Construction Of Porphyrin-Based Metal-Organic Framework Materials And Their CO₂ Capture/Catalytic Conversion Properties

In recent decades,the growing environmental problems caused by the excessive emission of greenhouse gases,mainly carbon dioxide,have seriously threatened the survival and development of every species on Earth.People have been working on ways to deal with carbon dioxide effectively.One of the new technologies is carbon dioxide capture and conversion,which allows the direct conversion of captured carbon dioxide molecules into chemicals with high added value,such as carbon monoxide,methane,methanol,formic acid or pharmaceutical intermediates.Each stage of the carbon cycle,such as carbon capture,regeneration and conversion,has its own requirements for materials.It has been shown that Porphyrin Metal-Organic Frameworks（PMOFs）inherit the unique optical properties,excellent thermal and chemosynthetic stability of porphyrin molecules,and combine the unique structural features,high surface area,and chemical tunability of Metal-Organic Frameworks,and thus the applicability of PMOFs in CO₂ capture and conversion technologies has been widely studied.The CO₂ capture capability of PMOFs materials greatly enhances their potential for CO₂ conversion.In addition,the well-defined structure of PMOFs greatly facilitates the understanding of their structure-property relationships and their role in CO₂ capture and conversion.In this study,we mainly utilized the diversity of coordination modes,unique optical properties and chemical stability of porphyrin organic building blocks.Oriented by the coordination chemistry,the porphyrin ligands were self-assembled with transition metal ions,and a series of PMOFs materials with pore structures synergized with active metal centers were constructed.The PMOFs constructed in this study achieved efficient capture of CO₂ and selective catalytic conversion.The research content of the study is as follows:Chapter 1 In this chapter,we summarize the origin and development of MetalOrganic Frameworks,especially Porphyrin Metal-Organic Frameworks,and summarize the research progress of PMOFs in the capture and conversion of CO₂ into value-added products,and present the subjects of this thesis.Chapter 2 Carbon dioxide capture,as a key step in the carbon cycle,affects the subsequent catalytic conversion.The CO₂ capture ability of MOFs is mainly affected by the shape/size of the pores and the chemical environment of the pores.In this chapter,we constructed five PMOFs materials with different pore environments using tetrakis（4-carboxyphenyl）porphyrin as the main organic building block,and utilized a mixed ligand strategy to introduce linear ligands with different numbers of Lewis basic sites into the structure of MOFs,and investigated their CO₂ adsorption properties.The results showed that Cd-PMOF 4 exhibited the most excellent CO₂ capture capacity in Cd-PMOFs 1-5 due to the special pore structure brought by the dual interpenetrating network and the pore chemical environment modified by the high-density Lewis basic sites.The CO₂ adsorption of Cd-PMOF 4 reached 46.19 cm³·g^-1 and 29.33 cm³·g^-1 at 273 K and 298 K（1 bar）,respectively.The selective separation coefficients of CdPMOF 4 for CO₂/CH₄（50/50）and CO₂/CH₄（5/95）binary gas mixtures calculated by IAST were 329.34 and 57.92,respectively.The work in this chapter highlights the constitutive relationship between material structure and CO₂ capture performance,providing guidelines for the construction of highly selective CO₂ capture materials.Chapter 3 Based on the research foundation in Chapter 2,the special pore structure brought about by the topological configuration of double interpenetrating layer-columnar network is conducive to the improvement of carbon dioxide adsorption performance of the material,and can effectively improve the stability of the material.The efficient CO₂ capture ability of MOFs can lay the foundation for catalytic conversion after CO₂ capture.Therefore,in this chapter,we successfully constructed three porphyrin metal-organic frameworks,Co-PMOFs 1-3,by introducing catalytically active cobalt metal centers into PMOFs,on top of the structure of dual interpenetrating layer columnar network topology obtained in Chapter 2.At zero loading,the enthalpies of adsorption of CO₂ by Co-PMOFs 1-3 were categorized as23.6 k J·mol^-1,18.2 k J·mol^-1,and 26.2 k J·mol^-1,respectively,which indicated that there was a good adsorption affinity between the materials and CO₂.The results of catalytic experiments show that this series of PMOFs can efficiently promote the cycloaddition reaction of CO₂ and epoxide under mild and solvent-free conditions（75 ℃,1 bar）.Among them,due to the larger pore size of Co-PMOF 3,the efficient catalytic activity is still maintained for larger-sized reaction substrates.More importantly,the introduction of photosensitive porphyrin and anthracene groups in the structure of this series of PMOFs makes the they show strong absorption peaks in the range of 200 nm to 800 nm.The catalytic activity of this series of PMOFs in the light-driven CO₂ cyclization reaction was investigated by taking Co-PMOF 3 as an example,and the results indicate that Co-PMOF 3 can efficiently utilize solar energy instead of thermal energy to drive the reaction.This study not only provides an excellent MOFs catalyst for the CO₂ cyclization reaction,but also provides new ideas for the rational design of high-quality photocatalysts for heterogeneous catalytic reactions in the future.Chapter 4 In order to truly participate in the natural carbon cycle,CO₂ molecules need to be reduced to produce compounds in lower valence states.Photocatalytic reduction of CO₂ into valuable small molecules such as CO is an effective strategy to solve the energy crisis and environmental problems.Therefore,on the basis of the study in Chapter 3,we selected tetrapyridyl porphyrin ligand as the light-trapping ligand,and used easily valence-alterable manganese metal ions as the metal nodes,and successfully constructed two-dimensional monolayer and two-dimensional bilayer Porphyrin MetalOrganic Frameworks,Mn-PMOF 1 and Mn-PMOF 2,by adjusting the reaction temperatures and solvent polarities.The bilayer PMOF is a low-dimensional MOF with a special structure in which the upper and lower layers are arranged in dislocation and are bridged by halogen ions.This bilayer Mn-PMOF 2 exhibits 100% ultra-high selectivity for the reduction of CO₂ to CO under simulated sunlight without any cocatalyst or photosensitizer and has excellent cycling stability.The intrinsic mechanism of this photocatalytic CO₂ reduction process is explored through experimental characterization and density functional theory（DFT）calculations.This work shows that the rational design of the number of layers in 2D MOF structures can tune the stability of these structures and opens a new avenue for the design of highly selective MOF photocatalysts.