Study On Preparation And Nanofiltration Properties Of Covalent Organic Framework Membranes

Membrane separation is the key technology to solve the problems of resource shortage,energy consumption and environmental pollution in China.High performance membrane materials are the key to achieve efficient membrane separation.Covalent organic frame materials（COFs）are porous crystalline polymers connected by covalent bonds,which are characterized by periodic regular pore structure and excellent chemical resistance and stability.Therefore,COFs are an ideal membrane material.However,COFs usually exist in solid powder and are almost insoluble in any solvent,which makes COFs poor in processability and membrane formation.In this study,a series of COFs membranes were prepared by the mediation of polydopamine layer,two-dimensional crystal seed layer and COFs interlayer,which showed excellent nanofiltration separation performance.It is expected to provide reference for the design,preparation and structure regulation of COFs nanofiltration membranes for molecular separation and salt ions/water molecular separation through the development of this study.The main research contents are summarized as follows:（1）Ionic TpPa-SO₃H/MPAN membranes were prepared on porous supports mediated by polydopamine（PDA）layer,based on biomimetic adhesion and in situ growth techniques.The PDA layer anchors the COF building blocks to the surface of the support through covalent/non-covalent interactions,and promotes heterogeneous nucleation and growth of TpPa-SO₃H.When the PDA deposition time is 1 h,the thickness of PDA layer is about 10 nm,which can provide abundant adsorption sites for building blocks without introducing additional water mass transfer resistance.The pure water flux of the TpPa-SO₃H/MPAN membrane prepared under the optimal conditions is 134.6 Lm^-2h^-1bar^-1,and the rejection rates for Chrome black T and Congo red are 99.2%and 99.1%,respectively.（2）Ionic TpPa-SO₃H/PAN membranes were prepared by the mediation of two-dimensional crystal seed layer,based on liquid-liquid interfacial polymerization,vacuum-assisted self-assembly and counter diffusion techniques.The TpPa-SO₃H seed layer regulated the diffusion-reaction behavior of COF building monomers,and induced the confined growth of TpPa-SO₃H,which is beneficial to better eliminate the intergranular defects of TpPa-SO₃H/PAN membrane.The charge of the channel wall inside the membrane can strengthen the electrostatic repulsion mechanism and realize efficient desalination.The thickness of TpPa-SO₃H crystal seed layer can be controlled by the volume of TpPa-SO₃H nanosheets dispersion.Under the optimal conditions,the pure water flux of prepared TpPa-SO₃H/PAN membrane is 1.4 Lm^-2h^-1bar^-1,and the rejection rate of Na₂SO₄ is 92.3%.（3）The ionic and non-ionic bilayer COFs membranes were prepared by the mediation of COF interlayer,based on biomimetic adhesion,in-situ growth and counter diffusion techniques.The COF interlayer provided confined space and template for the growth of COF top layer,and realized the controllable preparation of a variety of ionic/non-ionic bilayer COFs membranes.Interlaced aperture structure was created based on the interfacial confinement between the two COF layers.The non-ionic bilayer COFs membranes can realize the size screening mechanism strengthening,and the ionic bilayer COFs membranes can realize the size screening and electrostatic repulsion mechanism synergistic strengthening,and finally realize the efficient desalination.The pure water flux of the non-ionic TpPa/TpPa/MPAN membrane prepared under the optimal conditions is 2.0 Lm^-2h^-1bar^-1,and the rejection rate of Na₂SO₄ is 90.2%.The pure water flux of the ionic TpPa-SO₃H/TpPa-SO₃H/MPAN membrane prepared under the optimal conditions is 13.1 Lm^-2h^-1bar^-1,and the rejection rate of Na₂SO₄ is 98.3%,which is the best COF desalination membrane reported in the literature so far.