Hole new approach

French engineers have developed a technique for designing and making a porous compound
they claim is the most spacious solid material ever.

Nanoporous materials, riddled with networks of holes of different shapes and sizes, are extremely important in many branches of engineering. The chemical industry, for example, uses them to support catalysts; they form the basis for many types of filters; they are vital components for molecular sensors and other electronic components; and they are likely to be the basis for gas storage systems for fuel cells.

But making them is a tricky business, because their properties are dramatically altered by the sizes of their pores: this is the deciding factor in determining which substances can be trapped inside the structure, whether as part of a separation process or to act as their support. Large pores are particularly useful, but they’re also the most difficult to produce.

A team from the University of Versailles has combined computer simulation with synthetic chemistry to develop a new way to design and make some materials, and believes it has tailored a material with important applications across a variety of fields.

Gerard Férey and colleagues from the Lavoisier Institute at the university used chromium terephthalate, a compound containing both metals and organic chemistry, to make what they claim to be the most spacious solid material ever.

Going under the under the name of MIL-101, the compound consists of 90 per cent empty space, and has pores between 3–3.5nm across. Every gramme of the material has an internal surface area of 5,900m2, which means that a tablespoonful has a surface area equivalent to six football pitches.

The pores are of different, but regular sizes. This, Férey believes, means that it has potential in nanotechnology; different-sized materials with various functions can be trapped in the holes, creating a multifunctional nanomaterial, which could be used for applications such as drug delivery and diagnostic techniques.

The team had to devise three new techniques for making the substance. First, they made the building blocks from which the porous structure is constructed, then they developed the chemistry to put these together. T

he process to make the material turned out to be rather simple — the team heated a mixture of water, terephthalic acid (a main constituent of the common plastic PET), chromium nitrate and hydrofluoric acid to 220°C, and held it at the temperature for eight hours.

The trickiest step was to figure out exactly what they had made. Normally, X-ray diffraction would be used to determine the structure of a crystal, but MIL-101 can’t be grown as a single perfect crystal. So instead, the team applied the X-rays to a powder of the material.

Working with computer scientist Caroline Mellot-Drazneiks at the institute, the team worked out what would be the most likely structures to form from the various building-blocks, and simulated X-ray diffraction patterns from these. They then compared the real diffraction pattern to the simulation, and found that the MIL- 101 pattern was an exact match.

Férey has already started investigating some of the properties of their roomy solid. So far, they’ve filled the voids in the material with hydrogen gas, tungsten polyoxoanions, which can be used in water purification, semiconducting zinc sulphide, and the painkiller ibuprofen. And there are many more molecules that could fit the bill, said Férey.