Proceedings of the national academy of sciences of the united states of america vol:106 issue:45 pages:18908-18913
Driven by the depletion of crude oil, the direct oxidation of methane to methanol has been of considerable interest. Promising low-temperature activity of an oxygen-activated zeolite, Cu-ZSM-5, has recently been reported in this selective oxidation and the active site in this reaction correlates with an absorption feature at 22,700 cm(-1). In the present study, this absorption band is used to selectively resonance enhance Raman vibrations of this active site. O-18(2) labeling experiments allow definitive assignment of the observed vibrations and exclude all previously characterized copper-oxygen species for the active site. In combination with DFT and normal coordinate analysis calculations, the oxygen activated Cu core is uniquely defined as a bent mono-(mu-oxo)dicupric site. Spectroscopically validated electronic structure calculations show polarization of the low-lying singly-occupied molecular orbital of the [Cu2O](2+) core, which is directed into the zeolite channel, upon approach of CH4. This induces significant oxyl character into the bridging O atom leading to a low transition state energy consistent with experiment and explains why the bent mono-(mu-oxo) dicupric core is highly activated for H atom abstraction from CH4. The oxygen intermediate of Cu-ZSM-5 is now the most well defined species active in the methane monooxygenase reaction.