Clean Solvents - American Chemical Society

Chapter 3

Synthesis of Ionic Liquid and Silica Composites Doped with Dicyclohexyl-18-Crown-6 for Sequestration of Metal Ions

Downloaded by TUFTS UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: May 24, 2002 | doi: 10.1021/bk-2002-0819.ch003

Rajendra D. Makote, Huimin Luo, and Sheng Dai

*

Chemical Technology Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6181

Abstract A monolithic composite glass material that contains an ionic liquid, dicyclohexyl-18-crown-6, and sol-gel silica glass was synthesized and characterized . The selective extraction of Sr by this solid-state sorbent from aqueous solutions was demon -strated. 2+

Introduction Macrocyclic crown ethers that form strong complexes with alkali and alkaline earth metal ions have been the subject of numerous investigations. Various applications have been developed to make use of their selective complexati on capabilities. These applications include ion-selective electrodes, 1,

26

2,

3

© 2002 American Chemical Society

In Clean Solvents; Abraham, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

27 spectrophotometrical determinations, and ion-selective liquid membranes. In recent years, there has been extensive interestin the development of selective solidstate sorbents for toxic metal ions based an these macrocyclic crown ethers. ' Notably, sol-gel-derived membranes incorporating crown ether neutral carriers have been synthesized by Kimura, et a I. by means of sol-gel processing using tetraethoxysilane,diethoxydimethylsilane, and their corresponding alkoxysilylated 4

5,6

4

5

neutral carriers. Zhang and Clearfield have reported novel syntheses of crown ether-pillared a-zirconium phosphonate phosphates. Izatt, et al. have succeeded in covalently attaching a variety of crown ethers into a polymer matrix for chromato graphic separation.


6

Here, we report a methodology to immobilize crown ethers in a liquid nanophase that is entrapped in sol-gel silica for sequestration of metal ions. The nanophase is defined as any liquid domain with size in range of 1 nm to 50 nm. Here, the liquid nanophase is composed of a room-temperature ionic liquid. Ionic liquids are attracting increased attention worldwide because they promise significant environmental benefits. Unlike the conventional solvents currently in use, ionic liquids are nonvolatile and therefore do not emit noxious vapors, which can contribute to air pollution and health problems for process workers. Accordingly, no losses are induced by vaporization when these ionic liquids are entrapped in solid matrices, even at high temperatures. Unique intrinsic properties of these liquids are that they consist only of ions and can be made hydrophobic^. We have recently demonstrated that the novel dual properties of these new ionic liquidsmake them efficient solvents for the extraction of ionic species from aqueous solutions. Whereas conventional solvent extraction of Sr * and C s using crown ethers and related extractants can deliver practical R v a l u e of up to two orders of magnitude (10 ), tests with ionic liquids as extraction solvents delivered values of K on the order of 10 . These results clearly show the enormous potential that ionic liquids possess for increasing the extractive strength of ionophores such as crown ethers in fission-product separation applications. The same enhancement may hold also for ionic liquids entrapped in silica networks. 7

8

2

+

2

4

d

9.

Ionic liquid and organic polymer composites have been recently synthesized. Specifically, Carlin and Fuller reported a novel application of a gas-perm eable ionicliquid polymer gel composed of l-N-butyl-3-methylimidazolium hexafluoro phospha te and poly(vinylidene fluoride)-hexafluoropropylene copolymer in catalytic heterogeneous hydro genation reactions. Applications of these organic compo site polymers in a variety of electrochem ical devices have been d emonstrated by Fuller et al. and Kosmulski et a l . Our interest lies in the synthesis of ionicliquid and inorganic hybrid composite materials. These materials should be more 93

9b

10

stable under harsh environme nts because of the ceram ic networks. The temperature stability is limited only by that of the ionic liquid.


28

Experimental Dieyelohexyl-18-crown-6 (DC18C6), tetramethyl ortho-silicate (TMOS), and formic acid (99%) were used as received from Aldrich. The ionic liquid used in this experiment is 1 -ethyl-1 -methyl imidazolium bis(trifluoromethyl) sulfonamide (EtMeIm Tf N"), which was synthesized as reported previously and is known to be hydrophobic. Acid-catalyzed sol-gel processes were utilized to synthesize the composite materials doped with crown ether. ' In a typical run, 50 mg of crown ether was dissolved into 1 mL of EtMeIm Tf N" ionic liquid. To this mixture was added 1 mL of T M O S and 2 mL of formic acid. The solution gelled after 1 day and gelation continued for 1 week. The volatile hydrolysis products ( C H O H and H C O O C H 3 ) were vaporized. The final product is a monolith glass composite consisting of the ionic liquid-entrapped S i 0 network. The ionic liquid, which is hydrophobic in nature, is trapped inside the inorganic silica matrix. The crown ether (DC 18C6) is also retained in ionic liquid. The composite glass material was crushed for sorption experiments. Nitrogen adsorption isotherms were determined on a Micromeritics Gemini 2375 surface area analyzer. Mid IR spectra were measured using a Bio-Rad Excalibur FTS 4000 Spectrometer. +

2

11

12

13

+

2


3

2

Results and Discussion Figure 1 gives FTIR spectrum of the ionic liquid entrapped inside the silica matrix. The presence of C-H stretching and C-C framework vibrational features in Fig. 1 indicates the entrapment of the ionic liquid and crown ether. The ionic liquid can be removed from the silica network through solvent extraction using organic solvents. Figure 2 gives the pore-size distribution of the silica network after removal of the ionic liquid. As seen from Fig. 2, the pores formed by the removal of the ionic liquid from the silica network have an average domain size of around 150 A. Accordingly, the ionic liquid exists inside the sol-gel silica in the form of a nanophasic liquid. 11

13

13

2

1

1,2

Because DC18C6 is known to selectively complex Sr " *, adsorption studies of our composite glass materials were conducted with S resolutions. Strontium-90, which is generated in the nuclear fission process, is similar to calcium and presents a biological hazard because it accumulates in bone tissues. Therefore, various


29


80

Figure 1 FTIR spectrum of ionic liquid and crown ether entrapped in silica matrix. 2+

methods have been developed to extract Sr effectively from aqueous solutions. Table 1 presents Sr uptake as a function of the ionic-liquid content. As seen from the table, the sol-gel glass doped with the crown ether but without ionic liquid has a K value close to 0.5. The value of K sharply increases with the addition of the ionic liquid and levels off at concentrations of approximately 0.6 mL of the ionic liquid in 1 mL of TMOS. This clearly indicates that the ionic liquid plays a key role in the efficient adsorption of Sr * from the aqueous phase by the composite materials. The greatly enhanced extraction efficiency for the composite materials can be attributed to the unique solvation capability of the joom-temperature ionic liquids for the crown ether complex of Sr . This also indicates that most of the DC 18C6 molecules in the sol-gel glass without the ionic liquid are embedded in the silica matrix and are inaccessible to Sr"*". The incorporation of the nanophase ionic liquid in the silica matrix allows DC18C6 molecules to be solubilized in the ionic liquid instead of the silica network. The diffusion of DC18C6 in the liquid is much faster than that in the solid silica. Th erefore, D C18C 6 molecu les in the ionic liqu id are accessible for extraction. In order to determine whether or not the ionicity induced by doping the ionic liquid may contribute to the enhanced extraction, we 2+

d

d

2

2+

2



30

Figure 2 Pore-size distribution of the silica gel after removal of the ionic liquid. This distribution was calculated from N adsorption isotherm based on BJH (Barret-Joyner-Halenda) method. 2

prepared the sol-gel sorbent doped with NaCl instead of the ionic liquid. The rest of the components and sol-gel conditions were kept to be identical. Sorption tests indicated that the affinity (K « 1) of this sorbent toward •Sr is much less than that of the sorbents doped with the ionic liquid but close to that of the sorbent prepared without the ionic liquid. Accordingly, the ionicity of host matrixes plays no significant role in the enhanced extraction for our composites. 2+

d

To test the selectivity of our solid-state composite sorbents, we conducted competitive ion-binding experiments using a 1 mM S r solution containing excess amounts of competing ions. The competing ions used in this experiment wereCa , N a , and M g , which are common in waste sites. The sorbents used in this experiment were prepared using the following composition: 1 mL of ionic liquid, 1 mL of T M O S , 1 mL of formic acid, and 50 mg of DC18C6. The results given 2+

2+

+

2 +


31 +

in Table 2 indicate that neither Na or C a Mg

2 +

1 +

interferes with Sr

2+

extraction, while

causes some minor interference.

Table 1.

The partition coefficient (K ) as function of the ionic liquid content d

in sol-gel glasses. The compositions for other sol-gel precursors are 1 ml of TMOS, 1 ml of formic acid, and 50 mg of DC18C6.

In all experiments, each 3

sample was equilibrated with 10 mL of Sr(N0 )2 solution (1 * 10* M and pH 3

= 4) in stoppered plastic vials and these mixtures were stirred for an hour at room temperature. The uptake of Sr^by sorbents wa s measured with a Perkin


El mer Plasma 400 ICP/AE spectrophotometer.

Ionic liquid used in

Sr uptake

sol-gel synthesis (ml)

%

2+

0

10.43

0.5

0.2

47.64

3.6

0.4

92.35

48

0.6

99.10

430

0.8

99.19

480

1.0

99.27

530

*K = {(C - C) /C }* {Volume Solution (ml)}/ {Mass Gel (gram)}; where C and C represent the initial and final solution concentrations, respectively. d

t

Table 2.

f

t

The partition coefficient (KJ as function of excess competing metal ions.

Excess competing ions

2+

Sr

uptake %

K

d

2

5xlO' M +

97.68

170

2+

98.47

250

82.98

19

Na

Ca

2+

Mg

In conclusion, the hybrid composites consisting of a crown ether, an ionic liquid, and sol-gel silica have been successfufly prepared. The ionic liquid, which is hydrophobic and nonvolatile, is trapped in the silica network as a nanophase liquid and provides a diffusion medium for the crown ether molecules.

This

2

manifests itself in increased uptake of Si * with increased amounts of the ionic liquid. Ionic liquids have been previously used as effective solvents for extraction


f

32 14,

1 5 ,

1 6 ,

8

of organic and inorganic species. The solid-state composite sorbents presented here will make such applications more effective. Currently, we are exploring the use of these sol-gel composites as solid-state membranes for ionselective electrodes and removal of toxic metal ions.


Acknowledgment: The authors wish to thank Dr. Y . S. Shin andY. H . Ju for their initial assistance with the synthesis of molten salt precursors. This work was conducted at the Oak Ridge National Laboratory and supported by the Division of Chemical Sciences, Office ofBasic Energy Sciences, U.S. Department of Energy, under contract No. DE-AC05-00OR22725 with UT-Battelle, L L C .

References 1.

(a) Pedersen, C. J., J. Am. Chem. Soc. 1967, 89, 7017. (b) Pedersen, C. J.,

Angew. CHem. Int. Ed. Engl. 1988, 27, 1021. (c) Cram, D . J. Angew. Chem. Int. Ed. Engl. 1988, 27, 1009. (d) Lehn, J., Angew. Chem. Int. Ed. Engl. 1988,27, 89. 2.

(a) Hiraoka, M . , "Crown Compounds: Their Characteristics and Applica tions", Elsevier, Amsterdam, 1982. (b)Lindoy, L. F. "The Chemistry of Macrocyclic Ligand Complexes", Cambridge University Press, Cambridge, 1990. (c) Moyer, B. A. "Complexation and Transport," in Molecular

Recognition: Receptors for Cationic Guests, Gokel, G. W. Ed., Vol. 1, Comprehensive Supramolecular Chemistry, Atwood, J. L., Davies, J. E . D., MacNicol, D. D.; Vogtle, F.; and Lehn, J. - M . Eds., Pergamon, Elsevier, Oxford, 1996; Chap. 10, pp. 377-4 16. 3.

Izatt, R. M . , Powlak, K . , Bradshaw, J.S. , Bruening, R. L . , Chem. Rev. 91, 1721.

1991,

4.

Kimura, K . , Sunagawa, T., Yajima, S., Miyake, S., Yokoyama, M . Anal. Chem. 1998, 70, 4309.

5.

Zhang, B . and Clearfield, A. J. Am. Chem. Soc. 1997, 119, 2751.

6.

(a) Izatt, R. M B r a d s h a w , J. S., Bruening, R. L., Bruening, M . L., Tarbet, B. J., Christensen, J. J., Anal. Chem. 1988, 60, 1826.; (b) Izatt, R. M . , Bradshaw, J. S., Bruening, R. L . , Bruening, M . L . , Tarbet, B. J., Christensen, J. J., 7. Chem Soc, Chem Commun, 1988, 812; (c) Izatt, R. M . , Bradshaw, J. S., Bruening, R. L., Lamb, J. D., Christensen, J. J., J. Pure Appl Chem, 1988, 60, 453-460.


33 7.

(a) "Ionic Liquids Prove Increasingly Versatile", Chemical & Engineering News, January 4, 1999, p23-24. (b) "Designer Solvents-Ionic Liquids May Boost Clean Technology Development", Chemical & Engineering News, March 30, 1998,

p32-37. (b) Hussry, C. L., Adv. Molten Salt Chem., 1983,

5, 185; (c) Hussey, C. L . in Chemistry of Nonaqueous Solvents, Popov, A . and Mamantov, G. Editors, Chapter 4, V X H Publishers, New York (1994); (d) Carlin, R. T. and Wilkes, J. S. in Chemistry of Nonaqueous Solvents, Popov, A . and Mamantov, G . Editors, Chapter 5, V X H Publishers, New York (1994). (e) Seddon, K . R. J. Chem. TechnoL Biotechnol 1997, 351. (f) Chauvin, Y. and Olivier-Bourbigou, H. CHEMTECH


8.

68,

1995, 25, 26.

(a) Dai, S., Ju, Y . H., and Barnes, C . E., J. Chem. Soc. Dalton, 1999,

1201.

(b) Dai, S., Ju, Y . H . , and Luo, H. "Separation of Fission Products Based on Ionic Liquids" in International George Papatheodorou Symposium, Patras Science Park, 1999, p254-262. 9.

(a) Carlin, R. T. and Fuller, J. Chem. Comm. 1997, 1345; (b) Fuller, J., Breda, A. C., and Carlin, R. T. J. Electrochem. Soc. 1997, 144, L67.

10. Kosmulski, M . , Osteryoung, R. A . , and Ciszkowska, M. J. Electronchem. Soc. 2000, 147, 1454. 11. Bonhote, P., Dias, A . , Papageorigou, N . , Kalyansundaram, K., and Gratzel, M. Inorg Chem, 1996, 35, 1168. 12. Green, W. H., Le, K. P., Grey, J., and Sailor, M . J. Science, 1997, 1826. 13. Dai, S.,

276,

Ju, Y . H . , Gao, H . J., Lin, J. S., Pennycook, S. J , and Barnes, C.

E. Chem. Comm. 2000, 243. 14. Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.; Visser, A . E.; and Rogers, R. D. Chem. Comm. 1998,

1765.

15. Blanchard, L . A . , Hancu, D., Beckman, E . J., Brennecke, J. F. Nature 1999, 399, 28. 16. Dai, S.; Shin, Y . ; Toth, L. M . ; and Barnes, C. E. Inorg. Chem. 1997, 4900.


36,

Clean Solvents - American Chemical Society

Recommend Documents