(638g) Continuous Aerosol-Based Synthesis of Nanostructured Silica Supports

Mesostructured materials with a tunable pore structure have attracted great interest owing to their potential applications in catalysis. Mesostructured silica is usually synthesized in batch using the sol-gel technique, which requires several days. An alternative, more recent synthesis approach introduced by Brinker and co-workers, namely "evaporation-induced self-assembly" (EISA) of aerosols, enables continuous production of porous silica particles with a controlled morphology and pore size within a process time of only a few seconds. In order to probe the high-dimensional experimental parameter space, this paper applies a rigorous statistical methodology that allows to greatly reduce the number of experiments required to determine which experimental parameters have a significant effect on the textural properties.

A clear, homogeneous acidic precursor solution containing an organosilicate (TEOS) and a triblock copolymer (P123) as the template is atomized into an aerosol that is transported through a heated tubular reactor. Silica and surfactant micelles self-assemble with simultaneous hydrolysis-condensation of TEOS, resulting in organic-inorganic composites. High-temperature calcination results in nanoporous silica particles consisting of lamellar or hexagonal nanostructures.

Experiments conducted were based on a factorial design methodology for a comprehensive study into the effect of precursor composition and reactor temperature on the texture of synthesized materials. This methodology allows exploration over a wider range of conditions to highlight the true nature (global/local) of trends that are often misinterpreted as a universal occurrence in conventional experimental trials.

The analysis showed that all of variables were significant: P123/TEOS was identified as having the maximum effect on the BET surface area, followed by H₂O/TEOS, temperature, and EtOH/TEOS. Using contours, this method exclusively determined multiple conditions for achieving a required surface area and pore volume. N₂ adsorption measurements showed that a higher P123/TEOS results in particles with a broad pore size distribution. Lower ratios favour formation of a network with uniformly sized pores. The structural order and nanostructure were elucidated by X-ray diffraction (XRD) and transmission electron microscopy (TEM).