Covalent organic framework membranes with limited channels filling through in-situ grown polyaniline for efficient dye nanofiltration

Nanofiltration is a promising water treatment technology owing to its efficiency, energy-saving capabilities and environmental-friendly nature. Among the available nanofiltration membranes, polyamide membranes are commonly used to separate small organic molecules and divalent salts. Despite their high divalent salt rejection abilities, they are characterized by low porosity and dense structures, lowering their permeability, thus limiting their applications. Attempts to enhance the permeability of polyamide membranes have included incorporating nanomaterials, using new monomers, modifying the selective layers and the substrates, and adding new layers. However, the overall performance of these membranes is still limited. Thus, developing more effective and robust strategies for fabricating high-performance nanofiltration membranes is highly desirable.

Recently, covalent organic frameworks (COF) have been identified as a promising alternative to traditional polyamide membrane materials to fabricate nanofiltration membranes. Their advantages include high stability, high porosity, suitable pore sizes and regular tunnels. Several COF membrane fabrication strategies include blending, interfacial polymerization and in-situ growth. Among them, interfacial polymerization is generally considered the most simple and efficient approach for fabricating ultrathin layers. Several studies involving the fabrication of ultrathin and dense COF nanofiltration membranes have been proposed with remarkable permeance and rejection performance. Some use polyaniline (PANI), which has been widely used to fabricate different separation membranes. Nevertheless, fabricating dense and ultrathin COF nanofiltration membranes with high rejection and high permeance properties remains a big challenge.

To this note, a team of Tianjin University Researchers: Engineer Chenyue Mao, Professor Song Zhao, Mr. Pengpeng He, Professor Zhi Wang and Professor Jixiao Wang developed a novel strategy for fabricating COF membrane with limited pores and channels filling through in-situ polyaniline layer growth for efficient dye nanofiltration applications. They prepared COF membranes through interfacial polymerization of TpPa layer on hydrolyzed polyacrylonitrile (HPAN) substate. Following was the in-situ growth of polyaniline onto TpPa to form PANI-TpPa composite nanofiltration membrane. The membrane structure, optimal fabricating conditions and membrane separation performance were investigated and discussed. Their main aim was to achieve high permeance and dye rejection simultaneously. Their work is currently published in the research journal, Chemical Engineering Journal.

The research team demonstrated the improved PANI-TpPa/HPAN membrane performance. Its reaction parameters were reported to be 0.075M for the concentration of aniline, 30 s for the reaction of the TpPa layer and 1:3 for the APS: aniline molar ratio. The membrane also exhibited an excellent pure water permeance of 85.2 L.m^-2.h^-1.bar ^-1 with high rejection above 81% for acid fuchsin and 99% for congo red. In addition to high stability in organic solvent and acidic environment, it also maintained remarkable performance in the long-term test. Notably, the high dye rejection of the new membrane was attributed to the insertion of the PANI chains into the TpPa layer, which functioned to decrease the sizes of the TpPa pores to make the PANI layers denser. Similarly, the membrane amained high permeance due to its flexible structure and the hydrophilic surface of the PANI layer.

In summary, the authors developed a facile and efficient method for the fabrication of polymer-COF composite nanofiltration membrane consisting of in-situ PANI layer growth and interfacial polymerization of TpPa layer. Compared to other fabrication strategies, this approach was less time-consuming and was more efficient as it could be carried out at room temperature. It also exhibited high permeance and high rejection, making it a promising approach for dye separation applications. In a statement to Advances in Engineering, Professor Song Zhao, the corresponding author said the novel strategy contributes to designing and fabricating high-performance COF membranes for various applications.