An efficient process for converting methane into methanol would have significant economic implications, including access to cost-effective methanol fuel derived from natural gas. Cu- and Fe-containing zeolites are heterogeneous catalysts that exhibit promising activity in this transformation, hydroxylating methane rapidly under mild conditions. There is great interest in understanding the exceptional reactivity of these catalysts. The active sites of methanol synthesis are challenging spectroscopic targets, however, as they exist within a distribution of catalytically inactive ‘spectator’ sites. Site-selective spectroscopic methods developed and used in bioinorganic chemistry can overcome this problem. Whereas for metalloenzymes these techniques are used to focus on an isolated metal center within a dominant protein matrix, for metallozeolites, they enable us to focus on a single active site within a distribution of spectator sites. Coupled to electronic structure calculations, site-selective spectroscopy provides insight into active site geometric/electronic structure and reactivity that would otherwise be inaccessible. For the Cu-zeolites, the reactive intermediates in methanol synthesis exhibit intense absorption features that can be probed selectively using resonance Raman spectroscopy. This led us to define a new type of Cu/O active site – a mono-μ-oxo bridged Cu(II) dimer that forms from an O2 precursor resembling the active site of oxytyrosinase. For the Fe-zeolites, the active site exhibits only weak absorption features. We show that these can be probed selectively using magnetic circular dichroism spectroscopy (MCD). MCD defines the reduced active site, α-Fe(II), to be a mononuclear, S=2 square planar Fe(II) site. Oxidizing α-Fe(II) with N2O forms the reactive intermediate in methanol synthesis, α-O, shown here to be a mononuclear S=2 Fe(IV)=O species. α-O is therefore distinct from the methane-oxidizing bis-μ-oxo 2Fe(IV) core of intermediate Q in soluble methane monooxygenase, both in terms of its geometric/electronic structure, as well as its activation toward H-atom abstraction from CH4. The exceptional reactivity of α-O is shown to be a consequence of a constrained coordination geometry imposed by the zeolite lattice. In the context of metalloenzymes, this is known as an ‘entatic’ state, and it will be important to further explore how geometric constraints can be used to tune the reactivity of active sites in heterogeneous catalysts.

Defining the active sites of low-temperature methane hydroxylation in iron and copper zeolites

253rd National Meeting (2017)

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