Modeling Separation of Lanthanides via Heterogeneous Ligand Binding

Individual lanthanide elements have physical/electronic/magnetic properties that make each useful for specific applications. Several of the lanthanides cations (Ln(III)) naturally occur together in the same ores. They are notoriously difficult to separate from each other due to their "chemical similarity." Predicting the Ln(III) differential binding energies (DDE) or free energies DDG) at different binding sites will help design of materials with lanthanide selectivity. We apply ab initio molecular dynamics (AIMD) simulations and Density Functional Theory (DFT) to calculate DDG for Ln(III) coordinated to ligands in water and embedded in metal-organic frameworks (MOFs), and DDE for Ln(III) bonded to silica surfaces. Perturbative AIMD simulations of water-inundated simulation cells are applied to examine the selectivity of ligands towards adjacent Ln(III) in the periodic table. Static DFT calculations with a full Ln(III) first coordination shell, while less rigorous, show that all ligands examined with net negative charges are more selective towards the heavier lanthanides than a charge-neutral coordination shell made up of water molecules. Amine groups are predicted to be poor ligands for lanthanide-binding. We also address cooperative ion binding, i.e., using different ligands in concert to enhance lanthanide selectivity.<br/>This article has been authored by an employee of National Technology & Engineering Solutions of Sandia, LLC under Contract No. DE-NA0003525 with the U.S. Department of Energy (DOE). The employee owns all right, title and interest in and to the article and is solely responsible for its contents. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this article or allow others to do so, for United States Government purposes. The DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan https://www.energy.gov/downloads/doe-public-access-plan. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.