Facile synthesis of hierarchical porous γ-Al2O3 hollow microspheres for water treatment.

Hierarchical porous γ-Al2O3 hollow microspheres were synthesized by a modified spray drying method. Ageing the precipitated precursor and spray-drying assisted by NH4Cl salts are considered as two key steps for the synthesis of γ-Al2O3 hollow microspheres. The mechanism of the formation of hierarchical porous γ-Al2O3 hollow microsphere was proposed involving phase transformation from aluminum hydroxide to laminar boehmite during ageing and a following self-assembling process with NH4Cl as the template during spray drying. The meso-/macro-pores in γ-Al2O3 mainly arise from the stacking of the laminar boehmites which are obtained by ageing the precipitated precursors at 90°C. NH4Cl, which was the byproduct from the reaction between AlCl3·6H2O and NH3·H2O, was demonstrated to be an excellent template to act as the core and the barrier for separation of laminar boehmites. No extra NH4Cl was added. The as-synthesized hierarchical porous γ-Al2O3 hollow microsphere presented remarkably higher adsorption capacity, which is thirty times higher adsorption rate for Congo Red than the solid microsphere containing only small mesopores.