Enhanced Ion-Exchange Properties of a Complex Microporous Uranyl Borophosphate

An unusual microporous uranyl borophosphate has been prepared under mild hydrothermal conditions, namely, Cs₃(UO₂)₃[B(PO₄)₄](H₂O)_0.5 denoted as CUPB1. It crystallizes in the chiral space group <i>P</i>4₁2₁2. The three-dimensional (3D) open framework of CUPB1 is constructed by [B(PO₄)₄]^9- fundamental building blocks (FBBs) linked by UO₇ pentagonal bipyramids into nanoscale size (∼12.2 Å × 12.2 Å × 11.7 Å) uranyl borophosphate cages of U₁₂P₂₄B₈. The Cs⁺ cations are disordered and are located near the cage windows. Free void volume in CUPB1 is ∼59% based on the single-crystal X-ray diffraction data, which makes it a highly porous compound. Moreover, the porosity and flexibility of the Cs⁺ cations in the framework make it exchangeable with most monovalent and divalent cations in aqueous solutions. We have studied the ion-exchange properties in detail with the environmentally relevant cations Pb²⁺, Co²⁺, and Ni²⁺, as well as the key nuclear fission products Sr²⁺ and Ba²⁺, at both room temperature and ∼70 °C. The simple synthetic route, microporous structure, thermal stability, vibrational spectroscopy, and ion-exchange properties are discussed in detail.