The pyrolysis of methylcyclopentadiene: Isomerization and formation of aromatics

Abstract

The pyrolysis of methylcyclopentadiene (MCP) was examined in two shock tubes with a variety of techniques. Laser-schlieren (LS) experiments were performed over 1000–2000 K, and 60–600 Torr, in 2% MCP/Kr and 4% MCP/Kr. Incident-shock-quench-tank GC-MS product analyses (using a new method) were obtained over 800–1900 K in 4.5% MCP/Ar, and time-of-flight (TOF) mass spectra were recorded in 2% MCP/Ne over 1250–1600 K and 145–225 Torr. The LS experiments showed an initial endothermic reaction from C-C fission and subsequent exothermic reaction from methyl-radical recombination. Rate constants for the dissociation extracted from early gradients formed a consistent set with an apparent E a of 44 kcal/mol. This highly endothermic dissociation (Δ H 298 o =75 kcal/mol) is thus well into falloff, but the expected pressure dependence was not observed. A routine Gorin-modle Rice-Ramsperger-Kassel-Marus (RRKM) calculation, modified for the fact that only the 5-isomer can fission, produced the magnitude and temperature dependence but predicted detectable pressure variation. TOF and quench-tank product analyses showed benzene, acetylene, naphthalene, and methane as major low-temperature products. The presence of benzene at the lowest temperatures, as well as its rapid formation in the TOF experiments, suggests that H-atom-catalyzed decomposition to benzene is efficient in this pyrolysis, and the naphthalene production supports the c -C 5 H 5 dimerization path, in accord with recent theory. GC-MS analysis of the quench-tank experiments provided the first measurements of the distribution of the three MCP isomers at elevated temperatures, showing that 5-MCP is a very small but increasing fraction with temperature whose enthalpy is about 1.2 kcal/mol above that of the other two isomers.