Highly Efficient Uptake of Cs⁺ by Robust Layered Metal-Organic Frameworks with a Distinctive Ion Exchange Mechanism

¹³⁷Cs with strong radioactivity and a long half-life is highly hazardous to human health and the environment. The efficient removal of ¹³⁷Cs from complex solutions is still challenging because of its high solubility and easy mobility and the influence of interfering ions. It is highly desirable to develop effective scavengers for radiocesium remediation. Here, the highly efficient uptake of Cs⁺ has been realized by two robust layered metal-organic frameworks (MOFs), namely [(CH₃)₂NH₂]In(L)₂·DMF·H₂O (DMF = N,N'-dimethylformamide, H₂L= H₂aip (5-aminoisophthalic acid) for 1 and H₂hip (5-hydroxyisophthalic acid) for 2). Remarkably, 1 and 2 hold excellent acid and alkali resistance and radiation stabilities. They exhibit fast kinetics, high capacities (q _m ^Cs = 270.86 and 297.67 mg/g for 1 and 2, respectively), excellent selectivity for Cs⁺ uptake, and facile elution for the regeneration of materials. Particularly, 1 and 2 can achieve efficient Cs⁺/Sr²⁺ separation in a wide range of Sr/Cs molar ratios. For example, the separation factor (SF _Cs/Sr) is up to ∼320 for 1. Moreover, the Cs⁺ uptake and elution mechanisms have been directly elucidated at the molecular level by an unprecedented single-crystal to single-crystal (SC-SC) structural transformation, which is attributed to the strong interactions between COO^- functional groups and Cs⁺ ions, easily exchangeable [(CH₃)₂NH₂]⁺, and flexible and robust anionic layer frameworks with open windows as "pockets". This work highlights layered MOFs for the highly efficient uptake of Cs⁺ ions in the field of radionuclide remediation.