Preparation And Environmental Photocatalytic Properties Of Novel Porous Organic Polymer Composites

With the development of industrialization and the improvement of people’s requirements for environmental quality,high-efficiency,low-energy consumption,and non-polluting wastewater treatment technology has become one of the research focuses of water treatments.Among them,photocatalytic technology is driven by solar energy,has the characteristics of sustainability,high efficiency,and free secondary pollution,which received extensive attention as a new water treatment technology.As the core,the design and preparation of the photocatalysts play a key role in promoting the development of photocatalytic technology.Porous organic polymers（POPs）are new type of porous material formed by the connection of light elements such as C,N,and O through strong covalent bonds.It has the characteristics of high porosity,high stability,low density,designable structure,easy functionalization,and easy processing.In the meanwhile,the optical properties and catalytic activity of POPs can be regulated by energy band engineering.Selecting suitable monomers is of great significance for realizing the regulation at the molecular level and constructing POPs with high photocatalytic activity.In particular,POPs with conjugated structures have narrower band gaps and can more adequate utilization of visible light compare with other amorphous POPs.By further improving the crystallinity of POPs,covalent organic frameworks（COFs）with more stable structures,higher carrier mobility and charge separation efficiency can be obtained.In this paper,based on the imine POPs with conjugated structure,the photocatalytic properties of the composites were improved by constructing heterojunctions,improving crystallinity,dye sensitization,and substituent modification.FT-IR,BET,TGA,UV-Vis DRS,electrochemistry measure and other methods were used to explore the structure,morphology and mechanism of the material.This research mainly consists of the following parts:（1）Using 1,3,5-tris（4-aminophenyl）benzene（TPB）and 2,5-dimethoxy-1,4-dicarbaldehyde（TP-OCH₃）as monomers,TPB-TP-OCH₃POP was synthesized as the carrier to constructed g-C₃N₄/POPs heterojunctions using the normal pressure solvothermal method.The performance of the heterojunction composites in the photocatalytic Cr（Ⅵ）reduction under visible light condition were all improved to varying degrees.The fitted first-order kinetic rate constant for photocatalytic Cr（Ⅵ）reduction by the optimized g-C₃N₄/POP-2 was22.0 and 2.2 times that of g-C₃N₄and TPB-TP-OCH₃POP.（2）Using 1,3,5-tris（4-aminophenyl）benzene（TPB）and terephthalaldehyde（TP-H）as monomers,the crystalline TPB-TP-H COF was synthesized as a carrier to constructed PDI/TPB-TP-H heterojunction using the solvothermal method.PDI,as a kind of dye with narrow band gap,has high intensity and wide range of light absorption.Thus a large number of electrons can be photoexcited and then transfer to the conduction band of TPB-TP-H COF to participate in the photocatalytic Cr（Ⅵ）reduction reaction.Thus the performance of PDI/TPB-TP-H for the photocatalytic Cr（Ⅵ）reduction could be greatly improved.PDI/TPB-TP-H-5 with a PDI:TPB initial addition molar ratio of 32:1 had the highest photocatalytic reduction efficiency of Cr（Ⅵ）among the heterojunction,and the fitted first-order kinetic reaction rates are 12.3 and 3.6 times that of PDI and TPB-TP-H COF,respectively.（3）Using 1,3,5-tris（4-aminophenyl）benzene（TPB）and terephthalaldehyde monomers with different substituents,namely terephthalaldehyde（TP-H）,2,5-dimethoxy-1,4-and-formaldehyde（TP-OCH₃）and 2,5-difluoroterephthalaldehyde（TP-F）as monomers,synthesized the crystalline TPB-TP-COFs with different electronegativity substituents by solvothermal method（TPB-TP-H COF,TPB-TP-OCH₃COF and TPB-TP-F COF）.Substituents with different electronegativity can effectively improve the crystallinity,specific surface area,porosity,the range and intensity of light absorption,photocatalytic phenol oxidation,and Cr（Ⅵ）reduction activity compared with TPB-TP-H.The performance of TPB-TP-OCH₃COF with the electron-donating substituents of-OCH₃can be more effectively improved than that of TPB-TP-F COF with electron-withdrawing substituents of-F.The crystalline TPB-TP-COFs with different electronegativity substituents were further used as carriers to synthesize g-C₃N₄/TPB-TP-COFs heterojunctions.The heterojunctions synthesized of TPB-TP-COFs with different substituents can effectively improve the photocatalytic performance of phenol oxidation.However,due to the charge transfer efficiency between TPB-TP-COFs and g-C₃N₄would be affect by the electronegativity of the substituents,only g-C₃N₄/TPB-TP-OCH₃obtained the improved performance in photocatalytic Cr（Ⅵ）reduction.The superior generation,migration and separation of photogenerated electrons and holes make g-C₃N₄/TPB-TP-OCH₃have the best photocatalytic oxidation and reduction properties.