Over the last three years, researchers have been developing a sustainable technique for extracting essential rare-earth elements from liquid mixtures.
Rare-earth elements are increasingly common in our daily lives, appearing in everything from the smart device you’re currently using to the LED bulbs above you and neodymium magnets found in electric cars and wind turbines.
Extracting and refining these vital metals from complex ores is a challenging process that typically requires strong acids and hazardous solvents, primarily carried out in China. A dedicated team from Sandia National Laboratories has been working on an eco-friendly method to extract these rare-earth elements from aqueous mixtures.
The research began with the creation and modification of toy-like molecules known as metal-organic frameworks (MOFs) to assess their ability to absorb these crucial metals. Using computer simulations and X-ray experiments, the researchers studied how these rare-earth elements connect with the specially designed “sponges.” The ultimate aim is to engineer sponges that absorb specific rare-earth metals while leaving others behind. Their research has recently been shared in multiple scientific publications, including one in the journal ACS Applied Materials and Interfaces on August 26.
“We created MOFs with varying surface chemistries and demonstrated through adsorption experiments that these structures can selectively extract rare-earth elements from a mix of other metals,” explained Anastasia Ilgen, a geochemist at Sandia and the project leader. “This selectivity for rare-earths is advantageous. Notably, we showed that the metal absorption capabilities can be fine-tuned with the addition of different chemical groups on their surfaces.”
Creating stable sponges
The researchers focused on two MOFs made of zirconium. These structures are known for their stability in water and their adaptable nature, according to Dorina Sava Gallis, a materials chemist at Sandia involved in the study.
MOFs are composed of metal “hubs” and carbon-based “linker” “rods,” which can be swapped out to form nanosized “sponges” with distinct characteristics. Additionally, chemists can introduce various chemical groups into the MOFs to alter their properties or create structures with absent rods, said Sava Gallis.
In their research, published in the journal Chemical Communications, Sava Gallis and her team investigated two types of MOFs with zirconium hubs. They modified the linkers in one set of building blocks while attaching new groups to the metal hub in another.
The results indicated that MOFs with missing linkers absorbed more of the two rare-earth elements than those without gaps, which was expected. Adding an amino group to the linker had little effect on metal absorption, while incorporating a negatively charged chemical called phosphonate into the linker enhanced the absorption of all metals. Interestingly, in designs where the chemical groups were attached to the metal hubs, the added groups did not significantly affect the absorption of rare-earth elements but substantially increased nickel selectivity over cobalt, according to Sava Gallis.
“Both strategies we used successfully adjusted selectivity for different ions,” Sava Gallis remarked. “We are exploring new materials, combining insights gained from studying these two systems, to deliberately tailor the adsorption selectivity for each desired metal.”
Modeling molecular relationships
To assist in designing MOFs that selectively target specific rare-earth metals, Sandia computational materials scientist Kevin Leung utilized two modeling techniques. First, he ran molecular dynamics simulations to analyze the environment of rare-earth elements in water, both in the presence and absence of other chemicals, or within a MOF framework. Next, he conducted density functional theory modeling to calculate energy states for 14 rare-earth elements, ranging from cerium to lutetium, transitioning from water to various binding sites with different surface chemistries. These findings were shared in Physical Chemistry Chemical Physics.
In line with prior experimental results, Leung discovered that rare-earth elements did not show a preference for binding with amines compared to water, but did favor negatively charged species like sulfate or phosphate over water. This preference was more pronounced for heavier rare-earth elements like lutetium compared to lighter ones such as cerium and neodymium.
The objective was to identify a chemical that would allow for the selection of a single metal, but unfortunately, all modeled reactions exhibited a similar trend, Leung noted. He speculated that mixing slightly positively charged surface chemistry with negatively charged counterparts could potentially facilitate the selection of a specific metal. However, this idea has yet to be tested.
Investigating with X-ray technology and future plans
To examine how rare-earth metals interact with MOFs, Ilgen employed X-ray spectroscopy to investigate the chemical behavior of three rare-earth elements within zirconium-based and chromium-based MOFs. Using advanced X-ray absorption fine structure spectroscopy at Argonne National Laboratory, Ilgen discovered that the rare-earth elements chemically bonded with the metal hub in both types of MOFs. In MOFs featuring a phosphonate surface group, the rare-earth metals were attracted to the phosphonate rather than the metal hub.
“My spectroscopy research has uniquely identified the surface complexes formed by rare-earth elements in MOFs,” Ilgen stated. “This has not been achieved through X-ray spectroscopy before. Previous investigations inferred surface complexes based on adsorption patterns, but no one had visually identified them until now.”
Ilgen also noted that the binding of the rare-earth element to the metal hub occurred similarly in MOFs with missing linkers as in those with complete linkers. This observation is crucial because defect-free MOFs tend to be more stable and potentially reusable compared to their flawed counterparts.
In her paper, Ilgen suggested that by using metal hubs composed of a mixture of metals, it might be possible to design MOF sponges that preferentially adsorb one rare-earth element over others, although this approach remains untested.
With their extensive understanding of the interactions between rare-earth elements and MOFs, the team is ready to explore many potential paths in creating selective sponges.
“There are numerous design strategies for developing ion-selective MOFs, particularly for isolating individual rare-earth elements,” Ilgen explained. “One strategy could involve adjusting the chemistry of the metal hub to integrate various metals, optimizing the binding site for specific rare-earth elements. Another approach would focus on the chemistry of surface groups, where robust groups can outperform the metal hubs, creating ion-specific pockets associated with these groups. Finally, manipulating the pore sizes of the MOF itself could modify local chemistry, favoring certain elements.”
This project received funding from Sandia’s Laboratory Directed Research and Development program.
