High-surface-area alumina-silica nanocatalysts prepared by a hybrid sol-gel route using a boehmite precursor

Abstract

High-surface-area alumina-silica mixed oxide (Al(2)O(3):SiO(2)) nanocatalysts have been prepared by a hybrid sol-gel method using boehmite (synthesized from aluminum nitrate) as the source of alumina and tetraethyl orthosilicate as the source of silica. The gels, after calcination at 400 degrees C, result in mixed oxides with specific surface areas of 287 m2/g (Al(2)O(3):SiO(2)=3:1) and 262 m2/g (Al(2)O(3):SiO(2)=3:4). Further heating to 600 degrees C produces materials with specific surface areas of 237 and 205 m2/g, respectively. The larger specific surface areas characteristic of the 3Al(2)O(3):SiO(2) samples are attributed, via transmission electron micrograph investigations, to the presence of similar to 10 nm size, needle-like particles having an aspect ratio of 1:50. Further addition of silica leads to the formation of larger needles of 20-75 nm size. Calcination at 600 degrees C induced an approximately 5% decrease in the total pore volume for the 3Al(2)O(3):SiO(2) sample. In contrast, the material with Al(2)O(3):SiO(2)=3:4 showed an approximately 12% increase in pore volume when heated at 600 degrees C. The pore-size distribution was in the range 1-3.5 nm with r(max) at similar to 2 and similar to 2.5 nm at 600 degrees and 800 degrees C, respectively. Adsorption isotherms and pore-size distribution analyses are discussed in some detail for the aluminosilicates at different calcination temperatures. This discussion is supported by structural information determined from FTIR and 27Al MAS NMR studies. Relatively high acidity values (0.234 mmol/g for Al(2)O(3): SiO(2)=3:4) are observed for silica-rich compositions consistent with their application as efficient acid catalysts.