Methane C-H bond heterolysis versus homolysis on Cu-MFI and Au-MFI

The methane activation reaction is of fundamental importance for its transformation into high-value chemicals. Despite the fact that both homolysis and heterolysis are competitive mechanisms of C-H scission, experimental and DFT studies have revealed heterolytic scission of the C-H bond over metal-exchange zeolites. To rationalize the new catalysts, work must be done on the homolytic versus heterolytic scission of the C-H bond mechanism on these catalysts. We have performed the quantum mechanical calculations for the C-H bond homolysis versus heterolysis over Au-MFI and Cu-MFI catalysts. Calculations results showed that homolysis of the C-H bond is favorable both thermodynamically as well as kinetically over Au-MFI catalysts. However, over Cu-MFI, heterolytic scission is favorable. Both Cu(I) and Au(I) activate the CH₄ via electronic density back donation from filled nd¹⁰ orbitals, according to NBO calculations. Cu(I) cation has a higher electronic density back donation than Au(I) cation. This is also supported by the charge on the C-atom of Methane. Additionally, a greater negative charge on the O-atom in the active site in case of Cu(I), where proton transfer occurs, promotes heterolytic scission. Because of the larger size of the Au-atom and the smaller negative charge of the O-atom in the active site, where proton transfer occurs, C-H bond homolytic fission is preferable over Au-MFI.