Perfecting Porosity.

Better living through holey chemistry

Some chemists worry plenty about nothing.... Well, almost nothing. It's the holes inside solid materials that are on their minds.

Such porous materials can look like ordinary rocks, but they're infiltrated with invisible nooks and crannies. Like kitchen sponges, these materials can soak up liquids with a marked thirst. And they do so with an intriguing finesse.

The best known and most widely used porous solids today are crystals called zeolites. They occur naturally, but scientists also synthesize them. Zeolites' role in the transformation of raw petroleum into products like gasoline has made them central players in the oil-refining business. Labyrinthine channels in the crystals trap and help break down large molecules in oil.

More recently, zeolite crystals have served as mini-test tubes in which scientists have grown the smallest possible carbon nanotubes--microscopic cylinders of graphite that possess unusual electronic properties and could serve as the basis of future nanoscale electronics (SN: 12/16/00, p. 398).

Yet materials scientists know there's more scientific and technological gold to be found in porous materials. Chemists are eager to improve on the holes that make porous solids so valuable. Compared with the pores available today, bigger crystal cavities, for example, would be expected to trap larger molecules.

In their search for these payoffs, researchers have been combining geometry and chemistry to design their own porous solids. Such materials, they say, could be built with chambers of just the right size, shape, and chemistry to attract and break down pesticides, for example, or to capture radioactive pollutants. Large pores created with particular chemistries might act as minireactors to promote reactions that would never occur in a bench-top beaker.

There's yet another attractive aspect of designer porous solids. The huge amount of empty space in such materials enables them to absorb a nearly unimaginable amount of gas. "We have materials that can take a whole building full of gases and put it in one gram of material," says Omar Yaghi of the University of Michigan in Ann Arbor. "You could use it to store a fuel, let's say, and then release it later."

"The fact that you can vary the composition, structure, pore size, shape, and function [of porous materials] makes me think there's a great potential beyond zeolites," Yaghi says. "You can put together whatever building blocks you want and produce a material at will.... So, you've achieved the state that we've been trying to achieve for a long time. You've achieved tailored materials."

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Researchers designing porous solids are pursuing several strategies. For example, Achim Muller of the University of Bielefeld in Germany is creating porous materials by joining together smaller building blocks. He calls one of the types of block that he's created "a big wheel." Constructed of hundreds of molybdenum and oxygen atoms, it's 4 nanometers wide. Muller says that by stacking these wheels, he can form channels.

The space inside the channels is electron-rich, unlike zeolite pores, which are electron-poor, Muller notes. So, the big-wheel channels might catalyze chemical reactions that can't occur in zeolites. What's more, zeolites are monolithic structures that can't be broken into smaller units, whereas researchers could link big wheels to create materials with specific, predetermined internal structures, he says.

M. Ishaque Khan of the Illinois Institute of Technology in Chicago has developed a similar construction method, which he refers to as "Lego-chemistry."

"If you know how to glue molecules [together] of different kinds, you can come up with almost any kind of material to cater to the needs of industry and

society," Khan says.

The challenge is that researchers don't yet know how to stitch all their new building blocks together. Recently, Khan's team created compounds from inexpensive polyoxometalates, which are chemical clusters made from oxygen atoms and transition metal elements, such as vanadium and molybdenum. Polyoxometalates are a particularly good choice for the blocks, Khan says, because they come in many sizes and shapes suitable for building a variety of porous materials.

Khan and his coworkers are pursuing other synthesis methods involving interlinked oxometalates, as well. In one, they assemble a structure around template compounds, such as amines, that initially fill the spaces destined to be pores. Sometimes, however, the structure crumbles as the templates are removed, he notes.

In fact, such stability has been a universal problem for those trying to design materials with bigger pores. Crystals with very large pores generally aren't rigid enough to support themselves unless they have molecules, called guests, filling the cavities.

For a porous material to be as useful as possible, guests must be able to check in and out without the crystal collapsing. That way, both the porous material and its guests could even be reused. Such guests might be valuable materials being stored or contaminants extracted from the environment.

Chemists designing materials with large pores face another formidable challenge. Crystals with bigger pores require a framework of more atoms surrounding each pore, explains Gerard Ferey of the Institut Lavoisier in Versailles, France. But in a large framework, the chains of atoms twist, interlock, and generally clog up the would-be pores. "Nature hates a vacuum," Ferey recalls.

This tendency to clog space instead of forming b pores leaves little room for guests to enter or reside in a new material.

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These difficulties haven't dashed the hopes of pore designers. Yaghi and his colleagues have recently found a way to create porous materials that seems to alleviate the two challenges of instability and interlocking. Over the years, researchers have generally produced porous solids "through a process that many have called shake-and-bake or stir-and-wait," says Yaghi. "It was more of an art than a science. There was no design involved."

Not so for Yaghi, Michael O'Keeffe of the Arizona State University in Tempe, and their team. Their protocol begins with a choice of one out of a dozen simple, three-dimensional molecular topologies, such as the architecture of [Pt.sub.3][O.sub.4]. Then, they make substitutions for each platinum or oxygen atom by selecting complicated components. Only then do they determine the specific chemical steps required to stitch those complex parts together.

The team, "uses the same topology as the very usual structure from which it starts," says Ferey.

This technique--choosing a material's topology and characteristics before building it--is similar to a method often used already by medicinal chemists to design drugs, says O'Keeffe. "Organic chemists design a drug, and then they design a synthesis for it," he says.

Two years ago, Yaghi and O'Keeffe used the approach to create the first nexus of metallic and organic components, a metal-organic framework (MOF), that didn't collapse when guest molecules left the structure (SN: 11/20/99, p. 327). Due to its porous nature, one ounce of this material, called MOF-5, has 19 times the surface area of a football field.

"You're looking at a situation where one kilogram of MOF could contain several tanks of methane," Yaghi says.

The researchers have since demonstrated with an even newer material that interlocking frameworks don't always clog up. In the substance MOF-9, the metal-organic framework intertwines but still has accessible pores.

Even more recently, they've synthesized MOF-14, which embodies an intertwined metal-organic framework with extremely large, unclogged pores, says Yaghi. In the Feb. 9 SCIENCE, the researchers reported that the chains of atoms in the new material don't seriously block the crystal's pores, thus making available the biggest holes yet in a metal-organic framework--each more than 1.6 nanometers in diameter.

Now, the Yaghi team is busy using its technique to predict and create even more materials. "This is a breakthrough kind of chemistry," Yaghi claims. "You can design something unknown to inorganic or solid state chemists."

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