Synthesis And Gas Separation Application Of Functional Three-dimensional Covalent Organic Framework

Covalent organic frameworks(COF_S)are a new type of crystalline porous materials.Although there are more and more reports about COFs in the past ten years,most of them mainly facous on 2D COF_S with layered eclipsed structure.Therefore,the preparation of 3D COFs,especially new functional 3D COF_S is very important for enriching the diversity of COFs and expanding the application of COFs.Compared with traditional gas separation methods such as cryogenic distillation,membrane separation and adsorption separation have the advantages of cost-and energy-efficient,high separation efficiency and samll carbon footprint.In recent years,COFs stand out in the field of gas separation due to their unique properties.However,the research progress of COF-based in gas separation is extremely limited.On the one hand,the traditional Pyrex reaction vessel not only makes the COFs preparation process cumbersome,with great safety risks and high cost,but also restricts the production of COFs and the preparation of COF-based gas separation membrane.On the other hand,the fabrication of dense,continuous COF membranes with high selectivity for gas separation still remains a huge challenge.In addition,the narrow application range also seriously hinders the development of COFs in gas separation.For example,although the adsorption separation technology based on COFs adsorbent is widely used in CO₂separation,CH₄ purification and C₂H₂/C₂H₄ separation,the application of COFs in hydrogen isotope separation is rarely reported,especially,the application of COFs in hydrogen isotope separation at room temperature is still a blank.Based on the above problems,we prepared two novel kinds of functional 3D COFs,and studied the application of COFs in gas separation from two aspects of membrane separation and adsorption separation.The main contents are summarized as the following three aspects:(1)Two novel functionalized 3D COFs,named Me O-COF and CPOF-1,were prepared by Schiff-based condensation reaction using Teflon-lined autoclave and flask instead of Pyrex as reaction vessel.The flexible selection of reaction vessels not only reduces the potential safety hazards in the preparation process of COFS,but also greatly improves the production of COFs,and provides convenience for the fabrication of COF-based gas separation membrane.In addition,the ether oxygen groups and hydroxyl groups in the framework structure is beneficial to its subsequent application research.(2)According to the previous chapter,Me O-COF showed CO₂ adsorption affinity due to the strong quadrupole interaction between ether oxygen group and CO₂.In addition,the pore size of Me O-COF was only 1.06 nm.Based on this,we believe that MEO-COF will be an ideal material for gas separation membrane.In this chapter,dense and defect-free Meo-COF membranes were prepared on porous alumina substrates by secondary functionalization.Gas separation performance results showed that Me O-COF membrane had a high perm-selectivity of H₂/CO₂,the ideal separation factor and sepafaction factor of H₂/CO₂ were 9.72 and 8.33,respectively,with H₂permeance of up to 6.64×10^-7 mol m^-2 s^-1 Pa^-1.Through the study of structural-function correlation,it was found that the introduction of etheroxy functional group enhanced the H₂/CO₂ separation performance of Me O-COF membrane,which confirmed that the enhancement of adsorption-diffusion effect contributes to the improvement of selectivity of gas separation membrane.(3)In this chapter,after post-modification of CPOF-1,a palladium nanoparticle doped composite material(Pd@CPOF-1)was obtained,and the application of COFs in hydrogen isotope separation at room temperature was studied for the first time.By studying the mechanism of Pd@CPOF-1 hydrogen isotope separation,it was found that the confinement effect of CPOF-1 nanosize channel contributed to increased reaction activity of the Pd nanoparticles lowering the enthalpy thereby allowing the Pd react with D₂ at the room temperature.Therefore,Pd@CPOF-1 exhited strong D₂ adsorption selectivity.In addition,after ten cycles of experiments,the hydrogen isotope separation performance was not weakened.Moreover,the crystallinity and morphology of Pd@CPOF-1 was retained,which was indicative of the excellent regenerability and stability of Pd@CPOF-1.Pd@CPOF-1 is promising an ideal adsorbent for hydrogen isotope separation with low energy consumption,low cost and high efficiency.This work not only achieved the breakthrough of hydrogen isotope separation at room temperature,but also marked a significant milestone in the application of COFs in hydrogen isotope separation,and provided a new idea for the design and development of functional COFs.