F- mediated synthesis of mesoporous silica with ionic- and non-ionic surfactants. A new templating pathway

Mesoporous silica precursors were synthesized at room temperature from solutions in the presence of fluoride anions at a pH between 6 and 10 using the cetyltrimethylammonium cation as the surfactant. The solids were obtained from the hydrolysis of SiF62- or mixtures of TMOS and NH4F. When the pH decreases from 10 to 6, the Q(4)/Q(3) ratio increases. This increase is due to the polycondensation of the =SiOH groups which is catalyzed by the fluoride anions. Under these conditions, less surfactant S+ is incorporated. The decrease of the number of =SiO- groups is partially compensated by the incorporation of F- anions into the neutral framework I-0 (S+[(IF-)-F-0] mechanism). Too high an ionic strength gives rise to low-organized materials. Nevertheless, the increase of the surfactant concentration improves the organization of the materials. By using non-ionic surfactants under similar conditions, the presence of F- enables the formation of materials from reaction mixtures with low silica/surfactant ratios exhibiting characteristics close to those of the material obtained with conventional cationic surfactants.