Gas-Phase Mechanism of O center dot-/Ni2+-Mediated Methane Conversion to Formaldehyde

The gas-phase reaction of NiAl2O4+ with CH4 is studied by mass spectrometry in combination with vibrational action spectroscopy and density functional theory (DFT). Two product ions, NiAl2O4H+ and NiAl2O3H2+, are identified in the mass spectra. The DFT calculations predict that the global minimum-energy isomer of NiAl2O4+ contains Ni in the +II oxidation state and features a terminal Al-O center dot- oxygen radical site. They show that methane can react along two competing pathways leading to formation of either a methyl radical (CH3 center dot) or formaldehyde (CH2O). Both reactions are initiated by hydrogen atom transfer from methane to the terminal O.- site, followed by either CH3 center dot loss or CH3 center dot migration to an O2- site next to the Ni2+ center. The CH3 center dot attaches as CH3+ to O2- and its unpaired electron is transferred to the Ni-center reducing it to Ni+. The proposed mechanism is experimentally confirmed by vibrational spectroscopy of the reactant and two different product ions.