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

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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Figure 1: DR-UV-vis spectra of Fe-BEA in Kubelka–Munk units31.
Figure 2: MCD and VTVH-MCD of α-Fe(ii).
Figure 3: Computational evaluation of α-Fe(ii) cluster models.
Figure 4: Spectroscopic and computational elucidation of α-O.

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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.

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Authors

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.

Corresponding authors

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

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The authors declare no competing financial interests.

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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 figures and tables

Extended Data Figure 1 DR-UV-vis spectra of Fe-zeolites.

a, Ligand-field DR-UV-vis spectra of three Fe(ii)-zeolites that are known to contain α-Fe(ii): Fe(ii)-BEA, Fe(ii)-ZSM-5, and Fe(ii)-FER. Fe(ii)-MOR, which does not stabilize α-Fe(ii), is included for comparison. The lattice topologies that stabilize α-Fe(ii) have a conserved structural motif—the β-type six-membered ring (β-6MR). b, An example of a β-6MR is highlighted in this BEA lattice22.

Extended Data Figure 2 The BEA lattice.

a, The structure of the fundamental two-dimensional building unit of BEA. BEA is a layered structure built up from this unit. b, BEA is a disordered intergrowth of two polymorphs, BEA-A and BEA-B, which result from different layerings of the same fundamental two-dimensional building unit (highlighted in blue). Both polymorphs feature three-dimensional networks of 10 Å × 10 Å channels, large enough to accommodate CH4 and other small molecules22.

Extended Data Figure 3 Mössbauer features of Fe-BEA.

a, Room-temperature Mössbauer data were collected from a sample of Fe(ii)-BEA containing 0.3 wt% Fe. Three Fe components were resolved. Abs, absorption. b, Reacting Fe(ii)-BEA with N2O results in loss of the IS = 0.89 mm s−1 major species and appearance of a new major component (IS = 0.30 mm s−1; 78%). c, This new major species is eliminated upon reaction with CH4 at room temperature. It is therefore assigned to α-O. The IS = 0.89 mm s−1 component of Fe(ii)-BEA is thus assigned to α-Fe(ii). Similar Mössbauer features have also been observed in Fe-ZSM-5 and Fe-FER, but they have not been assigned to α-Fe(ii)9.

Extended Data Figure 4 Influence of β-6MR identity on predicted spectral features.

DFT-calculated structures of analogous Fe(ii) sites formed in each of the three types of β-6MR present in BEA (rings A1 and A2 in polymorph A, and B1 in polymorph B). Other atoms have been omitted for clarity. The table shows that the three sites are highly similar with respect to their metrical parameters, DFT-predicted Fe(ii) binding energies, and CASPT2-predicted spectral features.

Extended Data Figure 5 Influence of catalyst preparation on Fe speciation.

a, DR-UV-vis spectra (* = OH overtone) and b, Mössbauer spectra of Fe(ii)-BEA, showing the influence of the lattice Si/Al ratio and Fe loading on Fe speciation.

Extended Data Figure 6 Magnetic axes of the cluster models.

Orientation of the magnetic z axes of the T6/T6′ (left) and T8/T8′ (right) cluster models. Atoms have been omitted for clarity.

Extended Data Figure 7 MCD features of CH4-reacted Fe-BEA.

Comparison of MCD data, collected at a temperature of 2.6 K and a field of 7 T, from N2O-activated Fe-BEA before (black trace) and after (grey trace) reaction with CH4 at room temperature.

Extended Data Figure 8 Influence of Fe(iii)-OH geometry on O–H bond strength.

Shown are models of S = 5/2 Fe(iii)-OH sites and the associated S = 2 Fe(iv)=O species, with O–H bond strengths indicated on the arrows. a, Site 1 features a dianionic macrocyclic ligand resembling a β-6MR of a zeolite. b, Geometry optimization of the axial oxo structure in a shows that this site 1 conformation is destabilized by 4.5 kcal mol−1 (or 6.0 kcal mol−1, after correcting for strain of the macrocylic ligand). c, Site 2 is bound by two bidentate [AlH2(OH)2] ligands resembling Al T-sites. d, e, Sites 3 (d) and 4 (e) are bound by acac-like bidentate ligands (3-oxo-propenolate).

Extended Data Table 1 Excitation energies and oscillator strengths for S = 2 Fe(ii) candidate structures

Supplementary information

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). (PDF 374 kb)

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Snyder, B., Vanelderen, P., Bols, M. et al. The active site of low-temperature methane hydroxylation in iron-containing zeolites. Nature 536, 317–321 (2016). https://doi.org/10.1038/nature19059

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