Solubility of Fluorocarbons in Room Temperature Ionic Liquids

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Chapter 2

Solubility of Fluorocarbons in Room Temperature Ionic Liquids Mark B. Shiflett1 and A. Yokozeki2 1

DuPont Central Research and Development, Experimental Station 304, Wilmington, Delaware 19880-0304 2 DuPont Fluoroproducts Laboratory, Chestnut Run Plaza 711, Wilmington, Delaware 19880-0711

In this article, we summarise our investigation of the unique phase behaviour of fluorocarbons in room temperature ionic liquids. Gaseous absorption measurements (vapour-liquid equilibria, VLE) were performed using a gravimetric microbalance at various isothermal conditions (temperature between 283.15 and 348.15 K) and at pressures < 2 MPa. Experimental gas solubility data have been successfully correlated using both well-known solution models and a generic Redlich-Kwong (RK) type of cubic equation of state (EOS). Henry’s law constants (kH) have been calculated, and a linear correlation has been discovered between kH and fluorocarbon (methane and ethane series) critical temperature. The equation of state has been used to predict the global phase behaviour, which indicates that many of these systems belong to either type III or type V according to the Scott-van Konynenburg classification scheme. Vapour liquid-liquid equilibria (VLLE) measurements were conducted using simple mass-volume and cloud-point methods to verify the equation © 2009 American Chemical Society

In Ionic Liquids: From Knowledge to Application; Plechkova, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

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of state predictions. The unique lower critical solution temperature (LCST) behaviour for type-V systems, the large negative excess molar volumes in ionic liquid-rich side solutions, and the strong hydrogen-bonding capability are discussed. The large differences in the solubilities which exist among fluorocarbons have led us to consider using room temperature ionic liquids as extractants for separation and purification.

Introduction Fluorocarbons are important industrial compounds typically composed of a few carbon atoms and one or more fluorine atoms (1). Fluorocarbons may also contain other atoms such as hydrogen, oxygen and chlorine which lead to the following classifications: perfluorocarbons (PFCs), chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs) and hydrofluoroethers (HFEs). One of the most important applications of fluorocarbons in modern society has been their use as refrigerants in vapour compression systems (i.e. refrigerators, air-conditioners, chillers and heat pumps). Other applications include precision cleaning, foam blowing, and plasma etching, to name a few. Many of these applications require high purity such as refrigerants (>99.9%) and plasma etchants (>99.99995%). The processes for manufacturing fluorocarbons often result in co-products and undesired side-products which can require expensive, energy-intensive cryogenic distillation for separation. In some cases, these mixtures form homogeneous or heterogeneous azeotropes and methods such as extractive distillation are required to achieve separation. We have conducted the first comprehensive study of the phase behaviour of room temperature ionic liquids with a variety of fluorocarbons to develop a fundamental knowledge about their mixture properties and to assess new applications for separation and purification. Table 1 provides a list of common methane and ethane-series fluorocarbons which were studied, and includes the refrigerant abbreviation, chemical formula, CAS registry number, normal boiling point and critical point temperatures. Ionic liquids belong to a new class of compounds, which are molten salts with low melting points (< 373 K) (2). A unique feature of ionic liquids is a practical lack of vapour pressure which makes them useful as new solvents for extractive distillation. This article is a summary of our published works on the global phase behaviour of fluorocarbons in room-temperature ionic liquids (3-16).


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Experimental


Materials High-quality measurements begin with an analysis of the starting materials. In particular, the effect of impurities can have a large effect on thermophysical property measurements. For this reason, we have carefully analysed the purity of the fluorocarbons and ionic liquids used in our work (3-16). The majority of the fluorocarbons tested were obtained from DuPont Fluoroproducts (Wilmington, Delaware). The purities of the fluorocarbons were measured using a gas chromatography (GC) method (Agilent 6890N, Restek Rtx-200 column, 105 m x 0.25 mm). The purity for each fluorocarbon was reported in our previous works (3-16) and most fluorocarbons had a minimum purity of 99 mole percent. The fluorocarbons were used without additional purification; however, a molecular sieve trap was installed to remove trace amounts of moisture from the gases prior to entering the microbalance. Several commercially available ionic liquids were studied, as well as ten new ionic liquids with fluorinated anions which were synthesised (4,5). Two of the most common ionic liquids studied in our work were 1-butyl-3methylimidazolium hexafluorophosphate ([C4mim][PF6], CAS registry no. 174501-64-5) and 1-ethyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}amide ([C2mim][NTf2], CAS registry no. 174899-82-2). [C4mim][PF6] and [C2mim][NTf2] were obtained from Fluka, Sigma-Aldrich Chemie GmbH (Buchs, Switzerland) and Covalent Associates Inc. (Corvallis, OR), respectively. The stated purity of the [C4mim][PF6] and [C2mim][NTf2] were >97% and >99.5%, respectively. The ionic liquid samples were carefully analysed to verify the purity using a variety of analytical methods. The initial as-received mass fraction of water was measured by Karl Fischer titration (Aqua-Star C3000). The extractable ions (fluorine, chlorine, bromine) were measured by ion chromatography (column DIONEX AS17). The total chlorine ion content was measured using a Wickbold torch and ion chromatography method. Elemental analysis was performed by Schwarzkopf Microanalytical Laboratory, Inc. (Woodside, NY). Table 2 provides an example of the chemical analysis of the [C2mim][NTf2] ionic liquid (13).


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Table 1. Properties of Fluorocarbon Compounds Refrigerant Abbreviation

Chemical Formula

CAS Number

Tb /K

Tc /K

PFC-14

CF4

75-73-0

145.10 b

227.51 b

CFC-11

CFCl3

75-69-4

296.86 b

471.11 b

CFC-113

CFCl2-CF2Cl

76-13-1

320.74 a,b

487.21 b

CFC-113a

CCl3-CF3

354-58-5

319.31 c

483.42 c

CFC-114

CF2Cl-CF2Cl

76-14-2

276.74 b

418.86 d

CFC-114a

CFCl2-CF3

374-07-2

276.59 a

418.60 d

HCFC-123

CHCl2-CF3

306-83-2

300.97 b

456.83 b

HCFC-123a

CHClF-CF2Cl

354-23-4

301.35 f

461.60 e

HCFC-124

CHFCl-CF3

2837-89-0

261.19 a,b

395.43 b

HCFC-124a

CHF2-CF2Cl

354-25-6

261.38 a

399.90 d

HFC-23

CHF3

75-46-7

191.13 b

299.29 b

HFC-32

CH2F2

75-10-5

221.50 b

351.26 b

HFC-41

CH3F

593-53-3

195.03 b

317.28 b

HFC-125

CHF2-CF3

354-33-6

225.06 b

339.17 b

HFC-134

CHF2-CHF2

359-35-3

253.10 a

391.74 a

HFC-134a

CH2F-CF3

811-97-2

247.08 b

374.21 b

HFC-143a

CH3-CF3

420-46-2

225.91 b

345.86 b

HFC-152a

CHF2-CH3

75-37-6

249.13 b

386.41 b

HFC-161

CH2F-CH3

353-36-6

235.60 b

375.30 b

HFE-125

CHF2-O-CF3

3822-68-2

238.09 b

354.62 b

HFE-143a

CH3-O-CF3

421-14-7

249.24 b

377.98 b


25 a

(17); b (18); c (19); d (20); e (21); f (22)

Table 2. Chemical Analysis of [C2mim][NTf2] Ionic Liquid Units


Assay

Calculated/ Reported ≥ 99.5 a

Measured ≥ 99.4

Elemental C H F N S O

% % % % % %

24.55 2.83 29.13 10.74 16.39 16.35

24.60 3.02 29.70 10.75 17.05 NR b

H2O c

x 10-6

99 mol %) with reasonable economics (6). We have also studied the separation of isomer pairs (e.g., HFC-134a / HFC-134 and fluorinated benzenes) and found that [C2mim][NTf2] is an effective extractive solvent (13,41). A unique solubility study of two diastereomers was also investigated. HF-4310mee or 2,3-dihydrodecafluoropentane (CF3CHFCHFCF2CF3), is one of the chemical isomers of dihydrodecafluoropentane (C5H2F10) consisting of two diastereomers (threo- and erythro-isomers) which are further composed of two optical isomers (or R and S types of D/L enantiomers), as illustrated in Figure 5. Chemical Name

Tb /K

Structures

Erythro-2,3-dihydrodecafluoropentane 1

1

CF3

H

C

3 S

H F3C

2 R C

320.1 CF3

F

F

F

F

5 4

S 2 C

H

R 3 C

4 5

CF2

F2C

Erythro (2R,3S)

H CF3

Erythro (2S,3R)

Threo-2,3-dihydrodecafluoropentane

1

1

CF3

F H F3C

2 S C

3 S C

5 4

CF2

Threo (2S,3S)

328.3

H

H

F

F

CF3

R 2 C

R 3 C

4 5

F2C

F H CF3

Threo (2R,3R)

Figure 5. Schematic molecular structures of HFC-4310mee (erythro- and threo-2,3-dihydrodecafluoropentanes). R and S (rectus and sinister, or right and left) refer to the configuration about the chiral (asymmetric) carbon atom. Normal boiling point (Tb).



37 HFC-4310mee was developed more than a decade ago as a replacement for ozone-depleting solvents such as 1,1,2-trichloro-1,2,2-trifluoroethane (CFCl2CF2Cl, CFC-113). These diastereoisomers (or chemical isomers) possess two asymmetric (or chiral) carbon atoms at the positions 2 and 3, and the erythro-isomer is made of the two (50/50%) optical isomers labelled (2R, 3S) and (2S, 3R), while the threo-isomer is composed of the two (50/50%) optical isomers labelled (2S, 3S) and (2R, 3R). The thermophysical properties between the (2R, 3S) and (2S, 3R) optical isomers are identical and similarly true for the (2S, 3S) and (2R, 3R) isomers. However, the erythro- and threo-isomers have significantly different thermodynamic properties; e.g., the normal boiling points are 320.1 and 328.3 K for the former and the latter, respectively. Liquid-liquid equilibria of HFC-4310mee diastereoisomers in ionic liquids (e.g., [C4mim][PF6], [C4mim][BF4], [C2mim][BF4]) was measured using the mass-volume and cloud point methods (11). All mixtures showed limited miscibility; however, the solubility differences between the cases of the threoand erythro-isomers were quite dramatic, when considering the very similar chemical structures. Tx phase diagrams for threo- and erythro- isomers in [C4mim][PF6] are shown in Figure 6. The threo-isomer has a LCST and at T < 265 K is completely miscible. The erythro-isomer is only partially miscible (~10 mole % erythro-isomer) over the same temperature range. This means that not only the inclusion of a hydrogen atom, but the relative arrangement of the hydrogen atom on a carbon position is significantly important for the thermodynamic properties. Fluorocarbons (i.e., HCFCs and HFCs) possess strong hydrogen-bonding (H-F-H) capability and the hydrogen bonding is highly directional; therefore, the relative positions of hydrogen-bonding atoms are important.

Conclusions We have studied the solubility of an important class of compounds – fluorocarbons in a variety of ionic liquids. We have found amazingly different solubilities among fluorocarbons in RTILs. Although the mechanism of the solubility difference is not clear at an intermolecular level, the engineering applications projected from the present discovery are quite significant in the field of material separations among fluorocarbons, such as extractive distillation and extraction solvents. The observed VLE (P, T, x) behaviours of electrolyte (ionic liquid) solutions with fluorocarbons have been well correlated with conventional solution (activity coefficient) models and a cubic equation of state. The EOS model based on low pressure VLE data alone can be highly reliable when extrapolating thermodynamic properties. Fluorocarbon + RTIL mixture behaviour indicate that these systems belong to either type III or type V according to the Scott-van Konynenburg classification. Large negative excess


38 molar volumes indicate that the molecular size effect and hydrogen bonding capability are important. Future molecular dynamics calculations may provide further insight.


(a)

Threo-isomers mole % in [C4mim][PF6]

(b)

Erythro-isomers mole % in [C4mim][PF6]

Figure 6. Tx phase diagrams for LLE. (a) Threo-isomer + [C4mim][PF6] system and (b) Erythro-isomer + [C4mim][PF6] system. Broken line: calculated with the NRTL activity model. Symbols: experimental data (11); circles = VLLE experiments and triangles = the cloud-point method


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Acknowledgements The authors thank Mr. Brian L. Wells and Mr. Joe Nestlerode at the DuPont Experimental Station for their assistance with the gas solubility and liquid-liquid equilibria measurements, respectively. They also appreciate Drs. Allen Sievert and Mario J. Nappa (DuPont Fluorochemicals) who kindly provided samples of CFC-113, CFC-113a, HCFC-123, threo- and erythro-isomers of HFC-4310mee and Drs. Christopher P. Junk, Mark A. Harmer, and Thomas Foo (DuPont Central Research and Development) who synthesised new ionic liquids. They also appreciate Dr. Lam H. Leung (DuPont Corporate Center for Analytical Science) for the GCMS fluorocarbon analysis. They also thank Dr. Marcia L. Huber (National Institute of Standards and Technology, Boulder, Colorado) for kindly providing new fluorocarbon data files (HFC-41, HFC-161, HFE-125, HFE-143a) for the Refprop software. DuPont Central Research and Development supported the present work.

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Solubility of Fluorocarbons in Room Temperature Ionic Liquids

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