Design, Synthesis And Characterization Of Novel Metal-Organic Frameworks

Metal organic frameworks, or MOFs, have emerged as a new class of porouscrystalline materials with ultrahigh porosity and enormous internal surface areas.These properties, together with the extraordinary degree of variability and tunabilityfor both the organic and inorganic components of their structures, make MOFs ofinterest for potential applications in gas storage, separation, purification catalysis. Asa multifunctional materials, MOFs emerged approximately two decades ago and havesince quickly developed into a fruitful research field.Based on the principle of crystal engineering, we have focused our study on thesynthesis, structures and relationship between structures and properties of MOFs. Inthis dissertation, we have prepared nine new MOFs by using different organic ligands,and analyzed their structures as well as explored their multifunctional properties, suchas separation, fluorescence and ferroelectric properties. These results will beintroduced from the following four issues:（1） Separation processes play significant roles in industry and daily life and areused for three primary functions: concentration, fractionation, and purification. Theseprocesses usually include distillation, crystallization, extraction, absorption,adsorption, and membrane separations, of which distillation accounts for90-95%ofall processes in the chemical industry. Distillation is however not always feasiblebecause of some inherent limitations, such as the decompositions of some materials athigh temperatures that are often required for the system. Therefore adsorption basedseparation process was regarded as one of the low-carbon, environmentally friendlymethod. Bio-alcohols are considered to be sustainable alternatives for fossil fuels, andexpected to be commercialized during the next decade. Purification of bio-alcoholsfrom aqueous media is one of the key issues. Based on this, we synthesized two microporous MOFs, namely [Cd（Hthipc）2]·6H2O （1） and [Mn（Hthipc）2]·2H2O （2）,with1D hydrophilic channels, through H2thipc ligand. Two compounds are exhibitingunique selective adsorption of water over alcohols. The results of TGA and PXRDexperiments indicate that compound1and2show excellent hydrothermal stability.GC separation experiments demonstrate that alcohols can be efficiently separatedfrom aqueous mixtures by compound1. This microporous MOF could be used in thepurification procedures to separate and identify the purity of bio-alcohols andprovides the perspectives for efficient and economical production of bio-alcohols.（2） As a very important research field in both coordination chemistry and organicchemistry, in situ metal/ligand reactions have been extensively investigated for manydecades for the discovery of new organic reactions, elucidation of reactionmechanisms, as well as generation of novel coordination compounds or MOFs,especially those that are inaccessible in direct preparation from the ligands. Thismethod has been widely adopted in the preparations of new metal-organicframeworks （MOFs） that exhibit intriguing structural diversity and promisingphysical/chemical properties for potential applications in the recent years. Based onthe above issue, here we chose histidine derivates H2thipc as ligand, constructed themonomer [Cu（Hthipc）2]·3H2O·2HCl （3）, than we adopted the stepwise synthesisapproach to construct （3,3） connected2D MOFs [Cu（ipc）H2O]n（4），using compound3as processor linker. In situ dehydration and rearrangement reactions occurred in thisprocess. Meanwhile, in situ oxidation of Cu+was observed. Such kind of in situsolvothermal reactions rarely reported in the literature, and can be used asunconventional synthetic approach to construct novel MOFs or CPs, especially thosethat are inaccessible in conventional preparation from the ligands.（3） MOFs or coordination polymers through their tunable composition, structure,pore size, and volume, easy functionalization, flexible network and/or accessiblemetal sites, possess many advantages for the adsorption and release of biomoleculescompared to other carriers such as inorganic porous solids （zeolites or mesoporoussilica） or organic polymers. Their biodegradable character can also be modifiedthrough an adequate choice of the metal, linker and structure which results in adegradation in body fluid from a few minutes up to weeks. Among them, MOFs basedon endogenous linkers are of great interest even if real porous―BioMOFs‖are stillscarce. Recently, considerable effort has been devoted to preparation and structural characterization of metal purine complexes, because purine and its derivatives play apivotal role in the regulation of many biological processes. It should be noted thatmost of these complexes are monomers or oligomers, in which nitrogenous purines,such as guanine and adenine, act as ligands, and coordination polymers with purinesare quite limited. However, compared with the aromatic polycarobxylate ligandsgenerally used, purines are intriguing ligands in coordination polymers because oftheir biological compatibility, multiple metal coordination sites, molecular recognition,and self-assembling characteristics. Hypoxanthine （HYP,6-hydroxypurine）, anucleobase with one oxygen and four nitrogen donor sites, it can eliminate one or twoprotons, and coordinate metal ions through a combination of the N1, N3, N7, N9andO6coordination sites. Thus we introduced hypoxanthine as an endogenous ligand,reaction with Zn²⁺in solvothermal condition, constructed a novel none toxic2DMOFs with5.5×3.5aperture, in which the HYP ligand adopt a rare μ3-N1, N7, N9coordination mode.（4） MOFs are constructed from the self-assembly of bridging organic ligands andmetal ions or clusters. A judicious choice of these basic components （molecularbuilding blocks） prior to their synthesis is necessary to obtain highly functional MOFs.Their conventional direct synthesis often involves significant trial and error to achievea particular desired structure. Furthermore, the lack of control during the one-potdirect synthesis poses considerable difficulties, although it is not impossible toachieve crystalline porous materials that possess the preferred interior decoration witheither organic or metal functionalities. These drawbacks have led to the considerationof post-synthetic approaches as a viable option for the preparation of new analoguesof the porous structures. The importance of such post-synthetic modifications ofpre-assembled frameworks is that they offer better control over the structures of theseporous materials. Various post-synthetic-modification （PSM） processes have beenreported in the literature, such as the covalent attachment of new molecular units ontopre-existing organic functionalities or onto coordinatively unsaturated metal sites.Recently, Kim and co-workers demonstrated that metathesis, that is, the replacementor exchange of integral parts of the frameworks, can be applied to MOFs as PSMswithout altering the basic structure or topology of the frameworks. However, thisstrategy does not fit for all MOFs, there are only few MOFs can metal ion metathesis.Herein, we synthesized compound6-9using conjugated organic ligand H3ITC exhibiting complete and reversible exchange of metal ions constituting a robustframework while maintaining not only the structural integrity of the framework butalso single crystallinity. Because of their ultra high porosity, the structure informationof compound6-9cannot be obtained by CCD diffractometer. This metal ionmetathesis synthetic approach may open up a reliable route to produce highlyfunctional MOFs materials on demand.