Showing papers on "Reaction rate published in 1992"

Evidence for a sialosyl cation transition-state complex in the reaction of sialidase from influenza virus.

Abstract: The enzyme mechanism of sialidase from influenza virus has been investigated by kinetic isotope methods, NMR, and a molecular dynamics simulation of the enzyme-substrate complex. Comparison of the reaction rates obtained with the synthetic substrate 4-methylumbelliferyl-N-acetyl-α-D-neuraminic acid and the [3,3-2H]-substituted substrate revealed β-deuterium isotope effects for V/Km ranging over 1.09–1.15 in the pH range 6.0–9.5, whereas the effects observed for V in this pH range increased from 0.979 to 1.07. In D2O, βDV/Km was slightly increased by 2% and 5% at pD 6.0 and 9.5 respectively, while βDV was unchanged. Solvent isotope effects of 1.74 were obtained for both βDV/Km and βDV at pD 9.5, with βDV/Km decreasing and βDV remaining constant at acidic pD. 1H-NMR experiments confirmed that the initial product of the reaction is the α-anomer of N-acetyl-D-neuraminic acid. Molecular dynamics studies identified a water molecule in the crystal structure of the sialidase-N-acetyl-D-neuraminic acid complex which is hydrogen-bonded to Asp151 and is available to act as a proton donor source in the enzyme reaction. The results of this study lead us to propose a mechanism for the solvent-mediated hydrolysis of substrate by sialidase that requires the formation of an endocyclic sialosyl cation transition-state intermediate.

Accelerated imidization reactions using microwave radiation

Abstract: Microwave radiation has been clearly shown to result in enhancement of the rate of reaction for an imidization reaction. Analysis of the kinetic parameters showed that the apparent activation energy for the reaction was reduced from 105 to 55 kJ/mol, under the conditions of this experiment. The mechanism which has been proposed to explain this enhancement is based on the concept of a nonuniform temperature on a molecular scale, rather than a true reduction in the activation energy. © 1992 John Wiley & Sons, Inc.

Intrinsic and global reaction rate of methanol dehydration over .gamma.-alumina pellets

Abstract: Dehydration of methanol on y-Al,O, was studied in a differential fixed-bed reactor at a pressure of 146 kPa in a temperature range of 290-360 "C. A kinetic equation which describes a Langmuir-Hinshelwood surface controlled reaction with dissociative adsorption of methanol was found to fit the experimental results quite well. Coefficients in the equation follow the Arrhenius and the van't Hoff relation. The calculated value for the activation energy was found to be 143.7 kJ/mol, while calculated values for the heat of adsorption of methanol and water were 70.5 and 42.1 kJ/mol, respectively. The measured global reaction rates for 3-mm catalyst particles were compared to those calculated by means of intrinsic kinetics and transport processes within the particles. A reasonable agreement was found when the effective diffusion coefficients for reaction components were calculated using a parallel-pore model assuming that only Knudsen diffusion is important.

Partial liquid phase hydrogenation of benzene to cyclohexene over ruthenium catalysts in the presence of an aqueous salt solution: I. Preparation, characterization of the catalyst and study of a number of process variables

Abstract: A study has been made of the batch-wise partial hydrogenation of benzene to cyclohexene over ruthenium catalysts in the presence of an aqueous zinc sulphate solution. The reaction was performed at 423 K and at a pressure of 5.0 MPa in a stirred autoclave. The influence of a number of process variables on the performance of the catalyst, like the presence of water, the presence of salt-additives, the stirring speed, the temperature, the hydrogen pressure, the amount of catalyst, the catalyst particle size and the benzene/water ratio have been studied. It was found that it is extremely important that the catalyst is hydrophilic and therefore surrounded by a stagnant water layer; the presence of a stagnant water layer results in the suppression of the reaction rate and in a strong increase of the cyclohexene selectivity and yield. The high yield of the intermediate product, cyclohexene, which is of the order of 40 to 50 mol-%, is attributed to a physical effect, in casu severe mass transport limitation. Mass transfer calculations strongly support this conclusion.

Acceleration of the rate of nitrite oxidation by freezing in aqueous solution

Abstract: WHEN a dilute ionic solution freezes, differences in the partitioning of ions in the aqueous and ice phases can generate electric potentials which may influence electrochemical reactions1–3. Fitter and co-workers4,5 have studied the influence of freezing on the endothermic oxidation of sulphide to sulphate in growing ice crystals inside a cloud chamber. Here we report that the oxidation of nitrite by dissolved oxygen to form nitrate, which is a very slow process in solution, is accelerated markedly when it takes place in a solution undergoing freezing. At pH 4.5 and a temperature of 25 °C, the rate is increased by a factor of about 105 for a freezing rate of 0.2 g solution per minute; the reaction rate increases as the freezing rate increases. Although the mechanism of this acceleration is not yet clear, we are able to eliminate the possibilities of thennochemical, photochemical and simple electrochemical reactions, and of catalysis on the ice surface. Processes of this sort may be important for chemical reactions taking place in freezing cloud and fog droplets in the atmosphere.

A study of homogeneous methanol oxidation kinetics using CSP

Abstract: The homogeneous oxidation of methanol in air at constant pressure is examined using data generated by the method of computational singular perturbation (CSP). At any moment in time, the number of exhausted fast modes and the radicals (sometimes called the intermediaries) are computationally identified. The participation index, which quantifies the participation of any elementary reaction to an equations of state of the radicals, along with the importance index, which quantifies the importance of any elementary reaction to a particular species of interest, are computed and used to assess the sensitivities of the solution to the reaction rate constants. Every elementary reaction is classified so that it either belongs to the equilibrated set which contains fast reactions already equilibrated among themselves, and/ or the rate-controlling set which contains reactions controlling the current rate of activities, or neither of the above sets—in which case it is superfluous. A number of numerical experiments were performed to verify the assessments: (a) the relative effectiveness of the reaction rate constants of two reactions (#16, #160) in breaking up the fuel indicated by the importance index is verified, (b) that fuel breakup in an early time period can actually be slowed down by increasing the reaction rate constants of certain fuel breakup reactions (#156, #159) is verified. Numerical experiments also showed that species identified as radicals responded instantly to sudden changes in reaction rates, while the non-radicals responded more smoothly. The overall response of the unknowns to perturbations is always consistent with the CSP-derived effective stoichiometric coefficients. In addition, a minimum set of species is constructed with the help of the CSP data. This minimum set, which trims the original full set of 30 species to 15 species, generates numerical solutions in excellent agreement with solutions obtained with the full set.

Controlling bimolecular reactions: Mode and bond selected reaction of water with translationally excited chlorine atoms

Abstract: Vibrational overtone excitation prepares water molecules in the ‖13〉−, ‖04〉−, ‖02〉−‖2〉, and ‖03〉− local mode states for a study of the influence of reagent vibration on the endothermic bimolecular reaction Cl+H2O→OH+HCl. The reaction of water molecules excited to the ‖04〉− vibrational state predominantly produces OH(v=0) while reaction from the ‖13〉− state forms mostly OH(v=1). These results support a spectator model for reaction in which the vibrational excitation of the products directly reflects the nodal pattern of the vibrational wave function in the energized molecule. Comparison of relative OH product yield from Cl+H2O(‖04〉−) and the calibration reaction O(3P)+CH3OH shows that vibrational excitation of water to the ‖04〉− state leads to a near gas kinetic reaction rate (k(‖04〉−)=2×10−10 cm3 molecule−1 s−1). Relative rate measurements for the two vibrational states ‖03〉− and ‖02〉−‖2〉, which have similar total energies but correspond to very different distributions of vibrational excitation, demonstra...

Gas-Phase Rates of Alkane C-H Oxidative Addition to a Transient CpRh(CO) Complex.

Abstract: The gas-phase irradiation of CpRh(CO)2 (Cp = η5-C5H5) was examined in order to study the rates of reaction of the 16-electron intermediates presumed to be involved in the C-H oxidative addition of alkanes. "Naked" (unsolvated) CpRh(CO) was detected, and direct measurements of the rates of reaction of this very short-lived complex with alkane C-H bonds were made. Activation of C-H bonds occurs on almost every collision for alkanes of moderate size, and intermediates in which the alkanes are bound to the metal centers, without their C-H bonds being fully broken, are implicated as intermediates in the overall reaction.

Kinetics of phenol oxidation in supercritical water

Abstract: This paper reports tht aqueous solutions of phenol were oxidized in a flow reactor at temperatures between 300 and 420{degrees}C (0.89 {le} T{sub r} {le} 1.07) and pressures from 188 to 278 atm (0.86 {le} P, {le} 1.27). These conditions included oxidations in both near-critical and supercritical water. Reactor residence times ranged from 1.2 to 111 s. The initial phenol concentrations were between 50 and 330 ppm by mass, and the initial oxygen concentrations ranged from 0 to 1,100% excess. The oxidation experiments covered essentially the entire range of phenol conversions. Analysis of the kinetics data for phenol disappearance using a combination of the integral method and the method of excess revealed that the reaction was first order in phenol and 1/2 order in oxygen, and influenced by pressure. The global reaction order for water was taken to be nonzero, and the global rate constant was assumed to be independent of pressure so that the only effect of pressure was to alter the water concentration and hence the reaction rate.

Direct catalytic hydrodechlorination of toxic organics in wastewater

Abstract: The in-situ catalytic hydrodechlorination of chlorinated hydrocarbons in waste-water-generating HCl and a hydrocarbon-free chlorine is demonstrated as a viable wastewater remediation technique. Catalyst screening studies with a shaker-type hydrogenation reactor have shown that the commercial Pd/C catalyst is highly effective in hydrochlorinating various chlorinated hydrocarbons in synthetic wastewater at room temperature and near atmospheric pressure. 1, 1, 2-trichloroethane hydrodechlorination experiments in an autoclave reactor shows that initial rates are well correlated with first-order dependence of the reactant hydrocarbon adsorbed on carbon. Initial rates are also independent of hydrogen pressure, and adsorption on the carbon support is Langmuir type. Activation energies calculated at different catalyst loadings varied from 29 to 38 MJ/mol. 1,1,2-trichloroethane hydrodechlorination activity is much lower for Pd/Al2O3 than Pd/C because the reactant hydrocarbon does not adsorb on alumina. When the carbon support does not readily adsorb the reactant hydrocarbon, the hydrodechlorination rates dropped significantly. These results confirm the role of the carbon support in providing the major path to reaction and thereby significantly increasing reaction rates compared to direct adsorption from solution onto the palladium.

Rate of reduction of FeO in slag by Fe-C drops

Abstract: The rate of reduction of FeO in slags by Fe-C drops plays an important role in several metallurgical processes, including iron bath smelting. In this study, the rate of this reaction was determined by measuring the volume of CO generated as a function of time, and the reaction was observed by X-ray fluoroscopy. The drops entered the slag in a nearly spherical shape, remained as single particles, and for the major portion of the reaction remained suspended in the slag surrounded by a gas halo. The rate was found to decrease with carbon content for alloys with low sulfur contents. The rate decreased significantly with increasing the sulfur content. Based on the results and a comparison of the calculated rates, for the possible rate-controlling mechanisms, a kinetic model was developed. The model is a mixed control model including mass transfer in the slag, mass transfer in the gas halo, and chemical kinetics at the metal interface. At high sulfur contents (>0.01 pct), the rate is primarily controlled by the dissociation of CO2 on the surface of the iron drop. At very low sulfur, the rate is controlled by the two mass-transfer steps and increases as the gas evolution from the particle increases.

Atomic Hydrogen Driven Halogen Extraction from Si(100) -- Eley-Rideal Surface Kinetics

Abstract: : The interaction of atomic hydrogen with halogen-terminated Si(100) surfaces was studied by Auger electron spectroscopy (AES) and temperature programmed desorption (TPD) mass spectroscopy. Efficient removal of surface halogen has been observed when the halogen-terminated S1(100) surface was exposed to atomic hydrogen at a substrate temperature, 630 K. The reaction rate constants for halogen extraction on the S1(100) surface follow the trend kI greater than kBr greater than kCl. In addition, the halogen extraction kinetics are found to be first order in both the surface coverage of halogen and in the atomic hydrogen flux. Studies of the temperature dependence of the halogen extraction rate show the activation energies for the extraction of Cl and Br are 2.1 and 1.6 kcal mol-1, respectively. The extremely low activation energy for the reaction demonstrates that the H-extraction process follows an Eley-Rideal reaction mechanism where the surface reaction is mainly driven by the high internal energy of incident atomic hydrogen instead of thermal excitation from the S1(100) solid surface.

Enzymatic reaction kinetic: comparison in an organic solvent and in supercritical carbon dioxide

Abstract: Myristic acid esterification has been performed by an immobilized lipase from Mucor Miehei both in n-hexane and in supercritical carbon dioxide (SCCO2). The enzyme is stable in SCCO2 at 15 MPa and 323 K. The reaction rate is influenced by the concentration of water and by the reaction medium composition. A reaction mechanism is proposed, and kinetic parameters are determined at 12.5 MPa and 313 K. Maxium velocity appears 1.5-fold higher in SCCO2 than in n-hexane; however, as solubility of myristic acid is greater in n-hexane, it is not yet definitively clear that the supercritical medium is more favorable than the classical organic solvent for this type of enzyme reaction.

Illumination of a Black Box: Analysis of Gas Composition During Graphite Target Preparation

Abstract: We conducted a study of relative gas composition changes of CO2, CO and CH4 during the formation of graphite targets using different temperatures, catalysts and methods. Reduction with H2 increases the reaction rate without compromising the quality of the AMS target produced. Methane is produced at virtually any temperature, and the amount produced is greater at very low temperatures. The reduction of CO to graphite is very slow when H2 is not included in the reaction.

Role of the BRO + HO2 reaction in the stratospheric chemistry of bromine

Abstract: The impact of new laboratory data for the reaction BrO + HO2 → HOBr + O2 has been estimated in a one-dimensional photochemical modelling of bromine/ozone stratospheric chemistry. The reported 6 fold increase in the measured value for the rate constant of this reaction significantly increases both the HOBr mixing ratio and the global ozone depletion due to bromine compounds (the calculated ozone loss increases from 1.14% to 1.45% for a 20 ppt total bromine content). The higher rate constant makes the bromine partitioning and the ozone depletion very sensitive to the branching ratio of the potential channel forming HBr in the BrO + HO2 reaction. The influence of the existing uncertainty in the photolysis rate of HOBr is also analysed.

Kinetics of photocatalytic oxidation of phenol on TiO2 surface

Abstract: The kinetics of photocatalytic oxidation of phenol have been studied experimentally. A kinetic model based on the generation of OH . radicals as the rate-determining step is proposed. The results of the theoretical analysis can be used to explain the inverse initial concentration effect of phenol on the apparent rate constant. The model also can explain the experimental findings such as the pseudo-first-order dependence and the effect of the initial pH value on the initial reaction rate. Moreover, the addition of H 2 O 2 with ferric ions enhances the reaction rate significantly. It has also been discovered that the phenol oxidation rate is controlled by the decomposition rate of H 2 O 2 .

Investigation of the pH dependence of the kinetics of quartz dissolution at 25 °C

Abstract: The dissolution kinetics of two samples of commercial ground Fontainebleau sand consisting of > 99.6% quartz are studied at 25 °C in constant ionic strength conditions and in the pH range 4.5–10.5. The strong pH dependence of the reaction rate is attributed to changes in the surface speciation of quartz. The ionization of the surface silanol groups is investigated using potentiometric methods and the results are compared with literature data using a simple surface speciation model that accounts for the effects of the electrical double layer on the hydrogen ion activity. The surface speciation model produces titration curves in good agreement with the experimental data and offers an interpretation of the relationship between the density of ionized groups, [SiO–(s)], and pH.The rate constant, k+, for the dissolution reaction: SiO2(solid)+ 2H2O → Si(OH)4(aq) is determined from 25 experiments as: log k+=– 14.547 ± 0.162 + 0.361 ± 0.022 pH at 25 °C, ionic strength of 0.010 mol dm–3 and pH in the range 4.5–10.5. The constant, k+, is in units of mol m–2 s–1. The results are compared with the available literature data at 25 °C. Although there is general agreement between the experimental data and the surface speciation model concerning the pH dependence of the dissolution rates, the variation between the absolute values of the rate constants from different quartz samples is large. The pH dependence observed in this work and that of Brady and Walther, is consistent with a pK for the dissociation of surface silanol groups and surface density of terminal silanol groups of ca. 7.5 and 11 nm–2, respectively. A slope of ca. 0.33 obtained from the regression of pH and log k+ is predicted from the surface model. Similar gradients have been observed in other studies using quartz and also for a range of other silicate minerals for solution pH 7.