Amorphous materials precipitated silicas

Silica is one of the most abundant chemical substances on earth. It can be both crystalline or amorphous. The crystalline forms of silica are quartz, cristobalite, and tridymite [51,52]. The amorphous forms, which are normally porous [149] are precipitated silica, silica gel, colloidal silica sols, and pyrogenic silica [150-156], According to the definition of the International Union of Pure and Applied Chemistry (IUPAC), porous materials can be classified as follows microporous materials are those with pore diameters from 3 to 20 A mesoporous materials are those that have pore diameters between 20 and 500 A and macroporous materials are those with pores bigger than 500 A [149],... 

A brief description of the processes involved in biogenic-silica precipitation is useful in understanding many of the physical properties of this amorphous material. There have been significant advances in our understanding of the biochemical... 

It was shown that fine-size quartz particles readily dissolve in 1-2 M sodium carbonate or sodium hydroxide at 250°C. However, when quartz is extracted by a hot solution which is then cooled, an amorphous, acid-insoluble material is produced under some conditions. A likely possibility is that amorphous silica precipitates when the room temperature solubility limit of the material is exceeded. Formation of the amorphous material is prevented by using higher concentrations of alkali or a higher ratio of alkali to silica. 

In common use are three principal materials of silica one natural (both crystalline and amorphous) and two so-called synthetic, products of thermal process (fumed silica grades) and wet process (precipitated silica). Particles of silica commonly have a spherical shape. 

A more pronounced structural disorder often exists in freshly precipitated silica or metal hydroxides in soils, since these compounds typically are amorphous. Structurally disordered aluminosilicates, known collectively as allophane and imogolite, are common in the clay fractions of soils formed on volcanic ash parent material. ... 

Silica is one of the most popular mineral filler used in mbber technology. Three types of silica can be distinguished and namely precipitated, fumed and surface-modified silica. As an amorphous material with randomly placed functional silanol groups (Fig. 12.1), it readily generates hydrogen bonds with surrounding molecules [4]. 

Sparingly soluble materials are often found precipitated from aqueous solution in the form of amorphous particles of ill-defined structure. Precipitated silica is an obvious extreme example where fractal structures are formed (of dimension about 2.4) [171]. Such materials may be intrinsically hydrophobic or be rendered hydrophobic by surface treatment. The presence of rugosities on the surfaces of such particles can have a profound effect on their antifoam function. 

Amorphous silica in nature may originate from aquatic organisms, secreted as amorphous solid in the form of shells, plates, or skeletons. Amorphous silica also is found in volcanic ash or in precipitated material from the hot supersaturated waters of hot springs. 

This section shows, for four examples of increasing complexity, how precipitates are formed and how the properties of the precipitates are controlled to produce a material suitable for catalytic applications. The first two examples comprise silica, which is primarily used as support material and is usually formed as an amorphous solid, and alumina, which is also used as a catalytically active material, and which can be formed in various modifications with widely varying properties as pure precipitated compounds. The other examples are the results of coprecipitation processes, namely Ni/ AI2O3 which can be prepared by several pathways and for which the precipitation of a certain phase determines the reduction behavior and the later catalytic properties, and the precipitation of (VOjHPCU 0.5 H2O which is the precursor of the V/P/O catalyst for butane oxidation to maleic anhydride, where even the formation of a specific crystallographic face with high catalytic activity has to be controlled. 

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