Showing papers on "Catalysis published in 1990"

Enantioselective Epoxidation of Unfunctionalized Olefins Catalyzed by (salen)Manganese Complexes

Abstract: need to be sulfonated in order to produce the stable polysemiquinone form of the polymer. Indeed, additional sulfonation and consequent protonation of amine nitrogen atoms would convert some of the -(NH)to -(NHzf)groups and hence destabilize the polymer by reducing the extent of its T conjugation. The absorption maxima at 1080, 700, and 620 cm-' in the FTIR spectrum of compound IIIA,B are consistent with the presence9 of SO3groups attached to the aromatic rings. The absorption maxima at 820 and 870 cm-I indicative of 1,2,4-trisubstitution of the rings are out-of-plane bending of aromatic hydrogens. These absorptions are not present in the 1,2-disubstituted emeraldine base ( I I ) , from which compound IIIA,B was synthesized. The solubilities of compounds IIIA,B and IV differ markedly from those of the corresponding protonated and nonprotonated forms, respectively, of parent polyaniline. Compound IIIA,B dissolves appreciably in aqueous 0.1 M N H 4 0 H or NaOH to give the corresponding salts whereas emeraldine hydrochloride when treated in this manner is converted to the insoluble emeraldine base form (11). The anionic (SO,-) groups are presumably largely responsible for its solubility in water. Compound IIIA,B partly dissolves in DMSO, giving the dark green color of the protonated polyaniline, but is apparently deprotonated when it dissolves in N-methyl-2-pyrrolidinone (NMP), in which it has a blue-violet color, characteristic of compound IV. The high concentration of protons in the vicinity of the polymer backbone due to the presence of the attached SO3groups is not only responsible for the retention of doping of the polymer at the higher pH values, where the parent emeraldine base polymer (TI) is essentially a (nondoped) insulator, but is also consistent with the observed faster electrochemical redox reactions of compound IIIA,B in aqueous media.

5‐Hydroxymethylfurfural (HMF). A Review Focussing on its Manufacture

Abstract: The acid-catalysed dehydration of hexoses results in the formation of 5-hydroxymethylfurfural (HMF) Fructose and inulin are especially good starting materials A review is given of the many methods to produce HMF The reaction kinetics are dealt with in the sections: raw material, hydrolysis and reversion, catalysts, reaction temperature and time, the concentration and the risk of polymerization and the solvent and HMF stability Manufacturing processes are illustrated for fructose as the starting material: aqueous systems with homogeneous acid catalysis or those using ion-exchangers as catalyst, systems using dimethyl sulfoxide as the solvent and those using other organic solvents A short treatment is given on processes starting with glucose, work up procedures and the formation of levulinic acid and other in situ-formed HMF-derivatives 5-Hydroxymethylfurfural Ein Uberblick uber seine Herstellung Die saurekatalysierte Dehydratisierung von Hexosen resultiert in der Bildung von 5%-Hydroxymethylfurfural (HMF) Fructose und Inulin sind besonders gute Ausgangsmaterialien Es wird ein Uberblick uber die zahlreichen Methoden zur Herstellung von HMF gegeben Die Reaktionskinetik ist eingeteilt in die Abschnitte “Rohmaterial, Hydrolyse und Reversion, Katalysatoren, Reaktionstemperatur und -zeit, Konzentration und das Risiko der Polymerisierung und das Losungsmittel und die Stabilitat” Die Herstellungsverfahren werden fur Fructose als Ausgangsmaterial illustriert: Wasrige Systeme mit homogener Saurekatalyse oder unter Verwendung von Ionenaustauschern als Katalysatoren, Systeme unter Verwendung von Dimethyl-sulfoxid als Losungsmittel und solche, die andere Losungsmittel verwenden Ein kurzer Hinweis wird auf Verfahren gegeben, die mit Glucose beginnen, Aufarbeitungsverfahren sowie auf die Bildung von Lavulinsaure und andere in situ gebildeten HMF-Derivaten

Effects of common inorganic anions on rates of photocatalytic oxidation of organic carbon over illuminated titanium dioxide

Abstract: Effects of pH and common inorganic anions on the rates of photocatalytic oxidation of salicylic acid, aniline, and ethanol to CO{sub 2} over glass-supported near-UV illuminated TiO{sub 2} have been investigated by using conductivity detection of evolved CO{sub 2}. In the presence of chlorides and the absence of organics, lowering the pH of the solution below 3.6 caused an increase in conductivity with increasing circulation time. At pH 4.0 {plus minus} 0.2, the rate of oxidation decreased with increasing concentration of chlorides. Perchlorates and nitrates have very little effect. Sulfates or phosphates even at millimolar concentrations are rapidly adsorbed by the catalyst and reduce the rates of oxidation by 20-70%. The catalyst can be readily regenerated by washing with 0.1 M NaHCO{sub 3} solution. The data are considered in terms of kinetic salt effects, surface active sites having discrete adsorption properties, and Langmuir-Hinshelwood kinetic models. The implications for water purification processes based on photocatalytic oxidation using titanium dioxide are discussed.

Catalysts for the polymerization of olefins

Abstract: Catalysts useful in the polymerization of olefins, more particularly of ethylene and mixtures thereof with higher alpha-olefins and/or diolefins, are disclosed. The new catalysts are obtained by mixing a hydride or organometallic compound of a metal belonging to Groups I to III of the Mendelyeev Periodic System with the product obtained by contacting a titanium tetrahalide with a support comprising an anhydrous magnesium or zinc halide in an active form, under particular conditions.

Nickel: An Element with Wide Application in Industrial Homogeneous Catalysis

Abstract: The efficiency and future development of the chemical industry are closely linked to catalysis. It has been estimated, for example, that 60 to 70% of all industrial chemicals have involved the use of a catalyst at some point during their manufacture. In the past two decades the share of the market credited to homogeneous transition metal catalysis increasead to 10–15%. Besides cobalt, which is used mainly in hydroformylation reactions, nickel is the most frequently used metal. Many carbon–carbon bond formation reactions can be carried out with high selectivity if catalyzed by organonickel complexes. Such reactions include, inter alia, carbonylation reactions, cyclic and linear oligomerization and polymerization reactions of monoenes and dienes, and hydrocyanation reactions. It was Reppe and Wilke who pioneered and shaped the field of homogeneous nickel catalysis. Great impetus was also given to the development of organonickel chemistry by Wilke and his students. Research in this area has contributed immensely towards an understanding of the reactions involved in catalysis.—This review is primarily concerned with nickel-catalyzed reactions which are of interest both preparatively and industrially; some mechanistic aspects are also dealt with.

Infrared Spectroscopy of Adsorbed Species on the Surface of Transition Metal Oxides

Abstract: Nature of oxide surface centres characterization of the state of surface cations on oxide catalysts by IR study of the absorbed probe molecules CO and NO complexation of alkenes on oxide surfaces application of infrared spectroscopy to mechanistic studies of heterogeneous catalytic reactions

Dependence of catalytic rates on catalyst work function

Abstract: THE catalytic activity and selectivity of metals can be altered dramatically and reversibly by supplying or removing oxide anions, O2–, at the metal catalyst surface by interfacing the catalyst with an O2– conductor such as zirconia1–6. This effect, termed non-faradaic electrochemical modification of catalytic activity (NEMCA), leads to a steady-state catalytic reaction rate increase up to 3 × 105 times higher than the rate of supply or removal of O2– at the catalyst surface1,3,4. Here we report that β"-Al2O3, which is a Na+ conductor, can also induce the NEMCA effect. Furthermore we show that the origin of the NEMCA effect lies in the controlled variation of catalyst work function on polarization of the metal-solid-electrolyte interface, and demonstrate that the rates of metal-catalysed reactions depend exponentially on the average catalyst work function. Thus the influence of catalyst electronic structure, rather than surface geometry, is here the key factor in catalytic activity.

Oxygen in catalysis

Abstract: A description of catalytic systems commonly used as model systems in the laboratory and as industrial catalysts in large-scale operations, and a discussion of the mechanisms operating in these reactions. Attempts to describe the elementary steps by quantum chemical methods are also shown, as are rec

Hydrogen Bonding and Chemical Reactivity

Abstract: Publisher Summary This chapter discusses the hydrogen bonding, which has been recognized as the single most important intermolecular interaction, The chapter presents the hypothesis that there are three kinds of hydrogen bond: weak, strong, and very strong. These are determined by the shape of the potential energy well and the respective positions of hydrogen and deuterium within the well, which can be used to provide information about the well that applies in a particular example. The role of hydrogen bonding in catalysis has been discussed, although mainly in terms of the salicylate ion as a leaving group. With its strong intramolecular hydrogen bond playing an essential part in the reaction mechanism, it seems likely that this will herald other systems where the role of a strong hydrogen bond may serve as the key step in a catalytic process.

Oxyfunctionalization of alkanes with hydrogen peroxide on titanium silicalite

Abstract: BECAUSE of the chemical inertness of alkanes, the introduction of functional groups into them by oxidation has tended to require severe and thus unselective conditions. The homogeneous and heterogeneous catalysts used for alkene oxidation are ineffective for alkanes1,2. Cleavage of carbon–carbon bonds in the presence of transition metal complexes effects mainly complete oxidation to acids2, although some selectivity is occasionally possible3. Oxyfunctionalization without C–C bond cleavage has also been achieved2,4,5, and high selectivity may be obtained from natural or synthetic metalloporphyrin systems6,7. Here we show that titanium silicalite, a microporous crystalline Ti-silica8, is able to activate secondary and tertiary carbon atoms in alkanes, so that they may be oxidized to alcohols and subsequently to ketones using aqueous H2O2. The catalyst has low regioselectivity for alcohol formation but high reactant selectivity in the formation of ketones.

Conversion of Methane by Oxidative Coupling

Abstract: (1990). Conversion of Methane by Oxidative Coupling. Catalysis Reviews: Vol. 32, No. 3, pp. 163-227.

Oxidation of Alcohols by Activated Dimethyl Sulfoxide and Related Reactions: An Update

Abstract: The current review is intended to emphasize the most important developments in the use of activated DMSO for the oxidation of alcohols since 1981 through 1989, but includes sufficient coverage of the earlier literature so as to be self-contained. Only enough of the vast number of recent references are cited to highlight advances in the methodology and to provide recent examples of the application of these reactions.

Asymmetric hydroxylation, epoxidation, and sulfoxidation catalyzed by vaulted binaphthyl metalloporphyrins

Abstract: A chiral, vaulted binaphthyl porphyrin (1) has been prepared from 5α,10β,15α,20β-tetrakis(o-aminophenyl)porphyrin and (R)-(+)-2,2'-dimethoxy-1,1'-bi-6-naphthoyl chloride in 79% yield. Asymmetric oxygenations of alkanes, alkenes, and alkyl sulfides were catalyzed with the chloroiron(III) and chloromanganese(III) complexes of 1 with good yields and high stereoselectivities. The first catalytic asymmetric hydroxylations are reported for 1-Fe III Cl with enantiomeric excesses in the range of 40-72%. The corresponding manganese catalyst, 1-Mn III Cl, gave much lower enantiomeric excesses. For catalytic asymmetric epoxidations, enantiomeric excesses were in the range of 20-72%. Prochiral alkyl sulfides gave sulfoxides with 14-48% ee with 1-Fe III Cl

Palladium-catalyzed arylation of furan, thiophene, benzo[b]furan and benzo[b]thiophene

Abstract: Treatment of π-electron sufficient aromatic heterocycles such as title compounds with aryl bromides in the presence of tetrakis(triphenylphosphine)palladium gave the corresponding 2-aryl aromatic heterocycles.

The Structure and Activity of Pt‐Co Alloys as Oxygen Reduction Electrocatalysts

Abstract: This paper reports on carbon-supported Pt-Co (3:1 atom ratio) catalysts prepared in both acid and alkaline aqueous media, followed by heat-treatments to promote alloy formation. both preparations began with a commercial 10% Pt-on-carbon catalyst, with Pt particle sizes of 15--30 {Angstrom}. Significantly greater alloying was observed in the catalyst prepared in the acid medium. X-ray diffraction studies of the acid-prepared catalyst demonstrated lattice parameters tending away from Pt (3.927 {Angstrom}) and forward that for Pt{sub 3}Co (3.831 {Angstrom}), greatly increased particle sizes, and significant ordering evidenced by the presence of super-lattice reflections. In all cases, catalysts prepared in the alkaline medium were alloyed to a lesser extent, were of moderately increased particle size, and gave no indication of alloy ordering. Activity testing under phosphoric acid fuel cell conditions demonstrated that the most highly alloyed catalysts were not significantly more active than pure Pt catalyst of comparable crystallite size. Loss of cobalt in the phosphoric acid environment was the lowest in catalysts which were the most alloyed, and were the Pt{sub 3}Co ordered phase was present.