Gas-to-Liquid Phase Transition of PAH at Flame Temperatures

Significant evidence has shown that soot can be formed from polycyclic aromatic hydrocarbon (PAH) in combustion environments, but the transition of high molecular PAH from the gas phase to soot in a liquid or solid state remains unclear. In this study, the relationships between the boiling points of various planar PAHs and their thermodynamic properties are systematically investigated, to find a satisfactory marker for the phase transition event. Temperature-dependent thermodynamic properties, including entropy, specific heat capacity, enthalpy, and Gibbs free energy, are simultaneously calculated for PAHs, using density functional theory and three composite compound methods. Comparison of the results indicates that the individual G3 method, plus an atomization reaction approach, produces the most accurate thermochemistry parameters. Compared to entropy, enthalpy, and Gibbs free energy, the specific heat capacity at 298 K is found to be a better marker for the boiling point of PAHs due to the observed linear correlation, predictable characteristics, and fidelity of accuracy as a function of temperature. The correlation equation Y = 10.996X + 122.111 is proposed (where Y is the boiling temperature (K) and X is Cp at 298 K (cal/K/mol)). The standard deviation is as low as 16.7 K when comparing the calculated boiling points and experimentally determined values for 25 different aromatic species ranging from benzene to ovalene (C32H14). The effects of carbon number, structural arrangement, and partial pressure on the boiling point of large planar PAH are discussed. The results reveal that the carbon number in large planar PAH is the dominant factor determining its boiling points. It is shown that PAHs containing about 60–65 carbon atoms are likely to exist as liquids in flames, although the partial pressure of such species is very low.

大量研究证据表明，在燃烧环境中，多环芳烃（polycyclic aromatic hydrocarbon, PAH）可生成炭黑，但高分子量多环芳烃从气相转变为液态或固态炭黑的过程仍不明确。本研究系统探究了各类平面型多环芳烃的沸点与其热力学性质之间的关联，以期为该相变过程寻得合适的表征标志物。本研究采用密度泛函理论（density functional theory）与三种复合化学方法，同步计算了多环芳烃的温度依赖性热力学性质，包括熵（entropy）、比热容（specific heat capacity）、焓（enthalpy）及吉布斯自由能（Gibbs free energy）。结果对比表明，单独使用G3方法结合原子化反应途径，可得到最精准的热化学参数。相较于熵、焓与吉布斯自由能，298 K下的比热容因呈现出良好的线性相关性、可预测性以及随温度变化的精度稳定性，被证实为多环芳烃沸点的更优表征标志物。本研究提出了关联方程Y = 10.996X + 122.111（其中Y为沸点温度，单位为开尔文；X为298 K下的比热容，单位为卡/开尔文/摩尔）。针对从苯到卵苯（C32H14）的25种不同芳香族物质，计算沸点与实验测定值之间的标准偏差仅为16.7 K。本研究探讨了碳原子数、结构排布以及分压对大型平面型多环芳烃沸点的影响。结果表明，大型平面型多环芳烃的碳原子数是决定其沸点的主导因素。研究显示，尽管碳原子数约为60~65的多环芳烃分压极低，但它们在火焰中大概率以液态形式存在。