Roomy Zeolite

Like engineers who design superhighways with spacious lanes and broad on- and off-ramps, chemists have prepared a new crystalline material that features interconnecting channels and unusually large pores. The study may lead to porous catalysts that facilitate a smooth flow of molecular traffic and exhibit an exceptional degree of internal openness.

Crystalline porous materials are widely used in purification, gas separation, and catalysis. In particular, zeolites, a family of aluminosilicate-type compounds, are readily converted into solid acids and used as catalysts in the petroleum and chemical industries. For some applications, such as hydrocarbon cracking, such solid-acid catalysts are effective only with small molecules, because the solids' internal dimensions limit larger molecules' access to the acidic sites inside the pores.

Researchers have tried various synthesis strategies for expanding the size of zeolite pores but have achieved only limited success. Now, chemists at the Spanish National Research Council and Polytechnic University of Valencia, in Spain, have created a silicogermanate compound that answers the call for a large-pore zeolite.

The new material features straight channels with circular openings measuring 12.2 Å in diameter that are interconnected by smaller channels (5.6 Å diameter) that run perpendicular to the larger channels (Nature 2006, 443, 842). Dubbed ITQ-33, the new porous crystal was prepared by Avelino Corma, a professor in Valencia's Institute of Chemical Technology; María J. Díaz-Cabañas; José Luis Jordá; and their coworkers.

In a commentary appearing in the same issue of Nature, Raul F. Lobo, a professor of chemical engineering at the University of Delaware, points out that other research groups have prepared large-pore zeolites but that ITQ-33 stands out because of its superior properties. For example, aluminophosphates and other phosphate-based large-pore compounds have been reported, Lobo notes, but those materials exhibit low thermal stability and low acidity.

Likewise, a gallium silicate compound with large pores was prepared a few years ago, but compared with ITQ-33, the gallium material is relatively unstable under reaction conditions, Lobo says. Furthermore, the new Valencia zeolite offers the added benefit of a system of interconnecting channels. That feature, which qualifies ITQ-33 as the zeolite with the largest fraction of internal empty space, may enable molecules to bypass blocked passageways in favor of unobstructed routes.

On the basis of tests involving dealkylation of bulky aromatic compounds, cracking of petroleum oils, and other reactions, the Valencia team reports that the new zeolite is very active catalytically and, in some cases, outperforms commercial catalysts.

ITQ-33 has "great potential as an industrial catalyst," Lobo says. But he cautions that cost reductions and other improvements are needed to make the catalyst practical.