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

Taichang Zhang, Lidong Zhang, Xin Hong, Kuiwen Zhang, Fei Qi, Chung King Law, Taohong Ye, Pinghui Zhao, Yiliang Chen

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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 (C16H10), containing many radicals (CH3, C3H3, C5H3, C5H5, C7H5, C7H7, C9H7, C11H7 and C13H9) and isomers, such as C3H4 (propyne and allene), C4H4 (vinylacetylene and 1,2,3-butatriene), C5H5 (cyclopentadienyl radical and pent-1-en-4-yn-3-yl radical), C6H4 (3-hexene-1,5-diyne and benzyne), C6H6 (benzene and fulvene), C7H8 (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 C7H8 → C7H7 and the subsequent reactions are dominant at comparatively low temperature (<1440 K), while the reaction C7H8 → C6H5 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 C2H2 is so high that many aromatics are formed through the hydrogen-abstraction/C2H2-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 (C5H5) was determined to be a major product of the decomposition of toluene.

Original languageEnglish (US)
Pages (from-to)2071-2083
Number of pages13
JournalCombustion and Flame
Volume156
Issue number11
DOIs
StatePublished - Nov 2009

All Science Journal Classification (ASJC) codes

  • Chemistry(all)
  • Chemical Engineering(all)
  • Fuel Technology
  • Energy Engineering and Power Technology
  • Physics and Astronomy(all)

Keywords

  • Molecular-beam mass spectrometry
  • PAHs
  • Quantum mechanical calculation
  • Toluene pyrolysis
  • Tunable synchrotron VUV photoionization

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