(731g) Catalytic Activity of Porphyrin-Supported Iron Oxide Clusters for Methane Oxidation

During the last decade, natural gas production in the United States has significantly increased every year, and it is predicted to increase even more rapidly during the coming years. This has provided new incentives for selectively transforming natural gas components and their derivatives into more valuable chemicals through energy efficient and environmentally friendly processes. However, the current route for converting methane into more useful chemicals is through a complex, multi-step industrial process that requires multiple reactors, separations, and high temperatures and pressures¹. For this reason, there is a great interest in designing catalysts that accomplish this transformation directly while avoiding deeper oxidation to carbon dioxide.

Methane monooxygenase (MMO), an enzyme found in biological systems, is capable of oxidizing methane to methanol using O2 at mild conditions. MMO has two different forms: (1) particulate MMO (pMMO), which is postulated to have a dicopper-oxo active site, and (2) soluble MMO (sMMO), which contains a diiron site.² The identification of different iron and copper sites in these enzymes has inspired the synthesis of catalysts that mimic their activity towards the partial oxidation of methane.³

Using density functional theory, we have studied the growth and catalytic activity of iron oxide nanoclusters that can be grown via atomic layer deposition (ALD) on a porphyrinic substrate and mimic the structure of the active site in the enzyme for the oxidation of methane to methanol. The porphyrin-supported iron oxide active site consists of an Fe^IV₂(μ-O)₂(OH)₂ diamond core supported by a bridging Al^III(μ-O)₂(OH) moiety. After a series of ALD cycles to deposit the Al atom and the Fe^II sites on the porphyrin substrate, the bridging oxygen atoms that initiate the catalytic cycle can be incorporated via O₂ or N₂O activation. Preliminary results show that N₂O activation is more favorable, but may lead to the formation of sites that contain only one bridging oxygen and two Fe^III sites. Two mechanisms for the oxidation of methane using the Fe^IV₂(μ-O)₂(OH)₂ active site and N₂O as the oxidant are proposed: (1) a concerted mechanism in which the C-H bond is cleaved heterolytically and (2) a rebound mechanism in which a bridging oxygen or a terminal oxo group abstracts a hydrogen atom from methane. Our preliminary computational results show that the rebound mechanisms are more likely to occur due to lower activation barriers of about 70 kJ/mol. We also find activation barriers between 100-120 kJ/mol for N₂O activation and CH₃OH desorption. To further understand the effect of cluster composition on the activity for the various oxygen sites in the di-iron cluster, we replaced the aluminum atom and one of the iron atoms with other transition metals, calculated the main reaction barriers, and performed population analysis studies. Using these computational tools, we aim to understand the ways in which we can tune the activity of the proposed catalysts for the selective oxidation of methane.

 References

1. Crabtree, R. H. Chem. Rev. 95, pp 987–1007 (1995).

2. Lippard, S. J. and Feig, A. L. Chem. Rev. 94, pp 759–805 (1994).

3. Wulfers, M. J., Teketel, S., Ipek, B., and Lobo, R. F. Chem. Commun. 51, pp 4447-4450 (2015).