Showing papers on "Catalysis published in 1970"

Palladium: preparation and catalytic properties of particles of uniform size.

Abstract: A method has been developed for the preparation of uniform palladium particles of diameter from 55 to 450 angstroms. Uniform particles of gold layered on palladium were also synthesized. Hydrothermal treatment of aluminum hydroxide sol was used to prepare rods of alumina with uniform cross section from 100 to 500 angstroms and of varying lengths. The palladium was adsorbed as individual particles on alumina rods, both present in aqueous suspension. Then the suspension was dried to give a catalyst containing metal particles of uniform size dispersed in open pores produced by the intermeshing of the alumina rods. This procedure guaranteed the absence of diffusion control in the rate of reactions observed experimentally. All stages of the preparation were monitored with the electron microscope. The kinetics of the ethylene-hydrogen reaction were examined by means of a pulse technique. The number of active sites determined by carbon monoxide titration of the surface was equal to the number of surface atoms as determined by the calculation of the quantities of compounds involved in the synthesis and electron microscope examination. Furthermore, the activity per site depended on the method of preparation, being four times smaller when sodium formate was used as a reducing agent instead of sodium citrate. This may be due to the fact that the shape of particles makes certain crystallographic planes more favorable. Decrease in the size of particles to 56 angstroms produced no effect on catalytic activity beyond that due to the increase in the number of surface atoms. The activity of commercial 5 percent palladium on alumina diluted 100-fold with alumina gave 80.4 percent conversion with propylene and 82.7 percent conversion with ethylene. Thus there was little difference in the behavior of the two olefins.

Production of carboxylic acids and esters

Abstract: The present invention relates to a process for the preparation of carboxylic acids and esters, specifically by the reaction of alcohols and carbon monoxide in the presence of a supported catalyst comprising the decomposition products of rhodium nitrate on a carrier, the said reaction being conducted in the presence of a halide promoter. The process is also directed to the production of mixtures of organic acids together with organic esters.

Kinetics of catalytic cracking selectivity in fixed, moving, and fluid bed reactors

Abstract: A kinetic mathematical model of catalytic cracking is described which accounts for conversion and gasoline yield in isothermal fixed, moving, and fluid bed reactors The model has been tested and verified by using laboratory moving bed data with a commercial gas oil and catalyst It is shown that under certain conditions, the selectivity behavior and maximum gasoline yield of fixed, fluid, and moving bed reactors will be identical Maximum gasoline yield is defined in terms of both the kinetic parameters and the process variables for fixed, moving, and fluid bed reactors

Homogeneous hydroformylation of alkenes with hydridocarbonyltris-(triphenylphosphine)rhodium(I) as catalyst.

Abstract: The compound hydridocarbonyltris(triphenylphosphine)rhodium(I), RhH(CO)(PPh3)3, is an efficient catalyst for hydroformylation of alkenes at 25 °C at 1 atm. From alk-1-enes high ratios of straight chain to branched chain aldehydes can be produced. The dependence of rates and products on catalyst and substrate concentration, partial pressures of hydrogen and carbon monoxide, temperature, and added excess of triphenylphosphine are described and the suggested mechanisms discussed in relation to these results. Studies at higher temperature and pressure show that rapid and effective conversion of both a liquid alkene and also propene to give high yields of straight chain aldehyde can be achieved particularly when molten triphenylphosphine provides the solvent phase.

Micellar Catalysis in Organic Reactions: Kinetic and Mechanistic Implications

Abstract: Publisher Summary This chapter reviews the importance of micellar catalysis in relation to mechanistic physical–organic and bio-organic chemistry, to enable to design meaningful experiments in the area, and to guide one through the abundant literature of surfactants and solubilization. Although solubilization by surface active agents has found considerable utility in pharmaceutical and industrial processes, the attention is primarily focussed on the mechanistic aspects of micellar catalyzed organic reactions. The subject is in its rapidly growing infancy, and is scattered and involves interdisciplinary collaboration among colloid, electro, structural, biological, pharmaceutical and organic chemists. The effects of many charged macromolecules, such as polymers, on ionic reactions can be qualitatively predicted on the basis of simple electrostatic considerations analogous to those involved in Hartley's rule.

Carbon isotope fractionation in the Fischer-tropsch synthesis and in meteorites.

Abstract: Carbon dioxide and organic compounds made by a Fischer-Tropsch reaction at 400 degrees K show a kinetic isotope fractionation of 50 to 100 per mil, similar to that observed in carbonaceous chondrites. This result supports the view that organic compounds in meteorites were produced by catalytic reactions between carbon monoxide and hydrogen in the solar nebula.

Photosystem II and O2 Evolution

Abstract: We limit the scope of this review to some of the data pertaining directly to the O2 evolving reactions of photosynthesis and to the catalysts presumed to participate in these reactions. This therefore excludes many other factors which control and regulate photosystem II. Although the specific mechanism for the photosynthetic oxidation of water still remains to be elucidated, some inroads into this difficult problem have been made in recent years. We shall attempt to consolidate some data that have appeared since publication of a previous review of photosystem II and other reviews closely related to this topic (8, 34, 82). F or a discussion of the physical and theoretical chemistry of O2, the readers are referred to articles by George (37) and Samuel (100) . The natural history of oxygen was reviewed recently by Dole (25) . Reviews and pub­ lished symposia on related topics such as oxidases and related systems are abundant. The oxidation of H20 to molecular oxygen requires a collaboration of four oxidizing equivalents. With general acceptance of the Hill-Bendall scheme for photosynthesis (�8hv/02) ' quantum yields of 1 hv/eq reported for photosystem I (103) and inferred for photosystem II (24, 89), a four-quan­ tum process for O2 evolution appears to be likely. The fundamental questions inherent in the four-quantum mechanism that have been pursued are: 1. How and in what chemical species are the oxidizing equivalents for H20 oxidation collected from the quantum trapping centers? 2. Are there "free" oxygen intermediates in this process, or does the liberation of O2 result from some terminal concerted mechanism after collection of the four oxidizing equivalents? 3. What is involved in the regulation and control of the "up­ stairs" side of system II by kinetics of electron flux on the downstairs side of system II?

Catalytic Hydrogenolysis over Supported Metals

Abstract: The subject of catalytic hydrogenolysis is a broad one, referring in general to a class of catalytic reactions involving the rupture of chemical bonds via interaction with hydrogen.

Olites catalytic cracking of hydrocarbons with mixture of zsm-5 and other ze

Abstract: HYDROCARBONS ARE CRACKED TO PRODUCTS BOILING IN THE MOTOR FUEL RANGE BY USING A CATALYST MIXTURE COMPRISING A LARGE PORE SIZE ZEOLITE SUCH AS ZEOLITE Y AND A SMALLER PORE SIZE ZEOLITE OF THE ZSM-5 TYPE. IN A PREFERRED EMBODIMENT, A SILICEOUS MATRIX MATERIAL IS ALSO USED. THE USE OF THE ZSM-5 TYPE ZEOLITE RESULTS IN OBTAINING A FUEL HAVING AN INCREASED OCTANE NUMBER AND IN INCREASED YIELDS OF C3 AND C4 OLEFINS. THESE OLEFINS CAN BE UTILIZED IN MAKING ADDITIONAL GASOLINE OR THEY CAN BE USED TO PREPARE CHEMICALS IN ACCORDANCE WITH CONVENTIONAL TECHNOLOGY.

Manganese(III) complexes in oxidative decarboxylation of acids

Abstract: Manganese(II1) effects oxidative decarboxylation of a variety of acids in nonaqueous solutions. The products and stoichiometries of the decarboxylation are examined. The reduction of MnIII follows first-order kinetics. Autoretardation by the MnII formed in the reaction is attributed to mixed valence complexes between MnIII and Mn". Alkyl radicals (and carbon dioxide) formed by multibond homolysis of the MnIII carboxylates are oxidized by a second Mn"1 to alkenes and esters. The enhanced rate of decarboxylation and oxidation of alkyl radicals by MnIII in the presence of strong acids is ascribed to cationic Mn"1 species. Copper(I1) effectively traps alkyl radicals from the decarboxylation. The autoxidative decarboxylation of pivalic acid in the presence of oxygen is catalyzed by Mn"1 and produces high yields of t-butyl alcohol and di-r-butyl peroxide. variety of transition metal compounds have been A employed to catalyze the autoxidation of hydrocarbons. In many cases, these autoxidations involve a rather complex set of reactions and metastable intermediates. In particular, the role of free radicals and their interaction with the metal species are not clear. Manganese(II1) complexes have been used to oxidize a number of types of organic functional groups.2 When manganese compounds are employed in a catalytic capacity, the metal species is thought to alternate between the I1 and I11 oxidation states. MnlI1 in aqueous solutions, however, is especially prone to disproportionate (eq 1). The latter does not 2MnI11 e MnII + MnIV (1) appear t o be as severe a limitation in nonaqueous solutions, since MnlI1 complexes are relatively stable in these media. In this report, we demonstrate the use of M P complexes in the oxidative decarboxylation of acids. Products and kinetic studies are coupled in order to clarify the mechanism of the oxidation, the catalysis by strong acids, the retardation by Mn'I and the inter(1) For a review, see (a) N. M. Emanuel, E. T. Denisov, and Z. K. Maizus, "Liquid Phase Oxidation of Hydrocarbons," Plenum Press, New York, N. Y., 1967; (b) "Oxidation of Organic Compounds," Advances in Chemistry Series, No. 76, Vol. 2, American Chemical Society, Washington, D. C., 1968; (c) Angew. Chem. Intern. Ed. Engl., 8.97 (1969). (2) W. A. Waters and J. S. Littler, "Oxidation in Organic Chemistry," K. B. Wiberg, Ed., Academic Press, New York, N. Y., 1965, Chapter 3. (3) Except where it is pertinent to the discussion, the ligands associated with the manganese species will not be included. Octahedral coordination generally pertains. mediacy of alkyl radicals. Cu'I to oxidize alkyl radicals are also compared.

A source for the special catalytic power of enzymes: orbital steering.

Abstract: The velocities of acid catalyzed esterification and γ-lactonizations were studied to test the sensitivity of a chemical reaction to the orientation of the reacting atoms. Variation in orientation of the attacking oxygen atom relative to the carbon atom of the carboxylic acid was achieved by using bicyclic ring systems to limit the conformational mobility of the γ-hydroxy acids. Factors as high as 2 × 104 were observed for the acceleration of a reaction due to this „orientation factor” even after corrections for proximity and torsional strain have been made. The orientation factor is related to the shape of the electron orbitals and must have an angular preference far greater than previously estimated. Such sensitivity to orientation would provide factors large enough to explain the gap in our understanding of enzyme catalysis. It is suggested that the catalytic efficiency of enzymes depends on their ability not only to juxtapose the reacting atoms but also to „steer” their orbitals along a path which takes advantage of this strong directional preference.

Mechanisms of thiamine-catalyzed reactions. Decarboxylation of 2-(1-carboxy-1-hydroxyethyl)-3,4-dimethylthiazolium chloride.

Abstract: Previous investigations have indicated that the thiamine pyrophosphate-dependent enzymatic decarboxylation of pyruvate to acetaldehyde proceeds via the decarboxylation of 2-(l-carboxy-l-hydroxyethyl)thiamine pyrophosphate (Ia) to 2-(l-hydroxyethyl)thiamine pyrophosphate (IIa). This paper reports the synthesis of an analog of Ia, 2-(l-carboxy-l-hydroxyethyl)-3,4-dimethylthiazolium chloride (CHDT chloride), and the kinetics of the decarboxylation of CHDT to 2-(l-hydroxyethyl)-3,4-dimethylthiazolium ion in water, ethanol-water mixtures, and ethanol. We conclude from the dependence upon pH of the observed first-order rate constants for the decarboxylation of CHDT in water at 67 ' that the reactive species is the one in which the carboxyl group is ionized. The rate of decarboxylation of this dipolar ion is markedly increased in solvents less polar than water; the half-times for decarboxylation are 24.0 hr in water a t 45.6' and 3.2 min in absolute ethanol at 26.0'. Comparison of this model with the pyruvate decarboxylase reaction shows that the enzyme accelerates the decarboxylation of Ia in water by a factor of at least lo5. We propose that the enzymatic catalysis is effected through binding of the thiazolium portion of Ia in a region of the enzyme less polar than water and suggest that such an enzymatic solvent effect is a major cause of catalysis in many thiamine pyrophosphate-dependent enzymatic reactions. he decarboxylation of pyruvate to acetaldehyde by T the enzyme pyruvate decarboxylase is a reaction which requires the coenzyme thiamine pyrophosphate.2 On the basis of investigations of nonenzymatic model reactions, Breslow a proposed that the enzymic reaction proceeds by way of 2-(l-carboxy-l-hydroxyethy1)thiamine pyrophosphate (Ia), which is formed from thiamine pyrophosphate and pyruvic acid by reaction of the thiazolium ring, ionized at carbon 2, with the carbonyl group of pyruvic acid. Decarboxylation of this intermediate yields 2 4 1-hydroxyethy1)thiamine pyrophosphate (IIa), which can then lose acetaldehyde and so regenerate the thiamine pyrophosphate. Subsequently, the proposed intermediate H 0 > N x c H 3 Rl + 3J--cH3 HOZCC R.2 HC '4 & I CH3 I CH3

Adsorbed Atomic Species as Intermediates in Heterogeneous Catalysis

Abstract: Publisher Summary This chapter discusses the thermodynamic activity of an atomic species on the surface of a catalyst and its determination under steady-state conditions of a catalytic reaction. If the rate of an individual step of a catalytic reaction depends on the concentration of adsorbed atoms occurring as intermediates, it is imperative to determine this dependence. The determination of the surface concentration of molecules adsorbed on a catalyst is feasible under favorable conditions. In the case of atomic species, it has been found to be convenient to use the thermodynamic activity rather than the surface concentration as the relevant variable. The primary problem of chemical kinetics is the formulation of an empirical rate law that represents the rate of a reaction as a function of the concentrations or partial pressures of reactants, products, and catalysts present in a gaseous or liquid phase. The knowledge of the mechanism of a reaction may be used to formulate a rational rate law that may represent the empirical rate data in a more logical form than an empirical rate law.