Unimolecular decomposition of decalin and methyldecalin Subharaj Hossain 1,2 , Jagadeesh Gopalan 2 and Elangannan Arunan 1 * 1 Department of Inorganic and Physical Chemistry, Indian Institute of Science, India, 2 Department of Aerospace Engineering, Indian Institute of Science, India Decalin is an important component in the aviation/diesel fuel 1 . Trans-decalin has been selected as the surrogate component for investigation of transportation fuel 2 . Initial unimolecular decomposition reaction of cycloalkanes is a significant step in pyrolysis/combustion/oxidation of cycloalkanes. Initial reaction of alkylcyclohexane may proceed via the dissociation of C-H bond, elimination of side-chain alkyl group, or isomerization via ring-opening leading to alkene. Understanding of initial unimolecular reaction is important to develop complete pyrolysis/oxidation kinetic models of cycloalkane. Moreover, the competition between ring-opening isomerization and CH 3 elimination channel has a significant effect on aromatic formation. Several Studies on unimolecular reaction of cyclohexane are available in the literature and there is disagreement between these studies about the initial process (proposed initial process:- Tsang 3 : cC6H12=1-hexene, Aribike 4 et al.: cC 6 H 12 =3C 2 H 4 , cC 6 H 12 =2C 3 H 6 , cC 6 H 12 =C 4 H 6 +C 2 H 4 +H 2 ; Voisin 5 et al.: cC 6 H 12 =cC 6 H 11 +H). Finally, Kiefer et al. 6 theoretically calculated the reaction pathways for cyclohexane dissociation and confirmed that the initial decomposition of cyclohexane produces 1-hexene. Similar disagreement was observed for methylcyclohexane 7 as well. This discrepancy for the initial reaction of cyclohexane/methylcyclohexane motivate us to theoretically study the initial unimolecular decomposition of decalin and methyldecalin. Scheme1 shows all possible unimolecular reactions. Ring-opening isomerization of decalin/methyldecalin occurs in two steps.We used multi-reference calculation (CASSCF/MRCI) as these reactions proceeds through biradical intermediates. Figure1 shows the PES of C1-C2, C1-H bond dissociation of decalin, and C1-C2, C1-CH 3 bond dissociation of methyldecalin. Our preliminary result shows that for decalin, ring-opening isomerization channel is energetically favorable compared to the C-H dissociation channel and that can be the initial process. On the other hand, for methyldecalin, energy difference between CH 3 elimination channel and ring-opening isomerization channel is very small. We can see that the C-H dissociation and CH 3 elimination channel are barrier-less processes (without a saddle point) whereas ring-opening isomerization channel has a saddle point along the reaction coordinate. If we compare the ring- opening dissociation channel between decalin and methyldecalin, C-C bond(next to methyl group) dissociation energy decreases (~ 3 kcal/mol) due to the presence of CH 3 group in methyldecalin. We calculated the rate parameters for all the channels (using Transition state/variational transition state theory). The pressure-dependent rate constants for all the channels have been calculated using the RRKM/ME method. Results from these investigations will be presented in the meeting.
Scheme1 Decomposition and isomerization pathways of (a) decalin and (d) methyldecalin.
Figure1 PES of C1-H, C1-C2 bond of decalin and C1-CH3, C1-C2 bond of methyldecalin. Level of theory: CASSCF(2,2)/6-31+g(d,p).
References 1. Yu et al., Fuel , 2018, 212 , 41–48.
2. Mueller et al., Energy & Fuels , 2012, 26 , 3284–3303. 3. W. Tsang, Int. J. Chem. Kinet. , 1978, 10 , 1119–1138. 4. Aribike et al., Thermochim. Acta , 1981, 47 , 1–14. 5. Voisin et al., Combust. Sci. Technol. , 1998, 138 , 137–158. 6. Kiefer et al., J. Phys. Chem. A , 2009, 113 , 13570–13583. 7. Zhang et al., Energy & fuels , 2013, 27 , 1679–1687.
P11
© The Author(s), 2022
Made with FlippingBook Learn more on our blog