Direct oxidation of methane to methanol on single-site copper-oxospecies of copper porphyrin functionalized graphene: A DFT study

In this work, we discuss the study of large macro-cyclic molecules, copper porphyrins ( CuP), as the single-site active metal center for the methane catalytic oxidation. This work shows that single-site active metal center (metal-oxo species) can be created on a porous sheet of CuP deposited on graphene substrate. This active center can facilitate methane adsorption and initiate the C-H bond activation and subsequent two step methanol conversion process. We modeled this catalytic reaction process using ab initio density functional theory (DFT) to develop an understanding of functionalization, reaction energetics, and reaction mechanism at the atomic level. Our results indicate that O-CuP/graphene provides excellent reactivity for the methane to methanol conversion reaction. The first step, homolytic C-H bond scission, requires only 1.33 kJ/mol, which is unexpectedly very low value as compared to various catalysts reported for this process. This mechanism of metal-oxo active center acting catalytically to stabilize a radical intermediate is very likely a general phenomenon that can be used to control the kinetics of many homolytic reactions. The methanol is finally formed through the recombination of intermediates with a very small activation energy barrier of 0.29 kJ/ mol. Our results also indicate that graphene substrate can play an important role on catalytic activity of CuP surface by donating/backdonating necessary electrons from/to the substrate and create additional activity on the active metal sites during methane oxidation reaction. It is however important to point out that such a facile process requires a previous step of converting the CuP/graphene surface to metal-oxo species centered CuP/graphene surface by means of an oxidant. This information will be helpful to develop a novel catalyst that will have significant advantages over conventional catalysts for the selective oxidation of natural gas to methanol conversion process at mild conditions. (C) 2017 Elsevier B. V. All rights reserved.