Depolymerization, silicate

Prepolymerization of Silicate. From phosphate-buffered mixtures of identical chemical composition, different zeolites crystallize depending on the degree of prepolymerization of the silicate. Polymeric silicates tend to give ZK-15 or zeolites of chabazite structure [low-silica ZK-14 (17)] in the same pH range where metasilicate causes zeolite Y to crystallize (Table IV). Thus, either the depolymerization of waterglass is slower than generally thought or the precursors formed with partially depolymerized silicate are stabilized sufficiently to prevent further rapid depolymerization. 

Zeolite Y may be obtained either by using a sodium silicate solution or from silica sol (7, 8). The formation of zeolite Y depends on the time of action of NaOH on the sol before the introduction of aluminate (7). It has been believed that this phenomenon depends upon a depolymerization of the silica contained in the sol. The phenomena occurring during the synthesis of zeolite Y were also investigated by measuring changes in the liquid and solid phases of the hydrogel. 

The most important property of sodium and potassium silicate glasses and hydrated amorphous powders is their solubility in water. The dissolution of vitreous alkali is a two-stage process. In an ion-exchange process between the alkali-metal ions in the glass and the hydrogen ions in the aqueous phase, the aqueous phase becomes alkaline, due to the excess of hydroxyl ions produced while a protective layer of silanol groups is formed in the surface of the glass. In the second phase, a nucleophilic depolymerization similar to the base-catalyzed depolymerization of silicate micelles in water takes place. 

Figure 1 gives results obtained by Alexander et al. (I) and Baumann (2) by dissolving fine particles of commercially available vitreous silica powders in aqueous solutions. Similar data obtained in polymerization and depolymerization experiments by Scheel et al. (15) and Friedberg (10) indicate that the curve shown in Figure 1 represents an equilibrium concentration for oligomeric acid. It can be approached from the supersaturated state of monomeric silicic acid as well as from a solution of pure polymeric silicic acid. 

Previously, we have speculated on the possibility that some particularly highly condensed silicate anions, the double-n-ring (DnR) silicates, may be likely candidates for zeolite precursors (2,4,11). On the basis of an observed relationship between the extent of depolymerization of D4R silicates in the synthesis gel and the structure of the zeolites obtained from that gel, a possible precursor role of the D4R silicate has been discussed fll l. ... 

The depolymerization of silicate species under the influence of dilution or increased alkali seems to be unaffected by the presence of aluminate. However, even with enriched 29Si, the spectra take time to measure and so information about kinetics is lost and the spectra relate to the solutions some time after mixing. 

Vysotsky Z.Z., Gahnskaya V.I., Kolytchev V.I. et al. About the role of polymerization and depolymerization of silicic add in the processes of formation and rearrangement of the gel carcass. In Adsorption and Adsorbents. Kiev Naukova Dumka, 1972. 

Fine crystalline quartz or amorphous substances (gel, quartz glass, etc.) are used as one of initial components for the synthesis [19]. Depending on temperature, pressure, pH of the medium and the presence of salts, silica can exist in solution both as simple ions or molecules, and as more complicated polymer particles. Under normal conditions silica passes into solution in monomer form, as silicic acid Si(OH)4 at large pH, silicate ions SiOj - are formed. Monosilicic acid is a very weak acid however, at increased temperature its dissociation constant increases substantially. The amount of monomer form also increases with temperature. The dissolution of Si02 is due to hydration as well as to depolymerization. 

Soil microorganisms. Many reports have documented the ability of certain soil microorgEmisms (especially bacteria such as Bacillus siliceous, which is used as a fertilizer in some parts of the Soviet Union (Cooper, 1959)) to depolymerize silica and convert it to the soluble monomeric form (see e.g., Lauwers and Heinen, 1974, and references therein) thus, the presence or absence of these oi anisms can be expected to play an important role in determining the concentration of dissolved silica in soil solutions. 

The surfactant may be cationic, or anionic, even nonionic. The type most used for MCM-41 and MCM-48 is cationic quaternary ammonium surfactant. The inorganic resources are those monomers or oligomers that are polymerizable or condensable under certain conditions (concentration, temperature, pressure, pH, solvent, etc.) and form inorganic solid matter such as gel, ceramic, and glass. The depolymerization or hydrolysis is necessary sometimes, for example with TEOS, Ti(OC2H5)4, silicon sol, sodium silicate, and amorphous silica. 

To investigate whether the addition of a neutral alkali metal salt causes a redistribution of the silicate species, a 1.75 mol% Si02, R — 0.4 Cs silicate solution was doped with 46 wtZ LiCl solution to achieve lithium cation mole fractions of 0.42 and 0.59. The resulting connectivity distributions obtained from band integrals are shown in Figure 7. This figure shows that Li addition causes a moderate depolymerization of the silicate species present in the Cs silicate solution. 

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