Designing and Screening of Multi-Amino-Functionalized Ionic Liquid

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Designed and Screened of Multi-Amino Functionalized Ionic Liquids Solution for CO2 Capture by Quantum Chemical Simulation Guo-Hua Jing, Yuhao Qian, Xiaobin Zhou, Bihong Lv, and Zuoming Zhou ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03467 • Publication Date (Web): 10 Nov 2017 Downloaded from http://pubs.acs.org on November 13, 2017

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ACS Sustainable Chemistry & Engineering

Designed and Screened of Multi-Amino Functionalized Ionic Liquids Solution for CO2 Capture by Quantum Chemical Simulation Jing Guohua*, Qian Yuhao, Zhou Xiaobin, Lv Bihong, Zhou Zuoming (College of Chemical Engineering, Huaqiao University, Jimei Road, Xiamen, Fujian, 361021, People’s Republic of China. ) *Corresponding author, e-mail address: [email protected]

Abstract The structure and group combination of amino functionalized ionic liquids(AFILs), such as chain length, number of amino groups, interaction energy and activation barrier were the important factors affecting their performance of CO2 capture. As a forecasting tool, quantum chemical calculation can be used to screen and design the AFILs. In the present work, some different types of multi-AFILs (e.g. [DETAH][Lys], [TETAH][Lys], [DDAH][Lys] and [BTAH][Lys]) were firstly designed and their performance was forecasted under the DFT method，B3LYP 6-311++G(d,p) basis set. Based on the results of the quantum chemical comparing and excluding, [TETAH][Lys] and [DETAH][Lys] were chosen as the efficient absorbents for CO2 capture, which were expected to achieve a high CO2 loading more than 2.0 mol CO2/mol AFILs with a high regeneration efficiency. Then the experimental results confirmed the predictions of the quantum chemical calculation. The experimental results showed that the absorption capacity of [TETAH][Lys] and [DETAH][Lys] were 2.59 and 2.13 mol CO2/ mol AFILs, and the regeneration efficiency were 98.96% and 98.00%, respectively. Lastly the reaction mechanism of CO2 capture into these AFILs was explored by using

13

C NMR. The present work

proved that it was feasible to use quantum chemical calculation to design ILs, which 1

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could be an efficient way for the further AFILs research. Keyword Functionalized ionic liquids; CO2 absorption; Quantum chemical calculation;

Introduction Carbon dioxide (CO2) was one of the major greenhouse gases, and its capture and storage have got more and more attentions by scholars.1, 2 In the last decades, organic amine absorption method has been widely used in industry as the most effective and popular CO2 capture way.3-5 However, its nonnegligible shortcomings, such as foaming, solvent loss and equipment corrosion, not only greatly limited the industrial CO2 capture efficiency, but also contradicted the requirements of green economy and sustainable development.6 The last few years saw an explosion of interest in the use of amine-functionalized materials in CO2 capture and conversion. With negligible vapor pressures, high thermal stability, and virtually limitless chemical tenability, amino-functionalized ionic liquids (AFILs) were considered as a promising method for CO2 capture and separation.7 But Nelson et al.8 had question about the environmental friendliness of ILs and thought that the development of novel, effective ILs with reduced toxicity would still be a challenge. However, amino acid ionic liquids(AAILs) which was synthesized recently have been attractive to chemists for their low toxicity and high reactivity toward CO2.9-11 Then Bhattacharyya et al.12 synthesized a series of amino acid ionic liquids and found out the CO2 absorption capacity of single amino group was 0.5 mol CO2/mol, then the complete course of the 2


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carbamate formation reaction was given. Therefore, many researchers were interested in searching or synthesis new green AAILs for CO2 absorption.13-15 Based on those research results, more and more researchers realized that the most direct way to enhance the CO2 absorption capacity of the absorbent was to increase the number of amino group in AFILs.16 However, because of the restriction between the cation and anion, the number of amino groups in AFILs can not be increased infinitely. Meanwhile, the enormous possibilities of anion and cation combination make it a challenging task for designing and searching AFILs for a specific application.17 Therefore, in order to achieve excellent AFILs and to reduce the pollution caused by nonsensical synthesis, quantum chemical calculation was used to assist the absorbent development.18 By using the quantum chemical calculation to study the existed ILs at the electronic level, it was found that the interaction energy, especially the H-bond and van der Waals interactions could not be ignored.19 And the interaction energy of AFILs increased with the alkyl chain length of imidazolium ring decreased.20 Fernandes et al.21 found that the quantum chemical calculations could describe the trend obtained for the electrostatic cation-anion attraction potential. And they also found that the surface tension, which could impact the cation-anion interaction strengths, was dependence on the cation alkyl chain length and on the anion nature. It could be summarized from these studies that the interaction energy between cation and anion, Gibbs free energy and actiavation barrier could be treated as some influencing factors to predict the physical and chemical properties of ionic liquids.22, 23 But as it can be seen, quantum chemical calculation was only used to 3



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confirm the experimental results after designed and synthesized the AFILs in most of the above studies. Combined quantum chemistry simulation and experimental results, our previous work summarized a series of relationships between the structure and properties of AAILs, which provides a theoretical basis for the screening and synthesis of novel ionic liquids by quantum chemical calculations. It was found that both the CO2 absorption capacity and viscosity of AAILs were negatively correlated to their interaction energy, and the chain length of cation would influence the regeneration efficiency.24 Therefore, in this work, some novel AFILs were firstly designed, and their physical and chemical properties were predicted by the quantum chemical calculation. Based on the results of the theoretical results, the efficient AFILs was screened and synthesized for further investigation. The performance of CO2 capture into the AFILs was investigated, and their reaction mechanism was clarified by the 13

C NMR analysis.

Experiment Section Materials Diethylenetriamine(DETA), triethylenetetramine(TETA) and L-lysine were obtained from Xiya reagent. Ethanol was obtained from Shanghai Sinopharm Chemical Regent Co., Ltd., China. D2O was provided by J&K Scientific Ltd. All of these chemicals were analytical reagent. The gas of CO2 (>99.999%) was supplied by Fujian Nanan Chenggong Gas Co., Ltd., China. Synthesis of AFILs 4


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[DETAH][Lys] and [TETAH][Lys] used in this work were synthesized in laboratory. The synthesis reaction was in accordance with weak acid-base neutralization reaction: R1-NH2/R1R2-NH + R3-COOH → R1-NH3+/R1R2-NH2+ + R3-COO-

(1)

Take [TETAH][Lys] as an example, 0.5 mol of TETA was mixed with 0.5 mol of lysine. After mixing evenly, 10 mL of ethanol was added, and then the mixture was stirred until evenly. After that, distilled water were added dropwise to the mixture and stirred at the same time until the solution was clear. Then the solution was placed on a magnetic stirrer and stirred for 24h at 25 oC. Lastly, the compound was concentrated under vacuum at 70 oC in rotary evaporators and then diluted with water until the concentration of the solvent was 0.5 mol/L. Experimental Methods The CO2 capture performance of [DETAH][Lys] and [TETAH][Lys] solutions (0.5 mol/L, M) was measured in the absorption apparatus. The CO2 flow rate was controlled at 60 mL/min by mass flow meter. The temperature of the water bath was controlled at 30, 40, 50, 60 oC. The CO2 absorption loading of the solution was acquired by calculating the integrated characteristic value of absorption rate to absorption time. The absorption rate of the solution was calculated by the difference between the inlet and outlet CO2 flow rates of the bubble absorption bottle which were recorded by the soap film flowmeter. The formulas are shown as follows:

ra =

Qin − Qout P T ( act 0 ) n × 22.4 × 1000 P0Tact

(2)

t

CT = ∫ ra dt 0

5


(3)


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where ra is the instantaneous absorption capacity of CO2 (moL CO2/min), Qin and Qout are the inlet and outlet CO2 flow rates (mL CO2/min), CT is the total CO2 absorption capacity of AFILs (mol CO2/mol AFILs), t is the total absorption time of AFILs (min). After saturation, the CO2-saturated solutions were regenerated in a flask at varied regeneration temperature (110, 120, 130 oC) and regeneration time (80, 90, 100 min) in an oil bath. The pH value of the solution was measured by using the pH meter (FE20, Mettler Toledo Co., Ltd., China). 13

C NMR 13

C NMR data of [TETAH][Lys] and [DETAH][Lys] were obtained from

13

C

NMR (Bruker AVIII500 MHz), using an internal standard of 0.1 mL D2O for the deuterium lock. Computational Methods The quantum chemical simulation theory level and methods of [DETAH][Lys] and [TETAH][Lys] in this study were same to our previous work, which was the DFT method with the Becke’s three-parameter functional and the nonlocal correlation of Lee, Yang, and Parr (B3LYP) together with the 6-311++G(d,p) basis set.24 The interaction energy(△E), activation barrier(△Eact), enthalpy change(△H) and reaction energy(△Q) were given by Equation 4 and 5. △E (kJ/mol) = 2625.5×[Eion

pair

- (Ecation + Eanion)]

△Eact /△H /△Q (kJ/mol) = 2625.5×(Eproduct – Ereatant)

(4) (5)

where Eionpair is the energy of the ion pair and Ecation and Eanion are the energies of its cation and anion, respectively; Eproduct and Ereactant are the energies of product and 6


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reactant, respectively. Basis set superposition error (BSSE) and zero point energy correction(ZPE) were calculated, and all of them were expressed in units of au.

Results and Discussion Design and screen of the novel AFILs As mentioned above, the length of the alkyl chain, the number of amino groups interaction energy and activation barrier of the absorbents were the important factors affecting their performance of CO2 capture capacity.25 It was proved that an increasing alkyl chain length of the cations not only led to an increase in the CO2 absorption, but also in the regeneration efficiency,16 while an increasing amino functional group of cations and anions would also increase the CO2 absorption.6 By the way, interaction energy and activation barrier were also the important factors to the physical and chemical properties of the AFILs.24 Based on the factors, some different types of cations and anions were chosen to further structure design of the novel AFILs, which needed to own a long length of alkyl chain, high number of amino groups and suitable interaction energy and activation barrier. After preliminary screening, lysine was chosen as the best anion because of its high number of amino groups and great physical

chemical

properties.24,

triethylenetetramine(TETA),

26

Meanwhile,

diethylenetriamine(DETA),

1,12-Dodecanediamine(DDA)

and

1,3,5-Benzenetriamine(BTA) were chosen as the qualified cation for the AFILs, which had long length of alkyl chain or high number of amino groups. Their chemical structures are shown in Figure 1. As it can be seen, TETA, DETA and DDA had a long alkyl chain which meant their AFILs would be theoretically easy to regenerate. 7



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Although BTA was a ring structure, it had three amino groups on it, which meant it would have high CO2 absorption capacity. Based on these cations and anion, four novel AFILs [DDAH][Lys], [BTAH][Lys], [DETAH][Lys] and [TETAH][Lys] were formed virtually by quantum chemical simulation.

Figure 1. Chemical structures of (a)DDA; (b)TETA; (c)DETA; (d)BTA and (e)lysine. (Blue atom represented nitrogen; Red atom represented oxygen; Gray atom represented carbon; White atom represented hydrogen)

Though these four AFILs were all designed for CO2 capture, their performance would be different with each other. In order to select the best cation, both interaction energy and the CO2 absorption and desorption activation barrier of these four AFILs were calculated by quantum chemical tools. Their interaction energy is shown in Table 1.

8


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Table 1. Interaction energy and amino group number of four novel AFILs Viscosity(298.15K) [µ / mPa·s]

AFILs

△E [kJ/mol]

Amino groups a

[P6444][Lys]

-340.53

2

2.0324

[TETAH][Lys]

-371.77

5

--

[DETAH][Lys]

-387.79

4

--

[AEmim][Lys]

-398.81

3

1.2324

[APmim][Gly]

-444.50

2

1.0324

[BTAH][Lys]

-536.39

4

--

[DDAH][Lys]

-537.70

3

--

a: The protonated amino group in synthesis reaction was not included After calculation, the interaction energy was obtained. And the reactive amino groups number in [TETAH][Lys], [DETAH][Lys], [BTAH][Lys] and [DDAH][Lys] was 5, 4, 4 and 3, respectively. The interaction energy followed the order of [TETAH][Lys] < [DETAH][Lys] < [BTAH][Lys] < [DDAH][Lys]. Some interaction energy and viscosity data from literature were listed in Table 1 helping to understand the viscosity trend of AFILs. Because of the lysine anion, the trend of these four AFILs was similar with the AAILs that the single amino group CO2 loading, CO2 solubility and the viscosity of them were negatively correlated to the interaction energy24. Therefore, it could be predicted that the viscosity of [TETAH][Lys] and [DETAH][Lys] were between 1.23 and 2.03, and the viscosity of [BTAH][Lys] and [DDAH][Lys] were less than 1.03. However, although [BTAH][Lys] and [DDAH][Lys] would have a lower viscosity, their single amino group CO2 loading 9



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and the number of amino groups made their absorption performance greatly limited. Therefore， [TETAH][Lys] would still be a considerable AFILs. Besides, the activation barrier of CO2 absorption and desorption could be used to explain the difficulty of the CO2 absorption and desorption reactions. The higher the activation barrier, the harder the CO2 absorption and desorption reaction carried out. In order to further select the best substance, the activation barrier of CO2 absorption and desorption of these four AFILs was calculated. In the previous study,24 the activation barrier of the reaction between CO2 and lysine anion in AAILs had been studied, and the main CO2 absorption reaction of AFILs was as follows: CO2 + R-NH2 + OH- → R-NHCOO- + H2O

(6)

In this work, only the activation barrier of the reaction between these four cation and CO2 needed to be considered. The structure of these four cations in Figure 1 showed that TETA, DETA and DDA had an axisymmetric structure, and BTA was a centrosymmetric structure. Theoretically speaking, because of the different neighboring chemical groups of amino groups, there were two types of amino group in TETA and DETA molecule, and only one type in BTA and DDA molecule. However, after synthesis, one of the amino groups was protonated. Meanwhile, the other amino groups would hydrolyze before CO2 absorption. Therefore, took one hydrolyzed amino group and one protonated amino group per molecule as an example, there were six types of amino group in TETA, two in DETA, and still one type in BTA and DDA. The CO2 absorption activation barriers of the reaction between all types of amino groups and CO2 were calculated. The calculation results are shown in Table 2. 10


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Table 2. Activation barrier of four amino functionalized cations 3D model

△Eact

△Eact

3D model

[kJ/mol]

[kJ/mol]

TETA cation a

388.43

17.06

272.39

14.82

476.48

125.40

DETA cation a

12.50 DDA cation a

44.97 BTA cation a

469.18

473.01

a: Green atoms represented the protonated and hydrolyzed amino group. Yellow atom represented the reaction amino group. As shown in Table 2, the absorption activation barrier of BTA and DDA cation reached 473.01 and 469.18 KJ/mol, respectively. TETA had high activation barrier either, but it also had a low activation barrier pathway which was only 14.82 KJ/mol. All types of amino group reaction in DETA had low activation barrier. It was obviously that amine-CO2 reaction with low activation barrier would firstly carry out. 11



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Thus, in TETA and DETA, CO2 was firstly reacted with low activation amino groups, then the high activation amino groups. And if there were no promoting conditions such as high pressure or catalyzer, the high activation barrier amino groups could not be reacted with CO2 completely. It was proved by Xie et al. that the formation of the zwitterion was the rate-determining step.27 The higher the activation barrier, the harder and slower the reaction carried out. Therefore, because BTA and DDA had no low activation barrier reaction pathway, their reactions with CO2 were harder and slower than that of [TETAH][Lys] and [DETAH][Lys], reflected in the physical property was the decline of absorption capacity. It was indicated that [TETAH][Lys] and [DETAH][Lys] would be better than the other two. Therefore, all of the absorption reaction of DETA were low activation, but part of activation barrier in TETA were high, as saying above, the single amino group CO2 loading of [DETAH][Lys] would larger than [TETAH][Lys]. But it was contrary to the previous prediction which predicted the single amino group CO2 loading of [TETAH][Lys] was larger than [DETAH][Lys] according to the interaction energy. However, interaction energy only affected the steric hindrance in AFILs solution, which was negatively correlated to the CO2 physical absorption capacity of AFILs.24 But activation barrier could affect the chemical absorption capacity of AFILs, which was the mainly CO2 absorption way in AFILs. 28 Therefore, the single amino group CO2 loading of [DETAH][Lys] would larger than [TETAH][Lys], and also the CO2 absorption rate of [DETAH][Lys]. Generally speaking, CO2 regeneration reaction could be regarded as the reverse 12


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process of absorption reaction.29-31 However, unlike the absorption reaction (Equation 6), the protonated amino group would replace H2O as the hydrogen donor in regeneration reaction because the protonated amino group was more readily ionized than water. The total regeneration reaction was as follows:12 R1-NHCOO- + R1-NH3+ → 2R1-NH2 + CO2 ↑

(7)

These four novel AFILs had the same anion, so only the activation barrier of the four cations regeneration reaction needed to be considered. Based on this reaction, the activation barrier of regeneration reaction were calculated and the results are shown in Table 3. Table 3. Activation barriers of the regeneration reaction. AFILs

Reaction equation

△Eact a [kJ/mol]

[DETAH][Lys]

DETAH+ + DETACOO- → 2DETA + CO2↑

530.51

[TETAH][Lys]

TETAH+ + TETACOO- → 2TETA + CO2↑

485.95

[DDAH][Lys]

DDAH+ + DDACOO- → 2DDA + CO2↑

410.96

[BTAH][Lys]

BTAH+ + BTACOO- → 2BTA + CO2↑

394.48

a: The regeneration activation barriers were the lowest regeneration activation barrier of each AFILs. As it can be seen, the regeneration reaction activation barrier of these four AFILs followed [BTAH][Lys] < [DDAH][Lys] < [TETAH][Lys] < [DETAH][Lys], the lowest activation barrier of them was 394.48 kJ/mol and the highest was 530.51 kJ/mol. As saying above, an increasing alkyl chain length of the cations led to an increase in the regeneration efficiency. Meanwhile, the longer the chain length, the

13



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larger the steric hindrance, and a large steric hindrance would make the carbamate that produced by CO2 and AFILs unstable, which meant that the CO2 product of carbamate from longer chain length would decompose easier than the shorter chain length.24

Thus, for the same type of cation, the lower the activation barriers, and the

higher the regeneration efficiency. So the regeneration efficiency of these four AFILs followed [BTAH][Lys] > [DDAH][Lys] > [TETAH][Lys] > [DETAH][Lys]. According to the previous study, even the regeneration activation barrier of [AEmim][Lys] reached 578.10 kJ/mol, its regeneration efficiency was still over 94.07%.24, 32 Therefore, it can be predicted that all of these four AFILs in this work would have a good regeneration efficiency. In summary, the interaction energy and the CO2 absorption activation barrier calculation results of the four AFILs indicated that [TETAH][Lys] and [DETAH][Lys] had larger CO2 solubility and CO2 absorption capacity. All of these four AFILs had a high regeneration efficiency. All the quantum chemical calculation results showed that TETA and DETA were the best among the four substances, so they were chosen as the most suitable cation to synthesize the new AFILs in this work. Then the optimized configurations of [TETAH][Lys] and [DETAH][Lys] were calculated. Figure 1 showed the structure of TETA and DETA, both of them had primary and secondary amine. As saying above, TETA and DETA had two types of amino group in one TETA/DETA molecule. Both of the amino groups could react with CO2 in the solvents. The optimization results are shown in Figure 2 and Figure 3.

14


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(a)

(b)

Figure 2. Quantum chemical optimization structures of [TETAH][Lys]. Lysine reacted with (a) primary amine group and (b) secondary amine group

(a)

(b)

Figure 3. Quantum chemical optimization structures of [DETAH][Lys]. Lysine reacted with (a) primary amine group and (b) secondary amine group The optimization result in Figure 2 and Figure 3 showed that there were some special chemical and hydrogen bonds in the optimized structure of [DETAH][Lys] and [TETAH][Lys]. Then the calculation results of these special bond lengths were compared with the standard bond lengths in Table 4.

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Table 4. Comparison of partial bond length in optimal configuration. [TETAH][Lys] Bonds

[DETAH][Lys]

Standard[Å] Primary

Secondary

Primary

Secondary

N···Ha

< 2.7522

--

--

--

--

N-Hb

1.01

1.712

1.702

1.721

1.705

-COO-H

1.01

--

--

--

--

-COO···H

< 2.7222

1.019

1.020

1.012

1.020

a: N···H represented the hydrogen bond b: N-H represented the chemical bond As the result shown in Table 4, took [TETAH][Lys] as an example, the chemical bond N-H and hydrogen bond -COO···H at primary amine were 1.712 and 1.019 Å, respectively, and the standard values of them were 1.01 and 2.72 Å. It seems that the N-H bond length in the configuration was too long for a chemical bond. It was indicated that the protonated hydrion on the TETA/DETA amino group was not stable enough at the initial stage of the reaction, and the N-H chemical bond at primary amino was more like to be a hydrogen bond and -COO···H was more like to be a chemical bond. But it was common sense that the neutralization reaction would certainly take place, and the reaction between TETA/DETA and lysine may need a long time to react completely. In the present work, there were four reactive amino groups in [DETAH][Lys] and five in [TETAH][Lys], and both of the amino groups could chemical react with CO2 normally. Based on the zwitterionic mechanism, their predicted absorption capacity were 2.00 and 2.50 mol CO2/mol AFILs respectively, which were much higher than

16


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that of MEA process (0.5 mol CO2/mol MEA). Because of the weakly alkaline, amino groups made the pKa value of primary and secondary amino solution keeping at 10.50 and 10.73 respectively.33 Therefore, no matter how many amino groups in the solution, the pH value of the solution would be close to 10.50. Thus, it could be predicted that the pH value of [DETAH][Lys] and [TETAH][Lys] would keep at about 10.50. Through simulation and forecast of calculation, the efficient AFILs was screened, and the absorption capacity and regeneration efficiency of the novel AFILs was expected to be much higher than the AAILs reported before. Performance of the novel AFILs As saying above, the reaction between TETA/DETA and lysine may need a long time to react completely. Therefore, the reaction time was investigated. The result is shown in Figure 4.

absorption capacity mol CO2/mol AFILs

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.0 2.5 2.0 1.5 1.0

absorption immediately regeneration immediately absorption after stir 24h regeneration after stir 24h

0.5 0.0 0

10

20

30

40

50

60

time/min

Figure 4. Absorption and regeneration capacity of [TETAH][Lys] before and after stirring 24h. Take [TETAH][Lys] as an example, from Figure 4, it can be seen that the absorption capacity of [TETAH][Lys] reached 2.84 mol CO2/mol AFILs when TETA 17



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and lysine was only mixed together without stirring in synthesis process. According to the zwitterionic mechanism, it was obviously that nearly all amino groups in [TETAH][Lys] reacted with CO2, so it had such a high absorption capacity. However, the regeneration absorption capacity of [TETAH][Lys] without stirring was only 1.59 mol CO2/mol AFILs, only 55.99 % of the original absorption capacity. Both the experimental result showed that the reaction of TETA and lysine wasn’t complete. But in the case of stirring for 24h, although the original absorption capacity of [TETAH][Lys] dropped to 2.39 mol CO2/mol AFILs, the regeneration efficiency was much higher than that without stirring, which reached 99.10 %. It was indicated that TETA and lysine had completely reacted after stirred 24h, and only 5 amino groups could react with CO2, which was consistent with the predicted results. The difference in regeneration efficiency confirmed the necessity of stirring in synthesis process and the correctness of the previous prediction. Only when TETA/DETA and lysine reacted adequately, the ionic liquid was considered to be synthesized, and the regeneration efficiency could be significantly improved. Then it was predicted that the viscosity of AFILs was negatively correlated to the interaction energy, and their predicted viscosity was followed the order [TETAH][Lys] > [DETAH][Lys] > [BTAH][Lys] > [DDAH][Lys]. However, only TETA and DETA were the most suitable cation to synthesize the new AFILs in this work, and their viscosities at different temperature are shown in Figure 5.

18


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1.5 1.4 1.3

[TETAH][Lys] [DETAH][Lys]

1.2

viscosity / µ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.1 1.0 0.9 0.8 0.7 0.6 30

40

50

60

o

temperature / C

Figure 5. Viscosity of [DETAH][Lys] and [TETAH][Lys] at different temperature. From Figure 5, the viscosities of [TETAH][Lys] and [DETAH][Lys] at temperature 30, 40, 50, 60 oC were investigated. It showed clearly that the viscosity was decreased as the temperature increased, and the viscosity of [TETAH][Lys] was larger than [DETAH][Lys] at the same temperature. This experimental result was fully consistent with the viscosity prediction that [TETAH][Lys] > [DETAH][Lys]. According to the quantum chemical calculations, it was predicted that [TETAH][Lys] and [DETAH][Lys] had the best CO2 capture performance. Therefore, their absorption capacity under different absorption temperature was investigated after synthesized. The results are shown in Figure 6.

19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

absorption capacity / mol CO2/mol AFILs


Page 20 of 34


2.5

2.0

1.5

1.0

0.5

0.0

30

40

50

60

o

temperature / C

Figure 6. Absorption capacity of [DETAH][Lys] and [TETAH][Lys] at different temperature. The

absorption

capacity

and

absorption

rate

of

[TETAH][Lys]

and

[DETAH][Lys] at temperature 30, 40, 50, 60 oC were investigated in Figure 6. As shown in the results, both the absorption capacity of [TETAH][Lys] and [DETAH][Lys] were first increased and then decreased with the increase of temperature. In theory, high absorption temperature would accelerate molecular motion, thus speeding up the reaction rate. However, excessive absorption temperature would decrease the solubility of CO2 in solution, which would limit the CO2 absorption. Meanwhile, excessive high absorption temperature would also lead to solvent loss and accompanied by a little desorption reaction. Thus, the CO2 capture capacity had a sharp decrease at 50 and 60 oC. The highest absorption capacity of [TETAH][Lys] and [DETAH][Lys] were obtained at 40 oC, which were 2.59 and 2.13 mol CO2/mol AFILs, respectively.This experimental result was in good agreement 20


Page 21 of 34

with the predicted absorption capacity of [TETAH][Lys] and [DETAH][Lys], which was 2.50 and 2.00 mol CO2/mol AFILs, respectively. The comparison of absorption capacity and absorption rate of [TETAH][Lys] and [DETAH][Lys] at 40 oC is shown in Figure 7.

0.25 2.5 0.20 2.0 0.15 1.5


0.10

1.0

0.05

0.5

0.00

0.0 0

10

20

30

40

50

absorption capacity / mol CO2/mol AFILs

3.0

absorption rate / mol CO2/mol AFILs ⋅ min

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

time / min

Figure 7. Absorption capacity and absorption rate of [DETAH][Lys] and [TETAH][Lys]. As it showed, the absorption capacity of [TETAH][Lys] and [DETAH][Lys] gradually increased during the CO2 absorption reaction, while the absorption rate decreased gradually. After 60 minutes, the reaction of CO2 absorption tended to be stable and the solution became CO2-saturated. The absorption rate of [TETAH][Lys] and [DETAH][Lys] was 0.2392 and 0.2448 mol CO2/min, respectively. As saying above, there were 5 active amino groups in [TETAH][Lys] and 4 in [DETAH][Lys], so the single amino group CO2 loading of [TETAH][Lys] and [DETAH][Lys] was 0.5180 and 0.5325 mol CO2/mol AFILs, respectively. The experimental results were 21



clearly showed that the single amino group CO2 loading of [DETAH][Lys] was larger than [TETAH][Lys], and the absorption rate of [DETAH][Lys] was faster than [TETAH][Lys], which were fitted to the quantum chemical calculation predictions before. While the experimental results of the viscosity and absorption capacity of [TETAH][Lys] and [DETAH][Lys] were same to the quantum chemical calculation, the regeneration efficiency was investigated to further confirm the correctness of the quantum chemical predictions. The impacts of regeneration temperature and regeneration time on the regeneration efficiency of [TETAH][Lys] and [DETAH][Lys] are shown in Figure 8.

100


(a)

80

regeneration rate / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

60

40

20

0

110℃

120℃

130℃

22


Page 22 of 34

Page 23 of 34

100


(b)

80

regeneration rate / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

40

20

0

80min

90min

100min

Figure 8. Regeneration condition optimization of [TETAH][Lys] and [DETAH][Lys]. Figure 8-a was the impact of temperature under 90 minutes regeneration time.12 The result showed that the regeneration efficiency of [TETAH][Lys] and [DETAH][Lys] at 120 oC was 98.96% and 98.00%, respectively, and when the regeneration temperature reduced to 110 oC, the regeneration efficiency decreased sharply, the efficiency of [TETAH][Lys] and [DETAH][Lys] was decreased to 65.86 % and 64.93%. But when the regeneration temperature was up to 130 oC, there was little change in regeneration efficiency. So the best regeneration temperature was 120 o

C. Under this condition, the impact of regeneration time was investigated in Figure

8-b. The result showed a similar trend to the regeneration temperature. When the regeneration time reduced to 80 minutes, the efficiency of [TETAH][Lys] and [DETAH][Lys] was decreased to 81.61% and 82.23%, but there was a little change when the regeneration time increased to 100 minutes, so the optimal regeneration time was 90 minutes. Then the stability of [TETAH][Lys] and [DETAH][Lys] was

23



investigated under 120 oC and 90 min, the five times regeneration cycle is shown in Figure 9.

[TETAH][Lys]

[DETAH][Lys]

100

regeneration efficiency / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

80

60

40

20

0

fresh

cycle1

cycle2

cycle3

cycle4

cycle5

Figure 9. Five times regeneration cycle of [TETAH][Lys] and [DETAH][Lys]. After 5 times of regeneration cycle, the regeneration efficiency of [TETAH][Lys] still had 97.99%, but the value of [DETAH][Lys] decreased to 92.82%. The result showed that the stability of [TETAH][Lys] was better than [DETAH][Lys]. As saying in previous quantum chemical calculation, a high regeneration activation barrier which was impacted by chain length of cation, would reduce the regeneration efficiency. As excepted, the experimental result was in good agreement with the prediction in previous calculations. Reaction mechanism of the novel AFILs Reaction mechanism was an important basis to provide the possibility for industrial applications for a new compound. In order to get a better understanding of the new AFILs, the reaction mechanism of them was investigated. To gain a further 24


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Page 25 of 34

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insight into the absorption mechanism, the

13

C NMR spectra of them are shown in

Figure 10.

Carboxylate

(a)

(b) Carboxylate

(1) pH=10.67 LCO = 0

(1) pH=10.65 LCO = 0

2

Carboxylate

2

Carboxylate

Carbamate

Carbamate

(2) pH=10.02 LCO = 0.61

(2) pH=9.97 LCO = 0.63

2

2

Carboxylate

Carboxylate (3) pH=8.74 LCO = 1.40

(3) pH=9.05 LCO = 1.58

2

2

Carboxylate

Carboxylate

Carbonate (4) pH=7.93 LCO = 2.00

Carbonate

(4) pH=7.84 LCO = 2.37

2

2

Carboxylate

Carboxylate (5) pH=7.47 LCO = 2.13

(5) pH=7.46 LCO = 2.59

2

2

Figure 10. 13C NMR of (a)[DETAH][Lys] and (b)[TETAH][Lys] during the CO2 absorption.{LCO2 : CO2 loading (mol CO2/mol AFILs)} As shown in Figure 10, the pH value of [TETAH][Lys] and [DETAH][Lys] before CO2 absorption were 10.65 and 10.67, respectively. During the absorption, the signal of C=O on Lys-COO- at δ 182.5 ppm was right shift constantly until the solution was saturated, and the signal was at δ 174.9 ppm. The signals at δ 164.0 were attributed to the C=O in carbamate carboxyl carbon and the signals at 160.1 ppm was 25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 34

HCO3−/CO32− species.34 Therefore, it was clearly that the reaction mechanism of [TETAH][Lys] and [DETAH][Lys] was accorded with the zwitterionic mechanism, and because of the lysine anion, their reaction mechanism was also similar to AAILs: R1-NH2/R1R2-NH + H2O ⇌ R1-NH3+/ R1R2-NH2+ + OH-

(8)

CO2 + R1-NH2/R1R2-NH + OH- → R1-NHCOO-/R1R2-NCOO- + H2O

(9)

H+ + R1-NHCOO-/R1R2-NCOO- + H2O →R1-NH3+/R1R2-NH2+ + HCO3Take [DETAH][Lys] as an example, the reaction details are shown in Figure 11.

Protonation reaction H 2O

OH-

[DETAH][Lys]-NH3+

[DETAH][Lys]

CO2 absorption reaction OHCO2 H 2O [DETAH][Lys]-NHCOO-

Hydrolysis reaction

CO2

H 2O

H+ H2O

HCO3-

H+ HCO3-

[DETAH][Lys]-NH3+

Figure 11. Absorption mechanism of CO2 capture with [DETAH][Lys]. 26


(10)

Page 27 of 34

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However, because of the number and variety types of amino groups in [TETAH][Lys] and [DETAH][Lys], the signal at δ 164.0 ppm showed a cluster state. This phenomenon has been predicted in the quantum chemical calculation. When the CO2 loading of [TETAH][Lys] and [DETAH][Lys] were 0.63 and 0.61 mol CO2/mol AFILs respectively, both of the carbamate signal was centralized and simplex. That was because of the carbamate produced by the reaction between low activation barrier amino groups and CO2. When the CO2 loading of [TETAH][Lys] and [DETAH][Lys] were 1.58 and 1.40 mol CO2/mol AFILs respectively, the carbamate signal in [TETAH][Lys] became messy and clustered, which was different to that of [DETAH][Lys]. It could be explained by the quantum chemical calculation that there were six types of amino groups in TETAH+, but only two in DETAH+, so the carbamate signal in [TETAH][Lys] was more than [DETAH][Lys]. Meanwhile, the signal of HCO3−/CO32− appeared. It could be used to explain the right shift of the carboxylate signal. The signal at δ 182.5 ppm before CO2 absorption represented the C=O on Lys-COO-, its oxygen atom had a strong charge offset caused by the insufficient hydrogen ions in the solution. When the absorption reaction began, the pH value started to decrease, which meant the concentration of hydrogen ions started to increase. And the equilibrium reaction shown in Equation 11 would be started: Lys-COO- + H2O ↔ Lys-COOH + OH-

(11)

With the formation of Lys-COOH and the increase of hydrogen ions, the charge offset of the oxygen atom gradually decreased. Therefore, the signal of C=O would shift to the right side and finally stopped at δ 174.9 ppm. 27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Conclusions In this work, quantum chemical theoretical basis of previous studies such as alkyl chain length law, interaction energy law and activation barrier law were regarded as screening conditions for seeking the cation of novel AFILs. Four substances were chosen as the qualified cation after preliminary screening. Then the calculation result of interaction energy, CO2 absorption and regeneration activation barrier showed that [TETAH][Lys] and [DETAH][Lys] had the best physical chemical properties. They had more than 2.0 mol CO2/mol AFILs absorption capacity and good regeneration efficiency. The experimental result showed that their absorption capacity were 2.59 and 2.03 mol CO2/mol AFILs and the regeneration efficiency were 98.96% and 98.00%, respectively. After five times of regeneration, [TETAH][Lys] and [DETAH][Lys] showed good stability. All of the experimental results were consistent with the quantum chemistry predictions. Lastly,

13

C NMR was used to confirm the

reaction mechanism and found that the reaction mechanism of the novel AFILs was similar to AAILs because of the lysine anion. The result demonstrated the accuracy and reliability of quantum chemical simulation, which improved the efficiency of AFILs research.

Acknowledgments This work was sponsored by the National Natural Science Foundation of China (21576109 and 21676110), the Natural Science Foundation of Fujian Province (2017J01017), and the Foundation for Youth and Middle-aged Teacher of Fujian Educational Committee (JA15028). We also thank the Instrumental Analysis Center of 28


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Huaqiao University for analysis support.

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study on the regeneration heat requirement of an amine-based solid adsorbent process for post-combustion carbon capture. Appl. Energ. 2016, 168, 394-405. DOI: 10.1016/j.apenergy.2016.01.049 TOC [DETAH][Lys]/[TETAH][Lys] R-NHCOOH2O H+

3 4 CO2

OH-

2

HCO3OH-

CO2

R-NH3+

R-NH2

H2O

1

Synopsis This paper contributes to high efficiency simulation design of ionic liquids by quantum chemical calculation before synthesis.

34


Designing and Screening of Multi-Amino-Functionalized Ionic Liquid

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