Showing papers on "Reaction rate constant published in 2002"

Hydrodechlorination of Trichloroethylene to Hydrocarbons Using Bimetallic Nickel-Iron Nanoparticles

Abstract: High surface-area nickel−iron nanoparticles (1:3 Ni:Fe) have been studied as a reagent for the dehalogenation of trichloroethylene (TCE). Ni−Fe (0.1 g) nanoparticles reduced TCE from a 40-mL saturated aqueous solution (24 ppm) to <6 ppb in 120 min. The dehalogenation reaction, based on the surface area normalized rate constant, was 50−80 times slower using nanoiron or iron filings, respectively. On the bimetallic particles, the reaction occurs by nickel-catalyzed hydrodechlorination. As the iron actively corrodes, the cathodically protected nickel surface chemisorbs hydrogen ions. TCE adsorbed to the Ni surface is thus hydrogenated. This reaction competes kinetically with the evolution of molecular hydrogen. Hydrogenation of the C−Cl bond results in the formation of linear as well as branched saturated and unsaturated hydrocarbons. The final TCE degradation products are predominantly even-numbered saturated hydrocarbons, such as butane, hexane, and octane. The toxic dehalogenation products vinyl chloride ...

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.

Chlorination of Phenols: Kinetics and Formation of Chloroform

Abstract: The kinetics of chlorination of several phenolic compounds and the corresponding formation of chloroform were investigated at room temperature. For the chlorination of phenolic compounds, second-order kinetics was observed, first-order in chlorine, and first-order in the phenolic compound. The rate constants of the reactions of HOCl with phenol and phenolate anion and the rate constant of the acid-catalyzed reaction were determined in the pH range 1-11. The second-order rate constants for the reaction HOCl + phenol varied between 0.02 and 0.52 M(-1) s(-1), for the reaction HOCl and phenolate between 8.46 x 10(1) and 2.71 x 10(4) M(-1) s(-1). The rate constant for the acid-catalyzed reaction varied between 0.37 M(-2) s(-1) to 6.4 x 10(3) M(-2) s(-1). Hammett-type correlations were obtained for the reaction of HOCl with phenolate (log(k) = 4.15-3.00 x sigma sigma) and the acid-catalyzed reaction of HOCl with phenol (log(k) = 2.37-4.26 x sigma sigma). The formation of chloroform could be interpreted with a second-order model, first-order in chlorine, and first-order in chloroform precursors. The corresponding rate constants varied between k > 100 M(-1) s(-1) for resorcinol to 0.026 M(-1) s(-1) for p-nitrophenol at pH 8.0. It was found that the rate-limiting step of chloroform formation is the chlorination of the chlorinated ketones. Yields of chloroform formation depend on the type and position of the substituents and varied between 2 and 95% based on the concentration of the phenol.

Role of Crystalline Defects in Electrocatalysis: Mechanism and Kinetics of CO Adlayer Oxidation on Stepped Platinum Electrodes

Abstract: The kinetics of the electrochemical oxidation of a CO adlayer on Pt[n(111)×(111)] electrodes in 0.5 M H2SO4 has been studied using chronoamperometry. The objective is to elucidate the effect of the crystalline defects on the rate of the reaction by using a series of stepped surfaces. The reaction kinetics of the main oxidative process can be modeled using the mean-field approximation for the Langmuir−Hinshelwood mechanism, implying fast diffusion of adsorbed CO on the Pt[n(111)×(111)] surfaces under electrochemical conditions. The apparent rate constant for the electrochemical CO oxidation, determined by a fitting of the experimental data with the mean-field model, is found to be proportional to the step fraction (1/n) for the surfaces with n > 5, proving steps to be the active sites for the CO adlayer oxidation. An apparent intrinsic rate constant is determined. The potential dependence of the apparent rate constants is found to be structure insensitive with a Tafel slope of ca. 80 mV/dec, suggesting the...

Decomposition of hydrogen peroxide and organic compounds in the presence of dissolved iron and ferrihydrite.

Abstract: This work examines the contribution of solution phase reactions, especially those involving a chain reaction mechanism, to the decomposition of hydrogen peroxide (H2O2) and organic compounds in the presence of dissolved iron and ferrihydrite. In solutions at pH 4, where Fe was introduced as dissolved Fe(III), both H2O2 and 14C-labeled formic acid decomposed at measurable rates that agreed reasonably well with those predicted by a kinetic model of the chain reaction mechanism, using published rate constants extrapolated to pH 4. The ratio of the formic acid and H2O2 decomposition rates, as well as the dramatic effect of tert-butyl alcohol on these rates, confirmed that a solution chain reaction mechanism involving •OH controlled the decomposition kinetics of both compounds. In the presence of ferrihydrite as the iron source, the ratio of the rate of formic acid decomposition to that of H2O2 decomposition was significantly lower than that observed in the presence of only dissolved Fe. Moreover, neither rate...

Hydrogenation of carbon dioxide catalyzed by ruthenium trimethylphosphine complexes: the accelerating effect of certain alcohols and amines.

Abstract: A trace amount of alcohol cocatalyst and a stoichiometric amount of base are required during the hydrogenation of CO2 to formic acid catalyzed by ruthenium trimethylphosphine complexes. Variation of the choice of alcohol and base causes wide variation in the rate of reaction. Acidic, nonbulky alcohols and triflic acid increase the rate of hydrogenation an order of magnitude above that which can be obtained with traditionally used methanol or water. Similarly, use of DBU rather than NEt3 increases the rate of reaction by an order of magnitude. Turnover frequencies up to 95 000 h-1 have now been obtained, and even higher rates should be possible using the cocatalyst and amine combinations identified herein. Preliminary in situ NMR spectroscopic observations are described, and the possible roles of the alcohol and base are discussed.

Epoxidation of soybean oil in toluene with peroxoacetic and peroxoformic acids — kinetics and side reactions

Abstract: The kinetics of the epoxidation of soybean oil and the extent of side reactions were studied at 40, 60, and 80 °C. Epoxidation was carried out in toluene with “in situ” formed peroxoacetic and peroxoformic acid and in the presence of an ion exchange resin as the catalyst. The reaction was found to be first-order with respect to the double bond concentration. At higher temperatures and at higher conversions a deviation from the first-order kinetics was observed. The rate constants for the epoxidation with peroxoacetic acid were 0.118 (h−1) at 40 °C, 0.451 (h−1) at 60 °C and 1.278 (h−1) at 80 °C, while those for peroxoformic acid were 0.264, 0.734, and 1.250 (h−1). The activation energy was found to be 54.7 kJ/mol for the epoxidation with peroxoacetic acid and 35.9 kJ/mol for that with peroxoformic acid. Three factors indicated that side reactions did not occur on a large scale: The absence of an OH band in the IR spectra, the formation of less than 2% of higher molecular weight products from gel permeation chromatography and the selectivity values between 0.9 and 1.

The chemistry of amine-azide interconversion: catalytic diazotransfer and regioselective azide reduction.

Abstract: Azides have proven to be useful precursors to amines in organic syntheses. This report describes an improvement of the diazotransfer reaction and the first example of a regioselective azide reduction of compounds containing multiple azides. The use of a specific ratio of solvents and zinc chloride as a catalyst resulted in a more efficient diazotransfer reaction capable of delivering >90% conversion per amine with shorter reaction times than those previously reported. Azides can be reduced with good regioselectivity in moderate yields by a modification of the Staudinger reaction using trimethylphosphine at low temperatures. Electronic factors determine the selectivity for azide reduction, and the reaction is predictable by NMR analysis of the starting material. Several examples for the diazotransfer and regioselective azide reduction reactions are given, and a mechanistic hypothesis for both is proposed.

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.

Self-assembled monolayers supported on TiO2: Comparison of C18H37SiX3 (X = H, Cl, OCH3), C18H37Si(CH3)2Cl, and C18H37PO(OH)2

Abstract: The solution-phase reactions of octadecylsilanes with different headgroups (C18H37SiH3, C18H37Si(OCH3)3, C18H37SiCl3, and C18H37Si(CH3)2Cl) and of octadecylphosphonic acid (C18H37PO3H2) with titanium dioxide (anatase) were investigated. Chemical analysis and FTIR suggested that all the reactions, with the exception of that of C18H37Si(CH3)2Cl, yielded closely packed self-assembled monolayers (SAMs). SAMs were characterized with high grafting density (∼4.3−4.8 octadecyl groups/nm2) and with high degree of ordering of alkyl chains. Reaction of C18H37Si(CH3)2Cl yielded less ordered surfaces with grafting density ∼1.5 group/nm2. The kinetics plots were similar for all the reactions and illustrated two distinct regions, a rapid attachment followed by a slow growth of the grafting density. The uptake curves were adequately described by the first-order kinetics with two rate constants that differed from each other by 1−2 orders of magnitude. According to the rate constants, the following range of reactivity was ...

Esterification of different acids over heterogeneous and homogeneous catalysts and correlation with the Taft equation

Abstract: Esterification of acetic, propanoic and pentanoic acid with methanol, ethanol, 1-propanol, 2-propanol, butanol and 2-butanol was studied in the presence of a fibrous polymer-supported sulphonic acid catalyst, Smopex-101. The reaction temperature in the experiments was 60 °C. Comparative experiments were carried out on an ion-exchange resin, Amberlyst 15 and with a homogeneous catalyst, liquid HCl. The effect of different molar ratios between the reactants (acid and alcohol) on the reaction rate was investigated. The rate constants were related to the substituent effects of the reacting molecules according to the Taft equation. The substituent effects of alcohols were found to follow the Taft relationship, which was not the case for the acids. The experimental results were modelled according to a simple second-order model and a more advanced adsorption-based model. The adsorption-based model, which includes the adsorption of carboxylic acid and water and is consistent with structure–activity relationship, was superior to the second-order model.

The Rate Constant of the Reaction of Superoxide with Nitrogen Monoxide: Approaching the Diffusion Limit

Abstract: The reaction of superoxide with nitrogen monoxide has been reinvestigated. Photolysis of alkaline peroxynitrite solutions results in the formation of superoxide and nitrogen monoxide, which subsequently react at a rate of (1.5 ± 0.1) × 1010 M-1 s-1. When hydrogen peroxide is photolyzed in the presence of nitrogen monoxide, hydroxyl radicals are formed; these react with hydrogen peroxide to form superoxide, which then reacts with the nitrogen monoxide present. For the reaction of nitrogen monoxide with superoxide, a rate constant of (2.0 ± 0.4) × 1010 M-1 s-1 is derived. Photolysis of nitrite gives rise to hydroxyl radicals and nitrogen monoxide. In the presence of formate and oxygen, the hydroxyl radicals subsequently form superoxide radicals, which, in turn, react with nitrogen monoxide. From these experiments, a rate constant of (1.3 ± 0.2) × 1010 M-1 s-1 is derived. We thus report a rate constant of (1.6 ± 0.3) × 1010 M-1 s-1, the weighted average of these three photolysis experiments.

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.

OH-initiated oxidation of benzene

Abstract: The present work represents a continuation of part I of this series of papers, in which we investigated the phenol yields in the OH-initiated oxidation of benzene under conditions of low to moderate concentrations of NOx, to elevated NOx levels. The products of the OH-initiated oxidation of benzene in 700–760 Torr of N2/O2 diluent at 297 ± 4 K were investigated in 3 different photochemical reaction chambers. In situ spectroscopic techniques were employed for the detection of products, and the initial concentrations of benzene, NOx, and O2 were widely varied (by factors of 6300, 1500, and 13, respectively). In contrast to results from previous studies, a pronounced dependence of the product distribution on the NOx concentration was observed. The phenol yield decreases from approximately 50–60% in the presence of low concentrations ( 10 000 ppb) NOx concentrations. In the presence of high concentrations of NOx, the phenol yield increases with increasing O2 partial pressure. The rate constant of the reaction of hydroxycyclohexadienyl peroxyl radicals with NO was determined to be (1.7 ± 0.6) × 10−11 cm3 molecule−1 s−1. This reaction leads to the formation of E,E-2,4-hexadienedial as the main identifiable product (29 ± 16%). The reaction of the hydroxycyclohexadienyl radical with NO2 gave phenol (5.9 ± 3.4%) and E,E-2,4-hexadienedial (3.4 ± 1.9%), no other products could be identified. The residual FTIR product spectra indicate the formation of unknown nitrates or other nitrogen-containing species in high yield. The results from the present work also show that experimental studies aimed at establishing/verifying chemical mechanisms for aromatic hydrocarbons must be performed using NOx levels which are representative of those found in the atmosphere.

Kinetics and mechanism of the KOH — catalyzed methanolysis of rapeseed oil for biodiesel production

Abstract: The reaction of rapeseed oil with methanol catalyzed by KOH is described by a model consisting of two sequences of consecutive competitive reactions. The first sequence expresses the methanolysis of rapeseed oil to methyl esters (biodiesel) whereas the second sequence describes the always present side reaction-saponification of glycerides and methyl esters by KOH. The proposed chemical model is described (after rational simplifications) by a system of differential kinetic equations which are solved numerically by two independent computing methods. The thus obtained theoretical kinetic and equilibrium results are compared numerically and/or graphically with the experimental parameters. The latter were obtained by the determination of the relevant components in the actual reaction mixture by analytical methods. According to the experimental results, the proposed reaction scheme is fulfilled with the probability of ca. 78%. The optimal average rate constants and equilibrium constants of individual reaction steps of the discussed scheme are introduced. The limitations of the proposed reaction model are discussed.

Kinetics of Heterogeneous Catalytic Ozone Decomposition in Water on an Activated Carbon

Abstract: Ozone decomposition in water in the presence of an activated carbon has been studied. Variables investigated were agitation speed, carbon particle size, temperature and pH. In all cases, the presence of activated carbon improved the ozone decomposition rate. Between pH 2 and 7 the ozone decomposition rate due to both the homogeneous and heterogeneous mechanisms hardly varied while a significant increase was noticed with increasing pH. A kinetic study based on a Langmuir-Hinselwood type mechanism for the heterogeneous surface reaction was undertaken. According to this mechanism the heterogeneous ozone decomposition kinetics can be simplified to follow a first order process. Fit of experimental results to the kinetic equations derived from the mechanism allowed for the determination of the apparent first order rate constants of the ozone surface heterogeneous reaction and adsorption equilibrium constants.

Detailed Kinetics and Thermochemistry of C2H5 + O2: Reaction Kinetics of the Chemically-Activated and Stabilized CH3CH2OO• Adduct

Abstract: The kinetics of the chemically activated reaction between the ethyl radical and molecular oxygen are analyzed using quantum Rice−Ramsperger−Kassel (QRRK) theory for k(E) with both a master equation analysis and a modified strong-collision approach to account for collisional deactivation. Thermodynamic properties of species and transition states are determined by ab initio methods at the G2 and CBS-Q//B3LYP/6-31G(d,p) levels of theory and isodesmic reaction analysis. Rate coefficients for reactions of the energized adducts are obtained from canonical transition state theory. The reaction of C2H5 with O2 forms an energized peroxy adduct with a calculated well depth of 35.3 kcal mol-1 at the CBS-Q//B3LYP/6-31G(d,p) level of theory. The calculated (VTST) high-pressure limit bimolecular addition reaction rate constant for C2H5 + O2 is 2.94 × 1013T-0.44. Predictions of the chemically activated branching ratios using both collisional deactivation models are similar. All of the product formation pathways of ethyl...

Catalytic Regioselective Sulfonylation of α-Chelatable Alcohols: Scope and Mechanistic Insight

Abstract: This paper describes a convenient protocol for the regioselective sulfonylation of α-chelatable alcohols. Typically, the reaction of α-heterosubstituted alcohols with 1 equiv of p-TsCl and 1 equiv of Et3N in the presence of 2 mol % of Bu2SnO leads to rapid, regioselective, and exclusive monotosylation. The pKa of the amine was correlated to the reaction rate. A plausible mechanism for this reaction has been proposed on the basis of 119Sn NMR studies.

Reactivity of common functional groups with urethanes: Models for reactive compatibilization of thermoplastic polyurethane blends

Abstract: Two model urethane compounds, dibutyl 4,4′-methylenebis(phenyl carbamate) (BMB) and dioctyl 4,4′-methylenebis(phenyl carbamate) (OMO) were prepared by capping 4,4′-methylenebis(phenyl isocyanate) with n-butanol and n-octanol, respectively. The reactions of the two model urethane compounds with several small monofunctional compounds as well as two model poly(ethylene glycols) were carried out with neat mixtures at elevated temperatures. The ranking of reactivity of the functional groups with the urethanes was determined as follows—primary amine > secondary amine ≫ hydroxyl ∼ acid ∼ anhydride ≫ epoxide. Nuclear magnetic resonance spectroscopy (NMR) was used for the quantitative analysis. Fourier transform infrared spectroscopy was used to complement the NMR analysis. Conversions of carbamate in each reaction were monitored over time at constant temperature (200 °C). The reactions between OMO and primary amine were conducted at 170, 180, 190, and 200 °C and best described with a second-order bimolecular reaction model. The rate constant was estimated to be 1.8 × 10−3 L · mol−1 · s−1 and activation energy 115 kJ · mol−1. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2310–2328, 2002

Iron(III)−Salen Complexes as Enzyme Models: Mechanistic Study of Oxo(salen)iron Complexes Oxygenation of Organic Sulfides

Abstract: The oxidation of a series of para-substituted phenyl methyl sulfides was carried out with several oxo(salen)iron (salen = N,N‘-bis(salicylidine)ethylenediaminato) complexes in acetonitrile. The oxo complex [OFeIV(salen)]•+, generated from an iron(III)−salen complex and iodosylbenzene, effectively oxidizes the organic sulfides to the corresponding sulfoxides. The formation of [OFeIV(salen)]•+ as the active oxidant is supported by resonance Raman studies. The kinetic data indicate that the reaction is first-order in the oxidant and fractional-order with respect to sulfide. The observed saturation kinetics of the reaction and spectral data indicate that the substrate binds to the oxidant before the rate-controlling step. The rate constant (k) values for the product formation step determined using Michaelis−Menten kinetics correlate well with Hammett σ constants, giving reaction constant (ρ) values in the range of −0.65 to −1.54 for different oxo(salen)iron complexes. The log k values observed in the oxidatio...

n-Butyl Acetate Synthesis via Reactive Distillation: Thermodynamic Aspects, Reaction Kinetics, Pilot-Plant Experiments, and Simulation Studies

Abstract: The development of a heterogeneously catalyzed reactive distillation process for the production of n-butyl acetate by the esterification of n-butanol with acetic acid is presented. Thermodynamic aspects of the considered system are discussed, and UNIQUAC interaction parameters are given. The reaction was catalyzed heterogeneously by a strongly acidic ion-exchange resin (Amberlyst-15). The reaction kinetics was investigated, and the kinetic constants for a pseudohomogeneous kinetic model are presented. Reactive distillation experiments were performed using the structured catalytic packing Katapak-S. Several operation conditions were varied (reboiler duty, reactant ratio, total feed rate), and two different setups were realized experimentally. The experimental results are presented in comparison with simulation results. An equilibrium stage model is capable of describing the experiments quantitatively. n-Butanol conversions of 98.5% accompanied with n-butyl acetate purities of 96.9% were achieved using an e...