Absorption of n-butane in imidazolium and phosphonium ionic liquids and application to separation of hydrocarbon gases

Absorption of n-butane in imidazolium and phosphonium ionic liquids and application to separation of hydrocarbon gases

Accepted Manuscript Absorption of n-Butane in Imidazolium and Phosphonium Ionic Liquids and Application to Separation of Hydrocarbon Gases Takashi Mak...

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Accepted Manuscript Absorption of n-Butane in Imidazolium and Phosphonium Ionic Liquids and Application to Separation of Hydrocarbon Gases Takashi Makino, Mitsuhiro Kanakubo PII: DOI: Reference:

S1383-5866(17)33934-5 https://doi.org/10.1016/j.seppur.2018.04.032 SEPPUR 14528

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

30 November 2017 9 April 2018 14 April 2018

Please cite this article as: T. Makino, M. Kanakubo, Absorption of n-Butane in Imidazolium and Phosphonium Ionic Liquids and Application to Separation of Hydrocarbon Gases, Separation and Purification Technology (2018), doi: https://doi.org/10.1016/j.seppur.2018.04.032

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Absorption of n-Butane in Imidazolium and Phosphonium Ionic Liquids and Application to Separation of Hydrocarbon Gases Takashi Makino* and Mitsuhiro Kanakubo National Institute of Advanced Industrial Science and Technology (AIST), 4-2-1 Nigatake, Miyagino-ku, Sendai 983-8551, Japan *

Corresponding author. E-mail: [email protected], fax: +81-22-232-7002.

Abstract In this report, the density, viscosity, and heat capacity of the tetraalkylphosphonium ionic liquids, containing newly synthesized trihexyltetradecylphosphonium p-dodecylbenzenesulfonate and bis(2ethylhexyl)sulfobutanedioate, were measured at atmospheric pressure. Then, the n-butane absorption in the dialkylimidazolium and tetraalkylphosphonium ionic liquids were investigated at the n-butane pressure

of

0.101

MPa

and

the

temperatures

of

(298.15-353.15)

K.

In

a

series

of

bis(trifluoromethanesulfonyl)amide ionic liquids, the trihexyltetradecylphosphonium salt absorbed the largest amount of n-butane, followed by the triethyloctylphosphonium, 1-methyl-3-octylimidazolium, and 1-butyl-3-methylimidazolium salts. On the other hand, in the trihexyltetradecylphosphonium ionic liquids, the bis(2-ethylhexyl)sulfobutanedioate and p-dodecylbenzenesulfonate salts had the higher solubilities of n-butane than the bis(trifluoromethanesulfonyl)amide salt. In addition, the solubilities of water vapor in the trihexyltetradecylphosphonium salt were measured at 298.2 K and atmospheric pressure. Trihexyltetradecylphosphonium bis(trifluoromethanesulfonyl)amide was the most free from the moisture. We further performed the continuous gas absorption experiments of n-heptane and toluene using the present tetraalkylphosphonium ionic liquids. They removed successfully the vapors of hydrocarbons despite the very dilute concentrations of hydrocarbons, less than 1500 ppm. The initial absorption rates were calculated to discuss the absorption kinetics in the tetraalkylphosphonium ionic liquids.

Keywords Ionic liquid; Imidazolium; Phosphonium; Hydrocarbon; Absorption

1. Introduction Room temperature ionic liquids (ILs) are salts with their melting points being at or below ambient temperatures. ILs are commonly non-volatile, non-flammable, and thermally and chemically stable, and they have wide electrochemical window, high solubility of certain specific chemicals, and so on. Because of these unique characteristics, ILs have attracted much attention, for example, as solvents, catalysts, electrolytes, lubricants, and absorbents. Furthermore, physical and chemical properties of ILs can be optimized for individual applications by the chemical modification of ionic species as well as the combination of cation and anion. A large number of researches have been performed to understand the effects of chemical modifications and combinations on the natures of ILs. ILs can be promisingly utilized in separation technology [1-7], in particular, gas separation. Separation of hydrocarbon gases is an important process in petrochemical industry for the natural-gas purification and the olefin/paraffin separation and in paint industry for the reuse of volatile organic compounds. Several separation technologies, absorption, adsorption, membrane separation, and cryogenic separation, are available for hydrocarbon gases depending on the temperature, pressure, concentration, component and throughput of separation targets. Absorption is a versatile separation technique, however, conventional organic absorbents for hydrocarbons have some drawbacks such as volatility, flammability, and contamination into the products. ILs can overcome these problems because of their non-volatile and non-flammable natures, and thus, they are expected as potential absorbents. A primarily important factor in absorbents is the absorption property for hydrocarbon gases. There are a number of literatures on vapor-liquid equilibria for hydrocarbon + IL systems, in particular, the methane + IL and ethane + IL systems [8-24]. It is reported that the Henry constants of ethane in 1-butyl-3methylimidazolium bis(trifluoromethanesulfonyl)amid ([bmim][Tf2N]) is the smallest, followed by the hexafluorophosphate ([PF6]-), dicyanamide ([DCA]-), and tetrafluoroborate ([BF4]-) with the same cation [8-10]. The effect of anion species on the absorption of methane is similar with the absorption of ethane [10-12]. Moreover, ILs consisting of the cation with long alkyl chains absorb larger amounts of alkane. For

example,

the

solubilities

of

ethane

in

1-alkyl-3-methylimidazolium

bis(trifluoromethanesulfonyl)amid increases with increasing the carbon number of allyl chain in the order

of ethyl ([emim]+) < butyl ([bmim]+) < hexyl ([hmim]+) < octyl ([omim]+) [8,9,13,14]. Tetraalkylphosphoniums like tetrabutylphosphonium ([P4444]+), trimethyloctylphosphonium ([P1118]+) and trihexyltetradecylphosphonium ([P666,14]+) also improve the solubilities of alkanes [18,21-24]. The recent literature pointed out that the long alkyl chain strengthens the van der Waals interaction between the alkane and IL molecules [23]. One can speculate from these findings that the combination of cations and anions with long alkyl chains is the best way to increase the alkane absorption. Actually, Henry constants of

ethane

in

trihexyltetradecylphosphonium

bis(2,4,4-trimethylpentyl)phosphinate

and

tributylethylphosphonium octadecanoate are the first and second smallest among the reported ILs [22, 24]. On the other hand, the enlongation of alkyl chains in ILs often causes the slow mass transfer because of the high viscosity. It is proposed to use the ILs with somewhat long alkyl chains for industrial application [25]. Unlike the absorption of methane and ethane, the number of reports on the absorption of propane and nbutane is limited. Here, we focused on the n-butane absorption in ILs with long alkyl chains. The absorption

of

n-butane

was

investigated

in

dialkylmethylimidazoliums

with

[Tf2N]-

and

tris(pentafluoroethyl)trifluorophosphate ([FAP]-) [26-30]. The Henry constant of n-butane decreased with the enlongation of alkyl chains on the dialkylimidazolium as observed for methane and ethane [27,29]. 1Decyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([dmim][Tf2N]) showed the highest nbutane solubility among the reported ILs. [27]. Therefore, one can expect much higher solubilities of nbutane in ILs by further increase of alkyl groups. In the present study, solubilities of n-butane were measured in the dialkylimidazolium and tetraalkylphosphonium ILs with long alkyl chains at the n-butane pressure of 0.101 MPa. The ILs investigated were [bmim][Tf2N], 1-butyl-3-methylimidazolium octylsulfate

([bmim][C8SO4]),

[omim][Tf2N],

triethyloctylphosphonium

bis(trifluoromethanesulfonyl)amide ([P2228][Tf2N]), and [P666,14][Tf2N] in addition to newly synthesized trihexyltetradecylphosphonium

p-dodecylbenzenesulfonate

([P666,14][DBS])

and

trihexyltetradecylphosphonium bis(2-ethylhexyl)sulfobutanedioate ([P666,14][AOT]). The density, viscosity, and heat capacity of the tetraalkylphosphonium ILs were measured with the solubility of water vapor at atmospheric pressure. Furthermore, we performed the continuous gas absorption experiments of n-

heptane and toluene using the tetraalkylphosphonium salts to investigate whether they can remove hydrocarbon gases even at low partial pressures or not. Fig. 1 shows the chemical structures of ionic species constituting the ILs investigated.

2. Experimental 2.1. Materials Materials used in the present study are summarized in Table 1. 1-butyl-3-methylimidazlium bromide ([bmim]Br, purity: 99 %), 1-methyl-3-octylimidazolium chloride ([omim]Cl, purity: 99 %), and lithium bis(trifluoromethanesulfonyl)amide (Li[Tf2N], purity: 99 %) were obtained from Kanto Chemical Co., Inc. [bmim][C8SO4] (purity: >98 %) was supplied from Merck KGaA. [P666,14][Tf2N] (purity: >98 %) and trihexyltetradecylphosphonium chloride ([P666,14]Cl, purity: >95 %) were purchased from Iolitec GmbH. The triethyloctylphosphonium bromide ([P2228]Br, 50 wt%) aqueous solution was given by Nippon Chemical Industrial Co., Ltd. Sodium p-dodecylbenzenesulfonate (Na[DBS], purity: 99 %) and sodium bis(2-ethylhexyl)sulfobutanedioate (Na[AOT], purity: 99 %) were purchased from Sigma-Aldrich Co. N2 (purity >99.999 %), n-butane (purity: 99.95 %) and standard gases of n-heptane (524 ppm, N2-balance) and toluene (1480 ppm, N2-balance) were supplied from Iwatani Co. (N2), Takachiho Chemical Industrial Co., Ltd. (n-butane and n-heptane), and JFP Co., Ltd. (toluene), respectively. The chemicals and gases except for the ILs investigated were used without further purification. Any excess volatile compounds in ILs were removed by evacuation at 343 K (the [Tf2N]- salts) or 323 K (the other salts) for 30 h just before the measurements. Purified samples were transferred into closed cells or instruments by using an airtight syringe under dry N2 (dew point: < 258 K). The water contents (w/w) in ILs were measured with a Karl Fischer titration (KEM, MKC-520) as <50 ppm ([bmim][Tf2N], [bmim][C8SO4], [omim][Tf2N], and [P666,14][AOT]), 258 ppm ([P2228][Tf2N]), 554 ppm ([P666,14][Tf2N]), and 61 ppm ([P666,14][DBS]). The synthesized ILs were identified by 1H and

13

C NMR (Bruker Ascend400 NMR spectrometer with

benzene-d6 sealed in a capillary as the external standard at ~313 K). [P666,14][DBS] and [P666,14][AOT] were further analyzed with elemental analysis (Elementar, VarioMICRO with acetanilide as a standard) for H,

C, and N atoms. 2.2. Synthesis 2.2.1. [bmim][Tf2N], [omim][Tf2N], and [P2228][Tf2N]. The synthetic procedure was described in the previous studies in detail [31-33]. An aqueous solution of [bmim]Br was mixed and stirred for 12 h at room temperature with the equivalent molar amount of Li[Tf2N] aqueous solution. The thus-obtained organic phase was separated and washed with MilliQ water several times to remove LiCl. The halogen content of aqueous solution in contact with the sample was less than the detection limit of AgNO3 testing. After that, [bmim][Tf2N] was dried at 343 K under vacuum for 30 h to remove any volatile compounds. [omim]Cl and [P2228]Br were used instead of [bmim]Br to synthesize [omim][Tf2N] and [P2228][Tf2N], -

1

respectively. The purities were estimated as ~99 % for the synthesized [Tf2N] salts by H NMR spectra. 2.2.2. [P666,14][DBS] and [P666,14][AOT]. [P666,14]Cl was dissolved in dichloromethane, and the equivalent molar amount of Na[DBS] or Na[AOT] in MilliQ water. Then, the aqueous solution was added dropwise to the dichloromethane solution, and the mixture was stirred at room temperature for 12 h. The separated organic phase was washed with MilliQ water until the concentration of Cl- in the aqueous extract fell below the detection limit of AgNO3 testing. Then, dichloromethane was removed from the organic solution by using an evaporator. Finally, the products were dried under vacuum at 323 K for 30 h, and the slight yellowish ([P666,14][DBS]) and colorless ([P666,14][AOT]) viscous liquids were obtained. The estimated purities were > 99 % for the both [P666,14]+ ILs. The 1H and 13C NMR spectra are given in Figs. S1 and S2. Anal. Calc. (%) for [P666,14][DBS] (C50H97O3PS, M.W. 809.357): C, 74.20; H, 12.08; N, 0.00; Found: C, 73.81; H, 12.16; N, 0.00. Anal. Calc. (%) for [P666,14][AOT] (C52H105O7PS, M.W. 905.439): C, 68.98; H, 11.69; N, 0.00; Found: C, 68.68; H, 11.73; N, 0.00. 2.3. Density, viscosity, and heat capacity measurements The density, viscosity, and heat capacity of the tetraalkylphosphonium ILs were measured with the same apparatuses and procedures reported in our previous studies [34-37]. The densities were determined with a vibrating tube densimeter (Anton Paar, DMA 500M). The viscosities were measured using a

rotating-cylinder viscometer (Anton Paar, Stabinger SVM 3000). Each apparatus was calibrated with reference samples. The isobaric heat capacity was measured with a Calvet type calorimeter (Setaram, microDSC7), calibrated using the Joule effect module (Setaram, EJ3) and sapphire as a reference sample. The temperature was maintained within ±0.01 K. The instrumental accuracy for the density is less than ±0.05 kg m-3, whereas the expanded uncertainty is ±0.1% because of impurities. The expanded uncertainties for the viscosity and heat capacity are less than ±2 % and ±0.2 %, respectively. 2.4. n-Butane solubility measurement The experimental apparatus and procedure for the n-butane solubility measurement at atmospheric pressure was the same as described in detail elsewhere [38]. A desired amount of IL wIL was weighted with an electrical balance (Mettler Toledo, AB204-S). The weights of the IL and glass cell after saturated with N2 wN2 and n-butane wC4 were also determined gravimetrically. The amount of n-butane absorbed in IL w1 was calculated as the difference between wN2 and wC4, because the N2 solubility can be assumed to be negligible at the present condition. After the solubility measurement at 298.15 K, the temperature was changed in the following sequence, 313.15, 333.15, and 353.15 K. The n-butane solubility in mole fraction scale was given as x1 = (w1/M1) / (w1/M1 + wIL/MIL), where M1 and MIL stand for the molar masses of n-butane and IL. The n-butane pressure p1 was 0.101±0.001 MPa, and the temperature T was kept constant within ±0.01 K. The uncertainty of x1 is estimated to be less than 0.01. 2.5. Water vapor solubility measurement Approximately 1 g of the [P666,14]+ ILs were weighed out in glass sample bottles under dry N2 atmosphere. The initial total weight of the IL and bottle were determined with an analytical balance (Mettler Toledo, XS205). Then, the open bottles were set in a constant temperature and humidity chamber (Espec, SH-641). The temperature and the humidity were monitored using a thermo-hygrometer (Rotronic, Hygropalm HC2). The weight gain by the absorption of water vapor w2 was gravimetrically measured for a certain period until the weight change was less than ±0.001 g. Then, the solubilities of water vapor in equilibrium were obtained as the weight ratio W2 = w2 / wIL and the mole fraction x2 = (w2/M2) / (w2/M2 + wIL/MIL), where M2 is the molar mass of water. The uncertainties of temperature and

humidity are within ±0.1 K and ±0.8 %RH (±0.025 kPa at 298.2 K). The uncertainties for W2 and x2 were estimated as ±0.002, and ±0.01, respectively. 2.6. Continuous gas absorption experiment of hydrocarbon A flow sheet of an experimental apparatus for the continuous gas absorption experiment is shown in Fig. 2. A ~5.0 ml of the dried tetraalkylphosphonium IL was transferred into a glass cell and weighed with the electrical balance in the glove box filled with dry N2. The cell was attached to the experimental apparatus and immersed in the water bath kept at 298.2 ± 0.1 K. The temperature was monitored using a K-thermocouple, calibrated with a Pt-resistance thermometer (Fluke 1502A and 5626). A 10 ml min-1 of N2 was flowed through the cell for >30 min by a mass flow controller (Horiba-STEC, SEC E-40) to exchange the atmosphere in the apparatus with N2 completely. The solution was stirred at a fixed revolution speed during the experiment. Then, the flow channel was switched from the cell to the bypass. N2 was stopped and the n-heptane (524 ppm) or toluene (1480 ppm) standard gas was supplied at the same flow rate until a gas cell with ZnSe windows equipped in a FT-IR spectrometer (Thermo Fischer, Nicolet iS5) was filled throughout with the standard gas. The temperature of the gas cell was maintained at 423 K to avoid the adsorption of water and hydrocarbons on the inner surface of the gas cell. After that, the flow channel was switched back to the cell, and the standard gas started to flow into the IL phase in the glass cell. The concentration of hydrocarbon in the treated gas was analyzed using the FT-IR spectrometer at 5 minute intervals for 180 min. The calibration factor f for the concentration of hydrocarbon was determined from c0 = f·A using the standard gas just before each experiment, where the symbols c0 and A denote the concentration of hydrocarbon in the standard gas and the peak area in the wave number range of 2700-3200 cm-1. Typical IR spectra for the n-heptane and toluene standard gases are given in Fig. S3. The uncertainty of the concentration of hydrocarbon c was estimated within ±3 %.

3. Results and Discussion 3.1. Density, viscosity, and molar heat capacity

The experimental values of the densities  and viscosities  of newly synthesized [P666,14][DBS] and [P666,14][AOT], and the molar heat capacities Cp of the tetraalkylphosphonium ILs are summarized in Tables S1-S3. The densities and the heat capacities were fitted to quadratic equations, and the viscosities were correlated by the Vogel-Fulcher-Tammann (VFT) equation. The coefficients of the best fits for the equations are listed in Table S4. The physical properties of the dialkylimidazolium and tetraalkylphosphonium ILs at 313.15 K are summarized in Table 2 [39-46]. Here, we note the experimental data for [P666,14][DBS] and [P666,14][AOT] are reported for the first time. The density, viscosity, and molar heat capacity of each IL showed the ordinary temperature dependency. The densities and thermal expansion coefficients (/K-1;  ≡ (1/V)(V/T)p) of [P666,14][Tf2N], [P666,14][AOT], and [P666,14][DBS] decreased in this order. The van der Waals volumes of ions in vacuo, calculated using the ab initio calculations [47] with a B3LYP/6311G+(2d,p) basis set, were summarized in Table 3. The ratios of the molar mass to the van der Waals volume for the ILs decreased in the order of [P666,14][Tf2N] >> [P666,14][DBS] ≈ [P666,14][AOT]. This is roughly consistent with the order for the densities, [P666,14][Tf2N] >> [P666,14][AOT] > [P666,14][DBS], although the small difference in the densities between [P666,14][DBS] and [P666,14][AOT] cannot be explained. The free volumes Vfree in ILs are also listed in Table 3. Here, Vfree is defined as the difference between the molar volume Vm (≡ M/) and the van der Waals volume (Vvdw+ + Vvdw-), where Vvdw+ and Vvdw+ stand for the van der Waals volumes of cation and anion. [AOT]- showed the largest Vfree among the anions investigated followed by [DBS]-, [Tf2N]-, and [C8SO4]-. In addition, the Vfree in the [Tf2N]- salts +

+

+

+

increased in the following orders, [P666,14] > [P2228] > [omim] > [bmim] . The ions with long alkyl chains tend to increase the Vfree in the present ILs except for [C8SO4]-. The viscosities of the three [P666,14]+ ILs increased in [P666,14][Tf2N] < [P666,14][AOT] << [P666,14][DBS], different from the trend of the molar mass. The Mulliken charges on the oxygen atoms in [DBS]- and [AOT]- were -0.648 and -0.628, respectively, which are much smaller (larger in magnitude) than that in [Tf2N]- (-0.568). Thus, the viscosity increase for [P666,14][DBS] would be attributed to the stronger

interionic interaction. A similar trend was observed for the carboxylate ILs between 1-ethyl-3methylimidazolium acetate and trifluoroacetate [48,49]. Compared to the other [P666,14]+ ILs containing the anions with long alkyl chains [50,51], the viscosities increased in decanoate ([Dec]-) < [AOT]- < bis(2,4,4-trimethylpentyl)phosphinate ([TMPP]-) < [DBS]-. The calculated Mulliken charges on the oxygen atoms in [Dec]- (171.261 g mol-1) and [TMPP]- (303.447 g mol-1) were -0.640 and -0.751. It implies that the interionic interaction in [P666,14][Dec] is comparable or stronger than those in [P666,14][DBS] and [P666,14][AOT]. Thus, the low viscosity of [P666,14][Dec] would derive from the smaller molecular size of [P666,14][Dec]-. [P666,14][AOT] gave the highest heat capacity in the three [P666,14]+ ILs. The higher heat capacity of [P666,14][AOT] could be due to the larger degree of freedom of molecular motions of [AOT]-.

3.2. n-Butane and water vapor solubilities Table S5 lists the solubilities of n-butane in ILs at the n-butane pressures of 0.101 MPa. Fig. 3 presents the mole fraction scaled solubility x1 as a function of temperature (Fig. S4 is the same graph without error bars). The present experimental values in [bmim][Tf2N] and [omim][Tf2N] agree well with the literature values at the temperatures investigated [26,27]. The solubility of n-butane in each IL decreased with increasing the temperature as observed for other hydrocarbon gases. At 298.15 K, the solubility increased in [bmim][Tf2N] << [omim][Tf2N] ≈ [bmim][C8SO4] < [P2228][Tf2N] << [P666,14][Tf2N] < [P666,14][DBS] ≈ [P666,14][AOT]. The amount of n-butane in [P2228][Tf2N] was almost the same as that in [dmim][Tf2N] [27]. The trend indicates that the cations and anions with long alkyl chains are effective to increase the solubility of n-butane as reported in the earlier studies [8,9,13,14,18,21-24]. In particular, [P666,14]+ has the most significant effect in the ionic species used in the present study. [C8SO4]-, [DBS]-, and [AOT]- are superior to [Tf2N]- in the n-butane absorption. We suppose that the extension of alkyl chains increases the van der Waals interaction between n-butane and IL as mentioned in the introduction. Actually, the numbers of the methyl and methylene groups in the ILs are 5 for [bmim][Tf2N], 9 for [omim][Tf2N], 13 for [bmim][C8SO4], 14 for [P2228][Tf2N], 32 for [P666,14][Tf2N], 44 for [P666,14][DBS], and 48 for [P666,14][AOT], which coincides with the order of the n-butane solubility x1,298.15 at 298.15 K as plotted in

Fig. 4. In view of the slight convex behavior, the further extension of alkyl chains might be less effective to improve the n-butane solubility. Another possible factor in the solubility of n-butane is the free volume Vfree in ILs, where n-butane molecules can dissolve, as reported for the CO2 physisorption in ILs [52,53]. As listed in Table 3, the Vfree of [bmim][C8SO4], [bmim][Tf2N], [omim][Tf2N], [P2228][Tf2N], [P666,14][Tf2N], [P666,14][DBS], and [P666,14][AOT] increased in this order. This order is roughly consistent with the order of n-butane solubilities, except for [bmim][C8SO4]. Comparing the cations, the extension of alkyl chains increases both the van der Waals interaction and the free volume, resulting in the increase of n-butane solubility. We suppose that the exchange of the anions [DBS]- and [AOT]- with [Tf2N]- also improves both factors as in the cations. On the other hand, since the Vfree in [bmim][C8SO4] is smaller than that in [bmim][Tf2N], the larger solubility in [bmim][C8SO4] is attributable to the van der Waals interaction. Water vapor commonly exists in various gas separation sources. Since water is not a good absorbent for hydrocarbons, contamination of water in absorbents would affect the solubility of hydrocarbons. Therefore, ILs with stronger hydrophobicity will be desirable for the hydrocarbon separation. Fig. 5 presents the solubility (mole fraction) of water vapor in the [P666,14]+ salts at 298.2 K, and the experimental values are summarized in Table S6. Note that the tiny weight of water dramatically increases the mole fraction scaled solubility. [P666,14][Tf2N] absorbed the lowest amount of water vapor in the present [P666,14]+ +

ILs, followed by [P666,14][AOT] and [P666,14][DBS]. The values of x1 in the [P666,14] ILs were much smaller than those in hydrophilic ILs; see for example, in 1,3-dimethylimidazolium methylphosponate (x1 = ~0.76 at ~1.6 kPa and 298.2 K) and 1-ethyl-3-methylimidazolium thiocyanate (x1 = ~0.63 at ~1.6 kPa and 298.2 K) [54]. The spectroscopic and molecular dynamics studies pointed out that the hydrogen bonds between the charged atoms of anion and the hydrogen atoms of water are the dominant factor in the water vapor absorption [55,56]. It is suggested that [Tf2N]- has the lower affinity for the hydrogen atoms of water than [AOT]- and [DBS]-. [P666,14][Tf2N] is the most free from the moisture and has the highest selectivity of nbutane to water vapor in the present [P666,14]+ ILs.

3.3. Continuous absorption of hydrocarbon gas

Fig. S5 gives the time variations of the concentration of n-heptane/toluene in the gas cell. The concentration decrease from the initial values (n-heptane, 524ppm; toluene, 1480 ppm) contained the contributions of both gas absorption and gas replacement. We carried out the continuous gas absorption experiments without ILs (blank experiments in Fig. S5) and assumed that the replacement of N2 in the pre-heating coil and the glass cell with the n-heptane and toluene standard gases took 10 min and 35 min, respectively. Hereafter, we only refer to the experimental data after these periods for the following discussion. Fig. 6-(a) shows the concentration of n-heptane cC7H16 in the treated gas at 298.2 K. All the tetraalkylphosphonium ILs investigated continuously removed n-heptane from the original gas mixture (the content of n-heptane = 524 ppm) for 180 min. The cC7H16 in the treated gas did not reach the concentration of original gas during the gas absorpion experiment, which means that the investigated ILs were not saturated with n-heptane. Since the concentrations of n-heptane in the gas mixtures treated with the [P666,14]+ ILs were lower than [P2228][Tf2N], the former salts was more efficient than the latter. Among the [P666,14]+ ILs, [P666,14][AOT] and [P666,14][DBS] showed slightly better performances than [P666,14][Tf2N]. In order to discuss the absorption kinetics, we calculated the volume of n-heptane VC7H16 absorbed in IL as a function of time (Fig. 6-(b)) based on Fig. 6-(a). The VC7H16 at 10 min in Fig. 6-(b) is regarded as zero. Note that the integrated values are not the equilibrium solubility because the ILs were not saturated even at 180 min. All curves showed the slight concave behavior and the total volume of absorbed n-heptane at 180 min increased in [P2228][Tf2N] << [P666,14][Tf2N] < [P666,14][DBS] ≈ [P666,14][AOT]. We obtained the initial absorption rate vabs from the VC7H16 in the range of 10 – 40 min. The vabs for [P2228][Tf2N], [P666,14][Tf2N], [P666,14][DBS], and [P666,14][AOT] were 0.930 ml h-1, 1.36 ml h-1, 1.39 ml h-1, and 1.43 ml h-1, respectively. [P666,14][DBS] and [P666,14][AOT] are more viscous than [P2228][Tf2N] and [P666,14][Tf2N] as described in the section 3.1. Nevertheless, the absorption rate for [P666,14][AOT] was the fastest in the present tetraalkylphosphonium ILs. This suggests that the absorption equilibrium is still dominant in the present hydrocarbon separation experiments, and/or the mechanical stirring of absorbents can sufficiently facilitate the mass transportation. Similar experiments for a toluene mixture (1480 ppm) were carried out to investigate the performance

of the present tetraalkylphosphonium salts in the separation of aromatic hydrocarbon. Figs. 7-(a) and –(b) present the time profiles of (a) the concentration of toluene cC7H8 in the treated gas and (b) the volume of toluene VC7H8 absorbed in ILs. As shown in Fig. 7-(a), the cC7H8 was lower than 400 ppm at the ends of the experiments. This result indicates that the ILs can remove the toluene vapor as well as the n-heptane vapor, and still absorb toluene even after 180 min. The time profiles of the VC7H8 absorbed in the IL are shown in Fig. 7-(b), where the VC7H8 at 35 min is regarded as zero. Fig. 7-(b) reveals that the total volume of absorbed toluene at 180 min slightly increased in [P666,14][DBS] < [P666,14][AOT] ≈ [P2228][Tf2N] < [P666,14][Tf2N].The initial absorption rate vabs was calculated from the VC7H8 in the range of 35 - 65 min, and the vabs were 4.19 ml h-1 for [P2228][Tf2N], 4.25ml h-1 for [P666,14][Tf2N], 3.97 ml h-1 for [P666,14][DBS], and 4.15 ml h-1 for [P666,14][AOT]. Unlike the case for the n-heptane absorption experiment, the significant differences among the ILs were not observed under the present experimental conditions.

Conclusion We newly synthesized [P666,14][DBS] and [P666,14][AOT], and measured their densities, viscosities, and heat capacities at atmospheric pressure. [P666,14][DBS] and [P666,14][AOT] were less dense and more viscous than [P666,14][Tf2N]. The stronger interionic interaction in [P666,14][DBS] compared to [P666,14][AOT] would cause the higher viscosity. The molar heat capacities of [P666,14][AOT], [P666,14][DBS], and [P666,14][Tf2N] decreased in this order. The n-butane absorption in seven ILs were investigated at the n-butane pressure of 0.101 MPa and the temperature range from 298.2 K to 353.2 K. The experimental results indicated that the cations and anions with long alkyl chains were effective to increase the solubilities of n-butane. It is supposed that the extension of alkyl chains in the cations strengthens the van der Waals interaction and increases the free volume in the ILs, resulting in the solubility increase. Compared to [Tf2N]-, [DBS]- and [AOT]- would improve both the van der Waals interaction with n-butane and the free volume. On the other hand, the higher solubility of n-butane in [bmim][C8SO4] would be attributed to mainly the stronger van der Waals interaction. [P666,14][DBS] and [P666,14][AOT] were the best ILs for n-butane solubility in the

present study. On the other hand, [P666,14][Tf2N] showed the highest selectivity of n-butane to water vapor. Furthermore, we performed the continuous gas absorption experiments of n-heptane and toluene with [P2228][Tf2N], [P666,14][Tf2N], [P666,14][DBS] and [P666,14][AOT]. Each IL removed the vapors of hydrocarbons continuously for 3 h despite the dilute concentrations of hydrocarbons, less than 1500 ppm. In the case of the n-heptane absorption, [P666,14][AOT] and [P666,14][DBS] were superior in the absorption amount and kinetics to [P2228][Tf2N] and [P666,14][Tf2N] in spite of their higher viscosities. On the other hand, the present phosphonoium ILs showed the similar performance in the toluene absorption under the present conditions.

Nomenclature A

peak area [-]

Cp

isobaric molar heat capacity [J K-1 mol-1]

c

concentration of hydrocarbon [-]

c0

concentration of hydrocarbon in the standard gas [-]

f

calibration factor [-]

M

molar mass [kg mol-1]

p

pressure [Pa]

T

temperature [K]

t

time [s]

V

volume [m3]

Vm

molar volume [m3 mol-1]

vabs

initial absorption rate [m3 s-1]

W

weight fraction [-]

w

mass [g]

x

mole fraction [-]



thermal expansion coefficient [K-1]

η

viscosity [Pa s]

ρ

density [kg m-3]

subscript 1

component 1 (n-butane)

2

component 2 (water vapor)

IL

ionic liquid

C7H16

n-heptane

C7H8

toluene

Acknowledgement The authors would thank Ms. Eriko Niitsuma, Ms. Yuki Nagase, Mr. Atsuhiro Oguni, and Ms. Kaori Takeshita for their assistance with the measurements in the present study.

References

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Table 1. Materials used in the present study. Chemical 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide 1-butyl-3-methylimidazolium n-octylsulfate 1-methyl-3-octylimidazolium bis(trifluoromethanesulfonyl)amide triethyloctylphosphonium bis(trifluoromethanesulfonyl)amide trihexyltetradecylphosphonium bis(trifluoromethanesulfonyl)amide trihexyltetradecylphosphonium p-dodecylbenzenesulfonate trihexyltetradecylphosphonium bis(2-ethylhexyl)sulfobutanedioate 1-buty-3-methylimidazolium bromide 1-methyl-3-octylimidazolium chloride lithium bis(trifluoromethanesulfonyl)amide triethyloctylphosphonium bromide

Abbreviation [bmim][Tf2N]

CAS-number 174899-83-3

Supplier Synthesized

Purity ~99 %

[bmim][C8SO4]

445473-58-5

>98 %

[omim][Tf2N]

178631-04-4

Kanto Chemical Synthesized

[P2228][Tf2N]

1002754-38-2

Synthesized

~99 %

[P666,14][Tf2N]

460092-03-9

Iolitec

>98 %

[P666,14][DBS]

-

Synthesized

>99 %

[P666,14][AOT]

-

Synthesized

>99 %

[bmim]Br

85100-77-2

99 %

[omim]Cl

64697-40-1

Li[Tf2N]

90076-65-6

[P2228]Br

-

trihexyltetradecylphosphonium chloride sodium p-dodecylbenzenesulfonate sodium bis(2-ethylhexyl)sulfobutanedioate nitrogen n-butane

[P666,14]Cl

258864-54-9

Kanto Chemical Kanto Chemical Kanto Chemical Nippon Chemical Industry Iolitec

Na[DBS]

25155-30-0

Sigma-Aldrich

99 %

Na[AOT]

577-11-7

Sigma-Aldrich

99 %

N2 -

7727-37-9 106-97-8

>99.999 % 99 %

n-heptane

-

142-82-5

toluene

-

108-88-3

Iwatani Takachiho Chemical Industrial Takachiho Chemical Industrial JFP

~99 %

99 % 99 % 50 wt% (aqueous solution) >95 %

524 ppm (N2balance) 1480 ppm (N2balance)

Table 2. Physical properties of the present imidazolium and phosphonium ILs at 313.15 K.a) M/g mol-1

/kg m-3

Vm/cm3 mol-1

/10-4 K-1

/mPa s

Cp/J K-1 mol-1

419.364

1422.3 b)

294.84 b)

6.6633 b)

28.5 b)

575.1c)

[bmim][C8SO4] 348.504

1058.0 d)

329.40 d)

6.0385 d)

281e)

655 f)

[omim][Tf2N]

475.465

1304 g)

364.6 g)

6.754 g)

46.5 g)

756 h)

[P2228][Tf2N]

511.519

1231.3 i)

415.42 i)

6.5539 i)

56.5 i)

811.7

[P666,14][Tf2N]

764.006

1062.4 j)

719.13 j)

6.9119 j)

152 j)

1366

[P666,14][DBS]

809.343

925.50

874.49

6.5168

1744

1654

[P666,14][AOT]

905.426

940.82

962.38

6.8509

548

1820

[bmim][Tf2N]

a) Expanded relative uncertainties of present experimental data: Ur() = 0.001, Ur() = 2, Ur(Cp)=0.002 b) ref. 39, c) ref. 37, d) ref. 40, e) ref. 41, f) ref. 42, g) ref. 43, h) ref. 44. i) ref. 45, j) ref. 46.

Table 3. Van der Waals volumes of cation Vvdw+ and anion Vvdw- and free volumes Vfree of the present imidazolium and phosphonium ILs. Vm/cm3 mol-1

Vvdw+/cm3 mol-1

Vvdw-/cm3 mol-1

Vfree/cm3 mol-1

[bmim][Tf2N]

294.84 a)

123.9

125.6

45.3

[bmim][C8SO4]

329.40 b)

123.9

178.3

27.2

[omim][Tf2N]

364.6 c)

178.3

125.6

60.7

[P2228][Tf2N]

415.42 d)

219.6

125.6

70.2

[P666,14][Tf2N]

719.13 e)

496.2

125.6

97.3

[P666,14][DBS]

874.49

496.2

268.4

109.9

[P666,14][AOT]

962.38

496.2

353.1

113.1

a) ref. 39, b) ref. 40, c) ref. 43, d) ref. 45, e) ref. 46.

Figure 1. Chemical structures of cation and anion constituting the ILs investigated in the present study.

Figure 2. Flow sheet for the experimental apparatus for the n-heptane removal experiment. (a) N2 line, (b) n-heptane standard gas cylinder, (c) pressure regulator, (d) three-way valve, (e) mass flow controller, (f) valve, (g) pre-heating coil, (h) water bath, (i) stirrer controller, (j) glass cell, (k) stirrer bar, (l) magnetic stirrer, (m) thermocouple, (n) thermometer, (o) heater, (p) chiller, (q) gas cell, (r) FT-IR, (s) PC.

Figure 3. Solubilities of n-butane (mole fraction scaled) x1 in the ILs at p1=0.101±0.001 MPa. Filled circle, [bmim][Tf2N]; open circle, [bmim][Tf2N] (ref. 30); filled diamond, [bmim][C8SO4]; open diamond, [omim][Tf2N]; filled triangle, [P2228][Tf2N]; open square, [P666,14][Tf2N]; gray square, [P666,14][DBS]; filled square, [P666,14][AOT].

Figure 4. Relation between the number of the methyl (-CH3) and methylene (-CH2-) groups and the solubility of n-butane (mole fraction scaled) x1,298.15 at 298.15 K.

Figure 5. Solubilities of water vapor (mole fraction scaled) x2 in the [P666,14]+ ILs at 298.2 K. Open circle, [P666,14][Tf2N]; gray circle, [P666,14][DBS]; filled circle, [P666,14][AOT].

(a)

(b)

Figure 6. (a) Concentration of n-heptane cC7H16 in the gas mixture (n-heptane + nitrogen) treated with the phosphonium ILs and (b) volume of absorbed n-heptane VC7H16 in the phosphonium ILs (initial concentration of n-heptane in the gas mixture was 524 ppm). Gray dotted line, [P2228][Tf2N]; gray solid line, [P666,14][Tf2N]; dashed line, [P666,14][DBS]; solid line, [P666,14][AOT].

(a)

(b)

Figure 7. (a) Concentration of toluene cC7H8 in the gas mixture (toluene + nitrogen) treated with the phosphonium ILs and (b) volume of absorbed toluene VC7H8 in the phosphonium ILs (initial concentration of toluene in the gas mixture was 1480 ppm). Gray dotted line, [P2228][Tf2N]; gray solid line, [P666,14][Tf2N]; dashed line, [P666,14][DBS]; solid line, [P666,14][AOT].

Absorption of n-Butane in Imidazolium and Phosphonium Ionic Liquids and Application to Separation of Hydrocarbon Gases Takashi Makino* and Mitsuhiro Kanakubo National Institute of Advanced Industrial Science and Technology (AIST), 4-2-1 Nigatake, Miyagino-ku, Sendai 983-8551, Japan *

Corresponding author. E-mail: [email protected], fax: +81-22-232-7002.

Highlights · Solubilities of n-butane in the imidazolium and phosphonium ionic liquids. · New trihexyltetradecylphosphonium salts were the best for the n-butane solubility. · The tetraalkylphosphonium ionic liquids removed the diluted hydrocarbon gases.