Novel Ether-Functionalized Pyridinium Chloride Ionic Liquids for

Sep 30, 2014 - Xiaomin Zhang , Xi Feng , He Li , Jing Peng , Youting Wu , and Xingbang .... Chanele Rodriguez , Rahonel Fernandez , Nicole Zmich , Edd...
0 downloads 0 Views 631KB Size
Download PDF
Article pubs.acs.org/IECR

Novel Ether-Functionalized Pyridinium Chloride Ionic Liquids for Efficient SO2 Capture Jian Wang,†,‡ Shaojuan Zeng,†,‡ Lu Bai,†,‡ Hongshuai Gao,† Xiangping Zhang,*,† and Suojiang Zhang*,† †

Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of Multiphase Complex Systems, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ‡ School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Sulfur dioxide (SO2) emitted from the combustion of fossil flues is a major atmospheric pollutant that seriously threatens the environment and human health. The traditional methods for the removal and recovery of SO2 are irreversible and can result in secondary pollution or suffer from solvent loss. Therefore, it is urgent to explore new absorbents for the reversible, efficient, and environment-friendly capture of SO2. Ionic liquids (ILs) exhibit excellent performance for SO2 capture because of their unique physicochemical properties. To improve the absorption capacities of ILs for SO2, a series of novel etherfunctionalized pyridinium chloride ILs ([EnPy]Cl, n = 2−4) were designed and synthesized. The physiochemical properties of these ILs and their SO2 absorption performance under different conditions were investigated. In addition, the SO2/CO2 selectivities and reusabilities of the ILs and the absorption mechanism between SO2 and [EnPy]Cl (n = 2−4) were studied. It was found that 1-[2-(2-methoxyethoxy)ethyl] pyridinium chloride showed a relatively high absorption capacity of up to 1.155 (g of SO2)·(g of IL)−1 at 20 °C and 0.1 MPa. The SO2 absorption capacities of the studied ILs remained steady in absorption− desorption cycles, implying that [EnPy]Cl (n = 2−4) could be promising candidates for SO2 capture.

1. INTRODUCTION The increasing emissions of sulfur dioxide (SO2), mainly from the combustion of coal, oil, natural gas, and other materials, have made this compound one of the major atmospheric pollutants threatening human health, agriculture, and building materials.1−3 At the same time, SO2 is a useful resource as an intermediate in chemical production.4 Therefore, removal and recovery of SO2 is of critical importance. Although a number of traditional methods for this purpose have been developed and commercialized, they have several inherent disadvantages. For example, limestone scrubbing is irreversible and can produce huge amounts of calcium sulfate and wastewater, resulting in secondary pollution to the environment. In addition, amine scrubbing can suffer from solvent loss because of the volatility of amines.5−8 Accordingly, it is highly desired to develop a new method for the reversible, efficient, and environment-friendly capture of SO2. Ionic liquids (ILs) are one of the most promising alternatives for SO2 capture9−12 because of their unique properties, including high thermal stability, negligible vapor pressure, large liquid range, and tunable chemical properties.13−17 Wu et al.9 first reported that 1,1,3,3-tetramethylguanidine lactate ([TMG]L) could chemically absorb 0.978 (mol of SO2)·(mol of IL)−1 [0.305 (g of SO2)·(g of IL)−1] at 40 °C and 1.0 bar, forming guanidinum sulfurous acid cation. Subsequently, large numbers of studies were conducted on various highly efficient ILs for SO2 capture.18−25 For example, Hong et al.8 synthesized a series of ether-functionalized imidazolium methanesulfonates that could absorb 2.30−6.30 (mol of SO2)·(mol of IL)−1 [0.624−0.752 (g of SO2)·(g of IL)−1] at 30 °C and 0.1 MPa. They observed that the SO2 capacity increased with increasing number of tethered ether oxygen atoms. Cui et al.5 prepared © 2014 American Chemical Society

two kinds of ether-functionalized ILs with tetrazolate anion, and the SO2 capacities of tri-n-butyl{2-[2-(2-methoxyethoxy)ethoxy]ethyl}phosphonium tetrazolate ([P444E3][Tetz]) and 1{2-[2-(2-methoxy)ethoxy] ethoxy}ethyl-3-methyl imidazolium tetrazolate ([E3Mim][Tetz]) reached 5.00 (mol of SO2)·(mol of IL)−1 [0.765 (g of SO2)·(g of IL)−1] and 4.43 (mol of SO2)· (mol of IL)−1 [0.951 (g of SO2)·(g of IL)−1], respectively, through a combination of chemical and physical absorption mechanisms. Recently, Zhang et al.25 designed three etherfunctionalized imidazolium bis(trifluoromethylsulfonyl)imide ILs for SO2 capture, and the absorption capacity of SO2 in 3,3′-({[oxybis(ethane-2,1-diyl)]bis(oxy)}bis(ethane-2,1-diyl))bis(1-methyl-1H-imidazol-3-ium) bis(trifluoromethylsulfonyl)imide ([E3Mim2][Tf2N]2) reached 3.307 (mol of SO2)·(mol of IL)−1 [0.239 (g of SO2)·(g of IL)−1] at 20 °C and 1 bar. Although significant progress has been made in improving the absorption of SO2, the gravimetric capacities of these ILs for SO2 are relatively low. Actually, the gravimetric capacity for SO2 is more of a concern in industrial applications.7 Therefore, the design of novel ILs to further improve the gravimetric capacity for SO2 is very important. Additionally, most of the ILs studied for SO2 capture are imidazolium-based ILs, which are expensive, barely biodegradable, and highly toxic. Compared with imidazolium-based ILs, for ILs with the same anion and alkyl chain, pyridinium-based ILs are relatively cheaper,26 more biodegradable,27 and less toxic.28 For IL applications, there is a need to design new affordable ILs with excellent properties and Received: Revised: Accepted: Published: 16832

July 8, 2014 September 22, 2014 September 30, 2014 September 30, 2014 dx.doi.org/10.1021/ie5027265 | Ind. Eng. Chem. Res. 2014, 53, 16832−16839

Industrial & Engineering Chemistry Research

Article

under a N2 atmosphere. FT-IR spectra were obtained in the range of 400−4000 cm −1 on a Thermo Nicolet 380 spectrometer. 1H and 13C NMR spectra were recorded on a Bruker spectrometer (600 Hz) in deuterated dimethyl sulfoxide (DMSO-d6). High-resolution mass spectrometry (HR-MS) [electrospray ionization (ESI)] was performed on a Bruker Apex IV Fourier transform ion cyclotron resonance mass spectrometer. 2.4. Apparatus and Procedures. The SO2 absorption and desorption measurements were similar to those in our previous works.11,12,29 In a typical experiment, about 1.0 g of IL was charged into a glass container with an inner diameter of 10 mm; then, SO2 was introduced into the glass container and bubbled through the IL at a flow rate of 70 mL/min. The glass container was immersed in a circulating water bath (IKA C-MAG HS7) of the desired temperature. The weight change upon absorption of SO2 was monitored at regular intervals by an electronic balance (Mettler Toledo PL403) with an accuracy of 0.001 g. The measurements were performed at temperatures ranging from 20 to 80 °C in intervals of 10 °C. The effect of the SO2 partial pressure on SO2 absorption was investigated by changing the composition of SO2 and N2 with a total pressure of 0.10 MPa. Desorption of SO2 was carried out by bubbling N2 through the IL at 80 °C and a flow rate of 70 mL/min. The amount of SO2 released was also determined at regular intervals.

high absorption capacities. Consequently, we set out to design and synthesize pyridinium-based ILs with high SO2 gravimetric capacities to apply in the SO2 separation. In the present work, ether groups were introduced into pyridinium chloride ILs to increase the absorption capacities for SO2 of the ILs and improve their physiochemical properties. The physiochemical properties of these novel ether-functionalized pyridinium chloride ILs (denoted [EnPy]Cl, n = 2−4), including density, viscosity, thermal decomposition temperature, and glass transition temperature, were measured. Meanwhile, the SO2 absorption performances at different temperatures, SO2 partial pressures, and water contents were investigated and compared with those of their imidazoliumbased and nonfunctionalized analogues. In addition, the SO2/ CO2 selectivity and reusability of the ILs were studied. The absorption mechanism between SO2 and [EnPy]Cl (n = 2−4) was also explored by FT-IR and NMR spectroscopies.

2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals were used without further purification. SO2 (99.9%), CO2 (99.999%), and N2 (99.999%) were supplied by Beijing Beiwen Gas Factory. Diethylene glycol methyl ether (98.0%), triethylene glycol monomethyl ether (98.0%), tetraethylene glycol monomethyl ether (98.0%), and 1-chlorodecane (97.0%) were purchased from Sigma-Aldrich Co. N-Methylimidazole (98.0%) was obtained from Beijing Ouhe Technology Co., Ltd. Pyridine (99.5%) and other reagents were purchased from Beijing Chemical Company. All of the ILs used in this work were synthesized in our laboratory. 2.2. Synthesis of ILs. Compared with those of other functionalized ILs, the synthetic procedure of ether-functionalized pyridinium chloride is relatively simple. 1-[2-(2Methoxyethoxy)ethyl] pyridinium chloride ([E2Py]Cl), 1-{2[2-(2-methoxyethoxy)ethoxy]ethyl} pyridinium chloride ([E3Py]Cl), and 1-(2-{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}ethyl) pyridinium chloride ([E4Py]Cl) were synthesized by a two-step method. Taking [E3Py]Cl as an example, 2-[2-(2methoxyethoxy)ethoxy]ethyl chloride (E3Cl) was prepared from triethylene glycol monomethyl ether through a halogenation reaction according to the literature;5 then, pyridine and E3Cl were stirred with ethyl acetate as the solvent for 72 h at 120 °C and 0.1 MPa until a brown liquid was formed. The bottom phase was separated and washed with ethyl acetate several times. Finally, the product was dried under a vacuum at 70 °C for at least 48 h to remove the remaining organic solvent. In this work, 1-{2-[2-(2-methoxyethoxy)ethoxy]ethyl}-3-methyl-imidazolium chloride ([E3Mim]Cl) and 1-decylpyridinium chloride ([C10Py]Cl) were also prepared through a similar method to compare their absorption performances with that of the pyridinium analogue. 2.3. Characterization and Physical Properties. The densities of ILs were measured by a density meter (Anton Paar DMA 5000) with an accuracy of ±0.000 005 g/cm3. The viscosities of the ILs before and after uptake of SO2 were measured with a rheometer (Anton Paar MCR 302) at a shear rate of 10 s−1. Karl Fischer titration (Beijing Xianqu Weifeng, ZDJ-2S) showed that the water contents of the ILs were less than 0.05%. The decomposition temperature was tested by thermogravimetric analysis (TGA, Q5000 V3.15 Build 263) from room temperature to 873.15 K at a heating rate of 10 K/ min under a N2 atmosphere. Differential scanning calorimetry (DSC) was carried out on a Mettler Toledo DSC1 instrument between 123.15 and 298.15 K at a heating rate of 10 K/min

3. RESULTS AND DISCUSSION 3.1. Physicochemical Properties of ILs. The densities and viscosities of [EnPy]Cl (n = 2−4) as functions of temperature are presented in Table S1 (Supporting Information). The densities and viscosities of the ILs are slightly affected by the length of the alkyl chain. The density decreases with increasing length of the alkyl chain, and the viscosity decreases with increasing length of the alkyl chain. In addition, the densities of all three ILs were found to decrease slightly and the viscosities were found to decrease dramatically with increasing temperature, which is in good agreement with the results reported for other pyridinium-based ILs.29,30 The variations in the densities and viscosities of [EnPy]Cl (n = 2−4) before and after uptake of SO2 were also investigated (Table S2, Supporting Information). The densities of the ILs increased slightly, but the viscosities decreased dramatically after uptake of SO2. SO2 dissolved in the ILs might decrease the Coulombic interactions between the cations and anions of ILs, thus causing the viscosities of the ILs to decrease greatly.31,32 The thermal decomposition temperature (Td) of an IL gives an idea of the upper operating range of the fluid and is expressed as the temperature at which the fresh IL has lost 5% of its weight.33 The midpoint temperature of the heat-capacity change is regarded as the glass transition temperature (Tg).34 The TGA and DSC curves of [EnPy]Cl (n = 2−4) are shown in Figures S1−S4 (Supporting Information), and the results are summarized in Table 1. The thermal decomposition temperTable 1. Thermal Decomposition Temperatures and Glass Transition Temperatures of [EnPy]Cl (n = 2−4)

16833

IL

Td (K)

Tg (K)

[E2Py]Cl [E3Py]Cl [E4Py]Cl

483.4 477.8 469.5

216.7 217.3 214.5

dx.doi.org/10.1021/ie5027265 | Ind. Eng. Chem. Res. 2014, 53, 16832−16839

Industrial & Engineering Chemistry Research

Article

Table 2. Physicochemical Properties of [EnPy]Cl (n = 2−4) and Nonfunctionalized IL Analogues IL symbol [C4Py]Cl [C6Py]Cl [C8Py]Cl [C10Py]Cl [C12Py]Cl [C14Py]Cl [E2Py]Cl [E3Py]Cl [E4Py]Cl

IL name

IL formula

Nonfunctionalized 1-butylpyridinum chloride C9H14NCl 1-hexylpyridinium chloride C11H18NCl 1-octylpyridinium chloride C13H22NCl 1-decylpyridinium chloride C15H26NCl 1-dodecylpyridinium chloride C17H30NCl 1-tetradecylpyridinium chloride C19H34NCl Ether-Functionalized 1-[2-(2-methoxyethoxy)ethyl] pyridinium chloride C10H16O2NCl 1-{2-[2-(2-methoxyethoxy)ethoxy]ethyl} pyridinium chloride C12H20O3NCl 1-(2-{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}ethyl) pyridinium C14H24O4NCl chloride

MW (g/mol)

appearance

melting point (°C)

171.67 199.72 227.78 255.83 283.89 311.94

white solid white waxy solid light yellow waxy solid white solid solid solid

129−131,35 124.7536 38.3536 35.0536 68−70,35 38−3937 66−67.537 75−76.537

217.70 261.75 305.80

brown liquid brown liquid dark brown liquid

− − −

respectively. In addition, the molar absorption capacity of SO2 increased with increasing number of ether groups, and this trend is consistent with the results reported by Hong et al.8 The absorption of SO2 in [C10Py]Cl was measured and compared with that in [E3Py]Cl, considering that these two ILs have the same anion and a common alkyl chain except that three carbon atoms in [C10Py]Cl are substituted by ether oxygen atoms in [E3Py]Cl. Because [C10Py]Cl is a solid at room temperature, the absorption experiments were performed at 80 °C. As can be seen in Figure 2, absorption equilibrium

atures of ILs with the same anion decrease slightly as the alkyl chain length of the cation increase. All of the ILs did not have melting points, and the thermal decomposition temperatures of the three ILs were above 470 K, which indicates that these ILs have good thermal stability. To assess the effects of ether groups on the physicochemical properties of the ILs, the physicochemical properties of [EnPy] Cl (n = 2−4) and their nonfunctionalized IL analogues are summarized in Table 2. It can be seen that all of the nonfunctionalized pyridinium chloride ILs are solids at room temperature (25 °C), whereas the three ILs designed in this work are all liquids at room temperature. These results comprehensively reflect that fact that ether groups can make pyridinium chloride ILs liquid at room temperature, which makes these ILs more suitable for use in gas separations. 3.2. Absorption Performance of SO2 in Different ILs. The SO2 absorption performances of different ILs at 20 °C and 0.1 MPa are illustrated in Figure 1. It can be seen that the

Figure 2. Comparison of the SO2 absorption in [E3Py]Cl and [C10Py] Cl at 80 °C and 0.1 MPa.

was easily reached, which might be because of the low viscosities and low SO2 absorption capacities of the ILs at 80 °C. The absorption capacities of [E3Py]Cl and [C10Py]Cl reached 0.387 and 0.334 (g of SO2)·(g of IL)−1, respectively, at 80 °C and 0.1 MPa. [E3Py]Cl exhibited a relatively higher SO2 absorption capacity, suggesting that ether groups have a positive influence on the absorption of SO2. In addition, comparing pyridinium-based ILs with imidazolium-based ILs that have the same anion and common alkyl chain but different heterocyclic aromatic groups, for example, [E3Py]Cl and [E3Mim]Cl, the absorption of SO2 in the two ILs are remarkably similar (Figure 3), indicating that the heterocyclic aromatic group plays a minor role in determining the SO2 absorption capacities of ether-functionalized ILs.

Figure 1. SO2 absorption behaviors of [EnPy]Cl (n = 2−4) at 20 °C and 0.1 MPa.

absorption capacities at the beginning of the experiments were relatively low because of the high viscosities of the ILs. After the ILs had absorbed small amounts of SO2, their viscosities decreased, and the absorption processes accelerated. All three ILs exhibited excellent SO2 absorption performances; for instance, [E4Py]Cl and [E2Py]Cl could absorb 4.594 (mol of SO2)·(mol of IL)−1 [0.962 (g of SO2)·(g of IL)−1] and 3.924 (mol of SO2)·(mol of IL)−1 [1.155 (g of SO2)·(g of IL)−1], 16834

dx.doi.org/10.1021/ie5027265 | Ind. Eng. Chem. Res. 2014, 53, 16832−16839

Industrial & Engineering Chemistry Research

Article

3.3. Effects of Absorption Temperature and Partial Pressure of SO2. The effects of temperature on the absorption performance of SO2 in [EnPy]Cl (n = 2−4) at 0.1 MPa are displayed in Figure 4a. It can be seen that the SO2 absorption capacities of [EnPy]Cl (n = 2−4) decreased continuously with increasing temperature. For example, the molar ratios of SO2 absorbed by [E4Py]Cl and [E3Py]Cl decrease from 4.594 to 1.660 and from 4.289 to 1.581, respectively, when the temperature was changed from 20 to 80 °C. These results indicate that most of the absorbed SO2 can be easily stripped out of these ILs by heating. Figure 4b shows the effects of the SO2 partial pressure on the absorption performances of the ILs at 20 °C, and as can be seen, the molar ratios of SO2 to IL increased continuously with increasing partial pressure of SO2. For example, the molar ratio of SO2 absorbed by [E4Py]Cl increased from 2.101 to 4.594 when the SO2 partial pressure was increased from 0.02 to 0.10 MPa. The SO2 absorption capacities of [E3Py]Cl and [E2Py]Cl reached 1.855 and 1.650 (mol of SO2)·(mol of IL)−1, respectively, at low SO2 partial pressure (0.02 MPa). These results indicate that it is preferable to capture SO2 at low temperature and high SO2 partial pressure. 3.4. Effects of Water. The effects of a small amount of water on the absorption of SO2 in [E2Py]Cl was also investigated at 20 °C and 0.1 MPa because the presence of water is inevitable in real flue gases. [E2Py]Cl with different amounts of water was prepared by adding a specific weight of deionized water to the IL and mixing homogeneously. After the water contents were tested by Karl Fischer titration, the mixtures were used to absorb SO2. As can be seen in Figure 5, the absorption process can be divided into three sections. First,

Figure 3. Comparison of the SO2 absorption in [E3Py]Cl and [E3Mim]Cl at 20 °C and 0.1 MPa.

To compare the SO2 capacities in different ILs, we selected some typical ILs with high SO2 absorption capacities and some ether-functionalized ILs and listed their absorption data in Table 3 according to the gravimetric capacity of SO2. Among all of the ILs that physically and/or chemically absorb SO2, the three ether-functionalized pyridinium chloride ILs show very high SO2 capture gravimetric capacities. In particular, the SO2 absorption capacity of [E2Py]Cl reached 1.155 (g of SO2)·(g of IL)−1 at 20 °C and 0.1 MPa, which is slightly higher than the values for previously reported ILs under the same conditions.

Table 3. Comparison of the SO2 Gravimetric Capacities in Different ILs at 0.1 MPa SO2 capacity IL b

a

[E2Py]Cl [C2Mim][SCN]c [Et2NEMim][Tetz]c [E3Mim]Clb [E3Py]Clb [E4Py]Clb [E3Mim][Tetz]b,c [E3Mim]MeSO3b [P444E3][Tetz]b,c [C2Mim][C(CN)3] [E8Mim]MeSO3b [E2Mim]MeSO3b [E1Mim]MeSO3b [E3Py]Clb [C10Py]Cl [E3Mim2][Tf2N]2b [E2Mim2][Tf2N]2b [E1Mim2][Tf2N]2b

T (°C) 20 20 20 20 20 20 20 30 20 20 30 30 30 80 80 20 40 40

−1

g of SO2·(g of IL) 1.155 1.13 1.101 1.057 1.050 0.962 0.951 0.752 0.765 0.742 0.741 0.729 0.624 0.387 0.338 0.239 0.115 0.104

(mol of SO2)·(mol of IL)−1 3.924 2.99 4.32 4.367 4.289 4.594 4.43 3.81 5.00 2.33 6.30 3.19 2.30 1.581 1.334 3.307 1.506 1.291

ref this 38 7 this this this 5 8 5 38 8 8 8 this this 25 25 25

work

work work work

work work

a

[C2Mim][SCN], 1-ethyl-3-methylimidazolium thiocyanate; [Et2NEMim][Tetz], 1-(2-diethylaminoethyl)-3-methyl-imidazolium tetrazolate; [E3Mim][Tetz], 1-{2-[2-(2-methoxy)ethoxy]ethoxy}ethyl-3-methyl imidazolium tetrazolate; [E3Mim]MeSO3, 1-triethylene glycol monomethyl ether-3-methylimidazolium methanesulfonate; [P444E3][Tetz], tri-n-butyl{2-[2-(2-methoxyethoxy)ethoxy]ethyl}phosphonium tetrazolate; [C2Mim][C(CN)3], 1-ethyl-3-methylimidazolium tricyanomethanide; [E8Mim]MeSO3, 1-octaethylene glycol monomethyl ether-3-methyl-imidazolium methanesulfonate; [E2Mim]MeSO3, 1-diethylene glycol monomethyl ether-3-methylimidazolium methanesulfonate; [E1Mim]MeSO3, 1-ethylene glycol monomethyl ether-3-methylimidazolium methanesulfonate; [E3Mim2][Tf2N]2, 3,3′-({[oxybis(ethane-2,1-diyl)]bis(oxy)}bis(ethane-2,1diyl))bis(1-methyl-1H-imidazol-3-ium) bis(trifluoromethylsulfonyl)imide; [E2Mim2][Tf2N]2, 3,3′-[(ethane-1,2-diylbis(oxy))bis(ethane-2,1-diyl)]bis(1-methyl-1H-imidazol-3-ium) bis(trifluoromethylsulfonyl)imide; [E1Mim2][Tf2N]2, 3,3′-[oxybis(ethane-2,1-diyl)]bis(1-methyl-1H-imidazol-3ium) bis(trifluoromethylsulfonyl)imide; bIL contains ether groups. cSO2 is chemically absorbed by this IL. 16835

dx.doi.org/10.1021/ie5027265 | Ind. Eng. Chem. Res. 2014, 53, 16832−16839

Industrial & Engineering Chemistry Research

Article

of SO2 in [E2Py]Cl with water was lower than that in pure [E2Py]Cl after absorption for more than 5 min, mainly because the viscosity decreased dramatically after the uptake of SO2 and the viscosity was no longer a crucial factor so that the absorption capacity determined the absorption rate. Finally, all of the absorption capacities remained constant after 35 min, which means that absorption equilibrium was reached within this time. The absorption capacity of SO2 in [E2Py]Cl with water was still slightly lower than that in pure [E2Py]Cl, suggesting that there is a competition between the absorption of SO2 and that of water. Therefore, it is recommended to that exposure to moisture be avoided during IL preparation and preservation39 and that moisture in flue gas could be removed before passing the flue gas through an IL.25 3.5. SO2 Absorption−Desorption Cycles. The reusability of ILs is important in evaluating the economics of industrial applications, so five consecutive cycles of SO2 absorption and desorption were measured. SO2 absorption was conducted in [EnPy]Cl (n = 2−4) at 0.1 MPa and 20 °C, whereas the desorption of SO2 was carried out by bubbling N2 at a flow rate of 70 mL/min through [EnPy]Cl (n = 2−4) at 80 °C. Taking [E2Py]Cl as an example (Figure 6), only 5 min was required to

Figure 4. Effects of temperature at (a) 0.1 MPa and (b) a SO2 partial pressure at 20 °C on SO2 absorption by [EnPy]Cl (n = 2−4).

Figure 6. Five consecutive cycles of SO2 absorption and desorption of [E2Py]Cl at 0.1 MPa: (●) absorption, (○) desorption.

release about 90% of the total absorbed SO2, indicating that the desorption rate is very high and that SO2 is easily stripped out of the IL. In addition, the absorption capacity and desorption rate showed no significant drop in five cycles, suggesting that SO2 could be desorbed easily and that the SO2 absorption− desorption process is highly reversible. [E3Py]Cl and [E4Py]Cl exhibited similar performances as shown in Figures S5 and S6, respectively, of the Supporting Information. 3.6. SO2/CO2 Selectivity. SO2 and CO2 usually coexist in real flue gas, so a high SO2/CO2 selectivity is necessary for SO2 capture. The SO2/CO2 selectivity was investigated in this work by the same method as in most of the previously reported literature. 6,7,29 All three ILs exhibited high SO 2 /CO 2 selectivities, as shown in Table 4; for example, the SO2 and CO2 absorption capacities of [E3Py]Cl were 4.289 and 0.087 (mol of gas)·(mol of IL)−1, respectively, which implies that the SO2/CO2 selectivity reached 49 for this IL. The SO2/CO2 selectivity was also slightly higher than those of other functionalized ILs. This high selectivity, which mainly reveals differences in absorption capacity, demonstrates that this type

Figure 5. Effect of a small amount of water on the absorption of SO2 in [E2Py]Cl at 20 °C and 0.1 MPa.

the capacity of SO2 in [E2Py]Cl with water was slightly higher than that in pure [E2Py]Cl in the first 5 min. It is well-known that water can decrease the viscosity of ILs and decreased viscosity increases the mass-transport performance, thus increasing the absorption rate. Second, the absorption capacity 16836

dx.doi.org/10.1021/ie5027265 | Ind. Eng. Chem. Res. 2014, 53, 16832−16839

Industrial & Engineering Chemistry Research

Article

Table 4. SO2/CO2 Selectivities of ILs at 20 °C and 0.1 MPa gas capacity [(mol of gas)· (mol of IL)−1] ILa

SO2

CO2

SO2/CO2 selectivity

[E2Py]Cl [E3Py]Cl [E4Py]Cl [Et2NEMim][Tetz]7 [Et2NEMim][Tf2N]7 [P66614][Tetz]40 [C4Py][SCN]29

3.924 4.289 4.594 4.32 2.81 3.72 1.532

0.080 0.087 0.103 0.11 0.07 0.08 0.055

49 49 45 39 40 47 28

a

[Et2NEMim][Tetz], 1-(2-diethylaminoethyl)-3-methyl-imidazolium tetrazolate; [Et2NEMim][Tf2N], 1-(2-diethyl-aminoethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide; [P66614][Tetz], trihexyl(tetradecyl)phosphonium tetrazolate; [C4Py][SCN], N-butylpyridinium thiocyanate.

of IL could be a potential absorbent for the selective capture of SO2. 3.7. Mechanism of SO2 Absorption. The absorption mechanism of SO2 in the ILs was investigated by comparing the NMR and FT-IR spectra of [EnPy]Cl (n = 2−4) before and after the absorption of SO2. The FT-IR spectra of fresh [E2Py]Cl, of the IL after uptake of SO2, and of the IL after desorption of SO2 are shown in Figure 7. Two new peaks can

Figure 8. NMR spectra of [E2Py]Cl before and after absorption of SO2: (a) 1H NMR, (b) 13C NMR.

Figure 7. FT-IR spectra of [E2Py]Cl before and after absorption of SO2.

example, the typical peak of NCH2 in the pyridinium ring moved from 9.22 to 9.06 ppm upon uptake of SO2 (Figure 8a). The NMR spectra of ILs saturated with SO2 after desorption were almost unchanged compared with those of the fresh ILs, implying that the interactions between [EnPy]Cl (n = 2−4) and SO2 are very weak.

be observed in the FT-IR spectrum of the IL after absorption SO2. The broad peak at about 1289 cm−1 is attributable to the sulfate of the SO stretch. The small peak at 532 cm−1 can be ascribed to the presence of a ClS interaction. The two peaks disappeared from the spectrum when the SO2 dissolved in the IL was desorbed. A possible explanation for this behavior is that SO2 is absorbed through physical interactions between the electronegative ether oxygen atoms on the cation and the acid SO2,5,8 as well as the interaction between the Cl atoms of the IL and the S of SO2.41,42 Meanwhile, the NMR spectra of [EnPy]Cl (n = 2−4) before and after uptake of SO2 were further studied. It is clear that no distinct changes occurred in the chemical shifts for the peaks in the 1H NMR and 13C NMR spectra, as shown in Figure 8. In the 1H NMR spectra, the peaks exhibited a very slight shift; for

4. CONCLUSIONS A series of novel ether-functionalized pyridinium chloride ILs were designed by introducing ether groups into the pyridinium cation to improve the absorption capacities for SO2. The physicochemical properties and SO2 absorption performances of the ILs were investigated. These ILs were found to exhibit high SO2 gravimetric absorption capacities through physical interactions. In particular, the SO2 absorption capacity of [E2Py]Cl reached 1.155 (g of SO2)·(g of IL)−1 at 20 °C and 0.1 MPa, which is relatively higher than those of previously reported ILs under the same conditions. Meanwhile, the etherfunctionalized pyridinium chloride ILs were found to be as capable of capturing SO2 as their imidazolium-based analogues, and the SO2 absorption capacities of [EnPy]Cl (n = 2−4) increased with decreasing temperature and increasing SO2 16837

dx.doi.org/10.1021/ie5027265 | Ind. Eng. Chem. Res. 2014, 53, 16832−16839

Industrial & Engineering Chemistry Research

Article

(11) Yuan, X. L.; Zhang, S. J.; Lu, X. M. Hydroxyl ammonium ionic liquids: Synthesis, properties, and solubility of SO2. J. Chem. Eng. Data 2007, 52, 596−599. (12) Shang, Y.; Li, H. P.; Zhang, S. J.; Xu, H.; Wang, Z. X.; Zhang, L.; Zhang, J. M. Guanidinium-based ionic liquids for sulfur dioxide sorption. Chem. Eng. J. 2011, 175, 324−329. (13) Zhang, Y. Q.; Zhang, S. J.; Lu, X. M.; Zhou, Q.; Fan, W.; Zhang, X. P. Dual amino-functionalised phosphonium ionic liquids for CO2 capture. Chem.Eur. J. 2009, 15, 3003−3011. (14) Li, G. H.; Zhou, Q.; Zhang, X. P.; Wang, L.; Zhang, S. J.; Li, J. W. Solubilities of ammonia in basic imidazolium ionic liquids. Fluid Phase Equilib. 2010, 297, 34−39. (15) Zhang, X. P.; Zhang, X. C.; Dong, H. F.; Zhao, Z. J.; Zhang, S. J.; Huang, Y. Carbon capture with ionic liquids: Overview and progress. Energy Environ. Sci. 2012, 5, 6668−6681. (16) Zhang, J. Z.; Jia, C.; Dong, H. F.; Wang, J. Q.; Zhang, X. P.; Zhang, S. J. A novel dual amino-functionalized cation-tethered ionic liquid for CO2 capture. Ind. Eng. Chem. Res. 2013, 52, 5835−5841. (17) Lei, Z. G.; Dai, C. N.; Chen, B. H. Gas solubility in ionic liquids. Chem. Rev. 2014, 114, 1289−326. (18) Barrosse-Antlle, L. E.; Hardacre, C.; Compton, R. G. SO2 saturation of the room temperature ionic liquid [C2mim][NTf2] much reduces the activation energy for diffusion. J. Phys. Chem. B 2009, 113, 1007−1011. (19) Shiflett, M. B.; Yokozeki, A. Chemical absorption of sulfur dioxide in room-temperature ionic liquids. Ind. Eng. Chem. Res. 2009, 49, 1370−1377. (20) Yokozeki, A.; Shiflett, M. B. Separation of carbon dioxide and sulfur dioxide gases using room-temperature ionic liquid hmim Tf2N. Energy Fuels 2009, 23, 4701−4708. (21) Yang, J. T.; Hu, G. X. Absorption of SO2 from flue gas by aqueous fulvic acid solution. RSC Adv. 2012, 2, 11410−11418. (22) Tian, S. D.; Hou, Y. C.; Wu, W. Z.; Ren, S. H.; Zhang, C. Absorption of SO2 by thermal-stable functional ionic liquids with lactate anion. RSC Adv. 2013, 3, 3572−3577. (23) Liu, B. Y.; Wei, F. X.; Zhao, J. J.; Wang, Y. Y. Characterization of amide−thiocyanates eutectic ionic liquids and their application in SO2 absorption. RSC Adv. 2013, 3, 2470−2476. (24) Huang, K.; Wang, G. N.; Dai, Y.; Wu, Y. T.; Hu, X. B.; Zhang, Z. B. Dicarboxylic acid salts as task-specific ionic liquids for reversible absorption of SO2 with a low enthalpy change. RSC Adv. 2013, 3, 16264−16269. (25) Zhang, L. H.; Zhang, Z. J.; Sun, Y. L.; Jiang, B.; Li, X. G.; Ge, X. H.; Wang, J. T. Ether-functionalized ionic liquids with low viscosity for efficient SO2 capture. Ind. Eng. Chem. Res. 2013, 52, 16335−16340. (26) Pinto, A. M.; Rodríguez, H.; Arce, A.; Soto, A. Carbon dioxide absorption in the ionic liquid 1-ethylpyridinium ethylsulfate and in its mixtures with another ionic liquid. Int. J. Greenhouse Gas Control 2013, 18, 296−304. (27) Docherty, K. M.; Dixon, J. K.; Kulpa, C. F., Jr. Biodegradability of imidazolium and pyridinium ionic liquids by an activated sludge microbial community. Biodegradation 2007, 18, 481−493. (28) Couling, D. J.; Bernot, R. J.; Docherty, K. M.; Dixon, J. K.; Maginn, E. J. Assessing the factors responsible for ionic liquid toxicity to aquatic organisms via quantitative structure−property relationship modeling. Green Chem. 2006, 8, 82−90. (29) Zeng, S. J.; Gao, H. S.; Zhang, X. C.; Dong, H. F.; Zhang, X. P.; Zhang, S. J. Efficient and reversible capture of SO2 by pyridiniumbased ionic liquids. Chem. Eng. J. 2014, 251, 1385−8947. (30) Liu, Q. S.; Li, P. P.; Welz, U.; Chen, J.; Liu, X. X. Density, dynamic viscosity, and electrical conductivity of pyridinium-based hydrophobic ionic liquids. J. Chem. Thermodyn. 2013, 66, 88−94. (31) Ren, S. H.; Hou, Y. C.; Wu, W. Z.; Liu, Q. Y.; Xiao, Y. F.; Chen, X. T. Properties of ionic liquids absorbing SO2 and the mechanism of the absorption. J. Phys. Chem. B 2010, 114, 2175−2179. (32) Ando, R. A.; Siqueira, L. J.; Bazito, F. C.; Torresi, R. M.; Santos, P. S. The sulfur dioxide-1-butyl-3-methylimidazolium bromide interaction: Drastic changes in structural and physical properties. J. Phys. Chem. B 2007, 111, 8717−8719.

partial pressure; a small amount of water also slightly decreased their SO2 absorption capacities. Owing to the excellent absorption performances, simple synthetic procedure, high SO2/CO2 selectivity, and excellent reusability of these ILs, this type of IL could be a promising candidate for SO2 capture applications.

■

ASSOCIATED CONTENT

* Supporting Information S

Densities, viscosities, and TGA and DSC curves of [EnPy]Cl (n = 2−4); SO2 absorption−desorption cycles, NMR spectra, and FT-IR spectra of [E3Py]Cl and [E4Py]Cl. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.Z.). Tel.: +86-010-62558174. Fax: +86-10-82625243. *E-mail: [email protected] (S.Z.). Tel./fax: +86-01082544875. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported financially by the National Basic Research Program of China (No. 2013CB733506), the National Natural Science Foundation of China (Nos. 21036007, 21206169, and 21436010), the National High Technology Research and Development Program of China (No. 2013AA06540201), and the Key Program of Beijing Municipal Natural Science Foundation (No. 2141003).

■

REFERENCES

(1) Smith, S. J.; van Aardenne, J.; Klimont, Z.; Andres, R. J.; Volke, A.; Arias, S. D. Anthropogenic sulfur dioxide emissions: 1850−2005. Atmos. Chem. Phys. 2011, 11, 1101−1116. (2) Smith, S. J.; Pitcher, H.; Wigley, T. M. L. Global and regional anthropogenic sulfur dioxide emissions. Global Planet. Change 2001, 29, 99−119. (3) Philip, L.; Deshusses, M. A. Sulfur dioxide treatment from flue gases using a biotrickling filter−bioreactor system. Environ. Sci. Technol. 2003, 37, 1978−1982. (4) Huang, J.; Riisager, A.; Wasserscheid, P.; Fehrmann, R. Reversible physical absorption of SO2 by ionic liquids. Chem. Commun. 2006, 38, 4027−4029. (5) Cui, G. K.; Wang, C. M.; Zheng, J. J.; Guo, Y.; Luo, X. Y.; Li, H. R. Highly efficient SO2 capture by dual functionalized ionic liquids through a combination of chemical and physical absorption. Chem. Commun. 2012, 48, 2633−2635. (6) Yang, Z. Z.; He, L. N.; Zhao, Y. N.; Yu, B. Highly efficient SO2 absorption and its subsequent utilization by weak base/polyethylene glycol binary system. Environ. Sci. Technol. 2013, 47, 1598−1605. (7) Yang, D. Z.; Hou, M. Q.; Ning, H.; Ma, J.; Kang, X. C.; Zhang, J. L.; Han, B. X. Reversible capture of SO2 through functionalized ionic liquids. ChemSusChem 2013, 6, 1191−1195. (8) Hong, S. Y.; Im, J.; Palgunadi, J.; Lee, S. D.; Lee, J. S.; Kim, H. S.; Cheong, M.; Jung, K.-D. Ether-functionalized ionic liquids as highly efficient SO2 absorbents. Energy Environ. Sci. 2011, 4, 1802−1806. (9) Wu, W. Z.; Han, B. X.; Gao, H. X.; Liu, Z. M.; Jiang, T.; Huang, J. Desulfurization of flue gas: SO2 absorption by an ionic liquid. Angew. Chem., Int. Ed. 2004, 43, 2415−2417. (10) Anderson, J. L.; Dixon, J. K.; Maginn, E. J.; Brennecke, J. F. Measurement of SO2 solubility in ionic liquids. J. Phys. Chem. B 2006, 110, 15059−15062. 16838

dx.doi.org/10.1021/ie5027265 | Ind. Eng. Chem. Res. 2014, 53, 16832−16839

Industrial & Engineering Chemistry Research

Article

(33) Crosthwaite, J. M.; Muldoon, M. J.; Dixon, J. K.; Anderson, J. L.; Brennecke, J. F. Phase transition and decomposition temperatures, heat capacities and viscosities of pyridinium ionic liquids. J. Chem. Thermodyn. 2005, 37, 559−568. (34) Fredlake, C. P.; Crosthwaite, J. M.; Hert, D. G.; Aki, S. N.; Brennecke, J. F. Thermophysical properties of imidazolium-based ionic liquids. J. Chem. Eng. Data 2004, 49, 954−964. (35) Kharitonova, E. V.; Zhuravlev, O. E.; Chervinets, V. M.; Voronchikhina, L. I.; Demidova, M. A. Synthesis and antimicrobial activity of quaternary ammonium, pyridinium, and morpholinium tetrachloroferrates. Pharm. Chem. J. 2012, 46, 266−268. (36) Sastry, N. V.; Vaghela, N. M.; Macwan, P. M.; Soni, S. S.; Aswal, V. K.; Gibaud, A. Aggregation behavior of pyridinium based ionic liquids in waterSurface tension, 1H NMR chemical shifts, SANS and SAXS measurements. J. Colloid Interface Sci. 2012, 371, 52−61. (37) Galgano, P. D.; El Seoud, O. A. Surface active ionic liquids: Study of the micellar properties of 1-(1-alkyl)-3-methylimidazolium chlorides and comparison with structurally related surfactants. J. Colloid Interface Sci. 2011, 361, 186−194. (38) Wang, C. M.; Zheng, J. J.; Cui, G. K.; Luo, X. Y.; Guo, Y.; Li, H. R. Highly efficient SO2 capture through tuning the interaction between anion-functionalized ionic liquids and SO2. Chem. Commun. 2013, 49, 1166−1168. (39) Zhang, Z. M.; Wu, L. B.; Dong, J.; Li, B. G.; Zhu, S. P. Preparation and SO2 sorption/desorption behavior of an ionic liquid supported on porous silica particles. Ind. Eng. Chem. Res. 2009, 48, 2142−2148. (40) Wang, C. M.; Cui, G. K.; Luo, X. Y.; Xu, Y. J.; Li, H. R.; Dai, S. Highly efficient and reversible SO2 capture by tunable azole-based ionic liquids through multiple-site chemical absorption. J. Am. Chem. Soc. 2011, 133, 11916−11919. (41) Lee, K. Y.; Kim, C. S.; Kim, H.; Cheong, M.; Mukherjee, D. K.; Jung, K.-D. Effects of halide anions to absorb SO2 in ionic liquids. Bull. Korean Chem. Soc. 2010, 31, 1937−1940. (42) Cui, G. K.; Zheng, J. J.; Luo, X. Y.; Lin, W. J.; Ding, F.; Li, H. R.; Wang, C. M. Tuning anion-functionalized ionic liquids for improved SO2 capture. Angew. Chem., Int. Ed. 2013, 52, 10620−10624.

16839

dx.doi.org/10.1021/ie5027265 | Ind. Eng. Chem. Res. 2014, 53, 16832−16839