Synthesis And Applications Of Functional Covalent Organic Frameworks

Covalent organic frameworks(COFs),composed of light elements such as C,H,O,N,B are a class of crystalline organic porous polymers.These materials not only have low densities and high surface areas,but also have the characteristics of adjustable pore size,well-ordered internal structures,diversified topologies and wide sources of monomers,which endow them with advantages of excellent modifiability and diversified functions.This thesis mainly focuses on the synthesis and applications of imine-linked COFs,for which their applications in heavy metal ion adsorption,organic pollutant absorption,chemical sensing,and energy storage have been developed.In the first part of this thesis,we firstly gives a brief introduction on the background of organic porous materials.Then the progress achieved for COFs have been extensively reviewed,which includ es types of linkages,design strategies of topological structures,synthetic methods,and their applications.In the last part,the significance and necessity of the research involved in this thesis have been discussed.In the second chapter,we explored the application of COFs in removal of heavy metal ions from water.It is important to remove hexavalent chromium Cr(VI)from water because it poses a huge threat to the water ecological environment and human health.Adsorption of heavy metals by porous materials is a promising technique to address this issue.However,the design of adsorbents with rapid uptake and high capacity remains a challenge.A novel dual-pore COF bearing hydroxyl groups was synthesized and exploited to remove chromate from aqueous s olutions.It exhibits a fast removing process(272 Cr mg/g with 10 min),with a maximum adsorption capacity up to 384 Cr mg/g(p H = 1),which is among the highest value reported for organic porous materials up to now.On the basis of the mechanistic study,we found that the adsorption sites are mainly the hydroxyl groups on the backbone of the COF.Under acidic conditions,the hydroxyl group form hydrogen bonds with the oxygen atoms of chromate to enhance adsorption effect of the COF.In the third chapter,we report exploration of COFs as efficient adsorbents for removing organic pollutants from aqueous solutions.Bisphenol A(BPA),a compound widely used as a plastic additive and organic synthetic raw material,causes great interference to the human endocrine system.Therefore,it is necessary to develop new absorbents with excellent performance to remove BAP from water.In this section,we synthesized three stable COFs,COF-Tp-DAB,COF-Tp-TABPB and COF-Tp-DATP,and systematically evaluated their adsorption ability toward BPA.They exhibits high uptakes of BPA with the maximum values to be 542.07,556.33,535.59 mg/g for COF-Tp-DAB,COF-Tp-TABPB,COF-Tp-DATP respectively.Moreover,with the initial concentration of BPA and adsorbent to be 0.144 and 0.250 m g/m L,respectively,the removal efficiencies of the three COFs are larger than 80% within 5 mins,and excellent recyclability was observed.In the fourth chapter,we focus on exploring the application of COFs in chemical sensing.Acid gases,such as hydrogen chloride(HCl),pose a great threat to human health and industrial safety.In this context,developing chemsensors for their rapid and sensitive detection is necessary.In this section,we synthesized a tetraphenylethenebased 2D COF.It exhibits a weak yellow fluorescence emission and turns very quickly to a strong red emission(less than 1 sec.)upon exposure to HCl gas in the solid state,which is also apparent to the naked eye observation under visible light.In addition,the intensity of fluorescence emission of the COF suspension increases with the increase of HCl concentration,and the minimum detection limit can reach 4.7 ppm.Therefore,this COF can be used as a probe for visualization sensing of HCl.Further tests showed that the color can be restored after being fumigated with alkali steam.These experimental results show that the COF synthesized by this strategy has good repeatability.The mechanism study reveals that imine bonds are the recognizing sites and their protonation and deprotonation account for the change of optical property of the COF.In the fifth chapter,we explored the application of COFs in electrochemical energy storage.Lithium-Sulfur(Li-S)batteries have the characteristics of high energy density and low cost.However,the shuttle effect seriously affects their service life and efficiency.In order to solve this problem,we synthesized a dual-pore microporous COF.A sulfur loading of 74% can be reached in this COF.The as-prepared battery exhibits a capacity of 756 mAh/g at a current density of 1 C,and a capacity of 454 mAh/g can be maintained after 500 cycles,which shows a good cycling stability.