Solubility of sulfur dioxide in tetraglyme-NH4SCN ionic liquid: high absorption efficiency

An easily prepared ionic liquid was synthesized by a one-step method and applied in SO2 absorption efficiently. The cation of the ionic liquid is a supramolecular structure consisting of NH+4 and tetraglyme, similar to the structure of NH+4 and crown ether, and the anion is selected as SCN−. The ionic liquid has good thermal stability. Under the conditions of 293 K and 1 bar, one mol ionic liquid can absorb 2.73 mol SO2, which is about 30% higher than tetraglyme. The absorption mechanism was characterized using IR and NMR. And the results confirmed that the interaction mechanism between SO2 and the ionic liquid is a physical interaction rather than a chemical interaction.


Introduction
Sulfur dioxide (SO 2 ) is a common atmospheric pollutant mainly derived from the burning of fossil fuels. 1 SO 2 emitted from ue gas will cause serious harm to the natural environment 2 and human health. 3 In recent years, the theme of removing SO 2 gas from ue gas has attracted widespread attention around the world. At present, a limestone/lime based FGD method is the most widely used ue gas desulfurization technology in industrial practice. 4,5 However, this method produces a large amount of secondary pollution such as gypsum and industrial wastewater, along with low limestone utilization and poor selectivity, which is difficult to overcome. 6 Therefore, the development of new desulfurizers with good absorption capacity, selectivity, regenerability, thermal stability and an environmentally friendly nature has become one of the current research hotspots.
Recently, ionic liquids (ILs), as new candidate solvents to absorb SO 2 in ue gas, have been widely studied due to their low saturated vapor pressure, 7 high thermal and chemical stability, and excellent solubility to some substances. 8 The ionic liquids used for SO 2 absorption mainly include the following types: guanidinium ionic liquid, [9][10][11][12][13] hydroxyl ammonium ionic liquid, [14][15][16] imidazolium ionic liquid, 17-21 tetrabutyl ammonium ionic liquid [22][23][24] and quaternary phosphine ionic liquids. 25 However, these ionic liquids are relatively weak in their ability to absorb SO 2 . One reason is that they were not originally designed to absorb SO 2 and therefore are not optimized for SO 2 absorption. The ability of an ionic liquid to absorb SO 2 is closely related to the type of cation and anion of the ionic liquid itself.
For example, the absorption capacity of [C 4 Py] [BF 4 ] ionic liquid at 293 K and 0.1 MPa is 0.440 g g À1 ionic liquid, which is better than that of [C 8 Py][BF 4 ] (0.378 g g À1 ionic liquid) but weaker than [C 4 Py][SCN] (0.841 g g À1 ionic liquid). 26 In order to improve the absorption capacity of ionic liquids, researchers prepared a variety of functional ionic liquids. There are two main types of functional ionic liquids, including ether functional ionic liquids [27][28][29][30][31] and amine functional ionic liquids. [32][33][34] The latter generally has poor selectivity and desorption ability due to its chemical interaction with SO 2 . In contrast, the former have better absorption ability and superior selectivity than those of the original ionic liquids due to the physical interaction between the ether functional groups and SO 2 .
As a functional group, ether groups, mainly referred to herein as ethylene glycol and its derivatives, have good absorption capacity and good regenerability for SO 2 , thus giving it a wide range of potential industrial applications. 35,36 However, ethylene glycol and its derivatives alone have two disadvantages that are difficult to overcome. One is its less prominent SO 2 absorption capacity and the other is its relatively high vapor pressure. The latter causes it to be more volatile during the absorption-regeneration process, which in turn leads to excessive solvent loss. The introduction of ethylene glycol and its derivatives into ionic liquids could effectively solve the above two problems. As functional groups in the ether-based ionic liquid, ethylene glycol and its derivatives make the ether-based ionic liquids have many of the advantages mentioned above in absorbing SO 2 .
Conventional ether functional ionic liquids are difficult to industrialize due to their complicated synthesis and high cost. A simple one-step method by mixing and stirring was developed for ionic liquid synthesis. And this type ionic liquids have been used in nonaqueous electrolytes in Li batteries 37,38 and CO 2 absorption. 39 This method is dedicated to solving the problem that conventional ionic liquids are difficult to synthesize. We have previously prepared a series of glycine-lithium salt ionic liquids and studied their absorption of SO 2 . 40 These readily synthesized glyme-lithium salt ionic liquids have a greater thermal stability than glymes while maintaining similar SO 2 absorption capabilities. However, the higher price of lithium salt leads to higher cost of preparation of the corresponding ionic liquid, which severely limits the application of the ionic liquid in desulfurization. In order to solve this problem and further improve the absorption capacity of ionic liquids for SO 2 , a novel ether-based ionic liquid with advantages of simple synthesis, strong absorption capacity and low cost has been developed.
This ionic liquid was synthesized by tetraglyme (G4) and NH 4 SCN, and its structure is shown in Fig. 1. In order to increase the SO 2 absorption capacity of the ionic liquid, SCN À was chosen as the anion. The absorption and desorption performance of the ionic liquid were investigated. The interaction mechanism between the ionic liquid and SO 2 was also studied by infrared spectroscopy (IR) and nuclear magnetic resonance (NMR).

Chemicals
Tetraglyme (AR) was purchased from Shanghai Aladdin Bio-Chem Technology Co., LTD, and NH 4 SCN (AR) was purchased from Sinopharm Chemical Reagent Co., Ltd. All reagents were used without further purication. Chromatographic grade ethanol and distilled water are also used for this work. Certied standard pure SO 2 gas (>99.9%) and N 2 (99.9% purity) supplied by Beijing Gas Centre, Peking University (China) is used to determine the SO 2 absorption capacity of the ionic liquid.

Preparation of the ionic liquid
Tetraglyme and NH 4 SCN were mixed in a molar ratio of 1 : 1, and then the liquid mixture was stirred and heated at 303 K for 6 hours, so that [NH 4 -tetraglyme][SCN] ionic liquid can be obtained. The ionic liquid was then dried under vacuum for 48 hours at room temperature. The resulting ionic liquid is a clear, pale yellow liquid.

Absorption and desorption of SO 2
The absorption and desorption experiments of SO 2 were carried out in an absorption tube with an inner diameter of 15 mm. SO 2 at a ow rate of 100 mL min À1 was bubbled through the absorbent tube containing absorber sample. The constant temperature required for absorption and regeneration is maintained by a circulating water bath into which the absorber tube is immersed. The absorption capacity of SO 2 was determined by means of weighing. In the absorption experiments under different pressures, a mixed gas having different partial pressures of SO 2 was obtained by controlling the ow rates of SO 2 and N 2 . In the regeneration experiment, the temperature was maintained at 353 K, and the ow rate of nitrogen was 100 mL min À1 . And the analytical method was similar to that of absorption.

Properties of the ionic liquid
The conductivity of the ionic liquid was measured to be 1528 mS cm À1 . In contrast, tetraglyme itself has a conductivity of 0. The viscosity of ionic liquid is 104 mPa s at 298 K, which is 31 times that of tetraglyme (3.295 mPa s (ref. 41)) at the same temperature.
The ionic liquid was characterized by MS, NMR and IR. The result of MS is shown in Fig. S1 in ESI. † It can be clearly seen that the ionic liquid has a cationic molecular weight of about 240.2, which is the sum of the molecular weight of tetraglyme and the molecular weight of ammonium ion. It is reasonably speculated that cation in ionic liquid should consist of an ammonium ion and a tetraglyme molecule with a structure similar to that of crown ether-ammonium ion, as shown in Fig. 1.
An external reference (CDCl 3 ) method was used in 1 H-NMR and 13 C-NMR to avert the solvent effect by the deuterated reagents. The chemical shis of the H atoms are shown in Fig. 2. It can be seen from the gure that the chemical shis of the hydrogen atoms in the ether functional group move to the high eld, and the chemical shis change from 3.95, 3.85, and 3.69 of tetraglyme to 3.88, 3.81 and 3.61 of the ionic liquid, respectively. The main reason for the change in chemical shis is that the deshielding effect caused by oxygen atoms is inhibited in virtue of the interaction between NH + 4 and the oxygen atoms in tetraglyme group. The chemical shi of the H atom attached to the N atom appears at 7.05. Further, solvents having different molar ratio of tetraglyme and NH 4 SCN including 1 : 0.2, 1 : 0.4, 1 : 0.6 and 1 : 0.8, was prepared and the 1 H-NMR spectra is shown in Fig. S2 in ESI. † According to the spectra, H atoms of NH + 4 ions at different solvents above have similar chemical shis, which means that the interaction strength between NH + 4 ions and tetraglyme is close in several different solvents including the ionic liquid. The chemical shis of the H atom in tetraglyme group also moves to the high eld as the NH 4 SCN content increases in the solvents, which conrms the deshielding effect is inhibited in the ionic liquid. However, in the 13 C-NMR spectrum shown in Fig. 3, there is no signicant change in the chemical shis of the tetraglyme carbon atoms aer the formation of the ionic liquid. This indicates that there is no signicant interaction between NH + 4 and the carbon atoms of tetraglyme group in the ionic liquid. The existing ion-dipole interaction and hydrogen bonding between the two substances mainly occur between the hydrogen atoms of NH + 4 and the oxygen atoms of tetraglyme. Fig. 4 is an IR spectrum of tetraglyme and [NH 4 -tetraglyme] [SCN] ionic liquid before and aer SO 2 absorption. There is no signicant shi in the C-O vibration peak at 1110 cm À1 and the C-C vibration peak at 1430 cm À1 , when tetraglyme forms an ionic liquid with NH 4 SCN. A closer comparison of the spectra of the two materials reveals another difference: the ionic liquid has a distinct absorption peak of SCN À at 2064 cm À1 . Meanwhile, tetraglyme has a C-H vibration peak at 2876 cm À1 , but aer the formation of ionic liquid, the displacement of this absorption peak changes signicantly, which in turn produces a huge absorption peak between 2827 cm À1 and 3184 cm À1 . This phenomenon indicates that aer the formation of the ionic liquid, a very strong hydrogen bond is formed between the tetraglyme and the NH + 4 , resulting in a signicant change in the position of the C-H bond in the tetraglyme molecular.
The result of thermogravimetric analysis of tetraglyme and [NH 4 -tetraglyme][SCN] ionic liquid is shown in Fig. 5  that the initial decomposition temperature of tetraglyme and the ionic liquid is 371 K and 389 K, respectively. It can be seen that the thermal stability of the [NH 4 -tetraglyme][SCN] ionic liquid is signicantly better than that of tetraglyme itself. It was further noted that the temperature of the absorption experiment did not exceed 313 K, and the temperature of the desorption experiment was 353 K. This means that the normal operating temperature of the desulfurizer will generally not exceed 353 K. Therefore, we performed a constant temperature thermogravimetric experiment on [NH 4 -tetraglyme][SCN] ionic liquid and tetraglyme at 353 K. The results are shown in Fig. S3 in ESI. † As can be seen from the gure, tetraglyme has a higher volatility at 353 K, while the thermal stability of ether ionic liquid is rather low. This means that latter has satisfactory volatilization rate during the absorption-desorption process of the desulfurization experiment, avoiding excessive solvent loss in practical applications, thereby reducing the cost of desulfurization and avoiding environmental hazards caused by volatilization as much as possible.
Further thermal analysis experiments conrmed the strength of tetraglyme and NH 4 SCN, as shown in Fig. S4 in ESI. † According to DSC and TGA results, the interaction strength of tetraglyme and NH 4 SCN is about 4.03 kJ mol À1 at 373 K, which is close to the interaction strength between 15-crown-5 and NH 4 Cl. 42,43

Absorption capacity of ionic liquid
The absorption capacity of SO 2 at different temperatures and 1 bar is measured as shown in Fig. 6. It can be clearly seen that as the temperature increases, the absorption capacity of both the tetraglyme and the ionic liquid on the SO 2 gas is decreased. However, at all temperatures studied, the ionic liquid has an absorption capacity that is about 30% higher than that of tetraglyme. At the condition of 293 K and 1 bar, 1 mol this ionic liquid can absorb about 2.73 mol of SO 2 Correspondingly, the absorption capacity of tetraglyme is 2.10 mol. The result of the unit mass absorption capacity of tetraglyme and the ionic liquid is shown in Fig. S5 in ESI. † And it is apparent that the ionic liquid has a better unit mass absorption capacity than tetraglyme at the temperature exceeding 303 K.
The effect of SO 2 partial pressure on the absorption of SO 2 by ionic liquid has also been investigated. The results are shown in Fig. 7. As can be seen from the gure, when the SO 2 volume fraction is increased from 20% to 100%, the absorption of the ionic liquid is increased from 0.61 mol SO 2 pre mol ionic liquid to 2.73 mol SO 2 pre mol ionic liquid at 293 K. It can be seen that with the increase of the volume fraction of SO 2 gas, the absorption of SO 2 by the ionic liquid gradually increases, and both the two variables have a linear relationship. This suggests that the absorption between the ionic liquid and SO 2 should be dominated by physical interaction.
The SO 2 absorption capacities of solvents with different molar ratio of tetraglyme and NH 4 SCN were also test and the results are shown in Fig. 8. Under the condition of 303 K and 1 bar, the solvents have similar unit mass absorption from 0.41 g g À1 to 0.44 g g À1 . However, it should be noted that the absorption per mole of solvent increased from 1.50 mol to 2.02 mol, as the NH 4 SCN content increases from 1 : 0.2 to 1 : 1. The increase of molar absorption is presumed to originate from the inuence of SCN À in the solvents.

Regeneration
Based on these behaviors of the ionic liquid, aer the ionic liquid absorbs SO 2 , it can be desorbed by heating, nitrogen stripping or vacuum depressurization. Here, the desorption ability of the ionic liquid aer absorbing SO 2 was studied by a heating with nitrogen stripping method. The results show that aer ve absorption-desorption cycles, the absorption of SO 2 by [NH 4 -tetraglyme][SCN] ionic liquid still have 99% capacity of the initial absorption, and the desorption rate reaches 98% (see Fig. S6 in ESI †). It can be seen that the [NH 4 -  tetraglyme][SCN] ether ionic liquid has good absorption and desorption properties at the same time, which makes the absorbing-stripping process possible. Apparently, the negligible volatilization loss of the solvent facilitates its industrial application.

Mechanism
Tetraglyme has been proved that it has good absorption of SO 2 and regenerative capacity, due to the physical interaction between tetraglyme and SO 2 . 36,44 As the ionic liquid exhibits better absorption and similar regeneration performance than tetraglyme, it is speculated that there is a large similarity in the absorption mechanism between them. 1 H-NMR, 13 C-NMR and IR are further used in the study of absorption mechanisms. SO 2 has the characteristics of three absorption peaks in the infrared spectrum. 45 The antisymmetric stretching vibration peak around 1330 cm À1 and the bending vibration peak near 528 cm À1 can be seen clearly aer the ionic liquid absorbs SO 2 . However, the symmetric stretching vibration peak at 1150 cm À1 overlaps with the position of the C-O vibration peak, thereby covering the vibration peak of SO 2 . This result is quite consistent with the infrared spectrum before and aer SO 2 absorption by tetraglyme alone. There was no signicant change in the large absorption peaks of 2827 cm À1 to 3184 cm À1 before and aer absorption of SO 2 , indicating that NH + 4 itself had little effect on SO 2 absorption, and the supramolecular structure of the cation still retains aer absorbing SO 2 . Comparing the infrared spectrum of the ionic liquid before and aer SO 2 absorption, it can be seen that there is no signicant change in the position of all absorption peaks of ionic liquid, and no new absorption peaks were produced except for the absorption peak of SO 2 . This indicates that aer the ionic liquid absorbs SO 2 , there is no new chemical bond formation in the system, and the main interaction between SO 2 and the ionic liquid is the physical interaction.
As can be seen from the 1 H-NMR data of Fig. 2, when the tetraglyme and the ionic liquid absorb SO 2 , the chemical shis of all hydrogen atoms move toward the lower eld. Aer the absorption of SO 2 by tetraglyme, the chemical shis of hydrogen atoms move from 3.95, 3.85, 3.69 to 4.18, 4.09, 3.88, respectively. And the absorption of SO 2 by [NH 4 -tetraglyme] [SCN] ionic liquid causes the chemical shi of hydrogen atoms to move from 3.88, 3.81, 3.61 to 4.18, 4.10, 3.88, respectively. It can be seen that the two solvents have similar chemical shi changes aer SO 2 absorption and the moving directions of the two substances are consistent because the displacement change is caused by the magnetic susceptibility anisotropy due to the aromatic circulation effect of SO 2 . This means that the absorption mechanism of the SO 2 by the tetraglyme group of the ionic liquid is similar to that of the tetraglyme absorbing SO 2 alone, that is, the physical interaction plays a major role. Further, the chemical shi of H on NH + 4 also moves, and the single peak changes to a triple peak aer the absorption of SO 2 , which means that the structure of ionic liquid cations gets more rigid.
In the 13 C-NMR spectrum shown in Fig. 3, we can see that there is no signicant changes in the chemical shi of the tetraglyme group carbon atom aer the ionic liquid absorbing SO 2 . However, it should be noted that aer the absorption of SO 2 by the ionic liquid, the chemical shi of carbon atoms in SCN À has changed noticeable from 132.06 to 130.04. The ion-dipole interaction between SCN À and SO 2 is assumed to cause the nitrogen atom on the SCN À to reduce a deshielding effect on the carbon atoms on it, leading the chemical shi of its carbon atoms to move toward the high eld. The chemical shis of other carbon atoms are not signicantly changed. This indicates that the interaction that causes the ionic liquid to signicantly enhance the absorption capacity of SO 2 mainly occurs between SO 2 and the carbon atom of the SCN À , rather than the carbon atoms of tetraglyme.
In summary, based on the above spectral results and by comparing SO 2 absorption capacity of solvents with different molar ratios of tetraglyme and NH 4 SCN, it is considered that the charge transfer between SO 2 and the cation in the ionic liquid plays a major role in the process of SO 2 absorption by the ionic liquid, and the van der Waals force between SO 2 and SCN À also plays an important role, which make the molar absorption of ionic liquid to increase about 30% compared with that of tetraglyme.

Comparison of SO 2 absorption capacity in different ionic liquids
In order to evaluate the absorption performance of this ionic liquid, we compared it with other ionic liquids, as shown in Table 1. [NH 4 -tetraglyme][SCN] exhibits satisfactory SO 2 absorption capacity under the same conditions compared with other ionic liquids, and its ability to absorb SO 2 exceeds that of most non-functionalized ionic liquids. And it shows excellent regeneration ability. Moreover, compared with other traditional ionic liquids and functional ionic liquids, this kind of ionic liquid has the advantages of low cost and easy synthesis, making it more conducive to practical application.

Conclusion
[NH 4 -tetraglyme][SCN] ionic liquid was prepared and its absorption capacity and absorption mechanism of SO 2 were studied. Ammonium ion forms a relatively stable supramolecular structure through ion-dipole interaction and hydrogen bonding with tetraglyme, and this supermolecule structure is the cationic portion of the ionic liquid. This ionic liquid has remarkable features such as easy to prepare, low cost, and high thermal stability. This makes it possible for the ionic liquid to be a promising candidate to be applied in desulfurization absorbing-regenerating chemical process. [NH 4 -tetraglyme] [SCN] has strong absorption and desorption capacity for SO 2 and one mol the ionic liquid can absorb about 30% more SO 2 than tetraglyme. The results of IR and NMR experiments conrmed that the interaction mechanism between SO 2 and the ionic liquid is physical interaction rather than chemical interaction, which makes it easier to desorb SO 2 from the ionic liquid in the regeneration process.

Conflicts of interest
There are no conicts of interest to declare.