Accepted Manuscript

Investigation of CO2 Capture Efficiency and Mechanism in 2-Methylimidazole-

Glycol Solution

Huang Liu, Ping Guo, Guangjin Chen

PII: S1383-5866(16)32189-X

DOI: http://dx.doi.org/10.1016/j.seppur.2017.04.017

Reference: SEPPUR 13679

To appear in: Separation and Purification Technology

Received Date: 26 October 2016

Revised Date: 21 March 2017

Accepted Date: 17 April 2017

Please cite this article as: H. Liu, P. Guo, G. Chen, Investigation of CO2 Capture Efficiency and Mechanism in 2-

Methylimidazole-Glycol Solution, Separation and Purification Technology (2017), doi: http://dx.doi.org/10.1016/

j.seppur.2017.04.017

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1

Investigation of CO2 Capture Efficiency and Mechanism in 2-Methylimidazole-

Glycol Solution

Huang Liu1*

, Ping Guo1, Guangjin Chen

2*

1. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum

University, Chengdu 610500, China

2. State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, P.

R. China

Corresponding Authors: liuhuangswpu@sina.com (H. Liu); gjchen@cup.edu.cn (G. Chen)

Abstract

Here, we report the fabrication of a stable CO2 capture medium by dissolving 2-methylimidazole in

glycol. It was found that 2-methylimidazole dissolved in glycol well, and their mixture showed

good flowability. For single CO2 absorption in the 10 wt% 2-methylimidazole-glycol solution, the

molar ratio between CO2 and 2-methylimidazole molecules reached 0.79 at 293.15 K and 18.73 bar.

It was supposed that an alkylcarbonate salt formed upon contact between 2-methylimidazole, glycol

and CO2. This CO2 absorption mechanism was further characterized by using the Fourier-transform

infrared technology. For CO2/N2 (20.65/79.36 mol%) gas mixture separation in the 30 wt% 2-

methylimidazole-glycol solution, the equilibrium partial pressure of CO2 in the gas phase could be

decreased to as low as 0.10 bar with the selectivity of CO2 over N2 reaching 239 at 293.15 K and

4.55 bar. More importantly, the CO2 uptake in 2-methylimidazole-glycol solution was reversible,

CO2 was readily desorbed from the solution through vacuuming at room temperature and with no

apparent liquid loss. This work opens up the possibility of applying imidazole-alcohols mixtures to

CO2 capture.

Keywords: CO2 capture; 2-methylimidazole-glycol solution; alkylcarbonate salt

1. Introduction

It is increasingly recognized that the accumulation of CO2 in the atmosphere has resulted in many

serious environmental problems. Therefore, the development of efficient methods for capturing CO2

from industrial flue gases has become an important consideration for the energy and petrochemical

industries in recent years. Three types of technology-absorption separation [1,2], adsorption

2

separation [3,4], and membrane separation [5,6] have been used for CO2 capture to date. Among

them, because of its easy operation, high capacity, and separation efficiency, CO2 chemical

absorption in aqueous amines is the most widely used so far. However, although the use of these

aqueous amines is effective, there are some obvious drawbacks associated with them. For example,

because of the high volatility of amines, it is difficult to keep most of the amine solutes in the

solvent during the regeneration process; since CO2 is strongly bound to amines in the amine

solution, one needs to boil the solution to reverse the chemical bonding. The amine solution usually

contains 70% water or higher. As a result, uptake of water into the gas stream causes intensive

energy consumption, resulting in increased cost and corrosion problems.

In the past decade, room-temperature ionic liquids (RTILs) have been proposed as alternative or

next-generation CO2 absorption separation media [7-10]. As compared to traditional aqueous

amines, RTILs show some unusual properties, such as extremely low vapor pressure, wide liquid

temperature range, high thermal and chemical stability, and high CO2 solubility [11]. CO2

dissolution in most RTILs is a kind of physical absorption [12-16]. Sudhir et al. [16] measured the

CO2 solubility in 10 different imidazolium-based ionic liquids. They found that the CO2 solubility

in [bmim][Tf2N] reached over 0.7 mole fraction at 60 bar and that the solubility of CO2 was

strongly dependent on the choice of anion but not very dependent on the choice of cation. Cadena et

al. [14] performed both experiments and molecular modeling studies to investigate the mechanism

of high CO2 solubility in imidazolium-based ionic liquids. They too found that anion had the

greatest impact on the solubility of CO2. To further improve the CO2 absorption amount, amino-

functionalized ionic were synthesized [17-20], which were found to be very effective. The molar

uptake of CO2 per mole these ILs could approached 0.5, which is the theoretical maximum for CO2

sequestration as an ammonium carbamate salt. However, although the lack of an appreciable vapor

pressure is a quality associated with ILs, it comes at the expense of high viscosity. The viscosities

of virtually all RTILs are at least an order of magnitude lower than those of common organic

solvents. For these functionalized RTILs, their unreacted states show much higher viscosities

[19,20]. It is known that solvent viscosity is an important consideration for separation applications,

because high viscosity typically correlates with low gas diffusion rates, which negatively impact

both the mass and the heat transfer. Davis’s group [19] showed that their amine-tethered

imidazolium IL took three hours to reach saturation for CO2 absorption. Furthermore, Shannon et

al.[21] stated some ILs will likely remain inherently more expensive to manufacture than

conventional organic solvents because of the cost of starting materials, the number of reaction steps

3

needed, and, depending on the end use, the degree of purification required. In light of this, it is

desirable to develop suitable absorbents that can facilitate the sequestration of CO2 with low

synthesis cost and good flow characteristics and without concurrent loss of the capture agent or

solvent into the gas stream.

Recently, we [22] have proposed a kind of slurry system for CO2 capture, in which zeolitic

imidazolate framework-8 (ZIF-8), 2-methylimidazole, and glycol were mixed to form a flowable

slurry. This mixture was then used for CO2 capture. It was found that because of both the adsorption

effect of ZIF-8 and absorption effect of the glycol solution, the slurry system showed promising

high CO2 selectivity. We also found that the addition of 2-methylimidazole into the ZIF-8/glycol

slurry substantially increased the CO2 absorption amount, although the mechanism of action of 2-

methylimidazole in the slurry is not clear. Based on these preliminary findings, we here propose

using a 2-methylimidazole-glycol solution to perform CO2 capture process. Intuitively, because of

the high boiling points, low volatilities, and low cost of both glycol (boiling point: 197�) and 2-

methylimidazole (boiling point: 268�), the 2-methylimidazole-glycol solution may combine some

advantages of both aqueous amine solutions (e.g. easy to synthesize, good flowability, and high

CO2 absorption amount) and ionic liquids (e.g. low vapor pressure) for CO2 capture. As compared

to the ZIF-8/glycol-2-methylimdiazole slurry, although the adsorption effect of ZIF-8 is absent, the

2-methylimidazole-glycol solution has a lower apparent viscosity.

This paper is organized as follows: First, measurement of the basal physical properties, including

the solubility of 2-methylimidazole in glycol, and the density, pH, and viscosity of the 2-

methylimidazole-glycol solution is presented. Second, measurement of the CO2 solubility in the 2-

methylimidazole-glycol solution, as well as the corresponding CO2 absorption mechanism

determined by using Fourier-transform infrared analysis technology is presented. Third, the CO2

separation efficiency of the 2-methylimidazole-glycol mixture is reported.

2. Experimental

2.1. Experimental apparatus

CO2 capture. Both single CO2 absorption and CO2/N2 gas mixture separation experiments in this

work were performed by using the experimental apparatus as schematically illustrated in Figure 1.

The experimental apparatus was also used in some of our previous works [22,23]. The key parts of

the apparatus are a transparent sapphire cell and a steel-made blind cell, both of which are installed

in an air bath. The effective volume of the sapphire cell is 60 cm3 and that of the blind cell plus

connected tubes is 112 cm3. The maximum working pressures of these two cells are designed to be

4

20 MPa. To directly observe samples in the cell, a luminescence source of type LG100H is mounted

on the outside of the cell. A secondary platinum resistance thermometer (type-pt100) is used as the

temperature sensor. A calibrated Heise pressure gauge and differential pressure transducers are used

to measure the system pressure. The uncertainties of pressure and temperature measurements are ±

0.2 bar and ± 0.1 K, respectively. Real-time reading of the system pressure is recorded by a

computer.

Fig. 1. Schematic diagram of the experimental apparatus: resistance thermocouple detector (RTD), differential

pressure transducer (DPT), and data acquisition system (DAS).

Physical properties determination. The solubility of 2-methylimidazole in glycol was measured

by the static equilibrium method in a thermostatic water bath. The pH was measured using a Phs-3c

pH meter (Shanghai, China), the density with a 10 mL pycnometer, and the viscosity with a

modified Cannon-Ubelohde suspended level capillary viscometers, 0.6 mm in diameter [24].

2.2. Materials

The materials used in this work included 2-methylimidazole, glycol and feed gases. 2-

methylimidazole was purchased from Sigma-Aldrich. Glycol was purchased from Beijing Chemical

Reagents Company, China. Analytical grade carbon dioxide (99.99%) and nitrogen (99.99%) were

purchased from Beijing AP Beifen Gas Industry Company, China. The synthetic CO2/N2

(20.65/79.36 mol%) gas mixture was prepared in our own laboratory. A Hewlett-Packard gas

chromatograph (HP 7890) was used to analyze the composition of the prepared gas mixture.

5

2.3. Experimental procedures for CO2 capture

Before the experiment, the sapphire cell was dismounted from the apparatus, washed with distilled

water and dried, and then loaded with a known quantity (20 mL) of 2-methylimidazole-glycol

solution. After that, the cell was installed back into the apparatus. The system (sapphire cell + blind

cell + tubes connecting two cells) was then purged through vacuuming. The blind cell and the

sapphire cell were then disconnected by closing the top valve on the sapphire cell and a sufficient

amount of feed gas was injected into the blind cell. After that, the air bath was set to the desired

temperature. Once the temperature was reached and the pressure in the blind cell was stabilized, the

pressure was recorded as the initial pressure, P0. The top valve of the sapphire cell was opened

slowly, letting the desired amount of feed gas flow into the sapphire cell from the blind cell. This

valve was then closed and the magnetic stirrer around the sapphire cell was turned on. The pressure

of the residual gas mixture in the blind cell was recorded as P1. With the sorption of the injected gas

by the solution, the system pressure in the sapphire cell decreased gradually. During each

measurement, the pressure in the sapphire cell was recorded as a function of time. When the system

pressure remained constant for at least one hour, it was considered that equilibrium was achieved.

The equilibrium pressure of the sapphire cell was recorded as PE. The magnetic stirrer was turned

off. To measure the single CO2 solubility in the absorption media, the CO2 left in the blind cell was

again released to the sapphire cell to reach a higher desired initial pressure than before, the

magnetic stirrer was turned on again to obtain another CO2 equilibrium absorption pressure, and so

on. For the separation of CO2 in the gas mixture, the equilibrium gas phase needed to be sampled

under constant pressure by use of a hand pump. The obtained gas sample was analyzed by a HP

7890 gas chromatograph. The volume of the absorption liquid in the sapphire cell was calculated by

measuring its height. The inner radius of the sapphire cell is known to be 1.27 cm. The amount of

each gas species absorbed in the measured sample was determined through mass balance as

described below.

2.4. Data processing

The total number of moles of gases (nt) that was injected into the sapphire cell was calculated using

the following equation:

TRZ

VP

TRZ

VPn

××

×−

××

×=

1

t1

0

t0t

(1)

where T is the system temperature, P0 is the initial pressure in the blind cell, P1 is the equilibrium

pressure of the blind cell after injecting gases into the sapphire cell, Vt is the total volume of the

6

blind cell plus tubes connecting to it, and R is the gas constant. Compressibility factors Z0 and Z1

were calculated using the Benedict-Webb-Rubin-Starling equation of state. The total gas amount

(nE) in the equilibrium gas phase of the sapphire cell after absorption equilibrium was determined

using:

TRZ

VPn

××

×=

E

gE

E (2)

where PE is the equilibrium pressure of the sapphire cell and ZE is the compressibility factor

corresponding to T, PE, and the gas composition. Vg is the volume of equilibrium gas phase in the

sapphire cell at the end of each experimental run.

For single CO2 absorption, both the mole percent (x) of CO2 in the liquid phase and the mole ratio

(α) of CO2 to 2-methylimidazole (mIm) were used to indicate the CO2 absorption ability of the 2-

methylimidazole-glycol solution.

2

2 Im

CO

CO m glycol

nx

n n n=

+ +

(3)

'

2

Im

C O

m

n

nα = (4)

'

2 2CO CO E wn n P m H= − × (5)

where nCO2 is the total uptake of CO2 in the solution, nmIm and nglycol are respectively the molar

number of 2-methylimidazole and glycol in the fresh solution, mw (g) is the glycol mass used, and

H [(g bar)/mmol] is the Henry constant of CO2 in glycol, which is equal to 21.6 (g bar)/mmol at

293.15 K, as obtained in our previous work [22].

For separation of the CO2/N2 gas mixture in the 2-methylimidazole-glycol solution, the selectivity

(S) of CO2 over N2 was used to indicate the CO2 separation efficiency of the solution:

1 1

2 2

/

/

x yS

x y= (6)

where 1x and

2x are the mole percents of CO2 and N2, respectively, in the absorbed phase and 1y

and 2y are the mole percents of CO2 and N2, respectively, in the equilibrium gas phase.

Accordingly, the apparent mole percents of CO2 (x1) and N2 (x2) in the equilibrium liquid phase

can be obtained by the following formulas:

21

11

nn

nx

+= (7)

7

21

22

nn

nx

+= (8)

The total uptake of CO2 (n1) and N2 (n2) in solution was calculated as follows:

1E1t1 ynznn ×−×= (9)

2E2t2 ynznn ×−×=

(10)

where z1 and z2 are the mole percent of CO2 and N2, respectively, in the synthetic gas and y1 and y2

are the mole percent of CO2 and N2, respectively, in the equilibrium gas phase.

3. Experimental results and discussion

3.1.Physical properties of 2-methylimidazole-glycol solution

The solubility of 2-methylimidazole in glycol at 293.15, 298.15 and 303.15 K are reported in Table

1. As can be seen in the Table 1, the solubility of 2-methylimidazole in glycol was 35.1 mol% at

293.15 K, and it quickly increased to 42.2 mol% at 303.15 K, demonstrating that 2-methylimidazole

easily dissolved in glycol, especially at high temperatures. The corresponding solubility in weight

percent changed from 41.7 wt% to 49.1 wt% in the studied temperature range.

Table 1

Solubility of 2-methylimidazole in glycol at 293.15, 298.15 and 303.15 K

Solubility T (K)

293.15 298.15 303.15

mole percent (mol%) 35.1 38.8 42.2

weight percent (wt%) 41.7 45.6 49.1

Table 2 summarizes both the density and the pH value of the 2-methylimidazole-glycol solution at

303.15 K and at four different weight percents (mf) of 2-methylimidazole. For the density

measurement, it was found that because the density of solid 2-methylimidazole crystals (1.03 g/cm3)

is lower than that of glycol (1.113 g/cm3), the solution density decreased linearly with increasing 2-

methylimidazole concentration. For example, at mf = 40 wt%, the solution density decreased to

1.077 g/cm3 from the 1.113 g/cm

3 of pure glycol. As concerns pH, the pH of pure glycol is 7.79.

After adding 10 wt% 2-methylimidazole, the pH of the solution sharply increased to 10.12.

However, unlike to the change of density, further increases of mf increased the pH of the solution

only slightly. Higher pH values usually indicate stronger absorption ability of the absorbent for acid

gas components; thus, the results show that the 2-methylimidazole-glycol solution would be more

suitable for CO2 capture than pure glycol.

8

Table 2

Density and pH of 2-methylimidazole-glycol solution versus 2-methylimidazole weight percent (mf) at 303.15 K

and atmospheric pressure

mf

0 10 wt% 20 wt% 30 wt% 40 wt%

density 1.113 1.104 1.095 1.086 1.077

pH 7.79 10.12 10.39 10.71 10.93

As mentioned in the introduction, viscosity is an important property for CO2 absorbents because a

lower viscosity usually correlates to a higher gas-liquid mass transfer rate and lower transmission

cost during circulation in the absorption column. Table 3 presents the experimental viscosity (η) of

the 2-methylimidazole-glycol solution at four temperatures (293.15, 303.15, 313.15 and 323.15 K)

and six 2-methylimidazole percents (mf = 0, 10, 20, 30 and 40 wt%) at atmospheric pressure. The

viscosity of pure glycol at 293.15 K is 19.9 mPa·s, which is the same as that reported by others [25].

Regarding the 2-methylimidazole-glycol solution, the addition of 2-methylimidazole moderately

increased its viscosity, reaching only 27.4 mPa·s at 303.15 K at the maximum mf (40 wt%) studied

here. This viscosity is much lower than the viscosities of most RTILs reported in the literature.7

Furthermore, the viscosity of the solution decreased rapidly with increasing of temperature. As a

result, the solution easily circulated in the common industrial absorption column.

Table 3

Viscosities of 2-methylimidazole-glycol solution versus temperature (T) and 2-methylimidazole weight percent

(mf) in it at atmospheric pressure

T (K) η (mPa·s)

mf = 0 mf =5 wt%

mf =10 wt% mf =20 wt% mf =30 wt% mf =40 wt%

293.15 19.9 47.5

303.15 13.2 14.8 16.1 18.8 22.6 27.4

313.15 9.5 17.6

323.15 6.6 11.4

After that, we performed a series of pure CO2 dissolution and CO2/N2 gas mixture separation

experiments in the 2-methylimidazole-glycol solution to investigate the CO2 capture efficiency of

the solution. The corresponding experimental results are summarized in Tables 4-7 and are shown

in Figures 2-5.

3.2. CO2 solubility in 2-methylimidazole-glycol solution

Table 4 summarizes the solubility data of CO2 in 2-methylimidazole-glycol solutions at 293.15 K

and three weight percents of 2-methylimidazole (10, 30 and 40 wt%). To compare the results with

those of commonly employed aqueous amines, CO2 solubility in a 30 wt% aqueous

9

monoethanolamine (MEA) solution was also measured. The experimental results are presented in

Table 4, as well as in Figure 2. The solubility (x) of CO2 in the 2-methylimidazole-glycol solution

increased with increasing pressure and mf of 2-methylimidazole. For example, when mf increased

from 10 wt% to 30 wt% at 10 bar, the CO2 solubility in the solution increases from ~7.5 mol% to

~11.5 mol%. However, at low pressures (≤ 5 bar), a further increase of mf from 30 wt% to 40 wt%

did not lead to an apparent change of x. We think the reason for this is that when mf reached 30 wt%

or higher, the relatively high viscosity of the solution decreased the dissolution of CO2. In other

words, increasing the number of 2-methylimidazole molecules at both the gas-liquid surface and in

the bulk phase also hindered the dissolution of CO2 in the liquid at these low pressures. Increasing

the pressure further served to offset this inhibition because the solubility of CO2 in the 40 wt% 2-

methylimidazole-glycol solution was higher than that in the 30 wt% 2-methylimidazole-glycol

solution. It is interesting to note that for the 30 wt% 2-methylimidazole-glycol solution at 1.25 bar,

which is only slightly higher than atmospheric pressure, the CO2 solubility in the solution reached

4.59 mol%, which is higher than the CO2 solubility obtained in most RTILs.14

These results suggest

that the 2-methylimidazole-glycol solution may be more suitable for CO2 capture than these ILs14

under some practical conditions with low CO2 partial pressures, such as the capture of CO2 from

flue gas or biogas. However, the CO2 solubility in these ILs increased to the same level or even

higher than that in the 2-methylimidazole-glycol solution at pressures higher than 10 bar. As

compared to the 30 wt% MEA aqueous solution, the solubility of CO2 in the 2-methylimidazole-

glycol solution at low pressures was lower, whereas the CO2 solubility in the 2-methylimidazole-

glycol solution increased with increasing pressure much faster than that in the MEA solution. At

pressures higher than 6 bar, the solubility of CO2 in the 2-methylimidazole-glycol solution was

higher. More importantly, problems such as corrosion and solvent or solute loss during the

regeneration process for absorbents could be substantially weakened or even eliminated when using

the 2-methylimidazole-glycol solution as compared to aqueous amines because of the much lower

volatility of the former.

Table 4

CO2 solubility in both the 2-methylimidazole-glycol and aqueous MEA solutions at 293.15 K with different

weight percents (mf) of 2-methylimidazole and MEA

mf P (bar) x (mol%) α (mol/mol)

2-methylimidazole-glycol solutions 10 wt% 3.30 4.23 0.45

5.33 5.37 0.55

7.20 6.07 0.60

10

9.35 7.11 0.66

11.42 7.73 0.68

13.60 8.34 0.70

15.74 8.95 0.72

18.73 10.19 0.79

30 wt% 1.25 4.59 0.19

3.13 7.51 0.32

4.55 9.13 0.39

6.06 10.23 0.43

8.37 11.88 0.50

10.81 13.26 0.55

14.02 14.78 0.61

18.87 16.00 0.64

40 wt% 2.19 5.93 0.17

3.34 7.78 0.23

5.25 9.82 0.30

6.87 11.40 0.34

8.88 12.82 0.39

10.54 14.01 0.42

12.33 15.05 0.46

Aqueous MEA solution 30 wt% 2.17 9.32 0.81

4.18 10.29 0.89

5.86 10.67 0.92

7.79 11.02 0.94

10.11 11.57 0.98

11.67 11.68 0.99

13.19 11.89 1.00

Table 4 also presents the molar ratios (α) of CO2 to 2-methylimidazole dissolved in the solution at

different pressures and 2-methylimidazole contents. The ratio increases with increasing pressure but

decreased with increasing 2-methylimidazole weight percent in the solution. For mf = 10 wt% at

18.73 bar, the mole ratio between CO2 and 2-methylimidazole reached 0.79, which is much higher

than the theoretical maximum of 0.5 for CO2 sequestration as an ammonium carbamate salt in

amino-functionalized ILs and some aqueous amines [19,26-28]. The high CO2 solubility in the 2-

methylimidazole-glyocl solution should not be attributed to the formation of ammonium carbamate

salt as we guessed before [22]. Instead, it is more likely that the system studied here forms a so-

called “switchable organic liquid” [29-32], in which an alcohol and an amidine or guanidine base,

triggered by CO2, chemically binds with CO2 to form an amidinium or guanidinium alkylcarbonate

salt. Therefore, we propose the possible reaction of CO2 absorption by the 2-methylimidazole-

glycol solution shown in Schematic 1.

11

Schematic 1. Proposed CO2 absorption mechanism in 2-methylimidazole-glycol solution.

In this reaction, a certain 2-methylimidazole alkylcarbonate salt (or ionic liquid) is formed in the

presence of CO2, glycol, and 2-methylimidazole, in which CO2 acts as a “switch” to elicit this

change. It should be noted that the formed 2-methylimidazole alkylcarbonate salt shown in

Schematic 1 differs from common ionic liquids because it is only an ionic liquid after the CO2 is

chemically bound. It is also obvious from Schematic 1 that if enough pressure is applied, the

reaction of CO2 with 2-methylimidazole should terminate at 1:1 stoichiometry. Table 4 shows that

the molar ratio of CO2 to 2-methylimidazole decreases with increasing 2-methylimidazole weight

percent in the solution. The possible reason for this is that the formation of 2-methylimidazole

alkylcarbonate salt increased the viscosity of the solution to a certain extent, resulting in the

reduction of CO2 dissociation in the solution. As a result, a relatively low 2-methylimidazole

concentration may be more suitable for the reaction between 2-methylimidazole, glycol, and CO2.

However, a lower 2-methylimidazole content also means a lower CO2 absorption amount in the

solution, as shown in Table 4. Therefore, taking into account both the efficiency and CO2

absorption capacity, we think that 30 wt% is a suitable concentration for 2-methylimidazole in a 2-

methylimidazole-glycol solution to be used for CO2 capture.

12

0 4 8 12 16 20 24

4

8

12

16

aqueous MEA (30 w%) solution

mIm (10 wt%)-glycol solution

mIm (30 wt%)-glycol solution

mIm (40 wt%)-glycol solution

x (

mol%

)

P (bar)

Fig. 2. Comparison of CO2 solubility (mole percent) in 2-methylimidazole-glycol and aqueous MEA

solutions.

Then, the influence of temperature on the CO2 solubility measurement in the 2-methylimidazole-

glycol solution was studied. In this group of experiments, the weight percent of 2-methylimidazole

in the solution was set to 40 wt%. Generally, as shown in Table 5, the solubility of CO2 in the 2-

methylimidazole-glycol solution decreased with decreasing temperature, which is reasonable

because more CO2 was absorbed by glycol and reacted with glycol and 2-methylimidazole,

converting to the 2-methylimidazole alkylcarbonate salt at low temperature.

Table 5

CO2 solubility in 2-methylimidazole-glycol solution (40 wt% 2-methylimidazole) at three different

temperatures

T (K) P (bar) x (mol%)

293.15 2.19 5.83

3.34 7.58

5.25 9.82

6.87 11.40

8.88 12.82

10.54 14.01

12.33 15.05

303.15 2.23 4.75

3.42 6.18

4.98 7.84

8.00 10.12

13

10.28 11.41

12.55 12.66

15.30 13.97

18.44 15.33

313.15 2.21 3.28

3.77 4.80

5.86 6.41

8.00 7.72

10.30 8.96

13.05 10.29

15.69 11.44

19.26 12.62

In order to investigate the validity of the proposed CO2 absorption mechanism shown by

Schematic 1, we measured the Fourier-transform infrared (FT-IR) spectra for both the neat 2-

methylimidazole-glycol solution and the solution after treatment with CO2. As shown in Figure 3,

upon exposure to CO2, three new peaks at 1239 cm-1

, 1637 cm-1

and 2335 cm-1

emerged for the

CO2-2-methylimidazole-glycol mixture. The peak at 1637 cm-1

is consistent with the formation of a

new C=O group [33]. The new peak at 1239 cm-1

is due to the symmetric stretching vibration of the

-C-O-C- group [34]. The peak at 2335 cm-1

corresponds to the asymmetric and bending modes of

the CO2 molecules [35]. The existence of free CO2 molecules in the solution is attributed to the

physical dissolution of CO2 in glycol that has not reacted with 2-methylimidazole and CO2. These

results suggest that CO2 dissolution in the 2-methylimidazole-glycol solution includes both physical

and chemical absorption processes. Moreover, with the formation of 2-methylimidazole

alkylcarbonate salt, the intensity of the broad band between 3200 and 3400 cm-1

, which corresponds

to the hydrogen bonds formed between glycol molecules decreases because of the reduction in

single glycol content. In short, the FT-IR analysis results are in good agreement with the proposed

CO2 absorption mechanism.

14

4000 3000 2000 1000

1293 cm-1

1639 cm-1

tran

smit

tance

(%

)

wavenumbers (cm-1)

2-methylimidazole + glycol

2-methylimidazole + glycol + CO2

2335 cm-1

Fig. 3. FT-IR spectra of 2-methylimidazole-glycol solution before and after CO2 capture.

3.3. CO2 separation efficiency of 2-methylimidazole-glycol solution

Because CO2 showed high solubility in the 2-methylimidazole-glycol solution, further separation

experiments were carried out for a simulated flue gas (CO2/N2, 20.65/79.35 mol%) using the 2-

methylimidazole-glycol solution. Table 6 shows the separation results for this CO2/N2 gas mixture

in the 30 wt% 2-methylimidazole-glycol solution, where P0 and PE are respectively the initial and

equilibrium pressures in the sapphire cell and PE-CO2 is the partial pressure of CO2 in the equilibrium

gas phase. y1 and x1 are the mole percents of CO2 in the equilibrium gas phase and liquid phase,

respectively. S is the selectivity of CO2 over N2. As can be seen, the selectivity (S) of CO2 over N2

decreases with increasing temperature and pressure. For the experiment at 4.55 bar and 293.15 K,

after a single equilibrium separation stage, the mole percent of CO2 in the gas phase decreased from

20.65 mol% in the feed gas to as low as 2.32 mol%. The CO2 selectivity reached 239, a value that is

much higher than that for most absorbents reported in the literature. More importantly, the CO2

partial pressure in the gas phase was only 0.10 bar, much lower than the partial pressures of CO2 in

the common CO2 containing gas mixtures. Even at a PE of 18.06 bar at 313.15 K, S was still as high

as 112. In short, the high selectivity over a wide pressure range and the low CO2 equilibrium partial

pressures demonstrate that the 2-methylimidazole-glycol solution is suitable for capturing CO2 from

different kinds of feed gases such as flue gas (CO2/N2, ~1 bar), biogas (CO2/CH4, ~1 bar), and

15

integrated gasification combined cycle gas (CO2/H2, 3~5 MPa). At the same time, we think other

imidazole materials containing –NH bond, such as imidazole, 4-methylimidazole, benzimidazole,

and so on, should also be suitable for mixing with glycol to capture CO2 from gas mixtures.

Table 6

CO2/N2 (20.65/79.35mol%) gas mixture separation results in the 30 wt% 2-methylimidazole-glycol solution at

different pressures and temperatures

T (K) P0 (bar) PE (bar) PE-CO2 (bar) y1 (mol%) x1(mol%) S

293.15 5.83 4.55 0.10 2.32 85.08 239

7.71 5.99 0.14 2.37 83.94 216

16.82 13.34 0.41 3.05 84.87 178

303.15 6.72 5.23 0.14 2.68 83.73 186

11.76 9.22 0.27 2.88 83.93 176

16.70 13.48 0.46 3.39 85.23 164

313.15 6.41 5.09 0.18 3.48 85.78 167

11.66 9.25 0.34 3.72 84.23 138

16.64 13.60 0.54 3.96 84.15 128

21.68 18.06 0.89 4.93 85.35 112

Figure 4 shows the kinetics of the CO2/N2 (20.65/79.36 mol%) gas mixture separation processes in

the 30 wt% 2-methylimidazole-glycol solution at 303.15 K. Regardless of the initial pressures (P0)

in the reactor, all these experiments can reached nearly equilibrium pressure within approximately

10 min, showing the high CO2 capture rate of the 2-methylimidazole-glycol solution from gas

mixtures.

16

0 5 10 15 20 25 30

4

8

12

16

P (

bar

)

t (min)

P0=6.72 bar

P0=11.76 bar

P0=16.70 bar

Fig. 4. Kinetics of CO2/N2 gas mixture separation processes in the 30 wt% 2-methylimidazole-glycol

solution at 303.15 K with different initial pressures (P0) in the sapphire cell.

Essential to any CO2 capture media is the energy required for gas release. Here, it is important to

note that all measurements for the CO2/N2 (20.65/79.36 mol%) gas mixture separation equilibrium

listed in Table 6 were performed using the same solution through cycles of sorption/desorption (that

is, vacuum applied at room temperature for desorption). We observed no remarkable loss of the

volume and separation ability of the solution after nine cycles, demonstrating the high stability and

reusability of the 2-mehylimidazole-glycol solution. Figure 5 further presents the morphologies of

the 30 wt% 2-methylimidazole-glycol solution during CO2 capture processes at 303.15 K. Both the

fresh and the CO2 absorbed 2-methylimidazole-glycol solutions are transparent, showing that the 2-

methylimidazole alkylcarbonate salt formed during CO2 capture process was well dissolved in the

glycol. Moreover, it is important to note that although the formation of the 2-methylimidazole

alkylcarbonate salt might have increased the viscosity of the absorbent, after separation equilibrium,

the stirrer at the bottom of the sapphire cell easily started with the up-and-down motion of the

magnetic around the sapphire cell, showing that the equilibrium solution maintained acceptable

flowability. Figure 5c shows the regeneration process of the CO2 captured 2-methylimidazole-

glycol solution.When the pressure in the sapphire cell was decreased to subatmospheric pressure,

the CO2 in the solution desorbed quickly. It was found that with stirring and under vacuum (~0.2

bar), the solution could be regenerated in less than half an hour at room temperature. Meanwhile,

17

based on the CO2 absorption results under different temperatures listed in Table 6, we calculated

that the CO2 absorption enthalpy in the 2-methylimidazole-glycol solution was only approximately

32 kJ mol-1

(Figure 6), which is much lower than that in typical chemical absorption or chemical

adsorption processes [36,37]. This value also shows the easily regeneration phenomenon of the

solution.

(a) (b) (c)

Fig. 5. Morphologies of the 30 wt% 2-methylimidazole-glycol solution during CO2 capture at 293.15 K: (a) fresh

2-methylimidazole-glycol solution, (b) 2-methylimidazole-glycol solution after separation equilibrium for the

CO2/N2 (20.65/79.36 mol%) gas mixture, and (c) regeneration process for CO2 captured 2-methylimidazole-glycol

solution through evacuation.

4 6 8 10 12 14

-40

-35

-30

-25

-20

∆H

(kJ/

mo

l)

n (mol/L)

18

Fig. 6. Absorption enthalpy of CO2 in 2-methylimidazole-glycol solution.

Finally, the effect of the 2-methylimidazole content (mf) on the CO2 separation efficiency of the 2-

methylimidazole-glycol solution was also investigated. As shown in Table 7, a relatively higher mf

corresponds to a lower CO2 partial pressure in the equilibrium gas phase and higher CO2 selectivity

(S). However, it should be noted that a higher 2-methylimidazole content also means higher

viscosity for both the fresh and CO2-captured 2-methylimidazole-glycol solutions. Thus, taking

both the CO2 separation efficiency and flow behavior into consideration, the weight percent of the

2-methylimidazole in the solution should not be more than 30% in practical use.

Table 7

CO2/N2 (20.65/79.35 mol%) gas mixture separation results in different concentration of 2-

methylimidazole-glycol solution at 293.15 K

mf (wt%) P0 (MPa) PE (MPa) PE-CO2 (MPa) y1 (mol%) x1 (mol%) S

6 16.62 14.15 1.32 9.37 80.76 41

15 16.72 13.51 0.59 4.33 86.02 136

30 16.82 13.34 0.41 3.05 84.87 178

40

17.23 13.54 0.39 2.88 84.88 189

5. Conclusions

In this work, we propose to use a 2-methylimidazole-glycol solution for CO2 capture. First, some

basic physical properties, including the solubility of 2-methylimidazole in glycol and the density,

pH, and viscosity of the 2-methylimidazole-glycol solution were measured. It was found that 2-

methylimidazole crystals dissolved well in glycol, with a solubility of 42 mol% at 303.15 K. The

density of the 2-methylimidazole-glycol solution decreased and its pH and viscosity increased with

increasing 2-methylimidazole concentration. Then, the solubility of CO2 in the 2-methylimidazole-

glycol solution was measured. The experimental results showed that the CO2 solubility increased

with increasing pressure and increasing 2-methylimidazole weight percent. For the 10 wt% 2-

methylimidazole-glycol solution at 293.15 K and 18.73 bar, the molar ratio between CO2 and the 2-

methylimidazole molecules reached 0.79 after absorption equilibrium. Based on these results, a CO2

absorption mechanism in the solution was proposed, in which CO2 is bound with 2-

methylimidazole and glycol to form an alkylcarbonate salt. In this reaction, CO2 acts as a “switch”

19

to elicit the reaction. Further confirmation of the proposed CO2 absorption mechanism was made

using FT-IR analysis. Finally, a group of separation experiments for a CO2/N2 (20.65/79.36 mol%)

gas mixture in a 2-methylimidazole-glycol solution were performed to investigate the CO2

separation efficiency of the solution. The equilibrium separation results showed that the maximum

selectivity of CO2 over N2 reached 239 at 293.15 K and 4.55 bar; under these condition, the partial

pressure of CO2 in the gas phase was reduced to as low as 0.10 bar. The most suitable 2-

methylimidazole concentration in 2-methylimidazole-glycol solution was determined to be 30 wt%.

More importantly, CO2-capturing 2-methylimidazole-glycol solutions maintained good flowability

and could be easily regenerated through vacuuming at room temperature. Because both glycol and

2-methylimidazole have high boiling points, low volatilities, and are relatively inexpensive, the 2-

methylimidazole-glycol solution should be well applicable for CO2 capture. Furthermore,

considering the diversities of both imidazoles and alcohols, it is expected that the experimental

results obtained in this work would promote application of imidazole-alcohol mixtures in CO2

capture research.

Acknowledgements

The financial support received from the National Natural Science Foundation of China (21606184,

51374179) is gratefully acknowledged. We thank associate professor Wei Yan from the Department

of Chemistry, Technical University of Denmark, for improving the English of the paper.

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Highlights

1. 2-methylimidazole-glycol solution is proposed to capture CO2;

2. CO2 solubility in 2-methylimidazole-glycol solution was measured;

3. CO2 separation efficiency in 2-methylimidazole-glycol was investigated;

4. CO2 capture mechanism was determined through Fourier-transform infrared technology.

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