Effect of Boehmite crystallite size and steaming on alumina properties

Boehmite and pseudoboehmite were studied in order to find the effect of their individual properties upon the properties of transitional alumina and α-Al₂O₃ produced thereof; the influence of steaming on these properties was also considered. Samples were characterized by X-ray powder diffraction, crystalline structure refinement, scanning electron microscopy, thermoanalysis, and nitrogen adsorption. Boehmite and pseudoboehmite had the same crystalline structure but different crystallite dimensions, Differences between lattice parameters (only 0.0052 nm along the b axis) are so small that the assumption of many authors that pseudoboehmite has intercalated water in its crystalline structure cannot be supported. Small crystallites with larger active areas, increased by crystal faces perpendicular to (020) planes, explain the larger amounts of desorbed water that pseudoboehmite exhibits compared to boehmite while drying. The hydrogen-bond length in boehmite depends on the crystallite size and determines the temperature where transformation into transitional alumina takes place. The boehmite crystallite size also defined pore size and distribution in transitional aluminas as well as their grain and crystallite size. Since the transformation of the transitional alumina into α-Al₂O₃ depends on hydroxyls diffusion, the corresponding temperature is higher for the transitional alumina with a large grain size. Steaming promoted crystal growth, because water molecules favored migration of aluminum atoms. Nitrogen adsorption-desorption isotherms of transitional aluminas derived from pseudoboehmite exhibited hysteresis loops corresponding to less ordered structures associated with swelling of platelets during desorption of nitrogen molecules. Consequences on the properties of transitional aluminas used as cracking catalysts were inferred. As texture and acidity evolve in opposite directions, there is a need for controlling them independently.