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Версия для печати | Главная > Центр > Научные советы > Научный совет по катализу > ... > 1997 год > № 4

№ 4

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В научном совете по катализу

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Catalyst, branching improve polyethylenes
(Катализатор, улучшающий полиэтилены)

     Dow Plastics has introduced a series of polyethylene resins aimed at consumer and industrial packaging film uses. Whether blown or cast, the resins are easier to process, and the resulting films have lower heat-sealing temperatures than linear low-density polyethylenes. The new films have greater impact strength and toughness, so users can make them thinner. The films also have better puncture resistance, decreasing generation of waste at the source. Kurt W. Swogger, director of research and development for polyethylene at Dow Plastics in Freeport, Texas, attributes the properties of the Elite brand resins to small amounts of long-chain branching from incorporation of 1-octene comonomer and to control of molecular architecture by the monocyclopentadiethyltitanium-based catalyst. Also, as the polyethylene chains wind back and forth to become aligned in lamellar crystalline regions, the occurrence of branches leads either to inclusion of the branch in the crystal or to initiation of a new, adjacent crystalline domain. The resulting «tie-molecules» holding the crystalline lamellae together add to the strength of films, Swogger says.

Methane activation at low temperature
(Активация метана при низкой температуре)

    Researchers in India have converted methane to higher hydrocarbons at yields of up to 45% at temperatures of only 400 to 600oC. Using a nonoxidative process that combines methane and an alkene additive over a galloalumosilicate zeolite, Vasant R. Choudhary and coworkers at the national Chemical Laboratory in Pune have produced aromatic compounds with about 90% selectivity [Science, 275, 1286 (197)]. Other methane activation processes, based on oxidative or dehydrogenative coupling of methane to ethane or ethylene, are known to provide similarly high product selectivity. Those processes require much higher temperatures, however, and tend to produce unwanted carbon monoxide. The researchers propose a reaction mechanism based on hydrogen transfer between methane and the alkene additive on the catalyst surface. The zeolite, they explain, behaves as "a bifunctional catalyst with strong acid sites [and] dehydrogenation functions [caused by] zeolitic protons and extra framework gallium-oxide species. Support for their model is drawn from the enhanced methane conversion rates observed when using zeolites known to process the special gallium-oxide species. In addition, the researchers show that varying the methane-to-alkene reactant ratio markedly affects hydrogen production and conversion of methane to aromatic, as the reaction mechanism predicts.

(March 3, 1997 Chemical & Engeneering News)

 

     Methanol is a recognised alternative energy source as well as an important chemical feedstock. Its decomposition to hydrogen and carbon monoxide saves energy because the industrially produced waste heat can be used to increase a fuel's heating value. The decomposition is also important for increasing the fuel efficiency and reducing the amount of formaldehyde by-product. Catalysts containing nickel are active for this reaction, but further improvements are still needed - especially for the application to the recovery of waste heat. Such an improvement has recently been reported (Y. Matsumura, K. Kagawa, Y. Usami, M. Kawazoe, H. Sakurai and M. Haruta, Chem. Commun., 1997, 657). Methanol conversions of greater than 95%, with carbon monoxide selectivities of > 92%, are seen for methanol decomposition at 250oC over Pt/Ni-silica supported catalysts (Pt/Ni surface atomic ratio of 0.11) in fixed bed continuous flow reactors under atmospheric pressure.

     In a similar vein, catalytic reduction of carbon dioxide to more valuable compounds is obviously of great benefit from an environmental viewpoint. This reaction usually requires severe conditions of high pressure and/or high temperatures which has severely hindered its commercial application. But this has now changed thanks to Japanese chemists Y.Kohno, T.Tanaka, T.Funabiki and S.Yoshida (ibid, 1997, 841). Gaseous carbon dioxide is efficiently reduced by hydrogen to carbon monoxide at room temperature and low pressure (25 kPa) by light irradiation (<300nm) in the presence of a zirconium oxide photocatalyst.

The configuration of the electroluminescent cells

    Architectural control of poly(ferrocenylsilane)s is now possible for the first time. Regioregular rrocenylsilane)s (4), end group-functionalised polymers with controlled molecular weight, and novel graft polymers (5) are all accessible by transition metal-catalysed ring opening polymerisation of strained ferrocenophanes at ambient temperatures (P.Gomez-Elipe, P.M.Macdonald and I.Manners, Angew. Chem. Int. Ed. Engl., 1997, 36.762).


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