Under the supercritical pressure, the pyrolysis reactions of ethylbenzene and xylene were investigated by using the continuous flow device. At the pressure of 4 MPa and the different temperatures of 700, 750 and 780 ℃, the pyrolysis gaseous products were analyzed by online gas chromatography for ethylbenzene and xylene, while their liquid products were quantitatively analyzed by gas chromatography-mass spectrometry (GC-MS). It is found that the gaseous products of ethylbenzene decomposition are similar to those of xylene, while the liquid products of ethylbenzene and xylene decomposition are quite different, of which the major products are aromatic hydrocarbons. The experiment reveals that the higher the pyrolysis temperature is, the higher the conversion ratio will be for ethylbenzene and xylene. On the other hand, the conversion ratio of ethylbenzene is higher than that of xylene at the same temperature in experiment. From the experimental observation, we conclude that the pure aromatic hydrocarbon does not cause serious coking during the cracking process, owing to the low pyrolysis. Theoretical calculations are performed to obtain the bond energies for the different C—C or C—H bond types in side alkyl groups of ethylbenzene and xylene, by using density functional theory (DFT) method at the BHandHLYP/6-31+G(d, p) level. The bond energies calculated in this study agree well with those available from literature. It is found that the weakest bond is the C—C bond in the ethyl group of ethylbenzene, with a bond energy of 313.1 kJ/mol. This value is much smaller than the smallest bond energy of 393.2 kJ/mol of the C—H bond in the substituted methyl group in xylene. The weakness of the C—C bond in ethylbenzene thus leads to the fact that the pyrolysis of ethylbenzene is much easier than that of xylene. The theoretical results predict a higher conversion ratio of ethylbenzene than that of xylene. So the calculation gives a good explanation for the experimental phenomenon. Finally, the study in this work shows a new and important insight to the coking mechanism for the hydrocarbon fuels.

Key words: ethylbenzene, xylene, pyrolysis, supercritical, coke, bond energy