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Quantum Mechanical and Molecular Dynamics Simulations of Dual-Amino-Acid Ionic Liquids for CO Capture 2

Abdul Rajjak Shaikh, Hamed Karkhanechi, Eiji Kamio, Tomohisa Yoshioka, and Hideto Matsuyama J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07305 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 30, 2016

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

1

Quantum Mechanical and Molecular Dynamics

2

Simulations of Dual-Amino-Acid Ionic Liquids

3

for CO2 Capture

4

Abdul Rajjak Shaikh,1 Hamed Karkhanechi,1 Eiji Kamio,1 Tomohisa Yoshioka,2 and

5

Hideto Matsuyama1*

6

1Center

7

Engineering, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan

8 9

2Center

for Membrane and Film Technology, Department of Chemical Science and

for Membrane and Film technology, Graduate School of Science, Technology and

Innovation, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan

10 11

ABSTRACT

12

Global warming is occurring because of emission of greenhouse gases due to human

13

activities. Capture of CO2 from fossil-fuel industries and absorption of CO2 for natural

14

gas sweetening are crucial industrial tasks to address the threat from greenhouse gases.

15

Amino acid ionic liquids (AAILs) are used for reversible CO2 capture. In this study, the

1 ACS Paragon Plus Environment


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effect of CO2 chemisorption on tetramethylammonium glycinate ([N1111][GLY]),

2

tetrabutylammonium

3

glycinate ([aN111][GLY]) were analyzed using density functional theory (DFT) and

4

molecular dynamics (MD) studies. DFT studies predicted different reaction pathways

5

for CO2 absorption on [GLY]− and [aN111]+. The activation energy barriers for CO2

6

absorption on [GLY]− and [aN111]+ are 52.43 and 64.40 kJ/mol, respectively. The MD

7

results were useful for mimicking the reaction mechanism for CO2 absorption on AAILs

8

and its effect on physical properties such as the fractional free volume, diffusion

9

coefficient, and hydrogen bonding. Dry and wet conditions were compared to identify

10

factors contributing to CO2 solubility and selectivity at room temperature and elevated

11

temperature. Hydrogen bonding between ion pairs was used to understand the increase

12

in viscosity after CO2 absorption. The MD studies revealed that glycinate and related

13

products after CO2 absorption contribute the most to the increase in viscosity.

glycinate

([N4444][GLY]),

and

1,1,1-trimethylhydrazinium

14 15 16 17 18 2 ACS Paragon Plus Environment

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1


1. INTRODUCTION

2

Carbon dioxide is one of the primary greenhouse gases in the atmosphere. Human

3

activities such as increasing combustion of fossil fuels and industrial emittance

4

contribute to increasing greenhouse gases and thereby increasing the average

5

temperature at the earth's surface.1 Various methods have been proposed to limit

6

anthropogenic CO2 emissions. Current technology uses a dilute aqueous solution of

7

methanolamine in the sequestration process for selective capture of CO2.2,3 However, its

8

use is greatly limited by its high enthalpy of reaction and hence high energy demand for

9

regeneration, in addition to its susceptibility to oxidative degradation and concerns

10

regarding corrosion of equipment.4 Accordingly, the development of new materials that

11

can efficiently, reversibly, and economically capture CO2 and other flue gases is

12

vigorously sought.

13

Ionic liquids (ILs) are promising green solvents that have attracted attention from the

14

scientific and technological communities for CO2 absorption from various waste gases.5,6

15

With the tremendous choices available for selecting the cation and anion, they are

16

becoming essential components in green chemistry and are also materials with unique

17

and tunable properties that can be adjusted by selecting appropriate ions for specific

18

requirements.7 ILs are advantageous because they have low vapor pressure, high



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thermal stability, and wide liquidus windows; thus, the effects of the absorbent loss and

2

degradation are minimized.8,9 Some ILs are also recyclable, and this characteristic can

3

help to reduce environmental concerns over their use. Hence, ILs present a promising

4

route to energy- and cost-efficient separation of CO2 from industrial emissions.

5

The initial focus of interest on ILs as physical solvents for use in flue gas treatment

6

was due to the substantial stability of high-pressure CO2 in room-temperature ionic

7

liquids.10–12 More recently, task-specific ionic liquids (TSILs) have been an active area of

8

research.13 There are many types of TSILs, in particular those functionalized with

9

amine and amino acid groups, which contributed to the achievement of excellent CO2

10

capture performance.14–17 In TSILs, the reactive amino group is covalently tethered to

11

the cation or anion of the IL, leading to low-pressure chemical reactivity for CO2

12

absorption by ILs.18–24 These TSILs were found to capture CO2 at a 1:1 molar ratio in

13

dry conditions.21,25 However, the viscosity of these ILs in dry conditions is too high for

14

successful use in commercial process. Furthermore, it was proved that small quantities

15

of water can dramatically decrease the viscosity of ILs.26,27 The CO2 absorption capacity

16

of an aqueous solution of tetramethylammonium glycinate ([N1111][GLY]) increased from

17

0.169 to 0.601 mol CO2/mol IL as the mass fraction of [N1111][GLY] in the solution

18

decreased from 100% to 30%.28 Currently, numerous systems featuring amino and other


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1


functionalized ILs have also been reported.8,29–32

2

Membrane-based technology is under development with the goal of advancing toward

3

sustainable systems that minimize CO2 emissions.6 However, its use in industrial

4

applications was hindered by poor separation performance and typically low CO2

5

permeability. Our previous studies reported the use of amino acid ionic liquids (AAILs)

6

as a carrier for CO2 absorption in facilitated transport membrane (FTMs).19,33 The CO2

7

permeation properties of AAIL-FTMs are determined mainly by the physicochemical

8

properties of the AAILs with which the membrane is impregnated.34,35 Most ILs become

9

viscous after CO2 absorption, which greatly affects the way these materials might be

10

used for CO2 capture.36 Previous studies showed that the viscosity of ILs after CO2

11

absorption increased dramatically owing to formation of an intermolecular hydrogen

12

bond (HB) network among AAIL-CO2 complexes.37,38 The viscosity of AAILs was the

13

dominant factor in the CO2 permeation properties of AAIL-FTMs under dry conditions

14

at room temperature. On the other hand, the intermolecular hydrogen bonding

15

interaction becomes weak under humid conditions, elevated temperature, or both.39

16

Therefore, under these conditions, the marked increase in the viscosity of the AAIL-CO2

17

complex can be prevented.

18

The absorption capacity of most existing functionalized IL solutions with a single



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functional group is about 0.5 mol CO2/mol ILs, which is similar to that of most

2

amine-based solvents.40,41 To improve the absorption capacity of functionalized ILs, our

3

group

4

([aN111][GLY]) and compared its absorption capacity with that of single-functional-group

5

ILs such as tetramethylammonium glycinate ([N1111][GLY]) and tetrabutylammonium

6

glycinate ([N4444][GLY]).39 The designed dual AAIL, [aN111][GLY], shows excellent

7

CO2/N2 selectivity under humid conditions, elevated temperatures, or both. The effect of

8

water on that neat AAIL was studied using molecular dynamics (MD) simulations.42 It

9

was observed that [aN111][GLY] interacts more strongly with water molecules than

10

other AAILs and might contribute to high permeability in humid conditions. However,

11

the effect of CO2 absorption on that AAIL has not been studied.

12

has

Although

synthesized

many

the

dual

works related

AAIL

1,1,1-trimethylhydrazinium

to functionalized

glycinate

ILs have been reported,

13

understanding of the mechanism of CO2 absorption into functionalized ILs is incomplete

14

because of insufficient study. Most studies have focused on the existing reaction

15

between CO2 and alkanolamine as a reference and then proposed a mechanism for CO2

16

capture into functionalized ILs.43,44 Density functional theory (DFT) and MD

17

simulations are ideally suited to investigating the reaction mechanism and other

18

physical

properties

in

order

to

correlate

them

with

CO2

absorption

and


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1

permeability.37,45,46 Because there are two functionalized groups in dual-functionalized

2

ILs, both of which could react with CO2, the reaction would be more complex in this new

3

system. To date, few works have focused on the reaction mechanism of CO2 absorption

4

into dual-functionalized ILs.

5

In this paper, the effect of CO2 absorption on [N1111][GLY], [N4444][GLY], and

6

[aN111][GLY] in dry and wet conditions is studied using DFT calculations and MD

7

simulations. Two amino groups are present, one on the cation and one on the anion, in

8

[aN111][GLY]; hence, these dual functional groups can react with CO2, and it is

9

important to understand the CO2 absorption pathways on the two functionally different

10

amino groups. The transition state barrier for CO2 absorption on glycinate ([GLY]) and

11

[aN111] was investigated using DFT studies. MD studies were conducted to analyze the

12

HB network and other diffusion properties to understand the effect of CO2 absorption.

13

The viscosity of AAILs decreases with increasing humidity, temperature, or both; hence,

14

these factors are also studied using MD simulations in dry and wet conditions at 300

15

and 373 K. The free volume (FV) and fractional free volume (FFV) were also analyzed.

16

DFT studies complemented by MD simulations will be helpful for understanding the

17

selectivity of these AAILs toward CO2 and the effect of partial or full CO2 absorption on

18

other physical properties. This will make it possible to sort out the moieties contributing



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to the viscosity and thereby facilitate the design of new AAILs with improved CO2

2

absorption properties and barrier properties toward other gases.

3 4

2. COMPUTATIONAL DETAILS

5

2.1. Quantum Mechanical Calculation

6

All DFT calculations were performed using the Gaussian 09 suite of programs.47 DFT

7

calculations using the SMD48 continuum solvation model in combination with the

8

M062X density functional and the 6-31++G(d,p) basis set were performed. The

9

geometries of the resulting reactants, transition states, and products were optimized at

10

the same level of theory. The activation energy of a reaction was determined from ETS ―

11

ER, and that of the inverse reaction was determined from ETS ― EP, where ETS, ER, and

12

EP are the calculated total energies of the transition state, reactant, and products,

13

respectively. Zero-point energy corrections were calculated and taken into account in

14

the calculation of the energy barrier. The obtained transition states were confirmed

15

using one imaginary frequency along the reaction coordinate. The minimum energy

16

conformation for the reactant and product were also confirmed using positive

17

vibrational frequencies. The effect of an explicit water molecule was investigated by

18

comparing the calculated activation energies.


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1

2.2.

Molecular Dynamics Simulations

2

2.2.1.

Force Field

3

The Amber force field49 was used to determine the intra- and intermolecular force

4

constants for the glycinate, and the generalized Amber force field50,51 was used for the

5

cation of the ILs. Validation of the force field for pure AAILs was reported in our

6

previous paper.42 DFT calculations were performed using the Gaussian 09 program47

7

and the B3LYP and 6-31G(d) basis sets. The electrostatic potential surface was

8

generated by the Merz–Kollman method at the HF/6-31G(d) level of theory, and a

9

multiconfigurational two-stage restrained electrostatic potential (RESP) fitting was

10

performed. The Antechamber program was used for the RESP charge calculations.

11

2.2.2.

Simulations

12

The initial configurations of the systems were generated using the Packmol

13

program.52 All the simulations reported here were performed using the GROMACS 5.0.4

14

simulation program.53 The simulation details are similar to those described in our

15

previous work.42 Initially, an IL mixture was slowly heated to 500 K in 50 K increments.

16

The mixture was cooled to 300 K in 20 K decrements. During cooling, NVT dynamics

17

was performed for the first 3 ns, followed by 3 ns of NPT dynamics. The last

18

configuration obtained at 300 K was used as the initial configuration for the



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1

equilibration and production run. Equilibration simulations were performed using NVT

2

and NPT ensembles for 10 ns each. The temperature was maintained at 300 K using

3

velocity rescaling with a coupling time of 5 ps. The pressure was maintained at 1 atm

4

for all NPT simulations by a Parrinello–Rahman barostat with a coupling time of 1 ps.

5

The equations of motion were integrated using the leapfrog algorithm with a time step

6

of 2.0 fs. Production simulations were performed using an NVT ensemble for 50 ns

7

followed by NPT simulations for 30 ns. The total electrostatic interactions were

8

evaluated using the particle mesh Ewald summation. Coulomb and van der Waals

9

cutoffs of 1.2 nm were employed. Periodic boundary conditions in all directions were

10

employed to mimic the bulk behavior. Bond lengths were constrained with the LINear

11

Constraint Solver algorithm.54

12

Simulations were performed to study various mixture compositions to mimic the

13

reaction of CO2 with the AAILs from partially to fully reacted CO2. This work was

14

inspired by the research of Gutowski and Maginn,37 in which they studied the reaction

15

mechanism

16

bis(trifluoromethanesulfonyl)imide (Tf2N−), a TSIL, using an MD simulation. For

17

[N1111][GLY] and [N4444][GLY], only the glycinate chemical reacted with CO2, as the

18

cation has no amine group. However, for [aN111][GLY], three possibilities exist: (A) only

for

CO2

absorption

on

1-(3-aminopropyl)-3-methylimidazolium


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1

[aN111] reacted with CO2, (B) both [aN111] and [GLY] reacted with CO2, and (C) only

2

[GLY] reacted with CO2. The chemical structures of the cation ([N1111], [N4444], [aN111])

3

and anion ([GLY]), and the products formed after CO2 absorption ([GCO] and [ANC]),

4

are shown in Figure 1. The product formed after CO2 absorption on [GLY]− is denoted as

5

[GCO], and that for [aN111] is denoted as [ANC], as shown in parts e and f of Figure 1,

6

respectively. Tables S1 and S2 of the Supporting Information (SI) shows the number of

7

simulated systems. The effect of water on CO2 absorption was studied using 384 water

8

molecules inside the species for [aN111][GLY] and [N1111][GLY] and 820 water molecules

9

for [N4444][GLY]. In all the simulations, the weight percentage of water molecules was

10

10.8 to simulate partially saturated systems as observed experimentally. The number

11

was selected according to the number of cation–anion pairs in the mixture. A series of

12

MD simulations was performed to explore the thermodynamic and transport properties

13

of CO2 absorbed on the anion or cation. The effect of temperature was considered by

14

gradually increasing the temperature to 373 K and then conducting NVT and NPT

15

simulations as described above for 300 K.

16



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(a)

(b)

(c) CH3

CH3

H3C

N

N

N

[N1111]

[aN111]

[N4444]

O O

(e)

O

HO

-

(f)

NH O

-

O

[GLY]

[GCO]

1

CH3

H3C CH3

H3C

O

+

+

CH3

H2N

NH2

H3C

+

H3C

(d)

CH3

Page 12 of 45

H3C

HN +

N

OH CH3

CH3

[ANC]

2

Figure 1. Chemical structures of the AAILs used in this study. Cations: (a)

3

tetramethylamine [N1111], (b) tetrabutylamine [N4444], (c) 1,1,1-trimethylhydrazinium

4

[aN111], and anion: (d) glycinate [GLY]. Species after CO2 absorption are (e) (carboxy

5

amino) acetate [GCO] and (f) 2-carboxy-1,1,1-trimethylhydrazin-1-ium [ANC].

6 7 8 9

2.2.3.

Free Volume Calculation

The FV and FFV were calculated using the GROMACS FV analysis tool. The FV and FFV were calculated using the following equations:55,56

10 11

𝐹𝑉 = 𝑉m − 𝑉OC = 𝑉m − 1.3 𝑉vdW

12

𝐹𝐹𝑉 =

13

𝐹𝑉 𝑉m

=

𝑉m −1.3 𝑉vdW 𝑉m

= 1.3 𝐹𝑉 − 0.3

(1) (2)

where VOC is the occupied volume, VvdW is the van der Waals volume of the molecule,


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1

and Vm is the molar volume; VOC = 1.3VvdW, where the factor of 1.3 accounts for the

2

presence of some unoccupied space in the crystals even at 0 K.

3 4

3. RESULTS AND DISCUSSION

5

The force field parameters used in the present simulations for neat ILs were reported

6

in our previous paper.42 Here we explore the effect of CO2 absorption on these ILs using

7

DFT calculations and MD simulations. First, we discuss the reaction mechanism for

8

CO2 absorption on [GLY]− and [aN111]+ using DFT methods. Next, the MD results are

9

discussed.

10

3.1.

Reaction Mechanism

11

The reaction mechanism of CO2 absorption in amine has been reported in the

12

literature.57–60 It is generally accepted that amines react with CO2 via a zwitterionic

13

mechanism.61 It is commonly known that the amino acids in solution form stable

14

zwitterion (RNH2+COO−, eq 3) by transfer of acidic proton to the amine resulting in a

15

combination of carboxylate and an ammonium moiety (eq 4).

16

RNH2 + CO2 ⇄ RNH2+COO−

(3)

17

RNH2+COO− + RNH2 → RNHCOO− + RNH3+

(4)

18



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Page 14 of 45

1

This carbamate (RNHCOO−) formation reaction is exothermic and can be reversed

2

thermally. A number of studies18,62-64 have been reported for CO2 absorption in AAILs.

3

The molar uptake ratio of AAILs can be 1:1 or 1:2 molar depending on the anion and

4

cation. For example triethylbutylammonium L-alaninate ([N2224][ALA]) shows an

5

absorption capacity of 1 mole of CO2 for 2 moles of ILs (1:2), typical of

6

amino-functionalized

7

([P666,14][PRO]) exhibits equimolar absorption(1:1).63 It indicate two types of mechanism,

8

a maximum of 1:2 ratio implies the bimolecular mechanism as mentioned above (eq 3,4),

9

while 1:1 suggest a monomolecular mechanism involving one anion, possibly

10

terminating at the formation of carbamic acid group on the anion. Gurkan et al.63 have

11

reported that the reaction of CO2 with aminate-IL terminates at the formation of

12

carbamic acid (eq 5) without forming carbamate species (eq 6).

ILs,

whereas

trihexy(tetradecyl)phosphonium

13

RNH2 + CO2 ⇄ RNHCOOH

(5)

14

RNHCOOH + RNH2 → RNHCOO− + RNH3+

(6)

prolinate

15

Recently Sistla et al.62 studied several amino acid anion based ILs using a

16

combination of NMR and IR-spectroscopy and suggested an intramolecular carbamate

17

mechanism to explain the 1:1 mechanism with post-dimerization being proposed to

18

explain observation of the 1:2 capacity. The molar uptake ratio of AAILs under study is


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1

1:1 indicating that monomolecular mechanism could be favorable. Therefore only the

2

carbamate formation pathway is discussed in this article.

3

To investigate the role of the cation and anion with two functionally different amine

4

groups in [aN111][GLY] in CO2 absorption, DFT calculations were performed using the

5

Gaussian 09 program. Figure 2 shows the activation energy barrier for CO2 absorption

6

on [GLY]− and [aN111]+ in the presence and absence of a single water molecule. The

7

reaction pathway configurations for CO2 binding to the [GLY]− anion are shown in

8

Figure 2a. The first step for CO2 absorption is formation of a zwitterionic complex in

9

which strong interaction between CO2 and the N atom of [GLY]− exists. We were not

10

able to find the energy barriers for the formation of this zwitterionic complex.65 In the

11

zwitterionic complex, the C–N bond was found to be 1.55 Å long, and the O–C–O angle

12

was 133.6°, as shown in Figure 3a. This implies that CO2 interacts very strongly with

13

[GLY]−. The carbamate formation reactions are predicted to be exothermic and to have

14

ΔE values of about −42.42 kJ/mol in the absence of water molecules and −38.35 kJ/mol

15

in the presence of a single water molecule. In both cases, the reaction is exothermic. The

16

activation barrier for CO2 absorption on [GLY]− is 129.59 and 52.43 kJ/mol in the

17

absence and presence of an explicit water molecule, respectively. In Figure 2a, we can

18

see that incorporating a single water molecule lowered the activation barrier by up to



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Page 16 of 45

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~77 kJ/mol. Zhang et al.65 reported that the activation barrier for the [P(C4)4][GLY]

2

system was ~42 kJ/mol. Guo et al.66 conducted an experiment using a stopped-flow

3

ultraviolet/visible spectrometer to study glycinate in alkaline solution and reported an

4

activation barrier of 45.4 ± 2.2 kJ/mol. Recently, Mercy et al.67 reported activation

5

barrier for [N1111][ALA] of 30.80 kJ/mol using DFT methods. The geometry and

6

distances around the reacting site for the lowest energy zwitterion complex, the

7

transition state, and the final product for CO2 absorption in the absence of water

8

molecules are shown in Figure S1 of the SI. In Figure S1b, we can see that the

9

transition state resembles a highly strained four-membered structure, and hence CO2

10

absorption shows a high energy barrier for [GLY]−. Figure 3 shows the reaction

11

configuration in the presence of an explicit water molecule. In the transition state in the

12

presence of an explicit water molecule, we observed that water acted as a proton relay to

13

favor formation of a more flexible six-membered ring transition state structure (Figure

14

3b), which shows a very low energy barrier. In the product state, the C–N distance is

15

reduced to 1.38 Å, whereas the O–C–O bend angle is 123.9°(Figure 3c). In the product

16

state, the proton previously attached to the nitrogen in the anion is now shared between

17

the oxygen from CO2 and the carboxylate group, as shown in Figure 3c.


Page 17 of 45

140

TSGLY 129.59

(a)

180

52.43

60 20 -20

-100

0.00 Zwitterion

-38.35 -42.42 Product

Relative Energy (kJ/mol)

100

-60

1

[GLY] - + H2O

TSAN1 156.41

(b)

[aN111]+

[GLY] −

Relative Energy (kJ/mol)

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


140

[aN111]+ + H2O

100

64.40 60

20 -20

0.00 Pre-complex

4.09 -0.40 Product

-60

2

Figure 2. Energy profile for absorption of CO2 on (a) [GLY]− and (b) [aN111]+ in the

3

presence (dotted line) and absence (solid line) of a single water molecule.

4 5

Figure 2b shows the energy barrier for CO2 absorption on [aN111]+. The geometries of

6

the pre-complex, transition state, and product in the absence and presence of a single

7

water molecule are shown in Figure S1 (d–f) and Figure 3 (d–f), respectively. In contrast

8

to [GLY]−, which first forms a zwitterionic complex, [aN111]+ does not form a zwitterionic

9

intermediate. The main reason is the formation of two adjacent positively charged

10

groups (N+–N+), which is very unfavorable, as positive charges repel one another.

11

However, the transition state for [aN111]+ resembles that of [GLY]−, as shown in Figure

12

3b,e. In the presence of an explicit water molecule, the transition state resembles the

13

six-membered structure (Figure 3f) having a lower energy than the four-membered

14

structure without water molecules (Figure S1e). The activation barrier for [aN111] is

15

156.41 kJ/mol in the absence of water molecules and 64.40 kJ/mol in the presence of a 17 ACS Paragon Plus Environment


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Page 18 of 45

1

single water molecule. The energy barrier for CO2 absorption on [aN111]+ is higher than

2

that for [GLY]− by 27 kJ/mol in the dry condition and ~12 kJ/mol in the presence of

3

water molecule. In the presence of an explicit water molecule, the reaction is slightly

4

exothermic. In Figure 2, we can observe that CO2 absorption on [GLY]− is exothermic in

5

both conditions (with and without a single water molecule), whereas [aN111]+ is

6

endothermic in the absence of explicit water molecule and exothermic in the presence of

7

a single water molecule. These results confirm that CO2 absorption on [aN111]+ will be

8

favorable in the presence of a water molecule. This could be the reason that absorption

9

of CO2 on [aN111][GLY] is higher under humid conditions.39 Furthermore, as reported,

10

small quantities of water can dramatically decrease the viscosity of ILs.26,27 Moreover,

11

the flue gases emitted from current power plants contain a small amount of water

12

(~5%–10%) that could be used to advantage.68 Hence, it is important to design ILs that

13

do not affect CO2 solubility in the presence of small amount of water. Considering these

14

observations, [aN111] is better able to absorb more CO2 in wet or humid conditions and

15

hence can be an ideal candidate for CO2 absorption from flue gases. DFT results are

16

consistent with several experimental studies which suggest that CO2 absorption on

17

amino acid anion is better than amino-cation and non-functionalized ILs.62 The reason

18

for higher CO2 absorption on amino acid anion is due to lower activation barrier for CO2


Page 19 of 45

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1


absorption and might be lesser increase in visocosity.69 (a)

(b)

133.6°

(c) 1.06 1.41

1.23

1.28

1.55

1.28

1.24

2.16

1.46

1.38

1.44

1.04

123.9° 1.26 1.75

1.97

1.01

1.46

0.97 0.97

1.07

0.98

2.56

1.11

0.96

Zwitterion

(d)

Product (ANC + H2O)

TSGLY

(e)

1.16

(f) 1.22

1.16

1.34

1.23 2.83

2.31

1.03 1.45

1.44

1.03 1.02

2.00

0.97 0.97

0.97

1.37 1.48 1.26

1.85

1.43

1.04

3.31

1.72 1.76 1.00

1.02

0.98

2

Pre-complex

TSAN1

Product (GCO + H2O)

3

Figure 3. Structures of [GLY]− and [aN111]+ at various stages of reaction in the presence

4

of a single water molecule. (a) Zwitterion, (b) transition state, and (c) final product

5

structure for CO2 absorption on [GLY]−. (d) Pre-complex, (e) transition state, and (f)

6

final product for reaction of CO2 with [aN111]+. Distances are given in angstroms, and

7

angles are given in degrees.

8 9

3.2.

Molecular Dynamics

10

MD simulation is a useful tool to understand the dynamic and static properties of

11

AAILs. An MD simulation was conducted to understand the effect of CO2 chemisorption

12

on the density, FFV, diffusivity, and hydrogen bonding propensity of AAILs. These

13

analyses will help us to understand the effect of temperature and humidity on the 19 ACS Paragon Plus Environment


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1

diffusivity of AAILs, which is directly correlated with their permeability. The density

2

and hydrogen bonding results will be helpful for understanding the viscosity of AAILs

3

after CO2 absorption in dry and wet conditions.

4

3.2.1.

Equilibrium Density and Free Volume

5

The effect of temperature on the density and FV of the neat AAILs was calculated

6

using MD simulations and is shown in Figure 4. Figure 4a shows the effect of

7

temperature on the density of the pure ILs. The density decreases with increasing

8

temperature. The predicted density for the AAILs compares well with the experimental

9

density, and all the differences are less than 4% (~1% for [N1111][GLY], ~3%–4% for

10

[N4444][GLY], and ~2%–4% for [aN111][GLY]). Figure 4b shows the effect of temperature

11

on the FV of the AAILs. The FV, which represents the total interspace of ILs, is related

12

to many fundamental properties of ILs, including the solubility and diffusivity of

13

gases.42,70,71 The FV of the AAILs increased with increasing temperature. The FV

14

decreases in the order [N4444][GLY] > [N1111][GLY]  [aN111][GLY]. In all cases, the FV

15

increased by ~5% at 360 K and ~20% at 500 K, where [N1111][GLY] shows a higher FV

16

(22%) and [aN111][GLY] shows a slightly lower (18%) increase in FV at 500 K. The

17

increase in FV will be beneficial for absorption of more CO2, and it is expected that

18

higher temperatures will lower the activation energy for CO2 absorption. An


Page 21 of 45

1

experimental observation also found that the CO2 absorption capacity of AAILs

2

increased at elevated temperatures.39 1.25

41

(a) 1.20

39

1.15

37

Free Volume (%)

Density (g/cm3)

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


1.10 1.05 1.00

0.95

35 33 31 29

0.90

27 25

0.85

290

3

(b)

300

310

320

330

340

350

360

370

290

300

310

Temperature (K)

320

330

340

350

360

370

Temperature (K)

4

Figure 4. Effect of temperature on (a) density and (b) FV of [N1111][GLY] (),

5

[aN111][GLY] (), and [N4444][GLY] (). Open symbols represent experimental density.

6

After studying the reaction mechanism using DFT studies, we studied the effect of

7

CO2 chemisorption on the AAILs under dry and wet conditions using MD simulations. It

8

is observed experimentally that the viscosity of AAILs increased after CO2 absorption;

9

however, the viscosity decreases at higher temperatures, under humid conditions, or

10

both.39 Using MD simulations, we would like to mimic the reaction for CO2 absorption

11

and understand the effect of temperature and water (humidity) on the physical

12

properties, which are important for the solubility and permeability of CO2 in AAILs.

13

The mechanism of CO2 absorption on the three AAILs studied here is shown in Figure

14

S2. For [aN111][GLY], three possibilities were studied, as shown in Figure S2: routes A

15

(CO2 absorbed on [aN111]), B (CO2 absorbed on both [aN111] and [GLY]), and C (CO2 21 ACS Paragon Plus Environment


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Page 22 of 45

1

absorbed on [GLY]). In absence of experimental data for equilibrium concentration of

2

CO2 absorbed moieties (ex. GCO and ANC), above three possibilities remains feasible

3

for MD simulations. In addition, modeling such type of reaction to calculate free energy

4

of absorption is very difficult and requires much more computing power using quantum

5

mechanics. MD simulations can be very helpful in such scenario.

6

Figure 5 shows the change in density under dry and wet conditions with respect to

7

CO2 absorption. As shown in Figure 5a, the density of [N4444][GLY] is lower in the dry

8

condition than in the wet condition at 300 K. However, at a higher temperature (373 K),

9

the density is almost the same under both the dry and wet conditions before CO2

10

absorption increasing to higher value. The result could be understood using the fact

11

that the molar volume of [N4444][GLY] is higher than that of the other AAILs studied

12

here. Hence, the apparent change in density is not significant, as the IL has a larger

13

space for movement and to accommodate water molecules. For [N1111][GLY], no clear

14

trend was observed, as the density is similar in the range of 25%–75% CO2 absorption.

15

At higher CO2 absorption (complete saturation), the density increases in the wet

16

condition, as shown in Figure 5a. The density of [N1111][GLY] is higher in the wet

17

condition in the neat ILs and later decreases. After CO2 absorption, the hydrophilicity of

18

[N1111][GLY] increases, which could have the effect of lowering the density, as


Page 23 of 45

1

hydrophilic moieties have a favorable interaction with water molecules. As shown in

2

Figure 5b, at higher temperatures, the difference between the density in the dry and

3

wet conditions diminishes, indicating the important role of temperature in reducing the

4

density. In Figure 5a,b, we can see that the average density does not change

5

significantly for [N1111][GLY] and [N4444][GLY]. Tables S3 and S4 in SI shows the

6

obtained FV, total box volume, and density in the dry and wet conditions at 300 K for

7

studied AAILs.

8 1.4

1.4

(a)

1.3

Density (g/cm3)

Density (g/cm3)

373 K

(b)

300 K

1.3 1.2 1.1 1.0

1.2 1.1

1.0 0.9

0.9

0.8

0.8 0

25

50

75

0

100

25

CO2 Absorption (%) 1.40

1.40

(c)

Density (g/cm3)

1.30 1.25 1.20

1.30

1.25 1.20 1.15

1.15

1.10 0

25

50

75

100

0

25

[N1111][GLY] (dry) [N1111][GLY] (wet)

[N4444][GLY] (dry) [N4444][GLY] (wet)

50

75

100

CO2 Absorption (%)

CO2 Absorption (%)

9

11

100

1.35

1.10

10

50 75 CO2 Absorption (%)

373 K

(d)

300 K

1.35

Density (g/cm3)

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


[aN111][GLY]-A (dry) [aN111][GLY]-A (wet)

[aN111][GLY]-B (dry) [aN111][GLY]-B (wet)

[aN111][GLY]-C (dry) [aN111][GLY]-C (wet)

Figure 5. CO2 absorption versus density in dry and wet conditions for [N1111][GLY] and



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

Page 24 of 45

[N4444][GLY] at (a) 300 K and (b) 373 K and for [aN111][GLY] at (c) 300 K and (d) 373 K.

2

The effect of CO2 absorption on density of [aN111][GLY] at 300 and 373 K is shown in

3

parts c and d of Figure 5, respectively. The overall increase in density in case A is much

4

higher than that in case C. Reaction case A (Figure S2), where CO2 is absorbed on

5

[aN111], shows a decrease in density with increasing CO2 absorption in the wet condition

6

compared with that in the dry condition. Similar behavior was observed at 373 K, as

7

shown in Figure 5d. The density of neat [aN111][GLY] was 1.157 g/cm3 and increased to

8

1.374 g/cm3 after complete saturation with CO2 (case A) in the dry condition and 1.354

9

g/cm3 in the wet condition at 300 K. On the other hand, the density in case C (where

10

CO2 was absorbed on glycinate) shows a slight increase in the wet condition at 300 K.

11

The density increased from 1.157 g/cm3 to 1.198 g/cm3 in the dry condition and 1.211

12

g/cm3 in the wet condition at 300 K. The difference between the density in the dry and

13

wet conditions vanishes at higher temperature. The increase in density with CO2

14

absorption is much lower in case C. The density increase from the dry to the wet

15

condition in case A compared to that in case C is ~15% in the dry condition and ~13% in

16

the wet condition at 300 K. At the higher temperature (373 K), the density change in

17

case A compared to that in case C is ~15% in the dry condition and ~10% in the wet

18

condition. Overall, in Figure 5, we can see that the density increases in the wet


Page 25 of 45

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1

condition when CO2 is absorbed on glycinate and decreases when CO2 is absorbed on

2

[aN111]. In addition, the shift from lower temperature to higher temperature reduces the

3

overall density, which is consistent with an experimental observation.39 From Figure 5,

4

apparent change in density is higher in [aN111][GLY] as compared to [N1111][GLY] and

5

[N4444][GLY]. The existence of two reactive amino groups on [aN111][GLY] makes them

6

more hydrophilic and interaction of CO2 with amino groups increases density due to

7

formation of HBs. On the other hand, [N1111][GLY] and [N4444][GLY] have only one

8

amino group that can react with CO2; hence, the lower number of HBs between the ion

9

pairs as compared to [aN111][GLY] could be responsible for negligible change in density

10 11 12 13

after CO2 absorption. The effect of CO2 absorption on the total volume expansion in the wet condition was also investigated. The liquid volume change, ΔV/V0, is defined as ∆𝑉 𝑉0

(%) = 100 ×

𝑉m (𝑇,𝜌,𝜒𝐶𝑂2 )− 𝑉IL (𝑇,𝜌0 ) 𝑉IL (𝑇,𝜌0 )

(7)

14

where Vm(T,,χCO2) is the volume of the box in the mixture at a given percentage of CO2

15

absorption (χCO2), and VIL(T,0) is the molecular volume of the pure IL in the wet

16

condition. Figure S3 shows the effect of CO2 absorption on the liquid volume expansion

17

calculated using eq 7.

18

As shown in Figure S3 in SI, the maximum volume expansion for [N1111][GLY] was



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 45

1

observed at 75% CO2 absorption. In our previous studies,42 volume shrinkage was

2

observed after water molecules were added to [N1111][GLY] and [N4444][GLY]. This is the

3

reason for the large change in volume expansion for [N1111][GLY]. The initial ILs with

4

negative volume expansion (shrinkage) show more efficient packing, attractive

5

interaction, or both when they are mixed with water. Owing to the smaller size of the

6

cation in [N1111], the volume expansion is greater. In contrast, owing to the larger cation

7

in [N4444], the volume expansion is smaller for [N4444][GLY]. For [aN111][GLY], the

8

volume does not change significantly when CO2 is absorbed on [aN111] (case A). On the

9

other hand, the excess volume changed significantly when CO2 was absorbed on

10

glycinate (case C). At higher temperature, the volume expansion is larger for

11

[N4444][GLY] and [aN111][GLY]. Overall, the volume expansion increases with increasing

12

CO2 absorption, and it depends strongly on the size and hydrophilicity or

13

hydrophobicity of the IL.

14

3.2.2.

Fractional Free Volume

15

The FFV is the ratio of empty space to occupied space in a materials and is used to

16

characterize the efficiency of packing and free space in the IL matrix.70-71 Shannon et

17

al.71 reported the effect of the FV of imidazolium-based ILs and found good correlation

18

with the CO2/N2 selectivity. In our previous studies42, the effect of water on the FV and


Page 27 of 45

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1

FFV of neat ILs was found to depend on the hydrophilicity and hydrophobicity of

2

AAILs.42 Here, we analyzed the effect of CO2 absorption on the FFV for three cases of

3

CO2 absorption on [aN111][GLY]. Figure 6 shows the change in the FFV after CO2

4

absorption at 300 and 373 K in the dry (solid lines) and wet conditions (dashed lines). In

5

reaction A, the FFV decreased with increasing CO2 absorption in the dry and wet

6

conditions. The results are consistent with the density results shown in Figure 5c,d.

7

Case A shows an increase in density and a decrease in FFV with increasing CO2

8

absorption, indicating maximum packing in the IL matrix. On the other hand, case C

9

shows a lower density and higher FFV. The FFV is higher in the wet condition than in

10

the dry condition for case A. However, for case C, the FFV increased (or remained

11

constant) with increasing CO2 absorption and was lower at higher temperature. In all

12

cases, the FFV increased at higher temperatures, indicating higher space for CO2

13

absorption. Case B is between cases A and C; hence, it shows values intermediate

14

between those in these two cases. A significant change in the FFV after CO2 absorption

15

demonstrates less space for ion mobility, which will hinder the diffusion and

16

permeability of CO2. As in the DFT results, absorption of CO2 is favorable on GLY in

17

case C, as it does not increase the density and decrease the FV, unlike the results for

18

case A.



16.0

16.0

(a)

14.0

14.0

12.0

12.0

10.0

8.0

6.0

6.0

[aN111][GLY]-A (dry) [aN111][GLY]-A (wet) [aN111][GLY]-B (dry) [aN111][GLY]-B (wet) [aN111][GLY]-C (dry) [aN111][GLY]-C (wet)

4.0 0

25

50 CO2 Absorption (%)

75

100

373 K

10.0

8.0

4.0

1

(b)

300 K

FFV (%)

FFV (%)

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 28 of 45

0

25


75

100

2

Figure 6. Change in FFV with increasing CO2 absorption in [aN111][GLY] for three cases

3

at (a) 300 K and (b) 373 K in dry and wet conditions.

4

Explicit solvation models such as MD simulation provide an approximate enthalpy of

5

absorption. As MD simulation is performed at several stages of reaction, it can be used

6

qualitatively to understand stable system. More accurate results can be obtained using

7

ab initio and density functional theory calculations. However, such calculations are

8

computationally very expensive. Hence we have used MD simulation to understand

9

enthalpy of absorption which is approximate and qualitatively can be used for

10

comparison purposes between three different systems. Figure 7 shows the average

11

enthalpy versus the percentage CO2 absorption in the wet condition at 300 K. The

12

enthalpy is higher for [N4444][GLY] than for the other AAILs. In contrast, the enthalpy

13

for CO2 absorption in [aN111][GLY] is lowest for case C. For [aN111][GLY], the average

14

enthalpy is in the following order: case A > case B > case C. The results also indicate

15

that sequestration of CO2 from glycinate in [aN111][GLY] will require more energy. The 28 ACS Paragon Plus Environment

Page 29 of 45

1

results agree with the DFT results, which showed that CO2 absorption on GLY is more

2

exothermic, so stripping CO2 from GCO requires more energy (Figure 2a). From the

3

slope of the enthalpy, the absorption enthalpy (approximate) was calculated and is

4

presented in Table 1. 200000 100000

Avg. enthalpy (kJ/mol)

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


0 -100000 -200000 [N1111][GLY] [N4444][GLY] [aN111][GLY]-A [aN111][GLY]-B [aN111][GLY]-C

-300000 -400000 -500000 0

5

25

50

75

100

CO2 Absorption (%)

6

Figure 7. Graph of average enthalpy versus percentage CO2 absorption in wet condition

7

at 300 K.

8

The absorption enthalpy is higher for ILs in which CO2 is absorbed on glycinate. For

9

case A in [aN111][GLY], the absorption enthalpy is lower. These results also confirm that

10

CO2 absorption on glycinate is most strongly preferred. A higher absolute enthalpy

11

means a stronger IL–CO2 bond but also a higher heat requirement during CO2

12

desorption. The experimental results show that [N1111][GLY] and [N4444][GLY] exhibited

13

an absorption capacity of more than 0.7 mol/mol at 1 bar, whereas [aN111][GLY]

14

exhibited an absorption capacity of more than 1.2 mol/mol at 1 bar and 343 K.39 From 29 ACS Paragon Plus Environment


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Page 30 of 45

1

the experimental absorption results, it can be predicted that both amino groups of

2

[aN111][GLY] participate in absorption of CO2.39 Combining the experimental, MD and

3

DFT results, the most likely CO2 absorption is on [GLY] and then on [aN111], indicating

4

that case B might be dominant.

5

Table 1. Absorption Enthalpy for AAILs at 300 K in Wet Condition

6

3.2.3.

AAIL

ΔHwet (kJ/mol CO2)

[N1111][GLY]

−633.25

[N4444][GLY]

−654.15

[aN111][GLY]-A

−199.21

[aN111][GLY]-B

−452.57

[aN111][GLY]-C

−635.71

Hydrogen Bonds

7

We calculate the probability of HB formation between different groups. The

8

dynamics of HBs72 were qualified by calculating the time autocorrelation function,

9

which is defined as

10

𝐶(𝑡) =

〈ℎ(0)ℎ(𝑡)〉 〈ℎ(0)ℎ(0)〉

(8)

11

In the expression, represents the ensemble average over all hydrogen bonding

12

pairs in the system. The population variable h(t) is unity when a particular hydrogen

13

bonding pair is reserved for the entire time period from 0 to t; otherwise, it is zero. The

14

kinetics of HB breakage and re-formation can be derived from a chemical dynamics

15

analysis.73,74 Here, a forward rate constant k for HB breakage and a backward rate


Page 31 of 45

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1

constant k′ for HB formation are determined from the reactive flux correlation

2

function75 K(t):

3 4 5

𝐾(𝑡) = −

𝑑𝐶(𝑡)

(9)

𝑑𝑡

and 𝑑𝐶(𝑡) 𝑑𝑡

= 𝑘𝐶(𝑡) − 𝑘 ′ 𝑁(𝑡)

(10)

6

where N(t) is the probability that an HB that existed at t = 0 is broken but that the two

7

hydrogen bonding groups are still within hydrogen bonding distance. The HB lifetime in

8

this scheme is given by the inverse forward rate constant as follows:

9

τHB = 1/𝑘

(11)

10

Figure S4 shows representative autocorrelation profiles for hydrogen bonding

11

between GLY–GCO and GCO–WAT pairs for CO2 absorption by [N1111][GLY]. The plot

12

shows the slow dynamics of the GLY–GCO pair, indicating stronger hydrogen bonding

13

with increasing CO2 absorption. Figure S4b shows the autocorrelation function for

14

hydrogen bonding between GCO and water. The autocorrelation function slowly decays,

15

indicating stronger hydrogen bonding between GCO and water. Figure 8 shows the

16

lifetime of HBs calculated for various pairs. The HB lifetime is longest for the GCO–

17

WAT and GCO–GCO pairs. In addition, the HBs of GLY–WAT and WAT–WAT are also

18

longer than those of other pairs. From these observations, we can conclude that



1

interaction of GLY and GCO with themselves or with water molecules is dominant.

2

Thus, such interactions might be the main contributors to the higher viscosity observed

3

in experimental studies.39 7000

4500

[N1111][GLY]

(a)

[N4444][GLY]

(b)

4000

6000

3500 5000

3000

τHB (ps)

τHB (ps)

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 32 of 45

4000 3000

2500 2000

1500

2000

1000 1000

500

0

0 0

25

50

75

100

0

25

CO2 Absorption (%)

4

GLY-GLY GLY-GCO

50

75

100

CO2 Absorption (%) GLY-WAT

GCO-GCO

GCO-WAT

WAT-WAT

5

Figure 8. HB lifetimes 𝜏𝐻𝐵 versus CO2 absorption calculated at 300 K in wet condition

6

for (a) [N1111][GLY] and (b) [N4444][GLY].

7 8

A similar trend was observed for [aN111][GLY]. The lifetime of HBs for various pairs in

9

[aN111][GLY] is shown in Figure 9 for case A and B. The HB lifetime for case B in

10

[aN111][GLY] is shown in Table S5. HBs between GLY or GCO and AN1 ([aN111]) exhibit

11

a longer lifetime. In addition, the HB lifetime for the AN1–GLY, AN1–GCO, and GLY–

12

WAT pairs is longer in case B. In all three cases, the AN1–WAT interaction also exhibits

13

a longer HB lifetime, and the HBs are strongest in either GLY or GCO with other

14

moieties. Surprisingly, the interactions of AN1 and ANC with themselves or with each


Page 33 of 45

1

other exhibit lower HB lifetimes; hence, they do not contribute to stronger hydrogen

2

bonding. This trend reflects the fact that the higher viscosity observed after CO2

3

absorption is due to interaction of GLY or GCO with either itself or with other moieties,

4

which contributes to stronger hydrogen bonding. From Figure 9b, AN1-GCO shows

5

longer HB lifetime with increase in CO2 absorption. 7000

7000

(a) 6000

6000

5000

5000

4000 3000

3000 2000

1000

1000

0

[aN111][GLY]-C

4000

2000

0 0

25


GLY-GLY

6

(b)

[aN111][GLY]-A

τHB (ps)

τHB (ps)

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


AN1-AN1

75

100

0

AN1-GLY

GLY-WAT

GCO-GCO

ANC-GLY

ANC-WAT

WAT-WAT

25


GCO-WAT GLY-GCO

75

100

AN1-GCO

7

Figure 9. HB lifetimes 𝜏𝐻𝐵 versus CO2 absorption calculated at 300 K in wet condition

8

for (a) case A and (b) case C in [aN111][GLY].

9 10

3.2.4.

Diffusion Coefficient

11

Figure S5 shows the diffusion coefficient of water molecules at 300 and 373 K in the

12

AAILs. The diffusion coefficient of water increases with increasing temperature for

13

[N4444][GLY]. [N4444][GLY] shows the highest diffusion coefficient at 373 K because of the

14

large available FV (Table S6). The higher FV is responsible for high N2 absorption and



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 34 of 45

1

lower CO2/N2 selectivity. That is the reason that [N4444][GLY] shows much higher N2

2

absorption and lower CO2 absorption capacity. In contrast, the smaller ILs, [N1111][GLY]

3

and [aN111][GLY], show higher CO2 absorption and lower N2 absorption and hence

4

higher CO2/N2 selectivity.39 [N1111][GLY] and [aN111][GLY] show similar diffusion

5

coefficients at 300 and 373 K, and the coefficients are apparently much higher at 373 K.

6

No clear trend was observed for diffusivity with increase in CO2 absorption in

7

[N1111][GLY] and [aN111][GLY]. In all cases, the temperature affects the diffusion

8

coefficient.

9

Figure 10 shows the diffusion coefficients for ANC and GCO (the species obtained

10

after CO2 absorption on GLY and AN1, respectively) at 300 and 373 K. The diffusion

11

coefficient of ANC in [aN111][GLY] is higher than that of GCO at both temperature. With

12

increasing CO2 absorption, the diffusion coefficient is expected to decrease compared to

13

that of the pure IL and also observed in our simulation. From these results, we can

14

conclude that the diffusion coefficient is higher at high temperature, and CO2

15

absorption on AN1 shows a higher diffusion coefficient, leading to higher permeability,

16

as observed in experimental studies.39


Page 35 of 45

30

Diffusion Coefficient (× 10 -9) cm2/s

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


25 20 15 10 5 0 25

1

50 75 CO2 Absorption (%)

100

2

Figure 10. Diffusion coefficients of ANC and GCO molecules in [aN111][GLY]. Squares

3

and circles indicate results for ANC and GCO, respectively, at 300 K; dashed lines show

4

corresponding results at 373 K.

5 6

4. CONCLUSIONS

7

In this work we have examined mechanism of CO2 absorption on single and dual

8

functionalized AAILs, with the aim of elucidating aspects of absorption process which

9

may lead to the design of more effective dual-functionalized AAILs. Absorption of CO2

10

on [N1111][GLY], [N4444][GLY] and [aN111][GLY] was studied using DFT and MD studies.

11

The reaction mechanism for CO2 absorption on [aN111]+ and [GLY]− was studied using

12

DFT. The energy barrier was 52.43 kJ/mol for [GLY] and 62.54 kJ/mol for [aN111]. Before

13

CO2 absorption on [GLY]−, glycinate forms a zwitterionic complex, which is absent in

14

[aN111]. It has been observed that a single water molecule assists the reaction by

15

lowering the energy barrier. The reaction dynamics of CO2 absorption on the AAILs was



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 36 of 45

1

studied by mimicking the reaction mechanism. A reaction consisting of partial

2

absorption (25%) to full absorption (100%) was studied in dry and wet conditions at

3

room temperature and elevated temperatures. The density of the ILs after CO2

4

absorption does not increased significantly when CO2 is absorbed on glycinate. However,

5

for case A, in which CO2 chemisorption occurs on [aN111], shows a significant change in

6

density, which is greater than that in cases B and C. Consistent with density results,

7

the FFV was highest for case C and lowest for case A. In agreement with the DFT

8

results, the enthalpy of absorption was higher (more negative) for all the cases in which

9

GLY reacted chemically with CO2. However, it was lower for case A. Combining DFT

10

and MD results, CO2 absorption on [GLY] has lower activation energy and higher

11

enthalpy of absorption indicating preferred site for absorption is on [GLY] than that of

12

[aN111]. In normal cases, ratio of 1:1 CO2 absorption per IL is expected. However,

13

experimentally it is observed that 1 mole of [aN111][GLY] absorbs 1.2 moles of CO2.

14

Comparing with experimental absorption properties for [aN111][GLY], case B, in which

15

CO2 is absorbed on both the cation and the anion, is likely to dominate. Raising the

16

temperature was found to be useful for decreasing the density after CO2 absorption. The

17

results are consistent with an experimental observation in which the viscosity was

18

found to be lower at higher temperature and under humid conditions. The HB lifetimes


Page 37 of 45

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

for AN1–GLY, AN1–GCO, GCO–GCO, GCO–WAT, and GLY–WAT are much higher than

2

those for other pairs. These result indicates that interaction between GLY and GCO

3

with cation makes the greatest contribution to the viscosity. In addition, when CO2 is

4

absorbed on [aN111], resultant species become more hydrophilic and contribute for

5

hydrogen bonding with other species. The diffusion coefficient was higher in the wet

6

condition and at elevated temperatures; diffusion, in turn, is responsible for the higher

7

permeability of these moieties in the IL matrix.

8

In summary, CO2 absorption on [GLY] is higher than that on [aN111] based on DFT

9

results, and combining with experimental results, case B is likely the dominant reaction

10

pathway for [aN111][GLY]. A delicate balance is required to design new ILs with high

11

CO2 absorption capacity and lower viscosity. Although glycinate interacts more strongly

12

with CO2 than [aN111], it might be advisable to replace glycinate with another anion to

13

decrease the viscosity and increase the CO2 absorption. Glycinate has been widely used

14

in many studies and has excellent CO2 absorption; however, in this case, it is important

15

to explore [aN111] with other anions to determine their advantages. For example, it

16

might be possible to replace [GLY] with other amino acids that show similar properties.

17

Fine tuning might result in much higher CO2 absorption capacity and selectivity. The

18

regenerability and higher CO2 capture capacity of the AAILs suggest that judicious



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 38 of 45

1

choice of amino-acid anion with an appropriate cation can greatly enhance the CO2

2

absorption capacity.

3 4

ASSOCIATED CONTENT

5

Supporting Information Available: Simulation system, transition states for glycinate and

6

[aN111] without water, reaction mechanism, absorption enthalpy, volume expansion, HB

7

lifetime in [aN111][GLY], diffusion coefficient. This material is available free of charge

8

via the Internet at http://pubs.acs.org.

9 10

AUTHOR INFORMATION

11

Corresponding Author

12

*H.M.: Tel. /Fax: +81-78-803-6180; E-mail: [email protected];

13

A.R.S.: [email protected]

14 15

ACKNOWLEDGMENTS

16

This work was supported by research grant from the Organization of Membrane and

17

Film Technology, Japan. This research was also funded by Grants-In-Aid from the

18

Special Coordination Funds for Prompting Science and Technology, Creating of

19

Innovation Centers for Advanced Interdisciplinary Research Area (Innovative

20

Bioproduction, Kobe) from the Ministry of Education, Culture, Sports, Science, and 38 ACS Paragon Plus Environment

Page 39 of 45

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


Technology, Japan.

2 3

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