Separation of Nd³⁺ and Dy³⁺ Ions: A Molecular Approach towards Rare Earth Sustainability

The ongoing transition to green technologies demands for the development of new materials and systems to minimize our environmental impact. Rare Earth Elements (REEs) are fundamental in this transformation due to their unique magnetic, luminescent and catalytic properties [1]. These elements are essential in a wide range of technological applications, including lighting systems, catalyst production, medical imaging, ceramic and glass materials [2]. Among them, neodymium-iron-boron (NdFeB) permanent magnets play a key role in electronic devices, wind turbines, rechargeable batteries, and electric vehicles [3]. Despite all of the applications, the growing demand for REEs is also associated with significant geopolitical, economic and environmental challenges. The distribution of REEs-rich ores is highly uneven, with the global production dominated by a few countries, resulting in supply chain and market imbalances [4]. Furthermore, the extraction and processing of metals from primary ores can generate a large amount of environmental pollutants and hazardous waste [5]. As a result, the recovery and recycling of REEs from end-of-life (EOL) products is gaining increasing importance as a sustainable strategy to guarantee the supply and reduce the dependence on mining. Our work focuses on the development of a ligand-assisted strategy for the selective precipitation of REEs found in high-performance NdFeB magnets, which typically contain iron (~65%), neodymium (Nd, ~20%), and dysprosium (Dy, ~1%). A family of Salpen-based ligands were easily synthesized and designed to modify the steric hindrance and lipophilicity near the metal coordination sites, with the aim of influencing the precipitation or the dissolution of the corresponding Nd3+ and Dy3+ complexes in different organic solvents. Structural characterization of the resulting compounds was performed using Single-Crystal X-ray Diffraction (SCXRD), providing insight into the coordination environments and speciation of these metal-ligand systems. Five distinct Dy complexes were isolated and characterized, revealing different stoichiometries and nuclearities which were consistent with the speciation observed in solution through titration experiments followed by UV-vs spectroscopy. Promising results for the selective separation of rare earth cations were obtained by exploiting the ligand steric hindrance in combination with the subtle chemical differences between Nd3+ and Dy3+, such as their ionic radii and coordination number. Overall, this ligand-controlled separation approach offers a promising pathway toward a more efficient and scalable recycling of REEs from waste products. [1] S. Cotton, Lanthanide and Actinide Chemistry. 2006. [2] M. M Nkiawete, R. L Vander Wal J. Rare Earths, 2025, 43, 1-8. [3] J. J. Croat, J. F. Herbst, R. W. Lee, F. E. Pinkerton, Appl. Phys. Lett., 1984, 44, 148–149. [4] K. Binnemans, P. T. Jones, K. Van Acker, B. Blanpain, B. Mishra, D. Apelian, Jom, 2013, 65, 846–848. [5] K. Binnemans, P. T. Jones, B. Blanpain, T. V. Gerven, X. Yang, A. Walton, M. Buchert, J. Clean. Prod., 2013, 51, 1-22.