Fast determination of reaction kinetic parameters with the use of microwave heating. Kinetics of decomposition of organic substances with nitric acid

Abstract

A mathematical model for the kinetics of a chemical reaction under microwave irradiation in a close volume in the simplest approximation of the uniform temperature and pressure distribution in a system was proposed. The previously proposed dynamic method for determi nation of the activation energy of oxidation of organic substances with nitric acid was substan tiated. According to this method, the activation energy is determined from the experimental exponential plot for the pressure in the nonisothermic system vs. inverse temperature of the reaction mixture.An increase in the temperature of reaction mixtures seems to be the most popular way for acceleration of chemical reactions. One of the methods for heating (al ternative to the standard thermal method) is the absorp tion of electromagnetic microwave energy by a substance. The distinctive features of heating by microwave (MW) irradiation is its dependence on the dielectric characteris tics of components of a reaction mixture and the high time and volume gradients created in the system at the microlevel and providing fast and uniform heating of a solution. 1,2 Effects related to the orienting action of the electromagnetic field can also appear. 3 We have previously 4 proposed an approach to study microwave assisted chemical processes. The approach in cludes a comparison of reaction rates under conditions of thermal and microwave heating, analysis of the composi tion and yield of formed compounds, and calculation of the activation energy. The use of this approach for study ing hydrolysis of organic and inorganic compounds, com plexation in solutions and heterogeneous systems, and oxidation of organic compounds with acids showed that the MW irradiation does not change the route of reac tions, although considerably accelerates them and makes it possible to control purposefully the yield of reaction products. In the framework of this approach, we pro posed 5 the dynamic method to determine the activation energy from current parameters of the nonisothermic pro cess using the Arrhenius equation. An advantage of the dynamic method is that its use requires no particular isothermic kinetic experiments at different temperatures and the experiment itself takes much shorter time. The method of determination of the activation energy at con tinuously changing temperature was applied to the study of the kinetics of different processes, including those in enzymatic catalysis, 6 and to the mechanism of atomiza tion in electrothermal atomic absorption spectrometry. 7 We used 5,8 the method based on direct measurements of the pressure and temperature in a reaction system to study oxidation of organic substances with nitric acid. The oxi dation processes were demonstrated to be identical under thermal and microwave heating conditions. It was shown that the conditions and results of oxidation of organic substances with mineral acids can be simulated and pre dicted. However, an applicability of this method was sub stantiated only at a qualitative level. 5,8 This method should be jus...

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