Solvent extraction of Sr2+ and Cs+ based on room-temperature ionic liquids containing monoaza-substituted crown ethers

Article abstract of DOI:10.1021/ac035473d

A series of N-alkyl aza-18-crown-6 ethers were synthesized and characterized by NMR spectroscopy and mass spectrometry. These monoaza-substituted crown ethers in ionic liquids were investigated as recyclable extractants for separation of Sr²⁺ and Cs⁺ from aqueous solutions. The pH-sensitive complexation capability of these ligands allows for a facile stripping process to be developed so that both macrocyclic ligands and ionic liquids can be reused. The extraction efficiencies and selectivities of these monoaza-substituted crown ethers for Na⁺, K⁺, Cs⁺, and Sr²⁺ were studied in comparison to those of dicyclohexano-18-crown-6 under the same conditions. The extraction selectivity order for dicyclohexano-18-crown-6 in the ionic liquids investigated here was K⁺ ? Sr²⁺ > Cs ⁺ > Na⁺. The extraction selectivity order for N-alkyl aza-18-crown-6, in which the alkyl group is varied systematically from ethyl to n-dodecyl, was Sr²⁺ ? K⁺ > Cs⁺ > Na⁺ in 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]amide and 1-butyl-3-methylimidazolium bis[(trifiuoromethyl)suffonyl]amide and K⁺ > Sr²⁺ > Cs⁺ > Na⁺ in 1-hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl] amide and 1-octyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]amide. The strong dependence of selectivity on the type of ionic liquid indicates an important role played by solvation in solvent extraction processes based on ionic liquids. The optimization of macrocyclic ligands and ionic liquids led to an extraction system that is highly selective toward Sr²⁺.

Full text of DOI:10.1021/ac035473d

Anal. Chem. 2004, 76, 2773-2779
Solvent Extraction of Sr2+ and Cs+ Based on
Room-Temperature Ionic Liquids Containing
Monoaza-Substituted Crown Ethers
Huimin Luo,*,† Sheng Dai,‡ and Peter V. Bonnesen‡
Nuclear Science and Technology Division and Chemical Sciences Division, Oak Ridge National Laboratory,
Oak Ridge, Tennessee 37831
A series of N-alkyl aza-1 8 -crown-6 ethers were synthe-
sized and characterized by NMR spectroscopy and mass
spectrometry. These monoaza-substituted crown ethers
in ionic liquids were investigated as recyclable extractants
for separation of Sr2 + and Cs+ from aqueous solutions.
The pH-sensitive complexation capability of these ligands
allows for a facile stripping process to be developed so
that both macrocyclic ligands and ionic liquids can be
reused. The extraction efficiencies and selectivities of
these monoaza-substituted crown ethers for Na+, K+, Cs+,
and Sr2 + were studied in comparison to those of dicyclo-
hexano-1 8 -crown-6 under the same conditions. The ex-
traction selectivity order for dicyclohexano-1 8 -crown-6 in
the ionic liquids investigated here was K+ . Sr2 + > Cs+
> Na+. The extraction selectivity order for N-alkyl aza-
1 8 -crown-6 , in which the alkyl group is varied systemati-
cally from ethyl to n -dodecyl, was Sr2 + . K+ > Cs+ > Na+
in 1 -ethyl-3 -methylimidazolium bis[(trifluoromethyl)sul-
fonyl]amide and 1 -butyl-3 -methylimidazolium bis[(tri-
fluoromethyl)sulfonyl]amide and K+ > Sr2 + > Cs+ > Na+
in 1 -hexyl-3 -methylimidazolium bis[(trifluoromethyl)sul-
fonyl] amide and 1 -octyl-3 -methylimidazolium bis[(tri-
fluoromethyl)sulfonyl]amide. The strong dependence of
selectivity on the type of ionic liquid indicates an impor-
tant role played by solvation in solvent extraction pro-
cesses based on ionic liquids. The optimization of mac-
rocyclic ligands and ionic liquids led to an extraction
Figure 1. Structures of the two most common ionic liquid cations.
templates of the two most common classes of ambient-temperature
ionic liquids. The cation is usually a heterocyclic cation, such as
a dialkylimidazolium ion or an N-alkylpyridinium ion. The rela-
tively large size of these organic cations compared to simple
inorganic cations accounts for the low melting points observed
for these organic cations when paired with a variety of anions,
such as BF4-, PF6-, CF3SO3-, or other complex anions. These
ion pairs or salts can be liquids down to ∼-100 °C and are
thermally stable to ∼200 °C, depending on the specific structures
of the anions and cations. Unlike conventional solvents currently
in use for solvent extaction applications, these ionic liquids are
nonflammable, are chemically tunable, and have no detectable
vapor pressure.
We11 and others5,7,10,12 have been interested in the development
of IL-based solvent extraction methods for the separation of fission
products. In contrast to the high-temperature inorganic molten
salts, room-temperature ILs can be made hydrophobic while
retaining ionicity.12 This dual property forms the basis for using
ILs as unique separation media for the solvent extraction of ionic
species.11 Large distribution coefficients (DM) for the extraction
of metal ions have been observed by ionic liquids containing
complexing ligands.7-11 For example, whereas conventional sol-
vent extraction of Sr2+ using dicyclohexano-18-crown-6 can give
system that is highly selective toward Sr2 +
.
The application of ionic liquids (ILs) as replacement solvents
for various catalytic reactions and separation processes is being
extensively explored.1-11 Figure 1 gives the cationic structure
* To whom correspondence should be addressed. E-mail: luoh@ornl.gov.
† Nuclear Science and Technology Division.
‡ Chemical Sciences Division.
(1) Rogers, R. D.; Seddon, K. R. In Ionic Liquids: Industrial Applications to Green
Chemistry; Rogers, R. D., Seddon, K. R., Eds.; ACS Symposium Series 818;
American Chemical Society: Washington, DC, 2002.
(2) Wasserscheid, P., Welton, T., Eds. Ionic Liquids in Synthesis; Wiley-VCH:
Weinheim, 2 003.
(3) Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.; Visser, A. E.; Rogers, R.
D. Chem. Commun. 1 9 9 8 , 1756-1765.
(6) Blanchard, L. A.; Hancu, D.; Beckman, E. J.; Brennecke, J. F. Nature 1 9 9 9 ,
399, 28.
(7) Visser, A. E.; Swatloski, R. P.; Reichert, W. M.; Griffin, S. T.; Rogers, R. D.
Ind. Eng. Chem. Res. 2 0 0 0 , 39, 3596.
(8) (a) Armstrong, D. W.; He, L.; Liu, Y. S. Anal. Chem. 1 9 9 9 , 71, 3873. (b)
Carda-Broch, S.; Berthod, A.; Armstrong, D. W. Anal. Bioanal. Chem. 20 03 ,
375, 191.
(9) Chun, S.; Dzyuba, S. V.; Bartsch, R. A. Anal. Chem. 2 0 0 1 , 73, 3737.
(10) Dietz, M. L.; Dzielawa, J. A. Chem Commun 2 0 0 1 , 2124.
(11) Dai, S.; Ju, Y. H.; Barnes, C. E. J. Chem. Soc., Dalton Trans. 1 9 9 9 , 1201.
(12) Bonhote, P.; Dias, A. P.; Papageorgiou, N.; Kalyanasundaram, K.; Gratzel,
M. Inorg. Chem. 1 9 9 6 , 35, 1168.
(4) Blanchard, L. A.; Hancu, D.; Beckman, E. J.; Brennecke, J. F. Nature 1 9 9 9 ,
399, 28.
(5) Nakashima, K.; Kubota, F.; Maruyama, T.; Goto, M. Anal. Sci. 2 0 0 3 , 19,
1097.
10.1021/ac035473d CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/10/2004
Analytical Chemistry, Vol. 76, No. 10, May 15, 2004 2773

practical DM values of less than 1, our experiments11 with ionic
liquids as extraction solvents gave values of DM on the order of
104. The enhanced distribution coefficients can be attributed to
the unique solvation properties of ionic liquids for ionic species.
Ion-exchange processes also play an important role as revealed
by Dietz et al. and Visser et al. 7,10 However, the large DM values
observed for fission products using IL-based extraction processes
make it very difficult to strip extracted metal ions, so that crown
ethers could be recycled. Another drawback associated with all
solvent extraction processes based on dicyclohexano-18-crown-6
for extraction of Sr2+ is the significant coextraction of K+. The
affinity of dicyclohexano-18-crown-6 toward K+ is greater than that
toward Sr2+, limiting the direct application of dicyclohexano-18-
crown-6 in removal of Sr2+ in nuclear wastes. These deficiencies
associated with crown ether and IL-based extraction processes
for metal ions prompted us to explore other macrocycle ligands,
incorporating a switchable binding site. To allow stripping of
cations for recycle of macrocycle extractants, we have synthesized
several N-alkyl aza crown ethers where one of the oxygen atoms
is replaced by an aza group. The aza group provides a pH
switchable site for binding metal cations that can be readily
achieved via protonation and deprotonation of the aza group.13,14
The extraction abilities of these recyclable crown ethers and their
comparison with dicyclohexano-18-crown-6 in ILs are presented
here.
Table 1. Solubilities in Water, Water Contents, and
Densities of [Cnmim][NTf2]
water content
(ppm)
density
(g/ mL)
alkyl group in
solubility in
[Cnmim][NTf2] water (g/ 100 mL) dried
weta
dried weta
ethyl, C2
butyl, C4
1.77
0.80
2.46
0.34
0.21
350
240
513
167
33
19 400 1.50
13 600 1.42
20 700 1.37
10 900 1.33
8 150 1.31
1.46
1.39
1.35
1.30
1.25
[C4mim][PF6]b
hexyl, C6
octyl, C8
a
One milliliter of RTIL and 10 mL of DI H2O were shaken in a
vibrating mixer for 1 h to maximize the water content of the IL.
b [C4mim][PF6] was purchased from Aldrich.
of monoaza crown ethers were confirmed using a matrix-assisted
laser desorption/ ionization time-of-flight mass spectrometry (Voy-
ager DE MALDI-TOF). The matrixes used are 4-nitroaniline (99%,
Aldrich) and 2,5-dihydroxybenzoic acid (Sigma). Concentrations
of Na+, K+, Cs+, and Sr2+ were determined using a Dionex LC20
ion chromatograph equipped with an IonPac CS-12 analytical
column. In some cases, DSr values were verified by measuring
strontium concentrations using inductively coupled plasma-atomic
emission spectroscopy (ICP-AES).
Extraction Experiments. The extraction experiments were
performed in duplicate for each IL by contacting 1 mL of IL
containing various concentrations of the extractant with 10 mL of
cation-containing aqueous solution (1.5 mM) for 60 min in a
vibrating mixer. After centrifugation, the upper aqueous phase
was separated and the concentrations of cations were determined
by ion chromatography. In order not to complicate the extraction
systems, no buffer solution was added to control pH. The initial
pH of all three aqueous solutions is 6.4. The final pH values of
aqueous phases increase to 6.7 for the extraction system contain-
ing DCH18C6 but change to ∼11.2 for the systems containing
recyclable monoaza crown ethers because of the basicity associ-
ated with monoaza crown ethers.
EXPERIMENTAL SECTION
Materials and Methods. All chemicals and solvents were
reagent grade and used without further purification unless noted
otherwise. Dicyclohexano-18-crown-6 (DCH18C6) was purchased
from Aldrich and is a mixture of cis-syn-cis and cis-anti-cis
isomers. Four 1-alkyl-3-methylimidazolium bis[(trifluoromethyl)-
sulfonyl]amide ([Cnmim][NTf2]) ionic liquids, in which the alkyl
groups were ethyl (C2), n-butyl (C4), n-hexyl (C6), and n-octyl (C8),
were synthesized by modified literature procedures12 and the
details are given in Supporting Information. The solubility of ILs
in water was determined by combining 1 mL of IL and 10 mL of
deionized (DI) water and shaking for 60 min in a vibrating mixer.
A 0.5-mL aliquot of the aqueous phase was removed and diluted
to 5 mL with DI water. The absorbance at 211 nm was measured
using a Varian UV-visible-NIR spectrometer (model 5000).8b The
solubility of each IL in water was calculated by the comparison of
the measured absorbance with that obtained from dissolving a
known amount (2-20 mg) of IL in 5 mL of DI water. The density
of ILs was measured by filling a 1.0-mL volumetric flask to the
mark with each IL and weighing. The water content of the ILs
was measured using a Metronm 652 KF coulometer. These
physical properties of the ILs are listed in Table 1. Aqueous
solutions were prepared using DI water with a specific resistance
of 18 MΩ‚cm or greater. 1H, 13C, and 19F NMR spectra were
obtained in CDCl3 with a Bruker MSL-400 NMR spectrometer,
operating at 400.13 MHz for proton, 100.61 MHz for carbon, and
376.49 MHz for fluorine. Proton and carbon chemical shifts are
reported relative to tetramethylsilane. Fluorine chemical shifts are
reported relative to fluorotrichloromethane. The molecular weights
The distribution coefficients (DM) for extraction of Mn+ are
defined in eq 1 as
(Ci - Cf)
(Cf)
{volume of aqueous solution}
{volume of IL}
DM
)
(1)
{
}
where Ci and Cf represent the initial and final concentrations of
n+ in the aqueous phase. Although the value of DM depends on
M
the concentration of free extractants, the extraction trend reflected
in DM should be the same as that of the corresponding equilibrium
constant for a given initial extractant concentration. A volume ratio
is needed in calculation of distribution coefficients (eq 1) to
account for the difference in volume between two phases. Thus,
a distribution ratio for Mn+ greater than 1 (DM > 1) represents
an overall preference of Mn+ to the IL phase. The values of DM
were measured in duplicate with uncertainty within 5%.
Recycling Experiments. Following the extraction experi-
ments, one drop of 6 N HNO3 solution was added to the extraction
systems to protonate the monoaza crown ethers and the pH value
of the aqueous phase changed to 2.1. The mixture was agitated
(13) Beer, P. D.; Gale, P. A.; Smith, D. K. Supramolecular Chemistry; Oxford
University Press: New York, 1999.
(14) Matsushima, K.; Kobayashi, H.; Nakatsuji, Y.; Okahara, M. Chem. Lett. 1983,
701.
2774 Analytical Chemistry, Vol. 76, No. 10, May 15, 2004

for 60 min again in a vibrating mixer. After centrifugation, the
upper aqueous phase was separated, pH was checked to ensure
that remained around 2, and the concentrations of the recovered
cations were determined by ion chromatography. The IL phase
was rinsed with DI H2O several times until the aqueous phase
was neutral. This IL phase could be reused in subsequent
extraction experiments after the protonated monoaza crown ethers
were deprotonated by addition of one drop of saturated LiOH
solution into the extraction systems.
General P rocedure for Synthesis of N-Alkyl Aza-1 8 -
crown-6 Ethers. The protocol for synthesizing monoaza-
substituted crown ethers is based on the literature methodology.15
Briefly, 1-aza-18-crown-6, primary alkyl halide, sodium carbonate
(10 times of mole ratio of alkyl halide), and acetonitrile were mixed
in a flask. The mixture was heated to reflux at 80 °C for 3 days.
The resulting cloudy solution was filtered and the solvent removed
by rotary evaporation. A copious amount of DI water was added
to the residue, and the aqueous phase was extracted twice with
dichloromethane. The organic phase was dried (MgSO4) and the
dichloromethane removed to afford the desired compound as a
yellow oil.
N-Ethyl Aza-1 8 -crown-6 (1 ). From 1-aza-18-crown-6 (0.56 g,
2.13 mmol) and bromoethane (0.38 g, 3.49 mmol), 0.48 g (1.66
mmol) of N-ethyl aza-18-crown-6 was obtained (yield 78%): 1H
NMR δ, 3.86-3.60 (m, 20H), 2.76 (t, 4H, J ) 5.9 Hz), 2.60 (q, 2H,
J ) 7.1 Hz), and 1.04 (t, 3H, J ) 7.1 Hz); 13C NMR δ, 70.83 (CH2),
70.74 (CH2), 70.37 (CH2), 69.86 (CH2), 53.35 (CH2), 49.50 (CH2),
49.24 (CH2), and 11.89 (CH3). The NMR data agree with those
reported in the literature.16 MS m/ z (M+) calcd 291.41, found
291.32.
and 0.88 (t, 3H, J ) 7.0 Hz); 13C NMR δ, 70.46 (CH2), 69.86 (CH2),
69.58 (CH2), 68.58 (CH2), 55.05 (CH2), 53.28 (CH2), 31.59 (CH2),
29.34 (CH2), 29.03 (CH2), 27.25 (CH2), 26.20 (CH2), 22.41 (CH2),
and 13.87 (CH3). The NMR data agree with those reported in the
literature.19 MS m/ z (M+) calcd 375.56, found 375.11.
N-Dodecyl Aza-18-crown-6 (5). From 1-aza-18-crown-6 (0.73
g, 2.78 mmol) and 1-bromododecane (0.70 g, 2.79 mmol), 0.93 g
(2.15 mmol) of N-dodecyl aza-18-crown-6 was obtained (yield
77%): 1H NMR δ, 3.79-3.63 (m, 16H), 3.60 (t, 4H, J ) 5.9 Hz),
2.74 (t, 4H, J ) 5.7 Hz), 2.50 (m, 2H), 1.25 (br peak, 20H), and
0.88 (t, 3H, J ) 7.0 Hz); 13C NMR δ, 70.76 (CH2), 70.67 (CH2),
70.30 (CH2), 69.82 (CH2), 56.04 (CH2), 53.93 (CH2), 33.95 (CH2),
32.75 (CH2), 31.83 (CH2), 29.55 (CH2), 29.27 (CH2), 28.97 (CH2),
28.68 (CH2), 28.09 (CH2), 27.42 (CH2), 22.60 (CH2), and 14.04
(CH3). The NMR data agree with those reported in the literature.
MS m/ z (M+) calcd 431.68, found 431.36.
N-Hexadecyl Aza-1 8 -crown-6 (6 ). From 1-aza-18-crown-6
(0.392 g, 1.49 mmol) and 1-bromohexadecane (0.45 g, 1.49 mmol),
0.66 g (1.35 mmol) of N-hexadecyl aza-18-crown-6 was obtained
(yield 91%): 1H NMR δ, 3.69-3.60 (m, 20H), 2.74 (t, 4H, J ) 5.9
Hz), 2.50 (m, 2H), 1.43-1.19 (m, 28H), and 0.88 (t, 3H, J ) 7.0
Hz); 13C NMR δ, 70.77 (CH2), 70.69 (CH2), 70.32 (CH2), 69.85
(CH2), 56.05 (CH2), 53.96 (CH2), 49.14 (CH2), 33.86 (CH2), 32.75
(CH2), 31.83 (CH2), 29.59 (CH2), 29.27 (CH2), 28.68 (CH2), 28.09
(CH2), 27.41 (CH2), 27.07 (CH2), 22.59 (CH2), and 14.03 (CH3).
The NMR data agree with those reported in the literature.19 MS
m/ z (M+) calcd 487.78, found 487.67.
N-(1 H,1 H,2 H,2 H-P erfluorooctyl) Aza-1 8 -crown-6 (7 ).
From 1-aza-18-crown-6 (0.30 g, 1.15 mmol) and 1H,1H,2H,2H-
perfluorooctyl iodide (0.54 g, 1.15 mmol), 0.23 g (0.38 mmol) of
N-(1H,1H,2H,2H-perfluorooctyl) aza-18-crown-6 was obtained (yield
33%): 1H NMR δ, 3.71-3.60 (m, 24H) and 2.90-2.70 (m, 4H);
13C NMR δ, 70.55 (CH2), 70.45 (CH2), 70.26 (CH2), 69.64 (CH2),
58.49 (CF3), 53.79 (CH2), 49.14 (CH2), 47.97 (CF2), 46.65 (CF2),
and 28.71 (t, CH2 next to CF2); 19F NMR δ, -79.85 (t, 3F), -112.97
(m, 2F), -120.98 (br peak, 2F), -121.93 (br peak, 2F), -122.55
(br peak, 2F), and -125.19 (m, 2F). The NMR data agree with
those reported in the literature.20
N-Butyl Aza-1 8 -crown-6 (2 ). From 1-aza-18-crown-6 (0.59
g, 2.25 mmol) and 1-bromobutane (0.37 g, 2.70 mmol), 0.66 g (2.05
mmol) of N-butyl aza-18-crown-6 was obtained (yield 91%): 1H
NMR δ, 3.85-3.60 (m, 20H), 2.76 (t, 4H, J ) 5.9 Hz), 2.49 (m,
2H), 1.45 (m, 2H), 1.42(m, 2H), and 0.92 (t, 3H, J ) 7.3 Hz); 13
C
NMR δ, 70.83 (CH2), 70.74 (CH2), 70.38 (CH2), 69.90 (CH2), 55.79
(CH2), 54.01 (CH2), 29.28 (CH2), 20.61 (CH2), and 14.05 (CH3).
The NMR data agree with those reported in the literature.17 MS
m/ z (M+) calcd 319.46, found 319.21.
N-(9 -Anthracenylmethyl) Aza-1 8 -crown-6 (8 ). From 1-aza-
18-crown-6 (0.152 g, 0.58 mmol) and 9-(chloromethyl)anthracene
(0.13 g, 0.58 mmol), 0.22 g (0.49 mmol) of N-(9-methyl)anthracenyl
aza-18-crown-6 was obtained (yield 85%): 1H NMR δ, 8.55 (d, 2H,
J ) 8.9 Hz), 8.40 (s, 1H), 7.96 (d, 2H, J ) 8.8 Hz), 7.49 (m, 4H),
N-Hexyl Aza-1 8 -crown-6 (3 ). From 1-aza-18-crown-6 (0.63
g, 2.37 mmol) and 1-bromohexane (0.45 g, 2.72 mmol), 0.54 g
(1.69 mmol) of N-hexyl aza-18-crown-6 was obtained (yield 71%):
1H NMR δ, 3.85-3.52 (m, 20H), 2.76 (t, 4H, J ) 5.9 Hz), 2.48 (m,
2H), 1.42 (br peak, 2H), 1.30 (br peak, 6H), and 0.88 (t, 3H, J )
7.1 Hz); 13C NMR δ, 70.81 (CH2), 70.72 (CH2), 70.35 (CH2), 69.87
(CH2), 55.10 (CH2), 53.97 (CH2), 31.77 (CH2), 27.14 (CH2), 27.03
(CH2), 22.62 (CH2), and 14.04 (CH3). The NMR data agree with
those reported in the literature.18 MS m/ z (M+) calcd 347.52, found
347.08.
4.58 (s, 2H), 3.68-3.57 (m, 20H), and 2.89 (t, 4H, J ) 5.7 Hz); 13
C
NMR δ, 131.25 (C), 130.39 (C), 128.74 (CH), 127.26 (CH), 127.06
(C), 125.37 (CH), 125.17 (CH), 124.65 (CH), 70.31 (CH2), 70.65
(CH2), 70.54 (CH2), 70.06 (CH2), 70.02 (CH2), 53.75 (CH2), and
51.91 (CH2). The NMR data agree with those reported in the
literature.21 MS m/ z (M+) calcd 453.60, found 453.30.
N-Octyl Aza-1 8 -crown-6 (4 ). From 1-aza-18-crown-6 (0.68 g,
2.59 mmol) and 1- bromoctane (0.67 g, 3.48 mmol), 0.79 g (2.11
mmol) of N-octyl aza-18-crown-6 was obtained (yield 82%): 1H
NMR δ, 3.86-3.67 (m, 16H), 3.62 (t, 4H, J ) 5.5 Hz), 2.73 (t, 4H,
J ) 5.5 Hz), 2.52 (m, 2H), 1.44 (br peak, 2H), 1.27 (br peak, 10H),
RESULTS AND DISCUSSION
Synthesis. Syntheses of N-alkyl aza crown ethers have been
previously investigated.22-24 The typical synthesis in the literature
(19) Anelli, P. L.; Quici, S. J. Chem. Soc., Perkin Trans. II 1 9 8 8 , 1469.
(20) Nakatsuji, Y.; Shimizu, H.; Morita, H.; Masuyama, A.; Okahara, M. Chem.
Lett. 1 9 9 2 , 2185.
(21) Benco, J. S.; Nienaber, H. A.; Dennen, K.; McGimpsey, W. G. J Photochem.
Photobiol. 2 0 0 2 , 152, 33.
(15) Hu, J.; Barbour, L. J.; Gokel, G. W. Chem. Commun. 2 0 0 2 , 1808.
(16) Johnson, M. R.; Sutherland, I. O. J. Chem. Soc., Perkin Trans. I 1 9 7 9 , 357.
(17) Kuo, P. L.; Miki, M.; Ikeda, I.; Okahara, M. Tetrahedron Lett. 1 9 7 8 , 4273.
(18) Kuo, P. L.; Miki, M.; Ikeda, I.; Okahara, M. J. Am. Oil Chem. Soc. 1 9 8 0 ,
227.
(22) He, G. X.; Miato, T.; Ishibashi, N.; Shinkai, S.; Matsuda, T. Bull. Chem. Soc.
Jpn. 1 9 9 0 , 63, 401.
Analytical Chemistry, Vol. 76, No. 10, May 15, 2004 2775

Scheme 1
Table 2. Extraction Results of [C4mim][NTf2]Containing
Recyclable N-Alkyl Aza-18-crown-6 (0.1 M)a
DM in different aqueous
phases (1.5 mM)
R substituents in
N-alkyl aza-18-C6
CsNO3
CsCl
Sr(NO3)2
SrCl2
Figure 2. Effect of N-alkyl group variation of monoaza crown ethers
on extraction efficiency of [C4mim][NTf2] containing N-alkyl aza-18-
crown-6 (0.1M).
ethyl, C2
butyl, C4
hexyl, C6
octyl, C8
dodecyl, C12
hexadecyl, C16
9-anthracenylmethyl
fluorinated octyl
2.68
14.1
22.0
24.0
30.8
17.4
14.9
nm
2.48
13.7
22.0
25.7
30.5
20.2
nmb
17.5
56.0
333
470
919
793
453
3.77
129
55.8
381
518
1070
982
742
nm
results for the anthracenyl-substituted crown (8 in Scheme 1) and
the fluorinated substituted crown (7 in Scheme 1) gave relative
lower distribution coefficients despite the high hydrophobicity of
both substituents. This observation indicates that both aromatic
and fluorinated substituents may significantly influence the binding
capability of the monoaza-substituted crown ethers. Another point
that can be seen from Table 2 is that the counteranions of metal
cations have little influence on the values of both DSr and DCs.
This weak dependence on the counteranions is reminiscent of
the observation with DCH18C6-based IL systems9 and can be
rationalized with the ion-exchange model.10 As seen from Table
2, DCs values are in the range of 2-30, whereas DCH18C6 gave
a DCs value around 400 at the same conditions. Therefore, these
monoaza-substituted crown ethers have lower extraction efficiency
for Cs+ than DCH18C6.
215
a
Single-species extraction. b nm, not measured.
started with the alkylamine followed by several steps to reach the
desired monoaza crown ethers. The reaction used for synthesizing
recyclable monoaza crown ethers in this study is illustrated in
Scheme 1. Eight monoaza crown ethers were successfully
synthesized in good yield except for the fluorinated alkyl crown
ether 7 . The basic reaction involves the alkylation of the monoaza
group of 1-aza-18-crown-6. The hydrophobicity of the longer chain
alkyl group substituents should significantly reduce the solubilities
of the aza-substituted crown ethers in water.
Recycling Experimental Results. The cation-binding capa-
bilities of monoaza-substituted crown ethers are extremely pH
sensitive.14 At low pH, the aza crown will be protonated, and the
binding constant for metal cations will be severely reduced,
relative to that of the neutral aza crown, due to charge repulsion.
The binding affinity can accordingly be switched by varying the
pH, with metal ion binding taking place at neutral or high pH,
and metal ion release taking place at low pH. This switchable
binding property forms the main rationale for us to develop IL-
based separation processes using monoaza-substituted crown
ethers. The basic recycling strategy for the monoaza-substituted
crown ethers is summarized in Scheme 2. The recycling experi-
ments with N-octyl aza-18-crown-6 (4 ) or N-(9-anthracenylmethyl)
aza-18-crown-6 (8 ) in [C4mim][NTf2] for extraction of metal ions
from the aqueous solution containing CsNO3 or Sr(NO3)2 were
conducted. The experimental results are listed in Table 3. It is
clear from the table that more than 98% Cs or Sr cations were
recovered in each case. The IL phases containing N-alkyl aza
crown ethers could be reused for solvent extractions after
deprotonation with a base. No significant changes of extraction
efficiencies were observed, indicating that a stripping process
based on monoaza crown ethers and ionic liquids is feasible. In
addition, the NMR spectral results (see Supporting Information)
indicated that the IL’s NMR peaks did not change even after three
cycles, though the aza crown ether’s NMR peaks shifted to higher
Extraction Results of [C4min][NTf2] Containing Recyclable
Extractants. The extraction results with eight monoaza-substi-
tuted crown ethers for Sr2+ and Cs+ are listed in Table 2. Figure
2 shows the variation of DSr as a function of the alkyl chain length
on the aza group of crown ethers. As seen from Table 2 and Figure
2, the value of DSr increases as the alkyl chain length on the aza
group of crown ethers increases, peaking with octyl substitution.
This observation can be rationalized according to the hydropho-
bicity of the different alkyl-substituted crown ethers. The longer
the alkyl chain, the less partitioning of the crown ethers to the
aqueous phases. Therefore, the enhanced DSr values could be
expected for the crown ethers with longer alkyl chains. DSr values
as high as 1000 can be achieved with a concentration of 0.1 M
N-octyl aza 18-crown-6 in [C4mim][NTf2], which is comparable to
the extraction efficiency of DCH18C6 at the same condition.10,11
However, the DSr values decrease slightly as the alkyl chain length
on the aza group of crown ethers increases from C8 to C12 and
C16. This may be due to solubility problems associated with the
latter compounds. We noticed that some cloudiness occurred
when both N-dodecyl aza-18-crown-6 and N-hexadecyl aza-18-
crown-6 were dissolved in [C4mim][NTf2]. Also, the extraction
(23) Nakatsuji, Y.; Sunagawa, T.; Masuyama, A.; Kida, T.; Ikeda, I. Chem. Lett.
1 9 9 5 , 445.
(24) Kimura, K.; Sakamoto, H.; Koseke, Y.; Shono, T. Chem. Lett. 1 9 8 5 , 1241.
2776 Analytical Chemistry, Vol. 76, No. 10, May 15, 2004

Scheme 2
Table 5. Extraction Results of [Cnmim][NTf2]
Containing No Crown Ethersa
DM as a function of Cn
in [Cnmim][NTf2]
distribution
coefficient
ethyl,
C2
butyl,
C4
hexyl,
C6
octyl,
C8
DNa
DK
DCs
DSr
0.66
1.30
2.08
ne
0.14
0.48
0.065
0.45
neb
0.58
0.098
0.24
ne
ne
ne
ne
a
Competitive extractions for four metal cations. b ne, no extraction.
Table 3. Recycling Experimental Results of
[C4mim][NTf2]Containing Recyclable N-Alkyl
Aza-18-crown-6 (0.1 M)
DM in different aqueous phases (1.5 mM)
+ 6 N HNO3 + 6 N HNO3
R substituents in
N-alkyl aza-18-C6 CsNO3 (recovery)a Sr(NO3)2 (recovery)
octyl, C8
24.0 0.096 (99.1%)
919
3.77
0.078 (99.2%)
0.127 (98.7%)
9-anthracenylmethyl 14.9 0.115 (98.9%)
a
Calculated by following equation: Recovery ) Cs/ Ci, where Cs and
Ci represent concentration of Mn+ stripped back from IL phase and
initial concentration of Mn+ in aqueous phase.
Table 4. Effects of Different ILs on Extraction
Efficiency of N-Alkyl Aza-18-crown-6 and Comparison
with DCH18C6a
Figure 3. Effect of 1-alkyl group variation of ILs on efficiency of
competitive metal cation extraction from aqueous solutions by
DCH18C6 (0.1 M) in [Cnmim][NTf2].
DM as a function of Cn
in [Cnmim][NTf2]
aqueous
concn of crown
ethers in ILs
phase
ethyl,
C2
butyl, hexyl, octyl,
(1.5 mM)
C4
C6
C8
with the concentration of DCH18C6 in the ILs, reflecting the
important role played by the extractant. The DSr values obtained
using N-alkyl aza-18-crown-6 in different ILs exhibit the same
trend. In fact, the extraction efficiencies for Sr2+ for the IL systems
containing DCH18C6 and N-alkyl aza-18-crown-6 are very similar.
For instance, as can be seen from Table 4, N-octyl aza-18-crown-6
gave DSr values of 8430 in [C2mim-][NTf2] and 1070 in [C4mim]-
[NTf2], which are comparable to 10 700 in [C2mim][NTf2] and
935 in [C4mim][NTf2] for the corresponding DCH18C6-based
system. Accordingly, the extractive strength of N-octyl aza-18-
crown-6 for Sr2+ in ILs is very similar to that of DCH18C6.
Competitive Solvent Extraction of Four Metal Chlorides
by N-Alkyl Aza-1 8 -crown-6 and Their Comparison with
DCH1 8 C6 . Bartsch and co-workers reported very interesting
results9 in which the effect of varying the alkyl group in [Cnmim]-
[PF6] on the efficiency and selectivity of competitive extraction
of five alkali metal cations by DCH18C6 was reported. The reason
we chose [Cnmim][NTf2] instead of [Cnmim][PF6] in our study
is that the water solubilities of [Cnmim][NTf2] are significantly
lower than those of [Cnmim][PF6] and the former IL system is
more stable than the latter IL system.1 In a similar vein, we wanted
to understand the extraction efficiency of four metal chlorides by
ionic liquids alone, and thus, solvent extraction experiments using
aqueous solutions containing 2.5 mM of Na+ and K+ and 0.5 mM
of Cs+ and Sr2+ were performed with no crown ethers in ILs. As
can be seen from the results listed in Table 5, the extraction
efficiency with ILs alone is very limited and most distribution
0.02 M DCH18C6
0.10 M DCH18C6
CsCl
SrCl2
CsCl
SrCl2
CsCl
SrCl2
CsCl
SrCl2
CsCl
SrCl2
CsCl
SrCl2
CsCl
SrCl2
33.1
465
589
16.8
74.1
380
935
30.5
982
25.7
1070
22.0
518
13.7
381
12.1
15.1
66.0
82.2
19.4
209
14.3
169
14.3
135
2.69
2.06
8.35
3.95
6.66
17.5
3.22
5.29
4.31
4.47
2.74
2.47
1.74
0.38
10700b
45.5
3840b
25.2
8430b
24.4
2900
17.4
1000
6.02
799
0.10 M N-dodecyl
aza-18-crown-6
0.10 M N-octyl
aza-18-crown-6
0.10 M N-hexyl
aza-18-crown-6
0.10 M N-butyl
aza-18-crown-6
0.10 M N-ethyl
aza-18-crown-6
6.59
26.0
3.10
5.62
2.48
55.8
aSingle species extraction. b Determined by ICP-AE.
field because of protonation, implying that ILs are stable under
these experimental conditions.
Effects of 1 -Alkyl Group of ILs on Extraction Efficiency
of N-Alkyl Aza-1 8 -crown-6 and Their Comparison with
DCH1 8 C6 . Table 4 shows the comparison of DSr and DCs for
DCH18C6 and five N-alkyl aza-18-crown-6 in four different ILs.
Experiments were performed with single-species extractions,
using either SrCl2 or CsCl solution. The distribution coefficients
for both Sr and Cs increase with the decrease of the alkyl chain
length in ILs. Such dependence can be attributed to the ion-
exchange capability of the corresponding cations of a specific IL.
It is also clear from Table 4 that both DSr and DCs greatly increase
Analytical Chemistry, Vol. 76, No. 10, May 15, 2004 2777

Figure 6. Effect of 1-alkyl group variation of ILs on Sr2+/Na+, Sr2+
/
Figure 4. Effect of 1-alkyl group variation of ILs on Sr2+/Na+, Sr2+
/
Cs+, and Sr2+/K+ selectivities in competitive metal cation extraction
from aqueous solutions by N-octyl aza-18-crown-6 (0.1M) in [Cnmim]-
[NTf2].
Cs+, and Sr2+/K+ selectivities in competitive metal cation extraction
from aqueous solutions by DCH18C6 (0.1 M) in [Cnmim][NTf2].
Table 6. Selectivity Results of [Cnmim][NTf2]
Containing N-Dodecyl Aza-18-crown-6 (0.1 M)a
DM as a function of Cn
in [Cnmim][NTf2]
distribution coefficient
or selectivity ratio
ethyl,
C2
butyl,
C4
hexyl,
C6
octyl,
C8
DNa
DK
DCs
DSr
DSr/ DNa
DSr/ DK
DSr/ DCs
11.01
849
17.6
7446b
673
7.74
282
14.3
2600b
336
3.84
126
8.01
129
33.5
1.02
16.1
1.66
57.1
4.09
12.3
7.40
0.215
3.00
8.76
424
9.21
182
a
Competitive extractions for four metal cations. b Determined by
ICP-AES.
Figure 5. Effect of 1-alkyl group variation of ILs on efficiency of
competitive metal cation extraction from aqueous solutions by N-octyl
aza-18-crown-6 (0.1M) in [Cnmim][NTf2].
Table 7. Selectivity Results of [Cnmim][NTf2]
Containing N-Hexyl Aza-18-crown-6 (0.1 M)a
DM as a function of Cn
in [Cnmim][NTf2]
coefficients are less than 1. Such partitions are mainly induced
by the ion-exchange process. This observation indicates that the
enhanced extraction of the target metal ions is mainly through
the complexation reactions of the macrocyclic ligands.
distribution coefficient
or selectivity ratio
ethyl,
C2
butyl,
C4
hexyl,
C6
octyl,
C8
DNa
DK
DCs
DSr
DSr/ DNa
DSr/ DK
DSr/ DCs
8.74
273
17.4
1985
227
9.64
396
3.95
187
1.38
45.2
3.64
4.13
2.99
0.091
1.13
13.5
852
88.4
2.15
63.2
8.46
90.9
23.0
0.487
10.8
The extraction results for the competitive solvent extraction
of the aqueous solution with DCH18C6 in four ILs are illustrated
in Figure 3. (The data are corrected for the levels of metal cations
that are extracted by the ILs alone). As can be seen from Figure
3, there is a smooth decrease in the extraction efficiency for each
metal cation as the carbon chain length in ILs increases. Figure
4 shows the influence of varying the carbon chain length of ILs
on the selectivities of Sr2+/ Na+, Sr2+/ Cs+, and Sr2+/ K+ for
competitive extraction by DCH18C6 based in Figure 3. With the
exception of Sr2+/ Na+ in [C2mim][NTf2], the Sr2+/ Na+, Sr2+/ Cs+,
and Sr2+/ K+ selectivities decrease as the 1-alkyl group of ILs is
7.27
114
a
Competitive extractions for four metal cations.
The results for competitive extraction of the same aqueous
solutions with N-octyl aza-18-crown-6 in the same four ILs are
shown in Figure 5. With the exception of DK value for [C4mim]-
[NTf2], extraction efficiency for each metal cation decreases as
the carbon chain length in ILs increases. The key change from
the selectivity order of the DCH18C6 system is that the order of
extraction efficiency is Sr2+ . K+ > Cs+ > Na+ for the N-octyl
aza-18-crown-6 extraction systems based on [C2mim][NTf2] and
[C4mim][NTf2]. This observation clearly demonstrates the advan-
lengthened. The order of extraction efficiency is K+ . Sr2+
>
Cs+ > Na+ in all four ionic liquids. This observation is consistent
with the reported results9 for [Cnmim][PF6]. The high extraction
efficiency for K+ significantly hampers the applications of these
systems for extraction of Sr2+ in the presence of K+.
2778 Analytical Chemistry, Vol. 76, No. 10, May 15, 2004

Sr2+/ Na+, Sr2+/ Cs+, and Sr2+/ K+ for competitive metal cation
extraction by N-octyl aza-18-crown-6. The Sr2+/ Na+, Sr2+/ Cs+, and
Sr2+/ K+ selectivities increase as the carbon chain length in ILs
decreases. For [C2mim][NTf2], a Sr2+/ Na+ selectivity of more than
300 can be achieved. Similar trends were observed for other four
N-alkyl aza-18-crown-6, where the alkyl groups are n-dodecyl,
n-hexyl, n-butyl, and ethyl. The results are summarized in Tables
6-9. For the most part, DSr/ DK is relatively constant in a given
IL as the alkyl chain on the aza crown is varied.
Table 8. Selectivity Results of [Cnmim][NTf2]
Containing N-Butyl Aza-18-crown-6 (0.1 M)a
DM as a function of Cn
in [Cnmim][NTf2]
distribution coefficient
or selectivity ratio
ethyl,
C2
butyl,
C4
hexyl,
C6
octyl,
C8
DNa
DK
DCs
DSr
DSr/ DNa
DSr/ DK
DSr/ DCs
7.04
184
13.2
1510
214
2.34
87.8
7.09
157
67.1
1.79
22.1
1.42
72.0
4.84
23.8
16.8
0.33
4.91
neb
23.4
1.50
3.19
n/ ac
0.136
2.13
CONCLUSION
8.17
115
The synthesis and characterization of eight N-alkyl aza-18-
crown-6 ethers were reported. The extraction efficiency and
selectivity of these monoaza-substituted crown ethers in ionic
liquids have been studied with comparison to DCH18C6. These
monoaza-substituted crown ethers have lower extraction efficiency
for Cs than DCH18C6. However, N-octyl aza-18-crown-6 has a
comparable extraction efficiency for Sr by DCH18C6. The extrac-
tion selectivity order for DCH18C6 in all four ILs was K+ . Sr2+
> Cs+ > Na+. The extraction selectivity order for N-octyl aza-18-
crown-6 was Sr2+ . K+ > Cs+ > Na+ in [C2mim][NTf2] and [C4-
mim][NTf2]. Our recycling experiments clearly indicated that a
stripping process based on monoaza crown ethers and ionic liquids
is feasible. These observations demonstrated that the extraction
systems based on N-alkyl aza-18-crown-6 in ILs offer not only
a
Competitive extractions for four metal cations. b ne, no extraction.
c n/ a, not applicable.
Table 9. Selectivity Results of [Cnmim][NTf2]
Containing N-Ethyl Aza-18-crown-6 (0.1 M)a
DM as a function of Cn
in [Cnmim][NTf2]
distribution coefficient
or selectivity ratio
ethyl,
C2
butyl,
C4
hexyl,
C6
octyl,
C8
DNa
DK
DCs
DSr
DSr/ DNa
DSr/ DK
DSr/ DCs
1.15
42.5
2.10
271
235
6.38
129
0.271
24.2
0.124
46.5
172
neb
0.932
7.20
ne
1.20
1.28
0.166
n/ a
11.0
0.28
3.99
n/ ac
0.363
14.3
easier recycling strategy but also better selectivity toward Sr2+
.
1.92
376
ACKNOWLEDGMENT
a
Competitive extractions for four metal cations. b ne, no extraction.
This research was supported by the Environmental Manage-
ment Science Program of the Office of Science and Environmental
Management, U.S. Department of Energy, under Contract DE-
AC05-0096OR22725 with Oak Ridge National Laboratory, managed
by UTsBattelle, LLC. The authors thank Dr. Gregory B. Hurst
of Chemical Sciences Division at the Oak Ridge National Labora-
tory for performing MALDI-MS measurements.
c n/ a, not applicable.
tage of using N-alkyl aza-18-crown-6 for extraction of Sr2+ based
on ILs on the presence of other competitive cations. Interestingly,
the order of extraction efficiency is the same as that (K+ > Sr2+
> Cs+ > Na+) of DCH18C6 for the N-octyl aza-18-crown-6
extraction systems based on [C6mim][NTf2] and [C8mim][NTf2].
The reversal of the extraction efficiency for Sr2+ and K+ induced
by the variation of the IL cations further demonstrates the strong
role played by ILs in determining not only distribution coefficients
but also orders of the selectivities. Such an effect is unusual in
solvent extractions based on conventional solvents. Figure 6 shows
the influence of varying the carbon chain length of ILs on the
SUPPORTING INFORMATION AVAILABLE
Additional information as noted in text. This material is
available free of charge via the Internet at http:/ / pubs.acs.org.
Received for review December 12, 2003. Accepted March
3, 2004.
AC035473D
Analytical Chemistry, Vol. 76, No. 10, May 15, 2004 2779