Calix[4]arene-Decorated Covalent Organic Framework Conjugates for Lithium Isotope Separation

Lithium isotope separation has attracted extensive interest due to its important role in fusion and fission reactions. Up to now, it is still a great challenge to separate lithium isotopes (⁶Li and ⁷Li) in an efficient manner due to the low capture ability for lithium ions of related materials and highly similar physicochemical properties between lithium isotopes. In this work, three calix[4]arene-decorated crystalline covalent organic frameworks (COFs) with wave-like extension and AA-stacking configuration were designed and utilized for lithium adsorption and its isotope separation. Experimental studies show that these COFs exhibit an outstanding lithium adsorption capacity up to 94.66 mg·g^-1, which is about 2 times beyond that of adsorbents reported in the literature. The high adsorption capacity of COFs could be attributed to the abundant adsorption sites from calix[4]arene unit. More importantly, this study demonstrates for the first time that calixarene groups can separate lithium isotopes with an excellent separation factor up to 1.053 ± 0.002, comparable to the most successful solid-phase lithium separation adsorbent. The calculation based on density functional theory showed that calixarene played an important role in the lithium adsorption. Interestingly, the lithium isotope separation performance is mainly affected by the amine bridging units. This work demonstrated that calixarene COFs are promising adsorbents for lithium isotope separation.