Letter | Published:

The active site of low-temperature methane hydroxylation in iron-containing zeolites

Nature volume 536, pages 317321 (18 August 2016) | Download Citation

Abstract

An efficient catalytic process for converting methane into methanol could have far-reaching economic implications. Iron-containing zeolites (microporous aluminosilicate minerals) are noteworthy in this regard, having an outstanding ability to hydroxylate methane rapidly at room temperature to form methanol1,2,3. Reactivity occurs at an extra-lattice active site called α-Fe(ii), which is activated by nitrous oxide to form the reactive intermediate α-O4,5; however, despite nearly three decades of research5, the nature of the active site and the factors determining its exceptional reactivity are unclear. The main difficulty is that the reactive species—α-Fe(ii) and α-O—are challenging to probe spectroscopically: data from bulk techniques such as X-ray absorption spectroscopy and magnetic susceptibility are complicated by contributions from inactive ‘spectator’ iron. Here we show that a site-selective spectroscopic method regularly used in bioinorganic chemistry can overcome this problem. Magnetic circular dichroism reveals α-Fe(ii) to be a mononuclear, high-spin, square planar Fe(ii) site, while the reactive intermediate, α-O, is a mononuclear, high-spin Fe(iv)=O species, whose exceptional reactivity derives from a constrained coordination geometry enforced by the zeolite lattice. These findings illustrate the value of our approach to exploring active sites in heterogeneous systems. The results also suggest that using matrix constraints to activate metal sites for function—producing what is known in the context of metalloenzymes as an ‘entatic’ state6—might be a useful way to tune the activity of heterogeneous catalysts.

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Acknowledgements

B.E.R.S. acknowledges support from the National Science Foundation Graduate Research Fellowship Program under grant DGE-11474, and from the Munger, Pollock, Reynolds, Robinson, Smith & Yoedicke Stanford Graduate Fellowship. P.V. acknowledges Research Foundation–Flanders (FWO; grant 12L0715N) and KU Leuven for his postdoctoral fellowships and travel grants during his stay at Stanford University. S.D.H. acknowledges FWO for a PhD (aspirant) Fellowship. L.U. acknowledges FWO for a postdoctoral fellowship. Funding for this work was provided by the National Science Foundation (grant CHE-1360046 to E.I.S.), and within the framework of FWO (grants G0A2216N to B.F.S and G.0865.13 to K.P.). The computational resources and services used for the CASPT2 calculations were provided by the VSC (Flemish Supercomputer Center) and funded by the Hercules Foundation and the Flemish Government department EWI.

Author information

Author notes

    • Liviu Ungur

    Present address: Division of Theoretical Chemistry, Lund University, PO Box 124, 221 00 Lund, Sweden.

    • Benjamin E. R. Snyder
    •  & Pieter Vanelderen

    These authors contributed equally to this work.

Affiliations

  1. Department of Chemistry, Stanford University, Stanford, California 94305, USA

    • Benjamin E. R. Snyder
    • , Pieter Vanelderen
    • , Lars H. Böttger
    •  & Edward I. Solomon
  2. Department of Microbial and Molecular Systems, Centre for Surface Chemistry and Catalysis, KU Leuven – University of Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium

    • Pieter Vanelderen
    • , Max L. Bols
    • , Robert A. Schoonheydt
    •  & Bert F. Sels
  3. Department of Chemistry, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium

    • Simon D. Hallaert
    • , Liviu Ungur
    •  & Kristine Pierloot
  4. Photon Science, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA

    • Edward I. Solomon

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Contributions

E.I.S., B.F.S., R.A.S. and K.P. designed the experiments. B.E.R.S., P.V., M.L.B. and L.H.B. performed the experiments. B.E.R.S. performed the DFT calculations with help from L.H.B. S.D.H. and L.U. performed the CASPT2 calculations. B.E.R.S., P.V. and E.I.S. analysed the data. B.E.R.S. and E.I.S. wrote the manuscript with help from P.V., S.D.H. and L.U.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to Robert A. Schoonheydt or Bert F. Sels or Edward I. Solomon.

Reviewer Information Nature thanks A. Bell, E. Bill and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Extended data

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  1. 1.

    Supplementary Tables

    This file contains Supplementary Tables 1-3 showing coordinates of the DFT-optimized models of α-Fe(II) (Table 1), α-O (Table 2), and the α-O/CH4 H-atom abstraction transition state (Table 3).

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DOI

https://doi.org/10.1038/nature19059

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