Ethyl orthosilicate, hydrolysis

Cs2.5 in water is hardly separable by filtration because of its very fine particle size. Cs2.5/Si02 prepared by the hydrolysis of ethyl orthosilicate in the presence of colloidal Cs2.5 is efficient for the hydrolysis of ethyl acetate, although it is less active than Amberlyst 15 (200). 

There are many other methods of preparing active synthetic silica-alumina catalysts. A fair catalyst can be made by impregnating dried silica gel with an aluminum compound which is easily converted to the oxide by calcination, e.g., A1(NC>3)3. A preferred impregnation technique is to soak a sodium-free silica hydrogel in a solution of an aluminum salt and to follow this with an aqueous ammonia treatment to precipitate the hydrous alumina on the silica (Thomas, 16 Ryland and Tamele, 17). It should be noted that silica hydrogel can easily be freed of sodium ions by water washing, since it is not a zeolite. Exceptionally pure silica-alumina composites can also be prepared by the hydrolysis of mixtures of ethyl orthosilicate and aluminum alkoxides (Thomas, 18). 

Ethyl orthosilicate has been-used to prepare silicic acid that is not completely monomeric because the two-phase hydrolysis is not instantaneous. Disilicic acid was prepared from tetraethyl orthosilicate by the Brintzingers (18). 

T. Matsoukas and E. Gulari Dynamics of growth of silica particles from ammonia-catalyzed hydrolysis of tetra-ethyl-orthosilicate, J. Colloid Interface Sci., 124 (1988) 252-261... 

In practice the ester is mixed with the required amount of water and a suitable solvent (e.g., alcohols, esters, ketones). To obtain 100 Vo hydrolysis of 1 mol of an orthosilicate, 2 mol of water must be added. By maintaining a constant pH (for ethyl silicate ca. 2) the condensation reaction can be suppressed to such an extent that hydrolyzates of ethyl silicate containing 20% SiOj and with a degree of hydrolysis of 80-90% remain stable for about one year. If this equilibrium is disturbed (e.g., by shifting the pH value or evaporating the solvent), condensation commences and the solution solidifies after a short time. This mechanism is used in two-pack zinc dust paints. In the simplest case the binder is a hydrolyzate in alcoholic solution. Zinc dust and, if necessary, extenders, antisettling agents, colorants, etc. are then added. Addition of zinc consumes acid and the pH value increases. 

Historical Development. The importance and advantages of ethyl silicate zinc dust paints were recognized and intensively investigated in the late 1940s [2.187], [2.188]. The fundamental work of Lopata and Keithler [2.189] on the partial acid hydrolysis of tetraethyl orthosilicate and the use of the product as a zinc dust paint binder was followed by attempts to improve the application properties and quality of the coatings (e.g., by adding polyfvinyl butyral) [2.190], alkyl titanates [2.191], borate esters [2.192], or trialkyl phosphates [2.19. ]. 

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