Catalytic and luminescent applications of metal-organic frameworks
Creators: name: Deibert, Benjamin J.
Publisher: name: No Publisher Supplied
Year: 2017

description: Energy consumption is the highest in recorded history and will continue to increase as the population continues to grow and as nations continue developing. More than 85% of human energy consumption originates with the combustion of carbon-based fuels, resulting in the release of carbon dioxide, a greenhouse gas and contributor to anthropogenic climate change. Currently, renewable energy (including hydroelectric) accounts for no more than 20% of total energy consumption. Much of the scientific community is focused on developing cleaner and more efficient energy technologies to address this crisis.  Metal-organic frameworks (MOFs) are a relatively new type of coordination polymer that have been the subject of intense investigation since their popularization around the turn of the millennium. These materials are characterized by metal atoms or clusters connected in infinite crystalline arrays by multi-dentate organic ligands and often possess permanent porosity. The main focus of this project was to investigate the potential of these materials to function as catalysts for artificial photosynthetic reactions modeling natural photosystems (water oxidation and/or CO2 reduction). As part of this project, it was necessary to set up a lab capable of such catalytic testing. The auxiliary focus of this work was to study and develop MOFs as functional materials for energy efficient luminescent applications.  A manganese MOF bearing a Mn4O4 structural subunit was selected for evaluation as a water oxidation catalyst owing to its similarity to the active site in Photosystem II. When illuminated in a photoassay containing [Ru(bpy)3]2+ and S2O82-, Mn4(μ3-OMe)4(nic)4 (nic = nicotinate) undergoes transformation to a catalytically active mixed valence layered manganese (III/IV) oxide closely resembling the mineral birnessite. This work led to the hypothesis that the active sites were likely the Mn_interlayer^(3+) locations, which possess structural similarities to manganite (γ-MnOOH). Through collaborative work, manganite was prepared and tested for electrochemically driven water oxidation and was found to have one of the lowest overpotentials for of all reported manganese oxides, second only to bixbyite (Mn2O3), but suffers from chemical instability and subsequent deactivation.  The zirconium-porphyrin MOF, PCN-222, was chosen for evaluation as a CO2 reduction catalyst. PCN-222 boasts aqueous stability over a wide range of pH values, enabling catalytic testing in aqueous solution. Additionally, the porphyrin subunit is a well-studied organic macrocycle capable of housing a metal atom in its core, and shows catalytic activity for CO2 reduction when metallated with an appropriate metal (Fe, Co, Cu). After setting up a lab capable of CO2 reduction analysis, preliminary testing was ended when Jiang et al. published work mirroring our study. However, it was found that PCN-222 functions as a reversible colorimetric and fluorescent pH sensing material from a range of pH 0–3, making it one of the first solid-state pH sensors of its kind.   Finally, two new bismuth MOFs were synthesized based on reports of Bi3+ having catalytic activity. Bismuth MOFs are rare, with only about ten different varieties reported at the start of this work, and only three with permanent porosity. The rarity of these materials is ascribed to difficulties in obtaining single crystals, which was overcome in this work by employing modulating acids and metal-cluster precursors during synthesis. The first of these two new Bi MOFs features a large tetracarboxylate linker based on the tetraphenylethylene core, which is known as an aggregation-induced emission (AIE) fluorophore. The resulting structure is anionic and has charge balancing K+ ions, which form a 1-D chain with Bi. The as-made material has strong green luminescence that undergoes significant emission wavelength shifting upon solvent removal. When fully activated, the solvochromic material adopts yellow emission with a maximum wavelength of ~553 nm and a quantum yield of ~74% under blue-light excitation (455 nm). The high quantum yield and blue-excitable yellow emission makes this MOF a strong candidate for use in phosphor-converted white LEDs.   Utilizing the synthetic strategy developed in the former case, another new Bi MOF was prepared. This material is also constructed from a tetracarboxylate linker, but is based on the well-known macrocyclic porphyrin subunit. The resulting anionic MOF framework is charge balanced by dimethyammonium cations formed in-situ during synthesis. Structural changes occur upon guest-cation exchange with lithium, reducing the three-fold intercalation state to two-fold. Additional structural changes are observed upon solvent exchange, demonstrating framework flexibility due to the flexible coordination geometry of Bi3+. This material can be prepared with 2+ and 3+ transition metal cations (Cu2+, Ni2+, Fe3+, Mn3+) occupying the linker’s macrocyclic core, resulting in a trimetallic MOF. The Fe3+ containing phase was examined as a potential catalyst for electrocatalytic CO2 reduction under non-aqueous conditions. At an applied potential of -1.7 V vs. Ag/AgCl, CO and H2 are produced in roughly equal mole quantities. Further testing is underway to fully benchmark the MOF’s catalytic performance.