Regulating Activation Energy of Metal–Organic Framework Subnanochannels for High-Precision Anion Separation

Transition state theory effectively describes solute transport in membranes, where molecular-level mechanisms dictate the energy barriers involved. Based on this theory and insights from natural systems, our study centers on UiO-66-NH₂, a metal–organic framework (MOF) with specific binding sites for fluoride ions, resembling the structural features of biological fluoride ion channels. Using a secondary solvothermal growth method, we fabricate a dense and continuous polycrystalline UiO-66-NH₂ membrane on an anodic aluminum oxide substrate. This membrane features subangstrom pores (3.12 and 6.28 Å), which precisely sieve fluoride ions by facilitating selective dehydration and binding. Additionally, the high porosity and surface area of the membrane enhance ion flux while maintaining excellent selectivity. The strong Zr–F interactions within the channel play a pivotal role in reducing the activation energy required for F⁻ transport, resulting in efficient separation compared to other anions, with the F⁻/SO₄²⁻ selectivity reaching 169. This work sheds light on the fundamental ion transport mechanisms in subnanochannels and highlights the potential of MOF membranes for advanced ion separation applications.