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Showing papers on "Reaction rate constant published in 2008"


Journal ArticleDOI
TL;DR: In this article, an analysis of the performance of an updated hydrogen combustion mechanism is presented, with particular attention paid to different channels of reaction between H atoms and HO2 radicals, to pressure dependence of the recombination of HO 2 radicals, and to the anomalous rate constant.

308 citations


Journal ArticleDOI
TL;DR: An examination of thermodynamic parameters shows that the adsorption of arsenic (V) as well as arsenic (III) by GFH is an endothermic process and is spontaneous at the specific temperatures investigated.

293 citations


Journal ArticleDOI
TL;DR: In this paper, the rate constants of the hydrolytic conversion were determined for the resulting monomers (glucose and amino acids), and the values were found to strongly depend on the type of bond.
Abstract: The unique properties of water at elevated temperatures and pressures, namely the alterability of ionic product, dielectric constant, and density, make it an interesting and promising reaction medium. Several investigations have been made in order to obtain valuable products from biopolymers or, more general, biomass. Biopolymers can react with high-temperature water in very short residence times and with high rates of conversion. The achievable products are manifold and can be varied to a large extent by changing the operating conditions. The possibilities of sub- and supercritical water can even be significantly expanded by adding carbon dioxide since it reacts to carbonic acid, which serves as a catalyst. This work deals with the hydrolysis kinetics of different kinds of biopolymers, namely starch, cellulose (polysaccharides), and proteins (polypeptides). Kinetics was conducted over a broad range of experimental conditions using a continuous-flow reactor. In all three cases, the obtained experimental results could be described by a reaction model according to the approach of a single consecutive reaction following first order kinetics. The rate constants of the hydrolytic conversion were determined for the resulting monomers (glucose and amino acids), and the values were found to strongly depend on the type of bond. The peptide bonds in proteins exhibited a much higher stability compared to the β-1,4- and β-1,6-glycosidic linkages in cellulose and starch, respectively. The stability of the resulting monomers and their conversion to further degradation products were determined. The addition of carbon dioxide to water under hydrothermal conditions resulted in a significant increase in acid catalyzed reaction rates, which could be confirmed by the obtained rate constants.

239 citations


Journal ArticleDOI
Feng He1, Dongye Zhao1
TL;DR: In this paper, the effects of carboxymethyl cellulose (CMC) as a stabilizer on the reactivity of CMC-stabilized Fe-Pd bimetallic nanoparticles toward trichloroethene (TCE) were investigated.
Abstract: This study investigated the effects of carboxymethyl cellulose (CMC) as a stabilizer on the reactivity of CMC-stabilized Fe-Pd bimetallic nanoparticles toward trichloroethene (TCE). Overall, the particle stabilization prevented particle agglomeration and resulted in greater particle reactivity. The pseudo-first order TCE degradation rate increased from 0.86 h−1 to 6.8 h−1 as the CMC-to-Fe molar ratio increased from 0 to 0.0124. However, a higher CMC-to-Fe ratio inhibited the TCE degradation. Within the same homologous series, CMC of greater molecular weight resulted in more reactive nanoparticles for TCE hydrodechlorination. Hydrogen (either residual hydrogen from zero-valent iron (ZVI) nanoparticle synthesis or hydrogen evolved from ZVI corrosion) can serve as effective electron donors for TCE dechlorination in the presence of Pd (either coated on ZVI or as separate nanoparticles). Decreasing reaction pH from 9.0 to 6.0 increased the TCE reduction rate by 11.5 times, but enhanced the Fe corrosion rate by 31.4 times based on the pseudo-first order rate constant. Decreasing pH also shifted the rate controlling step of TCE reduction from Fe corrosion to hydrodechlorination. Ionic strength (<0.51 M) did not significantly affect the TCE reduction.

226 citations


Journal ArticleDOI
TL;DR: Ozonation of diclofenac in aqueous solution in the presence and absence of an *OH scavenger, tertiary butanol (t-BuOH), was studied, and the most important reaction intermediates and products were identified.
Abstract: Ozonation of diclofenac in aqueous solution in the presence and absence of an •OH scavenger, tertiary butanol (t-BuOH), was studied, and the most important reaction intermediates and products were identified. The second-order O3 rate constant was determined by competition with buten-3-ol and was found to be 6.8 × 105 M−1 s−1 at 20 °C. From this high rate constant, it has been concluded that O3 must initially add on the amino nitrogen. Decomposition of the adduct results in the formation of O3•− (→ •OH) and aminyl radical precursors. A free •OH yield of 30% was estimated based on the HCHO yields generated upon reaction of •OH with 0.01 M t-BuOH. Almost all diclofenac reacted when the molar ratio of O3/diclofenac was ∼5:1 in the presence of t-BuOH and ∼8:1 in its absence. As primary reaction products (maximum yield), diclofenac-2,5-iminoquinone (32%), 5-hydroxydiclofenac (7%), and 2,6-dichloroaniline (19%) were detected with respect to reacted diclofenac in the presence of t-BuOH. These primary products deg...

214 citations


Journal ArticleDOI
TL;DR: A comparative study of the degradation of Acid Orange 7 (AO 7) aqueous solutions in acidic medium of pH 3.0 by electro-Fenton process using Pt or boron-doped diamond (BDD) anode showed higher mineralization rate with BDD than Pt anode at the first hours of electrolysis because of the higher oxidizing power of this anode.

207 citations


Journal ArticleDOI
TL;DR: In this paper, the rate constant of CaO-CO 2 reaction was studied for two sorbents using an atmospheric thermogravimetric analyzer (ATGA) and a pressurised TGA, and the activation energies were 29 ± 4 and 24 ± 6 kJ / mol for the limestone and dolomite tested.

205 citations


Journal ArticleDOI
TL;DR: Density functional theory simulations were used to calculate activation barriers for different possible reaction mechanisms, including oxidation by hydroxyl radicals at different sites on the PFOS molecule, and direct electron transfer, and indicated that the most likely rate-limiting step for PFOS oxidation was direct electronTransfer.
Abstract: This research investigated the oxidative destruction of perfluorooctane sulfonate at boron-doped diamond film electrodes. Experiments measuring oxidation rates of PFOS were performed over a range in current densities and temperatures using a rotating disk electrode (RDE) reactor and a parallel plate flow-through reactor. The oxidation of PFOS yielded sulfate, fluoride, carbon dioxide, and trace levels of trifluoroacetic acid. Reaction rates in the RDE reactor were zeroth order in PFOS concentration. Reaction rates in the flow-through reactor were mass-transfer-limited and were pseudo-first-order in PFOS concentration, with a half-life of 5.3 min at a current density of 20 mA/cm2. Eyring analysis of the zeroth order rate constants at a fixed electrode potential yielded an apparent activation energy of 4.2 kJ/mol for PFOS oxidation. Density functional theory (DFT) simulations were used to calculate activation barriers for different possible reaction mechanisms, including oxidation by hydroxyl radicals at different sites on the PFOS molecule, and direct electron transfer. A comparison of the experimentally measured apparent activation energy with those calculated using DFT indicated that the most likely rate-limiting step for PFOS oxidation was direct electron transfer.

204 citations


Journal ArticleDOI
TL;DR: A kinetic study on the stability of (-)-EGCG in aqueous system was carried out using a HPLC-UV system and Matlab programming, indicating that the turning point of 82 degrees C reported in the literature for the reaction kinetics of catechins would need to be re-examined.
Abstract: (–)-Epigallocatechin gallate (EGCG) is the most abundant catechin in green tea, which has been linked with many health benefits. To ensure the conceivable health benefits from thermally processed products, a kinetic study on the stability of (–)-EGCG in aqueous system was carried out using a HPLC-UV system and Matlab programming. Simultaneous degradation and epimerization of (–)-EGCG were characterized during isothermal reactions at low temperatures (25–100 °C) combined with previously conducted experimental results at high temperature (100–165 °C); the degradation and epimerization complied with first-order reaction and their rate constants followed Arrhenius equation. Mathematical models for the stability of (–)-EGCG were established and validated by the reactions at 70 °C and with varied concentrations from different catechin sources. Two specific temperature points in the reaction kinetics were identified, at 44 and 98 °C, respectively. Below 44 °C, the degradation was more profound. Above 44 °C, the ...

197 citations


Journal ArticleDOI
TL;DR: Photodegradation experiments with various DOM sources including Pony Lake (Antarctica) fulvic acid and a synthetic estuarine sample, as well as Minnesota freshwater samples, demonstrated distinct reactivity patterns, indicating that DOM's 1O2-generation efficiency is not strongly coupled to its ability to promote other photooxidation pathways.
Abstract: The role of photochemically generated singlet oxygen (1O2) in the DOM-sensitized degradation of eighteen dissolved free amino acids was investigated. The fraction of total sensitized degradation due to reaction with 1O2 was determined through a kinetic analysis based on a measured reaction rate constant for each amino acid coupled with measured 1O2 concentrations and was confirmed through quenching experiments. Only four of the eighteen free amino acid residues examined were found to be photolabile under environmentally relevant conditions: histidine, methionine, tyrosine, and tryptophan. The fraction of Suwannee River Humic Acid (SRHA)-sensitized degradation due to reaction with 1O2 ranged from an upper value of 110 ± 10% for histidine to 8 ± 1% for tryptophan, with 26 ± 3% contribution for methionine and 33 ± 4% for tyrosine. In addition to degradation through reaction with 1O2, other reactive intermediates involved in the SRHA-photosensitized degradation of these amino acids were identified. Methionine...

196 citations


Journal ArticleDOI
07 Jul 2008-Polymer
TL;DR: In this paper, the effect of carboxyl and fluorine modified multi-wall carbon nanotubes (MWCNTs) on the curing behavior of diglycidyl ether of bisphenol A (DGEBA) epoxy resin was studied using differential scanning calorimetry (DSC), rheology and infrared spectroscopy (IR).

Journal ArticleDOI
TL;DR: A mechanism-based kinetic model has been developed to successfully describe the entire range of kinetic data for a total of 21 compounds of varying structural characteristics and improves the ability to quantitatively evaluate the kinetics of oxidative transformation of organic contaminants by manganese oxides in well-defined systems.
Abstract: Several groups of popular antibacterial agents (i.e., phenols, fluoroquinolones, aromatic N-oxides, and tetracyclines) were demonstrated in earlier studies to be highly susceptible to oxidation by manganese oxides, a common oxidant in soils. However, because of the high complexity, the reaction kinetics were not fully characterized. A mechanism-based kinetic model has now been developed to successfully describe the entire range of kinetic data for a total of 21 compounds of varying structural characteristics (with R2 > 0.93). The model characterizes the reaction kinetics by two independent parameters, the reaction rate constant (k) and total reactive surface sites (Srxn). The model fitting indicates that the reaction kinetics of antibacterials with MnO2 are controlled by either the rate of surface precursor complex formation (for tetracyclines) or by the rate of electron transfer within the precursor complex (for phenols, fluoroquinolones, and aromatic N-oxides). The effect of reactant concentration, pH, ...

Journal ArticleDOI
TL;DR: The adsorption kinetics of a cationic dye, methylene blue (MB), onto the silica nano-sheets derived from vermiculite via acid leaching was investigated in aqueous solution in a batch system with respect to contact time, initial dye concentration, pH, and temperature.

Journal ArticleDOI
Youji Li1, Shuguo Sun1, Mingyuan Ma1, Yuzhu Ouyang1, Wenbin Yan1 
TL;DR: In this article, the photocatalytic degradation kinetic characteristics were experimentally investigated under different reaction conditions (light intensity, initial organic content and TiO2 content of activated carbon, TA).

Journal ArticleDOI
TL;DR: In this article, the porosity of silica shells was readily controlled by introducing C18TMS as a porogen with heat treatment, which directly altered the turnover frequency and rate constant of the reaction.
Abstract: Nanoreactor frameworks have many advantages over bulk catalyst structures in terms of providing a regular reaction environment and conformational stability. In this work, Au@SiO2 nanoreactor frameworks were chemically modified to improve the catalytic efficiency of o-nitroaniline reduction. The porosity of silica shells was readily controlled by introducing C18TMS as a porogen with heat treatment. The diffusion rate of the silica layers was tuned from 5.9 × 10−19 to 2.1 × 10−18 m2 s−1, which directly altered the turnover frequency and rate constant of the reaction. Carboxylate functionality was introduced on the gold cores of Au@SiO2 nanoreactors by 3-MPA addition. The reaction rate was enhanced by a maximum of 2.4 times compared to unfunctionalized catalysts through a strong interaction between carboxylate anions and o-nitroaniline. Totally, the rate constant of Au@SiO2 yolk−shell nanoreactors exhibits a 13-fold enhancement by diffusion and surface functionality control. These results indicate that the r...

Journal ArticleDOI
TL;DR: In this paper, the reaction kinetics of reduction and oxidation of Mn and Zn iron oxides were determined using a thermogravimetric analyzer, and the reaction rate and activation energy for decomposition of water by oxidation of the metal oxides was also determined.

Journal ArticleDOI
Li Huang1, Liang Li1, Wenbo Dong1, Yan Liu1, Huiqi Hou1 
TL;DR: Investigation of the oxidation pathway and kinetics of ammonia oxidation by *OH shows that the *OH, generated by H2O2 photolysis, could oxidize NH3 to NO2- and further to NO3-.
Abstract: Many advanced oxidation technologies have been developed to remove ammonia in wastewater. All these technologies have one common characteristic, that is, the removal processes involve OH radical (*OH). In this research work, H2O2 was selected as *OH precursor. The removal of ammonia under 253.7 nm irradiation from low-pressure mercury lamp in the presence of H2O2 was studied to investigate the ammonia removal efficiency by *OH. Results show that the *OH, generated by H2O2 photolysis, could oxidize NH3 to NO2- and further to NO3-. Removal efficiencies of ammonia were low and were affected by initial pH value and ammonia concentration. Laser flash photolysis technique with transient absorption spectra of nanosecond was used to investigate the oxidation pathway and kinetics of ammonia oxidation by *OH. Results illustrate that *OH could oxidize NH3 to form *NH2 with a second-order rate constant of (1.0 +/- 0.1) x 10(8) M(-1) s(-1) (20 degrees C). *NH2, the main product of *OH with NH3, would further react with H2O2 to yield *NHOH. Since *NHOH could not stay stable in solution, it would rapidly convert to NH2O2- and consequently NO2- and NO3-. The rate constants for these elementary reactions were also given. The low removal efficiency of ammonia by *OH was mainly due to the slow reaction rate constant

Journal ArticleDOI
TL;DR: In this article, the removal of AB24 dye in aqueous solution by nano/micro-size zero-valent iron (ZVI) was investigated, and the degradation efficiency increases with increasing ZVI concentration and temperature but decreases with particle size of ZVI.

Journal ArticleDOI
TL;DR: In this paper, a detailed scheme for the Fe3+-catalyzed electro-Fenton mineralization of malachite green as a model triarylmethane dye is presented.
Abstract: A very detailed scheme for the Fe3+-catalyzed electro-Fenton mineralization of malachite green as a model triarylmethane dye is presented. Bulk electrolyses of 250-mL aqueous solutions of 0.5 mM malachite green with 0.2 mM Fe3+ as catalyst have been carried out at room temperature and pH 3.0 under constant current in an undivided cell equipped with a graphite-felt cathode and a Pt anode to assess the performance of the electro-Fenton system. In situ electrogeneration of Fe2+ and H2O2 from quick cathodic reduction of Fe3+ and dissolved O2 (from bubbled compressed air), respectively, allows the formation of the very oxidizing species hydroxyl radical ( OH) in the medium from Fenton's reaction. A pseudo-first-order decay kinetics with an apparent rate constant of k1,MG = 0.244 min−1 was obtained for total destruction of malachite green by action of OH at 200 mA, requiring 22 min for total decoloration of the solution. In the same experimental conditions, overall mineralization was reached at 540 min. Up to 15 aromatic and short-chain carboxylic acid intermediates were identified along the treatment. The evolution of current efficiency was calculated from the chemical oxygen demand (COD) removal. Based on the time course of most of the by-products and the identification of inorganic ions released, some plausible mineralization pathways are proposed and thoroughly discussed. It has been found that the electro-Fenton degradation of malachite green proceeds via parallel pathways, all of them involving primary splitting of the triaryl structure initiated by attack of OH on the central carbon, thus yielding two different N-dimethylated benzophenones. Successive cleavage of the aromatic intermediates generates oxalic acid as the ultimate short-chain carboxylic acid, whereas N-demethylation of some of them produces formic acid as well. Oxalic acid and its Fe2+ complexes, as well as formic acid, can be slowly but totally mineralized by OH.

Journal ArticleDOI
TL;DR: The relationship between mediator-enzyme overpotential and bimolecular rate constant is used to determine the optimum mediator redox potential for maximum power output of a hypothetical biofuel cell with a planar cathode and a reversible hydrogen anode.
Abstract: Oxygen-reducing enzyme electrodes are prepared from laccase of Trametes versicolor and a series of osmium-based redox polymer mediators covering a range of redox potentials from 0.11 to 0.85 V. Experimentally obtained current density generated by the film electrodes is analyzed using a one-dimensional numerical model to obtain kinetic parameters. The bimolecular rate constant for mediation is found to vary with mediator redox potential from 250 s−1 M−1 when mediator and enzyme are close in redox potential to 9.4 × 104 s−1 M−1 when the redox potential difference is large. The value of the bimolecular rate constant for the simultaneously occurring laccase−oxygen reaction is found to be 2.4 × 105 s−1 M−1. The relationship between mediator−enzyme overpotential and bimolecular rate constant is used to determine the optimum mediator redox potential for maximum power output of a hypothetical biofuel cell with a planar cathode and a reversible hydrogen anode. For laccase of T. versicolor (Ee0 = 0.82), the optimum...

Journal ArticleDOI
TL;DR: In this article, the authors investigated the ozonation of naproxen and carbamazepine during catalytic and non-catalytic semicontinuous oxidation experiments performed at 25°C and in the range of pH 3-7.
Abstract: This study investigates the ozonation of naproxen and carbamazepine during catalytic and non-catalytic semicontinuous oxidation experiments performed at 25 °C and in the range of pH 3–7. The results showed that naproxen and carbamazepine were completely consumed in the first few minutes of reaction. The extent of mineralization during non-catalytic runs reached about 50% and essentially took place during a period covering the first 10–20 min. Catalytic runs were carried out on a commercial catalyst consisting of fumed colloidal TiO2 particles. The catalyst increased the extent of mineralization by up to 75% of the initial organic carbon. The results showed that the catalyst enhanced mineralization both in acidic and neutral solutions, but the best results were obtained in a slightly acidic media. This effect was probably linked to the adsorption of reaction intermediates on Lewis acid catalytic sites. The catalyst enhanced the decomposition of ozone in an acid medium, but inhibited it in a neutral solution. This seems to exclude a mechanism based on the surface formation of hydroxyl radicals followed by their migration and bulk reaction with organic compounds. The evolution of the total organic carbon measured in samples taken during the run was modelled as a function of the integral ozone exposure. The kinetic regression model considered that the ozonation products from naproxen or carbamazepine consisted of either oxidizable or refractory compounds, where the latter were necessarily produced from the former. The model assumed a second order reaction between organic compounds and ozone. The higher non-catalytic rate constants for the first mineralization period were 1.048 × 10−2 ± 9.3 × 10−4 L mmol−1 s−1 for naproxen and 6.16 × 10−3 ± 5.6 × 10−4 L mmol−1 s−1 for carbamazepine, both at pH 7. The corresponding pseudohomogeneous catalytic rate constants were 7.76 × 10−3 ± 3.9 × 10−4 and 4.25 × 10−3 ± 9.7 × 10−4 L mmol−1 s−1 for naproxen and carbamazepine, respectively, at pH 5 and with a catalyst load of 1 g/L. The evolution of carboxylic acids during reaction revealed that the catalyst avoided the accumulation of oxalate especially in comparison with non-catalytic runs, in which it accounted for up to 30% of the final organic carbon. Specific ultraviolet absorbance at 254 nm was also followed during the run. The products from naproxen reached a high absorbance from the beginning of the ozonation that was maintained throughout the run. For carbamazepine, however, the absorbance rapidly decreased revealing a different chemical structure of reaction products.

Journal ArticleDOI
TL;DR: The performances of the 1 complexes in catalyzing H 2O 2 oxidations are shown to compare favorably with the peroxidases further establishing Fe (III)-TAML activators as miniaturized enzyme replicas with the potential to greatly expand the technological utility of hydrogen peroxide.
Abstract: Exceptionally high peroxidase-like and catalase-like activities of iron(III)−TAML activators of H2O2 (1: Tetra-Amidato-Macrocyclic-Ligand FeIII complexes [Fe{1,2−X2C6H2−4,5−(NCOCMe2NCO)2CR2}(OH2)]–) are reported from pH 6−12.4 and 25−45 °C. Oxidation of the cyclometalated 2-phenylpyridine organometallic complex, [RuII(o-C6H4py)(phen)2]PF6 (2) or “ruthenium dye”, occurs via the equation [RuII]+1/2 H2O2+H + →□FeIII−TAML [RuIII]+H2O, following a simple rate law rate = kobsper[1][H2O2], that is, the rate is independent of the concentration of 2 at all pHs and temperatures studied. The kinetics of the catalase-like activity (H2O2⁢ →□FeIII−TAML H2O+1/2 O2) obeys a similar rate law: rate = kobscat[1][H2O2]). The rate constants, kobsper and kobscat, are strongly and similarly pH dependent, with a maximum around pH 10. Both bell-shaped pH profiles are quantitatively accounted for in terms of a common mechanism based on the known speciation of 1 and H2O2 in this pH range. Complexes 1 exist as axial diaqua species ...

Journal ArticleDOI
TL;DR: Kinetic simulation results for a high temperature pyrolysis environment show that MB radicals are mainly produced through hydrogen abstraction reactions by H atoms, and the C(O)OCH(3) = CO + CH( 3)O reaction is found to be the main source of CO formation.
Abstract: In this paper, we report a detailed analysis of the breakdown kinetic mechanism for methyl butanoate (MB) using theoretical approaches. Electronic structures and structure-related molecular properties of reactants, intermediates, products, and transition states were explored at the BH&HLYP/cc-pVTZ level of theory. Rate constants for the unimolecular and bimolecular reactions in the temperature range of 300−2500 K were calculated using Rice−Ramsperger−Kassel−Marcus and transition state theories, respectively. Thirteen pathways were identified leading to the formation of small compounds such as CH3, C2H3, CO, CO2, and H2CO. For the initial formation of MB radicals, H, CH3, and OH were considered as reactive radicals participating in hydrogen abstraction reactions. Kinetic simulation results for a high temperature pyrolysis environment show that MB radicals are mainly produced through hydrogen abstraction reactions by H atoms. In addition, the C(O)OCH3 = CO + CH3O reaction is found to be the main source of C...

Journal ArticleDOI
TL;DR: In this paper, rate constant calculations and a reaction coordinate analysis of the Trp-cage mini-protein in explicit solvent using transition interface sampling were reported. But the results were not consistent with the observed folding and unfolding rate.

Journal ArticleDOI
TL;DR: The removal of a carbamate herbicide, propham, from aqueous solution by direct electrochemical advanced oxidation process using a boron-doped diamond (BDD) anode and its overall mineralization reaction follows a pseudo-first-order kinetics.

Journal ArticleDOI
TL;DR: In this paper, a condition-specific model was developed which uses a single entity as a representative of all Fe(II) species to overcome the difficulty of dealing with the large number of unknown kinetic constants.

Journal ArticleDOI
TL;DR: In this paper, the electrocatalytic properties of lead dioxide and boron-doped diamond (BDD) anodes were compared for the electrochemical incineration of methyl red, an azo dye, using an electrolytic flow cell with parallel-plate electrodes.
Abstract: The electrocatalytic properties of lead dioxide (PbO2) and boron-doped diamond (BDD) anodes were compared for the electrochemical incineration of methyl red, an azo dye, using an electrolytic flow cell with parallel-plate electrodes. The effects of several operating parameters such as current density, hydrodynamic conditions, and pH on the degradation rate and current efficiency were determined. The experimental data indicate that, on PbO2 and BDD anodes, methyl red oxidation takes place by reaction with hydroxyl radicals electrogenerated from water discharge. The electro-oxidation of methyl red was found to behave as a mass-transfer-controlled process, so that the removal rate and current efficiency were enhanced by high flow rates and independent of the pH in the range of 3.0−7.0. It was also observed that the methyl red decay reaction followed pseudo-first-order kinetics with a rate constant that increased slightly with applied current at the PbO2 anode but was essentially independent of current at the...

Journal ArticleDOI
TL;DR: In this paper, the authors apply simple probabilistic rules of particle interaction and combination to simulate chemical reactions without regard to local concentrations, and the results show that when low mixing limits the reaction probabilities, the reaction rates drop significantly, including the rate of approach to global equilibrium.
Abstract: [1] Chemical reactions may be simulated without regard to local concentrations by applying simple probabilistic rules of particle interaction and combination. The forward reaction A + B→ C is coded by calculating the probability that any A and B particles will occupy the same volume over some time interval. This becomes a convolution of the location densities of the two particles. The backward reaction is a simple exponential decay of C particles into A and B particles. When the mixing of reactants is not a limiting process, the classical thermodynamic reaction rates are reproduced. When low mixing (as by diffusion) limits the reaction probabilities, the reaction rates drop significantly, including the rate of approach to global equilibrium. At long enough times, the law of mass action is reproduced exactly in the mean, with some irreducible deviation in the local equilibrium saturations (the equilibrium constant divided by the mass action expression) away from unity. The saturation variability is not sensitive to numerical parameters but depends strongly on how far from equilibrium the system is initiated. This is simply due to a relative paucity of particles of some species as the reaction moves far to one side or the other.

Journal ArticleDOI
TL;DR: In this article, the kinetics of the epoxidation of soybean oil (SBO) by peroxyacetic acid (PAA) generated in situ in the presence of sulfuric acid as the catalyst was studied at various temperatures (45, 65 and 75 °C).
Abstract: In the present work, the kinetics of the epoxidation of soybean oil (SBO) by peroxyacetic acid (PAA) generated in situ in the presence of sulfuric acid as the catalyst was studied at various temperatures (45, 65 and 75 °C). It was found that epoxidation with almost complete conversion of unsaturated carbon and negligible oxirane cleavage can be attained by the in situ technique. The rate constant for epoxidation of SBO was found to be of the order of 10 -6 mol -1 s -1 and the activation energy of epoxidation is 43.11 kJ/mol. Some thermodynamic parameters: enthalpy, entropy and free activation energy of 40.63 kJ/mol, -208.80 J/mol and 102.88 kJ/mol, respectively, were obtained for the epoxidation of SBO. The kinetic and thermodynamic parameters of epoxidation obtained from this study indicate that an increase in the process temperature would increase the rate of epoxide formation. The epoxidation of corn oil and sunflower oil were also investigated under the same conditions. The results show that the reaction rate is in the order of soybean oil > corn oil > sunflower oil.

Journal ArticleDOI
TL;DR: A series of seven substituted 4,6-di-tert-butyl-2-(4,5-diarylimidazolyl)-phenols have been prepared and characterized, along with two related benzimidazole compounds, a valuable system in which to examine the key features of CPET.
Abstract: A series of seven substituted 4,6-di-tert-butyl-2-(4,5-diarylimidazolyl)-phenols have been prepared and characterized, along with two related benzimidazole compounds. X-ray crystal structures of all of the compounds show that the phenol and imidazole rings are close to coplanar and are connected by an intramolecular ArOH⋯N hydrogen bond. One-electron oxidation of these compounds occurs with movement of the phenolic proton to the imidazole base by concerted proton–electron transfer (CPET) to yield fairly stable distonic radical cations. These phenol–base compounds are a valuable system in which to examine the key features of CPET. Kinetic measurements of bimolecular CPET oxidations, with Erxn between +0.04 and −0.33 V, give rate constants from (6.3 ± 0.6) × 102 to (3.0 ± 0.6) × 106 M−1 s−1. There is a good correlation of log(k) with ΔG°, with only one of the 15 rate constants falling more than a factor of 5.2 from the correlation line. Substituents on the imidazole affect the (O–H⋯N) hydrogen bond, as marked by variations in the 1H NMR and calculated vibrational spectra and geometries. Crystallographic dO⋯N values appear to be more strongly affected by crystal packing forces. However, there is almost no correlation of rate constants with any of these measured or computed parameters. Over this range of compounds from the same structural family, the dominant contributor to the differences in rate constant is the driving force ΔG°.