Heavy-metal-ion capture, ion-exchange, and exceptional acid stability of the open-framework chalcogenide (NH4)(4)In12Se20

The hydrothermal synthesis of the purely inorganic open-framework indium selenide (NH4)(In12Se20)-In-4 (1) is reported. Compound 1 exhibits a unique three-dimensional open-framework structure. The framework of 1 shows an unusual, for a chalcogenide compound, rigidity arising from the unprecedented connection mode of its building blocks. Compound 1 possesses ion exchange capacity for Cs+, Rb+, NH4+, but it has selectivity against Na+ and Li+. It also showed exceptional stability in relatively concentrated hydrochloric acid. Ion exchange of 1 with hydrochloric water solutions can produce its solid acid analogue H-2(NH4)(2)In12Se20 The maximum cation-exchange capacity of I was found equal to two equivalents per mol, which is consistent with an exchange mechanism taking place in the ID-channels formed by the largest cavities. In addition, 1 can do ion-exchange with heavy-metal ions like Hg2+, Pb2+, and Ag+. The capacity of 1 to clean water solutions from heavy-metal ions was preliminarily investigated and found very high. Specifically, 1 can remove 99.9% of Hg2+, 99.8% of Ag+, and 94.9% of Pb2+ from aqueous solutions of each of these ions. Using different synthetic conditions, we isolated compound (NH4)(2)In12Se19 (2), which also has as good an acid stability as 1, but no ion-exchange properties. Overall, this work provides new hydrothermal synthetic approaches for isolation of all-inorganic open-framework chalcogenides.