Catalytic Activity of Porphyrin-Supported Iron Oxide Clusters for Methane Oxidation

No. 01 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). Catalytic Activity of Porphyrin-Supported Iron Oxide Clusters for Methane Oxidation Melissa Barona, Hieu A. Doan, Andrew S. Rosen, Omar K. Farha, Joseph T. Hupp, and Randall Q. Snurr a Department of Chemical and Biological Engineering, Northwestern University, Evanston, IL 60208, USA b Department of Chemistry, Northwestern University, Evanston, IL 60208, USA Current environmental concerns and the rising cost of fossil energy are driving the search for cleaner alternative feedstocks for chemical production with lower carbon dioxide emissions. The escalating production of natural gas is evidence that shale gas reserves surpass those of oil, and therefore, natural gas is likely to become one of the main sources of carbon-based chemicals in the next century. 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. 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. Using density functional theory, we have studied the 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 Fe2(μ-O)2(OH)2 diamond core supported by a bridging Al(μ-O)2(OH) moiety. After a series of ALD cycles to deposit the Al atom and the Fe sites on the porphyrin substrate, the bridging oxygen atoms that initiate the catalytic cycle can be incorporated via O2 or N2O activation. Two mechanisms for the oxidation of methane using the Fe2(μO)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. Population analysis reveals the addition of electrons from C-H activation and CH3OH formation to the lowest unoccupied molecular orbitals (LUMOs). Furthermore, the highest occupied molecular orbital (HOMO) has a significant contribution from the p orbitals of the oxygen active site. Understanding the nature and electronic structure of the active site can help us identify possible reactivity descriptors for the most important steps of the catalytic cycle (C-H activation and N2O activation) for a series of ALD-grown metal oxide clusters.