Highly Efficient SO2 Absorption and Its Subsequent Utilization by

Article pubs.acs.org/est

Highly Efficient SO2 Absorption and Its Subsequent Utilization by Weak Base/Polyethylene Glycol Binary System Zhen-Zhen Yang, Liang-Nian He,* Ya-Nan Zhao, and Bing Yu State Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, People’s Republic of China S Supporting Information *

ABSTRACT: A binary system consisting of polyethylene glycol (PEG, proton donor)/PEG-functionalized base with suitable basicity was developed for efficient gas desulfurization (GDS) and can be regarded as an alternative approach to circumvent the energy penalty problem in the GDS process. High capacity for SO2 capture up to 4.88 mol of SO2/mol of base was achieved even under low partial pressure of SO2. Furthermore, SO2 desorption runs smoothly under mild conditions (N2, 25 °C) and no significant drop in SO2 absorption was observed after five-successive absorption−desorption cycles. On the other hand, the absorbed SO2 by PEG150MeIm/PEG150, being considered as the activated form of SO2, can be directly transformed into value-added chemicals under mild conditions, thus eliminating the energy penalty for SO2 desorption and simultaneously realizing recycle of the absorbents. Thus, this SO2 capture and utilization (SCU) process offers an alternative way for GDS and potentially enables the SO2 conversion from flue gas to useful chemicals as a value-added process.

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reported by Han’s group in 2004, in which capacity of ∼1.0 mol of SO2/mol of IL was attained at 1 bar with 8 vol% SO2 in the gas.4 Recently, Wang’s group employed azole-based ILs containing tetrazolate anions together with phosphonium cation ([P66614][Tetz])25 or ether-functionalized cations ([P444E3][Tetz])12 for chemisorption of SO2, in which 1.54 and 1.58 mol of SO2/mol of IL can be absorbed at 0.1 bar SO2 pressure diluted by N2, respectively. Normally, traditional chemisorption of SO2 is based on the strong interaction between the absorbent and SO2,12,25 and thus has a high absorption enthalpy. Therefore, high energy is required for regeneration of the absorbent.27,29,33 Up to now, even though significant advances have been made, there are still inherent drawbacks such as tedious synthetic procedures of the absorbents, poor stabilities,4,27,29 slow sorption kinetics,4 and in particular, extensive energy consumption in the desorption.12,25,29 In this context, reducing the energy requirement for absorbents regeneration is an essential prerequisite for a breakthrough in GDS techniques. Wang et al. designed a concept of desulfurization by using aqueous solution for SO2 absorption and then converting the absorbed SO2 to valuable chemicals (i.e., NaHSO4) through an electrochemical method.34 In this work, we report an alternative approach of SO2 capture and utilization (SCU) to circumvent the energy penalty problem in the GDS process. The essence of our strategy is to

INTRODUCTION Sulfur dioxide, mainly emitted from petroleum refineries, fossil fuel burning power plants, sulfide-based metal smelters, and other SO2-emitting industries, causes harmful impacts on the environment, human health, livestock, and plants.1−3 Gas desulfurization (GDS) technology has been one of the most effective techniques to control SO2 emission from combustion of fossil fuels, such as coal and petroleum.4−8 On the other hand, removal of SO2 from other gases (e.g., CO2 and H2) is of vital importance to expand utility of such useful gases and avoid degradation of absorbents utilized for capture and sequestration of CO2.9−11 Accordingly, the development of efficient materials that can reversibly and economically capture SO2 is highly desirable. The conventional technology for GDS has been applied in industry, including limestone scrubbing, ammonia scrubbing, and organic solvents absorption, which could pose serious inherent drawbacks such as formation of byproducts, that is, calcium sulfate, wastewater, and solvent volatilization.5,6,8,12−16 In the past decades, numerous strategies have been proposed for physical and chemical absorption of SO2, for example, imidazolium/ammonium-based ionic liquids (ILs)17−21 and membranes-supported ILs22,23 have been developed as physical absorbents for SO2 scrubbing and separation. Chemical absorbents for GDS include guanidinium-based ILs,4,24 anion functionalized ILs,12,25−29 alkanol amine,30 and guanidiniumbased polymers.31,32 Actually, effective capture of SO2 from flue gas often requires strong chemical absorption because of the relatively low SO2 pressure (e.g., 0.2 vol% SO2 in the flue gas).13 In this respect, IL 1, 1, 3, 3-tetramethylguanidinium lactate ([TMG][lactate]) for chemisorption of SO2 was © 2013 American Chemical Society

Received: Revised: Accepted: Published: 1598

October 11, 2012 January 14, 2013 January 16, 2013 January 16, 2013 dx.doi.org/10.1021/es304147q | Environ. Sci. Technol. 2013, 47, 1598−1605

Environmental Science & Technology

Article

Scheme 2. Synthesis of PEG150MeIm and n-OctIm

use a binary system consisting of polyethylene glycol (PEG, proton donor)/PEG-functionalized base with suitable basicity for SO2 capture, and thereby substantial activation. Therefore, this system could be suitable for accomplishing chemical transformation of SO2, getting rid of the desorption step, and simultaneously realizing recycle of the absorbents (Scheme 1). Scheme 1. SO2 Absorption, Activation and Further Transformation Promoted by Weak Base (B)/Proton Donor (Nu-H) Binary System

a stirred solution of triethylene glycol monomethyl ether (0.3 mol) and pyridine (0.3 mol) in CHCl3 (200 mL), followed by refluxing the above reaction mixture at 100 °C for 4 h, and then a yellow liquid mixture is obtained, which was washed with water (4 × 125 mL), dried with MgSO4, and concentrated under reduced pressure at 60 °C to remove CHCl3. The crude product was purified under reduced pressure to give ClPEG150Me as a light yellow liquid. 1H NMR (400 MHz, CDCl3) δ 3.75 (t, 3J = 6 Hz, 2H), 3.61−3.69 (m, 8H), 3.53− 3.56 (m, 2H), 3.37 (s, 3H); 13C NMR (100.6 MHz, CDCl3) δ 71.8, 71.3, 70.6, 70.5, 59.0, 42.7; GC-MS: m/z (%):183.02 (100), 185.03 (33) [M+], 151.08 (26), 153.07 (8) [M+-CH3O], 103.13 (61) [M+-C2H4OCl]. Synthesis of PEG150MeIm and n-OctIm.60 ImNa (0.05 mol) and ClPEG150Me (0.045 mol) or 1-bromoctane (0.45 mol) were added in THF (30 mL). The mixture was stirred at reflux for 10 h and filtered; the filtrate was concentrated under reduced pressure to removed THF. Then CH2Cl2 (30 mL) was added to the residue, which was filtered again to remove the precipitation. The filtrate was concentrated under reduced pressure to remove CH2Cl2. The crude product was purified under reduced pressure to give PEG150MeIm or n-OctIm as colorless liquid. PEG150MeIm. 1H NMR (d6-DMSO, 400 MHz) δ 7.60 (s, 1H), 7.16 (s, 1H), 6.86 (s, 1H), 4.10 (t, 3J = 5.2 Hz, 2H), 3.66 (t, 3J = 5.62, 2H), 3.47−3.49 (m, 6H), 3.40−3.42 (m, 2H), 3.23 (s, 3H); 13C NMR (d6-DMSO, 10.6 MHz) δ 137.4, 128.0, 119.6, 71.2, 69.7, 69.6, 69.5, 58.0, 45.8; ESI-MS calcd for C10H18N2O3 214.13, found 215.3 [M+H]+. n-OctIm. 1H NMR (CDCl3, 400 MHz) δ 7.45 (s, 1H), 7.04 (s, 1H), 6.89 (s, 1H), 3.91 (t, 3J = 7.2 Hz, 2H), 1.74−1.79 (m, 2H), 1.25−1.27 (m, 12H), 0.86 (t, 3J = 6.4 Hz, 3H); 13C NMR (CDCl3, 100.6 MHz) δ 137.0, 129.3, 118.7, 47.0, 31.7, 31.0, 29.0, 28.9, 26.5, 22.5, 14.0; ESI-MS calcd for C11H20N2 180.16, found 181.3 [M+H]+. General Procedure for Absorption and Desorption of SO2. In a typical procedure, SO2 capture was carried out in a 10 mL Schlenk flask. The absorbents were charged into the reactor at room temperature. Then, the air in the flask was replaced by SO2 and a needle was used for SO2 bubbling, which was inserted in the bottom of the flask. The absorption reaction was conducted at 25 °C with a SO2 bubbling rate of 0.1 L/min. The amount of SO2 absorbed was determined by an analytical balance within an accuracy of ±0.0001 g every 5 minutes. During the absorption of SO2 under reduced pressure, SO2 was diluted with N2 in order to reduce the partial pressure of SO2 passing through the system. The SO2 partial pressure was controlled by changing the volume fraction of SO2. In a typical

Interest in employing PEG in GDS process stems from its distinctive properties,35 such as low cost, thermal stability, and SO2-philic property through interaction of its ether linkages with SO2.12,23 High SO2 capacity with good SO2/CO2 sorption selectivity was approached, and fast sorption kinetics can be attained even under low partial pressure of SO2 (1.35 and 1.06 mol of SO2/mol of base in 5 and 15 min under 0.1 and 0.02 bar SO2 partial pressure, respectively).4,12,25 Furthermore, SO2 desorption was achieved under very mild conditions (N2, 25 °C) adopting suitable base with moderate basicity, and no significant drop in SO2 absorption was detected after fivesuccessive absorption−desorption cycles. On the other hand, the SO2 molecule is presumably activated upon its capture, and thus the absorbed SO2 molecule could be directly used as a feedstock. As a result, the reaction of the absorbed SO2 with epoxides performed smoothly under mild conditions without utilization of any solvents or additives to afford the 5-membered cyclic sulfites (Scheme 4),36−42 which have been widely used as monomers for use in ring-opening polymerization reactions, solvents, and electrolyte additives.43−51 From the viewpoint of green chemistry and economic perspectives, synthesis of cyclic sulfites through the cycloaddition procedure utilizing the absorbed SO2 as a feedstock could be more attractive in comparison with the general procedure through the stoichiometric reaction of 1,2diol and thionyl chloride in the presence of an organic base (e.g., triethylamine or pyridine), which generates stoichiometric amounts of salts as waste and chlorine-containing impurities.52−58 In this way, SO2 in flue gas was eventually converted to valuable chemicals and the absorbents were recycled for the next run.

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EXPERIMENTAL SECTION Synthesis of PEG150MeIm and n-OctIm (Scheme 2).12,59,60 Synthesis of ImNa:59 Imidazole (0.5 mol) and sodium ethoxide (0.5 mol) were dissolved in ethanol (50 mL), stirred at 70 °C for 8 h, and then ethanol was removed by rotator evaporation under reduced pressure. The residue was washed with diethyl ether three times and dried in vacuum to give intermediate ImNa as a yellow solid. 1H NMR (D2O, 400 MHz) δ 7.69 (s, 1H), 7.06 (s, 2H); 13C NMR (D2O, 100.6 MHz) δ 136.5, 122.0. Synthesis of PEG150MeCl.12 A solution of thionyl chloride (0.45 mol) in CHCl3 (90 mL) was added slowly over 60 min to 1599

dx.doi.org/10.1021/es304147q | Environ. Sci. Technol. 2013, 47, 1598−1605


Article

desorption of SO2, N2 of atmospheric pressure was bubbled trough absorption system at 25 °C using the same equipment and procedure as SO2 capture. Absorption/desorption cycle was determined by several cycles of repeated experiments. General Procedure for Synthesis of Cyclic Sulfites Using Captured SO 2 and Its Recyclability. First, PEG150MeIm (2 mmol) and PEG150 (2 mmol) or choline chloride (2 mmol) were charged into a glass tube, in which SO2 was bubbled through a needle. Then, epoxide and NH4I (only for PEG 150 MeIm/PEG 150 system) was added after SO 2 absorption reached equilibrium. The tube was placed into a 25 mL stainless steel autoclave and then the mixture was stirred at predetermined temperature for 5 min to reach the equilibration. When the reaction finished, the reactor was cooled in ice−water and SO2 was ejected slowly. The product yields for catalyst and reaction parameters screening were determined by GC with a flame ionization detector and were further identified using GC-MS by comparing retention times and fragmentation patterns with authentic samples. For substrate scope, the desired products were purified column chromatography on silica gel (200−300 mesh, eluting with petroleum ether/ethyl acetone or petroleum ether/dichloromethane), and further identified by GC-MS and NMR, which are consistent with those reported in the literature42 and in good agreement with the assigned structures. For (Supporting Information (SI) Table S1, Entry 20) and (SI Table S2, Entry 14), a typical procedure is given as follows: PEG150 and NH4I/ Choline chloride, biphenyl (internal standard of GC) and propylene oxide were added successively into a glass tube. The suspension was cooled to −60 °C (liquid nitrogen/ethanol) and SO2 (8.0/10.4 mmol) was introduced into the vessel. The glass tube was placed in a stainless steel autoclave (25 mL inner volume). The reaction mixture was heated at 80 °C with stirring for 3/6 h. For recycle of the absorption systems, the binary systems were recovered after separation of propylene sulfite from the reaction mixture by distillation under reduced pressure and then reused for the next run without further purification.

Figure 1. (a) Results of in situ FT-IR spectroscopy monitoring SO2 capture by PEG150MeIm/PEG150 system at various times starting from SO2 bubbling at 25 °C under 1 bar SO2. (b) 1H NMR (d6-DMSO, 400 MHz), (c) 13C NMR (d6-DMSO, 100.6 MHz) spectrum for PEG150MeIm/PEG150 before and after reaction with SO2.

Table 1. SO2 Absorption by Base/PEG150 Binary Systema

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RESULTS AND DISCUSSION SO2 Absorption by PEG150MeIm/PEG150. We have reported a highly efficient binary system consisting of PEG

entry 1 2

Scheme 3. SO2 Capture by PEG150MeIm/PEG150 Binary System

3 4 5 6 7

absorbent (molar ratio) PEG150 PEG150MeIm/PEG150 (1:1) n-octIm/PEG150(1:1) DABCO/PEG150(1:2) HMTA/PEG150(1:6) Urea/PEG150(1:2) PEG150MeIm

pKa of the conjugated acidb

t/ mind

SO2 capacitye

7.1c

5 5

0.13 1.35

7.1c 8.7 6.2 0.2 7.1c

5 15 10 15 30

1.25 2.18 0.94 0.11 1.42

a Base, 1 mmol; reaction temperature, 25 °C; SO2 partial pressure, 0.1 bar, being diluted by N2. bpKa values in water, ref.63−65 cpKa value of 1methylimidazole (MIm). dTime required to reach absorption equilibrium. eMoles of SO2 captured per mole of base and absorption by (n-1) mmol PEG150 is subtracted, n = mole of PEG150 added per mole of base.

alkylsulfate salt (Scheme 3), being detected by NMR and in situ FT-IR. There are three distinct features in the in situ FT-IR spectrum (Figure 1(a)). The absorption intensity of O−H at 3459 cm−1 is decreased to one-half of original intensity after SO2 uptakeing, indicating half of hydroxyl groups in PEG150 are involved in the reaction. Characteristic peaks centered at 1549 cm−1 and 1579 cm−1 could correspond to [N−H]+ absorption

and a superbase, for example, amidine or guanidine superbase for CO2 absorption.61 In this regard, weaker bases are suitable for SO2 capture compared with uptake of CO2, thereby may lead to a good separation of SO2 from CO2. In the first place, SO2 underwent a reaction with PEG150 (triethylene glycol, Mw = 150 Da) and PEG150MeIm to form the liquid imidazolium 1600



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Table 2. Influence of Different Proton Donor on SO2 Capacitya entry

proton donor

t/minc

SO2 capacityd

1 2 3 4 5 6 7 8

ethylene glycol PEG150 PEG150b PEG200 PEG300 PEG400 PEG600 Glycerol

10 5 15 20 20 10 5 25

1.29 1.35 1.43 1.43 1.61 1.61 1.79 1.40

Table 3. SO2/CO2 Selectivity of PEG150 and PEG150MeIm/ PEG150a entry

absorbent

t/ minb

gas capacityc

1 2 3 4

PEG150 PEG150 PEG150MeIm/PEG150 PEG150MeIm/PEG150

15 15 15 5

1.32/SO2 0.05/CO2 4.88/SO2 0.18/CO2

SO2/CO2 selectivity 26 27

PEG150MeIm/PEG150 molar ratio: 1:1; reaction temperature, 25 °C; gas pressure, 1 bar. bTime required to reach absorption equilibrium. c Moles of SO2 or CO2 captured per mole of PEG150 (entries 1−2) or PEG150MeIm (entries 3−4). a

a

PEG150MeIm, 1 mmol; proton donor, 1 mmol; reaction temperature, 25 °C; SO2 partial pressure, 0.1 bar, being diluted by N2. b PEG150MeIm/PEG150 molar ratio, 2: 1. cTime required to reach absorption equilibrium. dMoles of SO2 captured per mole of PEG150MeIm.

Scheme 4. Reactions of Epoxides with SO2 Absorbed by (a) PEG150MeIm/PEG150 or (b) PEG150MeIm/Choline Chloride

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C NMR (Figure 1(c)) spectra upon SO2 sorption. After uptake of SO2, the typical proton peaks of PEG150MeIm move downfield for H-1 (7.60−8.75), H-2 (7.16−7.51), H-3 (6.86− 7.39), H-4 (4.10−4.31), H5 (3.66−3.75); and carbon absorptions shift upfield for C-1 (137.4−136.3), C-2 (128.0− 120.7), and downfield for C-3 (119.6−123.5), C-4 (45.8−49.4) in the 13C NMR spectrum, being further identified with 2D NMR (1H−13C HSQC), as shown in the SI. Effect of Weak Bases with Different Basicity. The effect of weak bases with different basicity on SO2 capture was examined in the presence of PEG150 at 25 °C under 0.1 bar SO2 partial pressure. SO2 absorption capacity, being defined as moles of SO2 captured/mol of weak base, is estimated from the weight increase. The basicity could play a key role in SO2 sorption performance. As shown in Table 1, the basicity decreases in the order: DABCO > n-OctIm ∼ PEG150MeIm > HMTA > Urea. In the cases of solid bases (DABCO, HMTA, Urea), more than one equivalent of PEG150 is needed presumably due to high viscosity upon SO2 absorption. In particular, PEG150 itself gave poor capacity, implying that only physical interaction between PEG and SO2 approached (entry 1). On the other hand, n-OctIm and PEG150MeIm was able to rapidly capture more than one equivalent of SO2, indicating that both chemical and physical absorption pathways are active.12,25 The capacity increased from 1.25 (n-OctIm) to 1.35 (PEG150MeIm), possibly being ascribed to additional physical absorption derived from interaction between the oxygen atom in the ether linkage of PEG150MeIm and acidic SO2 molecule (entry 2 vs 3).12,29 Notably, DABCO with two tertiary nitrogen atoms in one molecular can capture more than two equivalent of SO2 (2.18) (entry 4). However, HMTA with four tertiary nitrogen atoms only gave 0.94 mol of SO2 per mole of absorbent, presumably being ascribed to the relatively weak basicity, especially further reduced basicity of the monoprotonated HMTA upon SO2 uptaking (entry 5). In contrast, urea with weakest basicity only showed physical interaction with SO2, giving comparable capacity to PEG150 (entry 6). SO2affinity and low viscosities of the liquid bases could account for

Figure 2. The effect of (a) SO2 partial pressure (25 °C) and (b) temperature (SO 2 pressure: 1 bar) on SO 2 absorption by PEG150MeIm/PEG150 (1:1).

Figure 3. Five consecutive cycles of SO2 absorption (◆25 °C, SO2 partial pressure: 0.1 bar, 15 min) and release (◇25 °C, N2: 1 bar, 70 min) by PEG150MeIm/PEG150 (1:1).

of the protonated PEG150MeIm, being comparable to that of 1methyl-1H-imidazol-3-ium chloride (1551 cm−1, 1584 cm−1).62 New absorption bands at 939 cm−1 and 1321 cm−1 can be attributable to S−O and SO stretches, respectively, indicating chemisorption of SO2.25 The chemical shifts of PEG150MeIm/PEG150 vary in both 1H NMR (Figure 1(b)) and 1601



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Table 4. Cyclic Sulfites Synthesis from Epoxides and SO2 Absorbed by PEG150MeIm/PEG150 or PEG150MeIm/Choline Chloride

a

PEG150MeIm/PEG150 (molar ratio 1:1), 2 mmol; SO2 absorbed, 8.0 mmol under 1 bar SO2 pressure; epoxide, 5 mmol; NH4I, 5 mol % relative to epoxide; 80 °C; 3 h. bPEG150MeIm/choline chloride (molar ratio 1:1), 2 mmol; SO2 absorbed, 10.4 mmol under 1 bar SO2 pressure; epoxide, 10 mmol; 80 °C; 6 h. cIsolated by column chromatography on silica gel (see SI). dYields were determined by GC using biphenyl as an internal standard and that shown in the parentheses were conducted in the absence of PEG150MeIm, (1) PEG150, 4 mmol; SO2, 8.0 mmol (added at −60 °C); PO, 5 mmol; NH4I, 5 mol %; 80 °C; 3 h (19%); (2) PEG150, 2 mmol; Choline chloride, 2 mmol; SO2, 10.4 mmol (added at −60 °C); PO 10 mmol; 80 °C; 6 h (54%).

sorption kinetics, binary system PEG150MeIm/PEG was selected for further investigation. Influence of PEG Molecular Weight and Other Proton Donors. The influence of PEG molecular weight was investigated using PEG150MeIm as a weak base at 25 °C under 0.1 bar SO2 pressure. The results summarized in Table 2 reveal that the capacity increased from 1.29 to 1.79 in the order of PEG molecular weights: ethylene glycol < PEG150 < PEG200 < PEG300 ∼ PEG400 < PEG600 (entries 1, 2, 4−7), owing to increasing physical solubility of SO2 as more ethylene oxide linkages involved. When PEG150MeIm/PEG150 molar ratio is 2:1, comparable capacity was also attained (entry 3), hinting that both hydroxyl groups of PEG could be involved in the reaction with SO2. Glycerol was also found to be an efficient proton donor together with PEG150MeIm for SO2 absorption (1.40) (entry 8). Different sorption kinetics was related to various viscosities of the absorption system. Effect of SO2 Partial Pressure and Absorption Temperature. The effect of SO2 partial pressure and absorption temperature on the capacity is shown in Figure 2. The molar ratio of SO2 absorbed to PEG150MeIm increased from 1.35 to 4.88 as SO2 partial pressure went up from 0.1 to 1 bar, indicating high SO2 solubility in PEG and PEG derivatives. Notably, SO2 capacity can still reach 1.35 and 1.06 even under very low SO2 partial pressure (0.05 and 0.02 bar). Accordingly, this kind of SO2-philic PEG150MeIm/PEG150 system could have

Figure 4. Recyclability of the binary systems (◊ PEG150MeIm/PEG150, ⧫ PEG150MeIm/Choline chloride) for SO2 absorption and its further transformation into PS through reaction with PO. ◊ PEG150MeIm/ PEG150 (molar ratio 1:1), 2 mmol; SO2 absorbed, 8.0, 9.1, 9.0, 8.7, 9.3 mmol for cycle 1−5 respectively under 1 bar SO2 pressure; PO, 5 mmol; NH4I, 5 mol % relative to PO; 80 °C; 3 h. ⧫ PEG150MeIm/ choline chloride (molar ratio 1:1), 2 mmol; SO2 absorbed, 10.4, 12.1, 11.5, 11.0, 10.8 mmol respectively for cycle 1−5 under 1 bar SO2 pressure; PO, 10 mmol; 80 °C; 6 h.

the faster sorption rate, compared with other solid bases (entries 2, 3 vs 4, 6). In addition, weak base PEG150MeIm itself gave SO2 capacity of 1.42 mol of SO2/mol of base through the direct interaction of tertiary nitrogen with SO2, although with a slow absorption rate. Hence, considering both SO2 capacity and 1602



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penalty for SO2 desorption and simultaneously realizing recycle of the absorbents. This protocol could open up the possibility of transforming the captured SO2 from a waste into a chemical resource with harnessing the energy already spent on capture and compression of the SO2, and would find abundant applications in SO2 chemistry. We also believe this protocol can pave the way for the development of technological innovation toward efficient and low energy demanded practical process for GDS.

great potential application in industry (Figure 2(a)). In addition, SO2 absorption decreased steadily from 4.88 to 2.02 as the temperature rose from 25 to 100 °C (Figure 2(b)), implying that the captured SO2 can be easily stripped out. Recyclability of the Absorbents. Stability and reusability of the absorbent are two important factors to identify whether it finds potentially practical application in industry. As we know, extensive energy input is particularly needed for desorption of the relatively strong acidic SO2.12,25 Hence, reducing the energy requirement would be an essential prerequisite for a breakthrough in SO2 absorption techniques. As shown in Figure 3, absorbed SO2 by PEG150MeIm/PEG150 was easy to be stripped out just by bubbling N2 at room temperature, and no significant drop in SO2 absorption was detected after five successive absorption−desorption cycles. SO2/CO2 Selectivity. Removal of SO2 from CO2 has found wide application.9−11 To be delighted, PEG150 itself showed SO2 capacity of 1.32 mol SO2/mol of PEG150 under 1 bar SO2 pressure,66 together with SO2/CO2 sorption selectivity of 26, presumably being attributable to the higher acidity of SO2 (entries 1, 2, Table 3).9 Notably, SO2 capacity further went up from 1.32 to 4.88 by adding equimolar PEG150MeIm, whereas SO2/CO2 selectivity still kept nearly unchanged owing to the simultaneous increased physical sorption capacity of CO2 (entries 3−4). SO2 Capture and Utilization: Synthesis of Cyclic Sulfites from Captured SO2 and Epoxides. Inspired by the above findings, we performed subsequent reactions by using the captured SO2 as the starting material, NH4I or the absorbent (choline chloride) as the catalyst and energy input being supplied to chemical reactions in order to validate our strategy of SCU. The liquid imidazolium alkylsulfate salt formed upon SO2 absorption with PEG150MeIm/PEG150 (Scheme 4 (a)) or PEG150MeIm/choline chloride (Scheme 4 (b)) directly reacted with epoxides at 80 °C, successfully affording the target product (cyclic sulfites) with moderate yields (Table 4). Furthermore, the reaction of propylene oxide (PO) with the absorbed SO2 gave 64% and 62% yield of propylene sulfite (PS) using PEG150 or choline chloride as proton donor, respectively, much higher than that in the absence of PEG150MeIm (19% and 54%) (Table 4, entry 1). This is understandable because SO2 molecule can simultaneously be activated upon absorption by PEG150MeIm/PEG150 or choline chloride, thus rendering the reaction to proceed smoothly under mild conditions. Notably, SO2 capacity and PS yield were almost constant after five successive recycles of the binary system (Figure 4). In summary, we have developed an excellent system for rapid, selective, reversible SO2 capture using a binary system consisting of weak base with suitable basicity and a proton donor. PEG150MeIm/PEG150 gave high SO2 capacity of 4.88 mol of SO2/mol of base, and 1.06 of SO2 capacity can be attained even at SO2 partial pressure as low as 0.02 bar, owing to the strong SO2-philic properties of the PEG derivatives. Notably, the captured SO2 was easy to release by just bubbling N2 through the absorption system at room temperature, greatly reducing the energy requirement for SO2 desorption. Furthermore, this system also showed good SO2/CO2 sorption selectivity, thereby could be applied for separation of SO2 from CO2. The SO2 captured by PEG150MeIm/PEG150 or choline chloride, simultaneously leading to its activation, could directly be converted into useful organic compounds catalytically at mild conditions in lieu of free SO2, thus eliminating the energy

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ASSOCIATED CONTENT

S Supporting Information *

General experimental methods; Synthesis and characterization of the absorbents before and after SO2 capture; Reaction conditions screening for the synthesis of propylene sulfite, and characterization of cyclic sulfites. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone/Fax: (86)-22-2350-3878; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants No. 21172125), the “111” Project of Ministry of Education of China (Project No. B06005).

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REFERENCES

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Highly Efficient SO2 Absorption and Its Subsequent Utilization by

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