J. Chem. Eng. Data 2007, 52, 2007-2015
2007
Solubility Differences of Halocarbon Isomers in Ionic Liquid [emim][Tf2N] Mark B. Shiflett*,† and A. Yokozeki‡ DuPont Central Research and Development, Experimental Station E304, Wilmington, Delaware 19880, and DuPont Fluoroproducts Laboratory, Chestnut Run Plaza 711, Wilmington, Delaware 19880
Solubility behaviors of CFC-113 (CFCl2-CF2Cl), CFC-113a (CCl3-CF3), CFC-114 (CF2Cl-CF2Cl), CFC-114a (CFCl2-CF3), HCFC-123 (CHCl2-CF3), HCFC-123a (CHClF-CF2Cl), HCFC-124 (CHFCl-CF3), HCFC-124a (CHF2-CF2Cl), HFC-134 (CHF2-CHF2), and HFC-134a (CH2F-CF3) in room-temperature ionic liquid 1-ethyl3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf2N]) have been investigated using a gravimetric microbalance method from (283 to 348) K or volumetric and cloud-point methods. In the case of the perhalogenated compounds (CFC-113, CFC-114, and their isomers), the solubility behavior between isomers in the ionic liquid is practically identical with large immiscibility gaps. This suggests that the (present) ionic liquid cannot be used for these isomer separations. However, the monohydrogen substituted halocarbons (HCFC-123, HCFC-124, and their isomers) begin to show some difference (liquid-liquid immiscibility gap) in the ionic liquid. The isomer effect on the solubility in the ionic liquid becomes significant for the dihydrogen-substituted halocarbons (HFC134 and HFC-134a), and these isomers can be separated using [emim][Tf2N] as an entrainer in an extractive distillation. This observation is consistent with our earlier findings for various HFCs in ionic liquids.
Introduction Ionic liquids as a new type of solvent are being studied for a variety of separation applications.1-12 In particular, isomer separations4 are often required in the chemical and pharmaceutical industries. Often isomers have similar thermophysical properties (e.g., normal boiling point), which make it difficult to separate using conventional techniques such as distillation. The present study was initiated to find out whether roomtemperature ionic liquids (RTILs) could separate halocarbon isomers. In addition to the practical application, we are motivated to gain more fundamental understanding about the solubility behavior (or molecular interactions) of this class of compounds in RTILs. The present report is a continuation of our previous studies on the vapor-liquid equilibrium (VLE) and liquid-liquid equilibrium (LLE) of halocarbons in RTILs.13-21 In this work, we investigate three classes of halocarbons: chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs) in RTILs. We have systematically studied five isomer pairs and have prepared binary mixtures of 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113, CFCl2-CF2Cl) and 1,1,1-trichloro-2,2,2-trifluoroethane (CFC-113a, CCl3-CF3); 1,2-dichloro-1,1,2,2-tetrafluoroethane (CFC-114, CF2Cl-CF2Cl) and 1,1-dichloro-1,2,2,2tetrafluoroethane (CFC-114a, CFCl2-CF3); 1,1-dichloro-2,2,2trifluoroethane (HCFC-123, CHCl2-CF3) and 1,2-dichloro1,2,2-trifluoroethane (HCFC-123a, CHClF-CF2Cl); 1-chloro1,2,2,2-tetrafluoroethane (HCFC-124, CHFCl-CF3) and 2-chloro1,1,2,2-tetrafluoroethane (HCFC-124a, CHF2-CF2Cl); and 1,1,2,2-tetrafluoroethane (HFC-134, CHF2-CHF2) and 1,2,2,2tetrafluoroethane (HFC-134a, CH2F-CF3) with room-temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf2N]). The chemical structure for [emim][Tf2N] is shown in Figure 1. * Corresponding author. E-mail:
[email protected]. † DuPont Central Research and Development. ‡ DuPont Fluoroproducts Laboratory.
Figure 1. Chemical structure of ionic liquid [emim][Tf2N].
For the halocarbons with a sufficient vapor pressure (CFC114, CFC-114a, HCFC-124, HCFC-124a, HFC-134, and HFC134a), we use the gravimetric microbalance at a temperature range of (283 to 348) K for VLE measurements, whereas for low vapor pressure halocarbons (CFC-113, CFC-113a, HCFC123, and HCFC-123a), we use only the volumetric or cloudpoint method for LLE measurements. VLE data (CFC-114 isomers, HCFC-124 isomers, and HFC-134 isomers) have been well correlated with an EOS model.19,22 The EOS model has been used to predict the liquid-liquid-phase boundary which has been confirmed. Experimental LLE data (CFC-113 isomers and HCFC-123 isomers) have been well correlated by the NRTL (nonrandom two liquid) activity coefficient model.13,23,24
Experimental Materials. CFC-113 (CAS registry no. 76-13-1), CFC-113a (CAS registry no. 354-58-5), CFC-114 (CAS registry no. 7614-2), CFC-114a (CAS registry no. 374-07-2), HCFC-123 (CAS registry no. 306-83-2), HCFC-124 (CAS registry no. 2837-890), HCFC-124a (CAS registry no. 354-25-6), and HFC-134a (CAS registry no. 811-97-2) were obtained from DuPont Fluoroproducts (Wilmington, DE). HCFC-123a (product and lot no. 1100-7-11 Q9B-64, CAS registry no. 354-23-4) was obtained from SynQuest Laboratory, Inc. (Alachua, FL), and HFC-134 (lot no. 0221TA06, CAS registry no. 359-35-3) was obtained
10.1021/je700295e CCC: $37.00 © 2007 American Chemical Society Published on Web 08/21/2007
2008
Journal of Chemical and Engineering Data, Vol. 52, No. 5, 2007
Table 1. Properties of Halocarbon Compounds and Sample Purity Analysis compd
formula
Tb/K
Tc/K
purityg/%
impuritiesh/%
CFC-113 CFC-113a CFC-114 CFC-114a HCFC-123 HCFC-123a HCFC-124 HCFC-124a HFC-134 HFC-134a
CFCl2-CF2Cl CCl3-CF3 CF2Cl-CF2Cl CFCl2-CF3 CHCl2-CF3 CHClF-CF2Cl CHFCl-CF3 CHF2-CF2Cl CHF2-CHF2 CH2F-CF3
320.74a,b
487.21b
319.31c 276.74b 276.59a 300.97b 301.35f 261.19a,b 261.38a 253.10a 247.08b
483.42c 418.86d 418.60d 456.83b 461.6e 395.43b 399.90d 391.74a 374.21b
> 99.99 99.4 97.3 99.1 97.6 98.3 99.1 99.0 99.0 99.6
trace isomers 0.4 R-113, 0.1 R-114a, 0.1 other isomers 1.9 R-115, 0.4 R-12/R-22, 0.1 R-113, 0.1 R-123, 0.2 other isomers 0.6 R-123, 0.3 other isomers 1.0 R-123a, 1.0 R-4310, 0.4 other isomers 0.4 R-123, 0.3 R-114, 0.5 unsaturates, 0.5 other isomers 0.5 R-134, 0.2 R-22, 0.2 other isomers 0.5 R-134a, 0.2 R-245, 0.3 other isomers 0.9 R-124, 0.1 other isomers 0.3 R-143a, 0.1 other isomers
a Ref 23. b Ref 28. c Ref 29. d Ref 30. e Ref 31. f Ref 33. g Mole percent analysis by GCMS; see text for details. h R is the abbreviation for refrigerant: R-115, chloropentafluoroethane; R-12, dichlordifluoromethane; R-22, chlorodifluoromethane; R-4310, decafluoropentane; R-245, pentafluoropropane; R-143a, 1,1,1-trifluoroethane.
from Speciality Gases of America (Toledo, Ohio). The purity of these fluorocarbons was determined using a gas chromatography mass spectrometry (GCMS) method (Agilent 6890N, Restek Rtx-200 column, 105 m × 0.25 mm), and the results are shown in Table 1. The ionic liquid [emim][Tf2N] (EMIIm, electrochemical grade, assay g 99.5 %, C8H11F6N3O4S2, lot and catalog no. 259095 IL-201-20-E, CAS registry no. 174899-822) was purchased from Covalent Associates Inc. (Woburn, MA). Methyl alcohol (assay g 99.9 %, product and batch no. 6463771L 05441CE, CAS registry no. 67-56-1) was obtained from Sigma-Aldrich, Inc. (St. Louis, MO) and used as a reference fluid for volumetric calibration of the experimental LLE apparatus. The [emim][Tf2N] sample was analyzed to verify the stated purity. The initial as-received mass fraction of water was measured by Karl Fischer titration (Aqua-Star C3000, solutions AquaStar Coulomat C and A). The sample contained a mass fraction of about 413‚10-6 H2O which is greater than what is stated on the manufacturer’s website for electrochemical grade material (< 50 ppm).25 A 20 mL sample of water was used to extract fluorine, chlorine, and bromine from 0.2 g of [emim][Tf2N] at ambient temperature for 24 h. The extractable ions were measured by ion chromatography (column: DIONEX AS17; eluent: (0.4 to 50) mM NaOH; flow: 1.0 mL‚min-1; sample loop: 100 µL). The fluorine, chlorine, and bromine ions were found to be below the detection limit (< 5 µg‚mL-1). A 0.1886 g sample of [emim][Tf2N] was combusted in a Wickbold torch, and the combustion gases were collected in water (99.86 mL) and analyzed by ion chromatography for total chlorine content. Two separate samples were analyzed and found to contain a mass fraction of (440 and 480)‚10-6 chlorine (average (460 ( 20)‚10-6). Elemental analysis was performed by Schwarzkopf Microanalytical Laboratory, Inc. (Woodside, NY)26 for carbon, hydrogen, fluorine, nitrogen, and sulfur content. Combining the results from each of the techniques described and shown in Table 2, we conclude that the [emim][Tf2N] sample purity was g 99.4 % which compares closely with the stated purity (g 99.5 %) by the manufacturer. The [emim][Tf2N] was dried and degassed by first filling a borosilicate glass tube with about 10 g of the ionic liquid and pulling a coarse vacuum with a diaphragm pump (Pfeiffer, model MVP055-3, Nashua, NH) for about 3 h. Next, the [emim][Tf2N] was completely evacuated using a turbopump (Pfeiffer, model TSH-071) to a pressure of about 4‚10-7 kPa while simultaneously heating and stirring the ionic liquid at a temperature of about 348 K for 5 days. The final mass fraction of water was again measured by Karl Fischer titration, and the dried sample contained 188‚10-6 H2O. Experimental Method. A detailed description of the vaporliquid equilibria (VLE) and liquid-liquid equilibria (LLE)
Table 2. Chemical Analysis of [emim][Tf2N] Ionic Liquid [emim][Tf2N] units assay
calcd/reported
measured
g 99.5a
g 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 NRb
H2Oc
‚10-6
