Three-step Reaction Mechanism Research Articles

Metal–Halogen Exchange between Hydrazine Polyanions and α,α′-Dibromo-o-xylene

Aromatic-bridged bis(hydrazines) were found to be the main products in the reaction of hydrazine polyanions with α,α′-dibromo-o-xylene. It was confirmed that the reaction is driven by a metal–halogen exchange process. A three-step reaction mechanism is suggested.

Preparation and formation mechanism of silver gallium selenide powders prepared via the sol–gel method

The sol–gel method followed by selenization was developed to synthesize AgGaSe2 powders in this study. The molar ratio of gallium ions to silver ions was controlled to prepare pure AgGaSe2 powders. Two selenization processes were employed to investigate the formed phases of the obtained powders. When the sol–gel-derived precursors were mixed with selenium powders, pure-phase AgGaSe2 powders were formed after selenization. Without mixing selenium powders, AgGaSe2 was produced with the impurity phase of Ag9GaSe6. A three-step reaction mechanism for the formation of AgGaSe2 is proposed. Ag2Se is initially formed in the first step, and then reacts with constituent species as well as hydrogen to produce Ag9GaSe6. Finally AgGaSe2 is formed from the reaction among Ag9GaSe6, Ga2O3, Se and H2. When the selenium powders are mixed with the precursors, liquid selenium is formed in the selenization process and effectively promotes the reactions.

Trimerization of aldehydes with one α-hydrogen catalyzed by sodium hydroxide

AbstractTrimerization of 2-methylpropanal (isobutyraldehyde) is a simple and effective method to synthesize 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate and 2,2,4-trimethyl-1,3-pentanediol-3-monoisobutyrate which are often used as film forming auxiliaries in paints. The use of solid sodium hydroxide as a catalyst provides an excellent yield of above 85 % after the optimization of the reaction time and the catalyst dosage. Furthermore, trimerization of four other aldehydes with one α-hydrogen catalyzed by solid sodium hydroxide can also take place and the yield of 1,3-diol monoesters reaches 50–70 %. Trimerization of aldehydes with one α-hydrogen can be explained by a three-step reaction mechanism: (i) aldol condensation of aldehyde; (ii) crossed Cannizzaro reaction; and (iii) esterification of carboxylic acid and alcohol.

New High-Temperature Pb-Catalyzed Synthesis of Inorganic Nanotubes

A new procedure for the synthesis of MoS(2) nanotubes is reported, and additionally demonstrated for MoSe(2), WS(2), and WSe(2). Highly concentrated sunlight creates continuous high temperatures, strong temperature gradients, and extended hot annealing regions, which, together with a metallic (Pb) catalyst, are conducive to the formation of different inorganic nanotubes. Structural characterization (including atomic resolution images) reveals a three-step reaction mechanism. In the first step, MoS(2) platelets react with water-air residues, decompose by intense solar irradiation, and are converted to molybdenum oxide. Subsequently, the hot annealing environment leads to the growth of Pb-stabilized MoO(3-x) nanowhiskers. Shortly afterward, the surface of the MoO(3-x) starts to react with the sulfur vapor supplied by the decomposition of nearby MoS(2) platelets and becomes enveloped by MoS(2) layers. Finally, the molybdenum oxide core is gradually transformed into MoS(2) nanotubes. These findings augur well for similar syntheses of as yet unattained nanotubes from other metal chalcogenides.

Various carbon dust particles

Nano-sized carbon dusts are suspected of having negative effects on human health. An exact characterization of such particles is necessary to understand possible toxic effects, i.e. in the lung. Observed by transmission electron microscopy (TEM), the carbon dusts are a composite of very small primary particles and larger agglomerates of these. A differentiation of the primary particles and agglomerates according to source is not possible by TEM, however, thermogravimetry investigations in synthetic air atmosphere are helpful. Standardized carbon black and graphite show a single-step oxidation behaviour, whereas ethene soot and diesel soot, for example, show more complex-reaction mechanisms. The results of ethene soot exemplarily demonstrate the oxidation mechanism. In addition to the oxidation reaction to carbon dioxide, a sintering process takes place. To confirm the oxidation mechanism, thermal behaviour of ethene soot has been simulated by kinetic modulation using a three-step reaction mechanism of n-th order. The reaction order indicates a complex mechanism for the first-reaction step. For the second and third-reaction step, a phase boundary mechanism could be suggested.

Theoretical analysis of the conversion mechanism of acetylene to ethylidyne on Pt(111)

The conversion of acetylene to ethylidyne on Pt(111) has been comprehensively investigated using self-consistent periodic density functional theory. Geometries and energies for all of the intermediates involved as well as the conversion mechanism were analyzed. On Pt(111), the carbon atoms in the majority of stable C(2)H(x) (x = 1-4) intermediates prefer saturated sp(3) configurations with the missing H atoms substituted by the adjacent metal atoms. The most favorable conversion pathway for acetylene to ethylidyne is via a three-step reaction mechanism, acetylene → vinyl → vinylidene → ethylidyne. The first step, acetylene → vinyl, depends on the availability of surface H atoms: without preadsorbed H the reaction occurs via the initial disproportionation of acetylene, which resulted in adsorbed vinyl; with an abundance of preadsorbed H, acetylene could transform to vinyl via both the disproportionation and hydrogenation reactions. Conversions through initial dehydrogenation of acetylene and isomerizations of acetylene and vinyl are unfavorable due to high energy barriers along the relevant pathways. The conversion rate involving vinylidene as an intermediate is at least 100 times larger than that involving ethylidene.

Interaction of Mass Transport and Reaction Kinetics during Electrocatalytic CO Oxidation in a Thin-Layer Flow Cell

The interplay between electrocatalytic CO oxidation kinetics and mass transport in a thin-layer flow cell with a polycrystalline platinum electrode was experimentally studied and numerically simulated. The experiments were performed in a flow cell under controlled electrolyte flow. The computations are based on four different models for mass transport, coupled with a three-step reaction mechanism for CO oxidation that includes surface coverage effects. A zero-dimensional model neglecting mass transport effects on the overall reaction rate is applied to verify the effect of the kinetic parameters on the Faradaic current qualitatively. Transport models of increasing complexity, one-, two-, and three-dimensional, are used to analyze the impact of diffusion and convection on the total reaction rate in the flow cell. The numerical simulation, based on a time-resolved three-dimensional transport model coupled with the electrocatalytic kinetics, resolves profiles of temporal and spatial concentrations, velocities, and surface coverages. Mass transfer limitations of the electrocatalytic reaction rate are primarily caused by CO diffusion normal to the electrode. Both the two- and the three-dimensional models can quantitatively predict the Faradaic current as a function of potential measured in the experimental setup.

Modeling of uncertainties in biochemical reactions

Mathematical modeling is an indispensable tool for research and development in biotechnology and bioengineering. The formulation of kinetic models of biochemical networks depends on knowledge of the kinetic properties of the enzymes of the individual reactions. However, kinetic data acquired from experimental observations bring along uncertainties due to various experimental conditions and measurement methods. In this contribution, we propose a novel way to model the uncertainty in the enzyme kinetics and to predict quantitatively the responses of metabolic reactions to the changes in enzyme activities under uncertainty. The proposed methodology accounts explicitly for mechanistic properties of enzymes and physico-chemical and thermodynamic constraints, and is based on formalism from systems theory and metabolic control analysis. We achieve this by observing that kinetic responses of metabolic reactions depend: (i) on the distribution of the enzymes among their free form and all reactive states; (ii) on the equilibrium displacements of the overall reaction and that of the individual enzymatic steps; and (iii) on the net fluxes through the enzyme. Relying on this observation, we develop a novel, efficient Monte Carlo sampling procedure to generate all states within a metabolic reaction that satisfy imposed constrains. Thus, we derive the statistics of the expected responses of the metabolic reactions to changes in enzyme levels and activities, in the levels of metabolites, and in the values of the kinetic parameters. We present aspects of the proposed framework through an example of the fundamental three-step reversible enzymatic reaction mechanism. We demonstrate that the equilibrium displacements of the individual enzymatic steps have an important influence on kinetic responses of the enzyme. Furthermore, we derive the conditions that must be satisfied by a reversible three-step enzymatic reaction operating far away from the equilibrium in order to respond to changes in metabolite levels according to the irreversible Michelis-Menten kinetics. The efficient sampling procedure allows easy, scalable, implementation of this methodology to modeling of large-scale biochemical networks.

Effects of the Burner Diameter on the Flame Structure and Extinction Limit of Counterflow Non-Premixed Flames

Experiments and numerical simulations were conducted to investigate the effects of the burner diameter on the flame structure and extinction limit of counterflow non-premixed methane flames in normal gravity and microgravity. Experiments were performed for counterflow flames with a large inner diameter ( d) of 50 mm in normal gravity to compare the extinction limits with those obtained by previous studies where a small burner ( d < 25 mm) was used. Two-dimensional (2D) simulations were performed to clarify the flame structure and extinction limits of counterflow non-premixed flame with a three-step global reaction mechanism. One-dimensional (1D) simulations were also performed with the same three-step global reaction mechanism to provide reference data for the 2D simulation and experiment. For microgravity, the effect of the burner diameter on the flame location at the centerline was negligible at both high ( ag = 50 s−1) and low ( ag = 10 s−1) strain rates. However, a small burner flame ( d = 15 mm) in microgravity showed large differences in the maximum flame temperature and the flame size in radial direction compared to a large burner flame ( d = 50 mm) at low strain rate. In addition, for normal gravity, a small burner flame ( d = 23.4 mm) showed differences in the flame thickness, flame location, local strain rate, and maximum heat release rate compared to a large burner flame ( d = 50 mm) at low strain rate. Counterflow non-premixed flames with low and high strain rates that were established in a large burner were approximated by 1D simulation for normal gravity and microgravity. However, a counterflow non-premixed flame with a low strain rate in a small burner could not be approximated by 1D simulation for normal gravity due to buoyancy effects. The 2D simulations of the extinction limits correlated well with experiments for small and large burner flames. For microgravity, the extinction limit of a small burner flame ( d = 15 mm) was much lower than that of a large burner flame when ag ≤ 20 s−1. For normal gravity, the extinction limit of a small burner flame ( d = 23.4 mm) was also much lower than that of the large burner flame when ag ≤ 35 s−1. The effects of the burner diameter on the flame structure and extinction limit of counterflow non-premixed methane flames were more important in normal gravity than in microgravity.

Effect of potassium hydrogen phthalate (C8H5KO4) on the one-step electrodeposition of single-phase CuInS2 thin films from acidic solution

Abstract The effect of potassium hydrogen phthalate (C 8 H 5 KO 4 ) as a special additive on the one-step electrodeposition of single-phase CuInS 2 thin films from acidic solution (pH 2.5) was investigated in detail. The XRD, SEM and UV–vis–NIR characterization confirms that the addition of an adequate concentration of C 8 H 5 KO 4 (23 mM) to the electrolytic bath containing 12.5 mM Cu 2+ , 10 mM In 3+ , 40 mM S 2 O 3 2− and 100 mM LiCl can contribute greatly to the controllable growth of pure chalcopyrite CuInS 2 films with uniform surfaces and an ideal band gap of approximately 1.54 eV. Complexation studies of C 8 H 5 KO 4 with Cu 2+ and In 3+ in electrolytic solutions indicated that C 8 H 5 KO 4 can complex Cu 2+ more strongly than In 3+ and move the electrode potentials of Cu 2+ and In 3+ near each other as determined by polarization analysis. Furthermore, the potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) analysis performed in a series of solution systems revealed a three-step reaction mechanism for CuInS 2 deposition and considerable adsorption of C 8 H 5 O 4 − and Cu(C 8 H 5 O 4 ) + to the cathode surface. This deposition shows that the synergetic effects of complexation and adsorption originated from the additive on the Cu 2+ electro-reduction, thus promoting the co-deposition of copper, indium and sulfur in the form of single-phase CuInS 2 .

Thermische Isomerisierung eines Silenketazins zum Diazasilacyclopenten: Experimentelle und theoretische Untersuchungen

Lithium-tert-butylmethylhydrazonide II, Me3C(Me)C=N-NHLi, reacts with F2Si[N(CHMe2)2]2 to give Me3C(Me)C=N-NH-SiF[N(CHMe2)2]2 1. The lithium salt of 1, Me3 C(Me)C=N- N(Li)SiF[N(CHMe2)2]2, 1a, prepared in the reaction of 1 with n-C4H9Li, is substituted with F2BN(SiMe3)2 forming Me3C(Me)C=N-NBFN(SiMe3)2SiF[N(CHMe2)2]2, 2. Experiments to synthesise the silaketazine, Me3C(Me)C=N-N=Si[N(CHMe2)2], III, via LiF-elimination from 1a lead to the intramolecular formation of an N-functional 1,2-diaza-3-silacyclopentene, H2C-C(CMe3)=N- NHSi[N(CHMe2)2]2, 3, which is a structural isomer of III. The NH unit of 3 can be lithiated with n-C4H9Li. The lithium salt reacts with F2BN(SiMe3)2 forming the substituted ring compound 4. The rearrangement of the silaketazine III to the ring compound 3 is described by density functional calculations predicting a three-step reaction mechanism correlated with the experimental data. The structures of 3 and 4 are discussed in detail.

Kinetic study of the complexation of gallic acid with Fe(II)

Kinetic study on the complexation of gallic acid with ferrous sulfate was performed using UV–Vis absorption spectroscopy. Under the experimental conditions, the stoichiometric composition of the formed complex is 1:1. The complexation reaction was found to be a second-order one. The influences of temperature, ionic strength and solvents on the complexation reaction were investigated. According to the Arrhenius equation, the apparent activation energy of the complexation reaction was evaluated to be 71.64 kJ × mol −1. A three-step reaction mechanism was proposed, which can well explain the kinetic results obtained.

On the Mechanism of Iron(III)-Dependent Oxidative Dehydrogenation of Amines

Kinetic and structural data are presented for the iron-promoted dehydrogenation of the amine, [Fe(III)L3]3+ (1), L3 = 1,9-bis(2'-pyridyl)-5-[(ethoxy-2''-pyridyl)methyl]-2,5,8-triazanonane. Spectroscopic and electrochemical experiments under the exclusion of dioxygen helped to identify reaction intermediates and the final product, the Fe(II)-monoimine complex [Fe(II)L4]2+ (2), L4 = 1,9-bis(2'-pyridyl)-5-[(ethoxy-2''-pyridyl)methyl]-2,5,8-triazanon-1-ene. 2 is formed by disproportionation of the starting complex 1 by a three-step reaction mechanism, most likely via ligand-centered radical intermediates. The rate law can be described by the second-order rate equation, -d[(Fe(III)L3)3+]/dt = k(EtO)- [(Fe(III)L3)3+][EtO-], with k(EtO)- = 4.92 +/- 0.01 x 104 M(-1) s(-1) (60 degrees C, mu = 0.01 M). The detection of general base catalysis and a primary kinetic isotope effect (k(EtO)-(H)/k(EtO)-(D) = 1.73) represents the first kinetic demonstration that the deprotonation becomes rate determining followed by electron transfer in the oxidative dehydrogenation mechanism. We also isolated the Fe(II)-monoimine complex 2 and determined its structure in solution (NMR) and in the solid state (X-ray).

Electrochemical Characterizations of Germanium and Carbon-Coated Germanium Composite Anode for Lithium-Ion Batteries

The electrochemical reaction mechanism of germanium with lithium at room temperature was investigated. Two distinct voltage plateaus corresponding to the formation of Li 9 Ge 4 and Li 7 Ge 2 were observed, and the final products were identified as a mixture of Li 15 Ge 4 and Li 22 Ge 5 phases. A three-step reaction mechanism of germanium with lithium was suggested. Carbon-coated germanium composites prepared by mechanical milling and pyrolysis were evaluated as anode materials for lithium-ion batteries. The composite materials, Ge-mesocarbon microbeads and Ge-poly(vinyl alcohol), showed good cyclability and retained fairly large capacities of 473 and 579 mAh/g, respectively, up to 50 cycles when cycled within a limited voltage window.

Chemical kinetics and mass transport effects in solution-based selective-area growth of ZnO nanorods

We present a combined experimental and modeling study of the dependence of solution-based zinc oxide (ZnO) selective-area growth rates on pattern dimension. Selective growth is achieved by patterning a portion of the substrate with an organic template that inhibits growth. The density of ZnO nanorods and the mass grown per unit area of exposed surface increases as the distance between the exposed growth regions is increased and as the width of the exposed lines is decreased. A 2-D model was developed to calculate selective growth at the exposed surface regions, the loss of reactant material due to a competing reaction in solution, liquid-phase and surface diffusive mass transport to (or on) the growth surface, and the ZnO growth reaction at the surface. To explain the experimental results, we found it necessary to include a reaction by-product in the chemistry model, the desorption of which is the rate limiting step. A relatively simple, three-step reaction mechanism, combined with the species mass transport model, provides a good, semi-quantitative description of the experimental observations in the selective-area growth of ZnO from supersaturated solutions.

Flame retarding of wood by impregnation with boric acid – Pyrolysis products and char oxidation rates

The pyrolysis of fir wood impregnated with boric acid (0–5.4%) has been investigated for heating temperatures of 650 and 800 K by examining the yields of char, water, permanent gases (CO 2, CO, CH 4) and total organic products (together with 32 compounds). The yields of the last product class continuously decrease to the advantage of char and water, but the most significant modifications occur for acid contents below 2%. The formation of levoglucosan (with 2-acetylfuran, 5-methyl-2-furaldehyde and other minor species) first and levoglucosenone (with 2-furaldehyde) afterwards is favoured, whereas other compounds generated from the holocellulosic (hydroxyacetaldehyde, hydroxypropanone, acetic acid and minor carbohydrates) and lignin (phenols, cresols) fractions generally decline. Conversion times become longer and volatilization rates are reduced. The oxidation characteristics of char have been studied by means of thermogravimetric analysis and interpreted according to a three-step reaction mechanism. The boric acid treatment lowers the activation energy and reaction order of the most important step (145 versus 226 kJ/mol and 1.2 versus 0.86, respectively) which also shows lower rates and is slightly delayed.

Impact of radiation models in CFD simulations of steam cracking furnaces

The endothermic thermal cracking process of hydrocarbons takes place in tubular reactor coils suspended in large gas-fired pyrolysis furnaces. Heat transfer to the reactor tubes is mostly due to radiation from the furnace refractory walls and the flue gas. A three-dimensional (3-D) simulation of the flow in an industrial scale steam cracking furnace is performed. The renormalization group (RNG) k − ɛ turbulence model is used. The combustion kinetics is modeled by a three-step reaction mechanism, while turbulence–chemistry interaction is taken into account through the Finite Rate/Eddy-Dissipation model. The Discrete Ordinates model (DOM), the P-1 and the Rosseland Radiation model are used for modeling of the radiative heat transfer. The results of using the different radiation models are compared mutually with adiabatic simulation results. The absorption coefficient of the gas mixture is calculated by means of a Weighted-Sum-of-Gray-Gas model (WSGGM). The effect is discussed of the use of different radiation models on the predicted wall, tube skin and flue gas temperature profiles and heat fluxes towards the reactor tubes, as well as on the predicted species concentration profiles and structure of the furnace flames under normal firing conditions.

Influence of the Axial Dispersion on the Performance of Tubular Reactors during the Noncatalytic Supercritical Transesterification of Triglycerides

Transesterification of fats and oils by methanol is known to proceed by a three-step consecutive reaction mechanism in which the formation of the diglyceride is the rate-limiting step. At low conversion values (<25−35%), the system has mass transfer limitations due to the immiscibility of the oil−methanol system. Noncatalytic transesterication in supercritical methanol overcomes this problem by forming a one-phase reacting system. The implementation of this reaction mode in the continuous tubular reactor is needed for the evaluation of the effect of the backmixing that appears when methanol acquires a gaslike mobility in the supercritical state. Such a study was performed here, and it was found that supercritical transesterification reactors must be operated at an axial Péclet number equal or higher than 1000 in order to limit backmixing effects to a minimum and achieve batchlike conversions at residence times of 1 h or less. High conversion values at low Péclet numbers can only be obtained by working at inconveniently high temperatures and high methanol-to-oil ratios.

Structural Studies of FlaA1 from Helicobacter pylori Reveal the Mechanism for Inverting 4,6-Dehydratase Activity

FlaA1 from the human pathogen Helicobacter pylori is an enzyme involved in saccharide biosynthesis that has been shown to be essential for pathogenicity. Here we present five crystal structures of FlaA1 in the presence of substrate, inhibitors, and bound cofactor, with resolutions ranging from 2.8 to 1.9 A. These structures reveal that the enzyme is a novel member of the short-chain dehydrogenase/reductase superfamily. Additional electron microscopy studies show the enzyme to possess a hexameric doughnut-shaped quaternary structure. NMR analyses of "real time" enzyme-substrate reactions indicate that FlaA1 is a UDP-GlcNAc-inverting 4,6-dehydratase, suggesting that the enzyme catalyzes the first step in the biosynthetic pathway of a pseudaminic acid derivative, which is implicated in protein glycosylation. Guided by evidence from site-directed mutagenesis and computational simulations, a three-step reaction mechanism is proposed that involves Lys-133 functioning as both a catalytic acid and base.

Electrochemical Reactions Between Perovskite-Type LaCoO3 and Lithium

The perovskite-type composite metal oxide LaCoO3 was firstly used as an electrode material in rechargeable lithium cells. X-ray diffraction (XRD), scanning electron microscopy (SEM) and extended X-ray absorption fine structure (EXAFS) were employed to analyze the structures of synthesized and discharged LaCoO3 samples. Cyclic voltammetry and galvanostatic cell cycling were used to characterize the electrochemical performance of LaCoO3/Li cells. A stable reversible capacity from 110 to 130 mAh/g during up to 50 cycles can be achieved. Based on the analyses of ex-situ XRD and EXAFS of the lithiated/delithiated LaCoO3 electrode, a three-step electrochemical reaction mechanism was proposed.