Mass spectrometric study of the high temperature chemistry of benzene

Abstract

The high temperature pyrolysis of benzene was studied by monitoring the gas phase species flowing from a Knudsen reaction cell using modulated molecular beam mass spectrometry. The pyrolysis was studied at temperatures up to 1900°C and pressures to 5 Torr. At low pressures hydrogen and carbonaceous deposits (soot) on the reaction cell wall were the dominant products. The disppearance of benzene at the low pressure limit was found to have an apparent activation energy of 82 ± 5 kcal/mole. The rate of carbon formation was determined indirectly from the difference between carbon from benzene flow into the reactor and products leaving the reactor, and good agreement was noted for carbon recovered from the reactor for extended experiments at 1600°C. Carbon deposition is most efficient at low pressures; carbon deposition increases with pressure but the fraction of deposited carbon decreases as gas phase reactions become important. At higher pressures, where gas phase reactions dominate, C 2H 2, C 4H 2, C 6H 2, C 8H 6, C 12H 10, C 12H 8, H 2, and carbon were the major products. The results suggest that the pyrolysis of benzene primarily involves the reaction of an excited benzene molecule to form a C 12H 12 intermediate which decomposes to yield C 12H 10 and other products. Isotopic exchange in C 6H 6D 2 and C 6H 6C 6D 6 mixtures occurs at the same temperature as bimolecular reaction. At temperatures greater than 1100°C more direct processes involving C 6H 4 appear to contribute. The pyrolysis occurs primarily by molecular processes, possibly due to the effects of wall-related reactions.