Sivaramakrishnan Raghu, Michael Joe V, Harding Lawrence B, Klippenstein Stephen J
J Phys Chem A. 2015 Jul 16;119(28):7724-33. doi: 10.1021/acs.jpca.5b01032. Epub 2015 Apr 24.
The mechanism for the thermal decomposition of acetaldehyde has been revisited with an analysis of literature kinetics experiments using theoretical kinetics. The present modeling study was motivated by recent observations, with very sensitive diagnostics, of some unexpected products in high temperature microtubular reactor experiments on the thermal decomposition of CH3CHO and its deuterated analogs, CH3CDO, CD3CHO, and CD3CDO. The observations of these products prompted the authors of these studies to suggest that the enol tautomer, CH2CHOH (vinyl alcohol), is a primary intermediate in the thermal decomposition of acetaldehyde. The present modeling efforts on acetaldehyde decomposition incorporate a master equation reanalysis of the CH3CHO potential energy surface (PES). The lowest-energy process on this PES is an isomerization of CH3CHO to CH2CHOH. However, the subsequent product channels for CH2CHOH are substantially higher in energy, and the only unimolecular process that can be thermally accessed is a reisomerization to CH3CHO. The incorporation of these new theoretical kinetics predictions into models for selected literature experiments on CH3CHO thermal decomposition confirms our earlier experiment and theory-based conclusions that the dominant decomposition process in CH3CHO at high temperatures is C-C bond fission with a minor contribution (∼10-20%) from the roaming mechanism to form CH4 and CO. The present modeling efforts also incorporate a master-equation analysis of the H + CH2CHOH potential energy surface. This bimolecular reaction is the primary mechanism for removal of CH2CHOH, which can accumulate to minor amounts at high temperatures, T > 1000 K, in most lab-scale experiments that use large initial concentrations of CH3CHO. Our modeling efforts indicate that the observation of ketene, water, and acetylene in the recent microtubular experiments are primarily due to bimolecular reactions of CH3CHO and CH2CHOH with H-atoms and have no bearing on the unimolecular decomposition mechanism of CH3CHO. The present simulations also indicate that experiments using these microtubular reactors when interpreted with the aid of high-level theoretical calculations and kinetics modeling can offer insights into the chemistry of elusive intermediates in the high-temperature pyrolysis of organic molecules.
通过运用理论动力学对文献中的动力学实验进行分析，重新审视了乙醛热分解的机理。当前的建模研究是受近期观察结果的推动，这些观察结果来自于使用非常灵敏的诊断方法，对高温微管反应器中乙醛（CH₃CHO）及其氘代类似物CH₃CDO、CD₃CHO和CD₃CDO热分解实验中一些意外产物的观察。这些产物的观察结果促使这些研究的作者提出，烯醇互变异构体CH₂CHOH（乙烯醇）是乙醛热分解的主要中间体。目前对乙醛分解的建模工作纳入了对CH₃CHO势能面（PES）的主方程重新分析。该PES上能量最低的过程是CH₃CHO异构化为CH₂CHOH。然而，CH₂CHOH随后的产物通道能量要高得多，唯一能够通过热激发的单分子过程是重新异构化为CH₃CHO。将这些新的理论动力学预测纳入到关于CH₃CHO热分解的选定文献实验模型中，证实了我们早期基于实验和理论得出的结论，即在高温下CH₃CHO的主要分解过程是C-C键断裂，而漫游机制对形成CH₄和CO的贡献较小（约10 - 20%）。目前的建模工作还纳入了对H + CH₂CHOH势能面的主方程分析。这种双分子反应是去除CH₂CHOH的主要机制，在大多数使用高初始浓度CH₃CHO的实验室规模实验中，在高温（T > 1000 K）下CH₂CHOH会积累到少量。我们的建模工作表明，近期微管实验中观察到的乙烯酮、水和乙炔主要是由于CH₃CHO和CH₂CHOH与H原子的双分子反应，与CH₃CHO的单分子分解机制无关。目前的模拟还表明，当借助高水平理论计算和动力学建模对这些微管反应器实验进行解释时，可以深入了解有机分子高温热解中难以捉摸的中间体的化学性质。
