Fast and selective fluoride ion conduction in sub-1-nanometer metal-organic framework channels

Biological fluoride ion channels are sub-1-nanometer protein pores with ultrahigh F⁻ conductivity and selectivity over other halogen ions. Developing synthetic F⁻ channels with biological-level selectivity is highly desirable for ion separations such as water defluoridation, but it remains a great challenge. Here we report synthetic F⁻ channels fabricated from zirconium-based metal-organic frameworks (MOFs), UiO-66-X (X = H, NH₂, and N⁺(CH₃)₃). These MOFs are comprised of nanometer-sized cavities connected by sub-1-nanometer-sized windows and have specific F⁻ binding sites along the channels, sharing some features of biological F⁻ channels. UiO-66-X channels consistently show ultrahigh F⁻ conductivity up to ~10 S m⁻¹, and ultrahigh F⁻/Cl⁻ selectivity, from ~13 to ~240. Molecular dynamics simulations reveal that the ultrahigh F⁻ conductivity and selectivity can be ascribed mainly to the high F⁻ concentration in the UiO-66 channels, arising from specific interactions between F⁻ ions and F⁻ binding sites in the MOF channels.