(641f) Iron Carboxylate Metal-Organic Frameworks for Green Styrene Oxidation

Metal-organic frameworks (MOFs) are functionally modular nanoporous solids that have demonstrated notable reactivity and selectivity for hydrocarbon valorization applications, including alkyl and aryl alkene oxidations by green oxidants like hydrogen peroxide (H₂O₂) and air [1,2]. However, reaction mechanisms for many MOF-catalyzed oxidations are not well understood. Here, MIL-100(Fe), MIL-101(Fe), and NH₂-MIL-101(Fe), all Fe-based carboxylate MOFs, were synthesized without mineralizing agents [3-5] to ascertain effects of framework pore size, oxidation state, and coordination environment on reactivity for a probe aryl oxidation, styrene oxidation by H₂O₂. MIL-101 and MIL-100 have similar MTN zeotype framework structure with cages ranging from 2.5-3.4 nm, but MIL-100 has smaller hexagonal (0.86 nm) and pentagonal (0.55 nm) windows in contrast with 1.6 nm hexagonal and 1.2 nm pentagonal windows in MIL-101. These subtle differences in framework window sizes are relevant, as the kinetic diameter of styrene is 0.60 nm, indicating that styrene may diffuse through all MIL-101 cages, but only through MIL-100 cages accessible by the hexagonal window. This manifests in apparent reactivity differences, as MIL-101(Fe) demonstrates higher turnover rates for styrene oxidation compared to MIL-100(Fe) at 323 K based on lumped first-order kinetics normalized by moles of Fe. This disparity in reactivity is due to less accessible sites in MIL-100 compared to MIL-101 but could also depend on coordination environment or Fe valency distribution differences. NH₂-functionalized MIL-101(Fe) is also less reactive than MIL-101(Fe) despite similar pore sizes. The electron-donating group increases electron density in the MOF aromatic backbone and around Fe active sites; this decreases metal site and reactive intermediate electrophilicities, resulting in lower rates. Finally, all three frameworks are more selective for benzaldehyde pathways than for styrene oxide pathways at low (<10%) styrene conversion, though NH₂-MIL-101(Fe) eventually prefers styrene oxide-derived products by 7200 s.

References
[1] https://doi.org/10.1021/cr9003924
[2] https://doi.org/10.1039/C1CY00068C
[3] https://doi.org/10.1021/acs.cgd.6b01776
[4] https://doi.org/10.1016/j.jcat.2012.11.003
[5] https://doi.org/10.1007/s10854-018-9862-x