Greener Silica

MATERIALS PROCESSING

A new method for synthesizing mesoporous shells of silica combines surfactant templating and supercritical fluid processing.

"We have managed to use supercritical CO₂ as a benign internal phase and swelling agent in the formation of hollow spheres of silica with large mesopore wall structures," says Robert Mokaya, a reader in materials chemistry who led the University of Nottingham, England, group that developed the technique [Chem. Commun., published online Dec. 3, http://xlink.rsc.org/?doi=10.1039/b413820a].

"Hollow spheres of mesoporous silica are likely to find application in catalysis and drug delivery and as hosts for occlusion compounds in advanced composite materials," Mokaya notes. "With our technique, we are able to control both the pore size and morphology of the mesoporous silica by varying the pressure of the supercritical fluid."

The Nottingham chemists prepare the spheres, which have average pore diameters of 10 nm, by adding tetraethyl orthosilicate to an aqueous solution of a triblock copolymer: poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide). They then heat and pressurize the mixture in CO₂ in an autoclave to form an emulsion of CO₂ droplets in water. The copolymer acts as a surfactant at the CO₂/water interface. After pressure is reduced, the product--a white powder--is separated from the mixture by filtration. Organic material is removed by heating the powder in air to 500 C.

Other emulsion-templating methods for preparing hollow spherical materials are more complicated and less environmentally friendly, Mokaya points out. They typically employ large quantities of water-immiscible oil or an organic solvent, such as 1,3,5-trimethylbenzene, as an internal phase.

"Our synthesis method might be extended to mesostructured hollow spheres of other (nonsilica) metal oxides or even nonoxide frameworks," Mokaya suggests. "The challenge here is to find suitable precursors from which to construct the nonsilica frameworks."