Metal-organic Frameworks Based On Multicarboxylate Ligands

Metal-organic framework materials?MOFs?,also known as porous coordination polymers,have become a relatively new class of crystalline porous materials which are self-assembled via coordination bonds between metal cations or ionic clusters and organic ligands.Porous MOFs have received extensive attention from the researchers not only because of their unprecedented diversity in chemical composition and molecular structure,but also their great potential and development space in many fields,including but not limited to gas adsorption and separation,heterogeneous catalysis,molecular sensing and drug delivery.Gas adsorption is still a major research area in a variety of applications because of their advantages of high surface area,tunable pore size,and functional pore surface compared to traditional porous materials.In this thesis,the main research is to figure out the effect of different functional groups on the adsorption properties of MOFs,compare the gas adsorption properties of MOFs isomers,and optimize the gas adsorption properties of MOFs by ligand truncation strategy.1.Three kinds of curved tetracarboxylic acid ligands,5,5'-?2,6-diaminopyridine-3,5-diyl?diisophthalic acid,5,5'-?2,6-dimethylpyridine-3,5-diyl?diisophthalic acid,and5,5'-?2,6-dimethoxypyridine-3,5-diyl?diisophthalic acid,were designed and synthesized.Three copper-based MOFs were prepared by solvothermal method:ZJNU-87,ZJNU-88 and ZJNU-89.Single-crystal X-ray diffraction analyses show that the three MOFs are isoreticular,crystallizing in the hexagonal crystal system with space group of P6₃/mmc.The framework contains three different types of polyhedral cages.During the activation process,the framework of ZJNU-87 was collapsed due to the strong interactions between the amino group and the solvent molecules.The activated methoxy-functionalized ZJNU-89 has better adsorption properties than the methyl-functionalized ZJNU-88.At 298 K and 1 atm,C₂H₂,CO₂ and CH₄ uptake capacities of ZJNU-89 are 181.3,103.0 and 22.8 cm³·g^-1,respectively.This work not only reported two MOFs with excellent adsorption properties,but also provided a basic understanding of the substituent effcet on the framework stabilities and gas adsorption properties,which is essential for the design and comprehensive application of MOFs in the future.2.Two methoxy-functionalized ligand isomers,5,5'-?1,5-dimethoxynaphthalene-2,6-diyl?diisophthalic acid,5,5'-?2,6-dimethoxynaph-thalene-1,5-diyl?diisophthalic acid,were designed and synthesized,and two NbO-type copper-based MOFs isomers were synthesized by solvothermal method:ZJNU-58 and ZJNU-59.Single-crystal X-ray diffraction analyses show that although ZJNU-58 and ZJNU-59 crystallize in different space groups(C2/c for ZJNU-58 and R3^_<sub>m for ZJNU-59),they are isoreticular.Gas adsorption analyses show that the adsorption capacity of ZJNU-59 towards C₂H₂and CO₂ is higher than that of ZJNU-58,although the pore volume and surface area of ZJNU-59 are lower than that of ZJNU-58.At 298 K and 1 atm,C₂H₂ and CO₂ uptake capacities of ZJNU-59 are 199.5 and 109.4 cm3·g-1,respectively.Also,ZJNU-59 has better adsorption selectivities of C₂H₂/CH₄ and CO₂/CH₄ than ZJNU-58.Compared with their parent MOFs without methoxy group,the C₂H₂ and CO₂ adsorption capacities of ZJNU-59 are higher than that of its parent compound ZJNU-73,while the C₂H₂ and CO₂ uptake capacities of ZJNU-58 are slightly lower than its parent compound NOTT-103.However,the adsorption selectivities of C₂H₂/CH₄ and CO₂/CH₄ of these two MOFs are enhanced compared to their corresponding parent MOFs.This work not only reported two NbO-type MOFs for highly selective adsorption of C₂H₂ and CO₂ from CH₄,but more importantly provided a basic understanding of the impact of MOF isomerization and methoxy group functionalization on gas adsorption capacities,which is instructive for designing MOFs with superior properties.3.A hexacarboxylic acid ligand,2,4,6-tri?3,5-dicarboxyphenyl?-1,3,5-trimethyl-benzene,was synthesized and a novel copper-based MOF material ZJNU-100 was constructed.Single-crystal X-ray diffraction analyses show that ZJNU-100 belongs to the hexagonal system with the space group P6/mmm.There are seven different types of cages in the overall structure of ZJNU-100,and the range of pore size is 5.36-15.91?.At 298 K and 1 atm,the adsorption capacities of C₂H₂ and CO₂ of ZJNU-100 are 149.1and 83.1 cm3·g-1,respectively.In order to optimize its gas adsorption properties,a tetracarboxylic acid ligand,2,4-bis?3,5-dicarboxyphenyl?-1,3,5-trimethyl-benzene,was designed and synthesized by a ligand truncation strategy,and an ssa-type copper-based MOF material ZJNU-57 was constructed.Single-crystal X-ray diffraction analyses show that ZJNU-57 crystallizes in the hexagonal space group P6₃/mmc.The framework of ZJNU-57 contains three different types of polyhedral cages.Due to different structure and pore environment,the adsorption properties of ZJNU-57 were enhanced than ZJNU-100.At 298 K and 1 atm,the adsorption capacities of C₂H₂ and CO₂ of ZJNU-57 are 201.5 and 112.2 cm3·g-1,respectively.The quantum chemical calculations show that the methyl group in ZJNU-57 enhances the van der Waals interactions between the framework and the C₂H₂/CO₂ molecules due to the suitable pores and structures,which greatly improves the adsorption capacities of C₂H₂ and CO₂.This work not only reported a MOF material with a novel structure and a MOF material with high C₂H₂ and CO₂ adsorption capacity,but also demonstrated that ligand truncation strategy is an effective method for synthesizing MOFs with excellent properties.The results also provide some insights for designing MOFs with excellent adsorption properties.</sup>