Morphological and textural control of spray-dried mesoporous silica-based spheres

Micrometric ordered mesoporous silica spheres are synthesised by spray-drying alcoholic sols. For this synthetic route, the important effect of siloxane chemistry on both the morphological and the textural properties of spray-dried silica particles is demonstrated. This is achieved by correlating data obtained on the sols prior to atomisation (Si-29 liquid state NMR, SAXS) with data coming from a multi-scale characterisation of the resulting powders (SEM, XRD, H-1 high resolution solid state NMR). The morphology (agglomerated or separated spheres, 1 to 3 mum of diameter) and texture (disordered or ordered p6mm mesophases), as well as the silica-surfactant interface, are sensitive to variations in the proportion of solvent or in the ageing time of the sols. These effects are related to the relative extents of siloxane hydrolysis and condensation reactions as evidenced by the variations in the radii of gyration of the siloxane oligomers, and in the remaining nonhydrolysed Si-OR alkoxy groups. We propose a scheme for the formation of the spray-dried mesoporous spheres that takes into account the main role of these reactions, and their effects on the determining physicochemical processes: solid formation, fragmentation and aggregation of droplets, micellisation, mesophase formation and extension.