Thursday, November 8, 2007 - 5:18 PM
638g

Continuous Aerosol-Based Synthesis of Nanostructured Silica Supports

Raghuraman Pitchumani, Physical Chemistry and Molecular Thermodynamics and Nanostructured Materials, Department of Chemical Technology, Delft University of Technology, Julianalaan 136, Delft, 2628 BL, Netherlands, Marc-Olivier Coppens, The Isermann Department of Chemical and Biological Engineering, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12189, and Andreas Schmidt-Ott, Nanostructured Materials, Department of Chemical Technology, Delft University of Technology, Julianalaan 136, Delft, 2628 BL, Netherlands.

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).