Nuclear Waste Management - American Chemical Society

Chapter 8

Separation of Fission Products Based on Room-Temperature Ionic Liquids Downloaded by PENNSYLVANIA STATE UNIV on August 9, 2012 | http://pubs.acs.org Publication Date: September 21, 2006 | doi: 10.1021/bk-2006-0943.ch008

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Huimin Luo , Sheng Dai , Peter V. Bonnesen, and A. C. Buchanan, III 2

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Nuclear Science and Technology Division and Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831

Imidazolium-based ionic liquids containing cisdicyclohexano-18-crown-6 (DCH18C6) were tested for the extraction of Sr and Cs . For Sr extraction, the influence of the addition of a sacrificial cation exchanger was also investigated. A decrease in strontium distribution coefficients in ionic liquids was observed for the system using Na as a sacrificial ion. This observation is rationalized by the competition of Na for crown ethers. A slight increase in strontium distribution coefficients was observed for an ionic liquid system utilizing oleic acid as the cation exchanger. This moderate increase of the strontium distribution coefficient can be attributed to the proton-mediated ion exchanging process and the potential synergistic effect of the organic acid. A series of imidazolium-based ionic liquids containing 2-alkoxy2-oxoethyl substituents on one or both of the imidazole nitrogens was synthesized and characterized by NMR spectra. The solvent extraction results using calix[4]arene-bis(tertoctylbenzo-crown-6) [BOBCalixC6] in these ionic liquids are reported. 2+

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© 2006 American Chemical Society In Nuclear Waste Management; Wang, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

147

Introduction The applications of ionic liquids (ILs) as replacement solvents for various catalytic reactions and separation processes have been extensively explored (1-13). Figure 1 gives the structure templates of the two most common classes of ambient-temperature ionic liquids. The cation is usually a heterocyclic cation, such as a dialkyl imidazolium ion or an Af-alkylpyridinium ion. The relatively 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 BF ", PF ", CF S0 ", or other complex anions. These ion pairs or salts are usually liquids from around -100 °C and are thermally stable to around 200 °C, depending on the specific structures of the anions and cations. Unlike conventional solvents currently in use, these ionic liquids are nonflammable, chemically tunable, and have no detectable vapor pressure (/). We (13-15) and others (7-11) 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 ionic liquids (molten salts), roomtemperature ionic liquids can be made hydrophobic while retaining ionicity. This dual property forms the basis for using room-temperature ionic liquids as unique separation media for the solvent extraction of ionic species. Large distribution coefficients (D ) for the extraction of metal ions have been observed with ionic liquids containing complexing ligands (13-15). For example, whereas conventional solvent extraction of Sr * using dicyclohexano-18-crown6 can deliver practical D values of less than one, our experiments with ionic liquids as extraction solvents delivered values of D on the order of 10 (14). The enhanced distribution coefficients can be attributed to the unique solvation properties of ionic liquids for ionic species, with ion-exchange processes playing an important role as revealed in details by Dietz and Dzielawa (77), and Visser et al. (7-9). However, the large D values observed for fission products using IL-based extraction processes make it very difficult to strip the metal ions, and to recycle the crown ethers. Another drawback associated with IL-based extraction processes is the loss of ILs through ion-exchange reactions (7-11). These deficiencies associated with IL-based extraction processes for metal ions prompted us to explore IL systems with sacrificial cation exchangers that are also environmentally benign. Here, we report our results concerning the development of IL-based extraction systems containing Na and H* as sacrificial cations in the extraction of strontium and cesium ions. The extractant used in this investigation for Sr * is cis-dicyclohexano-18-crown-6 (DCH18C6), while the extractant used for Cs* is calix[4]arene-bis(^-octylbenzo-crown-6) [BOBCalixC6], Moyer and coworkers (17-18) have previously conducted extensive investigations into the use of BOBCalixC6 for extraction of Cs* in conventional solvents.

Downloaded by PENNSYLVANIA STATE UNIV on August 9, 2012 | http://pubs.acs.org Publication Date: September 21, 2006 | doi: 10.1021/bk-2006-0943.ch008

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In Nuclear Waste Management; Wang, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

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Ri

Imidazolium-Based

Pyridinium-Based

Figure 1. Structures of two most common ionic liquid cations.

Experimental Section Materials and Methods AH chemicals and solvents were reagent grade and used without further purification unless noted otherwise. DCH18C6 was purchased from Aldrich and is a mixture of cis-syn-cis and cis-anti-cis isomers. BOBCalixC6 was obtained from IBC Advanced Technologies (America Fork, UT) and used as received (97% stated purity). 1 -Alkyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]-amide [C mim][NTf ] ionic liquids, in which the alkyl groups were ethyl, butyl, hexyl, and octyl, were synthesized by modified literature procedures (19) and the details were reported elsewhere (20). Aqueous solutions were prepared using deionized water with a specific resistance of 18.0 megohm-cm or greater. H-, and C-NMR spectra were obtained in CDC1 with a Bruker MSL-400NMR spectrometer, operating at 400.13 MHz for proton, and 100.61 MHz for carbon. Proton and carbon chemical shifts are reported relative to tetramethylsilane (TMS). The UV spectra were measured using a Varian UV-VIS-NIR spectrometer (Model 5000). Concentrations of Cs and Sr * were determined using a Dionex LC20 ion chromatograph equipped with an IonPac CS-12 analytical column. n

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Extraction Experiments The extraction experiments were performed in duplicate for each room temperature ionic liquid (RTIL) by contacting 1 mL of RTIL containing various concentrations of the extractant with lOmLof cation-containing aqueous


149 solution (1.5 mM) for 60 min in a vibrating mixer. After centrifugation, the upper aqueous phase was separated and the concentration of cations was determined by ion chromatography. Metal ion distribution coefficients were calculated as: D = ([M ai]-[Mf i])/[M ] and uncertainty in D is ± 5%. M

iniri

ina

flnal

M


General Procedure for the Synthesis of l-(2-Alkoxy-2-oxoethyl)-3-methyl imidazolium bromide Equal molar amounts of 1-methylimidazole and alkyl bromoacetate were mixed in acetonitrile. The mixture was stirred at room temperature for 24 h. The product was obtained in nearly quantitative yield as a viscous liquid, which was of sufficient purity to be used in the preparation of the corresponding NTf salt. 2

Synthesis of l-(2-Ethoxy-2-oxoethyl)-3-methylimidazolium bromide (taskspecific ionic liquid [TSIL] 1) From 1-methylimidazole (4.11 g, 50 mmol) and ethyl bromoacetate (8.35 g, 50 mmol), 10.8 g of l-(2-ethoxy-2-oxoethyl)-3-methylimidazolium bromide was obtained (yield 87%). *H-NMR: δ, 10.08 (s, 1H), 7.83 (s, 1H), 7.71 (s, 1H), 5.51 (s, 2H), 4.25 (q, 2H), 4.12 (s, 3H), and 1.29 (t, 3H). C-NMR: δ, 165.75 (CO), 137.70 (CH), 123.67 (CH), 122.91 (CH), 62.63 (CH ), 50.03 (CH ), 38.40 (CH ), and 13.91 (CH ). 13

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Synthesis of l-(2-terf-Butoxy-2-oxoethyl)-3-methylimidazolium bromide (TSIL 2). From 1-methylimidazole (5.16 g, 62.9 mmol) and tert-butyl bromoacetate (12.26 g, 62.9 mmol), 14.8 g of l-(2-^r/-butoxy-2-oxoethyl)-3-methylimidazolium bromide was obtained (yield 85%). ^ - N M R : δ, 9.37 (s, 1H), 7.64 (s, 1H), 7.54 (s, 1H), 5.19 (s, 2H), 3.95 (s, 3H), and 1.52 (s, 9H). C-NMR: δ, 166.36 (CO), 138.45 (CH), 124.42 (CH), 123.96 (CH), 84.22 (C), 51.19 (CH ), 36.95 (CH ), and 27.99 (CH ). 13

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General Procedure for Synthesis of l,3-Bis(2-alkoxy-2-oxoethyl)imidazolium bromide The literature method (21) for preparing 1,3-dialkylimidazolium bromide was used. Imidazole (2.0 g, 29.4 mmol) in tetrahydrofuran (THF) (20 mL) was added dropwise to 95% sodium hydride powder (0.75 g, 29.4 mmol) in


150 THF (20 mL) at 0 °C. The ice bath was removed, and the mixture was stirred for 2 h at room temperature. Following dropwise addition of the alkyl bromoacetate (58.8 mmol) at room temperature, the mixture was heated to reflux for 18 h. The precipitate was filtered and thoroughly rinsed with THF, followed by dichloromethane. The filtrate was evaporated in vacuum, and the residue was rinsed with diethyl ether and dried under vacuum to give the desired product as a waxy solid.


Synthesis of l,3-Bis(2-ethoxy-2-oxothyl)-imidazoIium bromide (TSIL 3) From imidazole (2g, 29.4 mmol) and ethyl bromoacetate (9.82 g, 58.8 mmol), 7.8 g of l,3-di(ethyl acetate) imidazolium bromide was obtained (yield 82%). *H-NMR: δ, 9.98 (s, 1H), 7.72 (s, 2H), 5.41 (s, 4H), 4.25 (q, 4H), and 1.29 (t, 6H). C-NMR: δ, 165.84 (CO), 138.63 (CH), 123.30 (CH), 62.95 (CH ), 50.46 (CH ), and 14.07 (CH ). 13

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Synthesis of l,3-Bis(2-ter^butoxy-2-oxoethyl)-imidazolium bromide (TSIL 4) From imidazole (2 g, 29.4 mmol) and ter/-butyl bromoacetate (11.47 g, 58.8 mmol), 9.5 g of l,3-bis(2-/^-butoxy-2-oxoethyl)-imidazolium bromide was obtained (yield 85%). H-NMR: δ, 9.97 (s, 1H), 7.75 (s, 2H), 5.30 (s, 4H), and 1.50 (s, 18H). C-NMR: δ, 164.43 (CO), 138.23 (CH), 123.03 (CH), 84.26 (C), 50.53 (CH ), and 27.66 (CH ). l

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General Procedure for Synthesis of l-(2-Alkoxy-2-oxoethyi)-3-methyl imidazolium or l,3-Bis(2-alkoxy-2-oxoethyl)-imidazolium bis[(trifluoromethyl)sulfonyl]-imide The corresponding bromide was dissolved in 50 mL of deionized (D.I.) water and heated to 70 °C, and an equal molar amount of N-lithiotrifluoromethanesulfonimide (LiTf N) was dissolved in 100 mL of D.I. water and heated to 70 °C. The two solutions were combined and stirred to mix well. The resulting cloudy solution was cooled and then extracted with chloroform three times. The combined organic phases were washed four times successively with D.I. water to ensure that all the L i was removed. Evaporation of the solvent produced the desired compound as a light-yellow liquid, except one compound (TSIL 8) was obtained as a solid. 2

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151 Synthesis of l-(2-Ethoxy-2-oxoethyl)-3-methylimidazolium bis[(trifluoromethyi) sulfonylj-imide (TSIL 5) From the bromide, TSIL 1 (10.8 g, 43.3 mmol) and LiTf N (12.44 g, 43.3 mmol), 17.7 g of TSIL 5 was obtained (yield 91%). Ή-NMR: δ, 8.48 (s, 1H), 7.40 (s, 2H), 4.95 (s, 2H), 4.22 (q, 2H), 3.85 (s, 3H), and 1.27 (t, 3H). "C-NMR: δ, 166.97 (CO), 137.93 (CH), 124.37 (CH), 119.75 (CF , q, coupling constant of C-F is 321 Hz), 118.20 (CH), 63.36 (CH ), 50.74 (CH ), 37.08 (CH ), and 14.30 (CH ). 2

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Synthesis of l-(2-fcrr-Butoxy-2-oxoethyI)-3-methylimidazolium bis[(trifluoromethyl) sulfonyl]-imide (TSIL 6) From the bromide, TSIL 2 (6.28 g, 22.7 mmol) and LiTf N (6.5 g, 22.7 mmol), 6.3 g of TSIL 6 was obtained (yield 58%). Ή-NMR: δ, 8.75 (s, 1Η), 7.38 (s, 1H), 7.30 (s, 1H), 4.89 (s, 2H), 3.38 (s, 3H), and 1.48 (s, 9H). C-NMR: δ, 164.55 (CO), 137.34 (CH), 123.85 (CH), 123.12 (CH), 119.69 (CF , q, coupling constant of C-F is 321 Hz), 84.88 (C), 50.36 (CH ), 36.31 (CH ), and 27.67 (CH ). 2

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Synthesis of l,3-Bis(2-ethoxy-2-oxoethyl)-imidazolium bis[(trifluoromethyl) sulfonylj-imide (TSIL 7) From the bromide, TSIL 3 (3.6 g, 11.1 mmol) and LiTf N (3.21g, 11.1 mmol), 4.24 g of TSIL 7 was obtained (yield 73%). Ή-NMR: δ, 8.98 (s, 1Η), 7.40 (s, 2H), 5.04 (s, 4H), 4.30 (q, 4H), and 1.30 (t, 6H). C-NMR: δ, 165.35 (CO), 138.47 (CH), 123.31 (CH), 119.70 (CF , q, coupling constant of C-F is 321 Hz), 63.24 (CH ), 50.16 (CH ), and 13.87 (CH ). 2

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Synthesis of l,3-Bis(2-ter^butoxy-2-oxoethyl)-imidazolium bis[(trifluoromethyl) sulfonylj-amide (TSIL 8) From the bromide, TSIL 4 (3.8 g, 10.1 mmol) and LiTf N (2.91g, 10.1 mmol), 4.3 g of TSIL 8 was obtained as a yellow solid (yield 73%). Ή-NMR: δ, 8.89 (s, 1Η), 7.38 (s, 2H), 4.90 (s, 4H), and 1.48 (s, 18H). "C-NMR: δ, 164.29 (CO), 137.21 (CH), 123.31 (CH), 120.45 (CF , q, coupling constant of C-F is 321 Hz), 84.96 (C), 50.50 (CH ), and 27.68 (CH ). 2

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Results and Discussion Synthesis The TSILs containing the ester groups were synthesized to investigate the potential effect of ester groups on solvent extractions for Cs . The concept of TSILs was originally proposed by Davis and coworkers (7).The reaction used for synthesizing these TSILs is illustrated in Scheme 1. Eight ionic liquids (five are new) were synthesized in good yield. Compared to the reaction of 1-alkyl bromide with 1-methylimidazole, the reaction of alkyl bromoacetate with 1-methylimidazole is faster. This may be due to the electron withdrawing property of the ester group, making the nucleophilic reaction easier to proceed. The syntheses of TSILs 1,3, and 5 have been previously reported (22, 25). The procedure for TSILs 1 and 5 used in this paper is similar to the literature procedure except a different solvent (acetonitrile instead of THF) was used. There were no NMR data for TSILs 1 and 5 reported in reference 22; therefore, no comparison of the NMR data can be made. Our procedure for TSIL 3 involved only a one step synthesis, while the reported procedure needed two reaction steps. The NMR data for TSIL 3 are consistent with the literature data (23). The change of the proton chemical shifts for H2 on the imidazolium ring upon changing the anionfromthe bromide to NTf is listed in Table I. It is clear from Table I that the chemical shifts of H2 on the imidazolium ring move downfield when the bromides are converted into NTf salts. This observation is consistent with data for l-alkyl-3-methylimidazolium cations (19).


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Extraction Results

Effects of Different Anions According to the cation-exchange model proposed by Dietz and Dzielawa (7/), and Visser et al. (7-9), only cations are directly involved in the extraction processes so that the distribution coefficients are expected to only weakly depend on the counter anions of extracted cations. Table II shows the Ds values as a function of different concentrations of DCH18C6 and the counter anion. From these data, it is clear that the extraction efficiency for Sr was essentially unaffected by variation of the aqueous phase anion from nitrate to chloride at lower concentrations of DCH18C6. This finding is consistent with the results reported by Bartsch (10) and the ion-exchange model. At higher concentration of DCH18C6 the variation of the aqueous phase anion has some influence on the Sr * extraction efficiency. r

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r\

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LiNTfo

Scheme 1.

TSIL3:Risethyl TSIL 4: R is terr-butyl

Br~

+ ,.-Ν—CH C0 R

2 M e

2

+

NTff

2

TSIL 7: Ris ethyl TSIL8:Ristert-butyl

NTf>'

2

>N—CH CX)2R

r\

2

3n—CH C0 R

TSBL5:Risethyl TSIL 6: R is ter/-butyl

—Nc^

R 0 2 C H C —ISU

LiNTf

TSIL1: Ris ethyl TSIL2:Rister/-butyl

2

—CH2CX) R

Br

^

r\

Me—NU

M

RC^CHzC —ISU

RT, 18 h

Acetonitrile

(2) B r C H C 0 R (3) reflux 18h

(1) NaH/THF

2

+ BrCH C02R-


in

154 Table I. Comparison of Proton Chemical Shifts of H2 on Imidazolium Ring for l-(2-Alkoxy-2-oxoethyI)-3-methyl imidazolium and l 3-Bis(2-Alkoxy-2oxoethyl)-imidazoIium Cations 9


Proton Chemical Shifts (δ) of H2on Imidazolium Ring Imidazolium Cations

M e — +

M

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Tf N"

Δδ

10.08

8.48

1.60

9.37

8.75 0.62

9.98

8.95

1.03

9.97

8.89

1.08

10.32

8.74

1.58

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5

^ 2

H 5 C 2 O 2 C H 2 C —

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,>N— C H C 0 C ( C H 3 ) 3

M e — +

( C H

^ N — CH C0 C2H

Br

C 0

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C H

Me—N

Nuclear Waste Management - American Chemical Society

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