Reversible Amine-to-Imine Chemistry at a Covalent Organic Framework for Sustainable Uranium Redox Separation

The interconversion chemistry of amine-to-imine sites in a covalent organic framework (COF) was developed for the redox-based separation of uranium. Compared to traditional approaches using sacrificial reagents or material decomposition for the reduction and separation of uranium, amine-COF served as the electron donor and was regenerated repeatedly following the oxidation and uranium reduction/separation. The amine-COF, PI-3-AR, was formed from the sodium borohydride (NaBH4) reduction of the imine-linked COF, PI-3, prepared from the solvothermal synthesis of 1,3,5-triformyl benzene (TFB) and 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)trianiline (TTA). PI-3-AR could be converted back to PI-3 via oxidative amination using an excess of the oxidant iodine, I2, or in the photochemical reduction of uranyl ions (UO22+). In consecutive photochemical uranium reduction and separation cycling experiments, the reduced amine COF, PI-3-AR, underwent: (i) oxidation alongside uranium photoreduction and deposition; (ii) acid treatment and uranium extraction; and (iii) NaBH4 reduction and material recovery. The COF, PI-3-AR, and novel separation process involving amine-to-imine interconversion effectively removed uranium (maximum adsorption = 278 mg U/g COF) and maintained >98% uranium recovery over five recycling steps at pH 4.0.