Л. Юдина.
Катализ в интересах устойчивого развития. -
Российско-Голландский семинар
Премии и награды по химии
За рубежом
Показатели цитирования журналов по катализу
Iron-peroxide complex seen in lipoxygenase structure
The first X-ray crystal structure of the purple oxidized form of lipoxygenase supports the hypothesis that an iron(III)-peroxide complex is an intermediate in the reaction that the enzyme catalyzes, researchers at the University of Toledo report [J. Am. Chem. Soc., 123 10814 (2001)]. Lipoxygenases, which contain a nonheme iron coractor, catalyze the peroxidation of polyunsaturated fatty acids. Previous X-ray structures have been of "resting" forms of the enzyme, with its cofactor in the iron(II) state. Now, chemists Max O. Furik Jr., Ewa Skrzypczak-Jankun, and coworkers have obtained a crystal structure of the enzyme's purple active iron(III) form complexed to a lipid hydroperoxide. Although purple lipoxygenase is extremely sensitive to light and heat, the researchers were able to determine its structure by carrying out their experiments in the dark. The structure shows that the peroxide forms a covalent complex with iron via the peroxy group and occupies the sixth ligand position in the iron coordination sphere. The lipoxygenase that the Toledo team has been studying is from soybeans and differs in some significant ways from a lipoxygenase from rabbits, the researchers note. The factors that determine the enzymes' regio- and stereoselectivity likely will turn out to be different in the plant and mammalian cases, they suggest.
C&EN / November 12, 2001
Visible-light photocatalyst splits water
Sunlight-driven electrolysis of water to form O2 and H2 has been a longtime goal of inorganic photochemists, with the idea of efficiently producing hydrogen to power fuel cells. Transition-metal oxide semiconductors used as catalysts for this process generally absorb UV light, which accounts for only 4% of incoming sunlight, so the reactions aren't very efficient. Thus the greater goal has been to develop photocatalysts that absorb in the visible region, which is less energetic but accounts for about 43% of sunlight. In the latest step in that direction, Zhigang Zou and Hironori Arakawa of the National Institute of Advanced Industrial Science & Technology in Tsukuba, Japan, and coworkers report the solid-state synthesis of indium tantalum oxide semiconductors doped with nickel, In 1-x NixTaO4(x=0-0.2) [Nature, 414, 625 (2001)]. Nickel reduces the band-gap energy of the catalyst enough to push light absorption into the visible region but maintain the 1.23 eV needed to split water. The photocatalyst efficiency is less than 1%, however, which will need to be improved considerably to make the process practical.
New ligands for asymmetric catalysis
Metal complexes with simple phosphinite-oxazoline ligands are proving to be promising enantioselective catalysts. Ligands of the general structure shown can be prepared in a few steps from (S)-serine methyl ester. Chemists Chris Richards of Queen Mary University of London, and Geraint Jones of Cardiff University, in Wales, have explored the use of phosphinite-oxazolines as ligands in palladium-catalyzed allylic subsritutions [Tetrahedron Lett., 42, 5553 (2001)].With R2=H, they obtained the highest enantiomeric excesses (ee) when R1 is a relatively small ferrocenyl or phenyl group. At the University of Basel, in Switzerland, Andreas Pfaltz and Jorg Blankenstein are researching the same type of compounds as ligands for iridium-catalyzed enantioselective hydrogenation of alkenes [Angew. Chem.Int.Ed., 40, 4445 (2001)]. Their best results (up to 98% ee for unfunctionalized alkenes) were obtained with R1 = ferrocenyl or 3,5-bis(tert-butyl) phenyl and R2 = isopropyl, isobutyl or benzyl. "The modular, construction and the facile synthesis of these ligands should make it possible to tailor their structure for other substrates and other metal-catalyzed reactions," the Basel chemists write.
Simple setup yields 'nano-onions'
A simple new method yields high-quality "nano-onions", according to professor Gehan Amaratunga, research associate Manish Chhowalla, and coworkers in the department of engineering at the University of Cambridge [Nature, 414,506 (2001)]. The carbon particles could be useful as lubricants, Chhowalla says. The spherical nanoparticles, which range from 25 to 30 nm in diameter, have a C60 core surrounded by several carbon layers. Unlike other methods for generating carbon nanomaterials, this method does not require a vacuum system. The nanoparticles are generated by an arc discharge between two graphite electrodes submerged in water. The arc discharge is initiated by contacting a 5-mm pure graphite anode with a 12-mm cathode and generating a discharge voltage of 16 to 17Vand a discharge current of 30 amp. The nano-onions float on the water's surface, whereas other products sink to the bottom of the beaker. The production method could be adapted for industrial use by increasing the size of the apparatus, increasing the arc current, chilling and circulating the water, and automatically replenishing the consumable graphite anode, according to the researchers.
C&EN / December3, 2001
Siymyx furthers Dow,Rhodia collaborations
Symyx Technologies, which specializes in high-throughput combinatorial chemistry research, is expanding collaborarions with Dow Chemical and Rhodia. Symyx and Dow first teamed up in 1999, and the relationship has so far yielded two polyolefin catalysts that Dow is developing. Under the new agreement, Dow will have rights to market materials discovered by the companies and will pay Symyx royalties. Similarly, Symyx extended its collaboration with Rhodia in research related to specialty chemicals.
Trimerization catalyst boost
New ethylene trimerization catalyst systems based on diphosphine ligands offer an unprecedented combination of selectivity and activity, according to chemist Duncan F. Wass and coworkers at BP in the U.K. and the U.S. The team showed that, on activation with an alkyl aluminoxane, chromium complexes of ligands that bear ortho-methoxy-substituted aryl groups (such as the one shown) produce 1-hexene of almost 100% purity [Chem. Commun., 2002, 858]. 1-Hexene is an important comonomer for the production of linear low-density polyethylene. The purity of the olefin is a key factor in the process. "The higher selectivity of our systems is significant since it is difficult and costly to separate the various hexene isomers," Wass says. "Achieving more than 99.9% purity is virtually impossible by distillation." He points out that, at a given pressure, the new catalysts are some two orders of magnitude more productive than previous systems. The group hypothesizes that the potential for ortho substituents on the ligands to act as pendant donors and therefore increase the coordinative saturation of the chromium center is a critical factor in boosting catalyst performance.
C&EN / April22, 2002
Self-assembled carbon nanotubes
Self-assembled, honeycomb networks of carbon nanotubes for use in nanotechnology can be grown on oxidised silicon wafers, at around 8000C. The process uses a tunable chemical vapour deposition, with ferrocene (FeC10H10) and xylene (C8H10) as catalyst and carbon, precursors (catalyst: hydrocarbon ratio >0.2g/ml) see Figure. Prolonged deposition thickens the walls of the honeycombs, gradually filling the central holes and finally forming a packed him of vertically aligned carbon nanotubes (Z Zhang, B Wei & P M Ajayan, Chem Common 2002, 962).
Unprecedented telomerisation catalyst
The first reported monocarbene Pd(0) alkene complex (dmi)Pd0(dvds), available by treatment of the Pd(0) diallyl ether (dae) complex pd(dae)3 with dimesitylimidazol-2-ylidene carbene (dmi) in 1,1,3,3-tetramethyl-1,3-divinyl-disiloxane (dvds), at -300C, exhibits unprecedented high catalyst productivities and selectivities for a number of industrially important telomerisation reactions (R. Jackstell, M. G. Andreu, A. Frisch, K. Selvakumar, A. Zapf, H. Klein, A. Spannenberg, D. Rottger, O. Briel, R. Karch S. M. Beller, Angew Chem Int Ed Eng 2002,41.986). For example, yields and chemoselectivities greater than 98% and 99%, respectively, are observed for the (dmi)Pd0(dvds)-catalysed telomerisation of 1,3-butadiene with methanol, to give the 1-substituted 2,7-octadiene and a catalyst turnover number in excess of 98000 (900C, 1mol% NaOH, MeOH:butadiene ratio is 2: 1) see Scheme
NO remuval trom exhaust gases
Increasingly stringent environmental legislation to reduce gaseous pollutants, especially nitrogen oxides from exhaust gases, requires the development of new catalysts. Medium-pore zeolites containing transition-metal ions (for example, CeO2-ZSM-5) are very active in such selective catalytic reduction (SCK) reactions, using various hydrocarbon reductants. The major problems in using these catalysts are the deactivatlon caused by the segregation of the active metal oxide from the framework, and coke formation. The presence of water at higher temperatures also results in a decrease in catalytic activity due to delumination. No such problems are encountered with 15wt% CeO2-H ferrite catalysts (surface area 128m2/g), prepared by physically mixing the components. Very high NO removals (to N2), up to 85%, are observed (for example, from a synthetic exhaust gas of composition - 1000ppm NO, 1200ppm C3H6, 10vol%, O2, 10vol% H2O and balance N2) over a broad temperature range (150-6000C). Furthermore, unlike other systems, the presence of water actually retards and reduces coke formation, as well as improving NO removal.
N2O emission control
In related studies, silver-nanoparticles supported over cobalt oxide (Co3O4) are highly active and stable catalysts for N2O decomposition to N2 and O2 (>70% conversion at 250*C), even in the presence of excess oxygen and steam. This makes an attractive end-of-pipe solution to reduce N2O emissions (L Yan, X Zhang. T Ren. H Zhang, X Wang & J. Suo, Chem Commun 2002, 860).
Phase-transfer catalyst recovery
Finally phase-transfer catalysis is a technique for conducting reactions between reaction partners located in separate, contiguous immiscible phases, employing a phase-transfer catalyst (PTC) such as a quaternary salt. After reaction is complete, the moderately expensive and mildly toxic catalyst must be separated from the product and recycled. This is usually accomplished by one of four separate methods: extraction, distillation, adsorption or binding to an insoluble support. Extraction is probably the most common method used to separate a PTC in commercial processes. Though water soluble, many PTCs need large amounts of wash water to be removed from the organic phase. This requires the evaporation of large amounts of water to concentrate the catalyst for recycling, or results in the loss of catalyst plus treatment and disposal of many litres of water per kg of product produced.
American researchers (X. Xie, J. S. Brown, P. J. Joseph, C. L. Liotta & C. A. Eckert, Chem Commun 2002, 1156) have discovered that CO2 can be used to enhance the environmentally benign, and allow efficient recovery of PTCs by aqueous extraction. This method can alter the distribution of PTCs so dramatically that even in dilute solutions they can be separated selectively from an organic reaction mixture, with typically less than 55% of the water required in a traditional extraction. At pressures less than 6000kPa, CO2, drives the widely used PTC benzyl triethylammonium bromide into the aqueous phase, altering the distribution coefficient by about 200-fold.
Chemistry&Industry-1 July 2002
Fe catalyses chlorophenol destruction fast
A new catalytic method has been described for the rapid degradation of chlorophenols, a class of compounds widely found in certain pesticides and wood preservatives (S Gupta et al. Science 2002, 296, 326). The combination of a macrocyclic iron complex (1) and excess hydrogen peroxide degraded 99.5% of an alkaline (pH10) aqueous solution of tri- and pentachlorophenol, within less than ten minutes (see Figure). Most of the chlorine ended up as inorganic chloride. Some still remained bound in the form of chlorinated maleic and malonic acids, but these by-products could be further destroyed at neutral pH, albeit at a slower rate. Catalyst: substrate ratios as little as 1:2000 were possible. Neither dioxmes or other toxic by-products were detected. The low toxicity and high efficiency of the iron catalyst makes this procedure extremely attractive for treating chlorinated environmental pollutants in waste streams.
Chemistry&Industry-1 August 2002
