Crystallization of Sol–Gel Boehmite via Hydrothermal Annealing

Thin crystallites of boehmite were synthesized by annealing a sol–gel precursor under hydrothermal conditions. Samples were characterized with X-ray powder diffraction, thermogravimetry, transmission electron microscopy, and by refining the crystalline phases. Fresh samples consisted of boehmite sheets forming folded paper-like “crystallites,” which were transformed into thin flat crystalline plates perpendicular to crystallographic b-axis when they were annealed under hydrothermal conditions using water as mineralizer. Boehmite's crystallite size increased with the annealing time. The rhombic boehmite crystallites had their shortest diagonal parallel to crystallographic a-axis, and their lateral faces parallel to {101} planes forming an angle of 104.32° between each other; these planes contained active aluminum atoms responsible for the crystallites growing along them. The hydrogen bonding length, which decreased as crystallite size increased, determined the variation of boehmite's transition temperature into γ-alumina. Since this transformation is pseudo-morphic, both particle morphology and sample porosity of alumina were determined by the arrangement of crystallites in boehmite. γ-Alumina crystallite distribution had memory about boehmite crystallite dimensions and atomic arrangement: crystallites were oriented parallel to boehmite’s a axis, and were confined by boehmite's crystallite dimensions.