High Solubilities of Small Hydrocarbons in Trihexyl

Article pubs.acs.org/JPCB

High Solubilities of Small Hydrocarbons in Trihexyl Tetradecylphosphonium Bis(2,4,4-trimethylpentyl) Phosphinate Xiangyang Liu,†,§ Waheed Afzal,†,‡,⊥ Guangren Yu,† Maogang He,§ and John M. Prausnitz*,†,‡ †

Department of Chemical and Biomolecular Engineering, University of California, Berkeley, California, 94720-1462, United States Environmental Energy Technologies Division, Lawrence-Berkeley National Laboratory, Berkeley, California, 94720, United States § MOE Key Laboratory of Thermo-Fluid Science and Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi, 710049, People’s Republic of China ⊥ Institute of Chemical Engineering & Technology, University of the Punjab, Lahore, 54590, Pakistan ‡

S Supporting Information *

ABSTRACT: Experimental solubilities are reported for methane, ethane, ethylene, propane, and propylene in trihexyl tetradecylphosphonium bis(2,4,4trimethylpentyl) phosphinate [P(14)666][TMPP] from 313 to 353 K up to 6.7 MPa. A literature review on solubilities of small hydrocarbons in ionic liquids shows that solubilities in [P(14)666][TMPP] are appreciably larger than those in other ionic liquids. Contrary to solubilities in ionic liquids studied earlier, solubilities of paraffins (ethane and propane) in [P(14)666][TMPP] are larger than those of the corresponding olefins (ethylene and propylene). Because, at fixed temperature, the vapor pressure of an olefin is larger than that of the corresponding paraffin, the relative volatility of the olefin exceeds that of the corresponding paraffin, contrary to the relative volatility observed in conventional extractive distillation with polar solvents where the volatility of the paraffin exceeds that of the corresponding olefin.

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INTRODUCTION Although ionic liquids have been known for about one century, their properties were not extensively studied until about 2 decades ago. Most ionic liquids are stable when in contact with air and water. Ionic liquids can be partially or completely hydrophilic. Ionic liquids have negligible vapor pressure near ambient temperature; most are stable at elevated temperature.1,2 Because of the very large number of possible cations and anions, ionic liquids can be tuned for different properties; they are useful for separations, electrochemical processes, analytical and synthetic chemistry, nanotechnology, and other applications.3,4 We are interested in the separation of paraffins from olefins by selective absorption using this ionic liquid. Solubilities of gases in ionic liquids are required for designing separation processes. In this work, we have reviewed solubilities of small paraffins and olefins (methane, ethane, ethylene, propane, propylene) in ionic liquids.5−37 The literature review is summarized in Tables S1 and S2 (Supporting Information). We have measured solubilities of methane, ethane, ethylene, propane, and propylene in a partially hydrophobic38 ionic liquid trihexyl tetradecylphosphonium bis(2,4,4-trimethylpentyl) phosphinate [P(14)666][TMPP] from 313 to 353 K up to 6.7 MPa. We use the Krichevsky−Kasarnovsky (KK) equation to represent the experimental data. We choose [P(14)666][TMPP] because both of its ions have long alkyl chains with a total of 48 carbon atoms; long © 2013 American Chemical Society

alkyl chains may give [P(14)666][TMPP] hydrocarbon-like character. Although normal paraffins having 20 or more carbon atoms are solids at ambient temperature, [P(14)666][TMPP] is a liquid.39 For each gas, when we compare Henry’s constants in different ionic liquids with those in [P(14)666][TMPP], we observen the following: first, [P(14)666][TMPP] is a very good solvent for all hydrocarbon solutes, and second, this solvent exhibits solubilities of paraffins (ethane and propane) larger than those of the corresponding olefins (ethylene and propylene). High selectivity of paraffin/olefin in [P(14)666][TMPP] may be useful for a separation process. Because, at a fixed temperature, the vapor pressure of an olefin is higher than that of its corresponding paraffin, this ionic liquid enhances the separation factor. In a typical absorption process with a polar solvent, the volatility of ethane is larger than that of ethylene, but in [P(14)666][TMPP], it is the other way.

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EXPERIMENTAL SECTION All gases were purchased from Praxair or Matheson with a purity of ≥99.9 mass %. [P(14)666][TMPP] was purchased from Ionic Liquid Technologies (Io-Li-Tech) with a purity of >95 mass %. Information provided by the supplier showed that Received: April 8, 2013 Revised: July 10, 2013 Published: August 15, 2013 10534

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where V∞ 1 is the partial molar volume of gas in the liquid at infinite dilution. For an ionic liquid at normal temperature, PS2 is essentially zero. Equation 4 becomes

[P(14)666][TMPP] contained 2810 ppm chloride and 8030 ppm bromide ions. The ionic liquid was dried at 373 K for 24 h under vacuum. The water content for [P(14)666][TMPP] was found to be less than 0.6 mass %, measured using the Karl Fischer method (Aquaastar C2000 Tirator) before and after measurements. Figure S1 (Supporting Information) shows 1H NMR spectra (400 MHz in CDCl3) of [P(14)666][TMPP], consistent with the literature,40 and it shows no major organic impurities. Thermogravimetric analysis of [P(14)666][TMPP] at a heating rate of 5 °C/min under nitrogen in an aluminum pane shows that this liquid is stable up to at least 250 °C. We used an Anton-Paar vibrating-tube densimeter (model DMA 5000M) for measuring the density of the ionic liquid. Previous publications give experimental details.39,41 The solubilities of gases in [P(14)666][TMPP] were measured using an isochoric saturation method, as described elsewhere.42 Figure S2 (Supporting Information) shows our solubility apparatus; the main part of the apparatus is an equilibrium cell with known internal volume. The experiment began with filling the equilibrium cell with a known quantity of dried ionic liquid, removing any air and adding a known quantity of gas from a calibrated pressure vessel of known volume, temperature, and pressure. We fixed the temperature of the equilibrium cell and then measured the pressure using a calibrated pressure sensor. The pressure decreased due to solubility. Rapid equilibrium was achieved using magnetic stirring. The number of moles nlg of gas dissolved in the ionic liquid was calculated from ngl = VGC(ρi − ρf ) − ρg (VEC − Vl )

ln

f1 x1

= ln H1 +

V1∞P RT

(5)

At a fixed temperature and for a particular solute 1 in [P(14)666][TMPP], Henry’s constant and V∞ 1 are obtained from the intercept and slope of a plot of ln( f1/x1) versus pressure.

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RESULTS AND DISCUSSION Table S3 (Supporting Information) presents the density of dried [P(14)666][TMPP] from 298 to 363 K. Our density data reported in this work are about 0.2% higher than those that we reported in our earlier work.39 This disagreement may have resulted from two factors, (1) different batches of ionic liquid may have varying amounts of different impurities, and (2) extensive degasification of the sample used in this work may have caused removal of small quantities of low-density volatile solvents present as an impurity in the ionic liquid sample. The mass densities of [P(14)666][TMPP] from 298 to 363 K are given by ρ = −5.9125 × 10−4T + 1.0655

(6)

where ρ is in g/cm and T is in kelvins. Tables S4−S8 (Supporting Information) and Figures 1−5 present the solubilities of methane, ethane, ethylene, and 3

(1)

where VGC is the volume of the gas container, VEC is the volume of the equilibrium cell, Vl is the volume of ionic liquid in the equilibrium cell, ρi is the molar density of solute in the gas container before loading the solute into the equilibrium cell, ρf is the molar density of solute in the gas container after loading the solute into the equilibrium cell, and ρg is the molar density of solute in the vapor phase in the equilibrium cell when equilibrium is reached. ρi, ρf, and ρg can be calculated from the REFPROP computer program.43 Solubility of the gas in the liquid can be expressed as a mole fraction x1 =

ngl nl + ngl

(2)

nlg

where nl is the number of moles of ionic liquid and is the number of moles of gas absorbed in the ionic liquid. At temperature T and pressure P, the solubility of a gas i can be expressed in terms of Henry’s constant on the mole fraction basis, H, defined by Hi = lim

xi → 0

fi (T , P , xi) xi

= lim S P→P

Figure 1. Solubility of methane in [P(14)666][TMPP] (KK is Krichevsky−Kasarnovsky).

fi (T , P , xi) xi

propane in [P(14)666][TMPP] from 313 to 353 K up to 6.7 MPa from this work. We repeated each measurement three or four times. The average uncertainty in mole fraction is ≤0.002. Figure S3 (Supporting Information) shows that f i/xi for ethane is a linear function of pressure. The other four gases have the same trend. Table 1 and Figure 6 show Henry’s constants for the five gases at different temperatures. They can be represented by

(3)

where f i(T,P,x) is the fugacity of gas i calculated from the REFPROP computer program4 and PS is the vapor pressure of the solvent. A high Henry’s constant indicates a low solubility. Experimental solubilities are fitted to the KK equation.44 For gas 1 in solvent 2 ln

f1 x1

= ln H1 +

V1∞(P − P2S) RT

ln H =

(4) 10535

A +B T

(7)

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Figure 2. Solubility of ethane in [P(14)666][TMPP] (KK is Krichevsky−Kasarnovsky).

Figure 5. Solubility of propylene in [P(14)666][TMPP] (KK is Krichevsky−Kasarnovsky, and COSMO is the mole fraction calculated from COSMOThermX by using the vap file).

where H is in MPa and T is in kelvins. Coefficients A and B are listed in Table 2 with average absolute deviations less than 0.88%. Figure 6 shows that Henry’s constants for the five gases in [P(14)666][TMPP] rise as the carbon number of solute increases. The solubilities of ethane are larger than those of ethylene, opposite from solubilities in other ionic liquids.10,13−15,36 Table S9 (Supporting Information) and Figure 7 show solubilities of ethane and ethylene at standard temperature and pressure in some common solvents from the literature45−49 with solvent solubility parameters that vary from 14.4 to 29.7 MPa1/2. Solubilities of ethane are larger than those of ethylene when the solvent is a hydrocarbon (with a solubility parameter in the range of 14.4−18.7 MPa0.5), opposite from those when the solvent is an oxygenated hydrocarbon and other polar solvents (with a solubility parameter larger than 19.7 MPa1/2). Direct determination of the solubility parameters for ionic liquids is difficult because of their negligibly small vapor pressure. Indirect determination of solubility parameters depends upon the method.39,50 Strictly for comparison purposes, we use COSMOthermx to calculate the solubility parameter of [P(14)666][TMPP]; it is 18.2 MPa1/2 at 298.15 K. Solubility parameters for ionic liquids [emim][Tf2N], [bmim][Tf2N], [bmim][PF6], [bmim][BF4], [hmpy][Tf2N], [emim][DCA], and [emim][CF3SO3] are higher than 20 MPa1/2 at 298.15 K. For these ionic liquids, the solubilities of ethane are lower than those of ethylene.10,13−15,36 We can conclude that [P(14)666][TMPP] behaves like a nonpolar hydrocarbon. As expected, solubilities of gases rise with increasing pressure and fall with increasing temperature. The partial molar enthalpy of solution was calculated to consider the effect of temperature on solubilities of the five gases in [P(14)666][TMPP]. For infinitely dilute solutions8,51

Figure 3. Solubility of ethylene in [P(14)666][TMPP] (KK is Krichevsky−Kasarnovsky).

⎛ ∂ ln H ⎞ Δsol h = R ⎜ ⎟ ⎝ ∂(1/T ) ⎠ P

Figure 4. Solubility of propane in [P(14)666][TMPP] (KK is Krichevsky−Kasarnovsky, and COSMO is the mole fraction calculated from COSMOThermX by using the vap file).

(8)

Table 2 gives the partial molar enthalpies and the partial molar entropies of solution. As expected, the partial molar 10536

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Table 1. Henry’s Constants and Partial Molar Volumes of Gases at Infinite Dilution for Five Gases in [P(14)666][TMPP] Δa/%

T/K

Hexp/MPa

HKK/MPa

HCOSMO0/MPa

313 333 353

7.80 8.61 9.06

7.96 8.78 9.28

2.11 2.01 2.40

12.30 14.87 17.52

314 333 353

1.90 2.44 3.03

2.02 2.60 3.24

6.56 6.59 6.62

4.06 5.40 6.90

314 333 353

2.62 3.23 4.01

2.79 3.42 4.23

6.44 5.83 5.48

2.74 3.71 4.78

313 333 353

0.64 0.95 1.34

0.65 0.96 1.35

1.21 1.20 0.84

1.35 1.98 2.78

313 334 353

0.75 1.12 1.60

0.75 1.12 1.60

0.01 0.02 −0.01

0.88 1.33 1.87

Δb/%

HCOSMO1/MPa

Δc/%

V2∞/(cm3·mol−1)

57.69 72.71 93.38

12.10 14.65 17.27

55.13 70.15 90.62

223.2 212.0 230.0

113.68 121.31 127.72

3.98 5.30 6.78

109.47 117.21 123.76

496.0 428.3 381.3

4.58 14.86 19.20

2.67 3.63 4.69

1.91 12.38 16.96

374.5 343.7 298.7

110.94 108.42 107.46

1.31 1.93 2.71

104.69 103.16 102.24

17.36 19.26 16.65

0.86 1.30 1.83

14.06 16.23 13.98

Methane

Ethane

Ethylene

Propane 2039 1520 1195

Propylene 285.8 281.4 611.8

Δ = 100(HKK − Hexp)/Hexp; Hexp is the experimental Henry’s constant, and HKK is Henry’s constant calculated from the KK equation. bΔ = 100(HCOSMO0 − Hexp)/Hexp; HCOSMO0 is Henry’s constant for a solute in conformer 0 of [P(14)666][TMPP] calculated from COSMOthermx. cΔ = 100(HCOSMO1 − Hexp)/Hexp; HCOSMO1 is Henry’s constant for the solute in conformer 1 of [P(14)666][TMPP] calculated from COSMOthermx. a

Figure 6. Henry’s constants for five gases in [P(14)666][TMPP]. Figure 7. Solubilities of ethane and ethylene in common solvents at 298.15 K and 1 atm.

Table 2. Coefficients A and B in Equation 7 and the Partial Molar Enthalpy of Solution for Five Gases in [P(14)666][TMPP] gas

A

B

|ΔH|a/%

Δsolh/(kJ·mol−1)

methane ethane ethylene propane propylene

−417.7 −1324 −1198 −2059 −2106

3.393 4.865 4.780 6.127 6.435

0.87 0.10 0.88 0.22 0.61

−3.47 −11.01 −9.96 −17.12 −17.51

interaction between [P(14)666][TMPP] and dissolved gas decreases in the order propylene > propane > ethane > ethylene > methane because lower values for the partial molar enthalpy show stronger interaction between dissolved gas and ionic liquid. Henry’s constants for five gases in [P(14)666][TMPP] were compared with those in other ionic liquids.5−8,10−14,16,18,22,23,25−28,31,34,36 Tables S10−S14 (Supporting Information) provide Henry’s constants based on our literature review for solubilities of methane, ethane, ethylene, propane, and propylene in different ionic liquids. This review shows that the five gases have high solubility in [P(14)666][TMPP] and that Henry’s constants for the five gases in

|ΔH| is the absolute average deviation between experiment and calculation from eq 7.

a

enthalpies for gases in [P(14)666][TMPP] are negative, indicating that the solubilities for the five gases in [P(14)666][TMPP] decrease with rising temperature. The strength of 10537

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are grateful to Prof. Michael Manga for providing his density meter.

phosphonium-based ionic liquids are lower than those in ammonium-based and imidazolium-based ionic liquids. Average values were used when the literature gave more than one datum at the same condition. Henry’s constants in imidazolium-based ionic liquids decrease as the length of the cation’s hydrocarbon chain increases. Henry’s constants in ionic liquids with anion [Tf2N] are lower than those in ionic liquids with other anions. For the phosphonium-based ionic liquids, there are no significant differences for different anions. Parameters of the KK equation are shown in Tables 1 and S4−S8 (Supporting Information) and in Figures 1−6. The absolute average relative deviations in mole fractions calculated from the KK equation are less than 3.6%; absolute average relative deviations for Henry’s constants are less than 6.59%, indicating that the KK equation fits experimental data well. Table 1 and Figure 6 show a comparison between experimental Henry’s constants for gases in [P(14)666][TMPP] and COSMOthermx predictions. Henry’s costants for ethylene and propylene calculated from COSMOthermx agree with experiment within 20%. The relative deviations between Henry’s constants for methane, ethane, and propane calculated from the COSMOthermx and experiment are larger than 60%.

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CONCLUSIONS The solubilities of methane, ethane, ethylene, propane, and propylene in [P(14)666][TMPP] have been measured at 313, 333, and 353 K at pressures up to 6.7 MPa. Ethane and propane exhibit stronger solute−solvent interactions and larger solubilities than their corresponding olefins. Methane, ethane, ethylene, propane, and propylene have high solubilities in [P(14)666][TMPP] when compared to those in other ionic liquids. Experimental Henry’s costants for ethylene and propylene agree well with those calculated from COSMOthermx. However, COSMOthermx-predicted Henry’s constants for methane, ethane, and propane do not agree well with experiment.

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ASSOCIATED CONTENT

S Supporting Information *

Details concerning solubilities for methane, ethane, ethylene, propane, and propylene in [P(14)666][TMPP] from this work and those in different other ionic liquids reported in the literature; the NMR analysis of [P(14)666][TMPP]; density data for [P(14)666][TMPP]; schematic of the experimental apparatus; and calculations with the Krichevsky−Kasarnovsky equation and with COSMOthermX. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +1 510 642 3592. Fax: +1 510 642 4778. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Environmental Energy Technologies Division of the Lawrence Berkeley National Laboratory for financial support and to Prof. Scott Lynn and to Prof. Alexis T. Bell and co-workers for general assistance. We 10538

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dx.doi.org/10.1021/jp403460a | J. Phys. Chem. B 2013, 117, 10534−10539