A Europium-Organic Framework with Activation-Induced Open Metal Sites for Water Adsorption

Abstract

Understanding the role of open metal sites (OMS) in regulating guest-framework interactions is essential for the rational design of rare earth metal-organic frameworks (RE-MOFs) for sorption applications. Herein, we report a europium-based MOF, [Eu₂(TCPP)_1.5(DMF)₂(H₂O)]·DMF·H₂O (Eu-H₄TCPP), synthesized under solvothermal conditions. Single-crystal X-ray diffraction reveals a robust three-dimensional framework with one-dimensional channels, in which the Eu³⁺ centres are partially coordinated by labile solvent molecules. Eu-H₄TCPP exhibits high chemical stability over a wide pH range and thermal stability up to 250 ℃. Variable-temperature in situ powder X-ray diffraction demonstrates that the framework remains intact during activation, accompanied by the stepwise removal of coordinated DMF and H₂O molecules to generate accessible OMS. Gas sorption measurements using N₂ and water vapour reveal that the activation-induced coordinatively unsaturated Eu³⁺ centres serve as primary binding sites, dominating the initial water uptake through direct metal-water interactions. Water vapor sorption tests demonstrate that the activated Eu-MOF delivers a prominent water uptake capacity of 0.29 g g^-1, which is nearly twice that of the pristine non-activated counterpart. Moreover, the activated Eu-MOF maintains stable water adsorption performance for at least three consecutive sorption-desorption cycles, showing competitive water capture capability among state-of-the-art RE-MOFs. These results highlight the crucial role of OMS accessibility in modulating water adsorption behaviour in RE-MOFs.