Silicates precursors

As it was mentioned, the chemistry of the silica based sol-gel process comprises several steps. First, silicate precursor (e.g., tetraethylortosilicate TEOS or tetramethylosilicate TMOS) is mixed with water and catalyst and stirred for a few hours. This process leads to hydrolysis of the Si-O-R bonds. The hydrolysis reaction can be catalyzed by acids (HC1, HF, etc.) or bases (NH4OH, NaOH, etc.). The process is schematically described by equation ... 

The polymerization reaction are affected by the properties of the starting silicate precursors pH, catalyzator, silica concentration, temperature, presence or absence of salts and additives, aging and drying conditions. Among these various factors, pH and drying conditions are two of the most... 

In a reverse microemulsion, the hydrolysis and polymerization of the silicate precursor occur in the water droplet, therefore, to dope dyes in the silica nanoparticles they must be water soluble. However, a number of organic dye molecules are hydrophobic, requiring modifications prior to doping. Several methods are available to link a hydrophobic dye molecule to a water soluble group. A simple and effective example is to link a hydrophilic dextran to the dye molecules [8]. This modification can greatly enhance the water solubility of hydrophobic dye molecules, but will increase the cost of resultant DDSNs. 

The heats of reaction/solution of some reagents as hydrolysis/dissolution takes place can cause substantial elevation in slurry/solution temperatures, particularly at a large scale where heat transfer and radiative cooUng are not nearly as efficient as it is in small laboratory vessels. Other reagents, such as certain sodium alumi-nates and particularly reagents that are not freshly prepared, may need elevated temperatures for full dissolution in water. These hot or very warm solutions can adversely affect early nucleation conditions in some zeoHte syntheses. Hot reagent solutions and mixtures are sometimes cooled prior to their addition to other reagents to better control the early reactions and speciation of aluminosilicate and silicate precursors. 

Remarkably, in 2002, Inagaki and co-workers reported that, starting from 1,2-bis (triethoxysilyl)benzene as a siliceous precursor, mesoporous benzene-silica with crystal-like pore walls (Ph-PMO) can be prepared (Fig. 2) [35]. Owing to their crystallinity, these new hybrid organic-inorganic materials were much more stable in water than the amorphous mesoporous silica-supported sulfonic sites described above [36-39]. 

To delay crystallization, an additional element can be added, e.g. yttrium or magnesium76. Thus, Laine and coworkers80 have shown that ammonolysis of a cordierite (2Mg0 2Al203 5Si02) precursor system provides access to MgSiAlON materials. The development of a cordierite precursor is described below in the sections on silicate precursors. 

Because group I and II silicates are readily available from commercial inorganic, high temperature reactions, there is no obvious need for precursors to these materials. Nonetheless, several precursors have been synthesized and their reactivity explored. A whole series of group I silicate precursors can be made via the general reaction shown in equation 54150. 

The combination of micelle-forming species used in the preparation of meso-porous materials with silicate precursors of a variety of zeolites is a promising strategy to obtain mesoporous materials with zeolite-like acidity.[69] Although some progress has been made in this field, it has yet to be proven that catalytic materials with improved performance can be obtained in this manner. Strong evidence of the presence of crystallinity in the mesopore walls, combined with an increased acidic... 

Different mechanisms for the interactions between the silicate precursors and the organic template as well as the liquid crystal templating mechanisms have been discussed (Ying, Mehnert, and Wong, 1999 Tanev and Pinnavaia,... 

Since the use of alcohol in the sol-gel process can lead to aggregation of the colloidal metal, Au/SiC>2 catalysts have been prepared without alcoholic solution, using tetramethyl orthosilicate (TMOS) as a water-soluble silicic precursor and colloidal gold by reducing aqueous HAuCLi with magnesium citrate.114 However, the size of the gold particles was not reported, but reduction by citrate ion does not usually produce small particles. 

Organometallic precursors are used in the synthesis of a variety of semiconducting materials and superconducting cuprates. Organoaluminium silicate precursors have been used for the synthesis of aluminosilicates [25], while polymeric methylsilyl-amines have been used to obtain SiC-Si3N4 fibres [26], Silicon nitride can also be made using organometallic precursors. 

The general consensus is that mesoporous silica materials synthesized using surfactant templates can form by two distinct yet related reaction pathways. The first involves the hydrolytic poly-condensation of a silicatropic mesophase while the second works through co-assembly of silicate and surfactant micellar building blocks. For ionic surfactants, cooperative-assembly with silicate precursors is the result of charge density and geometry matching and multi-dentate... 

Hay R. L. (1968) Chert and its sodium-silicate precursors in sodium-carbonate lakes in East Africa. Contrib. Mineral. Petrol. 17, 255 -274. 

The Si NMR results provide molecular level evidence for the promoter (NaH2P04) induced nucleation in a clear liquid of Silicalite-1 synthesis mixture (composition is given Table 1), taken as representative simple (TEOS + TPAOH + H2O) system [6]. This sensitivity enhanced liquid state Si NMR studies afford monitoring the soluble silicate precursors (Q -Q ) [10] as a function of crystallization time spent during hydrothermal synthesis at 358 K. The relative concentration of Q - Q" silicate species when plotted against the crystallization time of Silicalite-1 in the presence and absence of promoter, the relative concentration of among total soluble Q - Q" silicate species decreases sharply at the onset of crystallization and vanishes after the completion of the crystallization [6], The corresponding crystallinity... 

Some phase diagrams of low-polydispersity amphiphilic block copolymers, exhibit areas of coexistence over a relatively wide range of composition (see Fig. 4) [32]. This is probably due to kinetic inertia or to the fact that at the borderline between two thermodynamically stable phases the energetic differences between two structures are marginal. Swelling these coexisting phases with a siliceous precursor affords a microphase-separated siliceous phase, which has the same structure as the binary mixture consisting of water and amphiphilic... 

Table 10.2 summarizes some examples of zeolites based on their classification by chemical composition. Low-silica zeolites (Si/Al < 5) are synthesized in basic conditions (pH >13) using a silicon source, an aluminum source, and alkali hydroxides at moderate temperatures, typically less than 120°C. The identity of the alkaU species used is a determining factor in which phase is obtained from synthesis, as the relative rates of (alumino)silicate hydrolysis and condensation reactions are dependent on the identity of the alkali cation. It is also believed that hydrated alkali cations effectively direct the assembly of (alumino)silicate precursors into fuUy connected three-dimensional structures. Sodium and potassium hydroxide have been used most frequently in low-silica zeolite syntheses due to their low cost and high solubility in... 

Hydrolysis of silicate precursors, typically alkoxides, Si(OR)4, to give silicate species in solution. 

It is also possible to add partially formed zeolite nanoparticles as a form of silicate precursor. If these are sufficiently reactive to cluster around the micelles and condense to form mesoporous solids, the resulting phase, once calcined, has both mesoporosity (between the walls) and microporosity (within the walls). This is discussed further in Chapter 10. 

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