After-treated synthetic amorphous silicas

Aerosilogel, definition, 7 Aerosol, definition, 7 After-treated synthetic amorphous silicas applications, 490 example, 489/... 

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. 

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 ... 

Aerosil particles from Degussa are fumed silica, a synthetic amorphous silicon dioxide manufactured by continuous flame hydrolysis of silicon tetrachloride. Siloxane and silanol groups are present on the surface of unmodified Aerosil particles, where silanol groups account for the hydrophilic behavior. Reactions of the silanol groups with various organosilanes and siloxanes can chemically modify the surface ofthe particles. Aerosil 200, Aerosil 7200 and Aerosil 972 were used in this study. Aerosil 200 is an unmodified silica, while Aerosil 7200 is a structure modified (3-methacryl-oxypropyl-trimethoxysilane) product based on Aerosil 200. The hydro-phobic Aerosil 972 is a fumed silica after-treated with dimethyldichlorosilane. Table 12.2 summarizes the properties of these three products. Importantly, the... 

Synthetic silica-alumina catalysts containing 25% alumina are converted to gamma-alumina and mullite when thermally treated at 700-1260°. The phase transformation is accompanied by loss of catalytic activity and by collapse of the porous structure of the catalyst. The gamma-alumina phase is formed apparently by crystallization of the amorphous alumina in the catalyst, while the mullite formation apparently results from a combined silica-alumina amorphous phase. At sufficiently high temperature all alumina is converted to mullite. Silica-alumina catalysts made from more stable silicas have a greater tendency to form gamma-alumina. Such catalysts have lower initial catalytic activity and maintain relatively high catalytic activity after steam deactivation. 

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