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CO2 absorption in mixed aqueous solution of MDEA and cholinium glycinate shengjuan yuan, Yang Zhuhong, Xiaoyan Ji, Yifeng Chen, yunhao sun, and Xiaohua Lu Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 06 Jun 2017 Downloaded from http://pubs.acs.org on June 7, 2017

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Fig. 1. Molecular structures of [Cho][Gly] and MDEA.

Fig. 2. Absorption loading of CO2 in the aqueous [Cho][Gly]-MDEA solutions at 308.2 K. , 30 % MDEA;

■

★

, 25 % MDEA + 5 % IL;

●

, 15 % MDEA + 15 % IL;

◆

, 20 % MDEA + 10 % IL;

▲

, 10 % MDEA + 20 % IL.


■

★

, 25 % MDEA + 5 % IL;

●

, 15 % MDEA + 15 % IL;

◆

, 20 % MDEA + 10 % IL;

▲

, 10 % MDEA + 20 % IL.

ACS Paragon Plus Environment

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■

★

, 25 % MDEA + 5 % IL;

●

, 15 % MDEA + 15 % IL;

◆

, 20 % MDEA + 10 % IL;

▲

, 10 % MDEA + 20 % IL.

Fig. 5. Apparent absorption rate constants of CO2 in the aqueous solutions with 30 wt % ([Cho][Gly]-MDEA). ■, 308.2 K;

, 318.2 K; ▲, 328.2 K.

●

Fig. 6. Temperature effect on the absorption loading of CO2 in aqueous solution with (10 wt % [Cho][Gly] + 20 wt % MDEA). ■, 308.2 K; ●, 318.2 K; ▲, 328.2 K.


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b

c

d


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e

Fig. 7. 13C NMR spectra of the aqueous solutions. (a) MDEA + H2O; (b) MDEA + H2O + CO2; (c) [Cho][Gly] + MDEA + H2O; (d) [Cho][Gly] + MDEA + H2O + CO2; (d) [Cho][Gly] + MDEA + H2O after the fourth regeneration.

Fig. 8. Mechanism of CO2 absorption into aqueous [Cho][Gly]-MDEA solution.

Fig. 9. Absorption loading of CO2 in the aqueous solution with (10 wt % [Cho][Gly] + 20 wt % MDEA) at 308.2 K.


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Fig. 10. Regeneration efficiency of the aqueous solution with (10 wt % [Cho][Gly] + 20 wt % MDEA).

Fig. 11. CO2 absorption performance of the four absorbents at 308.2 K and 400 kPa.


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CO2 absorption in mixed aqueous solution of MDEA and cholinium glycinate Shengjuan Yuana, Zhuhong Yanga,*, Xiaoyan Jib, Yifeng Chena,b, Yunhao Suna,b, Xiaohua Lua a

Key Laboratory of Material and Chemical Engineering, Nanjing Tech University,

Nanjing 210009, China b

Energy Engineering, Division of Energy Science, Lulea University of Technology,

97187, Lulea, Sweden 1

Corresponding author. E-mail: [email protected]; Tel./fax: +86 25 83172287

Abstract A new mixed solvent system that consists of cholinium glycinate ([Cho][Gly]) and aqueous N-methyldiethanolamine (MDEA) solution was developed in this work to serve as CO2 absorbent. The equilibrium absorption was carried out to investigate the effect of solution composition, pressure and temperature on CO2 absorption performance. The effect of CO2 absorption on the viscosity of the aqueous solutions was studied, and the regeneration efficiency of the aqueous solutions was also investigated. The results showed that the CO2 absorption loading decreased with increasing [Cho][Gly] concentration and temperature, and the absorption loading strongly depended on CO2 partial pressure. The reactivity of MDEA was significantly enhanced with the addition of [Cho][Gly]. The aqueous solution with (10 wt % [Cho][Gly] + 20 wt % MDEA) showed an optimal CO2 absorption and high regeneration efficiency. Furthermore, the CO2 absorption mechanism in the aqueous [Cho][Gly]-MDEA solution was explored by

13

C Nuclear Magnetic Resonance

(NMR), which indicated that the CO2 absorption in the aqueous [Cho][Gly]-MDEA solution was zwitterion mechanism.

Keyword: Carbon dioxide, MDEA, Amino acid ionic liquid, Absorption 1. Introduction Carbon dioxide (CO2) is one of the major contributors to the global warming, and the reduction of CO2 emission has become a hot issue1. CO2 capture technologies


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are in an urgent demand2. So far, technologies such as membrane separation, cryogenic separation, adsorption and absorption have been developed3. Among them, chemical absorption is the most widely used CO2 capture technology. The chemical materials used to absorb CO2 are mainly from five groups: alkanolamines, alkali metal carbonates, ammonia, amino acid salts and ionic liquids4. These chemicals are used in the form of aqueous solutions for CO2 capture. The aqueous solutions of amine,

such

as

monoethanolamine

(MEA),

diethanolamine

(DEA)

and

N-methyldiethanolamine (MDEA), or their mixtures are practically used absorbents5-7. It is worth noting that among the widely used amines, MDEA acquires the advantages of high CO2 loading (up to 1.0 mol CO2/mol MDEA), low regeneration cost and low corrosivity, but the absorption rate is low8. Therefore, in the industrial applications, the activators with fast reactivity such as MEA and DEA are added into the MDEA solution to overcome the challenge of slow absorption9. The mixture of amines enables both the high absorption rate (activators) and low regeneration energy (MDEA). However, the volatility of amine solutions leads to some other challenges such as environmental problems and equipment corrosion as well as high energy consumption during solvent regeneration10, it still needs to develop a cost-effective and environmental-friendly method for CO2 capture. Ionic liquids (ILs), the liquid organic salts composed of cation and anion, are considered as green solvents for CO2 absorption11. Compared to amines, the overwhelming advantages of ILs are negligible volatility and excellent thermal stability that greatly prevents the loss of absorbents in acidic gases removal. It was reported that the CO2 absorption loading can be enhanced by tuning the molecular structure of cation or anion of ILs12-14. Based on the reactive absorption mechanism between CO2 and amines, some special functional groups, such as amino groups and amino acid (AA), can be introduced to the anion or the cation of ILs. Amino acid anion has been demonstrated to improve the CO2 absorption loading of the ILs15. However, pure amino acid ILs mostly show very high viscosity and are not conducive to the actual absorption, making it still infeasible to use such kind of ILs as CO2 absorbent in the industrial applications.


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The required properties of practically useful CO2 absorbents are high CO2 absorption loading, fast reaction rate, excellent thermal stability and low environmental impact16-18. In 2008, Camper et al. proposed that mixing ILs with amines is an efficient and effective manner for CO2 gas capture, and more research is needed before IL-amine mixed solution can be viable replacements for aqueous amines for CO2 capture in continuous processes19. This mixing solvent has two major advantages: (1) synergistic combination of high absorption loading, fast reaction rate and excellent thermal stability from two individual components; (2) reduced energy consumption during regeneration. In recent years, CO2 absorption in the mixed amine and IL solutions has been widely investigated. Ahmaday et al. showed that the presence of the functionalized IL increased the initial rate of CO2 absorption in MDEA solution under certain conditions20. Enhanced CO2 absorption rate has also been observed in the aqueous solution

of

tetramethylammonium

1-butyl-3-methylimidazolium

glycinate

glycinate

([N1111][Gly])-MDEA21-24,

([Bmim][Gly])-MDEA25

and

1-butyl-3-methylimidazolium lysinate ([Bmim][Lys])-MDEA mixtures26. Not only the absorption rate, but the absorption loading can be simultaneously achieved by tuning the fraction of mixture components. For example, Fu et al.25 showed that when the mass fraction of [Bmim][Gly] ranged from 10 to 15 wt % and the total mass fraction of MDEA and [Bmim][Gly] was around 50 wt %, high absorption rate, large absorption loading and appropriate viscosity can be simultaneously achieved. Gao et al. showed that the regeneration efficiencies of the [N1111][Gly] activated MDEA solutions were all higher than 90 % at 378 K, and most of the gas was released before the boiling point of solutions27. Zhou et al. found that the reactivity of 2-amino-2-methyl-1-propanol (AMP) was significantly enhanced with the addition of 1-hexyl-3-methylimidazolium glycinate ([Hmim][Gly]) and the mixed solution can be regenerated efficiently after three cycles28. The selection of suitable ILs in the mixed solution is another challenge. Meanwhile, it is also desirable to synthesize ILs with renewable materials according to green chemistry purpose29. Traditional imidazolium- and pyridinium-based ILs has


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seen poor biodegradability and biocompatibility30. It is generally accepted that several molecular features strongly enhance biodegradability, such as the presence of esters, amides, hydroxyl, aldehyde, carboxylic acid groups, or linear alkyl chains31. The amino acids have low environmental impact, high biodegradability and high resistance to oxidative degradation32. Choline (Cho), an essential nutrient, is known to be non-toxic and biodegradable33. Most of the cholinium-based ILs was reported to be readily biodegradable and low toxicity34. Han et al. demonstrated that both cholinium prolinate ([Cho][Pro]) and [Cho][Pro]/polyethylene glycol (PEG200) mixture can capture CO2 effectively and be easily regenerated under vacuum or by bubbling N2 through the solution35. In our previous work, the [Cho][Pro]/PEG200 mixture (mass ratio = 1 : 2) showed a good selectivity for both CO2/CH4 and CO2/N2 mixtures36. Therefore, the cholinium-based amino acid IL solution is suggested as a promising candidate for CO2 separation. However, studies on the CO2 absorption with mixed green ILs and amine solutions has been rarely studied yet. The objective of this work is to study the CO2 absorption performance in the aqueous solution of [Cho][Gly]-MDEA. Green IL [Cho][Gly] was synthesized and then mixed with aqueous MDEA solution. CO2 absorption performance in the aqueous [Cho][Gly]-MDEA solutions was optimized by varying IL mass fraction from 0 to 20 wt %. Subsequently, regeneration efficiency was also evaluated on the optimized composition solution. The experimentally determined CO2 absorption loading in this work was correlated with CO2 partial pressure and these results were further compared with other absorbents. Meanwhile, the effect of CO2 absorption on solution viscosity was investigated. The CO2 absorption mechanism was explored as well based on the study of 13C NMR spectrum analysis. 2. Experimental 2.1. Materials Choline hydroxide solution (45 wt % in methanol) and MDEA were supplied by ALDRICH. Glycine (99 wt %) was produced by Chinese National Medicine Corporation. The purified water was made in our laboratory through reverse osmosis


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membrane. CO2 (99.99 vol %) was obtained from Nanjing Tianhong gas factory. 2.2. Absorbent preparation The molecular structures of [Cho][Gly] and MDEA are shown in Fig. 1. [Cho][Gly] was synthesized following previous report37, and the total yield was more than 95 %. [Cho][Gly] was vacuum dried at 343 K for 48 h before use. The water content was measured to be 0.14 wt % by using Automatic Karl Fischer Moisture Analyzer (Shanghai PEIOU V100). The structure of the synthesized [Cho][Gly] was identified by 1H NMR (Bruker AV-300). δ (ppm)：2.78 (2H, S, CH2NH2), 3.14 (9H, s, (CH3)3N), 3.42 - 3.45 (2H, m, CH2OH), 3.80 - 3.83 (2H, m, CH2CH2). These results were consistent with the previous literature38. The thermal stability of [Cho][Gly] was measured by TG (Netzsch TG209 F3). The decomposition temperature (Td) of [Cho][Gly] was measured as 448 K. The aqueous [Cho][Gly]-MDEA solutions were prepared with total 30 wt % amines (amino acid IL + MDEA) with different mass fractions of amino acid IL (0, 5, 10, 15 and 20 wt %). The total mass fraction of 30 wt % ([Cho][Gly]-MDEA) was chosen because this concentration is close to that used in the industrial processes.

Fig. 1. Molecular structures of [Cho][Gly] and MDEA.

2.3. Experimental procedure The experimental apparatus for measuring the CO2 absorption was described in our previous work36. Specifically, the volumes of the absorption tank and the gas reservoir are 53.0 and 488.6 mL, respectively. The accuracy of temperature is 0.1 K and the precision of the pressure sensors is 0.075 %. The CO2 absorption experiment was operated in following producer: firstly, the water bath was heated to the set temperature and maintained the temperature for one hour. The mixed aqueous solution (4 mL) was added into the absorption tank, and


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then the whole device was degassed by vacuum pump to remove the dissolved gas. A certain amount of pure gas was pumped into the absorption tank, and the gradually decreased pressure was recorded at different time. Once the pressure can be stabilized for 1 h, it was considered to reach the absorption equilibrium. The initial and final pressures of CO2 in the absorption tank were used to calculate the amount of absorbed CO2 in the aqueous [Cho][Gly]-MDEA solutions. The absorbed CO2 (mol CO2) by the aqueous solution was calculated by:

P0CO2 - PV V A -VL PeCO2 - PV VA - VL nCO2 = Z1 RT Z2 RT

(1)

where P0CO2 and PeCO2 are the initial and equilibrium pressures, respectively. Pv is the saturated vapor pressure of the solution. Z1 and Z2 are the compressibility factors corresponding to the initial and equilibrium states, respectively, and VA and VL represent volumes of the absorption bank and solutions, respectively. The absorption loading α (mol CO2/mol amines) is expressed as mole CO2 of mol [Cho][Gly]-MDEA with the expression: nCO2 α= nMDEA + nIL

(2)

According to the Damping-Film Theory39 and the equation of state22, the relationship between the mole of the gas versus the time for an isothermal gas absorption can be described as following equation:

ln

n0CO2 - neCO2 = Kt nCO2 - neCO2

(3)

where nCO2 represents the mole of CO2 at time t. n0CO2 and neCO2 are denoted to the moles of CO2 at the beginning and at the absorption equilibrium, respectively. K stands for the apparent absorption rate constant, and t is the time in minute. The regeneration was performed at 383.2 K and atmospheric pressure for 150 min and the evaporated water was condensed by a condenser. An appropriate amount of water was added to maintain the same water content in the solvent since a small amount of water was evaporated during the regeneration process. The regeneration efficiency can be expressed as following:


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η=

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ni n0

(4)

where n0 is the amount of CO2 absorbed by the fresh solution, and ni is the amount of CO2 absorbed after certain times (i) of regeneration. The viscosity of the aqueous [Cho][Gly]-MDEA solutions (1 mL) before and after CO2 absorption was measured by Brookfield Viscometer (DV-Ⅱ+ Pro) with an accuracy of 0.01 mPa·s. To gain an insight into the CO2 absorption mechanism, the solvents before and after CO2 absorption were characterized by

13

C NMR (Bruker AV-500), using an

internal standard of D2O for the deuterium lock. 3. Results and discussion 3.1. Viscosity of the various solutions The viscosity of the solution is an important parameter for modeling mass transfer rate because it significantly affects the liquid film mass transfer coefficient. Before measuring the viscosity of the aqueous [Cho][Gly]-MDEA solutions before and after CO2 absorption, the viscosities of the pure MDEA and the aqueous solution with 30 wt % MDEA at 313.2 K and 323.2 K was measured to calibrate the accuracy of the equipment. As shown in Table 1, the viscosities of the pure MDEA and the aqueous solutions with 30 wt % MDEA measured in this work and those taken from the literatures26,

40-41

are in good agreement, which implies that the experimental

measurements in this work are reliable. Table 1. Viscosity (µ) of pure MDEA and the aqueous solution with 30 wt % MDEA. wtMDEA (%)

μ (mPa·s)

313.2 K

323.2 K

exp.

lit.

exp.

lit.

100

34.7

34.8440

22.3

22.4740

30

1.96

1.9441

1.56

1.5426


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The viscosities of the aqueous [Cho][Gly]-MDEA solutions before and after CO2 absorption were determined in this work at temperatures from 308.2 to 328.2 K. The measured results are listed in Tables 2 and 3. It can be found that the viscosity of pure MDEA and pure [Cho][Gly] was much higher than that of the other aqueous [Cho][Gly]-MDEA solutions. It is obvious that the viscosity of the aqueous [Cho][Gly]-MDEA solution decreased with increasing temperature and [Cho][Gly] concentration. Moreover, the viscosity of the aqueous [Cho][Gly]-MDEA solutions increased after CO2 absorption. Since viscosity is one of the most important factors that determine the transfer property of CO2 absorption and low viscosity is often required

in

practical applications.

Therefore,

the

lower

viscous

aqueous

[Cho][Gly]-MDEA solutions are more favorable for industrial CO2 absorption. Table 2. Viscosity (µ) of the dry/aqueous [Cho][Gly]-MDEA solutions. Concentration

MDEA

100

30

25

20

15

10

0

(wt %)

IL

0

0

5

10

15

20

100

T (K)

μ (mPa·s)

308.2

43.9

2.32

2.27

2.20

2.06

1.97

84.3

313.2

34.6

1.96

1.95

1.87

1.86

1.78

50.2

318.2

27.4

1.81

1.75

1.70

1.64

1.59

29.4

323.2

22.1

1.56

1.49

1.44

1.41

1.37

19.9

328.2

14.7

1.42

1.41

1.38

1.36

1.31

14.3

Table 3. Viscosity (µ) of the aqueous [Cho][Gly]-MDEA solutions after CO2 absorption. Concentration

318.2 K

308.2 K

328.2 K

(wt %) α MDEA

IL

(mol

μ

CO2/mol

α

(mol

CO2/mol (mPa·s)

amines) 30

0

0.926

μ

(mol

0.892

μ

CO2/mol (mPa·s)

amines) 2.82

α

(mPa·s) amines)

2.39


0.786

2.23

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25

5

0.867

2.77

0.851

2.32

0.745

2.17

20

10

0.840

2.62

0.809

2.22

0.728

2.06

15

15

0.812

2.52

0.778

2.14

0.717

1.98

10

20

0.761

2.43

0.746

1.95

0.687

1.86

3.2. CO2 absorption in aqueous [Cho][Gly]-MDEA solutions 3.2.1 CO2 absorption loading The CO2 absorption was measured in the aqueous [Cho][Gly]-MDEA solutions of different [Cho][Gly] concentration at various pressure conditions. The total mass fraction of [Cho][Gly] and MDEA was kept at 30 wt %. Based on the amount of the CO2 absorbed in the solutions, the CO2 absorption loading could be obtained. The results are summarized in Table 4 and plotted in Figs. 2 - 4. In general, CO2 absorption loading increased with increasing pressure and decreasing concentration of [Cho][Gly]. For example, the absorption loading increased from 0.489 (29.16 kPa) to 0.926 mol CO2/mol amines (792.95 kPa) in the aqueous solution with 30 wt % MDEA at 308.2 K. The absorption loading decreased from 0.695 (229.39 kPa) to 0.629 mol CO2/mol amines (225.30 kPa) when the [Cho][Gly] concentration increased from 5 to 15 wt % at 308.2 K. The aqueous solution with 30 wt % MDEA showed the highest absorption loading under the same temperature and pressure condition. The absorption loading data was fitted by CO2 partial pressure as presented in Eq. (5)42:

lnPCO2 = Aln α2 + B ln α+ C

(5)

where PCO2 is CO2 partial pressure, α is CO2 absorption loading, A, B and C are the quadratic fitting regression parameters as presented in Table 5.


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■

●

, 25 % MDEA + 5 % IL;

, 15 % MDEA + 15 % IL;

★

▲

, 20 % MDEA + 10 % IL;

, 10 % MDEA + 20 % IL.

◆


■

●

, 25 % MDEA + 5 % IL;

, 15 % MDEA + 15 % IL;

★

▲

, 20 % MDEA + 10 % IL;

, 10 % MDEA + 20 % IL.

◆


■

●

, 25 % MDEA + 5 % IL;

▲

, 20 % MDEA + 10 % IL;


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, 15 % MDEA + 15 % IL;

★

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, 10 % MDEA + 20 % IL.

◆

Table 4. Absorption loading of CO2 in the [Cho][Gly]-MDEA solutions (wtIL, mass fraction). PCO2

α(mol

PCO2

α(mol

PCO2

α(mol

PCO2

α(mol

PCO2

α(mol

(kPa)

CO2/mol

(kPa)

CO2/mol

(kPa)

CO2/mol

(kPa)

CO2/mol

(kPa)

CO2/mol

amines)

amines)

amines)

amines)

amines)

308.2 K wtIL=0 %

wtIL=5 %

wtIL=10 %

wtIL=15 %

wtIL=20 %

29.16

0.489

34.35

0.496

39.04

0.517

39.75

0.533

44.21

0.509

83.43

0.634

98.07

0.627

117.95

0.611

123.34

0.608

129.33

0.587

201.90

0.724

177.20

0.679

274.15

0.677

225.30

0.629

204.13

0.610

317.08

0.748

229.39

0.695

371.45

0.699

330.26

0.664

376.87

0.642

393.98

0.775

410.49

0.744

428.71

0.723

429.07

0.705

434.64

0.674

591.04

0.818

628.56

0.796

645.46

0.775

641.74

0.737

636.92

0.708

792.95

0.926

858.27

0.867

858.52

0.840

855.92

0.812

836.78

0.761

318.2 K wtIL=0 %

wtIL=5 %

wtIL=10 %

wtIL=15 %

wtIL=20 %

36.59

0.443

34.14

0.450

40.68

0.480

51.27

0.488

53.31

0.476

94.93

0.582

109.00

0.568

120.36

0.565

127.98

0.561

134.30

0.545

154.30

0.641

214.35

0.652

209.34

0.622

195.08

0.594

218.25

0.588

225.14

0.689

328.03

0.695

275.65

0.648

323.76

0.652

341.95

0.625

425.55

0.743

447.84

0.726

430.75

0.695

458.79

0.680

440.58

0.657

601.63

0.808

581.41

0.767

646.59

0.752

636.75

0.728

605.71

0.701

827.68

0.892

794.00

0.851

855.55

0.809

816.17

0.778

836.78

0.746

328.2 K wtIL=0 %

wtIL=5 %

wtIL=10 %

wtIL=15 %

wtIL=20 %

48.93

0.397

37.15

0.368

59.44

0.432

54.30

0.412

72.44

0.440

94.95

0.499

94.73

0.487

119.81

0.511

123.34

0.501

142.47

0.494

162.34

0.579

156.77

0.550

185.75

0.560

182.54

0.545

200.97

0.523

268.21

0.653

263.94

0.615

272.06

0.601

274.15

0.579

292.54

0.559


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Energy & Fuels

382.90

0.695

398.61

0.657

394.53

0.641

371.11

0.612

415.51

0.595

542.56

0.738

564.96

0.699

549.30

0.687

549.62

0.671

573.95

0.641

760.75

0.786

793.69

0.745

729.71

0.728

758.40

0.717

789.79

0.687

Table 5. Coefficients of Eq. (5), the absorption loading as a function of pressure. (wIL, mass fraction). T (K)

wIL (%)

A

B

C

308.2

0

0.5466

6.0382

14.2408

5

3.2047

8.5627

14.9879

10

-3.3995

3.8251

14.5167

15

-10.1327

-1.0162

13.9224

20

-6.1780

1.7587

14.6464

0

1.5589

6.1629

14.4472

5

-0.5549

4.5991

14.4357

10

-2.3004

3.7532

14.5967

15

-2.9565

3.1994

14.6422

20

-2.9671

3.0942

14.8037

0

2.7936

7.25604

15.1270

5

2.52296

7.6294

15.6230

10

1.35297

6.4492

15.4391

15

0.8258

5.8623

15.4292

20

-2.2608

2.6962

14.9159

318.2

328.2

3.2.2. Apparent absorption rate constant The CO2 absorption rate in the aqueous [Cho][Gly]-MDEA solutions was measured by monitoring the pressure with absorption time. Based on which, the apparent absorption rate constant K was then calculated with Eq. (3) and the results are depicted in Fig. 5. The absorption rate constants increased first and then decreased with the increase of [Cho][Gly] concentration. Obviously, the absorption rate constants of aqueous [Cho][Gly]-MDEA solutions were all higher than those of aqueous MDEA solutions. By adding [Cho][Gly] into aqueous MDEA solutions, the


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anion of [Cho][Gly] chemically reacted with CO2 that leads to the increased absorption rate. Among all the studied solutions, the aqueous solution with (10 wt % [Cho][Gly] + 20 wt % MDEA) showed the highest absorption rate. In the aqueous solution with (10 wt % [Cho][Gly] + 20 wt % MDEA), all the amino acid anions in the solution can form zwitterion that maximizes the proton transfer to MDEA. This well explained the highest absorption rate in this solution. Further increasing [Cho][Gly] concentration resulted in partial participation of ILs in the deprotonation process and thus decreased the absorption rate.

Fig. 5. Apparent absorption rate constants of CO2 in the aqueous solutions with 30 wt % ([Cho][Gly]-MDEA). ■, 308.2 K;

, 318.2 K; ▲, 328.2 K.

●

3.2.3. Effect of temperature on CO2 absorption The CO2 absorption in the optimized aqueous solution with (10 wt % [Cho][Gly] + 20 wt % MDEA) was measured at different temperatures. The absorption loading and apparent absorption rate constants were then obtained. The CO2 absorption loading is shown in Fig. 6. Typically, the absorption loading decreased with increasing temperature. For example, absorption loading decreased from 0.611 to 0.511 mol CO2/mol amines when the temperature increased from 308.2 to 328.2 K at 117.95 kPa. Larger rate constant K is also observed at higher temperature. Fig. 5 showed that the

K increased with increasing temperature. The higher K value is favorable since it is beneficial to the gas diffusion. The K value in the aqueous solution with (10 wt % [Cho][Gly] + 20 wt % MDEA) goes up to 0.171 min-1 at 328.2 K, which is two times higher than that at 308.2 K.


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Fig. 6. Temperature effect on the absorption loading of CO2 in aqueous solution with (10 wt % [Cho][Gly] + 20 wt % MDEA). ■, 308.2 K;

, 318.2 K; ▲, 328.2 K.

●

3.3. Absorption mechanism To explore the CO2 absorption mechanism in the aqueous solution,

13

C NMR

spectrum of the aqueous solutions were characterized before and after CO2 absorption, and the results are shown in Fig. 7. In Fig. 7 (b), the new peak observed at 160 ppm -

2-

was attributed to HCO3 or CO3 after absorbing CO243-44. Therefore, the reaction mechanism between the tertiary amine MDEA and CO2 was proposed as follow: MDEA + CO2 + H2 O ↔ MDEAH+ + HCO-3

(6)

This is a base-catalyzed CO2 hydrolysis reaction and MDEA does not combine with CO2 directly8. Therefore, the CO2 absorption rate in the MDEA solution is very slow, though the absorption loading can be up to 1.0 mol CO2/mol MDEA. The peak at 164.0 and 160.1 ppm in Fig. 7 (d) was attributed to the formation of -

2-

the carbamate carboxyl carbon and HCO3 or CO3

species45, respectively.

Moreover, the carbonyl carbon of glycinate shifted from 180.7 to 172.5 ppm after absorbing CO2. In the aqueous [Cho][Gly]-MDEA solution, [Cho][Gly] completely dissociated into hydrated cation and [H2N-CH2-COO]-. Based on the above analysis, the CO2 absorption in the aqueous [Cho][Gly]-MDEA solution was similar to that of the primary amine46-47. That is, CO2 reacted with [H2N-CH2-COO]- to form a zwitterion and subsequently deprotonated by bases (described as B, including RNH2, H2O, OH- and MDEA) in the solutions. RNH2 + CO2 → RHN+2 COO-


7

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RNH+2 COO- + B → RNHCOO- + BH+

8

where R represented -CH2COO-. In the aqueous [Cho][Gly]-MDEA solution, fast reaction between [Cho][Gly] and CO2 could be expected to form zwitterion that transferred protons to MDEA. This is the major reason for the greatly enhanced absorption rate compared to the aqueous MDEA solution. From Fig. 7 (d), CO2 existed as carbamate, bicarbonate and carbonic acid in the aqueous [Cho][Gly]-MDEA solution, and thus the equilibrium reactions in the liquid phase were suggested as follow:

RNHCOO- + H2 O → RNH2 + HCO-3

(9)

RNH+3 → RNH2 + H+

MDEAH+ →MDEA + H+

(10)

CO2 + H2 O → H+ + HCO-3

H2 O → H+ + OH HCO-3 → H+ + CO23

11 (12)

(13)

(14)

The mechanism of the whole CO2 absorption into the aqueous solution with (10 wt % [Cho][Gly] + 20 wt % MDEA) is shown in Fig. 8. The mechanism of the aqueous [Cho][Gly]-MDEA solution in this work was consistent with the previous literature48, which stated that the amino or amino acid groups were found to react with CO2 according to zwitterion mechanism.

a


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b

c

d

e

Fig. 7. 13C NMR spectra of the aqueous solutions. (a) MDEA + H2O; (b) MDEA + H2O + CO2; (c) [Cho][Gly] + MDEA + H2O; (d) [Cho][Gly] + MDEA + H2O + CO2; (e) [Cho][Gly] + MDEA +


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H2O after the fourth regeneration.

Fig. 8. Mechanism of CO2 absorption into aqueous [Cho][Gly]-MDEA solution.

3.4. Regeneration study Besides the absorption characteristics, the regeneration performance is also an important criterion for CO2 absorbent selection, especially with respect to the energy penalty in the regenerator. The regeneration performance of the aqueous solution with (10 wt % [Cho][Gly] + 20 wt % MDEA) was investigated at atmospheric pressure and temperature of 383.2 K. The absorption loading after regeneration and the regeneration efficiency of the solution are shown in Figs. 9 and 10. With the increase of regeneration cycles, both absorption loading and regeneration efficiency decreased, while the regeneration efficiency remained above 95 %. Anderson et al. [10] reported a much smaller regeneration efficiency of 82 % in the aqueous solution with 30 wt % -

2-

MEA at high temperature of 393 K. Moreover, the resonances of HCO3 or CO3 in 13

C NMR spectra was not disappeared in Fig. 7 (e), which suggested that the

absorbent had not been regenerated completely. This explained the decrease of the regeneration efficiency after the regeneration experiments (four cycles).


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Fig. 9. Absorption loading of CO2 in the aqueous solution with (10 wt % [Cho][Gly] + 20 wt % MDEA) at 308.2 K.

Fig. 10. Regeneration efficiency of the aqueous solution with (10 wt % [Cho][Gly] + 20 wt % MDEA).

3.5. Comparison with other CO2 absorbents To further evaluate the commercial potential of the aqueous [Cho][Gly]-MDEA mixed solution, a few commercial CO2 absorbents, such as 30 wt % MEA, 30 wt % MDEA, and (15 wt % MEA + 15 wt % MDEA) solutions, were chosen to make a comparison. The absorption loading and the apparent absorption rate constant of these four absorbents at 308.2 K and 400 kPa are shown in Fig. 11. The aqueous solution with 30 wt % MEA has the highest absorption rate due to its strong alkaline nature. The aqueous solution with 30 wt % MDEA gives the highest absorption loading and lowest absorption rate. The aqueous solution with (15 wt % MEA + 15 wt % MDEA) shows compromised performance in both absorption loading and absorption rate. For the aqueous solution with (10 wt % [Cho][Gly] + 20 wt % MDEA), the absorption


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loading was much higher than the aqueous solution with (15 wt % MEA + 15 wt % MDEA), while its apparent absorption rate constant was slightly lower. Considering the green nature of [Cho][Gly] ILs and its high regeneration efficiency, the aqueous solution with (10 wt % [Cho][Gly] + 20 wt % MDEA) can be viewed as a promising absorbent for CO2 absorption and separation.

Fig. 11. CO2 absorption performance of the four absorbents at 308.2 K and 400 kPa.

4. Conclusions In this work, we studied the excellent CO2 absorption performance in a mixed solvent that contains optimized ratio of amino acid IL and amine solution. The unique properties of low viscosity, large absorption loading and high CO2 absorption rate were simultaneously achieved in the aqueous IL-MDEA mixture. It had been found that adding amino acid IL greatly reinforced the CO2 absorption rate in the aqueous MDEA solution, and aqueous solution with (10 wt % [Cho][Gly] + 20 wt % MDEA) had higher absorption rate than other aqueous [Cho][Gly]-MDEA solutions of total 30 wt % amines. Besides, over 95 % regeneration efficiency was achieved in the aqueous solution with (10 wt % [Cho][Gly] + 20 wt % MDEA). Zwitterion mechanism was explored by

13

C NMR spectrum between CO2 and aqueous [Cho][Gly]-MDEA

solution.

Nomenclature nCO2

the mole of CO2 absorbed by aqueous IL-MDEA solution (mol)

nIL

the mole of IL in the aqueous IL-MDEA solution (mol)


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nMDEA

the mole of MDEA in the aqueous IL-MDEA solution (mol)

P0CO2

the initial pressure of absorption bank (kPa)

PeCO2

equilibrium pressure of CO2 (kPa)

Pv

the saturated vapor pressure of the solution (kPa)

Z

compressibility factor

VA

the volume of the absorption bank (mL)

VL

the volume of the solution (mL)

T

the temperature in Kelvin (K)

R

gas constant (J mol-1 K-1)

α

the CO2 absorption loading (mol CO2/mol amines)

A, B, C

the parameters of quadratic fitting regression

n0CO2

the mole of CO2 in the absorption bank at initial pressure (mol)

neCO2

the mole of CO2 in the absorption bank at equilibrium pressure (mol)

K

the apparent absorption rate constant (min-1)

t

time (min)

μ

the viscosity (mPa·s)

n0

the mole of CO2 absorbed by the fresh solution (mol)

ni

the mole of CO2 absorbed by the solution after certain times (i) of regeneration (mol)

ƞ

regeneration efficiency

wt

mass fraction

Acknowledgements We would like to thank the National Basic Research Program of China (973 Program) (No.2013CB733500), National Natural Science Foundation of China (21136001, 21136004, 21476106, 21428601), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Jiangsu Natural Science Foundation (BK20130062). X. J. thanks Swedish Energy Agency for financial support.


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