Thermal decomposition of cyclohexane by flash pyrolysis vacuum ultraviolet photoionization time-of-flight mass spectrometry: a study on the initial unimolecular decomposition mechanism

• Published in: Physical chemistry chemical physics : PCCP Vol. 23; no. 16; pp. 984 - 9813
• Main Authors: Shao, Kuanliang, Liu, Xinghua, Jones, Paul J, Sun, Ge, Gomez, Mariah, Riser, Blake P, Zhang, Jingsong
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 28.04.2021
• Subjects: Benzene; Covalent bonds; Cyclohexane; Decomposition; Decomposition reactions; Ions; Mass spectrometry; Photoionization; Pyrolysis; Quantum chemistry; Scientific imaging; Spectroscopy; Thermal decomposition
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/d1cp00459j

Thermal decomposition of cyclohexane at temperatures up to 1310 K was performed using flash pyrolysis coupled with vacuum ultraviolet (118.2 nm) photoionization time-of-flight mass spectrometry. The experimental results revealed that the major initiation reaction of cyclohexane decomposition was C-C bond fission leading to the formation of 1,6-hexyl diradical. The 1,6-hexyl diradical could isomerize to 1-hexene and decompose into &z.rad;C 3 H 7 + &z.rad;C 3 H 5 and &z.rad;C 4 H 7 + &z.rad;C 2 H 5 . The 1,6-hexyl diradical could also undergo direct dissociation; the C 4 H 8 fragment via the 1,4-butyl diradical intermediate was observed, serving as evidence of the 1,6-hexyl diradical mechanism. Quantum chemistry calculations at UCCSD(T)/cc-pVDZ level of theory on the initial reaction pathways of cyclohexane were performed and found to be consistent with the experimental conclusions. Cyclohexyl radical was not observed as an initial intermediate in the pyrolysis. Benzene was produced from sequential H 2 eliminations of cyclohexane at high temperatures. Cyclohexane decomposes mainly via C-C bond rupture producing 1,6-hexyl diradical. The diradical leads to 1-hexene and can also directly dissociate. C 6 H 12 does not decompose to &z.rad;C 6 H 11 + H. Benzene can be formed via sequential H 2 eliminations of C 6 H 12 .