Belisario-Lara Daniel, Mebel Alexander M, Kaiser Ralf I
Department of Chemistry and Biochemistry , Florida International University , Miami , Florida 33199 , United States.
Department of Chemistry , University of Hawaii at Manoa , Honolulu , Hawaii 96822 , United States.
J Phys Chem A. 2018 Apr 26;122(16):3980-4001. doi: 10.1021/acs.jpca.8b01836. Epub 2018 Apr 18.
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.
已开展从头算G3(CCSD,MP2)//B3LYP/6 - 311G(d,p)势能面计算，以揭示三种CH异构体（正丁基苯、仲丁基苯和叔丁基苯）热解初始阶段的机理。计算得到的能量和分子参数已用于RRKM主方程计算，以预测这些分子一级单分子分解及其自由基碎片二级分解的温度和压力依赖性速率常数及产物分支比。结果表明，正丁基苯的一级解离主要产生苄基(CH) + 丙基(CH)和1 - 苯基 - 2 - 乙基(CHCH) + 乙基(CH)，它们的相对产率强烈依赖于温度和压力，同时还产生少量的1 - 苯基 - 丙 - 3 - 基(CH) + 甲基(CH)。预计在典型燃烧条件下，在纳秒尺度上发生的二级分解反应会使丙基(CH)分解为乙烯(CH) + 甲基(CH)，乙基(CH)分解为乙烯(CH) + 氢(H)，1 - 苯基 - 2 - 乙基(CHCH)主要分解为苯乙烯(CH) + 氢(H)，在较小程度上分解为苯基(CH) + 乙烯(CH)，1 - 苯基 - 丙 - 3 - 基(CH)主要分解为苄基(CH) + 乙烯(CH)。预计仲丁基苯的一级分解产生1 - 苯基 - 1 - 乙基(CHCHCH) + 乙基(CH)和少量的1 - 苯基 - 丙 - 1 - 基(CH) + 甲基(CH)，然后1 - 苯基 - 1 - 乙基(CHCHCH)和1 - 苯基 - 丙 - 1 - 基(CH)分别迅速分解为苯乙烯(CH) + 氢(H)和苯乙烯(CH) + 甲基(CH)。叔丁基苯几乎完全分解为2 - 苯基 - 丙 - 2 - 基(CH) + 甲基(CH)，并且进一步，2 - 苯基 - 丙 - 2 - 基(CH)迅速消除一个氢原子形成2 - 苯丙烯(CH)。如果有氢原子或其他活性自由基可使从丁基苯直接进行氢原子夺取成为可能，则分别可以从正丁基苯、仲丁基苯和叔丁基苯形成CH自由基（1 - 苯基 - 丁 - 1 - 基、2 - 苯基 - 丁 - 2 - 基和叔苯基 - 异丁基）作为主要产物。1 - 苯基 - 丁 - 1 - 基的二级分解导致苯乙烯(CH) + 乙基(CH)，而2 - 苯基 - 丁 - 2 - 基和叔苯基 - 异丁基分解为2 - 苯丙烯(CH) + 甲基(CH)。因此，这三种丁基苯异构体产生不同但重叠的初生热解碎片，这可能会影响这些JP - 8燃料组分的后续氧化机理和燃烧动力学。建议将丁基苯热解初始阶段生成的温度和压力依赖性速率常数用于动力学建模。
