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

Abstract

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 °C and 1000 °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 CC bond homolysis of n-CnH2n+2 to produce primary alkyl radicals (CmH2m+1, m < n) and termination by βC-H homolysis and mutual combination of radicals. The chain is propagated by the βC-C homolysis and H-abstraction of primary alkyl radicals to produce secondary alkyl radicals (ChH2h+1), and then the βC-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 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-CmH2m+1 radical are gradually dominated.