Investigation Of Design, Synthesis, Structures And Propetries Of Novel Metal-Organic Frameworks

In the last two decades, metal–organic frameworks (MOFs) constitute an emerging class ofnew kind of functional material useful in gas storage, purification and separation, luminescence,magnetism, heterogeneous catalysis, sensors and so on. They have attracted considerableattention for not only their intriguing topologies and higher surface area compared to 'classical'porous solids, but also the flexible tailor and decoration derived from the exsit of organic ligands.However, many crucial investigations are still in the initial stage, such as: how to realize directedsynthesis and controlled assemble according to the synthesis rule, how to optimize the functionalfeatures for realistic application and so on. Therefore, this field still leaves much to be desiredbefore the industrial application.Based on the principle of molecular engineering, we focused our research on the design,synthesis, structures and functional properties of new functional MOFs materials. With the aimof mastering synthesis regulation and improving the properties of MOFs materials, we weredevoted to investigate the influence of organic ligand on the target structures and the influence ofdifferent functional groups on the properties of pristine material. In this thesis, we introduce15new MOFs prepared with different organic ligands from two perspectives of structural designand functional design respectively. The synthesis method, the topology structures as well as theirfunctional properties are detailedly elaborated mainly in the following two aspects:(I) The aspect of structural design:(1) Aim at achievement of MOFs with novel charming topologies, new synthesisapproaches are continually explored as well as a number of efforts are contributed to surveyingthe synthesis rule in order to realize the directed synthesis. The groups of Kitagawa, Kim and Chen have been particularly interested in the frameworks with N-donor and O-donor mixedligands, which show aesthetic topological structures and extrusive functional features. Itdemonstrates that 'mixed-ligand' is an effective strategy to obtain new MOFs. Thus, we adoptedthis synthesis approach, with the accommondation of distictive aromatic di-carboxylate ligandsor di-pyridyl ligands owing different nature seperately in two synthesis systems to obtain newMOFs materials, meanwhile to investigate the influence of organic ligands on the targetstructures. The systematic variation of the aromatic di-carboxylate ligands mixed with flexibleN-donor ligand leads to compounds1–3based on uni-, di-, tri-nuclear secondary building units(SBUs) respectively. It is intresting to find that compound3possesses rare hex type net and anunprecedented phenomenon that there exist two kinds of pillars arrange alternately in the samelevel in an individual structure resulting in a special anionic framework. The systematicvariation of the di-pyridyl ligands mixed with asymmetrical aromatic carboxylate ligand leads tocompounds4–8. The di-pyridyl ligands play the role of the lewis base to adjust the pH of thesolvent during the synthesis process, meanwhile play the role of the ligand to construct theframework. The carboxylate ligand adopts various coordination modes generating distinctivetopological structures, which indicate the crucial influence of metal centers and organic ligandson the resulting structures. Finally, the strong fluorescence properties of those compoundsindicate that they may be suitable as candidates of potential photoactive materials. However,there is still a defect of this part, which is that these compounds show non-porosity due to theunstability or compactness.(2) Realizing that stability is the pre-requisite to the material for further applications, mucheffort has been devoted to the design and synthesis of new MOFs combining permanent porosityand remarkable stability. Recently, MIL-n and ZIFs respectively prepared from multicaboxylateand imidazolate ligands hold great promise to further application based on their remarkablestability. Considering the defect of first part, we switched our strategy to 'mixed type' organicligand taking advantage of the carboxylate groups and N atoms of the imidazole ring to design and synthesize stable and high dimensional porous MOFs for promising applications. Thus, weintroduced1–H–benzimidazole–5–carboxylic acid into the reaction systems. With Ni(II) andhydrothermal synthesis method, we obtained a novel3D MOF material compound9. In thestructure, there is1D channel formed by the parallel alignment of three infinite helical chainswith4.5×4.52aperture. Due to the high thermal stability, the activated compound9exhibitsgood porosity and selective adsorption property between nitrogen and hydrogen.(II) The aspect of functional design:(1) Flexible material MIL-53(M)(M=Cr, Al, Ga, et al.) manifests unprecedented verylarge breathing effect leading to the potential applications in gas separation, drug delivery and soon. However, such breathing effects are different based on the nature of the metal centers. Referto In, it is difficult to obtain the open form due to the strong interaction between the frameworkand the trapped molecules, which restricts the further porosity application. In another hand, it isproved experimentally and computationally that the functionalization with appropriatesubstitutions is an effective approval to achieve or improve exceptional features. Therefore, withthe intention of improving the porosity of MIL-53(In), we adopted 'pre-functionalization'strategy to implement different functional groups (-(OH)₂,-Br or-NO₂) into the prototypicalstructure. We successfully obtained the corresponding large scale single crystals and microcrystals. Through structure features and nitrogen adsorption behaviors, we investigated theinfluence of the functionalization on the breathing effects of MIL-53(In)_X. Notably,implemention of-Br and-NO2groups effectively improve the capability of the pore openning inthe case of pore capacity sustained, which encourages the possibility of the further application ofMIL-53(In)_X serie materials.(2) MOFs materials, as a new class of functional porous crystalline materials, have beenproved to be promising candidates for the adsorption or capture of strategic gases such as H2,CO2, due to their key features, such as very high internal surface areas and permanent tunable porosity. However, practical and industrial applications of those materials in realistic or non-ideal environments do require further improvement. Herein, with the intention of studying theinfluence of the polar functional groups on the adsorption properties, we still adopted 'pre-functionalization' strategy and successfully introduced different functional groups (-NH₂,-Br or-NO₂) into the prototypical structure MIL-68(In), which processes high thermal stability andquite high surface area as well as large channels sufficient to further implace the substituentgroups. The synthesis rule of these systems has been detailedly investigated. The result of gasadsorption indicates the implemention of polar functional groups can effectively enhance theenthalpy of H2and CO₂and then improve the adsorption capacity at the lower pressure, althoughthe final uptake amount at1atm is still dominated by the surface area or the pore volume. It ismeaningful especially for the gas adsorption or separation which is needed to be done at lowerpressure, such as CO₂capture in postcombustion (PCO2≈0.15bar).