A ReaxFF Molecular Dynamics Study of the Pyrolysis Mechanism of Oleic-type Triglycerides

The reactive force field (ReaxFF) method is employed in the molecular dynamics (MD) simulation of oleic-type triglyceride (OTG) pyrolysis for the first time. The complex pyrolysis mechanism of OTG at high temperature, especially focusing on the multichannel pyrolysis pathways of OTG and radical-related evolution mechanisms of products, is intensively investigated at the atomistic level by performing a series of ReaxFF MD simulations. On the basis of simulation trajectory analysis, we find that the initiation decomposition of OTG pyrolysis is through C–O bond fission to release the straight oleic acid radical (C18H33O2•). The decomposition of C18H33O2• radical is mainly started through multichannel pathways: the decarboxylation reaction to form long-chain hydrocarbon radical (C17H33•) and CO2, and C–C bond cleavages at α, β-C position to form hydrocarbon radicals and ester radicals. C–C bond β-scissions and conjugation reactions play important roles in the subsequent decomposition of the C18H33O2• radical. ReaxFF MD simulations lead to reasonable decomposition pathways for OTG pyrolysis compared with experimental results and were further confirmed by calculating the standard reaction enthalpies based on density functional theory. The temperature effect on distributions of various products is also analyzed. C2H4 is the most abundant stable product. Certain amounts of CO and H2O are first discovered at high temperature. The product evolution tendencies with temperature are reasonable compared with the experimental observations. On the basis of similar evolution characters, the dominant products are categorized into three groups: the stable products, the reactive radical products, and the temperature-dependent products. In particular, detailed radical-related evolution behaviors of three representative products (C2H4, CH3• radical, and CO) are discussed systematically at the atomistic level. Besides, the activation energy and pre-exponential factor for the pyrolysis of oleic-type triglycerides extracted from the ReaxFF MD simulations are in good agreement with the experimental results. This work demonstrates that the ReaxFF method is a computationally feasible and reliable approach to elucidate the intricate pyrolysis mechanism of oleic-type triglycerides.