Formation of uniform large SBA-15 microspheres via spray drying†
The effects of inlet drying temperature, initial solute content, hydrolysis time of the precursor, and the choice of solvent were analysed in the synthesis of large (∼50 μm) and uniform SBA-15 microspheres via spray drying. Higher temperatures led to microspheres with larger particles sizes due to a faster evaporation rate. Meanwhile, particle size was proportional to the solute content, thus SBA-15 microspheres with tuneable particle size over a wide range were obtained. Highly ordered mesostructures were assembled for SBA-15 microspheres obtained from initial solute contents between 5 and 12 wt%. Solute contents of lower than 5 wt% or higher than 12 wt% lead to less ordered mesostructures. Increasing hydrolysis time of the precursor solution from 1 hour to 2 hours allowed more cross-linking of silica, thus increasing surface tension forces and resulting in hollow, “bowl-like” particles. Notably, highly oriented mesopores were developed in the “bowl-like” particles. The use of water instead of ethanol as a solvent resulted in disordered “worm-like” mesostructure, with “bowl-like” particles due to the slower evaporation rate. The unique set up of our spray dryer enabled the collection of particles immediately after spray drying. Analyses via SAXS and TEM indicated that the mesostructures were still mobile immediately after spray drying, confirming the so-called “wet-pocket” phenomenon that determined the final mesostructures.