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

•Hierarchical porous γ-Al2O3 hollow microspheres were synthesized.•The mechanism for obtaining the γ-Al2O3 hollow microspheres containing macro-meso-micropores was proposed.•NH4Cl was demonstrated to be a template to act as the core and prevents the aggregation of laminar boehmites.•High adsorption capacity and adsorption rate of hierarchical porous γ-Al2O3 hollow microsphere for Congo Red.

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.

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