Hydrophobicity Tuning in Isostructural Urchin-Shaped Covalent Organic Framework Nanoparticles by Pore Surface Engineering for Oil–Water Separation

Accidental oil spills have released millions of barrels of petroleum hydrocarbons in oceans worldwide, causing severe damage to marine and coastal ecosystems. Materials to clean up these oil spills are therefore in high demand. Here, we report the rapid, scalable microwave-assisted synthesis of two isostructural imine-linked covalent organic frameworks (COFs; TAB-DFP and TAB-MPPD) from 2,6-diformylpyridine (DFP) or 4-(4-(methylthio)phenyl) pyridine-2,6-dicarbaldehyde (MPPD) and tris(4-aminophenyl) benzene (TAB) building blocks. The morphology, hydrophobicity, and hydrophilicity of the materials were changed by surface modification. The TAB-DFP COF exhibited a spherical sea urchin-like morphology and was hydrophilic in nature. In contrast, the TAB-MPPD COF exhibited a bowl-shaped hollow spherical morphology. Incorporation of a thioanisole moiety into the structure of DFP resulted in high hydrophobicity with a contact angle of 155 ± 2°. Thus, the TAB-MPPD showed excellent oil removal performance (>500 wt %) for various types of oils. The adsorbed oil could be desorbed by simply washing with ether, and the material could be reused beyond 10 consecutive cycles without any significant decrease in oil removal capacity.