Study of the Formation of the First Aromatic Rings in the Pyrolysis of Cyclopentene

The
thermal decomposition of cyclopentene was studied in a jet-stirred
reactor operated at constant pressure and temperature to provide new
experimental information about the formation of the first aromatic
rings from cyclic C5 species. Experiments were carried
out at a residence time of 1 s, a pressure of 106.7 kPa, temperatures
ranging from 773 to 1073 K and under diluted conditions (cyclopentene
inlet mole fraction of 0.04). Species were quantified using three
analytical methods: gas chromatography, synchrotron vacuum ultraviolet
photoionization mass spectrometry (SVUV-PIMS), and single photon laser
ionization mass spectrometry (SPI-MS). Several species could be quantified
using both methods allowing comparison of experimental data obtained
with the three apparatuses. Discrepancies observed in mole fraction
profiles of some large aromatics suggest that the direct sampling
in the gas phase (with a molecular beam or a capillary tube) provide
more reliable results. The main reaction products are 1,3-cyclopentadiene
and hydrogen. The formation of many unsaturated C2–C6 olefins, diolefins and alkynes was also observed but in smaller
amounts. Benzene, toluene, styrene, indene, and naphthalene were detected
from 923 K. SVUV-PIMS data allowed the identification of another C6H6 isomer which is 1,5-hexadien-3-yne rather than
fulvene. The quantification of the cyclopentadienyl radical was obtained
from SVUV-PIMS and SPI-MS data with some uncertainty induced by the
possible contribution to the signal for m/z 65 of a fragment from the decomposition of a larger ion.
This is the first time that a radical is quantified in a jet-stirred
reactor using non-optical techniques. SPI-MS analyses allowed the
detection of species likely being combination products of allyl and
cyclopentadienyl radicals. A model was developed for the pyrolysis
of cyclopentene. This model includes routes of formation of aromatics
from the cyclopentadienyl radical. The comparison of experimental
and computed data is overall satisfactory for primary reaction products
whereas discrepancies are still observed for aromatics.