Highly porous crystals are sought after for a variety of industrial applications, from storing methane and hydrogen in fuel cells to refining petroleum. Now researchers report the most porous crystals yet--with a surface area equivalent to that of 17 tennis courts in each gram of material. That's more than a five-tennis-court improvement over the previous best.
Industry has typically relied on porous minerals called zeolites since the 1930s. The crystals tend to have pores of limited shape and size, however, and are difficult to customize. One hurdle in building roomier crystals is coming up with molecular building blocks that don't intertwine. The corners of empty cubes, for example, invade each other, filling up the interiors of the cubes.
Trefoil-shaped carbon structures, on the other hand, can be arranged so as not to overlap, chemist Omar Yaghi of the University of Michigan, Ann Arbor, and colleagues report in the 5 February issue of Nature. To design the building blocks for their crystals they chose a shape that would expose as many crystal atoms as possible to the pores inside--one measure of surface area. These calculations suggested that crystals built of triangular subunits, each made of four hexagonal carbon rings, would have a large surface area and yet be relatively easy to synthesize. They linked the subunits with clusters of zinc acetate to create a scaffold riddled with wide channels, then soaked the crystals in various large organic molecules to test their sponginess and ability to sort by size.
"This is a major synthetic achievement," says chemist Raul Lobo of the University of Delaware in Newark. "What is really neat is [just] about every atom is exposed to molecules in the pores," compared to half or less in zeolites, he says. He adds that the crystal might serve as a protective layer for organic LEDs or act as a sieve for separating larger drugs and other organic molecules than is currently practical. "The applications for this thing are kind of endless," says chemist Christopher Cahill of George Washington University in Washington, D.C. Choosing the right linker molecule between subunits could lead to even roomier crystals or give them other useful chemical properties, he says.