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

TLDR: 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.

Abstract: 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.

Figures

Fig. 10 Mechanism of lignin stabilisation during formaldehyde-assisted lignin extraction, forming acid-stable dioxane structures and m-hydroxymethyl groups.226

Fig. 9 a-Alkoxylation during acid-catalysed organosolv fractionation in concentrated alcohols.

Fig. 7 Oxidative lignin chemistry. Proposed oxidation mechanisms are depicted in Fig. S1 (ESI†).

Fig. 25 Schematic representation of biocatalytic funneling strategies.

Fig. 16 Reductive lignin depolymerisation with hydrosilanes, followed by hydrolysis in a subsequent step.349

Fig. 19 Stabilisation mechanism during ethylene glycol-assisted ACD.97,204,205,375,376

Frequently asked questions

Q1. What are the contributions mentioned in the paper "Chemicals from lignin: an interplay of lignocellulose fractionation, depolymerisation, and upgrading" ?

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. 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. 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. 

Q2. What have the authors stated for future works in "Chemicals from lignin: an interplay of lignocellulose fractionation, depolymerisation, and upgrading" ?

Finally, to guide and improve future research on fractionation/ depolymerisation, the authors would like to point out a few critical remarks. As last, the authors would like to stimulate the lignin research community to take into account the entire ( native ) lignin fraction in future fractionation and lignin depolymerisation studies. When these prerequisites are fulfilled, fractionation and depolymerisation methods can be evaluated over different studies. 

Q3. What is the role of alkaline media in lignin cleavage?

Alkaline media enable the cleavage of lignin–carbohydrate bonds, the fragmentation of lignin via cleavage of the b-O-4 motifs, the solubilisation of the resulting fragments, and eventually lignin degradation/repolymerisation. 

Q4. What can be derived from depolymerisation studies?

While analytic methods such as 2D NMR and thioacidolysis make it possible to verify and compare the structural characteristics of lignins, valuable information regarding lignin reactivity can also be derived from depolymerisation studies. 

Q5. What is the prevailing pathway for cleavage of the b-O-4 motif?

In presence of a strong nucleophilic anion such as HS (Kraft pulping), the prevailing pathway is the cleavage of the b-O-4 motif via the formation of episulfide intermediates (11).82,85,86,89 

Q6. What are the common commercial lignins used in depolymerisation studies?

Lignin depolymerisation is frequently studied with commercial or technical lignins such as softwood kraft lignin (Indulin AT), Alcell lignin, and Protobind 1000, which are all derived from optimised fractionation processes and therefore represent industrially relevant lignin substrates. 

Q7. Why is a pretreatment step used to reduce biomass recalcitrance?

Because many physico-chemical factors of raw biomass hinder the direct biological deconstruction of (hemi)cellulose, a pretreatment step is usually applied to reduce biomass recalcitrance. 

Q8. What is the method for generating a small set of products?

Two acid-catalysed depolymerisation methods that have recently been developed include an in situ product stabilisation step and generate a small set of products. 

Q9. What is the effect of meta and orthocleavage pathways on pyruvate and?

It was also shown by Johnson et al. that employing meta and orthocleavage pathways of protocatechuate and catechol, two key central intermediates, result in different pyruvate, succinate, and acetyl-CoA yields, thus affecting product yields of products derived from central carbon metabolism (e.g. lactic acid (30) from pyruvate). 

Q10. How did Li et al. achieve the highest monomer yield in ethanol?

Li et al. performed liquid-phase reforming of softwood kraft lignin with MoC/C at 180 1C in various solvents (ethanol, methanol, isopropanol, and water), and also reached the highest monomer yield in ethanol (28 wt%; deoxygenated aromatics, methoxyphenols and benzyl alcohols). 

Q11. How can the authors prevent repolymerisation of reactive intermediates?

This can be accomplished by specifically aiming to prevent repolymerisation, whereas an alternative strategy is to enhance the reactivity of lignin towards depolymerisation, hereby increasing the rate of depolymerisation relative to the rate of repolymerisation (Fig. 23). 

Q12. What are the yields of deoxygenated aromatics and (methoxy)phenol?

The monomer yields (sum of deoxygenated aromatics and (methoxy)phenols) are generally below 20 wt%, with a few studies reporting higher yields.