Computational Study on the Unimolecular Decomposition of JP-8 Jet Fuel Surrogates III: Butylbenzene Isomers ( n-, s-, and t-CH).

Ab initio G3(CCSD,MP2)//B3LYP/6-311G(d,p) calculations of potential energy surfaces have been carried out to unravel the mechanism of the initial stages of pyrolysis of three CH isomers: n-, s-, and t-butylbenzenes. The computed energy and molecular parameters have been utilized in RRKM-master equation calculations to predict temperature- and pressure-dependent rate constants and product branching ratios for the primary unimolecular decomposition of these molecules and for the secondary decomposition of their radical fragments. The results showed that the primary dissociation of n-butylbenzene produces mostly benzyl (CH) + propyl (CH) and 1-phenyl-2-ethyl (CHCH) + ethyl (CH), with their relative yields strongly dependent on temperature and pressure, together with a minor amount of 1-phenyl-prop-3-yl (CH) + methyl (CH). Secondary decomposition reactions that are anticipated to occur on a nanosecond scale under typical combustion conditions split propyl (CH) into ethylene (CH) + methyl (CH), ethyl (CH) into ethylene (CH) + hydrogen (H), 1-phenyl-2-ethyl (CHCH) into mostly styrene (CH) + hydrogen (H) and to a lesser extent phenyl (CH) + ethylene (CH), and 1-phenyl-prop-3-yl (CH) into predominantly benzyl (CH) + ethylene (CH). The primary decomposition of s-butylbenzene is predicted to produce 1-phenyl-1-ethyl (CHCHCH) + ethyl (CH) and a minor amount of 1-phenyl-prop-1-yl (CH) + methyl (CH), and then 1-phenyl-1-ethyl (CHCHCH) and 1-phenyl-prop-1-yl (CH) rapidly dissociate to styrene (CH) + hydrogen (H) and styrene (CH) + methyl (CH), respectively. t-Butylbenzene decomposes nearly exclusively to 2-phenyl-prop-2-yl (CH) + methyl (CH), and further, 2-phenyl-prop-2-yl (CH) rapidly eliminates a hydrogen atom to form 2-phenylpropene (CH). If hydrogen atoms or other reactive radicals are available to make a direct hydrogen-atom abstraction from butylbenzenes possible, the CH radicals (1-phenyl-but-1-yl, 2-phenyl-but-2-yl, and t-phenyl-isobutyl) can be formed as the primary products from n-, s-, and t-butylbenzene, respectively. The secondary decomposition of 1-phenyl-but-1-yl leads to styrene (CH) + ethyl (CH), whereas 2-phenyl-but-2-yl and t-phenyl-isobutyl dissociate to 2-phenylpropene (CH) + methyl (CH). Thus, the three butylbenzene isomers produce distinct but overlapping nascent pyrolysis fragments, which likely affect the successive oxidation mechanism and combustion kinetics of these JP-8 fuel components. Temperature- and pressure-dependent rate constants generated for the initial stages of pyrolysis of butylbenzenes are recommended for kinetic modeling.