Two-Dimensional Fluorinated Covalent Organic Frameworks with Tunable Hydrophobicity for Ultrafast Oil–Water Separation

Covalent organic frameworks (COFs) hold great potentials for addressing the challenge of highly efficient oil–water separation, but they are restricted by the poor wettability and processability as crystalline membranes. Here we report the design and synthesis of two-dimensional (2D) robust COFs with controllable hydrophobicity and processability, allowing the COF layers to be directly grown on the support surfaces. Three 2D COFs with AA or ABC stacking are prepared by condensation of triamines with fluorine and/or isopropyl groups and perfluorodialdehyde. They all show excellent tolerance to water, acid, and base, with water contact angles (CA) of 111.5–145.8°. The two COFs with isopropyl and fluorine mixtures can grow as a coating on a stainless-steel net (SSN) substrate, whereas the one with only fluorine substituents cannot. The superhydrophobic COF@SSN coating with water CA of up to 150.1° displays high water-resistance and self-cleaning properties, enabling high oil–water separation performances with an efficiency of over 99.5 % and a permeation flux of 2.84×10⁵ L m⁻² h⁻¹, which are among the highest values reported for state-of-the-art membranes.