MOFs Based On Nitroazole Carboxylic Acid Ligands: Studies Of Synthesis And Properties

Metal organic frameworks（MOFs）have a good potential application value in gas adsorption and separation,catalysis,magnetism,fluorescence and other aspects due to their open metal sites,variable structure,high BET,porosity,modifiable functional sites and pore structure.The synthesis modification method can improve the properties of MOFs materials by introducing functional groups to form composites.Nitroazole carboxylic acids have been widely used in the construction of new MOFs structures due to their varied coordination methods.In this paper,we designed and synthesized an organic ligand 4’-（3,5-dimethyl-1-hydro-pyrazol-4-yl-）[1,1’-biphenyl]-4-carboxylic acid（HL）with carboxyl,pyrazole ring and biphenyl structural units,and explored the application of this ligand in the synthesis and post-modification of MOFs.The main results are as follows:1.HL,a new organic ligand,was designed and synthesized.The compound has fluorescence properties.When the excitation light wavelength is 360nm,the compound shows the best emission spectrum,and the maximum fluorescence emission peak is401nm.Using the compound as a flexible ligand and transition metal Co²⁺under hydrothermal conditions,a two-dimensional MOF material Co L₂ with one-dimensional rhombic aperture and triple interpenetrating structure was synthesized.The Co-MOF has good thermal stability and water stability.In addition,the compound has antiferromagnetism.Then,the metal center was replaced with Cu²⁺,and the two-dimensional layered MOF namely Cu L₂（H₂O）,was synthesized through self-assembly under hydrothermal conditions.2.The compound HL was introduced into the two-dimensional layered zirconyl MOF material Zr₆（μ₃-O）₄（μ₃-OH）₄（HCO₂）₆（BTB）₂（BTB=benzene-1,3,5-tribenzoate,in short of Zr-MOL）.As secondary building units（SBUs）)by post-synthetic modification.L^-forms coordination with the central ion Zr by replacing part of the formic acid in the secondary building unit Zr₆O₄（OH）₄（HCO₂）₆.The optical properties of modified Zr-MOL-L were found to be changed by ultraviolet absorption spectrum and fluorescence spectrum.The absorption range of different bands of Zr-MOL-L was broadened from the ultraviolet region to the visible region,and the absorption peak intensity was greatly enhanced.The optimal excitation wavelength and emission wavelength of the modified Zr-MOL-L were redshifted.According to the analysis of UV-Vis spectrum and X-ray photoelectron spectrum,Zr-MOL has been proved to have photocatalytic property by theoretical calculation and experiment.In the photocatalytic CO₂ reduction experiment,the unmodified Zr-MOL and the modified Zr-MOL-L were used as photocatalysts,respectively.Under the same reaction conditions,the UV-visible light source was used as the catalyst,and the CO₂ was reduced to CO by the compound.The experimental results showed that the rate of CO reduction catalyzed by Zr-MOL-L was increased by 3.6 times.It is proved that the introduction of the organic compound L^-significantly improves the photocatalytic performance of the compound.By adjusting the concentration of modified organic L^-in the post-modification process,it was found that when the molar ratio of organic matter to Zr-MOL was 0.75,the photocatalytic CO₂ reduction performance of Zr-MOL-L was the strongest.We also explored the photocatalytic degradation performance of rhodamine B,and the modified Zr-MOL-L photocatalytic degradation performance was significantly improved.We also added three different capture agents including IPA,BQ,EDTA-2Na to determine the reaction mechanism in the photocatalytic degradation process.The results showed that superoxide free radicals were the main active substances in the photocatalytic degradation process of dyes,and holes were also involved in the photocatalytic degradation process.