Reactive Molecular Dynamics Insights into Hydrogen and Carbon Coproduction during Methane and Propane Pyrolysis

This study investigates the pyrolysis behavior of methane (CH₄) and propane (C₃H₈) under high-temperature and high-density conditions using reactive molecular dynamics (ReaxFF-MD) simulations, with particular emphasis on the coproduction of hydrogen (H₂) and carbon-based byproducts. The results show that C₃H₈ decomposes more rapidly than CH₄ under similar conditions, primarily because its weaker C-C bonds have a lower activation energy for bond cleavage. In both systems, H₂ is primarily produced via hydrogen abstraction reactions involving H radicals formed during the early stages of the process. Acetylene (C₂H₂) arises through the stepwise dehydrogenation of C2 species. H₂ production progressively increases with pyrolysis time in both systems, driven by entropy effects. Notably, CH₄ yields more H₂ in the initial phase due to the early abundance of H radicals, whereas C₃H₈ exhibits a slower initial H₂ yield. Similarly, C₂H₂ formation in the CH₄ system requires more reaction steps, while C₃H₈ rapidly forms C₂H₅ intermediates that facilitate faster C₂H₂ generation, resulting in faster carbon condensation during C₃H₈ pyrolysis. The formation of carbon clusters proceeds through three distinct stages: feedstock fragmentation, carbon chain growth, and carbon cluster aromatization/graphitization. The final stage is characterized by the elimination of hydrogen and the formation of six-membered aromatic rings. In addition, the study provides a detailed analysis of the carbon nucleation process, suggesting that the Polycyclic Aromatic Hydrocarbon (PAH) model is likely more applicable at low densities and temperatures. In contrast, the polyyne model tends to dominate under high-density and high-temperature conditions. Overall, this study offers atomic-level insights into the pyrolysis of light hydrocarbons, highlighting the utility of ReaxFF-MD simulations in unraveling complex, coupled gas-phase, and condensation kinetics.