Selectivity, Kinetics, and Efficiency of Reversible Anion Exchange with TcO4− in a Supertetrahedral Cationic Framework

[ThB5O6(OH)6][BO(OH)2]·2.5H2O (Notre Dame Thorium Borate‐1, NDTB‐1) is an inorganic supertetrahedral cationic framework material that is derived from boric acid flux reactions. NDTB‐1 exhibits facile single crystal to single crystal anion exchange with a variety of common anions such as Cl−, Br−, NO3−, IO3−, ClO4−, MnO4−, and CrO42−. More importantly, NDTB‐1 is selective for the removal of TcO4− from nuclear waste streams even though there are large excesses of competing anions such as Cl−, NO3−, and NO2−. Competing anion exchange experiments and magic‐angle spinning (MAS)‐NMR spectroscopy of anion‐exchanged NDTB‐1 demonstrate that this unprecedented selectivity originates from the ability of NDTB‐1 to trap TcO4− within cavities, whereas others remain mobile within channels in the material. The exchange kinetics of TcO4− in NDTB‐1 are second‐order with the rate constant k2 of 0.059 s−1 M−1. The anion exchange capacity of NDTB‐1 for TcO4− is 162.2 mg g−1 (0.5421 mol mol−1) with a maximum distribution coefficient Kd of 1.0534 × 104 mL g−1. Finally, it is demonstrated that the exchange for TcO4− in NDTB‐1 is reversible. TcO4− trapped in NDTB‐1 can be exchanged out using higher‐charged anions with a similar size such as PO43− and SeO42−, and therefore the material can be easily recycled and reused. [ThB5O6(OH)6][BO(OH)2]·2.5H2O (NDTB‐1) is an inorganic supertetrahedral cationic framework material that exhibits facile single crystal to single crystal anion exchange with a variety of common anions. NDTB‐1 is selective for the removal of TcO4− from nuclear waste streams even though there are large excesses of competing anions. Anions trapped in NDTB‐1 can be exchanged using higher‐charged anions with a similar size and therefore the material can be easily recycled and reused.