Aqueous silicates hydrolysis

The stability of MCM-41 is of great interest because, from the practical point of view, it is important to evaluate its potential application as a catalyst or adsorbent. It is known that purely-siliceous MCM-41 (designated here as PSM) has a high thermal stability in air and in oxygen containing low concentration (2.3 kPa) of water vapor at 700 °C for 2 h [1], However, the uniform mesoporous structure of PSM was found to be collapsed in hot water and aqueous solution due to silicate hydrolysis [2], limiting its applications associated with aqueous solutions. After MCM-41 samples were steamed in 100% water vapor at 750°C for 5 h. their surface areas were found to be lower than amorphous silica-alumina and no mesoporous structure could be identified by XRD measurement [3]. In addition, PSM showed poor stability in basic solution [4]. 

With the addition of bentonite to a crushed basalt backfill, aqueous diffusion would be the most effective mass transfer process (31). Aagaard and Helgeson (32) state that at temperatures <200°C, aqueous diffusion rates are orders of magnitude greater than rates of silicate hydrolysis even in acid solutions. Therefore, the dissolution rate of backfill phases and the overall mass transfer process could be controlled by reactions at the mineral-fluid interface. As stated earlier, dissolution of basalt phases in the sampling autoclave experiments may also be controlled by interface reactions. 

Sakka, S., unpublished data.)- In the solutions, a number of methylsilsesquioxane species, formed by the hydrolysis of methyltriethoxysilane, with different structures are present even under the conditions where the cubic octamer is dominant in the aqueous silicate solutions. This indicates that the use of a silica source with tetra-functionality is required for the selective structure formation with the aid of organic quaternary ammonium ions. 

The general theory of nucleation and polymerization in aqueous systems, in which silica shows some solubility, is discussed in detail in Iler s book (3). However, very little was known at the time the book was published (1979) about the polymerization of silica when Si(OH)4 is formed in nonaqueous systems. Progress made up to 1990 in the understanding of the hydrolysis and condensation of silicon alkoxides that leads to silica gels or to silica sols of large particle diameter are lucidly discussed by Brinker and Scherer (8). Brinker s chapter in this book (Chapter 18) includes a clear explanation of the difference between hydrolysis and condensation of aqueous silicates and silicon alkoxides. 

Figure 6. Log of scattered intensity versus log K obtained by small-angle X-ray scattering (SAXS) for alkoxide-derived gels prepared under different hydrolysis conditions and a commercial aqueous silicate (Ludox SM). (Reproduced with permission from reference 44. Copyright 1985.)...

Earlier in this chapter we discussed the sol-gel processing of aqueous silicates and mentioned the preparation of Si02 gels from fine particles made by flame oxidation (2)-(4). The preparation of alumina sols from alkoxides has been characterized by Yoldas (45). Aluminum alkoxides such as aluminum sec-butoxide and aluminum isopropoxide are readily hydrolyzed by water to form hydroxides. Which hydroxide is formed depends on the conditions used in the hydrolysis. The initial hydrolysis reaction of aluminum alkoxides can be written... 

In our laboratory, we have recently conducted gelation studies of silica nanopardcles in microgravity during the STS-95 space shuttle mission (28). Stable silica nanoparticle dispersions may be form either by polymerization of silicic acids in an aqueous system or through hydrolysis and condensation of silicon alkoxides (the sol-gel or Stober route). Comparison of small-angle x-ray scattering (SAXS) measurements of Ludox, a commercial aqueous silicate with acid- and base-catalyzed alkoxides shows that only aqueous silicate sols are uniform, whereas alkoxides generate fractal particles. As Brinker and Scherer point out (29), these results illustrate that sols derived from aqueous silicates are... 

Aqueous Silicates provides a concise review of hydrolysis and condensation of aqueous silicate systems in which Tier s views [1] are augmented by more recent studies that employ Si NMR. 

Although the Si NMR spectra of aqueous silicate systems are quite complex, the speciation of polysilicates formed from tetraalkoxysilanes is more complicated, because hydrolysis and condensation occur concurrently. At the nearest functional-group level there are 15 distinguishable local chemical environments, which Kay and Assink have represented in matrix form in Fig. 22. The ordered triplet (X, Y, Z) represents the number of -OR, -OH, and -OSi functional groups attached to the central silicon [63,97]. 

Gelation is initiated in aqueous silicate systems by pH changes, and in alkox-ide precursor systems by addition of water (hydrolysis reactions to generate Si-OH groups). 

The H+ and NH forms of homoionic montmorillonite promote the hydrolysis of chloro-s-triazines to the hydroxy analogs (hydroxy-s-triazines) (73). Apparently, the surface acidity of these clays was extremely high, since no degradation was observed in control experiments conducted at pH 3.5 in homogeneous aqueous solution. Russell et al. (73) suggested that the hydroxy-s-triazine products were stabilized in the protonated form at the silicate surface. The IR spectra of these surface complexes agreed with the spectra obtained in 6N HC1, and it was inferred that the pH at the clay surface was 3 to 4 units lower than that measured in suspension. 

---