Silica modifications

Stober W (1967) Formation of silicic acid in aqueous suspensions of different silica modification. 

Borissova, M., Koel, M., and Kaljurand, M., Ionic liquids for silica modification Assessment by capillary zone electrophoresis, in Rogers, R. D. and Seddon, K. R. (Eds.), ACS Symposium Series 975, Ionic Liquids Not Just Solvents Anymore, ACS, Washington, DC., 35-46,2007. 

Formation of Silicic Acid in Aqueous Suspensions of Different Silica Modifications... 

When fine powders of vitreous silica, quartz, tridymite, cristobalite, coesite, and stishovite of known particle-size distribution and specific surface area are investigated for their solubility in aqueous suspensions, final concentrations at and below the level of the saturated concentration of molybdate-active silicic acid are established. Experimental evidence indicates that all final concentrations are influenced by surface adsorption of silicic acid. Thus, the true solubility, in the sense of a saturated concentration of silicic acid in dynamic equilibrium with the suspended silica modification, is obscured. Regarding this solubility, the experimental final concentration represents a more or less supersaturated state. Through adsorption, the normally slow dissolution rates of silica decrease further with increasing silicic acid concentrations. Great differences exist between the dissolution rates of the individual samples. 

Six different silica modifications were used vitreous silica, quartz, cristobalite, tridymite, coesite, and stishovite. Two of these—cristobalite and tridymite—were prepared from fine amorphous silica powder by tempering samples at 950°C. with 1% of a mineralizer (K2CO3 and KH2P04, respectively). Vitreous silica was obtained from fused rock crystals. All other samples were natural minerals pure specimens of Brazilian rock crystal were used as quartz coesite and stishovite were obtained as fine powders by isolation from Coconino sandstone of the Barringer Meteor Crater in Arizona (4). 

In view of this, the dissolution patterns of all other silica modifications were interpreted disregarding the condensation reaction. This seemed particularly adequate for stishovite, a high pressure, high temperature material, first produced in an autoclave by Stishov and Popova (17) in 1961 and detected at Meteor Crater one year later (6). The lattice... 

Figure 14. Concentration patterns of adsorption tests with different silica modifications. The exposed surface area is different for each sample initial silicic acid concentration is 78 ngrams Si02/ml.

Figure 15. Daily release of silicic acid from 1 sq. meter of silica surface area into 50 ml. of fresh solvent (0.9% NaCl plus 0.1% NaHCOt, pH = 8.4), repeated over 20 days for different silica modifications...

Figure 16. Dissolution patterns of different silica modifications 375 sq. meters of total surface area suspended in 500 ml. of a solution of 0.9% NaCl plus 0.1% NaHCOs at pH = 8.4...

Stober, W. Formation of silicic acid in aqueous suspensions of different silica modifications. In Equilibrium concepts in natural water systems (ed. R. F. Gould). Amer. Chem. Soc. 

The surface groups S with the highest free energy should be most efficient in the processes of chemical modification. In this case, one can expect the formation of a great variety of modification products (SR groups). For the compounds with the directional chemical bonds between the neighboring atoms in the main crystalline and amorphous silica modifications, these are the products of bond rupture and/or its rearrangement. 

The chemistry behind the modification of powders, fibres or thin layers is similar. Therefore fibres as well as thin oxide layers have repeatedly been used to model silica modification mechanisms. 

In various industrial applications, silica modification is used on thin layers of Si02, rather than on the powdered form. Monomolecular or thin layers of silicon oxide are thermally grown on Si-wafers for the production of high-tech materials. The surface chemistry of these layers is comparable to the powdered form, with the absence of a porous structure. 

For silica modification with APTS, the pH of the aqueous solution also has an effect on the eventual loading of the surface.12 The APTS surface loading on dehydrated silica, is given as a function of pH in figure 9.3. The loading was measured by integration of the CH2 vibration bands, after internal normalization. 

In this section, only the surface modification techniques will be discussed, and not the impregnation, sol-gel or co-precipitation techniques. Furthermore, it is not our aim to fully cover all chemical modifications on the silica surface. We merely want to present in introduction to and an insight in the fast expanding field of silica modifications, in order to create new catalysts, sensors and immobilizators. 

Stalder, K. and Stober, W., Haemolytic activity of suspensions of different silica modifications and inert dusts, Nature, 207, 874, 1965. 

Amazingly, under hydrothermal conditions in the temperature range of 373 K up to 493 K the hydrophilic high-alumina NaCaA and NaY zeolites are significantly more stable than the hydrophobic high-silica modifications DAY-S and DAY-Tg generated by dealumination. In steam of saturated pressure a shift of the stability region by 100 K and more can be observed... 

Zeolites (of which there are currently more than 200 types, 50 of which are naturally occurring) are differentiated by the Si/Al ratio in their anionic structure. This varies between 1 (in zeolite A) and °° in silicalite, an aluminum-free crystalline silica-modification. The industrially most important synthetic zeolites, apart from zeolite A, are zeolites X and Y with Si/Al ratios of 1 to 3, synthetic mordenite at ca. 5 and ZSM 5 at > 10. The thermal stability and acid resistance of zeolites increa.se as the Si/Al ratio increases. 

L.A.Belyakova, A.V.Simurov, V.V.Sidorchuk and V.A.Tertykh, The Method for Silica Modification, SSSR Patent No. 1742208 (1992). 

The support geometry is important with respect to Fischer-Tropsch synthesis performance (i.e. activity and selectivity) of the resultant catalyst based thereon. The modified alumina support after silica modification did indeed produce a catalyst with the desired Fischer-Tropsch synthesis characteristics [4]. 

The dissolution of titania supports was also investigated. Titania also (partially) dissolves in aqueous solutions and silica modification, using TEOS, effectively inhibited support dissolution [4]. 

Stober, W. (1967), Formation of Silicic Acid in Aqueous Suspensions of Different Silica Modifications, in Equilibrium Concepts in Natural Water Systems. (W. Stumm, Ed.) (ACS Symposium Series No. 67), American Chemical Society, Washington DC, Chapter 7, pp. 161-182. 

The modification of BEA zeolite by surface deposition of silica and impregnation with cerium oxide was studied as a tool to improve the selectivity of the reaction. The number of acid sites, particularly the strong ones, on BEA zeolite decreases with increasing amounts of silica deposited on its surface. Moreover, there is no severe pore blocking after deposition. On the contrary, cerium oxide impregnation affords a catalyst with decreased adsorption capacity because part of the cerium oxide is deposited in the channels of the zeolite crystals and blocks the porous system. In addition, cerium oxide modification creates new weak acid sites on the zeolite surface. Silica modification decreases catalytic activity but slightly increases selectivity with respect to all ortho-HAP, para-HAP and para-acetoxyacetophenone, in comparison to the unmodified BEA zeolite, and the stability of the catalyst is also improved after modification. The best reaction results are obtained over 16% cerium-oxide-modified catalyst, the selectivity with respect to the C-acetylated products being increased to about 70% while the conversion remains 60%-80%. 

The Inorganic filler. Many Inorganic minerals are transparent for UV light. This Is especially true for silica modifications. In addition the refraction Indices of matrix and filler are not very different. So It Is hardly revolutionary that silica fillers do not Interfere with UV curing. 

Fig. 12. SiiC edge XANES spectra of silica modifications, a Quartz b quartz glass c as-synthe-...

From the practical point of view with respect to the silica modification to obtain different functionalities, the electrophilic substitution reactions are very important additionally, they are interesting from the theoretical point of view due to a variety of possible reactants affecting the TS structure and third components (catalysts) promoting H transferring [ 1,2,11 ]. 

Disperse oxides unmodified or modified by organics (OC) or OSC are used as fillers, adsorbents, or additives [1-11]. OSCs are used as promoters of adhesion, inhibitors of corrosion, for the stabilization of monodisperse oxides and the formation of the nanoscaled particles. Oxide modification by alcohols or other OC is of interest for synthesis of polymer fillers, as such modification leads to plasticization and reinforcement of the filled coating, but in this case a question arises about hydrolyz-ability of the =M—O—C bonds between oxide surface and alkoxy groups, as those are less stable than =M—O— M= formed, for example, upon the silica modification by silanes or siloxanes. The high dispersity, high specific surface area, and high adsorption ability of fumed oxides have an influence on their efficiency as fillers of polymer systems. 

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