Pyrolysis of Alkanes: A Computational Approach

Area of Science:

• Computational Chemistry
• Chemical Kinetics

Background:

• Thermal cracking of paraffins is crucial for hydrocarbon processing.
• Understanding reaction mechanisms at high temperatures is essential for optimizing yields and product selectivity.

Purpose of the Study:

• To investigate the kinetic and thermodynamic aspects of thermal cracking in short unbranched alkanes using Density Functional Theory (DFT).
• To determine the most suitable DFT method for modeling these reactions by comparing with experimental data and CBS-QB3 calculations.
• To establish relationships between reaction enthalpy and activation energy for predicting kinetic parameters of longer paraffin chains.

Main Methods:

• Modeling thermal cracking reactions of short alkanes at 673 K via a free-radical mechanism.
• Employing various DFT functionals (B3LYP, M06-2X, PBE0, BMK, B3PW91) and basis sets (6-31G(d,p), 6-311+G(d,p)).
• Utilizing Evans-Polanyi (EP) relations to correlate reaction enthalpies and activation energies.

Main Results:

• Detailed thermodynamic and kinetic properties of initiation, hydrogen atom transfer (HAT), and β-scission reactions were analyzed.
• A linear relationship was established between reaction enthalpies and activation energies using EP relations.
• Pre-exponential factors for short-chain paraffins were calculated and categorized by radicalic products.

Conclusions:

• DFT methods accurately model thermal cracking of short paraffins, providing insights into high-temperature reaction pathways.
• The established Evans-Polanyi relations offer a predictive tool for kinetic parameters of longer paraffin chains.
• The findings facilitate the prediction of rate constants for long paraffin chains, aiding in process optimization.