Recent work has exploited the ability of metal-organic 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 Fe3-μ3-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 CH3OH 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 CH3OH(g) is formed via the radical-rebound mechanism on Fe(II) sites but that subsequent transport and dehydration steps are required to protect CH3OH(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.
Bibliographical noteFunding Information:
This work was supported by the Inorganometallic Catalyst Design Center, an Energy Frontier Research Center funded by the US Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) (DE-SC0012702). The authors acknowledge the Minnesota Supercomputing Institute (MSI) at the University of Minnesota for providing computational resources. Mössbauer spectroscopy was performed at the Institute for Rock Magnetism (IRM) at the University of Minnesota with the help of Peter Solheid. The IRM is a US National Multiuser Facility supported through the Instrumentation and Facilities program of the NSF, Earth Sciences Division, and by funding from the University of Minnesota. We acknowledge the Stanford Synchrotron Radiation Lightsource (SSRL) for access to beam time on Beamline 9-3. SSRL, SLAC National Accelerator Laboratory, is supported by DOE BES, under Contract DE-AC02-76SF00515, and the DOE BES-funded Consortium for Operando and Advanced Catalyst Characterization via Electronic Spectroscopy and Structure (Co-ACCESS) at SLAC National Accelerator Laboratory.
© 2021 American Chemical Society.
PubMed: MeSH publication types
- Journal Article