Liquid-state 29Si NMR study on the chemical kinetics of Stober synthesis

Stober syntheses, the ammonia-catalyzed reactions of tetraetboxysilane with water in low molecular weight (MW) alcohols, produce highly monodisperse, spherical silica nanoparticles with size ranging from 5 to 2000 nm. It is the initial hydrolysis and polycondensation process that have been investigated by liquid-state Si-29 NMR. We found that the intermediate species was Q(0)(1); the reaction equation of the rate-determining step was r = k(h) [TEOS] . [NH3](0.457)[H2O](0.051); the constant value was k(h) = 7.32 x 10(-3) mol(-0.5) . dm(1.5) . min(-1) (T=25 degreesC). In the meantime, the reaction rate constant values of those non-rate-determining steps by using the transition state theory were calculated, and the values of the activation energy and the Arrhenius constant were also derived. The results suggested that the reaction rates of the hydrolysis and condensation become faster with increasing the temperature, and the rate constant value of the rate-determining step increases larger than that of non-rate-determining step, indicating that high temperature is more favorable for the hydrolysis of TEOS. The molar ratios of H2O/TEOS and NH3/TEOS were adjusted to examine the effect of H2O and NH3 and we found that the reaction rates of hydrolysis and condensation of TEOS largely depended on the concentration of NH3 but not that of H2O.