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Ind. Eng. Chem. Res. 2009, 48, 7308–7313
Silica Materials Doped with Bifunctional Ionic Liquid Extractant for Yttrium Extraction Yinghui Liu,†,‡ Lili Zhu,†,‡ Xiaoqi Sun,† Ji Chen,*,† and Fang Luo§ State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, People’s Republic of China, Graduate School of the Chinese Academy of Sciences, Beijing 100039, People’s Republic of China, and Key Laboratory of Polyoxometalates Science of Ministry of Education, College of Chemistry, Northeast Normal UniVersity, Changchun 130024, People’s Republic of China
Two new silica-based organic-inorganic hybrid materials (B104SGs and O104SGs) doped with a binary mixture of imidazolium and phosphonium ionic liquids have been synthesized and used as sorbents in batch system for rare earths (RE) separation. Imidazolium ionic liquids 1-butyl-3-methylimidazolium hexafluorophosphate (C4mim+PF6-) or 1-octyl-3-methylimidazolium hexafluorophosphate (C8mim+PF6-) acted as porogens to prepare porous materials and additives to stabilize extractant within silica gel. Trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate (Cyphos IL 104), a low-molecular-mass organic gelator, can encourage the formation of porous silica materials and increase the rate of aggregation. Cyphos IL 104 not only acted as an excellent template for the preparation of silica sorbents but also as a bifunctional ionic liquid extractant (bif-ILE) for RE(III) extraction. By the electrostatic attraction between the SiO- groups and phosphonium cations along the surfaces of the silica gel, stable silica sorbents with high extractant content were formed. 1. Introduction Separation of rare earths (RE) is complicated due to their similar chemical properties.1-3 A variety of solid sorbents such as gels,4 polymers,5 and ion exchange/chelating resins6-8 have been used for separation of RE(III). Sol-gel derived silica particle is well-suited as a support matrix in that it could offer some advantages such as porous structure, improved mechanical strength, chemical stability, convenient preparation, and negligible swelling or shrinking.9-11 The use of functionalized silica support extractants to separate metal ions and organic compounds from aqueous media is gaining popularity.12-14 The functionalized silica combines the efficiency of solvent extraction with the stability of resins. The main method of incorporating organic functional groups into sol-gel materials is grafting or doping.15 Functional groups are generally covalently bounded to the sol-gel matrix using grafting technology, but this process also usually involved complicated synthesis, and sometimes additional functional groups are required to achieve selectivity or fast metal intake kinetics.16,17 Doping of extractant in sol-gel has been also extensively studied, and the doped molecules are often found to retain chemical activities.16 It has been demonstrated that silica materials doped with extractants can be successfully used as separation materials for their thermal stability and strong affinities for selected metal ions.18 Khan et al. have doped TAN (1-(2-thiazolylazo)-2-naphthol)) in sol-gel silica, which was used for the removal of zinc from aqueous solution; however, slow diffusion rate of extractant in silica matrices resulted in slow kinetics and low extraction efficiency of sol-gel sorbents.19 An alternative way is the use of additives to stabilize extractant within sol-gel matrices. Ionic liquids (ILs) are organic salts, which have received much attention as environmentally benign solvents in many * To whom correspondence should be addressed. Tel.: +86-4318526-2646. E-mail:
[email protected]. † Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences. § Northeast Normal University.
fields of chemistry and industry.20-26 As a new type of solvent, template, and a diffusion medium for extractant, ILs could be doped into the silica gel matrix and lead to higher metal removal efficiencies. However, few studies have been reported on encapsulation of ILs within silica gel for the extraction of metal ions. We have used trialkylphosphine oxides (Cyanex 923) and 1-octyl-3-methylimidazolium hexafluorophosphate (C8mim+ PF6-) to prepare porous silica materials for Y(III) uptake.27 The results indicated that the removal efficiencies of silica gel doped with Cyanex 923 and C8mim+PF6- were higher than that with only Cyanex 923. Imidazolium-based ILs were frequently used as a diluent for the extractant, while a new group of ILs with tetraalkylphosphonium cation was considered to be prospective for metal separation.28-31 Various asymmetrical tetraalkylphosphonium ILs such as tetradecyl(trihexyl)phosphonium chloride (Cyphos IL 101) and tetradecyl(trihexyl)phosphonium bistriflamide (Cyphos IL 109) have been used for the extraction of metal ions.32-37 In this study, we use sol-gel materials doped with the mixture of trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate (Cyphos IL 104) and imidazolium ILs to separate RE(III). High ionic strength brought from ILs could increase the rate of aggregation. Batch extraction studies are carried out to examine the influence of various parameters such as initial pH, sorbent dose, and time on uptake. Other influences of ILs on the preparation of B104SGs and O104SGs and their metal uptakes are also investigated. Both materials exhibit high thermal stability and can be repeatedly reused in extraction applications. 2. Experimental Section 2.1. Instrumentation. The morphology of the samples was observed using field-emission scanning electron microscope (FESEM, XL30, Philips) and transmission electron microscopy (TEM) JEM-2010. Thermogravimetric analysis (TGA) data were
10.1021/ie900468c CCC: $40.75 2009 American Chemical Society Published on Web 07/09/2009
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Figure 1. Structures of ILs. Table 1. Sol-Gel Sorbents Formulationsa sorbents 104SG O104SG-1 O104SG-2 O104SG-3 B104SG-1 B104SG-2 B104SG-3 B104SG-4
C4mim+PF6- (mL)
C8mim+PF6- (mL) 2.0 3.0 4.0
2.0 3.0 4.0 2.0
Cyphos IL 104 (mL) 0.437 0.437 0.655 0.873 0.437 0.655 0.873 0.529
a Densities of reactants: C8mim+PF6-, 1.220 g mL-1; C4mim+PF6-, 1.360 g mL-1; Cyphos IL 104, 0.886 g mL-1; DI water, 1.000 g mL-1; TEOS, 0.934 g mL-1.
recorded with Thermal Analysis Instrument (SDT 2960, TA Instruments, New Castle, DE). Measurements were conducted by heating the sample from room temperature to 800 °C at a heating rate of 10 °C min-1 under N2 atmosphere. N2 adsorption isotherms were recorded through the use of an ASAP-2010 adsorption instrument (Micromeritics). Surface areas were obtained using the BET (Brunauer-Emmett-Teller) equation. Pore sizes and pore volumes were obtained by applying the BJH (Barret-Joyner-Halenda) method. RE(III) ions recovered from B104SG-3 were determined using inductively coupled plasma optical emission spectrometers (ICP-OES, Thermo iCAP 6000). 2.2. Materials and Reagents. The ILs used in this experiment were 1-butyl-3-methylimidazolium hexafluorophosphate (C4mim+PF6-) and C8mim+PF6-, which were synthesized as previously reported.38 Cyphos IL 104 (>95.0%) was kindly supplied by Cytec Canada Inc. (Figure 1). Stock solutions of RE were prepared by dissolving their oxides (>99.9%) in nitric acid. Deionized water was used in the preparation of solutions, in sol-gel reactions, and for the washing of sorbents. All other chemicals used were of analytical grade (Beijing Beihua Fine Chemicals Co., Ltd.). 2.3. Preparation of Sol-Gel Sorbents. Tetraethoxysilane (TEOS, 25.0 mL) and formic acid (2.5 mL, 0.05 mol L-1) were mixed with deionized water (15.0 mL) under mild magnetic stirring at room temperature for 6 h. Then Cyphos IL 104 with or without Cnmim+PF6- (n ) 4, 8) was added, so that the gelation occurred within 2-10 min. The resultant solid material was dried in vacuum at 40 °C for 24 h. After cooling to room temperature, it was washed with deionized water to neutral pH. The sample was then kept dry in vacuum at 40 °C overnight. Sol-gel sorbents in this study are referred to as 104SG, O104SG-1-3, and B104SG-1-4, respectively. For comparison, blank sorbent was prepared by the same procedure, except that no Cyphos IL 104 and imidazolium IL s were added. The 0.075-0.150 mm particle size fraction was used in our experiments. The formulations of sol-gel sorbents were given in Table 1.
2.4. Extraction Experiments. A 0.040 g amount of each sorbent was placed in an Erlenmeyer flask and treated with 4 mL of aqueous solution of RE(III). The flasks were incubated for 60 min in a rotary shaker at room temperature. Solution samples were then centrifuged for 5 min, and the concentration of RE(III) left in the aqueous phase was analyzed by volume titration using ethylenediaminetretraacetic acid (EDTA). The amount of RE(III) absorbed by the sorbents was determined by the difference. The extraction efficiency (E) and distribution coefficient (Kd) were determined by the following formulas: E (%) )
C0 - C e × 100 C0
Kd (mL g-1) )
(C0 - Ce)/W Ce /V
(1)
(2)
where Co and Ce represent the initial and final concentrations (mol L-1) of RE(III) in aqueous phase, respectively, W is the mass (g) of the sol-gel materials, and V is the volume (mL) of the solution. All the determinations were performed in triplicate and results reported as mean ( standard deviation. 3. Results and Discussion 3.1. Characterization of Sol-Gel Sorbents. Imidazolium ILs have been widely used as templates for the preparation of micro-/mesoporous silica.39,40 Herein, we use the mixture of Cnmim+PF6- (n ) 4, 8) and Cyphos IL 104 to template silica. Figure 2a shows the scanning electron microscopy (SEM) image of the C4mim+PF6- and Cyphos IL 104 modified silica sorbent (B104SG-1), revealing that B104SG-1 possesses some macrochannels. Figure 2b shows the transmission electron microscopy (TEM) image of the pore morphology and structure of B104SG1, in which numerous mesopores are clearly noticeable. The result is further confirmed by BET measurements. The nitrogen sorption isotherm of B104SG-1 (Figure 3) exhibits the typical IV curve, which reflects the formation of the mesoporous structure. As shown in Table 2, the BET surface areas of B104SGs are higher than those of O104SGs when they were doped with equivalent volume of ILs. Accordingly, B104SGs were more porous than O104SGs. Moreover, the BET surface areas of B104SGs and O104SGs were much larger than the blank sorbent and 104SG, which was prepared without imidazolium ILs. It is a fact that surface areas of silica hybrid materials prepared by using ILs as template increase with the content of the ILs. High surface area is believed to enhance the metal binding capacity and uptake kinetics of porous materials.41 Blank sorbent and 104SG have similar surface area and average pore diameter. The results indicated that imidazolium ILs have great influence on the microstructure of sol-gel materials doped with binary IL mixtures.
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Ind. Eng. Chem. Res., Vol. 48, No. 15, 2009 Table 2. Results of Nitrogen Sorption Experiments of Sol-Gel Sorbentsa sorbents blank sorbent 104SG O104SG-1 O104SG-2 O104SG-3 B104SG-1 B104SG-2 B104SG-3 B104SG-4
SSAb av pore pore vol (m2 g-1) diamc (Å) (mL g-1) 148
34
0.127
156 214 716 990 580 885 1030 606
75 163 75 52 31 49 76 31
0.293 0.875 1.348 1.292 0.456 1.074 1.949 0.474
EY3+ (%)
KdY3+ (mL g-1)
4.5 ( 0.3
4.7 ( 0.4
25.8 ( 0.8 34.5 ( 1.5 42.1 ( 1.1 71.9 ( 3.2 70.9 ( 2.8 239.5 ( 29.4 72.8 ( 1.9 263.4 ( 23.9 52.9 ( 1.6 111.9 ( 6.8 82.3 ( 2.0 455.5 ( 55.3 97.3 ( 1.9 3528.0 ( 1493.8 67.4 ( 3.0 206.1 ( 25.7
a A 0.040 g amount of sorbents was mixed with 4.0 mL of Y(III) solutions (initial concentration, 0.201 mmol L-1; initial pH, 5.6) for 60 min. b BET specific surface area. c BJH.
Figure 4. TGA and DTA curves of sol-gel sorbents (a) blank sorbent, (b) O104SG-1, and (c) B104SG-1.
Figure 2. SEM (608× magnification) (a) and TEM (b) images of B104SG-1.
Figure 3. N2 adsorption-desorption isotherms for 104SG, B104SG-1, and O104SG-1.
The thermal stability of the prepared sorbents was investigated by TGA and differential thermal analysis (DTA) (Figure 4). B104SG-1 showed four main degradation stages. The initial weight loss which occurred in the temperature range of 27.3-165.8 °C corresponded to the loosely bound water molecules (about 9.1%), which was accompanied by an endothermic peak at about 75.1 °C in the DTA curve. The second step was from 165.8 to 262.9 °C, probably due to the evaporation of trapped water molecules in the silica (about 10.3%). A very broad peak (262.9-363.0 °C) that occurred in the third step was associated with the decomposition of C4mim+PF6- and Cyphos IL 104 covered on the surface of pores, which was in agreement with TGA curves
of C4mim+PF6- and Cyphos IL 104 (Figure 4). The fourth weight loss, which started from 363.0 °C, corresponded to the decomposition of C4mim+PF6- and Cyphos IL 104 within the matrix. The weight losses for the third and fourth steps were 13.6 and 7.6%, respectively. O104SG-1 exhibited a similar mass loss profile as B104SG-1. The total weight losses for the third and fourth steps of B104SG-1 and O104SG-1 were 21.2 and 18.7%, respectively. It is indicated that two binary IL mixtures were stably immobilized on the silica gel. The thermal decomposition temperature of C4mim+PF6-, C8mim+PF6-, and Cyphos IL 104 are 349, 376, and 371 °C, respectively. The stability of ILs doped in sol-gel matrix seems lower than that of the free ILs. In contrast, the blank sorbent shows a continuous weight loss with one endothermic peak at 113.0 °C, which was mainly attributed to physisorbed and structural water. When Cyphos IL 104 was added to the reaction mixture, the sol became viscid quickly and then gelated within 10 min. This indicated that the addition of Cyphos IL 104 encouraged the formation of porous silica materials. Cyphos IL 104, a lowmolecular-mass organic gelator, can accelerate the gelation of TEOS because the cation P6,6,6,14+ has strong electrostatic interaction with the growing anionic silicate polymers. Anionic silicate is attracted to the surface of phosphonium cation, and the negative charge is partially screened by phosphonium cation.42 When the sol-gel polymerization of TEOS proceeds in the organic gel state, the gelator strands act as a template which eventually creates a void in the resultant silica. 3.2. Effect of ILs on Y(III) Extraction. The effect of binary IL mixtures on sol-gel materials was investigated, and the results showed that the presence of ILs changed the characteristics of sorbents. The most interesting advantage of these silica
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Figure 5. Effect of initial pH on the extraction of Y(III) (initial concentration, 0.201 mmol L-1) with B104SG-1 and O104SG-1. Bars show standard deviations of measurements (n ) 3).
sorbents is their ability to combine the merits of the inorganic glasses with intrinsic properties of the organic reagents. As shown in Table 2, the sorption capacities of B104SG-1 and O104SG-1 are higher than that of 104SG. B104SG-1 and O104SG-1 with imidazolium ILs have more pores and the amount of phosphonium IL immobilized on the support surface was higher, allowing higher removal of metal ions from aqueous solution. The high difference in removal capacity levels between ILs-doped silica and blank sorbent clearly indicated that the extraction of Y(III) by B104SGs and O104SGs was mainly attributed to the doped ILs. Cyphos IL 104 without acidification is not suitable for the extraction of RE(III) because its aqueous solution is weakly alkaline. Cyphos IL 104 immobilized on sol-gel sorbents was acidified by formic acid in the gel formation process before it was doped in the silica matrix. The cations and anions of Cyphos IL 104 did not transfer to the equilibrium aqueous solutions. Such behavior was normally explained due to the formation of complicated complexes in the immobilized organic phase. With no anion and cation loss, B104SGs and O104SGs will not contaminate the aqueous solutions and retain their stability after extraction. Figure 5 shows the effect of initial pH on the removal of Y(III) with B104SG-1 and O104SG-1. There was an increase in Y(III) removal with increasing pH from 0.3 to 5.6. The maximum extraction efficiencies at pH 5.6 for B104SG-1 and O104SG-1 were 51.3 and 42.1%, respectively. The high concentration of H+ can inhibit the formation of RE complexes. The extraction of Y(III) by silica sorbents doped with Cyanex 923 (IL923SGs) was studied in our previous paper.27 The content of Cyphos IL 104 in B104SG-3 and O104SG-3 was 0.091 mmol g-1, while the maximum content of Cyanex 923 in stable IL923SGs was only 0.055 mmol g-1. Electrostatic interactions between the SiO- groups and cations of the phosphonium salts contribute to form a stable sol-gel material with a high dose of Cyphos IL 104. In this study, we compared the extraction by IL923SG27 with B104SG-4 under the condition of pH 5.6, and the results are given in Figure 6. Even though the dose of Cyanex 923 in IL923SG was equal to that of Cyphos IL 104 in B104SG-4, the removal rates of IL923SG and B104SG-4 were 46.3 and 91.9% in a solid/liquid ratio of 20.0 g L-1, respectively. B104SGs and O104SGs with Cyphos IL 104, which acted as a bifunctional ionic liquid extractant (bif-ILE) were more effective than IL923SG with molecular extractant Cyanex 923. The dose-dependent effect of the sorbent on the removal of Y(III) is shown in Figure 7. As expected, removal of Y(III)
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Figure 6. Comparison results of IL923SG and B104SG-4 for Y(III) (initial concentration, 0.201 mmol L-1; initial pH, 5.6) extraction. Bars show standard deviations of measurements (n ) 3).
Figure 7. Effect of sorbents dose on Y(III) (initial concentration, 0.201 mmol L-1; initial pH, 5.6) removal. Bars show standard deviations of measurements (n ) 3).
increased with increased adsorbent dose. The maximum removal rates of Y(III) for O104SG-1-3 and B104SG-1-3 were 0.10, 0.10, 0.08, 0.10, 0.08, and 0.05 g, respectively. B104SG-1 and O104SG-1, B104SG-2 and O104SG-2, and B104SG-3 and O104SG-3 have the same amount of Cyphos IL 104, respectively. The extraction capacities of sorbents with equivalent extractant were also compared. Evidently B104SGs are much better than O104SGs under the conditions used in this study, even when they contained equivalent Cyphos IL 104. For example, 2 mL of C4mim+PF6- (Mw, 284.18 g mol-1; density, 1.360 g mL-1) and C8mim+PF6- (Mw, 340.29 g mol-1; density, 1.220 g mL-1) were respectively used to template B104SG-1 and O104SG-1. The molar ratio of C4mim+PF6- to C8mim+ PF6- is about 1.33:1. As a result, B104SG-1 was more porous than O104SG-1. Metal ions have more chance to be complexed with extractant which was dispersed on the silica surface. The distribution ratios (Kd) of Y(III), Ho(III), Er(III), Dy(III), and Yb(III) were studied in individual RE(III) systems. A 0.21 mmol L-1 amount of Y(III), Ho(III), Er(III), Dy(III), or Yb(III) solution (4 mL) was shaken with 0.040 g of IL104SG-1. The separation factor (β) is the Kd of two metals measured under the same condition. Table 3 gives the β values between Y(III), Ho(III), Er(III), Dy(III), and Yb(III) by O104SG-1. The β values of Yb/Dy (37.82), Yb/Ho (20.83), Y/Dy (18.64), Y/Ho (10.27),
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Table 3. Separation Factors of RE(III) by O104SG-1 (β ) KdRE2/ KdRE1, [RE3+] ) 0.201 mmol L-1, CHNO3 ) 0.5 mol L-1) RE2 RE1
Ho
Er
Y
Yb
Dy Ho Er Y
1.82
7.71 4.24
18.64 10.27 2.42
37.82 20.83 4.91 2.03
and Er/Dy (7.71) are large enough that they can be effectively separated in the medium of HNO3 (0.5 mol L-1). Therefore, Cyphos IL 104 doped sol-gel sorbent has the potential to separate RE(III). 3.3. Kinetics of Y(III) Removed by Sol-Gel Sorbents. Figure 8 shows the variation of Y(III) adsorbed with time at pH 5.6. It is evident that the rate of removal was very fast initially and became slower with the lapse of time. It is clear from the results that 104SG and O104SG-1 exhibit slightly slower uptake kinetics for Y(III) as compared to B104SG-1. The extraction of Y(III) by B 104SG-1 occurred primarily within 30 min, and then the equilibrium was achieved. For 104SG and O104SG-1, the equilibrium was attained within 60 min. The metal ions have to diffuse into the pores of the silica through the interconnected three-dimensional network of pores before they reacted with the entrapped extractants. The porous characterization of the sorbents surface might facilitate metal extraction, causing a significant increase in Y(III) extraction kinetics. 3.4. Regeneration of Sol-Gel Sorbent. For economy and the purpose of extraction, the regeneration of the sorbent is absolutely necessary. The stripping agent used for the regeneration of B104SG-3 was 0.02 mol L-1 EDTA solution at pH 5.6. After each regeneration step the resin samples were extensively washed with deionized water to remove the residual chemicals. The result presented in Figure 9 revealed that there were no significant changes in four extraction cycles. After four extraction/stripping cycles, the regeneration efficiency of Y(III), Dy(III), Ho(III), Er(III), and Yb(III) for the regenerated B104SG-3 is 92.5, 92.2, 93.5, 92.7, and 93.2%, respectively. Both the cation and anion of Cyphos IL 104 were involved in the extraction, so there was no loss of IL during the separation process. The sorbent doped with binary mixture ILs can be regenerated and reused repeatedly.
Figure 8. Kinetic of extraction of Y(III) (initial concentration, 0.201 mmol L-1; initial pH, 5.6) for 104SG, B104SG-1, and O104SG-1. Bars show standard deviations of measurements (n ) 3).
Figure 9. Regeneration of B104SG-3 with 0.02 mol L-1 EDTA.
4. Conclusions In this study, we synthesized mesoporous silica sorbents doped with binary IL mixtures (C8mim+PF6-/Cyphos IL 104 or C4mim+PF6-/Cyphos IL 104) by the sol-gel method. These porous silica sorbents had greater thermal stability, higher specific surface area, and larger total pore volume than that prepared without imidazolium ILs. The Y(III) removal rates of B104SGs were higher than those of O104SGs and IL923SG. This process may also be used to develop efficient equipment (e.g., ILs immobilized on inorganic supports, such as particles and membranes) for separation of RE(III). Acknowledgment This project was supported by National Natural Science Foundation of China (Grant 50574080), Distinguished Young Scholar Foundation of Jilin Province (Grant 20060114), and SRF for ROCS, Ministry of Education of China. We thank Cytec Canada for the sample of Cyphos IL 104 and Cyanex 923. Literature Cited (1) Sun, X. B.; Zhao, J. M.; Meng, S. L.; Li, D. Q. Synergistic extraction and separation of yttrium from heavy rare earths using mixture of secoctylphenoxy acetic acid and bis(2,4,4-trimethylpentyl)phosphinic acid. Anal. Chim. Acta 2005, 533 (1), 83–88. (2) Gao, J.; Peng, B.; Fan, H.; Kang, J. Solvent extraction kinetics of rare earth elements. Talanta 1996, 43 (10), 1721–1725. (3) Reddy, M. L. P.; Bharathi, J. R. B.; Peter, S.; Ramamohan, T. R. Synergistic extraction of rare earths with bis(2,4,4-trimethylpentyl) dithiophosphinic acid and trialkyl phosphine oxide. Talanta 1999, 50 (1), 79–85. (4) Konishi, Y.; Shimaoka, J.; Asai, S. Sorption of rare-earth ions on biopolymer gel beads of alginic acid. React. Funct. Polym. 1998, 36 (2), 197–206. (5) Zhang, A. Y.; Kuraoka, E.; Kumagai, M. Group partitioning of minor actinides and rare earths from highly active liquid waste by extraction chromatography utilizing two macroporous silica-based impregnated polymeric composites. Sep. Purif. Technol. 2007, 54 (3), 363–372. (6) Jelinek, L.; Yuezhou, W.; Kumagai, M. Adsorption of Ce(IV) anionic nitrato complexes onto anion exchangers and its application for Ce(IV) separation from rare earths(III). J. Rare Earth 2006, 24 (4), 385–391. (7) Arai, T.; Wei, Y. Z.; Kumagai, M.; Horiguchi, K. Separation of rare earths in nitric acid medium by a novel silica-based pyridinium anion exchange resin. J. Alloys Compd. 2006, 408, 1008–1012. (8) Park, J. S.; Han, C.; Lee, J. Y.; Kim, S. D.; Kim, J. S.; Wee, J. H. Synthesis of extraction resin containing 2-ethylhexyl phosphonic acid mono2-ethylhexyl ester and its performance for separation of rare earths (Gd, Tb). Sep. Purif. Technol. 2005, 43 (2), 111–116. (9) Lee, S. H.; Doan, T. T. N.; Ha, S. H.; Koo, Y. M. Using ionic liquids to stabilize lipase within sol-gel derived silica. J. Mol. Catal., B 2007, 45 (1-2), 57–61. (10) Nassif, N.; Bouvet, O.; Rager, M. N.; Roux, C.; Coradin, T.; Livage, J. Living bacteria in silica gels. Nat. Mater. 2002, 1 (1), 42–44.
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ReceiVed for reView March 23, 2009 ReVised manuscript receiVed June 8, 2009 Accepted June 16, 2009 IE900468C
