Nickel oxide reduction under nonisothermal conditions

The interaction of nickel(II) oxide with carbon, polyethylene, and polystyrene in the temperature range 20–1000°C has been investigated by thermal analysis. The interaction of NiO with carbon and polyethylene under these conditions results in complete nickel reduction. In the case of polystyrene, because of its low decomposition temperature, the reduction process is incomplete. As compared to reduction with polyethylene, reduction with carbon takes place at a much higher temperature (750 versus 370°C) and requires a higher activation energy (68 ± 3 versus 41 ± 1 kcal/mol).     

Stress birefringence in amorphous polymers under nonisothermal conditions

The relationship of birefringence to stress in an amorphous polymer was studied, with emphasis on conditions of high stress and rapid cooling. The latter (nonisothermal) conditions are important in connection with studies of polymer processing operations. Polystyrene was pulled at a constant elongation rate (0.075 to 2 sec^?1 in the present and related work) under both isothermal conditions (in the range 120 to 157°C) and nonisothermal conditions (with cooling rates in the range 0.6 to 1.7°C/sec). Generally we conclude that stress in proportional to birefringence under a wide range of conditions, except that a nonlinear regime appears at stresses higher than about 10⁷ dyn/cm². In this regime, stress increases more rapidly with deformation than does birefringence.     

Kinetics of nonisothermal polymer crystallization

We adopt a cluster size distribution model to investigate the kinetics of nonisothermal polymer crystallization. The time dependencies of polymer concentration, number and size of crystals, and crystallinity (in Avrami plots) are presented for different cooling rates. The incubation period is also investigated at different cooling rates and initial temperatures. The relationship between cooling rates and incubation time is presented graphically and compared with experimental measurements. The initial temperature (relative to melting point) has a significant effect on nonisothermal crystallization. A comparison of moment and numerical solutions of the population balance equations shows the influence of Ostwald ripening. Agreement between modeling results and experimental measurements at different cooling rates supports the application of the distribution kinetics model for nonisothermal crystallization.     

In situ chemical and morphological characterization of copper under near ambient reduction and oxidation conditions

By using spatially resolved photoelectron spectromicroscopy, we have investigated the formation of Cu(I) oxide phases when a bulk Cu foil is exposed to a near ambient pressure of molecular oxygen. The experiment was performed by using a novel cell for in operando measurements capable to overcome the so called ‘pressure gap’. We have found that the oxidation of Cu proceeds through the formation of Cu(I) oxide domains in the micron and submicron range size, the evolution of which affects the overall morphology of the Cu surface as well. Chemical and morphological changes have been simultaneously detected. It was, surprisingly, found that during the oxidation process in many areas domains having +1 valence state do not spread on the metallic regions which homogenously oxidize; no chemical oxidizing wave was detected by the available lateral resolution and time frame.     

Direct reduction of copper oxide into copper under hydrothermal conditions

A new preparation method of Cu from CuO under hydrothermal conditions was investigated. Glucose was employed as reducing agent. Results showed that CuO can be reduced easily to Cu° with glucose as reductant at 220?C250 °C with NaOH. The reaction conditions such as reaction time, reaction temperature, sodium hydroxide concentration and water filling played key roles in the purity of the products. The proposed method provides an efficient and green conversion of CuO into Cu° without an expensive and toxic reducing agent at low temperatures.     

Effect of mechanical pretreatment on thermal decomposition of silver oxalate under nonisothermal conditions

The effect of mechanical pretreatment on the thermal decomposition of silver oxalate was investigated using mass-spectrometric thermoanalysis. Starting at different levels of predecomposition, kinetic data were obtained by analysis of the kinetics at the initial rise of decomposition. The observed mechanically induced increase in reactivity of the silver oxalate used is due to three effects: (i) A thermally more stable “phase” at the surface of the grains is rendered ineffective by the creation of new surfaces. (ii) Disintegration processes increase the specific surface by several orders of magnitude. (iii) The number of potential nuclei in the surface is considerably increased by the generation of lattice defects.     

Generalized composite degradation kinetics for polymeric systems under isothermal and nonisothermal conditions

A composite degradation methodology is extended to the conversion-dependence function in order to explain the importance of multiple reaction mechanisms which might be considered to be involved in degradation processes. Based on two elementary reaction mechanisms, a specific form of the model equation is derived, which is capable of describing various types of degradation behavior showing sigmoidal rate as well as deceleratory rate. The conversion-dependence function is derived to be independent of the Arrhenius-type reaction constant or temperature, and thus the kinetic parameters are determined by analytic methods that have been developed for isothermal and dynamic-heating experiments without any modification or additional assumptions. The developed model equation is tested by predicting the isothermal master curve of polyether-ether-ketone (PEEK), which is used as a model system in this study. The activation energies of the model system are analyzed using comparable methods for isothermal and dynamic experiments, which compare favorably in terms of the activation energy as a function of conversion. The resulting model equation, based on the kinetic parameters determined by isothermal experiments, can accurately predict both isothermal and dynamic-heating thermogravimetry utilizing the same constants and identical reaction mechanisms without additional assumption.     

Thermal characterization of polyester–melamine coating matrices prepared under nonisothermal conditions

Thermal analysis methods (differential scanning calorimetry, thermogravimetric analysis, and dynamic mechanical thermal analysis) were used to characterize the nature of polyester–melamine coating matrices prepared under nonisothermal, high‐temperature, rapid‐cure conditions. The results were interpreted in terms of the formation of two interpenetrating networks with different glass‐transition temperatures (a cocondensed polyester–melamine network and a self‐condensed melamine–melamine network), a phenomenon not generally seen in chemically similar, isothermally cured matrices. The self‐condensed network manifested at high melamine levels, but the relative concentrations of the two networks were critically dependent on the cure conditions. The optimal cure (defined in terms of the attainment of a peak metal temperature) was achieved at different oven temperatures and different oven dwell times, and so the actual energy absorbed varied over a wide range. Careful control of the energy absorption, by the selection of appropriate cure conditions, controlled the relative concentrations of the two networks and, therefore, the flexibility and hardness of the resultant coatings. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1603–1621, 2003     

Distribution of melamine in polyester–melamine surface coatings cured under nonisothermal conditions

The influence of experimental cure parameters on the diffusion of reactive species in polyester–melamine thermoset coatings during curing has been investigated with X‐ray photoelectron spectroscopy and attenuated total reflectance Fourier transform infrared. The diffusion of melamine plays a vital role in the curing process and, therefore, in the ultimate properties of coatings. At a low (<20%) hexamethoxymethylmelamine (HMMM) crosslinker concentration, the matrix composition is uniform, but at high HMMM concentrations, excess HMMM rapidly segregates to the air–coating interface. The rate of migration is governed by the difference in the surface free energies of polyester and HMMM and the concentration gradient of HMMM between the bulk and the surface. An increased rate of energy absorption also increases the rate of migration of HMMM to the surface. A physical model has been proposed to explain this surface segregation phenomenon in terms of cocondensation and self‐condensation reactions. It suggests that an appropriate amount of melamine can be segregated on the surface and allowed to self‐condense to form a desired thickness of a melamine topcoat through the control of the binder composition and cure conditions. This technique can be implemented to apply a melamine topcoat during cure. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 83–91, 2004     

Kinetics of nonisothermal calcination of ZnCO3 in air

Thermal curves obtained simultaneously at constant heating rates were used to study calcination of ZnCO₃ in air. Carroll and Manche's technique, Reich's equation, and Kissinger's method gave an average value for the activation energy of 23 kcal/mol. On the other hand, Coats and Redfern's technique showed the activation energy and preexponential factor to vary over wide ranges. They increase to a maximum and decrease again on increasing the temperature. Their values decrease as the heating rate was increased. One-dimensional diffusion mechanism, obeying parabolic law, may control the entire calcination process.     

Reaction of trifluoromethanesulfonamide with heterodienes under oxidation conditions

Reactions of trifluoromethanesulfonamide with divinyl sulfone, divinyl sulfoxide, divinyl sulfide, diphenyl sulfide, vinyl allyl and diallyl ethers was investigated in the presence of oxidation system t-BuOCl + NaI. The reaction with divinyl sulfone afforded a product of 1,5-heterocyclization, 2,6-diiodo-4-[(trifluoromethyl) sulfonyl]thiomorpholine 1,1-dioxide. The same product was obtained in the reaction with divinyl sulfoxide apparently due to its preliminary oxidation to sulfone. In reactions with divinyl sulfide and unsaturated ethers only the oxidation of substrates was observed accompanied with strong tarring; the products of a reaction with trifluoromethanesulfonamide were absent. With diphenyl sulfide a product was formed resulting from the oxidation at the sulfur atom [S(II) → S(IV)], N-(diphenyl-λ⁴-sulfanylidene)trifluoromethanesulfonamide.     

Selective oxidation of light alkanes under mild conditions

Methanotrophs mediate the conversion of methane (CH₄) into methanol selectively and efficiently near ambient conditions so we can learn from microbes to develop biomimetic catalysts capable of performing this difficult chemistry. This review highlights the development of a tricopper cluster catalyst that functions similar to the particulate methane monooxygenase enzyme in methanotrophic bacteria. The performance of this catalytic system formulated for quasi-heterogeneous catalysis is compared with other heterogeneous catalysts derived from Cu- and Fe-based zeolites and Cu mordenites known to activate CH₄ stoichiometrically near 200 °C. We also highlight a unique catalytic system, in which the oxidizing power of both O atoms of the O₂ molecule can be harnessed for oxidation of toluene to yield benzaldehyde at room temperature.     

Fast selective oxidation of alcohols under solvent-free conditions

A combination of acetic anhydride, H₂SO₄-nano silica, wet-SiO₂ (60 %), and K₂Cr₂O₇ as a new oxidizing system for the selective oxidation of different types of alcohols to the corresponding aldehydes and ketones at room temperature under solvent-free conditions is introduced. Mild reaction conditions, high yields of the products, short reaction time, no further oxidation to the corresponding carboxylic acid, and easy work-up make this new system a useful method for oxidizing alcohols.     

Aerobic oxidation of primary alcohols under mild aqueous conditions promoted by a dinuclear copper(II) complex

A sugar-discriminating dinuclear copper(II) complex was investigated for its ability to promote aerobic oxidation of primary benzylic alcohols in the presence of TEMPO and base. The transformation of benzyl alcohol to benzaldehyde was chosen as exploratory model reaction. The constitution of the catalytically active species was deducted from isothermal titration calorimetry and kinetic experiments, and the catalytic reaction was characterized both in aqueous organic and aqueous solution. The dinuclear complex is found to selectively oxidize primary over secondary alcohols in aqueous solution at ambient temperature with a turnover rate of 9 h⁻¹. A mechanism for the catalytic cycle is proposed.     

Kinetics of the oxidation of iodide by copper(III)-imine-oxime complexes

Summary The copper(III)-imine-oxime complexes [Cu^III(Enio)]⁺ and [Cu^III(Pre)]⁺ {EnioH₂ =N,N-ethylene bis(isonitrosoacetylacetoneimine) and PreH₂ = N,N-propylene bis (isonitrosoacetylacetoneimine)} react very rapidly with iodide. The rate law under fixed conditions for the reaction is given by the equation: –d[Cu^III]/dt = (2k₂[I^–] + 2k₃[I^–]²)[Cu^III] The [Cu^III(Enio)]⁺ reaction was pH-independent whereas the [Cu (Pre)]⁺ reaction rate increased with increasing pH. Both the k₂ and the k₃ pathways are believed to involve one-electron transfer. An inner-sphere mechanism may operate in the pathway, first-order in [I^–].     

Kinetics and mechanism of oxidation of copper(I) ions with thiuram disulfide

The kinetics of the oxidation of copper(I) perchlorate with thiuram disulfide (tds) in acetonitrile and its mixtures with benzene or ethyl bromide have been investigated. It has been shown that the rate of the reaction is limited by the inner-sphere transfer of an electron from the copper(I) ion to tds in a complex of the type [Cu^I(tds)]⁺. The subsequent dissociation of the anionradical tds —proceeds rapidly and leads to the formation of copper(II) dithiocarbamate which is recorded in the absorption spectra and by EPR. The rate constant of electron transfer has been determined (3·10^–4sec^–1) and it was found that it is practically independent of the polarity of the medium. The absence of such a correlation is attributed to the negligible energies of reorganization of the solvent in the inner-sphere electron transfer.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 4, pp. 450–455, July–August, 1988.