Controllable preparation of highly uniform γ-alumina microspheres via the sol–gel route for alkoxide in a coaxial microchannel

This study describes a novel and facile route for fabricating highly uniform γ-alumina microspheres with controllable size, morphology, and pore structure by integrating the sol–gel route for alkoxide with “temperature/pH-induced gelation” in a coaxial microchannel. Taking advantage of the controllability of the microfluidic devices and Al–O–Al chains formed by the sol–gel route of aluminum alkoxide, the uniformity of the microspheres was assured, and the mechanical strength of the microspheres was greatly improved to 18–27 N/mm². The effects of the continuous phase composition and two-phase flow rate on the size, morphology, and pore structure of the obtained microspheres were investigated. When triocylamine (TOA) was added to the continuous phase and the two-phase flow rate ratio was larger than 188, the as-prepared microspheres had a smooth surface and good sphericity; otherwise, the microspheres had poor spheriticy and included many ravines on the surface, which can be explained by the mutual influence of long-lasting movements of molecular chains resulted from low gelation speed and inner circulation of the droplets. Under different continuous phase compositions, the average pore diameter, pore volume, and specific surface area ranged from 7 to 12 nm, 0.89 to 1.58 mL/g, and 293.7 to 387.7 m²/g, respectively.