Department of Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, United States.
Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York, Buffalo, New Jersey 14226, United States.
J Phys Chem A. 2020 Jun 18;124(24):4905-4915. doi: 10.1021/acs.jpca.9b11898. Epub 2020 Jun 9.
Lignin is the most complex component of biomass, and development of a detailed chemical kinetic model for biomass pyrolysis mainly relies on the understanding of the lignin decomposition kinetics. -Coumaryl alcohol (-CMA, HOPh-CH═CH-CHOH), the focus of our analysis, is the simplest of the lignin monomers (monolignols) containing a typical side-chain double bond and both alkyl- and phenolic-type OH-groups. In parts I and II of our work (Asatryan, R. 2019, 123, 2570-2585; Hudzik, J. M. 2020, current issue), we created a detailed potential energy surface (PES) and performed a kinetic analysis of chemically activated, unimolecular, and bimolecular reactions pathways for -CMA + OH. Reaction pathways analyzed include dissociation, intramolecular abstraction, group transfer, and elimination processes. The α- and β-carbon addition reactions generate 1,3- () and 1,2-diol () adduct radicals, respectively. Well depths are approximately 29 and 41 kcal/mol below the -CMA + OH entrance level. Kinetic analysis aides in determining the major pathways for our conventional and fractional pyrolysis experiments. The current paper focuses on the H-abstraction reactions via H, OH, and CH light ("pool") radicals from -CMA. The thermochemical properties of all stable, radical, and transition-state species were determined using the ωB97XD density functional theory (DFT) and higher-level CBS-QB3 composite methods. Barrier heights from the prereaction complexes, for OH-radical abstractions, to the transition states for the propanoid side chain are compared to the model H-abstraction reactions of allyl alcohol (AA) with OH and -CMA with H and CH radicals. The lowest-energy, most stable, -CMA radical formed is at the C9 allylic position (-CMA-C9j) with exothermicity of 26.63, 41.32, and 27.34 kcal/mol for H, OH, and CH, respectively. For OH-radical abstraction at this position, our findings are consistent with corresponding data on AA + OH at 37.44 kcal/mol and similar to that of . A similar stable radical with an exothermicity of 34.95 kcal/mol occurs for the phenol hydroxyl group, generating the -CMA-O4j radical. H-abstraction pathways are considered in relation to other major pathways previously considered for -CMA + OH reactions including H-atom shifts, dehydration, and β-scission reactions. Derived rate coefficients for substituted phenols can be utilized in detailed kinetic models for lignin/biomass pyrolysis.
木质素是生物质中最复杂的成分，生物质热解详细化学动力学模型的发展主要依赖于对木质素分解动力学的理解。-愈创木基醇（-CMA，HOPh-CH═CH-CHOH）是我们分析的重点，是含有典型侧链双键和烷基型及酚型-OH 基团的最简单木质素单体（木质素单体）之一。在我们工作的第一部分和第二部分（Asatryan，R. 2019，123，2570-2585；Hudzik，J. M. 2020，当前问题）中，我们创建了一个详细的势能面（PES），并对-CMA+OH 的化学活化、单分子和双分子反应途径进行了动力学分析。分析的反应途径包括离解、分子内提取、基团转移和消除过程。α-和β-碳原子加成反应分别生成 1,3-（）和 1,2-二醇（）加合物自由基。势阱深度约为-CMA+OH 入口水平以下 29 和 41 kcal/mol。动力学分析有助于确定我们常规和分数热解实验的主要途径。当前论文侧重于-CMA 与 H、OH 和 CH 轻（“池”）自由基的 H 原子提取反应。使用 ωB97XD 密度泛函理论（DFT）和更高水平的 CBS-QB3 复合方法确定了所有稳定、自由基和过渡态物种的热化学性质。从预反应复合物到丙烷侧链的过渡态的 OH 自由基提取的势垒高度与烯丙醇（AA）与 OH 和-CMA 与 H 和 CH 自由基的模型 H 原子提取反应进行了比较。形成的最稳定、最低能的-CMA 自由基位于 C9 烯丙位（-CMA-C9j），与 H、OH 和 CH 的放热分别为 26.63、41.32 和 27.34 kcal/mol。对于此位置的 OH 自由基提取，我们的发现与 AA+OH 在 37.44 kcal/mol 上的对应数据一致，与类似。酚羟基也会生成一个放热 34.95 kcal/mol 的类似稳定自由基，生成 -CMA-O4j 自由基。考虑到与 -CMA+OH 反应的其他主要途径，包括 H 原子迁移、脱水和 β-断裂反应，考虑了 H 原子提取途径。取代酚的衍生速率系数可用于木质素/生物质热解的详细动力学模型。
