New considerations on the first steps for the conversion of chloromethane to olefins from a DFT perspective

This work investigates the initial stages of the reaction in the conversion of chloromethane to light olefins (CMTO) on NaZSM-5 zeolite. Density functional theory (DFT) calculations indicate that the overall reaction is endothermic in bulk but exothermic on the surface. The formation of the :CH₂ species is highly endothermic and has a high activation barrier. The migration of the CH₃ group between oxygen atoms within the zeolite structure is energetically less demanding than the formation of the :CH₂ species. Finally, the formation of ZOCH2CH3 from ZOCH3 + CH3Cl occurs through a two-step mechanism that includes a migration of the CH₃ group with energy barriers of +95.2 and +148.2 kJ mol^-1. The results obtained with ONIOM2 are qualitatively comparable to those obtained with VASP in terms of energy and geometry.

DFT calculations were carried out using Gaussian 09 and VASP code. For cluster models, ONIOM2(DF:PM3) was used with the ωB97x-D, PBE, and M062X density functionals (DF) along with the relativistic Stevens effective core potential and its corresponding basis set. QST2, IRC, and frequency calculations are performed to characterize the transition states. For periodic calculations, structure optimization was carried out using the PBE functional with the D3 dispersion of Grimme, a plane-wave basis set, and the PAW representation of the atomic cores. The electronic structures of the systems were analyzed using PDOS.