Hydrogel, silica-alumina

Porous texture modifications of a series of silica and silica-alumina hydrogels and xerogels a thermoporometry study. 

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. 

Figure I Comparison of the texture of silica and silica-alumina (hydrogels and xerogels) from cumulative volume curves...

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. 

Another interesting fact appears in table 3 textural modifications induced in silica hydrogel texture still exist in silica-alumina hydrogels. 

Table 3 Effect of pH gelation on the texture of stirred silica and silica-alumina hydrogels. ...

Figure 5 Effect of the alumina content on the pore radius distributions of silica-alumina hydrogels...

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. 

REHY dispersed in silica-alumina hydrogel. Base exchanged with (NH4)2S04 + RECI3. 

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... 

Fig. 13. Excess sodium ion required to maintain pH of 7 with varying alumina content of silica-alumina hydrogels.

Figure 2 Effect of the mixing mode on the texture of silica-alumina hydrogels (a) and...

Tables 1 and 2 give more details on the textural characteristics of silica and silica-alumina hydrogels (table 1), and of the corresponding xerogels (table 2). The main effect of the semi-continuous addition of the reactants is an increase of the mean pore radius of the silica-alumina xerogels when duration of the reactant addition increases.

Textural characteristics of silica and silica alumina hydrogels for each mixing mode (gelation time 150s)... 

Table 3 presents the textural characteristics of the silica-alumina hydrogels as evaluated from thermoporometry measurements. 

The Bingham viscosity remains small (few mPa.s). The effect of the solid content in silica-alumina hydrogels is illustrated on figure 9. The value of increases from 2 Pa to 20 Pa if the solid content in-... 

Pilot plant can also avoid pitfalls in equipment selection. The two rheograms on figure 11 show how a screw type pump can affect the rheological properties of silica-alumina hydrogel. The shear stress imposed by the rotor-stator couple of the pump results in an increase of the yield value of the hydrogel. In the pump, the size of the particles which compose the hydrogel is decreased by a milling effect and the forces of cohesion are increased. This can stop the flow if the pump characteristics have been underestimated. This results also in modification of the gel structure and ability to be spray dried. 

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