Catalytic Consequences of Pore Structure, Nodal Identity, and Coordination Environment on Styrene Oxidation by Hydrogen Peroxide over Fe MOFs

Selective hydrocarbon oxidation processes are central in fine and commodity chemical production and are sensitive to the nature of the active metal sites and their surrounding coordination environment. Isometallic Fe-based carboxylate MOFs (MIL-100, MIL-101, and NH₂-MIL-101) are employed to elucidate how Fe coordination environments and framework topology influence a probe aryl (styrene) oxidation mediated by hydrogen peroxide (H₂O₂). MIL-101 shows the highest oxygenate production turnover rates, as normalized by in situ thiophene titrations, followed by NH₂-MIL-101 and MIL-100. Post reaction ex situ Mössbauer spectroscopy elucidates Fe(II) formation under reaction conditions that underpin increased oxygenate production turnover rates; this Fe(II) formation was enabled by the reductive elimination of halide capping ligands unique to the MIL-101 family but not present in MIL-100 that only contains hydroxyls. Defect undercoordinated Fe sites promote unproductive H₂O₂ decomposition and secondary oxygenate formation but do not perturb primary oxygenate selectivities. Conversely, stabilizing hydrogen-bonding interactions between N-H donors and postulated benzaldehyde metallocycle transition state structures confer NH₂-MIL-101 100% selectivity for the primary oxygenate product benzaldehyde over styrene oxide at differential styrene conversions (<3%) compared to a maximum of 59% for MIL-101. Although a fraction of linker amine moieties oxidize to form nitro groups within NH₂-MIL-101, Fe leaching, identified as a contributor to catalyst deactivation for all frameworks, is reduced. Overall, this work showcases how seemingly subtle changes and perturbations to the coordination environments local to metal sites influence the observed reactivity, selectivity, and stability for oxidative (and other) transformations within MOF-catalyzed reaction systems throughout their lifetimes.