Abstract
The lanthanides (Ln) are an essential part of many advanced technologies. Our societal transformation toward renewable energy drives their ever-growing demand. The similar chemical properties of the Ln pose fundamental difficulties in separating them from each other, yet high purity elements are crucial for specific applications. Here, we propose an intralanthanide separation method utilizing a group of titanium(IV) butyl phosphate coordination polymers as solid-phase extractants. These materials are characterized, and they contain layered structures directed by the hydrophobic interaction of the alkyl chains. The selective Ln uptake results from the transmetalation reaction (framework metal cation exchange), where the titanium(IV) serves as sacrificial coordination centers. The “tetrad effect” is observed from a dilute Ln3+ mixture. However, smaller Ln3+ ions are preferentially extracted in competitive binary separation models between adjacent Ln pairs. The intralanthanide ion-exchange selectivity arises synergistically from the coordination and steric strain preferences, both of which follow the reversed Ln contraction order. A one-step aqueous separation of neodymium (Nd) and dysprosium (Dy) is quantitatively achievable by simply controlling the solution pH in a batch mode, translating into a separation factor of greater than 2000 and 99.1% molar purity of Dy in the solid phase. Coordination polymers provide a versatile platform for further exploring selective Ln separation processes via the transmetalation process.
Highlights
The lanthanide (Ln) series, collectively made up of a group of15 elements, has gained strategic importance in recent decades.The unique 4f electron structures entail distinctive and, in many cases, irreplaceable physical properties that make Ln essential components in advanced electronics, lasers, and permanent magnets.[1,2] Owing to the poor shielding of nuclear charge by 4f electrons, Ln3+ ions share extremely similar yet descending ionic radii across the series with an average difference between adjacent elements of only 1 pm (Ln contraction).[3]
Organic−inorganic hybrid materials utilizing an array of inorganic porous supports and organic functional ligands have been developed for selective Ln sorption.[16−20] Recently, we have shown that titanium alkyl-phosphate functionalized mesoporous silica possesses solvating extraction capability.[21]
We have demonstrated the basis of a Ln separation strategy utilizing a transmetalation reaction on layered titanium(IV) organophosphate materials, as opposed to the extraframework ion-exchange materials
Summary
15 elements, has gained strategic importance in recent decades. The unique 4f electron structures entail distinctive and, in many cases, irreplaceable physical properties that make Ln essential components in advanced electronics, lasers, and permanent magnets.[1,2] Owing to the poor shielding of nuclear charge by 4f electrons, Ln3+ ions share extremely similar yet descending ionic radii across the series with an average difference between adjacent elements of only 1 pm (Ln contraction).[3]. Organic−inorganic hybrid materials utilizing an array of inorganic porous supports and organic functional ligands have been developed for selective Ln sorption.[16−20] Recently, we have shown that titanium alkyl-phosphate functionalized mesoporous silica possesses solvating extraction capability.[21] The surface alkyl chains linked to titanium phosphate moieties mimic the structure of tri-n-butyl phosphate (TBP) and form complexes with Ln nitrates. These materials seem not to be perfect candidates for intralanthanide separation because of the similar complex formation constants across the Ln series.
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