An experimental and theoretical study of toluene pyrolysis with tunable synchrotron VUV photoionization and molecular-beam mass spectrometry

Abstract

An experimental study of toluene pyrolysis (1.24 vol.% toluene in argon) was performed at low pressure (1.33 kPa) in the temperature range of 1200–1800 K. The pyrolysis process was detected with the tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry (MBMS). Species up to m/ z = 202 (C 16H 10), containing many radicals (CH 3, C 3H 3, C 5H 3, C 5H 5, C 7H 5, C 7H 7, C 9H 7, C 11H 7 and C 13H 9) and isomers, such as C 3H 4 (propyne and allene), C 4H 4 (vinylacetylene and 1,2,3-butatriene), C 5H 5 (cyclopentadienyl radical and pent-1-en-4-yn-3-yl radical), C 6H 4 (3-hexene-1,5-diyne and benzyne), C 6H 6 (benzene and fulvene), C 7H 8 (toluene and 5-methylene-1,3-cyclohexadiene) and so on, were identified from near-threshold measurements of photoionization mass spectra, and the mole fraction profiles of the pyrolysis products were evaluated from measurements of temperature scan. Experimental results indicate that the reaction C 7H 8 → C 7H 7 and the subsequent reactions are dominant at comparatively low temperature (<1440 K), while the reaction C 7H 8 → C 6H 5 and subsequent reactions gradually become competitive and important with increasing temperature. Furthermore the barriers of the decomposition pathways of toluene and benzyl radical determined by quantum mechanical calculation are in good agreement with the initial formation temperatures of the species. Based on the mole fractions and formation temperatures of the detected pyrolysis species, a simple reaction network is deduced. At relatively high temperatures, H-abstraction is prevalent and the mole fraction of C 2H 2 is so high that many aromatics are formed through the hydrogen-abstraction/C 2H 2-addition (HACA) mechanism. Moreover the reactions of benzyl with toluene/benzyl/phenyl/propargyl radicals to directly produce larger aromatics should play an influential role in PAH formation. Meanwhile the five-member-ring recombination mechanism also plays an indispensable role in the aromatics growth, as cyclopentadienyl radical (C 5H 5) was determined to be a major product of the decomposition of toluene.