Catalytic Strategies and Mechanism Analysis Orbiting the Center of Critical Intermediates in Lignin Depolymerization

Abstract

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.

CdS–SH/TiO2 Heterojunction Photocatalyst Significantly Improves Selectivity for C–O Bond Breaking in Lignin Models

“…The formation of a heterojunction structure significantly enhances the separation of photogenerated electron–hole pairs. Two possible pathways for the depolymerization of PP-ol have been proposed, , differing mainly in the formation of a C α • radical or a C α –O • radical (which further forms a C α =O ketone intermediate). In our study, the time-course profiles of the conversion of PP-ol into aromatic monomers in Figure a,b show the production and consumption processes of PP-one intermediates, indicating that the pathway involving the C α  O ketone intermediate is more suitable for our studied CdS–SH/TiO 2 photocatalyst process.…”

“…Therefore, the TiO 2 nanosheets and CdS–SH were selected to construct the CdS–SH/TiO 2 heterojunction, which can solve the above-mentioned catalytic limitation of a single semiconductor and enable the close contact between lignin and the photocatalytic material to realize an efficient breakage of the C–O bond. In a recent review by Wang, a thorough analysis of different mechanisms involved in lignin depolymerization was conducted. Interestingly, two distinct pathways of photocatalytic lignin depolymerization were proposed: the self-hydrogen transfer process involving the C α O ketone intermediate and the C α • radical self-decomposition pathway .…”

Photocatalytic transfer hydrogenolysis of aryl ethers

“…Lignocellulosic biomass is an abundant and renewable carbon resource that is generated through CO 2 capture during photosynthesis, and its refinement is a sustainable approach for the production of basic chemicals and fuels. 1–3 Hydrogenation is the main route for the depolymerization and upgrading of biomass polymers into hydrocarbon products via C–O bond cleavage. 4,5 In lignin, aromatic units are linked by various ether bonds.…”

Uncovering the Structure of Lignin from Moso Bamboo with Different Tissues and Growing Ages for Efficient Ambient-Pressure Lignin Depolymerization

“…Lignin depolymerization is a promising valorization approach for lignin. − Recently, the application of ambient-pressure reductive catalytic depolymerization (ARCD) has received increasing attention due to its advantages, such as operating at mild reaction temperatures, not requiring hydrogen or protective gas, and being easy to scale up. , Furthermore, the ARCD method employs a hydrogen-free catalytic process that reduces excessive hydrogenation of the products, minimizes ash hydrogen consumption, reduces carbon dioxide emissions, and promotes a greener and more sustainable utilization of lignin. , From the perspective of the lignin structure, disclosing the effect of lignin’s native structure on catalytic depolymerization fosters a better understanding of the relationship between molecular properties of lignin and catalytic lignin depolymerization . Theoretically, the difference in the lignin structure of F and P during the different growth ages affects reductive catalytic depolymerization to produce the monoaromatic chemicals amenable.…”