A new model for thermal diffusion: kinetic approach

We present a new model for thermal diffusion, and we compare its results for both simple and real systems. This model is derived from a kinetic approach with explicit mass and chemical contributions. It involves self-diffusion activation free energies, following Prigogine's original approach. We performed, furthermore, both equilibrium and nonequilibrium molecular dynamics evaluations in order to compute respectively the self-diffusion activation free enthalpies and the Soret coefficient when no experimental data were available. Our model is in very good agreement with simulation data on Lennard-Jones mixtures, and a good behavior is noted for the water-ethanol mixture, where the composition dependence at which the Soret coefficient changes its sign is predicted very accurately. Finally, we propose a new water-ethanol experiment at higher temperature in order to check the validity of our model.     

Oxidative coupling of methane in a fixed bed reactor over perovskite catalyst： A simulation study using experimental kinetic model

The oxidative coupling of methane （OCM） to ethylene over a perovskite titanate catalyst in a fixed bed reactor was studied experimentally and numerically. The two-dimensional steady state model accounted for separate energy equations for the gas and solid phases coupled with an experimental kinetic model. A lumped kinetic model containing four main species CH4, O2, COx （CO2, CO）, and C2 （C2H4 and C2H6） was used with a plug flow reactor model as well. The results from the model agreed with the experimental data. The model was used to analyze the influence of temperature and feed gas composition on the conversion and selectivity of the reactor performance. The analytical results indicate that the conversion decreases, whereas, C2 selectivity increases by increasing gas hourly space velocity （GHSV） and the methane conversion also decreases by increasing the methane to oxygen ratio.     

Development of a new flow reactor for kinetic studies. Application to the ozonolysis of a series of alkenes

A new flow reactor has been developed to study ozonolysis reactions at ambient pressure and room temperature (297 ± 2 K). The reaction kinetics of O(3) with 4-methyl-1-pentene (4M1P), 2-methyl-2-pentene (2M2P), 2,4,4-trimethyl-1-pentene (tM1P), 2,4,4-trimethyl-2-pentene (tM2P) and α-pinene have been investigated under pseudo-first-order conditions. Absolute measurements of the rate coefficients have been carried out by recording O(3) consumption in excess of organic compound. Alkene concentrations have been determined by sampling adsorbent cartridges that were thermodesorbed and analyzed by gas-chromatography coupled to flame ionization detection. Complementary experimental data have been obtained using a 250 L Teflon smog chamber. The following ozonolysis rate coefficients can be proposed (in cm(3) molecule(-1) s(-1)): k(4M1P) = (8.23 ± 0.50) × 10(-18), k(2M2P) = (4.54 ± 0.96) × 10(-16), k(tM1P) = (1.48 ± 0.11) × 10(-17), k(tM2P) = (1.25 ± 0.10) × 10(-16), and k(α-pinene) = (1.29 ± 0.16) × 10(-16), in very good agreement with literature values. The products of tM2P ozonolysis have been investigated, and branching ratios of (21.4 ± 2.8)% and (73.9 ± 7.3)% have been determined for acetone and 2,2-dimethyl-propanal, respectively. Additionally, a new nonoxidized intermediate, 2-methyl-1-propene, has been identified and quantified. A topological SAR analysis was also performed to strengthen the consistency of the kinetic data obtained with this new flow reactor.     

Experimental and theoretical multiple kinetic isotope effects for an SN2 reaction. An attempt to determine transition-state structure and the ability of theoretical methods to predict experimental kinetic isotope effects

The secondary alpha-deuterium, the secondary beta-deuterium, the chlorine leaving-group, the nucleophile secondary nitrogen, the nucleophile (12)C/(13)C carbon, and the (11)C/(14)C alpha-carbon kinetic isotope effects (KIEs) and activation parameters have been measured for the S(N)2 reaction between tetrabutylammonium cyanide and ethyl chloride in DMSO at 30 degrees C. Then, thirty-nine readily available different theoretical methods, both including and excluding solvent, were used to calculate the structure of the transition state, the activation energy, and the kinetic isotope effects for the reaction. A comparison of the experimental and theoretical results by using semiempirical, ab initio, and density functional theory methods has shown that the density functional methods are most successful in calculating the experimental isotope effects. With two exceptions, including solvent in the calculation does not improve the fit with the experimental KIEs. Finally, none of the transition states and force constants obtained from the theoretical methods was able to predict all six of the KIEs found by experiment. Moreover, none of the calculated transition structures, which are all early and loose, agree with the late (product-like) transition-state structure suggested by interpreting the experimental KIEs.     

Consecutive,irreversible first-order reactions. Ambiguities and practical aspects of kinetic analyses

The general problem of eliciting reliable rate constants from experimental data is considered in detail for consecutive reactions. Practical aspects are emphasized.     

Effects of translational and rotational diffusion on the association in kinetic model of nucleation

Kinetics of an association and dissociation of single elements with the effects of translational and rotational diffusion and angular limitations is discussed. Separated clusters embedded in a solution of orientable single elements are considered.Steady-state positional and angular distribution of single elements is calculated from the equation of translational-rotational diffusion and the boundary conditions proposed for orientation-limited association. Although spherical orientable elements are assumed, the model can be used for non-spherical particles with aspect ratios close to unity.Diffusion-limited rate constants of association and dissociation are proposed which depend on translational and rotational diffusion constants of single elements, the tolerance angle of the association, and the cluster size.Effective concentration of single elements and effective rate constants are expressed by the equilibrium and diffusion-limited rate constants. Effects of finite diffusion rates and finite tolerance angle are discussed.The equations of the kinetic model of nucleation are modified due to the diffusion-limited rate of the association.     

Mixture diffusion of sulfonated dyes into cellulose membrane. III. Application of parallel diffusion model to binary system of direct dyes with different affinity

Mixture diffusion of two dyes (C.I. Direct Blue 15 (DB15) and C.I. Direct Yellow 12 (DY12)) with different affinity onto the substrate into cellulose membrane from the binary solution was studied at 55°C. Uptake curves and concentration–distance profiles were measured experimentally in the ratios (DB15:DY12) 1:0.5, 1:1 and 1:2. It was examined whether the diffusion of the dyes could be analyzed based on the parallel diffusion theory of surface and pore diffusion. It was revealed that the diffusion of DB15 with higher affinity could be analyzed based on the model in the ratios 1:0.5 and 1:1, although the theoretical value deviated slightly from the data in the concentration–distance profile in the ratio 1:1. On the other hand, the diffusion of DY12 with smaller affinity could not be described by the model, because the diffusivity of the dye changed during the adsorption process against the assumption of the model.     

The bromination of acetone. Application to multicomponent kinetic determinations

A very simple kinetic method is proposed for the resolution of mixtures of acetone and a second component. It is based on the reaction, at constant temperature, between bromine and acetone. This reaction can be regarded, under certain conditions, as pseudo-zero-order on the bromine concentration. To show the possibilities of the method, mixtures of acetone (between 2 and 20 x 10(-4) mol L(-1)) and either hydroquinone (between 0.4 and 2.2 x 10(-4) mol L(-1)) or resorcinol (between 0.1 and 10.7 x 10(-4) mol L(-1)) have been used with the concentration ratios [acetone]:[hydroquinone] ranging between 0.5 and 50 and [acetone]:[resorcinol] ranging between 2.8 and 200. No systematic errors were found to exist (90% confidence level) and random errors were mainly under 5%. The method can be extended to mixtures of acetone and a second component whose reaction with bromine is fast (phenols, aromatic amines, etc.).     

木质素降解模型化合物愈创木酚及苯酚原位加氢制备环己醇

以Raney Ni为催化剂,研究了甲醇水相重整制氢与木质素降解模型化合物愈创木酚／苯酚加氢的耦合反应.考察了反应前冷压、反应温度、反应时间、物料配比等条件对木质素降解模型化合物原位加氢反应性能的影响,并对影响机制进行了讨论.结果表明,在反应温度为220 ℃、反应前冷压0 MPa(表压)、物料比水／甲醇／模型化合物为20∶5∶0.8的条件下,反应7 h后愈创木酚转化率与环己醇选择性分别达99.00％和93.74％,反应12 h后苯酚的转化率与环己醇选择性分别达90.50％和99.29％.采用原位加氢反应,木质素降解的酚类模型化合物转化率和选择性明显优于外部供氢反应的转化率和选择性,同时,避免了外部供氢反应存在的氢气制备、储存、传输及加氢条件苛刻等问题,为木质素解聚产物制备化工品提供了新思路与实验基础.     

Morphologic and kinetic study of an epoxy-poly(ethyleneoxide) system. The fluorescence to predict miscibility

The kinetic of the curing process in the ethylenediamine (EDA)-poly (bisphenol A-co-epichlorohydrin) glycidyl end-capped (DGEBA) mixture modified with poly(ethylene oxide) (PEO) was studied. The epoxy component was labeled with a fluorescence group (dansyl) treating the DGEBA with the reactive dansyl derivative DNS-EDA. Dynamic DSC experiments were carried out and from their results the effect of the PEO composition on the epoxy curing was discussed. Furthermore, the effect of cure temperature and PEO composition on the morphology and crystallinity of the blend were studied as well. The morphologic study was carried out using complementarily optical transmission (TOM) and epifluorescence (EFM) microscopy. It was observed that: i) the addition of a non-reactive thermoplastic leads to a dilution effect of the reactive groups and therefore a decrease of the epoxy amine reaction rate; ii) the PEO composition does not seem to affect the non catalyzed process of the epoxy curing, while an increase in the PEO fraction within the epoxy/PEO mixture seems to change the mechanism of the cure reaction; iii) dynamic DSC scans, TOM and EFM images and steady state fluorescence spectra of the cured samples suggest that when the curing temperature increases there is an increase in the miscibility between PEO and the epoxy-amine reaction mixture; and iv) a reduction in the PEO/cured epoxy miscibility as the fraction of PEO increases was observed.     

Estimation of kinetic parameters in a fixed bed reactor: Application of fast fourier transform

The fast Fourier transform technique was introduced as a calculational tool for the estimation of parameters in the time domain. A complex fixed bed enzymatic reactor was selected as the model example. The results show that this is a suitable and effective technique in this regard.     

On the kinetic coupling and mechanism of aromatic ring hydrogenation

Synthesis of a mesoporous material by pillaring of magadiite with silica was investigated. The composite samples were characterized by XRD and BET methods. The acidity of surface OH groups was studied by adsorption of pyridime monitored by IR spectroscopy.     

Parametric analysis of kinetic models. 9. Catalytic triggers with diffusion exchange steps

Bimetallic catalysts consisting of Ru–Fe prepared on SiO₂ and carbon (sibunit) supports were found to be catalytically active and selective in the formation of C₄-aldehydes and alcohols in hydroformylation of propylene at 433 K and 2 atm. of CO+H₂+C₃H₆ (111). The addition of Fe to Ru altered the activity of monometallic Ru catalyst and favored the hydrogenation of CO to methanol.     

Comparison of kinetic Monte Carlo and molecular dynamics simulations of diffusion in a model glass former

We present results from kinetic Monte Carlo (KMC) simulations of diffusion in a model glass former. We find that the diffusion constants obtained from KMC simulations have Arrhenius temperature dependence, while the correct behavior, obtained from molecular dynamics simulations, can be super-Arrhenius. We conclude that the discrepancy is due to undersampling of higher-lying local minima in the KMC runs. We suggest that the relevant connectivity of minima on the potential energy surface is proportional to the energy density of the local minima, which determines the "inherent structure entropy." The changing connectivity with potential energy may produce a correlation between dynamics and thermodynamics.