Stringing covalent organic framework particles for preparing highly loaded mixed-matrix membranes for efficient and precise dye separation

Covalent organic frameworks (COFs) based mixed-matrix membranes (MMMs) combining the merits of processable polymers and ordered porosity are potential for molecular separation. However, it remains a challenge to control the interfacial compatibility to prepare high COFs loaded MMMs. Herein, we developed a facile strategy of "Stringing COF particles" for improving the interfacial compatibility to prepare high COFs loaded MMMs with high separation performance. The MMMs could be prepared by simply mixing COF powders with chitosan (CS) solutions, without needing the complicated nanosheetization or modification of COFs. Three typical COFs were synthesized and tested. The suitable large pore size and good hydrophilicity of COFs were confirmed crucial to the "Stringing" strategy. Characterizations indicated CS could enter the larger pores of the COFs (named COF-TADH) and firmly string COF particles together via strong H-bonding to obtain a tightly assembled COF-TADH@CS layer with COFs loading up to 67 wt%. The fabricated 67%COF-TADH@CS/nylon membrane exhibited excellent permeance and stable 100 % rejection to Coomassie brilliant blue (BB), highly competitive compared with other reported membranes. The high COFs loaded MMMs maintain the sieving merits of ordered COF pores, achieving precise separation of the mixed BB/Methyl orange (MO) solution with only MO penetrating through.