Solid Has the Most Spacious Pores

NANOMATERIALS

One could say that mil-101 is mostly nothing, but that's saying a lot: MIL-101 is a new, unusually porous material whose unit cell has an unprecedented volume of about 702,000 ų, meaning that the solid is about 90% empty space once the solvent molecules normally filling its pores are removed. It also boasts pores that are 29 or 34 Å across and an internal surface area of 5,900 m²/g (Science 2005, 309, 2040).

"A tablespoon of MIL-101 has the surface area of a half-dozen football fields, or about seven times the area of the most catalytically effective zeolites," notes a Science commentary authored by professors Joseph T. Hupp and Kenneth R. Poeppelmeier of Northwestern University's chemistry department.

For comparison, cloverite has held the record since 1991 for the largest pores (close to 30 Å across), but its cell volume is only about 125,000 ų.

Scientists are interested in highly porous materials for use in catalysis, separations, gas storage, sensors, and other applications. And MIL-101 appears to be a promising candidate for some of these uses, according to its discoverers--a team led by solid-state scientist Gérard Férey of the Lavoisier Institute at the University of Versailles, in France.

MIL-101--the letters stand for Matérial Institut Lavoisier--consists of inorganic chromium clusters linked by organic moieties. The key building block is a tetrahedral supercluster consisting of four smaller clusters (triads of oxochromium octahedra) that are held together by terephthalate (1,4-benzenedicarboxylate) groups. These superclusters form an extended framework containing cages of two different sizes.

The solid was prepared by heating terephthalic acid, chromium nitrate, hydrofluoric acid, and water at 220 °C for eight hours.

For a complex material having very large cells and pores like MIL-101, Férey explains, it's usually not possible to figure out the structure by using conventional single-crystal X-ray methods because the material is available only as a microcrystalline powder. So, with computer scientist Caroline Mellot-Draznieks, his team developed a computer program that provides structural information and simulated powder X-ray diffraction patterns for the architectures most likely to form (based on their energetic stability). By comparing the experimental powder X-ray pattern obtained for MIL-101 to the simulated patterns, the researchers found a match.

Férey is interested in using the pores of materials like MIL-101 as nanoreservoirs and nanoreactors. By introducing "nano-objects" into the pores, one could make a variety of nanomaterials consisting of uniform particles 2 to 3 nm in size. Larger species could be segregated in the large cages; smaller species, in the small cages. This approach could lead to new types of multifunctional nanomaterials.

The French team has already begun to explore some of these possibilities by filling MIL-101's pores with species as diverse as tungsten polyoxoanions, hydrogen gas, semiconducting zinc sulfide nanoparticles, and ibuprofen.

"It is the very beginning," Férey tells C&EN, "and the only limits, besides steric hindrance of the guests, are those of our imagination."