Values Of Reaction Rate Research Articles

The application of the radioactive tracer technique to study the kinetics of bromide isotope exchange reaction with the participation of strongly basic anion exchange resin indion FF-IP

In the present investigation, the 82Br radioactive isotope was used as a tracer to study the kinetics and mechanism of the exchange reaction between an ion exchange resin and an external solution of bromide ions. In an attempt to study the reversible bromide isotope exchange reaction kinetics, it was expected that whether the initial step was the exchange of radioactive bromide ions from the solution to the ion exchange resin (forward reaction) or from the ion exchange resin to the solution (reverse reaction), the two ion-isotope exchange reactions should occur simultaneously, which was further confirmed by the experimental values of specific reaction rates, 0.142 and 0.141 min−1, respectively. The results will be useful to standardize the process parameters so as to achieve optimum use of ion exchange resins in various industrial applications.

Catalytic properties of molecular sieves MCM41 type doped with heteropolyacids for ethanol oxidation

Keggin type heteropolyacids (HPAs) have been used in acid-catalysed reactions as well as oxidation reactions both in the heterogeneous and homogeneous systems. In order to be more effective for catalytic reactions, HPAs are usually impregnated on different porous materials with high surface area. The structure and texture of H 3PMo 12O 40 and H 4PVMo 11O 40 supported on Al-MCM41 and Fe-MCM41 were studied by XRD, low-temperature nitrogen adsorption and thermal analysis. Catalytic properties of prepared samples were studied by using a reactant pulse method at different temperatures. The ethanol conversion proceeds by two main pathways: an oxidehydrogenation reaction on redox catalytic centers, and a dehydration reaction on acidic centers, respectively. The favourable effect of HPAs deposition on Al-MCM41 and Fe-MCM41 supports for oxidehydrogenation pathway to acetaldehyde results from the examination of apparent activation energies E a and reaction rate values. An evidenced decreasing of E a was obtained for acetaldehyde formation in the case of Fe-MCM41-supported HPM (25 kJ/mole) comparatively with pure HPM (56.6 kJ/mole). For all reaction temperatures, the reaction rates of acetaldehyde formation are with one order of magnitude higher for supported samples than unsupported ones.

PHOTOCHROMIC KINETICS OF ASYMMETRICAL DIARYLETHENES IN DIFFERENT SOLVENTS AND IN PMMA FILMS

Three new asymmetric photochromic diarylethenes bearing fluorine atoms at the ortho-, meta-, or para-position of one terminal phenyl group were synthesized, and their photochromism and kinetics of the photochromic cyclization/cycloreversion both in different solvents and in PMMA films were investigated. The results showed that the cyclization/cycloreversion process of these compounds were determined to be zeroth/first order reaction, and the reaction rates (k) were obtained from the slope of every line. The substituent position effect on the cycloreversion process of the three compounds was remarkable and the solvent polarity enhanced this effect. The values of reaction rates in PMMA films were quite different from those in solvents indicating that the kinetics processes of the three compounds taking place in PMMA films differs from the same processes occurring in common solvent.

Charge emission during nitridation of a metal particle

Our model explaining and predicting generation of a temporal electric potential during nitridation of a single metal particle has been modified to take into account the surface emission of the electrons trapped by defects at deep energy levels. The model estimates have shown that the emission is a highly thermally activated process and its characteristic time correlates with that of the maximum particle temperature. One may neglect the emission at low particle temperatures: during the initial combustion stage or during all the combustion at the sufficiently high rates of heat removal and/or thick initial nitride shell. At the parameters considered the emission exerts no influence on the particle temperature, concentrations of neutral nitrogen interstitials, the reaction rate and other anelectric values. Nevertheless, it greatly affects the surface concentrations of electron holes and electric potential. An increase in the emission intensity leads to a non-equilibrium and formation of the second concentration maximum of the holes as well as to a switch in the potential polarity.

Effect of Carbonization Temperature on Combustion Reactivity of Bagasse Char

The combustion reactivity of bagasse chars was investigated under isothermal conditions at 400°C in air. The bagasse char samples were prepared by carbonizing bagasse in a fixed bed reactor at temperatures between 500°C and 800°C. It was observed that raising the carbonization temperature resulted in a significant decrease in reactivity of bagasse char. This was manifested by the decrease in the values of the maximum reaction rate, average rate based on 50% burnout and conversion achieved in 30 minutes with the increase in carbonization temperature. The decrease in reactivity of bagasse char with carbonization temperature was attributed to changes in the reactive components of bagasse.

Application of radiotracer technique to study the ion-isotope exchange reactions using a strongly basic anion-exchange resin duolite A-113

Short-lived radioactive 131I and 82Br tracers were used to study the kinetics of ion-isotope exchange reaction. The experiments were performed in the temperature range 26.0–43.0°C, ion concentration in solution in the range 0.0025–0.020 M, and amount of labeled ion-exchange resin varying from 0.250 to 1.250 g. For bromide ion-isotope exchange reactions, the calculated values of specific reaction rate, initial rate of bromide ion exchange, and amount of bromide ions exchanged were obtained higher than those for iodide ion-isotope exchange reactions under identical experimental conditions. The observed difference is attributed to the difference in the ionic size of bromide and iodide ions in solution.

Biofiltration of ketone compounds by a composite bead biofilter

In this study, the biochemical kinetic behaviors of ketone compounds in a composite bead biofilter were investigated. Both microbial growth rate k g and biochemical reaction rate k d would be inhibited at higher average inlet concentration. For the microbial growth process, the inhibitive effect was the least pronounced for acetone and the order of k g value was MEK > MIPK > acetone in the average inlet concentration range of 100–150 ppm. The degree of inhibitive effect was almost the same for three ketone compounds and the order of k g value was acetone > MEK > MIPK in the average inlet concentration range of 200–300 ppm. The values of half-saturation constant K s for acetone, MEK and MIPK were 26.80, 21.56 and 22.96 ppm, respectively. The values of maximum reaction rate V m for acetone, MEK and MIPK were 8.55, 9.06 and 7.55 g-C/h-kg packed material, respectively. The zero-order kinetic with the diffusion rate limitation could be regarded as the most adequate biochemical reaction model. For the biochemical reaction process, the inhibitive effect was the most pronounced for MEK and the order of k d value was MEK > acetone > MIPK in the average inlet concentration range of 100–150 ppm. The degree of inhibitive effect was MIPK > MEK > acetone and the order of k d value was acetone > MEK > MIPK in the average inlet concentration range of 200–300 ppm. The maximum elimination capacity of acetone, MEK and MIPK were 0.157, 0.127 and 0.101 g-C/h-kg packed material.

Determination of diffusion controlled reaction rates at a solid/liquid interface using scanning electron microscopy

A high-resolution method has been developed for the determination of localized values of interfacial reaction rate and mass transfer coefficient in aqueous solution. Scanning electron microscopy has been successfully applied to this problem through the measurement of electroplated film thickness formed under limiting current conditions. The method involves the calculation of local values of reaction rate via Faraday's laws and subsequent conversion of the data to absolute values of mass transfer coefficient. The technique has been verified in an undisturbed, turbulent flow regime (rotating cylinder electrode) through the use of Sherwood group dimensionless analysis. The resulting relationship shows comparable accuracy relative to electrochemical measurements. Favourable comparison has also been made with the generally accepted rotating cylinder correlation of Eisenberg, Tobias and Wilke. Differential rates of mass transfer to a single surface under conditions of disturbed flow have also been examined at a high spatial resolution using the stepped rotating cylinder electrode geometry. In this case, reaction rates have been measured as a function of circumferential distance within a recirculation zone situated immediately downstream of a backward-facing step.

Reactivity of Arenesulfinic Acids as Nucleophiles in the Addition Reaction to 2‐Haloacrylonitriles

AbstractThe nucleophilic addition reactions of both the non‐substituted and p‐substituted benzenesulfinic acids with 2‐chloro‐ and 2‐bromoacrylonitriles have been studied. The structures of the 3‐benzenesulfonyl‐2‐halogenopropanenitriles thus obtained were confirmed by microanalytical and spectral methods. The basic kinetic parameters of the reactions were determined and some local electronic descriptors concerning the reaction centers were calculated. For the interaction with benzenesulfinic acids, 2‐bromoacrylonitrile was found to have higher reactivity as a Michael acceptor than 2‐chloroacrylonitrile. The higher reactivity of 2‐bromoacrylonitrile is associated with the higher electrophilicity of its β‐carbon atom, which was proved by its pronounced acceptor delocalizability and increased reaction rate values with the corresponding benzenesulfinic acids.

Comparative study on iodide and bromide isotopic exchange reactions by application of radioactive tracer technique

In the present investigation, a short lived radioactive tracer isotopes 131I and 82Br were used to study the kinetics of isotopic exchange reactions. For bromide isotopic exchange reaction, the calculated values of specific reaction rate (min−1), and amount of bromide ions exchanged (mmol) were obtained higher than that for iodide isotopic exchange reaction under identical experimental conditions. For both the exchange reactions it was observed that when the concentration of ionic solution was increased twice, the␣percentage of ions exchanged increases sharply at nearly same specific reaction rate, indicating that concentration greatly influences the exchange reactions.

A kinetic study of isoamyl acetate synthesis by immobilized lipase-catalyzed acetylation in n-hexane

The objective of this work was to propose a reaction mechanism and to develop a rate equation for the synthesis of isoamyl acetate by acylation of the corresponding alcohol with acetic anhydride using the lipase Novozym 435 in n-hexane. The reaction between isoamyl alcohol and acetic anhydride occurred at high rate in first place. Then, if excess alcohol was used, produced acetic acid further reacted with remaining alcohol, leading to yields higher than 100% (based on initial acetic anhydride content). This reaction was much slower and took place only when acetic anhydride had been totally consumed. Optimal pH for Novozym 435 was 7.7. Acetic acid strongly inactivated the enzyme but it was partially caused by the pH drop in the biocatalyst aqueous microenvironment. Acetic anhydride also showed an important inhibition effect. On the contrary, isoamyl alcohol and isoamyl acetate had no negative effect on the lipase. The analysis of the initial rate data showed that reaction followed a Ping-Pong Bi-Bi mechanism with inhibition by acetic anhydride. The kinetic constants were obtained by multiple regression analysis of experimental findings. Equation predictions and experimental reaction rate values matched very well at conditions where acetic acid concentration in the medium was low.

Chlorination kinetics of ZnO with Ar-Cl2-O2 gas and the effect of oxychloride formation

The chlorination rate of ZnO with Ar-Cl2-O2 gas was measured from 1023 K to 1273 K and the effects of temperature and partial pressures of chlorine and oxygen were investigated. The rate-determining step of chlorination was considered to be the dissociation of intermediate between ZnO and Cl2 from linear relationship between reciprocal values of reaction rate and partial pressure of chlorine. The activation energy of chlorination was 58.2±2.5 kJ/mol. This comparatively low activation energy as a chemically controlled reaction was consistent with estimated rate-determining step of the dissociation of unstable compound. Increasing the partial pressure of oxygen slightly increased the chlorination rate, and this effect is considered to be caused by the increase in the formation rate of zinc oxychloride. To clarify the formation of zinc oxychloride, the equilibrium between Ar-Cl2-O2 gas and ZnO was investigated by the transpiration method at 1073 K. Calculated partial pressures of ZnOCl from experimental results were in the same order with or one order of magnitude larger than those estimated from reported Gibbs energy of formation of ZnOCl. Zinc oxychloride formation in ZnO chlorination must be taken into consideration as well as ZnCl2 and Zn2Cl4 formation.

Biochemical kinetic behaviors between n-butyl acetate and composite bead in biofilter

In this study, the kinetic behaviors between n-butyl acetate and composite bead were investigated. Both microbial growth rate and biochemical reaction rate would be inhibited with increasing average inlet concentration. The order of the inhibitive effect, which resulted from increased average inlet concentration for four operation temperatures, was 30>35>40>25 degrees C. Both microbial growth rate and biochemical reaction rate would be enhanced and inhibited with increasing operation temperature in the operation temperature ranges of 25 to 30 and 30 to 40 degrees C, respectively. The enhancing and inhibitive effects resulting from increased operation temperature were the most pronounced at the average inlet concentration of 200 ppm. The values of maximum reaction rate V (m) and half-saturation constant K (s) ranged from 0.011 to 0.047 g C h(-1) kg(-1) packed material and from 19.30 to 62.40 ppm, respectively. The zero-order kinetic with the diffusion rate limitation could be regarded as the most adequate biochemical reaction kinetic model. The values of maximum elimination capacity ranged from 0.51 to 0.20 g C h(-1) kg(-1) packed material, and the optimal maximum elimination capacity of biofilter occurred at the operation temperature of 30 degrees C.

Catalytic activity of versatile peroxidase from Bjerkandera fumosa in aqueous solutions of water–miscible organic solvents

Versatile peroxidase (VP) from the white rot fungus Bjerkandera fumosa was isolated and purified by ion exchange and gel filtration chromatography. Its primary catalytic activity in oxidation of Mn(II) was studied in aqueous solutions in the presence of varying concentrations (up to 8 M) of acetonitrile (MeCN), dimethylsulfoxide (DMSO), ethanol, and n-propanol. The observed maximum reaction rate values decreased with the addition of organic solvents in the order: MeCN < n-propanol < DMSO < ethanol. The observed Michaelis constant values decreased in the order: DMSO < MeCN < n-propanol. This lead to various shapes of the specific activity constant values ( k s = V max/ K M) of VP in oxidation of Mn(II) in solutions of different organic solvents. K M values of typical organic substrates of enzymatic reactions increase with addition of organic solvents, due to unfavorable partitioning of substrates between the medium and the active center of the enzyme. In case of inorganic ions, the decrease of the medium polarity caused by addition of the organic solvent should lead to the enhancement of substrate binding by the enzyme due to the variation in the activity coefficient/solubility of the substrates in the studied systems, and therefore to the decrease of K M values. The results of our experiments confirm this expectation. To further support this thesis, the effect of ethanol on oxidation of 2,6-dimethoxyphenol catalyzed by VP in the presence of Mn(II) was also studied. The observed V max values were exponentially decreasing with the solvent concentration, while the K M versus ethanol concentration curve passed through a minimum. The later effect could be described by two exponential relationships (the one decreasing for Mn(II) and the other one increasing for 2,6-dimethoxyphenol).

Application of artificial neural networks and DFT-based parameters for prediction of reaction kinetics of ethylbenzene dehydrogenase

Artificial neural networks (ANNs) are used for classification and prediction of enzymatic activity of ethylbenzene dehydrogenase from EbN1 Azoarcus sp. bacterium. Ethylbenzene dehydrogenase (EBDH) catalyzes stereo-specific oxidation of ethylbenzene and its derivates to alcohols, which find its application as building blocks in pharmaceutical industry. ANN systems are trained based on theoretical variables derived from Density Functional Theory (DFT) modeling, topological descriptors, and kinetic parameters measured with developed spectrophotometric assay. Obtained models exhibit high degree of accuracy (100% of correct classifications, correlation between predicted and experimental values of reaction rates on the 0.97 level). The applicability of ANNs is demonstrated as useful tool for the prediction of biochemical enzyme activity of new substrates basing only on quantum chemical calculations and simple structural characteristics. Multi Linear Regression and Molecular Field Analysis (MFA) are used in order to compare robustness of ANN and both classical and 3D-quantitative structure-activity relationship (QSAR) approaches.

Comparative Study on Bromide and Iodide Ion-Isotopic Exchange Reactions Using Strongly Basic Anion Exchange Resin Duolite A-113

Kinetics of ion-isotopic exchange reaction was studied using industrial grade ion exchange resin Duolite A-113. The radioactive isotopes {sup 131}I and {sup 82}Br were used to trace the ion-isotopic exchange reaction. The experiments were performed in the temperature range of 26.0degC to 43.0degC and the concentration of external ionic solution varying from 0.005 M to 0.100 M. For bromide ion-isotopic exchange reaction, the calculated values of specific reaction rate, initial rate of bromide ion exchange, and amount of bromide ions exchanged were obtained higher than that for iodide ion-isotopic exchange reaction under identical experimental conditions. The observed variation in the results for two ion-isotopic exchange reactions was due to the difference in the ionic size of bromide and iodide ions. (author)

Rh/γ-Al2O3–sepiolite monolithic catalysts for decomposition of N2O traces

The behaviour of Rh/γ-AlO3–sepiolite monolithic systems in the N2O decomposition reaction using catalysts with rhodium contents between 0.2 and 0.8wt.% were analysed. The observed increase of the catalytic activity with the metal loading was related to an enhancement in the number of active sites. The SEM–EDX results showed that rhodium was mainly located on the alumina particles. The variation of the reaction rate values with the inlet N2O and O2 concentrations, suggested an apparent first order equation with respect to N2O and zero order for O2. The presence of NH3 traces or water vapour in the feed decreased the catalytic activity, probably due to the competitive adsorption of NHx or OH species. The ammonia oxidation reaction took place, producing N2O as a secondary product. The monolithic catalysts developed operate with a low pressure drop achieving a N2O conversion higher than 90% at 773K in the presence of oxygen, ammonia or water vapour in the stream.

Direct simulation of non-premixed flame extinction in a methane–air jet with reduced chemistry

A three-dimensional direct numerical simulation (DNS) study of a spatially evolving planar turbulent reacting jet is reported. Combustion of methane with air is modelled using a four-step reduced mechanism in the non-premixed regime. A total of eight chemical species are integrated in time along with the fluid mechanical fields. The solution of the compressible Navier–Stokes equations is obtained numerically for moderately low Mach number. A large computational grid, with 100 million grid points, is required in order to resolve the flame. The cold flow Reynolds number is 3000. The focus of the study is to investigate the dynamics of extinction fronts in three-dimensional turbulent flows. A novel data reduction and identification algorithm was developed to postprocess the large DNS database and extract the shape of the evolving flame surface including its edges and their propagation velocity. The joint probability density function (p.d.f.) of edge velocity and scalar dissipation was obtained and the results indicate that the three-dimensional flame edges propagate with a velocity that is largely controlled by the local rate of scalar dissipation, or equivalently in terms of the local Damkohler number at the flame edge, as predicted by theory. Naturally, the effects of unsteadiness in this flow produce a broad joint p.d.f. The statistics collected also suggest that the mean value of the hydrogen radical reaction rate are very small in the turbulent regions of the flow owing to the functional form of the hydrogen radical reaction rate itself. The consequence of these results in the context of turbulent combustion modelling is discussed. Additional statistical and morphological information of the flame is provided.

Direct use of the mass output of a thermobalance for controlling the reaction rate of solid-state reactions

Sample controlled thermal analysis equipment has been developed constituted by an electrobalance in which the mass output (TG signal) is directly used for monitoring the temperature of thermal decomposition reactions under constant rate thermal analysis (CRTA) or stepwise isothermal analysis (SIA) control. The sample weight is programmed to follow a preset linear decrease as a function of the time by means of a conventional controller, that at the time control a second conventional temperature programmer. The CRTA control is achieved by controlling the temperature is such a way that if the mass input is higher than the setpoint, the temperature increases at a predefined heating rate; while if the mass input is lower than the setpoint, the temperature decreases at a predefined cooling rate. The SIA control is achieved by selecting the run–hold command from the menu of the digital input of the temperature programmer. In such a case, the programmed linear heating schedule is in progress while the sample weight is higher than the setpoint and an isothermal dwell is maintained as soon as the weight becomes lower than the setpoint. The direct use of the mass output for the control provides a higher sensitivity for selecting very low values of constant reaction rates than the more conventional methods using the DTG output as control parameter. The thermal degradation of polyvinye chloride (PVC) has been used for checking the behavior of the equipment here developed, showing that the dehydrochlorination of PVC is controlled either by a nucleation and growth of nuclei or by a random scission of the main chain of the polymer.

Autonomously oscillating biochemical systems: parametric sensitivity of extrema and period

This work addresses sensitivity analysis of autonomously oscillating biochemical systems. Building on results from the engineering literature, a general analysis is presented which addresses key features of oscillatory trajectories, namely the period and local maximum or minimum values of species concentrations and reaction rates. A discussion of sensitivity invariants generalises results from steady-state sensitivity analysis to this context. The results are illustrated by the application to a model of a circadian oscillator.