Ionic liquid anion effects in the extraction of metal ions by macrocyclic polyethers
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 () and its bis[(trifluoromethyl)sulfonyl] imide () 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 or 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 [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 ( vs. 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+ where n = 5, 8, or 10), along with the corresponding bis[(trifluoromethyl)sulfonyl]imide and bis[(perfluoroethyl)sulfonyl]imide ILs (designated as Cnmim+, and Cnmim+BETI−, respectively) were employed. The and BETI− ILs were prepared via either conventional or microwave synthesis, purified, and characterized according to published methods [23], [24], while the 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.
References (30)
- et al.
Thermal properties of imidazolium ionic liquids
Thermochem. Acta
(2000) - et al.
Room temperature ionic liquids of alkylimidazolium cations and fluoroanions
J. Fluorine Chem.
(2000) - et al.
Ionic liquids: applications in catalysis
Cata. Today
(2002) - et al.
Structural variations in room-temperature ionic liquids: Influence on metal ion partitioning modes and extraction selectivity
Sep. Purif. Technol.
(2012) - et al.
Extraction of uranium(VI) by 1.1 M tri-n-butylphosphate/ionic liquid and the feasibility of recovery by direct electrodeposition from organic phase
J. Alloys Comp.
(2008) - et al.
Effect of aqueous phase anion on the mode of facilitated ion transfer into room-temperature ionic liquids
Talanta
(2012) - et al.
Air and water stable 1-ethyl-3-methylimidazolium-based ionic liquids
J. Chem. Soc. Chem. Commun.
(1992) - et al.
Ionic liquids: innovative fluids for chemical processing
AIChE J.
(2001) - et al.
Influence of chloride, water, and organic solvents on the physical properties of ionic liquids
Pure Appl. Chem.
(2000) - et al.
Physicochemical properties and structures of room temperature ionic liquids. 2. Variations of alkyl chain length in imidazolium cation
J. Phys. Chem. B
(2005)
Characterization and comparison of hydrophilic and hydrophobic room-temperature ionic liquids incorporating the imidazolium cation
Green Chem.
Influence of gamma irradiation on hydrophobic room temperature ionic liquids [BuImMe]PF6 and [BuImMe](CF3SO2)2N
Dalton Trans.
Effect of tetraalkylphosphonium based ionic liquids as lubricants on the tribological performance of a steel on steel system
Tribol. Lett.
(Eco)toxicity of fluoro-organic and cyano-based ionic liquid anions
Chem. Commun.
Electrochemical aspects of ionic liquids
Cited by (27)
Selective recovery of rare earth elements from e-waste via ionic liquid extraction: A review
2023, Separation and Purification TechnologyCitation Excerpt :The fundamental disadvantage of IL-based extraction technique is that a portion of the IL is generally decomposed in the aqueous phase, which can be termed “sacrificial agent”. To determine the exchange process, the solubility of an IL towards aqueous medium is crucial because hydrophobic cations can attenuate these exchanges but hydrophilic anions exacerbate the phenomena [76,77]. For instance, improving the hydrophobic nature of an IL can transform anionic exchange process to neutral exchange process [78].
Ionic liquids-assisted extraction of metals from electronic waste
2021, Ionic Liquid-Based Technologies for Environmental SustainabilityApplications of ionic liquids in environmental remediation
2020, Green Sustainable Process for Chemical and Environmental Engineering and Science: Green Solvents for Environmental RemediationFlotation of quartz using ionic liquid collectors with different functional groups and varying chain lengths
2016, Minerals EngineeringCitation Excerpt :Ionic liquids which posses low volatility and high ionization temperature, have several applications in varied fields. They are used as phase-transfer catalysts, solvents, lubricants, gas capturing agents, coating materials, or chemical sensors (Bradaric et al., 2003; Weng et al., 2006; Pernak et al., 2006; Garvey and Dietz, 2014; Ferreira et al., 2014; Gao et al., 2014). The functional heads such as ammonium, imidazolium, pyridinium, piperidinium, pyrolidinium etc. which contain quaternary nitrogen atom are considered to be the most common cations of ionic liquids (Chen et al., 2013; Seddon, 1996; Lagrost et al., 2003; Blahušiak et al., 2013; Mancini et al., 2013; Tong et al., 2013).
Separation of BTEX from a naphtha feed to ethylene crackers using a binary mixture of [4empy][Tf<inf>2</inf>N] and [emim][DCA] ionic liquids
2015, Separation and Purification TechnologyCitation Excerpt :Therefore, the extraction of BTEX from this naphtha could reduce costs and could produce income by selling the aromatics extracted. Ionic liquids (ILs) have been studied as potential substitutes of conventional solvents in separation processes, highlighting their application in the aromatic extraction [5–17]. In addition to their good extractive properties, the most remarkable feature of ILs is their nonvolatile nature [18].
Use of the ionic liquid-tricaprylmethyl ammonium salicylate (TOMAS) as a flotation collector of quartz
2014, Separation and Purification TechnologyCitation Excerpt :They are environment friendly and possess typical properties like low-vapor pressure, high viscosity, dual natural polarity, good thermal stability and a wide range of miscibility with water and other organic solvents, and avoid many environmental and safety problems [6–11]. Some of the ionic liquids based on ammonium, phosphinium, imidazolium, pyridinium are used as phase-transfer catalysts, solvents, lubricants, gas capture agents, coating materials, or chemical sensors [12–17]. Tricaprylmethyl ammonium salicylate (TOMAS) is an ammonium based ionic liquid derived from the aliquat cation.