Selective separation of light rare-earth elements by supramolecular encapsulation and precipitation

Abstract

Supramolecular chemical strategies for Rare Earth (RE) element separations are emerging which amplify the small changes in properties across the series to bias selectivity in extraction or precipitation. These advances are important as the REs are crucial to modern technologies yet their extraction, separation, and recycling using conventional techniques remain challenging. We report here a pre-organised triamidoarene platform which, under acidic, biphasic conditions, uniquely and selectively precipitates light RE nitratometalates as supramolecular capsules. The capsules exhibit both intra- and intermolecular hydrogen bonds that dictate selectivity, promote precipitation, and facilitate the straightforward release of the RE and recycling of the receptor. This work provides a self-assembly route to metal separations that exploits size and shape complementarity and has the potential to integrate into conventional processes due to its compatibility with acidic metal feed streams.

Fig. 1. Encapsulation and precipitation of the rare-earth elements (REs).

a The tripodal amido-arene L used in this study, b the capsular hexanitratometalate complex, [{La(κ²-NO₃)₆}⊂(H₃L₂)]_n 1-La, c a schematic of the precipitation of rare-earths by L from a biphasic nitric acid/toluene mixtures and its stripping using a polar protic solvent such as water.

Fig. 2. X-ray crystal structure and DFT NCI analysis of 1-La [{La(NO₃)₆}⊂(H₃L₂)]_n.

a X-ray crystal structure (top-down view). For clarity, all hydrogen atoms except those involved in hydrogen bonding and a disorder component of the amide arm are omitted (where shown, thermal displacement ellipsoids are drawn at 50% probability). N-H and O-H hydrogen atoms were located in the difference Fourier map and (O1)H is 50% occupied on a crystallographic special position. Atom colours: La = green; oxygen = red; nitrogen = blue; carbon = silver; hydrogen = grey. b DFT structure showing non-covalent interactions. NCI attractive hydrogen bond interactions are represented as blue disks. Atom colours: La = green; oxygen = red; nitrogen = blue; carbon = silver; hydrogen = grey.

Fig. 3. Precipitation of REs by the tripodal amido-arene L.

Precipitation arising from a 0.0025 M mixed-RE solution in 2 to 8 M HNO₃/toluene equal-volume biphasic mixture after the addition of 3.5 mmol L (5-fold excess L relative to metal) at 298 K.

Fig. 4. Separation factors (SFs) between REs (La-Eu) on precipitation by L.

Determined from precipitations arising from a 0.0025 M mixed-metal solution in 8 M HNO₃/toluene equal-volume biphasic mixture after the addition of 3.5 mmol L (5-fold excess L relative to metal) at 298 K. Separation factors are calculated from pseudo-distribution ratios, where the metal lost from the aqueous phase (relative to the feed solution) is assumed to be entirely contained in the precipitate.