Showing papers on "Catalysis published in 1978"

Optimization of methods for aspartate aminotransferase and alanine aminotransferase.

Abstract: Conditions for accurate measurement of catalytic activity of aspartate aminotransferase and alanine aminotransferase in human serum have been reinvestigated. The basic variables (kind of buffer, buffer concentration, pH, ion effects, and the influence of pyridoxal-5-phosphate) can now be considered optimized. On this basis, the kinetic parameters of both aminotransferases were determined, i.e., Michaelis and inhibitor constants for substrates and reaction products. With a mathematical approach for two-substrate enzyme reactions the substrate concentrations were calculated from the viewpoints "most economical," "most convenient," and "lowest variability." Also the conditions for the indicator reactions have been newly defined with respect to a kinetic model. All calculated data were rechecked experimentally and it can be shown that both approaches fully agree. Furthermore, we show that the mathematical approach allows more precise recommendations for optimized methods. For technical reasons, the catalytic activity of aspartate aminotransferase in human serum can only be measured as a 0.96 fraction of its theoretical maximum velocity, the catalytic activity of alanine aminotransferase as a 0.91 fraction. The assay conditions for a Reference Method are finally described and recommendations are made for optimized routine methods for determination of the catalytic activity of these transferases in human serum.

Preparation of catalysts by metallic complex adsorption on mineral oxides

Abstract: The usefulness of metallic complex adsorption phenomena on oxides for the preparation of dispersed metallic supported catalysts have led us to analyze phenomena which occur at the interface—oxide solution. This analysis is based on simple principles such as surface polarization of oxides versus pH and adsorption of counterions by electrostatic attraction. The three most important parameters which seem to regulate these adsorption phenomena are : isoelectric point of the oxide, pH of the aqueous solution, and nature of the metallic complex. This simplified approach toward adsorption phenomena is in agreement with the published results in the literature concerning the fixation of chiorometallic and amine complexes of metals belonging to 7a, 8 and lb groups on alumina or silica carriers. An extension of this analysis to other mineral oxides is proposed. 1. IMPORTANCE OF METALLIC CATALYSTS The importance of metallic catalysts in our economic system need certainly no longer be demonstrated today. They are involved in such different fields as the oil refining, automobile, petrochemical and fine chemical industries. A list of the principal processes operating with metallic catalysts will be found in table 1. The properties of these catalysts in general and their activi y in particular are in close relation to the state of dispersion of the active elements. This explains why three—fourths of the processes listed in table 1 use catalytic systems in which the active phase is in the form of very small crystallites of about ten angstroms dispersed on the surface of a support. It is what we usually call dispersed metallic catalysts. The metals used in these catalysts belong generally to groups 7a, 8 and lb of the periodic table, very often such as platinum and palladium. Four supports are most often used in their preparation : alumina, silica-. alumina, active carbon and molecular sieves. It is to be noted that the Raney nickel which is still used nowadays in a lot of liquid phase hydrogenations constitutes a particular case of a dispersed metallic catalyst which is non supported. Il we consider the tonnages of catalyst involved, the most important processes are catalytic reforming, hydrogenations of different petrochemical streams, and above all, automobile post—combustion, which represents the greatest turnover for the industry.

Heterogeneous photocatalytic decomposition of saturated carboxylic acids on titanium dioxide powder. decarboxylative route to alkanes

Abstract: The heterogeneous photocatalytic decomposition of acetic acid/acetate mixtures was studied on n-type TiO/sub 2/ powder. The influence of solution composition, reaction temperature, light intensity, and semiconductor properties (crystallographic structure, doping) was investigated. For photodecarboxylation in the absence of molecular oxygen, anatase powders proved to be most efficient with increased reactivity due to doping and/or partial coverage of the powder particles with platinum. The products of the photodecomposition of acetic acid on platinized anatase powder were mainly methane and CO/sub 2/, with small amounts of hydrogen and ethane. Other saturated carboxylic acids (propionic, n-butyric, n-valeric, pivalic, adamantane-1-carboxylic acid) were also photocatalytically decarboxylated to the corresponding alkanes. An interpretation based on the photoelectrochemical properties of n-type TiO/sub 2/, with photooxidation and (dark) reduction reactions occurring on the particles on local cell processes, is proposed.

Conversion of a protein to a homogeneous asymmetric hydrogenation catalyst by site-specific modification with a diphosphinerhodium(I) moiety

Abstract: We wish to describe an approach to the construction of asymmetric hydrogenation catalysts based on embedding an (effectively) achiral diphosphinerhodium(l) moiety at a specific site in a protein: the protein tertiary structure provides thc chirality required for enantioselective hydrogenation. Although it is presently difficult to predict the enantioselectivity of any hydrogenation from knowledge of the structures of catalyst and substratc, phosphine-rhodium(l) complexes having rigid. conformationally homogeneous structures seem generally to be more effective catalysts than those which are conforma-tionally mobile .2 A globular protein modified by introduction of a catalytically active metal at an appropriate sitc could, in principle, provide an exceptionally wcll-defined steric environment around that mctal. and should do so for considerably smaller effort than would be required to construct a synthetic substancc of compa rable stereochemical complexi ty. Our initial efforts have focused on avidin. This well-characterized protein is composed of four ide ntical subunits, each of which binds biotin and many of its dcrivativcs sufficiently' tightly that association is effectively irreversible (K.r : l0-r2*10-rs M).1'a Biotin was converted to a chelating di-phosphine and complexed with rhodium(l) by the sequencc outlined in cq I (NBD = norbornadiene, Tf = triflate).5.6 The i n term ed ia te N,I/-bi s (2-d i ph en y I phos prh i noet h 1,'l) bi ot i n a m i dc (l) was fully characterizcd:s the rhodium complex l.

Theoretical study on a reaction pathway of Ziegler–Natta‐type catalysis

Abstract: Ab initio SCF–LCAO–MO all‐electron calculations have been performed for the catalyzed reaction, related to the Ziegler–Natta polymerization process TiCl4⋅Al(CH3)3+C2H4 →TiCl4⋅Al(CH3)2⋅C3H7. The energy of the Ti–Al–ethylene complex during the reaction pathway is associated to the following processes: First, the ethylene coordinates to the catalyst complex and subsequently it reacts with the methyl moiety producing an activation energy barrier mainly due to the breaking of the ethylene double bond. The computed barrier height (15 kcal/mole) is in reasonable agreement with experimental activation energies. The identification of the energy barrier is based on the analysis of the electron population, bond energies, and molecular orbitals of the system as they evolve during the reaction. A comparison with the qualitative picture resulting from the classical model of Cossee and with semiempirical calculations is presented. From these calculations a model is derived which leads to a tentative suggestion for the e...

Selective formation of methanol from synthesis gas over palladium catalysts

Abstract: It is reported that hydrogenation of CO over supported Pd catalysts produces methanol in high selectivity within the temperature-pressure regime where CH/sub 3/OH formation is thermodynamically allowed. The same behavior was found for Pt and Ir as well, but at lower rates. The mechanism of CH/sub 3/OH formation on Pd is discussed. 3 tables, 33 references. (DLC)

Hydrodesulfurization of dibenzothiophene catalyzed by sulfided CoO-MoO3γ-Al2O3: The reaction network

Abstract: Kinetics experiments have determined the reaction network in the hydrodesulfurization of dibenzothiophene catalyzed by sulfided CoO-MoO3/γ-Al2O3 at 573°K and 102 atm The predominant reaction is a direct sulfur extrusion, giving biphenyl and hydrogen sulfide; the biphenyl is subsequently hydrogenated slowly to give cyclohexylbenzene and then bicyclohexyl Dibenzothiophene also undergoes a primary hydrogenation reaction preceding sulfur removal, but it is about one thousand times slower than the sulfur extrusion reaction

Self-Restoring of the Active Surface in the Hydrogen Sponge La Ni 5

Abstract: Photoemission studies combined with measurements of the magnetic susceptibility show that La segregates to the surface of La${\mathrm{Ni}}_{5}$ in the presence of ${\mathrm{O}}_{2}$ or ${\mathrm{H}}_{2}$O, and the Ni precipitations are formed during hydrogenation. This represents a self-restoring mechanism of the active surface, since the highly reactive La removes the oxygen and keeps the Ni metallic. Dissociative adsorption of ${\mathrm{H}}_{2}$ may then occur at the Ni atoms.

Acid catalysis of the formation of the slow-folding species of RNase A: evidence that the reaction is proline isomerization.

Abstract: Unfolded RNase A is known to contain an equilibrium mixture of two forms, a slow-folding form (U(1)) and a fast-folding form (U(2)). If U(1) is produced after unfolding by the slow cis-trans isomerization of proline residues about X-Pro imide bonds, then the formation of U(1) should be catalyzed by strong acids. Therefore, the rate of formation of U(1) has been measured at different HClO(4) concentrations. After rapid unfolding of the native protein in concentrated HClO(4) at 0 degrees , the slow formation of U(1) was measured by use of refolding assays. Catalysis of its formation was found at HClO(4) concentrations above 5 M. The uncatalyzed reaction follows apparent first-order kinetics but, in the acid-catalyzed range, two reactions are found. The faster reaction produces two-thirds of the slow-folding species and shows acid catalysis above 5 M HClO(4). Catalysis of the slower reaction begins at 8 M HClO(4). The faster reaction shows a 100-fold increase in rate at 10.6 M HClO(4) over the rate of the uncatalyzed reaction of 5 M. The activation enthalpy of the uncatalyzed reaction has been measured in two sets of unfolding conditions: DeltaH(double dagger) is 21.5 kcal/mol (1 kcal = 4.2 x 10(3) J) in 3.3 M HClO(4) and 21.0 kcal/mol in 5 M guanidine HCl, pH 2.5.Both acid catalysis of the formation of U(1) and its high activation enthalpy are consistent with the rate-limiting step being cis-trans isomerization either of X-Pro imide bonds or of peptide bond. The rate of the uncatalyzed reaction is in the range expected for proline isomerization and is 0.1% of that of peptide bond isomerization; thus, the simplest explanation for the formation of U(1) is proline isomerization. Earlier data, showing that the kinetic properties of the U(1) right arrow over left arrow U(2) reaction in refolding conditions differ from those of proline isomerization, can be explained if there is kinetic coupling between early steps in the folding of U(1) and its conversion to U(2).The existence of two acid-catalyzed reactions that are distinguished by the HClO(4) concentration at which catalysis begins suggests that at least two essential proline residues produce slow-folding species of RNase A by isomerization after unfolding. Because protonation of imide bonds is responsible for acid catalysis of proline isomerization, the slower reaction probably involves an imide bond with a low pK. It may be the bond connecting Lys-41 and Pro-42, because the positive charge on Lys-41 could make this bond more difficult to protonate.

Palladium-catalyzed vinylic substitution with highly activated aryl halides

Abstract: p-Bromphenol (I) reagiert mit dem Acrylester (II) in Gegenwart von Phosphinen (III) und Palladium(II)-acetat zum p-Hydroxy-zimtsaureester (IV).