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Версия для печати | Главная > Центр > Научные советы > Научный совет по катализу > ... > 2000 год > № 15

№ 15

Академик М.А. Лавреньев

Содержание

Академик М.А. Лаврентьев и математические основы катализа
М.Г. Слинько, В. Н. Пармон.

Химическая промышленность на рубеже веков (Окончание)

О научных мероприятиях в области катализа
  II Всероссийское совещание
"Высокоорганизованные каталитические системы"

Зарубежные новости




Академик М.А. Лаврентьев и математические основы катализа

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Химическая промышленность на рубеже веков

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О научных мероприятиях в области катализа

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Зарубежные новости

Elizabeth Wilson
Better polymers from new, improved catalysts

Nickel-based catalysts, developed by California Institute of Technology chemistry professor Robert H. Grubbs and colleagues, produce high molecular weight polyethylene, can tolerate contaminants, work at low temperatures and pressures, and don' require a cocatalyst. Not only that, but the catalysts can polymerize olefins with functional groups, which could lead to polymers with new properties [Science, 287, 460 (2000)].

More than 85 million metric tons of polyolefins are produced each year. Since the 1950s, a good percentage of these have been made using Ziegler-Natta catalysts such as TiCl3/(CH3CH2)2AlCl.

During the past 15 years, however, metallocene catalysts have enjoyed increasing popularity in polyolefin production. Unlike Ziegler-Natta catalysts, metallocenes are very well characterized, offering an unprecedented level of control-chemists are able to tailor the properties of resulting polyolefins very specifically.

However, metallocene catalysts require scrupulously clean conditions to be effective. The electrophilic nature of the cationic early-transition-metal center in these catalysts makes them susceptible to inactivation by organic functional groups and impurities such as oxygen, nitrogen, or sulfur.

Another approach to olefin catalysis involves the use of late-transition-metal complexes, which have less of an affinity for oxygen than metallocenes. The Shell higher olefin process (SHOP) employs a neutral, nickel-based catalyst with a phosphorus-oxygen ligand chelate. The SHOP system can tolerate polar functional groups, but it primarily produces oligomers four to 20 units long and requires high temperatures and pressures.

Numerous researchers are working on surmounting these various problems. The strategy of Grubbs and colleagues was to start with the SHOP catalyst, replacing the PO-based species with one based on the more sterically bulky salicylaldimine - an approach developed by Maurice S. Brookhart, chemistry professor at the University of North Carolina, Chapel Hill.

The Caltech team synthesized a number of variants. They were able to produce polyethylene with a molecular weight of more than 250,000, narrow molecular weight distribution, and very little branching. They were also able to copolymerize ethylene and olefins functionalized with polar groups such as hydroxyl.

"These are exciting results than represent a significant step forward in our understanding of transition-metal olefin polymerization catalysis," says Tobin J. Marks, chemistry and materials science and engineering professor at Northwestern University, Evanston, Ill.

"It's impressive work and has meaningful industrial implications,"says John J. Murphy, director of metallocene and single-site catalyst programs at Catalyst Group in Spring House, Pa. However, he says, it's not an unanticipated development. "It's a very logical extension of where the technology is going in industry," he says.

The Grubbs group is continuing to work on its catalyst family. "It will be interesting to see how this area develops as the scope and mechanism are further explored," Marks notes.

JANUARY 24, 2000 C&EN

Michael Freemantle
Bulk silica imprinted with organic functional groups

Researchers have devised a new strategy for imprinting bulk microporous silica with organic functional groups that could lead to the design and synthesis of the materials with features modeled on those of enzymes and antibodies.

Mark E. Davis, professor of chemical engineering at California Institute of Technology, Passadena, and coworker Alexander Katz, now assistant professor of chemical engineering at the University of California, Berkeley, used molecular imprinting to generate bulk amorphous silica with specially tailored microcavities containing aminopropyl groups covalently attached to the pore walls [Nature, 403, 286 (2000)].

"This is a very interesting paper, demonstrating successful imprinting and control over the spatial distribution of organic functional groups in an inorganic support material," comments Andreas Stein, assistant professor of chemistry, University of Minnesota, Minneapolis, and an expert on porous inorganic materials. "I would expect this work to spawn a lot of new research."

Davis and Katz produced bulk amorphous silica by polymerizing tetraethoxysilane using an acid catalyst and a method for synthesizing three-dimensional networks from metal alkoxides known as sol-gel polymerization. During the polymerization process, they incorporated into the silica framework "imprint molecules" consisting of aromatic rings carrying one, two, or tree 3-aminopropyltriethoxilane side groups. Removal of the aromatic cores created framework cavities in which the aminopropyl groups were anchored. They than established the presence of the imprint-generated microporosity by solid-state nuclear magnetic resonance spectroscopy, physical adsorption experiments, catalytic transformations, and other studies.

"The novelty and thus the importance of the work is that the experiments show that imprinting is possible in bulk silica," remarks Kay Severin, senior researcher at Ludwig Maximilians University, Munich, Germany, who recently showed that organometallic catalysis can be enhanced by polymer imprinting with a transition-state analog ( C&EN, Jan.10, page 31). "Traditionally, molecular imprinting has been carried out with organic polymers or in some cases on silica surfaces."

According to Davis and Katz, their silica-imprinting concept can be extended to other types of chemical functionalities and imprints of different size and shape, thus making it plausible to incorporate more specificity and activity into imprinted sites.

"One can now think of using the transition-state analog approach, which is used in the area of catalytic antibodies, to attempt to create rationally designed heterogeneous catalysts," Davis says. "A major breakthrough would be to create a chiral system. To induce Chirality, at last three sites of interaction between solid and guest are needed," he explains. That "is why we developed a methodology for at least three functional groups."

Ketz tells C&EN that his group at UC Berkeley is aiming to develop imprinting as a general strategy for synthesizing materials-by-design. "One of the issues in particular that we wish to pursue is the amount of complexity it is necessary to incorporate into these materials in order to mimic some of the essential features of binding and catalysis found in biological systems," he says.

JANUARY 24, 2000 C&EN



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