Design,Synthesis,and Photocatalytic Performance Of Melamine-based Covalent Organic Framework And Its Composites

With the vigorous development of industry,rapidly increasing energy demand and global environmental degradation are identified as two major challenges.The prevalence of antibiotics in aquatic systems has become an environmental concern.Tetracycline（TC）is the second most widely used antibiotic and can be used for disease control and growth promotion in animals.The treatment of TC containing wastewater has attracted attentions,but it is still an unsolved problem.Photocatalysis with the use of inexhaustible solar energy may be an ideal approach.Photocatalyst plays a key role in the photocatalytic reaction process,and finding suitable photocatalyst is the key to achieve high photocatalytic performance.Compared to traditional photocatalyst,covalent organic framework（COFs）with the advantages of low density,high porosity,and stable structure,especially that the COFs like Lego blocks allow the precise connection of functional organic molecules through covalent bond and extend to porous crystalline structure,which can realize effective regulation between structure and function,are a new type of photocatalytic materials.Although some progress and achievements have been made in recent years,the application of COFs in photocatalysis is still in its infancy.Considering the limited light absorption range,low photoelectron-hole separation efficiency and high charge recombination rate of COFs,this work studied the photocatalytic properties of COFs-based materials by adjusting the combination sites of functional monomer molecules,adopting a variety of synthesis methods and constructing different types of heterojunctions.The performance and mechanism of removing TC from water based on COFs photocatalytic material were discussed.Also,considering that hydrogen peroxide（H₂O₂）is an important raw material for advanced oxidation technology,the photocatalytic production potential of H₂O₂ was analyzed and discussed.The detailed research contents and conclusions are as follows:（1）A typical class of melamine-based COFs is Schiff base network polymers（SNWs）,whose structure and properties can be regulated by selecting the types of aldehydes,binding sites of monomer molecules and steric hindrance.Therefore,in order to explore the influence of different binding sites and steric hindrance on the structure and performance of SNWs,a series of structurally related melamine-based SNWs were prepared by solvothermal method in Chapter 2.The structure of SNWs was precisely adjusted by selecting the connection sites of melamine and phenylaraldehyde.Due to the influence of steric hindrance,different structures（SNW-o,p,m）were obtained and their light absorption and photocatalytic properties were different.SNW-o with highly delocalizedπ-system exhibits better visible light absorption.In the visible light irradiation time of 60 min,the degradation efficiency of TC by SNW-o was 81.2%,and the degradation rate constant was 0.02516 min^-1.Moreover,the photocatalytic yield of H₂O₂ was 2.135 times higher than that of traditional carbon nitride（g-C₃N₄,CN）.In summary,this chapter successfully synthesized SNWs with good photocatalytic performance by using low-cost melamine and phenylaraldehyde as monomers,increasing the possibility of its industrial production.However,the SNWs obtained in the experiment are mainly amorphous structure,and it is necessary to further explore the preparation and synthesis of more stable COFs with better crystallinity.（2）In the above study,the synthesis of SNWs was based on solvothermal synthesis with the reversible reaction of Schiff base,which requires strict control of reaction conditions such as vacuum sealing and long reaction time to adjust the crystal structure.To solve these problems,microwave-assisted synthesis of Tp Ma COF based on melamine（Ma）and 1,3,5-triformylphloroglucinol（Tp）was conducted in Chapter 3 of the paper.Meanwhile,Tp Ma _TD COF was synthesized by solvothermal method to reveal the difference of photocatalytic properties caused by the synthesis method.Compared with three days of solvothermal reaction,the microwave-assisted method can successfully synthesize the materials in 20 min.The obtained TpMa _MW had better crystallinity and higher yield.The structure and photocatalytic activity of Tp Ma _MW did not change significantly after cycles experiments,indicating that Tp Ma _MW had good stability.However,Tp Ma COF still has the problem of fast electron-hole recombination rate and low photocatalytic degradation efficiency.Therefore,the photocatalytic performance of Tp Ma COF in subsequent experiments was further optimized.（3）Heterojunction construction is recognized to be an attractive way to achieve remarkable photocatalytic performance.To better improve the photocatalytic performance of Tp Ma,a series of metal-free Tp Ma/CN II heterojunctions were prepared by simple and controllable self-assembly method in Chapter 4.The absorption range of the prepared Tp Ma/CN heterojunction was more than 500 nm,which was higher than that of the pristine CN.The photocatalytic experiment showed that Tp Ma/CN-5 photocatalyst had the highest photocatalytic activity for TC degradation,and its photocatalytic degradation rate was 2.3 and 4.3 times higher than that of Tp Ma and CN,respectively.And the photocatalytic activity of TpMa/CN-5 sample to H₂O₂production(880.494μM h^-1)was 49 times higher than that of pristine CN.Experimental and theoretical studies showed that with the interaction of Tp Ma and CN,a built-in electric field was formed at the interface of the composites,and electrons transferred from the surface of CN to the surface of Tp Ma,and oxygen atoms in Tp played an important role as the transfer site.Upon the visible-light irradiation,driven by the built-in electric field at the interface and the type II band structures,the photogenerated electrons of Tp Ma could be rapidly transferred to the conduction of CN,accelerating charge separation.This study demonstrates that Tp Ma as a co-catalyst can not only effectively improve the performance of traditional photocatalysts,but also reduce the costs caused by expensive raw materials for COFs synthesis.（4）In order to further prove the applicability of TpMa as a cocatalyst,and considering that the improvement of charge separation efficiency in type II heterojunction is at the cost of reduction capacity,which is not conducive to the photocatalytic reaction,Tp Ma/NH₂-Ui O-66（TUN）S-type heterojunction was prepared by in-situ growth of NH₂-Ui O-66 on the surface of Tp Ma in Chapter 5.Density functional theory calculation showed that after the heterojunction was constructed,the electrons of Tp Ma were transferred to NH₂-Ui O-66 to form a built-in electric field.The photoelectrochemical experiments showed that the surface charge transfer efficiency,carrier lifetime and injected electron lifetime of the TUN heterojunction increased,while the photogenerated charge recombination rate decreased.The optimized TUN-3showed the highest photocatalytic activity and could achieved almost 100%degradation of TC within 90 min with adsorption and photocatalysis,and achieved a degradation rate constant of 0.02864 min^-1,which was 2.41 and 2.84 times higher than that of NH₂-Ui O-66 and Tp Ma,respectively.The H₂O₂ yield was 575.88μM h^-1 under visible light irradiation,and the apparent quantum efficiency(Φ_AQY)at 420 nm was4.15%.Moreover,the reaction path of two-electron reduction of O₂ to H₂O₂ was proved.In this paper,the design,synthesis and photocatalytic performance of melamine-based COFs and their composites were studied,and the effects of modification strategies such as synthesis precursor regulation,synthesis method selection and heterostructure construction on the photocatalytic tetracycline degradation and hydrogen peroxide production of were explored.This not only promotes the laboratory research,industrial production and practical application of COFs,provides a new perspective and technical support for the design and utilization of efficient and stable non-metallic photocatalysts,but also affords a theoretical basis and practical method for the application of visible light photocatalytic technology in practical water pollution control.