Article pubs.acs.org/EF
CO2 and CH4 Sorption by Solid-State [P4 4 4 4][NTf2] Ionic Liquid Based on Quartz Crystal Microbalance Experiments under Different Pressures Lanyun Wang,†,‡,§ Yongliang Xu,*,†,‡,§ Shaokun Wang,† and Yanan Wei† †
School of Safety Science and Engineering and ‡State Key Laboratory Cultivation Base for Gas Geology and Gas Control, Henan Polytechnic University, Jiaozuo, Henan 454003, People’s Republic of China § Collaborative Innovation Center of Coal Safety Production of Henan Province, Jiaozuo, Henan 454003, People’s Republic of China S Supporting Information *
ABSTRACT: Using a quartz crystal microbalance (QCM) measurement system, CO2 and CH4 capacity trapped by a solid-state ionic liquid, tetrabutylphosphonium bis(trifluoromethanesulfonyl)imide [P4 4 4 4][NTf2], which was coated on the surface of a quartz crystal under pressures from 0.1 to 3 MPa at 30 °C, was calculated on the basis of the Sauerbrey equation. The frequency data under the pressure of 4 MPa are not applicable for CO2 measurements, because the ionic liquid film was destroyed and the frequency signal was perturbed possibly as a result of the reduced evenness of the [P4 4 4 4][NTf2] film after CO2 absorption. Besides, it is shown that both the gas sorption and CO2/CH4 selectivity, especially the mass-ratio-based selectivity, increase with pressure in the case of this solid-state ionic liquid film. During the ambient pressure, CO2/CH4 selectivity in [P4 4 4 4][NTf2] was lower than those in some reported fluid room-temperature ionic liquids. The advantage of solid-state ionic liquids is an improved selectivity when the pressure increases, which is distinct from the results by fluid ionic liquids, for which the gas selectivity usually only slightly fluctuates with the pressure.
1. INTRODUCTION For the same volume, methane (CH4) has 64 times more potential than carbon dioxide (CO2) as a heat-trapping gas. The amount of CO2 and CH4 gases is released from coal mining activities, and abundant flue gases are emitted during coal combustion and petroleum industry. Consequently, the capture of greenhouse gases is urgently required in a consideration of retarding the exacerbation of global warming. Besides, it is also worth retrieving CO2 and CH4 because both of them are significant industrial stock feeds. For example, CO2 is usually used in refrigeration, photosynthesis, and oil recovery,1,2 and CH4 is a kind of pivotal clean fuel as well as an important precursor for syngas production.3 In addition, removal of CO2 from CH4 is another significant problem needing to be resolved because CO2 in natural gas causes corrosion and crystallization problems.4 Aqueous amines, e.g., methyldiethanolamine and diethanolamine, are relatively efficient chemical solvents for CO2 capture and separation from other unreactive gases, but the disadvantages of amine loss and vapor contamination after absorption enable them to become more cost-consuming and uneconomical.5 As potential alternatives, ionic liquids (ILs) have obtained burgeoning attention in gas solubility and separation as a result of their negligible volatility, thermal stability, non-corrosivity, and non-odor originating from their specific structural composition.6 Generally, ILs with high polarity, which have great affinity to some basic or acidic gases, such as SO2, H2S, NH3, and CO2, are avoided by unreactive or nonpolar gases, e.g., CH4, N2, CO, O2, and noble gases. It is clearly stated that polar ILs, e.g., [2mHEA][Prop], [C 4 C 1 im][CH 3 SO 3 ], [C4C1im][CH3SO4], and [C1C1im][CH3SO4], exhibit high © 2017 American Chemical Society
selective absorption with CO2/CH4 selectivities of 91.4 (20 °C), 46.7 (80 °C), 35 (30 °C), and 67.3 (30 °C), respectively.7 Besides, low-cost phosphonium ILs with unsymmetric structures, e.g., [P4 4 4 1][NTf2] and [P4 4 4 1][MeSO4], are competitive because of their great CO2 capture ability in their fluid bulk.8−10 However, absorption/desorption in these room-temperature ILs requires high energy consumption and long-time diffusion in liquid bulk.11 To conquer the disadvantages of absorption, solid-state ionic liquids (SoILs) here are considered as alternatives because it was reported that SoILs could greatly reduce diffusion difficulties and accelerate desorbing speed.12 Besides, these solid salts could be more stable when being applied in separation membranes. Therefore, fundamental works about gas sorption by these crystallized molten salts should be carried out for further application of ILs in gas capture and separation. Considering energy consumption in CO2 capture, a highpressure pre-combustion process is generally more prominent than a low-pressure post-combustion process. During carbon capture of natural gas pre-combustion, highly concentrated CO2 is produced with pressure over 4 MPa accompanied by H2 and uncombusted CH4, leading to a high-pressure gas mixture. From another perspective, a high-pressure measurement also provides useful information for gas separation during an efficient and low-cost pressure swing adsorption process using ILs. Received: November 22, 2016 Revised: February 13, 2017 Published: February 16, 2017 4179
DOI: 10.1021/acs.energyfuels.6b03104 Energy Fuels 2017, 31, 4179−4185
Article
Energy & Fuels
Figure 1. Experimental system of gas sorption in ILs by a QCM. According to refs 15 and 16, frequency shifts should be a sum of influences from additional mass, pressure and viscosity of gas, and roughness of the quartz crystal, i.e., Δf = Δf m + Δf P + Δfη + Δf r. The last two factors could be neglected for a polished quartz crystal, with a surface roughness of 3 nm root mean square (rms) in this work. Although numerous phosphonium-based ILs have been produced, even in ton scale, these groups of ILs have been more or less unpopular in the literature relative to their imidazolium or pyridinium counterparts. However, their enhanced thermal and chemical stabilities compared to pyridinium- and imidazolium-based ILs, unique miscible and solvating performance, and relatively simple preparation routes advance their promising application in a specific industry.17 [P4 4 4 4][NTf2] [molecular weight (MW) = 539.58 g mol−1] with a symmetric cation was chosen as a solid sorbent, the structure of which is illustrated in Figure 2. [P4 4 4 4][NTf2] IL with a purity of over 99%
Therefore, it is essential and applicable to study the sorption behaviors under great pressures. In this work, in combination of a high-pressure reactor, a quartz crystal microbalance (QCM) will be adopted to measure the gas sorption ability of a solid-state [P4 4 4 4][NTf2] IL from 0.1 to 4 MPa at a temperature of 30 °C. The QCM is able to detect the change in frequency of a quartz crystal resonator, monitoring small mass changes on an electrode with an accuracy of nanograms. Mineo et al.13 and Baltus et al.14 used the QCM to measure CO2 capacity in poly(IL)s and imidazolium ILs, stating that the QCM could display a fast and linear response as well as great data reproducibility. The goal of this work is to give some insight into the fascinating and fastsorption performance of gas capture and ideal separation by SoILs, especially the sorption differences from that by fluid room-temperature ILs.
2. EXPERIMENTAL SECTION 2.1. Main Instruments and Materials. A GHXF high-pressure reactor, provided by Xi’an Taikang Biological Technology Co., Ltd., is used to provide a high-pressure experimental condition for gas sorption. A fixed temperature is controlled by a BT-224S electricheated thermostat water bath from room temperature to 90 °C with an accuracy of ±0.5 °C. A CHI-400C QCM with an accuracy of ±0.001 Hz, a product of Shanghai Chenhua Instruments Co., Ltd., is a critical measuring facility for gas sorption in this paper. As depicted in Figure 1, a quartz crystal is displaced in the highpressure reactor, which is immersed into the thermostat water bath. The resonance of a QCM is disturbed by micro mass changes on the surface of a resonator, shown by frequency changes of a quartz crystal. According to the Sauerbrey equation in eq 1, the sorbed mass (Δm) of CO2 and CH4 by [P4 4 4 4][NTf2] film could be calculated from the recorded oscillation frequency change (Δf) of quartz crystals before and after being coated by [P4 4 4 4][NTf2] under different CO2 or CH4 pressures Δf = f − f0 =
− 2f0 2 A μq ρq
Figure 2. Chemical structure of [P4 4 4 4][NTf2] IL. was purchased from Lanzhou Zhongke Kate Industry & Trade Co., Ltd., China, and its differential scanning calorimetry (DSC) measurement result is shown in Figure S1 of the Supporting Information, presenting a melting point of 86.6 °C.18 In comparison to its ammonium analogue, the phosphonium IL has a weaker coulomb force between ions, leading to a lower melting point (melting point of one counterpart, [N4 4 4 4][NTf2], is reported to be 365−369 K19,20) and less viscous property,21 which provides a looser structure for the polarinduced polar interaction with stronger electrostatic interaction between CO2 and the anion [NTf2]−. Therefore, it is expected that [P4 4 4 4][NTf2] could give better gas capture results in comparison to its ammonium analogues. 2.2. Experimental Preparation. To load a very thin IL film on a quartz crystal, a dilute solution (10 mg/mL) of the IL was prepared by dissolving IL (50 mg) into dichloromethane (5 mL) with an analytical purity. A clean crystal (without coated IL) was carefully immersed vertically in the IL−dichloromethane solution for 5 min to guarantee a completely wetted crystal surface. Two IL-coated crystals were prepared by this dip-coating technique and were placed into a vacuum (0.01 MPa) oven at 40 °C for 4 h to evaporate dichloromethane to obtain IL-coated quartz crystals.
Δm (1)
where f and f 0 are the measured frequency and fundamental frequency of a quartz crystal, respectively, μq = 2.947 × 1011 g cm−1 s−2 is the shear modulus, and ρq = 2.648 g cm−3 is the density of quartz. A is the area of the gold electrode coated on the crystal, with values of 0.196 cm2 for two sides. It is observed that the frequency shift (Δf) has a linear relationship with the mass change (Δm) on the QCM crystal surface. For a new quartz crystal used in this study (AT-cut; f 0 = 7.995 MHz), a frequency change of 1 Hz corresponds to a mass change of around 1.357 ng on both sides of crystal electrodes. 4180
DOI: 10.1021/acs.energyfuels.6b03104 Energy Fuels 2017, 31, 4179−4185
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Energy & Fuels Respectively, two clean crystals in the higher pressure reactor were purged with dry CH4 and CO2, and then frequency changes under pressures of 0.1, 1, 2, 3, and 4 MPa were monitored. Then, gas pressure was gradually relieved for desorbing the trapped gas from the IL. The same procedure was repeated in the cases of IL-coated crystals. Once a constant value at each pressure stage was achieved, which is an indication of sorption equilibrium, frequencies of crystals were recorded with time.
3. RESULTS According to eq 1, the frequency change of the IL-coated quartz (Δfg) subtracted by that of the clean quartz crystal (Δfc) at the same gas atmosphere gives rise to a linearship with the mass amount of gas caught by IL, and hence, the sorbed gas mass (Δmg) is expressed in eq 2. Δmg = −
=
AΔfg 2.26 × 106f0 2
⎞ ⎛ AΔfc ⎟ − ⎜⎜ − 6 2⎟ ⎝ 2.26 × 10 f0 ⎠
Figure 3. Frequency shifts of quartz crystals before and after being coated by [P4 4 4 4][NTf2] with increasing CO2 pressures.
A(Δfc − Δfg ) 2.26 × 106f0 2
(2)
Then, the mole fraction of gases, xg, in the IL is arranged as eq 3 describes14 xg =
(Δfc − Δfg )/Mg ΔfIL /MIL + (Δfc − Δfg )/Mg
(3) −1
where MIL is the molecular weight of ILs (g mol ), Mg denotes the molecular weight of gas (g mol−1), and Δf IL represents frequency shifts of crystals before and after coating ILs measured in vacuum (Hz). Assuming the fundamental frequency of a clean quartz crystal resonator to be f 0 and that of an IL-coated resonator to be f 0IL, the frequency change caused by coating an IL is Δf IL = f 0IL − f 0, which is responsible for the mass of the IL film (ΔmIL) according to eq 1. The frequencies of crystals used for measuring CO2 and CH4 sorption are listed in Table 1.
Figure 4. Frequency shifts of quartz crystals before and after being coated by [P4 4 4 4][NTf2] with increasing CH4 pressures.
There are studies stating that the tetraalkylphosphonium IL with bistriflamide anion only physically absorbs CO2 without chemical interaction occurring,10,22 and it is expected that a good reversibility would appear. Actually, CO2 sorption under 4 MPa and desorption were conducted during our experiments. Beyond our expectation, a dramatic increase of the frequency shift under 4 MPa pressure appeared during the CO2 sorption experiment, which is demonstrated in Figure 5. Figure 6 exhibits the appearances of IL films before and after CO2 sorption, and it is clearly observed that the crystallized IL film was destroyed after CO2 sorption under 4 MPa, with a white and fluidized (seems like foam) substance instead of the previous immobilized film. It is stated that CO2 physisorption will decrease the viscosity of ILs possibly as a result of a destruction of cation−anion hydrogen bonding,23 which could explain this undesired phenomenon. The curve after 4 MPa in Figure 5 is not appropriate and applicable because the QCM only resonates properly for evenly coated films. Figure 7 presents an excellent sorption/desorption process of the CH4−[P4 4 4 4]NTf2] binary system, indicating a very weak physical interaction between CH4 and [P4 4 4 4][NTf2] with a nearly complete desorption performance.
Table 1. Frequencies of Crystals before and after Coating IL and the Weights of IL Films for CO2 and CH4 Sorption gas
f 0 (Hz)
f 0IL (Hz)
Δf IL (Hz)
ΔmIL (ng)
CO2 CH4
7986979 7986929
7982856 7982508
4123 4421
5594.911 5999.297
A plot of frequency shifts for a clean quartz film as a function of gas pressures is shown as a red line in Figure 3. Operating in the same way, the dependence of the frequency change response upon increasing CO2 pressures for the same crystal coated with [P4 4 4 4][NTf2] is presented as a blue line, presenting a negative frequency shift with the pressure. The same procedures were conducted for CH4 sorption results by the clean and IL-coated crystals, and the frequency change responses subject to different CH4 pressures are depicted in Figure 4. To minimize the interference of the frequency fluctuation on the mass calculation, an average frequency change at each constant pressure stage calculated by the median filter algorithm was obtained, and these frequency changes (including uncertainty u) with pressure were included in Table 2. In combination of the weights of IL films contained in Table 1, CO2 and CH4 capacities trapped by the [P4 4 4 4][NTf2] film in forms of mass ratio and mole ratio are included in Table 3.
4. DISCUSSION What is worth noting is that the sorption behavior includes CO2 and CH4 adsorption on the surface of the film as well as 4181
DOI: 10.1021/acs.energyfuels.6b03104 Energy Fuels 2017, 31, 4179−4185
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Table 2. Average Frequencies of the Clean and [P4 4 4 4][NTf2]-Coated Crystals under Different CO2 and CH4 Pressures CO2
CH4
P (MPa)
Δfg (Hz)
u(Δfg)
Δfc (Hz)
u(Δfc)
Δfg (Hz)
u(Δfg)
Δfc (Hz)
u(Δfc)
0.1 1 2 3
−0.933 −6.354 −8.336 −21.343
0.2396 1.0982 0.2562 2.2383
−0.346 30.069 72.509 120.331
0.3473 0.3307 0.8150 0.3317
−1.105 38.301 102.233 162.235
0.6115 0.3798 0.4966 0.7176
−0.569 50.486 116.220 178.069
0.2045 1.0979 1.9474 0.8628
Table 3. Solubility of CO2 and CH4 and Ideal Selectivity of CO2/CH4 in [P4 4 4 4][NTf2] under Different Pressuresa CO2
CH4
selectivity
P (MPa)
X
u(X) (×10−3)
Xm
u(Xm) (×10−4)
X
u(X) (×10−2)
Xm
u(Xm) (×10−4)
Sa
Sb
0.1 1 2 3
0.0017 0.1083 0.2405 0.4214
1.7456 4.2500 3.1861 7.6440
0.0001 0.0088 0.0196 0.0344
1.4235 3.4657 2.5981 6.2333
0.0040 0.0925 0.1061 0.1202
0.6224 1.1272 1.8643 1.2055
0.0001 0.0027 0.0031 0.0036
1.8457 3.3424 5.5281 3.5747
0.43 1.17 2.27 3.51
1 3.26 6.32 9.56
X is the gas capacity in the form of the mole ratio (mol mol−1), and Xm is the gas capacity in the form of the mass ratio (g g−1). Sa is the ratio of CO2 and CH4 capacity in the form of the mole ratio, and Sb is the ratio of CO2 and CH4 capacity in a form of the mass ratio. a
Figure 5. Frequency shifts of CO2 sorption by the [P4 4 4 4][NTf2] film during pressure increasing and decaying processes (0.1−4 and 0.1 MPa) at 30 °C.
Figure 7. Frequency shifts of CH4 sorption on the [P4 4 4 4][NTf2] film during pressure increasing and decaying processes (0.1−4−0.1 MPa) at 30 °C.
through van der Waals dispersion forces.11,25 These reasons may provide some explanations for the unusual results that the solid [P4 4 4 4][NTf2] film only sorbed 0.0017 mol of CO2 mol−1 of IL, a little less than CH4 capacity (0.004 mol mol−1) under ambient pressure. With a pressure increase, gases penetrated through the gas− IL interface and diffused in ILs and the gas capacity primarily depended upon the gas−IL affinity and their diffusion rate within ILs, although this process was more difficult than that in the fluid ILs because of the more compact structure of solidstate [P4 4 4 4][NTf2]. As reported, [NTf2]− has a strong interaction with CO2 through van der Waals and electrostatic interactions rather than CH4, which has no partial charge, leading to no electrostatic forces with ILs.25 Furthermore, the slightly smaller kinetic diameter of CO2 (0.33 μm) than that of CH4 (0.38 μm) enabled CO2 to diffuse faster through the IL film.26 That is why an increasing capacity gap between CO2 and CH4 was detected when the pressure increased. However, in comparison to those fluid ILs, CO2 capacity in this SoIL is evidently less, as shown in Figure 8 (detailed data are listed in Table S1 of the Supporting Information) and Figure 9, which dominantly originates from the limited cavities from the relatively compact SoIL film.
Figure 6. Appearances of [P4 4 4 4][NTf2] films before and after CO2 sorption at 30 °C under 4 MPa.
gas permeation into the thin film, which is related to the size of gas and affinity between gas and IL. When CO2 competed with CH4 for occupying the surface of IL under a low pressure, more CH4 molecules were adsorbed possibly because of the smaller van der Waals diameter of CH4 (0.436 nm) than CO2 (0.512 nm).24 Besides, as a result of the greater affinity with the anion [NTf2]−, a dense CO2 layer on the gas−IL interface would be formed, leading to a reduced CO2 diffusion rate through this interface, and, hence, make an obstacle for other CO2 molecule occupations. In contrast, CH4 could stay on the interface flexibly and be attracted by the apolar large alkyl chains on the phosphonium cation, orienting to the interface 4182
DOI: 10.1021/acs.energyfuels.6b03104 Energy Fuels 2017, 31, 4179−4185
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Figure 10. CO2/CH4 selectivities by [P4 4 4 4][NTf2] (this work) and [P6 6 6 14][N(CN)2] and [P6 6 6 14][phos].36,37 Figure 8. CO2 sorption in [P4 4 4 4][NTf2] (this work) compared to other room-temperature ILs at ca. 30 °C.22,27−34
Clearly, ILs with smaller molecular weights have an excellent selectivity based on the mass solubility ratio, and a high pressure improves CO2/CH4 selectivity possibly as a result of both gradually increasingly adsorbed CO2 on the interface and fast diffusion of CO2 in the IL film, although only a small amount got through it. Generally, gas sorption behavior related to this solid IL is greatly different from the absorption in the fluid roomtemperature ILs. Gas captured by SoILs mainly depends upon adsorption on the ion surface, especially under low pressure. When the pressure increases, a small amount of gas would be pushed into the limited void spaces of crystallized ILs. From ambient pressure to medium pressure, during which few gas molecules penetrate into the IL film, adsorption on the gas−IL interface dominates the capture process, which is dependent upon the size of molecules. Under higher pressures, with gas permeation, the gas−IL interaction and gas diffusion rate gradually control the sorption process. A smaller kinetic diameter of CO2 makes it diffuse quicker than CH4 in IL, leading to a larger CO2 capacity and increased ideal CO2/CH4 selectivity. From the sorption performance of solid-state [P4 4 4 4][NTf2] for CH4 and CO2, it takes a very short time to reach equilibrium compared to the case in large-scale and long-time liquid bulk. Another important reason why solid ILs could be alternatives in the application of the IL membrane for gas separation is their small molecular weights (one solid molten salt usually has a cation with short alkyl chains and a small anion),38 which is more efficient in the concept of mass capacity.
Figure 9. CO2 sorption in [P4 4 4 4][NTf2] (this work) compared to two phosphonium ILs.10,22
In comparison to the newly reported gas capture by some SoILs by Dowson et al.,12 CO2 sorption by several solid-state ammonium ILs, such as [N3 3 3 3][C1CO2] (1.12 g g−1), [N3 3 3 3][C1CO2] (1.4 g g−1), [N6 6 6 6][C1CO2] (1.78 g g−1), [N1 8 8 8][C1CO2] (2.89 g g−1), [N2 2 2 2]Br (0.59 g g−1), and [N4 4 4 4]Br (0.78 g g−1), CO2 capacity in [P4 4 4 4][NTf2] (0.88 g g−1) under 10 bar is better than [N4 4 4 4]Br and [N2 2 2 2]Br but weaker than those [C1CO2]−-based counterparts as a result of a probable chemical reaction of the [C1CO2]− anion with CO2 and a weaker cation−anion interaction, providing more void volume for CO2 molecules.35 Figure 10 illustrates the ideal selectivities of some reported fluid tetraalkylphosphonium ILs, e.g., trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate [P6 6 6 14][phos] and trihexyltetradecylphosphonium dicyanamide [P6 6 6 14][N(CN)2] (the detailed data are presented in Table S2 of the Supporting Information). It is evidently observed that [P4 4 4 4][NTf2] has an inferior mole-ratio-based selectivity compared to [P6 6 6 14][phos] and [P6 6 6 14][N(CN)2] corresponding to pressures. Greater resistance for gas permeation into this SoIL may be responsible for that, and more external promoter, such as pressure, is needed to increase the gas solubility and selectivity. Nevertheless, [P6 6 6 14][phos] and [P6 6 6 14][N(CN)2] present only a slight selectivity variance with pressure, while [P4 4 4 4][NTf2] created a great linearity relationship between selectivity and pressure, approaching a mass ratio selectivity of up to 9.7 under 3 MPa.
5. CONCLUSION CO2 and CH4 capacities in solid [P4 4 4 4][NTf2] IL under different CO2 and CH4 pressures from 0.1 to 4 MPa at 30 °C were measured by a QCM experimental system. It is found that a high pressure is favorable for enhancing gas sorption and CO2/CH4 selectivity. In comparison to those fluid ILs with large molecular weights, [P4 4 4 4][NTf2] presented linearly increased CO2/CH4 selectivity with pressures possibly as a result of the more CO2 adsorption on the gas−IL interface, assisted by a faster diffusion rate of CO2 in IL, although only a small amount of gases could be pushed into the compact structure. In comparison to the reported solid-state ammonium ILs with acetic anion, [P4 4 4 4][NTf2] does not possess advantages in CO2 capacity, and therefore, more delicately designed ILs could be used as sorbents. It is promising to use these solid molten salts because they usually have high thermal stability and compact structures, and thus, it is possible to reach 4183
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(11) Perez-Blanco, M. E.; Maginn, E. J. Molecular dynamics simulations of CO2 at an ionic liquid interface: Adsorption, ordering, and interfacial crossing. J. Phys. Chem. B 2010, 114 (36), 11827− 11837. (12) Dowson, G. R.; Reed, D. G.; Bellas, J. M.; Charalambous, C.; Styring, P. Fast and selective separation of carbon dioxide from dilute streams by pressure swing adsorption using solid ionic liquids. Faraday Discuss. 2016, 192, 511−527. (13) Mineo, P. G.; Livoti, L.; Lo Schiavo, S.; Cardiano, P. Fast and reversible CO2 quartz crystal microbalance response of vinylimidazolium-based poly(ionic liquid)s. Polym. Adv. Technol. 2012, 23 (11), 1511−1519. (14) Baltus, R. E.; Culbertson, B. H.; Dai, S.; Luo, H.; DePaoli, D. W. Low-Pressure Solubility of Carbon Dioxide in Room-Temperature Ionic Liquids Measured with a Quartz Crystal Microbalance. J. Phys. Chem. B 2004, 108 (2), 721−727. (15) Tsionsky, V.; Daikhin, L.; Urbakh, M.; Gileadi, E. Behavior of Quartz Crystal Microbalance in Nonadsorbed Gases at High Pressures. Langmuir 1995, 11 (2), 674−678. (16) Hu, A.; Shi, J.; Rong, Z.; Xue, P.; Gong, F.; Cheng, S. Adsorption behavior of collagen on the spin-coated PLA surface. Gaofenzi Xuebao 2010, 00 (11), 1262−1268. (17) Stojanovic, A.; Morgenbesser, C.; Kogelnig, D.; Krachler, R.; Keppler, B. K. Quaternary Ammonium and Phosphonium Ionic Liquids in Chemical and Environmental Engineering. In Ionic Liquids: Theory, Properties, New Approaches; Kokorin, A., Ed.; InTech: Croatia, 2011; Chapter 26, DOI: 10.5772/14340. (18) Kohno, Y.; Ohno, H. ChemInform Abstract: Ionic Liquid/ Water Mixtures: From Hostility to Conciliation. Chem. Commun. 2012, 48 (57), 7119−7130. (19) Sun, J.; Forsyth, M.; MacFarlane, D. R. Room-Temperature Molten Salts Based on the Quaternary Ammonium Ion. J. Phys. Chem. B 1998, 102 (44), 8858−8864. (20) Fagnant, D. P.; Desilva, M. A.; Brennecke, J. F. Solid−Liquid Equilibria Measurements of Mixtures of Lithium Bis(trifluoromethanesulfonyl)imide with Varying Alkyl Chain Length Ammonium Bis(trifluoromethanesulfonyl)imide Ionic Liquids. J. Chem. Eng. Data 2016, 61 (2), 958−967. (21) Scarbath-Evers, L. K.; Hunt, P. A.; Kirchner, B.; MacFarlane, D. R.; Zahn, S. Molecular features contributing to the lower viscosity of phosphonium ionic liquids compared to their ammonium analogues. Phys. Chem. Chem. Phys. 2015, 17 (31), 20205−20216. (22) Carvalho, P. J.; Á lvarez, V. H.; Marrucho, I. M.; Aznar, M.; Coutinho, J. A. P. High carbon dioxide solubilities in trihexyltetradecylphosphonium-based ionic liquids. J. Supercrit. Fluids 2010, 52 (3), 258−265. (23) Laurenczy, G.; Dyson, P. J. Determination of the viscosity of the ionic liquids [bmim][PF6] and [bmim] [TF2N] under high CO2 gas pressure using sapphire NMR tubes. Z. Naturforsch., B: J. Chem. Sci. 2008, 63 (6), 681−684. (24) Schicks, J. M. Gas hydrates. Annu. Rep. Prog. Chem., Sect. C: Phys. Chem. 2010, 106, 101−117. (25) Budhathoki, S.; Shah, J. K.; Maginn, E. J. Molecular Simulation Study of the Solubility, Diffusivity and Permselectivity of Pure and Binary Mixtures of CO2 and CH4 in the Ionic Liquid 1-n-Butyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide. Ind. Eng. Chem. Res. 2015, 54 (35), 8821−8828. (26) Mohshim, D. F.; Mukhtar, H.; Man, Z. Composite blending of ionic liquid-poly(ether sulfone) polymeric membranes: Green materials with potential for carbon dioxide/methane separation. J. Appl. Polym. Sci. 2016, 133 (39), 43999. (27) Jacquemin, J.; Husson, P.; Majer, V.; Costa Gomes, M. F. Influence of the Cation on the Solubility of CO2 and H2 in Ionic Liquids Based on the Bis(trifluoromethylsulfonyl)imide Anion. J. Solution Chem. 2007, 36 (8), 967−979. (28) Jalili, A. H.; Mehdizadeh, A.; Shokouhi, M.; Sakhaeinia, H.; Taghikhani, V. Solubility of CO2 in 1-(2-hydroxyethyl)-3-methylimidazolium ionic liquids with different anions. J. Chem. Thermodyn. 2010, 42 (6), 787−791.
a good CO2/CH4 selectivity by tuning pressure, especially in the form of mass-ratio-based selectivity when incorporated with variety of porous supporters.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b03104. DSC curve of [P4 4 4 4][NTf2] IL from 30 to 100 °C (Figure S1), mole ratio solubilities of CO2 in several phosphonium ILs close to ambient conditions (Table S1), and reported selectivities of two [P6 6 6 14]+ ILs at ca. 30 °C (Table S2) (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +0086-15993737902. E-mail:
[email protected]. ORCID
Lanyun Wang: 0000-0001-7862-5703 Yongliang Xu: 0000-0001-5484-4952 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the support of the research funding provided by the National Natural Science Foundation of China (51304073, 51304071, and U1361205) and the Program for Innovative Research Team in Ministry of Education of China (IRT_16R22). The authors also appreciate all of the reviewers and editors for their professional and constructive comments.
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REFERENCES
(1) Atkins, P. W. Chemistry: Principles and Applications; Longman Group, Ltd.: Hong Kong, 1988. (2) Austell, J. M.; Moore, M. E.; Hustad, C. W. CO2 for enhanced oil recovery needs enhanced incentives. Proceedings of the 4th Annual Conference on Carbon Capture and Sequestration; Alexandria, VA, May 2−5, 2005. (3) Iaccino, L. L.; Lattner, J. R. Production of aromatic hydrocarbons and syngas from methane. U.S. Patent 7,772,450 B2, Aug 10, 2010. (4) Baker, R. W.; Lokhandwala, K. Natural gas processing with membranes: An overview. Ind. Eng. Chem. Res. 2008, 47 (7), 2109− 2121. (5) Rafat, A.; Atilhan, M.; Kahraman, R. Corrosion Behavior of Carbon Steel in CO 2 Saturated Amine and Imidazolium-, Ammonium-, and Phosphonium-Based Ionic Liquid Solutions. Ind. Eng. Chem. Res. 2016, 55 (2), 446−454. (6) Ionic Liquids UnCOILed: Critical Expert Overviews; Kenneth, R., Seddon, N. V. P. E., Eds.; John Wiley & Sons, Inc.: Hoboken, NJ, 2013. (7) Carvalho, P. J.; Coutinho, J. A. P. The polarity effect upon the methane solubility in ionic liquids: A contribution for the design of ionic liquids for enhanced CO2/CH4 and H2S/CH4 selectivities. Energy Environ. Sci. 2011, 4 (11), 4614−4619. (8) Anthony, J. L.; Anderson, J. L.; Maginn, E. J.; Brennecke, J. F. Anion effects on gas solubility in ionic liquids. J. Phys. Chem. B 2005, 109 (13), 6366−6374. (9) Ramdin, M.; Vlugt, T. J. H.; de Loos, T. W. Solubility of CO2 in the Ionic Liquids [TBMN][MeSO4] and [TBMP][MeSO4]. J. Chem. Eng. Data 2012, 57 (8), 2275−2280. (10) Tagiuri, A.; Sumon, K. Z.; Henni, A. Solubility of carbon dioxide in three [Tf2N] ionic liquids. Fluid Phase Equilib. 2014, 380, 39−47. 4184
DOI: 10.1021/acs.energyfuels.6b03104 Energy Fuels 2017, 31, 4179−4185
Article
Energy & Fuels (29) Jalili, A. H.; Safavi, M.; Ghotbi, C.; Mehdizadeh, A.; HosseiniJenab, M.; Taghikhani, V. Solubility of CO2, H2S, and Their Mixture in the Ionic Liquid 1-Octyl-3-methylimidazolium Bis(trifluoromethyl)sulfonylimide. J. Phys. Chem. B 2012, 116 (9), 2758−2774. (30) Yim, J.-H.; Song, Ha Na; Lee, B.-C.; Lim, J. S. High-Pressure Phase Behavior of Binary Mixtures Containing Ionic Liquid [HMP][Tf2N], [OMP][Tf2N] and carbon dioxide. Fluid Phase Equilib. 2011, 308 (1−2), 147−152. (31) Shin, E. K.; Lee, B. C.; Lim, J. S. High-pressure solubilities of carbon dioxide in ionic liquids: 1-Alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. J. Supercrit. Fluids 2008, 45 (3), 282− 292. (32) Kim, S. A.; Yim, J. H.; Lim, J. S. High-pressure phase behavior of binary mixtures containing methylpyrrolidinium derivative ionic liquids and carbon dioxide. Fluid Phase Equilib. 2012, 332 (332), 28−34. (33) Carvalho, P. J.; Á lvarez, V. H.; Machado, J. J. B.; Pauly, J.; Daridon, J.-L.; Marrucho, I. M.; Aznar, M.; Coutinho, J. A. P. High pressure phase behavior of carbon dioxide in 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids. J. Supercrit. Fluids 2009, 48 (2), 99−107. (34) Yim, J. H.; Lim, J. S. CO2 solubility measurement in 1-hexyl-3methylimidazolium ([HMIM]) cation based ionic liquids. Fluid Phase Equilib. 2013, 352, 67−74. (35) Yokozeki, A.; Shiflett, M. B.; Junk, C. P.; Grieco, L. M.; Foo, T. Physical and Chemical Absorptions of Carbon Dioxide in RoomTemperature Ionic Liquids. J. Phys. Chem. B 2008, 112 (51), 16654− 16663. (36) Ramdin, M.; Olasagasti, T. Z.; Vlugt, T. J. H.; de Loos, T. W. High pressure solubility of CO2 in non-fluorinated phosphoniumbased ionic liquids. J. Supercrit. Fluids 2013, 82, 41−49. (37) Ramdin, M.; Amplianitis, A.; Bazhenov, S.; Volkov, A.; Volkov, V.; Vlugt, T. J. H.; de Loos, T. W. Solubility of CO2 and CH4 in Ionic Liquids: Ideal CO2/CH4 Selectivity. Ind. Eng. Chem. Res. 2014, 53 (40), 15427−15435. (38) Koel, M. Ionic Liquids in Chemical Analysis; CRC Press (Taylor & Francis Group): Boca Raton, FL, 2008; Vol. 4, pp 448, DOI: 10.1201/9781420046472.bmatt.
4185
DOI: 10.1021/acs.energyfuels.6b03104 Energy Fuels 2017, 31, 4179−4185
