The Dynamic Association Processes Leading from a Silica Precursor to a Mesoporous SBA-15 Material

Abstract

During the last two decades, the synthesis of silica with an ordered mesoporous structure has been thoroughly explored. The basis of the synthesis is to let silica monomers polymerize in the presence of an amphiphilic template component. In the first studies, cationic surfactants were used as structure inducer. Later it was shown that pluronic copolymers also could have the role. One advantage with the pluronics copolymers is that they allow for a wider variation in the radius of pores in the resulting silica material. Another advantage lies in the higher stability resulting from the thicker walls between the pores. Mesoporous silica has a very high area to volume ratio, and the ordered structure ensures surface homogeneity. There are a number of applications of this type of material. It can be used as support for catalysts, as templates to produces other mesoporous inorganic materials, or in controlled release applications. The synthesis of mesoporous silica is, from a practical point of view, simple, but there are significant possibilities to vary synthesis conditions with a concomitant effect on the properties of the resulting material. It is clear that the structural properties on the nanometer scale are determined by the self-assembly properties of the amphiphile, and this knowledge has been used to optimize pore geometry and pore size. To have a practical functional material it is desirable to also control the structure on a micrometer scale and larger. In practice, one has largely taken an empirical approach in optimizing reaction conditions, paying less attention to underlying chemical and physicochemical mechanisms that lead from starting conditions to the final product. In this Account, we present our systematic studies of the processes involved not only in the formation of the mesoporous structure as such, but also of the formation of structures on the micrometer scale. The main point is to show how the ongoing silica polymerization triggers a sequence of structural changes through the action of colloidal interactions. Our approach is to use a multitude of experimental methods to characterize the time evolution of the same highly reproducible synthesis process. It is the silica polymerization reactions that set the time scale, and the block copolymer self-assembly responds to the progress of the polymerization through a basically hydrophobic interaction between silica and ethylene oxide units. The progression of the silica polymerization leads to an increased hydrophobicity triggering an aggregation process resulting in the formation of silica-copolymer composite particles of increasing size. The particle growth occurs in a stepwise way caused by intricate shifts between colloidal stability and instability. By tuning reaction conditions one can have an end product of hexagonal prism composite particles with single crystal 2D hexagonal order.

Anatase TiO2 nanocrystals anchored at inside of SBA-15 mesopores and their optical behavior

“…8(i)). The heat treatment performed at 550 • C promoted the complete removal of organic surfactants as well as the growth by self-assemble of several hexagonal mesoporous along the caxis [82]. This procedure resulted in the formation of SBA-15 mesoporous silica/TiO 2 nanocrystals (Fig.…”

Effect of Time, Temperature and Stirring Rate Used in the First Step of the Synthesis of SBA-15 on Its Application as Reductor of Tars in Tobacco Smoke

“…The properties of SBA-15 mesoporous materials have been studied for years by different authors, who reported the effect on the variables of synthesis on the corresponding structure and morphology developed [26][27][28][29]. For instance, Lee et al [30] studied the effect of temperature and stirring rate on the morphology of the material.…”

Formation of block-copolymer-templated mesoporous silica

“…[15][16][17] Another study suggests that the spherical micelles aggregate into flocs of spherical micelles and that these form cylindrical micelles that rearrange into the hexagonal structure. 18 However, the synthesis conditions have varied for the different studies, for example which surfactant and silica precursor that were used, reaction temperature etc. A correlation between the synthesis conditions and micellar evolution is also lacking.…”