Study of surfaces properties of fumed alumina/silica materials

Highly-disperse fumed alumina/silica (AS) with different content of alumina was studied by X-ray, Al-27 MAS NMR, optical spectroscopy of adsorbed color indicator, electrophoretic and quantum chemical methods, in addition, original silica and alumina were studied for comparison with AS. The alumina phase formed in AS for C-Al203 above 20 wt%, when six-fold coordinated AI atoms appear, is amorphous. The alumina patches are covered by a silica shell in aqueous suspension as electrophoretic mobility of the AS particles is practically independent of the concentration of alumina and an effective diameter of the particles in suspension considerably larger than the size of primary particles due to aggregation and agglomeration. The Bronsted acidic sites =Al-O(H)-Si= are observed on the AS surface and the intensity of the corresponding bands in the adsorbed Hammet indicator spectra depends nonlinearly on the concentration of alumina. (C) 1997 Elsevier Science B.V.