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 + + CH 4 → OM + (CH 4 ) → [TS1] → HO−M + −CH 3 → [TS2] → M + (CH 3 OH) → M + + CH 3 OH, 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 + (CH 4 ) to HO−Mn + −CH 3 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 + −CH 3 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 + (CH 4 ) to HO−M + −CH 3 via TS1. We discuss in terms of qualitative orbital interactions why MnO + (d 4 oxo complex) is most effective for methane C−H bond activation. The activation energy from HO−M + −CH 3 to M + (CH 3 OH) 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.