Separation of light hydrocarbons with ionic liquids: A review

Separation of light hydrocarbons with ionic liquids: A review

Accepted Manuscript Separation of light hydrocarbons with ionic liquids: A review Yuqi Huang, Yuanbin Zhang, Huabin Xing PII: DOI: Reference: S1004-...

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Accepted Manuscript Separation of light hydrocarbons with ionic liquids: A review

Yuqi Huang, Yuanbin Zhang, Huabin Xing PII: DOI: Reference:

S1004-9541(18)30845-0 https://doi.org/10.1016/j.cjche.2019.01.012 CJCHE 1376

To appear in:

Chinese Journal of Chemical Engineering

Received date: Revised date: Accepted date:

13 December 2018 11 January 2019 11 January 2019

Please cite this article as: Y. Huang, Y. Zhang and H. Xing, Separation of light hydrocarbons with ionic liquids: A review, Chinese Journal of Chemical Engineering, https://doi.org/10.1016/j.cjche.2019.01.012

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ACCEPTED MANUSCRIPT Review

Separation of light hydrocarbons with ionic liquids: a review$ Yuqi Huang*, Yuanbin Zhang* and Huabin Xing†

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College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China

Abstract Light hydrocarbons (C1-C4) are fundamental raw materials in the petroleum and chemical

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industry. Separation and purification of structurally similar paraffin/olefin/alkyne mixtures are

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important for the production of high-purity or even polymer-grade light hydrocarbons. However, traditional methods such as cryogenic distillation and solvent absorption are energy-intensive and environmentally unfriendly processes. Ionic liquids (ILs) as a new alternative to organic solvents have

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been proposed as promising green media for light hydrocarbon separation due to their unique tunable structures and physicochemical properties resulted from the variations of the cations and anions such

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as low volatility, high thermal stability, large liquidus range, good miscibility with light hydrocarbons, excellent molecular recognition ability and adjustable hydrophylicity/hydrophobicity. In this review, the recent progresses on the light hydrocarbon separation using ILs are summarized, and some

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parameters of ILs that influence the separation performance are discussed.

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Keywords ionic liquids; hydrocarbons; absorption; separation; solubility

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1 Introduction

Separation of structurally similar light hydrocarbons (C1-C4 paraffins/olefins/alkynes)

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is one of the most important and challenging chemical processes in the petrochemical industry for the production of high-purity chemicals and clean energy.[1-4] Typically, cryogenic distillation and extractive distillation are the currently dominant methods to separate light hydrocarbon mixtures in industry.[5-8] However, cryogenic distillation is an energy-intensive process while extractive distillation usually needs organic extractants with high volatility that results in solvent loss and environment pollution.

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Supported by the National Natural Science Foundation of China (No. 21725603), Zhejiang Provincial Natural Science Foundation of China (LZ18B060001) and the National Program for Support of Top-notch Young Professionals (H. X.). * These authors contributed equally to this work. † To whom correspondence should be addressed. E-mail: [email protected] 1

ACCEPTED MANUSCRIPT Therefore, it is of urgent importance to exploit “green” extractants with low volatility, high stability and high solubility for light hydrocarbons. Recent years have witnessed a growing study in ionic liquids (ILs) due to their unique properties such as negligible vapor pressure, high thermal stability, tunable structures, excellent molecular recognition ability and large liquidus range.[9-11] Another intriguing characteristic is that one can finely tune the physical-chemical properties of by suitable

choice

of

cations

and

anions,

which

makes

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ILs

them

as

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“designer-solvents”.[10, 12] Due to these attributes, ILs have been widely explored in separation processes such as hydrocarbon separation,[13-16] metal separation[17, 18] and

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bioactive compound purification.[19-23] Moreover, ILs usually exhibit improved solubility for hydrocarbons due to the interaction between the ions and gases such as

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hydrogen-bonding interaction, van der Waals interaction, p-π interaction and π-π interaction.[11, 24-26] Therefore in the field of hydrocarbon separation such as paraffin

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purification,[27] paraffin/olefin separation[28-31] and acetylene/ethylene separation[32-34], ILs as efficient extractants were extensively investigated.

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In this review, the recent progresses on the hydrocarbon separation using ILs as

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novel solvents are summarized. Additionally, some parameters of ILs that influence the separation performance such as hydrogen-bonding basicity [35-38] and free volume [39, 40]

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are discussed. The chemical structures, names and abbreviations of cations and anions

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mentioned in this review are listed in Table 1.

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Table 1 Chemical structures of cations and anions summarized in this review

N

Cm

N

N

C

N

N

1-Cn-3-Cm imidazolium

1-butyronitrile-3-methylimidazolium

[CnCmIm]+

[(C2CN)C1Im]+

N

1-methyl-3-(propyn-3-yl)imidazolium [C1(C2H2CH)Im]+

N

N

N

N

N 1-(3-cyanopropyl)-3-methylimidazolium

N

C

N

NU

N

[C1(C3H5CH2)Im]+

[C1(C6H5CH2)Im]+

N

N

1,3-dibutyronitrile-imidazolium

1-butyl-1-methylpyrrolidinium

[(C2CN)2Im]+

[C4C1Pyrr]+

Cm Cq

P

1-hexyl-3-methylpyridinium

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C

N

1-methyl-3-(buten-3-yl)imidazolium

1-methyl-3-benzylimidazolium

[C1(C3CN)Im]+

[C6C1Py]+

Cm

Cn

Cq

N

Cn

Cp

Cp

[Pnmpq]+

CnCmCpCq ammonium

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CnCmCpCq phosphonium

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[Nnmpq]+

N

N

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1-(Z-octadec-9-enyl)-3-methylimidazolium [oleyl-C1Im]+

Anions

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N

N

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N

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Cn

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Cations

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ACCEPTED MANUSCRIPT F

CF3 S N

C2F5

O

F

C2H5

S

carboxylate

[CnH2n+1COO]-

[BF4]-

O

O

N

F

O

F P

N F

F

N

O

O

n

tetrafluoroborate

[FAP]-

N F

*

F

tris(pentafluoroethyl)trifluorophosphate

[Tf2N]-

F

B

F

bis(trifluoromethylsulfonyl)imide

F

F P

S

O

O

F

C2F5

O

F

F

O

O

trifluromethanesulfonate

dicyanamide

nitrate

[CF3SO3]-

[DCA]-

[NO3]-

F hexafluorophosphate

[PF6]-

nC12H25 O O

O

O

S

O

O

O dodecylbenzenesulfonate

acetate

[DBS]-

[OAc]-

[TMPP]-

O

methylsulfate

[MeSO4]-

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bis(2,4,4-trimethylpentyl)phosphinate

S

O

O

P

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O

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O F3C

O

O P

O

O

H

P

O O

O

H

butylphosphonate

metylphosphonate

[BuHPO3]-

[MeHPO3]-

O

P

O

O

O

diethylphosphate

dimethylphosphate

[Et2PO4]-

[Me2PO4]-

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2 Hydrocarbon separation 2.1 Separation of paraffins

P

O

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O

O

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Paraffin purification is an important process in industry, especially due to the rapid development on the exploitation of shale gas in recent years, which consists largely of methane (CH4), ethane (C2H6), propane (C3H8) and other paraffins.[41, 42] The solubility

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of paraffins with different carbon numbers is fairly varied in ILs, which makes them possible to be separated by ILs. So far, a plenty of fundamental investigation on

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paraffin solubility in ILs have been conducted for efficient purification of paraffins. The solubility data of CH4, C2H6 and C3H8 in various ILs is summarized in Fig. 1. Among imidazolium ILs, [C6C1Im][Tf2N][43-45] has the highest mass solubility of CH4, C2H6 and C3H8 that reaches 0.006, 0.026 and 0.083 mmol·g-1 at 313 K and 100 kPa, respectively while [C4C1Im][Tf2N][35, 46, 47] has the best selectivity for C3H8/CH4 (14.2) and C2H6/CH4 (4.5) under the same conditions. When the anion is [Tf2N]-, the mass solubility of CH4, C2H6 and C3H8 basically follows the order of [C6C1Im][Tf2N] > [C4C1Im][Tf2N] > [C2C1Im][Tf2N][48],which demonstrates that extending the alkyl side

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ACCEPTED MANUSCRIPT chains in imidazolium cations can improve the solubility of hydrocarbons. This is because that long alkyl side chains can improve the nonpolarity of ILs, which enhance the Van der Waals interaction between ILs and gases, thus promote gas dissolution in ILs. However, the mass solubility of hydrocarbons in imidazolium ILs is still not high. When compared, quaternary phosphonium ILs display increased mass solubility for all the three gases. Prausnitz et al.[48-50] have reported that phosphonium-based ILs such as

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[P4444][TMPP][48], [P(14)666][TMPP][49] and [P8111][TMPP][50] exhibit high mass

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solubility and excellent selectivity of light hydrocarbons. Among the three ILs, [P8111][TMPP] provides the best solubility for C3H8 at 313 K and 100 kPa and the best

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selectivity for C3H8/CH4 (13.5) and C2H6/CH4 (4.6). However, the high viscosity of [P8111][TMPP] (804 mPa·s at 298 K) limits its real application in industry.[51] Zhang et

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al.[27] have designed long-chain carboxylate-based ILs with asymmetric phosphonium

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cations to reduce the viscosity of ILs but retain the high solubility and selectivity for paraffin purification. In these carboxylate ILs ([P4442][CnH2n+1COO], n = 5, 11, 17), extension of the carbon numbers on the alkyl chain of carboxylate anions can enhance

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the solubility of paraffins. Therefore, [P4442][C17H35COO] has the highest solubility of

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C3H8 (0.204 mmol·g-1) and the selectivity of C3H8/CH4 and C2H6/CH4 is 13.4 and 5.6

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respectively at 308 K and 100 kPa.

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Fig. 1. Mass solubility of CH4, C2H6 and C3H8 (a), selectivity of C3H8/CH4 versus mass solubility of C3H8 (b) and selectivity of C2H6/CH4 versus mass solubility of C2H6 (c) in

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the ILs at 313 K (* at 308 K) [27, 35, 43-50, 52-54]

2.2 Separation of olefins and paraffins Separation of olefin and paraffin mixtures is one of the most energy-intensive

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separation processes in the petrochemical industry because of their similar molecular sizes. Recently, ILs have attracted increasing attention as solvents to absorb and

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separate olefins and paraffins due to their unique properties.[55-58] Herein, the separation performances of ILs for ethylene/ethane (C2H4/C2H6) and propene/propane (C3H6/C3H8) are summarized. 2.2.1 Separation of ethylene and ethane The mass solubility data of C2H4 and the selectivity for C2H4/C2H6 in different ILs are summarized in Fig. 2. The solubility of C2H4 and C2H6 in common imidazolium ILs was measured by Camper et al.[54], which showed that the selectivity declined with the increase of C2H4 solubility. In order to enhance the separation performance for C2H4

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ACCEPTED MANUSCRIPT and C2H6, some functional groups were introduced into ILs. However, this may enhance the polarity of ILs and usually resulted in decreasing the capacity of gases.[59-61] Xing et al.[30] designed a symmetrical dual nitrile-functionalized IL [(C2CN)2Im][Tf2N] to simultaneously weaken the polarity of the functionalized IL and enhance the selectivity. The results showed that [(C2CN)2Im][Tf2N] exhibited a higher selectivity for C2H4/C2H6 separation than the single nitrile-functionalized IL [(C2CN)C1Im][Tf2N]

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and non-functionalized IL [C4C1Im][Tf2N] while the solubility of C2H4 in

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[(C2CN)2Im][Tf2N] was only slightly decreased. This was in good agreement with the COSMO-RS calculation results that the polarity of ILs and their misfit interaction with

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gases played the major roles in the dissolution of C2H4 and C2H6. They also added

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silver ion (Ag+) into ILs to improve the separation performance of C2H4 and C2H6. The results exhibited that the selectivity of C2H4/C2H6 in Ag-[C4C1Im][Tf2N] improved

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significantly, which reached 10.7 at 0.56 bar and 303.15 K. However, Ag+ did not enchance the solubility of C2H4 significantly in [(C2CN)C1Im][Tf2N] and [(C2CN)2Im][Tf2N] because nitrile group could make a strong interaction with Ag+ and

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hindered the π-complexation of Ag+ with C2H4. Moura et al.[62] investigated the effect

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of imidazolium side chain unsaturation on the solubility of C2H4 and C2H6. The solubility of both gases in [C1(C6H5CH2)Im][Tf2N] and [C1(C3H5CH2)Im][Tf2N] were

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lower than that in saturated alkyl side-chain imidazolium ILs. This was explained by a less favorable enthalpy of solvation and the molecular level of solvation behaviors with aid

of

computational

simulations.

Other

unsaturated

ILs[28]

including

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the

[C1(C3CN)Im][DCA], [C1(C3CN)Im][DCA] [C4C1Im][DCA] and [C1(C3CN)Im][Tf2N] were also investigated, in which [C1(C3CN)Im][DCA] had the highest selectivity. In short, functionalized cations and anions could enhance the selectivity for C2H4/C2H6 separation but usually sacrificed the solubility. Green et al.[29] synthesized lipidic ILs [oleyl-C1Im][Tf2N] with “nonpolarlike” solvent properties to achieve high solubility of C2H4 and C2H6. However the selectivity of C2H4/C2H6 was only 0.987, which meant that C2H6 had slightly higher solubility than C2H4, in sharp contrast to the relatively higher C2H4 solubility in many conventional polar ILs. 7

ACCEPTED MANUSCRIPT Other types of ILs such as pyridinium and phosphonium ILs were investigated for the separation of C2H4 and C2H6 as well. [C6C1Py][Tf2N][63] had a modestly high solubility of C2H4 (0.030 mmol·g-1) while the selectivity was 1.21. Liu et al.[48-50] reported a class of phosphonium ILs with [TMPP]- anion, which showed improved solubility of C2H4 and C2H6 than other classes of ILs. Additionally, the selectivity of C2H4/C2H6 was below 1, indicating a favorable absorption of C2H6 in these

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phosphonium ILs. This phenomenon was similar to that of [oleyl-C1Im][Tf2N][29]

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owning “nonpolarlike” solvent properties.

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Fig. 2. Mass solubility of C2H4 and selectivity of C2H4/C2H6 in the ILs at 313 K (* at

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303 K) [28-30, 48-50, 54, 62, 63]

2.2.2 Separation of propene and propane Representive examples of C3H6/C3H8 separation using ILs are summarized in Fig. 3. It is obvious that both cations and anions have a great effect on the separation performance. When the cation is [C2C1Im]+, the solubility of C3H6 increases in the order of [C2C1Im][Tf2N] > [C2C1Im][DCA] > [C2C1Im][CF3SO3] while the selectivity declines.[54] However, it is interesting to find that both the solubility and selectivity follows the order of [C4C1Im][BF4] > [C4C1Im][PF6] > [C4C1Im][NO3], which means

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ACCEPTED MANUSCRIPT the absorption capacity and separation performance can be improved simultaneously when using the [BF4]- anion. Fallanza et al.[64] studied the influence of the alkyl chain length and number of substituents in the imidazolium cation on the gas solubility and found that increasing the alkyl chain length or the number of substituents can increase the solubility of C3H6 with the selectivity decreasing. This is because of that increasing the number of carbon atoms of aliphatic substituent can endow the ionic liquids with

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more organic characters, and thus enhance affinity for both gases. Ortiz et al.[65]

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prepared ILs containing Ag+ for C3H6/C3H8 separation and found that introducing Ag+ into [C4C1Im][BF4] could increasing the solubility of C3H6 while the solubility of C3H6

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was nearly the same. This improved the selectivity of C3H6/C3H8 which reached 16 at 1

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bar and 298 K and the chemical complexation effects between Ag-[C4C1Im][BF4] and C3H6 was responsible for the high selectivity

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Except for imidazolium ILs, other cations have also been investigated. When [C4C1Py][BF4] is changed to [C4C1Im][BF4], the solubility as well as selectivity for C3H6/C3H8 keep almost the same, indicating the similarity of [C4C1Py]+ and [C4C1Im] +

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in gas separation.[58] In quaternary ammonium ILs, increasing the alkyl chain length in

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cations is helpful to improve solubility and [N8881][Tf2N][66] has excellent solubility of C3H6 (0.195 mmol·g-1) but the selectivity is only 1.13. Liu et al.[49] synthetized

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[P(14)666][TMPP] with the highest solubility of C3H6, but the selectivity is below 1 which means C3H8 has a larger solubility. This is in accordance with the fact that

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[P(14)666][TMPP] displays better solubility of C2H4 than C2H6.

9

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ACCEPTED MANUSCRIPT

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Fig. 3. Mass solubility of C3H6 and selectivity of C3H6/C3H8 in the ILs at 313 K (* at

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308 K) [47, 49, 54, 58, 66]

2.3 Separation of other hydrocarbons

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2.3.1 Separation of acetylene and ethylene

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Acetylene/ethylene (C2H2/C2H4) separation is important to produce polymer-grade C2H2 or C2H4 in the petroleum industry and ILs have been proposed as benign media in the separation and purification processes.[34, 67-69] Palgunadi et al.[33] have investigated

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the solubility of C2H2 and C2H4 in imidazolium and pyrrolidinium ILs of different anions and found that a more basic anion is beneficial for higher C2H2 solubility due to

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the enhanced acid-base interaction between the acidic hydrogen atom on C2H2 and the basic anion of ILs while the solubilization of C2H4 is mainly relied on the extent of solvent-solvent cohesion energy. In addition, ILs with more basic anions such as [OAc]-, [Me2PO4]- and [MeHPO3]- have higher solubility of C2H2 and improved selectivity for C2H2/C2H4 than ILs with less basic anions such as [Tf2N]- and [BF4]-. To prove the theory, they evaluated the solubility of C2H2 in ILs using a linear solvation energy relationship on the basis of Kamlet-Taft parameters as the polarity descriptors.[38] It was shown that the solubility of C2H2 in these ILs could be linearly

10

ACCEPTED MANUSCRIPT correlated with the hydrogen-bond basicity (β) values. To further confirm the results above, we summarize the β values from literatures[11, 32, 38, 70-72]

and compare them with the C2H4 and C2H2 solubility data[30, 32, 33, 38, 54] in the

same ILs. β values are usually measured by solvatochromic experiments using 4-nitroaniline and N,N-diethyl-4-nitroaniline as probes.[73, 74] The results are shown in Fig. 4. It is interesting to find that as the β values increase, the Henry’s Law Constants

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(KH) of C2H2 decline and their change relation is approximately linear. This means that

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high β value improves the solubility of C2H2 in ILs, which is consistent with the conclusion proposed by Palgunadi et al.[38] The correlation equation between KH and β

R2 =0.945

(1)

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KH = 2.403×β - 1.430

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values is shown in Eq 1.

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Fig. 4. Plot of hydrogen-bond basicity of different ILs against their solubility of C2H2. 1, [C2C1Im][Tf2N]; 2, [C4C1Im][PF6]; 3, [C4C1Im][Tf2N]; 4, [C4C1Pyrr][Tf2N]; 5,

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[C4C1Im][BF4]; 6, [C2C1Im][MeSO4]; 7, [C2C1Im][EtSO4]; 8, [C4C1Im][CF3SO3]; 9, [C2C1Im][MeHPO3];

10,

[C4C1Im][OAc];

11,

[C4C1Im][C5H11COO];

12,

[C4C1Im][C7H15COO]; 13, [P4444][C5H11COO]; 14, [P4444][C7H15COO] [11, 30, 32, 33, 38, 54, 70]

Xu et al.[75] proposed that the basicity of ILs could be significantly enhanced via weakening the cation-anion interaction or employing an anion-tethered strategy. This explains the fact that the β value of [P4444][C7H15COO] is larger than [C4C1Im][C7H15COO] as [P4444]+ cation has a larger size, which leads to the weaker electrostatic interaction between the cation and the anion in [P4444][C7H15COO].

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ACCEPTED MANUSCRIPT Xing et al.[11, 24] investigated the differential solubility of C2H2 and C2H4 in ILs using quantum chemical calculation and molecular dynamics simulation, which indicated that the C2H2 solubility was dominant by the hydrogen-bonding interaction between the gas molecules and anions while C2H4 solubility was affected by the hydrogen-bonding

interaction and p-π interaction in C2H4-anions as well as π-π interaction in C2H4-cations. The optimized geometries of the ion pairs and gas-ion pairs (Fig. 5)

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clearly showed that the solute leans more toward the anion and the distances of

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Hgas···X (X = O or F in anions) between C2H2 and the anions ranged from 0.195 (C2H2-[bmim][OAc]) to 0.246 nm (C2H2-[bmim][Tf2N]) were much shorter than those

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between C2H4 and the same anions ranged from 0.230 (C2H4-[bmim][OAc]) to 0.264

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nm (C2H4-[bmim][Tf2N]). These results also demonstrated that the strength of hydrogen bonds is in the order of C2H2-ion pairs > C2H4-ion pairs and [bmim][OAc] >

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[bmim][BF4] > [bmim][Tf2N]. Moreover, the van der Waals and electrostatic contributions to the total energy for solute-solute interaction, solute-cation and solute-anion were calculated from isothermal-isobaric simulations, which indicated that

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the van der Waals interaction between C2H4 and cations dominates the dissolution of

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C2H4 in ILs while the hydrogen bonding interaction plays the most important role in

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C2H2 dissolution (Fig. 6).

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ACCEPTED MANUSCRIPT Fig. 5. optimized structures of (a) [bmim][BF4], (b) C2H4-[bmim][BF4], (c) C2H2-[bmim][BF4], (d) [bmim][OAc], (e) C2H4-[bmim][OAc], (f) C2H2-[bmim][OAc], (g) [bmim][Tf2N], (h) C2H4-[bmim][Tf2N], and (i) C2H2-[bmim][Tf2N]. The dotted lines represent the possible modes of interaction, with interatomic distances in

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angstroms [24]

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Fig. 6. Interaction energy of C2H4 (a) and C2H2 (b) in the five ILs at 313 K [11]

Zhao et al[32] reported a rapid screening strategy to look for optimal ILs with high C2H2 capacity and high C2H2/C2H4 selectivity using COSMO-RS calculation based on

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the prediction of KH. The essence was to improve the molecular free volume of ILs and

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enhance the basicity of anions via introduction of flexible asymmetric substituents. After screening 420 ILs, they found that the structure of anions had a great effect for both the separation selectivity and capacity of C2H2 while the cations mainly influenced

the

capacity

of

C2H2 in

ILs.

Based

on

that,

a

series

of

tetraalkylphosphonium-based ILs with long-chain carboxylate anions were designed. These long-chain carboxylate ILs not only exhibited strong hydrogen-bonding basicity, but also overcame common disadvantages of traditional ILs such as strong polarity, high melting point and high viscosity.[76] These unique physicochemical properties endowed the long-chain carboxylate ILs promising media for C2H2/C2H4 separation. 13

ACCEPTED MANUSCRIPT All the solubility data of C2H2 and C2H4 is summarized in Fig. 7 and the solubility of C2H2 in [P4444][C5H11COO] is 1.163 mmol·g-1, which is the highest among all the ILs

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that have been reported.

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Fig. 7. Mass solubility of C2H2 and selectivity of C2H2/C2H4 in the ILs at 313 K [32, 33]

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2.3.2 Separation of 1,3-butadiene and 1-butene 1,3-Butadiene (1,3-C4H6) is a widely used raw material obtained from C4 hydrocarbon mixtures for the production of various synthetic rubbers. Nowadays,

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extractive distillation is the major method for the separation of 1,3-C4H6 and 1-butene

8]

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(1-C4H8)while ILs as a green solvent can help improve the separation performance.[7, Scovazzo et al.[77-81] determined the solubility of 1,3-C4H6 and 1-C4H8 in

imidazolium-, ammonium-, and phosphonium-based ILs and the results can be seen in Fig. 8. Among all these ILs, 1,3-C4H6 always has larger solubility than 1-C4H8. Imidazolium ILs have high selectivity for 1,3-C4H6/1-C4H8 while the mass solubility of 1,3-C4H6 is moderate. In the ammonium ILs with the same anion [Tf2N]-, The solubility of 1,3-C4H6 follows the order of [N(10)311][Tf2N] > [N6222][Tf2N] > [N8881][Tf2N] > [N4311][Tf2N] > [N6311][Tf2N] > [N(10)111][Tf2N] > [N6111][Tf2N] > [N4111][Tf2N]. It is interesting to find that high asymmetry and large volume of cations 14

ACCEPTED MANUSCRIPT may improve the solubility of 1,3-C4H6. As for phosphonium ILs, the solubility of 1,3-C4H6 is extremely high and reaches 0.531 mmol·g-1 in [P(14)444][DBS]. However,

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the selectivity in all the phosphonium ILs is unsatisfactory and lower than 1.37.

Fig. 8. Mass solubility of 1,3-C4H6 and selectivity of 1,3-C4H6/1-C4H8 in the ILs at 303 K [77-81]

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Huang et al.[82] designed several tributylethylphosphonium-based carboxylate ILs ([P4442][CnH2n+1COO]) featuring large free volume, strong hydrogen bonding basicity, relatively low viscosity and good thermal stability through finely tuning the structure of

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carboxylate ILs (Fig. 9). Generally speaking, high β value can improve the gas solubility and free volume in ILs is important in determining the viscosity and ultimate

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solubility as the free volume can act as the hole carriers for molecular transport and decrease the viscosity.[83-85] Hu and Liu et al.[86] invoked a new mechanism to describe the dissolution behavior of gases in ILs: 1) create suitable cavities in ILs to accommodate the gas molecules; 2) introduce the gas molecules into the cavities, 3) interact between the gases and the ILs by multiple intermolecular interaction. Therefore, these carboxylate ILs with large free volume and strong hydrogen bonding basicity exhibit high mass solubility for 1,3-C4H6 ([P4442][C17H35COO]: 0.899 mmol·g-1, [P4442][HCOO]: 0.663 mmol·g-1) and excellent selectivity of 1,3-C4H6 over n-C4H8 at

15

ACCEPTED MANUSCRIPT 308.1 K and 100 kPa. To understand the reason why [P4442][CnH2n+1COO] has both high solubility and selectivity, molecular simulation (COSMO) was used to calculate the free volume of the ILs and a relationship between the free volume of unit mass (FVUM)/β value and the mass solubility of 1,3-C4H6 was built (Fig. 10).[82] The interaction energy between [P4442][HCOO] and 1,3-C4H6/n-C4H8 by quantitative calculation was also calculated

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(Fig. 11). The results demonstrated that large FVUM and high β values can improve the

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solubility of 1,3-C4H6 in ILs. A high β value can enhance the interaction between the ILs and 1,3-C4H6, and large FVUM can provide more free space to accomodate the

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gases. The equation of linear fitting between FVUM/β values and solubility of 1,3-C4H6

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is showen in Eq 2 and Eq 3. The quantitative calculation of interaction between [P4442][HCOO] and 1,3-C4H6/n-C4H8 indicates there exist two hydrogen bonds between [HCOO]- and 1,3-C4H6 but only one between [HCOO]- and n-C4H8. This difference

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resulted in a lower BSSE-corrected interaction energy (-43.25 kJ·mol-1) in [P4442][HCOO]-1,3-C4H6 than that of [P4442][HCOO]-n-C4H6 (-39.42 kJ·mol-1), which

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was consistent with the experimental results that 1,3-C4H6 has higher solubility in

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[P4442][HCOO] than than n-C4H8.

R2 =0.859

(2)

β = 2.032C1,3-C4 H6 -0.212

R2 =0.787

(3)

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FVUM = 0.232C1,3-C4 H6 +0.032

Fig. 9. The structure of cations and anions of tributylethylphosphonium carboxylate ILs (a) and the solubility of 1,3-C4H6 and selectivity to 1,3-C4H6 over n-C4H8 in these ILs (b) [82]

16

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ACCEPTED MANUSCRIPT

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Fig. 10. Plot of FVUM (red) and β values (green) of different ILs against their solubility of 1, 3-C4H6. The solid line stands for correlation with a linear equation. 1, 2,

[BMIm][BF4];

3,

[N4441][Tf2N];

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[EMIm][Tf2N];

4,

[BMIm][CH3COO]; 5,

[P(14)444][DBS]; 6, [P4442][HCOO]; 7, [P4442][CH3COO]; 8, [P4442][C3H7COO]; 9,

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CE

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[P4442][C5H11COO]; 10, [P4442][C11H23COO]; 11, [P4442][C17H35COO] [82]

Fig.

11.

Minimum-energy

structures

of

[P4442][HCOO]-1,3-C4H6

(a)

and

[P4442][HCOO]-n-C4H8 (b) calculated at the B3LYP/ 6-31++G(d, p) level. Dark grey represents C atom; light gray represents H atom; red represents O atom; orange represents P atom [82]

3 Conclusions and perspectives 17

ACCEPTED MANUSCRIPT Thanks to the unique properties such as negligible vapor pressure, high thermal stability, tunable structures, excellent molecular recognition ability and large liquidus range, ILs have been widely investigated as promising solvents/media for light hydrocarbon separation. In this review, solubility data and separation selectivity of light hydrocarbons (paraffins/olefins/alkynes) in representive ILs are summarized. When gases dissolves into the ILs, these with more carbon numbers usually have larger

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solubility. On the other hand, imidazolium ILs with longer alkyl side chains in cations

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show higher solubility for the same light hydrocarbons. Introducing Ag+ into ILs usually helps improve the separation performance, but large cost and high viscosity

SC

limit its application. Another interesting discovery is that the solubility of hydrocarbon

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in phosphonium ILs is usually higher than that in imidazolium ILs, which makes phosphonium ILs an excellent candidate to separate hydrocarbons. Moreover,

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Functional groups can be introduced into ILs to improve the separation selectivity but the absorption capacities decline inevitably. Hydrogen-bonding basicity and free volume of ILs are proposed as the potential factors that affect the solubility and

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separation performance for hydrocarbons. Strong hydrogen-bonding basicity and large

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free volume usually lead to better solubility for hydrocarbons. Both two factors are affected by the structures of cations and anions as well as cation-anion interaction. This

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may be utilized to help design ILs with excellent hydrocarbon separation performance. Despite the plenty of excellent properties in ILs, the high viscosity limits its real

AC

application, which should be properly solved in the future. Another research direction is to investigate the mechanism of interaction between ILs and gases, which can help researchers to design proper ILs with improved hydrocarbon solubility and selectivity.

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of gases in ionic liquids, Chem. Soc. Rev. 40 (2011) 3802-3823.

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