^(29)SiNMR研究Stber反应的化学动力学

Liquid-state ^(29)Si NMR Study on the Chemical Kinetics of Stber Synthesis

St ber反应以低分子量的醇为溶剂 ,由正硅酸乙酯 (TEOS)在氨催化下水解缩聚形成高度分散的尺寸为 5～ 2 0 0 0nm的球形SiO2 颗粒 ,是溶胶 -凝胶过程的典型反应 .通过液体2 9SiNMR研究St ber反应水解和缩聚的动力学过程 ,确定了反应过程的中间体是Q10 ,而不是文献所说的Q61.得到了TEOS碱性条件下决速步的水解速率方程r =kh[TEOS] [NH3 ] 0 .457·[H2 O] 0 .0 51,其速率常数为kh=7.3 2× 10 -3 mol-0 .5·dm1.5·min-1(T =2 5℃ ) ,利用过渡态理论得到决速步以外的反应方程的速率常数 .同时得到了反应活化能Ea 和指前因子A的值 .结果表明 ,温度升高 ,所有反应的速率均增大 ,决速步的反应速率增大得较多 ,说明升高温度更有利于Q10 的生成 ;水量的改变对所有反应速率几乎没有影响 ;催化剂的用量对其水解和缩聚速率影响较大 .

Stber syntheses, the ammonia-catalyzed reactions of tetraethoxysilane 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 29Si NMR. We found that the intermediate species was Q1 0; the reaction equation of the rate-determining step was r=k h· 0.457 0.051; the constant value was k h=7.32×10 -3 mol -0.5·dm 1.5·min -1 (T=25 ℃). 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 H 2O/TEOS and NH 3/TEOS were adjusted to examine the effect of H 2O and NH 3 and we found that the reaction rates of hydrolysis and condensation of TEOS largely depended on the concentration of NH 3 but not that of H 2O.