Preparation And Characterization Of Porous Sio₂ Microspheres

Porous silica microspheres are porous materials with special morphology, having great potential applications in many fields, such as catalyst carrier, adsorption and separation, controlled drug delivery and release etc. In this work, normal and hollow porous SiO₂ microspheres were prepared by a combined template route with sol-gel process in two tetraethyl orthosilicate-ethanol-ammonia-water media with distinct water/silica ratios. Factors influencing the formation, structure and properties of the obtained porous SiO₂ microspheres were studied by SEM, TEM, small angle XRD, IR, TG-DTA, etc. The formation mechanism of the porous microspheres is discussed. The adsorption capabilities of sorbic acid of the porous SiO₂ microspheres with different microstructures and pore characteristics were tested and compared.In the medium with a high H2O/TEOS ratio, surfactants, including dodecy-, tetradecyl- and cetyl-trimethylammonium bromide were added. Surfactant-silica composite microspheres were prepared and porous SiO₂ microspheres were obtained after the removal of surfactants by calcination. It is shown that the pore size increases and the nanopores become more ordered with increasing alkyl chain length of the surfactants. The pore volume and specific surface area also increases with the amount of surfactants. The microspheres prepared with 0.3mol cetyl-trimethylammonium bromide have the highest ordering and largest specific surface area, pore volume and pore diameter of 909 m2/g, 0.5749 ml/g and 2.02nm, respectively. In the medium with a low H2O/TEOS ratio, microporous silica microspheres with small surface areas and pore volumes were obtained. The aggregation of product is reduced with the increase in addition of ammonia solution. With addition of cetyl-trimethylammonium bromide in this medium, microspheres with combined mesoporous and microporous were prepared, however, the pores are not ordered.Hollow mesoporous SiO₂ microspheres were synthesized using polystyrene (PS) beads as core template and dodecy-, tetradecyl- and cetyl-trimethylammonium bromide as porogens. PS-surfactant-silica composite microspheres were formed. Hollow mesoporous SiO₂ microspheres were obtained after the removal of PS and surfactants by calcination. It is found that the thickness of shells and pore diameter increase with increasing the alkyl chain length and amount of surfactants. The thickness of the hollow SiO₂ shells could also be increased by repeated silica coatings. In the medium with a low H2O/TEOS ratio, hollow microporous SiO₂ microspheres were prepared without addition of surfactant via PS templating. Hollows were only templated when the amount of ammonia solution was less than 4.5mol. Increasing the amount of PS can increase hollow spheres among the product. With the increase in amount of TEOS, less hollow microspheres are found broken. The surface of hollow microspheres is more coarse and interparticle voids decrease.Tests on adsorption of sorbic acid of the prepared porous silica microspheres show that the adsorption behavior of porous microspheres mainly depends on their pore volumes and pore diameters. The larger the pore volume and pore diameter the microspheres have, the higher the adsorption capability. In addition, hollow porous microspheres exhibit much higher adsorption of sorbic acid than non-hollow ones.