Experimental and theoretical insight into the soot tendencies of the methylcyclohexene isomers

• Published in: Proceedings of the Combustion Institute 2019-01, Vol.37 (1), p.1083-1090
• Main Authors: Kim, Seonah, Fioroni, Gina M., Park, Ji-Woong, Robichaud, David J., Das, Dhrubajyoti D., John, Peter C. St, Lu, Tianfeng, McEnally, Charles S., Pfefferle, Lisa D., Paton, Robert S., Foust, Thomas D., McCormick, Robert L.
• Format: Article
• Language: English
• Subjects: 09 BIOMASS FUELS; DFT; Flow reactor; Methylcyclohexene; Particulate matter; Soot
• ISSN: 1540-7489; 1873-2704
• DOI: 10.1016/j.proci.2018.06.095

The formation of soot precursors during combustion of three positional isomers of methylcyclohexene was investigated in flow reactor experiments and through density functional theory simulations. As evidenced by a recently published structure-property model, the sooting tendencies of these compounds differ from those of structurally similar molecules – suggesting new or unusual reaction chemistry. It was demonstrated that 1-methyl-1-cyclohexene and 4-methyl-1-cyclohexene preferentially react via a retro-Diels–Alder pathway leading to ring opening and molecular weight reduction. 3-methyl-1-cyclohexene, which exhibits much higher yield sooting index, preferentially reacts via dehydrogenation to cyclohexadienes and toluene – consistent with higher soot formation. It was demonstrated that the relative stability of the first radical intermediate plays a considerable role in determining the branching ratio between formation of soot precursors and ring opened retro-Diels–Alder reaction products. This study underscores the importance that small structural features can have in determining the ultimate fate of carbon during combustion processes.