80~90 mPa·s at 40 ℃. Hence, the combination of ILs and molecular solvents to form novel IL-based solvents for NH

absorption is an effective way to overcome the above problem of kinetic absorbent performance.

Among those IL-based absorbents, IL-based deep eutectic solvents (DESs) composed of hydrogen-bond acceptors (HBAs) and hydrogen-bond donors (HBDs)

, have been widely used for gas separation such as CO

, SO

and NH

due to wide liquid range and low volatility like ILs, which are regarded as promising alternatives for gas separation

. At present, most of the reported IL-based DESs focused on choline chloride (ChCl) and ammonium-based DESs for NH

absorption. The ChCl based-DESs can be divided into IL-based binary and ternary systems. Duan et al

and Zhong et al

studied the NH

solubilities in the ChCl-based binary DESs at different temperature and equilibrium pressure, and found

Accepted Article

that the maximum capacity of the ChCl/EG (molar ratio of 1:2) is 46 mg NH

/g DES at 40 ℃ and 0.10 MPa, which is still unable to compete with the water method. In order to enhance NH

absorption in DESs, Li et al

and Zhong et al

investigated IL-based ternary DESs adding resorcinol (Res) and phenol (PhOH) to the ChCl-based DESs, respectively. It was observed that the ChCl/Res/Gly DES (molar ratio of 1:3:5) with flexible hydrogen-bonding supramolecular networks exhibits excellent NH

absorption of 130 mg NH

/g DES at 40 ℃ and 0.10 MPa due to hydrogen bonding between NH

molecules and the DES, and NH

/CO

selectivity of this DES also is up to 142. However, the viscosities of the ternary systems could gradually increase and even become solid during NH

absorption, which result in slow absorption and desorption process. Jiang et al

presented four DESs by mixing ethylamine hydrochloride (EaCl) and Gly with different molar ratios. It was found that all the EaCl/Gly DESs possess quick absorption rates due to the relatively low viscosity and all the absorption equilibrium time is about 60 s. The highest NH

capacity of 164 mg NH

/g IL at 25 ℃ and 0.11 MPa is obtained by EaCl/Gly (molar ratio of 1:2) due to the hydrogen bonding between the DES and NH

. Therefore, the development of new IL-based absorbents with low viscosity and outstanding absorption-desoption performance is of great importance.

In this work, three protic ILs (PILs), involving imidazolium nitrate ([Im][NO

]), 1-methylimidazolium nitrate ([Mim][NO

]) and 1, 2-dimethylimidazolium nitrate ([Mmim][NO

]), with acidic protic H and excellent affinity towards NH

, were synthesized, and further combined with EG containing abundant hydroxyl groups that

Accepted Article

can interact with NH

by hydrogen bonding, forming a series of novel PIL-based DESs for improving NH

absorption. The thermal stability, the effect of PILs types, the molar PIL/EG ratios, the temperature and pressure on NH

capacities, NH

selectivity towards other gases as well as recyclability of PIL-based DESs were investigated in detail. Furthermore, the absorption mechanism was studied and discussed by spectroscopic characterizations (NMR and FTIR).

Experimental

Materials

NH

(99.999%), N

(99.999%) and CO

(99.999%) were supplied by Langfang Langwei Gas Company. Imidazole (99%), 1-methylimidazole (98%) and 1,2-dimethylimidazole (98%) were purchased from Aladdin Industrial Corporation.

Besides, other chemical reagents of analytical grade, such as nitric acid (65 wt%), ethylene glycol, acetone and ethyl acetate were purchased from Beijing Chemical Company.

Preparation of PILs and PIL-based DESs

The three PILs involving imidazolium nitrate ([Im][NO

]), 1-methylimidazolium nitrate ([Mim][NO

]) and 1, 2-dimethylimidazolium nitrate ([Mmim][NO

]) were synthesized according to the reported method

, and their structures and NMR spectra were shown in Figure 1 and Figure S1. Taking [Im][NO

] as an example, imidazole and nitric acid with the molar ratio of 1.05:1 were mixed in an acetone solution and stirred in an ice bath for 24 h to form the crude PIL product, and then was treated by

Accepted Article

rotary evaporation at 70 ℃ under vacuum to remove the water and acetone. After that, adding ethyl acetate to the mixture removed impurities at least 5 times. Finally, the white solid [Im][NO

] was obtained after dried under vacuum at 60 ℃ for 48 h. The PIL-based DESs were obtained by stirring the mixtures consisted of PILs and EG with different molar ratios at 50 ℃ for 30 min until homogeneous liquid presented.

Besides, the

H NMR spectra of [Im][NO

], EG and [Im][NO

]/EG DES (1:3) were shown in Figure S2.

Characterizations and properties of PIL-based materials

All the DESs were dried under vacuum at 50 ℃ for 48 h before use. The Thermo Nicolet 380 Spectrometer was used to record FTIR spectra for confirming the structures of the studied DESs. Besides, the Bruker 600 spectrometer was also used to study

H and

C NMR spectra to confirm their structures. The Mettler Toledo React IR 15 was utilized to record the NH

absorption process at regular intervals. The density and viscosity of DESs were investigated in the temperature range from 20 to 70 ℃ with the Anton Paar DMA 5000. The TGA Q5000 was used to collect the decomposition temperatures from room temperature to 600 ℃ under N

atmosphere with heating rate of 10 ℃

min. The melting point of DESs was measured by Mettler Toledo DSC1 from -150 to 100 ℃ under N

atmosphere with a heating rate of 10 ℃

min.

NH

absorption and desorption

Accepted Article

The same procedures of our previous work

was followed to study the experiments of NH

absorption and desorption. In the process of NH

absorption, the NH

gas, with the flow rate of 80 ml/min at atmosphere pressure, was passed through about 3.0 g DESs, which was placed on the glass container with an inner diameter of 3.0 cm. The desired temperature within ± 0.1 ℃ of the glass container was controlled by a circulated water bath. The total amount of NH

absorbed by DES was finally determined according to the increase of the weight of mixture by an electronic balance with an accuracy of ± 0.1 mg at different times. For the process of NH

desorption, the DES with saturated NH

was heated with reducing the NH

partial pressure to release NH

, so the pure N

gas with the same flow rate was passed through the studied DES with NH

in the glass container at 80 ℃, and the amount of NH

desorption was also recorded at regular intervals until the weight was constant.

Results and discussion

Physicochemical properties of PIL-based DESs

Density and viscosity are important parameters for evaluating the properties of absorbent in gas separation. Therefore, densities and viscosities of PIL-based DESs in the temperature range from 20 to 70 ℃ were studied. It could be found that the densities and viscosities of all the PIL-based DESs are very close at the same experiemnt conditions, and the change of temperature has greater impact on viscosities than densities. The densities of all the DESs were about 1.2 g

cm

at the range from 20 to 70 ℃ in Figure 2(a). Because the neat PILs present as the state of

Accepted Article

solid under 70 ℃, their viscosities cannot be obtained due to out of the measurement range. As shown in Figure 2(b), the viscosity of EG is about 21 mPa

s at 20 ℃ higher than those of all the IL-based DESs, which are about 19 mPa

s under the same conditions. When the temperature is over 20 ℃, the viscosity of EG is in the range of 14.19 to 3.93 mPa

s at the temperature from 30 to 70 ℃, and the IL-based DESs possess the viscosity range from14.43 to 4.10 mPa

s under the same conditions.

In addition, we also investigated the thermal decomposition temperatures (T

) and melting points (T

) of PILs, and the curves and results were shown in Figure S3, Figure 3(a) and Table 1, respectively. The T

of all the PILs were above 140 ℃, which are higher than the temperature of the absorption and desorption of NH

Moreover, the T

of all PIL-based DESs were shown in Figure 3, and were lower than those of both pure PILs and EG, which confirms the formation of the PIL-based DESs due to the hydrogen bonding between PILs and EG

. Besides, the expanded curves from -35 to -20 ℃ of [Im][NO

]/EG with the molar ration of 1:5 and 1:6 were shown in Figure S4, and the T

of [Im][NO

]/EG of 1:5 and 1:6 could be clearly found at -29 and -26 ℃.

The effect of ILs on NH

absorption in PIL-based DESs

The absorption performance of these PILs was firstly investigated as shown in Table 2. It was found that the PIL [Im][NO

] with two protic hydrogens shows much higher NH

capacity up to 252 mg NH

/g IL than other two PILs with single protic hydrogen. This is because the acidic protic hydrogen of cations plays an important

Accepted Article

role in NH

absorption, and it can interact with the NH

molecules through the stronger hydrogen bonding, which was proved by our previous work

. The PIL [Bim][NTf

] could absorb more than 2 mole NH

molecules sequentially through the interaction between the protic H site on the cation and NH

. The N atom of the first NH

molecule interacted with the protic H atom on the imidazole ring by hydrogen bonding, then the second equivalent was more likely to interact with already absorbed NH

molecules, rather than with other H atoms on the cation. For the PILs with single protic H on cations, because the acidity of 2-C on the imidazolium ring by replacing the H atom with the methyl group decreased, the NH

absorption capacity decreased slightly from 194 mg NH

/g IL in [Mim][NO

] to 182 mg NH

/g IL in [Mmim][NO

] under the same conditions, indicating that the methyl substituent has a little effect on NH

absorption

. However, the equilibrium time of NH

absorption in the solid PILs is longer than that of the PIL-based DESs due to the phase transfer from initial solid to liquid, especially for [Mmim][NO

] that takes nearly 5 h to reach saturation.

Similarly, the desorption process of [Im][NO

] was very slow, which needs more than 6 h (Figure S5) to release the absorbed NH

.

Further, the NH

absorption performance of PIL-based DESs with different PILs and PIL/EG contents at 40 ℃ and 0.10 MPa were also measured and shown in Figure 4. The results indicated that the PIL-based DESs show much faster absorption rate of NH

than the pure PILs, and the equilibrium time is about 15 min. When the molar ratio of HBA/HBD was 1:3 and the cation of PILs varied from [Im] to [Mim] and [Mmim], the NH

capacity of PIL-based DESs decreased slightly from 172 to 152 and

Accepted Article

141 mg NH

/g DES in Figure 4(a), respectively, which is consistent with the order of PILs for NH

absorption. With increasing EG content of the DES, the NH

capacity obviously decreased as shown in Figure 4(b) under the same conditions. Besides, when the molar ratio of [Im][NO

] to EG is 1:3, 1:4, 1:5, 1:6 and 1:8, the mass fraction of [Im][NO

] in the corresponding DES mixture is 41%, 35%, 30%, 26% and 21%, respectively. When the mass fraction of [Im][NO

] in DES mixture decreased from 41% to 21%, the NH

capacity decreased from 172 to 146 mg NH

/g DES, which implied the dominance of the PILs in NH

absorption.

In addition, we listed the reported absorbents for NH

absorption in Table 3.

Compared with the pure ILs and ordinary IL-based DESs in the literature, the designed [Im][NO

]/EG (1:3) DES has very high NH

capacity up to 211 mg NH

/g DES at 30 ℃ and 0.10 MPa, which is the highest value reported up to date.

NH

selectivity in PIL-based DESs

Additionally, there are always other gases such as CO

and N

accompanied with NH

in real industries tail gases. Therefore, selectively separating NH

from gas mixtures becomes particularly important. The NH

and CO

absorption curves in the DES of [Im][NO

]/EG (1:3) were studied at 40 ℃ and 0.10 MPa shown in Figure S6, and the NH

/CO

selectivity was calculated and listed in Table 4. It was found that NH

capacity in the [Im][NO

]/EG (1:3) is much higher than CO

capacity under the same conditions, resulting in higher NH

/CO

selectivity up to 139.6. The results

Accepted Article

implied that the [Im][NO

]/EG (1:3) DES can selectively separate NH

from the gas mixtures containing NH

and CO

.

Effect of temperature and pressure on NH

absorption

Both the temperature and NH

partial pressure play the significant role in the capacity of NH

in DESs. Therefore, [Im][NO

]/EG (1:3) with higher NH

capacity and selectivity was selected to study the effect of temperature and NH

partial pressure on absorption performance in Figure 5. The Figure 5(a) presented that the mass capacity of NH

in the DES continuously increases from 49 to 252 mg NH

/g DES with the decreasing temperature from 80 to 20 ℃ at 0.10 MPa. When the temperature continuously decreases to -20 ℃, the superhigh NH

capacity of 655 mg NH

/g DES can be achieved.

The effect of NH

partial pressure on NH

capacity at 0 ℃ and 40 ℃ was shown in Figure 5(b). The results indicated that NH

capacity increases from 43 to 172 mg NH

/g DES with the increasing partial pressure of NH

from 0.01 to 0.10 MPa at 40

℃, and the capacity of [Im][NO

]/EG (1:3) under low pressure of 0 ℃ is almost 2

times higher than those at 40 ℃. Moreover, the absorption behavior of low concentration of NH

(2000 and 5000 ppm) in the [Im][NO

]/EG (1:3) DES was also studied at 40 ℃ as shown in Figure S7. When the concentration of NH

was 2000 and 5000 ppm, the NH

capacity was 16 and 46 mg NH

/g DES, respectively. Therefore, it can be concluded that the absorbed NH

can be easily released under higher temperature and lower NH

partial pressure.

Accepted Article

Regeneration and recycle of the [Im][NO

]/EG DES

In order to investigate the regeneration and recyclability of the [Im][NO

]/EG (1:3) DES after absorbing NH

, six consecutive absorption-desorption experiments were further investigated. The [Im][NO

]/EG DES-NH

system was exposed to N

atmosphere and 80 ℃, and the recyclability performance was shown in Figure 6. The results indicated that there is no significant loss in NH

capacity after six absorption-desorption cycles, and the FTIR spectra of [Im][NO

]/EG DES (Figure 8) before absorption and after desorption of NH

is basically consistent, which implied that the process is completely reversible.

The mechanism for NH

absorption

Further, the mechanism for NH

absorption in DES was explained by combining

H NMR,

C NMR and In-situ FTIR spectra.

NMR spectra of PILs-based DESs

The deuterated chloroform (CDCl

) as solvent was mixed with the [Im][NO

]/EG (1:3) before and after NH

saturation in tube, and the

H NMR and

C NMR spectra were collected at -5 ℃ as presented in Figures 7(a) and 7(b), respectively. In the

H NMR spectra of fresh [Im][NO

]/EG DES, the peaks at 3.69, 2.69 and 5.43 ppm were ascribed to H-7 and H-8, as well as H-6 and H-9 from EG

, respectively, and the bands at 7.53, 8.88 and 13.98 ppm were belonged to H-4 and H-5, H-2, as well as H-1 and H-3 on the imidazole ring, respectively

. Compared with the fresh IL-based DES, the protic- H-1 and H-3, as well as part of H-6 and H-9 atoms shifted together to 2.10

Accepted Article

ppm

and the rest of H atoms from hydroxyl groups shifted to 5.72 ppm after NH

absorption. In the

C NMR spectra of the DES with absorbed NH

, the peaks of C-2 on 135 ppm as well as C-4 and C-5 on 122 ppm became more noticeable than the fresh DES. All the changes indicated that there are multiple hydrogen bonding interactions between NH

and hydroxyl groups on EG as well as protic hydrogens on the [Im][NO

].

FTIR spectra of PILs-based DESs

In addition, FTIR spectra of [Im][NO

]/EG DES before and after absorption as well as after desorption were investigated as presented in Figure 8. It was found that there is no obvious difference between the fresh DES and NH

-saturated one, except that

the peak at 1591 cm

disappeared after the DES combined with NH

, and the structure of DES before NH

absorption and after regeneration is the same.

In order to better understand the NH

absorption process of the binary [Im][NO

]/EG system, In-situ FTIR was further used to investigate the variation in the structures of DES during NH

absorption in Figure 9. In the spectra of PIL-based DES with NH

there were six new peaks including 915, 931, 1064, 1258, 1531 and 1635 cm

. Among them, the bands at 915 and 931 cm

were assigned to out-plane bending vibration of N (1)-H and N (3)-H of the imidazole ring, respectively. The peaks at 1531 cm

and 1258 cm

attributed to C-H and C-N stretching vibration of the cation gradually became stronger, respectively, and the peak at 1635 cm

ascribed to the NH

molecule was also observed. Meanwhile, the peak at 1591 cm

Accepted Article

assigned to N-H deformation vibration disappeared after absorbing NH

. These changes suggested hydrogen bonding between NH

and the cation of the PIL

. Besides, the peak at 1064 cm

was attributed C-OH stretching vibration of EG

, suggesting the interaction between hydroxyl groups on EG and NH

. In summary, all the changes of peaks on the spectra further proved the NMR results.

Conclusion

In order to overcome the mass-transfer restrictions of ILs during NH

absorption, three PILs involving [Im][NO

], [Mim][NO

] and [Mmim][NO

] with acidic protic H were synthesized, and further combined with EG with abundant OH groups to form novel PIL-based DESs with different molar ratios for improving NH

absorption. The effect of PILs types, the molar PIL/EG ratios, as well as temperature and pressure on NH

absorption performance, recyclability of PIL-based DES, and the absorption mechanism were investigated in detail. The results showed that the protic H plays an important role in NH

absorption by PILs, and NH

absorption performance of the PIL-based DESs was improved with the increase of PIL/EG content. Among these PIL-based DESs, the [Im][NO

]/EG DES with the molar ratio of 1:3 not only exhibited faster absorption rate than pure PIL, but also possessed the highest capacity of 211 mg NH

/g DES at 30 ℃ and 0.10 MPa and great NH

/CO

selectivity of 139.6, along with good recyclability, and the reason is that multiple and reversible hydrogen bonding was formed between PIL-based DES and NH

molecules. This study implied that the PIL-based DESs possess great potentials as attractive adsorbents in NH

separation and recovery applications.

Accepted Article

Acknowledgments

This work was supported by National Key R&D Program of China (2017YFB0603401), the National Natural Science Foundation of China (21890764 and 21978306), Beijing Municipal Natural Science Foundation (2182071), and Hebei Natural Science Foundation (B2019103011).

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32. Zubeir LF, van Osch DGP, Rocha MAA, Banat F, Kroon MC. Carbon dioxide solubilities in

decanoic acid-based hydrophobic deep eutectic solvents. Journal of Chemical and Engineering Data.

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33. Ren HW, Lian SH, Wang X, Zhang Y, Duan E. Exploiting the hydrophilic role of natural deep

eutectic solvents for greening CO₂ capture. Journal of Cleaner Production. 2018;193:802-810.

Accepted Article

34. Yang DZ, Han YL, Qi HB, Wang YB, Dai S. Efficient absorption of SO₂ by EmimCI-EG deep

eutectic solvents. ACS Sustainable Chemistry & Engineering. 2017;5:6382-6386.

35. Yang DZ, Hou MQ, Ning H. Efficient SO₂ absorption by renewable choline chloride-glycerol deep

eutectic solvents. Green Chemistry. 2013;15:2261-2265.

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environmentally benign functional deep eutectic solvents. Journal of Hazardous materials.

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quaternary ammonium salt as an efficient SO₂ absorbent. New Journal of Chemistry.

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eutectic solvents as efficient and reversible SO₂ absorbents. Industrial and Engineering Chemistry

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39. Duan XZ, Gao B, Zhang C, Deng DS. Solubility and thermodynamic properties of NH₃ in choline

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Accepted Article

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Figure Captions

Figure 1. The structure of [Im][NO

], [Mim][NO

], [Mmim][NO

] and EG

Figure 2. Densities (a) and viscosities (b) of PIL-based DESs at different

temperatures

Figure 3. (a) The melting points of PILs and PILs-based DESs; (b) The melting points of [Im][NO

]/EG DESs with different molar ratios

Accepted Article

Figure 4. (a) NH

absorption behaviors in PIL-based DESs (1:3) at 40 ℃ and 0.10 MPa; (b) NH

capacity in PIL-based DESs with different mass fraction at 40 ℃ and 0.10 MPa

Figure 5. Effect of temperature (a) and pressure (b) on NH

absorption by [Im][NO

]/EG (1:3)

Figure 6. Six consecutive cycles of NH

absorption and desorption by [Im][NO

]/EG (1:3). Adsorption: 40 ℃ and 0.10 MPa. Regeneration: 80 ℃ and N

atmosphere Figure 7.

H NMR (a) and

C NMR (b) of [Im][NO

]/EG (1:3) before and after NH

absorption

Figure 8. FTIR spectra of EG, [Im][NO

], [Im][NO

]/EG (1:3) before and after absorption and after desorption

Figure 9. In-situ FTIR spectra of [Im][NO

]/EG (1:3) during NH

absorption

HN

NH

1 2

3

5 4

O

N O O

N

NH O

N O O H₃C

N

NH O

N O O H₃C

CH₃

HO

OH

6

7 8

9

[Im][NO

] [Mim][NO

] [Mmim][NO

] EG

Figure 1. The structure of [Im][NO3],[Mim][NO3], [Mmim][NO3] andEG

Accepted Article

Figure 2. Densities (a) and viscosities (b) of PIL-based DESs at different temperatures

Accepted Article

Figure 3. (a) The melting points of PILs and PILs-based DESs; (b) The melting points of

[Im][NO3]/EG DESs with different molar ratios

Accepted Article

Figure 4. (a) NH3 absorption behaviors in PIL-based DESs (1:3) at 40 ℃ and 0.10 MPa; (b) NH3

capacity inPIL-based DESs with different mass fraction at 40 ℃ and 0.10 MPa

Accepted Article

Figure 5. Effect of temperature (a) and pressure (b) on NH3 absorption by [Im][NO3]/EG (1:3)

Accepted Article

Figure 6. Six consecutive cycles of NH3 absorption and desorption by [Im][NO3]/EG (1:3).

Adsorption: 40 ℃ and 0.10 MPa. Regeneration: 80 ℃ and N2 atmosphere

Accepted Article

Figure 7. ¹H NMR (a) and ¹³C NMR (b) of [Im][NO3]/EG (1:3) before and after NH3 absorption

Accepted Article

Figure 8. FTIR spectra of EG, [Im][NO3], [Im][NO3]/EG (1:3) before and after absorption and

after desorption

Accepted Article

Figure 9. In-situ FTIR spectra of [Im][NO3]/EG (1:3) during NH3 absorption

Accepted Article

Table 1.Thermal decomposition temperatures and melting points of ILs

ILs ^aTd /℃ ^bTm /℃

[Im][NO3] 147.6 88.6

[Mim][NO3] 151.7 70.6

[Mmim][NO3] 143.1 86.5

Note: ^aTd is the temperature where a substance reduces 5% of the initial mass when heating from

25 to 600 ℃ with the heating rate of 10 ℃/min under N2 atmosphere. ^bTm is the temperature

where a substance melt when heating from -150 to 100 ℃ with the heating rate of 10 ℃/min

under N₂.

Accepted Article

Table 2. NH3 capacity in PILs at 40 ℃ and 0.10 MPa

ILs T/℃ P/MPa

NH3 capacity

mg NH3/g IL

[Im][NO3] 40 0.10 252

[Mim][NO3] 40 0.10 194

[Mmim][NO3] 40 0.10 182

Accepted Article

Table 3. NH3 capacity in ILs and IL-based DESs

Absorbents T/℃ P/MPa NH3 capacity (mg NH3/g solvent) Ref

[Im][NO3]/EG (1:3) 30 0.10 211

This

work

[Im][NO3]/EG (1:3) 40 0.10 172

[Mim][NO3]/EG (1:3) 40 0.10 152

[Mmim][NO3]/EG (1:3) 40 0.10 141

[Emim]2[Co(NCS)4] 30 0.10 198 ²³

[Bim][SCN] 40 0.97 183 ²²

ChCl:Res/Gly (1:3:5) 25 0.10 170 ¹⁰

EaCl/Gly (1:2) 25 0.11 164 ⁴⁵

ChCl/PhOH/EG (1:5:4) 25 0.10 162 ⁴⁴

[Bim][NO3] 30 0.10 135 ²²

ChCl/Res/Gly (1:3:5) 40 0.10 130 ¹⁰

ChCl/PhOH/EG (1:7:4) 40 0.10 130 ⁴⁴

ChCl/PhOH/EG (1:5:4) 40 0.10 119 ⁴⁴

[Bim][NTf2] 40 0.10 113 ¹⁴

[EtOHmim][BF4] 40 0.10 45 ²⁰

ChCl/Urea (1:2) 40 0.11 27 ¹

[Bmim][PF6] 25 0.10 21 ⁴⁷

[Bmim][MeSO3]/Urea (1:1) 40 0.17 18 ³⁶

[Bmim][BF4] 25 0.10 17 ⁴⁷

[Bmmim][DCA] 40 0.10 9 ²²

[Bmmim][NTf2] 40 0.10 8 ²²

[Bmim][NTf2] 26 0.10 5 ⁴⁷

Accepted Article

Table 4. NH3/CO2 selectivity in [Im][NO3]/EG (1:3)

Samples

Gas capacity^a Selectivity^a

mg gas/g DES NH3/CO2

[Im][NO3]/EG (1:3) 172/NH3 3.2/CO2 139.6

Note: ^a The experiment was performed at 40 ℃ and 0.10 MPa, M_(DES)=317.291.

Accepted Article