SINGLE-PULSE SHOCK-TUBE STUDY OF THE DECOMPOSITION OF TETRAETHOXYSILANE AND RELATED-COMPOUNDS

Tetraethoxysilane (TEOS) has been decomposed in single-pulse shock tub e experiments over the temperature range 1160-1285 K and pressures of about 150 kPa (1.5 bar). The main observed products are ethylene and e thanol. The yields of these products as a percentage of decomposed TEO S increase with temperature. Studies have also been carried out with t etra-n-propoxysilane (TPOS), dimethyldiethoxysilane (DMDEOS), and trim ethylethoxysilane (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 resul ts and the thermochemistry, the following rate expressions for the ele mentary processes are recommended: k[TEOS --> C2H4 + HOSi(OC2H5)(3)] = 1.04 x 10(10)T(1.1) exp(-30 950 K/T) s(-1); k[TEOS --> CH3 + CH2OSi(O C2H5)(3)] = 4 x 10(17) exp(-43 300 K/T) s(-1). The observed ethanol pr oduct is postulated to arise from decomposition of the silanol in a ga s phase reaction. A kinetic model which quantitatively accounts for th e observed products in the decomposition of TEOS, DMDEOS, and TMEOS ha s been developed. The model includes radical reactions as well as mole cular reactions of the silanol and subsequently formed products, inclu ding silicates and silyl acids. The model requires an activation energ y of less than or equal to 200 kJ mol(-1) for the reaction which forms ethanol from the silanol. Such a low barrier is apparently at odds wi th recently calculated values for the thermochemistry of some silicon compounds.