Design And Synthesis Of Covalent Organic Frameworks And Study On Their Photocatalytic CO₂ Reduction Performance

The massive consumption of fossil fuels has led to the excessive emission of CO₂ and other greenhouse gases,and the consequent energy shortage and environmental pollution have become two major crises that limit the sustainable development of human society.The conversion of CO₂ into hydrocarbon fuels or high value-added chemicals（CO,CH₄,CH₃OH,HCOOH,etc.）through artificial photosynthesis using renewable solar energy is one of the effective ways to solve the above problems,which can alleviate both the energy crisis and the greenhouse effect.Due to the high thermodynamic stability of CO₂,the design and development of efficient and stable photocatalytic system is the key to achieve photocatalytic reduction of CO₂.Covalent organic frameworks（COFs）are emerging porous crystalline materials with well-defined structure,tunable function,wide range of light absorption,strong gas adsorption ability and good thermal stability,which have received wide attention in the field of photocatalysis.In this thesis,a variety of Schiff-base COFs with superior performance are developed as photocatalysts for CO₂ reduction,and the mechanism of photocatalytic CO₂ reduction is investigated in depth by density functional theory（DFT）calculations.The main research contents are as follows:（1）Rapid recombination of photogenerated electron-hole pairs is one of the main factors affecting the efficiency of photocatalytic CO₂ reduction.The introduction of auxiliary functional groups in electron donor-acceptor（D-A）COFs may enhance their D-A effect,which is beneficial to improve the separation and transfer efficiency of photogenerated electron-hole pairs and thus their photocatalytic CO₂ reduction activity.To address this issue,COF-TVBT-Pa is synthesized by 2,4,6-tris（4-vinylbenzoyl）-s-triazine（TVBT）and p-phenylenediamine（Pa）through Schiff-base condensation reaction.COF-TVBT-Br and COF-TVBT-Me can be prepared by replacing Pa with 2,5-dibromo-p-phenylenediamine（Pa-Br）with electron-absorbing group and 2,5-dimethyl-p-phenylenediamine（Pa-Me）with electron-giving group,respectively.A series of comparative experiments demonstrate that COF-TVBT-Br has stronger electrical conductivity,longer surface electron survival time,higher photogenerated electron-hole pair separation and transfer efficiency and stronger photocatalytic CO₂ reduction activity compared with COF-TVBT-Pa and COF-TVBT-Me.This work proves that the introduction of auxiliary functional groups can change the light absorption range and energy band structure of COFs,as well as enhance the D-A effect of COFs,improve their electrical conductivity and photocatalytic CO₂ reduction efficiency, provding a theoretical basis for further design and synthesis of efficient COFs-based photocatalysts.（2）To overcome the difficulty of simultaneous CO₂ reduction and H₂O oxidation in the same photocatalytic system,TpBb-COF containing N and S heteroatoms in the framework is synthesized by benzo[1,2-d:4,5-d′]bisthiazole-2,6-diamine（Bb-NH₂）and 2,4,6-trihydroxybenzene-1,3,5-trialdehyde（Tp-CHO）to further enhance the separation and transfer efficiency of photogenerated electron-hole pairs.When TpBb-COF is used as a photocatalyst,CO₂ and H₂O can be converted into CO and O₂,respectively,without the addition of photosensitizers and sacrificial agents.When the reaction temperature is 80°C and the CO₂ concentration is 30.0%,TpBb-COF exhibits the highest photocatalytic CO₂reduction efficiency.DFT calculations prove that the electron transfer direction is from Bb-NH₂ to Tp-CHO.H₂O is preferentially adsorbed on TpBb-COF through hydrogen bonds and the adsorbed H₂O acts as the anchor to convert CO₂ to CO.This work provides new insights for further design and synthesis of COFs-based photocatalysts for artificial photosynthesis with H₂O as the electron donor.（3）The separation and transfer efficiency of photogenerated electron-hole pairs in COFs can be improved by introducing auxiliary functional groups or heteroatoms,but the efficiency of photocatalytic CO₂ reduction is still relatively low because metal-free COFs usually lack efficient catalytic active sites.The introduction of efficient metal active sites into COFs using coordination bonds may further improve the photocatalytic CO₂ reduction efficiency of COFs.Based on the above considerations,a novel bipyridine-based covalent organic framework COF-TVBT-Bpy is designed and synthesized,which can not only provide a coordination environment for the metal active center,but its highly conjugated structure can also facilitate the transfer of photogenerated charges.A series of metal active sites are introduced into COF-TVBT-Bpy to prepare M@COF-TVBT-Bpy（M=Co/Ni/Cu/Zn/Mn）as the photocatalysts for the conversion of CO₂ and H₂O to syngas（a mixture of CO and H₂）in different ratios.Among them,Co@COF-TVBT-Bpy exhibits the highest photocatalytic CO₂ reduction efficiency,the production rate of syngas is 2291.1μmol·g^-1·h^-1 and the ratio of CO and H₂ is about 1:1.DFT calculations explore the electron transfer pathway and mechanism of syngas generation,and reveal the reason for the 1:1 ratio of CO and H₂ when Co@COF-TVBT-Bpy is used as a photocatalyst.In addition,experimental results demonstrate that the ratio of syngas can be regulated in a wide range by adjusting metal ions,incident light wavelength and CO₂ concentration.This work proves the great potential of using COFs as platforms to anchor metal active centers for utilization as photocatalysts and provides a facile method for large-scale adjustment of syngas ratios.（4）The introduction of metal active sites into COFs through coordination bonds can improve the photocatalytic CO₂ reduction efficiency of COFs,but the reproducibility of the prepared photocatalyst materials will be affected because the loading amount of metal ions cannot be precisely controlled.To address this problem,the construction of covalent metal organic frameworks（CMOFs）by linking metal clusters and organic ligands through covalent bonds can combine the respective advantages of COFs and MOFs to obtain more efficient and stable photocatalyst materials.A Cu^I cluster-based CMOF（JNM-2）is used as a photocatalyst for efficient photocatalytic reduction of CO₂ to HCOOH and CO,in which the Cu^I cluster acts as the catalytic active center,while the triazine ring in the organic ligand has theπ-electron conjugation effect,which facilitates the intramolecular electron transfer.The reaction intermediate,reduction product generation pathway and photocatalytic CO₂reduction mechanism are investigated in depth using DFT calculations.CO₂ is converted to the reaction intermediate COOH by the first hydrogenation step and then to HCOOH and CO by two different hydrogenation steps.Since the energy required to form HCOOH is lower than that of CO,HCOOH is produced preferentially and in higher yields.This work is important for the further design and synthesis of CMOFs-based photocatalysts for CO₂reduction.