Improving SO₂ capture by tuning functional groups on the cation of pyridinium-based ionic liquids

Abstract

In this work, three kinds of novel functionalized ionic liquids (ILs) [NEt₂C₂Py][SCN], [C₄OPy][SCN] and [C₄CNPy][SCN] were developed by introducing a tertiary amino group, ether group and nitrile group on the pyridinium cation to improve SO₂ absorption performances. Among the investigated ILs, [NEt₂C₂Py][SCN] showed the highest absorption capacity of 1.06 gSO₂ gIL⁻¹ under ambient conditions due to a combination of chemical and physical absorption. By contrast, the enhancement in SO₂ capacity by [C₄CNPy][SCN] and [C₄OPy][SCN] is mainly ascribed to the stronger physical interaction between ILs and SO₂ than the conventional IL [C₄Py][SCN]. Meanwhile, higher SO₂/CO₂ selectivity was also obtained using these functionalized ILs, which was increased up to 41% comparing with that of [C₄Py][SCN]. Moreover, the effect of water on SO₂ capacity and the absorption mechanism were studied. The results indicated that the presence of water caused a slight decrease in SO₂ capacity of [C₄CNPy][SCN] and [C₄OPy][SCN] because of physical absorption, whereas a slight increase in SO₂ capacity by [NEt₂C₂Py][SCN] due to the formation of hydrogen sulfite salts through chemical absorption. In addition, three kinds of cation-functionalized ILs could remain the stable absorption performance after five cycles of absorption and desorption, implying these ILs show great potentials for SO₂ capture.

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1. Introduction

The increasing emission of SO₂ from the combustion of fossil fuels has inevitably brought about very adverse influences on the environment and human health. Up to now, some traditional methods for flue gas desulfurization (FGD) have been developed, such as limestone scribbling, ammonia scribbling and organic amines scribbling.^1–3 Among them, although limestone scribbling as one of the most attractive approaches has been applied widely in industries, this irreversible process usually results in huge amounts of byproducts and wastewater that doesn't comply with the sustainable principles. For amine scrubbing that can efficiently minimize additional pollutants and recover SO₂, yet they also possess inherent drawbacks, such as the loss of solvents due to their high volatility and degradation as well as high energy consumption for regeneration.^4–7 Therefore, the design and development of a proper absorbent for efficient and reversible capture of SO₂ is of critical importance in the absorption process.

Recently, ionic liquids (ILs) have drawn extensive attentions as competitive candidates in the field of acidic gases (CO₂,^8–15 SO₂,^16–23 and so on) capture and separation owing to their unique properties, such as extremely low vapour pressure, high thermal stability and tuneable properties.^24–26 It was reported that in general SO₂ has optimistic physical solubility in conventional ILs, and compared with the cation of ILs the anion plays a primary role in SO₂ capture.^27–29 Therefore, a great number of anion-functionalized ILs, such as azole-based ILs,^30–33 benzoate and phenolate-based ILs with halogen groups,³⁴ dicarboxylic acid-based ILs,^35,36 lactate-based ILs,³⁷ acetate-based ILs,³⁸ phosphate-based ILs,³⁹ were designed and synthesized for enhanced SO₂ capture by introducing different interaction sites on the anions. Among them, the phenyl-containing azole-based ILs showed the highest capacity of SO₂ absorption, which could reach 5.75 molSO₂ molIL⁻¹ (0.61 gSO₂ gIL⁻¹) at 20 °C and 0.1 MPa. However, there are very few types of cation-functionalized ILs for SO₂ capture, which mainly focused on ether-functionalized ILs^32,40,41 and the TMG-based ILs.^13,42,43 For example, Hong et al.⁴¹ developed the ether-functionalized imidazolium-based ILs as highly efficient SO₂ absorbents, which could exhibit very high capacity of up to 6.30 molSO₂ molIL⁻¹ (0.74 gSO₂ gIL⁻¹) at 30 °C and 0.1 MPa. Subsequently, Wang et al.³² designed a new ether-functionalized IL with tetrazolate anion for SO₂ capture, and the absorption capacity could achieve as high as 5.00 molSO₂ molIL⁻¹ (0.76 gSO₂ gIL⁻¹) at 20 °C and 0.1 MPa through chemical and physical absorption. Based on the above discussion, it was proved that in fact the functional groups on the cation also have a very positive effect on SO₂ absorption besides the anions. Meanwhile, although these functionalized ILs have extremely high molar capacity of SO₂ absorption comparing with conventional ILs, their gravimetric capacity of SO₂ is still relatively low.

In our previous work,²⁷ considering the advantages of pyridinium-based ILs such as high thermal stability, lower cost and higher biodegradability, a series of conventional pyridinium-based ILs were synthesized and used for SO₂ capture. It was found that [C₄Py][SCN] has very high gravimetric capacity of 0.84 gSO₂ gIL⁻¹ under ambient conditions, which is much higher than that of most reported functionalized ILs. As a successive work towards further exploring more highly efficient ILs for reversible capture of SO₂, a new method was developed by introducing different active sites, such as the tertiary amino group, the nitrile group and the ether group onto the cation of pyridinium-based ILs with the thiocyanate anion. In this study, three kinds of novel cation-functionalized ILs [NEt₂C₂Py][SCN], [C₄OPy][SCN] and [C₄CNPy][SCN] with high gravimetric capacity of SO₂ absorption were synthesized. The effect of different functionalized groups on their physicochemical properties and the performances of SO₂ absorption were systematically investigated. Meanwhile, the effect of different conditions (such as temperature, partial pressure of SO₂ and the water contents in ILs) on the absorption performances, SO₂/CO₂ selectivity and the regeneration and recyclability of ILs were also studied. Moreover, the role of the tertiary amino group, nitrile group and ether group in SO₂ capture was well explained through both the experiments (FT-IR and NMR spectroscopy) and Quantum Chemical calculations.

2. Results and discussion

2.1 Physicochemical properties

The physicochemical properties of four pyridinium-based ILs with the same anion [SCN] including the tertiary amino-functionalized IL ([NEt₂C₂Py][SCN]), the ether-functionalized IL ([C₄OPy][SCN]), the nitrile-functionalized IL ([C₄CNPy][SCN]) and the conventional IL ([C₄Py][SCN]) were studied. Density and viscosity of the ILs in this work as a function of the temperature were shown in Fig. S1 and S2,† and the effect of different substituent groups on the pyridinium cation on their physical properties were studied. Clearly, the groups on the cation of pyridinium-based ILs have an obvious effect on the density and viscosities of ILs. As seen in Fig. S1,† comparing with the conventional IL [C₄Py][SCN], two kinds of cation-functionalized ILs, such as [C₄OPy][SCN] and [C₄CNPy][SCN], showed the higher density due to the introduction of ether and nitrile groups on the pyridinium cation, while there was only a slight decrease in the density of the amino-functionalized IL [NEt₂C₂Py][SCN], which was very close to that of [C₄Py][SCN].²⁷ Similarly, as shown in Fig. S2,† the grafting of a tertiary amino group and a nitrile group onto the cation notably leaded to an increase in viscosity, while the incorporation of an ether group could effectively decrease the viscosity. For example, the viscosities of [NEt₂C₂Py][SCN] and [C₄CNPy][SCN] were 612.43 and 313.46 mPa s at 20 °C, respectively, which exhibited higher viscosities than that of [C₄Py][SCN] (107.34 mPa s at 20 °C) with only alkyl-based group on the cation. However, the viscosity of [C₄OPy][SCN] as 94.07 mPa s at 20 °C was obviously lower than the that of alkyl-substituted one, which showed the same viscosity reduction trend as that for other ether-functionalized ILs based on different cations.^44,45

In addition, the effect of different substituent groups on thermal stability was also studied. As shown in Fig. S3,† the thermal decomposition temperatures of [NEt₂C₂Py][SCN], [C₄OPy][SCN] and [C₄CNPy][SCN] were 160 °C, 232 °C and 228 °C, respectively. Compared with the non-functionalized counterpart [C₄Py][SCN] (220 °C), the incorporation of ether group and nitrile group could improve slightly the thermal stability of [C₄OPy][SCN] and [C₄CNPy][SCN], while [NEt₂C₂Py][SCN] exhibited a relatively lower thermal stability, which may be mainly attributed to the increased basicity due to the presence of a tertiary amino group.³¹

2.2 Effect of different cations on SO₂ absorption

Taking into account the high absorption capacity of SO₂ and excellent reversibility for the pyridinium-based IL [C₄Py][SCN],²⁷ therefore, the anion [SCN] was selected to pair with different cations containing a tertiary amino group, a nitrile group and an ether group on the pyridinium ring, and the effect of these groups on SO₂ capture was investigated. As shown in Fig. 1, the tertiary amino-functionalized IL [NEt₂C₂Py][SCN] exhibited the highest absorption capacity of SO₂ among the investigated ILs, which can capture 3.958 molSO₂ molIL⁻¹ at 20 °C and 0.1 MPa. Under the same conditions, the capacity of SO₂ absorption for [C₄OPy][SCN] and [C₄CNPy][SCN] could be as high as 2.956 and 2.917 molSO₂ molIL⁻¹, respectively, whereas the capacity in [C₄Py][SCN] was 2.553 molSO₂ molIL⁻¹. Obviously, the cation-functionalized ILs exhibited enhanced absorption of SO₂ in comparison with [C₄Py][SCN] containing non-functionalized group on the cation, indicating these functionalized groups on the cation have a positive impact on improving SO₂ capture.

Fig. 1 SO₂ absorption by cation-functionalized ILs at 20 °C and 0.1 MPa.

2.3 Effect of temperature and pressure on SO₂ absorption

The effect of temperature and SO₂ partial pressure on SO₂ absorption by the cation-functionalized ILs were shown in Fig. 2. As shown in Fig. 2(a), the SO₂ absorption capacity of [NEt₂C₂Py][SCN], [C₄OPy][SCN] and [C₄CNPy][SCN] at 0.1 MPa decreased continuously as the temperature increased. For instance, the molar ratios of SO₂ to IL for [NEt₂C₂Py][SCN], [C₄CNPy][SCN] and [C₄Py][SCN] decreased dramatically from 3.958, 2.917 and 2.553 to 1.411, 0.853 and 0.950 with an increase of temperature from 20 to 80 °C, respectively. It was found that comparing with [C₄Py][SCN] the temperature has much greater influence on SO₂ absorption for [C₄CNPy][SCN], which means that heating may be more favorable for SO₂ capture by [C₄CNPy][SCN] to be released completely due to the introduction of the nitrile group into the cation.

Fig. 2 Effect of temperature at 0.1 MPa (a) and SO₂ partial pressure at 20 °C (b) on SO₂ absorption by cation-functionalized ILs.

The pressure dependence of SO₂ absorption by [NEt₂C₂Py][SCN], [C₄OPy][SCN] and [C₄CNPy][SCN] at 20 °C was presented in Fig. 2(b). As expected, the SO₂ partial pressure also played a significant role in SO₂ absorption, and the SO₂ absorption capacity of [NEt₂C₂Py][SCN], [C₄CNPy][SCN] and [C₄Py][SCN] increased sharply with an increase of SO₂ partial pressure. For example, the molar ratios of SO₂ to [NEt₂C₂Py][SCN] and [C₄CNPy][SCN] increased from 1.363 to 3.958 and 0.652 to 2.917, respectively, when the SO₂ partial pressure changed from 0.01 to 0.1 MPa. Meanwhile, considering the low concentration of SO₂ in the real flue gas, the SO₂ absorption capacity for three kinds of cation-functionalized ILs at 5 kPa and 2 kPa of SO₂ partial pressure was also studied. The results showed that the absorption capacity of cation-functionalized ILs was less than 1 molSO₂ molIL⁻¹ as the partial pressure of SO₂ was lower than 5 kPa. For instance, the molar ratio of SO₂ to [NEt₂C₂Py][SCN] was 0.807 at 5 kPa and 0.126 at 2 kPa, respectively.

2.4 SO₂/CO₂ selectivity

As we know, there are usually other gases besides the low concentration of SO₂ in flue gas, such as CO₂, O₂ and N₂, so gas selectivity for judging the separation performance of a solvent becomes particularly important.¹¹ As reported by Brennecke et al.⁴⁶ and our previous work,²⁷ N₂ and O₂ solubility in ILs are generally much lower than CO₂ and SO₂ solubility, which suggests that the effective separation of two kinds of acid gases (SO₂ and CO₂) is the key procedure. Consequently, SO₂/CO₂ selectivity for [NEt₂C₂Py][SCN], [C₄OPy][SCN] and [C₄CNPy][SCN] was studied and the results were listed in Table 1. Comparing with [C₄Py][SCN], the remarkable improvement in SO₂/CO₂ selectivity for the three kinds of cation-functionalized ILs was achieved. Especially for [C₄OPy][SCN] and [C₄CNPy][SCN], SO₂/CO₂ selectivity substantially improved about 41% and 25%, respectively, which is more than 1.5 times that of [P₆₆₆₁₄][Tetz].³⁰ The nature may be that the increase of the ILs' polarity induced by an ether group and a nitrile group resulted in the enhanced absorption of polar SO₂ gas and slightly decreased absorption of non-polar CO₂ gas.^44,47,48

Table 1 SO₂/CO₂ selectivity for cation-functionalized ILs (20 °C, 0.1 MPa)

ILs	molgas molIL^−1		Selectivity
	SO2	CO2	SO2/CO2
[NEt2C2Py][SCN]	3.958	0.061	65
[C4CNPy][SCN]	2.917	0.037	79
[C4OPy][SCN]	2.956	0.042	70
[C4Py][SCN] (ref. 27)	2.553	0.045	56

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2.5 Effect of H₂O on SO₂ absorption

In practical applications, water is always accompanied with SO₂ in flue gas, and during SO₂ absorption the water can be also absorbed into absorbents. Therefore, the effect of water on SO₂ absorption by [NEt₂C₂Py][SCN], [C₄CNPy][SCN] and [C₄OPy][SCN] was investigated at 20 °C and 0.1 MPa and the results were shown in Fig. 3 and S4,† respectively. It was demonstrated that the presence of water has a different impact on SO₂ absorption for the three kinds of cation-functionalized ILs. For [C₄CNPy][SCN] (Fig. 3(a)) and [C₄OPy][SCN] (Fig. S4†), SO₂ absorption capacity in the two ILs decreased slightly with an increase of water content, while SO₂ absorption capacity in [NEt₂C₂Py][SCN] (Fig. 3(b)) increased a little after the addition of water. For example, when the water in ILs increased to about 1.0%, the molar ratio of SO₂ to [C₄CNPy][SCN] decreased from 2.917 to 2.851, while SO₂ absorption capacity in [NEt₂C₂Py][SCN] increased from 3.958 to 4.181 molSO₂ molIL⁻¹. The results indicated that the water present in [C₄CNPy][SCN] and [C₄OPy][SCN] plays a negative role in SO₂ absorption capacity, but is favorable for improving SO₂ absorption capacity in [NEt₂C₂Py][SCN]. The detail reason will be discussed in the section of the mechanism.

Fig. 3 Effect of water on SO₂ absorption capacity by [C₄CNPy][SCN] (a) and [NEt₂C₂Py][SCN] (b) at 20 °C and 0.1 MPa.

2.6 Regeneration and recycle of ionic liquids

In order to examine the reversibility of SO₂ absorption, multiple consecutive cycles of SO₂ absorption and desorption for [NEt₂C₂Py][SCN], [C₄OPy][SCN] and [C₄CNPy][SCN] were studied. As shown in Fig. 4, SO₂ captured by the three types of cation-functionalized ILs could be almost completely stripped out at 80 °C by bubbling N₂ through the ILs for 60 min, which was further proved by FT-IR spectra of [NEt₂C₂Py][SCN] (Fig. 5), [C₄CNPy][SCN] (Fig. 6) and [C₄OPy][SCN] (Fig. S5†) before absorption and after desorption of SO₂. Comparing with [C₄Py][SCN], the residual capacity of SO₂ absorption by [C₄CNPy][SCN] decreased from 0.099 to 0.055 molSO₂ molIL⁻¹, implying that the desorption of SO₂ is enhanced owing to the presence of the nitrile group, which agrees well with the former results. Meanwhile, these cation-functionalized ILs can be repeatedly used and still remain the stable absorption performances after five cycles of absorption and desorption, which indicated that the process of SO₂ absorption and desorption by these cation-functionalized ILs is highly reversible and the ILs are recyclable.

Fig. 4 SO₂ absorption and desorption by cation-functionalized ILs. SO₂ absorption was carried out at 20 °C, and SO₂ desorption at 80 °C under N₂. [NEt₂C₂Py][SCN], absorption ■, desorption □; [C₄OPy][SCN], absorption ●, desorption ○; [C₄CNPy][SCN], absorption ▲, desorption Δ.

Fig. 5 FT-IR spectra of [NEt₂C₂Py][SCN] before and after SO₂ absorption and after SO₂ desorption.

Fig. 6 FT-IR spectra of [C₄CNPy][SCN] before and after SO₂ absorption and after SO₂ desorption.

2.7 Comparison with other ionic liquids

To our knowledge, the mass of SO₂ absorbed per unit mass of IL is of great importance in industrial separation processes.^11,33 Therefore, in order to evaluate the potential of ILs for SO₂ capture, gravimetric absorption capacity of SO₂ for the three types of cation-functionalized ILs was compared with other ILs reported in literature in Table 2. It was clearly seen that although the molar absorption capacity of SO₂ in the investigated cation-functionalized ILs is slightly lower than some reported functionalized ILs, their corresponding gravimetric capacity of SO₂ is very high. Especially the tertiary amino-functionalized IL [NEt₂C₂Py][SCN] exhibited the highest absorption capacity of 1.06 gSO₂ gIL⁻¹ under atmospheric pressure in this work, which is very close to the maximum value (1.13 gSO₂ gIL⁻¹) reported to date,⁴⁹ and even could absorb as high as 0.37 gSO₂ gIL⁻¹ under low SO₂ partial pressure of 0.01 MPa. The results indicated the pyridinium-based IL [NEt₂C₂Py][SCN] will be a good candidate for SO₂ removal from flue gases.

Table 2 Comparison of SO₂ absorption capacity with other ILs

ILs	Temperature °C	SO2 absorption^a		Selectivity	References
		0.1 MPa	0.01 MPa	SO2/CO2
a gSO2 gIL^−1 (molSO2 molIL^−1).
[NEt2C2Py][SCN]	20	1.06 (3.958)	0.37 (1.411)	65	This work
[C4OPy][SCN]	20	0.90 (2.956)	0.23 (0.950)	70	This work
[C4CNPy][SCN]	20	0.85 (2.917)	0.20 (0.853)	79	This work
[C4Py][SCN]	20	0.84 (2.55)	0.16 (0.49)	56	27
[Emim][SCN]	20	1.13 (2.99)	0.37 (0.98)	—	49
[P66614][SCN]	20	0.38 (3.24)	0.13 (1.06)	—	49
[P66614][BenIm]	20	0.61 (5.75)	0.26 (2.59)	—	31
[P66614][Tetz]	20	0.43 (3.72)	0.18 (1.54)	47	30
[P66614][4-Br-phCOO]	20	0.38 (4.12)	0.15 (1.66)	—	34
[P444E3][Tetz]	20	0.76 (5.00)	0.29 (1.87)	—	32
[E3mim][Tetz]	20	0.95 (4.43)	0.34 (1.58)	—	32
[E8mim][MeSO3]	30	0.74 (6.30)	—	—	41
[Bmim][OAc]	20	0.62 (1.91)	0.21 (0.66)	—	38
[TMG]L	40	0.53 (1.70)	0.31 (0.98)	—	42
[TMG][BF4]	20	0.40 (1.27)	0.02 (0.06)	—	17

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2.8 The mechanism for SO₂ absorption

In order to understand the role of the different substituent groups on the cation in SO₂ absorption, FTIR and NMR spectroscopy were used to investigate the interaction between ILs and SO₂. The FT-IR spectra of fresh ILs, SO₂-absorbed ILs and ILs after desorption for [NEt₂C₂Py][SCN], [C₄CNPy][SCN] and [C₄OPy][SCN] were shown in Fig. 5, 6 and S5,† respectively. As shown in Fig. 5, compared with the FTIR spectrum of the fresh IL [NEt₂C₂Py][SCN], three new absorption bands centered at 1315, 1132 and 946 cm⁻¹ were observed in the FTIR spectrum of [NEt₂C₂Py][SCN] after uptake of SO₂. The peaks appearing at 1315 and 1132 cm⁻¹ could be assigned to the asymmetric and symmetric S=O stretches, and another new peak at 946 cm⁻¹ was attributed to the formation of S–O bonds as a result of SO₂ absorption, which indicated the chemical interaction exists between the amino-functionalized IL [NEt₂C₂Py][SCN] and SO₂ by the formation of a charge transfer complex^50,51 besides the physical interaction between the anion [SCN] and SO₂.^30–32

For the nitrile-functionalized IL [C₄CNPy][SCN], there were only two new bands at 1296 and 1124 cm⁻¹ for the asymmetric and symmetric S=O stretches in the FTIR spectrum of [C₄CNPy][SCN] saturated with SO₂ gas (Fig. 6), which indicated that only physical absorption of SO₂ occurs in the nitrile-functionalized IL [C₄CNPy][SCN]. Similarly, the changes in the spectra of the ether-functionalized IL [C₄OPy][SCN] before and after SO₂ absorption showed the same results (Fig. S5†), which is consistent with those previously reported by Hong et al.⁴¹ and Wang et al.³²

Furthermore, the FT-IR spectra of fresh ILs, SO₂-absorbed ILs and ILs after desorption for [NEt₂C₂Py][SCN], [C₄CNPy][SCN] and [C₄OPy][SCN] with water were shown in Fig. 7, 8 and S6,† respectively. Compared with FT-IR spectra of [NEt₂C₂Py][SCN] without water after SO₂ absorption, a new peak at 1034 cm⁻¹ corresponding to S–OH were observed in FT-IR spectrum of [NEt₂C₂Py][SCN] with water after SO₂ absorption (Fig. 7), which indicated that the enhanced absorption of SO₂ for [NEt₂C₂Py][SCN] in the presence of water could be ascribed to the formation of hydrogen sulfite salts. The results showed good agreements with that reported by Sayari et al.^4,5,52,53 As shown in Fig. 8 and S6,† the results indicated that the presence of water does not change the physical absorption process between [C₄CNPy][SCN]/[C₄OPy][SCN] and SO₂, and the structure of [C₄CNPy][SCN]/[C₄OPy][SCN] with water after absorption and desorption of SO₂ can also keep stable.

Fig. 7 FT-IR spectra of [NEt₂C₂Py][SCN] with water before and after absorption of SO₂ and after desorption.

Fig. 8 FT-IR spectra of [C₄CNPy][SCN] with water before and after absorption of SO₂ and after desorption.

The ¹H NMR and ¹³C NMR spectra of [NEt₂C₂Py][SCN], [C₄CNPy][SCN] and [C₄OPy][SCN] before and after SO₂ absorption were further compared in Fig. 9, 10 and S7,† respectively. As shown in Fig. 9, after the absorption of SO₂, typical peaks in the ¹H NMR spectrum of the cation in [NEt₂C₂Py][SCN] moved downfield from 2.44 and 2.86 ppm to 2.58 and 2.99 ppm, respectively. Meanwhile, the corresponding ¹³C NMR signals showed upfield shifting from 53.01 and 59.52 ppm to 52.53 and 58.72 ppm, respectively. These chemical shifts further proved that there exists the chemical interaction between the tertiary amino group on the pyridinium ring and SO₂. Additionally, neither new peaks or significant chemical shifts were observed after SO₂ absorption by the nitrile-functionalized IL [C₄CNPy][SCN] (Fig. 10) and ether-functionalized IL [C₄OPy][SCN] (Fig. S7†), which agrees well with the IR results.

Fig. 9 ¹H NMR and ¹³C NMR spectra of [NEt₂C₂Py][SCN] before and after SO₂ absorption.

Fig. 10 ¹H NMR and ¹³C NMR spectra of [C₄CNPy][SCN] before and after SO₂ absorption.

Moreover, the density functional theory calculations were performed using Gaussian 09 program to further investigate the interaction between ILs and SO₂. The optimized structures of IL–SO₂ systems for [NEt₂C₂Py][SCN], [C₄OPy][SCN], [C₄CNPy][SCN] and [C₄Py][SCN] were presented in Fig. 11. As shown in Fig. 11(a)–(c), the interactions between SO₂ and these three pyridinium-based ILs are very different. Compared with that of [C₄Py][SCN]–SO₂, the intermolecular distance between the sulfur atom of SO₂ and the sulfur atom of SCN anion in [C₄OPy][SCN]–SO₂ and [C₄CNPy][SCN]–SO₂ became slightly shorter. Meanwhile, three hydrogen bonds formed between SO₂ molecule and the cations of [C₄CNPy][SCN] and [C₄OPy][SCN] whereas only two hydrogen bonds formed between SO₂ molecule and the cation of [C₄Py][SCN], suggesting the stronger interaction between SO₂ and [C₄OPy][SCN] or [C₄CNPy][SCN]. The results may further prove the higher absorption capacity of SO₂ using [C₄OPy][SCN] and [C₄CNPy][SCN] than [C₄Py][SCN], which is consistent with the experimental results. However, for [NEt₂C₂Py][SCN] that physically and chemically absorb SO₂, the optimized structures of IL–SO₂ in Fig. 11(d) showed that the physical interaction of [NEt₂C₂Py][SCN] with SO₂ was almost similar to that of [C₄Py][SCN]–SO₂. Therefore, it was reasonably believed that the enhancement in SO₂ absorption for [NEt₂C₂Py][SCN] mainly originated from the strong chemical interaction between the tertiary amino group and SO₂.

Fig. 11 Optimized structures showing the interactions of ILs with SO₂ molecules: (a) [C₄Py][SCN]–SO₂ (ΔH = −67.3 kJ mol⁻¹), (b) [C₄OPy][SCN]–SO₂ (ΔH = −68.8 kJ mol⁻¹), (c) [C₄CNPy][SCN]–SO₂ (ΔH = −62.8 kJ mol⁻¹), (d) [NEt₂C₂Py][SCN]–SO₂ (ΔH = −63.9 kJ mol⁻¹). H: light gray, C: gray, O: red, N: blue, S: yellow.

In addition, the interaction enthalpies ΔH between ILs and SO₂ were calculated as −67.3, −68.8, 63.9 and −62.8 kJ mol⁻¹ for [C₄Py][SCN], [C₄OPy][SCN], [NEt₂C₂Py][SCN] and [C₄CNPy][SCN], respectively. It is clearly observed that ΔH of [C₄OPy][SCN]–SO₂ is comparable to that of [C₄Py][SCN]–SO₂, however, ΔH of [NEt₂C₂Py][SCN]–SO₂ and [C₄CNPy][SCN]–SO₂ is slightly lower than that of [C₄Py][SCN]–SO₂, which means that the cation-functionalized ILs may be more efficient for SO₂ absorption due to their higher SO₂ absorption capacity and comparable or lower interaction enthalpies.

3. Experimental

3.1 Materials

SO₂ gas (99.9%) was supplied by Beijing Beiwen Gas Factory. N,N-diethyl-2-amino-1-bromoethane hydrobromide (98%), 2-chloroethyl ethyl ether (98%), and 5-chlorovaleronitrile (97%) were purchased from TCI. Other chemical reagents of analytical grade, including pyridine, 1-bromobutane, ethyl acetate, dichloromethane, acetone, ethanol, acetonitrile, methanol, diethyl ether and sodium thiocyanate were purchased from Beijing Chemical Company. All reagents were used for the synthesis of the ILs without further purification.

3.2 Synthesis and characterizations of ILs

Three types of cation-functionalized ILs with the same anion [SCN] including 1-(2-diethylaminoethyl)pyridinium thiocyanate ([NEt₂C₂Py][SCN]), 1-ethyoxylethylpyridinium thiocyanate ([C₄OPy][SCN]) and 1-valeronitrilepyridinium thiocyanate ([C₄CNPy][SCN]) were synthesized in a two-step method, and the preparations of ILs were shown below. The structures of the ILs studied in this work were shown in Fig. 12.

Fig. 12 Structures of pyridinium-based ILs with different groups on the cation.

Synthesis of [NEt₂C₂Py][SCN]. The IL [NEt₂C₂Py][SCN] was synthesized following the preparation procedures of 1-(N,N-dimethylaminoethyl)-2,3-dimethylimidazolium trifluoro-methanesulfonate ([mammim][TfO]).⁵⁴ In the experiment, pyridine (7.62 g, 0.096 mol) and N,N-diethyl-2-amino-1-bromoethane hydrobromide (25.15 g, 0.096 mol) were dissolved in acetonitrile (200 ml) and heated under reflux at 80 °C for 48 h. After cooling to room temperature, the mixture was filtered and the remaining solid was washed with acetone (3 × 40 ml). After drying at 60 °C under vacuum for 48 h, [NEt₂C₂Py][Br][HBr] was obtained as white solid. [NEt₂C₂Py][Br][HBr] (29.65 g, 0.087 mol) and NaOH (3.49 g, 0.087 mol) were added in methanol (25 ml) at 0–5 °C and stirred for 12 h at room temperature. Then equimolar NaSCN (7.069 g, 0.087 mol) was added and stirred for 24 h. After that time the methanol was removed by rotary evaporation, and the mixtures were washed with dichloromethane (200 ml). After filtration and removal of solvents from filtrate, the yellowish oil [NEt₂C₂Py][SCN] was finally obtained under vacuum at 50 °C for 48 h.

Synthesis of [C₄CNPy][SCN]. Pyridine (10.614 g, 0.134 mol) and 5-chlorovaleronitrile (Cl(CH₂)₄CN, 15.778 g, 0.134 mol) were firstly mixed and stirred at 80 °C for 8 h, then the reaction mixture was stirred at 110 °C for 40 h. After cooling to room temperature, the reaction mixture was washed with diethyl ether (3 × 40 ml) and dried at 80 °C under vacuum for 48 h to obtain the pale yellow solid [C₄CNPy]Cl. The anion exchange reaction was performed by mixing [C₄CNPy]Cl (28.706 g, 0.146 mol) and equimolar NaSCN in acetone (200 ml). After stirring for 48 h at room temperature, the mixture was filtered and washed with acetone (3 × 100 ml). The organic layer was collected and concentrated by rotary evaporation to remove the solvent. The product [C₄CNPy][SCN] was finally obtained as the dark orange liquid under vacuum at 50 °C for 48 h.

Synthesis of [C₄OPy][SCN]. 2-Chloroethyl ethyl ether (20.524 g, 0.185 mol) was added to pyridine (14.654 g, 0.185 mol) while stirring at 100 °C for 48 h. After the reaction, ethanol (150 ml) and activated carbon (8 g) were added into the reaction solution at room temperature, and the mixture kept stirring for 3 h. After filtration and removal of the solvents by rotary evaporation, the resulting product was washed with ethyl acetate (3 × 40 ml), and dried under vacuum at 75 °C for 48 h to afford [C₄OPy]Cl as the brown solid. The IL [C₄OPy][SCN] was synthesized following the same procedures described above for [C₄CNPy][SCN], except for the use of [C₄OPy]Cl instead of [C₄CNPy]Cl. The final product [C₄OPy][SCN] was obtained as the brown liquid under vacuum at 50 °C for 48 h.

All the ILs were dried under vacuum at 50 °C for 48 h before use. The structures of these ILs were confirmed by ¹H and ¹³C NMR spectroscopy with Bruker 600 spectrometer and FT-IR spectroscopy by a Thermo Nicolet 380 Spectrometer. The water content in the ILs after drying were determined with volumetric Karl Fischer Titration (Metrohm, 787 KF Titrino) and found to be less than 0.1 wt%. The residual halide contents in the ILs were measured using PXSJ-226 Series Ion meter (INESA Scientific Instrument Co. Ltd), and found to be less than 0.3 wt%. Decomposition temperature was measured with a Thermogravimeric Analysis (TGA) 2050 meter. Densities and viscosities were measured on Paar DMA 5000 density and Paar AMVN viscosity meters, respectively.

3.3 Absorption and desorption of SO₂

The experiments for SO₂ absorption and desorption followed the same procedures of our previous work.²⁷ In a typical absorption of SO₂, the pure SO₂ was bubbled through about 5 g IL in the glass container with an inner diameter of 3.0 cm and a height of 12.0 cm at a flow rate of 140 ml min⁻¹. The temperature of the glass container was controlled at the desired temperature within ±0.1 °C by a circulated water bath. The amount of SO₂ absorbed in the IL was measured using an electronic balance with an accuracy of ±0.1 mg and was finally determined according to the increase of the weight. For the absorption of SO₂ under reduced pressure, the gases of different SO₂ partial pressure were obtained by changing the flow of SO₂ and N₂.

SO₂ desorption from saturated IL solutions was carried out in a similar way to the absorption method mentioned above. The pure N₂ with the same flow rate was bubbled through the IL that absorbed SO₂ in the glass container at 80 °C, the amount of SO₂ desorption was determined at regular intervals by an electronic balance until the weight kept unchanged. The reproducibility of the absorption and desorption experiments was less than ±3%, and it was estimated that the experimental error involved in the method was at the level of ±5%.

4. Conclusions

In this work, three kinds of novel cation-functionalized ILs [NEt₂C₂Py][SCN], [C₄OPy][SCN] and [C₄CNPy][SCN] were synthesized, and the effect of different functional groups on their physicochemical properties and the performances of SO₂ absorption were systematically investigated. The results demonstrated that these cation-functionalized ILs exhibited much better absorption capacity and SO₂/CO₂ selectivity than the conventional IL [C₄Py][SCN]. Among the investigated ILs, [NEt₂C₂Py][SCN] exhibited the highest absorption capacity of 1.06 gSO₂ gIL⁻¹ under ambient conditions. Meanwhile, SO₂/CO₂ selectivity was greatly improved up to 41% through the incorporation of these groups on the cation of ILs. The present of water causes a slight decrease in SO₂ capacity by [C₄CNPy][SCN] and [C₄OPy][SCN], whereas a slight increase in SO₂ capacity by [NEt₂C₂Py][SCN]. Moreover, spectroscopic characterizations and Quantum Chemical calculation results indicated that only physical absorption of SO₂ occurs in [C₄CNPy][SCN] and [C₄OPy][SCN], but a combination of the chemical and physical interaction coexists between [NEt₂C₂Py][SCN] and SO₂. The great enhancement in SO₂ absorption by [NEt₂C₂Py][SCN] mainly originates from the strong chemical interaction between the tertiary amino group and SO₂ besides the physical interaction of [SCN]–SO₂. In addition, the three kinds of cation-functionalized ILs could also keep the stable absorption performance after five cycles of absorption and desorption, implying these ILs show great potentials for the applications of SO₂ capture.

Acknowledgements

This work was supported by the key program of Beijing Municipal Natural Science Foundation (no. 2141003), the key program of the National Natural Science Foundation of China (no. 21436010), the External Cooperation Program of BIC, Chinese Academy of Sciences (no. GJHZ201301) and the National High Technology Research and Development Program of China (no. 2013AA06540201).

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Footnote

† Electronic supplementary information (ESI) available: NMR data, FTIR and NMR spectra of the investigated ILs before and after SO₂ absorption, densities and viscosities at different temperatures and TGA results of the ILs, and the effect of water on SO₂ absorption for [C₄OPy][SCN]. See DOI: 10.1039/c4ra13469a

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