Thermal stability of Al-modified silica aerogels through epoxide-assisted sol-gel route followed by ambient pressure drying

A novel method for preparing Al-modified silica aerogels with pretreated kaolin as the raw material is developed. Sols comprising silica and Al are prepared by an in situ reaction method from pretreated kaolin. The pretreatment step simplifies the process, saves preparation time, and reduces costs. A method for adjusting the Al content in the aerogel by changing the washing time of the sol during the ion impurity removal process is also presented. The volume stabilities and thermal properties of the Al-modified silica aerogels after high-temperature treatment are investigated. The results indicate that a small amount of modified Al improves the volumetric stability of an aerogel owing to viscous sintering of the Si-O-Al-IV-(O-Al-VI)(y) units, which increases the disorder of the SiO2 structure, building a barrier to viscous flow and hindering mass transfer. [GRAPHICS] A novel method for preparing Al-modified silica aerogel through epoxide-assisted sol-gel route followed by ambient pressure drying with pretreated kaolin. The Al content in the aerogel can be controlled by the washing times. Besides, most of Al-VI and a small amount of Al-IV in the aerogel has been clarified by Al-27 NMR in the modified aerogel. The Al-modified aerogel improves the thermal stability of the aerogel especially in the pore structure.