Showing papers on "Reaction rate published in 1981"

First practical method for asymmetric epoxidation

Abstract: OF THE DISCLOSURE Methods and compositions are provided for asymecrically donating an oxygen atom to a pair of electrons to produce an asymmetric product. Specifically, a metal alkoxide is used as a catalyst, where the metal has a coordination number of at least four, and at least one, usually two, of the alkoxide groups bonded to the metal are bonded to asymmetric carbon atoms. The metal catalyst is employed in conjunction with a hydroperoxide and an alkanol having a functionality with a pair of electrons capable of accepting an oxygen atom. The resulting product is enriched in one enantiomer due to the enantioselective introduction of an asymmetric center or an enhanced rate of reaction of one of the enantiomers of a chiral alkanol. Greatly enhanced yields of enantiomers are achieved as compared to prior enantioselective introduction of oxygen. This invention was made at least in part in the course of a grant from the U.S. National Institutes of Health (GM24551).

Simplified Reaction Mechanisms for the Oxidation of Hydrocarbon Fuels in Flames

Abstract: Simplified reaction mechanisms for the oxidation of hydrocarbon fuels have been examined using a numerical laminar flame model. The types of mechanisms studied include one and two global reaction steps as well as quasi-global mechanisms. Reaction rate parameters were varied in order to provide the best agreement between computed and experimentally observed flame speeds in selected mixtures of fuel and air. The influences of the various reaction rate parameters on the laminar flame properties have been identified, and a simple procedure to determine the best values for the reaction rate parameters is demonstrated. Fuels studied include n-paraffins from methane to n-decane, some methyl-substituted n-paraffins, acetylene, and representative olefin, alcohol and aromatic hydrocarbons. Results show that the often-employed choice of simultaneous first order fuel and oxidizer dependence for global rate expressions cannot yield the correct dependence of flame speed on equivalence ratio or pressure and can...

Transport and reactivity of hydrocarbon molecules in a shape-selective zeolite

Abstract: For the catalytic cracking of C6 to C9 hydrocarbons on ZSM-5, we demonstrate quantitatively the contributions of each of two mechanisms for molecular shape selectivity. Using crystallites of different sizes and activities, and classical methods for evaluating diffusion inhibition of the reaction rate, we separate the effects of mass-transport-induced selectivity from that created by steric inhibition by the size of a reaction complex. The selective cracking of n-paraffins compared to monomethyl paraffins (from C6 to C9) is due to a higher intrinsic rate constant of the n-paraffin, with diffusional mass transport playing no appreciable role. In contrast, dimethyl paraffin cracking is strongly diffusion-inhibited. The methyl paraffin/n-paraffin discrimination is a result of steric constraint on the sizeable methyl paraffin/carbonium ion reaction complex. This structural selectivity is shown to be absent for the corresponding olefins where such complexes do not arise. The diffusivities at reaction conditions have been determined. For the linear hydrocarbon, diffusivity notably exceeds that expected from the Knudsen model. This reminds us to review assumptions of conventional concepts of mass transport. The availability of zeolites now allows us to probe many basic phenomena in catalysis, molecular configuration and dynamics, including mass transport.

Mechanism of methanol conversion into hydrocarbons over ZSM-5 zeolite

Abstract: The conversion of methanol into hydrocarbons over ZSM-5 zeolites has been investigated using a gas-recirculation system. The conversion proceeds autocatalytically. The reaction rate is greatly enhanced by the addition of ethylene or cis-but-2-ene, indicating that the autocatalysis is caused by the reaction of olefin and methanol. Infrared studies have revealed that methoxyl groups (—OCD3) are formed by the reaction of surface hydroxyl groups and methanol molecules (CD3OH), and they decompose to reproduce hydroxyl groups (—OD), giving hydrocarbons. The reaction mechanism in which methyl carbonium ions attack the C—H bond of methanol or dimethyl ether molecules is proposed. Methyl carbonium ions are supposed to be released from extensively polarised methoxyl groups formed by the reaction of methanol molecules and strong Bronsted-acid sites. In fact, strongly acidic materials, Nafion H and heteropolyacids, were found effective for methanol conversion. A semiquantitative analysis of the autocatalytic phenomenon in methanol conversion is also presented.

Reaction kinetics of nitrogen dioxide with liquid water at low partial pressure

Abstract: The reaction 2NO/sub 2/(g) + H/sub 2/O(l) ..-->.. 2H/sup +/ + NO/sub 3//sup -/ + NO/sub 2//sup -/ (l) has been studied for 1 x 10/sup -7/ less than or equal to P/sub NO2/ less than or equal to 8 x 10/sup -4/ atm. Since this reaction involves the transfer of a reactant from the gas into the aqueous phase, the rate depends upon the following: (a) physical mass transfer of the reactant, (b) the equilibrium solubility of NO/sub 2/, and (c) homogeneous aqueous-phase kinetics. In order for the observed rate to yield information most sensitive to (c), the rate of (a) must be comparable to that of (c). In the case of a second-order reaction, this can be achieved by working at low partial pressure of NO/sub 2/ as well as a high rate of physical mixing of the two phases. To facilitate the latter, the gas was brought into contact with the liquid as finely dispersed bubbles produced by flowing through a disk-frit; the mass transfer time constant, determined by uptake of CO/sub 2/, was tau/sub m/ = 1.7 to 5.3 s. The rate of reaction 1, monitored by observing the electrical conductivity of the aqueous solution, exhibitedmore » dependence on p/sub NO2/ and tau/sub m/ consistent with second-order kinetics and a steady-state aqueous-phase NO/sub 2/ concentration. Values of H/sub NO2/, the Henry's law coefficient, and k/sub 1/, the second-order aqueous-phase rate constant, were determined to be (7.0 +- 0.5) x 10/sup -3/M atm/sup -1/ and (1.0 +- 0.1) x 10/sup 8/M/sup -1/s/sup -1/, respectively, at 22/sup 0/C. Self-consistency of the gas-liquid reaction model has been demonstrated in terms of the identity of the diffusing species and the extent of mass-transport limitation.« less

Qualitative model for dynamic combustion

Abstract: A qualitative model for studying shock-wave chemistry interactions in combustion theory is introduced. The model which we study bears the analogous relationship to reacting gas flow as Burgers’ equation does to ordinary compressible fluid flow. When the corresponding physical assumptions of the Chapman–Jouget and von Neumann–Zeldovich–Doring theories are introduced in this model, explicit and completely analogous phenomena occur. Without any approximations, combustion profiles with finite reaction rate and finite diffusion are examined in detail. In the context of this model, the validity of the approximate theories mentioned above depends on the relative size of two critical parameters—the energy liberated by chemical reaction and a parameter which measures the ratio of the width of the shock layer to the reaction zone.

Smectite to illite conversion rates; effects of solution chemistry

Abstract: Mixed-layer illite/smectite (I/S) was formed by reacting the Chambers or Polkville montmoril- lonite hydrothermally at 270 ~ and 350~ from several hours to more than 15 weeks. Reactions were con- ducted in closed vessels containing K or mixed K-Na, K-Ca, or K-Mg solutions of varying concentrations. The reaction rate and the rate of ordering of I/S for the reaction smectite + K § ~ mixed-layer I/S + SiO2 was inhibited by the addition of Na § Ca 2+, and Mg2+; the inhibitory strength of Na +, Ca 2§ and Mg 2+, on an equivalent basis, increased approximately in the ratio 1:10:30. The first order reaction-rate constants for the reactions at 270 ~ and 350~ indicate an activation energy of about 30 kcal/mole. In the experimental system studied, the reaction smectite -~ mixed-layer I/S appeared to proceed by solid state transformation, as suggested by: (1) rapid dissolution of large amounts of silica, probably creating an AI-enriched residue; (2) similarity of particle size and morphology of the mixed-layer products to those of the original montmorillonite, implying no extensive dissolution of A13+; and (3) relatively high activation energy compared to published values for silicate dissolution.

Reaction Model for Iron Dissolution Studied by Electrode Impedance II . Determination of the Reaction Model

Abstract: The electrode impedance determined experimentally shows a certain number of time constants as capacitive or inductive features. These impedances arise from various processes occurring at or near the electrode interface, such as charge transfer, adsorption‐desorption of reaction intermediate species, changes of the roughness factor, changes of the number of active sites or of volume concentration in the vicinity of the electrode, etc. The origins of impedances are examined on the basis of experimental results given in Part I . Then, hypotheses describing reaction rates, such as the Tafel law, reaction reversibility, and the adsorption isotherm law, are analyzed in order to translate reasonably the reaction models into mathematical expressions. With hypotheses retained, it is concluded that the experimental results should be interpreted by a model including three adsorbed reaction intermediate species. Forty possible reaction schemes, as the complete set of prospective models, were written and examined according to both steady‐state polarization curves and electrode impedances. The appearance of two current maxima implies at least two dissolution paths in the reaction models and allowed us to eliminate 10 of the 40 reaction schemes. On the other hand, the appearance of inductive impedance was found to constitute a very selective criterion. Only one model, given in Part I , was found to simulate suitably the whole set of experimental results.

Kinetic study of carbon dioxide reaction with tertiary amines in aqueous solutions

Abstract: Reaction kinetics of CO/sub 2/ with triethanolamine (TEA) and methyldiethanolamine (MDEA) in aqueous solution have been studied by using a stopped-flow technique with pH detection. Rate constants are obtained from the comparison of experimental and theoretical curves giving the optical density as a function of time. At concentrations of CO/sub 2/ well below the saturation limit, the results are consistent with the hydration reactions of the CO/sub 2/ molecules either with neutral water molecules or with hydroxide ions, depending upon the pH, itself governed by the ionization equilibrium of the dissolved amine. Moreover, a specific (catalytic) reaction, first order with respect to both carbon dioxide and amine (rate constant, 2.85 M/sup -1/ s/sup -1/ at 25/sup 0/C), has been shown to contribute significantly to the reaction rate in the case of the first amine (TEA) only.

Curing of compression molded sheet molding compound

Abstract: A recently developed kinetic model has been applied here to describe the polyester-styrene addition copolymerization. By assuming that the termination step is negligible and the reaction rate between inhibitor and initiator free radical is much, faster than any other reactions, the kinetic mechanism can be simplified to be expressed as a single equation. The parameters, rate constant of initiator decomposition and rate constant of propagation, are estimated from the induction time and the time to the peak exotherm of isothermal differential scanning calorimetry (DSC) curves. Temperature profiles inside plate sections of SMC parts during molding are predicted by a mathematical model in which addition polymerization is coupled with heat transfer. The predicted temperature profiles compare well with the experimental results. The model is also used to predict the cure time of different part thicknesses, mold temperature and initiator concentration. Glass fibers playa role as a heat sink as well as heat conductor during curing. Adding glass fibers to SMC not only lowered the maximum exotherm but also reduced the cure time.

Pressure and electron temperature dependence of H− density in a hydrogen plasma

Abstract: The density of negative ions in a low‐pressure hydrogen plasma has been investigated as a function of neutral gas pressure, plasma density, and electron temperature. The comparison of the experimental data, obtained by the photodetachment technique, with theoretical results derived from computed reaction rates, seems to indicate that hydrogen negative‐ion production occurs mainly through the process of electron attachment on vibrationally excited hydrogen molecules.

Quasiclassical trajectory studies of the H+H2 reaction on an accurate potential‐energy surface. III. Comparison of rate constants and cross sections with experiment

Abstract: Quasiclassical trajectories computed for the H+H2 reaction on the accurate Siegbahn–Liu–Truhlar–Horowitz potential‐energy surface are presented. Reaction rate constants as a function of temperature for H2 in the ground and first excited vibrational state are compared with experimental rate data. For v = 0, agreement is found to be excellent for all isotopic combinations. For v = 1, however, all theoretical results predict much smaller rate constants than are observed experimentally. This discrepancy cannot be ascribed to the absence of tunneling inherent in classical mechanics and is unlikely to be due to errors in the surface. Angular distributions in the laboratory frame have been computed from theoretical results for D+H2 and H+T2 and compared with recent experiments. Agreement is fairly good.

Chemically modified electrodes for electrocatalysis

Abstract: At a modified electrode, electrocatalysis is accomplished by an immobilized redox substance acting as an electron transfer mediator between the electrode and a reaction substrate. Such mediated electrocatalysis is possible with monomolecular and multimolecular layers of the redox substance. The electron transfer mediation can assume several special forms; these are identified and experimental examples are given. The differences between electrocatalytic behaviour of monomolecular and multimolecular layers are discussed; electrocatalysis in the latter circumstance can include reaction rate elements of electrochemical charge and substrate migration through the multilayer in addition to the chemical rate. Theoretical ideas are presented that interconnect these three rate elements, to show that either all of the multilayer sites can participate in the electrocatalytic reaction, or only about the equivalent of a monolayer, depending on the relative rates of the electrochemical charge transport, the diffusion of substrate, and the chemical reaction rate.

Transport Phenomena and Reaction in Fluidized Catalyst Beds

Abstract: Publisher Summary Flow properties of the fluidized catalyst bed (FCB) are different from those of other conventional fluidized beds. The different treatment required is very significant for research and development on fluidized catalytic beds. This chapter discusses the factors affecting the flow properties. These properties include particle size distribution, also heat and mass transfer, and mixing properties. In the FCB, the dilute phase plays an important role in advancing the catalytic reaction when reaction rates are high. This factor provides a basis for identifying the appropriate reaction model and clarifying the effect of the dilute phase on selectivity and stability. The simplified theory for a bubble column is applied to understand particle circulation in the FCB. As a result, it is possible to analyze flow patterns of bubbles and “emulsion.” This chapter considers the uses of fluidized beds for catalytic reactions, using a viewpoint quite different from studies directed toward the physical handling of solid particles or toward gas–solid noncatalytic reaction. Particle size distribution has a great effect on most aspects of fluidization. Fluid beds are usually operated in the turbulent-flow region to obtain good contact and sufficient throughput of reactant gas. The uniformity and contact efficiency of a fluid bed can be improved by immersion of a surface within the bed.

Platinum Chloride–Diphosphine–Tin(II) Halide Systems as Active and Selective Hydroformylation Catalysts

Abstract: The hydroformylation of 1-alkenes was efficiently catalyzed by PtCl2–diphosphine–SnX2 systems whose diphosphines were 1,4-bis(diphenylphosphino)butane derivatives with rigid ring skeletons. The effects of the structure of diphosphines, the P/Pt atomic ratio, the sort of tin(II) halide or solvent, the reaction variables, and the structure of olefins on the relative rate and the product distribution were investigated. A higher reaction rate than when using HRh(CO)(PPh3)3, and a linearity of aldehydes up to 99%, were attained. The coordination structure of the effective diphosphines as well as the reasons for the rate enhancement and for the excellent selectivity were discussed.

Microphotometric measurement of initial maximum reaction rates in quantitative enzyme histochemistry in situ.

Abstract: Final reaction product formation was recorded microphotometrically for succinate dehydrogenase in cross-sectioned muscle fibres at initial rate conditions and during prolonged incubations. Incubations with gel films and aqueous reaction medium both showed a decline of reaction rates. Maximum reaction rates could only be determined at initial rate conditions during the first minute of the incubation. Reaction rates recorded in different areas of the same tissue section were found to change with time to different degrees. From these results it was concluded that quantitative histochemical measurements of enzyme reactionsin situ can only be valid if measured under initial maximum velocity conditions.

On the frequency factor in electron transfer reactions and its role in the highly exothermic regime

Abstract: Consideration of current information on the dependence of the electron transfer rate on the radial separation distance and on the reactants′ radial distribution function suggests for adiabatic transfers a frequency factor closer to 10^(12)M^(−1) s^(−1) than to 10^(11)M^(−1) s^(−1). One effect is to raise the λ values estimated from self-exchange rate constants, and to extend thereby the range of ΔG°'s in which the “inverted region′” is masked by a diffusion-controlled reaction rate.

Kinetics of calcium carbonate (calcite)-seeded crystallization: Influence of solid/solution ratio on the reaction rate constant

Abstract: Calcium carbonate (calcite) crystallization follows a parabolic rate expression over a large range of solid solution ratios. Growth occurs by a uniform advance of macro steps across the major crystallographic faces. At low seed crystal concentrations surface nucleation contributes to the observed reaction rate. Over the range of solid solution ratios examined, calcite crystallization can be described by the parabolic growth rate equation dN dt = −ksN 2 , where N is the amount of calcite to be precipitated from solution to reach equilibrium, i.e., the theoretical crystal yield (moles/liter); dN dt is the rate of change of the crystal yield with respect to time (moles/liter · min); s is the concentration of added seed material (mg/liter); and k is the crystallization rate constant [(moles/liter)(mg/liter)(min)] −1 . The constant k is independent of the solid solution ratio over much of the range examined here, but it is higher at low seed concentrations. This phenomenon is explained by the competition between growth on the calcite seed crystal surface and surface nucleation. Our results demonstrate that the parabolic crystallization rate equation-and a surface reaction mechanism—satisfactorily describe calcite formation over the solid/ solution range encountered in many geochemical and technical situations.

Experimental study and numerical modeling of high temperature oxidation of propane and n-butane

Abstract: The oxidation of propane and n-butane was studied experimentally and analytically near 1000 K between 1 and 6 bars. Experiments were performed in a laminar flow quartz reactor on highly diluted mixtures (less than 2% of fuel by volume), over a large range of equivalence ratios (0.05–25). The extension of the reaction zone allowed chemical sampling of the major products (C2H4, C3H6, CH4, H2, CO, C2H6). It is shown that the initial oxygen concentration has very little influence on ethene and propene yields, but an appreciable one on methane yields. The observed accelerating action of oxygen on the reaction rate is attenuated at high equivalence ratios. A numerical model incorporating detailed chemical kinetics and thermal effects is proposed for the interpretation of these experiments. The reaction mechanism used for this simulation predicts most of the experimental results. Most important steps in this detailed mechanism are identified and assembled in a simplified kinetic scheme describing the reaction processes.