CO2 Selectivity by Ionic Liquids in Natural Gas

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CO2/CH4 and H2S/CO2 Selectivity by Ionic Liquids in Natural Gas Sweetening Lanyun Wang, Yongliang Xu, Zhendong Li, Ya-nan Wei, and Jianping Wei Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02852 • Publication Date (Web): 29 Nov 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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CO2/CH4 and H2S/CO2 Selectivity by Ionic Liquids in Natural Gas Sweetening Lan-yun Wang a,b,c, Yong-liang Xu a,b,c *, Zhen-dong Li a, Ya-nan Wei a, Jian-ping Wei a,b,c a. College of Safety Science and Engineering, Henan Polytechnic University, Jiaozuo 454003, China; b. Key Laboratory of Henan Province for Gas & Fire Prevention in Coalmines, Jiaozuo 454003, China; c. Collaborative Innovation Center of Coal Safety Production of Henan Province, Jiaozuo 454003, China

Abstract: CO2 and H2S in natural gas usually leads to low caloric value and corrosion of transportation pipeline, and hence separating CO2/CH4, H2S/CH4 and H2S/CO2 is essential for natural gas purification, desulfurization, as well as gas regeneration and reutilization. Trapping H2S, CO2 and CH4 not only eliminates their environmental contamination, but also increases feedstocks for industrial production. In order to avoid the contamination and causticity problems caused by the conventional alkali absorbents, ionic liquids have been proposed as alternatives to absorb and separate different gases for decades. In this paper, the investigations of pure ionic liquids in the selective absorption of CO2/CH4, H2S/CH4 and H2S/CO2 are reviewed. Important influencing factors including temperature, pressure, functionality, and properties of gas and ionic liquids were analyzed. Ionic liquids with alkali groups on cation and anion are promising CO2 and H2S solvents and competent separators for removing CO2 or/and H2S from CH4. However, it requires more careful structure and property adjustments in order to efficiently separate CO2 from the acidic H2S. It was observed that ionic liquids with moderate basicity exhibit much better selectivity because strong alkali groups could strongly bind with both H2S and CO2, leading to low H2S/CO2 selectivities. A statistic work was carried out and revealed that ionic liquids with small molecular weights and compact structures usually perform better selectivity; low temperature and pressure are favorable for increasing separation performance in physical selective absorption. As for the selectivity involving chemisorption, increasing temperature possibly enhances H2S/CO2 selectivity, which is caused by the reduction of CO2 capacity, dominated by physisorption, rather than increase of H2S capacity. In comparison with the ideal selectivity, real selectivity makes more application sense because it usually involves competitive absorption of binary gas mixtures rather than a pure gas component. According to the reported real selectivities combining experimental results and molecular dynamic simulation, it is concluded that feeding gas ratio, ionic liquid dosage, temperature and pressure are significant factors required to be adjusted to approach great real gas selectivity. Keywords: CO2; CH4; H2S; gas selectivity; ionic liquids; natural gas sweetening

*

Corresponding author at: College of Safety Science and Engineering, Henan Polytechnic University, Jiaozuo, Henan. E-mail address: [email protected]

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1 INTRODUCTION There are many technologies for gas separations, for example, pressure swing adsorption1-3, physical or chemical solvents scrubbing4,5, and gas membrane separation.6,7 Most technologies suffer from higher energy consumption, higher cost, and serious secondary pollutions.8-11 Several materials on gas separation involved in physisorption, i.e. organic solvents, porous carbonaceous materials, zeolites, silica gels and metal-organic frameworks, could only reach a limited capacity, and require a frequent sorption-desorption-regeneration performance.8,12-19 Physical absorbents may be a variety of organic liquids, used separately or as mixtures, and usually work at super-atmospheric pressure, yet with high risk of solvent loss during depressurizing cycles. In comparison, chemical absorption could efficiently achieve high gas capacity, but face a challenge of increasing H2S/CO2 selectivity and lowering energy consumption during solvent regeneration (i.e. desorption of the absorbed gases) with the need for heating.7 Membrane separation method could separate gases efficiently with much less energy consumption, but suffers from instable performance which needs to be solved urgently.8,11,20 For flammable and explosive gases, liquid solvents are optimal choices for gas separation avoiding explosion accidents, which is one of the significant reasons to separate and purify natural gases in liquid phase. Impurities in natural gases could be sweetened in an absorber-stripper configuration using either physical solvents, chemical solvents, or a mixture of both depending on the type and concentration of the impurities.21 Sour gases, such as CO2 and H2S, have to be removed in consideration of meeting customer specifications and avoiding technical problems during gas transportation. Low caloric value and the risk of dry ice formation during liquefaction of natural gas make it essential to eliminate CO2, and pipeline corrosion caused by H2S should be avoided. Aqueous alkanol amines are main chemical solvents which are used for CO2 and H2S capture on commercial level because of their high gas capacity. However, it suffers a number of serious drawbacks including solvent regeneration, solvent loss, low H2S/CO2 selectivity and corrosivity while using them in natural gas processing and capture of CO2 from different industrial effluents. In addition, some of the amines have also been found to have eco-toxicity. Therefore, it has become mandatory to search out alternative solvents to avoid these odious problems. Some physical solvents, e.g. Rectisol and Selexol, using methanol and dimethyl ethers of polyethylene glycol as solvents to separate H2S/CO2 streams, usually have comparable absorption and lower heat requirements during desorption, while suffering great solvent loss because of its very high volatility.22 In order to overcome these problems, ionic liquids have obtained burgeoning attention in solubility of gases, such as NH323-34, SO235-58, CO259-85, N2O86-94, CH492,95-101, other light hydrogen carbons95-97,99,102-104, and less soluble gases(i.e. CO, N2, O2)86,105-118, due to their negligible volatility, high thermal stability, non-corrosivity and non-odor originated from their specific structural composition which make them superior to most of the conventional organic solvents.119 Lei et al. reported an excellent review of gas solubility in ionic liquids120, and CO2 chemisorption in functionalized ionic liquids have been introduced in detail in the review by Zhang and his co-workers for carbon capture.121 Karadas, Ramdin, and Kumar respectively reviewed the application of ionic liquids in sweetening natural gas, especially CO2 capture, with some critical suggestions being proposed.61,122,123 All the above-mentioned reviews provide


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valuable information for carbon capture and sulfur sweetening technologies. However, selectivity of CO2/CH4 and CO2/H2S are of high requirements in practical industrial application, which motivates us to review CO2/CH4 and CO2/H2S selectivity in pure ionic liquids for providing some useful information in friendly solvents selection. This review excludes the investigations of gas capture and separation in blended ionic liquid-based solvents, in order to reveal the influences of experimental conditions, especially the functional structures of ionic liquids, on CO2/CH4, H2S/CH4 and CO2/H2S selectivity.

2. SELECTIVITY OF CO2/CH4 IN IONIC LIQUIDS Considering the difficulties of separately measuring gas components in a liquid phase, the ideal selectivity is the most concerned separation attribute by most literatures to evaluate the separating ability of ionic liquids. Ideal selectivity is expressed as the ratio of pure gas solubilities in liquid phase at certain temperature and pressure shown as Equation (1), or ratio of bubbling-point pressures at a constant temperature and composition expressed in Equation (2), or simply indicated as the ratio of Henry’s constants described in Equation (3).98

x  SB/A =  B   xA P,T P  SB/A =  A   PB T , x S B /A =

K HA K HB

(1)

(2)

(3)

In the above equations, xB, xA are the mole fraction of gas A and gas B dissolved in ionic liquids at a fixed pressure; KHA, KHB denote the Henry’s constants of gas A and gas B in ionic liquids; PA, PB are the bubbling pressures of gas A and gas B in ionic liquids at the same temperature. All the aforementioned parameters are related to single gas-ionic liquid binary systems. There would be some distinction among the ideal selectivities calculated by the three equations because they are available depending on different conditions. Equation (1) and (2) are suitable for comparing the selectivity under different pressures, whereas they require interpolations or extrapolations to obtain the solubility data under the same pressure or the same composition. Equation (3) is the most conveniently and commonly used method by researchers to get a fast impression which ionic liquid is a ready absorbent for gas separation.98

2.1 Ideal CO2/CH4 Selective Absorption in Ionic Liquids by Physisorption Table 1 contains CO2/CH4 selectivity in some organic solvents, where the maximum CO2/CH4 solubility selectivity is observed in propylene carbonate (S=17), slightly smaller than that in water. In Table 2 includes CO2/CH4 selectivity in several ionic liquids, surprisingly showing comparable


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and even better selectivity especially in non-fluorine ionic liquids. Table 1. CO2/CH4 Selectivity by Some Organic Solvents at 298 K124 Solvents

Molecular Formula

Molecular Weight

KH

SCO2/CH4

Ref.

（MW）） Acetonitrile

CH3CN

41.05

6.11852

14

124

Acetone

CH3COCH3

58.08

4.69019

12

124

Tetrahydrofuran

(CH2)4O

72.11

4.51798

10

124

Methyl acetate

CH3COOH

74.08

4.7611

11

124

Dimethylformamide

(CH3)2NCOH

73.09

7.15178

15

124

propylene carbonate

CH2OCOOCHCH3

102.09

6.77697

17

124

methanol

CH3OH

32.04

15.6002

7.2

124

n-hexane

C6H12

86.16

8.26608

2.4

124

water

H2 O

18.02

166.132

24

124

Table 2. CO2/CH4 Selectivity in Ionic Liquids at Different Temperatures MW

Name of ILs

Formula of ILs

T

S

Ref.

284.18

1-butyl-3-methylimidazolium hexafluorophosphate

[C4C1im][PF6]

283.15

38.24

110

142.13

N-methyl-2-hydroxyethylammonium propionate

[2mHEA][Prop]

293.15

91.37

125

419.36

1-butyl-3-methylimidazolium bis[(trifluoromethylsulfonyl]imide

[C4C1im][NTf2]

293.15

20.98

125

764

Trihexyl(tetradecyl)phosphonium bis[(trifluoromethylsulfonyl]imide

[P6 6 6 14][NTf2]

293.15

6.36

125

205.26

1-butyl-3-methylimidazolium dicyanamide

[C4C1im][N(CN)2]

298

13.82

126

206.18

1-ethyl-3-methylimidazolium methylphosphonate

[C2C1im][CH3OHPO2]

298

24.91

92

208.24

1-methyl-3-methylimidazolium methyl sulfate

[C1C1im][CH3SO4]

298.15

34.62

127

197.97

1-ethyl-3-methylimidazolium tetrafluoroborate

[C2C1im][BF4]

298.15

36.25

127

391.31

1-ethyl-3-methylimidazolium bis[(trifluoromethylsulfonyl]imide

[C2C1im][NTf2]

298.15

14.90

127

447.42

1-hexyl-3-methylimidazolium bis[(trifluoromethylsulfonyl]imide

[C6C1im][NTf2]

298.15

10.29

127

226.02

1-butyl-3-methylimidazolium tetrafluoroborate

[C4C1im][BF4]

298.15

11.86 a

94

213.97

1-(2-hydroxyethyl)-3-methyl-imidazolium tetrafluoroborate

[HO(C2H4)C1im][BF4]

298.15

4.35 a

94

a

94

391.31


[C2C1im][NTf2]

298.15

7.82

424.42

Trimethylhexylammonium bis[(trifluoromethylsulfonyl]imide

[N6 111][NTf2]

298.15

9.53a

94

250.31

1-butyl-3-methylimidazolium methyl sulfate

[C4C1im][CH3SO4]

303.15

35.00

127

447.42


[C6C1im][NTf2]

303.15

9.51

127

556.17

1-ethyl-3-methylimidazolium tris(nonafluoroethyl)trifluorophosphate

[C2C1im][eFAP]

303

11.58

100

177.21

1-ethyl-3-methylimidazolium dicyanamide

[C2C1im][N(CN)2]

313.15

20.83

128

189.22

1-allyl-3-methyl imidazolium dicyanamide

[Amim][N(CN)2]

313.15

21.80

98

318.46

1,2,3-tris(diethylamino)cyclopropenylium dicyanamide

[Cprop][N(CN)2]

313.15

11.10

98

206.18

1-ethyl-3-methylimidazolium methylphosphonate

[C2C1im][CH3OHPO2]

313

16.86

92

208.3

N-methyl-N-pentylpyrrolidinium dicyanamide

[C4C1pyrr][N(CN)2]

313.15

16.70

98

16.85

128

260.23

1-ethyl-3-methylimidazolium trifuoromethanesulfonate

[C2C1im][CF3SO3]


313.15

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Table 2 (continued) 264.26

1-ethyl-3-methylimidazolium Diethyl-phosphate

[C2C1im][DEPO4]

313.15

10.80

98

549.9

Trihexyl(tetradecyl)phosphonium dicyanamide

[P6 6 6 14][N(CN)2]

313.15

5.30

98

773.27

Trihexyl(tetradecyl)phosphonium [P6 6 6 14][TMPP]

313.15

14.00

98

bis(2,4,4-trimethylpentyl)phosphinate 648.85

Trioctylmethylammonium bis[(trifluoromethylsulfonyl]imide

[N8 8 8 1][NTf2]

313.15

7.00

98

399.39

Triethylsulfonium bis[(trifluoromethylsulfonyl]imide

[(C2H5)3S][NTf2]

313.15

10.40

98

391.31


[C2C1im][NTf2]

313.15

11.46

129

419.36


[C4C1im][NTf2]

313.15

10.24

129

447.42


[C6C1im][NTf2]

313.15

8.75

129

403.23

1-ethanenitrile-3-methylimidazolium

[NC-C1mim][NTf2] 313.15

18.28

313.15

15.96

313.15

13.25

129

bis[(trifluoromethylsulfonyl]imide 430.34

1-butanenitrile-3-methylimidazolium

129

[NC-C3mim][NTf2]


1-hexanenitrile-3-methylimidazolium

129

[NC-C5mim][NTf2]


1-propargyl-3-methylimidazolium bis[(trifluoromethylsulfonyl]imide

[HCC-C1mim][NTf2]

313.15

13.83

129

461.45

1-heptyl-3-methylimidazolium bis[(trifluoromethylsulfonyl]imide

[C7C1im][NTf2]

313.15

8.00

130

503.53

1-decyl-3-methylimidazolium bis[(trifluoromethylsulfonyl]imide

[C10C1im][NTf2]

313.15

8.00

130

436.43

1-butyl-1-methylpiperidinium bis[(trifluoromethylsulfonyl]imide

[C4C1pip][NTf2]

313.15

8.80

421.34

98

130

1-(2-Methoxyethyl)-3-methylimidazolium [P1C1im][NTf2]

313.15

13.00

[P2C1im][NTf2]

313.15

12.00

[P3C1im][NTf2]

313.15

12.00

[Cprop][NTf2]

313.15

6.80

313.15

18 b

313.15

16 b

313.15

13 b

313.15

14 b

129

b

129


130

1-(2-(2-Methoxyethoxy)ethyl)-3-methylimidazolium bis[(trifluoromethylsulfonyl]imide

509.44

130

1-(2-(2-(2-Methoxyethoxy)ethoxy)ethyl)-3-methylimidazolium bis[(trifluoromethylsulfonyl]imide

532.56

98

1,2,3-tris(diethylamino)cyclopropenylium bis[(trifluoromethylsulfonyl]imide

403.23

1-ethanenitrile-3-methylimidazolium

129

[NC-C1mim][NTf2]


1-butanenitrile-3-methylimidazolium

129

[NC-C3mim][NTf2]


1-hexanenitrile-3-methylimidazolium

129

[NC-C5mim][NTf2]


1-propargyl-3-methylimidazolium bis[(trifluoromethylsulfonyl]imide

[HCC-C1mim][NTf2]

391.31


[C2mim][NTf2]

313.15

12

419.36


[C4mim][NTf2]

313.15

10 b

129

447.42


[C6mim][NTf2]

313.15

8.7 b

129

419.36


[C4C1im][NTf2]

333.15

8.00

131

234.32

1-butyl-3-methylimidazolium mesylate

[C4C1im][CH3SO3]

353.15

46.74

125

Note: MW is molecular weight of ionic liquids; a

Selectivity is calculated by solubility under the pressure of 0.5 bar;


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b

Selectivity is calculated by solubility under 1 bar.

Temperature and functionality are two primary factors influencing CO2/CH4 solubility selectivity in ionic liquids. In view of CO2/CH4 selectivity in ionic liquids at different temperatures in Table 2, decreased selectivity with rising temperatures is observed for the same ionic liquid. For example, CO2/CH4 selectivity of 10.24 was observed for [C4C1im][NTf2] at 313.15 K, but only 8 at 333.15 K. Figure 1 demonstrates the CO2/CH4 selectivities of several ionic liquids at 313.15 K and lower temperatures ranging from 293 to 303 K. It is clear to see that selectivity below 20 were obtained for most ionic liquids at 313.15 K (the black dots), while bigger selectivities were shown from 293 ~ 303 K (the red dots). It is explained possibly by the facts that CO2 solubility decreases with increasing temperature, whereas some experiments manifested that an increase in temperature has a small or even negligible impact on CH4 solubility,127 possibly leading to decreased CO2/CH4 selectivities under higher temperatures.132 This rule is also observed for removing CO2 from other unreactive gases such as N2, O2 and H2, because these gases have been observed or predicted to have inverse dependence with increasing temperature, namely solubility increases with temperature increase, which is not a good characteristic towards selectivity enhancement.105-108,110,126,133,134

100 CO2/CH4 selectivity

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80

T=293~303 K T=313.15 K

60 40 20 0 100 200 300 400 500 600 700 800 Molecular weight / gmol-1

Figure 1. CO2/CH4 selectivity (KH ratio) with molecular weights at different temperature ranges92,125-127100,128-130 Another important finding is ionic liquids with short substituted chains (i.e. smaller molecular weights) are efficient for improving selectivities.135 Those with largely volumetric cations e.g. [P6 + + + 6 6 14] , [N8 8 8 1] and [Cprop] , which are confirmed to be good solvents for CO2 and CH4, only have a selectivity below 14.98 It is stated that with the alkyl chain length on the imidazolium cation decreases, selectivities increase in an order of [C6C1im][NTf2](8.75) < [C4C1im][NTf2](10.24) < [C2C1im][NTf2](11.45) at 313 K.129 The same regularity is observed for CO2/CH4 selectivity with a diminishing order of [C4C1im][NTf2](11) > [C7C1im][NTf2](8) = [C10C1im][NTf2](8) and [P1C1im][NTf2](13) > [P2C1im][NTf2](12) = [P3C1im][NTf2](12) at 313.15 K.130 When the length of substituted chain is elongated to a degree, the selectivity would verge to a constant, which is originated from the predominant role of large molar volume for both gases during physisorption.


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Relative to CO2, the solubility of CH4 (and other less soluble gases) is more dependent on the volume of ionic liquids based on dispersion forces, while CO2 solubility is a combined result from both the dispersion force and the electrostatic interaction. It is stated that Van der Waals interactions play much more significant role than electrostatic forces during CO2 absorption in ionic liquids with large nonpolar field.136 When the alkyl chain is large enough, therefore, both CO2 and CH4 are absorbed into ionic liquids mainly by virtue of free volume, relatively weakening the effort of electrostatic forces. It leads to a smaller gap of CO2 and CH4 capacity in the lengthened alkyl chains. Anyhow, it indicates that large molecular volume takes responsibility for gas solubility but not for increasing CO2/CH4 selectivity. Ionic liquids with high polarity, which have great affinity to CO2, are avoided by nonpolar CH4 and favourable for removing CO2 from CH4.16,17 It is also clearly supported by the fact that the highly polar ionic liquids, e.g. [2mHEA][Prop], [C4C1im][CH3SO3], [C4C1im][CH3SO4], [C1C1im][CH3SO4] as well as [C2C1im][CH3OHPO2] exhibit excellent CO2/CH4 selectivities of 91.4 (293.15K), 46.7 (353.15K), 35 (303.15K), 34.6 (298.15K), and 24.91(298.15 K) respectively,125,126 much higher than most non-functionalised imidazolium based ionic liquids with fluorinated anions with selectivities below 20. According to Zeng et al., the improvement of CO2/CH4 in ionic liquids with polar units is due to the decreased CH4 capacity rather than enhancement of CO2 solubility.137 However, polarity is not the only essential accelerator for CO2/CH4 selectivity. For instance, the polar [HO(C2H4)C1im][BF4] only performs low CO2/CH4 selectivity of 4.35, much less than 11.86 of [C4C1im][BF4] at 298.15 K and 0.5 bar.94 This contradiction is attributed to a strong cation-anion interaction produced between the tethered hydroxyl group in cation and [BF4]- anion, leading to a weak ion-CO2 interaction. The weakened CO2 affinity allows a low CO2 capacity which is possibly responsible for a decreased CO2/CH4 selectivity. Therefore, there is a trade-off between polarity and cation-anion interaction in order to achieve satisfactory selectivity. It is stated that in [C6C1im][PF6], the small [PF6]- anion with high charge density serves to CO2-anion electrostatic interaction, but the much higher cohesive energy of [C6C1im][PF6] results in a strong cation-anion interaction, which leads to less CO2 capacity in comparison with the larger [C6C1im][eFAP](tris(pentafluoroethyl)trifluorophosphate).136 Besides, it is reported that [NTf2]- possesses a greater molar polarizability than [BF4]-, [PF6]-, [CH3SO4]-, [N(CN)2]- and [CF3CO2]-,138 indicating more delocalised and flexible charge distribution in anion, resulting in a weak cation-anion electrostatic interaction during CO2 physisorption. However, from Table 2, it is observed that [C4C1im][PF6] and [C2C1im][BF4], with low CO2 capacity, achieve much higher CO2/CH4 selectivity of 38.24 and 36.25 at 283.15 K and 298.15 K respectively. As depicted in Figure 2, CO2 and CH4 solubility, represented by Henry’s constants, in three ionic liquids with [BF4]-, [PF6]- and [eFAP]- anions, showing much larger gaps in [C4C1im][PF6] and [C4C1im][BF4] compared to that in [C2C1im][eFAP]. Clearly, it is not dominated by CO2 solubility, possibly due to the effect of molar volume of ionic liquids. Large sized [C2C1im][eFAP] with low cohesive energy, could reach better solubility for both CO2 and CH4, leads to low CO2/CH4 selectivity.


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250 [C4C1im][BF4]-CH4

200 [C4C1im][PF6]-CH4

KH / MPa

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

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[C4C1im][BF4]-CO2 [C4C1im][BF4]-CH4 [C4C1im][PF6]-CO2 [C4C1im][PF6]-CH4 [C2C1im][eFAP]-CO2 [C2C1im][eFAP]-CH4

150

100 [C2C1im][eFAP]-CH4

50

0 280

300

320

340

360

Temperature / K

Figure 2. Henry’s constants for CO2 and CH4 in [C4C1im][BF4], [C4C1im][PF6] and [C2C1im][eFAP] with increasing temperatures To summarize what have been analysed above, a delicate balance between cation-anion interactions, polarity and molar volumes of ionic liquids is required to screening a highly selective solvent for CO2/CH4 separation.137

2.2 Real CO2/CH4 Selective Absorption in Ionic Liquids by Physisorption Although ideal selectivity of CO2/CH4 in ILs is comparable to those in conventional physical solvents, real CO2/CH4 selectivity in ILs is expected to be lower than the ideal selectivity. For example, real selectivity of [C4C1im][NTf2] and [C2C1im][(C2H5)2PO4] were reported to be approximately the same or slightly lower compared to their ideal selectivity.139 That is because in a real process typically contains more than one gaseous components in the feed. The co-solvent interaction and anti-solvent competition between different gases may influence the dissolution process. In order to reveal the competition mechanism, simulation methods are used to obtain the real selectivity involving gas mixtures, due to the fact that measuring gas components in a liquid phase is significantly difficult. Based on thermodynamics simulations, several research groups, e.g. Gomes et al., Ramdin et al., Brennecke et al., Maginn et al. and Shiflett et al., have been simulating ternary systems of IL-gas mixtures in order to get insight into the phase equilibrium behaviours and the gas-gas mutual interaction when they are simultaneously dissolved in ionic liquids. According to real selectivity calculation for [C4C1im][NTf2] and [C2C1im][dep], it was found the real CO2/CH4 selectivity decreases evidently with increasing temperatures in ionic liquids, which is similar to the case of ideal selectivity.140 High pressures are not able to enhance gas selectivity as Figure 3 illustrates. CO2/CH4 real selectivity in [C6C1im][NTf2] at 298.15 K was


Page 9 of 32

reported to diminish sharply first and then demonstrates a fluctuation with increasing pressures.127,141

12 10.3

CO2/CH4 selectivity

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

Energy & Fuels

CO2:CH4=50:50 CO2:CH4=10:90

10 8 6 4 2

2

4

6

8 10 12 14 16 18 Pressure / bar

Figure 3. CO2/CH4 real selectivity in [C6C1im][NTf2] with pressures and CO2 concentration in gas phase at 298.15 K141 (the blue dashed line is the ideal selectivity of the same set127) Generally, a better CO2/CH4 selectivity associates with lower gas phase pressures in contrast to lower selectivity at high pressures, implying high pressure enables the real selectivity to present distinct deviations and decrease from the ideal selectivity.142 It may be possibly due to the changed polarity of ionic liquids143 leading to an unstable CO2 capacity under very high pressures. It is reported that the alkyl chains on cation could be compressed with increasing pressures, providing smaller volume to accommodate gas molecules.144 Moreover, the dynamic diameter of CO2 is smaller than CH4, enabling less CO2 capacity than CH4 when the volume of ionic liquids shrinks. Besides, viscosity of ionic liquids is also susceptible with increasing pressures,144 and gas permeability could be reduced and gas diffusion be slowed down during gas absorption. Therefore, although high pressure usually substantially increases gas solubility in ionic liquids, the process could be retarded by the viscous solvents, leading to some fluctuations of gas capacity detection before phase equilibria. However, the selectivity decrease has no linearity with increasing pressures. It can be observed that CO2/CH4 real selectivity in [C6C1im][NTf2] presents quick increases near 14 bar after a gliding trend between 4 and 12 bar for CO2:CH4 feeding ratio of 50:50. Thereby, appropriately imposing a lower pressure is of great practical value in order to approach a satisfactory real selectivity, which has a similar pressure influence to ideal selectivity. In addition to temperature and pressure, the solute concentration in feeding mixtures also plays a non-negligible role in gas selectivity. Feeding concentration ratio does not make apparent efforts on selectivity, while increasing CO2 feeding helps to increase selectivity slightly.145,146 However, when CO2 concentration is very high, e.g. when mass ratios of CO2:CH4 are 42:58, 74:26 and 86:14, real CO2/CH4 selectivities at 333 K in [C4C1im][NTf2] were calculated to be 6.3, 5.6, and 5.4 respectively, presenting a slight decrease and a larger deviation from their corresponding ideal selectivity (6.44, which is Henry’s constant ratio).145 Nevertheless, the gas feeding composition generally only have a minor influence on the selectivity of CO2/CH4, and this is the reason why


Energy & Fuels 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 10 of 32

many researchers prefer using ideal selectivity to choose advantageous ionic liquids for removing CO2 from those sparsely soluble gases.

2.3

CO2/CH4

Selective

Absorption

in

Ionic

Liquids

by

Chemisorption As reviewed above, physisorption cannot accomplish a desired and efficient CO2 removal from CH4, let alone the soluble hydrocarbons. In order to increase CO2 solubility, chemical absorption with low absorption energy and improved absorption kinetics are desired in the optimising design of functionalised ionic liquids. In the recent review of Cui et al.121, the presence of amino group72-85,147, carboxylate anion79,148-150, aprotic anions including heterocyclic and phenolate ones64-70,151 as well as acidic protons150 etc., in ionic liquid structures has been validated to play an important role in CO2 chemical capture, and the reaction stoichiometry is mainly related to the interaction sites that conform the ionic liquids. The typical reaction paths of CO2 chemisorption in carboxylate, amino-functionalized, phenolic ionic liquids and those with amino-acid and aprotic heterocyclic anions are demonstrated in Figure 4(a)~(h). H3C

O

H

CH3

+

-

O

H3C

N

CH3

+

N

N

+

N CO2

O

O

H3C

O

H3C -

OH

(CO2: IL=1:1) (a) Chemical Reaction of CO2 with [C4C1im][C1CO2] by Substituting Acidic C2(H) on Imidazolium with 150

Carboxylate being Produced

O O

C14H29

-

C14H29

+

P

+

H13C6

O

O

+

P

CO2 H13C6

C6H13

H13C6

-

C6H13

H13C6

(b) CO2 Absorption Mechanism in Phenolic Ionic Liquids trihexyl(tetradecyl)phosphonium1-naphthol ionic liquid, [P6 6 6 14][1-Naph]152

-

O

O C O H3C

CH3 N

C4 H9

H3C

N

P

+

N

H

O

+

H O

-

H9 C4 H9 C4

CH3


C4 H9

O

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Energy & Fuels

(c) Absorption mechanism of CO2 with [TMGH][PhO], and CO2 with tetrabutylphosphonium phenolate ionic liquid [P4 4 4 4][PhO]153

C14H29

C14H29

+

P

-

N

C6H13 C6H13

-

O

+

N

C6H13

+

P

CO2

C

C6H13 C6H13

N

O

C6H13

N

(d) Reaction Pathway of CO2 Absorbed by a Typical Heterocyclic Ionic Liquid [P6 6 6 14][Im]151 H

H H3C

+

N

N

+

CH3

+

H3C

CO2

N

CH3

N O

-

-

Anion-CO2

O

N

N

N N

O

-

O

+

N

H3C

CH3

N

Cation-CO2

H N N

(e) Two Parallel Equilibrium Reaction Pathways, CO2-[2-CNPyr]- and CO2-[C2C1im] +, between CO2 and [C2C1im][2-CNPyr] with Carbamate and Carboxylic Formation Respectively65

C14H29

C14H29

+

P

-

N

C6H13 C6H13

O

+

N

C6H13

+

P

CO2

C

C6H13 C6H13

O

-

C6H13

N

O

N

O

H

H

O O

(f) Reaction Mechanism of CO2 Capture by Carbonyl-Containing Ionic Liquids [P6 6 6 14][4-CHO-Im] (blue dashed line is hydrogen-bonding)151

2

H3C

N

+

N

NH2

+

CO2

H3C

N

+

N

NH O O

-

+

H3N

(CO2: IL=1:2)83


+

N

N

CH3

Energy & Fuels 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 12 of 32

-

-

Br CH3 +

N

H2N

Br

CH3

N

+

NH2

+

+

N

H3N

CO2

N

NH

O -

O

(CO2: IL=1:1) 154 (g) CO2 Absorption Mechanism in Mono and Dual Amino-Functionalized Cation Based Imidazolium Ionic Liquids with Halogen Anions

+

+

NH2

HO O

NH2

HO

OH

+

-

CO2

O

OH

-

O

NH2

-

+

NH2

O

O

O

+

NH2

HO

O

OH -

O NH

O

-

O

+

O

H3N

O

-

(CO2: IL=1:2) 85

(h) 1:2 Reacting Mechanism of Amino Acid Ionic Liquid [DEA][Gly] Absorbing CO2

-OOC C6H13

- OOC

NH2

H29C14

CO2

+

P

H13 C6 C6H13

C14H29

C6H13 COO

S CH3

-

+

CO 2

P

H13C6

O

HO

C6H13

S CH3

H29 C14

C

P

H13 C6 C6H13

NH

+

H13 C6 C6H13

COO

-

O

+

P

NH

C6H13

C6 H13

H29C14

N

C OH

(CO2: IL=1:1) (i) 1:1 Stoichiometry Reaction Schematics of CO2 with [P6 6 6 14][Met] and [P6 6 6 14][Pro] (Producing Carbamic Acid)72

Figure 4. CO2 chemical interaction with several functionalized ionic liquids As Figure 4(a) illustrated, the acetate anion deprotonates the [C2C1im]+ cation at the C2 atom of the imidazolium ring. The dissolved CO2 reacts with the negative charged carbon atom to a


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Energy & Fuels

stable carboxylate. Figure 4(b) presents that as for CO2 dissolved by phenolic ionic liquids, carbonate could be created by attaching CO2 with O atom on phenolic anion. A newly published article stated different CO2 absorption paths in [TMGH][PhO] and [N4 4 4 4][PhO] ionic liquids, which are shown in Figure 4(c). The strong proton donation ability of [TMGH]+ overcomes CO2 to bind with the strong basic [PhO]-. In contrast, the [N4 4 4 4]+ cation has no proton donating ability, enabling the CO2-[PhO]- chemical reaction.153 In Figure 4(d~e) demonstrates the phosphonium ionic liquids with aprotic heterocyclic anions produce different CO2 capture mechanisms, presenting the heterocyclic ion would trap CO2 directly with carbamate formation, but if the cation is imidazolium, H(C2) would be deprotonated by aprotic anion and CO2 will bind with C2 with carboxylic production. When a carbonyl group is introduced in heterocyclic anion, CO2 solubility would be enhanced. Figure 4(f) proposes absorption mechanism of CO2 interacts with carbonyl-containing ionic liquid [P6 6 6 14][4-CHO-Im] including carbamate formation by CO2 binding with basic N atom on imidazole and hydrogen bonding between CO2 and CHO group. Due to the electron-withdrawing nature of the carbonyl group, the easier desorption and excellent reversibility was approached. 1: 2 and 1: 1 stoichiometry are reached when the imidazolium cation is respectively functionalized by mono and dual amino groups (see Figure 4(g)). The amino in anion [Gly]- dominates the CO2 chemisorption in [DEA][Gly], and the ammonium cation is ineffective in CO2 absorption due to the protonation of amino in [DEA]+, which is shown in Figure 4(h). Although some ambiguous speculation about [P4 4 4 4][AA]-CO2 system, Gurkan et al. confirmed an equimolar CO2 absorption by trihexyl(tetradecyl)phosphonium prolinate ([P6 6 6 72 14][Pro]) and methioninate ([P6 6 6 14][Met]) with carbamic acid produced as Figure 4(i) presents, which is over further reaction with another anion to make a carbamate.155 The most inferior problem is the large energy consumption, a very long time and high temperature are required to completely desorb CO2 from these active-site containing ionic liquids. For example, regenerations of [P6 6 6 14][Triz]156, [MTBDH][Im]64 and [P6 6 6 14][2-CN-Pyr]67 require N2 flowing under 80 oC lasting for 20, 30 and 60 minutes respectively for releasing CO2 completely, and even 5 hours for CO2 desorption of [C2C1im][2-CN-pyr].65

3. SELECTIVITY OF H2S/CH4 AND H2S/CO2 IN IONIC LIQUIDS H2S forms dynamic hydrogen-bonded network, with two hydrogen bond donors and two hydrogen bond acceptors per molecule. It has a smaller dipole moment of 0.9 D, which is smaller than water(1.8 D),157 and hence H2S shows much greater solubilities in organic solvents than that in water (see Table 3). Alcohols and glycols do not show a high enough capacity to be of any interest for industrial application, while dimethyl sulfoxide and tetrahydrofuran, with a high capacity of 0.1 mole of H2S per mole of solvent, are also far from ideal due to their high viscosity, boiling point and toxicity. Chemical solvents, such as aqueous alkanol amines have an improved H2S capacity over physical solvents with capacities up to 0.9 mole H2S per mole of amine, approaching the theoretical stoichiometry of 1:1 (H2S: amine). Nevertheless, these systems have to face high corrosion and amine loss through evaporation in a large scale of practical system. In order to conquer these disadvantages, ionic liquids are used in H2S absorption, including physisorption and chemisorption. This chapter will introduce the absorption selectivities of


Energy & Fuels 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 14 of 32

H2S/CO2 and H2S/CH4, which are extraordinarily concerned during the sulfur elimination of natural gas. Table 3. H2S Solubility in some Physical Polar Solvents and Chemical Solvents, given as the Mole Ratio (H2S: Solvent) Solvents

T/K

P / bar

X

Ref.

Water

298.15

1

0.00185

158

Propylene carbonate

298.15

1.1

0.0516

159

Propanone

298.15

1

0.0698

160

Dimethylmethanamide

303.15

1.28

0.0707

161

Dimethyl sulfoxide

313.15

0.98

0.1

161

Tetrahydrofuran

298.15

1

0.1014

160

Aqueous 46.78wt% methyldiethanolamine

313.16

1.2782

0.765

162

313.16

1.2790

0.794

162

313.17

1.8844

0.9

162

313.2

1.154

0.819

163

Aqueous 37.73 wt% methyldiethanolamine +7.64 wt% diethanolamine Aqueous 41.78 wt% diethanolamine Aqueous 5 wt % 2-amino-2-methyl-1-propanol + 40 wt % methyldiethanolamine

3.1 H2S absorption mechanism in ionic liquids Generally, hydrogen bonding acts as the main force between H2S and ionic liquids, where H2S could act as both bond-donor by H and bond-acceptor by S.164 For example, it is elucidated by Chen et al. that the [Lac]- anion has a strong bonding ability with H2S by forming -S-H… O(carboxylate group) bond, which is responsible for the high H2S solubility in carboxylate ILs.164 Relative to the conventional imidazolium carboxylate ionic liquids, the protic ones, such as [N1 0 (CH2OH)2][C1CO2], [N1 1 0(CH2OH)][C1CO2]165 as well as [HO(C2H4)C1im][BF4]33, could not enhance H2S capacity because of a stronger cation-anion interaction in the presence of protons in cation.33 Besides, basicity regulations were carried out for increasing chemisorption of acidic H2S. It is reported that H2S capacity of some ionic liquids with carboxylate anions decreases in an order of [C2C1im][Pro] > [C2C1im][Ac] > [C2C1im][Lac]166, which is consistent with the alkalinity of these three anions. Amino acids or amino salts usually optimal absorbents for H2S capture by a proton transfer reaction as Equation (4) expresses. HCOOCH2NH2 + H2S ⇋ [HS]- + [HCOOCH2NH3]+ (4) In order to enhance H2S capacity, Huang et al. designed several dual Lewis-base carboxylate ammonium ILs, and it shows that the tertiary amine (pKb=4.1) in the N,N-dimethyl-glycinate ([DMG]-) anion should account for 0.81 mole of H2S capacity in one mole of [N2 2 2 4][DMG] at 298.15 K by forming O(carboxylate)···H-S···N(amine) bonds.167 Recently, H2S capacity was reported to achieve 0.97 mol/mol (298.2 K, 1 bar) in Tetramethylguanidinium phenolate ([TMGH][PhO]) ionic liquid due to its strong basic [PhO]- anion.153 Its absorption mechanism for H2S are depicted in Figure 5, clearly presenting a proton transfer reaction between H2S and basic [PhO]- dominating H2S chemical absorption in [TMGH][PhO].


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Energy & Fuels

According to Density Function Theory calculation, the interaction between the basic phenolate anion and the active protons from H2S overcomes the cationic hydrogen-bond donation to H2S, which is consistent with the calculating results of H2S in ionic liquids with α‑amino acid anions and N7, N9‑dimethyladeninium cation (see Figure 6).168 It is computationally confirmed that there are four active sites, H2S-amine group(anion), H2S-carboxylate group(anion), H2S-amine group(cation) and H2S-adeninium ring(cation) when H2S is trapped in these ionic liquids, presenting a much stronger H2S-amine(anion) interaction comparing to H2S-amine(cation). CH3

H3C

N

N H3C

CH3

+

N H

H H

S

-

O

H

Figure 5. Absorption mechanism of H2S with [TMGH][PhO]153

S

H

r4 H +

N

N

H N

N NH2 H

S H

H

CH3

r3

O -

CH3

r1 S H NH2 R

O H

S H

r2

Figure 6. Four interaction paths, r1~r4, between H2S and [dMA][Gly]168 In addition to the conventional absorption mechanism, a reversible reaction, Michael addition, proposed by Gunaratne et al., could immobilize H2S chemically by the addition of H2S across the double bond with -SH being produced.169 However, the recovery condition is ambiguous and required to be studied further. In summary, hydrogen bonding and acid-base interaction are two dominant paths for improving H2S capacity, in addition to the usually known weak Van der Waals force and electrostatic interaction.


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Page 16 of 32

3.2 Ideal H2S/CO2 Selective Absorption in Ionic Liquids by Physisorption Ionic liquids were observed to have a greater H2S solubility than CH4 because of the stronger acidity and polarity of H2S, making them available to separate the two gases. [2mHEA][Prop] and [C4C1im][CH3SO3] with high polarity exhibited great ideal H2S/CH4 selectivity which are respectively 31 and 38 at 353 K, mainly due to the much less affinity of nonpolar CH4 with polar solvents.125 Separation of CO2 and H2S is the most concerned question during sulphur removal after natural gas sweetening. According to the reported investigations of CO2 and H2S solubility in ionic liquids, ideal selectivity of H2S/CO2 are calculated and listed in Table 4, where S(1/1) denotes the solubility selectivity calculated based on Equation (1) under 1 bar, and SKH is obtained from ratio of Henry’s constants in Equation (3). Table 4. Ideal Selectivity of H2S/CO2 in Different Ionic Liquids with Physisorption Nature MW/ g mol-1

Ionic liquids

236.29

[C2C1im][C2H5SO4]

T /K

S

Ref.

303.15

1.611203

170

313.15

1.56338

170

323.15

1.53012

170

333.15

1.515464

170

343.15

1.442478

170

353.15

1.345865

170

293.15

12.19512

55

333.15

4.081633

55

303.15

4.728045

171

313.15

4.534989

171

323.15

4.311927

171

333.15

4.071212

171

343.15

3.810433

171

353.15

3.520856

171

SKH

250.31

260.23

[C4C1im][CH3SO4]

[C2C1im][CF3SO3]

391.13

[C2C1im][NTf2]

333.15

2.7

167

419.36

[C4C1im][NTf2]

333.15

3.03

131


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Energy & Fuels

Table 4(continued) 556.17

213.97

222.26

272.13

407.31

[C2C1im][eFAP]

[HO(C2H4)C1im][BF4]

[HO(C2H4)C1im][CF3SO3]

[HO(C2H4)C1im][PF6]

[HO(C2H4)C1im][NTf2]

303.15

1.909412

172

313.15

1.850294

172

323.15

1.789916

172

333.15

1.708451

172

343.15

1.603232

172

353.15

1.50666

172

303.15

3.450479

33

313.15

3.114355

33

323.15

2.871094

33

333.15

2.659054

33

343.15

2.493113

33

353.15

2.490566

33

303.15

3.182087

34

313.15

3.12004

34

323.15

2.940881

34

333.15

2.684489

34

343.15

2.411275

34

353.15

2.295233

34

303.15

3.226581

34

313.15

3.067537

34

323.15

3.183921

34

333.15

3.123579

34

343.15

3.196448

34

353.15

3.207246

34

303.15

2.335033

34

313.15

2.367866

34

323.15

2.209465

34

333.15

2.206263

34

343.15

2.133818

34

353.15

2.088143

34

298.15

3.48

173

S(1/1) 219.12

[C4C1im]Br

H2S/CO2 selectivity at 303.15 K in 1-(2-hydroxyethyl)-3-methylimidazolium based ionic liquids increase in the order of [HO(C2H4)mim][NTf2] < [HO(C2H4)mim][CF3SO3] < [HO(C2H4)mim][PF6] < [HO(C2H4)mim][BF4] with selectivities of 2.33, 3.12, 3.22, 3.45 respectively, presenting a reverse sequence against CO2 and H2S solubility in these ionic liquids.33,34 Another example is [C2C1im][CF3SO3]171 which behaved a better selectivity up to


Energy & Fuels 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 18 of 32

4.73 than [C2C1im][eFAP] with 1.9 at 303.15 K, although inferior H2S and CO2 uptake in [C2C1im][CF3SO3] compared to that in [C2C1im][eFAP].172 Additionally, [C4C1im]Br, with a simple preparation and a smaller molecular mass, approached a satisfactory selectivity of 3.48 at 298.15 K and 1 bar, which is also a promising physical absorbent for selectively separating H2S/CO2.173 All the above results support again that a compact structure is more favourable for gas selectivity. Temperature is still an adverse external inhibitor for H2S/CO2 separation. As shown in Table 4, a dramatic decrease of H2S/CO2 selectivity in [C4C1im][CH3SO4] was observed from 12.2 to 4.08 when temperature increases from 293.15 to 333.15 K.55 [C2C1im][CF3SO3], [HO(C2H4)mim][BF4] and [HO(C2H4)mim][CF3SO3] show a relatively clear decrease up to 1.25, and others only slightly diminishes except for [HO(C2H4)im][PF6] which selectivity nearly keeps a constant with temperatures. The adverse effect of temperature may be a consequence of a deferred influence on exothermic solvation caused by heating. Another important reason is that the molecular size of H2S increases much more rapidly with temperature increase than that of CO2, resulting in less H2S accommodating in ionic liquid molecules.33

3.3 Real H2S/CO2 Selective Absorption in Ionic Liquids by Physisorption Based on a generic Redlich-Kwong type of cubic EOS, Shiflett et al. predicted the phase behaviours of ternary systems including CO2-H2S-[C4C1im][PF6]174 and 175 CO2-H2S-[C4C1im][CH3SO4] and obtained the real H2S/CO2 selectivity. The same ternary system, CO2-H2S-[C4C1im][CH3SO4], was also investigated by the Soft-SAFT equation.55 It is definitely seen that the gas feeding ratio slightly influenced the selectivity depending on the dosage of ionic liquids, and the selectivities of gas mixtures with different feeding ratios tended to reach a constant peak with increasingly adding ionic liquids. The similar behaviour was also found in CO2-H2S-[C2C1im][eFAP] ternary system predicted by the Redlich-Kwong Eos.172 Generally, low pressure and low temperature also serve to enhancement of gas selectivity during physisorption. However, in viscous ionic liquids, increasing temperature plays a positive role in improving gas permeation by reducing viscosity and enabling gases diffusion. Generally H2S played an anti-solvent effect on CO2 solubility in ionic liquids.120 The presence of H2S had a competitive and negative influence on CO2 retrieve especially in amine-mediated separation. In order to reveal the competitive chemisorption of H2S and CO2, ternary systems involving chemisorption in ionic liquids should also be investigated for providing more information for future industrial applications.

3.4

H2S/CO2

Selective

Absorption

in

Ionic

Liquids

by

Chemisorption Although strong alkaline groups in anions are beneficial for improving H2S capacity (see Table 5), their strong bonds with CO2 increases CO2 absorption and thus reduce the H2S/CO2 selective


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performance. Table 6 includes ideal selectivity of H2S/CO2 in ionic liquids with different alkalinity. It is observed the selectivity of H2S/CO2 in common amines is too low to compete with ionic liquids. Some substituted benzoate-based ionic liquids with strong basic anions were investigated in order to improve H2S capture and H2S/CO2 separation.176 The maximum selectivity, 20.6 at 333.15 K under ambient pressure, is obtained by [C6C1im][2-F-Ben] with pKa of 3.27, while the strong basic [C6C1im][4-CH3O-Ben] with pKa of 4.47 only performed a low selectivity of 5.6. The unexpected performance of strong basic ionic liquids may be ascribed to their simultaneously strong absorption for both H2S and CO2. Table 5. H2S Solubilities in Ionic Liquids with Alkaline Functionality near Ambient Pressure, given as the Mole Ratio (H2S: IL) Ionic liquids

T/K

P/ bar

X

Ref.

[C2C1im][C1CO2]

313.15

0.957

0.4521

165

[C4C1im][C1CO2]

293.15

1.053

0.7625

165

[C6C1im][C1CO2]

303.15

1.095

0.8011

165

[N2 2 2 4][C1CO2]

333.15

1

0.5

167

[C2C1im][C1CHOHCO2]

293.15

1.063

0.4800

166

[C2C1im][C2CO2]

293.15

1.059

0.8090

166

[C2C1im][C2CO2]

303.15

0.998

0.6310

166

[N2 2 2 4][NIA]

298.15

1

0.867

167

[N2 2 2 4][IMA]

298.15

1

0.846

167

[N2 2 2 4][DMG]

333.15

1

0.81

167

[N2 2 2 4][Gly]

333.15

1

0.67

167

[C6C1im][4-CH3O-Ben]

333.15

1

0.4203

176

[N1 1 0(C6H12NMe2)][NTf2]

313.2

1

0.673

177

[N1 1 0(C2H4OC2H4NMe2)][NTf2]

298.2

1

0.546

177

Another typical examples are [C2C1im][C1CO2], [N2 2 2 4][C1CO2] and [N2 2 2 4][Gly] which also have low selectivities similar to amines, due to the fact that both CO2 and H2S have strong bonding with the strong alkaline anions [C1CO2]- and [Gly]-, leading to a low solubility gap. Among three dual Lewis base functionalized ionic liquids, [N2 2 2 4][IMA] and [N2 2 2 4][NIA] presents better selectivity than [N2 2 2 4][DMG] due to their weak bonding with CO2 but stronger interaction with H2S. [DMG]- is able to attract CO2 by both the amine group and carboxylate group, while it is more difficult for [NIA]- and [IMA]- to interact with CO2 because of the steric hindrance of aromatic ring enabling CO2 to be far from carboxylate oxygen but only close to nitrogen atoms. All the above analysis implies that strong alkalinity is not essential for enhancing selectivity, and an elaborated structure design is required to enhance H2S solubility while weaken CO2 affinity.


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60 H2S/CO2 Selectivity

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 20 of 32

[N2 2 2 4][DMG] [N2 2 2 4][IMA] [N2 2 2 4][NIA] [HO(C2H4)mim][BF4] [HO(C2H4)mim][CF3SO3] [C2C1im][CF3SO3]

50 40 30 20 10 0

-10 290 300 310 320 330 340 350 360 370 Temperature / K Figure 7. Comparison of H2S/CO2 selectivity in chemisorption to physisorption with increasing temperature33,34,167,171

Table 6. Ideal Selectivity of H2S/CO2 in Ionic Liquids with Alkaline Functionality pKa

Solvents

T/K

S

Ref.

S(1/1) -

n-methyldiethanolamine(MDEA)

299.15

0.38

173

-

50% aqueous MDEA

298.2

1

178

4.47


333.15

5.6

176

4.34

[C6C1im][4-CH3-Ben]

333.15

6.4

176

4.24

[C6C1im][3-CH3-Ben]

333.15

6

176

4.2

[C6C1im][Ben]

333.15

7.7

176

4.14

[C6C1im][4-F-Ben]

333.15

5.8

176

4.09


333.15

7.3

176

4.09


333.15

7.2

176

3.99

[C6C1im][4-Cl-Ben]

333.15

11.2

176

3.91

[C6C1im][2-CH3-Ben]

333.15

6.2

176

3.87

[C6C1im][3-F-Ben]

333.15

8.1

176

3.83

[C6C1im][3-Cl-Ben]

333.15

9.2

176

3.27

[C6C1im][2-F-Ben]

333.15

20.6

176

2.94

[C6C1im][2-Cl-Ben]

333.15

18.4

176

0.23

[C6C1im][TFA]

333.15

5.9

176

9.89

[N2 2 2 4][DMG]

333.15

13

167

348.15

18

167

363.15

31

167

298.15

10

167

313.15

16

167

333.15

26

167

348.15

58

167

6.8

[N2 2 2 4][IMA]


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Table 6(continued) 298.15

13

167

313.15

17

167

333.15

16

167

348.15

30

167

[C2C1im][C1CO2]

333.15

1.3

167

[N2 2 2 4][C1CO2]

333.15

1.5

167

[N2 2 2 4][Gly]

333.15

1.1

167

-

[TMPDA][NTf2]

298.2

6.0

177

-

[BDMAEE][NTf2]

298.2

37.2

177

-

[TMHDA][NTf2]

313.2

29.5

4.76

4.76

9.6

[N2 2 2 4][NIA]

4.76

[N1 1 0 (CH2OH)][C1CO2]

303.2

9.8

3.75

[N1 1 0 (CH2OH)][HCO2]

303.2

11.2

9.95

[P4 4 4 4][PhO]

313.2

1.1

[C6C1im][PhO]

313.2

1.4

[DBUH][PhO]

313.2

3.8

[TMGH][PhO]

313.2

9.4

177 165,177 177

153 153 153 153

SKH 4.76

3.75

4.76

3.75

[N1 0 (CH2OH)2][C1CO2]

[N1 0 (CH2OH)2][HCO2]

[N1 1 0 (CH2OH)][C1CO2]

[N1 1 0 (CH2OH)][HCO2]

303.2

8.9

177

313.2

8.4

177

323.2

8.0

177

333.2

7.6

177

303.2

10.2

177

313.2

9.4

177

323.2

8.5

177

333.2

7.9

177

303.2

15.1

177

313.2

14.4

177

323.2

14.4

177

333.2

14

177

303.2

19.5

177

313.2

18.6

177

323.2

17.9

177

333.2

17.2

177

(pKa denotes the acidity of the acid corresponding to the anion.) Different from a decline trend of H2S/CO2 selectivity with increasing temperatures observed in physisorption for both H2S and CO2, some chemical solvents, e.g. the dual-based [N2 2 2 4][DMG], [N2 2 2 4][IMA] and [N2 2 2 4][NIA]167, present improved selectivities upon temperature increases(see Figure 7). It is caused by sharply reduced CO2 capacity rather than increased H2S


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solubility.167

70

[N2 2 2 4][DMG] [N2 2 2 4][IMA] [N2 2 2 4][NIA] [N2 2 2 4][C1CO2] [N2 2 2 4][Gly]

60

H2S/CO2 Selectivity

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 22 of 32

50 40 30 20 10 0 0.0

0.2

0.4 0.6 0.8 Pressure / bar

1.0

Figure 8. H2S/CO2 selectivity with increasing pressure at 333.15 K167 As for the influence of pressure, as Figure 8 presents, with increasing pressures, H2S/CO2 selectivities in [N2 2 2 4][C1CO2] and [N2 2 2 4][Gly] nearly keep constants, but decrease dramatically in some excellent absorbents, e.g. [N2 2 2 4][DMG], [N2 2 2 4][IMA] and [N2 2 2 4][NIA]. The different trends originate from their different CO2 absorption ability related to basicity of anions, as well as the compensation of CO2 physisorption under high pressures. For [N2 2 2 4][C1CO2] and [N2 2 2 4][Gly], CO2 could compete with H2S for their basic anions, and chemisorption could dominate both CO2 and H2S absorption in these two ionic liquids. In contrast, for [N2 2 2 4][DMG], [N2 2 2 4][IMA] and [N2 2 2 4][NIA], H2S overcomes CO2 to interact with these ionic liquids by chemisorption which is not very dependent on pressure. However, absorption of CO2 is mainly controlled by physisorption which is more sensitive to pressure, leading to a visible decrease tendency of H2S/CO2 selectivity with rising pressures. In consideration of the cooperation of anionic strong basicity and cationic hydrogen-bond donation to enhance H2S/CO2 selectivity, [TMGH][PhO] was proposed as one applicable separator because of its high H2S chemisorption but low CO2 binding ability. Although CO2 could act as hydrogen bond acceptor with cation, as well as form unstable non-polanar structure with the phenolic anion, it could not compete with the strong hydrogen-bonding between cation and anion and restructure the hydrogen-bonding networks.153 Nevertheless, [TMGH][PhO] does not perform very surprising S1/1 selectivity of H2S/CO2 compared with previously reported [BDMAEE][NTf2] and [TMHDA][NTf2]177, even only achieve half of S1/1 in [N2 2 2 4][NIA] at the same temperature.167

4. CONCLUSION Ionic liquids have received much attention to replace volatile organic solvents to sweeten natural gas by selective absorption processes of CO2, CH4 and H2S. (1) It is good scheme to introducing polar groups in anion, e.g. carboxyl, sulforic, phosphoric functional groups, which eventually improve the solubility of polar gases while suppress that of nonpolar gases, thus enhancing the selectivity of CO2/CH4 and H2S/CH4.


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(2) Ionic liquids with smaller molecular weights usually possess better solubility selectivity. For physisorption selectivity, low temperature and low pressure are favourable for improving gas separation. Nevertheless, for [N2 2 2 4][DMG], [N2 2 2 4][IMA] and [N2 2 2 4][NIA], high temperatures could improve H2S/CO2 selectivity, possibly due to different absorption regimes of H2S and CO2 in these dual base ionic liquids. (3) Excellent absorbents for CO2 and H2S, e.g. those carboxylated anions and dual Lewis base functionalised ones, present advantageous CO2 and H2S removing ability from CH4. However, inefficient H2S/CO2 separation was obtained in ionic liquids with strong alkaline functionality due to the simultaneous chemisorption of H2S and CO2. Experimental results prove that some ionic liquids with moderate basicity are more available for removing H2S from CO2 due to their prior chemical interaction with H2S rather than CO2. However, gas selectivity in ionic liquids is still in its infancy and a number of data should be reported to provide guidance for industrial applications. Therefore, several issues are needed to be resolved in the future at least including the following aspects. (1) Although high temperature is not favorable for gas selectivity, increasing temperature makes ionic liquids less viscous enabling gas diffuse more efficiently, and then curtails the separation time. Viscosity of some ionic liquids will increase after chemisorption of CO2 and H2S, which hinders the absorption process and rate. Hence, experimental investigations about relationships between viscosity, solubility and selectivity should be studied further. (2) Resulting from the difficulties of directly measuring the gas components in a liquid phase, based on multicomponent Equations of State, computational simulations should be carried out further in order to investigate the relation between gas selectivity and the feeding gas ratio, ionic liquid dosage, temperature and pressure. Real selectivity involving ionic liquids with chemisorption nature for CO2 and/or H2S should be simulated in order to get insight into gas competitive absorption mechanism. Moreover, as for the stated co-solvent role CO2 plays on sparsely soluble gases, such as CH4, N2, H2, CO and O2, further investigations should be carried out and a clear internal cause should be revealed. (3) Changes of gas selectivity when recycling ionic liquids should be paid more attention concerning the regeneration of ionic liquids. Energy consumption during desorption is another significant problem which should be considered when using these chemicals in pilot-scale applications. Although the present costs of the ionic liquids do not make them ready for commercial applications, current synthetic studies could overcome this barrier in the near future, making ionic liquids economically more viable. (4) Due to the high cost and viscosity of most functional ionic liquids, it is more economical to blend ionic liquids with other low-cost solvents, such as water, surfactant179 and amine solutions180-182 to reduce viscosity and enhance gas permeability and diffusion. In order to ameliorate the separation function of membranes and porous materials, supported ionic liquid membranes183-185, ionic liquid/ZIF-8 mixed-matrix membranes186, and ionic liquid/metal organic framework composites16,187,188 have been investigated for several years. Thermal stability, mechanical durability and mass transfer of membranes could be facilitated after adding ionic liquids.3,189,190 Incorporation of ionic liquids into the pores of metal organic frameworks are desired to change the physicochemical properties and gas affinities of MOFs, as well as improve the adsorption selectivity.188


Energy & Fuels 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

ASSOCIATED CONTENT

Supporting Information Supporting Information Available: Names, abbreviations and structures of cations and anions involved in this review are provided in supporting information.

AUTHOR INFORMATION Corresponding Author *Telephone: 0086-15993737530; E-mail: [email protected]

ORCID Yong-liang XU: 0000-0001-5484-4952

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

Authors are grateful to the supports from the National Natural Science Foundation of China (NO. 51304073 and NO. 51304071), Program for Innovative Research Team in Ministry of Education of China (NO. IRT_16R22), as well as China Postdoctoral Science Foundation (NO. 2017M612397 and NO. 2017M612396). We also appreciate all the reviewers and editors for their professional and constructive comments.

(1) (2) (3) (4) (5) (6) (7)

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CO2 Selectivity by Ionic Liquids in Natural Gas

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