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Metal-Organic Frameworks Based on Main Group Metals


Metal-organic frameworks (MOFs) represent a large family of crystalline porous materials which have been extensively studied during the past decade. Due to their great porosity, precise and uniform pore size and functional organic linkers, they have found applications in the area of gas storage, separation, ion-exchange, catalysis and sensors. While previous studies in this area emphasize more on transition metal based MOFs, the main group metal based MOFs have been relatively less explored, which, however, may bring improved properties and novel structures. This dissertation focuses on two categories of metal-organic frameworks based on the s-block metal Li and p-block metal In, respectively. The design, synthesis, structure and properties will be discussed.

Lithium is the lightest metallic element and is therefore desired for construction of ultralight MOFs with the expectation of improved gas sorption properties. In addition, the appropriate binding energy between lithium and H2 make it an ideal candidate for H2 storage. Nonetheless, almost all Li-MOFs to date have shown quite dense structures. There is no effective strategy for construction of lithium based open frameworks. In this work, we have intentionally introduced lithium cubane cluster into framework design as a versatile secondary building unit (SBU). Through this method, a series of open-framework materials were obtained, among which even a highly symmetric zeolitic framework was achieved. Surprisingly, except the coordination compounds, a series of H-bonded frameworks have also been observed in this system. In addition, a mixed multivalent ligand route has also been successfully applied for construction of 4-connected lithium imidazolate frameworks. H2 and CO2 sorption properties have been studied upon these compounds.

In-MOF represents another area that has recently attracted researcher's attention due to its higher charge and bigger coordination sphere that differs from the traditional divalent transition metals. In this work, we have developed a series of indium based charged frameworks in two opposite categories, indium trimer based cationic frameworks (ITCs) and indium monomer based anionic frameworks (IMAs). Especially, it is found that the introduction of intrinsically positively charged indium trimer SBUs can be an effective way for systematical synthesis of cationic frameworks, which are less common in the MOFs study. The ion-exchange of a series of organic dyes upon the nanogated cationic frameworks has been studied in detail. The size and charge dependent behaviors make the reported MOF a promising candidate for the stationary phase materials of ion-exchange based separation process.

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