Silica-alumina hydrosol

The formation of a hydrosol which sets to a true hydrogel in a short time is the basis for an important recent advance in the art of catalyst manufacture on a large scale. The gelation time of a silica-alumina hydrosol is a function of several variables (pH, concentration, degree of mixing of reagents, and temperature), the control of which permits a uniform dispersion of the hydrosol into an immiscible fluid, such as oil,... 

Fig. 2.3 Hydrosols via modification of the Al-organic protective shell. As modifiers can be used e.g. alcohols, carbonic acids, silanols, sugars, polyalcohols, polyvinylpyrrolidone, surfactants, silica, alumina. Advantages Tailoring of the dispersion in lipophilic and/or hydrophilic solvents (e.g. water) Anchoring onto surfaces via d-bonds.

The commercial alumina and silica gel sorbents are mesoporous, i.e., with pores mostly larger than 20 A (see Fig. 1). Activated alumina is produced by thermal dehydration or activation of aluminum trihydroxide, A1 (OH)3 (Yang, 1997), and is crystalline. Commercially, silica is prepared by mixing a sodium silicate solution with a mineral acid such as sulfuric or hydrochloric acid. The reaction produces a concentrated dispersion of finely divided particles of hydrated Si02, known as silica hydrosol or silicic acid ... 

Finally, intermediate gels are often used which derive from a mixture of a silica hydrosol with an aqueous solution of an aluminum salt. Although the silicon and aluminum bearing species are actually mixed at small scales in all these sol-gel precursors, the formation at relatively low temperatures of homogeneous and dense mullite bodies from amorphous alumina-silica... 

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