Resolving Intermediate Solution Structures during the Formation of Mesoporous SBA-15 (original) (raw)
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Contribution to the study of the formation mechanism of mesoporous silica type SBA3
Microporous and Mesoporous Materials, 2007
Dynamic light scattering (DLS) was used to follow the evolution of the micelles of two triblock copolymer surfactants, Pluronic F127 and Pluronic P123, in diluted aqueous reaction mixtures during the formation of respectively SBA-16 and SBA-15 type mesoporous silica particles at different temperatures, acid and salt concentrations. The silica source (tetraethoxysilane TEOS) is adsorbed by the micelles and undergoes hydrolysis. The resulting siliceous species coat the hydrophilic corona of the micelles, transforming those into composite colloids. The shape of the colloids formed from Pluronic F127 remains spherical but their size increases, whereas the spherical shape of the colloids with Pluronic P123 evolutes into an elongated shape. The decrease of the repulsion energy of these composite colloids allows their aggregation into ''liquid particles'' (spheres or polyhedra), as observed by optical microscopy. The ''liquid particles'', which can be re-dissolved in the case of the SBA-16 precursor by decreasing the temperature, transform into the final solid mesoporous silica particles by siloxane bonds formation within the coalesced silica walls.
Microporous and Mesoporous Materials, 2008
Dynamic light scattering (DLS) was used to follow the evolution of the micelles of two triblock copolymer surfactants, Pluronic F127 and Pluronic P123, in diluted aqueous reaction mixtures during the formation of respectively SBA-16 and SBA-15 type mesoporous silica particles at different temperatures, acid and salt concentrations. The silica source (tetraethoxysilane TEOS) is adsorbed by the micelles and undergoes hydrolysis. The resulting siliceous species coat the hydrophilic corona of the micelles, transforming those into composite colloids. The shape of the colloids formed from Pluronic F127 remains spherical but their size increases, whereas the spherical shape of the colloids with Pluronic P123 evolutes into an elongated shape. The decrease of the repulsion energy of these composite colloids allows their aggregation into ''liquid particles'' (spheres or polyhedra), as observed by optical microscopy. The ''liquid particles'', which can be re-dissolved in the case of the SBA-16 precursor by decreasing the temperature, transform into the final solid mesoporous silica particles by siloxane bonds formation within the coalesced silica walls.
Study of the Formation of the Mesoporous Material SBA-15 by EPR Spectroscopy
The Journal of Physical Chemistry B, 2003
SBA-15 is an hexagonal mesoporous material which is synthesized with nonionic poly(ethylene oxide)-poly-(propylene oxide)-poly(ethylene oxide) block copolymers (Pluronics, EO y PO x EO y ), templates. Pore diameters in the range of 2-30 nm can be obtained with a relatively thick silica wall (up to 6 nm). This material possesses both large, uniform, and ordered channels, along with a complementary net of micropores which provides connectivity between the ordered channels through the silica. This study focuses on the investigation of the formation mechanism of SBA-15 with emphasis on the PEO interactions with the silica and the initiation of the micropores. This was achieved using in situ X-band EPR spectroscopy in combination with electron spin-echo envelope modulation (ESEEM) experiments. The paramagnetic centers were introduced as spinlabeled Pluronic L62 (EO 6 PO 30 EO 6 ) where nitroxides replace the OH groups at the end of the polypropylene oxide (PEO) blocks (L62-NO). Initially, the acidic reaction conditions were adjusted to prevent the decomposition of the nitroxide radical, while still producing highly ordered SBA-15. Then, the locations of the nitroxides of L62-NO within the micelles of Pluronic P123 (y ) 20, x ) 70) and L64 (y ) 13, x ) 30) were determined through three-pulse ESEEM experiments on solutions prepared in D 2 O. In these experiments, the 2 H modulation induced by D 2 O was compared with that of a series of small spin-probes with known hydrophilic and hydrophobic characters that were introduced into the micelles. The NO group of L62-NO was found to be close to the core-corona interface in both types of Pluronics. The temporal evolution of the EPR spectrum during the reaction showed that for SBA-15 made with P123 the most significant changes in the L62-NO spectrum occur within the first 100 min. Furthermore, X-ray diffraction measurements of dried materials showed that the hexagonal structure of SBA-15 is also created within the first 2 h. A partitioning of the L62-NO between the precursors of the mesopores and micropores of the SBA-15 structure takes place at the very early stages of the reaction, and a continuous depletion of water within the corona-core interface was observed. In the final product obtained without a thermal stage, the majority of the PEO chains are located in the micropores. The extent of the PEO chains located within the silica micropores depends on the thermal stage temperature and on the Si/P123 molar ratio. In the L64 synthesis, practically all of the NO groups of L62-NO are located within the silica network and experience a single environment.
1998
onstrated their exciting properties in acidic and oxida-The mechanism of formation of micelle-templated silica (MTS) tive (10, 11) catalysis of large organic molecules, as hosts was investigated by means of computer-aided analysis of the elecfor polymeric structures (12, 13), in sorption studies (14, tron paramagnetic resonance (EPR) spectra of three different 15), and as minireactors for electron transfer reactions (16). probes inserted in the micelle. Information were obtained on (a) The discovery of these new materials led to the extension the kinetics of formation of MTS; (b) the interactions between the of the concept of ''templating'' from microporous to mesomicelle surface and the solid silica; and (c) the modifications of the porous molecular sieves. The micropore structure of zeolites micelle structure during the MTS formation. As a reference, the is templated by hydrated cations or individual organic moleformation of the micelles in the absence of silica, in neutral and alkaline media, and the interactions of the probes with already cules. The mesopore structure of M41s is templated by selfformed hexagonal mesoporous silica (MCM-41) in the absence of assembled micelle aggregates of surfactants. Many efforts surfactants were investigated. The MTS synthesis at 323 K was were devoted to determining the mechanism of mesophase shown by EPR analysis and by X-ray diffraction measurements to formation (2, 5, 17-20). Considering these results and their involve two steps, starting from a ''zero-time'' for addition of the likely interpretation, the most plausible mechanistic pathway silica solution to the micelle solution: (a) in the first few minutes, for mesophase formation implies a cooperative organization silicate species coated the micelles and ''froze'' them (decrease in of organic species and dissolved inorganic species. In this the rotational mobility evaluated from EPR analysis), forming a mechanism, silicate anions in solution neutralize the charge disordered silica-micelle aggregate. An order parameter was needed of the cationic surfactant and play an intimate role in directing to simulate spectra of probes in the CTAB micelles interacting with formation of the supramolecular micelle arrays, precursors the hydrophobic region of mesophases. In the second step (b), an increasing fraction of cationic head groups of the surfactant strongly of the mesostructures (5). Structures of the mesophases are interacted with the solid walls of the structured silica. The destabilisimilar to well-known lyotropic phases . Few studies zation of the first silicate precipitate and the formation of the more have examined the kinetics of formation of these mesophases. stable hexagonal mesophase corresponded to an Ostwald ripening Nevertheless, great variations of the rate of formation as a mechanism, which was monitored by the change of the order paramfunction of synthesis conditions clearly appear from the literaeter in the EPR spectra. The lengthening of micelles created an ture. The source of silica is of paramount importance, the anisotropic cylindrical structure evidenced by a tilting of the surfacduration of the synthesis being measured on a scale of minutes tant chains (tilt of the rotational axis with respect to the axis of the when tetraethoxysilane (TEOS) is used (4, 21-23) and on elongated micelle in the EPR spectra). ᭧ 1998 Academic Press a scale of several hours when soluble inorganic silicates are
Effect of Poly(vinyl alcohol) Concentration on the Micro/Mesopore Structure of SBA15 Silica
Materials
In this work, a series of micro/mesoporous SBA15 silica materials were synthesized using P123 and poly(vinyl alcohol) (PVA) as co-templates. The pore structure of the prepared SBA15 was observed to be a function of the PVA concentration. When the amount of PVA was relatively small, the specific surface area, micropore volume, and pore wall thickness of the synthesized SBA15 were considerably large. By contrast, when a large amount of PVA was added, the pore wall thickness was greatly reduced, but the mesopore volume and size increased. This is because the added PVA interacted with the polyethylene oxide (PEO) in the shells of the P123 micelles. Furthermore, when the amount of PVA was increased, the core polypropylene oxide (PPO) block also increased, owing to the enhanced aggregation of the P123/PVA mixed micelles. This research contributes to a basic comprehension of the cooperative interactions and formation process underlying porous silica materials, assisting in the rational des...
Science of Sintering, 2018
Mesoporous silica SBA-15 materials of high specific surface area (~700 m 2 g −1) were synthesized by using block copolymer Pluronic P123 as template and tetraethoxysilane as silica source. The obtained materials were characterized by XRD, nitrogen adsorptiondesorption measurements, SEM, EDS and AFM analysis. It was found that small modifications of synthesis conditions influenced the morphological features of the synthesized SBA-15 samples. The SEM analysis had shown that the SBA-15 synthesized at lower temperature and longer time of reaction (80 °C, 48 h) provided elongated rod-shaped grains about 1μm long. The other sample synthesized at higher temperature and shorter time of reaction (100 °C, 24 h) had rounded grains and grains of regular spherical shape with diameters ranging from 0.5 to 2 μm. The EDS analysis confirmed that the particles of both synthesized samples were of the SiO 2 content. In addition, the AFM analysis had shown different surface morphologies of the materials synthesized under various conditions.
Langmuir, 2005
Hydrothermal templating of mesoporous molecular sieves by nonionic triblock copolymers [poly-(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) (PEO-PPO-PEO)] at specific block lengths of EO20-PO70EO20 and selected 2 M HCl dosage (pH-0.3) caused the formation of micrometer-sized SBA-15 hexagons with well-ordered hexagonal pore channels (pore size and wall thickness of ∼6 nm and pore-to-pore distance of ∼12 nm) after template removal. For a beneficial lower surface energy, these {10} laterally coalesced hexagons tend to stack imperfectly over the base into towerlike entities, leaving dislocations and faults within the single domain thus formed. Evidence for the mechanism of Brownian motion/coalescence of the hexagonal-mesophase particulates, previously suggested for MCM-41 accretion in the presence of cationic surfactant, is more clearly identifiable in the present low-pH case of amphiphilic block copolymer templates and linear silica oligomers.
Tailoring properties of SBA-15 materials by controlling conditions of hydrothermal synthesis
Journal of Materials Chemistry, 2005
The influence of synthesis time and temperature on the properties of SBA-15 is investigated for the samples prepared by using two different silica sources, tetraethylorthosilicate (TEOS) and sodium metasilicate. The samples studied were obtained via two-step synthesis, which involved an initial self-assembly of polymer and silica species at 40 uC for 2 hours followed by longer hydrothermal treatment at higher temperatures. The SBA-15 samples obtained from both silica sources were highly ordered as evidenced by SAXS spectra showing up to seven reflection peaks characteristic of the P6mm symmetry group. While for the samples studied the specific surface area decreased with increasing time and temperature of the hydrothermal treatment, their mesopore size distributions became narrower and their mesopore widths and mesopore volumes showed a tendency to increase. In contrast, a slight decrease in the mesopore wall thickness and micropore volume was observed with increasing time of this treatment. Also, the samples obtained from sodium metasilicate exhibited thicker mesopore walls than those prepared from TEOS. It was shown that the adsorption properties of SBA-15 can be tailored by adjusting the time and temperature of the hydrothermal treatment and simultaneously reducing the time of the initial self-assembly process from 24 h to 2 h. { Electronic supplementary information (ESI) available: tabulated SAXS and N 2 adsorption data for the samples obtained by using different hydrothermal treatment times, temperatures and silica sources and those synthesized according to the original recipe. See