Theoretical study on the reaction mechanism of CH4 with CaO

The reaction pathways and energetics for the reaction of methane with CaO are discussed on the singlet spin state potential energy surface at the B3LYP/6-311+G(2df,2p) and QCISD/6-311++G(3df,3pd)//B3LYP/6-311+G(2df,2p) levels of theory. The reaction of methane with CaO is proposed to proceed in the following reaction pathways: CaO + CH₄ → CaOCH₄ → TS] → CaOH + CH₃, CaO + CH₄ → OCaCH₄ → TS] → HOCaCH₃ → CaOH + CH₃ or TS] → CaCH₃OH → Ca + CH₃OH, and OCaCH₄ → TS] → HCaOCH₃ → CaOCH₃ + H or TS] → CaCH₃OH → Ca + CH₃OH. The gas-phase methane–methanol conversion by CaO is suggested to proceed via two kinds of important reaction intermediates, HOCaCH₃ and HCaOCH₃, and the reaction pathway via the hydroxy intermediate (HOCaCH₃) is energetically more favorable than the other one via the methoxy intermediate (HCaOCH₃). The hydroxy intermediate HOCaCH₃ is predicted to be the energetically most preferred configuration in the reaction of CaO + CH₄. Meanwhile, these three product channels (CaOH + CH₃, CaOCH₃ + H and Ca + CH₃OH) are expected to compete with each other, and the formation of methyl radical is the most preferable pathway energetically. On the other hand, the intermediates HCaOCH₃ and HOCaCH₃ are predicted to be the energetically preferred configuration in the reaction of Ca + CH₃OH, which is precisely the reverse reaction of methane hydroxylation.