Hydrothermal Synthesis of Boehmite Plate Crystals

Large plate crystals of boehmite up to 4 μm were prepared from Al(NO3)3 solutions with the addition of NaOH under hydrothermal conditions at 250°C for 6 h. The conditions to obtain monodispersed boehmite large plate crystals were determined to be in the range of OH/Al molar ratio from 3.5 to 3.8 and Al concentration from 1.3 to 2.0 mol/kg. Bayerite formed by mixing the starting materials changed to gibbsite at 200°C, and at the same time boehmite started to crystallize from gibbsite. Even after complete phase change from gibbsite, small boehmite crystals grew larger by dissolution and precipitation mechanism. The key to obtain boemite large plate crystals is the preparation of homogenous fine boehmite crystals by quickly heating fine gibbsite crystals to the boehmite stable region, as well as the selection of OH/Al molar ratio and Al concentration.