Surfactant-free preparation of mesoporous solid/hollow boehmite and bayerite microspheres via double hydrolysis of NaAlO and formamide from room temperature to 180 °C.

The transformation from solid boehmite microspheres to hollow bayerite microspheres at room temperature (25 °C) was successfully realized via the double hydrolysis of NaAlO and formamide (FA) solution. Effects of reaction time, temperature and FA dos on the transformation process were studied in detail. The results show that hollow boehmite microspheres were obtained via increasing the temperature above 120 °C and the FA dos above 12 mL; amorphous alumina hydrate, solid boehmite and hollow bayerite microspheres were also obtained at 25 °C for 1 h, 2 h and 24 h reactions, respectively; solid bayerite microspheres were obtained by decreasing the FA dos below 4 mL at 25 °C. Because of the slow change of pH from 12.9 to 8.7, simultaneous dissolution and regeneration for different aluminum hydroxide were the key factors for forming hollow boehmite/bayerite microspheres. The Al yield for boehmite microspheres reached 41.4% at 25 °C for a 2 h reaction; when increasing the temperature to 180 °C, the Al yield for hollow boehmite microspheres with higher crystallinity increased to 82.7%. Moreover, the solid/hollow boehmite microspheres with high surface areas showed outstanding adsorption capacities of 751.9 mg/g and 694.4 mg/g, respectively, for Congo red. This significant transformation of structure and morphology provides an effective strategy for preparing mono-phase hydrated alumina with excellent adsorption performance.