Material Preparation And Separation Performance Study Of Imine-based Covalent Organic Framework

Covalent organic framework（COF）is an emerging crystalline porous polymer consisting of organic building units precisely interconnected through covalent bonds to form a two-dimensional（2D）or three-dimensional（3D）open network.COF exhibits permanent porosity,high specific surface area,precise pore size,good stability,and a controllable chemical environment,making it a promising material for a wide range of applications,including catalysis,energy storage,sensing,gas adsorption,and separation.In particular,the application of COF in separation has the potential to greatly alleviate environmental pollution and energy shortages,provided it meets commercial application standards.Currently,traditional industrial separation technologies,such as distillation,are no longer in line with the new development concept of green environmental protection and energy conservation due to their high energy consumption（consume half of the total energy used in the chemical industry and 10-15%of the world’s energy consumption）.Many industrial separation processes,such as:H₂/CO₂,CO₂/CH₄,C₃H₆/C₃H₈,seawater desalination,dye wastewater purification,etc.,require more advanced and energy-efficient methods to optimize the separation process,which is a critical enterprise towards a sustainable energy future.Membrane separation and adsorption-based separation,as alternative new separation technologies,have become hot topics in academic and industrial research.Imine-based COF is the most widely studied COF material in the field of separation because it has the most abundant organic building units,strong processability,excellent crystallinity and stability,but it is still in infancy and still face many challenges.Therefore,this paper focuses on some of the issues currently faced by COF in the field of separation,and selects imine-based COF as the research object to prepare and explore it separation performance.COF obtained by traditional solvothermal method in Pyrex tubes usually yields only a small amount of crystalline powder,and it does not melt at high temperatures or dissolve in solvents.These characteristics make the large-scale production and membrane preparation of COF become challenging which limits the development of COF in the field of separation.To address the aforementioned issue,in chapters 2 and3 of this paper,a reaction kettle is used respectively under conditions of room temperature and organic vapor induction to prepare appropriate imine-based COF as membranes,which are then applied to nanofiltration and H₂/CO₂ separation.In addition,in chapter 4 of this paper,the organic vapor-induced strategy is continued to prepare a large amount of a nitrogen-rich COF powder with ultra-microporous pore size as an adsorbent,and evaluated its separation ability for CO₂/CH₄ in a dynamic breakthrough manner.The specific contents are as follows:（1）To address the energy consumption disadvantage of traditional solvothermal preparation of COF membranes,chapter 2 of this paper successfully prepared 2D P-COF membrane at room temperature.The obtained P-COF membrane has a permanent porosity,high specific surface area(565 m²·g^-1),precise pore size（1.1nm）,and regular one-dimensional mass transfer channels,which ensure high permeance while achieving accurate size sieving performance of P-COF membrane.By testing the nanofiltration properties of dye molecules of different sizes,it was found that P-COF membrane can accurately reject dye molecules（rhodamine B and acid fuchsin）larger than the intrinsic pore size of membrane material,and at the same time,the water permeance is up to about 50 L·m^-2·h^-1·bar^-1,which has important potential value for the purification of wastewater containing various dyes produced by the dyeing and textile manufacturing industry.In addition,the method of preparing COF membranes at room temperature is more energy-efficient and easier to operate,which is of great significance for the development of COF membranes if it can be widely promoted.（2）Most of the COF membranes reported so far are grown on porous supports,the mutual shielding between the support and the membrane and the properties of the support itself（thickness,porosity,compatibility,etc.）can all affect the performance of the membrane.However,self-standing membranes can effectively avoid these problems,because self-standing COF membranes are composed entirely of COF itself,and their abundant porosity can be fully utilized during the separation process,which may improve the performance of COF membranes.Therefore,chapter 3 of this paper used a vapor-induced method to avoid direct contact of organic building units with solvents and catalysts,and fabricated two self-standing membranes in homemade molds:2D N-COF membrane and 3D COF-300 membrane.Both self-standing COF membranes have narrow pore sizes and abundant CO₂ adsorption sites.Therefore,they were applied to H₂/CO₂ separation,and the test results showed that the H₂permeance of the two COF membranes was up to 4319 GPU and 5160 GPU,respectively,at room temperature,which is very prominent among all the COF-based membranes used for H₂/CO₂ separation;At the same time,the separation selectivity of the two COF membranes is 13.8 and 11,respectively,and their comprehensive performance exceeds the Robeson upper bound revised in 2008.Moreover,the separation performance of the two COF membranes did not significantly decrease at high temperatures of 100℃,demonstrating outstanding thermal stability.This chapter also used gas adsorption testing and Grand Canonical Monte Carlo（GCMC）simulation to explain the separation mechanism of the two COF membranes.This method provides a convenient strategy for preparing self-standing COF membranes and promotes the development of COF membranes to some extent.（3）In order to overcome the problem of low efficiency of traditional solvothermal for preparing COF.Continuing the vapor-induced strategy,chapter 4 of this paper have successfully synthesized a sub-gram（0.83 g）amount of imine-based N-COF powder with ultra-microporous pore size and nitrogen-rich pore environment in a one-pot reaction vessel.Compared to the traditional solvothermal method,this preparation process is more convenient,and the obtained N-COF exhibits slightly improved crystallinity and specific surface area compared to N-COF synthesized by the solvothermal method reported previously.The nitrogen-rich pore environment enhances the CO₂ adsorption capacity of N-COF,while the narrow ultra-microporous pore size improves the kinetic selectivity of N-COF for gas separation.Therefore,this chapter believe that N-COF has the potential for dynamic breakthrough separation of CO₂/CH₄.Adsorption tests have shown that N-COF exhibits an ideal adsorption selectivity of 12.86 for CO₂/CH₄.Subsequent dynamic breakthrough tests have demonstrated significant separation performance of N-COF for CO₂/CH₄ gas mixtures.This batch preparation method for COF saves a significant amount of experimental time and may have practical applications in the future.