Thorium metal-organic framework crystallization for efficient recovery from rare earth element mixtures

Rare earth (RE) elements are critical materials that underpin many modern technologies, particularly in the clean energy industry. Despite their importance, these vital resources are difficult to obtain due to the presence of numerous metals and radioactive contaminants, such as thorium, that are present in RE ores. Current processing methods, which are dominated by homogeneous solvent extraction, are inefficient and produce substantial hazardous waste. In this work, we describe an alternative strategy to separate thorium from REs through metal-organic framework (MOF) crystallization. Starting from a mixture of thorium and rare earth ions in solution, we utilize the simple carboxylate ligand trimesic acid to selectively crystallize a novel thorium MOF, NU-2500, leaving the remaining rare earth ions in solution. By leveraging the increased oxophilicity of Th(iv) compared to RE(iii) ions, we observe the exclusive formation of the thermodynamically preferred Th-MOF product. This valence-selective crystallization strategy occurs rapidly (within 30 minutes) at mild temperatures (80 °C) with an environmentally-friendly ethanol/water solvent system to produce phase-pure NU-2500 containing >98% molar fraction of thorium. Sequestering the radioactive Th(iv) ions within a solid framework enables facile separation of REs through simple filtration. We demonstrate that our selective crystallization platform retains its high selectivity for Th crystallization even at low initial Th concentrations and in complex mixtures with multiple different REs. We anticipate that further insights into the kinetics and thermodynamics of MOF crystallization can be applied to additional challenging industrial separations.