Absorption of SO₂ at High Temperatures by Ionic Liquids and the Absorption Mechanism

Abstract

The capture of $SO_2$ at or close to the temperatures of real flue gas is much more attractive in application. In this work, two kinds of ionic liquids (ILs) based on lactate anion were used to absorb $SO_2$ at high temperatures from 100 to $120^{\circ}C$. The ILs show high absorption capacities of over one mol $SO_2$ per mol IL at $110^{\circ}C$. The absorption of $SO_2$ by the ILs based on lactate anion is reversible and the ILs can be reused for the capture of $SO_2$ at high temperatures with high absorption capacity and thermal stability. Furthermore, the absorption mechanism of $SO_2$ by the ILs was studied by FT-IR, $^1H$ NMR and $^{13}C$ NMR spectra. It has been found that there are strong chemical interactions between the ILs and $SO_2$. Also the absorption mechanism is different when there is water present in ILs compared to when there is not.

Introduction

The emission of sulfur dioxide (SO2), which is mainly from the burning of fossil fuels, has drawn much attention worldwide. It is essential to remove SO2 from the flue gas since SO2 is harmful to human body and causes serious environmental pollution. Up to now, flue gas desulfurization (FGD) is the most efficient way for the capture of SO2.1, 2

Currently, CaCO3 is the most commonly used material for flue gas desulfurization (FGD), but there are large amounts of by-product CaSO4 produced in the process. Liquid absorbents such as aqueous amines have also been used. However, this method may cause secondary pollution due to the high volatility of amines. As a result, recyclable liquid solvents with low volatility and high capacity are expected as excellent absorbents for the capture of SO2.

Recently, the absorption of acidic gases (CO2, SO2 and H2S) by ionic liquids (ILs) has been widely studied for their unique properties. For example, ILs have negligible vapor pressure, high thermal stability, tunable structure and high absorption capacity of acidic gas. Han et al. reported the first task-specific ILs 1,1,3,3-tetramethylguanidinium lactate ([TMG]L) for the capture of SO2. They found that the mole ratio of SO2 to [TMG]L could reach 1.0 at 40 ℃ with 8% SO2 in N2.3 From the viewpoint of application, Wu et al. studied the effect of water and O2 on the absorption of SO2 by functional ILs, and they proposed a way to regenerate the ILs.4-6 Zhang et al. reported new guanidinium-based ILs with quite low viscosities for the capture of SO2.7 Riisager et al. found that [TMG][BF4], [TMG][BTA], [TMGB2][BTA], [Bmim][BF4] and [Bmim][BTA] could absorb large amounts of SO2 through physical absorption.8 Other types of ILs, such as imidazolium-based,9-16 alkanolaminium-based,17-19quaternary phosphonium-based,16,20 ether-functionalized imidazolium-based15,16 ILs, amide-thiocyanates eutectic ILs21 and caprolactam tetrabutyl ammonium bromide,22-24 have been studied by different research groups. New technologies, such as ILs supported on membranes or porous silica particles and ILs polymers, have also been applied for the capture of SO2.25-27 Simultaneously, theoretical researches on the absorption of SO2 by ILs have attracted much attention.28-30

After removing dust, the temperature of real flue gas is around 140 ℃.31 The flue gas with high temperature should be cooled down to about 50 ℃ for the capture of SO2 in wet desulfurization process; and the purified flue gas should be heated to above 80 ℃ for emission according to the technical code for designing flue gas desulfurization plants of fossil fuel power plants of China (DL/T 5196-2004). Therefore, there would be large amounts of energy consumption during the process. The capture of SO2 at or close to the temperatures of real flue gas shows some advantages compared with the traditional method: (1) the energy cost of cooling down the high temperature flue gas would be decreased significantly; (2) heating the purified flue gas would no longer be needed. For their excellent properties, ILs can be used at high temperatures, and the ILs with high SO2 absorption capacities at high temperatures are much more attractive in application. However, the previous studies mainly focused on SO2 capture at low temperatures (some even at room temperature). These ILs cannot efficiently capture SO2 at high temperatures. For example, the solubility of 0.1 MPa SO2 in [TMG][BTA] at 120 ℃ is very low, just 0.1 mole SO2 per mole IL.8 Therefore, it is necessary to design new ILs with high capacities to capture SO2 at high temperatures. ILs cannot absorb large amounts of SO2 by physical interactions at very high temperatures. So, it is reasonable to design ILs that can chemically interact with SO2. It has been reported that ILs based on lactate, acetate and methyl sulfate anions can chemically absorb SO2.20 However, Lee et al. reported that the absorption of SO2 by [Bmim][Ac] is irreversible when there is water in the IL or in the gas.14 During the absorption process, the acetate anion in [Bmim][Ac] transforms into acetic acid and the IL transforms into [Bmim][HOSO2]. The acetic acid formed could be released from the IL easily due to its high volatility, which results in the non-renewable of [Bmim][Ac]. Considering the above result, lactic acid may be a wise choice as the anion of ILs due to its low vapor pressure.

In previous work, the absorption and desorption behaviors of SO2 by lactate anion-based ILs with different kinds of cations were studied.32 It was found that tetraethylammoniumbased IL shows the highest SO2 absorption capacity and imidazolium-based IL shows the highest thermal stability. From the TGA curves of tetraethylammonium lactate ([N2222]L) and 1-butyl-3-methylimidazolium lactate ([Bmim]L), we know that the decomposition temperatures (Td) for [N2222]L and [Bmim]L are 201 ℃ and 232 ℃, respectively.32 As a result, [N2222]L and [Bmim]L can be used at temperatures higher than 100 ℃. Although [N2222]L and [Bmim]L can absorb SO2 efficiently at 60 ℃, whether the ILs can be applied for the absorption of SO2 at higher temperatures was not studied; neither was the absorption mechanism. Therefore, the absorption of SO2 by [N2222]L and [Bmim]L at high temperatures from 100 ℃ to 120 ℃ were studied. Effects of temperature, SO2 partial pressure and water on the absorption of SO2 by the ILs were systematically investigated, and the absorption mechanism was proposed based on the FT-IR, 1H NMR and 13C NMR spectra.

 

Experimental

Raw Materials and Experimental Methods. SO2 (99.95%) and N2 (99.999%) were obtained from Beijing Haipu Gases Co., Ltd. (Beijing, China). N-Methylimidazole was obtained from Leadership Chemical Co., Ltd. (Shandong, China) and it was distilled before use. Analytical reagent lactic acid (85% in water), 1-chlorobutane (98%), sodium hydroxide (98%) and tetraethyl ammonium hydroxide (25% in water) were supplied by Aladdin Chemical Co., Ltd. (Shanghai, China). [N2222]L and [Bmim]L were synthesized and characterized following the literature.32

The absorption and desorption of SO2 were carried out at ambient pressure in a constant temperature oil bath. Before absorption, about 3.5 g IL was loaded in a glass tube, and the IL was treated with 100 cm3/min N2 at 110 ℃ for more than 12 h to remove volatile compounds.33 Then, some amount of the IL was drawn from the glass tube to ensure that 3.0 g IL was left in the glass tube. The absorption of SO2 was treated with 50 cm3/min SO2, and the desorption of SO2 was treated with 100 cm3/min N2 at desired temperatures. Solubility of SO2 in the ILs was determined by weight increase of the glass tube.

The absorption of SO2 in ILs + H2O (wIL = 83%) was carried out on the same device at 45 ℃ because it is difficult to control water steam, and the solubility of SO2 in ILs + H2O cannot be determined exactly at high temperatures. During the absorption, water might be released from the absorption glass tube. To capture the released water, the down stream gas from the absorption glass tube was bubbled through a concentrated sulfuric acid solution in a glass tube. The water absorbed by the concentrated sulfuric acid was also considered when determining the solubility of SO2 in the ILs with water.

 

Results and Discussion

Effect of Temperature and SO2 Partial Pressure on the Absorption of SO2. Figure 1 shows the absorption of SO2 in [N2222]L and [Bmim]L at 100, 110 and 120 ℃. It can be seen from the figures that the ILs can absorb SO2 with high efficiency and the absorption curves almost increase linearly in the initial 5 minute period. The ILs can absorb large amounts of SO2 even at high temperatures. For example, the mole ratios of SO2 to [N2222]L are 1.229, 1.138 and 1.075 at 100, 110 and 120 ℃, respectively. The SO2 absorption capacities of the ILs from 25 to 120 ℃ shows that [Bmim]L has a slightly smaller absorption capacity than [N2222]L does (Figure S1), such as at 100 ℃ 1.102 mole ratio of SO2 to IL for the former, and 1.229 for the latter. Effect of SO2 partial pressure on the absorption capacities of the ILs is shown in Figure 2, which shows that the mole ratios of SO2 to [N2222]L and [Bmim]L decrease from 1.138 and 1.033 to 0.547 and 0.456, respectively, when the partial pressure of SO2 decreases from 101 kPa to 3 kPa. As a result, the absoprtion capacities of the ILs decrease with the increase of temperature and decrease of SO2 partial pressure, which suggests that the ILs can be regenerated by increasing temperature and/or decreasing pressure.

Figure 1.Absorption of SO2 in (a) [N2222]L and (b) [Bmim]L at high temperatures: ■, 100 ℃; ●, 110 ℃; ▲ , 120 ℃.

Figure 2.Effect of SO2 partial pressure on the absorption capacities of the ILs at 110 ℃: ■, [N2222]L; □, [Bmim]L.

Desorption of SO2 and Regeneration of the ILs. The ILs saturated with SO2 at 110 ℃ were treated with 100 cm3/min N2 at 110 ℃. The absorbed SO2 in the ILs can be released easily from the ILs in the initial 60 minute period, but then over time the mole ratio of SO2 to IL changes slowly. Taking [Bmim]L as an example, the mole ratio of SO2 to IL is 1.033, 0.330 and 0.157 at 0, 60 and 360 min, respectively. That is to say about 68% and 85% of the absorbed SO2 can be released from the IL if treated with 100 cm3/min N2 at 110 ℃ for 60 and 360 min. Under this condition, it needs a long time to release SO2 from the ILs. However, the desorption efficiency can be enhanced by treating the SO2 absorbed ILs at higher temperatures and/or lower pressures. The FTIR spectra of the ILs before absorption and after desorption are shown in Figure S2, which confirms that the ILs can be regenerated by heating the ILs saturated with SO2.

Figure 3 shows the reuse cycles of [N2222]L and [Bmim]L for SO2 absorption/desorption. The absorption of SO2 was carried out at 110 ℃ for 60 min, and the desorption of SO2 was carried out at 110 ℃ for 360 min. It can be seen from Figure 3 that no obvious loss of absorption capacities were found for the ILs during the reuse cycles. Taking [Bmim]L as an example, the mole ratios of SO2 to IL of the 5 cycles are 1.033, 1.031, 1.043, 1.045, 1.029 after the absorption at 110 ℃ for 60 min. As a result, the absorption of SO2 by the ILs are reversible and the ILs can be reused for the capture of SO2 with high absorption capacity and stability.

Figure 3.The absorption and desorption cycles of SO2 in (a) [N2222]L and (b) [Bmim]L: , absorption; , desorption.

Effect of Water on the Absorption of SO2 and Regeneration of the ILs. There is some amount of water or moisture in flue gas, and the IL will absorb water from flue gas due to their hydrophilicity. Previously, Lee et al. reported that in the presence of water, the absorption of SO2 in [Bmim][Ac] is irreversible for the high volatility of acetic acid.14 As a result, lactic acid with low vapor pressure may solve the problem.

In order to verify our hypothesis, the absorption of pure SO2 in IL + H2O (wIL = 83%) and regeneration of the ILs were studied. The absorption of SO2 was carried out at 45 ℃ for 60 min and the desorption of SO2 was carried out at 45 ℃ for 60 min (It is very dangerous to put the glass tube with large amounts of SO2 into the oil bath at 110 ℃ directly) and then at 110 ℃ for 360 min, and the result is shown in Figure 4. The SO2 absorption capacities of ILs in the presence of water decrease a little. For example, the mole ratio of SO2 to [Bmim]L is 1.629 when there is 17 wt % water in the IL, compared with that of 1.940 when there is no water. There are strong interactions between hydrophilic ILs and water, and the interactions between the ILs and SO2 decrease with the addition of water. As a result, the competition between water and SO2 leads to the decrease of SO2 absorption capacities of ILs in the presence of water.

Figure 4.The absorption of SO2 in (a) [N2222]L and (b) [Bmim]L at 45 ℃: ■, IL; ● IL + H2O (wIL = 83%).

Figure 5 shows the absorption and desorption of SO2 by the two ILs in the pesence of 17 wt % water with 5 cycles. The results indicate that the absorption of SO2 by the ILs based on lactate anion are highly reversible in the presence of water. For example, the mole ratios of SO2 to [Bmim]L are 1.629, 1.637, 1.621, 1.638 and 1.614 of the 5 cycles of absorption. It is very interesting that the absoption of SO2 by [Bmim][Ac] is irreversible while that of [Bmim]L is highly reversible, though the two ILs have the same cation [Bmim]+. The reason is that after the absorption of SO2 by the ILs in the presence of water, the formed acetic acid could be easily removed under vacuum at high temperature, while the formed lactic acid would remain in the absorbent. After the absorbed SO2 is released from the absorbent, [Bmim]L can be regenerated by the combination of [Bmim]+ and lactate anion. The result suggests that an acid with low vapor pressure is favorable to form excellent ILs which could be used in real flue gas.

Figure 5.The absorption and desorption cycles of SO2 in (a) [N2222]L + H2O (wIL = 83%) and (b) [Bmim]L + H2O (wIL = 83%): , absorption; , desorption

The Absorption Mechanism. The high SO2 absorption capacities of the ILs at high temperatures suggests that strong chemical interactions existed between SO2 and the ILs. The absorption machanism of SO2 by ILs has been studied experimentally and theoretically. Han et al. demonstrated that both physical and chemical interactions existed between SO2 and [TMG]L.3 They suggested that SO2 reacted with the N–H group in [TMG]+ and a new N–S band was formed, so one mole IL can chemically absorb one mole SO2. Zhang et al. reported new ILs based on [TMG]+, and they suggested a similar absorption mechanism.7 Zhang et al.17 and Zhai et al.18 proposed that SO2 reacted with the N–H group on the cation of alkanolaminium-based ILs, while the O atom on S=O probably forms intramolecular hydrogen bond with the H atom of the amine. Simultaneously, a carboxylic acid such as formic acid, acetic acid and lactic acid was formed. Recently, Wu et al. demonstrated that the task-specific ILs, [TMG]L and [MEA]L, could chemically absorb 0.5 mol SO2 per mole IL,34 and they distinguished the physically and chemically absorbed SO2 and confirmed the absorption mechanism.35 Lee et al. found that [Bmim][Ac] would change into [Bmim][HOSO2] after the absorption of SO2 when there was some amount of water in the IL or in the gas.14 Wang et al. applied molecular dynamics simulations28 and quantum chemical calculations29 to investigate the solubility of SO2 in [TMG]L. The theoretical research shows that SO2 could form S···O interactions with lactate anion and form N–H···O interactions with [TMG]+. The S···O interactions is 3.5–8.8 times stronger than that of N–H···O hydrogen bonding, which suggests that the chemically absorbed SO2 was mainly due to the strong interactions between SO2 and electronegative oxygen atoms in the anion. Based on the theory, Wang et al. synthesized quaternary phosphoniumbased ILs with multiple electronegative nitrogen atoms in the anions.20 As expected, these ILs have extremely high absorption capacities. Although many groups have studied the absorption mechanism of SO2 by ILs, the mechanism is still unclear.

In order to understand the absorption mechanism of SO2 by [N2222]L and [Bmim]L, the ILs and ILs-SO2 were characterized by FT-IR (KBr), 1H NMR (CDCl3) and 13C NMR (DMSO-d6). The FT-IR, 1H NMR and 13C NMR spectra of [N2222]L and [N2222]L-SO2 are shown in Figure 6. Compared with the FT-IR spectrum of [N2222]L, the spectrum of [N2222]L-SO2 shows new absorption bands at 1224 cm–1 and 967 cm–1, which can be assigned to sulphate S=O and S–O stretches, respectively. This could support the existence of chemical interactions between SO2 and the IL. The newly appeared band at 1731 cm–1 shows the the formation of –COOH. Since there is no H+ in the system (IL or SO2) that can be supplied to –COO– to form –COOH, the production of H+ during the reaction of IL and SO2 is necessary. It can be seen from the 1H NMR spectra of [N2222]L and [N2222]L-SO2 that the typical peak of –CH(OH)– of lactate anion moves downfield from 3.84 ppm to 4.52 ppm, which suggests that the IL can chemically absorb SO2 and the strong interactions exist between SO2 and lactate anion. From the 13C NMR spectra of [N2222]L and [N2222]L-SO2, we can see that the peaks of CH3–, –CH(OH)– and –COO– of lactate anion moves upfield from 21.5, 67.3 and 177.9 ppm to 19.5, 63.2 and 175.6 ppm, respectively, which confirms that SO2 reacts with lactate anion. The results of FT-IR, 1H NMR and 13C NMR spectra of [Bmim]L before and after SO2 absorption (Figure S3) also show that the IL could chemically absorb SO2, and SO2 reacts with lactate anion. Based on the above results, we proposed that SO2 reacts with –OH to from –O–(S=O)–O– and H+ (Scheme 1). It is similar to the reactions of SO2 and alcoholic hydroxyl group to form zwitterionic compounds reported by other research groups.36-39 When there is water in the IL or in the gas, SO2 reacts with water and forms H2SO3. Since lactic acid (pKa = 3.86) is a weaker acid than H2SO3 (pKa1 = 1.89), the conjugated base (lactate anion) would be favorable to exist as lactic acid in the presence of H2SO3. As a result, the IL reacts with H2SO3 to form [IL]+HSO3 –/[IL]2 2+SO3 2– and lactic acid (Scheme 1).

Figure 6.The (a) FT-IR, (b) 1H NMR and (c) 13C NMR spectra of [N2222]L before and after absorption: A, [N2222]L; B, [N2222]L-SO2.

Scheme 1.The proposed absorption mechanism by the ILs based on lactate anion. R+ stands for [N2222]+ or [Bmim]+; L− stands for lactate anion; La stands for lactate acid.

 

Conclusion

In summary, two kinds of ILs based on lactate anion were used to absorb SO2 at high temperatures more than 100 ℃. Effects of temperature, SO2 partial pressure and water on the absorption of SO2 by [N2222]L and [Bmim]L were studied. Furthermore, the absorption mechanism of SO2 by the ILs was studied by FT-IR, 1H NMR and 13C NMR spectra. It was found that, the mole ratio of SO2 to the ILs can reach more than 1.00 at 110 ℃. The absorption/desorption cycles of SO2 by these ILs were tested for five runs, and no obvious loss of absorption capacities were found even when there is some amount of water in ILs during the cycles. The result shows that the ILs based on lactate anion are promising absorbents for the capture of SO2 at high temperatures with high absorption capacity and stability. The absorption mechanism has been proposed, indicating that there are strong chemical interactions between the ILs and SO2.