Resolving Intermediate Solution Structures during the Formation of Mesoporous SBA-15

Abstract

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 degrees 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 HCl and TMOS at 35 degrees 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.

Hierarchical fabrication of silica cocoon with hexagonally ordered channel constructed wall via an emulsion-assisted process

“…More importantly, decane enables the formation of O/W emulsion onto which the P123 micelles aggregate to generate the hierarchical structure. Being similar to the polymerization procedure described by Goldfarb et al [28] and Shen et al [29], the partially P123 micelles covered emulsion droplets assemble to produce parallel hexagonally ordered channels. After the removal of the template and the swelling agents, a hierarchically constructed material with hexagonally ordered channels arranges in a superstructure consisting of alternating protuberant and hollow walls of mesoporous silica.…”

Effect of Different Pluronic P123 Triblock Copolymer Surfactant Concentrations on SBA-15 Pore Formation

“…The mixed solution was then refluxed at 50-60C for 2 hours. Thirdly, in terms of the triblock copolymer surfactant concentration, the addition was made in accordance with the work of others, except for the concentrations of 86.2 mol (Ruthstein et al, 2006) and 689 mol , which were replaced by different weights of pluronic ® P123 triblock copolymer surfactant (BASF Corp., USA) to form different concentrations, namely 0.35, 2.50, 2.70, 3.00, and 3.30 millimoles. The surfactant was added to a mixture of 25 ml of ethanol and 10 ml of HCl.…”

“…One of the templates used for this purpose is pluronic P123, a triblock copolymer with a chemical formula of EO20PO70EO20, EO = ethylene oxide and PO = propylene oxide (Ruthstein et al, 2003, Ruthstein et al, 2006. This surfactant has been used for templating purposes due to its having similar properties to those of hydrocarbon surfactants.…”

Mesoporous Building Blocks: Synthesis and Characterization of Mesoporous Silica Particles and Films

“…SAXS, SANS, NMR, and cryo-TEM [30,102,103,[121][122][123][124][125][126][127][128], or by theoretical modelling [129]. The many different techniques used give results that can be difficult to interpret, and sometimes they even contradict each other.…”