(348f) Liquid-Liquid Equilibria of Rare Earth Elements with Protonated Betaine Bis(trifluoromethylsulfonyl)Imide

The demand for the rare earth elements (REEs) has gradually increased and is expected to reach 210,000 tons by 2025 due to their irreplaceable physicochemical properties in a variety of high technologies such as lasers, magnets, batteries, etc., where they are used as alloys or additives. This fact has spurred interest in the development of efficient and environmentally benign extraction processes on a global scale. Nevertheless, their similar physicochemical properties, which originate from their electron configurations, make separation processes from natural resources complicated and challenging.

The current general separation process for REEs has traditionally relied on liquid-liquid extraction since the 1960s, utilizing extractants such as D2EHPA, PC88A, etc. However, this method suffers from poor separation factors, instability of separation materials, and the generation of significant waste material, amounting to over 20 million tons per year. To address these drawbacks, ionic liquid (IL) have been proposed as an alternative due to their stability, low vapor pressure, unique solvent environment, and wide degree of freedom in tuning physicochemical properties by modifying their structure. In recent years, a multitude of ILs, ranging from nonfunctionalized to functionalized with extractants, have been tested and reported. Most of these ILs feature ammonium and phosphonium ions.

In the current work, protonated betaine bis(trifluoromethylsulfonyl)imide ([HBet][TFSI]) was chosen due to its ability to dissolve large quantities of metal oxides. Instead of following the conventional method of adding neutral betaine as an extractant to the system, we focused on providing REEs with the unique solvent environment of IL by dissolving high concentrations of REE oxides in the IL. We considered the system as a mixture of [REE₂(betaine)_y(H₂O)_z][TFSI]₆ and [HBet][TFSI] and are investigating charge transfer and polarizability effects of the IL on the solubility of these mixtures in water. We show that the addition of Ree₂O₃ to the system significantly lowers the upper critical solution temperature (UCST) for liquid-liquid phase separation and have measured the influence on the water/[HBet][Tf₂N] phase behavior through turbidity measurements by adding two different concentrations of La₂O₃, Nd₂O₃, Eu₂O₃, Gd₂O₃, Ho₂O₃, and Yb₂O₃. At the higher concentration, distinct differences are observed in the cloud point curves for the light and heavy REEs, suggesting that liquid-liquid extraction with [HBet][Tf₂N] may be a viable method for separation of REE compounds.