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We studied the structural evolution during the formation of large-pore cubic Ia3¯d silica-based mesoporous materials, synthesized with Pluroinc P123 and butanol as structure directing agents. We used cryogenic high resolution scanning electron microscopy (cryo-HRSEM) and freeze-fracture-replication (FFR) transmission electron microscopy (TEM). Typically a silica precursor is added to an acid-catalyzed solution of Pluronic P123 and butanol. The latter serves as a cosolute, which can be added either at the beginning of the reaction, or after precipitation and the formation of a hexagonal phase. In this study we focused on the structural evolution from the hexagonal phase to the final cubic phase in the two different reactions. The same structural evolution with different kinetics was detected for both reactions. Cryo-HRSEM and FFR-TEM images revealed that from the hexagonal phase a perforated layer (PL) phase is formed, which later evolves into a bicontinuous structure. The final cubic phase forms within the layers, maintaining their orientation. We suggest a formation mechanism involving cylinder merging for the hexagonal to PL transition. Upon additional polymerization of the silica, the PL phase relaxes into the stable Ia3¯d cubic phase. Another minor mechanism detected involves the direct transition between the hexagonal to the final cubic phase through cylinder branching. [ABSTRACT FROM AUTHOR]

The evolution of the solution microstructures during the formation of the hexagonal mesoporous material SBA-15 was studied by direct imaging and freeze-fracture replication cryo-TEM. A reaction mixture was sampled at different times after the addition of tetramethoxyorthosilane (TMOS) to an acidic solution of Pluronic P123 held at 50 °C. Solution microstructures were detected by direct imaging cryo-TEM in the time window of 6.5–40 min after the addition of the TMOS (t = 0). The micrographs revealed that the initial spheroidal micelles evolve into threadlike micelles, which become longer and straighter with time. Then bundles with the dimensions similar to those found in the final material appeared, although there was no sign of a hexagonal arrangement up to 40 min. Due to the appearance of a precipitate at 40 min the sample became too viscous, preventing clear observation of its content. To observe the structures present after 40 min, freeze-fracture replication was carried out as well. Such samples were collected also at 22 min and showed the presence of threadlike micelles in agreement with the direct imaging cryo-TEM micrographs. The 2 h samples showed some areas of hexagonal ordered structures, which become very clear at 2 h 50 min. The cryo-TEM measurements were carried out under the same reaction conditions used in earlier in situ EPR experiments, thus allowing us to correlate molecular level events with the microstructure shape evolutions. This showed that the elongation of the micelles is a consequence of a reduction of the polarity and the water content within the micelles due to silicate adsorption and polymerization. Similar experiments were carried out also on SBA-15 prepared with HCI and TMOS at 35 °C. The appearance of threadlike micelles, followed by clustering of the TLMs, was observed under these conditions as well, but the reaction rate was faster. This suggests that the observed mechanism for the formation of SBA-15 is general. [ABSTRACT FROM AUTHOR]