Kinetic Modeling of the Free-Radical Process during the Initiated Thermal Cracking of Normal Alkanes with 1-Nitropropane as an Initiator.

To elucidate the accelerating effect of 1-nitropropane on the thermal cracking of normal alkanes, ab initio and density functional theory calculations were performed to investigate the elementary reactions involved in the initiated thermal cracking of a 1-nitropropane/n-heptane mixture. The kinetic parameters were evaluated on the basis of standard transition state theory (TST) or variational transition state theory (VTST) with Wigner tunneling correction. The activation energy for the C–N bond rupture of 1-nitropropane to produce primary n-propyl and nitro radicals is calculated to be 234–269 kJ/mol, while the bond dissociation energy of the C–C bond within the n-heptane molecule is predicted to be at least 335 kJ/mol. These calculated results demonstrate that the presence of 1-nitropropane makes the free radical formation become relatively easier compared with single n-heptane cracking. Furthermore, compared with the C–H bond cleavage and 1,3-H intramolecular transfer, the unstable n-propyl radical mainly follows the C–C bond cleavage pathway to produce ethylene and secondary ĊH3radical. After formation of these free radicals, the H-abstraction of n-heptane with radicals occurs readily with considerably lower activation energy than the radical formation step to initiate the chain reaction. The analysis result indicates that the thermal cracking of n-heptane is accelerated mainly due to the change of the initial step from the C–C bond cleavage of n-heptane to the C–N bond rupture of 1-nitropropane. [ABSTRACT FROM AUTHOR]