Preparation Of Ultrathin Composite Nanofiltration Membranes Based On Covalent Organic Frameworks

The preparation process and structure properties regulation of separation layer are important to the preparation of ultra-thin composite nanofiltration membrane.The targets of this study are to reduce the thickness of the composite nanofiltration membrane and control the interfacial polymerization process.Based on the hybrid modification of polyamide layer and the optimization of supporting layer structure,we introduced the covalent organic frameworks?COFs?to prepare thin and compactness polyamide layers for thin-film composite?TFC?nanofiltration membranes.The separation performance,structure and morphology of nanofiltration membranes were investigated.The effect of introducing COFs through different ways on the process of interfacial polymerization was deeply explored.Firstly,a hybrid TFC membrane with high flux was prepared by incorporating hydrophilic CTF-1 into the polyamide separation layer.The introduction of CTF-1 in the polyamide layer increased the membrane hydrophilicity and controlled the adsorption/diffusion process of the amine monomers during the interfacial polymerization process.The thickness of separation layer was reduced from 238nm to164nm through the cross-section morphology.Besides the interspace between polyamide chains and CTF-1 brought about interface channels for water transfer.Meanwhile,the pore structure of the CTF-1 provided additional channels which could further enhance the water flux of the membranes.Therefore,the water permeation flux of CTF-1-modified TFC nanofiltration membrane rose to 259.84 L m^-2 h^-1 MPa^-1and the dyes rejection remained around 90%.Secondly,the ultrathin composite membranes with enhanced nanofiltration performance were prepared by introducing the pure COF interlayer to optimize the hydrophilicity and porous structure of the supporting layer.The pure COF interlayer showed good water adsorption capacity and improved the surface hydrophilicity of the pristine supporting layer.In addition,it provided sufficient space for the storage of amine monomers and controlled the adsorption/diffusion process of the amine monomers.The thickness of separation layer was reduced from 47nm to 43nm through the cross-section morphology.Therefore,the water permeation flux of optimized TFC nanofiltration membrane rose to 413.85 L m^-2 h^-1 MPa^-1,which was3.38 times that of the pristine PA/PES membrane.At the same time,the rejection of Na₂SO₄ and dyes remained above 90%.Thirdly,the ultrathin composite membranes with enhanced nanofiltration performance were prepared by interfacial polymerization on the polydopamine?PDA?-covalent organic framework?COF?interlayer.The interlayer showed good surface hydrophilicity and high porosity,which could control the adsorption/diffusion of amine monomers during the interfacial polymerization process,resulting in an ultrathin and compactness polyamide layer.The interlayer not only optimized the porous structure and hydrophilicity of the support layers,but also significantly improved the interfacial interaction between the polyamide separation layer and the PAN support layer.The thickness of separation layer was reduced from 79nm to 30nm through the cross-section morphology.The water permeation flux of that membrane reached up to 207.07 L m^-2 h^-1 MPa^-1 with high rejections of Na₂SO₄?93.41%?and Orange GII?94.47%?.Moreover,PDA-COF interlayer endowed the modified nanofiltration membrane with good structural stability after alcohol treatment for 16days.