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.
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