Facile Hydrothermal Synthesis of Hierarchical Boehmite: Sulfate-Mediated Transformation from Nanoflakes to Hollow Microspheres

An interesting morphological evolution from single nanoflakes to nanoflake assemblies, flower-like structures, and hollow microspheres is reported for the first time for boehmite prepared from Al(NO3)3·9H2O with addition of urea and different amounts of KAl(SO4)2·12H2O under hydrothermal conditions. The microstructure, morphology, and textural properties of the boehmite materials were characterized by X-ray powder diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption. It was found that the resulting hierarchical boehmite materials composed of nanoflakes were transformed from featureless material to irregular particles and solid microspheres. To better understand this transformation, different sulfate additives—AlNH4(SO4)2·12H2O, Al2(SO4)3·18H2O, Na2SO4, (NH4)2SO4, and MgSO4·7H2O—were used to synthesize boehmite and magnesia-incorporated boehmite hollow microspheres. This study shows that the sulfate-mediated transformation strategy can be extended for the preparation of other hierarchical metal oxides with controlled morphological and textural properties for catalysis, adsorption, and separation applications.