Silica-alumina hydrogel preparation

Intrinsic effects of pH change on gel texture are described by results of table 3 which gives the main textural characteristics of silica and silica-alumina hydrogels prepared at two basic pH according to the same preparation procedure acid is poured in silicate under continuous stirring. 

Second step preparation of silica-alumina gels. Aluminium sulfate solution (33 wt% Al2(S04)3-18H20) is added to the silica gel slurry. A further addition of an ammonia solution (20 wt% NHj) leads to the precipitation of alumina at pH 6. The obtained slurries of silica-alumina hydrogels are successively filtered under vacuum and washed several times to remove impurities, and spray-dryed in well defined conditions. Spray drying leads to solid spherical particles with reproducible physical characteristics, in particular pore size distribution, pore texture and particle size distribution. 

The formation of silica from a sol-gel transition is a low activation energy reaction. Thus, it was only expected a slight effect of the preparation temperature on the gel texture (silica-alumina as well as silica). However, as shown in table 5, the effect of preparation temperature on the texture of silica and silica-alumina hydrogels is important. 

Much can be learned concerning the structure of the catalysts, which are used in the calcined state, from the reactions which occur previously in the hydrogel state. A series of silica-alumina hydrogels was prepared... 

Thermoporometry measurements allow to evidence the effects of operating parameters of the preparation process on the texture of hydrogels of silica and silica-alumina. As the aim... 

The preparation of synthetic silica-alumina catalysts is a relatively simple one, involving the coprecipitation or cogelation of the two hydrous oxides from mixed solutions of sodium silicate and aluminum sulfate. Depending on how the solutions are mixed and on the pH and concentration of the resulting mixture, the combined hydrous oxides will be formed as a coprecipitate, which separates from a greater part of the aqueous phase, or as a true hydrogel, which embraces the entire solution volume. 

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). 

For the purpose of synthesizing structures mmilar to these minerals (Noll, 23), the conditions of preparation have to be quite different from those used to prepare hydrogel mixtures of silica-alumina. The same applies for their subsequent processing including calcination which is an essential phase in the preparation of satisfactory catalysts. These differences in preparation are obviously related to the phenomena described in the foregoing section. A special study was carried out by the authors to give a clearer picture of these relationships. 

Silica hydrogels prepared in acid conditions conduct to silica-alumina xerogels having pores smaller than those obtain in basic conditions. 

Silica/alumina catalysts were carefully prepared by precipitating alumina from an aluminum salt at pH 7 onto a freshly prepared silica hydrogel. The precipitate... 

The figure 7 shows the scale effect on the texture of hydrogels prepared at laboratory and on pilot plant. No differences can be observed on silica-alumina samples (curves 3 and 4). On the contrary P.S.D. of pilot plant silica sample is broader than P.S.D. of laboraioiy silica sample (curves 1 and 2). Differences in mixing efficiency of each reactor are probably the cause. 

The figure 8 shows hydrogel iheograms for silica and silica-alumina prepared at the pilot plant. Hydrogels are non-newtonian plastic fluids of Bingham type and can be described by the following relation ... 

Zeolite rho was prepared from aluminosilicate hydrogels containing sodium and cesium cations. The procedure is entirely comparable with the synthesis of faujasite except for substitution of CsOH for about 10% of the NaOH in the faujasite synthesis gel. Alumina trihydrate (Alcoa C-33 grade) was dissolved in 50% NaOH solution at 100°. After cooling to ambient temperature, the required amount of CsOH solution was added, and the resulting liquor was blended into 30% silica sol (duPont Ludox LS-30) with vigorous mixing. After 3-7 days incubation at 25°, the synthesis gel was held at constant temperature, 80, 90, or 100°, until crystals formed maximum crystallinity was usually achieved in 2-4 days. 

From this we concluded that the replaced feldspar preparation was mostly a method for preparing hydrous silica containing a little alumina but substantially free of other metallic ions due to the extensive acid treatment to remove the alkali and calcium ions. We also concluded that purified silica hydrogel could be made less expensively from water glass (sodium silicate). 

Starting with a large batch of undried, carefully purified silica hydrogel, highly active cracking catalysts were prepared by precipitating hydrous alumina on the silica. U. S. Patent 2,285,314 describing this method of preparation was filed October 22, 1938. Since there was no obvious stoichiometry... 

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