Gas-Phase Mechanism of O^.- /Ni²⁺ -Mediated Methane Conversion to Formaldehyde

The gas-phase reaction of NiAl₂ O₄ ⁺ with CH₄ is studied by mass spectrometry in combination with vibrational action spectroscopy and density functional theory (DFT). Two product ions, NiAl₂ O₄ H⁺ and NiAl₂ O₃ H₂ ⁺ , are identified in the mass spectra. The DFT calculations predict that the global minimum-energy isomer of NiAl₂ O₄ ⁺ contains Ni in the +II oxidation state and features a terminal Al-O^.- oxygen radical site. They show that methane can react along two competing pathways leading to formation of either a methyl radical (CH₃ ⋅) or formaldehyde (CH₂ O). Both reactions are initiated by hydrogen atom transfer from methane to the terminal O^.- site, followed by either CH₃ ⋅ loss or CH₃ ⋅ migration to an O^2- site next to the Ni²⁺ center. The CH₃ ⋅ attaches as CH₃ ⁺ to O^2- 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.