Mark L. Dietz

On the radiation stability of crown ethers in ionic liquids.

Crown ethers (CEs) are macrocyclic ionophores used for the separation of strontium-90 from acidic nuclear waste streams. Room temperature ionic liquids (ILs) are presently being considered as replacements for traditional molecular solvents employed in such separations. It is desirable that the extraction efficacy obtained with such solvents should not deteriorate in the strong radiation fields generated by decaying radionuclides. This deterioration will depend on the extent of radiation damage to both the IL solvent and the CE solute. While radiation damage to ILs has been extensively studied, the issue of the radiation stability of crown ethers, particularly in an IL matrix, has not been adequately addressed. With this in mind, we have employed electron paramagnetic resonance (EPR) spectroscopy to study the formation of CE-related radicals in the radiolysis of selected CEs in ILs incorporating aromatic (imidazolium and pyridinium) cations. The crown ethers have been found to yield primarily hydrogen loss radicals, H atoms, and the formyl radical. In the low-dose regime, the relative yield of these radicals increases linearly with the mole fraction of the solute, suggesting negligible transfer of the excitation energy from the solvent to the solute; that is, the solvent has a radioprotective effect. The damage to the CE in the loading region of practical interest is relatively low. Under such conditions, the main chemical pathway leading to decreased extraction performance is protonation of the macrocycle. At high radiation doses, sufficient to increase the acidity of the IL solvent significantly, such proton complexes compete with the solvent cations as electron traps. In this regime, the CEs will rapidly degrade as the result of H abstraction from the CE ring by the released H atoms. Thus, the radiation dose to which a CE/IL system is exposed must be maintained at a level sufficiently low to avoid this regime.

Anion Effects in the Extraction of Lanthanide 2-Thenoyltrifluoroacetone Complexes into an Ionic Liquid

The extraction of trivalent lanthanides from an aqueous phase containing 1 M NaClO4 into the room temperature ionic liquid 1-butyl-3-methylimidazolium nonafluoro-1-butanesulfonate by the β-diketone extractant 2-thenoyltrifluoroacetone (Htta) was studied. Radiotracer distribution, absorption spectroscopy, time-resolved laser-induced fluorescence spectroscopy, and X-ray absorption fine structure measurements point to the extraction of multiple lanthanide species. At low extractant concentrations, fully hydrated aqua cations of the lanthanides are present in the ionic liquid phase. As the extractant concentration is increased 1:2 and 1:3 lanthanide:tta species are observed. In contrast, 1:4 Ln:tta complexes were observed in the extraction of lanthanides by Htta into 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide.

Effect of aqueous phase anion on the mode of facilitated ion transfer into room-temperature ionic liquids

Measurements of the partitioning of various alkali and alkaline earth cations between solutions of hydrochloric acid and a series of 1,3-dialkylimidazolium-based ionic liquids (ILs) to which a crown ether has been added have revealed substantial differences in extraction behavior versus both conventional molecular solvents (e.g., 1-octanol) under the same conditions and the same ILs when nitric acid solutions are employed as the aqueous phase. These results can be rationalized by application of a three-path model for metal ion partitioning into ILs in the presence of a neutral extractant. Additionally, the results point to a significant role for anion hydration energy in determining the balance amongst the possible modes of partitioning and strongly suggest that ion exchange involving the cationic metal complex and the cationic constituent of the ionic liquid constitutes the default route for metal ion extraction in IL systems incorporating a neutral extractant.

Carbon Dioxide Solubility Enhancement through Silicone Functionalization: “CO2-philic” Oligo(dimethylsiloxane)-substituted Diphosphonates∗

Abstract Carbon dioxide has received significant attention as a potential environmentally benign medium to replace hazardous organic compounds, but is a relatively poor solvent. The addition of siloxane substituents provides an attractive and inexpensive means to solubilize a wide variety of compounds in CO2. By synthesizing and testing a family of gem-diphosphonate ligands that have been rendered CO2-philic by incorporation of a number of related, discrete dimethylsiloxane oligomers, we show that small variations in substituents have a significant effect on the CO2-philicity of the ligand. To our knowledge, this is the first systematic study of the effect of siloxane substituent size, branching, and position on the affinity of a ligand for CO2. In addition, we present a general approach to the preparation of novel gem-diphosphonate ligands.

Hydrogen-Bonding Interactions and Protic Equilibria in Room-Temperature Ionic Liquids Containing Crown Ethers

Nuclear magnetic resonance (NMR) spectroscopy has been used to study hydrogen-bonding interactions between water, associated and dissociated acids (i.e., nitric and methanesulfonic acids), and the constituent ions of several water-immiscible room-temperature ionic liquids (ILs). In chloroform solutions also containing a crown ether (CE), water molecules strongly associate with the IL ions, and there is rapid proton exchange between these bound water molecules and hydronium associated with the CE. In neat ILs, the acids form clusters differing in their degree of association and ionization, and their interactions with the CEs are weak. The CE can either promote proton exchange between different clusters in IL solution when their association is weak or inhibit such exchange when the association is strong. Even strongly hydrophobic ILs are shown to readily extract nitric acid from aqueous solution, typically via the formation of a 1:1:1 {H(3)O(+)•CE}NO(3)(-) complex. In contrast, the extraction of methanesulfonic acid is less extensive and proceeds mainly by IL cation-hydronium ion exchange. The relationship of these protic equilibria to the practical application of hydrophobic ILs (e.g., in spent nuclear fuel reprocessing) is discussed.

Rapid quantification of imidazolium-based ionic liquids by hydrophilic interaction liquid chromatography: Methodology and an investigation of the retention mechanisms.

The separation of nine N,N-dialkylimidazolium-based ionic liquids (ILs) by an isocratic hydrophilic interaction high-performance liquid chromatographic method using an unmodified silica column was investigated. The chosen analytical conditions using a 90:10 acetonitrile-ammonium formate buffer mobile phase on a high-purity, unmodified silica column were found to be efficient, robust, and sensitive for the determination of ILs in a variety of solutions. The retention window (k = 2-11) was narrower than that of previous methods, resulting in a 7-min runtime for the nine IL homologues. The lower limit of quantification of the method, 2-3 μmol L(-1), was significantly lower than those reported previously for HPLC-UV methods. The effects of systematically modifying the IL cation alkyl chain length, column temperature, and mobile-phase water and buffer content on solute retention were examined. Cation exchange was identified as the dominant retention mechanism for most of the solutes, with a distinct (single methylene group) transition to a dominant partitioning mode at the highest solute polarity.

Tribological Performance of Environmentally Friendly Ionic Liquid Lubricants

Presented in this study is a new class of greener lubricants, room temperature ionic liquids, that represent a promising potential solution to many of the problems associated with both conventional lubricants and those based on natural oils. In this study, friction and wear tests were carried out using a pin-on-disk tribometer under ambient conditions to characterize the performance of the ionic liquids as lubricants. Specifically, ionic liquids consisting of salicylate, benzoate (common food additives) and saccharinate (an artificial sweetener) anions with conventional phosponium cations were evaluated as lubricants and compared to petroleum-based lubricants and natural oils in regards to structure and performance. The ionic liquids generally demonstrated better tribological performance than either the petroleum-based lubricants or natural oils. The mechanisms governing the chemical composition and improved tribological performance are discussed while highlighting possible industrial applications of this new class of lubricants.

Ionic Liquid (IL) Cation and Anion Structural Effects on Metal Ion Extraction into Quaternary Ammonium-based ILs

ABSTRACT Numerous factors have previously been shown to influence the mode of extraction of alkali and alkaline earth cations from an acidic aqueous phase into 1, 3-dialkylimidazolium-based ionic liquids (ILs) by a crown ether, among them the hydrophobicity of both the IL anion and cation. To determine if this observation is “generic” and thus, could provide the basis for guidelines for the rational design of ILs to be used as solvents in metal ion extraction, other families of ILs must be studied. A series of quaternary ammonium-based ILs have therefore been examined as solvents for the extraction of various metal ions from acidic nitrate- and chloride-containing aqueous phases by dicyclohexano-18-crown-6 (DCH18C6). Although the overall metal ion extraction behavior in these systems is similar to that observed for 1, 3-dialkylimidazolium-based ILs, significant differences in metal ion separation factors (e.g., αSr/Na) are observed under certain conditions, differences that may be sufficient to influence the choice of IL in separations applications.

Extraction and Reductive Stripping of Pertechnetate from Spent Nuclear Fuel Waste Streams

An approach directed at rapid sequestration and disposal of technetium-99 from UREX (uranium extraction) liquid waste streams is presented. This stream is generated during reprocessing of light-water-reactor spent fuel to recycle the actinides and separate fission products for waste disposal. U and Tc are co-extracted from a nitric acid solution using tri-n-butylphosphate in dodecane, so that Tc(VII) is present in the strip solution after the actinide separations. The goal is to separate uranyl from the pertechnetate in this U-Tc stream and then sequester Tc in the metallic form. Our approach is based on reductive stripping of pertechnetate either from aqueous solution (for column extractions) or organic solvents (for liquid-liquid extractions). In both of these methods, metallic zinc in the presence of formic acid serves as a reducing agent, and 99Tc is recovered as a co-precipitate of Zn(II) hydroxide and hydrous Tc(IV) oxide, with a Zn:Tc ratio between 1:1 and 2:1 mol/mol. This solid residue can be reduced to a Zn-Tc alloy by high temperature (500–700°C) hydrogenation, and the resulting heterophase alloy can be added to a metallic Fe-Zr-Mo waste form that is processed at 1600°C, with subsequent loss of Zn by evaporation. Alternatively, Zn and Tc can be separated and 99Tc sequestered as NH4TcO4 for further reduction to Tc(0) metal. The aqueous Zn reduction process removes ∼90% of 99Tc per cycle. The nonaqueous Zn reduction in 1:1 methanol – formic acid removes 60–70% of 99Tc per cycle, depending on the extracting agent (such as a tetraalkylammonium nitrate). The extracting agent is recycled in the process. The pertechnetate is extracted from the aqueous phase into 1,2-dichloroethane, which is removed by evaporation and reused. The residue is either calcined and steam reformed to Tc(0) or processed by the nonaqueous Zn reduction method. These methods can be used not only to remove the pertechnetate from the U-Tc product stream, but also to sequester the pertechnetate from aqueous waste streams generated through the processes described in this paper, thereby closing the cycle. The same approaches can be used to close the 99Tc cycle for other methods that are currently being developed at Los Alamos and Argonne National Laboratories.

Thermal Properties of Macrocyclic Polyethers: Implications for the Design of Crown Ether-Based Ionic Liquids

The most commonly studied classes of ionic liquids (ILs) comprise relatively large and asymmetric heterocyclic cations (e.g., diakylimidazolium or N-alkylpyridinium) in combination with any of a wide variety of inorganic (e.g., BF4−, Cl−) or organic (e.g., bis[(trifluoromethyl- sulfonyl)imide], Tf2N−) anions. Recently it has been shown that ILs can also be formed by complexation reactions of metal cations (e.g., Li+, as its Tf2N− salt) with various neutral ligands (e.g., cyclohexano-15-crown-5 or alkylamines). Because the upper limit of the useful temperature range of any IL is governed by its thermal stability, and because the thermal stability of a neutral ligand (i.e., its propensity to either volatilize or decompose) is of obvious importance in determining that of an IL prepared from it, a systematic examination of the thermal properties of a series of macrocyclic polyethers of potential utility in the synthesis of new ILs has been undertaken. The results show that the temperature corresponding to the onset of mass loss upon heating (i.e., evaporation and/or decomposition) varies with the ring size, substitution, nature of the donor atoms, and stereochemistry of the macrocycle, but is most strongly influenced by the molecular weight and aromatic content of the compound.