Oxidative Dissolution of Metals in Organic Solvents.

Dissolution of metals in organic solvents is relevant to various application fields, such as metal extraction from ores or secondary resources, surface etching or polishing of metals, direct synthesis of organometallic compounds, and separation of metals from other compounds. Organic solvents for dissolution of metals can offer a solution when aqueous systems fail, such as separation of metals from metal oxides, because both the metal and metal oxide could codissolve in aqueous acidic solutions. This review critically discusses organic media (conventional molecular organic solvents, ionic liquids, deep-eutectic solvents and supercritical carbon dioxide) for oxidative dissolution of metals in different application areas. The reaction mechanisms of dissolution processes are discussed for various lixiviant systems which generally consist of oxidizing agents, chelating agents, and solvents. Different oxidizing agents for dissolution of metals are reviewed such as halogens, halogenated organics, donor-acceptor electron-transfer systems, polyhalide ionic liquids, and others. Both chemical and electrochemical processes are included. The review can guide researchers to develop more efficient, economic, and environmentally friendly processes for dissolution of metals in their elemental state.

Fluorine-free organic electrolytes for the stable electrodeposition of neodymium metal.

Electrowinning is regarded as a clean process to recover neodymium metal from secondary sources such as spent Nd-Fe-B permanent magnets, but the current methods are severely limited by a high energy consumption (molten salts), or by the high costs and environmental impact of the electrolyte components (ionic liquids). Therefore, there is a demand for more sustainable electrowinning methods for the recovery of neodymium metal. Inspired by our own previous work and the work of others, we developed new fluorine-free organic electrolytes that enable the electrodeposition of neodymium metal at room temperature. The electrolytes consist of solvated neodymium borohydride, Nd(BH4)3, dissolved in the ether solvents tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), 1,2-dimethoxyethane (DME) and diethylene glycol dimethyl ether (diglyme, G2), and these complexes can be prepared entirely from non-fluorinated precursors such as neodymium(III) chloride (NdCl3) and sodium borohydride (NaBH4). In contrast to our previous bis(trifluoromethylsulfonyl)imide-containing electrolytes, electrodeposition of neodymium proceeds over time without significant loss of current density, indicating a higher stability against unwanted side-reactions that lead to passivation of the deposit on the electrode. Characterization of the deposits by scanning electron microscopy (SEM), energy-dispersive X-ray fluorescence (EDX), and X-ray photoelectron spectroscopy (XPS) unambiguously indicated the presence of neodymium metal.

Electrochemical behavior and electrodeposition of gallium in 1,2-dimethoxyethane-based electrolytes.

The electrochemical behavior and electrodeposition of gallium was studied in a non-aqueous electrolyte comprising of gallium(iii) chloride and 1,2-dimethoxyethane (DME). Electrochemical quartz crystal microbalance (EQCM) and rotating ring disk electrode (RRDE) measurements indicate that reduction of gallium(iii) is a two-step process: first from gallium(iii) to gallium(i), and then from gallium(i) to gallium(0). The morphology and elemental composition of the electrodeposited layer were examined using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). Metallic gallium was deposited as spheres with diameters of several hundred nanometers that were stacked on top of each other. X-ray photoelectron spectroscopy (XPS) revealed that each gallium sphere was covered by a thin gallium oxide shell. Electrochemical experiments indicated that these oxide layers are electrically conductive, as gallium can be electrodeposited and partially stripped on or from the layer of spheres below. This was further evidenced by simultaneous electrodeposition of gallium and indium, using indium as a tracer. Electrodeposition of gallium from an O2-containing electrolyte resulted in spheres with smaller diameters. This was due to the formation thicker oxide shells, through which diffusion of gallium atoms that were electrodeposited on the surface, was slower. The concentration of gallium adatoms on top of the gallium spheres to form a new sphere therefore reaches the critical concentration for nucleating a new gallium sphere sooner, leading to smaller spheres.

Electrodeposition of neodymium and dysprosium from organic electrolytes.

A new class of organic electrolytes has been developed for the electrodeposition of rare-earth metals at room temperature. These electrolytes consist of a rare-earth bis(trifluoromethylsulfonyl)imide or chloride salt and a borohydride salt, dissolved in the ether solvents 1,2-dimethoxyethane or 2-methyltetrahydrofuran. In these electrolytes, a soluble lanthanide(iii) borohydride complex [Ln(BH4)4]- is formed, which allows for the electrodeposition of neodymium- or dysprosium-containing layers. The electrochemistry of these electrolytes was characterized by cyclic voltammetry. The deposits were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray fluorescence (EDX) and X-ray photoelectron spectroscopy (XPS), and the results suggest the presence of metallic neodymium and dysprosium.

Cation Effect of Chloride Salting Agents on Transition Metal Ion Hydration and Solvent Extraction by the Basic Extractant Methyltrioctylammonium Chloride.

The addition of a nonextractable salt has an important influence on the solvent extraction of metal ions, but the underlying principles are not completely understood yet. However, relating solute hydration mechanisms to solvent extraction equilibria is key to understanding the mechanism of solvent extraction of metal ions as a whole. We have studied the speciation of Co(II), Zn(II), and Cu(II) in aqueous solutions containing different chloride salts to understand their extraction to the basic extractant methyltrioctylammonium chloride (TOMAC). This includes the first speciation profile of Zn(II) in chloride media with the three Zn(II) species [Zn(H2O)6]2+, [ZnCl3H2O]-, and [ZnCl4]2-. The observed differences in extraction efficiency for a given transition metal ion can be explained by transition metal ion hydration due to ion-solvent interactions, rather than by ion-solute interactions or by differences in speciation. Chloride salting agents bearing a cation with a larger hydration Gibbs free energy reduce the free water content more, resulting in a lower hydration for the transition metal ion. This destabilizes the transition metal chloro complex in the aqueous phase and increases the extraction efficiency. Salting agents with di- and trivalent cations reduce the transition metal chloro complex hydration less than expected, resulting in a lower extraction efficiency. The cations of these salting agents have a very large hydration Gibbs free energy, but the overall hydration of these salts is reduced due to significant salt ion pair formation. The general order of salting-out strength for the extraction of metal ions from chloride salt solutions is Cs+ < Rb+ < NH4+ ≈ K+ < Al3+ ≈ Mg2+ ≈ Ca2+ ≈ Na+ < Li+. These findings can help in predicting the optimal conditions for metal separation by solvent extraction and also contribute to a broader understanding of the effects of dissolved salts on solutes.

Indium electrodeposition from indium(iii) methanesulfonate in DMSO.

The electrochemical behavior and electrodeposition of indium was investigated at 26 °C and 160 °C from a solution composed of indium(iii) methanesulfonate and dimethylsulfoxide (DMSO). Indium(iii) methanesulfonate was synthesized from indium(iii) oxide and methanesulfonic acid (MSA). Cyclic voltammetry, quartz crystal microbalance measurements and rotating ring disk electrode experiments indicated that reduction of indium(iii) to both indium(i) and indium(0) occurs. Yet, reduction to metallic indium was found to be the predominant process. Deposited indium could be stripped to indium(i). This unstable species disproportionated to indium(iii) and indium(0), leading to the formation of micron-sized metallic indium particles in the electrolyte. At 26 °C, indium deposited on glassy carbon as smooth, flat films whereas at 160 °C, it deposits as droplets.

Physicochemical study of diethylmethylammonium methanesulfonate under anhydrous conditions.

The protic ionic liquid diethylmethylammonium methanesulfonate ([DEMA][OMs]) was analyzed in depth by differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, Raman spectroscopy, and broadband dielectric spectroscopy (BDS) under anhydrous conditions. Karl Fischer titration, NMR, and FT-IR spectra confirmed the high purity of [DEMA][OMs]. The melting point (37.7 °C) and the freezing point (14.0 °C) obtained by DSC agree well with the values determined by BDS (40.0 °C and 14.0 °C). The dc conductivity (σdc) above the melting/freezing point obeys the Vogel-Fulcher-Tammann (VFT) equation well, and thus, the proton conduction in [DEMA][OMs] is assumed to be dominated by the vehicle mechanism. In contrast, the σdc below the melting/freezing point can be fitted by the Arrhenius equation separately, and therefore, the proton conduction is most likely governed by the proton hopping mechanism. The non-negligible influence of previously reported low water content on the physicochemical properties of [DEMA][OMs] is found, indicating the importance of reducing water content as much as possible for the study of "intrinsic" properties of protic ionic liquids.

Enhancing Metal Separations by Liquid-Liquid Extraction Using Polar Solvents.

The less polar phase of liquid-liquid extraction systems has been studied extensively for improving metal separations; however, the role of the more polar phase has been overlooked for far too long. Herein, we investigate the extraction of metals from a variety of polar solvents and demonstrate that, the influence of polar solvents on metal extraction is so significant that extraction of many metals can be largely tuned, and the metal separations can be significantly enhanced by selecting suitable polar solvents. Furthermore, a mechanism on how the polar solvents affect metal extraction is proposed based on comprehensive characterizations. The method of using suitable polar solvents in liquid-liquid extraction paves a new and versatile way to enhance metal separations.

Model for Metal Extraction from Chloride Media with Basic Extractants: A Coordination Chemistry Approach.

The metal extraction mechanism of basic extractants is typically described as an anion exchange process, but this mechanism does not correctly explain all observations. This paper introduces a novel model for the extraction of metals by basic extractants from chloride media supported by experimental data on methyltrioctylammonium chloride and Aliquat 336 chloride systems. This model relies on the hypothesis that the metal species least stabilized in the aqueous phase by hydration (i.e., the metal species with the lowest charge density) is extracted more efficiently than the more water stabilized species (i.e., species with higher charge densities). Once it is transferred to the organic phase, the extracted species can undergo further Lewis acid-base adduct formation reactions with the chloride anions available in the organic phase to form negatively charged chloro complexes, which than associate with the organic cations. Salting-out agents influence the extraction, most likely by decreasing the concentration of free water molecules, which destabilizes the metal complex in the aqueous phase. The evidence provided includes (1) the link between extraction and transition-metal speciation, (2) the trend in extraction efficiency as a function of the concentration of different salting-out agents, and (3) the behavior of HCl in the extraction system. The proposed extraction model better explains the experimental observations in comparison to the anion exchange model and allows the prediction of optimal conditions for metal extractions and separations a priori, by selecting the most suitable salting-out agent and its concentration.

Synthesis of Guerbet ionic liquids and extractants as ß-branched biosourceable hydrophobes.

This study investigates the synthesis of ß-branched amines and ß-branched quaternary ammonium chloride ionic liquids as novel extractants. The synthesis methodology was tailored to facilitate the reaction scale-up and the use of biorenewable starting materials. The developed process is an overall green, easy and straightforward synthesis of ß-branched amines, and ammonium salts, starting from linear aldehydes. In order to evaluate the potential of the synthesised materials in applications, the rheology, density, thermal stability, chemical stability, phase transitions, and mutual solubility with water of the novel extractants was studied.

Solvation structure of poly-m-phenyleneisophthalamide (PMIA) in ionic liquids.

Polyaramids are a class of high-performance polymers, known for their high mechanical strength and chemical and thermal stability. Their ability to create a network of intermolecular hydrogen bonds causes them to be very poorly soluble in conventional solvents. Hazardous solvents such as N-methylpyrrolidone (NMP) and dimethylacetamide (DMA), in combination with an inorganic salt such as CaCl2, are currently used for the synthesis and processing of polyaramids. Ionic liquids are proposed as suitable greener alternatives. In this work, we studied the solubility and dissolution mechanism of the meta-oriented polyaramid poly-m-phenyleneisophthalamide (PMIA) in a wide range of ionic liquids. It was found that, similarly to cellulose, PMIA could be dissolved readily and in large amounts in ionic liquids containing a strongly coordinating anion (such as chloride, acetate and dialkylphosphate) and an imidazolium cation. Hydrogen bonding between the anion and the amide NH of PMIA is the main solvent-solute interaction. An odd-even effect in solubility occurred when altering the length of the side chains on the imidazolium cation. Furthermore, it was found that the presence of hydrogen bond donating CH moieties on the cation is a necessary condition for dissolution. The exact role of these hydrogen bond donors was investigated by FTIR and 13C NMR spectroscopy. It was found that there is no significant interaction between the hydrogen atoms of the imidazolium ring and the amide carbonyl groups. Rather, the hydrogen bond donors are needed to stabilize the solvation shell around PMIA through alternating cation-anion interactions.

Low-Temperature Oxidation of Fine UO2 Powders: Thermochemistry and Kinetics.

The thermochemical behavior of low-temperature oxidation in fine UO2 powders has been investigated by simultaneous thermogravimetric analysis and differential scanning calorimetry. The evaluation of the thermochemical and kinetic data reveals a complex interplay between different mechanisms. The initial reaction concerns the rapid chemisorption of oxygen gas onto the surface of UO2 grains, having an activation energy of only 13.1 ± 0.6 kJ mol-1. The subsequent oxidation at temperatures between 40 and 100 °C occurs first at the surface via a field-assisted mechanism, which progresses via domain growth into the bulk. At more elevated temperatures, thermally activated diffusion becomes the dominant mechanism.

Ionic Liquid Crystals: Versatile Materials.

This Review covers the recent developments (2005-2015) in the design, synthesis, characterization, and application of thermotropic ionic liquid crystals. It was designed to give a comprehensive overview of the "state-of-the-art" in the field. The discussion is focused on low molar mass and dendrimeric thermotropic ionic mesogens, as well as selected metal-containing compounds (metallomesogens), but some references to polymeric and/or lyotropic ionic liquid crystals and particularly to ionic liquids will also be provided. Although zwitterionic and mesoionic mesogens are also treated to some extent, emphasis will be directed toward liquid-crystalline materials consisting of organic cations and organic/inorganic anions that are not covalently bound but interact via electrostatic and other noncovalent interactions.

Thermal stability of trihexyl(tetradecyl)phosphonium chloride.

Dynamic TGA studies of phosphonium ionic liquids have reported thermal stabilities of 300 °C or higher for these compounds. This is often an overestimation of the real thermal stability. The chosen technique as well as the experimental parameters can influence the thermal stability. In this paper, the thermal stability of commercially available Cyphos IL 101 is studied. The effect of the nature of the atmosphere (air or inert gas), the purity of the sample, the heating rate and presence of a metal on the short-term and long-term stability of commercial Cyphos IL 101 is investigated. The thermal decomposition products are characterized using thermogravimetric analysis coupled to mass spectrometry (TGA-MS). Impurities present and higher heating rates lead to an under- and overestimation of the thermal stability, respectively. The presence of oxygen leads to a lower thermal stability. In contrast, adding metal chlorides to the ionic liquid causes an increase in the thermal stability. The chloride anions are coordinated to the metal ion, so that the Lewis basicity of the anions is reduced. Also this paper gives insights in the behavior of Cyphos IL 101 at high temperatures, which is of relevance for possible application of this ionic liquid in high-temperature industrial processes.

Direct Analysis of Metal Ions in Solutions with High Salt Concentrations by Total Reflection X-ray Fluorescence.

Total reflection X-ray fluorescence (TXRF) is becoming more and more popular for elemental analysis in academia and industry. However, simplification of the procedures for analyzing samples with complex compositions and residual matrix effects is still needed. In this work, the effect of an inorganic (CaCl2) and an organic (tetraalkylphosphonium chloride) matrix on metals quantification by TXRF was investigated for liquid samples. The samples were spiked with up to 20 metals at concentrations ranging from 3 to 50 mg L-1 per element, including elements with spectral peaks near the peaks of the matrix elements or near the Raleigh and Compton scattering peaks of the X-ray source (molybdenum anode). The recovery rate (RR) and the relative standard deviation (RSD) were calculated to express the accuracy and the precision of the measured element concentrations. In samples with no matrix effects, good RRs are obtained regardless of the internal standard selected. However, in samples with moderate matrix content, the use of an optimum internal standard (OIS) at a concentration close to that of the analyte significantly improved the quantitative analysis. In samples with high concentrations of inorganic ions, using a Triton X-100 aqueous solution to dilute the sample during the internal standardization resulted in better RRs and lower RSDs compared to using only water. In samples with a high concentration of organic material, pure ethanol gave slightly better results than when a Triton X-100-ethanol solution was used for dilution. Compared to previous methods reported in the literature, the new sample-preparation method gave better accuracy, precision, and sensitivity for the elements tested. Sample dilution with an OIS and the surfactant Triton X-100 (inorganic media) or ethanol (organic media) is recommended for fast routine elemental determination in matrix containing samples, as it does not require special equipment, experimentally derived case-dependent mathematical corrections, or physicochemical removal of interfering elements.

Liquid Nickel Salts: Synthesis, Crystal Structure Determination, and Electrochemical Synthesis of Nickel Nanoparticles.

New nickel-containing ionic liquids were synthesized, characterized and their electrochemistry was investigated. In addition, a mechanism for the electrochemical synthesis of nanoparticles from these compounds is proposed. In these so-called liquid metal salts, the nickel(II) cation is octahedrally coordinated by six N-alkylimidazole ligands. The different counter anions that were used are bis(trifluoromethanesulfonyl)imide (Tf2 N(-) ), trifluoromethanesulfonate (OTf(-) ) and methanesulfonate (OMs(-) ). Several different N-alkylimidazoles were considered, with the alkyl sidechain ranging in length from methyl to dodecyl. The newly synthesized liquid metal salts were characterized by CHN analysis, FTIR, DSC, TGA and viscosity measurements. An odd-even effect was observed for the melting temperatures and viscosities of the ionic liquids, with the complexes with an even number of carbon atoms in the alkyl chain of the imidazole having a higher melting temperature and a lower viscosity than the complexes with an odd number of carbons. The crystal structures of several of the nickel(II) complexes that are not liquid at room temperature were determined. The electrochemistry of the compounds with the lowest viscosities was investigated. The nickel(II) cation could be reduced but surprisingly no nickel deposits were obtained on the electrode. Instead, nickel nanoparticles were formed at 100 % selectivity, as confirmed by TEM. The magnetic properties of these nanoparticles were investigated by SQUID measurements.

Assessment of the U3O7 Crystal Structure by X-ray and Electron Diffraction.

Polycrystalline U3O7 powder was synthesized by oxidation of UO2 powder under controlled conditions using in situ thermal analysis, and by heat treatment in a tubular furnace. The O/U ratio of the U3O7 phase was measured as 2.34 ± 0.01. The crystal structure was assessed from X-ray diffraction (XRD) and selected-area electron diffraction (SAED) data. Similar to U4O9-ε (more precisely U64O143), U3O7 exhibits a long-range ordered structure, which is closely related to the fluorite-type arrangement of UO2. Cations remain arranged identical to that in the fluorite structure, and excess anions form distorted cuboctahedral oxygen clusters, which periodically replace the fluorite anion arrangement. The structure can be described in an expanded unit cell containing 15 fluorite-like subcells (U15O35), and spanned by basis vectors A = ap - 2bp, B = -2ap + bp, and C = 3cp (lattice parameters of the subcell are ap = bp = 538.00 ± 0.02 pm and cp = 554.90 ± 0.02 pm; cp/ap = 1.031). The arrangement of cuboctahedra in U3O7 results in a layered structure, which is different from the well-known U4O9-ε crystal structure.

Low-Temperature Oxidation of Fine UO2 Powders: A Process of Nanosized Domain Development.

The nanostructure and phase evolution in low-temperature oxidized (40-250 °C), fine UO2 powders (<200 nm) have been investigated by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM). The extent of oxidation was also measured via in situ thermogravimetric analysis. The oxidation of fine powders was found to proceed differently as compared to oxidation of coarse-grained UO2. No discrete surface oxide layer was observed and no U3O8 was formed, despite the high degree of oxidation (up to O/U = 2.45). Instead, nanosized (5-15 nm) amorphous nuclei (interpreted as amorphous UO3), unmodulated and modulated U4O9, and a continuous range of U3O7-z phases with varying tetragonal distortion (c/a > 1) were observed. Oxidation involves formation of higher uranium oxides in nanodomains near the grain surface which, initially, have a disordered defect structure ("disordered U4O9"). As oxidation progresses, domain growth increases and the long-period modulated structure of U4O9 develops ("ordered U4O9"). A similar mechanism is understood to happen also in U3O7-z.

Magnetomigration of rare-earth ions in inhomogeneous magnetic fields.

The effects of external inhomogenous (gradient) magnetic fields on the movement of the rare-earth ions: Dy3+, Gd3+ and Y3+, in initially homogeneous aqueous solutions have been investigated. Differences in the migration of rare-earth ions in gradient magnetic fields were observed, depending on the magnetic character of the ions: paramagnetic ions of Dy3+ and Gd3+ move towards regions of the sample where the magnetic field gradient is the strongest, while diamagnetic ions of Y3+ move in the opposite direction. It has been showed that the low magnetic field gradients, such the ones generated by permanent magnets, are sufficient to observe the magnetomigration effects of the ions in solution. The present work clearly establishes the behavior of magnetically different ions in initially homogeneous aqueous solutions exposed to magnetic field gradients. To this avail, a methodology for measuring the local concentration differences of metal ions in liquid samples was developed.

Efficient separation of transition metals from rare earths by an undiluted phosphonium thiocyanate ionic liquid.

The ionic liquid trihexyl(tetradecyl)phosphonium thiocyanate has been used for the extraction of the transition metal ions Co(ii), Ni(ii), Zn(ii), and the rare-earth ions La(iii), Sm(iii) and Eu(iii) from aqueous solutions containing nitrate or chloride salts. The transition metal ions showed a high affinity for the ionic liquid phase and were efficiently extracted, while the extraction efficiency of the rare-earth ions was low. This difference in extraction behavior enabled separation of the pairs Co(ii)/Sm(iii), Ni(ii)/La(iii) and Zn(ii)/Eu(iii). These separations are relevant for the recycling of rare earths and transition metals from samarium cobalt permanent magnets, nickel metal hydride batteries and lamp phosphors, respectively. The extraction of metal ions from a chloride or nitrate solution with a thiocyanate ionic liquid is an example of "split-anion extraction", where different anions are present in the aqueous and ionic liquid phase. Close to 100% loading was possible for Co(ii) and Zn(ii) up to a concentration of 40 g L(-1) of the transition metal salt in the initial aqueous feed solution, whereas the extraction efficiency for Ni(ii) gradually decreased with increase in the initial feed concentration. Stripping of Co(ii), Zn(ii) and Ni(ii) from the loaded ionic liquid phase was possible by a 15 wt% NH3 solution. The ionic liquid could reused after extraction and stripping.