(690g) Effects of Coordination Environment on the Reactivity, Selectivity, and Stability of Iron Carboxylate MOFs for Peroxide-Mediated Alkene Oxidation

Metal-organic frameworks (MOFs) are porous catalysts active in diverse chemical transformations, including hydrocarbon oxidation by benign oxidants (e.g., hydrogen peroxide (H₂O₂) and air).^1,2 Here, Fe-based carboxylate MIL-100(Fe), MIL-101(Fe), and NH₂-MIL-101(Fe) were synthesized^3-5 to decouple the effects of framework pore size, Fe oxidation state, and coordination environment on reactivity, selectivity, and material stability for a probe aryl (styrene) oxidation utilizing H₂O₂. Mesoporous MIL-101 and MIL-100 have similar MTN zeotype framework structures, but MIL-100 has smaller pore-limiting diameters (0.86 nm, 0.55 nm) in contrast with those for MIL-101 (1.6 nm, 1.2 nm). Here, the kinetic diameter of styrene is 0.60 nm, indicating that styrene (and oxygenate products) may diffuse through all MIL-101 cages, but only through the larger MIL-100 cage. MIL-101(Fe) demonstrates higher lumped first-order rate constants (normalized by moles of Fe) for styrene oxidation compared to MIL-100(Fe) at 323 K. This disparity in reactivity is a result of less accessible sites in MIL-100 compared to MIL-101 but also depends on coordination environment, acidic/basic site densities, Fe valency distributions, and surface coverages by DMF-derived species originating from different synthetic procedures that all govern oxygenate selectivities and material stability. MIL-101(Fe) is also more reactive than NH₂-functionalized MIL-101(Fe) despite identical pore sizes. Inductive effects of the amine group decrease metal site and reactive intermediate electrophilicities and, thus, rates. Notably, NH₂-MIL-101(Fe) demonstrates fractional benzaldehyde selectivity near unity compared to ≤0.59 ±0.03 for MIL-101(Fe), which is likely rooted in disparate H₂O₂-derived reactive intermediate distributions imposed by first and second coordination effects. Finally, deactivation mechanisms are probed for all three frameworks, but the degree of metal leaching, Fe valency changes, and C_xH_yO_zN_m surface coverages differ in their relative contributions to catalyst inefficiencies. Overall, this work provides fundamental insight into structure-function relationships for transition-metal MOF oxidation catalysts.

References

1. [1] https://doi.org/10.1021/cr9003924
2. [2] https://doi.org/10.1039/C1CY00068C
3. [3] https://doi.org/10.1021/acs.cgd.6b01776
4. [4] https://doi.org/10.1016/j.jcat.2012.11.003
5. [5] https://doi.org/10.1016/j.cej.2018.09.122