Study on pyrolysis behavior of long-chain n-alkanes with photoionization molecular-beam mass spectrometer
To understand the pyrolysis behavior of long linear alkanes, six n-alkanes (n-C n H 2n+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 °C and 1000 °C. Lots of important alkyl and alkenyl radicals (CH 3 , C 4 H 9 , C 8 H 15 , C 9 H 19 , C 3 H 5 , C 5 H 9 , 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-C n H 2n+2 to produce primary alkyl radicals (C m H 2m+1 , m < n) and termination by β C-H homolysis and mutual combination of radicals. The chain is propagated by the β C-C homolysis and H-abstraction of primary alkyl radicals to produce secondary alkyl radicals (C h H 2h+1 ), and then the β C-C homolysis and H-abstraction of the secondary alkyl radicals to form alkyl/alkenyl radicals (C h H 2h+1 /C k H 2k-1 , h ≤ n, k < n-3) and alkenes (C k H 2k /C k H 2k-2 ) and the hydrogen-shift 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-C m H 2m+1 radical are gradually dominated.
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