Layered polysilicate

K. (1993) Synthesis of highly ordered mesoporous materials from a layered polysilicate./. Chem. Soc. Chem. 

Inagaki, S., Fukushima, Y. and Kuroda, K. Synthesis and characterization of highly ordered mesoporous material FSM-16, from a layered polysilicate, Stud. Surf. Sci. Catal., 1994, 84, 125-132. 

S. Inagaki, Y. Fukushima, and K. Kuroda, Synthesis of Highly Ordered Mesoporous Material from a Layered Polysilicate. J. Chem. Soc., Chem. Commun., 1993, 680-682. 

The discovery of ordered mesoporous materials (OMMs) [28-31] opened new perspectives in the field of synthesis and application of mesoporous silicas. OMMs were reported for the first time in 1990 by Kuroda and coworkers [28,29], who synthesized mesoporous molecular sieves using layered polysilicate precursors. However, it was the discovery of scientists from Mobil Oil published in 1992 [30,31] that attracted considerable attention and initiated a significant and still growing number of studies in the direction of synthesis, application, and characterization of ordered mesoporous materials [32-69]. 

The stabilizing adsorbed polysilicate layer is the actual exchange volume or layer, and addition of salt above 0.15 M must lead to further binding of Na+ to the stabilizing layer. It is therefore important to view the barrier at the surface as a M+-polysilicic acid coating, which will increase in thickness as the pH is decreased because of decreased -SiOH ionization conversely, it will decrease in thickness as the amount of bound cation increases (i.e., it will exchange ) in the language of Allen and Matijevic. 

Indeed, the presence of increasing amounts of bound Na+ must switch off the electrosteric contribution of the adsorbed polysilicate layer, and,... 

Viewed in terms of the electrosteric mechanisms outlined in Figures 6 and 7, the 1 1 electrolyte cation effects indicate that relative to Na+, Li+ enhances any stability imposed by the barrier of polysilicate plus bound cation, whereas Cs+ diminishes such effects. This observation is consistent with a hydration stabilization (6) mechanism. Thus, the more strongly hydrated the cations in the polysilicate exchange layer,... 

Synthetic Insoluble Silicates. Insoluble crystalline silicates, ie, mineral-type compounds, are synthesized from soluble silicates by precipitation, gelation, ion exchange, and hydrothermal techniques. Hydrothermal treatment of partially neutralized, high mole ratio (m = 12—50), sodium silicate solutions yields neutral alkali polysilicates that exhibit a layered structure and high ion-exchange capacity (135,136). These and other lamellar silicates can be utilized either alone or modified via pillaring (137) as adsorbents and catalysts. 

The stabilizing adsorbed polysilicate layer is the actual exchange volume or layer, and addition of salt above... 

Indeed, the presence of increasing amounts of bound Na" " must switch off the electrosteric contribution of the adsorbed polysilicate layer, and, for the pH 8.0 sol at its critical coagulation concentration, which is greater than O.lSAf, a secondary minimum well of >2kT must open up as a result of the compression of the thickness and screening length of the adsorbed layer (k, Boltzmann constant T, temperature). 

Crystalline layered sodium disUicates are the most recent development in the field of detergent silicates. In contrast to hydrous sodium polysilicates and metasilicates, they display a three-dimensional crystal lattice characterized by corrugated silicate layers, which are separated by sodium ions. The chemical formula is NajSijO,. This reflects that the material is anhydrous and has a formal molar ratio of 2. Several crystal phases differ in the kind of corrugation of the silicate layers [3,21,66,86-88]. 

The difference between depositing material molecularly and as particles has been noted in other systems. Thus Howard and Parfitt noted that in depositing silica on titania pigment either a layer, could be deposited as polysilicate ions by an isothermal, pH-dependent process or colloidal silica particles could be deposited by a coagulation mechanism (284a). 

As has been discussed in Chapter 2 relating to lithium silicates and polysilicates, the lithium ion is unique in that it is preferentially adsorbed at the silica interface forming an impervious layer which retards the dissolution of silica at high pH (206). 

When dehydrated, the crystals are submicroporous. being penetrated by water and small ions. They have a layerlike structure which, in some types, expands and contracts during ion exchange or hydration-dehydration like the layerlike clays. The layer structures thus differ from the zeolitetype aluminosilicates, which have a rigid, three-dimensional lattice. The unusual, extremely thin alica sheets from a synthetic sodium polysilicate are shown in Figure 5.24 (495). 

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