An adaptive supramolecular organic framework for highly efficient separation of uranium via an in situ induced fit mechanism

On the basis of the unusual coordination structure of UO22+ combined with the adaptive nature of supramolecular organic frameworks (SOFs), here we have designed and prepared a novel SOF-based solid phase extraction adsorbent (MA-TMA) using N-donor-containing melamine (MA) and O-donor-containing trimesic acid (TMA) as bifunctional building blocks mutually linked via hydrogen bonds. The as-prepared MA-TMA, with a rich N-/N- and N-/O-heterocyclic structure throughout its framework, provides an accessible coordination geometry and/or ligand environment for the uranyl ion, which builds the crucial structural basis for the pre-organized adaptive frameworks closely related to the induced-fit and selective recognition of uranyl ions. The main results are as follows: (1) the highest selectivity of 92%, so far unreported, and a considerable capacity of 324 mg g(-1) for uranium adsorption by MA-TMA are observed in weak acidic multi-cation solution (pH 2.5), accompanied by a distribution coefficient K-d value of 16 000 mL g(-1), 100-fold or more over other 11 competitive cations; (2) MA-TMA could reach its limiting saturation capacity of 1028 mg g(-1) at pH 4.5 in pure-U(VI) solution; (3) noteworthily, the morphology of MA-TMA changed from a ribbon-like structure with a nano-diameter before adsorption into aggregated granules with a size of tens of microns after adsorption, which would be much more favorable for subsequent solid-liquid separation. Furthermore, possible mechanisms for the selective recognition of uranyl ions and the morphological changes of MA-TMA after adsorption are explored based on experimental characterization and chemical rationale.