Single-phase and Diphasic Aerogels and Xerogels of Mullite: Preparation and Characterization

Single-phase mullite composition gels have been synthesized using tetraethoxysilane [Si(OC₂H₅)₄] and aluminum nitrate nonahydrate as precursors. Diphasic mullite gels have been prepared using colloidal silica and boehmite as precursors. Xerogels and aerogels of both the above gels have been obtained by ordinary drying in air at 60°C and critical point drying in methanol, respectively. Single-phase xerogels show an intense exothermic peak at about 980°C while their counterparts, aerogels, do not show any detectable exotherm by differential thermal analysis (DTA). These results suggest that the structure of single phase gels changed during critical point drying and the structural changes were investigated by solid-state ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. No differences between diphasic xerogels and aerogels could be detected by DTA because there was little or no effect of drying on the discrete silica and alumina phases. The tetrahedral coordination of Al in single-phase mullite gel changed to octahedral coordination as detected by MAS NMR during critical point drying, which suggests that the alumina component segregated. Both single-phase and diphasic aerogels of mullite composition showed very high surface areas in the temperature range 1000-1400° C and these may be useful for high temperature catalytic applications.