Effect of thermal treatment on the nanostructure of SiO2-Al2O3 xerogels

SiO2-Al2O3 xerogels of various compositions were prepared using the sol-gel process. Aluminum nitrate was mixed with prehydrolyzed tetraethylorthosilicate (TEOS), and gelation was initiated with NH4OH, with a final gelation pH of 8.5. The effect of calcination protocol on the surface area, pore volume, pore size distribution, skeletal density, and pore and particle structures of the resulting xerogels was studied. The results show that essentially mesoporous materials are produced by this synthesis route, and that it is possible to thermally treat these mixed oxide gels to tailor the pore size distribution without severely affecting surface area or pore volume. Low Al2O3 content gels remain amorphous and undergo viscous sintering with heat treatment, whereas heat treatment of high Al2O3 content gels causes crystallization which delays sintering. Thus it is possible to add small amounts of Al2O3 to impart thermal stability in SiO2-Al2O3 xerogels. (C) 1997 Elsevier Science B.V.