Design,Synthesis And Performance Of Porphyrin-based Porous Materials For Photocatalytic CO₂ Reduction

Against the backdrop of increasingly pressing global energy and environmental issues,the extensive extraction and utilization of fossil fuels have led to a sharp increase in greenhouse gas emissions such as CO₂,exacerbating the challenges of climate change.Photocatalytic CO₂ reduction,which emulates natural photosynthesis,has gained significant attention as a solution to this problem.Photocatalytic CO₂ reduction,driven by sunlight,converts carbon dioxide into high value-added compounds using photocatalysts,making it a suitable alternative for promoting carbon cycling.However,the design and synthesis of ideal catalysts for this reaction pose a challenge for researchers.Porphyrins and their derivatives,as important molecules with photosensitivity and chemical diversity,hold potential application value in the field of photocatalytic carbon dioxide reduction.In recent years,novel porous materials represented by metal-organic frameworks（MOFs）and covalent organic frameworks（COFs）have gained widespread attention due to their tunable structures,diverse pore topologies,high surface areas,and excellent chemical stability.Using porphyrins and their derivatives as building blocks,porphyrin-based porous materials can combine the benefits of both,accomplish tailored functionalization,and broaden their applications in energy and the environment.The thesis aims to explore the relationship between the structure and catalytic performance of porphyrin-based porous materials,aiming to provide crucial references for the development of efficient and practical photocatalysts.The primary research accomplishments of this study are as follows:1.By comparing the structures and catalytic properties of covalent triazine frameworks（CTFs）synthesized by different methods,the influence of synthesis strategies on the structure of CTFs and,ultimately,on catalytic performance is highlighted.Using tetra-（4-cyanophenyl）porphyrin as the reactant monomer,two porphyrin-based CTFs were synthesized using classic high-temperature ionothermal method and improved trifluoromethanesulfonic acid（Tf OH）catalysis method,respectively.The results demonstrate that the structure of CTF-Tf OH formed by Tf OH vapor catalysis is ordered,consisting of intact porphyrin units and triazine units,and CTF-Tf OH-Co formed after introducing Co²⁺into its porphyrin ring,containing single-electron Co sites.While CTF-Zn Cl₂-Co synthesized by ionothermal method exhibits relatively disordered structure,with the porphyrin units with the porphyrin units almost vanishing and the Co sites lacking single electrons.This work emphasizes the significant influence of different synthesis methods on the structure of CTFs,and demonstrates the relevance of catalyst structure on catalytic performance.2.While certain high-performance catalysts may be constructed starting from CTFs,crystalline materials represented by MOFs are the ideal objects for in-depth catalytic reaction investigations.In this study,we investigated porphyrin-based MOFs with ordered network structures and flexible tunable structures as the research subjects,examining whether the active sites inside MOF-based catalysts would be ineffective due to insufficient light absorption in the photocatalytic CO₂ reduction reaction.2D-MOF and 3D-MOF built from Zr₆ clusters and tetra-（4-carboxyphenyl）porphyrin（TCPP）with two-dimensional and three-dimensional structures were designed and synthesized,respectively,and the classical CO₂ photocatalyst Re（bpy）（CO）₃Cl was embedded to form catalytically active 2D-MOF-Re and 3D-MOF-Re.The large channels of 2D-MOF-Re and 3D-MOF-Re can minimize the impact of mass transfer processes on catalytic reactions.Photocatalytic CO₂ reduction experiments using 2D-MOF-Re and 3D-MOF-Re containing an equal amount of Re catalytic sites show that in a 6-hour reaction,the TON of 2D-MOF-Re reached 27.8,which was 50 times that of3D-MOF-Re.This indicates that although there are active catalytic sites within bulk MOFs,some of them cannot function due to the inability to absorb light.This work underlines the significance of rationally designing of MOF-based photocatalyst structures for fully exploiting their photocatalytic potential.3.In addition to incorporating metal-organic complexes as catalytic active centers,metal ions may also be added to porphyrin-based MOFs.There are two common methods for introducing metal ions:one is to synthesize metal porphyrin ligands first and then synthesize MOFs,and the other is to synthesize porphyrin-based MOFs first and then introduce metals into them.In this study,we focused on two-dimensional metal porphyrin-based MOFs to investigate the impact of different methods of introducing metal sites on the structure and photocatalytic performance of porphyrin-based MOFs.Firstly,a two-dimensional planar metal porphyrin MOF,NS-Co1,was created using TCPP-Co as the ligand and the Zr₆ cluster as the metal cluster.Simultaneously,reacting the two-dimensional porphyrin MOF formed by TCPP and Zr₆clusters with Co Cl₂ also yields a metal porphyrin-based MOF,NS-Co2.Experimental results show that besides Co²⁺being coordinated in the porphyrin ring,Co²⁺is also coordinated on the Zr₆ clusters in NS-Co2.Photocatalytic CO₂ reduction performance results demonstrate that the catalytic activity of post-metalized NS-Co2 is close to three times that of pre-metalized synthesized NS-Co1.This indicates that unsaturated sites on MOF metal clusters can also coordinate metal ions,and the catalytic effect they provide has a significant and non-negligible impact on enhancing photocatalytic performance.This work provides valuable insights for the design and optimization of metal porphyrin-based MOF photocatalytic materials.