Reactive molecular dynamics insight into the thermal decomposition mechanism of 2,6-Bis(picrylamino)-3,5-dinitropyridine

Abstract

2,6-bis(picrylamino)-3,5-dinitropyridine (PYX) has excellent thermostability, which makes its thermal decomposition mechanism receive much attention. In this paper, the mechanism of PYX thermal decomposition was investigated thoroughly by the ReaxFF-lg force field combined with DFT-B3LYP(6–311++G) method. The detailed decomposition mechanism, small-molecule product evolution, and cluster evolution of PYX were mainly analyzed. In the initial stage of decomposition, the intramolecular hydrogen transfer reaction and the formation of dimerized clusters are earlier than the denitration reaction. With the progress of the reaction, one side of the bitter amino group is removed from the pyridine ring, and then the pyridine ring is cleaved. The final products produced in the thermal decomposition process are CO2, H2O, N2, and H2. Among them, H2O has the earliest generation time, and the reaction rate constant (k3) is the largest. Many clusters are formed during the decomposition of PYX, and the formation, aggregation, and decomposition of these clusters are strongly affected by temperature. At low temperatures (2500 K–2750 K), many clusters are formed. At high temperatures (2750 K–3250 K), the clusters aggregate to form larger clusters. At 3500 K, the large clusters decompose and become small. In the late stage of the reaction, H and N in the clusters escaped almost entirely, but more O was trapped in the clusters, which affected the auto-oxidation process of PYX. PYX’s initial decomposition activation energy (Ea) was calculated to be 126.58 kJ/mol. This work contributes to a theoretical understanding of PYX’s entire thermal decomposition process.