Insight into congener substitution effects on the combustion chemistry of tetramethoxysilane and tetramethoxymethane: an experimental and kinetic modeling study

Methoxysilane precursors are widely used in the flame synthesis of nano-silica, while the unclear combustion reaction mechanism hinders the development of directional synthesis technology. To obtain an in-depth understanding of the methoxysilane combustion chemistry and to explore the differences between Si and its congener C, this work presents a comprehensive study on the congener substitution effects in the combustion kinetics of the flame synthesis precursor tetramethoxysilane (TMOS) and its oxygenated hydrocarbon counterpart tetramethoxymethane (TMOM). The rate constants of H-abstraction reactions, isomerization and β-scission reactions are calculated, and the congener substitution effects between TMOM and TMOS are theoretically revealed. Evident differences exist in both reaction rate constants and their branching ratios, particularly in the H-abstraction reaction by OH and radical decomposition reactions. Laminar burning velocities (LBVs) of TMOS and TMOM are measured at 423 K. The detailed combustion models for the TMOS and TMOM are developed and validated against the LBVs obtained from this work and flow reactor pyrolysis in the literature. Modeling analysis and modified fictitious diluent gas methods are performed to provide insight into the combustion chemistry and the congener substitution effects. The LBVs of TMOS are slightly lower than those of TMOM/air mixtures, which can be attributed to the combination of thermal and chemical effects. The pyrolysis reactivity of TMOM is extremely higher than that of TMOS due to its lower energy barrier for the methanol elimination reaction, while the differences in the C₁-C₂ pyrolysis product result from the variations in the contribution of H-abstraction and CH₃ recombination reactions.