Rare-earth elements are essential components of many modern technologies and are commonly found in devices such as smartphones, LED lightbulbs, electric vehicles, and wind turbines. However, the process of purifying these critical metals from ores is often conducted using environmentally harmful methods involving strong acids and hazardous solvents, primarily in China. To address this issue, a team of researchers from Sandia National Laboratories has been working on developing a more environmentally friendly method to separate rare-earth elements from complex mixtures.
The researchers began by creating and modifying metal-organic frameworks (MOFs), which are tinker-toy-like molecules, to test their ability to adsorb rare-earth elements. Using computer simulations and X-ray experiments, the team investigated how the rare-earth elements interacted with the synthesized “sponges.” Their goal was to design sponges that could selectively absorb one rare-earth metal while excluding others. The team’s findings were recently published in a series of scientific papers, including one in the scientific journal ACS Applied Materials and Interfaces.
The researchers selected two zirconium-based MOFs for the project, which are highly stable in water and easily adjustable. By adding different chemical groups within the MOFs, the researchers were able to modify their properties and engineer structures with missing rods. Through their experiments, the team found that the MOFs with missing linkers were more effective at binding rare-earth elements compared to those without missing linkers. Furthermore, incorporating negatively charged chemical groups into the MOFs improved the adsorption of all metals.
To further guide the design of MOFs selective for specific rare-earth metals, a computational materials scientist performed molecular dynamics simulations and density functional theory modeling. The simulations revealed that rare-earth elements show a preference for negatively charged chemicals over water, especially for heavier rare-earth elements. The goal was to find a chemical that would allow selective binding of one metal, but no uniform trend was observed, suggesting the need for further research and experimentation.
Using X-ray spectroscopy, the researchers examined the chemical environment of rare-earth elements in zirconium-based and chromium-based MOFs. They observed how the rare-earth elements interacted with the metal hubs and surface groups within the MOFs, providing valuable insights into the adsorption mechanisms. The team proposed several design strategies for creating MOFs that selectively adsorb specific rare-earth elements, including tuning the chemistry of the metal hub and adjusting the pore dimensions of the MOF structure.
Overall, the research conducted by the team at Sandia National Laboratories represents a significant step towards developing environmentally friendly methods for separating rare-earth elements from complex mixtures. By gaining a deeper understanding of the interactions between rare-earth elements and MOFs, the team has laid the groundwork for designing selective sponges that can effectively extract specific metals while excluding others. Their findings hold promise for advancing the field of rare-earth element separation and reducing the environmental impact of their extraction and purification processes.