Effect of SO2 on CO2 Absorption in Flue Gas by Ionic Liquid 1-Ethyl-3

Article pubs.acs.org/IECR

Effect of SO2 on CO2 Absorption in Flue Gas by Ionic Liquid 1‑Ethyl-3methylimidazolium Acetate Xiaoshan Li, Liqi Zhang,* Ying Zheng, and Chuguang Zheng State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, Hubei Province, People’s Republic of China Ind. Eng. Chem. Res. Downloaded from pubs.acs.org by KTH ROYAL INST OF TECHNOLOGY on 08/22/15. For personal use only.

S Supporting Information *

ABSTRACT: Significant efforts are being made to develop several novel solvents or materials for postcombustion CO2 capture technology. Traditional amine solvents suffer from mass loses due to its volatility and poisoning by flue gas impurities. Ionic liquids (ILs) are considered to be promising alternatives for CO2 capture due to their unique features, such as negligible vapor pressure. Despite the extensive research on CO2 capture by ILs, few studies have investigated the effect of flue gas components on CO2 absorption performance. Because of the large differences between CO2 and SO2 in the absorption capacity and partial pressure in flue gas, it is essential to study the role of SO2 in CO2 capture using ILs. This work focused on studying the effect of SO2 with low concentration on postcombustion CO2 capture by 1-ethyl-3-methylimidazolium acetate [C2mim][OAc] and explaining the microscopic mechanism through quantum chemical calculation. Results showed that the CO2 absorption capacity was largely decreased by 25% in the presence of SO2. After 5 cycles of regeneration, the initial absorption capacity of CO2 in [C2mim][OAc] remarkably decreased by 48%. Thus, the ionic liquid lost its excellent reversibility and regeneration property for CO2 capture under continuous exposure to SO2. The quantum chemical calculation indicated that CO2 could hardly compete with SO2 for the main active sites in [C2mim][OAc]. In addition, the net charge transfer amount from acetate anion to CO2 significantly decreased from −0.537 to −0.039 e in the presence of SO2, which explained the decreased absorption capacity of CO2 in [C2mim][OAc].

1. INTRODUCTION

capture process, demonstrating that ILs present an energysaving and cost-effective way for postcombustion CO2 capture. It is known that the typical postcombustion flue gas from coal contains not only approximately 10−15% vol CO2 but also other impurities, such as 10% H2O, 0.05−0.2% vol SO2, 0.15− 0.25% vol NOx and particulates.12 Currently, almost all the previous studies focused on the absorption of pure CO2 (100% vol) in ILs, without considering the role of other flue gas impurities.13−22 The effect of water on CO2 absorption by ILs has been studied, indicating that water only slightly decreased the CO2 absorption capacity.23−26 However, few studies investigated the effect of other flue gas impurities on the absorption performance of CO2 by ILs.4 In fact, these impurities lead to several problems in CO2 capture processes. The stripped CO2 gas will be contaminated, causing low product purity. Moreover, previous studies using the amine sorbents for CO2 capture revealed that the presence of SO2 inhibits CO2 absorption due to its strong interaction with the sorbents, leading to rapidly decreased CO2 absorption capacity over multiple cycles.27−29 However, it remains unclear whether ILs will also show this phenomenon like the amine sorbents. It is also reported that ILs showed high solubility of SO2 due to the strong interaction energy.30−34 All the studies investigated the solubility of single gas (CO2 or SO2) in ILs and showed that the ILs showed much higher SO2 absorption

It is predicted that coal has dominated the global energy market for a long time, which will lead to the continuous increasing tendency of the atmosphere CO2 concentration. It is well accepted that CO2 capture and storage (CCS) technology has been proved to be an effective method for the reduction of greenhouse gas emission.1 Currently, there are three main processes for CCS, including precombustion capture, postcombustion capture as well as oxy-fuel combustion.2 In terms of the traditional pulverized coal power plants, the postcombustion capture process has been considered to be more competitive because this process could keep the current energy infrastructure largely intact. However, several problems such as unpredictable environmental impacts, high costs and energy consumption have become the great obstacles to its industrial application. Great efforts are being made to separate CO2 from the flue stream by developing several novel CO2-capturing solvents or materials.3 Among the emerging novel solvents or materials, ionic liquids (ILs) have attracted remarkable interests, which are considered as promising candidates for CO2 capture.4−9 ILs have several unique advantages in gas separation of CO2, one of which is the negligible vapor pressure causing lower energy consumption in regeneration process due to no contamination of the gas stream and little mass losses of ILs.10 Shiflett and coworkers11 reported that ionic liquid 1-butyl-3-methylimidazolium acetate [C4mim][OAc] could reduce the energy losses by 16% and provide 11% reduction in investment in comparison with commercial monoethanolamine (MEA) solvent for CO2 © XXXX American Chemical Society

Received: June 17, 2015 Revised: August 9, 2015 Accepted: August 18, 2015

A

DOI: 10.1021/acs.iecr.5b02208 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Ind. Eng. Chem. Res. Downloaded from pubs.acs.org by KTH ROYAL INST OF TECHNOLOGY on 08/22/15. For personal use only.

Industrial & Engineering Chemistry Research capacity than CO2. Zhang and co-workers32 made comparisons between CO2 absorption and SO2 absorption in three guanidinium-based ILs. At the same conditions, pure CO2 solubilities in these ILs were lower than 0.5 mol/mol, whereas the absorption capacities of pure SO2 were larger than 2.6 mol/ mol. Wang and co-workers proposed a novel strategy for CO2 and SO2 capture through tuning the anions with different pKa values and multiple-site chemical absorption. The absorption capacity of pure CO2 in eight tunable basic azole-based ILs at 23 °C varied from 0.08 to 1.02 mol/mol,20 whereas two of those ILs, [P66614][Tetz] and [P66614][Im], showed extremely higher absorption capacity of pure SO2 (>3.5 mol/mol, at 20 °C).33 In addition, the solubility of pure SO2 in ionic liquid [C4mim][OAc] was measured to be 0.6 mol/mol34 whereas that of pure CO2 was 0.09 mol/mol18 at 50 °C. It is well accepted that SO2 has stronger interactions with ILs than CO2 does. The absorption capacity of pure SO2 in ILs showed almost 1 order of magnitude larger than that of pure CO2 at the same condition (usually 0.1 MPa). It is also well accepted that low pressure results in low absorption capacity. In the case of postcombustion flue gas, the partial pressure of SO2 (0.0002 MPa) was about 2 orders of magnitude lower than that of CO2 (0.015 MPa), which may lead to a lower absorption capacity for SO2. Thus, we tried to investigate how the presence of SO2 with such low partial pressure would affect CO2 absorption capacity by ILs. To the best of our knowledge, the absorption behavior of CO2 in ILs under the exposure to SO2 has not been well investigated. Therefore, it is essential to study the role of SO2 with low concentration on CO2 capture. The imidazolium acetate ILs were commonly used solvents for CO2 capture and several reports have suggested that these ILs were promising solvents for highly efficient and reversible CO2 capture.11,18,19,35−37 Yokozeki and co-workers35 measured the solubilities of CO2 in 18 kinds of ILs. It turned out that 1ethyl-3-methylimidazolium acetate [C2mim][OAc] showed high solubility for reversible CO2 capture, especially at low pressure. Chen and co-workers36 tested 11 kinds of ILs for CO2 capture and proposed that ILs with an acetate anion presented the superiority in absorption rate and capacity, which are promising substitutes for conventional organic solvents in CO2 capture because of its high absorption capacity, simple synthesis process and low cost compared with other ILs. Therefore, our work focused on studying how the presence of SO2 with low concentration affects CO2 absorption behavior in ionic liquid 1ethyl-3-methylimidazolium acetate [C2mim][OAc]. The absorption performance of CO2 in [C2mim][OAc] was measured at the simulated flue gas atmospheres of 15% CO2 under exposure to 0.2% SO2. The effect of SO2 on the regeneration properties of the CO2−[C2mim][OAc] system and the interplay between CO2 and SO2 in ionic liquid absorption were also studied. Moreover, the sorption mechanism was analyzed through the characterization of FTIR, NMR and quantum chemical calculation.

before use. The molecular structure of [C2mim][OAc] is presented in Figure S1. The viscosity and water content of [C2mim][OAc] were measured through the Brookfield DV-C viscometer and the Mettler Karl Fischer coulometer. The viscosity was 83 mPa·s at 303 K and 0.1 MPa, lower than the reported value38 (105 mPa·s). The reason was that viscosity measurement was greatly affected by the presence of impurities (particularly water). The water content in our sample measured was 0.74 wt %, larger than the reported one of 0.12 wt %.38 Thus, more water content resulted in the lower viscosity. 2.2. Absorption Measurement. The absorption apparatus is similar to our previous work.13 Each experiment was repeated at least three times with the experimental error at a level of ±5%. First, the CO2 absorption performance in [C2mim][OAc] was measured at the simulated flue gas atmospheres of 15% vol CO2 under exposure to 0.2% SO2. The absorption experiment was performed at ambient pressure and at the temperature of 30 °C. The gas stream with flow rate of 50 mL/ min was introduced into an absorption tube with the ILs partly immersed in a thermostatic bath. CO2 concentration in outlet gas stream was analyzed by gas chromatography (GC, FL9070) and recorded at regular intervals of time. The amount of absorbed CO2 was determined from the difference in the CO2 concentrations between the inlet and outlet integrated over time by using eq 1 ACCO2 (mol CO2 /mol IL) t2

=

MIL × ρCO × Q ∫ (C0 − CCO2(t ))dt 2

t1

mIL × MCO2

(1)

where ACCO2 is the molar absorption capacity of CO2 in IL; Q (mL/min) is the flow rate of the gas stream; C0 is the initial CO2 concentration of the inlet stream; CCO2 (t) is the measured CO2 concentration of the outlet stream during the whole absorption process; t1 is the beginning time of the absorption process and t2 refers to the time when CO2 concentration of the outlet stream returns to the initial concentration; mIL is the mass weight of the ILs used for absorption; ρCO2 is the density of CO2 at the experiment condition that can be calculated by NIST Refprop software. Then, the regeneration properties of the CO2−[C2mim][OAc] system were tested over 5 cycles of absorption and desorption. The absorption process was conducted at 30 °C for 180 min and the desorption process was carried out at 90 °C with N2 purging for 60 min. In addition, solubility of SO2 in fresh [C2mim][OAc] and [C2mim][OAc] after 5 cycles of CO2 absorption/desorption was also measured. SO2 concentration in outlet gas stream was continuously detected by the handheld flue gas analyzer and the amount of SO2 absorbed was calculated similarly to that of CO2. Next, to investigate the effects of one component on the other’s absorption, the interplay between CO2 and SO2 was studied through three groups of sequential absorption experiment. For groups 1 and 2, pure CO2 was first introduced into the absorption tube with ionic liquid [C2mim][OAc] until the absorption equilibrium. Then the CO2-saturated ILs continued to absorb SO2 at the atmosphere of 0.5% SO2/N2 (group 1) or desorb at the atmosphere of N2 (group 2). For group 3, the ionic liquid first absorbed SO2 at the atmosphere of 0.5% SO2/ N2 and then the SO2-saturated ILs continued to absorb pure CO2. The interplay between CO2 and SO2 in [C2mim][OAc]

2. EXPERIMENTAL SECTION 2.1. Materials. All the gases were supplied by Minghui Gas Technology Co., Ltd., China. The mixture gas was dried using CaCl2 particles as a desiccant and the gas composition was measured through the hand-held flue gas analyzer (Multilyzer STe M60, AFRISO). The ionic liquid 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc], 170.21 g/mol) with the purity over 99% were obtained from Lanzhou Greenchem ILs, LICP, CAS, China. The IL was dried at 80 °C under vacuum for 72 h B


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Industrial & Engineering Chemistry Research could be obtained by comparing group 1 with group 3, whereas the effect of SO2 on the purge of CO2 saturated ILs could be obtained by comparing group 1 with group 2. 2.3. Characterization of FTIR and NMR. The complex formation was investigated through the characterization of FTIR and NMR technology. The FTIR spectrum of three samples, fresh [C 2mim][OAc] before CO2 absorption, [C2mim][OAc] after CO2 absorption with or without SO2 were analyzed on a FTIR spectrometer (Bruker, VERTEX 70). Moreover the structure changes of [C2mim][OAc] after absorption and desorption were also presented. 1H NMR and 13 C NMR spectra of two samples, fresh [C2mim][OAc] and [C2mim][OAc] after CO2 absorption with SO2, were also collected on a Bruker AV400 NMR spectrometer using heavy water (D2O) as a solvent and tetramethylsilane (TMS) as an internal standard. 2.4. Quantum Chemical Calculation. To investigate the interaction between the ionic liquid used in this work with CO2 and SO2, quantum chemical calculation was performed through the Gaussian 09 program.39 The interaction between the acetate anion and gases were calculated, including the [OAc]− CO2, [OAc]−SO2 and [OAc]−CO2−SO2 complex. Geometry optimization and vibrational frequency calculation of all the complexes were carried out at the B3LYP/6-31+G(d,p) level with dispersion correction by the Grimme40 method. Moreover, the basis set superposition error (BSSE) was corrected using the counterpoise41 method when calculating the interaction energies. For the initial configurations of these complexes, different gas molecule locations around the acetate anion were taken into consideration. These calculations were performed on the gas phase model without considering the solvent effects. Thus, the interaction energies of the [C2mim][OAc]−CO2 and [C2mim][OAc]−SO2 system were performed through the continuum universal solvation model (SMD). The SMD universal solvation model for the generic ILs was proposed by Truhlar42 and Li43 employed the SMD model to investigate the extractive desulfurization mechanisms of ILs. Once the solvation free energies were calculated, the interaction energy of the [C2mim][OAc]−CO2 and [C2mim][OAc]−SO2 system in the solvation state could be obtained.

Figure 1. CO2 absorption performance of ionic liquid [C2mim][OAc] at the atmosphere of 15%CO2/N2 and 15%CO2/0.2%SO2/N2.

detrimental effect of SO 2 was irreversible during the regeneration process.27−29 In our investigation, the presence of SO2 in flue gas was identified to be an obstacle for CO2 separation by ILs. Because SO2 reacts strongly with ionic liquid and hinders CO2 absorption, the long-term absorptivity of ILs over multiple cycles is of great concern. CO2 absorption performance of [C2mim][OAc] during 5 cycles of regeneration was measured at the atmosphere of 15% CO2/N2 and 15%CO2/0.2%SO2/N2, respectively (Figure 2).

3. RESULTS AND DISCUSSION 3.1. Effect of SO2 on the Absorption Performance of CO2 in [C2mim][OAc]. Figure 1 shows the absorption performance of CO2 in [C2mim][OAc] at the atmosphere of 15%CO2/N2 and 15%CO2/0.2%SO2/N2, respectively. It can be seen that the absorption capacity of CO2 without the presence of SO2 was 0.221 mol CO2/mol IL. However, it was obvious that the CO2 absorption capacity under exposure to 0.2% SO2 was largely decreased to 0.167 mol CO2/mol IL with the reduction percentages of 25%. Despite the low inlet concentration, SO2 was hardly detected in the outlet gas stream, indicating that SO2 was completely absorbed in [C2mim][OAc] during the absorption process. Because CO2 and SO2 are both acidic gases in flue gas, the ionic liquid can absorb these gases simultaneously. Because of the limited sorption active sites of the ionic liquid, some of its occupation by SO2 resulted in lower CO2 absorption capacity. Moreover, in comparison with CO2 molecule, SO2 has high molecular polarity and dipole moment resulting in strong affinity with the sorbent. Several reports have indicated that the presence of SO2 in flue gas also had a competitive and negative influence on amine-mediated separation of CO2 and the

Figure 2. CO2 absorption performance of [C2mim][OAc] during 5 cycles of regeneration at the atmosphere of 15%CO2/N2 and 15% CO2/0.2%SO2/N2.

[C 2mim][OAc] could maintain 99% of the initial CO2 absorption capacity without SO2 after 5 cycles of regeneration, showing a high-efficient and reversible CO2 absorption. However, obviously the initial absorption capacity of CO2 largely decreased by 48% under continuous exposure to 0.2% SO2, meaning that the presence of SO2 with low concentration would reduce the usable lifetime of ILs. In addition, SO2 absorption performance of fresh [C2mim][OAc] and [C2mim][OAc] after 5 cycles of CO2 absorption/ desorption was also investigated at the atmosphere of 15% CO2/0.2%SO2/N2. It can be seen in Figure 3 that [C2mim][OAc] showed high solubility for the first SO2 absorption and C


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Figure 3. SO2 absorption performance of [C2mim][OAc] and [C2mim][OAc] after 5 cycles of CO2 absorption/desorption at the atmosphere of 15%CO2/0.2%SO2/N2. (a) Outlet SO2 concentration vs time; (b) absorption capacity vs time.

SO2 was completely separated in 300 min. However, the absorption capacity of SO2 in [C2mim][OAc] after 5 cycles of CO2 absorption/desorption largely decreased from 0.521 to 0.368 mol SO2/mol IL and the separation of SO2 was unsatisfactory even at the beginning, indicating the irreversibility of the [C2mim][OAc]−SO2 system. The interactions between [C2mim][OAc] and CO2/SO2 were further studied through FTIR characterization (Figures 4 and 5). In comparison with the FTIR spectrum of fresh

Figure 5. FTIR spectrum of [C2mim][OAc]−CO2/SO2 (a), [C2mim][OAc]−CO2/SO2 desorption at 90 °C (b) and [C2mim][OAc]−CO2/SO2 desorption at 180 °C (c).

anion with the proton at the C(2) position of the cation to form the carbene species,45 high water content in ILs may also lead to low concentration of carboxylate salt. It is interesting to see that some new absorption bands at 1710, 1277, 1241, 1092, 961 and 879 cm−1 appeared for [C2mim][OAc] after 15%CO2/0.2%SO2 absorption (Figure 4c). The appearance of carbonyl band at 1710 cm−1 indicated that the acetate anion was partly converted into the acetate acid. The bands at 1092 and 961 cm−1 could be assigned to HSO3 −. Interestingly, the carbonyl band at 1710 cm−1 disappeared during desorption at the temperature of 180 °C (Figure 5c), which demonstrated that the acetate acid was evacuated at high temperature. The results were in good agreement with previous FTIR reports on [C4mim][OAc]− SO2, suggesting that the acetate anion with H2O strongly reacts with SO2 with the new products of acetate acid CH3COOH and new IL [C4mim][HSO3].34 In addition, it can be seen from the 1H NMR and 13C NMR spectrum (Supporting Information, Figure S2 and Figure S3) that the chemical shift attributed to the imidazolium cation remained without any change. However, the peaks of acetate anion had quite a few changes. The chemical shift attributed to H in CH3COO− in 1H NMR spectrum moved downfield from 1.815 to 1.990 ppm (Table S1). Similarly, the chemical shift

Figure 4. FTIR spectrum of fresh [C2mim][OAc] (a), [C2mim][OAc] after 15%CO2 absorption (b) and [C2mim][OAc] after 15%CO2/0.2% SO2 absorption (c).

[C2mim][OAc], [C2mim][OAc] after 15% CO2 absorption had minimal changes. The chemical reaction between [C2mim][OAc] and CO2 has been proposed by Gurau,44 suggesting that the cation does participate in the reaction to form the carboxylate salt. However, Shiflett18 stated that the amount of such a chemical reaction must be minor and reversible. In our cases, the FTIR spectrum of the cation showed minimal changes which may be due to the minor amount of the reaction of CO2 with the carbene species. Because [C2mim][OAc] absorbed CO2 at relatively low concentration of 15 vol %, such small amount of carboxylate salt in [C2mim][OAc]−CO2 sample could not be detected within the detection limits. Moreover, because water inhibits the interaction of the acetate D


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Figure 6. SO2 absorption performance of fresh [C2mim][OAc] and CO2 saturated [C2mim][OAc] .(a) Outlet SO2 concentration vs time; (b) absorption capacity vs time.

Figure 7. CO2 absorption performance of fresh [C2mim][OAc] and SO2 saturated [C2mim][OAc]. (a) Outlet CO2 concentration vs time; (b) absorption capacity vs time.

attributed to the carboxylate and methyl in acetate anion in 13C NMR spectrum moved upfield from 23.302 to 20.522 ppm and 180.843 to 176.456 ppm, respectively (Table S2). These changes indicated the interaction between the acetate Lewis base and the CO2/SO2 Lewis acid. The 13C NMR spectrum in our investigation (Figure S3 and Table S2) agreed well with that of Carvalho,46 which indicated a preferential interaction of the acid carbon of the CO2 molecule with the carboxylate group of the acetate anion based on the NMR results and quantum chemical calculations. Thus, the acetate anion dominated the interaction, with the cation playing a secondary role. It has been already found that CO2 was reversibly soluble in 1-butyl-3-methylimidazolium acetate [C4mim][OAc].18 The absorbed SO2 in [C4mim][OAc] had much difficulty in desorption due to its strong interaction with the anion [OAc], leading to only about 30% of the absorbed SO2 stripped out at 120 °C for 4h.34 The irreversibility of the [OAc]−SO2 complex inhibited not only SO2 separation but also CO2 capture. More SO2 molecules will be left combining with the acetate anion over multiple cycles until all the sorption active sites are occupied, severely affecting the subsequent absorption capacity of CO2. Thus, ILs lost its excellent

reversibility and regeneration property for CO2 capture under continuous exposure to SO2. It is worth noting that [C2mim][OAc] is not the only IL poisoned by SO2. The amino acid functional ILs are regarded as the most popular solvents for CO2 capture and the main functional group reacting with CO2 is the active alkaline group (−NH2). It is well accepted that amine solvents or aminemediated sorbents suffer from thermal and chemical stabilities because of poisoning by SO2.27−29 The NH2-containing ILs can react with SO2 in a similar way as [C2mim][OAc]. The strong and irreversible reaction between SO2 and other amino acid ILs due to stronger acidity and polarity of SO2 in comparison with CO2 will result in much difficulty in completely regenerating the ILs. It is postulated that the usable lifetime of a sorbent plays a key role in the cost of using the sorbent. Good regeneration property can compensate for high cost of ILs, which is one of the most competitive advantages in comparison with commercial MEA solvents. Once ILs are poisoned by SO2, losing this advantage will bring a risk to its competitiveness potential. 3.2. Interplay between CO2 and SO2 Absorption by [C2mim][OAc]. The SO2 absorption performance of fresh [C2mim][OAc] and CO2 saturated [C2mim][OAc] was investigated to make clear the influence of CO2 on SO2 E


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Industrial & Engineering Chemistry Research absorption (Figure 6). Fresh [C2mim][OAc] displayed a high solubility of SO2 under 0.5% concentration with 0.875 mol SO2/mol IL, presented in Figure 6 (the black curve). SO2 absorption by anion-functionalized ILs under low partial pressure has been investigated by Zhang47 and Wang.48 The absorption capacity of SO2 in [C2mim][OAc] was comparable to that in the anion−functionalized ILs synthesized by Wang48 due to the large absorption enthalpies between the anions and SO2 (−89.2 to −109.2 kJ/mol). Surprisingly, CO2 saturated [C2mim][OAc] still maintained a high SO2 absorption capacity of 0.835 mol SO2/mol IL, meaning that the presence of CO2 showed little effect on subsequent SO2 absorption. When the gas stream with 0.5% SO2/N2 flowed through the CO2 saturated IL, SO2 was completely absorbed while CO2 was released and detected in the outlet stream. As mentioned above, [C2mim][OAc] mainly physically reacted with CO2 and the regeneration of the [C2mim][OAc]−CO2 system was easy and complete. Most of the active sorption sites could be recovered and continued bonding with SO2 so that [C2mim][OAc] still absorbed SO2 after its saturation in the presence of CO2. The CO2 absorption performance of fresh [C2mim][OAc] and SO2 saturated [C2mim][OAc]was investigated to make clear the influence of SO2 on CO2 absorption (Figure 7). Fresh [C2mim][OAc] displayed a high solubility of CO2 with 0.248 mol SO2/mol IL, presented in Figure 7 (the black curve). A summary of CO2 absorption capacity by acetate-based ILs is given in Table 1. The solubility of CO2 in imidazole acetate-

saturated [C2mim][OAc] dramatically decreased from 0.248 to 0.052 mol CO2/mol IL, about 80% reduction. The reason could be that almost none of the sorption sites were left for subsequent CO2 capture after its saturation of SO2. It was difficult to recover the chemically absorbed [C2mim][OAc]− SO2 complex due to their strong interactions. Desorption of CO2 saturated [C2mim][OAc] solution under the purge of two gas streams was presented in Figure 8. The amount of stripped CO2 in N2 atmosphere was 0.073 mol CO2/mol IL, accounting for nearly 30% of the absorbed CO2. The reason for incomplete desorption was the low temperature of 30 °C. It is worth noting that the presence of 0.5% SO2 in the purge gas stream could greatly promote CO2 release with the desorption capacity of 0.175 molCO2/mol IL, nearly 70% of the absorbed CO2. Guo and co-workers49 indicated that SO2 and NOx can also affect the mutual absorption. SO2 hinders NO absorption by blocking the reaction sites while NO improves the chemical adsorption of SO2. Physically adsorbed NO can be replaced and desorbed by SO2 due to the weaker van der Waals force of the initially adsorbed NO in comparison with SO2. Moreover, they also suggested that SO2 adsorbs on the sorbent before NO because SO2 with higher boiling points shows easier vapor−liquid phase transitions due to greater intermolecular forces. In our cases, SO2 also showed higher boiling points and stronger interactions with ILs than CO2. Thus, the reason for the higher desorption capacity in the presence of SO2 could be that SO2 replaced most of the already physically absorbed CO2. Because the vast majority of absorbed CO2 reacted with ILs physically, the enhancement of SO2 replacing CO2 could be more obvious. It could be concluded that CO2 had little influence on SO2 absorption whereas the SO2-saturated ionic liquid inhibited the subsequent CO2 absorption. There was a competition relationship between CO2 and SO2, and the ILs showed better selectivity and preferential absorption toward SO2 than CO2. 3.3. Quantum Chemical Calculation on the Interaction of [C2mim][OAc] with CO2 and SO2. As observed in the results of FTIR and NMR spectrum, the acetate anion dominated the interaction with gases. Deeply investigating the interaction between the acetate anion with CO2 and SO2 through quantum chemical calculation could help to well understand the interplay between CO2 and SO2. In this work, the structures of acetate anion with CO2, SO2 and CO2−SO2 was fully optimized based on the DFT−D3 calculation. Each

Table 1. Summary of CO2 Absorption Capacity by Acetatebased Ionic Liquids ILs [C4mim][OAc] [C2mim][OAc] [C4mim][OAc] [N2222][OAc]− H2O [C2mim][OAc]

T (K)

absorption capacity (mol CO2/mol IL)

ref

298 298 323 298

0.274 0.267 0.201 0.386

Shiflett18 Shiflett35 Carvalho46 Wang21

303

0.248

this work

based ILs was measured by Shiflett18,35 of almost 0.3 mol CO2/ mol IL. It can be seen that in this work the CO2 absorption capacity of [C2mim][OAc] was comparable to those reported values. It is interesting that in comparison with fresh [C2mim][OAc], the absorption capacity of CO2 by SO2

Figure 8. Stripped CO2 at the atmosphere of 0.5% SO2/N2 and N2. (a) Outlet CO2 concentration vs time; (b) desorption capacity vs time. F


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acetate anion, the location of the CO2 molecule around the acetate anion shows large difference from that in the [OAc]− CO2 complex. It seems that the main active site interacting with the CO2 molecule has to be changed to the O atom in the carbonyl group (CO) due to the occupation of the negatively charged O atom by SO2. The structural parameters of CO2 in the [OAc]−CO2−SO2 complex approach to those in an isolated CO2 which could explain the largely decreased absorption capacity of CO2 in SO2-saturated [C2mim][OAc] in Figure 7. Moreover, the distance between CO2 and the anion [OAc] is significantly extended from 1.62 to 2.75 Å, indicating a weakened interaction. Therefore, it can be seen that CO2 rarely affects the absorption state of SO2 whereas SO2 greatly weakens the interaction between CO2 and the anion. Thus, the presence of SO2 in flue gas inhibits CO2 separation and leads to lower CO2 absorption capacity, which is in accordance with the above experimental results. Because the interaction energy and the absorption enthalpy are important to determine the gas absorption capacity, the thermochemical parameters are listed in Table 3. The strong interaction between [OAc] and SO2 was confirmed by comparing the interaction energy and absorption enthalpy of the [OAc]−CO2 and [OAc]−SO2 complexes. The binding energy and absorption enthalpy for the [OAc]−CO2−SO2 complex was less than the sum of those for the [OAc]−CO2 and [OAc]−SO2 complexes, indicating the competitive interplay between CO2 and SO2. In addition, the absorption reaction also results in the changes of charge distribution. Thus, the negative charge transfer from the acetate anion to CO2 and SO2 was calculated from the charge distribution results. The net charge transfer amount from [OAc] to SO2 had little change in the [OAc]−SO2 and [OAc]−CO2−SO2 complexes, allowing the acetate anion to maintain the strong interaction with SO2. However, the net charge transfer amount from [OAc] to CO2 significantly decreased from −0.537 e in the [OAc]−CO2 complex to −0.039 e in the [OAc]−CO2−SO2 complex in the presence of SO2, which could also explain the decreased absorption capacity of CO2 in [C2mim][OAc] in Figure 1. Because the ions of [C2mim][OAc] cannot always be considered independently, the quantum chemical calculation was also performed on the ion pair of [C2mim][OAc] and its interaction with CO2 and SO2. The optimized structures of [C2mim][OAc]−CO2 and [C2mim][OAc]−SO2 are presented in Figure 10. Interestingly, the structure of the ion pair of [C2mim][OAc] remained unchanged when reacting with CO2. However, it is obvious that SO2 significantly disturbed the structure of the ion pair of [C2mim][OAc] due to the strong interaction with the anion. The thermochemical parameters for the complexes of [C2mim][OAc]−CO2 and [C2mim][OAc]− SO2 are listed in Table 4. The interaction energies of the ion pair of [C2mim][OAc] with gases were lower than the acetate anion with gases, because the cation competed with the gases for the anion. The absorption enthalpy of [C2mim][OAc]−


initial configuration was taken into consideration and the complex with the lowest energy was regarded as the optimized structure, presented in Figure 9. The structure parameters of the optimized geometries were listed in Table 2.

Figure 9. Optimized structure of the complexes of acetate anion with (a) CO2, (b) SO2 and (c) CO2 and SO2.

Table 2. Structural Parameters for the Complexes of OAc− CO2, OAc−SO2 and OAc−CO2−SO2 structural parameters a

C−O (Å) O−C−O (deg)b O(AC)−C(CO2) (Å)c S−O (Å)a O−S−O (deg)b O(AC)−S(SO2) (Å)c

OAc−CO2

OAc−SO2

OAc−CO2−SO2

1.49 113.4 2.06

1.17 172.6 2.75/2.83 1.49 113.8 2.11

1.22 139.0 1.62

a Refers to the bond length of C−O in CO2 and S−O in SO2. bRefers to the ∠OCO angle in CO2 and ∠OSO angle in SO2. cRefers to the distance between O atom in AC and C atom in CO2 or S atom in SO2.

Organization of gas molecules around the anion is related to the absorption reaction. As presented in Figure 9a, the negatively charged oxygen (O atom) in acetate anion shows strong interaction with the C atom in CO2 with the distance of about 1.62 Å. Because of the strong complexation, the average CO2 angel bends to 139° and the bond length of C−O is elongated to about 1.22 Å, demonstrating a largely bent CO2 configuration compared with an isolated CO2 molecule. Similarly, the optimized structure of the [OAc]−SO2 complex shows the average distance of 2.06 Å between the negatively charged O atom in [OAc] and S atom in SO2, the bending of 113.4° and the elongated bond length of 1.49 Å for SO2 configuration. Figure 9a,b demonstrates that the negatively charged O atom in acetate anion is the main active site for single CO2 and SO2 absorption. In the case that an acetate anion interacts with one CO2 and one SO2 molecule (Figure 9c), SO2 configuration in [OAc]− CO2−SO2 is similar to that in [OAc]−SO2, which could explain remained large solubility of SO2 in CO2-saturated [C2mim][OAc] in Figure 6. Interestingly, because the SO2 molecule strongly binds with the negatively charged O atom in the

Table 3. Thermochemical Parameters and Net Charge Transfer Amount for the Complexes of OAc−CO2, OAc−SO2, and OAc− CO2−SO2 ΔE (kJ/mol) ΔH (kJ/mol) ΔG (kJ/mol) net charge transfer amount (e)

OAc−CO2

OAc−SO2

OAc−CO2−SO2

−40.7 −47.6 −1.2 −0.537

−114.8 −124.2 −71.3 −0.370

−139.8 −153.4 −71.1 −0.039(CO2)/−0.316(SO2)

G


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relationship between CO2 and SO2, and the ILs showed better selectivity and preferential absorption toward SO2 than CO2. Quantum chemical calculation results showed that the negatively charged O atom in acetate anion is the main active site for single CO2 and SO2 absorption. However, the main active site interacting with CO2 molecule has to be changed to the O atom in the carbonyl group (CO) due to the occupation by SO2, indicating that CO2 could hardly compete with SO2 for the main active site. In addition, the large difference of structure parameters and charge distribution of CO2 in the [OAc]−CO2 and [OAc]−CO2−SO2 complexes also confirmed the negative effect of SO2 on CO2 interacting with [C2mim][OAc]. In this work, we employed the acetate-based ILs (AcILs) to investigate the effect of SO2 on CO2 absorption. Because of the tunability of the cations and anions, this conclusion may not apply to the whole ILs family. Other kinds of ILs should be taken into consideration. In our cases, the irreversibility of SO2−AcILs resulted in the unfavorable regeneration property for CO2 capture. It could be predicted that ILs could remove CO2 and SO2 simultaneously for multiple cycles if SO2 absorption in ILs is reversible or completely released during the desorption process. However, easy desorption results from low absorption enthalpy. A lower absorption enthalpy of the IL−CO2 system means low absorption capacity. Thus, the trade-off between the capacity and the effect of SO2 and other impurities should be balanced considering the development of new CO2 capture technology using ILs. In addition, because the presence of SO2 had a detrimental effect on the absorption performance of the AcILs−CO2 system, it is essential to upgrade the FGD unit for more effective SO2 removal. Also, more stringent local and regional air pollution policies on SO2 and other impurities could be beneficial to reduce the cost of CO2 capture.


Figure 10. Optimized structure of the complexes of (a) [C2mim][OAc]−CO2 and (b) [C2mim][OAc]−SO2.

Table 4. Thermochemical Parameters for the Complexes of [C2mim][OAc]−CO2 and [C2mim][OAc]−SO2 ΔE (kJ/mol) ΔH (kJ/mol) ΔG (kJ/mol) a

[C2mim][OAc]−CO2

[C2mim][OAc]−SO2

−24.2 (−19.0)a −29.1 8.1

−78.9 (−67.3) −91.5 −37.1

Values in brackets were obtained based on the SMD model.

CO2 (−29.1 kJ/mol) presented a good agreement with the partial molar enthalpy of solvation from an appropriate correlation of Henry’s constant (−32.4 ± 3.0 kJ/mol) reported by Carvalho.46 In addition, the continuum universal solvation model (SMD) was employed to consider the solvent effects. The interaction energies of the [C2mim][OAc]−CO2 and [C2mim][OAc]− SO2 system based on the SMD model were −19.0 and −67.3 kJ/mol respectively, which were lower than those in gas phase (−24.2 and −78.9 kJ/mol). However, the difference between the interaction energies ([C2mim][OAc]−CO2 and [C2mim][OAc]−SO2) in gas phase (54.7 kJ/mol) showed a good agreement with the difference between the solvation energies based on the SMD model (48.3 kJ/mol). Also, in the solvation state, the interaction between [C2mim][OAc] and SO2 is stronger than [C2mim][OAc]−CO2, indicating that [C2mim][OAc] prefers to interact with SO2 rather than CO2.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b02208. Molecular structure of 1-ethyl-3-methylimidazolium acetate [C2mim][OAc] is presented in Figure S1. 1H NMR and 13C NMR spectrum and data of fresh [C2mim][OAc] and [C2mim][OAc]−CO2−SO2 are given in Figures S2 and S3 and Tables S1 and S2 (PDF).

4. CONCLUSIONS The absorption performance of CO2 in ionic liquid 1-ethyl-3methylimidazolium acetate [C2mim][OAc] under the exposure to SO2 with low concentration was investigated through experimental method and theoretical calculation. The ionic liquid showed high efficient and reversible absorption to CO2 in gas stream without SO2. However, it was found that the occurrence of SO2 in flue gas inhibited CO2 absorption due to the occupation of the sorption active sites. The initial absorption capacity of CO2 in [C2mim][OAc] remarkably decreased by 48% after 5 cycles of regeneration. The interplay between CO2 and SO2 absorption by [C2mim][OAc] was also investigated through three groups of sequential absorption experiments. The CO2-saturated [C2mim][OAc] could continue to absorb SO2 because the active sorption sites interacting with CO2 could be easily recovered and continued bonding with SO2. Whereas the SO2-saturated ionic liquid no longer absorbed CO2 due to the difficulty in recovering the active sorption sites occupied by SO2. There was a competition

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AUTHOR INFORMATION

Corresponding Author

*L. Zhang. E-mail: [email protected]. Tel: +86 27 87542417. Fax: +86 27 87545526. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the National Natural Science Foundation of China (51076056), the National Basic Research Program of China (2011CB707301), the Specialized Research Fund for the Doctoral Program of Higher Education (20130142130009) and the Foundation of State Key Laboratory of Coal Combustion. The authors also acknowledge the extended help from the Analytical and Testing Center of Huazhong University of Science and Technology. H


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Effect of SO2 on CO2 Absorption in Flue Gas by Ionic Liquid 1-Ethyl-3

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