Average Composition of Silica Alumina Gels Prepared by Cogelation using Instantaneous Mixing Method, and Surface and Bulk Compositions of their Primary Particles

Abstract

Silica alumina gels with various compositions were prepared by cogelation using instantaneous mixing of aqueous solutions of sodium silicate and aluminium nitrate, followed by washing, drying at 30° C for 24 h and calcining at 500° C for 2 h. The washing is composed of the two processes: the first is dispersion of the hydrogel into ion-exchanged water, and the second is separation by filtration of the dispersing element. The hydrogels and filtrates were sampled the whole quantity in every washing through five times. The amounts of elution of Na, Si and Al ions from the cogels into their filtrates were examined through the repetitive washing process. The chemical compositions of each gel and each filtrate obtained were determined by chemical analysis and ICPE spectroscopy. In order to know the effect on the surface composition by the calcination, one of the cogels was calcined at the temperatures up to 500° C, and the alumina content of these samples was examined by XRF analysis.
As the results, the decrease of Na ion content in these hydro-cogels due to repetition of washing could be recognized. Further, the elution of Si was found to be more than that of Al for all the cogels, and the elution occurs on the surface of primary particles of the co-gels. It was known that the calcination enhances the surface alumina composition. The comparison of the alumina content of the cogels known by chemical analysis with that known by XRF analysis showed that the former is the same as the stoichiometrical composition, and the latter is larger than that.
We concluded that the surface Al₂O₃ composition of primary particles had become markedly higher than the bulk composition through washing and calcining the hydrogels.