Structure, Dynamics, and Reactivity for Light Alkane Oxidation of Fe(II) Sites Situated in the Nodes of a Metal-Organic Framework

Matthew C Simons; Jenny G Vitillo; Melike Babucci; Adam S Hoffman; Alexey Boubnov; Michelle L Beauvais; Zhihengyu Chen; Christopher J Cramer; Karena W Chapman; Simon R Bare; Bruce C Gates; Connie C Lu; Laura Gagliardi; Aditya Bhan

doi:10.1021/jacs.9b08686

Structure, Dynamics, and Reactivity for Light Alkane Oxidation of Fe(II) Sites Situated in the Nodes of a Metal-Organic Framework

J Am Chem Soc. 2019 Nov 13;141(45):18142-18151. doi: 10.1021/jacs.9b08686. Epub 2019 Oct 31.

Authors

Affiliations

¹ Department of Chemical Engineering and Materials Science , University of Minnesota , 421 Washington Avenue SE , Minneapolis , Minnesota 55455 , United States.
² Department of Chemistry, Chemical Theory Center, and Supercomputing Institute , University of Minnesota , 207 Pleasant Street SE , Minneapolis , Minnesota 55455 , United States.
³ Department of Chemical Engineering , University of California , Davis , California 95616 , United States.
⁴ SSRL , SLAC National Accelerator Laboratory , Menlo Park , California 94025 , United States.
⁵ Department of Chemistry , Stony Brook University , 100 Nicolls Road , Stony Brook , New York 11794 , United States.

PMID: 31670511
DOI: 10.1021/jacs.9b08686

Abstract

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)═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).