ACIDITY AND CATALYTIC ACTIVITY OF SILICA- ALUMINA GELS PREPARED BY COGELATION USING INSTANTANEOUS MIXING METHOD .2. AVERAGE COMPOSITION OF SILICA-ALUMINA GELS PREPARED BY COGELATION USING INSTANTANEOUS MIXING METHOD, AND SURFACE AND BULK COMPOSITIONS OF THEIR PRIMARY PARTICLES

Silica alumina gels with various compositions were prepared by cogelat ion using instantaneous mixing of aqueous solutions of sodium silicate and aluminium nitrate, followed by washing, drying at 30 degrees C fo r 24 h and calcining at 500 degrees C for 2 h. The washing is composed of the two processes: the first is dispersion of the hydrogel into io n-exchanged water, and the second is separation by filtration of the d ispersing element. The hydrogels and filtrates were sampled the whole quantity in every washing through five times. The amounts of elution o f Na, Si and Al ions from the cogels into their filtrates were examine d through the repetitive washing process. The chemical compositions of each gel and each filtrate obtained were determined by chemical analy sis 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 degrees C, and the alumina content of these sam ples was examined by XRF analysis.As the results, the decrease of Na i on 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 pr imary particles of the co-gels. It was known that the calcination enha nces the surface alumina composition. The comparison of the alumina co ntent of the cogels known by chemical analysis with that known by XRF analysis showed that the former is the same as the stoichiometrical co mposition, and the latter is larger than that. We concluded that the s urface AI,OB composition of primary particles had become markedly high er than the bulk composition through washing and calcining the hydroge ls.