Ammonia-catalyzed hydrolysis kinetics of mixture of tetraethoxysilane with methyltriethoxysilane by 29Si NMR

In situ liquid Si-29 NMR was used to investigate the ammonia-catalyzed hydrolysis and condensation of tetraethoxysilane (TEOS) and methyltriethoxysitane (MTES) mixed systems with methanol as solvent. Varying the molar ratio of TEOS, MTES, water and ammonia in original solution, the initial hydrolysis rate constants of TEOS and MTES were calculated as well as the corresponding reactive orders. Under ammonia catalysis, the hydrolysis of TEOS and MTES in mixed system all retained the first-order reaction, similar to the cases of their individual systems. The hydrolysis rate constant of TEOS in mixed system is larger than that in its single system. Meanwhile, the hydrolysis rate constant of MTES in mixed system was smaller than that in its single system. Furthermore the condensation rate constants of TEOS and MTES in mixed system all decreased when compared to the individual condensation rate constants of MTES and TEOS. Another important result was: the reactive orders of both ammonia and water increased to different extent for TEOS and MTES in mixed system. Hydrolysis kinetics shows more compatible hydrolysis-condensation relative rates between TEOS and MTES. Assisted by the solid Si-29 MAS NMR and XPS, the obtained hybrid gel showed weak cross-condensation between TEOS and MTES. With the small angle X-ray scattering (SAXS), the double fractal structure in particulate networks was disclosed to be resulted from the more compatible hydrolysis-condensation relative rates between TEOS and MTES. (c) 2005 Elsevier B.V. All rights reserved.