Mechanisms of Heterogeneous Processes in the System SiO2+ CH4: II. Methylation of >Si=O Groups

Abstract

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 CH4 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)(CH3) 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 (≡Si–O)2Si(O⋅)(CH3). This radical carries a free-radical process of silanone group methylation. The main channel for the pyrolysis of (≡Si–O)2Si(OH)(CH3) 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.