Effect of hydrolysis conditions of the silicate precursor on the synthesis of siliceous MCM-48

Abstract

Publisher Summary This chapter examines the effect of specific synthesis variables, such as concentration and mode of addition of both base and surfactant (CTAB) on its synthesis. When the silicate source (TEOS) is hydrolyzed with a dilute base solution, and the surfactant solution is added at the early stages of hydrolysis, MCM-48 was obtained at CTAB/SiO2 ratios higher than 0.65. Lower ratios led to the formation of a lamellar material. When the base concentration is high (Na20/H20 = 0.049) a poorly crystallized mixture of the cubic and lamellar materials was obtained, even at a CTAB/H20 ratio equal to 0.65. By varying the base concentration, both the degree of condensation of the silicate oligomers and the relative proportion of monomers in solution, changes. As the cubic phase is obtained with dilute base solutions, it is probably the result of the interaction of the surfactant with monomeric silicate species, while the lamellar material results from the CTAB interaction with the more condensed ones. The delay time between addition of the base and the surfactant, and the mode of addition of the base, also influence the type of inorganic material obtained. Short delay times (less than 2 min), and a gradual addition of the base favor the formation of MCM-48, otherwise, a mixture of the lamellar and cubic phases is observed. The interaction of the surfactant with diverse distributions of silicate species in solution could also explain these results.