In pursuit of more sustainable and competitive biorefineries, the effective valorisation of lignin is key. An alluring opportunity is the exploitation of lignin as a resource for chemicals. Three technological biorefinery aspects will determine the realisation of a successful lignin-to-chemicals valorisation chain, namely (i) lignocellulose fractionation, (ii) lignin depolymerisation, and (iii) upgrading towards targeted chemicals. This review provides a summary and perspective of the extensive research that has been devoted to each of these three interconnected biorefinery aspects, ranging from industrially well-established techniques to the latest cutting edge innovations. To navigate the reader through the overwhelming collection of literature on each topic, distinct strategies/topics were delineated and summarised in comprehensive overview figures. Upon closer inspection, conceptual principles arise that rationalise the success of certain methodologies, and more importantly, can guide future research to further expand the portfolio of promising technologies. When targeting chemicals, a key objective during the fractionation and depolymerisation stage is to minimise lignin condensation (i.e. formation of resistive carbon–carbon linkages). During fractionation, this can be achieved by either (i) preserving the (native) lignin structure or (ii) by tolerating depolymerisation of the lignin polymer but preventing condensation through chemical quenching or physical removal of reactive intermediates. The latter strategy is also commonly applied in the lignin depolymerisation stage, while an alternative approach is to augment the relative rate of depolymerisation vs. condensation by enhancing the reactivity of the lignin structure towards depolymerisation. Finally, because depolymerised lignins often consist of a complex mixture of various compounds, upgrading of the raw product mixture through convergent transformations embodies a promising approach to decrease the complexity. This particular upgrading approach is termed funneling, and includes both chemocatalytic and biological strategies.

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Chemicals from lignin: an interplay of lignocellulose fractionation, depolymerisation, and upgrading

Lignin, a major component of lignocellulose, is the largest source of aromatic building blocks on the planet and harbors great potential to serve as starting material for the production of biobased products. Despite the initial challenges associated with the robust and irregular structure of lignin, the valorization of this intriguing aromatic biopolymer has come a long way: recently, many creative strategies emerged that deliver defined products via catalytic or biocatalytic depolymerization in good yields. The purpose of this review is to provide insight into these novel approaches and the potential application of such emerging new structures for the synthesis of biobased polymers or pharmacologically active molecules. Existing strategies for functionalization or defunctionalization of lignin-based compounds are also summarized. Following the whole value chain from raw lignocellulose through depolymerization to application whenever possible, specific lignin-based compounds emerge that could be in the fu...

Bright Side of Lignin Depolymerization: Toward New Platform Chemicals

Unique low-molecular-weight lignin with high purity extracted from wood by deep eutectic solvents (DES): a source of lignin for valorization

The lignin-first concept offers an opportunity to utilize the entire lignocellulosic biomass efficiently However, current conversion strategies rely on high-temperature hydrogenolysis by supported metal catalysts, leading to low-functionalized products or difficulty in separation of solid catalyst from cellulose/hemicellulose Here, we report the fractionation and valorization of lignocellulose via solar energy-driven conversion of native lignin at room temperature We found that cadmium sulfide quantum dots not only catalyse the cleavage of β-O-4 bonds in lignin models quantitatively but also are efficient for the conversion of native lignin within biomass into functionalized aromatics under visible light, while cellulose/hemicellulose remain almost intact Further, the colloidal character of quantum dots enables their facile separation and recycling by a reversible aggregation–colloidization strategy The β-O-4 bond in lignin is cleaved by an electron–hole coupled photoredox mechanism based on a Cα radical intermediate, in which both photogenerated electrons and holes participate in the reaction Lignin-first approaches, which prioritize lignin upgrade over cellulose, can open the way to full biomass valorization, but are still hampered by the need of harsh reaction conditions and difficulties in catalyst recovery Now, a photocatalytic strategy based on the use of cadmium sulfide quantum dots is reported that overcomes these limitations

Solar energy-driven lignin-first approach to full utilization of lignocellulosic biomass under mild conditions

Lignocellulosic biomass is the most abundant organic carbon source and has received a great deal of interest as renewable and sustainable feedstock for the production of potential biofuels and value-added chemicals with a wide range of designed catalytic systems. However, those natural polymeric materials are composed of short-chain monomers (typically C6 and C5 sugars) and complex lignin molecules containing plenty of oxygen, resulting in products during the downstream processing having low-grade fuel properties or limited applications in organic syntheses. Accordingly, approaches to increase the carbon-chain length or carbon atom number have been developed as crucial catalytic routes for upgrading biomass into energy-intensive fuels and chemicals. The primary focus of this review is to systematically describe the recent examples on the selective synthesis of long-chain oxygenates via different C-C coupling catalytic processes, such as Aldol condensation, hydroalkylation/alkylation, oligomerization, keto...

Hongxi Zhang

Papers

Oxidative conversion of lignin and lignin model compounds catalyzed by CeO2-supported Pd nanoparticles

Selective activation of the C–O bonds in lignocellulosic biomass for the efficient production of chemicals

Mesoporous H-ZSM-5 as an efficient catalyst for conversions of cellulose and cellobiose into methyl glucosides in methanol

A New Approach To Explore The Catalytic Depolymerization of Lignin Via Samarium Oxide