Surface charge enhanced synthesis of TpEB-based covalent organic framework (COF) membrane for dye separation with three typical charge properties

As a novel crystalline polymeric material, covalent organic framework (COF) has been paid the most attention by membrane separation researchers all over the world due to its pre-designable, regular pore structures, and reliable chemical stability. However, the fabrication of a homogenous, stable, thin COF layer on the support membrane remains challenging. In this work, the mechanism of electrostatic interaction between amino monomer and support was used to generate a stable and clear interface between the aqueous phase and the organic phase through the interfacial polymerization (IP) process. Specifically, the hydrophilicity and large pore size of polyacrylonitrile (PAN) mixed matrix membrane (MMM) allowed ethidium bromide monomers (EB) from the aqueous phase to penetrate and reach the negatively charged porous surface of the MMM. This assisted the fixation of the EB monomers at the interface with highly electrostatic interaction between the MMM’s surface and the EB monomers. The Tp (1,3,5-triformylphloroglucinol) monomer of organic phase was polymerized with surface entangle EB, resulting in the TpEB COF with very thin, hydrophilic, stable, homogeneously distributed tiny pores, and cationic surface charge. The prepared TpEB COF composite membrane had a water permeability of 32.34 L·m−2.h−1.bar−1, highly selective sieving performances for dye molecules of different charges and sizes. This membrane could reject ∼ 99.99 % of anionic dyes like Potassium permanganate (PP), Congo red (CR) and Coomassie brilliant blue G-250 (CBB), meanwhile maintain high solvent permeability for a long time. But for cationic or neutral dyes, the rejection rates mainly depended on their molecular sizes.