Pyrolysis is a promising technology for chemical recycling
of waste
plastics, since it enables the generation of high-value chemicals
with low capital and operating cost. The calculation of thermodynamic
equilibrium composition using the Gibbs free energy minimization approach
can determine pyrolysis operating conditions that produce desired
products. However, the availability of thermochemical data can limit
the application of equilibrium calculations. While density functional
theory (DFT) calculations have been commonly used to produce accurate
thermochemical data (e.g., enthalpies of formation) of small molecules,
the accuracy and computational cost of these calculations are both
challenging to handle for large, flexible molecules, exhibiting multiple
conformations at elevated (i.e., pyrolysis) temperatures. In this
work, we develop a computational framework to calculate accurate,
temperature-dependent thermochemistry of large and flexible molecules
by combining force field based conformational search, DFT calculations,
thermochemical corrections, and Boltzmann statistics. Our framework
produces accurately calculated thermochemistry that is used to predict
equilibrium thermal decomposition profiles of octadecane, a model
compound of polyethylene. Our thermochemistry results are compared
against literature data demonstrating a great agreement, and the predicted
decomposition profiles rationalize a series of pyrolysis experimental
observations. Our work systematically addresses entropic contributions
of large molecules and suggests paths for accurate and yet computationally
feasible calculations of Gibbs free energies. The first-principles-based
thermodynamic equilibrium analysis proposed in this work can be a
significant step toward predicting temperature-dependent product distributions
from plastic pyrolysis and guide experimentation on chemical plastic
recycling.