(57b) Tracking the Elementary Kinetics and Molecular Structures during Polyolefin Pyrolysis

Plastics have integrated into nearly all human activities since their invention 100 years ago, yet the recycling of plastics remains a difficult challenge. Herein, we examine the pyrolysis processes to chemically upcycle polyolefin plastics into monomers and other value-added chemicals. Polyolefin upcycling faces challenges that stem from the complex molecular composition of the feedstock, the large number of coupled chemical reactions and the dynamic changes that occur in the condensed phase chemical environment that forms, making it difficult to elucidate elementary kinetics and control the selectivity to products. These effects manifest in macroscopic phenomena such as differences in kinetics and changes in product distribution with changes in reaction temperature and feedstock composition. Understanding the molecular processes and developing a robust predictive kinetic model in polyolefin upcycling is crucial for the design and implementation of feasible plastic upcycling facilities.

Herein we present the development and application of an ab initio-based kinetic Monte Carlo and Molecular Dynamics (kMC+MD) simulation approach that can model the kinetics and product distribution of polyolefin pyrolysis. This approach tracks detailed atomic information including 3D coordinates of atoms and connectivity of chemical bonds in the feedstock molecules. It uses Stochastic Simulation Algorithm (SSA) to perform elementary reaction steps and uses classical MD simulations to follow the dynamics of the feedstock and the reaction environment as reactions proceed. The elementary step kinetics for the KMC simulations were established from density functional theory calculations.

The atomic-structural information is retained throughout the simulation, allowing us to follow molecular transformations and the local molecular compositions during the reaction. As such, the simulations capture the unique kinetic manifestations that would otherwise be lost in coarse-grained deterministic models. This KMC+MD simulation approach is used herein to examine the thermal pyrolysis reactions of polyethylene and polypropylene with some success in predicting kinetics and product distribution.