Design,Preparation,and Application Of Ionic Covalent Organic Framework(COF) Nanosheet Membranes

Membrane separation is a new separation technology that emerged rapidly after the 1960s,in which its core is the thin-film materials with separation function.As a potential alternative to replace the traditional separation methods driven by thermal energy,membrane separation technology offers new opportunities to solve the major issues encountered by mankind,such as environmental pollution,water shortage and energy crisis.With the increasing industrialization and the global population explosion,these problems in terms of environment and resources are becoming more and more serious and complex,which puts higher demands on membrane separation technology.The permeability and selectivity of porous membranes based on conventional polymeric materials often often are interinhibitive due to the classic trade-off effect.One of the primary reasons of this problem is the inhomogeneous pore size distribution in these membranes.Therefore,it is an important study interest for the development of membrane materials to normalize the pore size.Hence,the porous crystalline materials with uniformal aperture are considered as one of the ideal materials for the preparation of high-performance separation membranes.Covalent organic frameworks（COFs）are a class of organic porous crystalline materials constructed by the building blocks of organic small molecule,which are arranged in a highly ordered and periodic network through strong covalent bonds.The pore size of COFs can be precisely adjusted in the range of 0.5-10 nm.In addition,the excellent physicochemical properties of COFs are perfectly meet the requirements for the development of high-performance separation membranes in which the highly ordered pore channels,rich pore structure,and excellent stability are conducive to high selectivity,high permeability,and good operational stability,respectively.This thesis devotes to the design,preparation and application of high-performance separation membranes based on the COF materials,and mainly focuses on the rapid preparation of COF nanosheet membranes and the enhancement of their separation performance.In this thesis,a series of COF nanosheet membranes with enhanced separation performance are constructed by introducing monodisperse charge sites in the pore channels of COF nanosheets,and their applications in ion separation,nanofiltration and gas separation are systematically investigated as follows.（1）A room-temperature salt template method was developed for the preparation of ionic COF nanosheets.Then,the dense cationic COF nanosheet membranes with regular structures were fabricated by the positive pressure-assisted self-assembly method with constant driving force.A charge-induced cation confined transport mechanism within the positively charged nanochannels was proposed to achieve efficient and rapid sieving of mono-/multi-valent ions.The transport mechanism proposed here was analyzed using an approach combined theory with experiment.（2）A diffusion and solvent co-mediated modulation approach was employed to synthsize the anionic COF nanosheets.Then,a novel two-cells electrophoretic deposition（EPD）method with constant driving force was developed to rapidly prepare ultrathin COF nanosheet membranes in the hydrous system.Moreover,this method exhibited universality towards different substrats and COF nanosheet with different charge.The charge-enhanced sieving properties of anionic COF membranes with regular structures for negatively charged molecules were explored.The experimental results indicated that a synergistic effect of size sieving and electrostatic repulsion jointly contribute to the excellent nanofiltration performances.（3）A new room-temperature liquid-phase method with scalability was developed for the preparation of ionic COF nanosheets.Then,the sandwich-type（cationic|anionic|cationic）COF composite nanosheets were constructed by the electrostatic self-assembly of two kind of ionic COF nanosheets with opposite charge.Finally,the composite nanosheets with net surface charge were assembled into multi-interfacial heterojunctions COF membranes by the EPD method with constant driving force.Based on the multi-level sieving for the gas molecules with different kinetics size at the heterojunction interface,multi-interfacial heterojunctions COF membranes exhibited enhaced selectivity of H₂/CO₂.In this thesis,a series of ionic COF nanosheets were synthesized by the salt-template method and liquid-phase synthesis method at room temperature.Then,the ionic COF membranes with regular structure were prepared on the porous substrates by the positive pressure-assisted self-assembly method and EPD method with constant driving force.By introducing abundant and monodisperse charge sites into the pore walls of COF nanosheets,the pore environment and stacking model of COF membranes were precisely regulated,which further endowed COF membranes enhanced separation performances in nanofiltration,ion separation and gas separation,demonstrating the great potential and application prospects of COF materials in the membrane separation.