he solution being such as to comply with the following values of the following molar ratios: ______________________________________SiO2 /Al2 O3 from 30:1 to 500:1;TAA--OH/SiO2 from 0.05:1 to 0.2:1;H2 O/SiO2 from 5:1 to 40:1;______________________________________ (b) heating the so-obtained solution in order to cause the gelification to occur; (c) drying the obtained gel;