Work Toward a Cleaner Way to Purify Critical Metals

The team found that the MOFs with missing linkers bound more of the two rare-earth elements compared to 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. However, incorporating a negatively charged chemical group called phosphonate into the linker improved the adsorption of all the metals. Interestingly, in the MOF structure where the chemical groups were attached to the metal hubs, the additional chemical groups did not make much of a difference on the adsorption of the rare-earth elements. However, they greatly increased the selectivity for nickel over cobalt, Dorina said.

“We are seeing that both approaches we implemented effectively tune the selectivity for different ions,” Dorina said. “We’re looking into designing new materials, combining the knowledge we have gained from studying these two material systems, to intentionally tailor the adsorption selectivity for each 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 understand the environment of rare-earth elements in water, with or without other chemicals, or within 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 earlier experimental work, Kevin found that rare-earth elements do not exhibit a preference for binding with amines over water. However, they do show a preference for negatively charged chemicals like sulfate or phosphate compared to water. Kevin found this preference is stronger for heavier rare-earth elements such as lutetium compared to lighter elements like cerium and neodymium.

The goal was to find a chemical that would allow them to select one metal, but unfortunately everything modeled had a uniform trend, Kevin said. He hypothesized that combining a slightly positively charged surface chemical with a negatively charged surface chemical would be able to select for one metal. However, this approach has not yet been attempted.

X-ray Illumination and Next Steps
To see precisely how the rare-earth metals interact with MOFs, Anastasia used X-ray spectroscopy to examine the chemical environment of three rare-earth elements in zirconium-based MOFs and chromium-based MOFs. Using synchrotron-based X-ray absorption fine structure spectroscopy at Argonne National Laboratory, Anastasia observed that the rare-earth element chemically bonded to the metal hub in both zirconium and chromium MOFs. In the MOF with a phosphonate surface group, the rare-earth metals bound to the phosphonate instead of the metal hub.

“My spectroscopy work is the first to identify the surface complexes formed by rare-earth elements in MOFs,” Anastasia said. “No one had done X-ray spectroscopy before. Previous studies inferred surface complexes based on adsorption trends, but no one had ‘seen’ them. I saw them with my X-ray eyes.”

Anastasia also saw that the rare-earth element bound to the metal hub in the same manner in MOFs with missing linkers as in MOFs with all the linkers. This is significant because MOFs without defects are more stable and potentially more reusable than MOFs with missing linkers.

In the paper, Anastasia proposed that metal hubs with a mixture 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 numerous 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 one another,” Anastasia said. “One strategy involves tuning the chemistry of the metal hub, potentially incorporating multiple types of metals to optimize the binding site for a specific rare earth. Another strategy focuses on surface group chemistry, where strong surface groups outcompete the metal hubs, creating ion-specific pockets associated with the surface groups. Lastly, the pore dimensions of the MOF itself can be adjusted, as nanosized pores alter local chemistry to favor specific elements.”

Revolutionary Metal Spong (in the Making)
Teamwork between experimentalists and computational experts, materials scientists and geochemists has allowed them to push the envelope on an environmentally friendly method to separate critical rare-earth elements.

“This is a really excellent example of how Sandia’s breadth of capabilities come together to tackle this kind of complex problem,” said Dorina Sava Gallis, a Sandia materials chemist on the project. “Because we have this expertise all in one place, it allows us to rapidly and iteratively advance the discoveries.”

Since October 2021, Dorina as well as project lead geochemist Anastasia Ilgen and computational expert Kevin Leung have been making and studying tinker-toy like metal-organic frameworks for their ability to absorb these important metals, studying the details of how the rare earths chemically interact with the MOFs, and thinking of new ways to “tune” them to be specific for one metal over another.

“The bottom line is that the materials are robust, they’re great platforms and they’re actually tunable,” Anastasia said. “With the surface chemistry you can tweak up or down how much of a certain element is absorbed. There are two pieces connecting experiment to modeling. One is that linkers do not have to be missing for a rare-earth element to coordinate to the metal hub. The second one is that the phosphonate group outcompetes the metal hub. We have very good agreement between models and experiments, which doesn’t often happen.”

Mollie Rappe is Principal Corporate Communications Specialist at Sandia National Laboratories. The article was originally posted to the website of Sandia National Laboratories.