Anionic silicates

There are also some reactions of allylic silanes that proceed through anionic silicate species. These reactions usually involve activation by fluoride and result in transfer of an allylic anion. 

ESI-MS has shown to be able to determine semi-quantitatively the initial dissolution species in silicalite-1. Their identities are further substantiated by monitoring selected fragmentation patterns. Doubly charged species consisting of aggregation of identical anionic silicates were evidenced in very low concentration in agreement with the NMR. 

It is found in this study that an adjustment of pH value of solution by acid (HF or HC1) to 10.5 is very important for the effective formation of uniform mesopores. However, the acid should be added into the mixture solution after the addition of surfactant otherwise, the formation of the ordered mesoporous structure would be affected. The explanation is that when acid is added to a mixture solution without surfactant, the pH value of system will reduce and subsequently influence the interaction between cationic surfactant and anionic silicate species in the mixture, leading to the poor polymerization of inorganic silicate species. In addition, when HF is used prior to the addition of surfactant, the formation of stable NajSiFg can deactivate the polymerization of silicate species, further terminating the growth of mesoporous framework. 

TABLE 20. 29Si NMR data and ligand exchange barriers (determined by 19F NMR) for hexacoordi-nated anionic silicates... 

The main advantage of the zirconium family of columns is their stability from pH 1 to 10 and at temperatures from ambient to 200°C. Their separating character also differs from silica-based columns due to the lack of ionizable surface molecules. Silica above pH 3.0 loses a proton to form anionic silicate moieties, giving the bonded-phase silica column some anionic as well as nonpolar organic column characteristics (Fig. 6.8a). 

In naturally occurring LDHs, the most commonly found interlayer anion is carbonate. In practice, however, there is no significant restriction to the identity of the charge balancing anion that can occupy the interlayer region. Examples include inorganic (such as halides, oxo-anions, silicates and polyoxometalates) as well as organic anions (such as alkyl or aryl carboxylates and sulfonates). 

The two main types of infinite chain anions are the pyroxenes, which contain singlestrand chains of composition (SiO ) (Fig. 8-4) and the amphiboles, which contain double-strand, cross-linked chains or bands of composition (SLOif),. Note that the general formula of the anion in a pyroxene is the same as in a silicate with a cyclic anion. Silicates with this general stoichiometry are often called metasilicates, especially in older literature. There is actually neither metasilidc add nor any discrete metasilicate anion. With the exception of the few metasilicates with cyclic anions, such compounds contain infinite chain anions. 

Hypervalent silicon componnds have found wide utility in organic synthesis. In general, pentacoordinated anionic silicates are more reactive toward nucleophiles than are tetracoordinated silanes. For example, Mes2SiF2 is nmeactive toward water, while (the 18-crown-6 potassium salt of) Mes2SiF3 is completely hydrolyzed within minutes. Similarly, the pentacoordinate anion HSi(OEt)4 is an effective reducing agent for aldehydes, ketones, and esters at or below room temperature (Scheme 2) no snch reaction occurs with HSi(OEt)3. The difference in relative reactivities of hypervalent and nonhypervalent species is relevant to the intermediates proposed in Section 7.6. 

Mesostructure syntheses can be carried out under conditions in which the silicate alone would not condense to solid (at pH 12 14 and low silicate concentration) and the surfactant CTAB (concentration < 2%) alone would not form a lyotropic liquid-crystal phase. The rapid formation of MCM-41 when surfactant solution and silicate solution are combined indicates that there is strong interaction between the cationic surfactant and anionic silicate species in the formation of mesophases. 

The resin in the chloride form exchanges chloride against tungstate and thiomolybdate. In the case of tungstate the equation is idealized, because in reality isopolytungstates are present Sodium ions as well as the anions silicate, phosphate, and arsenate are not retained and pass the column. The pH is too high for heteropolytungstate formation with these anions. 

Spirocyclic pentaoxy anionic silicates have been synthesised and their molecular structures, as determined by X-ray crystallography, have been compared with related five-co-ordinate silicon and phosphorus compounds to support their use as model compounds in nucleophilic substitution reactions. 

Cationic detergents (invert soaps) form strong solid complexes with anionic silicates (e.g., montmorillonite). Depending on the concentration and sol-... 

Silicates containing chains or sheets of silicate tetrahedra (extended anion silicates)... 

A list of different types of mechanism is given in Table 5.7, based on the species present in the final composite mesophase. The isoelectric point of silica is around 2, so that syntheses at pH values above this will involve anionic silicates, whereas under strongly acidic conditions the silica will carry a positive charge. The inorganic matrix, I, is represented as 1 or I" ", respectively, and the surfactant as S" (cationic), (anionic) or S° (uncharged). represents anions within the solution. 

In 1992 scientists of the Mobil Oil Corporation published the surfactant controlled synthesis of mesoporous silicate material, designated MCM-41 [1]. Previous studies concerning the preparation and characterization [2-4] and the mechanism of MCM-41 formation [5] have been published. Based on the synthesis mechanism the idea was further developed to design mesostructured transition metal oxides analogous to the MCM-41 structure. The mechanism is based on ionic interactions of the cationic surfactant headgroups with the anionic silicate species. In the synthesis of MCM-41 related materials the charged silicate species are substituted by metal oxides which are able to form polyanions. The metal oxide polyanions perform an analogous function to the silicate species in the synthesis of MCM-41. 

Separations at high pH common anions, silicate, borate, cyanide, formate, acetate... 

Binary rare earth borates 216 4.8. Mixed anion silicates 276... 

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