ne of the most active areas of chemistry is new materials, and one of the chemical elements that continues to surprise is silicon, and especially its oxide, silica. Modified forms of this are important for living organisms, from the nettle’s sting to the marine sponge, as well as for industry, where it can be used to make extremely tough ceramics or, as a zeolite catalyst, to convert waste gas into gasoline. Its catalytic activity stems from its having an open framework of pores, channels, and cavities into which small molecules can penetrate, undergo complex reactions, and emerge as useful products.

   What limits the use of porous silica is the size of the channels, because this restricts the molecules that can gain access to the active sites in the cavities. So-called mesoporous silica has channels up to 10 Å in diameter, but paper #6 in the current Hot List reports a version in which they are more than 30 times wider. It was made in the laboratories of Brad Chmelka and Galen Stucky, who are among the world’s leading chemists in the field.

   Stucky’s and Chmelka’s group is at the University of California, Santa Barbara, and it was there that the remarkable material, code name SBA-15, was tailored to give pores of uniform sizes, from 75 to 320 Å, depending on the temperature at which they were grown (between 35 and 80° C), and the solvent medium from which they crystallized. The secret of Stucky’s process lies in the addition of a few percent of poly(alkylene oxide) block copolymers to direct the construction of the new material.

   A typical copolymer consisted of a central chain of poly(propylene oxide) with add-on chains of poly(ethylene oxide) at either end. The nature of this tri-block polymer partly decides the pore size, although this is also influenced by adding trimethylbenzene to the medium from which the crystals grow. The structures of the various types of SBA-15 were analyzed by X-rays, scanning electron microscopy, and transmission electron microscopy, which showed highly ordered structures. Paper #6 reported that the block polymer could be extracted and reused, and that the new mesoporous silica is robust enough to withstand prolonged heating in boiling water, or temperatures up to 500 °C, and still retain its wide channel structure.

   Stucky and Chmelka liken what they are doing with silica to the biological processes that Nature uses to make silica systems, and they are investigating the role that biological polymers play in the assembly of natural 3D structures. In paper #6 they showed that Nature’s strategy could be applied using low-cost block polymers, literally to open up the range of mesoporous silicas, with the added bonus of finding that these too were remarkably stable.

   More recently Stucky has published on other silica systems: mesoporous fibers that act as a new laser material (Adv. Mater., 2 June 1999), membranes (Chem. Mater., 11:1174, 1999) and foams (J. Amer. Chem. Soc., 121:254, 1999). Meanwhile his group has been investigating how extracts from a marine sponge direct the way in which silica polymerizes (Proc. Natl. Acad. Sci. USA, 96:361, 1999).

   Asked what uses the large-pore silica of paper #6 might have, Stucky tells Science Watch: "Companies are exploring their use for selectively separating large biological molecules, as biocatalyst supports, for environmental remediation, as computer chip resists, as catalysts, as sealants, and as optical coatings, but these materials were first made only a little over a year ago, so there are no commercial applications on the market yet."

   Paper #7 also describes a newly modified mesoporous silica-type material, in the cavities of which are propyl mercaptan molecules. These are directly bonded to the silicon atoms at the propyl end of the molecule, leaving the mercaptan (SII) unit free to react, for example to capture a mercury or a lead atom. This research, under the direction of J. Liu at the Battelle Memorial Institute’s Pacific Northwest National Laboratory in Washington state, has environmental implications since it offers a way of dealing with contaminated waste-waters. A single treatment with their mesoporous material reduced mercury- or lead-polluted water to drinking-level standards–and it could be regenerated by washing with acid to remove the trapped metal.