Chemists create the first of a new class of catalysts to handle big molecules

November 16, 1999

Chemistry may not grab the kind of headlines that computers do, but it's what makes modern life happen, from gasoline to plastics to the material of computer chips. Chemical catalysts are special molecules that encourage chemical processes and are one of the most critical components of our technological culture. These chemical movers and shakers are the unsung heroes of our chemistry-based civilization.

A team of chemists working at Arizona State University has designed and synthesized the first stable example of an important new class of materials that are expected to be of special use as catalysts. The new material, and other materials that can now be created following its model, should be useful in isolating and modifying molecules that previously known catalysts have not been able to handle, particularly larger molecules.

The team, composed of ASU chemists Hailian Li and Michael O'Keeffe and University of Michigan chemists Mohamed Eddaoudi and Omar M. Yaghi, report the synthesis in the November 18 issue of the journal Nature.

The new material, formed from zinc oxide and terephthalic acid, is a "porous material" -- essentially a chemical framework with large box-like spaces between its components though which other molecules can enter. The material belongs to a class of materials known as "metal-organic frameworks." Though metal-organic frameworks have been studied by chemists for some time, this is the first such material ever developed that is stable in the absence of other molecules.

Metal-organic framework materials are synthesized substances believed by chemists to have many of the functional capabilities of zeolites, a very important class of porous materials used as catalysts by industry in a wide variety of applications that range from petroleum refinement to laundry detergents. As they have similar framework structures, metal-organic framework materials are sometimes referred to as "metal-organic zeolites" although they are not true zeolites.

"A major push in chemistry right now is to make new porous materials -- materials with open frameworks," O'Keeffe said. "An important use of framework materials is to separate molecules -- they're sometimes called 'molecular sieves' -- a big molecule won't go through, but a little one does. A pressing current problem is what do you do with the big ones?

"There's a natural barrier to the size of the openings in zeolites, so chemists have looked to other materials, to other ways of making frameworks," explained O'Keeffe. "One avenue being explored is using metal atoms and organic molecules -- so called metal-organic frameworks -- which could have much larger pores.

"The problem has been that the frameworks made previously have been unstable. If you remove material from the central pores, they begin to collapse. The trick is to find something that's stable."

The new material, which has a double bond linking the organic acid to the zinc oxide, is exceptionally stable. "You can pump out the material and leave it completely empty and it's still stable," said O'Keeffe. "It's even stable in a vacuum. All sorts of molecules can now be put in and out of its structure. It's a molecular sieve that can handle quite large molecules."

The material retains its integrity at temperatures of up to 300 degrees centigrade. "This is the first metal-organic material of this sort that is stable," said O'Keeffe. "It's really spectacular -- it's very stable and yet it's very nearly the lightest crystalline material ever made. It's also very practical: the component chemicals are common and cheap and non-toxic."

Though the usefulness of this particular material is still incompletely determined, O'Keeffe points out that it is potentially a major breakthrough for the chemical industry. The synthesis of a stable metal-organic framework opens the way to developing a whole new generation of catalytic materials capable of performing chemical tasks that, until now, have been impossible. Possible applications for this new chemical technology include advanced petrochemical refinement, pollution remediation, and cheaper storage methods for natural gas.

"If you can develop more open materials like this one, the range of things you can do chemically will be greatly increased. A lot of chemists have been striving for something exactly like this for some time now, so this is a moment that everyone's been waiting for," O'Keeffe said.

The new material was developed by the Materials Design and Discovery Group at ASU's Department of Chemistry and Biochemistry. The research was funded by the National Science Foundation.

Arizona State University

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