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Metal organic frameworks (MOFs), with their crystalline, porous structures, can be synthesized to incorporate a wide range of catalytically active metals in tailored surroundings. These materials have potential as catalysts for conversion of light alkanes, feedstocks available in large quantities from shale gas that are changing the economics of manufacturing commodity chemicals. Mononuclear high-spin (S = 2) Fe(II) sites situated in the nodes of the MOF MIL-100(Fe) convert propane via dehydrogenation, hydroxylation, and overoxidation pathways in reactions with the atomic oxidant N2O. Pair distribution function analysis, N2 adsorption isotherms, X-ray diffraction patterns, and infrared and Raman spectra confirm the single-phase crystallinity and stability of MIL-100(Fe) under reaction conditions (523 K in vacuo, 378-408 K C3H8 + N2O). Density functional theory (DFT) calculations illustrate a reaction mechanism for the formation of 2-propanol, propylene, and 1-propanol involving the oxidation of Fe(II) to Fe(III) via a high-spin Fe(IV)a•O intermediate. The speciation of Fe(II) and Fe(III) in the nodes and their dynamic interchange was characterized by in situ X-ray absorption spectroscopy and ex situ Mössbauer spectroscopy. The catalytic relevance of Fe(II) sites and the number of such sites were determined using in situ chemical titrations with NO. N2 and C3H6 production rates were found to be first-order in N2O partial pressure and zero-order in C3H8 partial pressure, consistent with DFT calculations that predict the reaction of Fe(II) with N2O to be rate determining. DFT calculations using a broken symmetry method show that Fe-trimer nodes affecting reaction contain antiferromagnetically coupled iron species, and highlight the importance of stabilizing high-spin (S = 2) Fe(II) species for effecting alkane oxidation at low temperatures (408 K).
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. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from the National Science Foundation (NSF) through the MRSEC program. 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 No. 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. Data for pair distribution function analysis were collected at beamline11-ID-B at Argonne National Laboratory, and use of the Advanced Photon Source, a User Facility operated for the US DOE, Office of Science, by Argonne National Laboratory, was supported by DOE Contract No. DE-AC02-06CH11357. We acknowledge Prof. R. Lee Penn, University of Minnesota, for helpful technical discussions.
Copyright © 2019 American Chemical Society.
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Reporting period for MRSEC
- Period 6
PubMed: MeSH publication types
- Journal Article