Radicals and molecular products from the gas-phase pyrolysis of lignin model compounds: Coniferyl alcohol, theory and experiment
Radicals and molecular products from the gas-phase pyrolysis of lignin model compounds: Coniferyl alcohol, theory and experiment
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DOI：
10.1016/j.jaap.2021.105413
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发表时间：
2022-01-01
影响因子：
6
通讯作者：
Asatryan, Rubik
中科院分区：
文献类型：
--
作者：
Barekati-Goudarzi, Mohamad;Khachatryan, Lavrent;Asatryan, Rubik
松柏醇（CFA）是木质素的主要结构单元之一，也是木质素的模型化合物.本文从两个方面研究了CFA热解过程中木质素热解的分子机理：（i）CFA真空热解产生的中间自由基的性质;（ii）分散在气相中的CFA热解产物的分布。采用低温基体隔离电子顺磁共振（LTMI-EPR）技术研究了CFA气相低压热解过程中的中间产物自由基。在液氮温度下捕获的自由基的各向异性EPR光谱在整个研究的温度范围（400-700 ℃）内记录，其特征在于高g值（< 2.0100）和宽EPR线宽（类似于13.5-13.0 G），规定为O-CH 3和OH末端键断裂产生的O-中心自由基的混合物。冷冻自由基的热退火建议的主要贡献的甲基过氧（CH 3 O2）自由基所产生的释放的CH 3和微量的氧。CFA的热解，第一次，在分散的气相条件下进行，以尽量减少在常规反应器中典型的凝相热解的颗粒表面介导的非均相二次反应。几个独特的产品，包括扁桃酸和一些羧酸化合物的多羟基化衍生物，除了那些通常在常规热解过程中产生的松柏醛，香草醛（及其氧化形式高香草酸），丁香酚及其异构体被确定。在wB 97 XD和M06-2X杂化密度泛函理论水平上对产物的形成机制进行了全面的分析，特别是对生成丰富羟基化合物的途径进行了分析，并研究了微量氧的作用。
Coniferyl alcohol (CFA) is one of lignin's main building blocks and a relevant model compound. Two aspects of CFA pyrolysis are studied in the current work to gain insight into the molecular mechanism of lignin thermolysis: (i) the nature of intermediate radicals generated from vacuum pyrolysis of CFA, and (ii) the product distribution from pyrolysis of CFA dispersed into the gas phase. Low temperature matrix isolation electron paramagnetic resonance (LTMI-EPR) spectroscopy is employed to investigate the intermediate radicals associated with gas phase low-pressure pyrolysis of CFA. An anisotropic EPR spectrum of radicals trapped at liquid nitrogen temperature was registered throughout the studied temperature range (400-700 degrees C) characterized by high g-value (< 2.0100), and broad EPR line-width (similar to 13.5-13.0 G) prescribed to a mixture of O-centered radicals produced from cleavage of O-CH3 and O H terminal bonds. Thermal annealing of frozen radicals suggested a major contribution of methylperoxyl (CH3O2) radicals produced by the liberated CH3 and trace amounts of oxygen. The pyrolysis of CFA, for the first time, was conducted in a dispersed, gas-phase conditions to minimize the heterogeneous secondary reactions mediated by particle surfaces typical for condensed-phase pyrolysis in conventional reactors. Several unique products including polyhydroxylated derivatives of mandelic acid and some carboxylic compounds were identified in addition to those normally produced during the conventional pyrolysis coniferyl aldehyde, vanillin (and its oxidized form homovanilic acid), eugenol and its isomers. A comprehensive analysis of product formation mechanisms, particularly the pathways for generating abundant hydroxylated compounds is performed and the role of trace oxygen is studied at the wB97XD and M06-2X hybrid density functional theory levels.