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 is an abundant biopolymer with a high carbon content and high aromaticity. Despite its potential as a raw material for the fuel and chemical industries, lignin remains the most poorly utilised of the lignocellulosic biopolymers. Effective valorisation of lignin requires careful fine-tuning of multiple "upstream" (i.e., lignin bioengineering, lignin isolation and "early-stage catalytic conversion of lignin") and "downstream" (i.e., lignin depolymerisation and upgrading) process stages, demanding input and understanding from a broad array of scientific disciplines. This review provides a "beginning-to-end" analysis of the recent advances reported in lignin valorisation. Particular emphasis is placed on the improved understanding of lignin's biosynthesis and structure, differences in structure and chemical bonding between native and technical lignins, emerging catalytic valorisation strategies, and the relationships between lignin structure and catalyst performance.

Paving the Way for Lignin Valorisation: Recent Advances in Bioengineering, Biorefining and Catalysis.

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

During the past decade, a growing scientific community is eagerly seeking for effective lignin valorisation approaches. Thought-out utilisation of the world's most abundant resource of bio-aromatics could substantially augment the profitability of future lignocellulosic biorefineries. From a multitude of complementary valorisation opportunities (e.g., composite materials, dispersants, carbon fibres), harnessing lignin as renewable feedstock for chemicals forms an alluring challenge. However, a root cause that hampers its full exploitation, is an historically grown and deeply ingrained (mis)conception, stating that lignin is merely considered as a subordinate opportunity to derive some extra added-value, without being of primary concern. Unfortunately, this mind-set doesn't reckon with the fact that lignin is prone to irreversible degradation, leading to recalcitrant condensed structures which are difficult to disassemble into a handful of chemicals. In response, new biorefinery schemes are being developed, wherein the valorisation of lignin is regarded as one of the primary targets. At the heart of these alternative biorefineries are fractionation strategies that aim to prevent structural lignin degradation, hereby enabling an efficient and selective lignin-to-aromatic conversion. Of particular interest are fractionation methods that implement active stabilisation mechanisms that prohibit the problem of lignin condensation, without compromising the structural integrity of the carbohydrates. This new and emerging biorefinery paradigm is termed lignin-first, and includes two distinct strategies to actively prevent structural degradation during biomass fractionation, namely (i) tandem depolymerisation–stabilisation of native lignin, and (ii) active preservation of β-O-4 bonds.

Lignin-first biomass fractionation: the advent of active stabilisation strategies

Conversion of lignin into well-defined aromatic chemicals is a highly attractive goal but is often hampered by recondensation of the formed fragments, especially in acidolysis. Here, we describe new strategies that markedly suppress such undesired pathways to result in diverse aromatic compounds previously not systematically targeted from lignin. Model studies established that a catalytic amount of triflic acid is very effective in cleaving the β-O-4 linkage, most abundant in lignin. An aldehyde product was identified as the main cause of side reactions under cleavage conditions. Capturing this unstable compound by reaction with diols and by in situ catalytic hydrogenation or decarbonylation lead to three distinct groups of aromatic compounds in high yields acetals, ethanol and ethyl aromatics, and methyl aromatics. Notably, the same product groups were obtained when these approaches were successfully extended to lignin. In addition, the formation of higher molecular weight side products was markedly supp...

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Aromatic Monomers by in Situ Conversion of Reactive Intermediates in the Acid-Catalyzed Depolymerization of Lignin

The first examples of reductive depolymerization of lignin are reported under metal-free conditions. Using hydrosilanes as reductants and B(C6F5)3 as a Lewis acid catalyst, wood lignin is efficiently converted to a narrow distribution of phenol derivatives at room temperature. A three-step methodology based on the selection of the wood species and the lignin extraction method followed by a convergent reductive depolymerization enables the production of four structurally defined aromatic compounds. The phenol products were successfully isolated in 7–24 wt% yield from lignin and 0.5–2.4 wt% yield from wood. The strategy is found to be robust and is applied to 15 different wood plants, including gymnosperm and angiosperm species. The efficiency of this novel methodology has been evaluated based on spectroscopic characterization of the lignin preparations and isolated yields of mono-aromatic products.

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Convergent reductive depolymerization of wood lignin to isolated phenol derivatives by metal-free catalytic hydrosilylation

Unprecedented organocatalytic reduction of lignin model compounds to phenols and primary alcohols using hydrosilanes

The reductive depolymerization of a variety of polymeric materials based on polyethers, polyesters, and polycarbonates is described using hydrosilanes as reductants and metal-free catalysts. This strategy enables the selective depolymerization of waste polymers as well as bio-based polyesters to functional chemicals such as alcohols and phenols at room temperature. Commercially available B(C6 F5)3 and [Ph3 C(+),B(C6 F5)4(-)] catalysts are active hydrosilylation catalysts in this procedure and they are compatible with the use of inexpensive and air-stable polymethylhydrosiloxane and tetramethyldisiloxane as reductants. A significant advantage of this recycling method is derived from its tolerance to the additives present in waste plastics and its ability to selectively depolymerize mixtures of polymers.

Room temperature organocatalyzed reductive depolymerization of waste polyethers, polyesters, and polycarbonates.

Lignin is the most abundant source of renewable ready-made aromatic chemicals for making sustainable polymers. However, the structural heterogeneity, high polydispersity, limited chemical functionality and solubility of most technical lignins makes them challenging to use in developing new bio-based polymers. Recently, greater focus has been given to developing polymers from low molecular weight lignin-based building blocks such as lignin monomers or lignin-derived bio-oils that can be obtained by chemical depolymerization of lignins. Lignin monomers or bio-oils have additional hydroxyl functionality, are more homogeneous and can lead to higher levels of lignin substitution for non-renewables in polymer formulations. These potential polymer feed stocks, however, present their own challenges in terms of production (i.e., yields and separation), pre-polymerization reactions and processability. This review provides an overview of recent developments on polymeric materials produced from lignin-based model compounds and depolymerized lignin bio-oils with a focus on thermosetting materials. Particular emphasis is given to epoxy resins, polyurethanes and phenol-formaldehyde resins as this is where the research shows the greatest overlap between the model compounds and bio-oils. The common goal of the research is the development of new economically viable strategies for using lignin as a replacement for petroleum-derived chemicals in aromatic-based polymers.

Elias Feghali

Papers

Convergent reductive depolymerization of wood lignin to isolated phenol derivatives by metal-free catalytic hydrosilylation

Unprecedented organocatalytic reduction of lignin model compounds to phenols and primary alcohols using hydrosilanes

Room temperature organocatalyzed reductive depolymerization of waste polyethers, polyesters, and polycarbonates.

Thermosetting Polymers from Lignin Model Compounds and Depolymerized Lignins

Toward Bio-Based Epoxy Thermoset Polymers from Depolymerized Native Lignins Produced at the Pilot Scale.