Sandia National Laboratories intern Kadie Marie Mei Sanchez conducts research on rare-earth element separation using advanced analytical equipment. [Craig Fritz/Sandia National Laboratories]
Exploiting metal-organic frameworks
The crux of the new research focuses on exploiting metal-organic frameworks (MOFs), a class of customizable, porous materials, to selectively extract rare-earth elements from aqueous solutions. Currently, the purification of these elements, found in everything from smartphones and LED lights to electric vehicles and wind turbines, relies on harsh acids and hazardous solvents.
“We synthesized MOFs with variable surface chemistry and were able to show through adsorption experiments that these MOFs can pick out rare-earth elements from a mixture of other metals,” said Anastasia Ilgen, a Sandia geochemist and project lead.
Sandia’s team, under the leadership of geochemist Anastasia Ilgen and materials chemist Dorina Sava Gallis, is designing MOFs with specific surface chemistries and pore sizes to act as selective “sponges” for individual rare-earth elements. To aid this design process, computational materials scientist Kevin Leung employs advanced modeling techniques, including molecular dynamics simulations and density functional theory calculations. These simulations help understand the atomic-level interactions between rare-earth elements and various MOF structures, guiding the team towards creating MOFs with enhanced selectivity and efficiency.
Next focus areas
Further analysis of these interactions is conducted using X-ray absorption fine structure spectroscopy at Argonne National Laboratory. This technique allows Ilgen to directly observe the chemical bonding between rare-earth elements and specific sites within the MOFs, such as the metal hubs or the attached chemical groups. These observations provide information for optimizing the MOF design for targeted rare-earth element capture. For example, Ilgen’s work revealed that rare-earth elements can bind to both the metal hubs and specific surface groups like phosphonates.
Ultimately, this research aims to develop a viable alternative for rare-earth element purification. By precisely tailoring the MOF structure and surface chemistry, the team seeks to create materials capable of selectively adsorbing individual rare-earth elements from complex mixtures. Successful development of this technology could lead to a more environmentally benign and potentially cost-effective purification process. It could also help decrease reliance on current methods that employ harsh chemicals and contribute to geopolitical supply chain concerns. Future research will focus on exploring different MOF designs, including incorporating mixed-metal hubs and fine-tuning pore dimensions.
