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Версия для печати | Главная > Центр > Научные советы > Научный совет по катализу > ... > 2002 год > № 24

№ 24

Обложка номера


В.Н. Пармон. С наступающим Новым годом

Г.М. Жидомиров, В.И. Авдеев, Л.Я. Старцева. VI Российская конференция "Механизмы каталитических реакций"

В.А. Собянин. Теория - практике. По следам VI Российской конференции "Механизмы каталитических реакций"

Л.Б. Юдина. Крупное событие в каталитическом мире

Л.Б. Юдина. Чтобы ориентироваться в современных реалиях

За рубежом.


В.Н. Пармон.


VI Российская конференция "Механизмы каталитических реакций"


По следам VI Российской конференции "Механизмы каталитических реакций"


Крупное событие в каталитическом мире


Чтобы ориентироваться в современных реалиях


За рубежом

За рубежом

Suzuki coupling catalyzed by palladium shells

Hollow palladium spheres fabricated in the laboratory have been successfully used as a recyclable heterogeneous catalyst for Suzuki coupling reactions {J. Am. Chem. Soc., 124, 7642(2002)}. Taeghwan Hyeon, an associate professor of chemical engineering at Seoul National University, in South Korea, and coworkers prepared spherical shells that are 300 nm across and consist of 10-nm palladium particles. The large surface area of these shells makes them highly active catalysts in Suzuki cross-coupling reactions such as the one shown here. In previous studies, other groups reported that palladium nanoparticles used in these reactions agglomerated after one cycle, resulting in a loss of catalytic activity. УTo our surprise," Hyeon and coworkers write, "[our Pd spheres] maintained their catalytic activities even after seven recycles. In addition, simple filtering can retrieve the catalyst from the reaction pot." Hyeon Says this heterogeneous catalyst is nearly as active as the most popular homogeneous palladium catalyst used for Suzuki coupling. Furthermore, he believes the new catalyst could also be applied to other important carbon-carbon coupling reactions.

C & en /JuLY 1, 2002

In search of actinide catalysts

In recent years, researchers at Los Alamos National Laboratory's Chemistry Division, led by Carol J. Burns and Jaqueline L. Kiplinger, have been exploring the potential of actinide metallocene complexes as "frameworks for chemical transformations." The additional bonding possible by actinide metal f orbitals means that appropriately designed complexes could display unique catalytic activity not afforded by transition metals, Burns notes. In their latest effort, the Los Alamos chemists report the synthesis of the first f-element ketimido complex (shown) [Organometallics, 21, 3073 (2002)]. The uranium (IV) complex has unusually high thermodynamic stability, similar to uranium imido complexes the researchers have previously prepared, which they ascribe to nitrogen's π-electron donation to uranium's f orbitals. Although not reactive enough to serve as catalyst, the new complex sheds light on actinide metal orbital involvement in ligand bonding, Burns says, and will help guide future work to discover useful catalysts.

C & en /JuLY 22, 2002

Sumitomo has new process

Sumitomo has developed a process for producing fatty acid methyl esters by reacting methanol with vegetable oil heated to over 240 *C. Traditional methods for producing fatty acid methyl esters require the use of alkaline catalysts that create undesirable by-products. Fatty acid methyl esters are mostly used to make higher alcohols that, in turn, are raw materials for surfactants. Sumitomo hopes to license its technology to other companies.

Metal-catalyzed cycloadditions add up to eight-membered rings

Last spring, chemistry professor Paul A. Wonder's team at Stanford University described how to build eight-membered rings with rhodium-catalyzed [5+2+1] cycloadditions [J. Am. Chem. Soc., 124, 2876 (2002)]. Now, in back-to-back papers in JACS, two other research groups show how to do the math a bit differently by using [4+2+2] cycloadditions. Like other medium-sized rings, those with eight carbons occur in many natural products and drugs but are notoriously difficult to prepare. In the approach of chemistry professor P. Andrew Evans and coworkers at Indiana University, Bloomington, intermolecular cycloaddition of heteroatom-tethered enyne derivatives with butadiene yields bicyclic heterocycles, as shown [J. Am. Chem. Soc., 124, 8782 (2002)]. Meanwhile, at Washington University, St. Louis, chemistry professor Scott R. Gilbertson's group finds that cyclization of alkynes with dieneynes gives eight-membered-ring products (page 8784). Both groups use rhodium catalysts modified with silver salts, and both note that the exact nature or amount of the silver salt profoundly affects the reactions' outcomes.

C & en /JuLY 29, 2002

Large-scale carbon nanofibre synthesis

Carbon nanotubes and their counterparts with no central channel, carbon nanofibres, are being investigated for their special electronic and structural properties in device applications, and as nanotips in atomic force microscopy. In many cases, it is not enough to make a bunch of carbon nanotubes/nanofibres. More importantly, they have to be grown at predetermined locations on a substrate, with control over length, diameter, shape, chemical composition and orientation. It was recently noted that the angle of alignment is in fact influenced by an applied electric field. V. Merkulov et al have devised a process where an electric field can help in the fabrication of oriented carbon nanofibres in a large-scale synthesis (Appl Phys Lett 2002, 80, 4816).

Electric fields lines control the direction a nanofibre grows during FECVD
Electric fields lines control the direction a nanofibre grows during FECVD

The authors set out by making catalyst nanodots (composed of a Ni-Fe alloy) on a silicon wafer using electron-beam lithography and metal evaporation. The carbon nanofibres were then grown from these catalyst sites by plasma-enhanced chemical vapour deposition (PECVD) of acetylene gas, in the presence of ammonia. As the plasma is generated between two electrodes, the carbon nanofibres tend to grow perpendicular to the cathode, which is normally the substrate. When an additional sample holder (with the substrate on top) is positioned in between the electrodes, the field lines located far enough from the edges will still be aligned perpendicular to the substrate's surface. However, around the corners the electric field lines bend significantly, and carbon nanofibres that are allowed to grow there will do so at an angle to the surface (see Figure). So, by choosing where to place the substrate, the alignment angle can be varied by up to 40*. Even kinked nanostructures are possible if the substrate is repositioned half-way through the PECVD process.

Another strategy how to control the fabrication of aligned carbon nanofibres has been devised by S. Huang, L. Dal and A. Mau (Adv Mater 2002, 14, 1140). By patterning a specially developed metal-containing photoresist followed by a calcination/reduction step, they were able to generate catalyst nanodots anywhere they wanted on a substrate. Aligned carbon nanofibres grew from these dots during acetylene pyrolysis. Having demonstrated the principle, the authors became a bit more imaginative with their choice of photomask, in their case a conventional black-and-white film. Rather than using a regular array of dots, they drew chemical structures, formulae and orbital shapes, photographed a high-quality printout on paper, then replicated a reduced image with carbon nanofibres on a substrate.

Chemistry&Industry - 1 October 2002



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