Showing papers on "Elementary reaction published in 1993"
TL;DR: In this article, a detailed kinetic model is derived using Hougen-Watson rate expressions for the Fischer-Tropsch reactions and the water gas shift reaction on a precipitated promoted iron catalyst.
Abstract: A detailed kinetic model is derived using Hougen-Watson rate expressions for the Fischer-Tropsch reactions and the water gas shift reaction on a precipitated promoted iron catalyst. The model discrimination and the parameter estimation are performed according to the integral method of kinetic analysis on experiments described in part 1 of this series. In the construction of the kinetic model, the information gained from in situ catalyst characterization is used. The model proposed assumes different active sites for the water gas shift reaction on one side and for the reactions leading to n-paraffins and 1-olefins on the other side. The rate expressions for the hydrocarbon-forming reactions are based on elementary reactions corresponding to the carbide mechanism, in the assumption that two kinds of elementary reactions, the ones describing the adsorption of carbon monoxide and these describing the desorption of the hydrocarbon products, are not at equilibrium. The assumption that the active sites are almost completely occupied with surface hydrocarbon intermediates greatly simplified the kinetic expressions. The rate expression for the water gas shift reaction is based on elementary reactions involving a formate surface intermediate. The two-site reaction describing the formation of the formate intermediate is proposed to be rate determining. Themore » values of the activation energies derived correspond well with data reported in the literature.« less
203 citations
108 citations
TL;DR: In this article, the kinetics of hydrogen oxidation in supercritical water were determined in an hermal, ug-flow reactor at temperatures between 495 and 600°C and at a pressure of 246 bars.
Abstract: The kinetics of hydrogen oxidation in supercritical water were determined in an hermal, ug-flow reactor at temperatures between 495 and 600°C and at a pressure of 246 bars. Inlet hydrogen and oxygen concentrations ranged from 0.4 × 10−6to 4.6 × 10−6 mol/cm3, with fuel equivalence ratios varying from 0.04 to 4.0. Over the range of conditions studied, the kinetics were independent of oxygen concentration and exhibited a first-order dependence on hydrogen concentration, with an activation energy of 372±34 kJ/mol and a pronounced induction time. An elementary reaction model, based on existing gas-phase models and modified to account for the high-pressure environment, was able to reproduce closely the experimental results. including the overall concentration dependencies (reaction orders) and activation energy. Based on the model. important elementary reactions were identified and details of the oxidation mechanism were inferred.
84 citations
TL;DR: In this article, the conversion versus time data for the elementary reactions and for the overall reaction were obtained by monitoring the changes in concentration of epoxy, primary and secondary amine groups during cure using near-infra-red spectroscopy.
54 citations
TL;DR: The overall Gibbs free energy change (ΔG) of a chemical reaction is often termed the driving force of the reaction as discussed by the authors, which is strictly true for elementary reactions-reactions that pass through only one local maximum (the transition state) along the reaction coordinate connecting reactant and product states.
Abstract: The overall Gibbs free energy change (ΔG) of a chemical reaction is often termed the driving force of the reaction. The sign of ΔG defines the direction of spontaneous reaction, and the condition ΔG=0 defines the point of chemical equilibrium. This is strictly true for elementary reactions-reactions that pass through only one local maximum (the transition state) along the reaction coordinate connecting reactant and product states. However, under many circumstances it is also true for reactions that involve one or more intermediates, particularly if the steady state intermediate concentrations are very small
50 citations
TL;DR: In this paper, a pressure-jump relaxation technique was used to investigate the kinetics of Ca-Na and Ca-K exchange on montmorillonite, and single relaxations were observed in both systems.
Abstract: Batch and flow methods are usually not suitable for measuring kinetics of ion-exchange reactions on soils and soil constituents. These reactions are usually very fast. Thus, a rapid kinetic method must be employed. A pressure-jump relaxation technique was used in this study to investigate the kinetics of Ca-Na and Ca-K exchange on montmorillonite. Single relaxations were observed in both systems. Forward (k,) and backward (£_,) rate constants for the cation-exchange reactions were determined and were greater for Ca-Na exchange (k, = 953 L mols~', *., = 643 L mol' s-) than those for Ca-K exchange (k, = 385 L mols-, t, = 432 L mol5-'). The values of the exchange equilibrium constant obtained kinetically were in good agreement with those obtained statically. The exchange isotherms from the static studies showed a preference of K over Ca on montmorillonite, but no preference was observed in the Ca-Na exchange system. A VOLUMINOUS AMOUNT of research has been conducted on cation exchange on soils and soil constituents. Since the early work of Way (1850), who studied NH4-Ca exchange on soils, it has been generally assumed that exchange processes on soils and soil colloids are rapid, often instantaneous. This may explain in part why most studies on cation exchange have dealt with equilibrium aspects, and only recently have studies appeared on the kinetics and mechanisms of exchange phenomena (Sparks, 1989). Another reason that the kinetics of cation exchange have not been extensively investigated is the lack of appropriate techniques to measure rapid reaction rates. With most batch and flow techniques, one cannot measure the kinetics of chemical reactions that are faster than about 15 s (Sparks, 1989). Moreover, traditional batch and flow methods primarily measure transport-controlled reactions and, thus, apparent rate parameters are determined. Consequently, chemical kinetics are not determined. Cation-exchange reactions on some clay minerals are particularly rapid, usually occurring within minutes or seconds. Malcom and Kennedy (1969), using a specific ion electrode technique, found that >70% of Ba-K exchange on kaolinite, illite, and montmorillonite occurred in 3 s. Sparks and Jardine (1984), using a continuous flow method, found that adsorption of K on kaolinite and montmorillonite was completed in 40 and 120 min, respectively. With the technique of Sparks and Jardine (1984), a combination of chemical exchange and diffusion phenomena were being measured. Pressure-jump relaxation is a useful technique for Dept. of Plant and Soil Sciences, Univ. of Delaware, Newark, DE 19717-1303. Received 3 Feb. 1992. *Corresponding author. Published in Soil Sci. Soc. Am. J. 57:42-46 (1993). measuring rapid reaction rates on soil constituents (Sparks, 1989; Sparks and Zhang, 1991). This technique is based on the principle that chemical equilibrium is pressure dependent. The pressure pulse created by an autoclave perturbs a system from equilibrium by a small amount, and then the system decays back to the original equilibrium. The time required for the decay process (from the nonequilibrium to equilibrium state) is referred to as the relaxation time (T), which is related to the specific rates of the elementary reactions involved. With the pressure-jump method, one can measure the chemical kinetics of reactions in millisecond or microsecond time scales. Pressure-jump, as well as other chemical relaxation techniques, have been used to study cation adsorption-desorption kinetics (Hachiya et al., 1979; Negishi et al., 1984; Hayes and Leckie, 1986) and, recently, to investigate anion adsorption-desorption on goethite (Zhang and Sparks, 1989, 1990a, b). In most of these studies, reactions on the oxide-water interface were investigated. No one has used pressure-jump relaxation to study cation-exchange kinetics on clay surfaces. Accordingly, the objective of this study was to investigate the rates of Ca-Na and Ca-K exchange on montmorillonite using pressure-jump relaxation, and to compare the equilibrium constants calculated via kinetics with those determined using a traditional thermodynamic approach. MATERIALS AND METHODS Theoretical Considerations For the Ca-Na and Ca-K exchange reaction,
46 citations
TL;DR: In this article, the kinetics of the elementary reaction steps in surface-limited thin film growth of germanium from digermane were investigated by utilizing surface differential reflectance.
Abstract: The kinetics of the elementary reaction steps in surface‐limited thin film growth of germanium from digermane was investigated by utilizing surface differential reflectance. Separation of the elementary reaction steps of chemisorption and reaction product desorption was achieved by using a pulsed molecular beam to modulate the digermane delivery to the heated substrate. Both elementary reaction steps were found to be single exponential first‐order processes. The chemisorption reaction on Ge(100) was rapid (k1=500±50 s−1), and independent of the substrate temperature between 680 and 810 K. On the other hand the desorption step in the same substrate temperature range is strongly temperature dependent with an activation energy of 1.7±0.1 eV and a prefactor of 2×1013±1 s−1. A simple kinetic model comprised of two opposing first‐order elementary steps is found to be consistent with the experimental results. It is concluded that the kinetic data are compatible with the pairing mechanism for molecular hydrogen d...
39 citations
TL;DR: In this paper, the rates of gasification by CO2 have been determined for three laboratory produced chars derived from U.K. coals with varying ash contents using an isothermal thermogravimetric technique.
Abstract: The rates of gasification by CO2 have been determined for three laboratory produced chars derived from U.K. coals with varying ash contents. Data for the temperature range 900 to 1233K were obtained using an isothermal thermogravimetric technique. Reaction rates were determined in atmospheres of either CO2, O2 or CO2/O2 mixtures for which the CO2 partial pressure varied between 10 to 101 kPa. Pre-exponential factors and activation energies for the elementary reaction steps were evaluated using the Langmuir-Hinshelwood expression. High temperature data for the range from 1400 to 1800K were obtained by projecting the char particles through a flat methane-air flame. Particle temperatures in the flame were measured using a two-colour pyrometer system and the rates of char gasification at various CO2 partial pressures were calculated from energy balance equations and then used to provide limited information on the char/CO2 reaction rate above 1400K. The activation energy obtained for the transfer of o...
35 citations
01 Mar 1993
TL;DR: In this article, the structure and concentration of peroxide groups on the surface of the working catalysts were investigated at elevated temperatures as high as 800°C by varying the partial pressures of the reactant gases.
Abstract: Oxidative coupling of methane is an important route to formation of ethane and ethylene. The reaction is catalyzed by a broad range of basic oxides, including lanthanum oxides, that are modified with group IA or group IIA ions. — Laser Raman spectroscopy has been used to characterize surface peroxide structures on La2O3 and Na- and Sr-modified La2O3. These surface peroxide groups are formed on these catalysts by calcination in O2 at temperatures between 500°C and 800°C. By high temperature in situ Raman spectroscopy it was possible to follow the formation of these surface peroxides and their reaction with methane. Changes in the structure and concentration of peroxide groups on the surface of the working catalysts were investigated at elevated temperatures as high as 800°C by varying the partial pressures of the reactant gases. The results provide evidence for the role of peroxide ions for the activation of methane during the oxidative coupling reaction.
32 citations
TL;DR: In this paper, numerical simulations of premixed flames burning mixtures of methane and methyl chloride in air are conducted at atmospheric pressure in order to understand the effect of hydrocarbon bound chlorine on hydrocarbon-air flames.
28 citations
TL;DR: In this article, the growth rate of a gas-solid deposition reaction is derived on the basis of collision theory of heterogeneous unimolecular elementary reaction and classical statistical physics, which can explain many experimental phenomena including low and atmospheric pressure, hydrogen and inert ambient, amorphous, polycrystalline and single-crystal silane deposition.
TL;DR: In this article, the authors present a detailed analysis of the most recent mechanism of the oscillatory chlorite-iodide reaction proposed by Citri and Epstein, using stoichiometric network analysis to determine the complete set of major reaction pathways (extreme currents) of the mechanism under CSTR conditions.
Abstract: We present a detailed analysis of the most recent mechanism of the oscillatory chlorite-iodide reaction proposed by Citri and Epstein. First, we use stoichiometric network analysis (SNA) to determine the complete set of major reaction pathways (extreme currents) of the mechanism under CSTR conditions. Two qualitatively different types of unstable extreme currents are predicted within the mechanism. At three different steady states the dominant extreme currents (EC) are singled out. Subgroups of elementary reactions («dynamic elements»), dominant at different times during a cycle of oscillation, are identified
01 Jun 1993
TL;DR: In this paper, the thermal dissociation of silane was measured behind reflected shock waves using the Atomic Resonance Absorption Spectroscopy (ARAS) for detecting Si, H, and O atoms.
Abstract: In the present investigation the thermal dissociation of silane was measured behind reflected shock waves using the Atomic Resonance Absorption Spectroscopy (ARAS) for detecting Si, H, and O atoms. The experiments were performed at temperatures 1250 K ≤ T ≤ 2115 K and pressures between 0.7 and 1.5 bar. Initial mixtures of 0.15 to 5 ppm SiH4 diluted in Ar and 5 ppm SiH4 with 500 ppm O2 diluted in Ar were studied. The H atom measurements show formation rates, which are much less than the respective rates for Si atom formation. This indicates that the H atoms were formed by secondary reactions and are not primary dissociation products of silane or silylene. A reaction mechanism suitable to describe all measured H atom concentration profiles is given in this paper.
Si atoms also measured during the silane thermal decomposition were found to be useful to understand the kinetics of this process. From the Si concentration profiles together with computer simulations rate coefficients for the reactions
SiH4 + M ⇋ SiH2 + H2 + M (1)
SiH2 + M ⇋ Si2 + H2 + M (2)
were derived, which can be summarized by the following Arrhenis expressions:
k1 = 9.9×1015exp(−24000 K/T) cm3mol−1 s−1
k2 = 9.1×1013exp(−15100 K/T) cm3mol−1 s−1.
In the case of the SiH4/O2/Ar mixtures no detectable Si atom concentrations were found, while the observed O atom formation rates were identical to that of Si atoms in mixtures without O2 under comparable reaction conditions. This means that Si originating from the thermal decomposition of silane is directly converted into O by reaction with O2, i.e. O is in this system a measure of Si.
01 Jan 1993
TL;DR: In this paper, the rotational energy of the transition state in unimolecular reactions has been used as a probe for the structure of transition states, which is to be expected for a concerted elimination reaction (tight transition state).
Abstract: A threshold photoelectron photoion coincidence experiment has been setup which allows to measure breakdown curves of energy selected ions at different sample temperatures. These studies reveal the rotational energy of the transition state in unimolecular reactions, which in turn is used as a probe for the transition state structure. According to a model recently suggested this should allow the distinction of tight and loose transition states. The breakdown curves of methane and ethane ions in the region of the lowest reaction channel have been measured at 300 K and close to 0 K. In the case of the H loss from methane ions the shift between these two breakdown curves reveals that the dissociation energy Eo varies significantly with the rotational temperature as expected for a simple bond fission (loose transition state). In the case of H2 loss from ethane ions the shift between the breakdown curves at 300 and close to 0 K shows that E0 (J) does not vary significantly with the rotational temperature. This is to be expected for a concerted elimination reaction (tight transition state).
TL;DR: In this article, the connection between hydride coverage and thin-film growth rate was investigated by a kinetic model consisting of the elementary reaction steps of source gas chemisorption and hydrogen desorption in silicon and germanium epitaxial thin film growth.
Abstract: The connection between the hydride coverage and thin‐film growth rate was investigated by a kinetic model consisting of the elementary reaction steps of source gas chemisorption and hydrogen desorption in silicon and germanium epitaxial thin‐film growth from silanes and germanes. A generalized form of the model for steady‐state conditions was used to extract the kinetic parameters of the elementary reaction steps from experimental film growth data in the literature. Three‐dimensional plots of the growth rate as a function of the substrate temperature and the source gas flux were used to summarize the trends in steady‐state surface‐limited thin‐film growth using the kinetic parameters extracted by the model. The three‐dimensional plots show that: (1) each of the elementary reaction steps is dominantly dependent only on a single external growth parameter, and (2) in the transition region where neither of the elementary steps is clearly dominant the growth rates exhibit a complex dependence on the growth par...
TL;DR: The catalytic decomposition of polypropylene was studied in this article, where the most important elementary reaction is the intramolecular rearrangement of chain-end secondary carbonium ions in the liquid fraction to inner tertiary carbon atoms; the C9 fraction was produced by β-scission of the rearranged ions.
TL;DR: In this paper, the authors performed diamond growth experiments in a microwave plasma ball reactor on silicon wafers or on a molybdenum sheet provided with cones (stamped into the sheet with a punch).
Abstract: Diamond growth experiments were performed in a microwave plasma ball reactor on silicon wafers or on a molybdenum sheet provided with cones (stamped into the sheet with a punch). All substrates had been treated by scratching with diamond powder in advance. The gas mixture used was CH4/H2, sometimes with the addition of CO. Substrate temperatures ranged from 953 to 1428 K, pressures from 100 to 400 mbar, and microwave powers from 250 to 700 W. A strong preference of diamond growth was observed on the cones in the molybdenum substrates. This is interpreted as being caused by gas transport hindrance. The resulting deposition coefficient of the “active” species is about 0.1 under all conditions investigated. The deposition experiments on silicon substrates are numerically modeled in two steps. In the first step, temperature fields and electron density and energy distributions in pure hydrogen are calculated following the method described previously. The output of this first simulation step is taken as input data for the second step. The condition is applied that chemical reaction rates due to thermal or electronic activation and diffusional flows compensate each other at every point of the reactor. In this way stationary concentrations of the 13 species in 29 elementary reactions are computed and, from these, the expected deposition profile of diamond on the silicon substrate, assuming one of the carbon-containing species to be the “active” one. When the experimental deposition profiles are compared with the calculated ones, C2H as the “active” species gives the best match to all the experimental results. CH3 and C2H2 (and perhaps others) might contribute to the diamond growth to a limited extent only.
TL;DR: An algorithm is introduced that tests whether a proposed elementary reaction can be realized within a specified number of cleavages and formations of covalent bonds, related to the problem of computing the minimum chemical distance of a given stoichiometry.
Abstract: An algorithm is introduced that tests whether a proposed elementary reaction can be realized within a specified number of cleavages and formations of covalent bonds. This is related to the problem of computing the minimum chemical distance of a given stoichiometry, but differs from it in important ways that are exploited in the algorithm design. One application of the algorithm is as a filter in MECHEM—a computer aid for the elucidation of reaction pathways. In that application, reaction steps implying more changes to covalent bonds than a given threshold are ruled implausible, and in practice such tests need to be carried out many thousands of times. Future applications of the algorithm can be expected because the question addressed is a fundamental one: What elementary reactions can occur? © 1993 by John Wiley & Sons, Inc.
TL;DR: In this article, the reaction of CF 3 O(X 2 A′) with O( 3 P) has been studied using ab initio molecular orbital theory and it was shown that the products for this reaction are surface dependent.
TL;DR: In this paper, the role of the solvent has been analyzed using a self-consistent reaction field method in which the solvation energy is obtained using a multipole expansion of the molecular charge distribution.
Abstract: The ketene + formaldimine cycloaddition reaction to give 2-azetidinone has been studied as a model of one of the most interesting methods for the synthesis of β-lactams. The role of the solvent has been analyzed using a self-consistent reaction field method in which the solvation energy is obtained using a multipole expansion of the molecular charge distribution. Substantial solvent effects are predicted both for the reaction mechanism and for the reaction rate. Zero-point energy corrections have been shown to also be important.
01 Nov 1993
TL;DR: In this paper, products of the reaction of the NO3 radical with monoalkenes were identified using a fast flow system with mass spectrometric detection, and Oxiranes were found to be the predominant hydrocarbon compound produced at low pressures with yields of (90 ± 10)% for 2,3-dimethyl-2-butene and (80 ± 10%) for cis-2butene.
Abstract: Products of the reaction of the NO3 radical with monoalkenes, i.e., 2,3-dimethyl-2-butane, 2-methyl-2-butene, cis-2-butene, isobutene and propene were identified using a fast flow system with mass spectrometric detection. The reaction mixtures were analyzed in situ without applying enrichment procedures. Oxiranes were found to be the predominant hydrocarbon compound produced at low pressures with yields of (90 ± 10)% for 2,3-dimethyl-2-butene and (80 ± 10)% for cis-2-butene. For the unsubstituted butenes and for propene formation of products containing a carbonyl group was also suggested.
TL;DR: In this paper, a modified excimer formation mechanism using an encounter complex, in which the main reaction takes place, is presented and the rate coefficients of the elementary reaction steps are discussed, and the slope of measured decay curves are in better accordance with the calculated curves of the modified mechanism and the strong increase of the ratio of excimer to monomer quantum yield can be verified experimentally.
Abstract: A modified excimer formation mechanism using an encounter complex, in which the main reaction takes place, is presented and the rate coefficients of the elementary reaction steps are discussed. The kinetic description by ordinary rate equations fails in the case of excimer formation with a time-dependent rate coefficient caused by non-stationary diffusion. Convolution kinetics remain valid in that case and provide decay curves and quantum yields.Classical and the modified excimer formation mechanisms were investigated to obtain some information about the influence of the molecular kinetic parameters (molecular dimensions, temperature, solvent viscosity). The difference between the rate equation treatment and convolution kinetics becomes smaller when the modified mechanism of excimer formation is used. The influence of temperature on the ratio of excimer to monomer quantum yields shows interesting effects; over a temperature range of 40 K the classical excimer formation mechanism increases by a factor of two, but the modified mechanism increases by a factor of ten.The slope of measured decay curves are in better accordance with the calculated curves of the modified mechanism and the strong increase of the ratio of excimer to monomer quantum yield can be verified experimentally.
TL;DR: In this article, a reaction path Hamiltonian is used to describe the dynamics of a model isomerization reaction, based on the basic concepts of the Davis-Gray analysis of unimolecular reaction rate.
Abstract: We show that a reaction path Hamiltonian can be used, with the basic concepts of the Davis–Gray analysis of unimolecular reaction rate, to generate an accurate description of the dynamics of a model isomerization reaction.
TL;DR: In this paper, the effect of solvents or reaction components on thermal stability is discussed, and the typical combinations of primary and secondary reactions which often occur are discussed, as well as some examples of this very extensive type of secondary reactions are discussed.
01 May 1993
TL;DR: In this article, the rates of the reaction of styrene with atomic oxygen have been measured in a discharge flow system with mass spectrometric detection, and the measurements were carried out under oxygen atom excess in the temperature range 298 K < T < 873 K.
Abstract: The rates of the reaction of styrene with atomic oxygen have been measured in a discharge flow system with mass spectrometric detection. The measurements were carried out under oxygen atom excess in the temperature range 298 K < T < 873 K. The main reaction channel at room temperature is O atom addition to the vinyl side chain, yielding as main product (identified by mass spectrometric product analysis) styrene oxide. Towards higher temperatures O atom addition to the aromatic ring as a second reaction channel becomes more important. Using Hammett plots the following Arrhenius expressions for the addition to the aromatic ring (kar) and to the alkene double bond (kdb) of the side chain can be derived:
First experiments with indene give similar results, showing a somewhat higher reactivity for this molecule compared to styrene.
01 Jun 1993
TL;DR: In this article, the rate of the reaction NH(X3Σ−) + O2 at low and high temperatures was investigated in the intermediate temperature range and the results confirmed a strongly curved Arrhenius plot for the speed of this reaction.
Abstract: Measurements of the rate of the reaction NH(X3Σ−) + O2 at low and high temperatures show strongly different apparent energies of activation In order to check the consistency of both sets of measurements the reaction was investigated in the intermediate temperature range These experiments confirm a strongly curved Arrhenius plot for the rate of this reaction
01 Apr 1993
TL;DR: In this article, a kinetic investigation of the collisional behaviour of ground state atomic carbon, C(2p2 (3PJ)), with unsaturated cyclic compounds was presented, and the following absolute second-order rate constants for the reactions of C(23PJ) with the following cyclic collision partners were reported.
Abstract: We present a kinetic investigation of the collisional behaviour of ground state atomic carbon, C(2p2 (3PJ)), with unsaturated cyclic compounds. Atomic carbon was generated by the repetitive pulsed irradiation (λ > ca. 160 nm) of C3O2 in the presence of excess helium buffer gas and the added reactant gases in a slow flow system, kinetically equivalent to a static system. C(23PJ) was then monitored photoelectrically by time-resolved atomic resonance absorption in the vacuum ultra-violet (λ = 166 nm, 33PJ23PJ) with direct computer interfacing for data capture analysis. The following absolute second-order rate constants for the reactions of C(23PJ) with the following cyclic collision partners are reported (errors 2σ), demonstrating rapid reaction in all cases:
These results constitute the first reported body of absolute rate data for reactions of ground state carbon with these reactants. Whilst a large body of absolute rate data for atomic silicon in its Si(3p2 (3PJ)) ground state has now been determined directly in recent years by time-resolved atomic resonance absorption spectroscopy in the ultra-violet itself with a wide range of collision partners, including unsaturated reactants, there are no analogous data for these cyclic reactants. Comparison is made with rate data for C(23PJ) with benzenoid and fluorinated benzenoid collision partners, and the results also discussed in the context of nucleation and soot formation in flames.
TL;DR: In this article, the potential energy profiles of the radical substitution reactions of H2S with some atomic radicals X (X = H, O, F) and of CH3SH with the hydrogen atom have been investigated by ab initio molecular orbital calculations employing the 6-31G** basis sets.
Abstract: Potential energy profiles of the radical substitution reactions of H2S with some atomic radicals X (X = H, O, F) and of CH3SH with the hydrogen atom have been investigated by ab initio molecular orbital calculations employing the 6-31G** basis sets. All the reactions studied have proved to be elementary reactions with no intermediate adducts involved. At the MRD-CI level using the 6-31G** plus the Rydberg s and p orbitals on sulfur, the activation energies of the homolytic substitution reactions H2S + X → H + HSX are calculated to be 50, 127, and 104 kJ mol−1 for X = H, O, and F, respectively, which are uniformly higher than those calculated for the hydrogen abstraction reactions H2S + X → SH + XH. For the substitution reaction CH3SH + H → CH3 + H2S, however, the activation energy calculated is as low as 19 kJ mol−1, which is even slightly lower than that for the abstraction CH3SH + H → CH3S + H2. The predicted prevalence of the homolytic substitution reaction of CH3SH over the hydrogen abstraction is qua...
TL;DR: In this paper, the difference of reaction activation barriers of carbon chain growth and termination in Fischer-Tropsch (FT) synthesis is determined from experiments on Fe/Zn catalyst.
Abstract: A method is established, by which the difference of the reaction activation barriers of carbon chain growth and termination in Fischer-Tropsch (FT) synthesis can be determined from experiments. A FT synthesis is carried out on Fe/Zn catalyst. We apply the method to analyze the experimental result and obtain the difference of reaction activation barriers of carbon chain growth and termination of α-olefins on the catalyst.
TL;DR: In an aqueous solution, cyclodextrins resulted from enzymatic degradation of starch are cyclic oligosaccharides consisting of six, seven, or eight glucose units.
Abstract: In an aqueous solution, ƒ¿-and ƒÁ-cyclodextrin (ƒ¿-and ƒÁ-CD) reacted with inorganic sodium cyclo—triphosphate hexahydrate (P3m), Na3P3O9•E6H2O, to form a triphosphate derivative and three monophosphate derivatives of ƒ¿-and ƒÁ-CD, of which the secondary hydroxyl groups were phosphorylated. The maximum yields of phosphorylated products of ƒ¿-and ƒÁCD were 22.6 and 19.0 %, respectively. INTRODUCTION Cyclodextrins resulted from enzymatic degradation of starch are cyclic oligosaccharides consisting of six, seven, or eight glucose units, and called ƒ¿-,ƒÀand ƒÁ-CD, respectively. Received November 12, 1993; Accepted December 20, 1993