高钛硅比分子筛溶胶凝胶水热合成动力学模型研究

Modeling Crystallization Kinetics:Synthesis of the High Ti/Si Silicotitanium Molecular Sieve

高钛硅比分子筛具有独特的催化和离子交换性能,可用于处理处置高污染放射性废物。利用溶胶凝胶水热法研究了除铯用新型高钛硅比分子筛孔道结构化合物Na4Ti4Si3O10(NaTS)的合成工艺,并讨论了温度、时间、不同钛硅比对合成产物的影响。研究结果表明,NaTS的合成反应属吸热反应,温度升高利于钛源进入分子筛骨架。当合成温度为200℃时,产物结晶完整,表观形貌为规则的四方颗粒,晶体尺寸为20nm,钛硅比为1.33,铯交换分配系数为36500mL/g。NaTS的合成属于液相生成机理,整个过程包括硅酸盐物种的溶解和缩合反应,晶体成核,生长及凝胶溶解几个步骤。动力学模型研究表明合成过程属自发成核机理。合成诱导期长,20h左右才开始形核结晶,120h,晶体基本生长完全。利用Runge-Kutta和单纯型法计算求解,得NaTS的晶体生长速率常数仅为5.6×10-7。因此,促进产物晶体生长是提高NaTS合成速度的关键因素。

The synthetic technology of the novel high Ti/Si pore tunnel framework compound Na4Ti4Si3O10 （NaTS） for cesium removal through sol-gel hydrothermal method was studied systematically. Also, the effects of the temperature, reaction time and various Ti/Si on the properties of the product were discussed. The results show that the reaction is endothermic, which suggests that the increment of the temperature contributes more titanium in the framework of NaTS. Under the condition of 200 ℃, the product is well-crystallized regular tetragonal particle with an average grain size of about 20 nm and the ratio of Ti/Si is 1.33, the cesium adsorption can be up to 36500 mL/g. The crystallization of NaTS, which is composed of silicate dissolution and condensation reaction, nucleation, crystal growth, and gel dissolution process, belongs to the liquid phase transformation mechanism. The crystallization kinetics model for a spontaneous nucleation system of NaTS had been established, considering the spontaneous nucleation and crystal growth. By Runge-Kutta and simplex methods, the crystal growth constant of NaTS is calculated to be only 5.6× 10^-7. It can be concluded that the key increasing the synthesis of NaTS is to improve the speed of crystal growth.