Study on the formation of mesoporous molecular sieves in the presence of various anions

This work describes the mesophases obtained when mesoporous silicates are synthesized in the presence of hydrofluoric, hydrochloric, hydroiodic, acetic or nitric acids. The results show that when the silicate precursor is varied different phases or mixture of phases can be obtained. Thus pure-silica MCM-41 is obtained from sodium silicate or [(TMA)SiO2.5](8) and cetyltrimethylammonium bromide in the presence of acetic, hydrofluoric and hydrochloric acids. Hydroiodic acid affords a mixture of phases from both sources of silica after either mild thermal (347-349 K) or hydrothermal (423 K) treatments. Upon removal of the organic counterpart, MCM-41 is formed from tetramethylammonium silicate and magadiite from sodium silicate. The presence of nitric acid yields MCM-41, for sodium silicate and when HNO3 is used, for tetramethylammonium silicate, after mild thermal treatment. Upon hydrothermal treatment, MCM-41 converts to magadiife. The presence of aluminosilicate anions causes the appearance of phases mixture for nitric and hydrofluoric acids in high surfactant:silicon molar ratios. These results show that the mesophase formation might be the product of competition reactions involving the surfactant, silicate anions and the acid anions present in the reaction mixture. There seems to be a compromise between silicate and aluminosilicate charge density and basicity on the process of assembling and polymerizing (alumino)silicates species on CTA(+) arrangements. Charge density is likely to direct the choice of species that are drawn to the organo-inorganic interface: the highly charged silicate species or the aluminosilicate ones are the first assembled. The polymerization process affects the basicity of species, turning them more or less capable of competing with acid anions for the CTA(+) molecules. When acid anions bind more effectively with CTA(+), they displace silicate or aluminosilicate species and might remain in the solid as contaminating phases. (C) 1999 Elsevier Science B.V. All rights reserved.