Chemist Stephen Lee is exploring new building blocks to the future

Professor Stephen Lee in his office in Spencer T. Olin Laboratory. Charles Harrington/University Photography

By David Brand

Stephen Lee is seeking materials that some say can't exist, but if they did would have extraordinary properties. They would be crystal structures that could bind to any specific molecule, whether it be a metal or a gas.

"One of my colleagues mentioned that if you could just make a site that would bind to gold preferentially to other ions, you could separate out gold in sea water," he said.

More feasibly, Lee is looking for materials that would seek out and remove organic compounds, including such hazardous chemicals as dioxins and biphenyls, or would be able to hold catalytically active metals such as palladium and silver. But, Lee confessed, "there are many people who still think this is hopeless."

Lee recently joined Cornell as a professor of solid state chemistry in the chemistry and chemical biology department from the University of Michigan, where he had been associate professor of chemistry since 1993 and where he was a both a MacArthur and a Sloan fellow. It is something of a homecoming for Lee: He was a visiting scientist at Cornell in 1995.

Until four years ago, Lee's background was entirely in solid state chemistry, but he became interested in the field of organic -- carbon-based -- building blocks and began investigating how such molecular units are packed together. One potential advantage of organic molecules is that the researcher has much more control of both size and shape than with their inorganic counterparts. If a way could be found of constructing an organic porous solid, Lee reasoned, it could be tailored to a great number of industrial uses. Its pores, for example, could be fashioned into the size and shape of a specific molecule. The problem was, and is, it has never been done.

Industry makes wide use of inorganic porous solids, called zeolites, for catalysis, separation and storage. Just one example is their use in separation of petroleum feed stocks. The big problem with these inorganic solids, said Lee, is that the porosity can't be manipulated for specific uses. Organic molecules, because of their greater diversity, would give researchers many more tools in the control of their flow. Lee uses the example of trying to channel water from a high elevation to a lower one. The intention is to build a channel that will guide the water in a controlled flow.

The problem in making an organic porous solid is that it hasn't yet been produced as a stable crystal. Inorganic zeolites, by contrast, are as stable as glass. Current organic porous solids still dissolve in a fair number of solvents.

In his Cornell lab, Lee's research team is following a two-step approach. In the first step, researchers D. Venkataraman and Geoffrey Gardner, together with Jeffrey Moore's research group at the University of Illinois, linked together organic molecules containing benzene rings with silver ions and benzene itself. When the benzene was heated and boiled off, uniform-size pores were left. These pores, however, are still not sufficiently stable for many applications. The problem is that the links of the silver to the organic molecule are too weak to hold up to many applications. Said Lee: "It's like people making a chain by holding hands; if one person loses a grip, the chain is broken and the network falls apart."

Now Lee and graduate students Yuan-Hon Kiang and Zhengtao Xu are taking the second step by adding alcohol molecules to the initial organic molecule. These alcohol molecules can be used to react with cross-linking agents. Such agents are widely used in polymers to make the material more rigid and more stable. Lee's group hopes to see the same effect in the new porous materials.

"This is only the first generation. By the second or third generation we hope to be able to generate the first truly robust porous organic solid," he said. "If you can really succeed in making organic solids stable enough to do reactions in, I see no reason why applications shouldn't be at least as big as with current inorganic solids. And potentially you would have much more control."

Lee's work is supported by a grant from the National Science Foundation and by Cornell's ambitious materials research program, which drew him to the campus in the first place.