Spatial arrangement of fibrils in concentrated aqueous sols of fibrillar pseudoboehmite

The particle size, aggregation state and spatial arrangements of the particles in concentrated sols of boehmite and of fibrillar pseudoboehmite are important parameters for determining the final properties of powders of γ-Al2O3 for use as catalysts, adsorbents, coatings and alumina derived materials of desired porosity and mechanical strength. However, these parameters are very difficult to evaluate, even qualitatively, in concentrated sols especially by conventional transmission electron microscopy. In this paper, we report the use of freeze-etching and critical point drying methods applied to transmission and scanning electron microscopy to describe the spatial organization of fibrils in concentrated aqueous sols of pseudoboehmite. For this purpose, a series of mono-, two- and three-layer sols of fibrillar pseudoboehmite containing 70 g/l of AlOOH were produced by reaction of aluminum powder with aqueous solution of acetic acid. A three-dimensional interwoven organization of short, single fibrils was observed in the mono-layer sol. Similar arrangement was seen in the upper layer of two- or three-layer sols, although in these sols, the fibrils are long and aggregate-forming thin bundles. In the bottom layer, the fibrils are aggregated in thick bundles aligned parallel to their lengths and forming plates ordered inside the whole layer. In the third layer of three-layer sols, macroscopically constituted by white precipitates, a three-dimensional structure constituted by the stacking of the fibril plates was observed. A model is proposed for the three-dimensional arrangement of the fibrils in the sols of pseudoboehmite according to their aging time at 90 °C.