Synthesis Of Triazine-based Covalent Organic Frameworks For Photocatalytic CO₂ Reduction

Photocatalytic reduction of CO₂ to simulate artificial photosynthesis is a green and environmental carbon fixation technology,and the development of high-efficiency photocatalytic materials with visible light respons e is the key to realizing this new environmental friendly green techno logy.Triazine-based covalent organic frameworks（COFs）have several advantages as photocatalysts,such as high stability,excellent light absorption and electron-transporting property,easily tuned structure,high CO₂ adsorption capacity,and so on.Thus,it is a new type of carbon-fixing material with great potential in photocatalytic reactions.At present,the research of COFs in photocatalysis is still in its infancy,and there are problems such as low photocatalytic activity,low utilization of sunlight,and easily recombination of photogenerated electrons and holes,which limit its practical application in the field of photocatalysis.To this end,in this paper,triazine-based covalent organic frameworks TTA-TFP and BAO-TFP/PT with electron-rich largeπ-conjugated systems were synthesized by the reaction of amine-aldehyde condensation to form Schiff bases,and their physicochemical properties were systematically studied,and the performance of photocatalytic reduction of CO₂ under visible light was investigated.The results are as follows:1.The electron-rich largeπ-conjugated system COFs TTA-TFP was synthesized by taking 2,4,6-tris（4-aminophenyl）-1,3,5-triazine（TTA）as amino monomer and 1,3,5-tris（p-formylphenyl）benzene（TFP）aldehyde-based monomer,and CuInS₂ semiconductor was grown in situ with TTA-TFP as a carrier,and CuInS₂/TTA-TFP composite materials with different ratios were synthesized.Compared with pure TTA-TFP,the CuInS₂/TTA-TFP material exhibits more excellent photocatalytic performance for CO₂ reduction,among which,10 wt.%CuInS₂/TTA-COF exhibits the highest yields of HCOOH and CO,respectively171.6 and 35.2μmol g^-1 h^-1.Combined with various characterization techniques,the introduction of CuInS₂ can not only enhance the visible light response range of the composite material,but also effectively suppress the recombination of photogenerated electron and hole pairs,promote the separation and migration of photogenerated carriers,and then enhance its activity of photocatalytic CO₂reduction.2.Using 2,5-bis（4-aminophenyl）-1,3,4-oxadiazole（BAO）as the amino monomer,1,3,5-tris（4-formylphenyl）benzene（TFP）and2,4,6-tris（4-formylphenyl）-1,3,5-triazine（PT）as aldehyde monomers,two new COFs with largeπ-electric conjugated system（BAO-TFP and BAO-PT）were successfully constructed.Compared with BAO-TFP,BAO-PT with a planar triazine structure exhibits strongerπ-πconjugation,better visible light response,and better photo-generated electron-hole pair migration efficiency,leading to more excellent photocatalytic CO₂ reduction performance using ethanolamine as the electron sacrificial agent.Combining with inorganic semiconductor materials and constructing a largeπ-conjugated electron system are two effective ways to improve the photocatalytic CO₂ reduction performance of triazine-based covalent organic frameworks.The study not only can provide a new way for the development of novel visible-light-induced photocatalysts,but also can broaden the application of COFs and enrich the theory of the photocatalytic CO₂ fixation.