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

• Published in: Journal of analytical and applied pyrolysis 2021-10, Vol.159, p.105324, Article 105324
• Main Authors: Liu, Fanggang, Jin, Lijun, Ban, Yanpeng, Hu, Haoquan
• Format: Article
• Language: English
• Subjects: In-situ detection; Long-chain n-alkanes; Mechanism; Py-PI-MBMS; Pyrolysis
• ISSN: 0165-2370; 1873-250X
• DOI: 10.1016/j.jaap.2021.105324

Proposed reaction pathways of six n-alkanes’ pyrolysis process consist of three stages: Firstly is the CC bond homolysis of n-CnH2n+2 to produce primary alkyl radicals (CmH2m+1, m < n) in the thermal field. Then, the βC-C homolysis and H-abstraction of primary alkyl radicals form secondary alkyl radicals (ChH2h+1). Subsequently, partial secondary alkyl radicals will undergo further decomposition via βC-C homolysis and H-abstraction to form alkyl/alkenyl radicals (ChH2h+1/CkH2k-1, h ≤ n, k < n-3) and alkenes (CkH2k/CkH2k-2). Lastly, the chain reaction is terminated by mutual combination and βC-H scission of radicals. [Display omitted] •In-situ detection of alkyl and alkenyl radicals from n-alkanes pyrolysis via Py-PI-MBMS.•The position of formed free radicals in alkyls related to the pyrolysis temperature.•Detected stable products, alkyl and alkenyl radicals reveal the pyrolysis process.•A proposed radical chain reaction mechanism for cleavage of long-chain n-alkanes. 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.