3 The extraction efficiency of metal complexes can also be controlled by the pH

3 the extraction efficiency of metal complexes can

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(3) The extraction efficiency of metal complexes can also be controlled by the pH of the system. 23 Among IL extractions of metal ions, the extraction of radioactive metals (lanthanides and actinides) has particular industrial significance for the handling of nuclear materials. Earlier studies in the area included the behavior of uranium species in various ILs includ- ing mixtures of chloroaluminate ILs and butylpyri- dinium chloride, 24 , 25 mixtures of chloroaluminate ILs and [C 2 MIM]Cl, 26 chloroaluminate ILs, 27 and sev- eral RTILs. 18 , 19 , 28 Recent work primarily focused on two aspects: (1) the fundamental understanding of ILs in nuclear chemistry such as radiochemical sta- bility of ILs, 29 superoxide ion electrochemistry in ILs, 30 and electrochemical properties of actinides in ILs; 31 (2) applications of ILs in nuclear extractions of lanthanides 32 and actinides (Table 1). 29 , 33 , 34 However, the disadvantages of using extractants are the difficulty of separating them from ILs and the complexity of the system. 28 As a new concept introduced in the late 1990s, Task-Specific ILs (TSILs) are designed to have targeted functionality and, in this case, TSILs are those ILs containing metal ion-ligating functional groups. Therefore, TSILs serve as the hydrophobic solvent and the extractant at the same time. TSILs containing side-chains of thiourea derivatives and urea derivatives could dramatically increase the partitioning of Hg 2 + and Cd 2 + ions from aqueous solutions. 28 , 35 Moreover, as indicated in a recent review, 36 TSILs are not limited to the extraction processes; they are also versatile solvents/catalysts used in organic catalysis, solid phase synthesis, and even production of ‘liquid Teflon’ and emulsions. IL EXTRACTIONS OF ORGANIC/BIO/BIOFUEL MOLECULES The high solubility of charged organic molecules in ILs has stimulated the development of organic prod- uct recovery by these ‘green’ media. As illustrated in Table 2, phase partitions of many phenolic com- pounds are investigated in the IL–water biphasic sys- tems. The distribution coefficient is highly influenced by the pH value which determines the existing form of a phenolic compound as a conjugated acid or base. 41 , 44 The same principle applied to the extraction of amino acids through [BMIM][PF 6 ] where the concentration of the crown ether (DC18C6) also played an impor- tant role on the amino acid recovery (Table 2). 45 Meanwhile, IL biphasic systems were also used to separate many other biologically important molecules 1090 J Chem Technol Biotechnol 80 :1089–1096 (2005)
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Use of ionic liquids as ‘green’ solvents for extractions Table 2. Examples of IL extractions of organic/bio/biofuel molecules Substances IL Extractant Reference Phenolic compounds phthalic acid, aniline, 4-hydroxybenzoic acid, benzoic acid, p -toluic acid, benzene, chlorobenzene, 1,2,4-trichlorobenzene, 1,4-dichlorobenzene, 4,4 -dichlorobiphenyl [BMIM][PF 6 ] None 44 Phenol, tyrosol, p -hydroxybenzoic acid [C n MIM][BF 4 ] ( n = 1, 3, 6, 8, 10) [C n MIM][PF 6 ] ( n = 6, 10) None 41 Chlorophenols [C 4 MIM][PF 6 ], [EMIM][Beti] None 58 Amino acids Tryptophan, glycine, alanine, leucine, lysine, arginine [BMIM][PF 6 ] DC18C6 45 Carbohydrates
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