| Abstract |
Metal organic frameworks (MOFs) represent a unique class of functional crystalline materials resulting from the combination of organic linkers and metal ions or clusters with tunable chemical composition, diverse structures and exceptional porosities. They demonstrate remarkable sorption and chemical properties and for this reason MOFs are important candidates for key technological applications related to energy and environment, including gas storage/separation, catalysis and sensing.
More specifically, the linker’s topology as well as the metal clusters (i.e. secondary building unit, SBU) are the key factors that will dictate the structure of the material’s crystal network. Thus, the strategically choice of the above enables the design and the successful synthesis of a plethora of new materials with targeted topologies, pores sizes and chemical properties.
However, MOFs despite their unique tunable character and intriguing properties they often show poor stability due to the nature of the coordination bond that is formed between the linker and the secondary building unit. Great efforts have been made in order to face this drawback and recently a significant number of new works has been published focusing on high valence metal cations Zr(IV)-, Hf(IV)- and Y(III))-based MOFs, which successfully deal with this challenge. In detail, these materials show notable stability due to the strong affinity between the high oxidation state metals and the oxygen atom from the carboxylate linkers, which are widely used in the synthesis of MOFs. Furthermore, these materials are very promising, since beside their exceptional stability they also show rich structural characteristics and fascinating properties.
In this work, we focused in the design, synthesis and study of Metal Organic Frameworks (MOFs) based on high valence metals cations including Zr(IV), Hf(IV) and RE(III) (Rare Earth) with tetratopic carboxylate linkers of different geometries (rectangular and tetrahedral). In total 21 new MOFs were synthesized, presenting various topologies such as csq, she, scu, shp and flu. All of the materials were structurally characterized using X-ray diffraction experiments. Although all of the new MOFs were obtained in single crystal form, in eleven (11) of them we were able to determine the atomic structure using single crystal X-ray crystallography. The corresponding single crystal structures were analyzed in detail are compared with literature data. Gas sorption studies were carried out for five (5) of the new materials. Their porosity along with gas sorption properties were studied in detail, providing important findings.
In particular, Y-csq-MOF-1 and Zr-she-MOF-1 were found to present high Xe uptake with high selectivity of Xe over Kr at 298Κ and 1 bar. The corresponding data places these MOFs among the top five materials for Xe/Kr separation.
The CO2 and CH4 sorption properties of Zr-sulfone-flu-MOF-1 were studied and compared with the non-functionalized analogue, revealing an increase in the CO2 uptake (115cm3g-1 at 273K for Zr-sulfone-flu-MOF-1 and 63.6 cm3g-1 for PCN-521at 273K) as well as in the Qst0 which is increased by 33% in the sulfone MOF in comparison to that of PCN-521 indicating the important role of the polar –SO2 groups in this system.
The catalytic activity of Zr-she-MOF-1 in CO2 cycloaddition reactions with epoxides was evaluated, providing further insights into the chemical properties of this kind of solids. The observed catalytic activity is comparable with some of the best performing MOFs, including Hf-NU-1000.
The results of the present Thesis provide important information for the targeted synthesis of novel MOFs based on Zr(IV)/Hf(IV) and RE(III) clusters, using reticular chemistry rules. Our findings help to better understand how these system forms and the evaluation of their gas sorption and catalytic properties provide important knowledge towards understanding the structure-property relationship.
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