Formaldehyde formation and its impact on the MTO initiation investigated with ab initio molecular dynamics simulations

Formaldehyde plays an important role in the methanol to olefins (MTO) process as a hydrogen-acceptor and also as a precursor to carbon monoxide. Therefore, it is discussed both as a central intermediate in the
initiation of the MTO process as well as during deactivation. Here, we compute activation barriers for hydrogen transfer reactions that eventually yield formaldehyde and CH4 from the reaction of DME or methanol and a surface methoxy species (SMS). Analogous reactions lead to the formation of CO from formaldehyde. Using the intermediate hard sphere model, we compute anharmonic adsorption free energies and then apparent free energies using molecular dynamics (MD) simulations. Additionally, we perform high level electronic structure calculations using the random phase approximation (RPA) evaluated with free energy perturbation theory (FEPT). Together with recently reported methylation barriers for CO and ketenes, this provides highly accurate barriers for the initiation of the MTO process via the CO-catalyzed mechanism. Combining the data provides a seamless Gibbs free energy profile where the kinetically relevant parts – including adsorption – are computed
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explicitly with MD simulations and corrected with RPA calculations. Formaldehyde formation occurs more readily than the second dehydrogenation step that yields CO, which is thus the rate-limiting step. This analysis is only possible with the availability of anharmonic adsorption free energies at the surface methoxy species. At 400 °C and 1 bar reference pressure adsorption is generally unfavorable in terms of Gibbs free energy for the adsorbates considered.