Lignin, as a precious resource given to mankind by nature with abundant functional aromatic structures, has drawn much attention in the recent decade from academia to industry worldwide, aiming at harvesting aromatic compounds from this abundant and renewable natural polymer resource. How to efficiently depolymerize lignin to easy-to-handle aromatic monomers is the precondition of lignin utilization. Many strategies/methods have been developed to effectively degrade lignin into monomers, such as the traditional methods of pyrolysis, gasification, liquid-phase reforming, solvolysis, chemical oxidation, hydrogenation, reduction, acidolysis, alkaline hydrolysis, alcoholysis, as well as the newly developed redox-neutral process, biocatalysis, and combinatorial strategies. Therefore, there is a strong demand to systemically summarize these developed strategies and methods and reveal the internal transformation principles of the lignin. Focusing on the topic of lignin depolymerization to aromatic chemicals, this review reorganizes and categorizes the strategies/methods according to their mechanisms, orbiting the center of critical intermediates during the lignin linkage transformation, which includes the critical anionic intermediates, cationic intermediates, organometallic intermediates, organic molecular intermediates, aryl cation radical intermediates, and neutral radical intermediates. The corresponding introduction involves the generation and the transformation chemistry of the critical intermediates via the corresponding C–H/O–H/C–C/C–O chemical bond transformations, leading to the cleavage of the C–C/C–O linkage bonds. Accompanying the brief introduction of lignin chemistry and the final concluding remarks and perspectives on lignin depolymerization, this review aims to provide a current research process of lignin depolymerization, which may provide useful suggestions for this vigorous research field.

中文翻译：

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以木质素解聚关键中间体为中心的催化策略与机理分析

木质素作为大自然赋予人类的宝贵资源，具有丰富的功能性芳香结构，近十年来引起了全球学术界和工业界的广泛关注，旨在从这种丰富的可再生天然高分子资源中获取芳香族化合物。如何高效地将木质素解聚为易于处理的芳香族单体是木质素利用的前提。已经开发了许多策略/方法来有效地将木质素降解为单体，例如传统的热解、气化、液相重整、溶剂分解、化学氧化、氢化、还原、酸解、碱解、醇解，以及新的方法开发了氧化还原中性过程、生物催化和组合策略。所以，迫切需要系统地总结这些已开发的策略和方法，并揭示木质素的内部转化原理。本综述围绕木质素解聚为芳香族化学品的主题，根据其机制对策略/方法进行了重组和分类，围绕木质素键合转化过程中的关键中间体中心，包括关键的阴离子中间体、阳离子中间体、有机金属中间体、有机分子中间体、芳基阳离子自由基中间体和中性自由基中间体。相应的介绍涉及关键中间体的生成和转化化学，通过相应的 C-H/O-H/C-C/C-O 化学键转换，导致 C-C/C-O 键的断裂债券。