International Journal of Chemical Kinetics
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Kinetics of the esterification of acetic acid with 2‐propanol: Impact of different acidic cation exchange resins on reaction mechanism Abstract The kinetics of the esterification of acetic acid with the secondary alcohol, 2‐propanol, catalyzed by the cation exchange resins, Dowex 50Wx8‐400, Amberlite IR‐120, and Amberlyst 15 has been studied at temperatures of 303, 323, and 343 K; acid to alcohol molar ratios of 0.5, 1, and 2; and catalyst loadings of 20, 40, and 60 g/L. The equilibrium constant was experimentally determined, and the reaction was found to be mildly exothermic. External and internal diffusion limitations were absent under the implemented experimental conditions. Systems catalyzed by gel‐type resins (Dowex 50Wx8‐400 and Amberlite IR‐120) exhibit some similarities in their reaction kinetics. Increase in reaction temperature, acid to alcohol ratio, and catalyst loading is found to enhance reaction kinetics for the three catalysts. The pseudohomogeneous (PH), Eley Rideal (ER), Langmuir Hinshelwood (LH), modified Langmuir Hinshelwood (ML), and Pöpken (PP) models were found to predict reaction kinetics with mean relative errors of less than 5.4%. However, the ML model was found to be better for predicting reaction kinetics in the systems catalyzed by gel‐type resins, while the PP model was better for the system catalyzed by the macroreticular catalyst, Amberlyst 15. The E act for the forward reaction is found to be 57.0, 59.0, and 64.0 kJ/mole for the systems catalyzed by Dowex 50Wx8‐400, Amberlite IR‐120, and Amberlyst 15, respectively. For these three catalysts, the adsorption equilibrium constants of the components present in the system increase in the same order as do the solubility parameters of the component. Nonideality in the system is successfully accounted for by the UNIFAC model. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 593–612, 2006
International Journal of Chemical Kinetics - Tập 38 Số 10 - Trang 593-612 - 2006
Kinetics study of propyl acetate synthesis reaction catalyzed by Amberlyst 15 Abstract The reaction kinetics of esterification of acetic acid with n ‐propanol was investigated. The reaction was catalyzed by the commercial cation‐exchange resin Amberlyst 15, and the kinetic data were obtained in a batch reactor within the temperature range 338–368 K. The chemical equilibrium constant, K eq , was first determined experimentally; the result shows that K eq is about 20 and slightly temperature dependent. Altogether 14 sets of kinetic data were then measured. The influences of operating parameters such as temperatures, initial molar ratios, and catalyst concentrations were checked. The pseudo‐homogeneous (PH), Rideal–Eley (RE), and Langmuir–Hinshelwood–Hougen–Watson (LHHW) kinetic models were developed to interpret the obtained kinetic data. The parameters of the kinetic models were identified by the software DIVA, and the confidence interval of each parameter was also estimated. Both the chemical equilibrium constant and kinetic models were formulated in terms of the liquid phase activity, which was described by the nonrandom two‐liquid (NRTL) model. The LHHW model gives the best fitting result, followed by the RE model and the PH model, whereas the confidence intervals rank in the reverse order. In addition, an effective solution was proposed to overcome a convergence problem occurring in the LHHW model parameter identification, which has been reported several times in the literature. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 245–253, 2007
International Journal of Chemical Kinetics - Tập 39 Số 5 - Trang 245-253 - 2007
Kinetics and mechanism of oxidation of diethyl ketone by <i>N</i>‐bromosuccinimide Abstract Oxidative kinetics of diethyl ketone in perchloric acid media in the presence of mercuric acetate have been studied by using N ‐bromosuccinimide (NBS) as oxidant in the temperature range of 25°‐50°C. It has been found that the order with respect to NBS is zero while with respect to diethyl ketone and [H+ ], it is unity. Succinimide, sodium perchlorate, and mercuric acetate have an insignificant effect on the reaction rate, while the dielectric effect was negative. A solvent isotope effect (k 0D 2 O /k 0H 2 O = 1.6–1.8) at 35°C has been observed. On the basis of the available evidences a suitable mechanism consistent with the experimental results has been proposed in which it is suggested that the mechanistic route for NBS oxidation in an acidic medium is through the enol form of the ketone. The magnitude of the solvent effect also supports the mechanism. Various activation parameters have been calculated, and the 1,2‐dicarbonyl compound has been identified as the end product of the reaction.
International Journal of Chemical Kinetics - Tập 10 Số 9 - Trang 995-1002 - 1978
A Chemical Kinetic Mechanism for 2‐Bromo‐3,3,3‐trifluoropropene (2‐BTP) Flame Inhibition ABSTRACT In this work, we report a detailed chemical kinetic mechanism to describe the flame inhibition chemistry of the fire‐suppressant 2‐bromo‐3,3,3‐trifluoropropene (2‐BTP), under consideration as a replacement for CF3 Br. Under some conditions, the effectiveness of 2‐BTP is similar to that of CF3 Br; however, like other potential halon replacements, it failed an U.S. Federal Aviation Authority (FAA) qualifying test for its use in cargo bays. Large overpressures are observed in that test and indicate an exothermic reaction of the agent under those conditions. The kinetic model reported herein lays the groundwork to understand the seemingly conflicting behavior on a fundamental basis. The present mechanism and parameters are based on an extensive literature review supplemented with new quantum chemical calculations. The first part of the present article documents the information considered and provides traceability with respect to the reaction set, species thermochemistry, and kinetic parameters. In additional work, presented more fully elsewhere, we have combined the 2‐BTP chemical kinetic mechanism developed here with several other submodels from the literature and then used the combined mechanism to simulate premixed flames over a range of fuel/air stoichiometries and agent loadings. Overall, the modeling results qualitatively predicted observations found in cup‐burner tests and FAA Aerosol Can Tests, including the extinguishing concentrations required and the lean‐to‐rich dependence of mixtures. With these data in hand, in a second phase of the present work, we perform a reaction path analysis of major species under several modeled conditions. This analysis leads to a qualitative understanding of the ability of 2‐BTP to act as both an inhibitor and a fuel, depending on the conditions and suggests areas of the kinetic model that should be further investigated and refined.
International Journal of Chemical Kinetics - Tập 47 Số 9 - Trang 533-563 - 2015
Nickel peroxide: A more probable intermediate in the Ni(II)‐catalyzed decomposition of peroxomonosulfate Abstract The Ni(II) ion catalyzed thermal decomposition of peroxomonosulfate (PMS) was studied in the pH range 3.42–5.89. The rate is first order in [PMS] and Ni(II) ion concentrations. At pH greater than or equal to 5.23, the reaction becomes zero order in [PMS] and this changeover in the order of the reaction occurs at a higher concentration of nickel ions. The first‐order kinetics in PMS can be explained as a rate‐limiting step and is the transformation of nickel peroxomonosulfate into nickel peroxide. This peroxide intermediate reacts rapidly with another PMS to give oxygen and Ni(II). The formation of nickel peroxide is associated with a small negative or nearly zero entropy of activation. The zero‐order kinetics in [PMS] can be explained by the fact that the hydrolysis of aquated nickel(II) ions into hydroxocompounds is the rate‐limiting step. The turnover number is 2 at pH 3.42 and increases with pH. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 320–237, 2007
International Journal of Chemical Kinetics - Tập 39 Số 6 - Trang 320-327 - 2007
A pyrolysis mechanism for ammonia Abstract The mechanism of NH3 pyrolysis was investigated over a wide range of conditions behind reflected shock waves. Quantitative time‐history measurements of the species NH and NH2 were made using narrow‐linewidth laser absorption. These records were used to establish an improved model mechanism for ammonia pyrolysis. The risetime and peak concentrations of NH and NH2 in this experimental database have also been summarized graphically. Rate coefficients for several reactions which influence the NH and NH2 profiles were fitted in the temperature range 2200 K to 2800 K. The reaction and the corresponding best fit rate coefficients are as follows:
with a rate coefficient of 4.0 × 1013 exp(−3650/RT ) cm3 mol−1 s−1 ,
with a rate coefficient of 1.5 × 1015 T −0.5 cm3 mol−1 s−1 and
with a rate coefficient of 5.0 × 1013 exp(−10000/RT ) cm3 mol−1 s−1 . The uncertainty in rate coefficient magnitude in each case is estimated to be ±50%. The temperature dependences of these rate coefficients are based on previous estimates. The experimental data from four earlier measurements of the dissociation reaction
were reanalyzed in light of recent data for the rate of NH3 + H → NH2 1 + H2 , and an improved rate coefficient of 2.2 × 1016 exp(−93470/RT ) cm3 mol−1 s−1 in the temperature range 1740 to 3300 K was obtained. The uncertainty in the rate coefficient magnitude is estimated to be ± 15%.
International Journal of Chemical Kinetics - Tập 22 Số 5 - Trang 513-535 - 1990
Formation and decay of the ABTS derived radical cation: A comparison of different preparation procedures Abstract Bleaching of a preformed solution of the blue‐green radical cation 2,2′‐azinobis (3‐ethylbenzothizoline‐6‐sulfonic acid) (ABTS+· ) has been extensively used to evaluate the antioxidant capacity of complex mixtures and individual compounds. The reaction of the preformed radical with free‐radical scavengers can be easily monitored by following the decay of the sample absorbance at 734 nm. The ABTS radical cation can be prepared employing different oxidants. Results obtained using MnO2 as oxidant show that the presence of manganese ions increases the rate of [ABTS]+· autobleaching in a concentration‐dependent manner. The radicals can also be obtained by oxidizing ABTS with 2,2′ ‐azobis(2‐amidinopropane)hydrochloride (AAPH) or peroxodisulfate (PDS). The oxidation by AAPH takes place with a large activation energy and a low reaction order in ABTS. The data support a mechanism in which the homolysis of AAPH is the rate‐limiting step, followed by the reaction of ABTS with the peroxyl radicals produced after the azocompound thermolysis. On the other hand, the low activation energy measured employing PDS, as well as the kinetic law, are compatible with the occurrence of a bimolecular reaction between the oxidant and ABTS. Regarding the use of ABTS‐based methodologies for the evaluation of free radical scavengers, radical cations obtained employing AAPH as oxidant can be used only at low temperatures, conditions where further decomposition of the remaining AAPH is minimized. The best results are obtained with ABTS derived radicals generated in the reaction of PDS with an ABTS/PDS concentration ratio equal (or higher) to two. However, even with radicals prepared by this procedure, stoichiometric coefficients considerably larger than two are obtained for the consumption of the radical cation employing tryptophane or p ‐terbutylphenol as reductants. This casts doubts on the use of ABTS‐based procedures for the estimation of antioxidant capacities. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 659–665, 2002
International Journal of Chemical Kinetics - Tập 34 Số 12 - Trang 659-665 - 2002
Reactions of NO<sub>2</sub><sup>+</sup> and solvated NO<sub>2</sub><sup>+</sup> ions with aromatic compounds and alkanes Abstract The rate constants and modes of reaction of NO2 + and C2 H5 ONO2 NO2 + with aromatic compounds and alkanes have been determined in a pulsed ion cyclotron resonance mass spectrometer. Both ions undergo competing charge transfer and substitution reactions (NO2 + + M → MO+ + NO; C2 H5 ONO2 NO2 + + M → MNO2 + + C2 H5 ONO2 ) with aromatic molecules. In both cases, the probability that a collision results in charge transfer increases with increasing exothermicity of that process. The C2 H5 ONO2 NO2 + ion does not undergo charge transfer with molecules having an ionization potential greater than about 212 kcal/mol (9.2 eV); this observation leads to an estimate of 13 kcal/mol for the binding energy between NO2 + and C2 H5 ONO2 . The importance of the substitution reaction depends on the number of substituents on the aromatic ring and the molecular structure, and, in the case of C2 H5 ONO2 NO2 + ions, on the energetics of the competing charge transfer process. Both NO2 + and C2 H5 ONO2 NO2 + undergo hydride transfer reactions with alkanes. For both these ions, k (hydride transfer)/k (collision) increases with increasing exothermicity of reaction, but in both cases the rate constants of reaction are unusually low when compared with other hydride transfer reactions of comparable exothermicity which have been reported in the literature. This is interpreted as evidence that the attack on the alkane preferentially involves the nitrogen atom (where the charge is localized) rather than one of the oxygen atoms of NO2 + .
International Journal of Chemical Kinetics - Tập 10 Số 7 - Trang 657-667 - 1978
An updated comprehensive kinetic model of hydrogen combustion Abstract A comprehensively tested H2 /O2 chemical kinetic mechanism based on the work of Mueller et al. 1 and recently published kinetic and thermodynamic information is presented. The revised mechanism is validated against a wide range of experimental conditions, including those found in shock tubes, flow reactors, and laminar premixed flame. Excellent agreement of the model predictions with the experimental observations demonstrates that the mechanism is comprehensive and has good predictive capabilities for different experimental systems, including new results published subsequent to the work of Mueller et al. 1 , particularly high‐pressure laminar flame speed and shock tube ignition results. The reaction H + OH + M is found to be primarily significant only to laminar flame speed propagation predictions at high pressure. All experimental hydrogen flame speed observations can be adequately fit using any of the several transport coefficient estimates presently available in the literature for the hydrogen/oxygen system simply by adjusting the rate parameters for this reaction within their present uncertainties. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 566–575, 2004
International Journal of Chemical Kinetics - Tập 36 Số 10 - Trang 566-575 - 2004
Kinetic and mechanistic aspects of the NO oxidation by O<sub>2</sub> in aqueous phase Abstract The oxidation kinetics of NO by O2 in aqueous solution was observed using a stopped flow apparatus. The kinetics follows a third order rate law of the form k · [NO]2 · [O2 ] in analogy to gas‐phase results. The rate constant at 296 K was measured as (6.4 ± 0.8) · 106 M−2 s−1 with an activation energy of 2.3 kcal/mol and a preexponential factor of (4.0 ± 0.5) · 108 M−2 s−1 . The rate constant displays a very slight pH dependence corresponding to less than a factor of three over the range 0 to 12. The system NO/O2 in aqueous solution is an efficient nitrosating agent which has been tested using phenol as a substrate over the pH range 0 to 12. The rate limiting step leading to formation of 4‐nitrosophenol is the formation of the reactive intermediate whose competitive hydrolysis yields HONO or NO2 − . The absence of NO3 − in the autoxidation of NO, the exclusive presence of NO2 − as a product of the nitrosation reaction of phenol, and the kinetic results of the N3 − trapping experiments point towards N2 O3 as the reactive intermediate. © 1994 John Wiley & Sons, Inc.
International Journal of Chemical Kinetics - Tập 26 Số 12 - Trang 1207-1227 - 1994
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