Showing papers on "Reaction rate published in 1993"

Catalytic activity of various salts in the reaction of 2,3-epoxypropyl phenyl ether and carbon dioxide under atmospheric pressure

Abstract: Reaction of 2,3-epoxypropyl phenyl ether (1) with carbon dioxide was carried out under atmospheric pressure in N-methylpyrrolidinone (NMP) at 100 o C in the presence of 5 mol% of various salts to obtain a five-membered cyclic carbonate, 4-(phenoxymethyl)-1,3-dioxolan-2-one (2), selectively. Only side salts showed high catalytic activity, and the order of intrinsic activity was found to be as follows: chloride>bromide>iodide which is the order of nucleophilicity of the anion. Furthermore, the order of the activity was found to be lithium salt>sodium salt> benzyltrimethylammonium salt, which is in accord with the order of Lewis acidity of the cation. Kinetic analyses show that the reaction rate can be represented by -d[1]/dt=k[1][cat.], where the carbon dioxide pressure shows no effect on the reaction rate

Understanding the acid behavior of zeolites from theory and experiment

Abstract: ZEOLITES are microporous aluminosilicates which, in their protonated form, act as solid catalysts1, and are widely used in the oil and petrochemical industries for processes such as cracking, isomerization and alkylation if hydrocarbons2. The proposed mechanisms3–5 of these processes mostly involve proton transfer and formation of carbenium or carbonium ions as reactive intermediates, but the detailed function of the zeolite and in particular the relation between acidity and catalytic activity is not well understood. Here we report experimental and theoretical studies of denterium–hydrogen exchange between deuterated methane and protonated zeolites — a prototypical hetero-geneous catalytic reaction between a hydrocarbon and an acid zeolite. We monitored this slow exchange reaction in two different zeolites using infrared spectroscopy, and used ab initio quantum chemistry calculations to determine both the reaction mechanism and the acidity–activity relationship. Combining our theoretical results with recent estimates8–11 of the acidity differences within zeolites enables us to reproduce the experimentally observed reaction rates and thus to obtain a detailed microscopic picture of this heterogeneous catalytic process.

Dynamic Solvent Effects on Electron-Transfer Reactions

Abstract: Electron-transfer processes in solution are among the most important reactions in chemistry and biology. The huge number of redox reactions of transition metal ions and complexes, many preparatively important oxidations and reductions of organic compounds, photosynthesis, and metabolism are only a few examples where electron-transfer reactions play a pivotal role. This ubiquity, as well as their relative simplicity, makes them excellent models for the study on a molecular level of chemical reactions in solution. A particularly important question in chemical reaction dynamics in solution is the influence of the solvent on the reaction rate. In this context one distinguishes between static and dynamic solvent effects. Static effects refer to the stabilization of reactants, transition state, and products, that is, how the solvent affects the free energies of these species and the energy of activation. This interpretation of solvent effects on all kinds of chemical reactions is well established. A more recent development is the investigation of the influence of solvent dynamics on the rate of a reaction. The transfer of an electron is usually thought to be triggered by a fluctuation of the dielectric polarization in the surrounding solvent. The dynamics of such fluctuations is determined by the finite response time of the orientational polarization of the solvent. Under certain conditions this dielectric response time can become the rate-determining factor of the reaction. In this article I intend to give a review of these modern developments in the theory and experimental study of electron-transfer processes. We shall see that solvent dynamics may lead to a whole plethora of phenomena in reaction dynamics. The concepts needed for their description are not limited to electron transfer but bear relevance to many other chemical reactions in solution.

Photocatalytic oxidation in aqueous titanium dioxide suspensions: the influence of dissolved transition metals

Abstract: The influence of type, species distribution, standard reduction potential, and concentration of several transition metals on the rate of photocatalytic oxidation of toluene was investigated. A significant increase in reaction rate was observed in the presence of 10−5 M Cu(II), Fe(III), and Mn(II) at pH 3, with decreased rates at higher concentrations and pH values. There was no clear correlation between reaction rate and aqueous metal species distribution, nor did the oxidation states of Cu or Fe alter their effects on the reaction rate. Neither Ni(II) nor Zn(II) had a significant influence on the rate of organic oxidation. Negligible adsorption of metals onto TiO2 was measured at the metal concentrations and pH values for which the highest reaction rates were observed, indicating that dissolved metals increase the reaction rate via a homogeneous pathway rather than a TiO2 surface reaction. A mechanism involving formation of a reactive complex between the metal, the organic or its oxidation intermediate, and an oxygen-containing species is proposed to explain the experimental data. The rate of the photocatalytic reaction is described by a Langmuir—Hinshelwood rate form, modified to account for homogeneous catalytic pathways and decreased UV transmittance in the presence of dissolved metals.

Solvent effects on the mechanism and selectivities of asymmetric Diels-Alder reactions

Abstract: The electrostatic effect of the solvent on the reaction of cyclopentadiene with methyl acrylate has been studied with the help of a Self-Consistent Reaction Field model by means of ab initio computations. The effect on the reaction mechanism, endo/exo selectivity, and activation energy has been analyzed. Moreover, the computations allow a discussion of the solvent effect on the diastereofacial selectivity when chiral acrylates are used and a omparison with experimental data for the reaction of cyclopentadiene with (-)-menthyl acrylate. The results are in agreement with the experimentally observed increase of the endo/exo and diastereofacial selectivities as a function of solvent polarity, but hydrophobic effects and hydrogen-bond formation are necessary to account for changes in reaction rates

Effects of charged amino acid mutations on the bimolecular kinetics of reduction of yeast iso-1-ferricytochrome c by bovine ferrocytochrome b5.

Abstract: The reduction of wild-type yeast iso-1-ferricytochrome c (ycytc) and several mutants by trypsin-solubilized bovine liver ferrocytochrome b5 (cytb5) has been studied under conditions in which the electron-transfer reaction is bimolecular The effect of electrostatic charge modifications and steric changes on the kinetics has been determined by experimental and theoretical observations of the electron-transfer rates of ycytc mutants K79A, K'72A, K79A/K'72A, and R38A (K' is used to signify trimethyllysine (Tml)) A structurally robust Brownian dynamics (BD) method simulating diffusional docking and electron transfer was employed to predict the mutation effect on the rate constants A realistic model of the electron-transfer event embodied in an intrinsic unimolecular rate constant is used which varies exponentially with donor-acceptor distance The BD method quantitatively predicts rate constants over a considerable range of ionic strengths Semiquantitative agreement is obtained in predicting the perturbing influence of the mutations on the rate constants Both the experimentally observed rate constants and those predicted by BD descend in the following order: native ycytc > K79A > K'72A > K79A/K'72A Variant R38A was studied at a different ionic strength than this series of mutations, and the theory agreed with experiment in predicting a smaller rate constant for the mutant In all cases the predicted effect of mutation was in the correct direction, but not as large as that observed The BD simulations predict that the two proteins dock through essentially a single domain, with a distance of closest approach of the two heme groups in rigid body docking typically around 12 A Two predominant classes of complexes were calculated, the most frequent involving the quartet of cytb5/ycytc interactions, Glu48-Arg13, Glu56-Lys87, Asp60-Lys86, and heme-Tml72, having an average electrostatic energy of -130 kcal/mol The second most important complexes were of the type previously postulated (Salemme, 1976; Mauk et al, 1986; Rodgers et al, 1988) with interactions Glu44-Lys27, Glu48-Arg13, Asp60-Tml72, and heme-Lys79 and having an energy of -64 kcal/mol The ionic strength dependence of the bimolecular reaction rate was well reproduced using a discontinuous dielectric model, but poorly so for a uniform dielectric model

A model for heterogeneous chemical processes on the surfaces of ice and nitric acid trihydrate particles

Abstract: The study presents a model that incorporates the physics and physical chemistry of ice surfaces relevant to polar stratospheric clouds. Surface concentrations of H2O, HCl, HOCl, ClONO2, and N2O5 on ice and nitric acid trihydrate (NAT) crystals are computed, and surface reaction rates and reaction probabilities (sticking coefficients) are determined. For gas pressures of about 10 exp -7 torr and temperatures in the range of 180-200 K, HCl completely coats ice and water-rich NAT surfaces, while HOCl, ClOHO2, and N2O5 may cover 0.01-1 percent of these surfaces. The energy parameters are used to calculate surface temperatures such as adsorption and desorption constants, surface coverages, reaction rate coefficients, surface diffusion coefficients, and reaction probabilities for various species and chemical interactions on ice and NAT surfaces. Implications for chemical processing on polar stratospheric clouds are discussed.

Simulations of the NO+NH3 and NO+H2 reactions on Pt(100): Steady state and oscillatory kinetics

Abstract: A model for the reactions of NO+NH3 and NO+H2 has been developed for simulating the reduction of NO on Pt(100) in the 10−6 mbar pressure range for temperatures between 300 and 700 K. The model consists of seven ordinary differential equations for describing the coverage changes of six adsorbed species as well as an equation for describing the 1×1⇄hex phase transformation. Simulations of the N2 and H2O reaction rates for both reaction mixtures reproduced the hysteresis effects and the existence range for kinetic oscillations, which were found in the experiments. In addition, the occurrence of the so‐called ‘‘surface explosion’’ in both reaction systems is well described by the model. In contrast to the NO+CO reaction on Pt(100), where oscillations may also take place on a pure 1×1 substrate, the 1×1⇄hex phase transition occurs during oscillations for the NO+NH3 and NO+H2 reactions. The transitions between different adsorbate/substrate phases during one oscillatory cycle which are predicted by the model are...

Homogeneous nitrous oxide formation and destruction under combustion conditions

Abstract: N2O decomposition and formation during the oxidation of NH3 and HCN were studied in a quartz flow reactor in the presence of CO, NO and other gases. The emphasis is on the influence of CO and NO. In addition, the homogeneous nitrogen chemistry of fluidized bed combustion and the selective noncatalytic reduction of NO (SNR) are discussed. The rate of N2O decomposition in N2 agrees with a first-order rate expression. The presence of CO or H2 increases the decomposition rate regardless of the additional presence of O2 For the formation of N2O, HCN oxidation is more efficient than NH3 oxidation. The presence of NO increases the amount of N2O formed during the oxidation of HCN or NH3.CO moves the N2O formation toward lower temperatures. H2O increases the reaction rate where few components are present, whereas H2O has little influence in the presence of large amounts of a combustible component such as CO. There are indications that NO is a necessary intermediate for any significant formation of N2O during the oxidation of NH3 and HCN. NO reduction is obtained when NO is initially present during oxidation of both NH3 and HCN. These results are comparable to the respective SNR results with reductant ammonia and urea.

Liquid-phase oxidation of cyclohexane using CoAPO-5 as the catalyst

Abstract: In this study, CoAPO-5 was found to be an effective heterogeneous catalyst for the liquid-phase oxi- dation of cyclohexane with glacial acetic acid as the solvent without any promotors being added to the reaction mixture. The effects of cobalt content, BET surface area and dissolved cobalt ions and the kinetics of the reaction were investigated at 5–15 kg/cm2 and 115–135°C in a semi-batch autoclave reactor. Under these reaction conditions, no induction period was observed. The products of reaction were cyclohexanone, cyclohexanol, dibasic acids, and caprolactone, etc. The ratio of the concentration of adipic acid to succinic acid had a maximum value at the middle stage of reaction. The initial rate of reaction was found to be proportional to the 2.0 power of the cyclohexane concentration and to the 0.5 power of the catalyst loading. The effects of oxygen pressure on the reaction rate depended on the cobalt content and the reaction conditions. The cobalt ions of CoAPO-5 are mainly in the lattice positions and they can transform reversibly between the oxidizing states of CoII and CoIII. Reaction mechanism and rate expressions were proposed. Since the conversion and selectivity reported in this study were mod- erately good, and because CoAPO-5 has the advantage of being a durable heterogeneous catalyst, it could prove useful as a catalyst for the one-step liquid-phase oxidation of cyclohexane.

Laboratory flow reactor measurements of the reaction SO3 + H2O + M → H2SO4 + M: Implications for gaseous H2SO4 and aerosol formation in the plumes of jet aircraft

Abstract: The reaction SO3 + H2O + M has been investigated with a tubular flow reactor at a pressure of 85 Torr. Sulfuric acid has been identified as the most probable product of the reaction. Both, reactant SO3 as well as the product H2SO4 have been measured quantitatively using chemical ionization mass spectrometry. An upper limit for the reaction rate coefficient of 2.4 × 10−15 cm³s−1 has been inferred from the measurements. Implications for sulfuric acid and aerosol formation in the plumes of jet aircraft are discussed.

Liquid state 29Si NMR study on the sol-gel reaction mechanisms of ormosils

Abstract: Sol-gel reaction mechanisms of organically modified silicates (ormosils) were conducted by liquid state 29Si NMR (nuclear magnetic resonance) spectroscopy. Samples were prepared by mixing tetraethoxysilane (TEOS), polydimethylsiloxane (PDMS) and water with isopropanol, tetrahydrofuran (THF) and hydrochloric acid (HCl). Several series of solutions with different reaction times were examined by liquid state 29Si NMR spectroscopy. A new proposed site, D(Q), was assigned from the spectrum of the solution of the dimethyldiethoxysilane (DMDES)/TEOS system. By the reaction between hydrolyzed TEOS and OSi(CH3)2O in the middle of PDMS chains as well as the silanol and groups, HOSi(CH3)2O, of PDMS, bonds between PDMS and TEOS were formed and the PDMS chains were found to exist also as chains (D) and/or cyclic D4 tetramers (D4C) during the reaction. Results of this study also display the expected effects of temperature, acid and water on the reaction rates. The higher the reaction temperature (room temperature to 70°C), the more copolymerization between PDMS and TEOS occurs. The water content (H2O/TEOS = 2 to 4) had little effect on the rate of copolymerization but significantly increased the rate of condensation of TEOS. Increasing acid content (HCl/TEOS = 0.1 to 0.3) increased the rates of both copolymerization and condensation of TEOS. Structural models of the sol-gel reaction mechanisms of ORMOSILs are proposed.

Dispersion and reaction in two‐dimensional model porous media

Abstract: Darcy‐scale convective–diffusive–reactive phenomenological coefficients characterizing the transport of a reactive solute through the interstices of a two‐dimensional, spatially periodic, model porous medium (on whose surfaces the solute undergoes a first‐order, irreversible chemical reaction) are herein determined numerically as functions of the microscale Peclet (Pe), Damkohler (Da), and Reynolds (Re) numbers. The role of bed porosity e and (circular) cylindrical array configuration are also studied, the latter encompassing square, staggered, and hexagonal arrays. Calculations are effected via generalized Taylor dispersion theory. The Darcy‐scale reactivity coefficient K* characterizing the effective (first‐order, irreversible) volumetric reaction rate is found, inter alia, to be (approximately) inversely proportional to Pe, a conclusion confirmed by analytical results for the limiting case of small Da. Configurational properties of the porous medium are observed to significantly influence K*, especially for small porosities and large Da. Moreover, it is found that the mean interstitial velocity vector Ū* of the reactive solute generally differs (often dramatically) from the comparable velocity vector (1/e)Ū of the (inert) solvent as a consequence of the chemical reaction occurring at the surfaces of the cylindrical bed particles. These data reveal that the mean solute speed ‖Ū*‖ through the interstices may be larger or smaller than the comparable solvent speed (1/e)‖Ū‖, depending upon the existence and nature of a diffusive boundary layer adhering to the cylindrical bed‐particle surfaces. Finally, the longitudinal and lateral components of the solute’s (transversely isotropic) dispersivity dyadic D*, parallel and perpendicular, respectively, to the direction of mean flow, are generally observed to decrease with increasing Da. This behavior stems from the fact that, in the diffusion boundary‐layer limit, an increasing proportion of the total depletion of solute (via microscale reaction at the cylinder surfaces) arises from those interstitial zones characterized by the existence of large velocity gradients.

A Chemical Kinetics Model for Glass Fracture

Abstract: The authors utilize a chemical-kinetics-based model to describe the rate of crack extension in vitreous silica as a function of the applied stress and the presence of reactive species. Their approach builds upon previous fracture models that treat the atomic bond rupture process at the crack tip as a stress-enhanced hydrolysis reaction. They derive the stress dependence for siloxane hydrolysis from measurements of hydrolysis rates for strained silicate ring structures. The stress dependence determined for siloxane hydrolysis yields an activation volume of 2.0 cm[sup 3]/mol, which is in good agreement with the stress dependence determined for silicate glass fracture. This result supports previous fracture models that are based on absolute reaction rate theory and predicts an exponential dependence of crack extension rate on applied stress intensity.