A combined theoretical and experimental study of the pyrolysis of pyrrolidine

Abstract

Pyrrolidine is a suitable model substance featuring a five-membered N-heterocycle representing the typical structure of the N-containing compounds in biomass. Previous studies have provided ambiguous arguments on the reaction mechanism of pyrrolidine thermal decomposition. Knowledge on the fate of the most dominant decomposition product, the unstable diradical ·CH2NHCH2·, is lacking. In this work, a high-level potential energy surface of the unimolecular reactions of ·CH2NHCH2·, including isomerization and decomposition channels, was explored. Then, the rate coefficients of various channels were obtained by the RRKM/master equation method over 500–2000 K and 0.001–100 atm. The results show that the thermal stabilization of cyc-C2H5N is highly favored over other isomerization and decomposition channels. The channels isomerizing to CH3NCH2, cis-HNCHCH3 and trans-HNCHCH3 compete with each other, and the rate constants are at least two orders of magnitude lower than the formation of cyc-C2H5N. Being thermodynamically unstable, cyc-C2H5N will mainly isomerize back into the diradical at temperatures ≤ 1200 K at 1 atm or isomerize to cis-HNCHCH3 when the temperature is higher. To validate the postulated reaction pathways, a pyrolysis experiment of pyrrolidine was conducted in a SiC reactor with a short residence time (40–60 μs) at 1050 K and 0.263 atm. The experimental result confirms the collisional stabilization of H2NCHCH2 and cyc-C2H5N + CH3NCH2. The diradical ·CH2NHCH2· was not readily detectable due to its low concentration, which falls below the detection limit of current analytical techniques, while the stabilization of cis-HNCHCH3 and trans-HNCHCH3 was not sure because of their extremely low photoionization cross section under the studied energy range. The rate constants of the isomerization and decomposition reactions of diradical ·CH2NHCH2· and cyc-C2H5N are provided, which are valuable for developing the mechanism for pyrrolidine and deepening our understanding of the mechanism of N-heterocyclic compounds pyrolysis/combustion.