Poplar wood was rapidly fractionated via a flow-through reaction using aqueous solutions of an acid hydrotrope (AH), p-toluenesulfonic acid (p-TsOH), at temperatures below 98 °C. 13C–1H two-dimensional nuclear magnetic resonance (NMR) spectroscopic analyses demonstrated that the AH-solubilized lignins (AHLs) from a range of fractionation conditions with yields up to approximately 80% had a very high content of β-aryl-ether linkages compared to milled wood lignin (MWL) with a low enough condensation to facilitate subsequent reductive catalytic depolymerization resulting in a lignin monomer yield of over 30%. Gel-permeation chromatographic (GPC) and differential scanning calorimetric (DSC) analyses showed that the AHLs have high molecular weights and low glass transition temperatures T g. These AHLs also have a pinkish color suitable for applications such as cosmetics and dye dispersants. AH fractionation (AHF) preserved the cellulose fraction as solid fibers also with a light pinkish color for the materials market and solubilized up to approximately 90% of xylan which can be converted to furfural using p-TsOH in the spent liquor without additional catalysts. The advantages herein are the use of one recyclable industrial chemical such as p-TsOH in an aqueous system below water boiling temperature to valorize all three major fractions of lignocelluloses in a short time frame, with very promising yields and well-preserved lignin and cellulose structure.

Poplar wood was fractionated under a range of conditions using an inexpensive and recyclable commercial aromatic acid, p-toluenesulfonic acid (p-TsOH), as an acid hydrotrope (AH). The fractionated cellulose-rich solid fraction demonstrated excellent physical and optical properties as papermaking fibers. The dissolved xylose can be dehydrated into furfural using the p-TsOH in the spent liquor without additional catalysts. The acid hydrotrope dissolved lignin (AHL) has high β-aryl ether linkage content (∼60%) and high molecular weight (∼4000 Da), similar to that of mill wood lignin (MWL). Increasing acid hydrotrope fractionation (AHF) severity increased AHL yield, but reduced β-aryl ether bonds, decreased molecular weight and increased AHL glass transition temperature. An AHL yield of 50% can be obtained while retaining approximately half of the β-O-4 linkages, beneficial for lignin valorization as a polymer in composites as well as for production of monomers or aromatics through subsequent depolymerization.

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.