Preparation And Photocatalytic Properties Of Porphyrin-Based Covalent Organic Frameworks

How to efficiently develop and utilize renewable energy is an important issue facing human in the future.Converting solar energy into usable chemical energy through photocatalysts is one of the feasible methods at present.Among all kinds of photocatalytic materials,porphyrins covalent organic framework materials have been widely used in the field of photocatalysis because of their advantages such as strong light absorption,good stability,narrow band gap and adjustable functional groups.In this paper,the synthesis of hollow tubular covalent organic framework and the influence of its special morphology on photocatalytic reduction of CO₂ were studied.Secondly,constructing the heterojunction based on g-C₃N₄ and porphyrin covalent organic framework to tune the valency band of COF and improve its photocatalytic activity of reduction of CO₂.This paper is mainly divided into the following two parts:（1）Synthesis of hollow rod-like porphyrin COF and its photocatalytic reduction of CO₂.Porphyrin covalent organic framework（COF）was synthesized from 5,10,15,20-tetra（4-aminophenyl）-porphyrin（Tph）and 2,5-dihydroxy p-phenyldialdehyde（Dha）by schiff base condensation reaction.Hollow DhaTph Tubes and DhaTph-Cu Tubes were prepared by using 2,4,6-trimethylbenzaldehyde as competitive reagent under solvothermal condition.The formation mechanism of hollow tubular morphology is revealed in detail.The COF self-assembly formed at the beginning is a solid rod,and the COF microcrystals migrated out of the inner center due to the unstable part of the inner center and changed to a tubular shape as time increase.The hollow DhaTph-Cu Tubes showed high photoreduction efficiency for CO₂ with a rate constant of 15.9μmol g^-1 h^-1,which was 3 and 2 times of that of DhaTph Tubes and DhaTph Tubes,respectively.DhaTph-Cu Tubes have high specific surface area,strong visible light absorption,fast separation and transfer of electrons and holes,which are the main reasons for their high activity of photocatalytic reduction of CO₂.（2）Preparation of g-C₃N₄-DhaTph-Cu and its photocatalytic reduction of CO₂.g-C₃N₄ was modified by 2,5-dihydroxy p-benzaldehyde,and then DhaTph-Cu COF was in-situ grown on the surface of g-C₃N₄"from bottom to top"by schiff base reaction.It was found that the crystallization effect of g-C₃N₄-DhaTph-Cu was the best when the feed ratio of g-C₃N₄to Tph-Cu is 1.5:1.The rate of CO formation in the photocatalytic reduction of CO₂ by g-C₃N₄-DhaTph-Cu（1.5:1）is 6.4 times and 4.7 times that of g-C₃N₄ and DhaTph-Cu Tubes,respectively.The g-C₃N₄-DhaTph-Cu forms an S-type heterojunction,which reduces the recombination rate of photogenerated electrons and holes,and makes the valence band and conduction band have better redox ability.The photocatalytic performance of g-C₃N₄-DhaTph-Cu（1.5:1）is mainly due to the improvement of crystallinity and the construction of heterojunction structure.