Silica species

Time-resolved in situ Small Angle Neutron Scattering (SANS) investigations have provided direct experimental evidence for the initial steps in the formation of the SBA-15 mesoporous material, prepared using the non-ionic tri-block copolymer Pluronic 123 and TEOS as silica precursor. Upon time, three steps take place during the cooperative self-assembly of the Pluronic micelles and the silica species. First, the hydrolysis of TEOS is completed, without modifications of the Pluronic spherical micelles. Then, when silica species begin to interact with the micelles, a transformation from spherical to cylindrical micelles takes place before the precipitation of the ordered SBA-15 material. Lastly, the precipitation occurs and hybrid cylindrical micelles assemble into the two-dimensional hexagonal structure of SBA-15. 

Figure 6 Comparison of the contrast with SANS and SAXS. Typical values of the SLD (cm 2) for SANS and of the electron densities (e /nm3) for SAXS inside the core, shell and solvent are given. The volume fractions inside the shell are estimated for PEO) = water) = 0.5 without silica species and PEO) = Si02) = 0.25 and water) = 0.5...

The reason why the hybrid micelles evolve from sphere to cylinder is not yet completely understood, but it results from the fact that when silica species are adsorbed onto the surface of the micelles, the average curvature of the micelles is decreasing [9], Polymerisation of silica species by condensation leads to precipitation of the ordered hexagonal mesoporous material. 

Results of 29Si NMR spectroscopy of the Ti-Beta precursor gels are shown in Figure 1. The spectra exhibit peaks that belong to four major types of silica species, Q°, Q1, Q2, and Q3. Here Q" denotes a silicon environment with n Si-O-Si bonds. By comparison of the measured spectra with the 29Si NMR spectra found in the literature [4, 5] we were able to determine that the peak with the chemical shift of -72.9 ppm corresponds to Si monomer Q°, while the peaks at -80.9 ppm and -81.4 ppm are the peaks of Q1 linear trimer and dimer, respectively. The peaks from -83 ppm to -89.8 ppm were contributed to Q2 silicon oligomers, while the peaks at the chemical shifts from -90.3 ppm to -102 ppm were denoted as the part of the Q3 silica species. 

The first phase in the process is the formation of the sol . A sol is a colloidal suspension of solid particles in a liquid. Colloids are solid particles with diameters of 1-100 nm. After a certain period, the colloidal particles and condensed silica species link to form a gel - an interconnected, rigid network with pores of submicrometer dimensions and polymeric chains whose average length is greater than one micrometer. After the sol-gel transition, the solvent phase is removed from the interconnected pore network. If removed by conventional drying such as evaporation, so-called xerogels are obtained, if removed via supercritical evacuation, the product is an aerogel . 

Preparation of X-ray amorphous ZSM-5 crystallites according to procedure BT It is important that the gel formation takes place as homogeneously as possible. Because of the particular sensitivity of various silica and alumina species to the pH (63,64), the pH range between 4.5 and 8.5 was avoided. Nucleation was performed at pH 3.5-4, where a low viscous gel containing essentially monomeric silica species is rapidly formed (65).The, pH is theii raised to about 9, in order to form tetrahedral A1(0H) entities and to favour the further A1 incorporation within the zeolitic framework. NaCl was used to increase the (super)saturation of the gel, which will flocculate into a macromolecular colloid (V) and initiate the nucleation. This procedure yields 100 % crystalline zeolite after... 

The aggregation of the silica species in solution continues upon dilution with water [3], With TPA, the main final products at room temperature are nanoslabs (8), counting 396 Si atoms and composed of twelve precursors (5) (Fig.l). With TPA, it was experienced that the timing of the water addition was not very critical. The yield of Silicalite-1 nanoslabs on silica basis is typically 80% [1]. With TBA, there is a violent hydrolysis and gel formation when the water is added from the beginning. To avoid this, it is preferred to add water after complete hydrolysis. The MEL nanoslabs with occluded TBA rapidly dimerize at room temperature to result in the formation of slabs with dimensions of 1.3 x 4.0 x 8.0 nm [3], The product yield on silica basis is similar to the TPA system. With TEA, the amount of extractable silicate was much smaller. 1R spectra of the extracts recorded after 24 h stirring of the solutions are shown in Fig.3. 

After some time the colloidal particles and condensed silica species link together to become a three-dimensional network. The physical characteristics of the gel network depend greatly on the size of particles and extent of cross-linking prior to gelation. At gelation, the viscosity increases sharply, and a solid object results in the shape of the mold. 

To each of these systems a measured amount (s 5g) of crushed (< 125p) quartz was added in a stainless-steel beaker, and the systems were heated to their boiling points at atmospheric pressure for 5 hr. Quartz was chosen for these experiments because of its tight crystalline structure and relative insolubility in relation to other silica species (2). Solubility values for this mineral would represent a minimum for a given solvent system with respect to silica. 

If the reaction takes place under basic conditions then the silica species are present as anions, that is, deprotonated silanol groups (Si-O-) in this case the surfactants have to be charged positively to ensure interactions between both components commonly cationic quaternary ammonium surfactants are used as the SDA this synthesis pathway is termed S+I (Fig. 3.6a). 

The preparation can also take place under acidic conditions below the isoelectric point of the Si-OH-bearing inorganic species (pH 2) then the silica species are positively charged, that is, protonated silanol groups (Si-OHT) in order to produce an interaction with cationic surfactants, it is necessary to add a mediator ion X- (usually a halide), which gives rise to the S+X I+ pathway (Fig. 3.6b). 

Thus, the dominating interactions in pathways (a) to (d) in Figure 3.6 ate of electrostatic nature moreover, it is possible that the attractive interactions are mediated through hydrogen bonds this is the case when nonionic surfactants are used (e.g., S° a long-chained amine N° polyethylene oxide), whereby uncharged silica species (S°I° pathway Fig. 3.6e) or ion pairs [S°(XI)° pathway Fig. 3.6f] can be present. 

It is also remarkable that the total amount of Na+is very low in samples synthesized from Si(OEt)4, irrespective of the synthesis temperature (TableVII). Presumably the Si(OH)4 species progressively released by hydrolysis have time to form the adequate TEA aluminosilicate precursors and few Si-O-Na defects are created, as indicated by 29si NMR (61). Conversely, a large amount of reactive silica species stemming from Aerosil are immediately available and are randomly neutralized either by TEA+ or Na+. A highly defected structure is therefore more easily generated. 

The 27A1 and 29Si NMR measurements (7) showed that after treatment with 0.01 molar HC1 most of the amorphous silica-containing material is removed from the parent catalyst A. This can be understood easily since the maximum solubility of silica (16) is reached at pH = 2. Although the improved performance of the treated catalyst cannot be entirely explained by the removal of less active material, i.e. the increase of the number of Lewis acid sites per mass unit, it is believed that these silica species block most of the catalytically active centers, i.e. the highly dispersed Lewis acidic alumina sites in the micro- and mesopores of the parent US-Y zeolite. 

The formation of disordered mesotunnels may be related to the change of hydrophobic/hydrophilic property of the block copolymers with temperature. When the hexagonal mesostructure is formed at low temperature, the hydrophilic PEO chains have strong interaction with the silica species and are partially occluded into silica wall [15]. The high temperature process would result in volume expansion of the block copolymer because the PEO chains become hydrophobic. The microporous void space in the wall would be attacked by the copolymer and become expanded, resulting in formation of the mesotunnels. It is also expected that by addition of TMB, the number and size of the mesotunnels increase because of larger volume expansion of the block copolymer caused from TMB. 

It has been found that organic agents present in the gel can have a profound influence on syntheses, especially for zeotypes and high-silica species. Organic cations can interact with the reaction gel in a number of ways. They can... 

The direct interaction between surfactants and inorganic precursors was later found to be not the only pathway for the formation of mesophases. A major discovery following Mobil s work is the synthesis of mesophases through the assembly of cationic inorganic species with cationic surfactants in acidic solutions. Here, the interaction between cationic silica species and cationic surfactant headgroups is suggested to be mediated by halide anions. 

Showed in Figure 1 are the NMR spectra of the surfactant-collected precursor before and after 150°C steaming. For both samples, there was approximately an equal distribution of and Q silicon environment. TTie steaming produced only a small increase of species, suggesting the hydration of surface silica species. The NMR spectra are very similar to that obtained by Kremer et al. recently. 

Figure 7.5. Species in equilibrium with amorphous silica. Diagram computed from equilibrium constants (25°C, I = 0.5). The line surrounding the shaded area gives the maximum soluble silica. The mononuclear wall represents the lower concentration limit below which multinuclear silica species are not stable. In natural waters the dissolved silica is present as monomeric silicic acid.

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