CO2 and CH4 Sorption by [N4 4 4 4][NTf2] Ionic Liquid Using Quartz

Article pubs.acs.org/jced

CO2 and CH4 Sorption by [N4 4 4 4][NTf2] Ionic Liquid Using Quartz Crystal Microbalance Experiments under Different Pressures Lanyun Wang,†,‡,§ Yanan Wei,† Shaokun Wang,† and Yongliang Xu*,†,‡,§ †

College of Safety Science and Engineering, Henan Polytechnic University, Jiaozuo 454003, China Collaborative Innovation Center of Coal Safety Production of Henan Province, Jiaozuo 454003, China § State Key Laboratory Cultivation Base for Gas Geology and Gas Control, Henan Polytechnic University, Jiaozuo 454003, China ‡

S Supporting Information *

ABSTRACT: Using a quartz crystal microbalance system, the sorbed gas mass on tetrabutylammonium bis(trifluoromethanesulfonyl)imide [N4 4 4 4][NTf2] ionic liquid film coated on the surface of a quartz crystal under pressures from 0.1 to 4 MPa at 303 K was calculated based on the Sauerbrey equation. The experimental results are qualitatively close to the sorption simulation results, showing the rationality of experimental data. Although the amount of the sorbed gas on [N4 4 4 4][NTf2] ionic liquid film increases with pressure, CO2/CH4 selectivity only presents a slight fluctuation from 1.13 to 1.53, which is much lower than that in most fluid ionic liquid bulks possibly due to the dominant adsorption regime.

1. INTRODUCTION Increasingly additional carbon dioxide (CO2) and methane (CH4) released into the atmosphere have severe impacts on global warming, which has become one of the most important environmental issues. CO2 contained in natural gas causes corrosivity and crystallization problems, and hence removing CO2 from natural gas is desired in order to meet the requirements of practical application.1 Besides, during carbon capture of natural gas precombustion, it is essential to remove CH4 from CO2 because even a small amount of CH4 will affect the efficiency of purification, and moreover the existence of CH4 is against regulations of the allowed composition of CO2 for storage.2 Overall, the capture and separation of CO2 and CH4 is of high importance, not only because it realizes carbon emission reduction but also because both are important industrial and agricultural materials which could be used in refrigeration, photosynthesis, and oil recovery, providing important stockpiles for effective carbon utilization.3 Aqueous amines, e.g., methyldiethanolamine and diethanolamine, are commonly used as efficient chemical solvents for CO2 capture and/or separation from other unreactive gases. Nevertheless, the disadvantages, e.g., amine loss and vapor contamination after absorption, enable them to become more costly and uneconomical. As a popular potential alternative, ionic liquids have obtained burgeoning attention in gas solubility and separation due to their negligible volatility, thermal stability, noncorrosivity, and nonodor owing to their specific structural composition.4 Jacquemin et al.5 reported experimental solubilities of CO2, C2H6, CH4, O2, N2, H2, Ar, and CO in [C4C1im][BF4] and concluded that CO2 is two orders of magnitude more soluble © XXXX American Chemical Society

than CH4. Figure 1 shows the Henry’s law constants, KH, of different gases in conventional pure [C6C1im][NTf2] at 313.15

Figure 1. Henry’s law constants of gases in [C6C1im][NTf2] ionic liquid at 313.15 K6,8−11

K.6−11 It can be observed that KH spans over more than three orders of magnitude, depending on the nature of various gases. Acidic gases, e.g., SO2, H2S, and CO2, are more soluble than other nonpolar gases, and CO2 is nearly 10 times more soluble than CH4 at these conditions. Received: September 27, 2016 Accepted: February 10, 2017

A

DOI: 10.1021/acs.jced.6b00840 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Figure 2. Measurement system of gas sorption in ionic liquids based on quartz crystal microbalance.

ionic liquids, such as [C3C1im][NTf2], [C4C1im][NTf2], and [C6C1im][NTf2], determined by QCM at 298.15 K with CO2 pressures up to 0.1 MPa.44 In this work, combining a highpressure reactor, QCM would be used to measure oscillation frequency changes of quartz crystals before and after being coated by solid-state ionic liquids under high pressures with an aim of obtaining CO2 sorption capacity and CH4 on ionic liquid.

Due to the large solubility differences, it is promising to use ionic liquids to separate reactive gases from unreactive gases. There are numerous investigations reported on the chemisorption of CO2 in ionic liquids, such as those with amino groups, carboxylate anion, and aprotic hetercyclic anion based on CO2−amine reaction,12−41 absorbing CO2 even up to 2 mol ratio at the ambient condition. Nevertheless, high energy consumption is required during regeneration processes of ionic liquids (namely, the process of complete desorption of gas) from chemisorption. Another important concern is that, to date, thousands of research works have been reported focusing on gas capture into fluid ionic liquid bulks which demand longtime equilibrium and diffusion. There are few articles related to gas sorption behaviors on ionic liquids which present solid in room temperature conditions. Solid-state ionic liquids (SoILs), usually with melting temperature points higher than room temperatures, were targeted because of their advantages in enabling gas access to large ionic surfaces with reduced diffusion difficulties and desorbing speeds. They were reported to only slightly diminish the maximum CO2 capacity but adsorb/desorb CO2 thousands or tens of thousands of times faster than room temperature ionic liquids, based on the investigations involving [N3 3 3 3][AC], [N 6 6 6 6][AC], [N1 8 8 8][AC], [N2 2 2 2]Br, and [N4 4 4 4]Br ionic liquids.42 Outside of influences from their structures, a high pressure and a low temperature are favorable for gas capture. In consideration of energy consumption during CO2 capture, high-pressure precombustion is generally more prominent than the low-pressure postcombustion. During carbon capture of natural gas precombustion, natural gas is converted into CO and H2 by autothermal re-forming and then CO is converted into CO2 through a shift reaction. This procedure tends to produce high CO2 concentrations with high total pressure even over 4 MPa. Therefore, during CO2 separation from other gases (including H2 and uncombusted CH4), it is necessary to operate the separation under high pressures. From another perspective, a high-pressure measurement also provides data for gas separation during an efficient and low-cost pressure swing adsorption process related to ionic liquids. Quartz crystal microbalance (QCM) is capable of measuring ultrasensitive mass and testing microweight caused by gases sorption by a very thin ionic liquid film with accuracy up to nanogram. Mineo et al. used QCM to measure CO2 sorption and desorption in a series of poly(ionic liquid)s based on the 1vinyl-3-hexylimidazolium polymerizable cation, indicating QCM could display fast and linear response, and the sorption results present great reproducibility.43 Baltus et al. reported CO2 capacity of several imidazolium-based room-temperature

2. EXPERIMENTS 2.1. Apparatus. The gas sorption capacity measurements were made by a system combining QCM and a high-pressure reactor. As Figure 2 depicted, a quartz crystal is put into the high-pressure reactor which is immersed into the water bath for keeping different temperatures. CHI-400C quartz crystal microbalance with an accuracy of 0.001 Hz, a product of Shanghai Chenhua Instruments Co., Ltd., is used to measure oscillation frequency changes of quartz crystals before and after being coated by ionic liquids under various pressures. The QCM contains a quartz crystal oscillator, a frequency counter, a fast digital function generator, a high-resolution and high-speed data acquisition circuitry, a potentiostat, and a galvanostat. CHI-400C data acquisition software was used to record the frequency at 0.1 s intervals. An 8 MHz AT-cut quartz crystal (around 1.37 cm diameter), including a golden electrode with a diameter of around 0.51 cm which is placed in the center of the crystal on each side, is used as the resonator. In order to minimize the viscous effects, polished crystals with roughness of around 3 nm rms were used as ionic liquid film loaders. An enhanced pressure stainless steel (SS316L) high-pressure container capable of operation to 10.0 MPa and 573.15 K was linked with QCM. A BT-224S electrically heated thermostatic water bath could provide various temperatures with an accuracy of ±0.5 K. 2.2. Material. Considering the superiority of solid-state ionic liquids and avoidance of water influence, a hydrophobic and solid-state ionic liquid, [N4 4 4 4][NTf2] (MW = 522.61 g mol−1) with a symmetric cation (see Figure 3), was used to trap CO 2 and CH 4 in a small scale. It is reported that [N4 4 4 4][NTf2] has a melting point of 365−369 K,45,46 determining its crystallized nature at room temperature, possibly due to the predominant Coulombic force between cation and anion.47 [NTf2]− anion was selected originally from an expectation of increasing CO2 uptake because of the fluorinated methyl groups which have good affinity with CO2 molecules by electrostatic forces.48 [N4 4 4 4][NTf2] in this work with purity over 99% was purchased from Lanzhou Zhongke Kate Industry & Trade Co., Ltd. of China. B



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where f and f 0 are the measured frequency and the fundamental frequency of a clean quartz crystal, respectively; μq = 2.947 × 1011 g cm−1 s−2 and ρq = 2.648 g cm−3 are the shear modulus and density of quartz, respectively. 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 1.35 ng on both sides of the crystal electrodes when the area A of the gold disk coated onto the crystal equals 0.196 cm2 for the two sides. In real gas atmosphere, departing from the additional mass Δm, several other influencing factors should be considered, including pressure, viscosity of gas, and the roughness of a quartz crystal. The frequency shifts caused by these effects are defined as Δf m, Δf p, Δfη, and Δf r, and the whole shift is expressed in eq 2.

Figure 3. Chemical structure of [N4 4 4 4][NTf2] ionic liquid.

2.3. Experimental Section. 2.3.1. Computational Methodology. In order to qualitatively calculate CO2 capacity sorbed by [N4 4 4 4][NTf2], computer simulations were carried out using the sorption module of Material Studio software (a commercial software for MD simulations), which gives sorption isotherms for CO2 gas caught by ionic liquid surface and bulk through a series of fixed pressure simulations. Before conducting the Sorption calculation, the ion pairs of [N4 4 4 4][NTf2] and CO2 were optimized using Dmol3 on the basis of GGA-BLYP/DNP(3.5). Based on the amorphous cell module, a cell containing 30 ionic liquid molecules was built. After cleaving a surface from the cell, a vacuum slab with a thickness of 10 Å was built from the build/crystals/build vacuum slab. A supercell including 270 ionic liquid molecules was used to trap gas molecules. Under the COMPASS II force field, five different sorption isotherms, i.e., systems at 303 K under 0.1, 1, 2, 3, and 4 MPa, were computed via the sorption module, which allows displacements, creations, and destructions of sorbate species. 2.3.2. Preparation of Ionic Liquid-Coated Films. In order to apply a very thin ionic liquid film on the quartz crystal, a dilute solution (10 mg mL−1) of the ionic liquid was prepared by dissolving ionic liquid (50 mg) into dichloromethane (5 mL) with analytical purity. The ionic liquid films were prepared using a dip-coating technique. A clean crystal was immersed carefully and vertically in the ionic liquid−dichloromethane solution for 5 min to guarantee a stationary solution and a completely wetted crystal surface. Two ionic liquid-coated (ILcoated) films were prepared for measuring CO2 and CH4 sorption, respectively. Two sets of crystals coated with ionic liquid−dichloromethane solution were placed into a vacuum oven at 313.15 K for 4 h to evaporate the dichloromethane from the ionic liquid film, and finally two dry IL-coated quartz crystals were prepared. 2.3.3. Experimental Process. Two clean crystals in the higher pressure reactor were purged with dry CH4 and CO2 to obtain the frequency change under pressures of 0.1, 1, 2, 3, and 4 MPa at 303.15 K. Frequencies of the crystal were recorded once constant values were achieved. The same procedure was repeated in the case of IL-coated crystals. 2.3.4. Experimental Calculation. Due to gas sorption, the frequency of the IL-coated quartz crystal presents a shift which is related to the mass change of the crystal. According to the Sauerbrey equation, in vacuum condition, the frequency shift, Δf, has a linear relationship with the mass change, Δm, on the QCM surface, which is described in eq 1: Δf = f − f0 =

−2f0 2 A μq ρq

Δf = f − f0 = Δfm + Δfp + Δfη + Δfr

(2)

The pressure effect increases the frequency of the quartz crystal resonator linearly as described in eq 3. Δfp = [(1.06 × 10−6)f0 ]P

(3)

The slope of eq 3 is independent of the nature of the gas and is proportional to the fundamental frequency of the crystal, f 0. The viscosity term, Δfη, describes the interaction of the vibrating crystal with a viscous medium and is expressed in eq 4:49 ⎡ nf 3/2 ⎤ ⎥ ρη Δfη = −⎢ 0 ⎢ πρ μ ⎥ g g q q ⎦ ⎣

(4)

where n (=1 or 2) is the number of faces of the crystal in contact with the gas; μg and ρg are the shear modulus and density of the gas, respectively. As for the roughness term, it is reported that Δf r could be minimized or neglected if surface roughness of a polished crystal is less than 10 nm rms.50 The surface roughness of the crystals in this work is around 3 nm rms, and hence the roughness effect here could be ignored.51 Considering the influences of pressure and viscosity on the frequency of quartz crystal, a corresponding clean crystal was adopted as a comparative set to compensate these two effects. The frequency shift of the clean crystal under a certain gas pressure is defined as Δfc. Therefore, the frequency change Δfg of the IL-coated quartz subtracted by the frequency change Δfc of the clean quartz crystal at the same gas atmosphere gives the amount of gas caught by the ionic liquid film which is expressed in eq 5: Δmg = −

=

AΔfg 2.26 × 106f0 2

⎞ ⎛ AΔfc ⎟ − ⎜⎜ − 6 2⎟ ⎝ 2.26 × 10 f0 ⎠

A(Δfc − Δfg ) 2.26 × 106f0 2

(5)

Therefore, the mole fraction of gases in the ionic liquid is arranged as eq 6 shows,44 xg =

Δm

(Δfc − Δfg )/Mg ΔfIL /MIL + (Δfc − Δfg )/Mg

(6)

where MIL is the molecular weight of the ionic liquid and Mg is the molecular weight of the gas, g mol−1.

(1) C



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The ionic liquid loading was calculated from the frequency difference (Δf IL) measured in vacuum before and after coating with the ionic liquids. Assuming the fundamental frequency of clean quartz crystal resonator to be f 0 and that of the IL-coated resonator be f 0IL, so the frequency change caused by ionic liquid loading is Δf IL = f 0IL − f 0, and thus producing the mass of ionic liquid film, ΔmIL, according to eq 1.

Table 3. Frequencies of Crystals before and after Being Coated by [N4 4 4 4][NTf2] and the Mass of Ionic Liquid Films under Vacuum Condition

f 0/Hz

f 0IL/ Hz

Δf IL/Hz

ΔmIL/ng

7987531

7965849

21682

29413.8

f 0IL/ Hz

Δf IL/ Hz

ΔmIL/ng

7986979 7986295

7981262 7980306

5717 5989

7758.51 8129.03

No. 1 crystal is used for CO2 solubility measurement, and No. 2, for CH4.

Figure 4 respectively noted by the red line and the blue one. One can clearly observe that, in the case of IL-coated crystal, decreased frequency shifts relative to that from the clean one are presented indicating increased gas sorption on the crystals. With a pressure increase, the gap between red and blue lines becomes large, implying an increased gas sorption with pressure. The frequency changes, as well as the CO2 and CH4 capacity with pressures, are contained in Tables 4−6, respectively. Figure 5 illustrates that CO2 and CH4 capacities in a form of mole fraction generally linearly correlate with pressure following the expected Henry’s law behavior, which is a common phenomenon in physisorption. It is seen that CO2 mole fraction trapped by the solid [N4 4 4 4][NTf2] film is only 0.0006, close to the situation of CH4, at ambient pressure, which is two orders of magnitude less than that in fluid analogues, e.g., [N1 4 4 4][NTf2] and [N2 1 1 3][NTf2] as Table 7 lists. One reason is due to the dominant gas adsorption behavior on the ionic surface. This sorption behavior was also demonstrated by computational simulation. According to the simulated results from the sorption module in Material Studio (see Table S1), the experimental CO2 capacity is qualitatively consistent with the theoretical CO2 capture ability by solid-state [N4 4 4 4][NTf2] (Figure 6). From the CO2 distribution shown in Figure S5a−e, it is observed that,under ambient pressure, almost all CO2 molecules locate on the surface of the ionic liquid, and only a few CO2 molecules penetrate into the ionic liquid bulk when pressure rises. Therefore, when the pressure increases, gas capacity in [N4 4 4 4][NTf2] increases gradually because of multilayer adsorption on the ion surface as well as a small amount of gas penetration into the ionic liquid bulk under the great pressure gap. Another important reason is the different physical properties and void spaces of sorbents. There is qualitative conclusion stating that interaction energy between ions is in proportion to the melting point of ionic liquids. That means ionic liquids with higher melting points would be more difficult to interact with gas molecules because of stronger intermolecular interactions between cations and anions.52 Furthermore, the solid-state ionic liquids usually are tightly packed with much less microdomain/voids to accommodate gas molecules compared to their liquid analogues.53 With pressure, selectivities only change slightly with a minor fluctuation, which is also observed in supported membranes

Table 1. Fundamental Frequencies of Crystals before and after Being Coated by [N4 4 4 4]Br and the Mass of Ionic Liquid Film system

f 0/Hz

1 2 a

3. RESULTS AND DISCUSSION In order to obtain the deviation of crystal vibration frequency, the fundamental frequency changes of the clean crystal were measured respectively under vacuum and atmospheric and pure CO2 at ambient pressure at 303 K, and the average fluctuation of the frequency change is close to ±0.058636 Hz/min. All three experimental figures are presented in the Supporting Information (Figures S1−S3). In order to minimize the interference of the frequency fluctuation on the mass calculation, an average frequency change at each pressure based on the median filter algorithm was obtained. First, an experiment for verifying the reliability of the QCM measurement system was conducted. A hydrophilic [N4 4 4 4]Br, a solid-state ionic liquid reported in the literature,42 was chosen as the CO2 sorbent by this QCM system. Frequency change curves of clean crystal and the [N4 4 4 4]Br-coated one under different CO2 gas pressures are shown in Figure S4 of the Supporting Information. The fundamental frequency under vacuum condition and the mass of [N4 4 4 4]Br spreading on the crystal are included in Table 1.

[N4 4 4 4]Br film

itema

The gas solubility uncertainty (u) originates from the uncertainty of frequency of a quartz crystal resonator. The uncertainty of CO2 and CH4 capacity in an ionic liquid was obtained by calculating the standard deviation of frequency combining the Sauerbrey equation at each pressure stage (see Table 2). According to the Sauerbrey equation, the average trapped CO2 in a form of mass ratio is determined to be 0.0048 g g−1. Although this capacity has the same order of magnitude with the reported 0.0078 g g−1, it is slightly lower possibly due to different experimental scales and the unavoidable moisture uptake during experimental operations. However, the close results compared to the reported ones indicate a reliability of the measuring system in this work. Based on the same experimental procedure, the frequencies of crystals used in measuring CO2 and CH4 sorption by [N4 4 4 4][NTf2] are listed respectively in Table 3. Plots of observed frequency shifts for a clean quartz crystal and an IL-coated one as a function of gas pressure are shown in

Table 2. Average Frequencies of [N4 4 4 4]Br-Coated and Clean Crystals and CO2 Capacity (Mass Ratio, g g−1) by [N4 4 4 4]Br under Different Pressures at ca. 298 K P/MPa

Δfg/Hz

u( fg)/Hz

Δfc/Hz

u( fc)/Hz

XCO2

u(XCO2)/10−5

ref

0.1 1 1

−54.23 −75

1.8096 1.0436

−1.97 32.05

0.2711 0.3752

0.0024 0.0048 0.0078

9.5990 6.5456

this work this work 42

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Figure 4. Frequency shifts of the quartz crystals before and after being coated by [N4 4 respectively.

4 4][NTf2]

with increasing CO2 and CH4 pressure,

Table 4. Average Frequencies of the IL-Coated and Clean Crystals, and CO2 Capacity (Mole Fraction) under Different Pressures P/MPa

Δfg/Hz

u( fg)/Hz

Δfc/Hz

u( fc)/Hz

x

u(x)/10−3

0.1 1 2 3 4

−0.6083 −29.4725 −42.8124 −61.3785 −91.5374

0.2661 0.3895 0.3888 0.4688 2.5098

−0.3375 30.0866 72.2078 124.7081 160.4632

0.1541 0.8864 0.9817 0.3305 0.4835

0.0006 0.1101 0.1929 0.2788 0.3436

0.8723 2.6436 2.8390 1.6577 6.1799

Table 5. Average Frequencies of the IL-Coated and Clean Crystals, and CH4 Capacity (Mole Fraction) under Different Pressures P/MPa

Δfg/Hz

u( fg)/Hz

Δfc/Hz

u( fc)/Hz

x

u(x)/10−2

0.1 1 2 3 4

−0.5431 41.9044 93.3457 136.3224 193.0665

0.2507 0.5091 0.7791 2.2072 0.4072

−0.45865 60.4488 119.7610 196.3642 268.9241

0.2194 0.8266 0.5828 0.7562 1.2408

0.0005 0.0918 0.1259 0.2466 0.2926

0.2557 0.7230 1.4984 1.5901 0.8906

Table 6. CO2 and CH4 Sorption (Mass Ratio) and Ideal Selectivity of CO2/CH4 in [N4 4 4 4][NTf2] under Different Pressures P/MPa

XCO2

u(XCO2)/10−4

XCH4

u(XCH4)/10−4

SCO2/CH4

0.1 1 2 3 4

0.000047 0.0104 0.0201 0.0325 0.0441

0.7351 2.2316 2.3971 1.3980 5.2354

0.000014 0.0031 0.00441 0.0100 0.0127

0.7848 2.2298 4.6573 4.9468 2.7511

1.22 1.20 1.53 1.13 1.17

coated by some ionic liquids, e.g., [C2C1im][CF3SO3],56 [1,3bis(3-methylimidazolium)propane bis(trifluoromethylsulfonyl)imide] (pr[mim]2[NTf2]2) and [(1,6-bis(3methylimidazolium)hexane bis(trifluoromethylsulfonyl)imide)] (h[mim]2[NTf2]2),57 stating that the pressure has a slight effect on CO2/CH4 selectivity. Also, it is easy to observe that [N4 4 4 4][NTf2] only possesses very weak selectivity of SCO2/CH4, only from 1.13 to 1.53, which is much lower than those in room temperature ionic liquids with the same anion, such as imidazolium-based ionic liquid [C4C1im][NTf2] with a great ideal selectivity of 11 at 313.15 K.58 It is possibly due to the strong cation−anion interaction in [N4 4 4 4][NTf2] which reduces the interaction strength between CO2 and the [NTf2]− anion. In contrast, the highly unsymmetric structure of [C4C1im][NTf2] leads to a weaker

Figure 5. Mole fraction of CO2 and CH4 sorbed by [N4 4 4 4][NTf2] as a function of gas pressures at 303 K.

Table 7. Mole Fraction Solubility of CO2 in Several Ammonia Ionic Liquids ionic liquids

T/K

P/MPa

X

ref

[N1 4 4 4][NTf2] [N2 1 1 3][NTf2] [N2 1 1 3][NTf2]

298.15 313.15 313.15

0.0998 0.0999 1.9

0.023 0.034 0.317

54 55 55

cation−anion interaction and provides more free volume for gases compared to the more symmetric [N4 4 4 4][NTf2] which E



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[N4 4 4 4][NTf2] ionic liquid at 303 K under different pressures (Table S1) (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel.: +0086 15993737902. E-mail: [email protected]. ORCID

Lanyun Wang: 0000-0001-7862-5703 Funding

We acknowledge gratefully the support of the research funding provided by the National Natural Science Foundation of China (Grant Nos. U1361205, 51304071 and 51304073), as well as the Program for Innovative Research Team in Ministry of Education of China (Grant No. IRT_16R22). We also appreciate all of the reviewers and editors for their professional and constructive comments.

Figure 6. Simulated and experimental sorption isotherms of CO2 by [N4 4 4 4][NTf2] at 303 K.

Notes

The authors declare no competing financial interest.

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is an aprotic coulomb-dominated system with a high melting point.59 Moreover, the nonpolar domain in the alkyl chains on the cation [N4 4 4 4]+ surrounding the positive ammonium center enables this ionic liquid to attract more nonpolar alkane CH4.46 The similar kinetic diameters of CO2 (0.33 μm) and CH4 (0.38 μm)60 indicate a similar diffusion coefficient of the two gases in the ionic liquid film and may be responsible for the small selectivity of SCO2/CH4 with an average value of 1.39. Accordingly, gas sorption behavior on the solid ionic liquid film is greatly different from the absorption in the fluid ionic liquids, mainly due to the tighter ion interactions and less free volumes. However, the solid ionic liquid easily forms a thermally stabilized film incorporated with various supported porous membranes. In order to approach an enhanced selectivity, unsymmetric structure should be designed in to consider the favoring of increased free volume and weakening cation−anion intramolecular interaction.

4. CONCLUSIONS A self-built high-pressure and quartz crystal microbalance measurement system was applied to measure the gas (i.e., CO2 and CH4 here) capture by solid-state [N4 4 4 4][NTf2] ionic liquid film coated on a quartz crystal under different CO2 and CH4 pressures from 0.1 to 4 MPa at 303 K. Capacities of CO2 and CH4 increase with pressure, but the selectivity of CO2/CH4 only presents a fluctuation due to a similar pressure effect on CO2 and CH4 sorption under high pressure. Compared with fluidized ionic liquids, this solid [N4 4 4 4][NTf2] has much lower gas capacity and selectivity, because it is hard for gases to permeate into the compact structure due to a strong cation− anion interaction through Coulomb forces.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.6b00840. Frequency change curves of a clean crystal and its first deviation curve under vacuum, atmospheric, and ambient CO2 pressure (Figures S1−S3), frequency change curves of clean and [N4 4 4 4]Br-coated crystals at ca. 298 K under different pressures (Figure S4), simulated CO2 capacity of [N4 4 4 4][NTf2] ionic liquid at 303 K under different fixed pressures based on Sorption of Material Studio (Figure S5), and simulated CO2 capacity on F



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CO2 and CH4 Sorption by [N4 4 4 4][NTf2] Ionic Liquid Using Quartz

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