(673d) Reaction and Structural Dynamics in Aqueous Pollutant Degradation By Earth-Abundant Metal-Organic Frameworks

Increased water consumption with concomitant surging water pollution drive a continued need to improve the efficacy of wastewater treatment. Metal-organic frameworks (MOFs), consisting of modular networks of organic linkers coordinated to metal-containing nodes, are potential materials for application in the adsorptive and oxidative removal of aqueous pollutants. MOFs made from earth-abundant metals, particularly iron (Fe), are attractive given their low toxicity and metal cost and have demonstrated water pollutant degradation.[1] This work focuses on hydrogen peroxide oxidation of methylene blue, a medication and dye, to understand connections between structure and function of Fe-based MOFs in the catalytic removal of pollutants pertinent to industrial effluent as well as limitations of MOFs in aqueous phase systems broadly. Under excess oxidant conditions, MIL-101(Fe) demonstrates a lumped first order rate constant (normalized by total Fe) three times that of a contrasting Fe-terephthalate with the same secondary building units in different crystallographic arrangement (MTN zeotype vs. acs topology, respectively), [2,3] possibly indicating disparate ligands coordinated to metal nodes. However, significant structural changes are evident, including loss in crystallinity and apparent iron 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. In contrast, MIL-100(Fe), isostructural to MIL-101(Fe) but constructed from a different carboxylate linker, demonstrates similar degrative reactivity to these Fe-terephthalate MOFs after thermal activation but without apparent leaching of active species, suggesting its potential for longer catalyst lifetime. This exemplifies the need to understand the timescale and extent of structural changes during reaction to properly contextualize material performance across aqueous oxidation catalysts.

References

1. https://doi.org/10.1021/acsami.7b02563
2. https://doi.org/10.1021/cm051870o
3. https://doi.org/10.1039/d1mh01663f