STABILITY AND REACTIVITY OF EARTH-ABUNDANT METAL–ORGANIC FRAMEWORKS FOR LIQUID PHASE REACTIONS

Abstract

Metal–organic frameworks (MOFs), which consist of highly tunable networks of organic linkers coordinated to metal-containing nodes, hold significant potential for use in adsorptive and (photo)catalytic liquid-phase applications. MOFs made from earth-abundant metals (e.g., zirconium (Zr) and iron (Fe)) are attractive for wastewater treatment given their low metal cost and previously demonstrated abatement of aqueous pollutants. This work predominately focuses on the oxidative degradation of methylene blue (MB), a medication and dye, by hydrogen peroxide using Zr- and Fe-based MOFs to elucidate connections between structure and function in the (photo)catalytic removal of pollutants pertinent to industrial effluent. However, findings from this thesis are critical for assessment of MOF stability in other (catalytic) applications. Under excess oxidant conditions, contrasting Fe-MOFs MIL-101(Fe), MIL-100(Fe), and MOF-235 are explored as catalysts for MB breakdown. MIL-101(Fe) demonstrates a lumped first-order rate constant (Fe-normalized) three times that of MOF-235 despite their being synthesized from the same source materials (MTN zeotype vs. acs topology arrangements, respectively), possibly resulting from disparate coordination around their metal nodes. However, significant structural changes are evident after MB oxidation, including loss in crystallinity and apparent Fe leaching. MIL-100(Fe) (MTN zeotype, with a different linker) demonstrates improved MB degradation upon thermal vacuum activation, driven by removal of moieties coordinated to its metal trimers. Moreover, leaching of active species from MIL-100(Fe) is limited in comparison to MIL-101(Fe), suggesting its potential for longer catalyst lifetimes. Exploration of pollutant degradation is extended to photocatalytic-driven oxidation by thermochemically stable UiO(Zr) MOFs, which possess semiconductor-like characteristics. Addition of Fe-(as oxide nanoparticles and dispersed moieties) to UiO-67 and pore size modulation through UiO-68 synthesis (longer linker) decrease MOF band gap energy, enhancing oxidation of MB at longer wavelengths. Although degradation rates are higher on the Fe-loaded material, it exhibits a lower oxidant utilization efficiency compared to UiO-68. The thermochemical stability and high specific surface areas of UiO-derived materials also motivate their use as scaffolds for highly dispersed metals (e.g., palladium) to catalyze pharmaceutically relevant carbon-coupling. Continued development of hybrid structures using earth-abundant MOFs represents a promising route toward refining their use in liquid-phase catalytic applications more broadly.