Methane Over‐Oxidation by Extra‐Framework Copper‐Oxo Active Sites of Copper‐Exchanged Zeolites: Crucial Role of Traps for the Separated Methyl Group

Copper‐exchanged zeolites are useful for stepwise conversion of methane to methanol at moderate temperatures. This process also generates some over‐oxidation products like CO and CO 2 . However, mechanistic pathways for methane over‐oxidation by copper‐oxo active sites in these zeolites have not been previously described. Adequate understanding of methane over‐oxidation is useful for developing systems with higher methanol yields and selectivities. Here, we use density functional theory (DFT) to examine methane over‐oxidation by [Cu 3 O 3 ] 2+ active sites in zeolite mordenite MOR. The methyl group formed after activation of a methane C−H bond can be stabilized at a μ‐oxo atom of the active site. This μ‐(O−CH 3 ) intermediate can undergo sequential hydrogen atom abstractions till eventual formation of a copper‐monocarbonyl species. Adsorbed formaldehyde, water and formates are also formed during this process. The overall mechanistic path is exothermic, and all intermediate steps are facile at 200 °C. Release of CO from the copper‐monocarbonyl costs only 3.4 kcal/mol. Thus, for high methanol selectivities, the methyl group from the first hydrogen atom abstraction step must be stabilized away from copper‐oxo active sites. Indeed, it must be quickly trapped at an unreactive site (short diffusion lengths) while avoiding copper‐oxo species (large paths between active sites). This stabilization of the methyl group away from the active sites is central to the high methanol selectivities obtained with stepwise methane‐to‐methanol conversion.