(67h) Stability and kinetics of Fe-based MOFs with diverse structures in aqueous pollutant degradation

Global access to clean water is an essential and growing need due to concomitant increased water consumption and pollution. Thus, advanced, effective materials, such as metal-organic frameworks (MOFs), that can remediate water contaminates either through their absorption or degradation are of mounting importance. MOFs made from earth-abundant metals, particularly iron (Fe), are attractive given their low toxicity and metal cost and have demonstrated water pollutant degradation at moderate conditions in catalytic and photocatalytic processes. This work focuses on hydrogen peroxide oxidation of methylene blue, a probe molecule used to understand catalyst performance in the context of the breakdown of medication and dyes from pharmaceutical and textile industries, respectively. We aim to elucidate structure-functional relationships driving trends in catalytic reactivity and stability on two polymorphic Fe-MOFs—MIL-101(Fe) and MOF-235. Under excess oxidant conditions, MIL-101(Fe) demonstrates a larger lumped first order rate constant (normalized by total Fe) than MOF-235 with the same secondary building units in different crystallographic arrangement, indicating the role disparate ligands coordinated to metal nodes play in reactivity. However, significant structural changes are evident, including loss in crystallinity and apparent Fe leaching, even though the catalysts continue to demonstrate degradative reactivity upon recovery and reuse. Moreover, leaching occurs even when catalyst is exposed to water in the absence of reactants; thus, solvent interactions drive material changes that require careful interpretation to clarify the nature of active sites and reactivity. MOF stability is extended to other systems, probing effects of different solvents (polar, aprotic (acetonitrile) and polar, protic (water)) on deactivation and dominant surface species. This work exemplifies the need to understand the timescale and extent of structural changes during reaction to properly contextualize material performance across aqueous oxidation catalysts.