Beyond Radical Rebound: Methane Oxidation to Methanol Catalyzed by Iron Species in Metal-Organic Framework Nodes

Recent work has exploited the ability of metalorganic frameworks (MOFs) to isolate Fe sites that mimic the structures of sites in enzymes that catalyze selective oxidations at low temperatures, opening new pathways for the valorization of underutilized feedstocks such as methane. Questions remain as to whether the radical-rebound mechanism commonly invoked in enzymatic and homogeneous systems also applies in these rigid-framework materials, in which resisting the overoxidation of desired products is a major challenge. We demonstrate that MOFs bearing Fe(II) sites within Fe-3-mu(3)-oxo nodes active for conversion of CH4 + N2O mixtures (368-408 K) require steps beyond the radical-rebound mechanism to protect the desired CH3OH product. Infrared spectra and density functional theory show that CH3OH(g) is stabilized as Fe(III)-OCH3 groups on the MOF via hydrogen atom transfer with Fe(III)-OH groups, eliminating water. Consequently, upon addition of a protonic zeolite in inter- and intrapellet mixtures with the MOF, we observed increases in (CHOH)-O-3 selectivity with increasing ratio and proximity of zeolitic H+ to MOF-based Fe(II) sites, as methanol is protected within the zeolite. We infer from the data that (CHOH)-O-3( g) is formed via the radical-rebound mechanism on Fe(II) sites but that subsequent transport and dehydration steps are required to protect (CHOH)-O-3( g) from overoxidation. The results demonstrate that the radical-rebound mechanism commonly invoked in this chemistry is insufficient to explain the reactivity of these systems, that the selectivity-controlling steps involve both chemical and physical rate phenomena, as well as offering a strategy to mitigate overoxidation in these and similar systems.

Automated summary

Recent work has shown that MOFs can isolate metal sites mimicking enzymes to catalyze selective oxidations at low temperatures, offering new pathways for the valorization of underutilized feedstocks like methane. It has been demonstrated that protecting the desired product CH3OH in MOFs requires steps beyond the radical-rebound mechanism, with further transport and dehydration steps necessary to prevent overoxidation. The selectivity-controlling steps involve both chemical and physical rate phenomena, providing a strategy to mitigate overoxidation in these systems.