Custodis Victoria B F, Hemberger Patrick, Ma Zhiqiang, van Bokhoven Jeroen A
Department of Chemistry and Applied Biosciences, Institute for Chemical and Bioengineering , ETH Zurich, HCI E 127, Vladimir-Prelog-Weg 1, 8093 Zurich, Switzerland.
J Phys Chem B. 2014 Jul 24;118(29):8524-31. doi: 10.1021/jp5036579. Epub 2014 Jul 1.
Fast pyrolysis of lignin is one of the most promising methods to convert the complex and irregular structure of lignin into renewable chemicals and fuel. During pyrolysis the complex set of radical reactions, rearrangements, and eliminations is influenced by temperature, pressure, and the lignin origin and structure. This model compound study aims to understand reaction pathways and how primary intermediates lead to the observed product selectivity. The pyrolysis microreactor directly connected to the gas chromatograph with a mass spectrometer (py-GC/MS) detects the final products, while imaging photoelectron photoion coincidence (iPEPICO) with VUV synchrotron radiation shows primary decomposition radicals. The tested model compounds, diphenylether (DPE) and ortho-methoxyphenol (guaiacol), represent a common lignin linkage and the most present subunit in lignin, respectively. Radical fragments, such as the hydroxycyclopentadienyl radical in guaiacol decomposition, are identified by mass-selected threshold photoelectron spectra (ms-TPES) in excellent agreement with the Franck-Condon simulation. While homolysis produces phenoxy-, phenyl-, and hydroxyphenoxy radicals, which are observed in high vacuum, radically initiated reactions are dominant in ambient conditions and produce recombination and rearrangement products, such as 2-hydroxybenzaldehyde in the case of guaiacol. The degree of substitution plays a dominant role in both the stabilization of the intermediate radical and the following degree of recombination. The recombination of phenoxy radicals is enhanced compared to hydroxy-phenoxy radicals.
木质素的快速热解是将复杂且不规则结构的木质素转化为可再生化学品和燃料最具前景的方法之一。在热解过程中，自由基反应、重排和消除等一系列复杂反应受温度、压力以及木质素来源和结构的影响。本模型化合物研究旨在了解反应途径以及初级中间体如何导致所观察到的产物选择性。直接与配有质谱仪的气相色谱仪相连的热解微反应器（py-GC/MS）可检测最终产物，而利用真空紫外同步辐射的成像光电子光离子符合技术（iPEPICO）则可显示初级分解自由基。所测试的模型化合物二苯醚（DPE）和邻甲氧基苯酚（愈创木酚）分别代表了木质素中常见的一种连接方式和最主要的亚基。愈创木酚分解过程中产生的自由基碎片，如羟基环戊二烯基自由基，通过质量选择阈值光电子能谱（ms-TPES）得以鉴定，与弗兰克-康登模拟结果高度吻合。虽然均裂会产生在高真空条件下观察到的苯氧基、苯基和羟基苯氧基自由基，但在环境条件下，自由基引发的反应占主导地位，并产生重组和重排产物，例如愈创木酚会生成2-羟基苯甲醛。取代度在中间体自由基的稳定性以及随后的重组程度方面均起主导作用。与羟基苯氧基自由基相比，苯氧基自由基的重组得到增强。
