Volume 6, Number 4, 1997

Large-pore catalyst shows promise for processing fuel or pharmaceuticals

A new material-developed by a research team including a University of Delaware faculty member-may someday result in more efficient, less costly catalysts for generating gasoline, medicines and other products.

Raul F. Lobo, assistant professor of chemical engineering, recently helped synthesize a catalytic material with unusually large atomic pores or "cages" that might prove useful for capturing bigger molecules in hydrocarbon fuels and pharmaceuticals.

Lobo and his colleagues at the University of Texas-Dallas, the California Institute of Technology and the University of Massachusetts are excited about the new aluminosilicate "zeolite" because it features pores outlined by 14-atom rings-possibly the largest to date.

The material also remains stable at temperatures up to 900 degrees Celsius, roughly two times hotter than the temperatures required for industrial catalysis, Lobo reported in the May 1996 journal, Nature. Researchers have previously made very large-pore zeolites by combining alumina with phosphates (rather than silicates), he adds, but those materials tend to fall apart in such hot environments.

"Some aluminophosphate zeolites have been made with larger pores than aluminosilicate zeolites," he says, "but these phosphate-based structures are thermally unstable. To make a zeolite react, you have to heat it up."

Catalysis, from A to Zeolite

In Greek, the word zeolite means "boiling stone." That's because "many zeolites appear in nature, and they're full of water, which makes steam," Lobo explains. Engineers now use synthetic zeolites, sometimes also described as molecular sieves, to capture desirable molecules
of a certain size.

Heating a water-laden zeolite triggers chemical reactions that pull molecules into the material's pores. For this reason, zeolites are used for "cracking" the large molecules in crude oil, to generate desirable products such as gasoline, diesel fuel and kerosene. "The size of the molecule you can crack depends on the pore size of your zeolite," Lobo noted. "The market for high-quality gasoline is much bigger than the market for diesel fuel and kerosene, but existing catalysts produce larger amounts of those lower-grade products."

Rugged, thermally stable zeolites with larger pores would make it easier and less expensive to produce high-octane gas, he says. Other applications for the zeolites may include pharmaceuticals based on very large molecules.

Lobo helped synthesize the large-pore zeolite by using a material called bis-(pentamethyl-cyclopentadienyl)- cobalt(III) hydroxide as a kind of "template." First, this template material was heated to 175 degrees C. in a sealed oven. Then, it was hardened and converted to cobalt oxide by exposing it to air at 550 degrees C. Finally, the cobalt oxide was scrubbed from the template using hydrogen chloride, leaving behind a zeolite containing a significant amount of silica, the rugged crystalline material in glass. An evaluation of the resulting material, based on X-ray diffraction patterns and other analyses, revealed wide atomic rings.

Before the material can be developed for use in an industrial setting, the resesarch team must still overcome several technical hurdles, Lobo says. The template material is expensive, he says, and "we can't reuse it because we're essentially burning it." Also, the researchers have "solved" most of the structure, meaning that they understand what it looks like at the microscopic level. But, some aspects of the zeolite's structure remain a mystery, and Lobo concedes that "it could take years" to fully develop the material.

Lobo and his colleagues are optimistic about the work, however. "Given the current price of the structure-directing agent that we're using," he says, "this material would probably be most useful in the pharmaceutical industry, at least initially. Later, we hope it could be developed for use in petrofuel production."

--Ginger Pinholster