Traditional Extractants in Nontraditional Solvents: Groups 1 and 2

The crown ethers 18-crown-6 (18C6), dicyclohexano-18-crown-6 (DCH18C6), and 4,4'-(5')-di-(tert-butylcyclohexano)-18-crown-6 (Dtb18C6) were dissolved i...
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Ind. Eng. Chem. Res. 2000, 39, 3596-3604

Traditional Extractants in Nontraditional Solvents: Groups 1 and 2 Extraction by Crown Ethers in Room-Temperature Ionic Liquids† Ann E. Visser, Richard P. Swatloski, W. Matthew Reichert, Scott T. Griffin, and Robin D. Rogers* Department of Chemistry and Center for Green Manufacturing, The University of Alabama, Tuscaloosa, Alabama 35487

The crown ethers 18-crown-6 (18C6), dicyclohexano-18-crown-6 (DCH18C6), and 4,4′-(5′)-di-(tertbutylcyclohexano)-18-crown-6 (Dtb18C6) were dissolved in 1-alkyl-3-methylimidazolium hexafluorophosphate ([Cnmim][PF6], n ) 4, 6, 8) room-temperature ionic liquids (RTILs) and studied for the extraction of Na+, Cs+, and Sr2+ from aqueous solutions. In the absence of extractant, the distribution ratios for the metal ions indicate a strong preference for the aqueous phase. With the crown ethers as extractants in RTIL-based liquid/liquid separations, the resulting metal ion partitioning depends on the hydrophobicity of the crown ether and also on the composition of the aqueous phase (e.g., concentration of HNO3 vs Al(NO3)3). Aqueous solutions of HCl, Na3 citrate, NaNO3, and HNO3 (the latter at low concentrations) decrease the metal ion distribution ratios and also decrease the water content of the RTIL phase. High concentrations of HNO3 decompose PF6- and increase both the water content and the water solubility of the RTIL phase. Highly hydrated salts such as Al(NO3)3 and LiNO3 salt out both the RTIL ions and the crown ethers; thus, when the aqueous phase contains Al(NO3)3, the trend more closely resembles traditional solvent extraction behavior where DSr > DCs and the most hydrophobic extracting phase produces the highest partitioning. When [C8mim][PF6] is used as the extracting phase, the metal ions can be loaded from Al(NO3)3 and stripped using water. Dtb18C6 forms 1:1 complexes with Cs+ and Sr2+ and also yields the highest distribution ratios out of the three crowns examined. In comparison to traditional solvent extraction behavior, the metal ion partitioning in these systems exhibits exceptional behavior and, in certain instances, suggests a complicated partitioning mechanism, which necessitates a more thorough understanding of RTILs as solvents before interpretation of the results. Introduction 1967,1

the class of cyclic Since their discovery in polyether molecules known as crown ethers has been intensely investigated for metal ion extraction in liquid/ liquid separations. Of particular interest is the affinity of alkali metal cations2-7 and alkaline earth cations8-11 for these molecules. Throughout his career, Professor Izatt has demonstrated how these macrocycles can be used as the platform for designing more elaborate molecules that are capable of complexing metal ions12-15 and how crown ethers and related macrocycles can be applied in various separations processes.16 The cationselective nature of crown ether extraction facilitates their implementation in the removal of Cs+ and Sr2+ from solutions containing high concentrations of Na+, and thus, they have been extensively studied for applications to nuclear waste treatment.10,17-21 Crown ethers obtain their selectivity through several factors,12,22 and certain modifications can be made to produce a molecule designed for optimal results for specific applications. The ability to fine-tune and reorganize the crown ether structure and the resulting properties of the crown by, for example, changing the ring size and rigidity, changing the number and type of donor atoms, appending ionizable groups, and modifying * To whom correspondence should be addressed. Phone: 205-348-4323.Fax: 205-348-0823.E-mail: [email protected]. † This paper is written in honor of Professor Reed M. Izatt for his outstanding contributions to macrocyclic chemistry.

the lipophilicity offers broad appeal for applications of these molecules in liquid membranes,23 solvent extraction,4,9,24,25 chromatographic applications,10,26 and many others. In a typical liquid/liquid extraction experiment, the crown ether resides in the hydrophobic extracting phase and serves to dehydrate and complex the metal ions while removing them from the aqueous phase.12,27 To enhance the efficiency of such a process, an organic solvent is selected to sustain the biphasic system while maximizing the hydrophobic and complexing properties of the extracting phase. However, the volatile organic compounds (VOCs) commonly employed in traditional liquid/liquid separations can have high hazard ratings and low flash points, indicating associated health and safety concerns. While liquid/liquid separations utilizing VOC diluents have extensive applications in industry from chemical synthesis to hydrometallurgy, the end result of such practices often generates volumes of contaminated solvent. Ending the reliance on separations that consume large quantities of VOCs has become a target area for emphasis in the development of “Green” industrial processes that are inherently safer and are less polluting.28-32 Alternative Separations Media-Room-Temperature Ionic Liquids. Our research group is investigating alternatives to traditional liquid/liquid solvent extraction approaches. Previously, for example, we examined the use of crown ethers as metal ion extractants in poly(ethylene glycol)-based aqueous biphasic

10.1021/ie000426m CCC: $19.00 © 2000 American Chemical Society Published on Web 09/14/2000

Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000 3597

Figure 1. Common cations used in RTILs: N-alkylpyridinium (a) and 1-alkyl-3-methylimidazolium (b) and peak assignments for the H NMR spectra of [C4mim]+ (c).

systems (ABSs), where both hydrophilic and hydrophobic phases are aqueous.33,34 We are now turning our attention to new hydrophobic phases based on roomtemperature ionic liquids (RTILs). RTILs are a class of novel compounds that are composed entirely of ions and, initially, may be thought to resemble molten metallic ionic melts such as NaCl at 800 °C. In contrast to the high-temperature melts, certain RTILs exhibit favorable characteristics (e.g., moisture stability, nonmeasurable volatility, and water immiscibility) that merit their consideration as solvent alternatives in liquid/liquid extraction.35-37 As a class of solvents, RTILs are highly solvating and several are noncoordinating, which makes them suitable for catalysis and synthesis.38-41 The RTILs currently under active study in the literature are composed of large asymmetrical cations, 1-alkyl3-methylimidazolium ([Cnmim]+) and N-alkylpyridinium ([Cnpy]+), as shown in Figure 1. The alkyl group is usually an alkane, where by increasing the length of the alkane chain one can change the resulting properties (e.g., viscosity, hydrophobicity, and melting point).36,42,43 Along with these cations, several anions (e.g., PF6-, BF4-, N(CF3SO2)2-, CuCl2-, Cl-, Br-) are used to form a class of liquids capable of dissolving a wide array of materials.40,44-49 In current studies, the anion is used to primarily control the water miscibility of the resulting RTIL; 1-alkyl-3-methylimidazolium salts of PF6- are water immiscible, BF4- salts are water miscible depending on the alkyl chain length, and tetrahaloaluminate salts are moisture sensitive. The choice of cation and anion, along with the type of substituent group(s) on the cation, allows fine-tuning of the melting points, hydrophobicity, and solvent properties of the RTILs.36,42,43,49 Liquid/liquid extraction may be carried out with RTILs containing PF6- as the anion because they can take the place of the hydrophobic extracting phase in biphasic systems. We recently reported the partitioning of simple benzene derivatives and found a direct correlation between uptake to the RTILs and hydrophobicity (as measured by log P) of the solute. When the organic solutes contained ionizable groups, there was a pH dependence of the phase affinity according to the charge on the molecules.46,49 Thymol blue, a large aromatic dye, exhibited phase preference for [Cnmim][PF6] RTIL in the zwitterionic red and monoanion yellow forms but was quantitatively retained in the aqueous phase at high pH in the dianion blue form. Distribution ratios of the red form increased when going from [C4mim][PF6] to [C6mim][PF6] to [C8mim][PF6] RTIL, which prompted classification of the C8 derivative as the most hydrophobic RTIL in that series.

We have also studied the use of RTILs in liquid/liquid extraction of metal ions47,50 where, in the absence of extractant molecules, the hydrated metal ions remain in the aqueous phase. However, traditional metal ion extractants such as 1-(2-pyridylazo)-2-naphthol (PAN) and 1-(2-thiazolylazo)-2-naphthol (TAN) have been shown to enhance the extraction of Fe3+, Ni2+, and Co2+ in RTIL-based liquid/liquid separations.50 The incorporation of PAN and TAN produces pH-dependent, reversible metal ion partitioning from aqueous solutions above pH 8. Also, inorganic anions (e.g., CN-, OCN-, SCN-, and halides) form complexes with certain metal ions and increase their hydrophobicity and partitioning to the RTIL. In particular, Hg2+ distribution ratios increased dramatically in the presence of I- and SCN-.50 Other work in the literature has focused on the partitioning of specific solutes in these systems. Brennecke and co-workers have used [C4mim][PF6] to demonstrate how supercritical CO2 can be used to strip aromatic molecules from the RTIL phase.44 Dai et al. recently published a communication using dicyclohexyl18-crown-6 to produce “unprecendentedly large” liquid/ liquid distribution ratios for Sr2+ to both [C2mim][bis(triflyl)amide] and [C3mim][bis(triflyl)amide] when the aqueous phase contained low concentrations of NO3-.45 In this later study, it was intriguing to us that the highest Sr2+ distribution ratios were from low concentrations of HNO3. This behavior is contrary to that expected from traditional solvent extraction, where organic phases are usually loaded at high concentrations of NO3- or HNO3 and stripped at low acidity or NO3concentrations.51-53 Our previous results had not found a great difference between RTIL and traditional VOC solvents in solvent extraction studies, perhaps suggesting a more complicated partitioning mechanism at work here. For example, in light of the high concentration of ions in both phases, there is the possibility of ion exchange or leaching of the ions or solutes from the RTIL phase, which could be exacerbated under high acid concentrations. We were asked to prepare a manuscript to honor Professor Izatt, and in light of the interesting results published by Dai et al.,45 we chose to investigate in more detail the utilization of 18C6, DCH18C6, and Dtb18C6 in [Cnmim][PF6], n ) 4, 6, and 8, for the extraction of Na+, Cs+, and Sr2+. As discussed below, this investigation leads to a cautionary note regarding the utilization of RTILs as solvents. The added level of complexity of ionic versus molecular solvents requires an increased awareness of the total system, including leaching of the RTIL components to the aqueous phase and the effect of salts or acids in the aqueous phase on RTIL composition. Experimental Section The crown ethers DCH18C6 and 18C6 were obtained from Aldrich (Milwaukee, WI) and Dtb18C6 was obtained from Eichrom Industries (Darien, IL). All were used without further purification. HPF6 was supplied by Ozark-Mahoning (Tulsa, OK) and was used as received. All other chemicals were obtained from Aldrich (Milwaukee, WI), were of reagent grade, and were used without further purification. 22NaCl, 85SrCl2, and 137CsCl were obtained from Amersham Life Sciences (Arlington Heights, IL). γ-ray emission analysis was used for all isotopes and carried out on a Packard Cobra

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II Auto-Gamma spectrometer (Packard Instrument Co., Downers Grove, IL). All aqueous solutions were prepared with deionized water that was purified with a Barnsted deionization system (Dubuque, IA) and polished to 18.3 MΩ‚cm. Aqueous solutions of HNO3, HCl, KSCN, and Al(NO3)3 were prepared as molar concentrations by transferring a known amount of material to a volumetric flask and diluting to the specified volume with deionized water (or 0.001 or 0.5 M HNO3 from a stock solution, where appropriate). When needed, pH adjustments of the aqueous phase were made using H2SO4 or NaOH. Each crown ether solution was prepared at molar concentrations by weighing out a known amount of material, transferring it to a volumetric flask, and diluting to the specified volume with the appropriate ionic liquid. Ionic Liquid Synthesis. The ionic liquids were synthesized using the methods described in previously published material46,49 and stored in contact with DI water to equilibrate the water content. For analysis of the resulting RTILs, NMR (H at 360.13 MHz and 19F at 470.56 MHz) were used to determine the purity of each ionic liquid and ensure complete reaction (however, before NMR analysis, the majority of the water was removed from the RTIL by heating to 70 °C while on a vacuum line). Each H NMR spectrum contained peaks corresponding to 1-alkyl-3-methylimidazolium cation and indicated no residual reactants. The chemical shifts (ppm, neat sample) for the H NMR of [C4mim][PF6] were assigned to the hydrogen atoms on the [C4mim]+ cation as they are labeled in Figure 1: 0.72 (triplet, HA), 1.15 (sextet, HB), 1.68 (quintet, HC), 2.25 (singlet, broad, water), 3.73 (singlet, HE), 4.05 (triplet, HD), 7.22 (singlet, HG), 7.30 (singlet, HH), and 8.26 (singlet, HF). The 19F NMR spectra consist of two peaks, one at -74 ppm and the other at -72.5 ppm, corresponding to the splitting of 19F (I ) 1/2) by 31P (I ) 1/2). The H NMR analyses were performed on a Bruker AM 360 instrument (Houston, TX) while 19F NMR analyses used a Bruker AM 500 instrument (Houston, TX). Crown Ether Partitioning. H NMR was used to monitor the partitioning of each crown ether between [C4mim][PF6] and various aqueous phases. Each experiment was carried out as follows using Dtb18C6 as an example. A 3-mL aliquot of 0.1 M Dtb18C6 in [C4mim][PF6] was contacted with 3 mL of DI water at pH ∼ 5. The systems were vortexed (2 min) and centrifuged (2000 g, 2 min), mixed once again, and finally, centrifuged (2000 g, 2 min) to ensure that the phases were disengaged. (On the basis of previous work with these biphasic systems, the time allotted for vortexing and centrifuging was assumed to be sufficient to reach equilibrium.) The individual phases were separated and placed into shell vials from which 0.5 mL of the aqueous phase was transferred to an NMR tube. In a similar manner, H NMR was utilized to determine whether contacting aqueous solutions of HNO3, HCl, Sr(NO3)2, or Al(NO3)3 would affect the partitioning of crown ethers to the aqueous phase. To measure the distribution ratios for each crown ether, a series of crown ether standard solutions (0.0010.1 M) were prepared in [C4mim][PF6] from which a 0.5mL aliquot was taken for analysis by H NMR (neat samples). The peak area (triplet, 3.7 ppm, assigned to O-CH2) was integrated for each sample and the Beer’s Law plot was linear over the concentration range investigated. For these systems, a 1-mL aliquot of 0.1

M crown ether solution in [C4mim][PF6] was contacted with 1 mL of water, vortexed (2 min) and centrifuged (2000 g, 2 min), mixed once again, and finally, centrifuged (2000 g, 2 min) to ensure that the phases were fully separated. A 0.5-mL aliquot was removed from the aqueous phase and analyzed (neat) by H NMR for determination of peak area by integrating the peak, if any, at 3.7 ppm. Any peak at 3.7 ppm would be attributable to the signal from the O-CH2 groups; hence, the integrated area was divided by 24 to account for the 24 hydrogens around the 18-crown-6 ring. The resulting number (the area per mole of the crown ether) was converted to the molarity of the crown ether in either phase. In these calculations, the density of each phase was assumed to be constant. Finally, the distribution ratios were determined (to approximately 95% accuracy) using eq 1:

(initial concentration in RTIL final concentration in aqueous) D) final concentration in aqueous

(1)

Metal Ion Distribution Ratios. Metal ion distribution ratios were determined by mixing 1 mL of RTIL and 1 mL of an aqueous phase followed by vortexing (2 min) and centrifuging (2000 g, 2 min) to equilibrate the phases. Addition of the metal ion tracer (ca. 0.005 µCi, 5 µL) was followed by two intervals of vortexing (2 min) and centrifuging (2000 g, 2 min) to ensure that the phases were fully mixed and separated. The phases were separated and dispensed into shell vials from which 100 µL of each phase was removed for radiometric analysis. Because equal volumes of both phases were removed for analysis, the distribution ratio for the metal ions was determined as in eq 2:

D)

activity in the RTIL lower phase activity in the aqueous upper phase

(2)

Each experiment was done in duplicate and the results agreed to within 5%. Imidazolium Partitioning. UV/vis spectroscopy was used to monitor the partitioning of the 1-butyl-3methylimidazolium cation to the aqueous phase. All spectra were obtained on a Cary 3C spectrophotometer (Varian Optical Spectroscopy, Mulgrave, Victoria, Australia). Prior to the recording of any spectra, a baseline was obtained for DI water (and automatically subtracted for each experiment) using quartz cells and the reference cell for all experiments contained DI water. For each experiment, 3 mL of RTIL and 3 mL of a specific aqueous phase were contacted, vortexed (2 min), centrifuged (2000 g, 2 min), and followed by another interval of vortexing (2 min) and centrifuging (2000 g, 2 min) to ensure that the phases were fully separated. A 200-µL aliquot was removed from the aqueous phase, placed into a quartz cell, and diluted with 3 mL of DI water. The concentration of [C4mim]+ in the aqueous phase was quantified by measuring the absorbance for a series of solutions of [C4mim][Cl], a water-soluble RTIL whose cation absorbs at 289 nm. The associated Beer’s Law plot was formed for 0.01-2.5 M [C4mim][Cl] solutions prepared by weighing out a known amount of [C4mim][Cl], transferring it to a volumetric flask, and diluting to the specific volume with DI water. The plot of the absorbance as a function of concentration is linear over this concentration, and at a [C4mim][Cl] concentration

Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000 3599 Table 1. Distribution Ratios between RTIL/Aqueous Phases as a Function of pH ion

calc. ∆Ghyd (kJ/mol)a

aqueous pH

[C4mim][PF6]

Cs+

-245

1 7 13

Na+

-385

Sr2+

Cl-

a

[C6mim][PF6]

[C8mim][PF6]

0.084 0.067 0.072

0.071 0.068 0.066

0.075 0.072 0.077

1 7 13

0.025 0.023 0.023

0.013 0.011 0.013

0.010 0.011 0.011

-1385

1 7 13

0.033 0.048 0.054

0.040 0.029 0.035

0.028 0.026 0.044

-270

1 7 13

0.0031 0.0017 0.0025

0.0011 0.0014 0.00094

0.00024 0.00041 0.00079

From Marcus ref 57.

of 0.14 M, the molar absorptivity is 2.2. Using the [C4mim]+ concentrations in the aqueous phase determined with the Beer’s Law plot, the distribution ratios were calculated using eq 1. PF6- Partitioning. The presence of PF6- in the aqueous phase was examined after contacting 1 mL of [C4mim][PF6] with 1 mL of various aqueous phases, thoroughly mixing them as described above, and transferring 0.5 mL of the aqueous phase to an NMR tube for 31P NMR analysis with a Bruker 500 AM MHz instrument (Houston, TX). Each sample was analyzed neat. A sample of 0.1% H3PO4 was used as an internal standard in the tubes for each experiment. The singlet peak for 31PO43- was set to 0 ppm and the other peaks were determined relative to that position. When PF6is present in the resulting spectra, there is a heptet of peaks centered at -150 ppm, corresponding to the splitting of the 31P by the six neighboring fluorine atoms. Water Content of RTIL. The water content of each RTIL was determined using a volumetric Aquastar Karl Fischer titrator (EM Science, Gibbstown, NJ) with Composite 5 solution as the titrant and anhydrous methanol as the solvent. Each sample was at least 1 g and duplicate measurements were performed on each sample. Duplicate measurements agreed to within 100 ppm. Results and Discussion Table 1 shows the partitioning of each metal ion in [Cnmim][PF6] from various aqueous phases and indicates the low affinity of the metal ions for the RTIL phase in the absence of extractant. There is little if any change with pH and, in general, the distribution ratios for these metal ions are approximately the same in these RTILs. 18-Crown-6, dicyclohexyl-18-crown-6, and 4,4′-(5′)-di(tert-butylcyclohexano)-18-crown-6 were dissolved in each RTIL and the extraction of Na+, Cs+, and Sr2+ from aqueous solutions examined. Of these crown ethers, Dtb18C6 produces the highest distribution ratios for Sr2+ (∼100) in traditional solvent extraction, although the distribution ratios do depend on which organic solvent is used.9,17,54 When Dtb18C6 is employed in chromatographic separations using solvent-impregnated resins from acidic media, the uptake of Sr2+ is 4 orders of magnitude higher than that for Na+ or Cs+, both of which remain below 1.20 As Dai et al. observed, extraction of metal ions using crown ethers in RTILs can occur at low HNO3 concen-

Figure 2. Distribution ratios for Na+, Cs+, and Sr2+ with increasing concentrations of crown ethers in [C4mim][PF6]/water systems. The concentrations of crown ethers represent preequilibration concentrations in the RTIL phase. Open symbols correspond to Na+. Solid lines correspond to Sr2+ and Na+. Dashed lines correspond to Cs+. Table 2. Distribution Ratios for Crown Ethers in [C4mim][PF6]/Aqueous Systems crown, M

water

8M HNO3

8M HCl

2M Sr(NO3)2

2M Al(NO3)3

0.1 M 18C6 0.1 M DCH18C6 0.1 M Dtb18C6

1.4 2.1 a

1.5 2.3 a

1.0 9.0 a

9.0 10 a

16 99 a

a

No crown ether detected in aqueous phase.

trations. In fact, when [C4mim][PF6] is used, the extraction of these metal ions can occur directly from water, with distribution ratios increasing with concentration of the crown ether (Figure 2). Dtb18C6 produces the highest distribution ratios from water, increasing to DNa ) 3.3, DCs ) 92, and DSr ) 105 at 0.1 M crown ether. (These values represent the highest D values obtained for these metal ions with the three crown ethers studied in [C4mim][PF6]/water systems.) Slope analysis suggests an extraction stoichiometry of 0.8:1 for each metal ion with Dtb18C6, near the expected 1:1 value. With the other crown ethers, a clear and concise trend for the metal ion partitioning is difficult to ascertain. When 0.1 M DCH18C6 is used, Cs+ and Sr2+ have D values of 10 and 3.3, respectively, although Na+ is not extracted and remains in the aqueous phase (D ) 0.021). The presence of 18C6 produces D values slightly above 1 for Sr2+ at 0.1 M 18C6, although there is little extraction of Cs+ and Na+ to the [C4mim][PF6] phase with D values remaining near 0.1. It is clear from Figure 2 that when Dtb18C6 and DCH18C6 are utilized as extractants, Sr2+ and Cs+ distribution ratios are very similar and at least an order of magnitude higher than those for Na+. In contrast, the distribution ratios for Sr2+ with 18C6 are an order of magnitude higher than those for both Cs+ and Na+. The relative efficacy of the three crown ethers is consistent with their reported relative hydrophobicity (Dtb18C6 > DCH18C6 > 18C6) in traditional solvents, where Dtb18C6 is the most hydrophobic, produces the highest Sr2+ partitioning,9 and gives little uptake of Na+.20 Table 2 supports a similar trend for crown ether hydrophobicity in RTILs, indicating distribution ratios of the crown ethers to the RTIL from water increase from 1.4 for 18C6 to 2.1 for DCH18C6, to quantitative partitioning for Dtb18C6.

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Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000 Table 3. Water Content of Equilibrated and Dried RTIL; Density of Equilibrated RTIL ionic liquid [C4mim][PF6] [C6mim][PF6] [C8mim][PF6]

water content, density, g/mL water content, M (equilibrated)b (equilibrated) M (dried)a 0.72 0.02 0.02

0.92 0.68 0.46

1.37 1.29 1.22

a Each RTIL was dried on a vacuum line while heating the material to 70 °C. The material was considered to be dry after it had stopped bubbling and the outside of the flask was no longer cold to the touch. Such conditions are attainable after 3-4 h. b Each RTIL was equilibrated with water during the washing procedure, which incorporates vigorous agitation and mixing. The RTIL remained in contact with water after the washing was completed.

Figure 3. Distribution ratios for Cs+ and Sr2+ with 0.1 M Dtb18C6 in [Cnmim][PF6] with increasing HNO3 concentrations.

Contrary to traditional solvent extraction results, several observations from Figure 2 are unique to the behavior of Dtb18C6 in RTILs. Usually, extraction to the hydrophobic phase occurs in the presence of aqueous anions that facilitate the transfer of a neutral complex or ion pair with the extracted metal ion.5,27 As shown in Figure 2, extraction occurs from water without the addition of anions. Also, similar distribution ratios for Cs+ and Sr2+ are noteworthy here, especially since previous work has shown that Dtb18C6 can extract Sr2+ much more effectively than it does Cs+.17,26 (The equilibrium constants for each metal ion with Dtb18C6 are 5.2 × 103 (Sr2+), 8.1 (Na+), and 8.6 (Cs+).26) Horwitz et al. have also demonstrated that Dtb18C6 extracts Na+ better than Cs+,26 although the results here show that the trend in extraction behavior is different in RTILbased systems and suggests perhaps different extraction mechanisms need to be considered, as discussed further below. There are significant differences between metal ion partitioning in [Cnmim][PF6] as n is changed from 4 to 8. Distribution ratios for Sr2+ decrease from 92 (0.1 M Dtb18C6, [C4mim][PF6]) to 4.6 ([C6mim][PF6]) to 0.007 ([C8mim][PF6]). Similar results are observed for Cs+ whose D values decrease 105, 9.6, and 0.006. The next step was to study the dependence of distribution ratios on HNO3 concentration using 0.1 M Dtb18C6 and all three RTILs. The results (Figure 3) initially appear to be quite surprising. As observed from water, DSr and DCs are approximately the same, but the distribution ratios decrease as the concentration of HNO3 increases from 0.001 to 1 M. In addition, the most hydrophobic phase, [C8mim][PF6], results in the lowest distribution ratios followed by [C6mim][PF6] and finally [C4mim][PF6]. These results would initially appear contrary to traditional solvent extraction found in the literature; thus, a more detailed examination of the RTIL solvents was conducted. Although the extracting phase in traditional solvent extraction is termed the hydrophobic phase, certain solvents allow a large amount of water to be present in that phase.54,55 Hydrogen-bonding organic solvents can contain several moles of water, which serve to enhance the extraction of both Sr2+ and Cs+ with certain crown ethers.54,55 When a series of n-alcohols or carboxylic acids was used as the diluents for various crown ethers, (e.g., DCH18C6, di-(tert-butylbenzo)-24-crown-8, and di(tert-butylbenzo)-21-crown-7) in liquid/liquid extraction,

Figure 4. Distribution ratios for Na+, Cs+, and Sr2+ with 0.1 M Dtb18C6 in [C4mim][PF6] with increasing concentrations of HNO3. The acid concentrations refer to precontact concentrations in the aqueous phase.

those solvents with shorter alkyl chains contained more water in the organic phase and produced higher D values.54,55 The results were attributed to the increased concentration of water in the organic phase, which created an environment well-suited for the hydrated nitrate anion, thus facilitating the extraction by the crown ether.54 (Horwitz et al. have shown that the amount of nitrate extraction depends on which crown ether is present, but the resulting HNO3 concentration in the organic phase can be quite high.9) The water content of the three RTIL solvents was measured and the trend in Table 3 follows from the hydrophobicity of the liquids. The concentration of water in equilibrated [C4mim][PF6] is 0.92 M, which decreases to 0.62 M in [C6mim][PF6] and to 0.46 M in [C8mim][PF6]. On the basis of these results, the higher distribution ratios of Sr2+ in [C4mim][PF6] versus [C8mim][PF6] are consistent with the literature; however, the decrease in DSr with increasing HNO3 concentrations is not. The decrease in DM with increasing concentrations of HNO3 is not as easy to rationalize. In traditional liquid/liquid extraction of Sr2+, increasing concentrations of the complexing nitrate anion result in higher partitioning to the extracting phase.51-53 A more detailed study of our RTIL phase ([C4mim][PF6]) was thus carried out over a wider range in HNO3 concentrations. The results (Figure 4) indicate a minimum DM at 1 M HNO3, followed by a sharp increase at higher acid concentrations. Data in Table 4 reveal that, above 1 M HNO3, rather dramatic increases in the water content of the [C4mim][PF6] are observed. In addition, after a

Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000 3601

H+ + PF6- + 6H2O + 3HNO3 f H3PO4 + 6HF + 3HNO3 + 2H2O (3)

Table 4. Water Content of [C4mim][PF6] (with and without 0.1 M Dtb18C6) versus Aqueous Phase Composition water (M), no crown

water (M), w/crown

0.9 0.8 0.8 0.9 0.9 1.3 1.8 2.4 3.2

1.2 1.1 1.0 0.9 1.3 1.4 1.7 1.7 1.8

0.8 0.8 0.7 0.7 0.6

1.3 1.1 0.9 0.9 0.8

4.0 M HCl 6.0 M HCl 8.0 M HCl 10 M HCl

0.6 0.6 0.6 0.5

1.3 0.8 0.8 0.7

0.010 M NaNO3 0.50 M NaNO3 1.0 M NaNO3 3.0 M NaNO3 6.0 M NaNO3

1.3 1.3 1.3 1.0 0.8

2.4 2.1 1.8 1.0 0.9

aqueous phase 0.0010 M HNO3 0.010 M HNO3 0.10 M HNO3 1.0 M HNO3 2.5 M HNO3 4.0 M HNO3 6.5 M HNO3 8.0 M HNO3 10 M HNO3 0.10 M Al(NO3)3 0.50 M Al(NO3)3 1.0 M Al(NO3)3 1.5 M Al(NO3)3 2.0 M Al(NO3)3

Table 5. Aqueous Phase Concentration of [C4mim]+ from [C4mim][PF6] after Contact with Various Aqueous Phases aqueous phase

no crown

0.1 M 18C6

0.1 M DCH18C6

0.1 M DB18C6

0.1 M Dtb18C6

water 2 M Al(NO3)3 8 M HNO3 8 M HCl

a 0.08 1.2 a

a 0.07 1.2 a

a 0.08 1.2 a

a 0.07 1.2 a

a 0.07 1.2 a

a

Not detected in aqueous phase.

4-day contact time with 6.5 M HNO3, the water content of [C4mim][PF6] increased from 1.8 to 2.9 M. Short contacts (2-4 min) of 8-10 M HNO3 with [C4mim][PF6] resulted in large increases in the water content, up to 3.2 M, and [C4mim][PF6] in contact with 8 or 10 M HNO3 becomes monophasic within several hours. The effect of HNO3 on the RTIL is even more obvious when considering the leaching of [C4mim]+ to the aqueous phase (Table 5). Short (2-4 min) contacts of [C4mim][PF6] with water does not lead to detectable concentrations of [C4mim]+ in the aqueous phase; however, similar contacts with 8 M HNO3 result in large concentrations (1.2 M) of the RTIL cation in the aqueous phase. The presence or absence of crown ether extractant does not appear to change the leaching of the cation. In addition to the leaching of [C4mim]+ to the aqueous phase, the presence of high HNO3 concentrations also promotes the degradation of PF6- to PO43-. In an aqueous solution of HPF6 there are several other species present by weight percent, including HF (2-12%), HPO2F2 (2-12%), H2PO3F (2-12%), and H3PO4 (212%).56 In the production of ionic liquids, species other than PF6- are removed from the RTIL in the washing step or are present in concentrations too small to be detected by NMR. When a 0.1 M HNO3 aqueous phase after contact with [C4mim][PF6] is analyzed, 31P NMR indicates the presence of only PF6-. However, HNO3 may act as a catalyst as in eq 3:

NMR spectra taken after contact with aqueous phases of increasing HNO3 concentrations (0.1-8 M) indicate a conversion of PF6- to PO43-, and at 8 M HNO3, there are no peaks centered around -150 ppm and the signal at 0 ppm (set as the H3PO4 standard) is the only one visible, thus indicating the conversion of PF6- to PO43is accelerated at high acidities. Because PO43- is hydrophilic, this also helps to explain why the [C4mim][PF6] RTIL becomes monophasic rather quickly when contacted with high concentrations of nitric acid. (The HNO3-catalyzed formation of PO43- from PF6- also is accelerated in the presence of SiO2, which forms SiF4 and H+, further driving the reaction toward production of H3PO4.) The increase in D values for each metal ion at higher HNO3 concentrations can thus be attributed to the major changes in the water content and composition of the RTIL, which would facilitate partitioning of both the hydrated Na+, Cs+, and Sr2+ cations and their accompanying NO3- anions. An appreciable amount of water in the extracting phase creates an environment that is acceptable for the hydrated nitrate ion and reduces the energy needed to transfer the Sr(NO2)3‚ crown complex into the extracting phase.54 Similar arguments can be made for the extraction of Cs+ and Na+. Thus, the partitioning here is noticeably different from traditional solvent extraction in two respects: the observed anion dependence of Sr2+ extraction with Dtb18C6 shows a minimum around 1 M HNO39 and also, as in Figure 2, the extraction of Cs+ is remarkably close to Sr2+. Because of the obvious effect of HNO3 acid on the ionic composition and thus solvent properties of [Cnmim][PF6], additional studies were conducted with Na3citrate, HCl, NaNO3, Al(NO3)3, and LiNO3 (Figure 5). The partitioning of Sr2+ to 0.1 M Dtb18C6 [C4mim][PF6] from aqueous solutions of HCl, Na3citrate, and NaNO3 all exhibit similar trends to that observed for HNO3, decreasing DM with increasing salt or acid concentrations in the aqueous phases. (The partitioning of Cs+ in the same systems was similar.) If, as observed on increasing hydrophobicity of the RTIL, DM decreases with decreasing water content, the results upon addition of these salts are internally consistent. Water content decreases in the RTIL phase with increasing HCl and NaNO3 concentrations (Table 4). Even HCl at 10 M and NaNO3 at 6 M do not result in the same increase in water content or changes in ionic composition as observed at high HNO3 concentrations and thus there is no upturn in DM versus concentration with HCl or NaNO3. In addition, Dtb18C6 is not detectable in the aqueous phase under any of the conditions studied here (Table 2) and there is no detectable leaching of the C4mim+ cations after short (2-4 min) contacts of [C4mim][PF6] with 8 M HCl (Table 5). These results suggest a possible salting out of the RTIL phase where there is little tendency for the hydrophobic [C4mim]+ or [PF6]- ions to partition to the aqueous phase. To further investigate the possible salting-out effect for RTILs, aqueous phases of Al(NO3)3 were studied, with rather dramatic results. As indicated above, the loss of both [C4mim]+ and crown ethers from the RTIL is dependent on the aqueous phase composition and

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Figure 5. Distribution ratios for Sr2+ with 0.1 M Dtb18C6 in [C4mim][PF6] with various aqueous phases. Distribution ratios from Al(NO3)3 are represented by the following symbols: [ (No HNO3), b (0.1 M HNO3), and 9 (0.5 M HNO3). The dashed line corresponds to partitioning from aqueous solutions of NaNO3. All concentrations correspond to precontact concentrations.

suggests that the presence of highly hydrated salts prevents leaching of the RTIL to the aqueous phase. Somewhat surprisingly, in the presence of Al(NO3)3, even under acidic (0.001 or 0.5 M HNO3) conditions, distribution ratios of Sr2+ to [C4mim][PF6] with 0.1 M Dtb18C6 remain above 100. Increasing Al(NO3)3 concentrations result in small increases in DSr. Utilization of the highly hydrated Li+ cation (as LiNO3) results in similar behavior, although the DSr values are approximately an order of magnitude lower (Figure 5). The lower values for Li+ (for a given NO3- concentration) may result from its smaller hydration number (5.2) compared to Al3+ (20.4)57 or from the increased acidity of Al3+ in aqueous solutions. Review of the data in Tables 2, 4, and 5 reveals that Al(NO3)3 may salt out both the crown ethers and the RTIL ions. Distribution ratios of 18C6 and DCH18C6 between [C4mim][PF6] and 2 M Al(NO3)3 are much higher than those observed from water, 8 M HNO3, or 8 M HCl (Table 2). Distribution ratios of the crown ethers from 2 M Al(NO3)3 also increase in an order similar to that of the lipophilicity of the crown ether molecules. 18C6 (DCE ) 16) < DCH18C6 (DCE ) 99) < Dtb18C6 (not detectable in the aqueous phase). Increasing concentrations of Al(NO3)3 tend to slightly decrease the water content of [C4mim][PF6], as observed for HCl, NaNO3, and low concentrations of HNO3 (Table 4). In addition, leaching of the [C4mim]+ cation is dramatically reduced from 2 M Al(NO3)3 compared to 8 M HNO3 (Table 5). Given the exceptional results from Al(NO3)3, we reinvestigated the partitioning of Sr2+ and Cs+ from aqueous solutions of Al(NO3)3 using all three RTIL solvents ([Cnmim][PF6] n ) 4-8). The results depicted in Figure 6 are rather dramatically different from earlier results and now appear to be consistent with extraction of Sr2+ and Cs+ with Dtb18C6 from traditional solvents. First, the distribution ratios of Sr2+ are over an order of magnitude larger than those observed for Cs+. Second, distribution ratios increase in all three solvents with increasing Al(NO3)3 concentrations. Finally, the variable hydrophobicity of the three ionic solvents is evident by higher distribution ratios for [C6mim][PF6] and [C8mim][PF6] versus [C4mim][PF6].

Figure 6. Distribution ratios for Sr2+ and Cs+ with 0.1 M Dtb18C6 in [Cnmim][PF6] with increasing concentrations of Al(NO3)3 in 0.001 M HNO3.

A comparison of Figures 3 and 6 reveals that [C8mim][PF6], the most hydrophobic of the three ionic solvents, produces results most similar to traditional solvents. In the presence of 0.1 M Dtb18C6, but absence of Al(NO3)3, Sr2+ prefers the aqueous phase (DSr ) 0.026). Addition of Al(NO3)3 promotes partitioning to the RTIL phase with DSr as high as 645 (2 M Al(NO3)3). This suggests that reversible partitioning, needed to load and strip Sr2+ (or Cs+) in a solvent extraction process, is possible using [C8mim][PF6]. In addition, the additional hydrophobicity of the [C8mim]+ cation should lower the leaching of the cation to the aqueous phase. Conclusions The metal ion partitioning results indicate important differences between the efficacy of crown ethers for metal ion separations in traditional solvent extraction versus RTIL-based liquid/liquid systems. For a series of 18C6 derivatives, the highest Cs+ and Sr2+ distribution ratios were observed with the most hydrophobic crown ether, Dtb18C6, although there was not a significant difference in the distribution ratios for Cs+ and Sr2+. When the aqueous phase contains hydrated anions such as Cl- or NO3-, the highest D values for Sr2+ and Cs+ were obtained with the lowest acid concentrations. Aqueous solutes can drastically affect the stability and composition of the ionic liquids and leaching of component ions. Increasing aqueous concentrations of HNO3, HCl, or NaNO3 do not produce increased distribution ratios for Sr2+ or Cs+; instead, distribution ratios decrease as the water content of the RTIL phase decreases. In the presence of increasing concentrations of Al(NO3)3, the water content of the RTIL also decreases; however, the distribution ratios for the crown ethers increases, and leaching of [Cnmim]+ cations decreases. Most importantly, in the presence of Al(NO3)3, the trends in DM follow predictions based on traditional solvent extraction and [C8mim][PF6] can actually be loaded from Al(NO3)3 and stripped into water. Room-temperature ionic liquids may indeed replace certain VOCs in some solvent extraction processes; however, extreme caution is needed in interpretation of the results before a more fundamental understanding of these neoteric solvents is available. The complexity

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of ionic versus molecular solvents dictates review of possible ion exchange as well as partitioning mechanisms. It is also clear that one must not focus on just the chemistry of the anion or the cation, but both must be considered in the context of the aqueous phase composition. Fundamental studies to understand the relationship between ionic composition and physical and chemical properties of RTIL are currently underway in our laboratories. Acknowledgment This work was supported by funding from the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S. Department of Energy (Grant DE-FG02-96ER14673) and the PG Research Foundation. The authors appreciate the chemicals supplied by Eichrom Industries (Dtb18C6) and OzarkMahoning (HPF6). The authors are indebted to Dr. K. A. Belmore for assistance in obtaining and interpreting the NMR spectra. Literature Cited (1) Pederson, C. J. J. Am. Chem. Soc. 1967, 89, 7017-7036. (2) Hankins, M. G.; Bartsch, R. A.; Olsher, U. Solvent Extr. Ion Exch. 1995, 13, 983-995. (3) Marcus, Y.; Asher, L. E. J. Phys. Chem. 1978, 82, 12461254. (4) Sachleben, R. A.; Deng, Y.; Bailey, D. R.; Moyer, B. A. Solvent Extr. Ion Exch. 1996, 14, 995-1015. (5) Talanova, G. G.; Elkarim, N. S. A.; Talanov, V. S.; Hanes, R. E., Jr.; Hwang, H.-S.; Bartsch, R. A.; Rogers, R. D. J. Am. Chem. Soc. 1999, 121, 11281-11290. (6) Talanova, G. G.; Elkarim, N. S. A.; Hanes, R. E., Jr.; Hwang, H.-S.; Rogers, R. D.; Bartsch, R. A. Anal. Chem. 1999, 71, 672677. (7) Bryan, J. C.; Sachleben, R. A.; Lavis, J. M.; Davis, M. C.; Burns, J. H.; Hay, B. P. Inorg. Chem. 1998, 37, 2749-2755. (8) Tsurubou, S.; Mizutani, M.; Kadota, Y.; Yamamoto, T.; Umetani, S.; Sasaki, T.; Le, Q. T. H.; Matsui, M. Anal. Chem. 1995, 67, 1465-1469. (9) Horwitz, E. P.; Dietz, M. L.; Fisher, D. E. Solvent Extr. Ion Exch. 1990, 8, 557-572. (10) Horwitz, E. P.; Dietz, M. L.; Fisher, D. E. Anal. Chem. 1991, 63, 522-525. (11) Lamb, J. D.; Nazarenko, A. Y.; Hansen, R. J. Sep. Sci. Technol. 1999, 34, 2583-2599. (12) Bradshaw, J. S.; Izatt, R. M.; Bordunov, A. V.; Zhu, C. Y.; Hathaway, J. K. In Comprehensive Supramolecular Chemistry, Volume 1: Molecular Recognition: Receptors for Cationic Guests; Gokel, G. W., Vol. Ed.; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Vo¨gtle, F., Exec. Eds.; Lehn, J.-M., Ed. Bd Chair; Pergamon: Oxford, 1996; pp 35-95. (13) Bradshaw, J. S.; Izatt, R. M. Acc. Chem. Res. 1997, 30, 338-345. (14) Izatt, R. M.; Bradshaw, J. S.; Nielsen, S. A.; Lamb, J. D.; Christensen, J. J. Chem. Rev. 1985, 85, 271-339. (15) Izatt, R. M.; Clark, G. A.; Christensen, J. J. Sep. Sci. Technol. 1986, 21, 865-872. (16) Izatt, R. M.; Lamb, J. D.; Bruening, R. L. Sep. Sci. Technol. 1988, 23, 1645-1658. (17) Dietz, M. L.; Horwitz, E. P.; Rogers, R. D. Solvent Extr. Ion Exch. 1995, 13, 1-17. (18) Rogers, R. D.; Bauer, C. B.; Bond, A. H. Sep. Sci. Technol. 1995, 30, 1203-1217. (19) Horwitz, E. P.; Dietz, M. L.; Diamond, H.; Rogers, R. D. In Chemical Pretreatment of Nuclear Waste for Disposal; Schulz, W. W., Horwitz, E. P., Eds.; Plenum: New York, 1995; pp 81-99.

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Received for review April 28, 2000 Revised manuscript received July 31, 2000 Accepted July 31, 2000 IE000426M