CAREER: The solution structure of lanthanide-extractant complexes across liquid-liquid interfaces

Rare earth elements, including lanthanides, are vital to clean energy, defense, and consumer technologies, such as smart phones, wind turbines, lasers, guidance systems, and medical contrast agents. For most applications, specific lanthanides are needed in near-purified form. The purification process requires separating the elements from each other.

The separation is particularly challenging due to the chemical similarity of the rare earth elements and thus requires substantial amounts of solvent and energy. Efficient rare earth separation processes are needed to ensure a reliable, sustainable supply of rare earth elements for the United States. Liquid-liquid, or solvent, extraction is the separation process by which most rare earth elements are obtained.

The effectiveness of solvent extraction to separate rare earth elements lies in the selectivity of extractants, or ligands, to bind a particular rare earth element. Enhancing ligand selectivity and extraction capacity for particular lanthanides will improve the efficiency of the separation. This project seeks to resolve the structure of lanthanide-ligand complexes in solution and determine the mechanism by which ligands transport lanthanides from the aqueous to organic phase in solvent extraction.

The connection between molecular structures and mechanisms at aqueous-organic interfaces with the extent and rate of rare earth solvent extraction will be determined. This CAREER project will incorporate rare earth separation research into an undergraduate chemical engineering separation course and will expand efforts to promote chemical engineering in Nevada high schools.

The educational and outreach components of the project aim to bridge graduate student research and chemical separations instruction, introduce undergraduate students to the concepts of rare earth separations, and strengthen the chemical engineering workforce development pipeline in support of Nevada’s continued expansion toward technology-based employment.

With joint support from the Interfacial Engineering program and the Established Program to Stimulate Competitive Research (EPSCoR), this project intends to generate fundamental knowledge to determine the connection between lanthanide-ligand complex structures in solution with their relative aqueous-organic solubility (separation extent), as well as to resolve how ligand structure affects the transport mechanisms of lanthanide ions (separation rate). The selectivity of most current lanthanide extractants can be improved because it is based on size exclusion alone, and lanthanide ion size differences due to contraction are small.

This project will investigate multi-acidic ligands as lanthanide-specific extractants, leveraging their tunable binding strength arising from multiple protonation sites. A combination of ab initio molecular dynamics simulations with spectroscopy measurements will resolve lanthanide-ligand complex structures in aqueous and organic phases at varying protonation states.

Binding specificity is necessary but not sufficient for effective lanthanide extraction; therefore, the transport of lanthanide ions across the aqueous-organic interfaces will be investigated as well. Classical molecular dynamics simulations with rare event simulation techniques will generate free energy profiles of lanthanide-ligand complex transport across aqueous-organic interfaces, which will be used to determine relative aqueous-organic solubilities and mechanisms of ligand-mediated lanthanide transport across aqueous-organic interfaces.

Extraction measurements will be performed as well to verify predictions. Structural, mechanistic knowledge of lanthanide solvent extraction is limited, especially for multi-acidic ligands whose protonation state change with acidity. It is expected that the work in this project will identify the molecular characteristics that could make multi-acidic ligands selectively bind particular lanthanide ions and transport them to the organic phase.

More broadly, structures in solution are understudied in liquid-liquid separations, and structural and mechanistic insights can lead to fundamentally understanding structure-function connections in solvent extraction, an important unit operation with a need for higher selectivity to improve efficiency and sustainability.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.