Development of mesoporous materials by solid-liquid reactions within gel network

研究了不同老化条件下具有共连续大孔的烷氧基湿硅胶的介孔形成行为。采用溶胶-凝胶相分离法制备起始凝胶，并在中性至弱碱性条件下通过外加成或内生成氢氧化铵进行老化。尿素存在下制备的凝胶母液经热分解后pH值升高至10。在非水热条件下，老化过程中介孔演化仅依赖于pH值和温度，外部和内部方法在干燥收缩和热演化方面没有明显差异。在含脲凝胶的附加水热老化过程中，随着老化温度的升高，介孔演化速率显著加快。200。C时，中孔最终尺寸达到50nm，体积分数明显减小。200以上。C，柱状凝胶骨架的内部部分被洗脱出来并在其外表面重新沉淀，从而导致空心柱的壁由颗粒状建筑单元组成，其聚集密度降低。然而，可能由于凝胶样品的高纯度，从未观察到凝胶样品的结晶。相反，无定形二氧化钛凝胶在水热条件下更容易结晶。

The mesopore formation behavior in alkoxy-derived wet silica gels having co-continuous macropores has been investigated under varied conditions of aging. The starting gels were prepared via sol-gel route which include phase separation, and were aged under neutral to weakly basic conditions either by external addition or internal generation of ammonium hydroxide. The pH values of mother liquor of gels prepared in the presence of urea increased up to 10 by the thermal decomposition. Under non-hydrothemal conditions, the mesopore evolution depended only on pH values and temperatures during aging, and no substantial differences were recognized in the drying shrinkage and heat-evolution between the external and internal methods. In the additional hydrothermal aging performed with urea-containing gels, a significant acceleration of mesopore evolution rate was observed with an increase of aging temperature. At 200。C, the final size of mesopores reached 50nm with a marked decrease of their volume fraction. Above 200。C, the inner part of columnar gel skeletons eluted out and reprecipiated on their outer surface, thus resulted in the hollow columns with their walls consisting of granular building units less densely aggregated. The crystallization of the gel sample, however, was never observed possibly due to high purity of the gel samples. In contrast, the amorphous titania gels crystallized more easily under hydrothermal conditions.