Rare Earths (REs) are critical materials, and within their production, the most challenging step is the separation from primary and secondary sources, as well as individual separations. Conventional liquid-liquid extraction methods require several cycles with large volumes of solvents because of the close chemical similarities shared between them. Metal-Organic Frameworks (MOFs) are attractive candidates for solid-liquid separation: adsorption of solvated Rare Earths onto the linkers of MOFs can provide a versatile and reusable separation strategy.
This work presents insights into interactions between selected RE3+ cations and some de novo MOFs, highlighting relative binding stabilities of MOF-REs pairs for separations.
De novo framework structures are built and relaxed on atomic positions and lattice parameters. Structural analysis is done with Voronoi cell decomposition via Zeo++ software. The equilibrated structures were used to construct cluster and periodic models to study REs binding. Relative adsorption energies are estimated with density functional theory (DFT) and semiempirical methods (GFN1-xTB), as implemented in Amsterdam Modeling Suite.
Most frameworks exhibit channels ranging from 5 to 20 Å, suitable for the diffusion of solvated rare-earth ions. The modeling of REs–MOF interactions points to preferential binding as a function of linker chelating angle and ionic radius. Differences in relative binding energies rise to 10 kcal/mol with broader chelating angles favoring larger REs (La, Ce) and narrower chelating angles for smaller REs (Lu, Y, Sc).
This work on REs adsorption on MOFs is valuable as it provides insights into the separation potential of MOFs. Development of greener REs separations is increasingly important to meet their growing demand. The goal of this work correlates with Sholl and Lively statement in "Seven chemical separations to change the world": “Producing rare earths economically is a problem of separations, not availability ”.