Synthesis mechanism of cationic surfactant templating mesoporous silica under an acidic synthesis process

The presence of various counteranions in the interfacial region of the silicate - surfactant mesophase introduces opportunities for manipulation of the phase structure. Well-ordered three-dimensional hexagonal P6₃/mmc, cubic Pm3n, two-dimensional hexagonal p6mm, and cubic Ia3d mesoporous materials have been synthesized with the same surfactant (cetyltriethylammonium bromide), depending on the kind of acids. The counteranions of acidic media have resulted in increasing surfactant packing parameter g in the order SO₄ ^2- < Cl^- < Br^- < NO₃ ^-, which leads to different formation routes to the mesostructures. It has been found that the mesophases are always transformed from the lower curvature one into the higher curvature one in the acidic synthesis gel. The combination of X-ray diffraction patterns, scanning electron microscope images, and high-resolution transmission electron microscope images presented visible evidence for the mesostructural constructions. In particular, the synthesis of a cubic Pm3n mesoporous molecular sieve was studied in the presence of trimethylbenzene (TMB) isomers. The rate of transformation is greatly affected by the structure of the TMB isomers and their content; 1,2,3-TMB was the most favorable to the stabilization of the p6mm hexagonal mesophase, 1,3,5-TMB was the least favorable, and 1,2,4-TMB showed intermediate behavior.