Preparation Of Two-dimensional Covalent Organic Framework Films And Their Application In Organic Nanofiltration

Membrane separation technology exerts unique advantages in the fields of water treatment,and has become the most promising separation and purification technology.Designing and preparing separation membranes with high selectivity and high permeability is the key to improving membrane separation efficiency and expanding the application of membrane separation technology.Two-dimensional covalent organic framework（2D COF）film is a new type of organic crystal film materials with regular pore structure,which possesses the merits of designable porous and chemical structures,high crystallinity,good stability.2D COF films have shown promising potential and brilliant prospective in the preparation and application of separation membranes with high permeability and high selectivity.Based on the highly ordered pore structure and designable and modifiable molecular structure characteristics of 2D COF films,this dissertation focuses on the liquid-liquid interface synthesis method to efficiently and low-costly prepare imine 2D COF/PTFE membranes at room temperature.The dominant role of surface charge characteristics in nanofiltration separation is illustrated based on the molecular structure design to obtain a high efficiency to remove the organic dye molecules from aqueous solutions.These researches provide the important research foundation for the preparation of highly selective and highly permeable nanofiltration membranes and the expansion of the application of 2D COF membranes in membrane separation technology.This dissertation mainly contains the following parts:（1）The COF_TAPA-TFPA films with a smooth surface,ordered structure and controllable thickness are synthesized from tris（4-aminophenyl）amine（TAPA）and 2,3,5,6-tetrafluoro-1,4-dibenzaldehyde（TFPA）via the water/organic interfacial polymerization at room temperature under the catalysis of acetic acid.By improving the method to remove the reaction solution,the prepared COF_TAPA-TFPA film is uniformly attached to the surface of the polytetrafluoroethylene（PTFE）film,obtaining a high-quality COF_TAPA-TFPA/PTFE membrane.Under the optimized conditions（4 m M TAPA,6 m M TFPA,room temperature for 8 h）,the COF_TAPA-TFPA/PTFE membranes show the higher rejection rate（>98%）,high stability and high selectivity to the four dye molecules:Rose Bengal,Methylene Blue,Congo Red and Chrome Black T.Based on the intramolecular charge transfer between TAPA and TFPA as electron donator and acceptor,respectively,and surface negative characteristics of COF_TAPA/TFPA film,the electrostatic repulsion of COF_TAPA-TFPA/PTFE film plays a dominant role in the nanofiltration to obtain excellent separation effect on dye molecules with negative charge.（2）The ordered COF_TTA-DHTA films and COF_TTA-DHTA/PTFE membranes are also synthesized at the water/organic interface at room temperature via the Schiff base condensation reaction between 4,4’,4’’-（1,3,5-triazine-2,4,6-triyl）triphenylamine（TTA）and 2,5-dihydroxy-1,4-phthalaldehyde（DHTA）catalyzed by acetic acid.The hydroxyl group and protonable triazine motifs in COF_TTA-DHTA films endow the hydrophilic pore structure,facilitating the rapid passage of polar solvents such as water and alcohol.The negatively charged surface features the rejection of negatively charged organic dye molecules in the nanofiltration separation procedure.Under the optimized conditions（1 m M TTA,1.5 m M DHTA,room temperature for 4 h）,the prepared COF_TTA-DHTA/PTFE membrane has a separation efficiency of～100%and good separation stability for the three negatively charged dye molecules of Rose Bengal,Congo Red and Chrome Black T.