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Porous Materials Contains Catalytically Active Channels
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August 20, 2001
Volume 79, Number 34
CENEAR 79 34 p. 15
ISSN 0009-2347
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Molecular sieve pore contraction can be finely tuned for gas separation


A series of microporous titanosilicates with finely tunable pore sizes is suitable for commercially important separations of mixtures of gas molecules of nearly identical size, according to the researchers who developed the materials.

In what the group calls the "molecular gate" effect, the pores of the materials can be systematically adjusted to allow access to molecules of one gas while shutting out slightly larger molecules of another. The work was carried out by research fellow and manager Steven M. Kuznicki and research associate Valerie A. Bell at Engelhard Corp., Iselin, N.J; associate professor of chemical engineering Michael Tsapatsis at the University of Massachusetts, Amherst; and coworkers [Nature, 412, 720 (2001)].

"The work is both novel and significant," notes Colin S. Cundy, associate director of the Centre for Microporous Materials at the University of Manchester Institute of Science & Technology (UMIST), in England. "It promises to be a new departure in molecular sieve technology and an intriguing new area for academic investigation."

The materials are prepared by dehydrating a titanosilicate known as ETS-4 (Engelhard TitanoSilicate-4) that was discovered by Kuznicki in Engelhard's laboratories and patented in 1990. The ETS-4 framework is built of tetrahedrally coordinated silicon atoms and octahedrally coordinated titanium atoms. ETS-4 normally collapses to an amorphous material on heating. However, by exchanging sodium cations for strontium cations and then carefully heating the material to control the dehydration, the group is able to produce stable materials with reduced pore sizes.

"We show that ETS-4 undergoes a systematic, uniform, and controllable pore contraction with structural dehydration to form a new series of materials, which we denote as contracted titanosilicates, or CTS," Kuznicki tells C&EN. "The range of effective pore contraction is governed by cationic content."

Tsapatsis points out that CTS framework contraction can be systematically manipulated to separate molecules with dimensions in the range of 3 to 4 Å. CTS can be used, for example, to separate mixtures of nitrogen and oxygen molecules with lengths of 4.1 and 3.9 Å, respectively.

"Although similar pronounced framework flexibility has been observed in some aluminosilicate zeolites, this is the first time that it has been reported for a mixed octahedral/tetrahedral molecular sieve," Tsapatsis says.

One of the initial targets of the work has been the separation of nitrogen from methane in natural gas.

Nitrogen occurs commonly in natural gas, to the extent that it renders many natural gas reservoirs unusable, the authors note. They add that by manipulating CTS pore size to exclude methane while allowing adsorption of the smaller N2 molecules, it is possible to reduce initial N2 content of natural gas at wellhead pressures from 18% to less than 5%, with a methane recovery of at least 90%.

"This system for the purification of natural gas is currently under commercialization by Engelhard," Kuznicki explains. "Nitrogen molecules are only about 0.2 Å smaller than methane molecules. The separation is possible because of the extreme precision with which we can tune the pore size of CTS."

MOLECULAR SIEVE Framework of titanium (blue), silicon (green), and oxygen (red) atoms contracts on heating--at room temperature (left), d=4.27Å; at 250 °C (right), d=3.94Å.

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