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.