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Recent Advances in Ionic Liquid-Mediated SO2 Capture Shaorui Yan, Feng Han, Qingning Hou, Shuai Zhang, and Shiyun Ai Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b01959 • Publication Date (Web): 22 Jul 2019 Downloaded from pubs.acs.org on July 22, 2019

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Industrial & Engineering Chemistry Research

Recent Advances in Ionic Liquid-Mediated SO2 Capture Shaorui Yan,† Feng Han,† Qingning Hou,† Shuai Zhang*,† and Shiyun Ai*,†

†

College of Chemistry and Material Science, Shandong Agricultural University, Taian, Shandong, 271018, China

Abstract: The capture of SO2 from flue gases is a worldwide concern, which is closely related to environmental, economical, and technological issues. Ionic liquids (ILs), as a kind of promising acidic gas absorbents, have been attracting increasing attention for innovative SO2 capture approaches recently, burgeoning in literatures. In this paper, we give an overview of the relevant current progresses on ILs-mediated SO2 capture, focusing on promising strategies of improving SO2 absorption capacity, reducing desorption enthalpy, the design of green ILs absorbents, and technologies of SO2/CO2

ACS Paragon Plus Environment

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selective capture, eutectic ILs, supported ILs, ILs membranes, based on depiction of the corresponding SO2 absorption mechanism

1. INTRODUCTION

The emission of sulfur dioxide (SO2), which is mainly from the burning of fossil fuels, has drawn worldwide attention. It is essential to remove SO2 from the flue gas since SO2 is harmful to human body and causes serious environmental pollution. To capture SO2 efficiently, a variety of processes, such as wet flue-gas desulfurization (FGD), dry FGD, and semidry FGD processes, have been commercially widely adopted in industrial production.1 Up to now, FGD has been deemed as one of the most effective techniques to control SO2 emissions. However, the inherent disadvantages of these technologies could not be ignored, such as the production of large quantities of wastewater and useless byproducts, which is not consistent with sustainability principles. Organic absorbents such as aqueous amines have been applied for the capture of acidic gases (SO2, CO2 and H2S). However, in the case of large-scale industrial application for SO2 capture, amines can evaporate into the gas stream due to their volatility. What’s more, it is difficult to


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recycle SO2 from less-valuable ammonium sulfite, causing waste of resources and secondary pollution.2 The most attractive FGD approaches for SO2 gas separation are pressure swing absorption (PSA) or temperature swing absorption (TSA) technologies, which are energy-saving and avoid less-valuable byproducts by simultaneously allowing the SO2 to be reused as a direct source for sulfuric acid production.3 For practical use it is nevertheless difficult to find a material for reversible and selective absorption of SO2. Furthermore, strong chemical absorption processes are required for effective capture of SO2 from flue gas because of the relatively low partial pressure (e.g., 0.2 vol % SO2). Accordingly, new materials for efficient, reversible, and economical capture of SO2 are of critical importance for environmental protection and highly desirable to be developed. Ionic liquid (IL) is a kind of room temperature molten salts composed of specific different cations and anions. Additionally, ILs are capable of facilely tuning and thereby can be designed for task-specific applications through subtle combination of the respective cations and anions. Nowadays, ILs have been extensively investigated as environmentally benign solvents for a number of chemical processes, such as separations4-6 and reactions,7-16 because of their limitless attractive properties, such as


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wide liquid temperature range, excellent thermal stability, high ionic conductivity, and good solvation interactions with both polar and nonpolar compounds. Especially, due to their distinctive properties such as non-volatility, high loading capacity, easy recyclability, high thermal stability and various structure/property modulation, ILs have earned themselves an important place in the field of gas (SO2, CO2) separation in the scientific community. 17 In the past decades, design and synthesis of functionalized ILs as green absorbents in gas separation offers a new opportunity for novel capture systems that are capable of reversibly capturing CO2 and SO2 with a high capacity and absorption rate.18-21 Using ILs as absorbent in the desulfurization process has many advantages. ILs exhibit low vapor pressure and negligible volatile, without the shortcoming of volatilization using the traditional amine absorbent. For example, almost no waste generates during the cycling process of SO2 absorption and desorption without the problem of secondary pollution. ILs can be regenerated and reused, and the desorbed SO2 can be used as a sulfur source of valuable chemical products. More importantly, functionalized ILs could be designed


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according to the actual needs of gases capture process with high capacity and energysaving for acid gas absorption. In the past few years, lots of progresses have been made in the field of SO2 absorption by ILs. The first example of SO2 capture by ILs was reported by Han and co-workers, in which [TMG]L (1,1,3,3-tetramethylguanidinium lactate) was synthesized by directly neutralization of 1,1,3,3-tetramethylguanidine (TMG) and lactic acid (L).22 From then on, a series of functionalized ILs have been synthesized to the meet the need of SO2 absorption, such as increasing SO2 absorption capacity, reducing the desorption enthalpy, designing greener ILs and improving selectivity. Besides the direct application of ILs for SO2 capture, various promising techniques, such as supported ILs, ILs membranes, and selective absorption between SO2 and CO2, are developed to meet the SO2 capture. In this review, the relevant current progresses on ILs-mediated SO2 capture are discussed closely. Various promising strategies and technologies are introduced, such as strategies of improving SO2 absorption capacity, reducing desorption enthalpy, selectively capturing SO2 and technologies of supported ILs, eutectic ionic liquids and ILs


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membranes, hoping to provide some insight to design and measurement of ILs for capturing gaseous SO2 from fossil-fuel gas. 2. IONIC LIQUID FOR SO2 ABSORPTION

As a kind of novel and efficient materials, ILs can be used for the capture of acidic gases (SO2) from flue gas. A series of ILs have been employed for SO2 absorption. However, their SO2 solubility and absorption mechanism are quite different, which greatly influences the ILs design for industrial SO2 capture. According to the interaction mechanism, ILs can be divided to normal and functionalized ILs/task-specific ionic liquids (TSILs). Wu group employed the pKa of organic acids forming the anion of ILs to distinguish whether ILs are functional or not for the capture of SO2.23 If the pKa of an organic acid is larger than that of sulphurous acid, the ILs formed by the organic acid can be called as functional ILs. If not, the IL is just a normal IL. However, for some functionalized ILs, using pKa of organic acids may do not work. For example, IL with a free amino on the cation belongs to a functional IL for SO2 capture, which can’t be distinguished employing pKa of organic acids.


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Functional ILs or TSILs have been experimentally demonstrated to absorb more SO2 than normal ILs. SO2 absorption capacity in TSILs is contributed from both chemical interaction and physical interaction. The amount of SO2 chemical absorption follows the chemical equilibrium. Normal ILs only physically absorb SO2, following Henry’s law. Both TSILs and normal ILs can absorb SO2 effectively at high partial pressures of SO2, while only TSILs can absorb SO2 on the condition of relatively low SO2 partial pressure.24 2.1 Normal ionic liquids for SO2 absorption

[BMIm]BF4, [BMIm]Tf2N, [TMG]BF4, [TMG]Tf2N, [TMGB2]Tf2N, [TMGPO]BF4 and [TMGPO2]BF4 (Scheme 1) have been proved to absorb a large amount of SO2 gas, whose molar ratios of SO2 to ILs are 1.33, 1.50, 1.27, 1.18, 1.60, 1.62, 2.01, (wt%: 20.4%, 40.0%, 40.1%, 19.2%, 20.1%, 27.3% and 40%), respectively at ambient pressure and room temperature without any chemical transformation (Table S1, Scheme 1).25-26 However, under simulated flue gas condition (10% SO2 gas, 10 mole percent in N2), only 0.007, 0.005, 0.064, 0.061, 0.080 (SO2 molar ratio to ILs) SO2 gas is absorbed respectively. The solubility of SO2 in the [HMIm]Tf2N and [HMPy]Tf2N are 0.916, 1.092 mol SO2 per mol ILs


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at 25 oC, approximately 1 bar SO2 by simple physical absorption (Table S1, Scheme 1).27 Similar results are obtained in pyridinium-based ILs for SO2 capture. For [C4Py][SCN] IL, the absorption capacity of SO2 (0.841 gSO2 gIL-1) is mostly attributed to the stronger electrostatic interaction between the anion and SO2.28 For guanidinium-based ILs, as shown

in

Table

S1,

2-ethyl-1,1,3,3-tetramethylguanidinium

ethyl

sulfate

([C22(C1)2(C1)23gu][C2OSO3]) and 2,2-diethyl-1,1,3,3-tetramethylguanidinium ethyl sulfate ([(C2)22(C1)2(C1)23gu][C2OSO3])

show

good

performance

for

SO2

capture.29

[(C2)22(C1)2(C1)23gu][C2OSO3] has an high SO2 absorption capacity of 3.93 mol SO2 per mol of IL at 1 bar of SO2 and 20 °C. At simulated flue gas condition (0.1 bar, 20 oC), 0.71 and 1.08 mol SO2 per mole of IL can be captured by [C22(C1)2(C1)23gu][C2OSO3] and ([(C2)22(C1)2(C1)23gu][C2OSO3], respectively (Table S1, Scheme 1). On the other hand, SO2 capture capacity by [C22(C1)2(C1)23gu][NTf2] and ([(C2)22(C1)2(C1)23gu][NTf2] turn out to be only 0.17 and 0.24 mol SO2 per mole of IL under the same conditions, suggesting that the anionic moieties of the ILs play an important role in SO2 capture.


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N

N

N

N

N

BF4

N

Tf2N [BMIm]Tf2N

[BMIm]BF4

N

[TMG]BF4

N

N Tf2N

N N [TMGB2]Tf2N

N

[C6Py]BF4

N

N N

N H

OSO32 [C2 (C1)2(C1)23gu] [C2OSO3]

N N

[C8Py]BF4

[C4Py]SCN

N C8H17 BF4[C83MPy]BF4 N N

N H NTf2-

OSO3[(C2)22(C1)2(C1)23gu] [C2OSO3]

N C4H9 SCN-

N C8H17 BF4-

N C6H13 BF4[C63MPy]BF4

OH BF 4 OH

[TMGPO2]BF4

N C6H13 BF4-

NHC4H9 BF4[C43MPy]BF4

N

[TMGPO]BF4

N C4H9 BF4[C4Py]BF4

N

BF4

NH2 Tf2N

[TMG]Tf2N

N

OH

N

N

NH2 BF4

[C22(C1)2(C1)23gu]NTf2

N C4H9 NTf2[C4Py]NTf2 N N N NTf2[(C2)22(C1)2(C1)23gu]NTf2

Scheme 1. Several normal ILs for SO2 absorption.

Interestingly, the contact between minor amounts of SO2 and crystalline halidecontaining ILs, such as 1-butyl-3-methyl-imidazolium bromide ([BMIm]Br) leads to an evident melting as well as a dramatic decrease in viscosity compared with the pure molten phase.30 This valuable feature probably facilitates the SO2 absorption capacity, which could also be used in the design of TSILs. SO2 solubility in several halide-containing ILs


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are listed in Table S1, which lie in the range 1.91 - 2.11 mol ratio of SO2/ILs. SO2 solubility in ILs follows the order Br > Cl > I. These drastic changes on the physical properties of [BMIm]Br, such as melting, increased ionic conductivity, are attributed to shielding effects on ionic interactions due to Br-SO2 interactions.31 The SO2 molecules promote a breakdown of the crystal structure of solid [BMIm]Br--primary interaction of halide is with the imidazolium C2-H and SO2 is interacting with halide anion.32-33 Notably, although relatively high SO2 absorption capacity could be obtained at high partial pressures of SO2, only physical absorption exists in normal ILs, which is not favorable to capture SO2 in flue gas with low SO2 content. 2.2 Task-specific ionic liquids for SO2 absorption

TSILs and normal ILs present significant differences in the process of SO2 absorption.34,35 Different behaviors are found in viscosity, conductivity, and density between TSILs and normal ILs. Taking the example of [TMG]L, a chemical SO2 absorption process occurs when the mol ratio of SO2/IL is lower than 0.5 at 1 bar SO2, the increase of viscosity and density and the decrease of conductivity are observed. Subsequently, when the mol ratio of SO2/IL is higher than 0.5, a physical absorption takes place, accompanied


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by a decrease of viscosity and an increase of conductivity and density. For normal ILs, such as [BMIM][BF4], only physical absorption takes place in the whole SO2 absorption process, which results in a decrease of viscosity and an increase in the conductivity and density.[35] Han and coworkers reported the first example of the absorption of SO2 by TSILs [TMG]L, which could effectively absorb SO2 from a simulated flue gas with a SO2 content of 8% by volume, a high absorption capacity up to 0.978 : 1 (the molar ratio of SO2 to IL) or 0.305 g SO2 g-1 IL was obtained at ambient pressure and at 313.15 K.22 Based on results of FTIR and NMR experiments, it was proposed that SO2 reacted with the NH2 group of the cation, while the O atom on S=O probably formed intramolecular hydrogen bond with the H atom of the amine (Scheme 2). Gas phase ab initio calculations showed that the anion played a key role in the chemical interaction between [TMG]L and SO2, the S atom was bonded to the N atom on NH2 of [TMG], and some products with aminosulfate or aminosulfinic acid fragment may be formed.36 Various lactate-based ILs, such as [N2222]L, [Bmim]L, [Hmim]L, [TMG]L and [MEA]L also show good SO2 capture capacity, and the corresponding mole ratios of SO2 to ILs are 0.791, 0.676, 0.653, 0.414 and 0.230, respectively, suggesting that the cations of ILs have a significant influence on the


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absorption behaviors of SO2 while no obvious influence of the length of the alkyl chains is found on the solubility of SO2 in lactate-based imidazolium ILs.37 Guanidinium-based ILs guanidinium phenol ([TMG][PHE], Scheme 3), guanidinium 2,2,2-trifluoroethanol ([TMG][TE]) and guanidinium imidazole ([TMG][IM]), have also been examined for SO2 capture,38 and their SO2 capture capacity could reach up to 2.580, 4.132, 3.765 molar ratio of SO2 to IL under atmospheric pressure and room temperature, respectively.

N N

NH2

OH C COO H

+ SO2

N

- SO2

N

H O N S OH

OH C COO H

Scheme 2. Proposed reaction between [TMG]L and SO2.

N C NH2 N

O +SO2 -SO2

N C N H N S O OH

O

Scheme 3. The possible SO2 absorption mechanism in [TMG][PHE].

Different opinions about the mechanism of SO2 absorption of lactate-based ILs have also been put forward.22,39-41 Based on results of Molecular dynamics (MD) simulation and Quantum chemical calculations (QM), SO2 preferential interaction with the anion is


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revealed in several ion pairs complexed with SO2.42 And the lactate anion can interact with one, two or three SO2 molecules forming the S-O interactions, and the S-O interactions was 3.5 - 8.8 times than the [TMG]+-SO2 interactions. Therefore, the strong interactions between SO2 and electronegative oxygen atoms in the anion is proposed to be the main factor to affect the absorption of the SO2 gas in the [TMG]L. 2.3 Improving SO2 absorption capacity of ILs SO2 absorption capacity is a key measure of ILs in the process of SO2 capture. To improve the SO2 absorption capacity of ILs, various strategies have been proposed. Multiple-site strategy is the most promising one and has been well developed. Multiple anion-sites, multiple electronegative atoms in the anion, introducing SO2-philic groups (including halide, cyano, formyl, cyano and phenyl groups) on the anion and introducing SO2-philic groups (including polyethylene glycol, and amine groups) on the side chain of the cations are well-developed efficient methods for high SO2 absorption capacity.43 The directly mixing of super bases (1,8-diazabicyclo-[5.4.0]-undec-7-ene, DBU; TMG) and glycerol/binary acid generates multiple anion-sites ILs for SO2 capture, such as [DBU][glycerol], [TMG][PBE], [TMG][SUB], [TMG][SUC] and [TMG][DOD] (Scheme 4),


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and their corresponding SO2 solubility are 3.0, 8.74, 5.96, 4.76, 5.96 mol SO2 per mole ILs at 25 °C, respectively. 44-45 Anions of the ILs, as the main factor, greatly influence the solubility of SO2. In these TSILs, two or three anions are formed in one molecule IL, and each anion can react with one SO2, greatly contributing to the high SO2 absorption capacity. Beside anions, SO2 also interacts with the carboxyl and adjacent methylene on the anion as well as the amino group on the cations. For [TMG][PBE], the existence of SO2-philic ether chain can improve SO2 solubility by S-O ligand interaction. For [TMG][SUB] and [TMG][DOD], the increased chain length in anion enlarges the asymmetry of ILs, decreases the space resistance, avoiding SO2 crowding in the interstice, and causing the hydrogen atom of methylene to interact with SO2 easily.44-45

O

N O

NH 3

NH2 2

S O

O

O

[DBU][glycerol]-3SO2

[TMG][SUB], 5.96

O O O

NH2 2

O

O

m

O

O

[TMG][PBE], 8.74

O

N N

O O

O

N N

O

O

N N

S O

O S

O

NH2 2

O

n

O

n= 1, [TMG][SUC], 4.76 n= 5, [TMG][DOD], 5.96


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Scheme 4. Structure of multiple anion-site ILs and their SO2 solubility

Anions are crucial for the SO2 capture. Dispersing the negative charge into several adjacent electronegative atoms, such as nitrogen, oxygen atoms, by highly delocalized conjugated system, can generate multiple sites to interact with SO2 molecules, what significantly improves the SO2 solubility in ILs. Imidazole, azole, benzimidazole, acylamido, acetylacetonate anions are ideal delocalized conjugated ones to produce functionalized ILs based their structures for SO2 capture, which are shown in Scheme 5. And the corresponding SO2 absorption ability of ILs based on these anions are listed in Table S2. As shown as Table S2, high SO2 solubilities (from 3.37 ([C10mim][Tetz]) to 5.75 ([P66614][BenIm]) mol SO2/ILs) are obtained. Being ascribed to multiple-site interactions between SO2 and anions by N•••S, C=O•••S and π•••S interactions between the anion and SO2, the SO2 absorption capacities are significantly enhanced. For example, [P66614][Tetz] can react with SO2 via the interaction between multiple electronegative nitrogen atoms in the anion and the sulfur atom with positive charge in SO2.46 The proposed interaction of electronegative nitrogen atoms with SO2 is depicted in Scheme


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6. [P66614][Phth] reacts with 5 SO2 molecules through multiple-site N•••S, C=O•••S and π•••S interactions between the anion and SO2, resulting in a high SO2 capacity of up to 4.40 mole per mole ILs.47 Phenyl-containing azole-based ILs [P66614][BenIm] presents an extremely high SO2 capacity of up to 5.75 mole per mole ILs.48 Promisingly, the absorption enthalpy can be facilely tuned by the interaction between the anion and SO2, providing excellent reversibility. For example, 28 absorption/desorption cycles of [P66614][Tetz] proves its well-maintained high absorption capacities and the rapid absorption rates.46 Furthermore, the combination of ether-functionalized cations and multiple-site tetrazole anions, such as [P444E3][Tetz] and [E3mim][Tetz], would further improve their SO2 solubility.49 [P444E3][Tetz] and [E3mim][Tetz] absorbe 5.0 and 4.43 moles of SO2 per mole of IL at 20 oC and 1 bar, respectively, while [P44410][Tetz] and [C10mim][Tetz] exhibited 4.0 and 3.37 moles of SO2 per mole of IL, respectively. On the other hand, multiple site in one anion shares the negative charge, leading to reduced reactivity and low gas capacity, especially at low SO2 concentration. Therefore, adequately decreasing the number of sites, but increasing their activity, can thus result in the increased number of active sites in practical. For [P4442][BTFA], [P4442][TTFA] (Scheme 5) with limited potential sites, high


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SO2 capacities up to 4.27/4.05 and 1.82/1.80 mol SO2 per mol IL can be achieved under 1 and 0.1 bar, respectively, compared with 1.43 and 0.24 mol SO2 per mol [P4442][NTf2]. Even at 0.01 bar, 1.34, 1.26 mol SO2 per mol IL could be obtained for [P4442][BTFA], [P4442][TTFA].49 These high SO2 absorption capacities are ascribed to the multiple-site interactions between SO2 and limited number of active sites in the anions.48-49 Cations: C 4H 9

C14H29 P

C6H13

C6H13

O 3

[E3mim]

[P44410]

N C10H21

N

C10H21

C 4H 9

[P444E3]

N

P

C 4H 9

O 3

C 4H 9

[P66614] N

P

C 4H 9

C6H13

C 4H 9

N

[C10mim]

C 4H 9 P

N

C 2H 4 C 4H 9

C 4H 9

[P4442]

[Bzmim]

Anions:

N

N

N

[Im]

N

N

N N

N

N N

[Tetz]

N

N

N

N

[2-Ph-Im]

[BenIm]

N

[BenTriz]

[Indz]

O O

N

O

N

O

N

O

O

O

N

O

O

O

N

N

O [Mal]

[Phth] O

O

[Glu] O

CF3 [BTFA]

O CF3

S [TTFA]

[DAA]

[Suc]

O OO O S S F3C N CF3 [NTf2]

[NPA]

O F3C

O CF3

[HFA]


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Scheme 5. Structures of ILs with multiple electronegative atoms in anions.

O

N

O + 5SO2

[P66614]

O O S O S O [P66614] O H

O

S O N

O

O S O S O O

H

[P66614][Phth] H [P66614]

N N N N

+ 4SO2

[P66614]

O S O

H N N N N

O S

[P66614][Tetz]

O S

O

S O O

O

Scheme 6. Proposed interactions between [P66614][Tetz] / [P66614][Phth] and SO2. Introducing SO2-philic groups, such as ether, amino groups on the side chain of the cations is a promising strategy, because it increases the absorption capacities of ILs for SO2, improves their physiochemical properties.51 As shown in Scheme 7, ether groups (polyethylene glycol, PEG) were introduced into pyridinium chloride ILs to generate [EnPy]Cl (n = 2-4), and their absorption capacity are up to 3.924, 4.289, 4.594 mol SO2 / mol IL at 20 °C and 1 bar, and 1.650, 1.855, 2.101 mol SO2/mol IL at 20 °C and 0.2 bar.52 PEG-functionalized ILs derived from DABCO were also proved to be highly efficient absorbents for SO2 absorption (Scheme 7).54 For example, PEG150MeDABCONTf2 has a


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high SO2 capacity (4.38 mol SO2/mol IL), even under 0.1 bar SO2 partial pressure (1.01 mol SO2/mol IL). The high capture capacities of these ether-functionalized ILs are presumably owing to the strong SO2-philic characterization of the PEG chain. As listed in Table S3, dual ether-functionalized protic ILs on both the cations and anions, such as [BDMAEEH][MOAc], [BDMAEEH][EOAc], [BDMAEEH][MEAAc], [DMDEEH][MOAc], [DMDEEH][EOAc], [DMDEEH][MEAAc], [EDBEAH][MOAc], [TMPDAH][BAc] (Scheme 7), exhibit low viscosities and remarkable SO2 loading capacities (up to 6.12 mol of SO2 per mol of IL and 1.34 g of SO2 per g of IL at 1.0 bar). 55 The possible absorption sites and absorption mechanisms of [BDMAEEH][MEAAc] are shown in Scheme 8. As shown in Scheme 9 and Table S3, PEG can also coordinate with an alkali metal to generate metal chelating cations, which could be employed for functionalized ILs with high SO2 absorption capacity up to 6.65 mol of SO2 per mol of IL at 1 bar SO2 and 2.79 mol of SO2 per mol of IL at 0.1 bar SO2.56 Their ability to absorb SO2 increases as the radius of the metal chelate cation increases, because the increase in ionic radius results in a weakening of the interaction between the anion and cation, allowing the oxygen atom to absorb more SO2.56a Tris(3,6-dioxaheptyl) amine (TDA-1) could also serve as


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environmental friendly polydentate ligands in synthesis of chelate-based ILs, such as [Li(TDA-1)][SCN], [Li(TDA-1)][Tf2N], which presented excellent SO2 absorption capacities. The absorption capacities of [Li(TDA-1)][SCN] and [Li(TDA-1)][Tf2N] were 5.50 and 4.00 mole SO2 per mole IL under 20 °C and 1.0 bar, 2.36 and 1.31 mole SO2 per mole IL under 20 °C and 0.1 bar, respectively.56c As shown in Scheme 9b, the excellent SO2 capture ability of [Li(TDA-1)][SCN] can be ascribed to the dual-site chemical interactions between SO2 and two active sites (N and S). Introducing amino groups could effectively improve SO2 capture capacity of ILs, because of the strong interaction between amino groups and SO2. [Et2NEmim][PF6],2 [Et2NEMim][Tf2N]) and [Et2NEMim][Tetz],57 had been synthesized for SO2 capture (Scheme 7). [Et2NEmim][PF6], [Et2NEMim][Tetz] and [Et2NEMim][Tf2N] can absorb 2.11, 4.32 and 2.81 mol SO2 / mol IL at 1 bar SO2, respectively. What’s more, [Et2NEmim][PF6] can absorb 0.94 mol SO2 per mole IL (30 oC, 3% SO2), the [Et2NEMim][Tetz] can capture 0.47 g SO2/g IL at 0.1 bar SO2 partial pressure. The main reason for these high capacity is that both the cation and the anion could capture SO2 chemically.58-59 Interestingly, [Et2NEmim][PF6] is hydrophobic, indicating that the SO2 absorption mechanism is


20

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different in the presence or absence of water, as shown as in Scheme 10.2,

51

In the

presence of water, SO2 can dissolve in water to generate H2SO3, which is acidic and ionizes to H+ and HSO3-. Subsequently, H+ can react with the basic moieties of ILs, such as amine, anions, to form [ILH]+HSO3- (Scheme 10). Accumulation of water in IL, the SO2 absorption capacity would be decreased dramatically based on the total mass of absorbent. Therefore, the water absorbed by ILs should also be taken into account in the process of desulfurization. N+ 2 Cl-

O

N+ 3 Cl-

O

[E3Py]Cl

[E2Py]Cl

[E4Py]Cl

O

O N

N

NH

O

HN

N

NH2 [BDMAEEH]

[DMDEEH]

O

O

O [MOAc]

n

N N

NTf2

O

N

N

N

O O F3C S N S CF3 O O [Et2NEMim][Tf2N]

O

O

[MEAAc] 3

Tf2N

NTf2

PEG150MeDABCOA

n=7,11 Cn+1DABCOA

O O

[EOAc] O

N N

[TMPDAH]

O O

N

N

NH

H 3N

[EDBEAH]

O

O

N+ 4 Cl-

O

N N

[BAc]

NTf2

N N

N

C8(DABCO)2(NTf2)2

N

N

[Et2NEMim][Tetz]

5

NTf2

HexMImNTf2

N

N

N N N N

N

PF6 [Et2NEmim][PF6]

Scheme 7. Structures of PEG or amino-functionalized Lewis basic ILs.


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O 2S O

O N

NH

O O

O

+ 6SO2

O

N O

S

+

N O

O

O S O

O

O

HO

S

O 2S O SO

O

2

Scheme 8. The mechanism of action of [BDMAEEH][MEAAc]

a) M=Li,Na,K O O

O M

O R1

O C9H19

R 1= O

N

F F F

N Im

PhO

O O N S S O O

O O

SCN

[M(TX-10)] R2=(C2H4O)5H O

SCN

Na+

O

[M(TX-7)] R2=(C2H4O)2H

O R2

S C N

O

-

[Na-tetraglyme][SCN]

F F F

Tf2N

b) N O O

N active site

O O Li+ O

O

N O

SCNS active site

[Li(TDA-1)][SCN]

O

O O Li+ O

O Tf2N-

O

[Li(TDA-1)][Tf2N]

O

O Li+

O

Tf2N[Li(G3)][Tf2N]

Scheme 9. ILs of PEG-coordinated metal chelating cations.


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In the absence of water: [Et2NEmim][PF6]

SO2

[Et2NEmim][PF6] . SO2

In the presence of water:

N

N

N

H2SO3

N

N

PF6

H N

HSO3-

PF6

Scheme 10. SO2 absorption mechanism of [Et2NEmim][PF6] in the presence or absence of water.

In order to increase the absorption capacity of SO2, various methods have been proposed, in which SO2 can be chemically absorbed through an adjustable multi-site. Introducing SO2-philic groups, such as formyl or cyano groups on the basic anion of ILs can effectively improve SO2 solubility. As shown in Scheme 11, formyl-containing ILs ([P66614][3-CHO-Indo],

[P66614][2-CHO-Pyro],

[P66614][4-CHO-Indo],

[P66614][4-CHO-

PhO]),59 cyano-containing ILs ([P66614][2-CNPyro], [P66614][4-CNC6H4O], [P66614][4CNC6H4COO])61 were designed for SO2 capture, and their SO2 absorption capacities are listed in Table S4. Similar results can also be obtained by introducing cyano groups on cations.62 By analyzing data in Table S4, it can be concluded that the introduction of formyl or cyano significantly increases the SO2 capture ability of ILs.


23


O

N

N

O

O [2-CHO-Pyro]

[4-CHO-PhO]

[4-CNC6H4O]

O [4-CHO-PhCO2]

COO O CN

CN

O

O

O [3-CHO-Indo]

O

Page 24 of 78

N

CN

CN [4-CNC6H4COO]

[OCN]

[2-CNPyro]

Scheme 11. ILs with SO2-philic groups (formyl or cyano groups) on the basic anions.

Besides these strategies for improving SO2, cavity-based IL is a kind of novel materials. Anion-functionalized pillararenes (Scheme 12) prepared by combining pillararenes with functional anions, have exhibited excellent SO2 absorption capacity. For example, 3BenIm presents an absorption capacity for SO2 of 15.9 mmol/g (42.1 mol/mol) and excellent reversibility by tuning the basicity of the anion and the size of the cavity.63 The high SO2 capacity originates from multiple sites interaction between SO2 and the anion, where SO2 chemical absorption is significant strengthened by the cavity because the anion is confined in the window of the cavity and the window is electron-deficient. Multiple sites on polymer can also provide excellent SO2 capture capacity. With multiple azolyl


24

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groups serving as binding sites, imidazolium poly(azolyl)borate 1-methyl-3-nbutylimidazolium hydrotris(imidazolyl)borate ILs exhibits high SO2 absorption capacity (up to 5.8 mol/mol of IL or 1.05 g/g of IL).64

N

O

H 5

O

H N 10

N

N 3BenIm

Scheme 12. Structure of anion-functionalized pillararenes 3BenIm.

2.4 Reducing the desorption enthalpy Recyclability with low desorption enthalpy is a critical factor for selecting ILs in SO2 capture process. Most ILs usually presents good recyclability, and this characteristic would not be discussed here. The chemisorption of SO2 has a high capacity for gas absorption along with a high absorption enthalpy, always followed by high energy demand for the regeneration of ILs, which hinders the practical application of ILs in SO2 absorption.65 To reduce the desorption enthalpy, several methods, including reducing the


25


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basicity66 and introducing an electron-withdrawing substituent on the anion,67 have been developed. One strategy to reduce the strength of interaction between the anion and SO2 for highly reversible SO2 capture is to decrease the electronegativity of the interaction site in the anion.68 Two nitrile-containing anion-functionalized ILs, [Emim][SCN] and [Emim][C(CN)3], as shown in Scheme 13, exhibited high absorption capacity (2.99 mol SO2/mol IL and 2.33 mol SO2/mol IL) as well as rapid absorption kinetics.69 Compared with chemisorptions of SO2 by traditional anion-functionalized ILs. The absorption enthalpies by [Emim][SCN] and [Emim][C(CN)3] are relatively low, resulting in excellent reversibility due to the weak interaction between the less electronegative sulfur or carbon in the anion and SO2.70 N N

N

S

N

N

N N

[Emim][SCN]

N

[Emim][C(CN)3]

Scheme 13. The structures of [Emim][SCN] and [Emim][C(CN)3].


26

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However, these methods for easy desorption of SO2 often led to reduced capacity, owing to a decrease in the interaction between the IL and SO2. Increasing the capacity and simultaneous reducing the enthalpy of desorption is promising, and can be realized by introducing an electron-withdrawing interaction site (such as halogen groups) on the anion (Table S5).71 The effect of different ILs with halogen groups on the absorption of SO2 was investigated (Table S5). It was found that the SO2 capture capacities of [P66614][4-BrC6H4COO], [P66614][4-ClC6H4COO] and [P66614][4-FC6H4COO] were 4.12, 3.93, and 3.96 mol SO2 per mol IL, respectively, whereas that of [P66614][PhCOO] was 3.74 mol SO2 per mol IL. Compared to non-halogencontaining benzoate-based ILs, benzoate-based ILs with a halogen group exhibit higher SO2 absorption capacities, being ascribed to halogen-sulfur interaction between the halogen group on the anion and SO2. On the other hand, the absorption enthalpy decreases relatively, which means the captured SO2 can release more easily from the ILs. The halogen group is an electronwithdrawing group, which dispersed the negative charge of the O atoms on the anion and decreased the enthalpy for SO2 absorption (Table S5), resulting in the improved desorption.


27


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Relatively weak chemical interactions exist between the -COO- of the carboxylic acid anion and SO2, which can be used for designing ILs with ease of desorption (Scheme 14). In addition, when the carboxylic acid anions have a hydroxyl group72a or a halogen atom,72b highly efficient capture of SO2 would be observed. The -OH group can not only be an added interaction site to enhance SO2 capture, but also serve as an electronwithdrawing group, which weakens the interaction between SO2 and -COO-, improving the desorption efficiency of SO2.72 For example, [N2222][Malate], [N2222]2[Malate] have desorption efficiencies higher than those of [N2222][Succinate], [N2222]2[Succinate] because of the existence of -OH of the carboxylate anion (Scheme 14).72a O [N2222]+

O

O OH

[N2222]+ O

O

[N2222]

+

OH O

O [N2222]P

[N2222]

O

O [N2222]

O OH

[N2222]L

[N2222][Malate]

[N2222][Succinate]

O +

+

2

OH

O

O

O

[N2222]

O [N2222]2[Succinate]

+

2

O

O OH O

[N2222]2[Malate]

Scheme 14. Carboxylic anion-based ILs for SO2 capture.


28

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2.5 Green ILs for SO2 absorption ILs has attracted worldwide attention as substances for absorbing SO2 efficiently. On the other hand, under the call of green chemistry and sustainable development strategies, more and more are moving toward green, non-toxic and efficient, and the preparation of ILs is getting closer to green chemistry. Biodegradable TSILs are increasingly popular. As shown in Scheme 15, green, non-toxic, biodegradable active ILs based on betaine ([C4bet][NTf2],

[C4bet][SCN]),73

furoate

([P4442][FA],

binary

absorbent

[N2222][FA]/PEG200),74-75 amino acid ([Emim][Ala]/water) have been employed for SO2 capture. These functionalized ILs have higher biodegradability than ordinary ILs because of the presence of biodegradable moieties in their structure. [C4bet][SCN] exhibits SO2 absorption capacity of 0.93 mol per mol ILs at 0.1 bar SO2.73 At 20 oC, the maximum absorption capacity are 0.69 and 0.24 g SO2 per g [P4442][FA] under 1.0 and 0.1 bar, respectively. 74 The binary absorbent [N2222][FA]/PEG200 can reach an absorption capacity of 7.224 mol per kg at a temperature of 30 oC and 0.1 bar SO2.75 Moreover, these ILs exhibit the advantages of good reversibility, recyclability, biodegradability and low energy consumption.76


29


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Quaternary ammonium inner salts betaine (Bet) and L-carnitine (L-car), as novel kinds of environmentally benign and stable absorbents, were also applied to efficiently and reversibly capture SO2 (Scheme 15). And their capacities of SO2 with 50% inner salt mass fraction are 0.155 mol SO2/mol of Bet for Bet aqueous solutions ， 0.599 mol of SO2/mol of L-car for L-car aqueous solutions, at 40 °C with a SO2 concentration of 2%. What’s more, the absorption capacities of absorbents maintained after 5 absorption/desorption cycles.77 O

N

NTf2

O

O

N

-

[C4bet] [SCN]

[C4bet] [NTf2]

C 4H 9

C 4H 9 P C 2H 5 C 4H 9

O

O C 2H 5

O

C 2H 5 N C 2H 5 C 2H 5

[P4442][FA]

CO2

N

O O

OH

-

[Emim][Ala]

O N

Bet

O

O O

[Ch][FA]

O O

O

[N2222][FA]

NH2

N

N

SCN-

O

OH N

O L-car

Scheme 15. The structure of several green ILs.


30

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2.6 Selective absorption between SO2 and CO2 SO2 and CO2, two kinds of acid gases, are both from the burning of fossil fuels together, and threat the environment of our world, especially SO2. Solubility studies of SO2 and CO2 in many RTILs have been reported.78 Unexpectedly, for ILs designed for CO2 absorption, a gradual reduction was observed in the CO2 capacity of ILs on exposure to SO2, which was ascribed to the inability of the regeneration conditions to release the SO2 captured due to the strong interaction of ILs with SO2.78b What’s more, SO2 and CO2 can be used as sulfur source or carbon source in industry. Therefore, there is significant practical value to selectively capture SO2 or CO2 using different ILs from the flue gas mixture, then release SO2 or CO2 respectively to gain pure SO2 or CO2 gas. For normal ILs, such as [HMIm]Tf2N, could selectively absorb SO2 with relatively higher capacity than that of CO2.79 When the two acidic gas mixture are present at low concentrations, there is little competition between them in ILs. However, as more gas dissolve in ILs, the available free volume in the liquid decreases, SO2 and CO2 compete reciprocally. The calculated selectivity value of SO2 over CO2 in [HMIm]Tf2N, is in the range from 10 to 12, close to the ideal value of 14 obtained from pure component Henry’s


31


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law constants (Table 1). Although the solubility of SO2 is usually much greater than that of CO2 in [HMIm][Tf2N], the solubility are actually similar at the reduced partial pressure, impeding the selectively absorption of SO2 / CO2 gases.80,27 For TSILs, according to the criterion of distinguish whether the ILs are functional for SO2, CO2 capture, there is a difference between their pKa values of the corresponding acids (H2SO3, H2CO3), which could be employed to design TSILs with selective capacity toward SO2. The pKa of H2SO3 is 1.8, while the pKa of H2CO3 is 6.36. Therefore, by tuning the pKa of ILs between 1.8 and 6.36, a high SO2/CO2 selectivity could be obtained when ignoring the reactivity of SO2 with other functional groups. [TMG]L could absorb the SO2 and CO2 gases selectively. The capacity of the [TMG]L with CO2 was quite lower than that of SO2. CO2 has relatively poor association with [TMG]L, and SO2 is highly associated with TMG, which explained well the selectivity of the [TMG]L toward the SO2 and CO2 (Table 1). 38-39 Table 1. The selectivity of SO2/CO2 in several ILs

ILs

SO2(mol)/CO2(mol)

T / oC, P / SO2 bar

Ref.


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[HMIm]Tf2N

10 (5 bar)- 12 (10 60 oC

79-80, 27

bar) [DMPANH][MOAc]

119

30 oC, 1.0 bar

62

[DMAPNH][EOAc]

107

30 oC, 1.0 bar

62

[HexMIm]NTf2

45

25 oC, 1.0 bar

54

[PEG150MeDABCO]NTf2

110

25 oC, 1.0 bar

54

[TMG]L

84

40 oC, 1.0 bar

22

[Hmim][2-Cl-Ben]

33.9

60 oC, 0.01 bar

81

[Hmim][2-NO2-Ben]

227.6

60 oC, 1 bar

81

As depicted in Scheme 16, introducing SO2-philic groups, such as PEG, cyano groups, rather than CO2-philic groups could effectively improve SO2 selective absorption. PEGfunctionalized Lewis basic ILs are also good absorbents for selective separation of SO2 and CO2. The absorption capacity ([PEG150MeDABCO]NTf2) of SO2 and CO2 was 4.38 and 0.04 mol of gas/mol of IL at 25 oC, 1.0 bar SO2, respectively.54 Compared with the non-functionalized imidazolium IL ([HexMIm]Tf2N) with a SO2/CO2 selectivity of 45, SO2/CO2 selectivity of [PEG150MeDABCO]NTf2 (Scheme 16) increased to 110 due to the high SO2 capacity and low CO2 capacity of PEG moiety. Cyano-containing ILs [DMPANH][MOAc] and [DMAPNH][EOAc] exhibit good selectivities of SO2/CO2


33


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absorption, (119 and 107, respectively), at 30 oC and 1.0 bar, primarily due to the existence of the cyano group on the cation of ILs.62 Substituted benzoate-based ILs have been employed to investigate the effect of alkalinity of ILs on the selective absorption of SO2/CO2. Their alkalinity of substituted benzoate-based ILs can be easily tuned through altering the substitutions on the benzene ring. The maximal value of S(0.01/1) (mSO2 (0.01)/mCO2 (1)) is 33.9 for [Hmim][2-Cl-Ben], and the optimized pKa of ILs is around 3.00 to enable maximized S(0.01/1). The largest value of S(1/1) (mSO2 (1)/mCO2 (1)) is 227.6 for [Hmim][2-NO2-Ben].81


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a) O N N

3

NTf2

[PEG150MeDABCO]NTf2 b) N

O

O

CN [DMAPNH]

O

O

Et

O

O O

O

CN [MAPNH]

Me

[MOAc]

[EOAc]

H N H

O

O

Et

[EOAc]

O

O

Me

[MOAc]

c) O N

O

N nC6H13

[Hmim]

R [R-Ben]

R = 2-Cl, [Hmim][2-Cl-Ben] R = 2-NO2, [hmim][2-NO2-Ben]

Scheme 16. ILs with SO2-philic groups for selective SO2 absorption.

[Hmim][NTf2] and [Emim][OAc] can be immobilized into the porous structure of an activated carbon substrate, either by physical imbibition, or chemical grafting. Physically immobilized [Emim][OAc] within the pore structure of an activated carbon support resulted in astonishing SO2/CO2 selectivities with values of 306 (25 oC), 396 (40 oC) for gas mixture compositions simulating dehydrated combustion effluents, but could not preserve their


35


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enhanced absorption and separation performance after regeneration.82 Materials prepared with [Hmim][NTf2] endured stability tests with regeneration at 200 oC, albeit with reduced performance.82 Recently, a highly crosslinked non-porous polyionic liquid xerogel were synthesized by quick photopolymerization of Gemini IL monomers (Scheme 17) in aqueous solution without other comonomers and crosslinkers.83 This novel dense nonporous ionic xerogels/microgels exhibits a very high selective absorption for SO2/CO2 (614, P(D[VImC6]Br), SO2/CH4 (1992, P(D[VImC6]Br) (SO2/N2 and SO2/CO2 mixtures with 2000 ppm SO2), a high absorption capacity for SO2 (489-514 mg·g−1), and excellent reproducibility performance, proving the potential application of this type of immobilized ILs as SO2 sorbent in gas desulfurization.

N

N

N

X

N

N D[VImC6] N

X X = Br-, SCNBr

[VBIm]Br

Scheme 17. The structure of monomers D[VImC6]Br and [VBIm]Br.

2.7 Eutectic ionic liquids application in SO2 absorption


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Recently, some eutectic solvents with the similar properties of IL, sometimes also defined as “eutectic ionic liquids (EILs)” have aroused wide concern.84-85 EILs is usually obtained by mixing two components, one of which acts as a hydrogen bond acceptor (HBA) and the other as a hydrogen bond donor (HBD) but the EILs obtained has high purity and low melting point. Most EILs exhibit liquid at room temperature mainly because of the formation of intermolecular hydrogen bonds between the components. EILs such as ChCl-urea (ChCl: choline chloride),86-87 LiTf2N-acetamide,88 [EMIm]Cl-urea89 and CPLTBAB (caprolactam - tetrabutylammonium bromide)90 have been reported, presented similar properties to traditional ILs and favourable application prospects in electrochemistry, organic synthesis and isolation process. These EILs were prepared from organic quaternary ammonium/phosphonium salts and organic low molecular weight compounds, such as urea, CPL, acetamide, imidazole, carboxylic acid, alcohol or phenol as hydrogen donors. Compared with traditional ILs, the preparation process of this kind of EILs is very simple and environmentally benign, and the raw materials are easily available with low cost. From view point of green chemistry, they are suitable for largescale production and industrial application.


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CPL-TBAB,91-92 CPL-acetamide (1 : 1), CPL-imidazole (1 : 1), CPL-furoic acid (1 : 1), CPL-benzoic acid (1:1), CPL-ο-toluic acid (2 : 1), acetamide-KSCN (3 : 1), CPL-KSCN (3 : 1),93 acetamide-NH4SCN (3 : 1), CPL-NH4SCN (3 : 1) urea-NH4SCN (3 : 2)94 EU/BmimCl (1:2) , EU/EmimCl (1:1), EU/BmimCl (1:1) and TEG/EmimCl (1:6)95 have been reported for SO2 capture, and their SO2 absorption capacity are listed in Table S6.[91-105] For CPLTBAB in SO2 absorption, the mole fraction solubility is up to 0.680 mol SO2/mol IL at 25 oC

and ambient pressure.91 The solubility (g/100 g solutions) of SO2 in CPL-TBAB/water

(4 mol L−1) mixture is 52.29 at 20 oC.92 CPL-imidazole (1:1) can capture SO2 with 0.624 g/g of mass fraction at 30 °C and 1 atm of pure SO2.93 The SO2 solubility of acetamideKSCN (3 : 1), CPL-KSCN (3 : 1), acetamide-NH4SCN (3 : 1), CPL-NH4SCN (3 : 1) and urea-NH4SCN (3 : 2) were 0.588, 0.607, 0.579, 0.595, 0.372 (30 oC) g g-1 of mass fraction at 20 oC and 0.1 bar, respectively.94 Under the environmental conditions of 293.15K, 1atm, the SO2 absorption capacity of EU/BmimCl (1:2) can reach 1.18 gSO2/gDESs, and even at 0.1 atm, its absorption capacity can reach 0.42 gSO2/gDESs.95As depicted in Scheme 18, the SO2 absorption processes of these EILs are practically reversible, following a physical or chemical absorption process.Alcohol compounds, such as ethylene glycol


38

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(EG), triethylene glycol (TEG), phenols (guaiacol, GC; cardanol, CL), glycerol (Gly), are usually employed in EILs. As shown in Scheme 18, various EILs have been designed for SO2 absorption, including [Emim]Cl-EG,96 [Emim]Cl-TEG,97 Bet-EG, L-car-EG,98-99 ChClGC, ChCl-CL,100 Im-Gly and [Bmim]Cl:4-methylimidazole (4CH3-Im) (1:2).105-106 For Lcar-EG (1 : 5), the molar ratio of SO2 to L-car can reach 0.859 at 40 ◦C with a SO2 partial pressure of 0.02 atm.98-99 GC-CC (3 : 1) has the SO2 absorption capacity up 0.528 g SO2 per g DES.100 The available absorption of 2000 ppm SO2 in Im-Gly (1:2) is up to 0.634 mol of SO2/mol of Im (0.161 g of SO2/g of DES) at 40 oC.105 [Bmim]Cl:4-methylimidazole (4CH3-Im) (1:2)could absorb 1.42 and 0.613 gSO2/gDES at 1.0 bar and 0.1 bar SO2 partial pressure, respectively.106b [Emim]Cl-EG (2 : 1) has high SO2 absorption capacity of 1.15 g SO2/g solvent (53 wt%).96 [Emim]Cl-TEG (6 : 1) has the excellent SO2 absorption capacity of 0.54 g SO2/g of solvent at 0.1 atm and 20 °C.97 Furthermore, the high absorption capacity of [Emim]Cl-EG, [Emim]Cl-TEG is partially ascribed to the charge transfer when SO2 interacts with Cl-. Some EILs also exhibit good performance for SO2 absorption selectivity, such as GC-CC (3:1). Under the environmental conditions of 293.15 K and 1 bar, the selectivity of SO2/CO2 is 258 for GC-CC (3:1).100 It is worth noting


39


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that the most of EILs have almost no loss of activity of recyclability and selectivity after undergoing

many

N

cycles

of

absorption

and

N

N

desorption.

N

HO

HO OH

Cl

O N

O N

O OH

HO

OH

Cl SO2

SO2 O OH

HO SO2 absorption

OCH3

OH

OH

HO

H3CO

N

OH

SO2 desorption

OCH3

OH

OH

N

N

N

Cl

H

Cl N

HO

OH OCH3

H

N H

CH3 O

SO2

Cl

Cl

N

OCH3

N

H N

N

N

O O S O S O N N H H

N

Scheme 18. The mechanism of SO2 absorption in several DESs.

2.8 Supported ionic liquids for SO2 absorption


40

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ILs exhibit many promising properties for absorbing SO2. However, from the viewpoint of industrial practice, the high viscosity of ILs and the slow sorption rate could be operational drawbacks due to relatively small gas-liquid interface and slow diffusion rate of gas molecules in viscous media. In addition, the high viscosity and other inherent disadvantages such as large investment and high operation cost of wet desulfurization make ILs inferior to solid absorbents used for dry desulfurization. Solid-state or solidsupported IL absorbents could be more attractive in practicability than pure ILs for gas separation and purification. To obtain solid-state IL absorbents, polymerization and immobilization of IL on supported materials by physical or chemical methods are commonly used. Supported materials, such as porous silica particles (SiO2), mesoporous molecular sieve, activated carbon, polystyrene resins and silica nanoparticles, are very popular. As shown in Scheme 19, solid polymers could be synthesized by monomeric- or copolymerization of TMGA(1,1,3,3-tetramethylguanidine acrylate) and MBA (N,Nmethylenebisacrylamide) to provide poly(TMGA)107 and Poly(TMGA-co-MBA) for SO2 absorption.108 Poly(TMGA) can absorb SO2 with high capacity (up to 1.75 mole of SO2


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per mole of TMG) and rate.107 However, it experiences volume shrinkage and shape deformation during the cycling. Poly(TMGA-co-MBA) exhibits excellent SO2 absorption properties (up to 1.35 mole of SO2 per mole of TMGA unit), and keeps the particle morphology (shape and volume) and the SO2 absorption properties unchanged during the SO2 absorption/desorption cycles.108 O HC O CH2

H2 N

N N

O H 2C

C H

TMGA

N H

H2 C

O N H

C H

CH2

MBA

Scheme 19. Structure of TMGA and MBA monomer.

Supported ionic liquid phase absorbents (SILPs) usually consist of thin layers of ILs, physically or chemically stabilized on the internal (pore) and external surface of porous solids and nanoparticles with large specific area, achieving of the highest possible gas/IL interface, enhancing the absorption/desorption rate, benefiting their application in continuous flow processes for flue gas cleaning as the solid form of the absorbents. [TMG]L supported onto porous silica particles showed high capacity of 0.6 g SO2 /g TMGL in 15 - 30 min with pure SO2 gas, and 0.15 g SO2/g TMGL in 17 h with a N2 / SO2 mixture


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gas containing 2160 ppm SO2.109 [C3O1Mim][H3CSO3] (1-ethylene glycol monomethyl ether-3-methylimidazolium methanesulfonate) supported on silica gel SiO2 (0.1/1) can absorb 0.11 g SO2/g [C3O1Mim][H3CSO3]-SiO2 (0.1/1) at 25 oC and atmospheric pressure, which is equivalent to 1.1g SO2 per gram of [C3O1Mim][H3CSO3].110 [TMG]L supported on mesoporous molecular sieve MCM-41 ((MCM-41-TMGL-10%) exhibits a high absorption rate of 0.682 mol SO2 / mol IL.111 Low IL loading level (~10 wt % supported on SiO2 and MCM-41) provides relatively stable performance during cycling experiments (~5% losses in both cases),110-111 while higher loading over MCM-41 yields a continuous activity loss, probably being attributed to strong binding of SO2 at absorption sites.111 1-Ethyl-3methylimidazolium acetate ([Emim]Ac) supported on activated carbon exhibits a sorption capacity of 12.6 mg SO2/g sample.112 The loading of ILs on the these porous materials not only maintains the original liquid state but also increases the gas diffusion rate because of their loose porosity. The diffusion of gas relies on mesopores and micropores, which also has extremely high thermal stability and reproducibility. These immobilized IL materials can be reused in several absorption / desorption cycles, without significant loss of their sorption capacity and rate. Diamine-anchored merrifield resins, as shown in


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Scheme 20, were found to exhibit excellent performance as SO2 adsorbents under both hydrous and dry conditions, besides excellent water stability, thermal stability, and reproducibility.113 The SO2 absorption capacity of [TMEDAMR]Cl reaches up to 3.40 mol of SO2 per mol of IL. XN

N

[TMEDAMR]X

N

X-

X-

XN

[DMPMR]X

N

N

[DABCOMR]X

N [TEAMR]X

X = Cl, CH3SO3, NTf2

Scheme 20. Diamine-anchored merrifield resins for SO2 absorption.

Inverse supported IL absorbents, denoted as “inverse SILPs”, can be easily prepared in the form of flowing powder via a phase inversion technique and consist of tiny IL droplets enclosed into an ultrathin, porous solid sleeve of pyrogenic silica nanoparticles. [Bmim][Cl]-based “inverse SILP”, shows slightly enhanced absorption capacity by dissolving 5 wt % of chitosan to form the respective ionogel. The material’s performance is stable in repeated absorption/regeneration cycles at 60 °C under helium flow, exhibiting SO2/CO2 selectivity of above 300 at 25 °C, in a gas stream of 1 bar composed of 0.13 vol % SO2, 13 vol % CO2, 11.5 vol % O2 and N2 (balance).114 1,3-Bispropyltriethoxysilane-


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imidazolium chloride (FILs)-modified SBA-15 adsorbents have been synthesized for SO2 capture according to Scheme 21. 10% FILs@SBA-15 exhibits a good absorption over 36.0 mL g−1 at 30 oC, which is contributed to both support and FILs-based sites, and the combination of surface area and pore volume.114

N

N

Cl O Si O O

O O Si O

P123 + 40 ml 0.25 mol L-1 HCl

N

48 h, 100 oC

N

Cl [O]

O O O Si O

Si [O] [O]

[O] Si [O] [O]

SBA-15

Scheme 21. The synthesis of 1,3-bispropyltriethoxysilane-imidazolium chloride (FILs)modified SBA-15 adsorbents

2.9 ILs membranes for SO2 absorption Selective permeation through supported ionic liquid membranes (SILMs) is one of the most attractive approaches for gases separation. ILs can be impregnated in the porous supports of membranes. The non-volatility and incredible stability of ILs enable the membrane separation process to avoid the loss of the supported liquid and the contamination of the gas streams.116 Therefore, integrated SILM systems can be


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employed to remove and recover SO2 from flue gas simultaneously. [Emim][BF4], [Bmim][Tf2N], [Bmim][BF4], [Hmim][BF4], [Bmim][PF6] have been supported on the polyethersulfone (PES) microfiltration membranes, and the SO2 permeabilities versus driving force (cross-membrane SO2 partial pressure difference) for the SILMs tested at 25 °C are shown in Table S7.117 [Emim][BF4] has the highest SO2 permeability, 9350 ± 230 barrers, while [Bmim][PF6] has the lowest SO2 permeability, 5200 ± 117 barrers (measured at 20 kPa of SO2 driving force). The relative SO2 permeabilities appear to be related to the viscosities of the ILs, with the tendency of the highest to lowest SO2 permeabilities, [Emim][BF4] > [Bmim][Tf2N] > [Bmim][BF4] > [Hmim][BF4] > [Bmim][PF6]. Triethylbutylammonium dimalonate ([N2224][dimalonate]) supported on hydrophilic PES membrane with a pore size of 0.22 μm can facilitate the permeation of SO2 with a permeability of 7208 barrers (0.05 bar transmembrane pressure difference, 40 oC) with good permselectivities of SO2/CO2.118 Physical imbibition of 1-alkyl-3-methylimidazolium tricyanomethanide

([RMIM][TCM])

and

1-ethyl-3-methylimidazolium

trifluoromethanesulfonate ([EMIM][OTf]) ILs into nanoporous ceramic bearing a mesoporous separation layer, provides excellent SILMs materials for SO2/CO2 capture.


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The [OMIM][TCM] supporting membrane showed the highest SO2/CO2 selectivity values, reaching up to 30.7 at 25 oC under isobaric conditions, even though it was markedly reduced with rising temperature and applied pressure gradient. On the other hand, due to the contribution of chemical interaction between [OTf] and SO2, the [EMIM][OTf] membrane showed the highest SO2/CO2 selectivity (9.5) at 70 oC.119 3. CONCLUSIONS AND OUTLOOK The excellent applications of ILs as promising absorbents for SO2 absorption are ascribed to their attracting characteristics, such as negligible volatility, ease of structure tenability and so on. In this brief review, the recent investigations on SO2 absorption with common ILs, TSILs, eutectic ILs, supported ILs and IL membranes have been reviewed. Strategies of improving SO2 absorption capacity, reducing desorption enthalpy, and SO2/CO2 selectivity have been discussed. From the viewpoint of absorption process, recent progresses on improving SO2 absorption capacity and reducing desorption enthalpy have been addressed. On the other hand, it should be notable that the current advances on SO2 absorption is still relatively few, mainly limited in the form of laboratory experiment right now. To satisfy industrial demands, lots of issues need to be deeply


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investigated. Therefore, more related studies and lots of efforts are urgently desired to realize commercial application of IL-based SO2 absorption technology. First, the structure-activity relationship between IL structures-physical properties-SO2 absorption/desorption performances is one of the challenges, which is key for the efficient, reversible IL-based SO2 capture process. Although the structure-activity relationship have been widely investigated and discussed in this review, more compressive information about the SO2 absorption mechanism, SO2/CO2 selectivity need to be well investigated. The lack of availability of cheap and green ILs is another important obstacle in practical utilization of IL systems for SO2 absorption on large scales. As ILs are being used on laboratory scale with complex synthesis and purification steps currently, mass production of ILs with increased stability could decrease the cost of the ILs based SO2 capture system. Second, the combination of ILs with promising technologies to avoid the high viscosity of ILs offers a more practical strategy for SO2 separation. The introduction of ILs into aqueous solution, membranes or porous materials is one excellent strategy for SO2 absorption. In spite of rapid development of IL-based membranes, porous materials-


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supported ILs in recent years, there is still an enormous gap between the lab-scale and the real application. Lots of efforts are highly desired to address these issues. Additionally, taking into consideration the complex species and environments in the real flue gas, including temperature, pressure, humidity, impurity gases (ash, N2, O2, CO2 and so on), more information and data under more realistic operating conditions should be collected. For example, the presence of moisture, CO2 and O2 probably has significant effects on the SO2 absorption capacity, recyclability and stability of ILs. Therefore, more investigations on the SO2 absorption in the practical flue gas should be developed. Although the existence of difficulties hinders the commercial application of ILs for SO2 capture, it is believed that these barriers would be overcame in the near future, making SO2 capture by ILs economically more viable. ASSOCIATED CONTENT The following files are available free of charge. Table S1 - S7 are placed in the Supporting Information.

AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected] (S. Zhang); phone/fax: +86-0538-8241570

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

We are grateful to the Natural Science Foundation of Shandong Province (ZR2017BB025, J16LC15), and Youth Science and Technology Innovation Fund of Shandong Agricultural University (24166) for financial support.

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