Porous solids with enormous cavities might well solve two problems confronting chemists this century: greener catalysts for industry and better ways of storing gases, not least of which is hydrogen. Paper #1 reports what is currently the structure with the largest cavities of all, and at 700,000 ų it is several times larger than any previously known compound. It has an unprecedented surface area of 5,900 m² per gram and, moreover, it is a crystalline solid, which means its exact atomic structure can be measured. The material can be made from readily available chemicals.

Paper #1 is a joint effort led by Gérard Férey, based at the Institut Universitaire de France, plus collaborators at the Institut Lavoisier of the University of Versailles, the Royal Institution in London, and the European Radiation Synchrotron Facility at Grenoble.

The compound in question is coded MIL-101 (short for Matériaux de l’Institut Lavoisier no. 101) and consists of a framework of chromium ions interconnected with terephthalate groups. The compound was made by heating an aqueous solution of chromium(III) nitrate, terephthalic acid, and hydrofluoric acid at 220^oC for 8 hours. The green crystals which formed contained a multitude of water molecules, some of which were integral to the structure but most of which were there as guest molecules and easily removed. MIL-101 is stable in air and even when it was heated to 350^oC, and the guest waters evaporated, its framework did not collapse.

MIL-101 has two kinds of cages within its structure and into which other molecules can penetrate. The larger of these cages has an internal diameter of 34 Å, the smaller 29 Å. The volumes of these cavities are 20,600 ų and 12,700 ų respectively. The openings which allow access to them have different shapes, the smaller cavities have pentagonal windows with a span of 12 Å, while the larger ones have hexagonal access with the widest dimension being 16 Å.

Férey and coworkers were naturally keen to discover how large a unit might be able to enter MIL-101, and they chose a tungsten-phosphate polyion—formula K₇PW₁₁O₄₀—which has an overall dimension of around 13 Å. This was left in contact with MIL-101 in aqueous solution for two hours and the product analyzed by various techniques, including ³1P NMR, which showed large amounts of the polyion had indeed penetrated its pores. (The ³1P NMR showed a single peak thereby confirming that the polyion was still a discrete entity.)

So what might MIL-101 be used for? Clearly it could accommodate lots of small molecules such as gases. Larger molecules, even nano-sized ones such as proteins or enzymes, might well be enclosed. Alternatively it could provide an environment in which chemical reactions might be promoted by bringing molecules closer together and so act as a catalyst.

The strategy that led Férey to MIL-101 can be read in three back-to-back papers in a single issue of Angewandte Chemie International Edition (43[46]: 6285, 6290, and 6296; 2004). The last of these is the key paper, he says, producing as it did the solid MIL-100. This was the forerunner to MIL-101 but had smaller cavities. Speaking to Science Watch, Férey hints of an even more spectacular solid with a volume of 1,700,000 Å that might soon be announced.

Currently his group is looking at ways in which these solids can be used for hydrogen storage, or for the confinement of organic molecules, as well as continuing to create new solids. "I would like to better understand the interactions between guests and hosts, and we are developing different methods—structural, spectroscopic, computational—for doing this," he says. Meanwhile they have discovered another remarkable solid which can "breathe"—in other words, increase its volume threefold without breaking its chemical bonds.