An experimental and kinetic modeling study of cyclohexane pyrolysis at low pressure

The pyrolysis of cyclohexane at low pressure (40 mbar) was studied in a plug flow reactor from 950 to 1520 K by synchrotron VUV photoionization mass spectrometry. More than 30 species were identified by measurement of photoionization efficiency (PIE) spectra, including some radicals like methyl, propargyl, allyl and cyclopentadienyl radicals, and stable products (e.g., 1-hexene, benzene and some aromatics). Among all the products, 1-hexene is formed at the lowest temperature, indicating that the isomerization of cyclohexane to 1-hexene is the dominant initial decomposition channel under the condition of our experiment. We built a kinetic model including 148 species and 557 reactions to simulate the experimental results. The model satisfactorily reproduced the mole fraction profiles of most pyrolysis products. The rate of production (ROP) analysis at 1360 and 1520 K shows that cyclohexane is consumed mainly through two reaction sequences: cyclohexane → 1-hexene → allyl radical + n-propyl radical, and cyclohexane → cyclohexyl radical → hex-5-en-1-yl radical that further decomposes to 1,3-butadiene via hex-1-en-3-yl and but-3-en-1-yl radicals. Besides the stepwise dehydrogenation of cyclohexane, C3 + C3 channels, i.e. C3H3 + C3H3 and C3H3 + aC3H5 also have important contribution to benzene formation. The simulation reveals that C3H3 + C3H3 = phenyl + H reaction is the key step for other aromatics formation, i.e. toluene, phenylacetylene, styrene, ethylbenzene and indene in this work.