Construction And Adsorption Properties Of Metal-organic Frameworks Based On Nitrogen Heterocyclic Carboxylate Ligands

Metal-organic frameworks（MOFs）usually have ultra-high porosity and large specific surface area,and become catalysis,luminescence,chemical sensing,gas storage,proton conduction,magnetism,biomedicine and many other scientific fields research hotspots,especially in gas adsorption and separation.Due to the advantages of the pore size,the microporous MOFs can adsorb and separate gas molecules of different sizes,which reflects the role of gas molecular sieve.At present,the ligands used to construct microporous MOFs are mainly carboxylic acid or azoles.This article uses nitrogen heterocyclic carboxylate ligands and utilizes the advantages of the two groups to synthesize a series of new types of MOFs by solvothermal method.And the gas adsorption and separation properties were tested according to their corresponding structures.The specific results are as follows:1.Compounds 1-7 are obtained by assembling combining the nitrogen heterocyclic polycarboxylate ligand,pyrazole-3,4,5-tricarboxylate（L）and terephthalate derivative ligands with transition metal Cu²⁺and lanthanide rare earth metals in solvothermal method.Both 1-2 and 3-5 have binuclear Cu chelating plane,which is formed by the chelation reaction between transition metal Cu²⁺and the pyrazole nitrogen atom of L.The difference in the formation of the structure is caused by the difference in the spatial position of the rare earth ions connected to the carboxyl group on the chelating plane.The N2,CH4,and CO2 adsorption tests of 1were carried out at different temperatures,and the breakthrough test of N2/CH4 was carried out.The results show that 1 has high adsorption of CO2 and CH4,but the adsorption capacity of N2 is very low.The breakthrough curve analyzer proves that 1has a good separation effect on methane and nitrogen.The N2 and CO2 adsorption tests of 3-5 were carried out at different temperatures.The results show that there is a significant difference in the amount of CO2 adsorbed by three MOFs.Among them,5synthesized from 2-methylterephthalate shows the highest amount of CO2 adsorbed,which may be due to the distortion of the spatial structure caused by the reduced symmetry of the space group.As a result,the metal sites inside the pores and CO2form a stronger interaction force,indicating that 5 has potential application value in gas adsorption and separation.6 replaced the 2-aminoterephthalate in 3 with2,3,5,6-tetrachloroterephthalate and changed the p H of the solvent to synthesize a new one-dimensional porous structure.7 is a new structure produced during the synthesis of 6.Although only Dy³⁺is coordinated with 2,3,5,6-terephthalate,the structure has narrow pores through analysis.2.Compound 8 is obtained by assembling the nitrogen heterocyclic polycarboxylate ligand,2,3-pyrazine dicarboxylate（PZDC）and Cd²⁺in solvothermal method.After a series CO2 and N2 adsorption tests at different temperature,it is proved that the 8 has a temperature-dependent structural change.Because 8 shows a lower adsorption capacity for N2 at any temperature,coupled with its pore size advantage,it may have a more obvious screening effect on the adsorption and separation of different gas molecules.