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March 25, 2002
Volume 80, Number 12
CENEAR 80 12 p. 8
ISSN 0009-2347
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Benzene-silica hybrid assembles into framework with crystalline pore walls


Whether plant breeder or chemist, hybridizers hope to blend the best features of the parents into optimal offspring. That's what a team of scientists in Japan has accomplished with the creation of a benzene-silica hybrid that assembles itself into a crystalline mesoporous material.

The new material exhibits the periodic array of medium-sized pores that have made inorganic mesoporous molecular sieves so useful. Most striking, the pore walls themselves are completely regular, composed of orderly layers of benzene rings linked to alternating layers of silicate chains. The benzene rings can be functionalized, opening the door to the vast range of organic transformations that can lead to specially tailored properties [Nature, 416, 304 (2002)].

"To date, all conventional mesoporous materials have had amorphous pore-wall structures that restrict their applications," notes chemist Shinji Inagaki, leader of the Frontier Research Group at Toyota Central R&D Laboratories, Nagakute. Inagaki synthesized and characterized the new material with visiting researcher Shiyou Guan and Tohoku University physics professors Tetsu Ohsuna and Osamu Terasaki.

"Our discovery of a synthetic approach to formation of mesoporous materials with crystalline pore walls will open a new field of materials science due to their meso- and molecular-scale hierarchically ordered structures," Inagaki tells C&EN. "The result will impact a wide range of research fields--not only catalysis and adsorption, but also electrical, magnetic, and optical devices."

Professor Ferdi Schüth of the Max Planck Institute for Coal Research agrees that the Toyota research represents a significant advance. Schüth is chair of the International Zeolite Association's new Commission on Ordered Mesoporous Materials. "Since researchers all over the world have been looking for materials with periodicity in the walls," he says, "this finding can be considered a breakthrough."

To make the hybrid, Inagaki and coworkers begin with an organosilane monomer, (C2H5O)3Si–C6H4–Si(C2H5O)3, that they heat in basic solution with a surfactant template. They think interactions among individual precursor molecules, as well as among the precursors and the surfactant, direct the assembly into an ordered framework.

"We can control the pore size using different-sized surfactants, such as alkyltrimethylammonium compounds with various alkyl chain lengths and triblock copolymers," Inagaki says. By varying the surfactant, the group has synthesized hybrids with pore diameters ranging between 30 and 50 Å. No matter what the pore size, all the materials had the same regular arrangement in the walls.

In a step that could prove of practical importance in the near term, Inagaki's team sulfonated the material without destroying its meso- or molecular-scale periodicity. The resulting sulfonic acid-functionalized mesoporous material has "great potential as a solid acid catalyst for eco-friendly chemical processes and as an electrolyte for fuel cells," Inagaki says. Both the sulfonated and unsulfonated materials are stable up to 500 °C in air or nitrogen.

"The future of these kinds of materials will be very great since they are opening the possibility of controlling chemical and physical properties in the pores and in the framework at the same time," notes chemistry professor Robert J. P. Corriu of the University of Montpellier, France, who also carries out research in the area. "They are opening wide possibilities in materials science and catalysis."

HIGH ORDERS Alternating rings of hydrophobic benzene layers and hydrophilic silicate layers make up crystalline walls of mesoporous solid. The channels, in turn, line up in a hexagonal array. In this model of the pore surface, silicon atoms are shown as orange; oxygen, red; carbon, white; and hydrogen, yellow.
© NATURE 2002


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