Kinetics of acetic acid esterification with 2-ethoxyethanol over an Al-pillared clay catalyst

The Al-pillared clay catalyst obtained by exposing activated clay powder to sulfuric acid and aluminium salts and calcining in air at 373-673 K, was found to be highly active for the title reaction. The results indicated that pillared layer clay of the mixed oxide has been employed as parent catalysts for their definite structure and special properties which can be modified by the substitution of L and B acid sites cations. Solid acid catalyst of Supported aluminium was found to be highly active and selective at the 373-473 K temperature range for heterogeneous esterification. The activity is mainly attributed to the Lewis (and a considerably small number of Bronsted) acid sites whose number and strength increased due to pillaring. The water produced in the esterification can be induced by Al3+, which makes the catalyst surface to form strong B acid. Their acidities are obtained by pH measurement. If only B acid sites are > 70%, and pH < 1 in the 2-ethoxyethanol, there exists an activity of esterification. The used catalyst gave identical results with that of the fresh one. X-ray diffraction spectra show that the composition and active phase of the used catalysts are the same as the fresh ones. The kinetic study of the reaction was carried out by an integral method of analysis. The kinetic equation of surface esterification is y = 2.36x - 0.98.

Transient state of chiral recognition in a binary mixture of cyclodextrins in capillary electrophoresis

The transient state (as the defined point where no enantioseparation is obtained in a dual chiral selector system) of chiral recognition of aminoglutethimide in a binary mixture of neutral cyclodextrins (CDs) was studied by capillary electrophoresis (CE). The following three dual selector systems were used: alpha-cyclodextrin (alpha-CD) and beta-cyclodextrin (beta-CD); alpha-CD and heptakis(di-O-methyl-beta-cyclodextrin) (DM-beta-CD); alpha-CD and heptakis(tri-O-methyl-beta-cyclodextrin) (TM-beta-CD). The S-(-) enantiomer of the analyte was more strongly retained in the presence of either alpha-CD or TM-beta-CD at pH 2.5, 100 mM phosphate buffer, while the R-(+) enantiomer was more strongly retained in the presence of either P-CD or DM-P-CD. In the more simple case, the elution order is invariably kept if the enantiomers have the same elution order in either one of the two hosts of the binary mixture. In contrast, the elution order may be switched by varying the concentration ratio of two hosts that produce opposite elution order for this particular analyte. In such a dual selector system, the enantioselectivity will disappear at the transient state at a certain ratio of host,:host, Moreover, the migration times of the two enantiomers with host, alone (diluted in buffer) is approximately equal to the migration times at the corresponding concentration of host, alone (diluted in buffer), where the ratio of concentrations of host,:host, is the same as in the binary mixture at the transient state. As found by nuclear magnetic resonance experiments, the analyte is forming a 1:1 complex with either one of the CDs applied. From this finding, a theoretical model based on the mobility difference of the two enantiomers was derived that was used to simulate the transient state. (C) 2000 Elsevier Science B.V. All rights reserved.

Crystallization Behaviors of n-Octadecane in Confined Space: Crossover of Rotator Phase from Transient to Metastable Induced by Surface Freezing

In this paper, the confined crystallization and phase transition behaviors of n-octadecane in microcapsules with a diameter of about 3 Pm were studied with the combination of differential scanning calorimetry (DSC), temperature dependent Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD).

Crystallization Kinetics of Syndiotactic Polypropylene Studied by Time-Dependent Light Attenuation

Crystallization kinetics of syndiotactic polypropylene ( sPP) was observed by light attenuation measurements. The initial stages of temperature dependent sPP crystallization fall in the range of Rayleigh scattering and Rayleigh-Debye-Gans scattering. Initial time and growth time of crystallization were obtained, and the trend of crystallization temperature dependent linear attenuation coefficient on the radius and the index of the refraction of the spherulite were evaluated.

Mass transfer kinetics of neodymium(III) extraction by calix[4]arene carboxylic acid using a constant interfacial area cell with laminar flow

BACKGROUND: Thermodynamics and kinetics data are both important to explain the extraction property. In order to develop a novel separation technology superior to current extraction systems, many promising extractants have been developed including calixarene carboxylic acids. The extraction thermodynamics behavior of calix[4]arene carboxylic acids has been reported extensively. In this study, the mass transfer kinetics of neodymium(III) and the interfacial behavior of calix[4]arene carboxylic acid were investigated.

Extraction kinetics of thorium(IV) with primary amine N1923 in sulfate media using a constant interfacial cell with laminar flow

Extraction kinetics of thorium(IV) with primary amine N1923 in sulfate media has been investigated by a constant interfacial cell with laminar flow. Studies of interfacial tension and effects of the stirring rate, temperature, and specific interfacial area on mass transfer rate show that the most probable reaction zone takes place at the liquid-liquid interface. According to the experimental data correlated as a function of the concentration of the relevant species involved in the extraction reaction, the rate equation of extracting thorium has been obtained as follows: -d[Th(IV)]((o))/dt = 10(-3.10)center dot[Th(IV)](0.89)center dot[(RNH3)(2)SO4](0.74).

A quantitative HPLC method for determining lactide content using hydrolytic kinetics

A new method for quantitative analysis of lactide has been developed by applying chemical kinetics to a HPLC system. The most important advance is its practical approach to the quantification of analytes that are unstable in the HPLC mobile phase. In HPLC analysis, anhydrous mobile phases cannot separate lactide from impurities, and only mixtures of water and organic solvent can achieve effective separation. By selecting conditions for testing and studying the kinetics of lactide hydrolysis, extensive experiments revealed that lactide degradation can be treated as a pseudo-first-order reaction under the given HPLC conditions, and lactide content or purity can be quantitatively determined. This method is practical for measuring the purity of the intermediate lactide in polylactic acid (PLA) production and the lactide content in PLA.

Kinetics and Statistical Distributions of Single-Molecule Conformational Dynamics

We developed a coarse-grained yet microscopic detailed model to study the statistical fluctuations of single-molecule protein conformational dynamics of adenylate kinase. We explored the underlying conformational energy landscape and found that the system has two basins of attractions, open and closed conformations connected by two separate pathways. The kinetics is found to be nonexponential, consistent with single-molecule conformational dynamics experiments. Furthermore, we found that the statistical distribution of the kinetic times for the conformational transition has a long power law tail, reflecting the exponential density of state of the underlying landscape. We also studied the joint distribution of the two pathways and found memory effects.

Switch-on transient behavior of vanadium phthalocyanine based organic transistors

The authors investigated the switch-on transient properties of p-type vanadium phthalocyanine (VOPc) transistors, which were fabricated by weak epitaxy growth on ordered para-sexiphenyl (p-6P) layer. The overshoot phenomenon of drain current had been observed in the VOPc/p-6P transistors, which was explained by the filling of carriers in traps of organic films. The small overshoot value of about 35% and transient duration time of 2 ms demonstrated the low trap concentration in organic films, which were comparable to the reported hydrogenated amorphous-silicon thin-film transistors. Therefore, the VOPc/p-6P transistors can be applied in active matrix liquid crystal display as switch elements.

Extraction kinetics of cerium(IV) from sulfuric acid medium by the primary amine N1923 using a constant interfacial area cell with laminar flow

BACKGROUND: Thermodynamic studies on Ce(IV) extraction with primary amine N1923 demonstrate that primary amine N1923 is an excellent extractant for separation of Ce(IV) from Re(III). In order to clarify the mechanism of extraction and to optimize the parameters in practical extraction systems used in the rare earth industry, the extraction kinetics was investigated using a constant interfacial area cell with laminar flow in the present work.RESULTS: The data indicate that the rate constant (k(ao).) becomes constant when stirring speed exceeds 250 rpm. The apparent forward extraction rate is calculated to be 10(-1.70). The activation energy (E.) was calculated to be 20.5 kJ/mol from the slope of log kao against 1000/T. The minimum bulk concentration of the extractant necessary to saturate the interface (C-min) is lower than 10(-5) mol L-1.CONCLUSION: Studies of interfacial tension and the effects of stirring rate and specific interfacial area on the extraction rate show that the extraction rate is kinetically controlled, and a mass transfer model has been proposed. The rate equation has been obtained as: -d[Ce(IV)]/dt = 10(-1.70)[Ce(IV)] [(RNH3)(2)SO4](1.376). The rate-controlling step has been evaluated from analysis of the experimental results.

Chain propagation kinetics on melt grafting reaction

Graft copolymerization in the molten state is of fundamental importance as a probe of chemical modification and reactive compatibilization. However, few grafting kinetic studies on reactive extrusion have been carried out because of the inherent difficulties, as expected. In this work, we have studied chain propagation kinetics on melt grafting using pre-irradiated linear low density polyethylene (LLDPE) and three monomers, acrylic acid (AA), methacrylic acid (MAA), and methyl methacrylate (MMA), as the model system. We measured the apparent chain propagation rate coefficients of grafting (k(p,g)) and homopolymerization (k(p,h)) at an initial stage for the melt grafting by FT-IR spectroscopy and electron spin resonance spectroscopy. It was observed that the convective mixing affected the rate coefficients. The magnitude of k(p,h) and k(p,g) were in the same order, but k(p,h) was slightly larger than k(p,g) The k(p,g) of the three grafting systems increased in the order: LLDPE/MMA < LLDPE/MAA < LLDPE/AA. These results are explained in terms of phase separation, solubility, and inherent reactivity of the monomer.