Metal–Organic Frameworks Composed of Metal Nanocrystals and Metal Complexes for Heterogeneous Catalysis and Hydrogen Storage
- Author(s): Rungtaweevoranit, Bunyarat
- Advisor(s): Yaghi, Omar M.
- et al.
The assembly of organic and inorganic building units into porous crystalline structures has given rise to metal-organic frameworks (MOFs). The wide variety of these building blocks has propelled the discovery of MOFs with new structures, topologies and chemical properties. In retrospect, MOFs itself can be used as a building unit for subsequent synthesis to impart functionalities for specific purposes. In this dissertation, I used MOFs as a building unit for functionalizing metal nanocrystals (NCs) and for post-synthetic modifications of their interior to synthesize metal complexes for heterogeneous catalysis and hydrogen storage.
Following this direction, a zirconium-based MOF, UiO-66, was used to encapsulate copper NCs to affect the catalytic activity of copper NC for CO2 hydrogenation to methanol. In this construct, a periodic array of zirconium oxide clusters is situated on the Cu surface resulting in high interfacial contact between copper NC and zirconium oxide clusters. We found that this catalyst is highly active and selective for the synthesis of methanol. Systematic investigations and X-ray photoelectron spectroscopy suggest the presence of the interaction between copper NC and zirconium oxide cluster at the interface, also known as strong–metal support interaction (SMSI), that leads to such catalytic performance.
We have used the same interaction to anchor a single atom of copper on the missing linker defect site of zirconium oxide clusters of UiO-66. The resulting catalyst is highly active for CO oxidation with high stability and selectivity for CO oxidation. In-situ X-ray absorption spectroscopy and in-situ infrared spectroscopy show that the single copper site remains isolated throughout the heat treatment under different gases including nitrogen and hydrogen even at 350 °C.
With regard to modifications of MOF interior, MOFs have a wide range of structures, topologies and chemical functionalities primed for creating a complexity akin to the active sites of enzymes. We employed this diversity to create an active site inspired by particulate methane monooxygenase (pMMO), an enzyme that oxidizes methane to methanol in nature with high activity and high selectivity. By judicious selection of MOF with desired geometric parameters, we used MOF-808 as a precursor for post-synthetic modifications to install ligands bearing imidazole units and metalation with Cu(I) in the presence of dioxygen. The catalysts show high selectivity for methane oxidation to methanol under isothermal conditions at 150 °C. Combined spectroscopies and density functional theory calculations reveal bis(-oxo) dicopper species as the active site of the catalysts.
Finally, we have employed the post-synthetic modification of MOFs to install metal sites functioning as hydrogen adsorption sites for gas storage purpose. In this work, IRMOF-74-III equipped with primary amines was functionalized to install metal-binding ligands including Schiff base and catecholate ligands. The MOFs were subsequently metalated with Ni(II) and tested for hydrogen adsorption properties.