Kinetic analysis of complex chemical reactions by coupling model-free and model-fitting analysis

Three different complex multi-step reaction mechanisms of parallel-competing, consecutive and parallel -independent reactions were simulated for two different sets of kinetic parameters. The two sets were chosen in such a way to represent two very different situations. An advanced non-linear isoconversional method (NLN) and a multiple heating rates model-fitting method using nonlinear optimization (NLO) were applied to the data. The errors in kinetic parameters were evaluated for each set of data and for each method. It is shown that analysis of the thermoanalytical curves and of the E alpha-dependencies can be used in complementary with the model-fitting approach for the elucidation of the reaction mechanisms and estimation of the kinetic parameters of each step. In most cases, NLO improved the accuracy of the kinetic parameters obtained from NLN method, but only if the mechanism was known. However, selecting the right mechanism is an essential problem of NLO. The study showed that it was possible to accurately fit the mechanism of two consecutive steps with a model of two independent steps that resulted in evaluating invalid kinetic parameters. This example also highlights an important limitation of the so-called deconvolution kinetic analysis which consists in fitting of various multi-step reaction mechanisms with a model of independent steps.

Automated summary

This study simulated three different complex multi-step reaction mechanisms, including parallel-competing, consecutive, and parallel-independent reactions, using two sets of different kinetic parameters. The analysis of thermoanalytical curves and E alpha-dependencies, combined with model-fitting approach, can elucidate reaction mechanisms and estimate kinetic parameters for each step.