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

Iron-containing zeolites have an exceptional ability to convert methane into methanol, but their active site have been hard to study; now, magnetic circular dichroism has been used to explore the reactive species, providing a technique that should be generally applicable, and revealing the value of constraining active sites within a lattice to improve catalyst functionality. Iron-containing zeolites have an exceptional ability to convert methane into methanol, but the mechanism involved is unclear as it has proved difficult to study the active site. Here Edward Solomon and colleagues demonstrate that magnetic circular dichroism techniques, more commonly used to study enzymes, can be used to reveal the structure of the active α-Fe(II) site of these heterogeneous catalysts and the reactive intermediates. An efficient process converting methane into methanol would have broad application in research and in industry. Although the catalyst studied here is not practically relevant because it oxidizes methanol further, the data show that the exceptional reactivity of the system derives from a constrained coordination geometry enforced by the zeolite lattice — which might offer a useful route for tuning the activity of other heterogeneous catalysts. 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.