(574g) A New Method to Synthesize Micrometer-Sized Silica Spheres with Highly Ordered Mesostructure

The mesoporous materials with controlled nanostructures and macroscopic morphologies has attracted much attention because of their emerging applications in the areas of catalysis, adsorption, chromatography, and controlled release of drugs. Among the particles with all kinds of morphologies, micrometer-sized silica spheres are very promising because they can be easily packed into existing reactors, columns, or fixed and fluidized beds. However, for most of the existing synthesis methods, the formation of spheres usually has to be sacrificed for ordered mesostructure. Therefore, it is still a challenging work to synthesize micrometer-sized silica spheres with both uniform spherical morphology and ordered mesostructure.

Benefiting from the hydrogen bonding assistant assembly of PEG (poly(ethylene glycol)), micrometer-sized silica spheres with uniform spherical morphology and highly ordered mesostructure have been successfully synthesized with a two-stage synthesis process: the prehydrolysis of tetraethyl orthosilicate (TEOS) in an aqueous acidic solution (pH=1) using F127 as the template to obtain a stable sol, and afterward the fabrication of silica spheres by direct hydrothermal treatment. PEG is a water-soluble hydrogen-bonding polymer, whose hydrophilicity decreases with the increase of temperature. During the hydrothermal treatment, the stretching structure of PEG will change to constrictive structure, and consequently the formation of spherical morphology is facilitated by the hydrogen bond between PEG and colloid particles. The effect of the reactant composition on the particle morphology was investigated by scanning electron microscopy (SEM) observation, and the results showed that the particle morphology was sensitive to the pH and the PEG concentration.

The prepared silica spheres characterized by nitrogen adsorption and desorption, small-angle X-ray diffraction (XRD) and transmission electron microscopy (TEM), exhibited high surface areas (~760-1040m2/g), large pore volumes (~0.48-0.84mL/g), extremely narrow pore size distributions and highly ordered mesostructure. Finally, the mechanism of this synthesis route is discussed and a ‘hydrothermally induced colloidal coagulation and precipitation' mechanism is established.