Effects of single and double active sites of Cu oxide clusters over the MFI zeolite for direct conversion of methane to methanol: DFT calculations

In this work, we investigate the effect of various species of Cu oxide clusters including single and double active sites incorporated in the MFI zeolite framework for the direct conversion of methane to methanol. An M06-2X density functional calculation is employed to fine-tune the suitable number and species of active sites and to provide insights into the effect of the active sites on the reaction mechanism of methane to methanol. Two models, single and double active sites of Cu oxide clusters, have been chosen, in which the single active site of Cu oxide clusters, (mono(μ-oxo)dicopper(II)), is located at the Al1′–Al12′ pair ([Cu(μ-O)Cu]2+@Al1′–Al12′/MFI) or at the Al6–Al7 pair ([Cu(μ-O)Cu]2+@Al6–Al7/MFI) in the MFI framework. For the double active sites of Cu oxide clusters, two species of double active sites of Cu oxide are considered. The first one is the double active site of mono(μ-oxo)dicopper(II) containingtwo Al–Al pairs (Al1′–Al12′ and Al6–Al7 pairs) in the MFI framework (2[Cu(μ-O)Cu]2+/MFI) and the other is the double active site of trans-μ-1,2-peroxo dicopper(II), which occupies two Al–Al pairs (Al1′–Al12′ and Al6–Al7 pairs) in the MFI framework (2[Cu(μ-1,2-peroxo)Cu]2+/MFI). Furthermore, the activation energy for C–H bond dissociation in direct methane conversion to methanol is considered. Compared with the single active site of [Cu(μ-O)Cu]2+/MFI, the double active sites, in particular (2[Cu(μ-O)Cu]2+/MFI), exhibited the lowest activation energy, approximately 12.5 kcal mol−1. The high charge transfer between activated methane and Cu oxide active sites and also the high negative partial charge at the bridging oxygen of Cu oxide active sites, which directly interact with the methane molecule and abstracts its H atom, are considered as the important factors which affect the catalytic activity of Cu oxide clusters for direct methane conversion to methanol. These findings strongly support that the number and species of Cu oxide active sites incorporated in the MFI framework can highly affect the reaction mechanism of methane to methanol.