Polyacrylonitrile Nanofibers Coated with Covalent Organic Frameworks for Oil/Water Separation

The demand for oil cleanup is one of the most urgent issues and has been involved in global sustainable and green economic development. Existing industrial separation techniques are inefficient and time-consuming, and flexible membranes with selective wettability are currently advantageous for oil/water separation. Here, we report an effectual strategy to prepare composite membranes by combining covalent organic frameworks (COFs) with electrospun nanofibers for oil/water separation. A dip-coating strategy was developed to prepare two different Schiff base COFs [2,4,6-triformylphloroglucinol–4,4′-diaminobiphenyl (Tp-BD) and 3,5-tris(4-aminophenyl)benzene–terephthalaldehyde (TAPB-TPA)] on polyacrylonitrile (PAN) nanofibers at room temperature, in which Tp-BD and TAPB-TPA COFs could grow in situ on the surface of single-lined PAN nanofibers, structurally forming rough structures on the nanoscale. The thickness of the COF layer could be controlled by the concentration and volume of the ligand solution. Furthermore, on the basis of the rough structure, lauryl groups were linked to COF nanofibers to simultaneously obtain excellent hydrophobic COF nanofibers and maintain oleophilicity. Finally, superhydrophobic PAN/COF nanofibers with a water contact angle of 167° were applied to separate water-in-oil mixtures. The separation efficiencies of different samples with different oil and water concentrations were all greater than 95%, and the recycling performances of the materials were also satisfactory. The results show that the as-prepared nanofiber membranes have great potential for separating water-in-oil mixtures.