Alumina foam microspheres by emulsion drop-casting in aqueous ammonium chloride solution

A facile method for the preparation of alumina foam microspheres from hydrogenated vegetable oil-in-aqueous alumina slurry (HVO-in-AAS) emulsions is reported. The HVO-in-AAS emulsions drop-cast in cold aqueous NH4Cl solution undergo fast gelation due to the freezing of HVO, physical cross-linking of carrageenan gelling agent and coagulation by NH4Cl. Emulsions of HVO to AAS volume ratios of 1.5-2.5 prepared from 10 and 20 vol% AAS are used for drop-casting. Microspheres with sphericity close to 1 are obtained by drop-casting in 5 wt% aqueous NH4Cl solution at −3 °C. The well-defined interconnected cellular structure observed in the core of the sintered alumina foam microspheres is distorted towards the surface due to emulsion destabilization by the NH4Cl. The porosity and cell size of the sintered alumina foam microspheres are independent of drop-casting bath conditions but depend on the emulsion composition. The microspheres have diameter, open pore volume and average cell sizes in the ranges of 2.38-2.49 mm, 1.22-2.42 cm3/g and 15.66-6.87 µm, respectively.