不饱和结构对碳氢燃料低温氧化过程的影响机制

Advanced engine technologies depend greatly on low temperature combustion of fuels. Reactions of hydroperoxides and derived radicals lie at the core of the mechanism of low temperature oxidation. Experimental detection of these species is rather difficult due to the extremely short-lived character. The unsaturated structures of alkenes could greatly enhance the buildup of hydroperoxides and derived radicals, and therefore promote the probability of experimental detection. In this proposal, we will choose linear hexene isomers and n-hexane as our target systems to study the influence of unsaturated structures on low temperature oxidation of hydrocarbons. The synchrotron vacuum ultraviolet photoionization mass spectrometry will be coupled with jet stirred reactor to conduct the experiments. Ab initio transition state theory based master equation methodology will be used to generate temperature- and pressure-dependent rate coefficients of key elementary reactions. Accurate low temperature oxidation model of target alkenes will be developed based on experimental measurements and theoretical.investigations. Experiment, theory and modeling together improve our understanding on the influence of (1) unsaturated structures on low temperature oxidation of hydrocarbons by comparing reaction kinetics of n-hexane and hexenes; (2) the position of double bond and the length of saturated carbon-chain on low temperature oxidation by comparing reaction kinetics of hexene isomers. As the important component and key intermediate in combustion, the low temperature chemistry of alkenes helps to promote the model development for that of hydrocarbon fuels. We hope that this work can provide importance reference to the development of advanced engines.

低温氧化燃烧是先进发动机的设计需求，过氧化物及其自由基参与的系列反应是低温氧化机理的核心，然而其实验探测却极为困难。烯烃中的不饱和键能够提高关键自由基浓度，从而增加实验探测可能性。本项目选取直链己烯系列异构体和正己烷作研究对象，利用同步辐射真空紫外光电离质谱技术开展射流搅拌反应器低温氧化实验研究；利用基于从头算过渡态理论的主方程方法对关键反应开展宽压力动力学理论研究；结合实验测量和理论计算，最终发展出完善的己烯低温氧化模型。实验、理论和模型三者相辅相成，通过(1)对比烷烃和烯烃反应机理，研究不饱和结构对碳氢燃料低温氧化过程的影响；(2)对比己烯各异构体反应机理，研究双键位置和饱和碳链长度的影响。烯烃作为碳氢燃料代表性组分和重要燃烧中间体，对其低温氧化机理的研究有助于促进碳氢燃料低温燃烧模型的发展与完善，为先进发动机研发提供重要参考。

本项目围绕三个己烯异构体，结合实验探测的产物（包括中间产物）信息和理论预测的化学反应能量信息，分别明确了各己烯体系低温氧化过程中的主要反应类型和详细反应路径。通过对比不同己烯异构体、己烯与己烷，揭示了双键及其位置对反应类型及竞争关系、产物种类和反应活性的变化，从微观层面上解释了不同己烯低温氧化反应特性的差异。通过理论计算获得的反应动力学机理为低温氧化模型的发展与优化提供了重要基础数据，是提高模型准确性的有力保障。本项目关于己烯低温氧化反应机理的研究可作为今后更大分子烯烃的前提和参考，同时对于发动机燃油组分的设计与调控具有重要实用价值。此外，对不饱和双键作用机理的研究亦有助于深入理解包含大量不饱和脂肪酸的生物柴油的燃烧过程。