Metal-organic Frameworks Based On Polycarboxylate Ligands:Solvothermal Construction,structure Modulation,and Struture-Property Relationship Investigations

Metal-organic frameworks?MOFs?,also known as porous coordination polymers?PCPs?,are a newly emerging kind of porous materials formed by self-assembly of metal ions/clusters and organic ligands through coordination bonds and intermolecular forces.Compared with traditional porous materials?zeolites,activated carbon?,MOFs have the advantages of large specific surface area,considerable porosity,and adjustable pore size.Also,MOFs have superior synthetical controllability.For example,different coordination numbers of metal ions and different types of organic ligands can be used;the functional groups complementary to the target guests as well as open metal sites can be introdueced to the pore surface;and a charged skeleton can be built up.Therefore,MOFs have shown great potential as a relatively new type of porous materials in the fields including but not limited to gas storage and separation,drug delivery,fluorescence,and heterogeneous catalysis.In this dissertation,the research mainly focuses on the influence of positional isomerism of functional groups as well as different functional groups on the topological structures and gas adsorption properties of the resultant MOFs,and the optimization of adsorptive separation performances of MOFs by employing a ligand functionalization strategy.1.Three copper-based ligand-originated MOF isomers?ZJNU-81?ZJNU-82 and ZJNU-83?derived from methyl-functionalized V-shaped diisophthalate ligands were solvothermally synthesized and structurally characterized.Single-crystal X-ray diffraction studies revealed that the position of the methyl group in the central phenyl spacer of diisophthalate ligands exerted a significant effect on determining the final structures of the resulting MOFs,which was rationalized to result from the steric effect imposed by the methyl group controlling the conformational structures of the ligands during the self-assembly process.Furthermore,their gas adsorption properties with respect to C₂H₂,CO₂,and CH₄ were systematically investigated and comparatively analyzed.Gratifyingly,the three MOFs exhibited respectable C₂H₂ and CO₂ uptake capacities as well as impressive C₂H₂/CH₄ and CO₂/CH₄ adsorption selectivities.Although the disparity is small,the different gas uptake capacities and adsorption selectivities exhibited by the three MOFs demonstrate that the position of the methyl group has a certain impact on gas adsorption properties.Later,three isoreticular NbO-type copper-based MOFs based on dimethyl-functionalized linear diisophthalate ligands were solvothermally constructed,which were designated as ZJUN-94,ZJNU-95 and ZJNU-96.However,there is no significant difference in the adsorption of C₂H₂,CO₂ and CH₄,so we speculate that the effect of positional isomerism on gas adsorption properties is related to the topology of the parent MOF compound investigated.2.Tailoring the structures and properties of metal-organic frameworks?MOFs?is very important for developing porous MOFs for targeted applications.Bent diisophthalate ligands offer rich opportunity to construct MOFs with variable topologies because of their conformation flexibility and diversity.In this work,by employing substituent-induced ligand conformation regulation strategy,we designed and synthesized three bent diisophthalate ligands bearing different substituents,and sucessfully constructed their corresponding dicopper paddlewheel-based MOFs under suitable solvothermal conditions.As revealed by single-crystal structural analyses,they feature distinct topological structures,depending on the attached substituents.Interestingly,the methoxy-functionalized MOF exhibits a novel topology,thus enriching structural diversity of copper-bent diisophthalate frameworks.Furthermore,their gas adsorption properties with respect to C₂H₂,CO₂,and CH₄ were systematically investigated,revealing their promising potential for industrially important C₂H₂/CH₄and CO₂/CH₄ separations.More significantly,their separation performance can be greatly tailored by changing the substituents.At ambient conditions,C₂H₂ and CO₂uptake capacities vary from 157.7 to 184.5 cm³?STP?g^-1,and 90.8 to 101.3 cm³?STP?g^-1,respectively,while C₂H₂/CH₄?50/50,v/v?and CO₂/CH₄?50/50,v/v?adsorption selectivities range from 30.0 to 36.2,and 5.27 to 6.01,respectively.The amine-functionalized MOF performed better than the other two counterparts,which might be due to synergistic effect arising from Lewis basic amine groups,higher density of open copper sites,and more suitable pore size.The work not only reported three MOFs for preferable adsorption of C₂H₂ and CO₂ over CH₄,but also demonstrated the substituent-driven ligand conformation regulation is a facile way to modulate the structures and thus gas adsorption properties of MOFs.3.A lactam-functionalized bent diisophthalate linker,5,5'-?1-methylpyridin-2?1H?-one-3,5-diyl?diisophthalic acid,was judiciously designed and utilized to construct a dicopper paddlewheel-based MOF under solvothermal conditions.Single-crystal X-ray diffraction revealed that the resulting solid is a three-dimensional mfj-type network incorporting Lewis-acidic copper sites and accessible lactam groups as two different kinds of functional sites.Furthermore,gas adsorption studies indicated that the material exhibited great promise for C₂H₂/CH₄ and CO₂/CH₄ separations.In particular,compared to the unmodified parent compound,the resultant MOF exhibits9.4%and 12.9%higher C₂H₂ and CO₂ uptake capacities,as well as 48.0%and 28.3%higher C₂H₂/CH₄ and CO₂/CH₄ adsorption selecitivies for the equimolar gas mixtures at 298 K and 1 atm,despite lower specific surface area and pore volume.This work reported the first lactam-functionalized MOF displaying significantly enhanced adsorption of CO₂ and C₂H₂ over CH₄?4.By employing a ligand heterobifunctionalization strategy,we designed and synthesized an aminopyridine-functionalized diisophthalate ligand,and successfully targeted its corresponding copper-based NbO-type MOF ZJNU-98.Gas adsorption studies revealed that ZJNU-98 exhibited significantly enhanced adsorption of C₂H₂ and CO₂ over CH₄ compared to its parent MOF NOTT-101.At 298 K and 1 atm,C₂H₂ and CO₂ uptakes of ZJNU-98 are 10.0%and 16.7%higher than the corresponding values of NOTT-101,while 14.2%and 18.8%increase in C₂H₂/CH₄ and CO₂/CH₄ adsorption selectivities were observed for the equimolar gas mixtures in ZJNU-98 compared to NOTT-101.Furthermore,the contribution of the functional group effect on gas adsorption has been assessed,demonstrating that the amine group plays a more important role than pyridinic-N atom despite its lower Lewis basicity.This work provided an effective way and significant experimental evidence for the design of new porous MOFs with highly enhanced gas adsorption performance.