Directed Synthesis And Photocatalytic CO₂ Reduction Of Stable Multivariate Transition Metal–Organic Frameworks

In today’s society,with the rapid development of the economy,environmental pollution,energy shortage and the greenhouse effect are becoming increasingly serious.The reuse of greenhouse gases,such as CO₂,using“carbon-negative technology”is a cutting-edge scientific field.Among them,photocatalytic CO₂reduction technology is of great significance as it can convert inexhaustible solar energy into chemicals with higher added value and solve the problem of greenhouse effect.Metal–organic frameworks（MOFs）are widely used in photocatalytic CO₂reduction performance studies because of their regular pore structure,large specific surface area,high pore size and special microenvironment,and tunable structural properties.Compared with conventional semiconductors,MOFs have the following unique advantages in photocatalysis:（1）MOFs have fewer self-generated structural defects and reduced complexation of photogenerated electrons and holes;（2）the high porosity of MOFs allows faster transfer of photogenerated charges and accelerates catalysis,while making them expose more effective catalytic active centers,effectively preventing the reduction of catalytic activity triggered by the agglomeration of catalytic centers;（3）the homogeneous pore characteristics make them have the same active centers,geometric structure and electronic structure,while the porous structure can increase the interfacial contact area,reduce the surface work function of heterogeneous materials,and improve the electron and active species transport ability at the surface interface;（4）the tunability of the structure of MOFs（e.g.,the introduction of-NH₂functional group in organic ligands）can obtain more photogenerated electron-hole pairs,so MOFs are ideal catalysts for photocatalytic reduction of CO₂.However,most MOFs cannot achieve good stability and catalytic activity at the same time.For example,low-valent metal MOFs have high catalytic activity but are vulnerable to attack by guest molecules,resulting in insufficient stability;high-valent metal MOFs have good stability,but stronger coordination bonds and high linkage numbers cause their insufficient activity.Thus,this dissertation is centered on the synthesis of stable multivariate transition metal MOFs and their performance in photocatalytic CO₂ reduction:（1）A series of titanium-based mixed metal-oxygen clusters of isostructurally stable multifunctional transition metal-organic framework materials were synthesized based on soft and hard acid-base（HSAB）theory with 3,3’,5,5’-azobenzenetetracarboxylic acid（H₄abtc）as the organic ligand by a solvothermal method.The structure,morphology,and pore environment of the synthesized samples were characterized by X-ray single crystal diffraction,in situ molecular probe neutron diffraction,powder X-ray diffraction（PXRD）,scanning electron microscopy（SEM）,and N₂ adsorption,which demonstrated that multiple metal atoms were uniformly distributed on the metal nodes.The X-ray single crystal diffraction and in situ molecular probe neutron diffraction of PCN-250-Fe₂Ti samples verified that the strong coordination bonds of high-valent metals are difficult to break under conventional conditions,the peripheral coordination oxygen of highly reactive metal centers is easy to dissociate and the skeleton remains after dissociation,and the exposed metal sites have a strong trapping effect on CO₂ and can form coordination bonds with metal sites.In addition,the samples exhibited high stability under immersion in various boiling organic solvents for 12 h and in aqueous solutions at p H=2-12 for 7 d,respectively,due to the formation of titanium-based mixed-metal oxygen clusters.（2）With the formation of titanium-containing mixed metal-oxygen clusters,not only the light absorption range of the stable multivariate transition metal-organic framework materials is extended,but also their photocatalytic reaction activity and selectivity are improved by the change of LUMO and HOMO positions.The ordered porous structure can reduce the influence of the external field environment on the electronic state at the surface interface of the mixed-metal unit and facilitate the targeted regulation of the selective activation of the material surface interface and the targeted conversion of CO₂.The synthesized series of samples showed high activity and selectivity for photocatalytic CO₂ reduction under visible light,and the product was CO.The material with the best reduction performance was PCN-250-N₆,whose photocatalytic reduction of CO₂ to CO yielded:39.35 mmol/g with a selectivity of:87%.This thesis revolves around the key scientific problem that good stability and high catalytic activity in MOFs cannot coexist,and the multi-component MOFs materials are prepared by introducing traditional weak Lewis acids into strong Lewis acid clusters to form new metal clusters.Such metal clusters can greatly reduce the binding rate of the system during crystallization,while solving the difficult problem of low catalytic activity and easy deactivation of the active catalytic center in material synthesis due to the chemical bond inertness of stable strong Lewis acid MOFs.This paper uses a simple and convenient method to synthesize multifunctional MOFs materials that combine catalytic activity and stability.By constructing mixed-metal oxygen clusters in the synthesis of MOFs,the stability of metal–organic framework materials and the activity and selectivity of photocatalytic CO₂ reduction have been enhanced,providing a certain degree of guidance and reference for improving the stability of MOFs and the activity of photocatalytic CO₂ reduction.