Methane-to-Methanol Conversion by First-Row Transition-Metal Oxide Ions: ScO+, TiO+, VO+, CrO+, MnO+, FeO+, CoO+, NiO+, and CuO+

Abstract

The reaction pathway and energetics for methane-to-methanol conversion by first-row transition-metal oxide ions (MO + s) are discussed from density functional theory (DFT) B3LYP calculations, where M is Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu. The methane-to-methanol conversion by these MO + complexes is proposed to proceed in a two-step manner via two transition states: MO + + CH 4 → OM + (CH 4 ) → [TS] → OH−M + −CH 3 → [TS] → M + (CH 3 OH) → M + + CH 3 OH. Both high-spin and low-spin potential energy surfaces are characterized in detail. A crossing between the high-spin and the low-spin potential energy surfaces occurs once near the exit channel for ScO +, TiO +, VO +, CrO +, and MnO +, but it occurs twice in the entrance and exit channels for FeO +, CoO +, and NiO + . Our calculations strongly suggest that spin inversion can occur near a crossing region of potential energy surfaces and that it can play a significant role in decreasing the barrier heights of these transition states. The reaction pathway from methane to methanol is uphill in energy on the early MO + complexes (ScO +, TiO +, and VO + ); thus, these complexes are not good mediators for the formation of methanol. On the other hand, the late MO + complexes (FeO +, NiO +, and CuO + ) are expected from the general energy profiles of the reaction pathways to efficiently convert methane to methanol. Measured reaction efficiencies and methanol branching ratios for MnO +, FeO +, CoO +, and NiO + are rationalized from the energetics of the high-spin and the low-spin potential energy surfaces. The energy diagram for the methane-to-methanol conversion by CuO + is downhill toward the product direction, and thus CuO + is likely to be an excellent mediator for methane hydroxylation.