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Journal of Thermal Analysis and Calorimetry
Authors: M. Dantas, A. Almeida, Marta Conceição, V. Fernandes Jr, Iêda Santos, F. Silva, L. Soledade, and A. Souza

Abstract  

This work presents the characterization and the kinetic compensation effect of corn biodiesel obtained by the methanol and ethanol routes. The biodiesel was characterized by physico-chemical analyses, gas chromatography, nuclear magnetic resonance and thermal analysis. The physico-chemical properties indicated that the biodiesel samples meet the specifications of the Brazilian National Agency of Petroleum, Natural Gas and Biofuels (ANP) standards. The analyses by IR and 1H NMR spectroscopy indicated the ester formation. Gas chromatography indicated that biodiesel was obtained with an ester content above 97%. The kinetic parameters were determined with three different heating rates, and it was observed that both the methanol and ethanol biodiesel obeyed the kinetic compensation effect.

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On the basis of theoretical TG curves it has been shown that the kinetic compensation effect observed in thermal decomposition reactions is not due to the special form of the Arrhenius equation. Formally, the validity of a linear kinetic compensation law implies the existence of a characteristic temperature at which the rate constants of all reactions have the same value, but this temperature can be higher or lower than the temperatures at which the decomposition takes place.

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Abstract  

This paper presents the results obtained in the investigation of the reactions of potassium carbonate with some transition metals oxides (TiO2 , V2 O5 , Cr2 O3 , MnO2 , Fe2 O3 ). The reactions were carried out under non-isothermal conditions, and thermogravimetric analysis was used to monitor the transformation degree a. Experimental data indicated that the reaction of potassium carbonate and iron(III) oxide occurs in one stage, whereas the reactions of the oxides of titanium, vanadium, chromium and manganese are more complex, involving two-stage processes. Activation energies and pre-exponential factors were determined for all the processes taking place in the investigated systems. For the second stage of the reaction of K2 CO3 with Cr2 O3 , and V2 O5 the obtained values of activation energy were 59.2 and 512 kJ mol−1 respectively. Based on the values of activation energy and pre-exponential factor, the existence of a kinetic compensation effect was postulated for the three homologous series of reactions.

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The kinetic study of thermal degradation takes into account the validity of the Arrhenius equation. From TG data, the activation energy,E a and pre-exponential factor,A, are evaluated. These results are interpreted by using the ‘kinetic compensation effect’ as basis. A linear correlation between In(A) andE a is obtained in all cases studied. However, in a plot of the logarithm of the rate constant as a function of reciprocal temperature for the same series of reactions, the thermal oxidative degradations of Nylon-6 and PVC display a point of concurrence and one isokinetic temperature, whereas those of HIPS and PC do not. Therefore, in the thermal oxidative degradations of Nylon-6 and PVC a ‘true’ compensation effect occurs, which could be related to the bulk properties of metal oxides, such as different valence states, whereas for other polymers it displays only an ‘apparent’ compensation effect. This means that degradation is largely independent of the bulk properties of oxides, but may be related to the distribution of different kinds of active links in the polymer surface having different activation energies.

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Some specific factors which may cause the kinetic compensation effect (k.c.e.) during the decomposition of CaCO3 are identified. The role of the CO2 equilibrium pressure is examined in relation to the k.c.e. The article also shows why non-isothermal experiments must sometimes necessarily yield a value of activation energy different from the value obtained from isothermal experiments.

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A “true” kinetic compensation effect was established using the most appropriate kinetic functionF(α) for the non-isothermal decomposition of solids at various heating rates. It is likely that the correct kinetic mechanismF(α) is responsible for the “true” kinetic compensation effect, whereas an inappropriateF(α) would lead to “false” one.

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Two causes for the kinetic compensation effect (KCE) were recognized for a given solidstate reaction at various heating rates. One is due to any change in the range of reaction. This KCE is quantitative and meaningful, provided thatF(α) remains constant under the given conditions. The other is due to misestimation of the appropriate rate law, which in turn leads to a superficial KCE. It was also shown that the existence of an isokinetic point does not necessarily imply the occurrence of a meaningful KCE.

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The evaluation of kinetic parameters from chemical reaction curves implies datafitting procedures. Direct search methods are studied to minimize the Chi Square Function with respect to the activation energy and pre-exponential factor. The geometrical shape of the Chi Square Function can be related to the kinetic compensation effect as discussed in the literature. The minimization with respect to the pre-exponential factor can be solved analytically if the Chi Square Function includes integrated forms of the reaction mechanisms.

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The criticisms raised by J. Zsakó concerning an earlier paper by this author [J. Thermal Anal. 9 (1975) 101] are in turn examined. The lack of physical significance ofn, E, andZ for thermal decompositions is recognized by both authors. The disagreement lies in the value of using these parameters when the complete rate equation is not known.

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