Investigation Of Adsorption And Photocatalytic Performance Based On Porphyrin Organic-Metal Frameworks

In order to solve the serious energy and environmental crisis caused by rapid development of industry,we strive to find a sustainable environmental governance strategy and energy conversion method.Photocatalytic technology is considered to have great potential in this field because of its excellent optical performance,which contributes to the conversion of renewable energy and the remediation of environmental pollution,and has become one of the most attractive technologies today.the porous structures of MOF materials provide additional exposed active sites and catalytic substrate(product)transmission channels,which promotes the rapid transfer and utilization of photo-generated charges.At the same time,MOFs have structural adjustability,which can enhance the light response range and increase the number of photo-generated electron-hole pairs.In addition,various compound structures make MOFs easily combine with other compounds,such as other photosensitizers or cocatalysts to form a heterojunction structure,which promotes the generation and separation of photo-generated electrons and holes.However,compared with traditional semiconductors,MOFs still have the problems of too narrow band gap and too fast recombination of electrons and holes,which make them need to be combined with other materials(such as semiconductors and COF materials)to obtain higher photocatalytic efficiency.Therefore,improving the separation efficiency of photogenerated hole electron pairs of MOFs and retaining the good adsorption properties of MOFs materials,achieving synergy between adsorption and catalysis has become the focus of our workPorphyrin is an organic molecule with good absorption of visible light and direct conversion of solar energy to chemical energy,which is considered as an excellent photoactive material.In this thesis,a series of visible-light driven photocatalytic materials of metal-organic frameworks(MOFs)were constructed by modulating the morphology of MOFs,introducing a second ligand to adjust the energy band and pore structure,and constructing MOF-ON-MOF heterostructures.Not only improved the adsorption capacity of MOFs,but also promoted the separation of photogenerated hole-electron pairs,and successfully constructed MOFs materials with integrated adsorption and efficient catalysis for photocatalytic applications.(1)Investigating morphology control of porphyrin-based lanthanide MOFs and the adsorption photocatalytic performance:Compared to bulk MOFs,MOFs nanosheets have a larger surface area and more accessible active centers,which can reduce diffusion potential barriers,facilitate substrate contact with active centers,and provide rapid mass transfer and charge transfer,allowing for higher performance in catalytic applications.Microwave heating involves the interaction between electromagnetic radiation and dipole moment of molecules.Microwave makes the temperature of the reaction system rise rapidly and then chemical reaction occurs.This method is suitable for preparing small-sized MOFs crystals.Series of porphyrin-based lanthanide MOFs Nd-TCPP,Er-TCPP and Tm-TCPP were prepared by microwave method and solvothermal method respectively.The results showed that the MOF nanosheets materials prepared by microwave method have smaller sizes and larger specific surface areas.And their adsorption and photocatalytic degradation efficiency of BPA are higher than those of block MOFs prepared by hydrothermal method.Furthermore,different lanthanide metals will have obvious influence on catalytic efficiency as well.Among the prepared materials,the nanosheets of Nd-TCPP can remove more than 96%of 50 mg/L BPA under visible light irradiation in 60 minutes,which is obviously higher than that of Er-TCPP and Tm-TCPP.Because the energy level of triplet excited state is higher than that of Nd3+,the energy can transfer from triplet state to lanthanide metal,which can extend the excited state life of TCPP,improving the separation efficiency of photo-generated hole-electron pairs and exhibited stronger photocatalytic activity than Er-TCPP and TmTCPP.(2)Investigating preparation and the adsorption photocatalytic performance of porphyrin-based dual-ligand metal-organic Framework(MOF)Ti-TCPP-TzBIPY:Whether 2D or 3D MOFs materials,MOFs with a single ligand suffer from the defect of too fast photogenerated electron-hole pair complexation,which limits the photocatalytic activity of MOFs materials.Therefore,in this chapter,based on the reported porphyrin-based MOF material DGSIT-1(Ti-TCPP),the second ligand 2,5-bis(pyridin-4-yl)thiazolo[5,4-d]thiazole(TzBIPY)was introduced,and TzBIPY was introduced as a pillar in the framework between the two-dimensional layers formed by TCPP and the metal,to construct the dual-ligand metal-organic framework Ti-TCPP-TzBIPY.The introduction of the second ligand increases the pore size of the MOF,improves the adsorption and facilitates the adequate contact between the substrate and the active site.The charge transfer mechanism from TCPP to metal nodes(Ti-oxo)exists in both DGSIT-1 and Ti-TCPP-TzBIPY under light excitation.In addition,an energy transfer mechanism from the ligand TzBIPY to the ligand TCPP exists in the dual-ligand MOF material Ti-TCPP-TzBIPY,which can effectively prolong the TCPP excited state lifetime and promote the separation of photogenerated electron-hole pairs.Under visible light,the degradation rate of Ti-TCPP-TzBIPY reached more than 99.5%of BPA at 50 mg/L within 60 min,which was 6.2 times higher than that of DGSIT-1,and still maintained good stability after four repeated applications.(3)Investigating preparation of porphyrin-based MOF-on-MOF heterojunction and the adsorption photocatalytic performance:Compared to the structural modulation of a single MOF material,the construction of MOF-ON-MOF heterostructures by combining two MOF materials exhibits unprecedented synergistic effect and enhanced photocatalytic performance.By purposeful design,MOF-ON-MOF can synthesize the characteristics of different MOFs,while the formation of heterostructures increases the transport channels of photogenerated electrons and suppresses the compounding of photogenerated electron-hole pairs.In this chapter,the structure of MOF-ON-MOF PCN-222@NU-1000 was constructed by selecting NU-1000 that matches the lattice of PCN-222 and growing PCN-222 on both ends of NU-1000 by seed epitaxial growth method,the photocatalytic activity of the material was evaluated by the adsorption and photocatalytic degradation experiments of BPA.The MOF-ON-MOF structure increases the specific surface area of the material,improves the adsorption of BPA,and promotes the full contact of BPA with the active site.Meanwhile,the heterogeneous structure can improve the separation efficiency of photogenerated holeelectron pairs and produce more active species.With the synergistic effect of adsorption and photocatalysis,PCN-222@NU-1000 could achieve the complete degradation of 100 mg/L BPA within 60 min,and the performance remained stable after four cycles.