Porous properties of polymeric Al₂O₃-SiO₂ xerogels

Polymeric Al₂O₃-SiO₂ xerogels with chemical compositions of 9.4, 25, 60 and 80 mol% Al₂O₃ were prepared by a sol-gel method and their porous properties were examined by N₂ gas adsorption. Calculated amounts of aluminium nitrate nonahydrate (ANN) and tetraethylorthosilicate (TEOS) were dissolved in ethanol at a concentration of ANN+TEOS=0.3 mol/l with varying amount of added H₂O, and hydrolyzed at 60°C for 1 month. The resulting wet gels were dried at 110°C then calcined at 300°C to remove residual nitrates and organic matter. The specific surface areas of the four xerogels changed greatly with H₂O/TEOS but the relation was different in each xerogel. The pores in all these xerogels were less than 2 nm in radius and were thus very different from colloidal xerogels, i.e. they can be tuned between 1 and 30 nm by changing the preparation conditions. The differences in pore sizes are caused by the difference in the pH of the solution and also by the sedimentation conditions which form agglomerated structures.