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.