Scientific Questions for Lignin Conversion and a Brief Summary of Methods for Lignin Depolymerization

a discussion about the opportunity and challenge for lignin valorization, especially for the aromatic chemicals preparation, is firstly provided (Chapter 4.1). Then, focusing on the topic of lignin (catalytic) depolymerization to aromatic monomers, the involved scientific questions are briefly summarized (Chapter 4.2) as follows: (1) promoting the lignin depolymerization via selective cleavage of the ubiquitous lignin C–O/C–C bonds, increasing the conversion; and (2) restraining the lignin fragments recondensation with more stable chemical bonds generation or multimers formation, increasing the selectivity and final monomer yield. Then, two different approaches for the foundation of lignin depolymerization strategies are introduced: (1) direct lignin conversion; and (2) the bottom-up approach (Chapter 4.3). To provide a macro understanding of the research field of lignin conversion, the classification of lignin conversion methods by reaction types is summarized (Chapter 4.4), during which the corresponding catalyst types are also briefly provided. Then, a brief list of some representative systems reported in the last 10 years for native/technical lignin conversion is provided (Chapter 4.5). Based on the brief introduction and summary of the lignin conversion research in this chapter, we hope to provide a basic understanding of the lignin conversion process, which can make the acceptance of the following mechanism discussion easier.