(178d) Reactive Stability of Polymorphic Iron-Based Metal-Organic Frameworks in Green Oxidant Driven Aqueous Pollutant Degradation

Continued development of efficient wastewater treatment strategies remains critical with increasing water consumption and pollution of natural water sources worldwide. Metal-organic frameworks (MOFs) represent a highly modular class of porous materials, often possessing high surface areas that make them promising agents for water treatment.¹ Iron(Fe)-based MOFs, made by facile syntheses using earth-abundant, low toxicity metal, have demonstrated aqueous phase pollutant degradation at moderate conditions in catalytic and photocatalytic processes using chemical oxidants.¹ This work aims to elucidate structure-functional relationships driving trends in catalytic reactivity and stability on two polymorphic Fe-MOFs—MIL-101(Fe) and MIL-88B(Fe)—to better understand stability limitations on these materials to inform catalyst design and use. While MIL-101(Fe) and MIL-88B(Fe) consist of the same Fe-oxo node and terephthalate linkers, varied synthetic conditions lead to significant differences in their pore structure and size (MTN zeotype topology, 3.4 nm maximum and acs topology, 1.3 nm maximum, respectively).^2,3 Using methylene blue (MB), a medication and dye, as a representative organic pollutant and excess hydrogen peroxide as a green chemical oxidant, MIL-101(Fe) demonstrates a metal content normalized pseudo-first order rate constant over three times that of MIL-88B(Fe) at ambient, dark conditions, possibly owing to enhanced active site access via the more open pore structure of MIL-101(Fe). Though these MOFs show recyclability promise through demonstrated regenerable reactivity using mild washing and drying treatments, some crystallinity loss and leaching of active species is evident, suggesting the need to protect active sites to use them more effectively in application. Overall, this study adds to the body of knowledge surrounding Fe-MOF use for oxidation catalysis through kinetic and deactivation analysis that clarifies limitations on their potential use in wastewater treatment systems.

References

1. https://doi.org/10.1021/acsami.7b02563.
2. https://doi.org/10.1021/cm051870o
3. https://doi.org/10.1021/ja206936e