Room-Temperature Activation of the C-H Bond in Methane over Terminal Zn ^II -Oxyl Species in an MFI Zeolite: A Combined Spectroscopic and Computational Study of the Reactive Frontier Molecular Orbitals and Their Origins

Oxygenase reactivity toward selective partial oxidation of CH4 to CH3OH requires an atomic oxygen-radical bound to metal (M-O•: oxyl intermediate) that is capable of abstracting an H atom from the significantly strong C-H bond in CH4. Because such a reaction is frequently observed in metal-doped zeolites, it has been recognized that the zeolite provides an environment that stabilizes the M-O• intermediate. However, no experimental data of M-O• have so far been discovered in the zeolite; thus, little is known about the correlation among the state of M-O•, its reactivity for CH4, and the nature of the zeolite environment. Here, we report a combined spectroscopic and computational study of the room-temperature activation of CH4 over ZnII-O• in the MFI zeolite. One ZnII-O• species does perform H-abstraction from CH4 at room temperature. The resultant CH3• species reacts with the other ZnII-O• site to form the ZnII-OCH3 species. The H2O-assisted extraction of surface methoxide yields 29 μmol g-1 of CH3OH with a 94% selectivity. The quantum mechanics (QM)/molecular mechanics (MM) calculation determined the central step as the oxyl-mediated hydrogen atom transfer which requires an activation energy of only 10 kJ mol-1. On the basis of the findings in gas-phase experiments regarding the CH4 activation by the free [M-O•]+ species, the remarkable H-abstraction reactivity of the ZnII-O• species in zeolites was totally rationalized. Additionally, the experimentally validated QM/MM calculation revealed that the zeolite lattice has potential as the ligand to enhance the polarization of the M-O• bond and thereby enables to create effectively the highly reactive M-O• bond required for low-temperature activation of CH4. The present study proposes that tuning of the polarization effect of the anchoring site over heterogeneous catalysts is the valuable way to create the oxyl-based functionality on the heterogeneous catalyst.