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Catalysis

Elusive active site in focus

Nature volume 536, pages 280281 (18 August 2016) | Download Citation

The identification of the active site of an iron-containing catalyst raises hopes of designing practically useful catalysts for the room-temperature conversion of methane to methanol, a potential fuel for vehicles. See Letter p.317

On page 317, Snyder et al.1 describe how they have attacked two of the most challenging problems in the field of catalysis using methods more common to the study of metalloenzymes. The first problem is how to pick out from an assembly of potential candidates the active site of a heterogeneous catalyst — a solid that can accelerate reactions of chemical species in the gas phase or in solution — and to determine its structure. The second problem is more specific: how to design an efficient process for selectively converting methane, the main component of natural gas, to a more valuable product. The authors' approach combines powerful spectroscopic techniques with computational modelling, and leads to a detailed picture of a catalytic site that is probably responsible for activating methane so that it can react at room temperature.

There is great interest in transforming methane into more useful liquid fuels such as methanol. But methane is notoriously unreactive, and most transformations require conditions, such as high temperatures, that are unfavourable for the selective formation of a desired product. One prominent exception occurs in microorganisms: certain bacteria use methane as a source of carbon and energy by first converting it to methanol, using enzymes known as methane monooxygenases (MMOs).

In 1997, an iron-containing structure that could be generated in certain zeolites was reported2 to convert methane to methanol, even at room temperature (Fig. 1). (Zeolites are crystalline materials containing ordered arrays of pores that can house the active sites of catalysts.) One particularly intriguing aspect of this discovery was that soluble versions of MMO are also based on iron, and feature bimetallic ferrous oxide (Fe2O2) cores at their active site3.

Figure 1: Conversion of methane to methanol at room temperature.
Figure 1

An iron-containing structure2 called α-Fe(II) converts methane (CH4) to methanol (CH3OH) and can be generated in microporous crystalline materials called zeolites. Treatment of α-Fe(II) with nitrous oxide (N2O) at 250 °C generates α-O, which contains a highly reactive form of oxygen (red atom, which originates from N2O) that can readily remove a hydrogen atom from methane at 25 °C. The resulting methyl radical (CH3•) combines with that oxygen to give a bound form of methanol, which can then be extracted from the zeolite by water. Snyder et al.1 report that the iron atom in α-Fe(II) is constrained to an unusual 'square-planar' geometry within a ring of atoms in the zeolite; ring atoms are shown around the iron atom in α-Fe(II), but are not shown for the other iron-containing structures. Fe, iron; Si, silicon; Al, aluminium.

By using various spectroscopic techniques, several research groups have proposed4,5 that the iron species of the zeolite system contains an analogous bimetallic structure, known as the α-Fe(II) centre. But such studies are complicated by the non-uniform nature of heterogeneous catalysts; it is difficult to determine whether a particular spectroscopic feature is associated with the actual active site, rather than reflecting an inactive 'spectator' site, or is a blurred-out average of both. From their interpretation of results using a suite of methods, Snyder et al. propose a quite different structure for the α-Fe(II) centre.

As a first step, the authors observed changes in the optical spectrum of the iron-containing zeolite as the material was cycled through the various stages of the methanol-forming reaction. This allowed them to assign particular peaks to the reactive states of the iron species α-Fe(II) and α-O, an intermediate species that forms during the reaction. They could then identify the corresponding features in the spectrum obtained using magnetic circular dichroism (MCD), a technique that is highly sensitive to the molecular and electronic structure of transition-metal centres such as iron. By comparing parameters determined by MCD — as well as the parameters' dependence on temperature and magnetic-field strength — with those of structurally well-characterized model compounds, the authors concluded that the most probable structure for α-Fe(II) contains a highly unusual, monometallic iron centre that has a 'square-planar' geometry (Fig. 1), whereas α-O is a square-pyramidal Fe(IV)=O structure resulting from the attachment of an oxygen ligand to the top of the square-planar species.

Snyder and colleagues' conclusions were further supported by Mössbauer spectroscopy — another technique often used in bioinorganic chemistry for structural characterization of iron centres — along with computational results. As in earlier work4, Mössbauer spectroscopy on the iron-containing zeolite gave an overlay of several signals. This was potentially problematic, because it can be hard to find a priori grounds for assigning different Mössbauer signals to particular metal species. Fortunately, Snyder and co-workers were able to correlate the relative intensity of the largest signal quite precisely with the relative concentration of the active species, as determined from the amount of reaction product ultimately obtained. This provides more confidence that the authors have assigned the correct chemical structures to the active centres, and helps to demonstrate the potential of this multifaceted spectroscopic approach for characterizing heterogeneous catalysts.

There is one caveat: the system under study is particularly well suited to the authors' approach, for two reasons. First, the active site constitutes a large fraction of the total iron species in the zeolite, about 80% or more; and second, the reaction steps can be carried out one at a time because they occur under different conditions, allowing them to be correlated with the spectral changes they engender. One or both of those advantages probably will not apply in most heterogeneous catalytic systems of interest.

The proposed structure of α-Fe(II) is intuitively pleasing to chemists because it makes sense for this species' unusual reactivity (cleavage of the strong carbon–hydrogen bond of methane at low temperatures) to be associated with an unusual structure (square-planar geometry is rare for Fe(II) centres). The authors suggest that this structure is enforced by the rigid zeolite environment, in much the same way that proteins often constrain the active sites in metal-containing enzymes to abnormal geometries6. Furthermore, the fact that the structure is apparently quite different from that of the iron centre in MMOs, despite the similar reactivities, is an encouraging sign, because it suggests that various different iron species can solve the difficult problem of converting methane into methanol.

However, this iron-zeolite system is far from being a practical methane-conversion catalyst. Indeed, it is not really a catalyst at all: the various steps of the reaction each require very different conditions; and a complete reaction cycle, including release of the product from the zeolite, is completed only by using an extraction step, affording impractically low levels of methane conversion.

Furthermore, although the iron-containing zeolite successfully activates methane to undergo reactions, that is by no means the only prerequisite for an overall methane-conversion scheme, and often not even the most challenging one. Both thermodynamic and kinetic aspects of these reactions make it extremely difficult to oxidize methane selectively to methanol, without oxidizing it further and ultimately producing carbon dioxide7. In the zeolite, selectivity is achieved because the methanol is effectively immobilized by strong binding to the active site, which prevents it from undergoing over-oxidation at another zeolite site — but this also restricts methane conversion to extremely low levels. Operation of the system in a truly catalytic mode would expose the methanol to further oxidation, almost certainly decimating selectivity, as has been observed8 when the zeolite is used at high temperatures (greater than 200 °C). Nonetheless, the insights gained by Snyder et al. provide information that may well aid the design of catalysts for the highly desirable conversion of methane to methanol.

Notes

References

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  1. Jay A. Labinger is at the Beckman Institute, California Institute of Technology, Pasadena, California 91125, USA.

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Correspondence to Jay A. Labinger.

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