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.

The reaction pathways and the energetics for the direct methane−methanol and benzene−phenol conversions that occur on the surface of Fe−ZSM-5 zeolite are analyzed from B3LYP DFT computations. We propose a reasonable model for “α-oxygen”, a surface oxygen species responsible for the catalytic reactivities of Fe−ZSM-5 zeolite. Our model involves an iron−oxo species on the AlO4 surface site of the zeolite as a catalytic active center and as a source of oxygen. The essential features of the reaction pathways for the methane−methanol and benzene−phenol conversions are identical, especially in bonding characters. In the initial stages of each reaction, methane or benzene comes into contact with the active iron site of the “α-oxygen” model, leading to the reactant (methane or benzene) complex. After the initial complex is formed, each reaction takes place in a two-step concerted manner, via neither radical species nor ionic intermediates. The concerted reaction pathway for the methane (benzene) hydroxylation involves an H atom abstraction and a methyl (phenyl) migration at the iron active center. From computed energetics for the reaction pathways, we predict that the benzene hydroxylation should be energetically more favorable than the methane hydroxylation.

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Two kinds of H-atom abstractions from methane by iron−oxo complexes with different charges are discussed from density functional theory calculations. A concerted H-atom abstraction via a four-centered transition state is shown to be energetically more favorable than a direct H-atom abstraction via a transition state with a linear C−H−O array. Iron(IV)−oxo complexes appear to be the most effective for the cleavage of the C−H bond of alkanes in the concerted mechanism, which is rationalized from qualitative orbital interaction analyses. The results of this paper support the establishment of the two-step concerted mechanism that we have proposed for alkane hydroxylations by iron−oxo species. The proposed reaction mechanism may have relevance to our understanding of some catalytic and enzymatic processes concerning alkane hydroxylations if coordinatively unsaturated transition-metal oxides are responsible for such important chemical reactions.

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