A theoretical study of the oxidation of benzene by manganese oxide clusters: formation of quinone intermediates

Abstract

Manganese oxides (Mn_xO_y) have been widely applied in various chemical industrial processes owing to their long lifetime, low cost and high abundance. They have been used as co-reactants for the elimination of volatile organic compounds (VOCs); however, their oxidation mechanism is not clearly established. In this theoretical study, interaction capacities between benzene (C₆H₆) and Mn_xO_y clusters, which were modeled with MnO₂ and Mn₂O₃ molecules, were investigated by quantum chemical computations using density functional theory (DFT) with the PBE-D3 functional. The interaction capacity between C₆H₆ and Mn_xO_y was evaluated, and the probing of the initial stage of the C₆H₆ oxidation at a molecular level offers an in-depth oxidation reaction path. Interaction energies computed in several spin states, along with the analysis of the electron distribution using the quantum theory of atoms in molecules, natural bond orbital and Wiberg bond index techniques as well as local softness values and MO energies of fragments, point out that the interaction between C₆H₆ and Mn₂O₃ is stronger than that with MnO₂, amounting to −43 and −35 kcal mol⁻¹, respectively, and the metal atom is identified as the primary active site. During the oxide cluster-assisted oxidation, benzene firstly undergoes an oxidation reaction by active oxygen to generate intermediates such as hydroquinone and benzoquinone. The pathway involving p-benzoquinone as the product (noted as PR1) is the most energetically favored one through a transition structure lying at 19 kcal mol⁻¹, below the energy reference of the reactants, leading to an energy barrier significantly lower than that of 36 kcal mol⁻¹ found for the gas phase oxidation reaction with molecular oxygen without the assistance of the oxide clusters. Potential energy profiles illustrating the reaction paths and molecular mechanisms were described in detail.