Covalent organic framework membranes reconstructed pore channels through metal-phenolic complexation for dye/salt separation

Abstract

Covalent organic frameworks (COFs) characterized by high porosity, ordered channels, and remarkable chemical stability are potential candidates for the fabrication of nanofiltration separation membranes. In general, COFs membranes, generated through the process of interfacial polymerization, exhibit superior permeance without comparatively poor selectivity toward small molecules. Hereby, it was addressed with a simplified route to construct an extra selective layer on the COFs membranes. Concretely, COFs (TpPa/HPAN) membranes were first fabricated via interfacial polymerization on the hydrolyzed polyacrylonitrile (HPAN) membranes by utilizing monomers of 1,3,5-triformylphloroglucinol (Tp) and p-phenylenediamine (Pa). Then, a selective layer of metal-phenolic complex was constructed on the TpPa/HPAN membranes by electrostatic adsorption and metal-phenolic complexation. The introduction of a metal-phenolic complex selective layer masked the intrinsic pore size of TpPa and enhanced the hydrophilicity and electronegativity of the membrane, which improved the rejection property of the membranes for dyes larger than 350 Da. The fabricated M-TpPa/HPAN membranes showed exceptional performance in dye desalination with a water permeance of 92.1 L‧m−2‧h−1‧bar−1, rejections of Congo red and Eriochrome black T (EBT) over 99 %, 78.6 % of acid fuchsin rejection, 85.3 % of acid orange 7 rejection, low rejection of salts (e.g., 6.2 % for NaCl rejection), and the NaCl/EBT separation factor up to 243. Additionally, the M-TpPa/HPAN membranes maintained excellent stability during long-term filtration. The strategy of building an extra selection layer with eco-friendly, cost-effective, and scaled-up methods is also extensible for the other COFs and framework-based membranes.