Structural Dependence and Spectroscopic Evidence of Methane

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B: Liquids; Chemical and Dynamical Processes in Solution

Structural Dependence and Spectroscopic Evidence of Methane Dissolution in Ionic Liquids Tingyu Huang, Pei-Fang Yan, Zhanwei Xu, Xiumei Liu, Qin Xin, Haitao Liu, and Zongchao Conrad Zhang J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b03178 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on May 1, 2018

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The Journal of Physical Chemistry

Structural Dependence and Spectroscopic Evidence of Methane Dissolution in Ionic Liquids Tingyu Huang1,2, Peifang Yan1, Zhanwei Xu1, Xiumei Liu1, Qin Xin1, Haitao Liu3, Z. Conrad Zhang1* 1

State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Science, 457 Zhongshan Road, Dalian 116023, China

2

University of Chinese Academy of Sciences, Beijing 10049, China

3

Saudi Basic Industries Corporation, Shanghai 201319, China

*To whom correspondence should be addressed. E-mail: [email protected]; Tel: +86 8437 9462

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT High methane dissolution capacity in a liquid is important for methane storage and transformation. In this work, methane solubility in different ionic liquids (ILs) was studied and was found associated with IL’s structural and physical properties. In imidazolium-based ILs, ILs containing C-F and long alkyl chain showed high methane solubility mainly due to lower surface tension and molar

density.

Reducing

the

surface

tension

of

solvent

by

adding

0.16

moles

of

trimethyl-1-propanaminium iodide (FC-134) with respect to [Bmim][NTf2] increased methane solubility by 39.3%. In-situ high pressure attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopic results indicated a reversible process of methane dissolution in the ILs. The antisymmetric C-H stretching band of dissolved methane in ILs showed highly prominent rotational-vibrational bands with high intensity and narrow half-peak-width compared to gaseous methane. Induced interaction between methane and IL resulted in increased dipole variation strength and reduced methane molecular symmetry. The constant antisymmetric C-H stretching peak at 3016.85 cm-1 revealed an unconstrained methane rotation in the stable physical and chemical environment of IL. Methane insertion into the IL’s intra-network space needs activation energy to overcome the interaction of cation-anion network. Kinetic analysis of methane in [Bmim][NTf2] and [Bmim][HSO4] at different temperatures indicated that methane dissolution in these two ILs was a reversible first order and very weakly endothermic process and that methane dissolution required high activation energy in ILs with stronger cation-anion interaction.

2


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1. INTRODUCTION Methane (CH4) production has been largely increased in the last decade thanks to the excavation and release of shale gas. Methane densification for its storage, transportation and utilization is some of the important research subject. Methane transformation has been recognized as one of the most difficult challenges in chemistry. Low solubility of methane in water or organic solvents is a key limiting factor for homogeneous catalytic conversion of methane to methanol. So, materials with high methane solubility may facilitate the development of methane utilization technologies. Ionic liquids (ILs) are composed of cations and anions with melting points around or below ambient temperature.1 ILs have been investigated as potential media for methane transformation,2-8 storage,9-11 and separation,12-22 owning to their unique properties, such as negligible vapor pressure, weak coordination and good solubility. A number of ionic liquids based on imidazolium,10-13,15,17-22 phosphonium,14-16 and ammonium9 have been reported for their high methane solubility. Liu and co-workers studied phosphonium-based ILs with different Cn alkyl chains. Long alkyl chains in the cations were reported to favor methane dissolution.14 Kou and co-workers suggested that low polarity of [C4min], [C8min] and ammonium based ILs are good solvents for methane storage.9 Chen and co-workers indicated that ILs containing fluorinated anions are beneficial for methane dissolution, especially imidazolium-based [NTf2]-.13 There were also works that aimed at improving methane solubility through reducing viscosity of ILs.15 However, these previous works offered little understanding on the dissolution mechanism and on the detailed interaction between methane and ILs. FTIR is a powerful technique for the understanding of gas absorption by providing molecular level insight.23,24 In-situ ATR-FTIR has been widely used in studying IL structures. Kazarian and co-workers used ATR-FTIR to study the interaction of supercritical carbon dioxide with 1-butyl-3-methylimidazolium hexafluorophosphate and 1-butyl-3-methylimidazolium tetrafluoroborate.25 Ohlin and co-workers used in-situ ATR-FTIR to investigate CH4 in zeolite Na-ZSM-5.26 Scarano et al used FTIR to analyze methane adsorbed on zinc oxide and observed the formation of H3CH-Zn structrure.27 However, spectroscopic characterization of methane dissolved in ILs has not appeared in the literature. In this work, we report methane dissolution capacities, FTIR evidence and kinetic/thermodynamic results of methane dissolution to get mechanistic understanding of methane dissolution in ILs. ILs used in this work vary from imidazolium, piperidinium, pyrrolidinium, ammonium to phosphonium based cations and a variety of anion as well. We screened such a large number of ILs with vastly diverse structures for their methane dissolution capacities in order to establish a universal model to predict and improve methane solubility, despite of the types of ILs. Representative imidazolium based ILs were chosen for in-situ ATR-FTIR and kinetic/thermodynamic analysis because of their wide utilization on methane sensing,19 gas capture, 21-22 and gas separation.28,29

2. EXPERIMENTAL SECTION 2.1 Materials Methane was supplied by a local supplier with purity of 99.99%. 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide

([Emim][NTf2]),

1-butyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)amide ([Bmim][NTf2]), 1-butyl-3-methylimidazolium trifluorosulfonate 3



([Bmim][OTf]),


1-butyl-3-methylimidazolium ([Bmim][Ac]),

nitrate

1-butyl-3-methylimidazolium tetrafluoroborate

tetrafluoroborate

([Emim][BF4]),

hydrogensulfate

([Bmim][NO3]),


Page 4 of 19

([Bmim][HSO4]),

1-buthyl-3-methylimidazolium hexafluorophosphate

([Bmim][BF4]),

1-ethyl-3-methylimidazolium

acetate

([Bmim][PF6]),

1-ethyl-3-methylimidazolium trifluoro(trifluoromethyl)borate

([Emim][BF3CF3]), 1-butylpyridinium tetrafluoroborate [BPy][BF4], 1-ethyl-3-methylimidazolium trifluorotris(pentafluoroethyl)phosphate ([Emim][FAP]) were provided by Lanzhou Institute of Chemical Physics with purity > 99.0%. 1-butyl-3-methylimidazolium perfluorobutanesulfonate ([Bmim][PFBS]),


1-butyl-2,3-dimethylimidazolium

bis(trifluoromethylsulfonyl)amide

1-butyl-2,3,4,5-tetramethylimidazolium synthesized

as

reported.30-32

perfluorohexylsulfonate


1-hexyl-3-methylimidazolium

([Bmim][PFHS]), ([Bm2im][NTf2]),

([Bm4im][NTf2])

were


([Hmim][NTf2]), 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([Omim][NTf2]), 1-butyl-1-methylpiperidinium


([Pp14][NTf2]),

1-butyl-1-methylpyrrolidinium


([Py14][NTf2]),

methyltributylammonium bis(trifluoromethylsulfonyl)amide ([N1444][NTf2]), tetrabutylphosphonium bis(trifluoromethylsulfonyl)amide

([P4444][NTf2]),

bis(2,4,4-trimethylpentyl)phosphinate

[P(16)444][TMPP],

trihexyltetradecylphosphonium 1-methyl-3-(4-sulfobutyl)imidazolium

bis(trifluoromethylsulfonyl)amide ([C4SO3Hmim][NTf2]) were synthesized as reported.33 The ILs were further purified by drying under vacuum at 333 K for 24 h. All ILs were confirmed by 1H and 13C NMR spectroscopy with Bruker 400 and 100 spectrometers. The structures of all these ILs are shown in Figure 1.

Figure 1. Structure of ILs for methane dissolution METTLER TOLEDO DM40, DZY-005A, KRUSS-K100C were used to measure the density, 4


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viscosity and surface tension of ILs. 2.2 Apparatus and procedure The solubility of CH4 in ILs was determined by isochoric saturation method.9 Methane can dissolve in

ionic liquids during the gas charging process. So, gas container chamber was used to calculate the mole of methane into the absorption chamber. The apparatus in Figure 2 contains a gas container chamber (V1) and an absorption chamber (V2). The two chambers are made of stainless steel, and can afford pressure up to 12 MPa. These chambers are equipped with digital pressure sensors that are accurate to 1 KPa (0.025% accuracy in full scale) and with Pt100 thermo-sensors with ±0.15K uncertainty. The digital pressure sensors and thermo-sensors are supplied by Dalian Leidaer Co. Ltd. Absorption chamber was first filled with dried ILs. At a specified temperature (T), all chambers were purged with gaseous CH4 for three times, and degassed under vacuum. Next, the gas container was charged with CH4 (99.99%) to a specified pressure (P1). Then, the three-way-valve was opened and CH4 was allowed to flow into absorption chamber until pressure (P2) was stable in gas container chamber. The three-way-valve can also be used to wash the absorption chamber directly. The mole of methane in gas container and absorption chamber was calculated according to the real gas equation (1) through Z (compressibility factor),

Z .

(1)

P1V1/Z1RT is the total mole of methane in gas container chamber. P1V1/Z1RT - P2V1/Z2RT is the methane into the absorption chamber. Then, the three-way-valve was closed. When stirring was started, the gas pressure in absorption chamber dropped and reached a steady pressure (P3). 800 rpm was high enough to avoid the influence of mass transfer coming from stirring speed. The dissolved CH4 was calculated by the methane into the absorption chamber minus the residual methane. The number of mole of dissolved CH4 was calculated according to equation (2) n CH dissolution

1 P V P V P V ! 2 ∙ RT Z Z Z

Figure 2. Apparatus for CH4 dissolution

5



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An in-situ ATR-FTIR with Golden Gate ATR-FTIR accessary (Specac UK) was used to characterize the interaction between methane and IL in dissolution process. The Golden Gate ATR-FTIR consists of a diamond plate and a high pressure (up to 20 MPa) IR-reactor which can be heated to 473 K. The FTIR study was performed on a Thermo Scientific Nicolet iS50. During each FTIR measurement, after 0.2 ml IL (about 1~2 mm thickness) was added into the high pressure IR-reactor, methane was charged and allowed to equilibrate for 30 mins. FTIR spectra were recorded with a mercury cadmium telluride (MCT) detector. The pressure was measured every 5 minutes until it reached the equilibrium state. All spectra of the samples and the background were scanned for 16 times at one setting. 3. RESULTS AND DISCUSSION 3.1 Solubility of CH4 in ILs The solubility of CH4 was determined at 363 K, 2 MPa. All experimental data, including solubility and physical properties, were repeated for three times. The average results are shown in Table 1 with uncertainty of solubility in less than ±0.7%. The decomposition temperature (Tonset) of studied ILs are much higher than the temperature at which methane dissolution was measured and characterized in this work.34 For example, ILs containing C-F bonds in the anions have Tonset ≥ 673 K. Therefore, the environmentally friendly ILs studied in this work are stable and reusable under the conditions of this work. Although ILs containing fluorinated compound are relatively toxic, there no bad influence because of the reversible methane dissolution process, high IL’s stability and recyclability. At experimental temperature, ILs with [NTf2]- anion are stable under CO2, H2O, SO2 and H2S atmosphere that may appear together with methane in shale gas. ILs with [BF4]-, [PF6]- anions are unstable when containing water. Table 1. Solubility (Mol %) of methane in different ILs at 363 K, 2 MPa Entry

ILs

Solubility (mol %)

Molar density (mol/cm3)

Mw (g/mol)

Surface tension (mN/m)

Viscosity

（mm2/s2）

1

[Emim][NTf2]

5.7

3.6

391

33.2

4.8

2

[Bmim][NTf2]

6.2

3.2

419

29.4

5.2

3

[Hmim][NTf2]

7.4

2.8

447

27.9

6.3

4

[Omim][NTf2]

8.1

2.6

475

26.0

7.7

5

[Bm2im][NTf2]

4.9

3.0

433

36.0

14.0

6

[Bm4im][NTf2]

6.3

2.8

461

29.3

13.0

7

[Bmim][OTf]

4.5

4.3

288

31.9

8.3

8

[Bmim][NO3]

3.0

5.4

201

45.3

9.1

9

[Bmim][HSO4]

2.1

5.8

208

52.1

50.0

10

[Bmim][Ac]

2.9

6.0

170

39.0

13.0

11

[Bmim][PFBS]

6.5

3.1

438

30.1

8.5

12

[Bmim][PFHS]

7.3

2.6

538

27.2

8.1

13

[Bmim][PF6]

4.1

4.5

284

40.0

18.6

14

[Emim][BF4]

2.8

5.9

198

50.2

5.0

15

[Bmim][BF4]

3.5

5.0

226

40.6

8.9

6


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16

[BPy][BF4]

3.2

5.2

223

44.0

14.5

17

[Emim][BF3CF3]

3.6

5.0

248

40.1

5.0

18

[Emim][FAP]

6.0

2.9

556

33.0

10.2

19

[Pp14][NTf2]

4.4

3.0

436

32.0

11.3

20

[Py14][NTf2]

5.0

3.0

422

33.0

7.7

21

[N1444][NTf2]

8.3

2.4

480

28.1

15.3

22

[P4444][NTf2]

9.2

2.1

539

26.0

11.2

23

[P(16)444][TMPP]

17.1

1.1

773

24.4

68.0

24

[C4SO3Hmim][NTf2]

5.5

2.8

513

34.0

274.3

Methane solubility was measured in ILs of various types of cations and anions. For imidazolium cations, the long alkyl chains favored methane solubility. The molar fraction of methane dissolved in ILs increased from 5.7 mol% to 8.1 mol% when one of the N-groups of imidazolium cations was changed from C2 to C8 (see Table 1 entry 1-4). Several studies35,36 showed that neat ILs may form extended stacking structure. The intimacy of interaction between imidazolium cation and anion is influenced by both the type of anion and chain length of N-alkyl group on the imidazolium cation. The distance between cation and anion is shorter and the stacking structure is tighter in ILs with short alkyl imidazolium chain. Substitution of protons at C(2,4,5)-H of the [Bmim]+ cation by methyl group (C(2,4,5)-CH3) was investigated as well. The markedly decreased methane solubility when the H in C(2)-H was replaced by a methyl group (see entry 5 in Table 1, [Bm2im][NTf2]) suggests that C(2)-CH3 substitution weakened the interaction between methane and ILs. Two possibilities may arise from the CH3 substitution: one could be the reduced intra-network space around C(2) because CH3 group is bulkier than substituted H, and the other could be the dominating C(2)-H interaction of IL with CH4. The interaction between C(2)-CH3 and CH4 appears weaker than that between C(2)-H and CH4. When all three H in C(2,4,5)-H were replaced by methyl groups (see entry 6 in Table 1, [Bm4im][NTf2]), the methane solubility would be affected by two factors; on the one hand, C(2)-CH3 substitution negatively affects CH4 dissolution, and on the other hand, C(4,5)-substitution by -CH3 would positively favor methane dissolution. When comparing anions with the same [Bmim]+ cation, methane solubility decreased in the order of [Bmim][PFHS]> [Bmim][PFBS]> [Bmim][NTf2]> [Bmim][OTf]> [Bmim][PF6]> [Bmim][BF4]> [Bmim][NO3]> [Bmim][Ac] > [Bmim][HSO4]. When comparing anions with the same [Emim]+ cation, methane solubility decreased in the order of [Emmim][FAP]> [Emim][NTf2]> [Emim][BF3CF3]> [Emim][BF4]. Chen and co-workers suggested that ILs containing more fluorines are beneficial for methane dissolution, especially imidazolium-based [NTf2]-.13 However, although [BF4]- and [PF6]- have 4 and 6 fluorines, respectively, their methane solubilities are less than that of [OTf]- and [NTf2]- based ILs which have only 3 and 6 fluorines, respectively. Therefore, methane solubility was not well correlated with the number of fluorine. It appeared that anion containing more C-F rather than fluorine favors methane dissolution. Five physical properties, polarity, molar density, viscosity, surface tension, molecular weight, were investigated to determine the key factors that affect methane solubility in different ILs. Although methane is a nonpolar molecule, no correlation was found between methane solubility and polarity of anions. For examples, the polarity of ILs decrease in the order of [Bmim][HSO4]> [Bmim][NO3]> [Bmim][OTf]> [Bmim][PFBS]> [Bmim][PFHS]> [Bmim][PF6]≈[Bmim][BF4], but methane solubility followes

[Bmim][PFHS]>

[Bmim][PFBS]>

[Bmim][OTf]>

[Bmim][PF6]>

[Bmim][BF4]>

[Bmim][NO3]> [Bmim][HSO4]. Methane solubility can not be simply explained by the rule of similar 7



polarity. The molar density, viscosity, surface tension, and molecular weight of the ILs are plotted against the measured methane solubility as shown in Figure 3. Methane solubility evidently doesn’t correlate to the viscosity of the ILs. The increased molar fraction of dissolved methane roughly followed increasing molecular weight of the ILs, although poorly correlated in the methane molar fraction range between 4.5 and 9, possibly due to the increased dispersion force. Instead, methane solubility showed better correlation with surface tension and molar density. ILs with lower surface tension and molar density showed higher methane solubility. In the process of methane dissolution, it is conceivable that methane insertion at least caused disturbance to the cation-anion intra-network of ILs. Higher surface tension and molar density correspond to a stronger cation-anion network interaction and smaller intra-network space. The higher surface tension implies a stronger cation-anion network interaction, thus corresponding to larger energy for CH4 insertion in the space of IL’s network. The high strength of cation-anion interaction reflected by large surface tension is consistent with the conclusion of previous works35,37 based on 1H NMR study. For detailed analysis of imidazolium-based ILs, higher methane solubility in [Bmim][NTf2], [Bmim][PFBS] and [Bmim][PFHS] can be attributed to the weaker interaction between the cation and the fluorinated anions, consistent with their low surface tension and molar density. The weaker interaction gives rise to more space within the cation-anion network to accommodate methane without the need for much energy to overcome the cation-anion interaction in the ILs. The ILs with low methane solubility on the contrary, taking [HSO4]- anion as an example, not only have strong Coulombic interaction, but also have strong H-bonding network. The surface tension (52.1 mN/m) of the [Bmim][HSO4] showed in Table 1 are extremely high, thus bringing about the lowest observed methane solubility. In the work of Hardacre38, solubility of gases such as CO2, CO, SO2, N2, H2, O2 and CH4 et al was related to molar volume by regular solution theory (RST) model. Our results are therefore consistent with the work of Hardacre et al. in the effect of intra-network space on the dissolution of gases in ILs. The work of Hardacre38 was focused on the dissolution of CO2 and SO2. It should be pointed out, however, that the results of our work specifically based on methane dissolution indicate that molar density is not the only factor that determines methane solubility. Other factors including intra-network space, C(2)-R (R=H and alkyl) interaction, cation-anion network interaction and H-bonding all contribute to methane solubility. For ILs without Br φsted acid and alkyl-substitution at C(2) position, both molar density and surface tension reflect the space and interaction in the cation-anion network of ILs. For ILs containing acidic groups and C(2)-alkyl substituted cations, such as [C4SO3Hmim]+, [HSO4]- and [Bm2im]+, using molar density is not sufficient to predict methane solubility, as revealed by the results in Table 1. In fact, [Bm2im][NTf2] was reported to be one of the exceptions in the RST model by Hardacre as well.38 Instead, surface tension was a more comprehensive parameter to describe the methane dissolution in a broader range of ILs, including [Bm2im][NTf2]. An empirical equation (3) for methane solubility at 363K, 2 MPa based on surface tension is given by: +,

+,

y 56.45444 ∗ ) * ..----/ + 3.58854 ∗ 10 ∗ ) * .4.--5 6/ + 1.73365 3 Where 8 is the surface tension of a IL, and y is molar fraction of dissolved methane in the IL. Equation (2) has the highest fitting R-square (0.9607) among the 18 fitted models (Table S1) in establishing the relationship between methane solubility and surface tension. Detailed fitting curve of equation (2) is shown in Figure S1. This empirical model is broadly applicable to predict methane solubility in diverse sets of ILs as shown by the results of this work. It is therefore proposed as a universal tool in preliminary assessment of suitable ILs for a specific task without complicated experimental measurements. 8


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Figure 3. Plots of density, surface tension, viscosity and molecular weight against methane solubility in all studied ILs at 363K, 2 MPa. Table 2. Effect of surfactant on methane solubility at 363K, 2 MPa

FC-134: [Bmim][NTf2] (mol)

Solubility (mol %)

Enhancement of solubility

Surface tension (mN/m)

0 0.1 0.16

6.1 6.9 8.5

/ 13.1% 39.3%

29.4 28.6 25.3

FC-134 is trimethyl-1-propanaminium iodide Inspired by the preliminary understanding of the results in Table 1 and Figure 3, we hypothesized that using a suitable fluorinated surfactant in an IL to reduce surface tension of a solvent may help enhance methane solubility. Experiments were therefore designed and carried out to investigate the solubility enhancement of FC-134 on methane dissolution in [Bmim][NTf2] at 363K and 2 MPa (Table 2). The solubility enhancement percentage was calculated using the following equation:

Enhancement

?@ +?A ?A

∗ 100% .

Where cj is the molar fraction of methane in pure [Bmim][NTf2] solvent, ci is the molar fraction of 9



methane in the mixture of surfactant FC-134 and [Bmim][NTf2]. When 1/10 moles of FC-134 with respect to IL was added, the surface tension of mixed solvent was reduced to 28.6 mN/m and the methane solubility was enhanced by 13.1% compared with the methane solubility in pure [Bmim][NTf2]. Along with the increasing amount of FC-134, the methane solubility was increased. When 0.16 moles of FC-134 with respect to the IL was added, a dramatic enhancement (39.3%) in methane solubility was observed due to a much reduced surface tension (25.3 mN/m). Furthermore, these two experimental values coincide with equation (3). No more FC-134 could be added, because 0.16 moles of FC-134 in [Bmim][NTf2]reached saturation. The adding of FC-134 won’t change the properties of ionic liquids. It is only a physical mixture. Cations and anions of ILs still stay as ionic state. The results further confirmed the important effect on methane dissolution by decreasing surface tension. This surface tension based model is particularly important as a new method for predicting improved methane solubility in ILs. Furthermore, neither the molar volume model nor the molar density model is capable of predicting or describing the enhanced methane solubility. 3.2 In-situ high pressure ATR-FTIR analysis To gain a molecular-level insight into the CH4 dissolution in ILs and to understand the fundamental origin of CH4 solubility, we studied [Bmim][NTf2] in greater details by using the in-situ high pressure ATR-FTIR. Golden Gate in-situ ATR-FTIR spectroscopy with diamond ATR accessory (Specac, Ltd., UK) and the high pressure reactor were used as described in the Apparatus and procedure subsection. The FTIR resolution was typically set to 4 cm-1, although higher resolution up to 0. 25 cm-1 was also used to verify the accuracy of the spectra, because the noise was too serious under higher resolution to distinguish rotational-vibrational band position. Gaseous methane was first measured. As shown in Figure S2, there are two infrared-active gaseous methane peaks. The bands at 1304 cm-1 and 3016 cm-1 are v4 C-H deformation and antisymmetric C-H stretching mode, respectively. The band of antisymmetric C-H stretching at 3016 cm-1 was used as the main probe to follow the variation in C-H bond strength during CH4 dissolution in this work.28 The band at 1304 cm-1 overlaped with the absorption of the anion of the IL. Regular rotational-vibrational bands were clearly observed with 9.3cm-1 interval on both sides of the antisymmetric stretching mode (3016 cm-1) of gaseous CH4 at a lower pressure. These bands can be ascribed to molecular rotation energy change when the free rotation of methane occurs. Along with increasing pressure, rotational-vibrational bands disappeared and were replaced by P, R and Q bands. With increasing pressure, the distance between gaseous methane was decreased, and molecular rotational energy level was disturbed due to the collision of molecules. Then, there were only P, R and Q bands observed instead of the fine spectrum of rotational-vibrational bands.

10


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Figure 4. The FTIR spectra of (a) pure [Bmim][NTf2], (b) 7 MPa CH4 in pure [Bmim][NTf2], (c) CH4 degassed in [Bmim][NTf2], (d) charged 7 MPa N2 again after CH4 was completely degassed in [Bmim][NTf2], and (e) pure gas CH4 at 7 MPa. Table 3. Infrared intensity of fine CH4 rotational-vibrational bands before and after dissolved in [Bmim][NTf2] Intensity of gaseous CH4 (*10-3) Pressure

Rotational band at -1

Intensity of dissolved CH4 (*10-3)

Vibrational band at

Rotational band at -1

-1

Vibrational band at

(MPa)

3085.67 cm

3014.82~3016.84 cm

3085.67 cm

3016.85 cm-1

1

0.50

2.24

1.01

7.33

2

0.61

3.21

1.33

9.02

3

0.57

4.51

2.02

9.34

4

0.52

5.42

2.45

11.15

5

0.44

6.27

2.97

13.57

6

0.44

6.78

4.08

16.67

7

0.42

7.45

4.27

17.69

Figure 4 shows the FTIR absorbance spectrum of CH4 and that of the cation of [Bmim][NTf2] with dissolved gases. There are five major cation peaks for the IL from low wavenumber to high wavenumber.39 Peaks at 2967.62 and 2940.74 cm-1 are antisymmetric stretching of terminal –CH3 and – CH2 of N-alkyl chains, respectively. The peak at 2880.33 cm-1 is the symmetric stretching mode of all N-alkyl chains. The shoulder peak at 3105.09 cm-1 was assigned to C(2)-H, and was used as a main signal to probe the strength of the H-bonding between cation and anion of the ILs. Vibrations at 3121.56 and 3157.31 cm-1 are ascribed to the symmetric and antisymmetric stretching modes of C(4,5)-H, respectively. The C(4)-H and C(5)-H are similar in property as C(2)-H as elements of the conjugated imidazolium ring.40,41 Figure 4(a) is the spectrum of pure [Bmim][NTf2]. When CH4 was dissolved in [Bmim][NTf2] (see Figure 4(b)), the spectrum showed strong CH4 antisymmetric and rotational-vibrational bands.29 The rotational-vibratioanl bands intensity of dissolved CH4 increased with increasing pressure from 0.1 MPa to 7 MPa (see Figure S3 in Supporting information) rather than transferring to P and Q bands as observed for gaseous CH4

(see Figure S2). In gaseous CH4, only when the methane pressure is lower 11



than 1 MPa to guarantee enough space for free rotation of molecular CH4, subtle and regular rotational-vibrational bands can be observed. However, during dissolution process, even when pressure of CH4 was up to 7 MPa, rotational-vibrational bands still remained clear in regularly spaced wave numbers. In Table 3, the most sensitive rotational band at 3085.67 cm-1 was chosen to represent the intensity of rotational bands. The peak at about 3016 cm-1 is the vibrational band of methane. Because the FTIR bands observed in this study are very narrow, the absorbance peak height was simply taken as the band intensity. In gaseous methane, vibrational band intensity increased with increasing pressure, but the rotational bands intensity increased a little and then decreased due to increased molecular collision at higher pressure that restricts free molecular rotation. When methane was dissolved in [Bmim][NTf2], however, both rotational and vibrational band intensity increased along with increasing pressure, and the high methane concentration in IL didn’t affect the free rotation of methane. The intensity of dissolved methane is highly prominent compared to that of gaseous methane. The density of dissolved methane at 7 MPa in IL (0.8 mmol/ml) is similar to that of gaseous methane at 2-3 MPa. Dissolved methane at 7 MPa showed absorbance of 4.27*10-3 for the 3085.67 cm-1 band and 17.69*10-3 for the vibrational band. In gaseous methane, however, the absorbance of methane between 2 and 3 MPa are march smaller, at only 0.61-0.57*10-3 for the 3085.67 cm-1 band and 3.21-4.51*10-3 for the vibrational band. At the same pressure of 7 MPa, the absorance of rotational-vibrational band of dissolved methane was higher than gaseous methane (see Figure 4(b,e)). When isolated methane was inserted into the space of IL network during dissolution, the cage space of cation-anion network in the IL was large enough for methane to freely rotate. The enhanced intensity also indicated the chemical environment change of methane which resulted in bigger CH4 dipole variation strength and reduced CH4 molecular symmetry. Methane and IL mutually interact through induced dipole so that methane is stabilized in IL under pressure. Methane dissolution needs to overcome the cation-anion network interaction of IL before it can be dispersed in the liquid cage of IL. Therefore, ILs that have a lower molar density and surface tension would have bigger network space for easy CH4 insertion. After degassing, the FTIR band of degassed [Bmim][NTf2] recovered completely to that of pure IL and the CH4 peaks disappeared (see Figure 4(c)), indicating reversible CH4 dissolution and evolution. After 30 min of degassing to make sure that CH4 was fully degassed, 7 MPa N2 was charged. The bands kept smooth with no shift and no new peak (see Figure 4(d)) as compared to that of neat IL. Experimental observation of N2 dissolution clarified that the peaks corresponding to CH4 dissolved in [Bmim][NTf2] under 7 MPa are unambiguously the FTIR absorption of dissolved CH4 and did not come from pressure disturbance or noise.

12


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Figure 5. The FTIR spectra of (a) antisymmetric C-H stretching mode of gaseous CH4 and (b) antisymmetric C-H stretching mode of dissolved CH4. From bottom to top, the FTIR spectra correspond to increasing pressure from 0.1 MPa to 7.0 MPa. Figure 5 displays the antisymmetric C-H stretching of CH4 before and after dissolved in [Bmim][NTf2]. A 2.02 cm-1 red shift of the antisymmetric C-H with increasing pressure from 0.1 MPa to 7 MPa was observed in gaseous methane, indicating a weakened C-H bond at high pressure resulted from the dynamic coupling at higher gas density. The FTIR spectra (b) of CH4 dissolved in [Bmim][NTf2], however, didn’t show such a shift with increasing pressure. Antisymmetric C-H stretching of dissolved CH4 at pressure 1-7 MPa appeared at the same wavenumber as that of gaseous CH4 at 0.1 MPa. When CH4 was dissolved in IL, induced dipole interaction stabilized the isolated CH4 in the IL-network space in which CH4 was in free rotation like gaseous CH4 at low pressure. The liquid cages of IL restrict CH4 molecules from colliding with each other. In gaseous CH4, half peak width increased from 6.1 to 9.5 cm-1 during 0.1-7 MPa. For dissolved CH4, the half peak width was only 4.2 cm-1 even at 7 MPa due to a regular rotational energy across the vibrational energy level. IL offered a stable chemical space, i.e. liquid cage, for CH4, thus allowing an unconstrained rotation. The ionic environment of ILs induced a dipole interaction with CH4, but didn’t change the methane bond-force constant. Therefore, the ATR-IR spectroscopic analysis indicated that dissolved CH4 showed a constant stretching wavenumber, a narrow half peak width with high band intensity. Infrared spectra of methane dissolved in ILs are different from that of methane adsorbed on the surface of metal oxides. On cerium oxide42 and magnesium oxide43 surfaces, asymmetric interaction of methane adsorbed on the surface of metal oxide resulted in disordered methane Td symmetry. Red shift of two infrared-active methane stretching bands was reported. In addition, an infrared-inactive band was turned to infrared-active at 2875~2900 cm-1 after methane adsorption on the metal oxide surfaces. In ILs, methane was surrounded by ILs network in all direction. Induced dipole interaction is symmetrical to all four methane C-H bonds, and therefore no red shift and deformation of methane was observed. Therefore, interaction between ILs and methane found by this work is totally different from traditional adsorption on a solid surface. The ILs not only provided a stable liquid environment for concentrated methane, but also maintained methane in free-molecule like state. The rotational and vibrational band of methane dissolved in ILs can also be observed obviously in other ILs with high 13



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methane solubility such as [Bmim][PFBS], [Hmim][NTf2], and [Bmim][OTf]. The signal of rotational and vibrational band of methane dissolved in [Bmim][HSO4] is very low because of the low methane solubility. The mechanism of methane dissolved in different ILs in table 1 is similar. 3.3 Kinetic analysis The heat change involved in CH4 dissolution in ILs is too small and slow to be detected by micro DSC. Kinetic experiments were designed to understand the energy involved in CH4 dissolution. Figure 6 shows the results of in-situ measurement of CH4 pressure change when CH4 was dissolved in [Bmim][NTf2] at 313 K, 333 K, 373 K. The measurements were terminated after the pressure reached steady in 5 to 10 minutes. After IL was added into the absorption chamber, about 2 MPa of CH4 was charged at 313 K, 333 K, 373 k, separately. Methane pressure decreased a little before stirring, and decreased rapidly under vigorous stirring until a kinetic equilibrium was reached between CH4 dissolution and evolution. Figure 6 shows the detailed pressure changes of CH4 dissolution process in [Bmim][NTf2] (more detail data in Table S2). The rate of decreasing CH4 pressure followed the order of 373 K> 333 K> 313 K, revealing that dissolution rate of CH4 increased with increasing temperature. At 373 K, it only took 300 s to reach the dissolution equilibrium. But 600 s and 1400 s were needed to reach the equilibrium at 333 K and 313 K. Slow dissolution at the beginning several seconds came from the methane dissolution before stirring stated. According to Equation (2), methane solubility in [Bmim][NTf2] was measured to be 6.135%, 6.148%, 6.182%, separately to temperature at 313 K, 333 K, and 373 K, These three data almost remains constant. Only a little improvement can be found, indicating an extremely weakly endothermic process. The same observation was made for [Bmim][HSO4]. Detailed data are given in Table S2.

Figure 6. Methane dissolution pressure versus time at different temperature in [Bmim][NTf2]. There exists a reversible process for gaseous CH4 to dissolve and for dissolved CH4 to evolve. It is assumed that the dissolution of CH4 in ILs fit the apparent reversible first-order reaction kinetics. The reaction equation of first-order reaction is given below.

CH4(g)

k1 k2

CH4(l)

DE

C DF G HF G H4 HF

(4) 14


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where t is the dissolution time; p0 is the initial charged CH4 pressure; pt is the pressure of gaseous methane at dissolution time t; k1 is the forward reaction rate constant; k2 is the reverse reaction rate constant. k1 and k2 of [Bmim][NTf2] and [Bmim][HSO4] were calculated and related parameters are shown in Table S3. The results were well fitted to the reversible first-order rate hypothesis (Equation 4). Accordingly, the activation energy of absorption and desorption were obtained by Arrhenius equation (in Table 4). Table 4 Activation energy of CH4 dissolved in [Bmim][NTf2] and [Bmim][HSO4] K

Equation: IJG L + IJG4 5

[Bmim][NTf2]

[Bmim][HSO4]

E1 (kJ/mol)

21.86

43.26

R-square of E1

0.9999

0.9664

E2 (kJ/mol)

19.36

42.39

R-square of E2

0.9952

0.9555

∆E = E1- E2 (kJ/mol)

2.50±0.3

0.87±0.5

k is the rate constant (calculated in Table S3); T is the absolute temperature (in Kelvins); R is the universal gas constant; Ea is the activation energy for the reaction; k0 is the pre-exponential factor; E1 and E2 are absorption and desorption activation energy, respectively. Calculated activation energies of methane dissolved in [Bmim][NTf2] and [Bmim][HSO4] are shown in Table 4. Activation energy of absorption (E1) are larger than desorption (E2) in both two ILs, despite of its small ∆E, indicating that CH4 dissolution is a very weakly endothermic process and high temperature favors CH4 dissolution. This is consistent with the measured methane solubility. Endothermic energy (∆E) of [Bmim][NTf2] (2.05 kJ/mol) was higher than [Bmim][HSO4] (0.87 kJ/mol), showing a more endothermic evolution in [Bmim][NTf2]. E1 and E2 of methane in [Bmim][NTf2] was smaller than in [Bmim][HSO4], indicating a smaller energy barrier of CH4 insertion into the [Bmim][NTf2] network. In the process of CH4 dissolution, CH4 inserted into and caused disturbance to the cation-anion network of ILs by overcoming the energy of weak Coulombic interaction and weak H-bonding network interaction between the cations and anions of the ILs. The strong cation-anion interaction in [Bmim][HSO4], especially a strong H-bond of [HSO4]- with imidazolium C(2)H as suggested by its high surface tension is consistent with a high activation energy of absorption (43.26 kJ/mol) and desorption (42.39 kJ/mol). The stronger cation-anion interaction is responsible for the higher activation energy barrier, so it makes more difficult for methane to dissolve. The results of this work support that surface tension is a universal physical parameter to evaluate the network interaction of ILs. 4. CONCLUSION In this work, the measured methane dissolution, in-situ ATR-FTIR characterization, and kinetic/thermodynamic analysis have led to the understanding of the factors governing the ranking of ILs for the solubility of CH4. From the structural perspective, ILs containing long alkyl chains and C-F bonds showed higher CH4 solubility. The common properties of these ILs are lower surface tension and molar density, indicating that weaker interactions between cations and anions in the IL network favored CH4 dissolution. The absorption and desorption energies of CH4 in the space of IL’s network during the dissolution process depends on the strength of the cation-anion network interaction. Reducing the 15



surface tension by adding 0.1 and 0.16 moles of FC-134 with respect to [Bmim][NTf2] resulted in 13.1% and 39.3% improvement in CH4 solubility, respectively. ILs with C(2)-H in the cation substituted by C(2)-CH3 resulted in reduced CH4 solubility, suggesting that C(2)-H is a position interacting with CH4. Full methyl substitution of the H at the C(2,4,5) positions by -CH3 group resulted in substantially reduced surface tension and increased solubility as compared to C(2) substituted IL, indicating a considerably weakened interaction between the [Bm4im]+ cation and [NTf2]- anion compared to that of [Bm2im]+ and [NTf2]-. It is determined that the C(2)-H is not the only site governing the solubility. The intra-network space and interaction of cation-anion network in ILs are also important factors. ATR-FTIR spectroscopy was successfully used to study methane dissolution by revealing the methane dissolution mechanism. Peak at 3016.85 cm-1 exhibited antisymmetric C-H stretching of CH4 dissolved in [Bmim][NTf2]. The ILs as a medium allows reversible CH4 dissolution and evolution. Comparing ATR-FTIR results of dissolved CH4 in [Bmim][NTf2] with gaseous CH4, rotational-vibrational bands of dissolved CH4 became considerably intense than that of gaseous CH4 under the same pressure; half peak width of antisymmetric C-H stretching band of dissolved CH4 became much narrower than that of gaseous CH4; antisymmetric C-H stretching didn’t shift along with pressure changes. For the first time the results of this work revealed that CH4 was dissolved as isolated molecule into physically and chemically stable IL’s network and exhibited in free and regular rotation without collision with other methane molecules. Induced interaction between CH4 and IL can result in increased CH4 dipole variation strength and reduced CH4 molecular symmetry. Methane dissolution was a reversible first order and very weakly endothermic process in [Bmim][NTf2] and [Bmim][HSO4] resulting from kinetic/thermodynamic analysis. AUTHOR INFORMATION Corresponding Author *Tel: +86 8437 9462. Email: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by the Chinese Central Government “Thousand Talent Program” Funding and by Saudi Basic Industry Corporation funding. SUPPORTING INFORMATION. Infrared spectra of gaseous methane and methane dissolved in [Bmim][NTf2] at different pressures; detailed methane dissolution data in [Bmim][NTf2] and [Bmim][HSO4] at different temperatures; and detailed kinetic analysis process.

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TABLE OF CONTENTS IMAGE

The FTIR spectra of (a) pure [Bmim][NTf2], (b) 7 MPa CH4 in pure [Bmim][NTf2], (c) CH4 degassed in [Bmim][NTf2], (d) charged 7 MPa N2 again after CH4 was completely degassed in [Bmim][NTf2], and (e) pure gas CH4 at 7 MPa. ILs provided a stable liquid environment for concentrated methane, and maintained methane in free-molecule like state.

19


Structural Dependence and Spectroscopic Evidence of Methane

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