Unraveling combustion chemistry of tetramethoxysilane in flow reactor pyrolysis and laminar flame propagation

Abstract

Tetramethoxysilane (TMOS, Si(OCH3)4) is a widely used precursor in the vapor-fed aerosol flame synthesis (VAFS) of silica nanoparticles. This work reports the first attempt to investigate the flow reactor pyrolysis and laminar flame propagation of TMOS. Pyrolysis species were detected using gas chromatography (GC) at 1.09 atm and 930–1250 K, such as methane, methanol, ethylene, ethane and carbon monoxide, and their mole fraction profiles versus the heating temperature were also measured. The laminar flame propagation of TMOS/air mixtures was investigated in a constant-volume cylindrical combustion vessel at the initial pressure (Pu) of 1 atm, initial temperature (Tu) of 373 K and equivalence ratios from 0.7 to 1.4. TMOS has generally lower laminar burning velocities (LBVs) than many hydrocarbon fuels due to the high oxygen content and consequently low adiabatic flame temperature of TMOS. A kinetic model of TMOS combustion was developed and validated against both the new data in this work and the shock tube pyrolysis data in literature. Rate of production (ROP) analysis and sensitivity analysis were performed to provide insight into the key pathways in TMOS decomposition. In the flow reactor pyrolysis, the methanol elimination reaction and the H-abstraction reactions dominate the decomposition of TMOS. Among the primary decomposition products, CH2OSi(OCH3)2 mainly isomerizes to CH3OSi(=O)OC2H5, while CH2OSi(OCH3)3 and OSi(OCH3)3 mainly decompose via β-Si-O scission reactions producing OSi(OCH3)3 and OSi(OCH3)2, respectively. The primary decomposition pathways of TMOS can all eventually lead to the formation of SiO2. In the laminar flame propagation, the modeling analyses indicate formaldehyde and methyl radical are abundantly produced, making many reactions involving the two intermediates influential on the laminar flame propagation of TMOS.