Density Functional Theory (DFT) Study To Unravel the Catalytic Properties of M-Exchanged MFI, (M = Be, Co, Cu, Mg, Mn, Zn) for the Conversion of Methane and Carbon Dioxide to Acetic Acid

The conversion of greenhouse gases, such as CO₂ and CH₄, to value chemicals is a major challenge, because of the high stability of both molecules. In this study, density functional theory (DFT) calculations with long-range corrections and ONIOM were used to analyze the reaction mechanism for the conversion of CO₂ and CH₄ to acetic acid with MFI zeolite exchanged with Be, Co, Cu, Mg, Mn, and Zn cations. Our results demonstrate that (a) the highest reaction barrier on the reaction mechanism is CH₄ dissociation, and the transition state energy in that step is directly related to the energy of the lowest unoccupied molecular orbital and the electronegativity of the metal exchanged zeolites; (b) a charge transfer between CH₄ and the metal cation occurs simultaneously to CH₄ dissociation; (c) CO₂ insertion has a low energy barrier, and the protonation of the acetate species is spontaneous; (d) dispersion interactions are the main contributions to CH₄ adsorption energies, whereas, in the rest of the steps of the reaction mechanism, the contribution of dispersion to the energies of reaction is almost negligible; (e) desorption of acetic acid could be promoted by the coadsorption of water; and (f) CH₄ dissociation on Cu-MFI has an apparent activation energy of 11.5 kcal/mol, and a forward rate constant of 1.1 s–1 at 398 K.