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This book comprehensively details the applications of ionic liquids in rare earth green separation and utilization based on the unique interactions of ionic liquids with rare earth ions. It consists of nine chapters demonstrating the synthesis and properties of ionic liquids, coordination chemistry of ionic liquids and rare earth, ionic liquids as diluents, extractants, adsorption resins for rare earth extraction and separation, electrodeposition of rare earth metals in ionic liquids, and preparation of rare earth material with the aid of ionic liquids. It is both interesting and useful to chemists, metallurgists and graduate students working on fundamental research of ionic liquids as well as professionals in the rare earth industry. It provides considerable insights into green chemistry and sustainable processes for rare earth separation in order to meet the environmental challenge of rare earth metallurgy around the globe, especially in China.

Ji Chen is a Professor of Chemistry at the Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, China.





Chapter 1. Ionic Liquids in the Context of Rare Earth Separation and Utilization

With the rapid development of science and technology, rare earths (REs) are playing a vital role in economic and social development due to their unique electronic structure and physicochemical properties. REs are widely used in lighting, electrical, and magnetic field as a crucially strategic resource. Being the upstream of the RE industry supply chain, RE separation is crucial throughout the whole industry. The primary issues about RE separation are reviewed in this chapter. Conventional organic solvent extractants expose some inherent questions in the long-term application, while ionic liquids as “green solvents” show great potential in the separation processes. This chapter also overviewed RE resources in the world and the issues associated with these separation processes. A brief introduction to ionic liquid-based RE functional materials has been also made.
Ji Chen, Jiangling Gao

Chemistry of Ionic Liquids with Rare Earth


Chapter 2. Using Crystal Structures of Ionic Compounds to Explore Complexation and Extraction of Rare Earth Elements in Ionic Liquids

Despite only being able to directly measure the solid state, single crystal X-ray diffraction is of use in understanding how the fully ionic environment of an ionic liquid (IL) affects complexation of rare earth element ions. Here we examine crystal structures of ionic lanthanide complexes as case studies in this context. The complexation of a dithiophosphate IL is compared to non-IL systems, where the crystal structure illustrates the formation of hydrophobic domains despite the presence of highly charged ions. The crystal structure of a lanthanum complex with 1,2-di(4-pyridyl)ethylene illustrates how a large, neutral ligand in a fully ionic system interrupts the ionic packing, leading to the inclusion of noncoordinating ligands in the outer coordination sphere. The crystal structure of a salt composed of discrete terbium and thorium complex ions illustrates how in two metal ions in a fully ionic environment need not be bridged directly with a ligand but can be linked entirely through noncovalent interactions. Because ILs are fully ionic systems, these effects can be interpreted in the context of rare earth element behavior in ILs.
Steven P. Kelley, J. Seth Nuss, Robin D. Rogers

Ionic Liquids for the Extraction and Separation of Rare Earth


Chapter 3. Separating Rare-Earth Elements with Ionic Liquids

The rare-earth elements (REEs) are a group of 17 chemically similar metallic elements; this group consists of scandium, yttrium, and 15 lanthanides. Due to their essential role in permanent magnets, lamp phosphors, catalysts, and rechargeable batteries, the REEs have become an essential component of the global transition to a green economy. Currently, with China producing over 90 % of the global REE output and its increasingly tightening export quota, the rest of the world is confronted with the potential risk of REE shortage. As such, many countries will have to rely on recycling REEs from pre-consumer scrap, industrial residues, and REE-containing end-of-life products. Over the course of the last two decades, ionic liquids have been increasingly used to separate REEs in the recycling process. Ionic liquids (ILs) are a class of molten salts that are liquid at temperatures below 100 °C. ILs are amenable to the recycling of REEs because the cation and anion components are readily tailored to a given process, and they offer numerous advantages over typical organic solvents, such as low volatility, low flammability, a broad temperature range of stability, the ability to dissolve both inorganic and organic compounds, high conductivity, and wide electrochemical windows. In this chapter, we discuss the performance of several IL-based extraction systems used to separate and recycle REEs.
Nada Mehio, Huimin Luo, Chi-Linh Do-Thanh, Xiaoqi Sun, Yinglin Shen, Jason R. Bell, Sheng Dai

Chapter 4. Ionic Liquid-Based Extraction and the Application to Liquid Membrane Separation of Rare Earth Metals

Separation and recycling of rare-earth metals are attracting continuous attention worldwide. Liquid-liquid extraction is a conventionally employed technique for the separation of rare-earth metals. In recent years, growing attention has focused on room temperature ionic liquids as alternatives to conventional organic solvents.
The development of an efficient extraction system based on the ionic liquids (ILs) depends on the employment of an appropriate combination of the extractant and an ionic liquid. In the IL-based extraction system with neutral extractants such as CMPO and TOPO dissolved in the imidazolium ILs, the extraction efficiency and selectivity for rare-earth metals are greatly improved compared to that in an organic solvent system, and the stripping, however, is unfavorably difficult. The use of acidic extractants such as PC-88A is limited in the poor solubility in ILs, though the extraction is controllable by the acid concentration in the aqueous phase.
An extractant, recently developed N,N-dioctyldiglycol amic acid (DODGAA), seems to combine the advantages of a neutral and an acidic extractant. That is, DODGAA is soluble in the ILs and shows the high separation performance for rare-earth metals compared to that in n-dodecane. Furthermore, the stripping is possible by an acid solution. Due to the extremely high affinity of DODGAA for rare-earth metals, the extraction system is applicable in the recycling of rare-earth metals. Taking advantage of the feature of ILs, a supported liquid membrane system can be constructed using DODGAA as a carrier.
In this chapter, we summarize the IL-based extraction and separation of rare-earth metals and the application to the advanced technique, focusing on our research results.
Fukiko Kubota, Jian Yang, Masahiro Goto

Chapter 5. Application of Ionic Liquid Extractants on Rare Earth Green Separation

A green solvent extraction process for the separation of rare earth elements (REEs) in different media (such as hydrochloric acid media, nitric acid media, and sulfuric media) was developed using bifunctional ionic liquid extractants (Bif-ILEs) especially containing quaternary ammonium and phosphonium-based ILs. The characteristics of the Bif-ILEs are simple synthesis, high extraction ability and selectivity, low extraction acid and base consumption, and easy stripping. Bif-ILEs could be synthesized by acid/base neutralization, and this reaction process was under mild condition with easier purification. The extraction thermodynamics showed that the separation factor (β) values for REEs were changed with the different Bif-ILEs or reaction media. The inner synergistic effect between cations and anions in Bif-ILEs has greatly facilitated the β values of REEs and also avoided the production of ammonia nitrogen caused by the acidity extractant saponification. The kinetic experimental data on the separation of REEs by Bif-ILEs, with a constant interfacial area cell with laminar flow, showed that there exist real possibilities for increasing the efficiency of the separation by the use of kinetic factors. The kinetics model could be deduced from the rate-controlling step. Serving as extractants, additives, and templating materials, ILs were used in the preparation of separation materials, including silica-supported ionic liquids (S-SILs), membrane-supported ionic liquids (M-SILs), and polymer-supported ionic liquids (P-SILs). ILs were usually attached or doped into the solid support and offered high extraction efficiency and excellent stability for REEs separation. Using such separation materials may indeed enable us to substantially reduce the steps for the separation of REEs and thus decrease both the separation time and waste production.
Hualing Yang, Ji Chen, Hongmin Cui, Wei Wang, Li Chen, Yu Liu

Electrodeposition of Rare Earth Metal in Ionic Liquids


Chapter 6. Electrodeposition of Rare Earth Metal in Ionic Liquids

It is very important to develop the recovery process of Nd and Dy metals using ionic liquids from a standpoint of establishing an environmental harmonization system and a recycling-oriented society. For this purpose, the solubility, the solvation structures, the electrochemical behaviors, the diffusive properties, the nucleation, and the electrodeposition behaviors were investigated for Nd(III) and Dy(III) in ionic liquids on this study.
As for the solvation analysis of Nd(III) and Dy(III) by Raman spectroscopy, the solvation numbers of Nd(III), Dy(II), and Dy(III) in [P2225][TFSA] were 5.1, 3.8, and 5.0, respectively. These results also revealed that the solvation structures of Nd(III), Dy(II), and Dy(III) were [Nd(TFSA)5]2−, [Dy(TFSA)4]2−, and [Dy(TFSA)5]2−, respectively.
According to the electrochemical analyses with semi-integral and semi-differential methods, the reduction process of [Nd(TFSA)5]2− or [Dy(TFSA)5]2− proceeded in one step, [Nd(TFSA)5]2− + 3e → Nd(0) + 5[TFSA], or two steps, [Dy(TFSA)5]2− + e → [Dy(TFSA)4]2− + [TFSA] and [Dy(TFSA)4]2− + 2e → Dy(0) + 4[TFSA], respectively. The activation energies of the diffusion coefficients for [Nd(TFSA)5]2− and [Dy(TFSA)5]2− were 52.8 and 53.4 kJ mol−1, respectively, and these behaviors were related to the similar solvation structures. The nucleation behaviors of [Nd(TFSA)5]2− and [Dy(TFSA)5]2− were altered from instantaneous to progressive nucleation by increasing the overpotential. The potentiostatic electrodepositions of Nd and Dy were also carried out and the recovered blackish electrodeposits were Nd and Dy metals evaluated by XPS. Finally, we demonstrated that the recovery process of Nd metal from spent Nd–Fe–B magnets by wet separation and electrodeposition using ionic liquids was effective.
Masahiko Matsumiya

Utilization of Ionic Liquids on Rare Earth Materials


Chapter 7. Ionic Liquids and Rare Earth Soft Luminescent Materials

Soft luminescent materials resulting from the introduction of rare earth compounds to ionic liquids favorably combine the properties of ionic liquids with unique optical properties of rare earth compounds such as sharp emission band, long decay time of the excited state, and the tunable emission color over the entire spectral range of interest from UV to infrared spectral region. This chapter summarizes the progress ever made in such kind of soft luminescent materials, especially those that have been made in our research group. Our emphasis is put on the preparation strategies for the soft luminescent materials, the luminescent ionogels prepared from the immobilization of rare earth-containing ionic liquids by organosilica and polymers, and the unexpected luminescent enhancement of Eu3+-β-diketonate complexes hosted in zeolite L nanocrystals and nanoclay upon addition of ionic liquid bearing bulky triethoxysilane groups in water.
Huanrong Li, Yige Wang, Tianren Wang, Zhiqiang Li

Chapter 8. Photofunctional Rare Earth Materials Based on Ionic Liquids

This chapter mainly focuses on recent research progress in photofunctional rare earth materials based on ionic liquids, with an emphasis on the photofunctional rare earth hybrid materials using ionic liquid compounds as both double functional linkers and matrices. It covers photofunctional rare earth compounds with ionic liquids, photofunctional rare earth compounds dispersed in ionic liquids, and photofunctional rare earth hybrid materials based on ionic liquid with special groups (organically modified siloxanes, carboxyl groups, and thiol groups). Herein we focus on the work of our group in the recent years.
Bing Yan

Chapter 9. Ionic Liquid-Assisted Hydrothermal Synthesis of Rare Earth Luminescence Materials

The shape control of nano- and microcrystals has received considerable attention because the morphology, dimensionality, and size of materials are well known to have great effects on their physical and chemical properties, as well as on their applications in optoelectronic devices. Ionic liquids were found to be very advantageous in synthetic nanochemistry, especially as templates and capping agents. Their particular phase behavior and unique physicochemical properties, including complex solvation interactions with organic and inorganic compounds, can provide various growth pathways for nanocrystals with novel morphologies and properties. This chapter gives an overview of existing ionic liquid-assisted hydrothermal method for the synthesis of rare earth luminescence materials. Particular attention is given to synthesis mechanism and the role of ionic liquids in the formation of specific morphology, as well as the effect of morphology on the luminescent properties.
Yanhua Song, Yuefeng Deng, Ji Chen, Haifeng Zou
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