Metal Organic Frameworks: From Structure to Property
Porous Materials are an important class of materials due to their large-scale industrial applications in gas adsorption, gas separation, heterogeneous catalysis and so on. Among many kinds of porous materials, metal organic frameworks (MOFs) have emerged as one of the favorites in the past fifteen years. MOFs are crystalline porous materials that are mainly made of inorganic metal containing units (secondary building units, also called SBUs), organic ligands and templates.
Compared to commercial inorganic porous materials, zeolites, which are based on Al3+ and Si4+, MOFs can accommodate most types of metal ions and a large number of organic ligands. This makes it possible for MOFs to adopt some fantastic features: (1) in principle the pore size and the surface area of MOFs can be tuned by changing the organic linkers, which allows MOFs to have a wider range of pore size and surface area than zeolites; (2) functionality can be built into linkers; (3) in MOFs, the solvents have weak interaction with frameworks and therefore can easily leave the structure at relatively low temperature, which will provide useful and readily accessible porosity. Furthermore, the removal of coordinated solvent molecules can provide unsaturated metal centers, which are quite meaningful for gas adsorption and catalysis.
We focus on developing MOFs with zeolite topologies and analyzing how different factors affect the construction of the final products. We also evaluated the potential applications of synthesized MOFs in gas adsorption/separation, proton conductivity and oxygen reduction reaction.
In chapter 2, we reported a rare MOF in which two zeolitic frameworks (SOD and ACO) are within one MOF. The bent angle of the ligand plays an important role in the construction of this fantastic structure. This MOF showed selective adsorption of CO2 over H2. The work also proved how important the bent angle of ligands is in the synthesis of MOFs.
In chapter 3, by using the same ligand, and varying synthetic conditions (especially In3+/FDA ratio), we obtained three unique building blocks of indium, demonstrating charge-switching from positive trimer to negative monomer and leading to synthesis of In-MOFs with tunable framework charge. We further demonstrated the variations of gas sorption properties and proton conduction behaviors in such materials.
In chapter 4, we developed a series of zirconium-porphyrin frameworks. The frameworks are constructed by Zr4+ and TCBPP. Different metal ions (Zn, Co, Fe) are trapped within phorphyrin rings. The frameworks have exceptional stability and uniform pores. These make them become excellent candidates for being the precursors of catalysis for oxygen reduction reaction (ORR). This work demonstrated a novel method to develop noble metal free catalysts for electrochemical reactions.