Rare-earth elements are in every single place in modern life, present in all the pieces from the smart device you are reading this on to the LED lightbulbs overhead and neodymium magnets in electric vehicles and wind turbines.
Nonetheless, purifying these critical metals from ores with complex mixtures is a nasty business involving strong acids and dangerous solvents, and is primarily conducted in China. Over the past three years, a team of researchers from Sandia National Laboratories has been pioneering an environmentally friendly method to separate these rare-earth elements from watery mixtures.
Initially, the team made and modified tinker-toy-like molecules called metal-organic frameworks or MOFs to check their ability to adsorb these vital metals. They then used computer simulations and X-ray-based experiments to analyze how the rare-earth elements interact with the synthesized “sponges.” The team’s ultimate goal is to design sponges that selectively absorb one rare earth metal while excluding others. Their findings were recently published in a series of scientific papers, including one within the scientific journal ACS Applied Materials and Interfaces on August 26.
“We synthesized MOFs with variable surface chemistry and were capable of show through adsorption experiments that these MOFs can select rare-earth elements from a combination of other metals,” said Anastasia Ilgen, a Sandia geochemist and project lead. “They’re more selective for the rare earths — that is good. Importantly, we illustrated that their ability to select metals might be fine-tuned by adding chemical groups on their surfaces.”
Synthesizing stable sponges
The researchers chosen two zirconium-based tinker-toy-like MOFs for the project. These MOFs are highly stable in water and simply adjustable, based on Dorina Sava Gallis, a Sandia materials chemist involved within the project.
MOFs consist of metal “hubs” and carbon-based linker “rods,” which might be interchanged to create nanosized “sponges” with different properties. Also, chemists can add different chemical groups inside MOFs to change their properties or engineer structures with missing rods, Sava Gallis said.
Of their study, published within the scientific journal Chemical Communications, Sava Gallis and her team experimented with two varieties of MOFs featuring zirconium hubs. They attached recent chemical groups to the linkers in a single MOF constructing block, while attaching them to the metal hub in one other.
The team found that the MOFs with missing linkers certain more of the 2 rare-earth elements in comparison with those without missing linkers, as expected. The addition of an amino group to the linker had minimal impact on the adsorption of any of the metals. Nonetheless, incorporating a negatively charged chemical group called phosphonate into the linker improved the adsorption of all of the metals. Interestingly, within the MOF structure where the chemical groups were attached to the metal hubs, the extra chemical groups didn’t make much of a difference on the adsorption of the rare-earth elements. Nonetheless, they greatly increased the selectivity for nickel over cobalt, Sava Gallis said.
“We’re seeing that each approaches we implemented effectively tune the selectivity for various ions,” Sava Gallis said. “We’re looking into designing recent materials, combining the knowledge we now have gained from studying these two material systems, to intentionally tailor the adsorption selectivity for every metal of interest.”
Modeling molecular interactions
To further guide the design of MOFs selective for specific rare-earth metals, Sandia computational materials scientist Kevin Leung used two different computer modeling techniques. First, he conducted molecular dynamics simulations to know the environment of rare-earth elements in water, with or without other chemicals, or inside a MOF structure. Then he performed detailed density functional theory modeling to calculate the energy for 14 rare-earth elements from cerium to lutetium going from water to a binding site with various surface chemistries. These findings were published in Physical Chemistry Chemical Physics.
Consistent with the sooner experimental work, Leung found that rare-earth elements don’t exhibit a preference for binding with amines over water. Nonetheless, they do show a preference for negatively charged chemicals like sulfate or phosphate in comparison with water. Leung found this preference is stronger for heavier rare-earth elements resembling lutetium in comparison with lighter elements like cerium and neodymium.
The goal was to search out a chemical that might allow them to pick out one metal, but unfortunately all the pieces modeled had a uniform trend, Leung said. He hypothesized that combining a rather positively charged surface chemical with a negatively charged surface chemical would find a way to pick out for one metal. Nonetheless, this approach has not yet been attempted.
X-ray illumination and next steps
To see precisely how the rare-earth metals interact with MOFs, Ilgen used X-ray spectroscopy to look at the chemical environment of three rare-earth elements in zirconium-based MOFs and chromium-based MOFs. Using synchrotron-based X-ray absorption tremendous structure spectroscopy at Argonne National Laboratory, Ilgen observed that the rare-earth element chemically bonded to the metal hub in each zirconium and chromium MOFs. Within the MOF with a phosphonate surface group, the rare-earth metals certain to the phosphonate as a substitute of the metal hub.
“My spectroscopy work is the primary to discover the surface complexes formed by rare-earth elements in MOFs,” Ilgen said. “Nobody had done X-ray spectroscopy before. Previous studies inferred surface complexes based on adsorption trends, but nobody had ‘seen’ them. I saw them with my X-ray eyes.”
Ilgen also saw that the rare-earth element certain to the metal hub in the identical manner in MOFs with missing linkers as in MOFs with all of the linkers. This is critical because MOFs without defects are more stable and potentially more reusable than MOFs with missing linkers.
Within the paper, Ilgen proposed that metal hubs with a combination of metals could create MOF sponges that prefer to adsorb one rare-earth element over others, but she said this approach has not been attempted yet.
Armed with their extensive knowledge of rare-earth elements’ interactions with MOFs, the team has quite a few avenues to explore in designing selective sponges.
“There are several possible design strategies for ion-selective MOFs, specifically for separating individual rare-earth elements from each other,” Ilgen said. “One strategy involves tuning the chemistry of the metal hub, potentially incorporating multiple varieties of metals to optimize the binding site for a selected rare earth. One other strategy focuses on surface group chemistry, where strong surface groups outcompete the metal hubs, creating ion-specific pockets related to the surface groups. Lastly, the pore dimensions of the MOF itself might be adjusted, as nanosized pores alter local chemistry to favor specific elements.”
The project was funded by Sandia’s Laboratory Directed Research and Development program.
