Methane-methanol conversion by MnO+, FeO+, and CoO+: A theoretical study of catalytic selectivity

The entire reaction pathway for the gas-phase methane-methanol conversion by late transition-metal-oxide ions, MnO⁺ FeO⁺, and CoO⁺, is studied using an ab initio hybrid (Hartree-Fock/density-functional) method. For these oxo complexes, the methane-methanol conversion is proposed to proceed via two transition states (TSs) in such a way MO⁺ + CH₄ → OM⁺(CH₄) → [TS1] → HO-M⁺-CH₃ → [TS2] → M⁺(CH₃OH) → M⁺ + CH₃OH, where M is Mn, Fe and Co. A crossing between high-spin and low-spin potential energy surfaces occurs both at the entrance channel and at the exit channel for FeO⁺ and CoO⁺, but it occurs only once near TS2 for MnO⁺. The activation energy from OMn⁺(CH₄) to HO-Mn⁺-CH₃ via TS1 is calculated to be 9.4 kcal/mol, being much smaller than 22.1 and 30.9 kcal/mol for FeO⁺ and CoO⁺, respectively. This agrees with the experimentally reported efficiencies for the reactions. The excellent agreement between theory and experiment indicates-that HO-M⁺-CH₃ plays a central role as an intermediate in the reaction between MO⁺ and methane and that the reaction efficiency is most likely to be determined by the activation energy from OM⁺(CH₄) to HO-M⁺-CH₃ via TS1. We discuss in terms of qualitative orbital interactions why MnO⁺ (d⁴ oxo complex) is most effective for methane C-H bond activation. The activation energy from HO-M⁺-CH₃ to M⁺(CH₃OH) via TS2 is computed to be 24.6, 28.6, and 35.9 kcal/mol for CoO⁺, FeO⁺, and MnO⁺ respectively. This result explains an experimental result that the methanol-branching ratio in the reaction between MO⁺ and methane is 100% in CoO⁺, 41% in FeO⁺, and < 1% in MnO⁺. We demonstrate that both the barrier heights of TS1 and TS2 would determine general catalytic selectivity for the methane-methanol conversion by the MO⁺ complexes.