Chemical Kinetics and Combustion Modeling
Chemical Kinetics and Combustion Modeling
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DOI：
10.1146/annurev.pc.41.100190.002021
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发表时间：
1990
影响因子：
14.7
通讯作者：
James A. Miller;R. Kee;C. Westbrook
中科院分区：
文献类型：
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作者：
James A. Miller;R. Kee;C. Westbrook
来自燃料或其它物质的H原子离子通过R 02产生烷基氢过氧化物ROOH，其然后分解以产生RO和OH自由基。然而，对于比正丁烷更复杂的烃类燃料，R 02的更快速的过程是通过内部提取H原子的异构化（194）。该R 02异构化理论的一般特征为内燃机中发动机爆震的总体反应方案提供了动力学基础（1996 - 1998）。主要步骤示意性地由R 02 += tQOOH（内部H原子提取）QOOH → QO + OH（0-0均裂）组成。在足够低的温度下，分子氧可以进一步加入到QOOH自由基中，最终导致总反应QOOH + O2 =产物+ OH + OH。这两种替代方案都很重要，因为它们通过具有相对较低能量势垒的反应序列产生OH自由基。RO 2异构化速率主要由环状中间过渡态的大小、内部提取的H原子的键能和RO 2加成反应的平衡常数决定。对于较大烃的燃料，许多异构化是可能的，并且异构化反应的QOOH产物的0-0均裂对于每个异构化反应产生不同的稳定含氧物质。因此，对于正烷烃，1，4-H -原子夺取，接着0-0键分裂，导致3元含氧环，环氧乙烷。类似地，1，5-过程产生氧杂环丁烷，1，6-提取产生四氢呋喃，并且1，7-提取产生四氢吡喃。目前的模型通常使用Baldwin等人（199）列出的活化能，但A因子略低于1 0 1 2。Baldwin等人推荐的1 S 1，更接近1 0 1 1的值。5 Benson（189）推荐的用于涉及环状过渡态的单分子反应的S1。异构化反应是可逆的，并且逆异构化的活化能容易地从正向（吸热）反应的活化能和反应的AH计算（194）。通过从与C-O键相邻的位点内部提取H原子，随后断裂C-O键的ROz异构化将导致共轭烯烃和H 02。Gutman和同事已经讨论了导致相同产物的直接提取路径（200，201），倾向于通过RO 2异构化进行的路径。瓦格纳等人（20 I）的当前工作提供了对该反应的困难的一些了解，但这是R 02异构化中最简单的一种，并且存在更多的这样的反应，需要对其进行复杂的分析以完全理解详细的反应速率和机理。反应的产物环氧化物和其他含氧物种必须包括在动力学模型，但很少有定量研究的H原子提取或其他反应，这些物种已被报道。目前的模型必须估计环氧化物反应的速率和产物，这主要归因于OH或H 02对H原子的提取。
ion of H atoms from fuel or other species by R02 produces alkyl hydroperoxides ROOH that then decompose to produce RO and OH radicals. However, for hydrocarbon fuels more complicated than n-butane, a more rapid process for R02 is isomerization via internal abstraction of H atoms ( 1 94). The general features of this R02 isomerization theory provide the kinetic basis for global reaction schemes for engine knock in internal combustion engines ( 1 96-1 98). The major steps consist schematically of R02 +=t QOOH (internal H atom abstraction) QOOH � QO + OH (0-0 homolysis). At sufficiently low temperatures, molecular oxygen can add further to the QOOH radicals, leading eventually to an overall reaction QOOH + 02 = products + OH + OH. Both alternatives are important since they produce OH radicals through reaction sequences with relatively low energy barriers. ROz isomerization rates are determined primarily by the size of the ringlike intermediate transition state, by the bond energy of the H atom being abstracted internally, and by the equilibrium constant of the R02 addition reaction. For fuels of larger hydrocarbons many isomerizations are possible, and 0-0 homolysis of the QOOH product of the iso merization reaction yields a different stable oxygenated species for each isomerization reaction. Thus for n-alkanes, a 1 ,4-H -atom abstraction, followed by 0-0 bond fission, leads to a 3-membered oxygenated ring, an oxiran. Similarly, 1 ,5-processes lead to oxetans, 1 ,6-abstractions produce tetrahydrofurans, and l ,7-abstractions produce tetrahydropyrans. Cur rent models generally use activation energies tabulated by Baldwin et al ( 199), but with A factors slightly lower than the 1 0 1 2. 1 S 1 recommended by Baldwin et aI, closer to the value of 1 0 1 1 . 5 S 1 recommended by Benson ( 1 89) for unimolecular reactions involving a cyclic transition state. The isomerization reactions are reversible, and activation energies for the reverse isomerizations are easily computed from the activation energy of the forward (endothermic) reaction and the AH of the reactions ( 1 94). ROz isomerization through internal abstraction of an H atom from a site adjacent to the C-O bond, followed by breakage of the C-O bond, will lead to a conjugate olefin and H02_ Direct abstraction paths leading to the same products have been discussed by Gutman and co-workers (200, 20 1 ), favoring a path proceeding through R02 isomerization. The current work of Wagner et al (20 I) provides some insight into the diffi382 MILLER, KEE & WESTBROOK culties of this reaction, but this is one of the simplest of the R02 iso merizations, and there are many more such reactions for which complex analyses are needed to understand fully the detailed reaction rates and mechanisms. Reactions of the product epoxide and other oxygenated species must be included in kinetic models, but very few quantitative studies of H atom abstraction or other reactions for these species have been reported. Current models must estimate both the rates and products for reactions of the epoxides, primarily attributed to H atom abstraction by OH or H02•