Ab Initio Investigation of Primary Fuel Reactions of Monoaromatic Hydrocarbons under Pyrolytic Conditions: Anisole, Phenetole, and the 2‑, 3‑, 4‑Methylanisole Isomers

Wooden biomass contains high amounts of lignocellulose, which is one of the main fuel components during wildfire events. Furthermore, its properties in the context of alternative energy carriers are of interest in recent research. In order to better analyze and understand these highly complex molecules and their fundamental combustion properties, a complexity reduction by using model compounds can be applied. Monoaromatic oxygenated hydrocarbons (MAHs) are an option to map these systems on a more accessible level. In the present study, the MAHs anisole, phenetole, 2-methylanisole, 3-methylanisole, and 4-methylanisole were investigated by means of quantum chemical calculations. To this end, the DLPNO–CCSD(T)/CBS(cc-pVTZ, cc-pVQZ)//B3LYP-D3BJ/def2-TZVP levels of theory were utilized to derive a range of important physical and chemical quantities. These include bond dissociation energies (BDEs), one-dimensional representations of the potential energy surface, thermodynamic properties, and reaction rate parameters. As previously demonstrated, reactions of the aromatic ring structure and the attached hydrogen atoms are energetically unfavorable. This prompted the investigation of only the reactions affecting the methyl and alkoxy side chains. The reactions examined in this study are the primary fuel reactions that are relevant to pyrolysis. This set of 47 reactions includes the H atom abstraction by Ḣ and ĊH3, the unimolecular bond fissions, and the internal H atom migration reactions on the methoxy or ethoxy side chain. For all five molecules, the C–O bond on the alkoxy side chain is the weakest bond by BDEs, and the respective bond fissions are dominant. Besides the general importance of H atom abstractions, these dominant bond fissions have the highest impact on the overall reactivity among the investigated reactions. Due to the comprehensive amount of available literature for anisole, it is included as a benchmark molecule. The available literature on phenetole is limited, and the present study provides fundamental data for this species. For methylanisole, a recent publication focused on experimental and modeling efforts for these isomers. The importance of the C–O bond breaking, and the other determined reactions in this work were tested by including the calculated rate parameters in a validated chemical kinetic mechanism for methylanisole isomers from literature. The modified model was subsequently assessed in comparison to the initial version of the published model and experiments. Shock tube and rapid compression machine experiments were performed in the temperature range between 880 and 1220 K for pressures of 10 and 20 bar at stoichiometric conditions. This assessment yielded two notable findings. First, it confirmed the significant impact of the C–O bond fission. However, a comparison with recent high-level ab initio calculations revealed significant deviations in the rate constants. Second, it emphasized the importance of the subsequent phenoxy/methylphenoxy radical chemistry and the associated thermodynamic properties. Further refinement of the model descriptions of MAHs is warranted and necessary to improve the understanding of these important reference molecules.木质生物质（Wooden biomass）含有大量木质纤维素（lignocellulose），而后者是野火（wildfire）过程中的主要燃料组分之一。此外，其作为替代能源载体（alternative energy carriers）的相关特性在近期研究中广受关注。为更好地分析与理解这类高度复杂的分子及其基本燃烧特性，可通过采用模型化合物（model compounds）实现体系复杂度的简化。单环芳香含氧化合物（Monoaromatic oxygenated hydrocarbons, MAHs）便是一类可将该类体系转化为更易研究对象的理想选择。 本研究针对单环芳香含氧化合物中的苯甲醚（anisole）、苯乙醚（phenetole）、2-甲基苯甲醚（2-methylanisole）、3-甲基苯甲醚（3-methylanisole）以及4-甲基苯甲醚（4-methylanisole）开展了量子化学计算（quantum chemical calculations）。研究采用DLPNO–CCSD(T)/CBS(cc-pVTZ, cc-pVQZ)//B3LYP-D3BJ/def2-TZVP理论水平，推导得到一系列重要的物理与化学参数，包括键解离能（bond dissociation energies, BDEs）、势能面（potential energy surface）一维表征、热力学性质（thermodynamic properties）以及反应速率参数（reaction rate parameters）。 已有研究表明，芳香环结构与连附氢原子的反应在能量上并不有利，因此本研究仅聚焦于影响甲基与烷氧基侧链的反应。本研究考察的47种反应均为与热解（pyrolysis）相关的核心燃料反应，涵盖氢自由基（Ḣ）与甲基自由基（ĊH₃）介导的氢原子夺除反应、单分子键断裂反应，以及甲氧基或乙氧基侧链上的分子内氢迁移反应。 对于这五种分子而言，烷氧基侧链上的C-O键是键解离能最低的最弱键，对应的键断裂反应占据主导地位。除氢原子夺除反应的普遍重要性外，这类主导性的键断裂反应在所有考察反应中对整体反应活性的影响最为显著。 鉴于苯甲醚已有大量相关文献报道，将其作为基准分子纳入研究。苯乙醚的现有文献较为有限，本研究为该物种提供了基础性研究数据。针对甲基苯甲醚，近期已有文献针对这些异构体开展了实验与模拟研究。本研究通过将计算得到的反应速率参数纳入已验证的甲基苯甲醚异构体化学动力学机理（chemical kinetic mechanism）中，验证了C-O键断裂以及本研究中确定的其他反应的重要性。 随后将修改后的模型与已发表的初始模型及实验结果进行对比评估。实验在880~1220 K的温度区间、10 bar与20 bar的压力下，以化学计量比条件开展了激波管（shock tube）与快速压缩机（rapid compression machine）测试。 本次评估得到两项重要发现：其一，证实了C-O键断裂的显著影响，但与近期高精度从头算计算结果对比后发现，反应速率常数存在显著偏差；其二，凸显了后续苯氧基/甲基苯氧基自由基（phenoxy/methylphenoxy radical）化学过程及其相关热力学性质的重要性。未来仍需进一步优化单环芳香含氧化合物的模型描述，以加深对这类重要参考分子的理解。