Direct Separation of UO22+ by Coordination Sieve Effect via Spherical Coordination Traps

Molecule sieve effect (MSE) can enable direct separation of target, thus overcoming two major scientific and industrial separation problems in traditional separation, coadsorption, and desorption. Inspired by this, herein, the concept of coordination sieve effect (CSE) for direct separation of UO22+, different from the previously established two-step separation method, adsorption plus desorption is reported. The used adsorbent, polyhedron-based hydrogen-bond framework (P-HOF-1), made from a metal-organic framework (MOF) precursor through a two-step postmodification approach, afforded high uptake capacity (close to theoretical value) towards monovalent Cs+, divalent Sr2+, trivalent Eu3+, and tetravalent Th4+ ions, but completely excluded UO22+ ion, suggesting excellent CSE. Direct separation of UO22+ can be achieved from a mixed solution containing Cs+, Sr2+, Eu3+, Th4+, and UO22+ ions, giving >99.9% removal efficiency for Cs+, Sr2+, Eu3+, and Th4+ ions, but <1.2% removal efficiency for UO22+, affording benchmark reverse selectivity (S-M/U) of >83 and direct generation of high purity UO22+ (>99.9%). The mechanism for such direct separation via CSE, as unveiled by both single crystal X-ray diffraction and density-functional theory (DFT) calculation, is due to the spherical coordination trap in P-HOF-1 that can exactly accommodate the spherical coordination ions of Cs+, Sr2+, Eu3+, and Th4+, but excludes the planar coordination UO22+ ion.