Effect of Boehmite Crystallite Size and Steaming on Alumina Properties

Abstract

Boehmite and pseudoboehmite were studied in order to find the effect of their individual properties upon the properties of transitional alumina and R-Al 2 O 3 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 R-Al 2 O 3 depends on hydroxyls diffusion, the corresponding temperature is higher for the transitional alumina with a larger 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.

Binary Oxides with Defined Hierarchy of Pores in the Esterification of Glycerol

“…The TG curves became a plateau at temperatures above to 450 • C, suggesting the thermal stability of the samples. A similar trend is observed in the weight loss curves of the sol-gel derived samples indicating the dehydroxylation and crystallization of the solids [34].…”

Surface characterization and catalytic activity of sulfate-, molybdate- and tungstate-promoted Al2O3–ZrO2 solid acid catalysts

“…A similar phenomenon was reported by Hua et al [27] in the case of sulfated alumina-zirconia catalyst. An endothermic peak was also observed for SO 4 2− /Al 2 O 3 -ZrO 2 sample at higher temperatures due to phase transformation of alumina from gamma to alpha as reported in the literature [28]. No significant information was obtained from the TGA/DTA profiles of molybdenum oxide and tungsten oxide-promoted samples.…”

Electrophoretic deposition and thermal treatment of boehmite coatings on titanium

“…This is in accordance with data from the literature, 30 which showed that the mass loss in pseudoboehmite is higher than the mass loss in boehmite (about 1 wt. %.…”