Study on pyrolysis behavior of long-chain n-alkanes with photoionization molecular-beam mass spectrometer

摘要

To understand the pyrolysis behavior of long linear alkanes, six n-alkanes (n-CnH2n+2, n = 10, 12, 13, 14, 15, 16) were conducted by pyrolysis coupled with photoionization molecular-beam mass spectrometer at pressure below 10 Pa and temperature between 200 degrees C and 1000 degrees C. Lots of important alkyl and alkenyl radicals (CH3, C4H9, C8H15, C9H19, C3H5, C5H9, etc.), which are the direct evidence to understand the pyrolysis reaction pathway, were real-time detected by mass spectral detector. The results suggested that the predominant pyrolysis mechanism of n-alkanes is a free-radical chain mechanism, beginning with the C-C bond homolysis of n-CnH2n+2 to produce primary alkyl radicals (CmH2m+1, m n) and termination by beta C-H homolysis and mutual combination of radicals. The chain is propagated by the beta C-C homolysis and H-abstraction of primary alkyl radicals to produce secondary alkyl radicals (ChH2h+1), and then the beta C-C homolysis and H-abstraction of the secondary alkyl radicals to form alkyl/alkenyl radicals (ChH2h+1/CkH2k-1, h = n, k n-3) and alkenes (CkH2k/CkH2k-2) and the hydrogenshift to form free-radical isomers. During the process, H-abstrction, and hydrogen-shift reactions exist as a synergistic reaction. In addition, the position of free radical in alkyl isomers is closer to the central carbon at low temperature. With the increase of temperature, 2-CmH2m+1 radical are gradually dominated.