The CO2 Absorption in Flue Gas Using Mixed Ionic Liquids.

Because of the appealing properties, ionic liquids (ILs) are believed to be promising alternatives for the CO2 absorption in the flue gas. Several ILs, such as [NH2emim][BF4], [C4mim][OAc], and [NH2emim[OAc], have been used to capture CO2 of the simulated flue gas in this work. The structural changes of the ILs before and after absorption were also investigated by quantum chemical methods, FTIR, and NMR technologies. However, the experimental results and theoretical calculation showed that the flue gas component SO2 would significantly weaken the CO2 absorption performance of the ILs. SO2 was more likely to react with the active sites of the ILs than CO2. To improve the absorption capacity, the ionic liquid (IL) mixture [C4mim][OAc]/ [NH2emim][BF4] were employed for the CO2 absorption of the flue gas. It is found that the CO2 absorption capacity would be increased by about 25%, even in the presence of SO2. The calculation results suggested that CO2 could not compete with SO2 for reacting with the IL during the absorption process. Nevertheless, SO2 might be first captured by the [NH2emim][BF4] of the IL mixture, and then the [C4mim][OAc] ionic liquid could absorb more CO2 without the interference of SO2.


Introduction
The CO 2 absorption of flue gas is an important process for reducing greenhouse gas [1]. To date, most flue absorptions are performed by using amine solvents. However, conventional absorption methods usually have some disadvantages: High equipment corrosion rate, high absorbent make-up rate due to the amine degradation by SO 2 , NO 2 , and O 2 in the flue gas, and high energy consumption during the regeneration process [2]. In the last decades, ionic liquids (ILs) have been used in many fields. Multifunctional ionic liquids are easily prepared, and the vapor pressure of ionic liquids can be neglected [3]. The other attractive properties of ILs include: High thermal stability, large electrochemical window, and high dissolve ability of compounds [4]. Blanchard et al. [5] have reported that certain ILs can considerably dissolve CO 2 gas. Since then, ILs for CO 2 capture have attracted much attention. For example, multi-N-containing ionic liquids can absorb much more SO 2 /CO 2 in the flue gas than that of the limestone solvent [6]. Shiflett et al. [7,8] have found that the imidazolium-based ionic liquid [C 4 mim] [OAc] can markedly reduce the energy losses of CO 2 absorption comparing with those of the commercial monoethanolamine solvent. Guanidinium salt ILs (e.g., [TMG][L]) and functional ILs (e.g., [NH 2 p-bmim][BF 4 ] and 2-(2-hydroxyethoxy) ammonium acetate) all show high efficiency for CO 2 and SO 2 capture [9,10].
The typical flue gas from coal-burning usually contains about 15 vol% CO 2 , 10 vol% H 2 O, and more than 2 vol% SO 2 [11]. Apart from CO 2 , the effects of SO 2 on the flue gas absorption should be taken into account [12]. Most researchers consider that water has a little influence on the CO 2 capture, but the effects of SO 2 would not be neglected. For the impurities of the flue gas, it is found

CO 2 and SO 2 Absorption Performance of the Single ILs
When the simulated flue gas (15% CO 2 /85% N 2 ) only contains the CO 2 impurity, one mole [C 4 mim][OAc] could absorb 0.298 mol CO 2 (Table 1). However, when SO 2 was mixed in the simulated flue gas (15% CO 2 /2% SO 2 /N 2 ), the CO 2 absorption capacity of [C 4 4 ], respectively. In short, the single ionic liquids all exhibit the CO 2 absorption capacities, but this capacity would be greatly weakened by the interference of SO 2 . The researchers believed that CO 2 and SO 2 of the flue gas would be absorbed simultaneously [13,20].  4 ] during the absorption process. Additionally, an extreme case was investigated in which the simulated flue gas contained 80% CO 2 , 2% SO 2 , and 18% N 2 . However, SO 2 was also not found at the gas stream of the outlet before 15 min. Compared with CO 2 , SO 2 has higher dipole moments and molecular polarity, which often results in the strong affinity of SO 2 with ionic liquids [21,22].  The researchers believed that CO2 and SO2 of the flue gas would be absorbed simultaneously [13,20] Additionally, an extreme case was investigated in which the simulated flue gas contained 80% CO2, 2% SO2, and 18% N2. However, SO2 was also not found at the gas stream of the outlet before 15 min. Compared with CO2, SO2 has higher dipole moments and molecular polarity, which often results in the strong affinity of SO2 with ionic liquids [21,22].  The presence of SO2 in flue gas usually leads to a competitive and negative influence on the separation of CO2. Figure 2 (Figure 1b). These results agreed well with previous studies [23][24][25].
FT-IR can investigate the interaction between IL and CO2/SO2 [26]. The spectra of [C4mim][OAc] showed the changes after 15% CO2 and 2% SO2 absorption, respectively (Figure 3a−c). However, the [C4mim][OAc] spectrum had minimal changes when it was used to remove pure CO2. Although the appearance of the carbonyl band at 1720 cm −1 shows that the acetate anion might be partly converted into the acetate acid [27], Shiflett considered that the amount of such a chemical reaction was minor and reversible [24]. Thus, the other reactions between CO2 and the cation species in this work might The presence of SO 2 in flue gas usually leads to a competitive and negative influence on the separation of CO 2 . Figure 2 shows the CO 2 (Figure 1b). These results agreed well with previous studies [23][24][25].
group, which also indicates that most of the acetate ions were no longer associated with [C4mim] cations. The intense band at 950 cm −1 should be assigned to the vibrational mode of SO3 2− or S2O5 2− . It once again suggests that the interaction between the SO2 and [C4mim][OAc] ionic liquid was strong. Similarly, the peak intensity at 885 and 1543 cm −1 changed markedly when the [NH2emim][BF4] absorbed SO2. Particularly, two new peaks appeared at 968 and1367 cm −1 , which can be attributed to the interaction between the N elements of the IL and SO2 [6]. FT-IR can investigate the interaction between IL and CO 2 /SO 2 [26]. The spectra of [C 4 mim][OAc] showed the changes after 15% CO 2 and 2% SO 2 absorption, respectively (Figure 3a−c). However, the [C 4 mim][OAc] spectrum had minimal changes when it was used to remove pure CO 2 . Although the appearance of the carbonyl band at 1720 cm −1 shows that the acetate anion might be partly converted into the acetate acid [27], Shiflett considered that the amount of such a chemical reaction was minor and reversible [24]. Thus, the other reactions between CO 2 and the cation species in this work might not be detected within the wavenumber of 800−1600 cm −1 , as Figure 3a absorbing SO2. Meanwhile, the peaks at 1720, 1321, 1254, 1144, and 950 cm −1 newly appeared in the spectrum. The bands at 1321 and 1144 cm −1 can be attributed to the stretching of SO2 absorbed by the ionic liquid [26]. After SO2 absorption, the new peak at 1720 cm −1 shows the formation of a carbonyl group, which also indicates that most of the acetate ions were no longer associated with [C4mim] cations. The intense band at 950 cm −1 should be assigned to the vibrational mode of SO3 2− or S2O5 2− . It once again suggests that the interaction between the SO2 and [C4mim][OAc] ionic liquid was strong. Similarly, the peak intensity at 885 and 1543 cm −1 changed markedly when the [NH2emim][BF4] absorbed SO2. Particularly, two new peaks appeared at 968 and1367 cm −1 , which can be attributed to the interaction between the N elements of the IL and SO2 [6].    The bands at 1321 and 1144 cm −1 can be attributed to the stretching of SO 2 absorbed by the ionic liquid [26]. After SO 2 absorption, the new peak at 1720 cm −1 shows the formation of a carbonyl group, which also indicates that most of the acetate ions were no longer associated with [C 4 mim] cations. The intense band at 950 cm −1 should be assigned to the vibrational mode of SO 3 2−  (a) (b)

Quantum Chemical Calculation on the Interaction of IL Mixture with CO2/SO2
The absorption capacity of CO2 in the IL mixtures was higher than that of the single ionic liquid. This may be related to the interactions between ILs and CO2/SO2 molecules.   (Figure 5a), new resonance peaks at 8.10 ppm were found after SO 2 absorption, which indicates the formation of S . . . N [28]. According to this result, it was considered that the interaction between SO 2 and [NH 2 (Figure 5b), a typical peak of −COOH in the 1 H NMR spectrum moved from 12.75 to 11.83 ppm, and a new resonance peak was observed at 7.52 ppm after SO 2 absorption. These results suggest that the interaction between [NH 2 emim][OAc] and SO 2 had occurred [25]. That is, the interaction between [OAc] and SO 2 leads to the moving from 12.75 ppm to 11.83, while the reaction of [NH 2 emim] and SO 2 makes the new peak 7.52 ppm appearance.
In order to further investigate the effects of SO 2 on the CO 2 absorption capacity of ionic liquids, the CO 2 absorption performance of fresh IL and after SO 2 -saturated IL are illustrated in Figure 6 (a) (b) (a) (b)

Quantum Chemical Calculation on the Interaction of IL Mixture with CO2/SO2
The absorption capacity of CO2 in the IL mixtures was higher than that of the single ionic liquid. This may be related to the interactions between ILs and CO2/SO2 molecules.

Quantum Chemical Calculation on the Interaction of IL Mixture with CO 2 /SO 2
The absorption capacity of CO 2 in the IL mixtures was higher than that of the single ionic liquid. This may be related to the interactions between ILs and CO 2 /SO 2 molecules. Thus, the interaction of the [C 4 4 ] with CO 2 and SO 2 were also investigated ( Figure 7). The structural parameters for the IL−CO 2 /SO 2 complexes are listed in Table 2.
Molecules 2020, 25, 1034 7 of 13 [NH2emim][BF4] with CO2 and SO2 were also investigated ( Figure 7). The structural parameters for the IL−CO2/SO2 complexes are listed in Table 2.    In general, the CO 2 /SO 2 gas molecules around the anions and cations were related to the absorption reaction. As Figure 7a shows, there was a strong interaction between the N atom and the S atom of  (Table 3). It was found that the interaction energy and absorption enthalpy of [OAc]−CO 2 −SO 2 complex were less than the sum of the energy and the enthalpy for [OAc]−CO 2 and [OAc]−SO 2 , suggesting that CO 2 and SO 2 would competitively react with [OAc] anion.

Materials
The simulated flue gas was obtained by pure gas CO2, SO2, and N2 (purity of >99.99 wt%). They

Materials
The simulated flue gas was obtained by pure gas CO 2 , SO 2 , and N 2 (purity of >99.99 wt%). They were all purchased from Beifen (China) Gas Technology Company.  [17,18,29]. First, the [NH 2 emim] cation was prepared by the reaction of 1-methylimidazole and 2-bromoethylamine hydrobromide under reflux for 12 h. Second, the [NH 2 emim]-based IL was simply synthesized by ion exchange with NaBF 4 or NaOAc/CH 3 COOH in ethanol, and then the ethanol was removed in vacuum. The structures of the ILs were confirmed by proton nuclear magnetic resonance ( 1 H NMR, Bruker WB400 AMX spectrometer, Billerica, MA, USA). Here, deuterated chloroform (CDCl 3 ) was used as a solvent, and tetramethylsilane (TMS) was employed as an internal standard for 1 H NMR measurement.

CO 2 and SO 2 Absorption
As Figure 9 shows, CO 2 and SO 2 absorption experiments were performed in a 30 mL reactor immersed with a water-bath temperature controller. The temperatures were controlled at 293 K for absorption and 353 K for desorption, respectively. The simulated flue gas was a mixture of N 2 , CO 2 , and SO 2 in accordance with a certain proportion. As a typical absorption process, 10 mL IL or IL mixtures were added to the reactor at first. Subsequently, 15 vol% CO 2 , 2 vol% SO 2 and 83 vol% N 2 were mixed in storage. The intake speed of the mixed gas was controlled at 60 mL/min, and the absorption pressure was controlled at 101.3 kPa. The concentrations of CO 2 and SO 2 were analyzed by a gas analyzer (MRU NOVA2000) at the outlet. To investigate the IL regeneration, CO 2 or SO 2 saturated IL was also loaded in the reactor. Desorption was performed at 353 K under a pure N 2 gas atmosphere for 30 min. The amount of absorbed CO2 or SO2 was calculated by the following equation.
where Agas is the molar amount of CO2 or SO2 in the ionic liquid; Q is the flow rate of the gas stream; The amount of absorbed CO 2 or SO 2 was calculated by the following equation.
where A gas is the molar amount of CO 2 or SO 2 in the ionic liquid; Q is the flow rate of the gas stream; C 0 and C gas are the CO 2 or SO 2 concentrations at the inlet and the outlet streams, respectively; t 1 refers to the beginning time of the absorption process; when the CO 2 and SO 2 concentration at the outlet stream returns to the initial concentration, the time is t 2 ; M IL and M gas are the molecular weight of IL and CO 2 (or SO 2 ), respectively; m IL is the weight of the ILs, and ρ gas is the density of CO 2 or SO 2 .
After the IL was saturated by CO 2 and SO 2 , the complex structure was investigated by the FTIR and NMR technologies. The FTIR spectra of the samples were analyzed on an FTIR spectrometer (PerkinElmer, Frontier 2500). In addition, the structure changes of the [NH 2 emim]-based IL after absorption were also detected by the NMR spectrometer (Bruker WB400 AMX, 300 MHz) using chloroform-d (CDCl 3 ) as a solvent and tetramethylsilane (TMS) as an internal standard.

Theoretical Calculation
A quantum chemical calculation was used in this work to study the interaction between the ionic liquid and CO 2 with SO 2 . All calculations were carried out by the Gaussian 16 program [30]. For IL calculations, Li et al. [31] suggested that the density function of the Minnesota family [32] (e.g., M06-2X) with a diffusion function basis set (e.g., 6-311++G(d,p)) might give reasonable results. If dispersion-corrected density functionals (e.g., gd3bj, DFT-D3) were used, more reliable results could be obtained [33]. Therefore, the geometry optimization and frequency analysis of all ILs and IL mixtures were performed at the M06-2X/6-311++G(d,p) level and correction with Grimme's method. In order to calculate the interaction energy of the IL complexes, the basis set superposition error (BSSE) method was employed to correct the energy results [34]. The effect of the solvent should be taken into consideration in the theoretical calculation of the ionic liquids. It was found that the SMD solvation model proposed by Truhlar et al. can be used for the IL calculation very well [35,36]. Thus, the density functional theory (M06-2X and dispersion-corrected method) with the SMD model was also used to calculate the interaction energy of IL−CO 2 /SO 2 .

Conclusions
The CO 2 and SO 2 absorption of the flue gas in ionic liquids were investigated by the experimental method and theoretical calculation. The single ionic liquids, such as [NH 2 emim][BF 4 ] and [C 4 mim][OAc], all showed good CO 2 absorption performance for the simulated flue gas without SO 2 interference. However, SO 2 was more likely to react with the active sites of the ILs. When SO 2 was in the flue gas, the CO 2 absorption capacity of the single ionic liquid would be significantly inhibited. It was found that the interference of SO 2 on the CO 2 absorption performance might be markedly reduced by using the ionic liquid mixtures. The CO 2 absorption capacity of the IL mixture [C 4 4 ] was about 0.4 mol CO 2 /mol IL even at an atmosphere of 15% CO 2 /2%SO 2 /83% N 2 , which was greater than that of single [C 4 mim][OAc] (0.204 mol CO 2 /mol IL). There was a competitive relationship between CO 2 and SO 2 during the absorption process. The single ILs prefer to capture SO 2 rather than remove CO 2 , due to the stronger interaction energy of SO 2