Porous properties of coprecipitated Al₂O₃-SiO₂ xerogels prepared from aluminium nitrate nonahydrate and tetraethylorthosilicate

Al₂O₃-SiO₂ xerogels with various chemical compositions have been prepared by coprecipitation and their porous properties investigated by nitrogen gas adsorption, transmission electron microscopy (TEM), small angle X-ray scattering (SAXS), and ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR). The precipitates were formed by adding conc. aqueous ammonia to an ethanol solution of Al(NO₃)₃ · 9H₂O and Si(OC₂H₅)₄ (TEOS). The syntheses were performed both with and without added H₂O(H₂O/TEOS≥18/1). The xerogels were dried and calcined at 300°C for 4 h. The porous properties [specific surface area (S(A)), pore volume (V(p)) and pore size (r(p))] varied with their chemical composition, especially the SiO₂ rich xerogels. The experimental S(A) values ranged from ca. 10 to 560 m²g^-1, the V(p) values from 0.02 to 1.55 ml g^-1 and the r(p) values from 2 to 34 nm. The addition of H₂O during the preparation markedly influenced the porous properties of the xerogels with SiO₂ rich composition. The structure and microtexture of the xerogels are discussed on the basis of the NMR, SAXS and TEM data.