High Temperature Pyrolysis of O-Terphenyl: Evidence for Kinetic Control in the Benzene Polymerization Pathway and Importance of Arene Aggregation/ Condensation Reactions in the Formation of Polycyclic Aromatic Hydrocarbons

Abstract

The pyrolysis of o-terphenyl was effected in a drop tube furnace at 5 temperatures between 1248–1352 K, a range over which a significant change in product composition occurs. the use of atmospheric pressure chemical ionization mass spectrometry with a heated nebulizer interface permitted product characterization up to a molecular mass of 900 u. Least-condensed benzene polymers dominate the product spectrum at the lower temperatures. A shift to more condensed PAH, many of which are total resonant sextet (TRS) compounds, is seen at the higher temperatures, in agreement with previously established theory. However, the low abundance of the TRS compounds, the absence of a rate-limiting critical species, and the high abundance of o-terphenyl dimers, together suggest that the product suite is not at equilibrium but is instead kinetically determined. the abundance of several ring-rupture products, such as benz[a]anthracene, cyclopent[hi]acephenanthrylene and chrysene is also indicative of a kineti-cally-controlled pathway to molecular weight growth where thermodynamically-favored but slow condensation reactions compete with parallel ring-rupture and rearrangement mechanisms. A pathway to non-TRS dibenzopyrene isomers that does not involve sequential small-chain aliphatic additions has also been observed, and the dominance of the benzene polymerization series relative to acetylene addition products provides more evidence that arene aggrega-tion/condensation pathways to soot growth are important and merit additional study.

Key Words:

• o-Terphenyl Pyrolysis Products
• Benzene Polymerization Pathway
• Arene Aggre-gation
• Formation of PAH