(25b) Thermal Decomposition of Decalin: An Ab Initio Study

Decahydronaphthalene, also known as decalin (C10H18), is a bicyclic organic compound which is the saturated analog of naphthalene and occurs in cis- and trans- forms. Decalin has been employed as reference component of the multi-ring naphthene class for JP-8 surrogates. Previous researches that have different decalin composition showed good agreement with experimental results on sooting tendency of pool fire and matching the auto-ignition and combustion behavior hydrocarbon fuel. Moreover, it has the high thermal stability and shows attractive feature as potential hydrogen donor to prevent the formation of various harmful thermal deposits during combustion process. For these reasons, great interest is devoted to the use of decalin for surrogate jet fuels, as well as advanced fuel additive to suppress fuel deposits. However, the kinetic modelings of naphthene groups are less defined because of their complex reaction mechanisms and large number of possible reactions. Prompted by this need, we report on the breakdown mechanism for decalin, representative of the naphthene class. Experimental results previously reported on the thermal cracking of decalin in a steam pyrolysis environment measured the relative abundance of five aromatic species. Drawing on these experimental evidences together with our density functional theory (DFT) calculations, we identified a series of new reaction pathways leading to the formation of aromatic products, such as toluene, benzene, styrene, ethylbenzene and xylene. Using the calculated potential energy surfaces (PES) of the identified pathways, high pressure limit and atmospheric first order rate constants for unimolecular reactions were calculated using the RRKM and Master Equation (ME) method respectively. For bimolecular reactions, the transition state theory (TST) was employed to calculate second-order rate constants. A kinetic mechanism for decalin pyrolysis was constructed from the calculated rate constants and the thermodynamic data for species involved. This mechanism is used to study the time-dependent chemical kinetics behavior of decalin in a closed homogeneous gas mixture system in an atmospheric condition, using CHEMKIN software package. The simulation results show consistency in yielding benzene, styrene, xylene and ethylbenzene with limited experimental data. We expect that our RRKM and TST computed rate constants for the decalin breakdown can be further incorporated in existing kinetic schemes for flame combustion and pyrolysis to improve the prediction of aromatic concentrations.