Molecular-sized metal-organic frameworks – MOFs – could help in the search for new critical mineral supply sources.
A team of researchers from Sandia National Laboratories in the United States is working on a more straightforward and cleaner way to separate rare earth elements – with synthesized molecular “sponges.” Rare earth elements (REEs) are a set of 17 elements, each with different chemical properties and key ingredients for many high-tech applications. However, they are difficult to separate from each other, and this involves numerous, highly complicated steps. While REEs are, despite their name, not rare on Earth, downstream processing and separation are highly concentrated geographically and currently dominated by China.
For the last three years, the scientists have studied different approaches to separating REEs from each other in water-based mixtures. The team used modified molecules with the technical name metal-organic frameworks, or MOFs. These synthesized miniature “sponges” consist of interchangeable metal hubs and carbon-based rods onto which metal ions can bind. The Sandia team used zirconium and chromium-based MOFs in their research with the goal of finding ways to selectively “filter out” individual rare earth elements. The researchers found that tweaking the MOFs’ chemical makeup could enhance their selectivity for different metals. Experiments showed that certain chemical groups improved metal absorption, while computer simulations and X-ray spectroscopy further clarified how these interactions work. While some avenues were ultimately unsuccessful, the team made promising progress in others and hopes that this research could open new possibilities for designing MOFs to efficiently separate individual REEs in a cleaner and more straightforward way.
More on sponges in material science: Researchers from Northwestern University in Evanston, Illinois, have studied the use of regular sponges coated with nanoparticles that can remove hazardous metals like lead from polluted water, leaving potable water behind. The team hopes to apply the results to recovering critical minerals in the future.
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