Studies on Conversion of Metal-Organic Frameworks Toward Synthesis of Functional Nanomaterials

Abstract

Recently, the utilization of metal-organic frameworks (MOFs) has attracted great interest as precursors for synthesizing functional nanostructured materials, which are difficult to obtain by conventional methods. Since MOFs comprise the alternating array of organic and inorganic components, nanostructured materials such as porous metal oxides, assembled metal/metal oxide nanoparticles, porous carbon and their composites can synthesize through transformation of well-tailored MOFs. Those converted materials have been studied due to diverse applications including energy conversion and storage, gas sorption, and catalysis. We synthesized the Mg-based MOF constructed from aliphatic carboxylate ligands which are thermally less stable and much labile than aromatic ligands. The thermal conversion of the aliphatic-ligand based MOF (Mg-aph-MOF) leads to the formation of nanoporous metal oxides. During the thermal decompositions of MOF, aliphatic ligands transformed into organic vesicles which acted as self-templates, and finally were converted to porous structure. Furthermore, MOFs also appear as promising templates for the fabrication of well-defined microstructures that are difficult to obtain from conventional approaches. In this regard, we prepared two different Co-based MOFs composed of the same building blocks. Through proper heat treatment of MOFs, the two kinds of Co3O4 were obtained with the retention of parent MOF morphology such as plate-like and rod-like shapes. Along with metal oxide nanomaterials, three-dimensional mesoporous graphitic carbon (3D mesoG) was synthesized via the thermal conversion of NiII coordination compounds. During the thermolysis of [Ni2(EDTA)] (EDTA4- = ethylenediaminetetraacetate), NiII ions are transformed into metallic Ni nanoparticles (Ni NPs) which act as a catalyst for graphitization of decomposed ligands and a template for generating mesopores. By varying the type of ligands and metal centers of the starting compounds, we can examine the role of in-situ generated species from those components during thermolysis and manipulate the structure of resultant 3D mesoG. Lastly, understanding the conversion phenomena of a Zn-based MOF could be exploited for synthesizing metal-carbon composites. Zn-based MOFs have been adopted for the preparation of nanoporous carbon, evaporating an intermediate Zn metal via carbothermic reduction during thermolysis. Therefore, we utilized the Zn metal in situ evolved in Zn-based MOF conversion, which play an important role as a reducing agent for foreign metal oxides, or zincothermic reduction. In the reaction system of a Zn-MOF with GeO2, in situ evolved Zn reduce GeO2, producing Ge and ZnO. Interestingly, ZnO is automatically reduced to Zn via a carbothermic reduction during the conversion process, which returns reducing agent to the reaction. Thus, the repeated occurrence of the zincothermic and carbothermic reduction reaction promotes a recyclable redox-metallothermic reaction.