Synthetic amorphous silicas

John Gyann in his US Patent (4,764,384, 1988) proposed a blend of amorphous silica, synthetic amorphous magnesium silicate, diatomaceous earth and synthetic silica alumina for rejuvenating spent frying oil. In this invention, the inventor claims that the adsorbent would remove some free fatty acids and other oil breakdown products. It does not clearly specify if the adsorbent also removes the soap that is formed during adsorption due... 

Amorphous silica, ie, silicon dioxide [7631-86-9] Si02, does not have a crystalline stmcture as defined by x-ray diffraction measurements. Amorphous silica, which can be naturally occurring or synthetic, can be either surface-hydrated or anhydrous. Synthetic amorphous silica can be broadly divided into two categories of stable materials (1) vitreous silica or glass (qv), which is made by fusing quart2 at temperatures greater than approximately 1700°C (see Silica, vitreous silica), and microamorphous silica, which is discussed herein. 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

Approximately 40% of synthetic amorphous silica production is in Europe, followed by North America at 30%, and Japan at 12%. Although deposits of naturally occurring amorphous silicas are found in all areas of the wodd, the most significant commercial exploitation is of diatomaceous earth in industrialized countries (see Diatomite). This is because of the high cost of transportation relative to the cost of the material. Woddwide manufacturers of amorphous silica products are listed in Table 2. 

Silica gel is synthetic amorphous silica consisting of a compact network of spherical colloidal silica particles. Its surface area is typically between 300 and 850 m2/g. The predominant pore diameters are in the range 22-150 A. Silica gel is produced via the following procedure a sodium silicate solution reacts with a mineral acid, such as sulfuric acid, producing a concentrated dispersion of finely divided particles of hydrated Si02,... 

II. SYNTHETIC SILICAS (mostly amorphous) Surface area, pore volume, pore size and particle size are to some extent independently controllable (commercially interesting). 

Superglycerinated Fully Hydrogenated Rapeseed Oil, 45 Synthetic Amorphous Silica, 51... 

Physical-Chemical Features of Synthetic Amorphous Silicas and Related Hazard and Risk Assessment... 

Keywords synthetic amorphous silicas, particle sizes, dissolution kinetics, hazard and risk assessment... 

Synthetic Amorphous Silica (SAS) is highly pure, crystalline-fr-ee, silicon dioxide, which may be... 

Silicon dioxide, silica, can be natural or synthetic, crystalline or amorphous. This book is concerned mostly with synthetic amorphous silica in the colloidal state. 

The term synthetic amorphous silicas (SAS) dates to 1985, when a number of people from the American Society for Testing Materials (ASTM) formed a working group called SAS. When discussing amorphous silicas, researchers differentiate between... 

Synthetic amorphous silicas were characterized by Von Buzagh (37) as early as 1937. In 1953, Iler (38) wrote about the convenience of differentiating between precipitated silicas and silica gels. Stauff elabo-... 

The suggestions are of course given only in a rough and simplified form in the preceding list. For example, after-treatment with fluorinated compounds was also suggested (44, 45). A silica gel that has been after-treated with hydrofluoric acid develops hydrophobic characteristics to the same extent as Aerosil R 972, which is after-treated with dichlorodimethyl-silane. Figure 3 (46) demonstrates the different behavior of Aerosil 130 (the untreated synthetic amorphous silica) and Aerosil R 972 for water adsorption. 

As a rule, 3-7% of synthetic amorphous silica is required for applications in the coatings sector, whereas 0.2-2% is needed in the other... 

Special effects have also been established during the improvement of the free-flow properties of products in powder form. Unfortunately, it is not yet possible to predict under which conditions a hydrophobic synthetic amorphous silica will produce better results than a hydrophilic (i.e., untreated) one. A similar situation exists in the area involving the thickening of technically important resins [e.g., special unsaturated polyester or epoxy resins (63)]. 

In the past, non-after-treated SAS bore the Chemical Abstracts Service (CAS) registry number 7631-86-9, aluminum silicates bore the number 58425-86-8, and calcium silicates the number 1344-95-2. SAS that were after-treated with dichlorodimethylsilanes were given the number 60842-32-2 (formerly 68611-44-9), and SAS after-treated with polysi-loxanes the number 67762-90-7. Thanks to efforts by SASSI (Synthetic Amorphous Silica and Silicates Industrial Association), new numbers have been assigned since June 7, 1989, which are designed to allow a better differentiation between the amorphous and crystalline substances. The following numbers are now valid for the different types of SAS ... 

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