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Версия для печати | Главная > Центр > Научные советы > Научный совет по катализу > ... > 2001 год > № 18

№ 18

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СОДЕРЖАНИЕ

Памяти академика В.М. Грязнова

Катализ: взгляд сквозь годы (продолжение)
М.Г. Слинько.
О становлении и развитии математического моделирования
каталитических процессов

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Памяти академика В.М. Грязнова

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Катализ: взгляд сквозь годы

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SCIENCE & TECHNOLOGY
Concentrates

Graphite nanofibers for fuel-cell electrodes
Graphite nanofibers may serve as effective support materials for fuel-cell electrodes, according to a new study [J. Phys. Chem. B, 105, 1115 (2001)]. The high cost of precious-metal catalysts needed to oxidize fuels remains a key obstacle to widespread commercialization of fuel cells. Researchers are trying to find ways to reduce the dependence on platinum without sacrificing fuel cell performance. Moving toward significant cost savings, Northeastern University chemistry professors R. Terry, K. Baker and Nelly M. Rodriguez and coworkers report that in methanol oxidation studies, fuel-cell anodes fashioned from graphite nanofibers with platelet and ribbon-type structures require a platinum loading of just 5% by weight to function as effectively as carbon electrodes loaded with five times more platinum. In addition, the graphite-nanofiber-supported catalysts are found to be much more resistant to CO poisoning than traditional catalysts. The group proposes that the enhanced activity is linked to crystallographic orientations adopted by the platinum particles when they are dispersed on the fibers.

C & EN /February 19, 2001

How a platinum complex rouses methane bonds
The cationic platinum(II) complex shown, which activates the unreactive carbon-hydrogen bonds of methane, does not dissociate prior to reaction with methane. Chemists Lars Johansson and Mats Tilset at the University of Oslo, in Norway, find that the approach of methane to the platinum center helps to expel the ligand that it replaces [J. Am. Chem. Soc., 123, 739 (2001)]. If the complex were to dissociate by releasing water before methane binds, a three-coordinate, 14-electron platinum(II) cation intermediate should form. But the evidence of Johansson and Tilset indicates that methane associates with the tetracoordinated platinum(II) and that it exchanges with the coordinated water prior to
C-H bond activation. This clarification of a key step in the mechanism of methane activation will help in the design of better catalysts, which in tern will promote the use of methane as a raw material for value-added products.

C & EN/ January 29, 2001

Mild cross metathesis of alkynes
Alkyne metathesis may prove to be as versatile as olefin metathesis, thanks to a catalysts that's been discovered to efficiently promote cross reactions. The molybdenum complex shown, when activated by methylene chloride, promotes cross metathesis reactions of functionalized alkynes that decompose or file to react with better known catalysts [Org. Lett., 3, 221 (2001)]. In cross reactions, homodimerization of the starting alkynes competes with the desired cross reaction. But according to chemistry professor Alois Fűrnster and coworker Christian Mathes at the Max Planck Institute of Coal Research, Műlheim, Germany, the selectivity of the new reagent combination for cross metathesis is excellent. In addition, the researchers were surprised to find that
C-silylated alkynes undergo cross reactions smoothly; they report what they believe are the first examples of metathesis reactions involving such starting materials. The mild method tolerates a variety of polar functional groups. And double bonds in the starting materials are not affected.

C & EN / January 29, 2001

"Merry-go-round" alkylation mechanism
A new catalytic reaction adds multiple alkyl groups to aromatic rings via an unusual "merry-go-round" mechanism, according to associate professor of chemistry Masahiro Miura and coworkers at Osaka University in Japan [J. Am. Chem. Soc., 122, 10464 (2000)]. For example, the reaction of phenylboronic acid and 2-norbornene in the presence of a rhodium catalyst yields primarily the tetranorbornylbenzene shown. On the basis of experiments with deuterated starting materials, the researchers propose a sequential mechanism that starts with generation of a phenylrhodium species. Norbornene next inserts into the phenylrhodium bonds. The rhodium then inserts into an adjacent phenyl-hydrogen bond to form a cyclic intermediate. Subsequent intramolecular reductive elimination yields a new phenylrhodium species having one norbornyl group on the phenyl ring. The process repeats itself, proceeding around the ring adding norbornyl groups until steric factors interfere. "The sequence provides a straightforward method for the synthesis of a unique class of sterically encumbered aromatic molecules", the researchers write.

C & EN/ October 30, 2000

Novel zeolite enhances catalytic epoxidation
Mesoporous crystalline zeolites prepared by a novel technique could find a wide variety of applications in heterogeneous catalysis, according to Claus J.H. Jacobsen, senior scientist at Haldor TopsØe Research Laboratories, Lyngby, Denmark, and coworker. They show that large titanium-containing zeolite crystals with a mesopore system are catalytically active in the epoxidation of oct-1-ene and significantly more active in cyclohexene epoxidation than conventional microporous titanium-containing zeolites [Chem. Commun., 2000, 2157]. After reaction, the crystals are removed from the product mixture by simple filtration. The group uses an excess of zeolite gel and an inert carbon matrix to prepare the crystals. [J. Am. Chem. Soc., 122, 7116 (2000)]. The carbon is removed by combustion.

C & EN/ November 13, 2000

Russian researchers achieve synthesis of high reactivity lanthanide catalysts

A breakthrough by researchers at the Institute of Organometallic Chemistry in Nizhny Novgorod looks set to move the development of lanthanide complexes as catalysts for polymerization nearer to reality.

According to a report on Chemweb.com chemists have focused on hydride complexes of lanthanides as potential catalysts for the last decade because they have extremely high reactivity and catalytic activity in reactions with non-saturated compounds, such as reactions of hydrogenation, polymerization, hydroxylation and cyclization. To date hydride complexes have been synthesized almost for all lanthanides (Ln), but these compounds usually contain a cyclopentadienyl moiety, which increases their stability but decreases catalytic activity and reactivity of the Ln-H group.

The Russian researchers report that they have pioneered, the synthesis of molecular hydrides of bivalent europium and samarium that do not contain carbocyclic ligands.The synthesized compounds have an unsaturated coordination sphere and therefore are extremely active. The researchers not only developed the method of synthesis of the new compounds, but also studied their catalytic activity in polymerization of alpha-olefins. They described their results in the paper recently published in Proceedings of Russian Academy of Sciences: Chemistry (2000, No 5, p.947).

Applying the original synthetic procedure, the chemists obtained the lanthanide hydrides bound with tetrahydrofuran (THF). The hydrides are amorphous powders of dark-red (for samarium) or orange (for europium) colours. A composition of hydrides was established by using infra-red spectroscopy and NMR methods.

The obtained hydrides with the general formula LnH2(THF)2, where Ln is Eu or Sm, really contain active hydrogen. The chemists proved this by studying the reduction of bromine-containing compounds.

They also investigated the activity of complex hydrides of Eu and Sm as well as of the earlier obtained Yb hydride as catalysts in polymerization. It was found that these compounds show selective catalytic activity. They are inert with respect to propylene even at pressures as high as 20 atmospheres, but extremely active in styrene polymerization. It is interesting that the average molecular mass of the obtained polymer depends on the complex-forming atom. It is equal to 150 000 for samarium, 350 000 for ytterbium and 1 million for europium.

For further information contact: Mikhail N. Bochkarev,
e-mail: mboch@imoc.sinn.ru.

Materials Today 1, 2001

White House Issues Nanotechnology Plan
An implementation plan for the federal National Nanotechnology Initiative has been released by the White House National Science & Technology Council. It outlines the Administration's strategy for investigating in nanoscale research. In a letter to Congress that accompanies the report, Office of Science & Technology Policy Director Neal F. Lane writes: "The President has made the National Nanotechnology Institute (NNI) a top priority. Nanotechnology thrives from modern advances in chemistry, physics, biology, engineering, medical, and materials research and will contribute to cross-disciplinary training of the 21st century science and technology workers". The Administration's 2001 budget request includes $225 million for NNI. The initiative will support long-term nanoscale R&D leading to potential breakthroughs in areas such as materials and manufacturing, nanoelectronics, medicine, the environment, energy, chemicals, biotechnology, agriculture, information technology, and national security.

C & EN, May 1, p. 41.

NMR Analysis of 'rea' catalysts
A new experimental apparatus enables NMR analysis of catalysts that have been subjected to the high-temperature conditions of actual catalytic processes. Professor Xinhe Bao and coworkers at the Dalian Institute of Chemical Physics, Dalian, China, designed an apparatus in which an actual fixed-bed reaction can be carried out at temperatures as high as 1123 K. The catalyst can then be transferred, still under the reaction atmosphere, to a magic-angle spinning rotor for NMR spectroscopy experiments. The researchers used the technique to study a molybdenum-modified zeolite that catalyzes aromatization of methane at 973 K. In the proton spectra, they observed variations in the number of Brønsted acid sites and the formation of water, benzene, and aromatic carbonaceous deposits. Other in situ methods for studying catalysts by NMR spectroscopy did not work at temperatures higher than 800 K, the researchers point out.

Angew. Chem. Int. Ed., 39, 2928 (2000)

Nanoporous yield to Nmr Spectroscopy
Hyperpolarized xenon NMR spectroscopy can be used to probe the behavior of xenon gas atoms as they fill the channels of a porous crystalline solid, according to a new study. In this technique, a laser is used to optically pump xenon nuclei, causing them to collectively flip their spins up or down. The increased population of aligned spins greatly enhances traditionally weak NMR signals. Chemistry professors Piero Sozzani at the University of Milan and Alexander Pines at the University of California, Berkeley, along with their colleagues crystallized an inclusion compound from a benzene solution of tris(o-phenylenedioxy) cyclophosphazene, and evacuated it to obtain a novel material with hexagonal, axially symmetric nanochannels. NMR spectra of hyperpolarized xenon as it entered the pores of the solid showed a change in chemical shift anisotropy with increasing concentration of gas inside the channels. The authors's analysis of the spectra indicates that the xenon is diffusing, single file, into the channels, forming one-dimensional stacks. This technique could be used in supramolecular engineering, in developing models for single-file diffusion, and in studying atomic confinement.

Angew. Chem. Int. Ed., 39, 2928 (2000)


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