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

Abstract

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.