Showing papers on "Reaction rate published in 1984"

Intramolecular long-distance electron transfer in radical anions. The effects of free energy and solvent on the reaction rates

Abstract: A homologous series of eight compounds of the general structure A-Sp-B where B is 4-biphenylyl, Sp is a rigid saturated hydrocarbon spacer, the steroidal 5..cap alpha..-androstane skeleton, and A is one of a series of 8 different molecular groups with ..pi.. electron networks was synthesized. A liquid solution of these molecules in either 2-methyltetrahydrofuran or isooctane at room temperature was subjected to a 30-ps pulse of solvated electrons. Data are presented that demonstrate in these systems the dependence of the electron transfer rates on the exothermicity of the reaction and on solvent polarity. It is also noted that the electron transfer reactions are free ion reactions.

Rate Coefficients in the C/H/O System

Abstract: This chapter is a critical survey of reaction rate coefficient data important in describing high-temperature combustion of H2, CO, and small hydrocarbons up to C4. A recommended reaction mechanism and rate coefficient set is presented. The approximate temperature range for this mechanism is from 1200 to 2500 K, which therefore excludes detailed consideration of cool flames, low-temperature ignition, or reactions of organic peroxides or peroxy radicals. Low-temperature rate-coefficient data are presented, however, when they contribute to defining or understanding high-temperature rate coefficients. Because our current knowledge of reaction kinetics is incomplete, this mechanism is inadequate for very fuel-rich conditions (see Warnatz et al., 1982). For the most part, reactions are considered only when their rates may be important for modeling combustion processes. This criterion eliminates considering many reactions among minor species present at concentrations so low that reactions of these species cannot play an essential part in combustion processes. The philosophy in evaluating the rate-coefficient data was to be selective rather than exhaustive: Recent results obtained with experimental methods capable of measuring isolated elementary reaction rate parameters directly were preferred, while results obtained using computer simulations of complex reacting systems were considered only when sensitivity to a particular elementary reaction was demonstrated or when direct measurements are not available. Theoretical results were not considered.

Curing reaction of epoxy resins with diamines

Abstract: The curing reaction of a commercial bisphenol A diglycidyl ether (BADGE) with ethylenediamine (EDA) was studied by differential scanning calorimetry. Different kinetic expressions were found with isothermal (low temperature range) and dynamic (high temperature range) runs. Two competitive mechanisms are shown to be present: an autocatalytic one (activation energy E = 14 kcal/mol) and a noncatalytic path characterized by a second-order reaction with E = 24.5 kcal/mol. At low temperatures both mechanisms took place simultaneously, showing a significant decrease in the reaction rate after the gel point. At high temperatures only the noncatalytic reaction was present, without showing a noticeable rate decrease in the rubber region. Also, a third-order dependence of the glass transition temperature on reaction extent is shown.

Pyrolysis of methyl chloride, a pathway in the chlorine-catalyzed polymerization of methane

Abstract: The reaction of CH4 + Cl2 produces predominantly CH3Cl + HCl, which above 1200 K goes to olefins, aromatics, and HCl. Results obtained in laboratory experiments and detailed modeling of the chlorine-catalyzed polymerization of methane at 1260 and 1310 K are presented. The reaction can be separated into two stages, the chlorination of methane and pyrolysis of methylchloride. The pyrolysis of CH3Cl formed C2H4 and C2H2 in increasing yields as the degree of conversion decreased and the excess of methane increased. Changes of temperature, pressure, or additions of HCl had little effect. In the absence of CH4 C2H4 and C2H2 are formed by the recombination of ĊH3 and ĊH2Cl radicals. With added CH4 recombination of ĊH3 forms C2H6, which dehydrogenates to C2H4 + H2. C2H4 in turn dehydrogenates to C2H2 + H2. While HCl, C, CH4, and H2 are the ultimate stable products, C2H4, C2H2, and C6H6 are produced as intermediates and appear to approach stationary concentrations in the system. Their secondary reactions can be described by radical reactions, which can lead to soot formation. ĊH3 - initiated polymerization of ethylene is negligible relative to the Ċ2H3 formation through H abstraction by Cl. The fastest reaction of Ċ2H3 is its decomposition to C2H2. About 20% of the consumption of C2H2 can be accounted for by the addition of Ċ2H3 to it with formation of the butadienyl radical. The addition of the latter to C2H2 is slow relative to its decomposition to vinylacetylene. Successive H abstraction by Cl from C4H4 leading to diacetylene has rates compatible with the experimental values. About 10% of Ċ4H5 abstracts H from HCl and forms butadiene. Successive additions of Ċ2H3 to butadiene and the products of addition can account for the formation of benzene, styrene, naphthalene, and higher polyaromatics. The following rate parameters have been derived on the basis of the experimentally measured reaction rates, the estimated frequency factors, and the currently available heat of formation of the Ċ2H3 radical (69 kcal/mol):

Engineering plastics from lignin. I: Synthesis of hydroxypropyl lignin

Abstract: Hydroxypropylation of lignin in a batch reactor under alkaline conditions at 180°C was studied using propylene oxide (PO) by itself, and PO in combination with several ligninlike model compounds and with kraft lignin. While the PO homopolymerization rate increased rapidly at temperatures above 85°C, and was too fast to be determined accurately at 180°C, the addition of model compounds and lignin was found to delay homopolymerization in relation to the presence of ionizable functional groups. The observations are consistent with a reaction mechanism involving first order kinetics with regard to each alkoxide and PO concentrations. Where the reaction rates toward PO increase with increasing pKa values, the reaction sequence proceeds in the order of declining basicity. Thus lignins with high acidity were found to be subject to greater degrees of modification than those with more neutral character. This explains the earlier observed beneficial effect of lignin carboxylation on the properties of lignin–PO reaction mixtures.

Mass transport and reaction kinetic parameters determined electrochemically for immobilized glucose oxidase.

Abstract: Mass-transfer resistances often have pronounced effects on the overall reaction rates of enzymes immobilized at interfaces or in polymeric matrices. In the present work glucose oxidase was immobilized on the surface of a platinum disk electrode by one of three attachment techniques: silane-glutaraldehyde, allylamine-glutaraldehyde, and albumin-glutaraldehyde. In one group of studies the electrodes were rotated, and methods were employed to determine the diffusion and shielding coefficients for transport of a model electroactive compound, i.e., potassium ferrocyanide, through the enzyme matrix. A model electrochemically active compound was used because glucose exhibits a very slow rate of electron transfer at a platinum surface. The diffusion coefficient for ferrocyanide was reduced 7% by the silane-enzyme and 25% by the allylamine-enzyme matrices. In a second group of studies the electrodes were held stationary. Marked internal diffusional resistance was noted for the albumin-glutaraldehyde-enzyme matrix. The calculated flux of ferrocyanide was decreased by a factor of 2000-8500 for transport through albumin-enzyme matrices 0.21-0.063 cm thick, as compared to transport through free solution. In a third group of studies the rotating enzyme-matrix electrode was utilized in determining apparent values of the Michaelis constant for glucose. The velocity of the reaction was determined by amperometric measurement of the concentration of hydrogen peroxide reaching the ring electrode. The results, determined from Eadie-Hofstee type plots of reaction current and substrate concentration, gave values between 12 and 36 mM for the three methods of immobilization.(ABSTRACT TRUNCATED AT 250 WORDS)

Cure kinetics of epoxy formulations of the type used in advanced composites

Abstract: An analysis of the cure kinetics of three different formulations composed of tetraglycidyl 4,4′-diaminodiphenyl methane (TGDDM) epoxy resin and diaminodiphenyl sulfone (DDS) was performed. A series of isothermal tests was run, and the experimentally obtained results were checked against the proposed kinetic model. An autocatalyzed mechanism with the overall reaction order of 2 was found to adequately describe the cure kinetics. An increase in reaction rate was observed at higher temperature and higher DDS concentration. For a given formulation, the extent of reaction corresponding to the maximum reaction rate was independent of temperature. A secondary exotherm was detected, particularly in formulations with low DDS concentration, at approximately 40% conversion. At that point, the rate of primary amine–epoxide reaction decreases, and other reactions dominate the curing process. Such a mechanism is likely to cause a formation of an inhomogeneous thermoset morphology.

Instantaneous equilibrium approximation analysis

Abstract: The issue of “how fast” reversible chemical reactions must be before “instantaneous equilibrium” may be assumed in ground‐water contamination problems is investigated. An example multicomponent model is used to study the requirements that must be placed on a system's Damkohler numbers to guarantee that reaction mechanisms are not kinetically limited. Analysis is presented for multicomponent transport problems using soluble complexation, ion exchange, and surface complex formation reactions. Results are presented to illustrate that instantaneous equilibrium is an excellent approximation when nondimensionalized reaction rates exceed 100.

Catalytic hydrodenitrogenation of quinoline in a trickle-bed reactor. Comparison with vapor phase reaction

Abstract: Studies over a commercial NiMo/Al/sub 2/O/sub 3/ catalyst at 350, 375, and 390/sup 0/C and 6.9 MPa using quinoline or various intermediate reaction products showed that the reaction rate constants for the various hydrodenitrogenation (HDN) reactions in the presence of an inert paraffin liquid are very similar to those for the same reactions in the vapor phase, although the liquid tends to equalize the adsorptivities of the various N-compounds present. Over a wide range, for a specified pressure and temperature, the percent conversion as a function of contact time is remarkably similar for liquid- and vapor-phase processing. In both cases, the overall HDN reaction is essentially zero order under the conditions studied. Equilibrium between quinoline and 1,2,3,4-tetrahydroquinoline is rapidly attained in both vapor-phase and liquid-phase processing. Homogeneous reactions were negligible except for the hydrogenation of quinoline to 1,2,3,4-tetrahydroquinoline.

On the Initiation of a Spherical Flame Kernel

Abstract: Using matched asymptotic expansions for large activation energies of the reaction rates, we have studied the structure and stability properties of steady spherical premixed flames which, in this geometry, need not be supported by any source (or sink) of fresh (or burned) gases. The radius of such unsupported flames strongly depends on the differential diffusion of heat and reactant and is interpreted as a flame initiation criterion which corresponds to an unstable equilibrium. This radius must be exceeded for the flame to self-propagate outward. We then focus attention on two means to modify, or even suppress, such a critical flame size, viz.: (i) the effect of a central heal source of constant power, and (ii) the addition of small amounts of a hydrogen-like reactive species with large molecular diffusivity. Multiple steady solutions-including stable, non-propagating flames-or the disappearance of equilibrium solutions are exhibited as are qualitative changes in flame dynamics.

Control of crosslinking reaction rate of aqueous fracturing fluids

Abstract: A method of controlling crosslinking reaction rate in a water based polymer fracturing fluid that involves using a mixture of a crosslinker, in combination with a crosslinking rate accelerator or a crosslinking rate retarder at a ratio such that the crosslinking reaction rate is controlled. This decreases the shear effects upon the polymer fracturing fluid during pumping which affects downhole stability of the fluid and decreases pumping and frictional pressures. The ratio of rate accelerator or rate retarder to crosslinker for a desired crosslinking reaction time is dependent upon pH, surface temperature, polymer concentration, salt concentrations, fluid loss additives, surfactant types and concentrations, mutual solvents and alcohol. The preferred polymer fracturing fluid is a high pH aqueous salt solution including a hydroxypropyl guar polymer. The preferred crosslinkers are transition metal chelates. The preferred crosslinking rate accelerators are polyamines. The preferred crosslinking reaction rate retarders are glyoxal and triethanolamine. Batch mixing of the polymeric fracturing fluid, crosslinker and rate accelerator or rate retarder is possible; however in the preferred embodiment the crosslinker and accelerator or retarder are injected simultaneously with the polymeric fracturing fluid so that the crosslinking reaction rate can be continuously adjusted in response to changing conditions.

Oxidation rate of iron sulfides as affected by surface area, morphology, oxygen concentration and autotrophic bacteria

Abstract: The relationship between surface area and rate of oxidation of Fe sulphides (pyrite and marcasite) separated from Texas lignite was studied. The reaction kinetics with respect to Fe sulphide morphology and particle size were evaluated. The oxygen concentration and the presence of autotrophic Fe and S-oxidizing bacteria (thiobacillus ferro-oxidans) on the rate of oxidation were also evaluated. The formation of sulphate from Fe sulphide was selected to measure the rate of oxidation. Relative reaction rates for different morphological forms of Fe sulphide were: marcasite > framboidal pyrite > massive pyrite. As the surface area of pyrite doubled, reaction rate increased by a factor of 1.5. Sulphate production for the 5 to 2 ..mu..m fraction was twice that of the 50 to 20 ..mu..m fraction. Reaction rate was approximately fivefold greater for non-inoculated treatments at 20% O/sub 2/ compared with 0% O/sub 2/ and was approximately ninefold greater for the same treatment inoculated with T. ferro-oxidans.

Surface area of calcium oxide and kinetics of calcium sulfide formation

Abstract: Details are presneted of a new limestone injection multistage burner as an economical and practical method of reducing emissions from existing power plants by 50%. The process involves injection of pulverized limestone into coal burners that are designed to minimize NO/sub x/ formation by staged combustion. The reaction of CaO with SO/sub 2/ following combustion is the principal mechanism of sulfur capture in the LIMB process. The relationship between surface area and reaction rate is evaluated. It was found that the rate of CaS formation increases exponentially with the specific surface area of CaO in micrometer size particles. The effect of surface area on reactivity is at least as great as that expected for pure diffusion control. The activation energy of CaO reaction with H/sub 2/S or COS is 31 kcal/mol for the limestone studied. On the basis of the activation energy measured between 600/sup 0/ and 900/sup 0/C, and the reaction rates measured for CaO of 40 m/sup 2//g surface area, efficient sulfur capture would be predicted at 1250/sup 0/C in a dispersed-particle system if the kinetics of CaS formation is the only limitation.

Catalytic oxidation of carbon monoxide over ruthenium/silicon dioxide. An in situ infrared and kinetic study

Abstract: The partial deactivation of a series of Ru/SiO/sub 2/ catalysts during the CO oxidation reaction has been studied by using in situ infrared spectroscopy. The decrease in catalytic activity was fast in an oxygen-rich reactant gas mixture. At a CO/O/sub 2/ ratio greater than that required for stoichiometry, partial deactivation was slow, continuing for several hours. When the rate of the oxidation reaction was decreased following 100% conversion in the diffusion-controlled kinetic regime, a significant decrease in the reaction rate was observed. This reaction rate hysteresis was considerably larger when the catalyst was exposed to an oxygen-rich reactant gas mixture. During catalyst deactivation, the absorbance of the IR doublet centered at 2130 and 2060 cm/sup -1/ was observed to increase. This IR doublet, asssigned to CO absorbed on an oxidized Ru surface species, increased very rapidly when the reactant gas mixture was either net oxidizing or close to stoichiometry. The results of a temperature-programmed desorption (TPD) and a temperature-programmed oxidation study suggest that partial deactivation occurs as the result of the formation of a more tightly bound oxygen species, perhaps lattice oxygen which reacts more slowly with adsorbed CO than oxygen in the reactant gas mixture. Multiple steady-state oscillations weremore » observed following the attainment of steady-state reaction rates. The results of a temperature-programmed reduction study show that the CO species responsible for the CO absorption band at 2010 cm/sup -1/ reacts preferentially with H/sub 2/ to form CH/sub 4/.« less

Catalytic reactions of hydrocarbon with carbon dioxide over metallic catalysts

Abstract: The catalytic reaction between hydrocarbons and carbon dioxide, e.g. CH4+ +CO2→2CO+2H2 and C6H6CH3+CO2→C6H6+2CO+H2, has been investigated over various metallic catalysts under atmospheric pressure. In general, Rh−Al2O3 catalyst was found to be excellent in activity and selectivity. The reaction rates were moderately dependent on the pressure of carbon dioxide, whereas it was little influenced by the pressure of hydrocarbon.

Prediction of the Abiotic Degradability of Organic Compounds in the Troposphere

Abstract: A statistically relevant correlation between the reaction rate coefficient, k OH, for the OH radical reaction with 161 organic compounds in the gas phase at 300 K, and the corresponding vertical ionisation energies E i,v, reveals two classes of compounds: aromatics where −log(k OH/cm3s-1)≃3/2E i,v(eV)−2 and aliphatics where −log(k OH/cm3s-1)≃4/5E i,v(eV)+3. The prediction of the rate coefficient, k OH, for the reaction of OH with organic molecules from the above equations has a probability of about 90%. Assuming a global diurnal mean of the OH radical concentration of 5×105 cm3, the upper limit of the tropospheric half-life of organic compounds and their persistence can be estimated.

Electron transfer between flavodoxin semiquinone and c-type cytochromes: correlations between electrostatically corrected rate constants, redox potentials, and surface topologies.

Abstract: We have measured the ionic strength dependence of the rate constants for electron transfer from the semiquinone of Clostridium pasteurianum flavodoxin to 12 c-type cytochromes and several inorganic oxidants using stopped-flow methodology. The experimental data were fit quite well by an electrostatic model that represents the interaction domains as parallel disks with a point charge equal to the charge within this region of the protein. The analysis provides an evaluation of the electrostatic interaction energy and the rate constant at infinite ionic strength (k affinity). The electrostatic charge on the oxidant within the interaction site can be obtained from the electrostatic energy, and for most of those reactants for which structures are available, the results are in good agreement with expectation. The k affinity values were found to correlate with redox potential differences, as expected from the theory of adiabatic (or nonadiabatic) outer-sphere electron-transfer reactions. Deviations from the theoretical curves are interpreted in terms of the influence of surface topology on reaction rate constants. In general, we find that electrostatic effects, steric influences, and redox potential all exert a much larger effect on reaction rate constants for the flavodoxin-cytochrome system than has been previously observed for free flavin-cytochrome interactions. The implications of this for determining biological specificity are discussed.

Method for making alkylhalosilanes

Abstract: A method is provided for making alkylhalosilanes by effecting reaction between an alkyl halide, such as methyl chloride and powdered silicon in the presence of a copper-zinc-tin catalyst. Significant improvements in reaction rate and product selectivity are achieved when copper is employed at a critical weight percent relative to silicon and critical weight ratios of tin and zinc are employed relative to copper.