Single-Pulse Shock Tube Study of the Decomposition of Tetraethoxysilane and Related Compounds

Abstract

Tetraethoxysilane (TEOS) has been decomposed in single-pulse shock tube experiments over the temperature range 1160−1285 K and pressures of about 150 kPa (1.5 bar). The main observed products are ethylene and ethanol. The yields of these products as a percentage of decomposed TEOS increase with temperature. Studies have also been carried out with tetra-n-propoxysilane (TPOS), dimethyldiethoxysilane (DMDEOS), and trimethylethoxysilane (TMEOS). Evidence is presented that in all cases the main initial reaction is a 1,2-elimination to form the olefin and the corresponding silanol. A smaller contribution from C−C bond-breaking channels is also observed. In combination with lower temperature results and the thermochemistry, the following rate expressions for the elementary processes are recommended: k[TEOS → C2H4 + HOSi(OC2H5)3] = 1.04 × 1010T1.1 exp(−30 950 K/T) s-1; k[TEOS → CH3 + CH2OSi(OC2H5)3] = 4 × 1017 exp(−43 300 K/T) s-1. The observed ethanol product is postulated to arise from decomposition of the silanol in a gas phase reaction. A kinetic model which quantitatively accounts for the observed products in the decomposition of TEOS, DMDEOS, and TMEOS has been developed. The model includes radical reactions as well as molecular reactions of the silanol and subsequently formed products, including silicates and silyl acids. The model requires an activation energy of ≤200 kJ mol-1 for the reaction which forms ethanol from the silanol. Such a low barrier is apparently at odds with recently calculated values for the thermochemistry of some silicon compounds.