Pyrolysis of Alkanes: A Computational Approach.

Abstract

In this study, we investigate the kinetic and thermodynamic aspects of thermal cracking reactions of short paraffin chains by density functional theory (DFT) methods. The thermal cracking reactions have been modeled for a series of shorter unbranched alkanes at 673 K by following a free-radical mechanism. Benchmark calculations have been carried out with different functionals (B3LYP, M06-2X, PBE0, BMK, B3PW91) and basis sets (6-31G(d,p), 6-311+G(d,p)) to determine the most suitable DFT method, and the results were compared to the available experimental data. Computations were also performed at the CBS-QB3 level to evaluate the accuracy of the DFT method. The thermodynamic and kinetic properties of the initiation, hydrogen atom transfer (HAT), and decomposition (β-scission) reactions are intensely discussed to better understand the trends in product distributions at high temperatures. Evans-Polanyi (EP) relations have been used to build a linear relationship between the enthalpy of reactions and their activation energies; this process may be useful for the determination of the kinetic parameters of longer paraffin chains as well. Finally, the preexponential factors of short-chain paraffin have been calculated and classified based on the identity of the radicalic products. The latter, together with the activation energies derived from the EP relations can be used safely for the prediction of the rate constants for long paraffin chains.