Silica-alumina hydrogel structure

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... 

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. 

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. 

The silica hydrogel is a compliant material whose network building-up is not completed when alumina is added. So, alumina precipitation can easily modify the silica gel structure and induce drastic texture modifications. 

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