Mechanisms of Heterogeneous Processes in the System SiO_2 + CH_4: II. Methylation of >Si=O Groups

摘要

The methods of optical and IR spectroscopy and quantum chemistry were used to obtain data on the direction and kinetics of the reaction of a silanone (≡SiO)_2Si=O with a CH_4 molecule and a methyl radical. Two mechanisms of methylation of silanone groups, molecular and free-radical, are studied. Both processes are accompanied by the formation of (≡SiO)_2Si(OH)(CH_3) groups. The rate constant of the molecular process is determined and its activation energy is estimated (17 kcal/mol). A methyl radical adds to the silicon atom in a silanone group to form the oxy radical (≡SiO)_2Si()(CH_3). This radical carries a free-radical process of silanone group methylation. The main channel for the pyrolysis of (≡SiO)_2Si(OH)(CH_3) groups is their decomposition with the abstraction of a methane molecule. The activation energy of this process is ～70 kcal/mol. Quantum chemical methods were employed to obtain data on possible intermediates in the processes studied and these results are used to interpret spectral and kinetic data.