Elsevier

Separation and Purification Technology

Volume 123, 26 February 2014, Pages 145-152
Separation and Purification Technology

Ionic liquid anion effects in the extraction of metal ions by macrocyclic polyethers

https://doi.org/10.1016/j.seppur.2013.12.005Get rights and content

Highlights

  • Cation extraction from acidic phases into several Cnmim+ ILs by DCH18C6 is examined.

  • The balance between neutral complex extraction and ion exchange varies with IL anion.

  • Hydrophobic IL anions favor metal ion partitioning via ion-exchange processes.

  • IL anion effects on extraction efficiency/selectivity vary with aqueous acidity.

Abstract

As part of a broader effort to determine the factors governing the relative contributions of ion-exchange processes and neutral complex/ion-pair partitioning in the extraction of metal ions into room-temperature ionic liquids by neutral extractants, the effect of ionic liquid (IL) anion on the extraction of alkali and alkaline earth cations by a crown ether from acidic nitrate and chloride media has been systematically examined. Results for a series of 1,3-dialkylimidazolium (Cnmim+)-based ILs incorporating hexafluorophosphate (PF6-), bis[(trifluoromethyl)sulfonyl] imide (Tf2N-), or bis[(perfluoroethyl)sulfonyl]imide (BETI) as the counterion indicate that the choice of anion, in particular its hydrophobicity, can influence the balance among the various possible pathways for extraction, as well as both the extraction efficiency and selectivity. Overall, higher IL anion hydrophobicity is accompanied by a greater tendency toward ion exchange as the predominant mode of extraction. The effect upon extraction efficiency and selectivity is more complex, however, varying markedly with aqueous acidity.

Introduction

In the nearly two decades that have passed since the introduction of the first air- and water-stable ionic liquids (ILs) [1], countless studies have explored the influence of cation and anion structure on the physicochemical properties of these unique solvents and on their utility in a wide variety of applications. It is by now well-established, for example, that 1,3-dialkyl-imidazolium ionic liquids can be made progressively more water soluble by shortening the alkyl moieties appended to the imidazolium cation [2]. Similarly, incorporation of a sufficiently hydrophilic anion (e.g., Cl) into these same ionic liquids can render them completely water miscible [3]. Analogous studies on these and a host of other ionic liquids have demonstrated the importance of the precise nature of the cationic and anionic constituents of the ionic liquid in determining such properties as density [4], water content [5], thermal [5], [6] and radiolytic [7] stability, lubricity [8], toxicity [9], and conductivity [10], among others. In addition, various IL cation and anion characteristics have been found to be critical in determining the suitability of these solvents as media for electrochemistry [11], synthesis [12], and catalysis [13].

Our own work has examined the relationship between the properties of ionic liquids and their performance as solvents in the extraction of metal ions from aqueous solution by various neutral extractants (e.g., crown ethers). In contrast to extraction into conventional (i.e., molecular) organic solvents, metal ion partitioning into ILs in the presence of a neutral extractant is a complex process comprising multiple pathways, the balance among which is determined by the characteristics of the metal ion (e.g., charge density) [14], the extractant [15], and the ionic liquid itself [16], [17]. It has been shown for 1,3-dialkylimidazolium ILs, for example, that increasing the hydrophobicity of the IL cation can induce a change in the mode of partitioning of alkaline earth cations in the presence of crown ethers from one in which ion-exchange processes are favored to one in which neutral complex/ion-pair extraction is the predominant mode of metal ion partitioning [18]. As has recently been shown, such changes can have a significant influence on metal ion extraction efficiency and selectivity [14].

In contrast to IL cation effects on metal ion partitioning, which have been the subject of a number of investigations, the influence of IL anions has remained incompletely explored. Domańska and Rękawek [19], for example, examined the extraction of Ag+ by dithizone into 1-butyl-3-methylimidazolium hexafluorophosphate (PF6-) and its bis[(trifluoromethyl)sulfonyl] imide (Tf2N-) analog, noting only that the latter yields slightly higher (99.3% vs. 98.6%) extraction efficiency under the experimental conditions. Similarly, in a pair of papers by Giridhar et al. [20], [21], the suitability of 1-butyl-3-methylimidazolium-based ILs incorporating either PF6- or Tf2N- as solvents for the extraction of uranium by TBP was evaluated. No attempt was made to examine the influence of IL anion on anything other than the extraction efficiency, however. More recently, the possible effect of IL anion on the nature of the partitioning species in the extraction of lanthanides (i.e., Nd3+ and Eu3+) by thenoyltrifluoroacetone (HTTA) into 1,3-dialkylimidazolium-based ILs incorporating Tf2N- [16] or nonaflate (NfO) [17] has been considered by Jensen et al. The observed effects, in particular the greater propensity toward the extraction of various cationic lanthanide complexes when the latter solvents are employed, were attributed to the far higher concentrations of dissolved water present in the NfO IL following equilibration with aqueous buffer solutions. Along these same lines, work by Luo et al. [22] has explored IL anion (PF6- vs. Tf2N- vs. BETI, where BETI represents the bis[(perfluoroethyl)sulfonyl]imide anion) effects in the extraction of Sr2+ and Cs+ by crown ethers in several 1,3-dialkylimidazolium-based ILs. An important aspect of this work too was exploring the impact of a change in anion on the efficiency of metal ion extraction, with increasing IL anion hydrophobicity typically being accompanied by higher values of the strontium and cesium distribution ratios. Extraction selectivity for Sr2+ over several monovalent cations (e.g., Na+) was also found to improve with increasing anion hydrophobicity. The practical utility of these results was limited, however, as all extraction studies were carried out using only water as the aqueous phase.

In the present study, we present a detailed examination of the effect of IL anion on the extraction of several alkali (e.g., Na+) and alkaline earth (e.g., Sr2+) cations by dicyclohexano-18-crown-6 (DCH18C6) into a series of 1,3-dialkylimidazolium-based ILs from acidic nitrate and chloride media, with emphasis on the effect of IL anion hydrophobicity on the predominate mode(s) of metal ion partitioning observed.

Section snippets

Materials

For this study, three 1-alkyl-3-methylimidzolium hexafluorophosphate ionic liquids (abbreviated as Cnmim+PF6- where n = 5, 8, or 10), along with the corresponding bis[(trifluoromethyl)sulfonyl]imide and bis[(perfluoroethyl)sulfonyl]imide ILs (designated as Cnmim+Tf2N-, and Cnmim+BETI, respectively) were employed. The Tf2N- and BETI ILs were prepared via either conventional or microwave synthesis, purified, and characterized according to published methods [23], [24], while the PF6- ILs were

Results and discussion

To fully understand metal ion partitioning behavior in extraction systems using ionic liquids as replacements for the conventional organic solvents ordinarily employed, the effect of each constituent of the ionic liquid on the extraction process must be considered. As noted above, although considerable effect has been expended to determine the effect of the ionic liquid cation on this process, must less attention has been directed at the ionic liquid anion. In an earlier report, Luo et al. [22]

Conclusions

The results of this study, by elucidating the effect of a change in IL anion upon the predominant mode(s) of alkali and alkaline earth cation extraction by a crown ether into imidazolium-based ILs from acidic aqueous phases, represent another step toward clarifying the factors governing the “balance of pathways” in metal ion extraction by various extractants in ionic liquids. In contrast to changes in the IL cation, which have typically been found to have a pronounced impact on metal ion

Acknowledgements

The authors thank Dr. Sheng Dai (Oak Ridge National Laboratory) for providing samples of the BETI ionic liquids necessary to complete this study. The financial support of the Office of Basic Energy Sciences of the United States Department of Energy under the Single Investigator Small Group Research (SISGR) Program through sub-contract with Brookhaven National Laboratory is gratefully acknowledged.

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