Direct growth of covalent organic framework nanofiltration membranes on modified porous substrates for dyes separation

Covalent organic frameworks (COFs) are emerging as crystalline porous materials for advanced membranes, due to their regular pore structures and high porosity. The fabrication of defect-free ultrathin COFs membranes is still very challenging, since the poor COF-substrate affinity hampers the nucleation of COF crystallites. In this work, a facile direct growth method was developed to deposit continuous imine-based COF layer (TpPa-1, synthesized by the Schiff-base reactions of 1,3,5-triformylphloroglucinol (Tp) with p-phenylenediamine (Pa-1)). Aldehydes groups were firstly immobilized on polyacrylonitrile (PAN) porous substrate as the nucleation sites to promote the growth of continuous TpPa-1 layer via a good COF-to-substrate adhesion. Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM) and X-ray diffractometer (XRD) confirmed the formation of COF layer. The TpPa-1/HPAN membrane exhibited high water flux of 418.5 L/(m² h MPa) under the operation pressure of 0.1 MPa and excellent dyes rejection (93.91% for Orange GII and over 99% for Methyl blue, Congo red and Alcian blue). The high separation performance, coupled with its facile fabrication and remarkable stability, may render imine-based COFs membrane promising for wastewater treatment.