Showing papers on "Reaction rate published in 2002"

Ionic reactions and pyrolysis of glycerol as competing reaction pathways in near- and supercritical water

Abstract: Experimental results of the decomposition of glycerol in near- and supercritical water are presented considering measurements in the temperature range of 622–748 K, at pressures of 25, 35, or 45 MPa, reaction times from 32 to 165 s, and different initial concentrations. The reaction was carried out in a tubular reactor and a conversion between 0.4 and 31% was observed. The main products of the glycerol degradation are methanol, acetaldehyde, propionaldehyde, acrolein, allyl alcohol, ethanol, formaldehyde, carbon monoxide, carbon dioxide, and hydrogen. The results are compared with the studies of other working groups. The non-Arrhenius behavior of the overall degradation, as well as the pressure dependence of the reaction rate, and furthermore, the product distribution indicates the occurrence of two competing reaction pathways. One pathway consists of ionic reaction steps, which are preferred at higher pressures and/or lower temperatures. The second reaction pathway is a free radical degradation and dominates at lower pressures and/or higher temperatures. For reaction modeling, both mechanisms, the ionic and the free radical reaction network are compiled into one reaction model. The computer software package chemkin was used for the model calculations. The reaction model and the kinetic parameters were optimized in order to describe the experimental results for glycerol and the main products at 450 bar and all temperatures. This reaction model, consisting of the ionic and the free radical sub-mechanism satisfactorily describes the complex reaction at 450 bar.

Copper-Catalyzed Halogen Exchange in Aryl Halides: An Aromatic Finkelstein Reaction

Abstract: A mild and general method for the conversion of aryl, heteroaryl, and vinyl bromides into the corresponding iodides was developed utilizing a catalyst system comprising 5 mol % of CuI and 10 mol % of a 1,2- or 1,3-diamine ligand. A variety of polar functional groups are tolerated, and even N−H containing substrates such as sulfonamides, amides, and indoles are compatible with the reaction conditions. Both the reaction rate and the equilibrium conversion of the aryl bromide depend on the choice of the halide salt and the solvent. The best results were obtained using NaI as the halide salt and dioxane, n-butanol, or n-pentanol as the solvents.

A SN2 Reaction That Avoids Its Deep Potential Energy Minimum

Abstract: Chemical dynamics trajectory simulations were used to study the atomic-level mechanisms of the OH- + CH3F --> CH3OH + F- SN2 nucleophilic substitution reaction. The reaction dynamics, from the [OH...CH3...F]- central barrier to the reaction products, are simulated by ab initio direct dynamics. The reaction's potential energy surface has a deep minimum in the product exit channel arising from the CH3OH...F- hydrogen-bonded complex. Statistical theories of unimolecular reaction rates assume that the reactive system becomes trapped in this minimum and forms an intermediate, with random redistribution of its vibrational energy, but the majority of the trajectories (90%) avoided this potential energy minimum and instead dissociated directly to products. This finding is discussed in terms of intramolecular vibrational energy redistribution (IVR) and the relation between IVR and molecular structure. The finding of this study may be applicable to other reactive systems where there is a hierarchy of time scales for intramolecular motions and thus inefficient IVR.

Astrophysical Reaction Rate of 12C(α, γ)16O

Abstract: A new astrophysical reaction rate of 12C(α, γ)16O has been calculated based on our recent determination of the E1- and E2-capture cross sections. The R-matrix method has been applied to describe the SE1- and SE2-factor functions as well as the data of elastic α scattering and the β-delayed α decay of 16N from other experiments. The resulting reaction rate for stellar temperatures of T9 = 0.04-10 is presented in both tabular form and an analytic expression. A new temperature dependence of the reaction rate was obtained when compared with reported evaluations. The associated uncertainties were reduced considerably in comparison to previous determinations.

Investigation of Mass-Transport Limitations in the Solid Polymer Fuel Cell Cathode I. Mathematical Model

Abstract: In this paper, a one-dimensional, steady-state agglomerate model was used to describe the functioning and the mass transport limitations of the cathode in the solid polymer fuel cell (SPFC). This mathematical model is then compared to experimental results obtained on cathodes in an SPFC. The following processes were considered: Tafel kinetics of the oxygen reduction reaction, proton migration, oxygen diffusion in the agglomerates, and diffusion of a ternary gas mixture vapor in the pores of the active layer and of the gas backing. The model shows that limitation by proton migration in the active layer or by oxygen diffusion in the agglomerates leads to a doubling of the Tafel slope at higher current densities. For those two types of transport limitations, the dependence of the reaction rate on the active-layer thickness, oxygen partial pressure, and relative humidity of the gas were simulated. When additional limitation due to slow gas phase diffusion appears, the double Tafel slope is distorted. A mathematical expression for the limiting current density due to this process is presented. By using this expression, it is possible to correct the polarization curves for slow gas phase diffusion. © 2002 The Electrochemical Society. All rights reserved.

Photocatalytic degradation of 4-nitrophenol in aqueous TiO2 suspensions: Theoretical prediction of the intermediates

Abstract: The kinetics of the photocatalytic degradation of 4-nitrophenol (4-NP) in the presence of TiO2 has been investigated experimentally and theoretically. The effects of the catalyst loading, the initial concentration of 4-NP, H2O2 and the added Cu2+ ions on the degradation rate have been examined. A pseudo-first order kinetic model has been used to describe the results. A linear dependence of the rate constant upon the reciprocal of the initial 4-NP concentration has been obtained. The addition of H2O2 increases the reaction rate while Cu2+ ions has a detrimental effect. With the intention of predicting the primary intermediates, geometry optimizations of the reactants, the products and the transition state complexes have been performed with the semi-empirical PM3 method. The molecular orbital calculations have been carried out by an SCF method using RHF or UHF formalisms. Based on the results of the quantum mechanical calculations, the rate constants of the two possible reaction paths have been calculated by means of the transition state theory, and 1,2-dihydroxy-4-nitro-cyclohexadienyl radical which then forms 4-nitrocatechol has been determined as the most probable primary intermediate by the application of three different theoretical shortcut methods.

Role of the Catalytic Triad and Oxyanion Hole in Acetylcholinesterase Catalysis: An ab initio QM/MM Study

Abstract: The initial step of the acylation reaction catalyzed by acetylcholinesterase (AChE) has been studied by a combined ab initio quantum mechanical/molecular mechanical (QM/MM) approach. The reaction proceeds through the nucleophilic addition of the Ser203 O to the carbonyl C of acetylcholine, and the reaction is facilitated by simultaneous proton transfer from Ser203 to His447. The calculated potential energy barrier at the MP2(6-31+G*) QM/MM level is 10.5 kcal/mol, consistent with the experimental reaction rate. The third residue of the catalytic triad, Glu334, is found to be essential in stabilizing the transition state through electrostatic interactions. The oxyanion hole, formed by peptidic NH groups from Gly121, Gly122, and Ala204, is also found to play an important role in catalysis. Our calculations indicate that, in the AChE−ACh Michaelis complex, only two hydrogen bonds are formed between the carbonyl oxygen of ACh and the peptidic NH groups of Gly121 and Gly122. As the reaction proceeds, the distan...

Esterification of maleic acid with ethanol over cation-exchange resin catalysts

Abstract: Diethyl maleate is an important intermediate extensively used in the production of latex emulsion polymers, thermoplast and thermoset plastics. The current paper delineates the efficacy of several heterogeneous catalysts, such as Indion-170, Amberlyst-36, Amberlyst-15, Amberlite IRA 120, 20% DTP/K-10 (dodecatungstophosphoric acid supported on K-10 clay) in the esterification of maleic acid with ethanol at reflux. Amongst these Indion-170, Amberlyst-36, Amberlyst-15 were observed to be the most effective. It was observed that the catalyst chosen has excellent reusability and was not deactivated. The effects of various parameters on the rate of reaction demonstrated that the reaction was intrinsically kinetically controlled and there were no intraparticle and interparticle mass transfer limitations. A pseudo-homogeneous kinetic model was developed from the experimental data and activation energy was found to be 14.2 kcal/mol, which also suggested that this reaction was intrinsically kinetically controlled.

Reactive Uptake of Ozone by Oleic Acid Aerosol Particles: Application of Single-Particle Mass Spectrometry to Heterogeneous Reaction Kinetics

Abstract: The technique of single-particle mass spectrometry has been coupled to a reaction flow tube to measure the uptake coefficient, γ, of ozone (O3) by oleic acid (9-octadecenoic acid) aerosol particles. The reaction was followed by monitoring the decrease of oleic acid in the size-selected particles as a function of O3 exposure. The reactive uptake coefficient is found to depend on the size of the particle, with γmeas ranging from (7.3 ± 1.5) × 10-3 to (0.99 ± 0.09) × 10-3 for particles ranging in radius from 680 nm to 2.45 μm. It is suggested that the decrease in γmeas with increasing particle size results from the reaction being limited by the diffusion of oleic acid within the particle, and based on our measurements we estimate the value of γ to be (5.8−9.8) × 10-3 for particles that are not limited by oleic acid diffusion. A reaction model that includes simultaneous diffusion and reaction of both O3 and oleic acid is developed and used to fit the observed rates of reaction. Solutions obtained from this mo...

Rate acceleration of the Baylis-Hillman reaction in polar solvents (water and formamide). Dominant role of hydrogen bonding, not hydrophobic effects, is implicated.

Abstract: A substantial acceleration of the Baylis-Hillman reaction between cyclohexenone and benzaldehyde has been observed when the reaction is conducted in water. Several different amine catalysts were tested, and as with reactions conducted in the absence of solvent, 3-hydroxyquinuclidine was found to be the optimum catalyst in terms of rate. The reaction has been extended to other aldehyde electrophiles including pivaldehyde. Attempts to extend this work to acrylates was only partially successful as rapid hydrolysis of methyl and ethyl acrylates occurred under the base-catalyzed and water-promoted conditions. However, tert-butyl acrylates were sufficiently stable to couple with relatively reactive electrophiles. Further studies on the use of polar solvents revealed that formamide also provided significant acceleration and the use of 5 equiv of formamide (optimum amount) gave faster rates than reactions conducted in water. Using formamide, further acceleration was achieved in the presence of Yb(OTf)(3) (5 mol %). The scope of the new conditions was tested with a range of Michael acceptors and benzaldehyde and with a range of electrophiles and ethyl acrylate. The origin of the rate acceleration is discussed.

Screening System for the Maillard Reaction Inhibitor from Natural Product Extracts

Abstract: An assay for the Maillard reaction has been developed to screen efficiently inhibitors from natural resources such as extracts of plants. The fluorometric analysis of fluorescent material based on advanced glycation endproducts (AGEs) was applied to measurement of an inhibitory index of the Maillard reaction to detect all of the inhibitors at each step of the complicated reaction. To solve the two major problems, slowness of the reaction rate and the existence of interfering substances such as quencher and fluorescent material in the screening sources, we devised the following procedures. Slowness of the reaction rate was solved by raising the reaction temperature to 60°C at which the conformation of bovine serum albumin (BSA) did not change greatly. To remove the interfering substances, AGEs-BSA was precipitated from the reaction mixture. Thus, an efficient assay system by measuring the fluorescent intensity based on AGEs was established to isolate the glycation inhibitors from natural product extracts.

Autocatalyzed and Ion-Exchange-Resin-Catalyzed Esterification Kinetics of Lactic Acid with Methanol

Abstract: The esterification kinetics of lactic acid with methanol without addition of catalyst and catalyzed by ion-exchange resins are presented in this work. The effect of catalyst type, stirrer speed, catalyst size, catalyst loading, initial reactant ratio, and temperature on reaction kinetics was evaluated. Experimental reaction rates were correlated by some models based on homogeneous and heterogeneous (dual- and single-site mechanisms) approaches. Nonideality of the liquid phase was taken into account by using activities instead of mole fractions. Prediction of the activity coefficients was made by UNIFAC. Parameters of the different models were obtained by the simplex search method.

On the theory of the strange and unconventional isotopic effects in ozone formation

Abstract: The strange mass-independent isotope effect for the enrichment of ozone and the contrastingly unconventional strong mass-dependent effect of individual reaction rate constants are studied using statistical (RRKM)-based theory with a hindered-rotor transition state. Individual rate constant ratios of recombination reactions and enrichments are calculated. The theory assumes (1) an "eta-effect," which can be interpreted as a small deviation from the statistical density of states for symmetric isotopomers, compared with the asymmetric isotopomers, and (2) weak collisions for deactivation of the vibrationally excited ozone molecules. A partitioning effect controls the recombination rate constant ratios. It arises from small differences in zero-point energies of the two exit channels of dissociation of an asymmetric ozone isotopomer, which are magnified into large differences in numbers of states in the two competing exit channel transition states. In enrichment experiments, in contrast, this partitioning factor disappears exactly [Hathorn and Marcus, J. Chem. Phys. 112, 9497 (2000)], and what remains is the eta-effect. Both aspects can be regarded as "symmetry driven" isotopic effects. The two experiments, enrichments and rate constant ratios, thus reveal markedly different theoretical aspects of the phenomena. The calculated low-pressure ozone enrichments, the low-pressure recombination rate constant ratios, the effects of pressure on the enrichment, on the individual recombination rate constant ratios, and on the recombination rate constant are consistent with the experimental data. The temperature dependence of the enrichment and of the recombination rate constant ratios is discussed and a variety of experimental tests are proposed. The negative temperature dependence of the isotopic exchange rate constant for the reaction 16O + 18O18O-->16O18O + 18O at 130 K and 300 K is used for testing or providing information on the nature of a variationally determined hindered-rotor transition state. The theory is not limited to ozone formation but is intended to apply to other reactions where a symmetrical stable or unstable gas phase molecule may be formed.

Oxidation, ignition and combustion of toluene: Experimental and detailed chemical kinetic modeling

Abstract: The oxidation of toluene was studied in a jet-stirred reactor at 1 atm. New experimental results were obtained over the high temperature range 1000–1375 K, and variable equivalence ratio (0.5 ⩽ φ ⩽ 1.5). Concentration profiles of reactants, stable intermediates and final products were measured by probe sampling followed by on-line and off-line GC analyses. These experiments were modeled using a detailed kinetic reaction mechanism (120 species and 920 reactions, most of them reversible). This kinetic scheme was also used to simulate the ignition of toluene–oxygen–argon mixtures and the burning velocities of toluene–air mixtures. The presently proposed mechanism has already been validated by simulating the oxidation of benzene at 0.46 to 10 atm under stirred-reactor conditions, the ignition of benzene–oxygen–argon mixtures and the combustion of benzene in flames. Sensitivity analyses and reaction path analyses, based on species rates of reaction, were used to interpret the results. The routes involved in toluene oxidation have been delineated: toluene oxidation proceeds via the formation of benzyl, by H-atom abstraction, and the formation of benzene, by H-atom displacement yielding methyl and benzene; benzyl oxidation yields benzaldehyde, that further reacts yielding phenyl whereas benzyl thermal decomposition yields acetylene and cyclopentadienyl; further reactions of cyclopentadienyl yield vinylacetylene.

Kinetics and mechanism of cellobiose hydrolysis and retro-aldol condensation in subcritical and supercritical water

Abstract: Reactions of cellobiose in subcritical and supercritical water were studied. Kinetic study on the cellobiose decomposition clarified that the contribution of hydrolysis to the overall cellobiose decomposition rate decreased and that of retro-aldol condensation greatly increased with decreasing pressure in near-critical and supercritical water. It was found that the rate of retro-aldol condensation was expressed as a first-order reaction rate law and the kinetic parameters of this reaction were estimated. With regards to hydrolysis of cellobiose, it was indicated that the rate of hydrolysis was a second-order reaction (first-order reaction of the water concentration) and its activation energy and preexponential factor were determined. Mechanisms of these reactions were discussed based on the experimental findings. It was suggested that hydrolysis of cellobiose mainly took place by the nucleophilic attack of the oxygen atom of the water molecule or by the attack of a proton ion dissociated from supercritica...

Reaction mechanism of combustion synthesis of NiAl

Abstract: Based on precise temperature measurements during combustion and microstructural analysis of quenched samples, the evolution of reaction of the NiAl combustion synthesis has been studied. The combustion reaction of a multilayer Ni/Al system takes place in a thermal explosion mode under near adiabatic conditions. The experimental results clearly show that the combustion reaction starts right after the melting of Al. From the start to completion, the reaction goes through three stages. In the first stage, the temperature rises from the melting point of aluminum to the decomposition temperature of the intermediate phase NiAl 3 , i.e. 854 °C. The reaction is the dissolution of nickel in liquid aluminum, with the formation of small amounts of intermediate phases NiAl 3 and Ni 2 Al 3 at the solid–liquid interface. In the second stage, the temperature of the system increases from 854 to about 1300 °C. The reaction is still the dissolution of nickel in liquid aluminum solution. However, due to supersaturation, solid NiAl precipitates out at about 1300 °C, generates a great deal of heat and increases the temperature suddenly. The third stage starts at about 1300 °C, and ends at the maximum reaction temperature. The reaction rate of this stage is much higher (two orders higher) than that of first and second stages. The final product, liquid NiAl, forms at this stage.

Kinetic modeling of reactions in heated monosaccharide-casein systems.

Abstract: In the present study, a kinetic model of the Maillard reaction occurring in heated monosaccharide-casein systems was proposed. Its parameters, the reaction rate constants, were estimated via multiresponse modeling. The determinant criterion was used as the statistical fit criterion instead of the familiar least squares to avoid statistical problems. The kinetic model was extensively tested by varying the reaction conditions. Different sugars (glucose, fructose, galactose, and tagatose) were studied regarding their effect on the reaction kinetics. This study has shown the power of multiresponse modeling for the unraveling of complicated reaction routes as occur in the Maillard reaction. The iterative process of proposing a model, confronting it with experiments, and criticizing the model was passed through four times to arrive at a model that was largely consistent with all results obtained. A striking difference was found between aldose and ketose sugars as suggested by the modeling results: not the ketoses themselves but only their reaction products were found to be reactive in the Maillard reaction.

Kinetics of Absorption of Carbon Dioxide into Aqueous Solutions of Monoethanolamine + Triethanolamine

Abstract: Kinetics of the absorption of CO2 into monoethanolamine (MEA) + triethanolamine (TEA) + water were investigated at 30, 35, and 40 °C using a laboratory wetted wall column. Ten systems of which 0.5 and 1.0 kmol m-3 TEA mixed with various MEA concentrations (0.1, 0.2, 0.3, 0.4, and 0.5 kmol m-3) were studied. Densities and viscosities of 10 blended-amine systems and solubilities and diffusivities of N2O in the solutions were measured. The N2O analogy was applied to estimate the solubilities and diffusivities of CO2 in amine systems. On the basis of the fast pseudo-first-order rate for CO2 absorption, the overall pseudo-first-order reaction rate constants were determined from kinetic measurements. The addition of small amounts of MEA to TEA results in a significant enhancement of CO2 absorption rates. A hybrid reaction rate model, a zwitterion mechanism for MEA and a first-order reaction for TEA, was used to model the kinetic data. The model is satisfactory to represent CO2 absorption rates in TEA + MEA aque...