Outer-Sphere Water Clusters Tune the Lanthanide Selectivity of Diglycolamides

Area of Science:

• Coordination Chemistry
• Separation Science
• Sustainable Energy

Background:

• Selective separation of f-block elements is critical for nuclear fuel reprocessing and rare-earth metal refining.
• Current research primarily focuses on inner-sphere metal-ligand interactions, neglecting outer-sphere effects on selectivity.
• The solvating extractant N,N,N',N'-tetraoctyl diglycolamide (TODGA) is relevant for separating adjacent lanthanides.

Purpose of the Study:

• To investigate the fundamental origins of TODGA's selectivity for adjacent lanthanides in liquid-liquid extraction.
• To elucidate the role of outer-sphere interactions in the separation mechanism.
• To establish a link between coextracted water and lanthanide extraction efficiency.

Main Methods:

• Distribution studies to quantify extraction behavior.
• Quantum mechanical calculations to probe electronic interactions.
• Classical molecular dynamics simulations to model molecular arrangements and interactions.
• Integration of experimental and computational approaches.

Main Results:

• A direct relationship was established between coextracted water and the extraction of lanthanides by TODGA across the series.
• Outer-sphere interactions, particularly hydrogen bonding between nitrate ions and water clusters, are crucial for selectivity.
• The nonpolar environment significantly influences these outer-sphere interactions.

Conclusions:

• Selectivity in TODGA-lanthanide systems is governed by both inner- and outer-sphere interactions.
• Tuning outer-sphere hydrogen-bonding networks is essential for controlling selectivity in biphasic extraction.
• Findings provide insights for designing advanced separation systems for f-block elements.