Highly Selective Capture of CO2 by Ether-Functionalized Pyridinium

Article pubs.acs.org/EF

Highly Selective Capture of CO2 by Ether-Functionalized Pyridinium Ionic Liquids with Low Viscosity Shaojuan Zeng,†,‡ Jian Wang,†,‡ Lu Bai,† Binqi Wang,§ Hongshuai Gao,† Dawei Shang,†,‡ Xiangping Zhang,*,† and Suojiang Zhang*,† †

Downloaded by UNIV OF NEBRASKA-LINCOLN on September 5, 2015 | http://pubs.acs.org Publication Date (Web): September 1, 2015 | doi: 10.1021/acs.energyfuels.5b01274

Beijing Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green Process and Engineering, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China ‡ College of Chemical and Engineering, University of Chinese Academy of Sciences, Beijing, 100049, China § State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, 102200, China S Supporting Information *

ABSTRACT: In this work, three kinds of ether-functionalized pyridinium-based ILs [EnPy][NTf2] with low viscosity were designed and synthesized and used for highly selective separation of CO2 from CH4. It was found that the ether groups play an important role on physicochemical properties and CO2/CH4 selectivity in these three ILs. Compared with the nonfunctionalized analogues [CmPy][NTf2], the viscosities of [EnPy][NTf2] are lower and obviously decrease with the increasing number of ether oxygen atoms. The presence of ether groups on the cation has weak impacts on CO2 solubility of the ILs, but it contributes to a much lower CH4 solubility, which leads to the great increase of CO2/CH4 selectivity using [EnPy][NTf2]. Moreover, CO2/CH4 selectivity in all investigated ILs greatly decreases with the increasing temperature due to the weaker temperature dependence of CH4 solubility. In addition, the thermodynamic properties including the Gibbs free energy, enthalpy, and entropy of CO2 and CH4 in these ILs were also obtained, and the CO2 and CH4 dissolution mechanisms were further analyzed. The results demonstrated that the gas−IL interaction plays a dominate role in CH4 solubility in the investigated ILs, but CO2 dissolution in ILs is determined by both the IL−gas interaction and free volume of ILs. This work will offer new insights into designing more competitive ILs for selective separation of CO2 from CH4.

1. INTRODUCTION Natural gas has attracted significant attention as an alternative clean energy source to coal and oil in view of energetic and environmental problems. Besides the main component methane (CH4), natural gas always contains some other impurities such as hydrogen sulfide (H2S), carbon dioxide (CO2), and so on. In order to prevent pipeline corrosion and the decrease in heat value of natural gas, CO2 must be removed from it before liquefaction for the safety and efficiency of fuel utilization.1 Up to date, one of the most widely used technologies for CO2 capture is amine scrubbing by chemical reaction. Although this method has the excellent absorption perforamces of CO2 even under low partial pressure of CO2, there are some inherent drawbacks, such as loss of solvent due to thermal or chemical degradation, equipment corrosion, and high energy demands during the regeneration.2,3 In the past decade, ionic liquids (ILs) have emerged as a promising candidate for the separation of various gases owing to their outstanding properties, such as negligible vapor pressures, high thermal stability, and tunable structures.4−6 Since the conventional IL 1-N-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]) that can efficiently and physically absorb 0.75 mol of CO2 per mole of IL at 8.30 MPa was first reported,7 a large number of researchers have focused extensively on designing the novel functionalized ILs for highly efficient CO2 capture. Therefore, dozens of CO2 solubility data are available in the literature.8−14 However, in practice, for a gas separation process involving two or more gas mixtures, only © XXXX American Chemical Society

solubility data is not enough to judge the separation performance of an absorbent; the selectivity data are more important. Unfortunately, the data about CO2/CH4 selectivity in ILs and related systematical research are very limited.15−22 Noble et al.15,17−19 and Zhang et al.21 reported CO2 and CH4 solubility data in some conventional imidazolium-based ILs, such as [Bmim][BF4], [Bmim][NTf2], [Bmim][NO3], and [Bmim][N(CN)2]. The results showed that CH4 solubility is much lower than CO2 solubility in these ILs, but the CO2/CH4 selectivity in ILs is still unable to compete with the traditional physical absorbents, such as Rectisol and sulfolate. Similarly, Ramdin et al.22 studied CO2 and CH4 solubility and CO2/CH4 selectivity in 10 kinds of ILs such as piperidinium, pyrrolidinium, ammonium, and phosphonium. However, compared with the imidazolium-based ILs mentioned above, CO2/CH4 selectivity in most ILs is not significantly improved due to the simultaneously great increase of CO2 and CH4 solubility. For example, Henry’s law constants of CO2 and CH4 in the imidazolium-based IL [Bmim][NTf2] was 41 and 420 bar at 313.15 K, respectively,19 whereas Henry’s law constants of CO2 and CH4 in the piperidinium-based IL [bmpip][NTf2] decreased drastically to 5.01 and 43.93 bar, respectively.22 Therefore, it is vital to explore new task-specific ILs for improving the selective separation of CO2 from CH4. Received: June 8, 2015 Revised: August 25, 2015

A

DOI: 10.1021/acs.energyfuels.5b01274 Energy Fuels XXXX, XXX, XXX−XXX

Article


Energy & Fuels Previous work23−28 demonstrated that pyridinium-based ILs have many advantages, such as high thermal stability, lower cost, and higher biodegradability than imidazolium-based ILs, and excellent absorption performances of acid gases like CO2 and SO2, implying the great potentials of pyridinium-based ILs for gas separation. Bara et al.29 and Freeman et al.30 found that the presence of polar groups like ether groups is very effective for selective separation of CO2 from CH4. Therefore, in this work, a series of ether-functionalized pyridinium-based ILs [EnPy][NTf2] and the corresponding nonfunctionalized analogues [CmPy][NTf2] for comparison were synthesized. The effect of ether groups on physicochemical properties (e.g., density, viscosity, thermal decomposition temperatures, and glass transition temperatures), as well as CO2 and CH4 solubilty and CO2/CH4 selectivity under different temperatures was systematically investigated. Furthermore, the thermodynamic properties including the Gibbs free energy, enthalpy, and entropy of CO2 and CH4 in these ILs were also calculated, and CO2 and CH4 dissolution mechanisms in ILs were further analyzed.

dropped into the water phase. Finally, the bottom phase was evaporated under vacuum and dried at 70 °C for 48 h. [C7Py][NTf2] and [C10Py][NTf2] were synthesized in the same method except that 1-bromobutane was substituted by 1-bromoheptane and 1-bromodecane, respectively. Preparation of ether-functionalized pyridinium ILs [E1Py][NTf2], [E2Py][NTf2], and [E3Py][NTf2]: Taking [E1Py][NTf2] as an example, pyridine and 2-chloroethyl methyl ether (slight stoichiometric excess) were stirred with ethyl acetate as the solvent for 48 h at 80 °C. After cooling to room temperature, the brown solid precipitate formed. Then, the solid was washed with ethyl acetate for several times and dried at 70 °C under vacuum for 48 h to obtain the brown solid 1(2-methoxyethyl)pyridinium chloride ([E1Py]Cl). [E1Py][NTf2] was synthesized through anion exchange reaction following the same procedures described above for [C4Py][NTf2], except for the use of [E1Py]Cl instead of [C4Py]Br. The final product [E1Py][NTf2] was obtained as the brown liquid under vacuum at 70 °C for 48 h. [E2Py][NTf2] and [E3Py][NTf2] were synthesized in the same method except that 2-chloroethyl methyl ether was substituted by E2Cl and E3Cl, respectively. E2Cl and E3Cl were synthesized according to the reported methods.25 All the ILs were dried under vacuum at 70 °C for 48 h before use. 1 H NMR and 13C NMR spectra were recorded on a Bruker spectrometer (600 Hz) using d6-DMSO as the solvent, and FTIR spectra were collected by a Thermo Nicolet 380 spectrometer, which are used to confirm the structures of these ILs. The results are given in the Supporting Information. The water contents in the ILs after drying were determined with volumetric Karl Fischer Titration (Metrohm, 787 KF Titrino) and found to be less than 0.03 wt %. The residual halide contents in the ILs were measured using a PXSJ-226 Series Ion meter (INESA Scientific Instrument Co. Ltd.) and found to be less than 0.03 wt %. 2.3. Measurement of Physicochemical Properties. The density and viscosity of ILs were measured at temperatures from 293.15 to 353.15 K with a density meter (Anton Paar DMA 5000) and an automated microviscometer (Anton Paar AMVn), respectively. The accuracy of the densimeter was ±0.000005 g·cm−3, while the reproducibility of the microviscometer was 0.5%. Thermogravimetric analysis (TGA Q5000) was used to determine the decomposition temperature of ILs by heating from room temperature to 873.15 K under a nitrogen atmosphere at a heating rate of 10 K min−1. The glass transition temperature of ILs was measured by a differential scanning calorimeter (Mettler Toledo DSC1), which was carried out from 123.15 to 323.15 K under a nitrogen atmosphere at a heating rate of 10 K min−1. 2.4. Gas Solubility Measurement. CO2 and CH4 solubilities in ILs under different conditions were measured by the apparatus shown in Figure 2, which was similar to that in our previous work.32,33 In a typical experiment, about 10 g of IL was placed into the absorption vessel with a magnetic stirrer. After the absorption vessel was under vacuum, CO2 or CH4 gas was injected from the gas chamber to the absorption vessel of desired pressure. It was supposed that the absorption equilibrium was reached when the pressures of the two chambers remained constant for more than 2 h, and then more gas was introduced into the absorption vessel to reach another new equilibrium. The absorption vessel and the gas chamber were immersed in a circulated water bath to keep the temperature constant. CO2 and CH4 solubilities were calculated with the Peng−Robinson (PR) equation of state13 by the variation of the pressure in the absorption vessel and the gas chamber. The Peng−Robinson equation is given as follows

2. EXPERIMENTAL SECTION 2.1. Materials. Both CO2 (99.999%) and CH4 (99.999%) were purchased from Beijing Beiwen Gas Factory. 1-Bromobutane (99.0%), 1-bromoheptane (99.0%), 1-bromodecane (98.0%), and 2-chloroethyl methyl ether (98.0%) were supplied by J&K Chemical Ltd. Triethylene glycol monomethyl ether (98.0%) and diethylene glycol methyl ether (98.0%) were purchased from Sigma-Aldrich Co., Ltd. Bis(trifluoromethane)sulfonimide lithium salt (LiNTf2, 99.0%) was obtained from Shanghai Chengjie Ltd. 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4], 99.0%) was supplied by Linzhou Keneng Material Technology Co., Ltd. Pyridine (99.5%) and other solvents were supplied by Beijing Chemical Company. 2.2. Synthesis and Characterizations of ILs. Six pyridiniumbased ILs with the same anion [NTf2], including 1-butylpyridinium bis(trifluoromethane)sulfonamide ([C4Py][NTf2]), 1-heptylpyridinium bis(trifluoromethane)sulfonamide ([C7Py][NTf2]), 1-decylpyridinium bis(trifluoromethane)sulfonamide ([C10Py][NTf2]), 1-(2methoxyethyl)pyridinium bis(trifluoromethane)sulfonamide ([E1Py][NTf2]), 1-(2-(2-methoxyethoxy)ethyl)pyridinium bis(trifluoromethane)sulfonamide ([E2Py][NTf2]), and 1-{2-[2-(2-methoxyethoxy)ethoxy]ethyl}pyridinium bis(trifluoromethane)sulfonamide ([E3Py][NTf2]), were synthesized in a two-step method, and the preparations of ILs are shown below. The structures of the ILs studied in this work are shown in Figure 1.

Figure 1. Structures of [CmPy][NTf2] and [EnPy][NTf2]. The conventional pyridinium-based ILs [C4Py][NTf2], [C7Py][NTf2], and [C10Py][NTf2] were synthesized following the reported methods.23,31 Using [C4Py][NTf2] as an example, pyridine and 1bromobutane (slight stoichiometric excess) were stirred under reflux with ethyl acetate as the solvent for 48 h at 70 °C and ambient temperature until a white solid precipitate formed; then, the solid phase was separated and recrystallized from acetonitrile. The bromide salt was finally dried under vacuum at 70 °C for at least 48 h to remove the organic solvent. Then, [C4Py]Br was mixed with LiNTf2 and stirred with the deionized water as the solvent for 24 h. After the reaction, dichloromethane was added to the mixture, and the bottom phase was separated and washed with deionized water for several times until no white precipitation formed when AgNO3 solution was B

P=

RT a − Vm − b Vm(Vm + b) + b(Vm − b)

(1)

a=

0.45724R2Tc 2 · α(T ) Pc

(2) DOI: 10.1021/acs.energyfuels.5b01274 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 2. Schematic apparatus diagram of gas absorption: a, gas cylinder; b, valve; c, storage vessel; d, magnetic stir bar; e, absorption vessel; f, vacuum pump. b=

0.0778RTc Pc

Figure 3. CO2 solubility in [Bmim][BF4] at 298.15 K: ■, Yokozeki et al.;34 ●, Brennecke et al.;35 Δ, this work.

(3)

α(T ) = [1 + k(1 − Tr 0.5)]2

(4)

k = 0.3746 + 1.54226ω − 0.26992ω2

(5)

SCO2 /CH4 =

xgas =

HCO2

Δsol H = − RT 2

∂[ln(H /p0 )] ∂(T )

Δsol G = RT ln(H /p0 ) (6)

Δsol S =

ngas ngas + nIL

(7)

(11) (12)

Δsol H − Δsol G ∂[ln(H /p0 )] = − RT − R ln(H /p0 ) T ∂T (13)

where p0 is the standard state pressure.

where VS is the volume of the storage vessel, VA is the volume of the absorption vessel, and VIL is the volume of ILs. The volume variation of ILs before and after uptake of gas is ignored, which means that the volume of ILs is regarded as a constant. ngas is the amount of CO2 or CH4 absorbed in the ILs, and xgas is the molar fraction of CO2 or CH4 in ILs. In order to validate the absorption measurement, CO2 solubility in 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) was measured at 298.15 K. As shown in Figure 3, the experimental values are in good agreement with the literature values,34,35 which means that the experimental values are reliable and the measurements are creditable. 2.5. Thermodynamic Properties. Henry’s law constants of CO2 and CH4 in ILs were calculated as follows ⎡ f (T , P) ⎤ P eqϕ1(T , P) ⎥= H = lim ⎢ 1 P → 0⎣ x1 ⎦ x1

(10)

The thermodynamic properties of CO2 and CH4 in ILs were directly related to the Henry’s law constants. The enthalpy (ΔsolH), the Gibbs free energy (ΔsolG), and the entropy (ΔsolS) of solvation can be calculated by eqs 11−13 according to the literature,36,37 respectively.

where P is the pressure, T is the temperature, a, b, and α are correlation coefficients, Tc is the critical temperature, Tr is the relative temperature, Pc is the critical pressure, and ω is the acentric factor. The molar volume Vm at various temperatures and pressures was calculated from the above equations, and the amount of CO2 and CH4 absorbed in the ILs can be calculated as follows ⎛ V ⎞ V −V IL ngas = Δ⎜⎜ S ⎟⎟ = A Vm,A ⎝ Vm,S ⎠

HCH4

3. RESULTS AND DISCUSSION 3.1. Physicochemical Properties. Densities and viscosities of ether-functionalized ILs [EnPy][NTf2] and the nonfunctionalized analogues [CmPy][NTf2] as a function of the temperature are shown in Figures 4 and 5. As expected, the densities and viscosities of all the ILs decrease with the increase of temperature. Meanwhile, the side chain length including alkyl chains and the number of ether oxygen atoms on the pyridinium cation also play an important role in densities and viscosities of ILs. It was found that the increase of the side chain length on the cation can cause a slight decrease in densities, while it can lead to a great increase in their viscosities. For example, when the cation was changed from [E1Py] to [E2Py] and [E3Py], the densities of [EnPy][NTf2] varied from 1.52 to 1.47 and 1.44 g·cm−3 at 20 °C, respectively, and the viscosities of [EnPy][NTf2] increased from 75.25 to 82.13 and 120.71 mPa·s at 20 °C, respectively. For the ILs with the same length of side chain on the cation, the ether-functionalized ILs [EnPy][NTf2] had higher densities and lower viscosities than the nonfunctionalized analogues [CmPy][NTf2] due to the introduction of ether groups on the cation, and the viscosities of [EnPy][NTf2] obviously decreased with the increasing number of ether oxygen atoms. For instance, comparing with [C4Py][NTf2], the viscosities of [E1Py][NTf2] at different

(8) eq

where f1(T, P) is the fugacity of the gas, P is the equilibrium partial pressure, and ϕ1(T, P) is the fugacity coefficient calculated from PR equation of state.

P(Vm − b) V + ( 2 + 1)b a − 1.5 ln m RT Vm − ( 2 − 1)b 2 bRT (9) The ideal selectivity of CO2/CH4 in ILs can be obtained according to the reported literature,1 and calculated by the ratio of Henry’s law constant of CH4 to that of CO2 under the same temperature. ln ϕ1 = Z − 1 − ln

C


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Table 1. Molecular Weight, Thermal Decomposition Temperatures, Glass Transition Temperatures, and Melting Points of [EnPy][NTf2] and [CmPy][NTf2] ILs

M/g·mol−1

[E1Py][NTf2] [E2Py][NTf2] [E3Py][NTf2] [C4Py][NTf2] [C7Py][NTf2] [C10Py][NTf2]

418.34 462.39 506.44 416.37 458.45 500.53

a

Td/K

Tg/K

618 604 602 655 649 644

198 202 207 197 202

Tm/K

292 272 274

a


Td is defined as the temperature at which the IL loses 5% of its initial mass while heating from room temperature to 873.15 K with a heating rate of 10 K min−1 under a N2 atmosphere.

3.2. Effect of Cation on CO2 Solubility. Comparing with nonfunctionalized ILs [CmPy][NTf2] with the same anion, the ether-functionalized ILs [E nPy][NTf2] show the lower viscosities, which will be favorable to enhance mass transfer in reactions and separations with biphasic IL/CO2 systems. Therefore, CO2 solubility in [EnPy][NTf2] and [CmPy][NTf2] in the pressure range of 2−20 bar at 313.15 K was measured in Figure 6, and the effect of ether groups on the cation on CO2

Figure 4. Variation in the densities of pyridinium-based ILs with temperature: ■, [E1Py][NTf2]; ●, [E2Py][NTf2]; ▲, [E3Py][NTf2]; □, [C4Py][NTf2]; ○, [C7Py][NTf2]; Δ, [C10Py][NTf2].

Figure 5. Variation in the viscosities of pyridinium-based ILs with temperature: ■, [E1Py][NTf2]; ●, [E2Py][NTf2]; ▲, [E3Py][NTf2]; □, [C4Py][NTf2]; ○, [C7Py][NTf2]; Δ, [C10Py][NTf2]. Figure 6. CO2 solubility in [CmPy][NTf2] and [EnPy][NTf2] at 313.15 K: ■, [E1Py][NTf2]; ●, [E2Py][NTf2]; ▲, [E3Py][NTf2]; □, [C4Py][NTf2]; ○, [C7Py][NTf2]; Δ, [C10Py][NTf2].

temperatures decreased slightly and were very close to that of [C4Py][NTf2]. However, the viscosities of [E3Py][NTf2] decreased dramatically about 42% compared with that of [C10Py][NTf2]. The reason may be ascribed to the high rotational flexibility of the alkoxy chain, which can offer the convenience for the adjacent molecules of transport.38,39 In addition, the thermal decomposition temperatures (Td), glass transition temperatures (Tg), and the melting points (Tm) of six pyridinium-based ILs were measured. As shown in Table 1, the thermal decomposition temperatures of [EnPy][NTf2] and [CmPy][NTf2] decrease slightly with the increase of the side chain length on the cation, and are all above 600 K, indicating that all of these ILs have excellent thermal stability. The midpoint temperature of the heat capacity change is regarded as the glass transition temperature,40 and all the investigated ILs except [C4Py][NTf2] (not detected under the measurement conditions) have glass transition temperatures around 200 K. The nonfunctionalized ILs [CmPy][NTf2] have the melting points in the range of 272−292 K, whereas no melting points were detected for the ether-functionalized ILs [EnPy][NTf2], which is consistent with the reported results.25

solubility was investigated. As shown in Figure 6, CO2 solubility in these six ILs follows the sequence: [C10Py][NTf2] > [E3Py][NTf2] > [C7Py][NTf2] > [E2Py][NTf2] > [C4Py][NTf2] > [E1Py][NTf2]. When the side chain length on the cation increased, CO2 solubility in [EnPy][NTf2] under the same conditions increased within a narrow range. The similar results are also observed for CO2 solubility in [CmPy][NTf2]. Moreover, for the ILs with the same side chain length on the cation, [EnPy][NTf2] has a slightly lower solubility of CO2 than [CmPy][NTf2], and the reduction of CO2 solubility in [EnPy][NTf2] becomes more obvious with the increasing number of ether oxygen atoms. The results clearly indicated that the presence of ether groups on the cation has a little negative influence on CO2 solubility in ILs. The reason may be that the introduction of polar ether groups decreased the interaction between nonpolar CO2 gas and ILs to some extent, therefore resulting in a small reduction of CO2 solubility.41,42 3.3. Effect of Temperature and Pressure on CO2 Solubility. CO2 solubility in [EnPy][NTf2] at temperatures D


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from 298.15 to 343.15 K and pressures from 2 to 20 bar is presented in Figures 7, 8, and 9, and the effect of temperature

and pressure was also studied. As expected, the temperature and the pressure play a significant role in CO2 dissolution in [EnPy][NTf2]. CO2 solubility in the ILs increases with the decreasing temperature and the increasing pressure, and the results show a good linear relation between the molar fraction of CO2 and the pressure under different temperatures. For example, the molar fraction of CO2 in [E3Py][NTf2] decreased continuously from 0.42 to 0.30 with an increase of temperature from 298.15 to 343.15 K at 20 bar, and increased greatly from 0.07 to 0.42 with the increasing pressure from 2 to 20 bar at 298.15 K. The results implied that it is favorable for CO2 absorption in ILs at lower temperatures and higher pressures and easier for CO2 desorption at higher temperatures and lower pressures. 3.4. Effect of H2O on CO2 Solubility. As we know, water vapor is always accompanied by some gas mixtures, like natural gas in the practical applications. Therefore, the effect of water contents on CO2 solubility in [EnPy][NTf2] at 333.15 K was also studied, and the results are shown in Figure 10. The three

Figure 7. CO2 solubility in [E1Py][NTf2] at various temperatures: ■, 298.15 K; ●, 313.15 K; ▲, 323.15 K; ▼, 333.15 K; ★, 343.15 K.

Figure 10. Effect of water on CO2 solubility in [EnPy][NTf2] at 333.15 K. Figure 8. CO2 solubility in [E2Py][NTf2] at various temperatures: ■, 298.15 K; ●, 313.15 K; ▲, 323.15 K; ▼, 333.15 K; ★, 343.15 K.

kinds of ether-functionalized ILs [EnPy][NTf2] have very low contents of saturated water due to their hydrophobicity, and saturated water contents of [E1Py][NTf2], [E2Py][NTf2], and [EnPy][NTf2] at 333.15 K are 0.794, 1.088, and 1.215% by mass ratio, respectively. Comparing with pure ILs, the ILs [EnPy][NTf2] with saturated water have the lower CO2 solubility, and the reduction of CO2 solubility in IL−water systems becomes more obvious with the increasing pressures. The results demonstrated that, although the content of saturated water in [EnPy][NTf2] is low, water plays a great role in CO2 solubility in [EnPy][NTf2], especially at higher pressures. The results are consistent with the work of Brennecke et al.43 3.5. Henry’s Law Constant and Gas Selectivity. CO2 and CH4 solubility and their Henry’s law constants in [EnPy][NTf2] and [CmPy][NTf2] at different temperatures are listed in Tables 2, 3, and 4, respectively. As shown in Table 4, Henry’s law constants of CO2 in the six ILs at 313.15 K varied from 44.0 to 52.8 bar, with the lowest for [C10Py][NTf2] and the highest for [E1Py][NTf2]. By contrast, Henry’s law constants of CH4 in these ILs at 313.15 K were of a magnitude higher than that of CO2, which clearly indicated that CO2 solubility in the investigated ILs is much higher than CH4

Figure 9. CO2 solubility in [E3Py][NTf2] at various temperatures: ■, 298.15 K; ●, 313.15 K; ▲, 323.15 K; ▼, 333.15 K; ★, 343.15 K. E


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Energy & Fuels Table 2. CO2 and CH4 Solubility in [EnPy][NTf2] at Various Temperatures [E1Py][NTf2]


P/bar

xCO2

2.46 5.06 8.02 11.05 13.99 17.06

0.0449 0.1110 0.1622 0.2216 0.2640 0.3030

2.09 5.07 7.94 11.01 14.06 17.10

0.0403 0.0919 0.1352 0.1852 0.2283 0.2661

2.13 5.06 8.04 11.04 14.08 17.14

0.0236 0.0765 0.1198 0.1610 0.2062 0.2371

P/bar

xCH4

14.93 19.39 23.38 27.61

0.0168 0.0216 0.0254 0.0297

15.87 20.88 23.90 28.06

0.0167 0.0218 0.0254 0.0284

16.16 21.68 28.03 29.00

0.0164 0.0220 0.0272 0.0285

[E2Py][NTf2] P/bar

xCO2

T = 313.15 K 2.09 0.0486 5.00 0.1207 8.06 0.1784 11.06 0.2405 14.11 0.2806 17.11 0.3260 T = 323.15 K 2.09 0.0442 4.99 0.0999 8.14 0.1513 11.02 0.2026 14.02 0.2467 17.08 0.2813 T = 333.15 K 2.03 0.0313 5.01 0.0881 7.99 0.1361 11.00 0.1875 14.05 0.2311 17.01 0.2680 P/bar

xCH4

T = 313.15 K 13.82 0.0166 20.07 0.0248 27.81 0.0319 30.01 0.0335 T = 323.15 K 21.37 0.0235 23.58 0.0273 26.01 0.0283 29.92 0.0310 T = 333.15 K 15.18 0.0177 19.97 0.0212 23.88 0.0257 27.84 0.0288

Table 3. CO2 and CH4 Solubility in [CmPy][NTf2] at Various Temperatures

[E3Py][NTf2] P/bar

[C4Py][NTf2] P/bar

xCO2

xCO2

1.99 5.02 7.84 10.83 14.23 16.83

0.0555 0.1295 0.1904 0.2507 0.3123 0.3542

2.49 5.24 8.03 11.29 13.89 17.16

0.0518 0.1172 0.1664 0.2309 0.2708 0.3132

2.04 5.00 8.17 10.97 13.94 16.93

0.0505 0.1134 0.1742 0.2361 0.2816 0.3317

2.57 4.68 8.90 10.95 14.73 16.92

0.0430 0.0930 0.1564 0.1928 0.2380 0.2689

2.00 5.02 8.05 11.07 14.07 17.01

0.0391 0.0918 0.1568 0.2058 0.2467 0.2913

1.96 6.23 8.93 12.00 15.88 17.19

0.0305 0.0968 0.1359 0.1752 0.2265 0.2463

P/bar

xCH4

P/bar

xCH4

16.90 20.41 24.29 28.36

0.0237 0.0269 0.0303 0.0325

14.86 19.91 24.41 28.98

0.0223 0.0284 0.0339 0.0385

15.45 20.05 23.91 27.81

0.0188 0.0237 0.0269 0.0319

15.97 20.36 23.88 28.09

0.0212 0.0270 0.0317 0.0364

14.87 19.46 23.48 27.25

0.0171 0.0220 0.0264 0.0303

14.92 19.86 24.54 28.05

0.0200 0.0245 0.0308 0.0347

[C7Py][NTf2] P/bar

[C10Py][NTf2]

xCO2

T = 313.15 K 2.11 0.0568 4.78 0.1176 7.85 0.1717 11.16 0.2455 14.60 0.2937 17.13 0.3364 T = 323.15 K 1.91 0.0422 6.74 0.1292 8.34 0.1648 11.24 0.2125 15.99 0.2838 17.60 0.3054 T = 333.15 K 2.01 0.0376 5.59 0.0967 8.58 0.1467 10.89 0.1906 15.34 0.2485 17.00 0.2691 P/bar

xCH4

T = 313.15 K 14.64 0.0287 20.53 0.0385 23.78 0.0450 27.65 0.0506 T = 323.15 K 14.01 0.0268 19.24 0.0357 23.07 0.0422 27.64 0.0493 T = 333.15 K 15.00 0.0246 20.38 0.0333 23.95 0.0399 28.09 0.0483

P/bar

xCO2

2.19 5.16 8.09 11.64 14.01 17.02

0.0611 0.1321 0.2052 0.2809 0.3391 0.3758

2.29 5.52 9.85 12.47 14.88 17.38

0.0492 0.1213 0.1958 0.2428 0.2942 0.3387

2.13 5.07 7.59 11.21 14.89 17.15

0.0427 0.0993 0.1465 0.2129 0.2586 0.2960

P/bar

xCH4

13.92 19.39 23.78 30.33

0.0330 0.0438 0.0544 0.0639

17.73 19.87 26.91 28.81

0.0394 0.0434 0.0545 0.0581

15.08 19.91 24.53 27.72

0.0328 0.0409 0.0492 0.0540

in [EnPy][NTf2] increases slightly with the increasing number of ether oxygen atoms. Among the investigated ILs, [E3Py][NTf2] exhibits the highest CO2/CH4 selectivity at a given temperature, with a maximum value of 17.2 at 313.15 K. Meanwhile, the effect of temperature on CO2/CH4 selectivity was also studied. It was found that CO2/CH4 selectivity in all investigated ILs greatly decreases with the increase of the temperature, which shows the similar trend to the reported results.21 The reason is mainly attributed to the fact that the temperature has a more significant effect on CO2 solubility than CH4 solubility in these ILs. For instance, CO2 solubility decreased obviously about 27% on average from 313.15 to 333.15 K, but only 8.6% reduction for the CH4 solubility at the same temperature range. 3.6. Thermodynamic Properties of CO2 and CH4 Dissolution in ILs. In order to further understand the dissolution behaviors of CO2 and CH4 in [CmPy][NTf2] and [EnPy][NTf2], the thermodynamic properties of CO2 and CH4 in these six ILs were obtained and are summarized in Table 6.

solubility. Meanwhile, compared with the nonfunctionalized ILs [CmPy][NTf2], Henry’s law constants of CO2 in etherfunctionalized ILs [EnPy][NTf2] only mildly increased, while Henry’s law constants of CH4 greatly increased under the same temperature. For example, Henry’s law constants of CO2 and CH4 in [C10Py][NTf2] at 313.15 K were 44.0 and 467.4 bar, respectively, but Henry’s law constants of CO2 and CH4 in [E3Py][NTf2] at 313.15 K were 46.2 and 835.0 bar, respectively. The results demonstrated that the introduction of ether groups on the cation of ILs has no substantial influence on CO2 solubility, whereas it contributes to a more appreciable reduction in CH4 solubility, which implied that the etherfunctionalized ILs [EnPy][NTf2] are more effective for selective separation of CO2. Subsequently, CO2/CH4 selectivity in the six ILs under different temperatures was calculated and is presented in Table 5. Clearly, the ether-functionalized ILs [EnPy][NTf2] exhibit the higher CO2/CH4 selectivity than the corresponding nonfunctionalized ILs [CmPy][NTf2], and CO2/CH4 selectivity F


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Energy & Fuels Table 4. Henry’s Law Constants of CO2 and CH4 in ILs at Different Temperatures HCO2/bar

HCH4/bar

ILs

313.15 K

323.15 K

333.15 K

313.15 K

323.15 K

333.15 K


52.8 49.1 46.2 50.8 47.2 44.0

60.6 54.7 50.5 58.1 53.2 48.3

67.3 60.5 56.7 65.9 58.6 54.2

891.2 844.8 794.5 704.0 520.5 442.2

924.5 866.3 835.0 732.5 535.2 467.4

948.7 910.5 863.7 768.3 570.4 486.1

values, which means that the interaction between CH4 and ILs may dominate in CH4 solubility. Comparing with [CmPy][NTf2], the absorption enthalpy values of CH4 in [EnPy][NTf2] containing polar ether groups are slightly smaller, therefore resulting in the weaker CH4−IL interaction and lower CH4 solubility. Similarly, as the side chain length on the cation of ILs increases, the ILs’ nonpolarity generally increases and the absorption enthalpy values of CH4 become greater, which means that the higher CH4 solubility is mainly abscribed to nonpolar−nonpolar interactions. 3.7. Recycling of Ionic Liquids. In order to examine the recyclability of the ether-functionalized ILs [EnPy][NTf2] for CO2 absorption, [E3Py][NTf2] with the higher CO2/CH4 selectivity was selected to study five consecutive cycles of CO2 absorption and desorption. CO2 absorption in [E3Py][NTf2] was conducted at 313.15 K and 2 bar, and CO2 desorption was at 343.15 K under a vacuum of about 0.1 kPa for 40 min. As shown in Figure 11, it was observed that

Table 5. CO2/CH4 Selectivity in [EnPy][NTf2] and [CmPy][NTf2] at Different Temperatures


CO2/CH4 selectivity ILs

313.15 K

323.15 K

333.15 K


16.9 17.1 17.2 13.8 11.0 10.1

15.3 15.8 16.5 12.6 10.1 9.7

14.1 15.0 15.2 11.7 9.7 8.9

As shown in Table 6, the enthalpy values of CO2 and CH4 absorption are all negative, indicating that the dissolution process of CO2 and CH4 is exothermic. As we know, the absorption enthalpy is an important parameter to estimate the interaction between gas and liquid solvent. Compared with those of CH4, the higher absolute enthalpy values of CO2 dissolution in [CmPy][NTf2] and [EnPy][NTf2] imply the stronger interaction between CO2 and the ILs, which is consistent with the higher solubility of CO2. For CO 2 dissolution in ILs, when CH2 groups of the side chain on the cation of [CmPy][NTf2] were partly replaced by ether oxygen atoms, the lower absorption enthalpy values of CO2 in [EnPy][NTf2] were obtained, suggesting the weaker interaction between [EnPy][NTf2] and CO2 and easier desorption of CO2 from ILs. However, the trend of CO2 solubility in six ILs cannot show good agreement with the gas−IL interactions. For example, CO2 solubility in [E3Py][NTf2] is greater than that in [E1Py][NTf2], but the absorption enthalpy of CO2 in [E1Py][NTf2] is higher. The results indicated that, as the cations of ILs are very similar in structures, but different in the side chain length, CO2 solubility in these ILs cannot be solely explained by the gas−IL interactions, and in fact, the free volume of ILs plays a more important role in CO2 solubility in this case. That means the increase of the side chain length on the cation can result in the greater free volume, which can increase CO2 solubility in ILs.26 Unlike CO2 dissolution in ILs, the trend of CH4 solubility in the six ILs is perfectly consistent with the absorption enthalpy

Figure 11. CO2 absorption and desorption in [E3Py][NTf2]. CO2 absorption: 313.15 K and 2 bar, and CO2 desorption: 343.15 K and 0.1 kPa for 40 min.

Table 6. Thermodynamic Properties of CO2 and CH4 in [EnPy][NTf2] and [CmPy][NTf2] at 313.15 K CO2

CH4

ILs

ΔsolG (kJ mol−1)

ΔsolH (kJ mol−1)

ΔsolS (J mol−1 K−1)

ΔsolG (kJ mol−1)

ΔsolH (kJ mol−1)

ΔsolS (J mol−1 K−1)


10.29 10.10 9.94 10.19 10.00 9.82

−9.97 −8.84 −7.57 −10.13 −9.02 −7.96

−64.71 −60.50 −55.94 −64.90 −60.73 −56.77

17.65 17.51 17.35 17.04 16.25 15.83

−2.72 −3.26 −3.63 −3.79 −3.95 −4.11

−65.04 −66.32 −66.99 −66.49 −64.52 −63.66

G


Article

Energy & Fuels


Table 7. Comparison of Henry’s Law Constants of CO2 and CH4 and CO2/CH4 Selectivity in Different ILs ILs

T/K

HCO2/bar

HCH4/bar

CO2/CH4 selectivity

references

[E1Py][NTf2] [E2Py][NTf2] [E3Py][NTf2] [C4Py][NTf2] [C7Py][NTf2] [C10Py][NTf2] [C3CNmim][NTf2] [C5CNmim][NTf2] [HmPy][NTf2] [Emim][NTf2] [Bmim][NTf2] [Hmim][NTf2] [Tes][NTf2] [bmpip][NTf2] [Toa][NTf2] [Cprop][NTf2]

313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15

52.8 49.1 46.2 50.8 47.2 44.0 51 44 46.2 48 41 43 5.03 5.01 2.28 4.26

891.2 844.8 794.5 704.0 520.5 442.2 730 540 500 560 420 352 52.34 43.93 15.84 29.03

16.9 17.1 17.2 13.8 11.0 10.1 14 12 10.8 11.7 10.2 8.2 10.4 8.8 7.0 6.8

this work this work this work this work this work this work 19 19 44, 45 17 19 17 22 22 22 22

dissolution behaviors were further illustrated. The results demonstrated that the gas−IL interaction plays a dominate role in CH4 solubility in the investigated ILs, but CO2 dissolution in ILs is determined by both the IL−gas interaction and the free volume of ILs.

[E3Py][NTf2] can be repeatedly used, and CO2 solubility in [E3Py][NTf2] keeps almost no change after five absorption− desorption cycles, indicating that this process is highly reversible. 3.8. Comparison with Other Ionic Liquids. As we know, both gas solubility and selectivity of an absorbent play an important role in selective separation of one component from others. Therefore, we compared them with other kinds of ILs based on the same anion. Some relevant data on the Henry’s law constants of CO2 and CH4 and CO2/CH4 selectivity in ILs are listed in Table 7. As seen in Table 7, it was found that, although they have relatively low solubility of CO2 and CH4, all the ether-functionalized ILs [EnPy][NTf2] show the much higher CO2/CH4 selectivity than other kinds of ILs with the same anion [NTf2]. Moreover, the high CO2/CH4 selectivity in [EnPy][NTf2] is mainly attributed to the extremely low solubility of CH4 due to the presence of ether groups, which will offer a new strategy into designing more effective ILs for selective separation of CO2 from CH4.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b01274. NMR data, FTIR spectra, TGA results, density, and viscosity of the investigated ILs at different temperatures (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel/Fax: +86-010-82544875 (X.Z.). *E-mail: [email protected]. Tel/Fax: +86-010-82627080 (S.Z.).

4. CONCLUSIONS In this work, a series of ether-functionalized pyridinium-based ILs [EnPy][NTf2] and the corresponding nonfunctionalized analogues [CmPy][NTf2] for comparison were synthesized, and the effect of ether groups on physicochemical properties (e.g., density, viscosity, thermal decomposition temperatures, and glass transition temperatures), as well as CO2 and CH4 solubilty and CO2/CH4 selectivity under different temperatures, was systematically investigated. The results indicated that the introduction of ether groups has an important impact on physicochemical properties, gas solubility, and selectivity of ILs. Compared with that of [CmPy][NTf2], the viscosities of [EnPy][NTf2] are lower and obviously decrease with the increasing number of ether oxygen atoms. The incorporation of ether groups on the cation has no substantial influence on CO2 solubility, whereas it contributes to a more appreciable reduction in CH4 solubility, resulting in the higher CO2/CH4 selectivity using [EnPy][NTf2]. Moreover, CO2/CH4 selectivity in all investigated ILs greatly decreases with the increasing temperature due to weaker dependence of CH4 solubility on temperature. In addition, the thermodynamic properties including the Gibbs free energy, enthalpy, and entropy of CO2 and CH4 in these ILs were also calculated, and gas

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the program of Beijing Municipal Natural Science Foundation (Nos. 2141003 and 2142029), the National Natural Science Fund for Distinguished Young Scholars (No. 21425625), the National Natural Science Foundation of China (No. 21436010), and the Science and Technology Innovation Team of Cross and Cooperation of Chinese Academy of Sciences. The authors also thank Prof. Jianling Zhang and Prof. Zhaofu Zhang from the Institute of Chemistry of Chinese Academy of Sciences for valuable discussion and suggestions.

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Highly Selective Capture of CO2 by Ether-Functionalized Pyridinium

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