Lignin Functionalization for the Production of Novel Materials

Lignin is the largest natural source of aromatic molecules and thus represents a promising renewable chemical feedstock.Lignin has a complicated chemical structure and generally undergoes rapid condensation during its isolation from biomass. Condensation makes lignin more recalcitrant to further upgrading and to incorporation into materials.New analytical techniques are allowing a better understanding of native and isolated lignin’s structure, which in turn is allowing the development of improved isolation methods that minimize or prevent condensation.As a result of better analytics, improved chemical modification techniques are being performed on the lignin backbone to introduce new functional groups that were not present in the native structure. The functionalization of lignin allows improved blending of lignin with polymers and the development of new biobased materials. Lignin, a major constituent of lignocellulosic biomass, is the largest natural source of aromatic molecules and thus is an attractive feedstock for renewable chemical production. Direct incorporation of isolated lignin into materials has long been researched due to the idea’s simplicity and the scheme’s potentially high atom economy. However, due to its high chemical reactivity lignin is difficult to isolate without having it undergo uncontrolled condensation and repolymerization, which greatly hinder its ease of incorporation into polymers and other materials. Therefore, controlled chemical modifications have been and are being developed to increase lignin’s compatibility with existing materials. This review presents the latest advances in lignin extraction and functionalization and their potential for improving the production of lignin-based materials. Lignin, a major constituent of lignocellulosic biomass, is the largest natural source of aromatic molecules and thus is an attractive feedstock for renewable chemical production. Direct incorporation of isolated lignin into materials has long been researched due to the idea’s simplicity and the scheme’s potentially high atom economy. However, due to its high chemical reactivity lignin is difficult to isolate without having it undergo uncontrolled condensation and repolymerization, which greatly hinder its ease of incorporation into polymers and other materials. Therefore, controlled chemical modifications have been and are being developed to increase lignin’s compatibility with existing materials. This review presents the latest advances in lignin extraction and functionalization and their potential for improving the production of lignin-based materials. Due to increasing concerns of rising concentrations of atmospheric carbon dioxide, many countries have established programs and strategies to mitigate climate change [1.Hook M. Tang X. Depletion of fossil fuels and anthropogenic climate change – a review.Energy Policy. 2013; 52: 797-809Crossref Scopus (873) Google Scholar, 2.McMichael A.J. et al.Climate change and human health: present and future risks.Lancet. 2006; 367: 859-869Abstract Full Text Full Text PDF PubMed Scopus (1694) Google Scholar, 3.Urban M.C. Accelerating extinction risk from climate change.Science. 2015; 348: 571-573Crossref PubMed Scopus (1157) Google Scholar]. These concerns are notably leading to demands for sustainable alternatives to crude oil and are encouraging the scientific community to explore new renewable feedstocks for producing energy and materials [4.Carlsson A.S. 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Until now, lignin has been mostly used as a source of renewable energy by the pulp and paper industry, where lignin-containing black liquors derived from pulping processes are still largely burned to recover inorganics and energy for the mills [8.Bajpai P. Basic overview of pulp and paper manufacturing process.in: Green Chemistry and Sustainability in Pulp and Paper Industry. Springer, 2015: 11-39Crossref Google Scholar]. However, as the production of lignin exceeds the demand for its use as a fuel and as interest for renewable chemical has increased [9.Olsson M.R. et al.Exporting lignin or power from heat-integrated Kraft pulp mills: a techno-economic comparison using model mills.Nord. Pulp Pap. Res. J. 2006; 21: 476-484Crossref Google Scholar], increasing attention is being given to lignin valorization (see Glossary) into new chemicals and materials, since it is by far the most abundant natural source of renewable aromatic molecules on earth. Given the prominence of aromatic functionalities in important chemical sectors such as fragrances, flavors, polymers, coatings, and resins, these areas have all explored the use of lignin [10.Calvo-Flores F.G. Dobado J.A. Lignin as renewable raw material.ChemSusChem. 2010; 3: 1227-1235Crossref PubMed Scopus (669) Google Scholar,11.Ponnusamy V.K. et al.A review on lignin structure, pretreatments, fermentation reactions and biorefinery potential.Bioresource Technol. 2019; 271: 462-472Crossref PubMed Scopus (276) Google Scholar]. Although lignin has an extraordinary potential as a renewable feedstock, its use poses several challenges including its isolation and valorization. First, due to the biosynthetic process, its structure is variable especially across different types of plant species, in terms of monomer composition as well as in the type and order of the chemical bonds that hold these monomers together [12.Vanholme R. et al.Lignin biosynthesis and structure.Plant Physiol. 2010; 153: 895-905Crossref PubMed Scopus (1633) Google Scholar]. Second, the functional groups present in lignin make it a very reactive compound under most extraction conditions, limiting its further upgrading. Lignin’s original structure is incredibly difficult to maintain intact during most pulping and isolation procedures [13.Gierer J. Chemical aspects of Kraft pulping.Wood Sci. Technol. 1980; 14: 241-266Crossref Scopus (365) Google Scholar]. Moreover, despite significant improvements in the past decade, the qualitative and especially quantitative determination of lignin’s structural features remains challenging [14.Hatfield R. Fukushima R.S. Can lignin be accurately measured?.Crop Sci. 2005; 45: 832-839Crossref Scopus (317) Google Scholar,15.Lupoi J.S. et al.Recent innovations in analytical methods for the qualitative and quantitative assessment of lignin.Renew. Sust. Energ. Rev. 2015; 49: 871-906Crossref Scopus (238) Google Scholar]. Nevertheless, recent improvements in both lignin analytics and lignin functionalization have created new opportunities for the production of controlled lignin structures and the incorporation of these structures into materials, which we describe later. Most of the mass of lignocellulosic biomass is found in the macro- and micro-fibrils of the rigid walls of its plant cells, which essentially comprise three biopolymers: cellulose, hemicellulose, and lignin (Figure 1) [16.Isikgor F.H. Becer C.R. Lignocellulosic biomass: a sustainable platform for the production of bio-based chemicals and polymers.Polym. Chem. 2015; 6: 4497-4559Crossref Google Scholar]. The cellulose is in the form of bundled fibrils that are bound together by the hemicellulose and lignin giving the overall rigidity to the cell wall. Lignin is formed during radical coupling of phenylpropanoid units, such as p-coumaryl, coniferyl, and sinapyl monolignols [17.Higuchi T. Lignin biochemistry: biosynthesis and biodegradation.Wood Sci. Technol. 1990; 24: 23-63Crossref Scopus (451) Google Scholar]. The consequence of this is a random copolymer comprising syringyl (S), guaiacyl (G), and p-hydroxyphenyl (H) subunits [18.Ralph J. et al.Lignins: natural polymers from oxidative coupling of 4-hydroxyphenyl-propanoids.Phytochem. Rev. 2004; 3: 29-60Crossref Scopus (1127) Google Scholar], although the monomer composition varies across wood species. Softwoods like pine or spruce are composed mainly of guaiacyl units, while hardwoods such as birch or eucalyptus contain both guaiacyl and syringyl functionalities but with a higher fraction of syringyl units. para-Hydroxyphenyl monomers, together with the syringyl and guaiacyl units, are mostly found in grass lignin [19.Rinaldi R. et al.Paving the way for lignin valorisation: recent advances in bioengineering, biorefining and catalysis.Angew. Chem. Int. Ed. 2016; 55: 8164-8215Crossref PubMed Scopus (1269) Google Scholar]. Additional structural heterogeneity comes from the radical biosynthesis of lignin, not described in this review, but widely described in the literature [12.Vanholme R. et al.Lignin biosynthesis and structure.Plant Physiol. 2010; 153: 895-905Crossref PubMed Scopus (1633) Google Scholar,20.Sangha A.K. et al.Radical coupling reactions in lignin synthesis: a density functional theory study.J. Phys. Chem. B. 2012; 116: 4760-4768Crossref PubMed Scopus (81) Google Scholar]. The driving force for the synthesis of lignin is the formation of monolignol radicals, which have several resonance structures leading to the formation of several different C–O and/or C–C linkages [21.Rodrigues Mota T. et al.Plant cell wall composition and enzymatic deconstruction.Bioengineering. 2018; 5: 63-77Crossref Google Scholar, 22.Zakzeski J. et al.The catalytic valorization of lignin for the production of renewable chemicals.Chem. Rev. 2010; 110: 3552-3599Crossref PubMed Scopus (3312) Google Scholar, 23.Li Y. et al.An “ideal lignin” facilitates full biomass utilization.Sci. Adv. 2018; 11eeau2968Crossref Scopus (138) Google Scholar] (see potential model structures in Figure 2). Due to the complexity, heterogeneity, and variety that is present in lignin, elucidating its structure and associated properties is a nontrivial task [24.Yuan T.-Q. et al.Characterization of lignin structures and lignin–carbohydrate complex (LCC) linkages by quantitative 13C and 2D HSQC NMR spectroscopy.J. Agr. Food Chem. 2011; 59: 10604-10614Crossref PubMed Scopus (426) Google Scholar]. One of the main recurring issues is that lignin is difficult to isolate from its other components without modifying its structure. This leads to a trade-off where lignin must either be characterized in the presence of other biomass constituents or analyzed in a modified form. One example of these recurring issues is faced during gravimetric isolation techniques such as the Klason method, which is known to lead to significant lignin degradation due to the high acidity that is used [25.Hatfield R.D. et al.A comparison of the insoluble residues produced by the Klason lignin and acid detergent lignin procedures.J. Sci. Food Agric. 1994; 65: 51-58Crossref Scopus (246) Google Scholar]. Moreover, the high acidity necessary for the isolation of lignin can also lead to polysaccharide degradation and the formation of humins that are sometimes erroneously included with the lignin (often termed ‘pseudolignin’). A few types of isolated lignin can overcome these challenges. Among the most important are milled wood lignin (MWL) and cellulolytic enzyme lignin (CEL). The first is based on fine ball-milling of wood followed by extraction of the lignin using a solution of water and dioxane. The second also begins with ball-milling, but this is followed by several treatments with cellulase enzymes that can hydrolyze the polysaccharides leaving behind the lignin as a solid residue. However, these methods have several limitations. Notably, the extraction of lignin is less efficient compared with chemical methods, leaving some lignin behind, while the intensive ball-milling has the effect of partially depolymerizing lignin and inducing chemical modifications on its structure [26.Chang H. et al.Comparative studies on cellulolytic enzyme lignin and milled wood lignin of sweetgum and spruce.Holzforschung. 2009; 29: 153-159Crossref Scopus (326) Google Scholar,27.Capanema E. et al.How well do MWL and CEL preparations represent the whole hardwood lignin?.J. Wood Chem. Technol. 2015; 35: 17-26Crossref Scopus (34) Google Scholar]. Analysis of isolated lignins can thus be complicated by these structural modifications, which make the analytical results difficult to trace back to the native lignin structure. To overcome the need for lignin extraction, many researchers have focused on analyzing native lignin embedded in plant cell walls via nuclear magnetic resonance (NMR) spectroscopy. However, these techniques require the reduction of plant material to a gel-like structure through ball-milling and then finding suitable solvents capable of solubilizing whole-cell walls without yielding chemical modifications [28.Lu F. Ralph J. Solution-state NMR of lignocellulosic biomass.J. Biobased Mater. Bio. 2011; 5: 169-180Crossref Scopus (44) Google Scholar]. Due to lignin’s complex structure, the information provided by 1H NMR is often limited because the signals of the various protons overlap. 2D NMR, and particularly heteronuclear single quantum coherence (HSQC) NMR, leads to much more straightforward identification of the various lignin and carbohydrate functionalities, as the overlapping of peaks is completely avoided or largely minimized. The output of this NMR experiment is a bidimensional spectrum where each signal corresponds to a unique C–H bond in the analyzed sample. Similar to its use for whole-cell-wall characterization, NMR is frequently used to analyze isolated lignin. Despite the absence or reduced signal of carbohydrates, 2D NMR techniques still need to be used to resolve the different functionalities that would otherwise overlap in 1D techniques. Nevertheless, advances have been made to quantify chemical bonds in lignin via spectroscopy. 31P NMR is a powerful tool that can be used to quantify hydroxyl and phenolic groups, but still requires a functionalization step where the hydroxyl and phenolic groups of lignin are commonly phosphitylated via a reaction with 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (TMDP) [29.Meng X. et al.Determination of hydroxyl groups in biorefinery resources via quantitative 31P NMR spectroscopy.Nat. Protoc. 2019; 14: 2627-2647Crossref PubMed Scopus (176) Google Scholar]. Recently, Talebi Amiri and colleagues applied the HSQC zero (HSQC0) technique, more commonly used on proteins, to quantify chemical bonds present on the lignin backbone and predict the behavior of the polymer under catalytical hydrolytic conditions [30.Talebi Amiri M. et al.Establishing lignin structure–upgradeability relationships using quantitative 1H–13C heteronuclear single quantum coherence nuclear magnetic resonance (HSQC-NMR) spectroscopy.Chem. Sci. 2019; 10: 8135-8142Crossref PubMed Google Scholar]. Together with NMR methods, functional groups can also commonly be determined by infrared spectroscopy. Although powerful, the two techniques are not always optimized for a complex substrate like lignin and, particularly in whole-cell experiments, are commonly used to provide qualitative rather than quantitative information on the functionalities present in the biopolymer. Specifically, peak ratios can be used to provide some information on the relative ratios of the different functionalities present but not their actual amounts. Despite the challenges and limitations, several analytical techniques have been applied in the study of the isolated biopolymer. Among the properties of isolated lignin that are commonly reported is molecular weight. Gel permeation chromatography (GPC) is the typical tool for this kind of measurement, although it often requires functionalization to make the polymer soluble for analysis. Moreover, due to variable structures, no calibration standards for lignin exist. Although it is common to use polystyrene as a calibration standard, it cannot be considered to give an accurate measure of the molecular weight as the hydrodynamic volumes for the two polymers differ. In addition to this, an extra source of error can come from the detection systems used for the determination of molecular weights. An example of this was presented recently by Zinovyev and colleagues [31.Zinovyev G. et al.Getting closer to absolute molar masses of technical lignins.ChemSusChem. 2018; 11: 3259-3268Crossref PubMed Scopus (50) Google Scholar], who reported that when size exclusion chromatography was combined with multiangle light- scattering detectors (MALSs), molecular weight could be incorrectly estimated due the autofluorescence of lignin’s aromatic structure. Lignin molecule weight has also been investigated by mass spectroscopy, particularly matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). However, the heterogeneous nature of lignin and the challenge of ionizing all fractions equally severely limit the use of this technique for accurate characterization of lignin [32.Bowman A.S. et al.Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analysis for characterization of lignin oligomers using cationization techniques and 2,5-dihydroxyacetophenone (DHAP) matrix.Rapid Commun. Mass Sp. 2019; 33: 811-819Crossref PubMed Scopus (16) Google Scholar]. Taken together, the dramatic progress made in the past 15 years, especially with NMR techniques, has allowed the research community to have a much clearer understanding of lignin’s structure. This has translated into the ability to better relate lignin’s structure to its chemical reactivity and physical properties. This has been at the root of recent progress in the ability to control and tune lignin’s behavior for specific applications. Before valorizing, modifying, or incorporating lignin into materials, the biopolymer must be isolated [33.Hassan S. Williams G. Emerging technologies for the pretreatment of lignocellulosic biomass.Bioresource Technol. 2018; 262: 310-318Crossref PubMed Scopus (441) Google Scholar]. Several processes and methods have been developed over the years to achieve this separation. The pulp and paper industry is the predominant large-scale producer of lignin, where the biopolymer is a byproduct of the isolation and purification of cellulose [34.Carvajal J.C. et al.Comparison of lignin extraction processes: economic and environmental assessment.Bioresource Technol. 2016; 214: 468-476Crossref PubMed Scopus (84) Google Scholar]. The two most common and well-known methods for the industrial extraction of lignin are the Kraft (Figure 3, top pathway) and the sulfite processes (Figure 3, light-blue/gray pathway) [7.Sixta H. Handbook of Pulp. Wiley, 2006Crossref Scopus (445) Google Scholar], which are summarized in Box 1.Box 1Current Industrial Methods for Lignin IsolationThe two most industrially relevant processes to isolate lignin are the Kraft and sulfite processes [7.Sixta H. Handbook of Pulp. Wiley, 2006Crossref Scopus (445) Google Scholar]. These were developed and optimized mainly by the pulp and paper industry, which isolates pure cellulose from wood for further use in paper, cardboard production, and other applications.The Kraft process (Figure 3, top pathway) relies on the use of a basic aqueous solution of sodium sulfide at temperatures of 150–180°C. Under these conditions lignin breaks into oligomers that are substantially modified from their native structure but become soluble at pH values above 12, due to the deprotonation of the phenolic functional groups. Hemicellulose, rosin soaps, and inorganic compounds are also solubilized in the same aqueous solution, forming so-called black liquors. The liquors (after being dried) are normally burned in the recovery boiler of the industrial plant to isolate inorganic salts, while the thermal energy that is liberated from the combustion of lignin is then recovered to fulfil the energetic demand of the pulp mills. However, when the production of lignin exceeds the need for energy, the recovery of Kraft lignin can be achieved through a process called Lignoboost that leads to the isolation of a solid lignin after acidification with CO2 and sulfuric acid and subsequent filtration [101.Tomani P. The Lignoboost process.Cell. Chem. Technol. 2010; 44: 53-58Google Scholar].In the sulfite process (Figure 3, light-blue/gray pathway), lignin is removed using an aqueous solution of different sulfite or bisulfite salts with pH values ranging from 1 to 5. During the process, lignin is dissolved into the aqueous liquor together with hemicellulose, leaving cellulose behind as a solid. As for Kraft lignin, it is possible to recover lignin from sulfite pulping. In these cases, the ‘Magnefite’ process is used to recover magnesium lignosulfonates [102.Kvarnlöf N. Germgård U. Oxygen delignification of acid sulfite and bisulfite softwood pulps.Bioresources. 2015; 10: 3934-3947Crossref Scopus (4) Google Scholar], while the Howard process allows the isolation of calcium lignosulfonates by precipitation and filtration with CaO [103.Qureshi N. et al.Biorefineries: Integrated Biochemical Processes for Liquid Biofuels. Elsevier, 2014Google Scholar]. The two most industrially relevant processes to isolate lignin are the Kraft and sulfite processes [7.Sixta H. Handbook of Pulp. Wiley, 2006Crossref Scopus (445) Google Scholar]. These were developed and optimized mainly by the pulp and paper industry, which isolates pure cellulose from wood for further use in paper, cardboard production, and other applications. The Kraft process (Figure 3, top pathway) relies on the use of a basic aqueous solution of sodium sulfide at temperatures of 150–180°C. Under these conditions lignin breaks into oligomers that are substantially modified from their native structure but become soluble at pH values above 12, due to the deprotonation of the phenolic functional groups. Hemicellulose, rosin soaps, and inorganic compounds are also solubilized in the same aqueous solution, forming so-called black liquors. The liquors (after being dried) are normally burned in the recovery boiler of the industrial plant to isolate inorganic salts, while the thermal energy that is liberated from the combustion of lignin is then recovered to fulfil the energetic demand of the pulp mills. However, when the production of lignin exceeds the need for energy, the recovery of Kraft lignin can be achieved through a process called Lignoboost that leads to the isolation of a solid lignin after acidification with CO2 and sulfuric acid and subsequent filtration [101.Tomani P. The Lignoboost process.Cell. Chem. Technol. 2010; 44: 53-58Google Scholar]. In the sulfite process (Figure 3, light-blue/gray pathway), lignin is removed using an aqueous solution of different sulfite or bisulfite salts with pH values ranging from 1 to 5. During the process, lignin is dissolved into the aqueous liquor together with hemicellulose, leaving cellulose behind as a solid. As for Kraft lignin, it is possible to recover lignin from sulfite pulping. In these cases, the ‘Magnefite’ process is used to recover magnesium lignosulfonates [102.Kvarnlöf N. Germgård U. Oxygen delignification of acid sulfite and bisulfite softwood pulps.Bioresources. 2015; 10: 3934-3947Crossref Scopus (4) Google Scholar], while the Howard process allows the isolation of calcium lignosulfonates by precipitation and filtration with CaO [103.Qureshi N. et al.Biorefineries: Integrated Biochemical Processes for Liquid Biofuels. Elsevier, 2014Google Scholar]. These two procedures allow the isolation of lignin at industrial scales using aqueous solutions at different pH, but the severity of the preceding lignin isolation methods invariably leads to a highly condensed and chemically modified biopolymer due to side reactions. In both cases, the harsh conditions of basicity or acidity favor the formation of unstable intermediates on the lignin backbone via the elimination of water molecules (Figure 3). Condensation of these intermediates occurs through intra- or intermolecular reactions with other lignin oligomers, creating new, highly stable C–C linkages. These intermediates can also react with other chemical species present in solution [35.Crestini C. et al.On the structure of softwood Kraft lignin.Green Chem. 2017; 19: 4104-4121Crossref Google Scholar,36.Dimmel D. Gellerstedt G. Chemistry of alkaline pulping.in: Lignin and Lignans: Advances in Chemistry. CRC Press, 2010: 350-392Crossref Google Scholar]. The combination of all of these side reactions, as we discuss later, is one of the key factors that limits the formation of controlled chemical structures and, ultimately, the use of industrial lignins [37.Gellerstedt G. et al.Towards a new concept of lignin condensation in Kraft pulping. Initial results.C. R. Biol. 2004; 327: 817-826Crossref PubMed Scopus (67) Google Scholar,38.Balakshin M.Y. et al.Elucidation of the structures of residual and dissolved pine Kraft lignins using an HMQC NMR technique.J. Agr. Food Chem. 2003; 51: 6116-6127Crossref PubMed Scopus (142) Google Scholar]. In addition to the industrial methods described earlier, which use aqueous solutions for the extraction of lignin, another class of emerging pulping and lignin valorization procedures use organic solvents at temperatures ranging from 80°C to 250°C to fractionate the biomass. These novel methods are usually referred to as Organosolv processes. The resulting Organosolv lignins can be obtained using several solvent mixtures and conditions [39.Nitsos C. et al.Organosolv fractionation of softwood biomass for biofuel and biorefinery applications.Energies. 2017; 11: 50Crossref Scopus (81) Google Scholar]. Alcohols or dioxane are the most frequently utilized organic solvents for this process and are often combined with water in concentrations generally varying from 40% to 100%. To help the delignification, inorganic acid catalysts (e.g., H2SO4, HCl) or organic acids (e.g., formic acid) are added at low concentrations (0.1–2%) [40.Lancefield C.S. et al.Pre-treatment of lignocellulosic feedstocks using biorenewable alcohols: towards complete biomass valorisation.Green Chem. 2017; 19: 202-214Crossref Google Scholar, 41.Zhang Z. et al.Organosolv pretreatment of plant biomass for enhanced enzymatic saccharification.Green Chem. 2016; 18: 360-381Crossref Google Scholar, 42.Das A. et al.Lignin conversion to low-molecular-weight aromatics via an aerobic oxidation-hydrolysis sequence: comparison of different lignin sources.ACS Sustain. Chem. Eng. 2018; 6: 3367-3374Crossref Scopus (102) Google Scholar]. These acid-catalyzed processes, although with a lesser extent, often lead to condensation and repolymerization reactions similar to those described earlier, hindering lignin’s subsequent upgrading or use [43.Gordobil O. et al.Assessment of technical lignins for uses in biofuels and biomaterials: structure-related properties, proximate analysis and chemical modification.Ind. Crop. Prod. 2016; 83: 155-165Crossref Scopus (160) Google Scholar] (Figure 3, red pathway). Recently, Shuai and colleagues [44.Shuai L. et al.Formaldehyde stabilization facilitates lignin monomer production during biomass depolymerization.Science. 2016; 354: 329-333Crossref PubMed Scopus (762) Google Scholar] reported that a modified acid-catalyzed Organosolv process with the addition of aldehydes such as formaldehyde could greatly improve the quality and upgradeability of the resulting lignin. Acetal formation is highly favorable and maintains intact a large majority of the ether bonds originally present in the lignin. Because of this functionalization, the lignin obtained can be catalytically depolymerized to monomers at near-theoretical yields (i.e., based on the cleavage of the number of β–O–4 linkages originally present in the lignin), which demonstrated near-complete functionalization [44.Shuai L. et al.Formaldehyde stabilization facilitates lignin monomer production during biomass depolymerization.Science. 2016; 354: 329-333Crossref PubMed Scopus (762) Google Scholar]. To produce aromatic chemicals, many recent studies have explored the catalytic depolymerization of lignin to aromatic monomers that could be used as a source of fuels or as building blocks for polymers and functional materials. These approaches are beyond the scope of this review, but several recent articles cover this subject [45.Schutyser W. et al.Chemicals from lignin: an interplay of lignocellulose fractionation, depolymerisation, and upgrading.Chem. Soc. Rev. 2018; 47: 852-908Crossref PubMed Google Scholar, 46.Zhu J. et al.Catalytic activation of unstrained C(aryl)–C(aryl) bonds in 2,2′-biphenols.Nat. Chem. 2019; 11: 45-51Crossref PubMed Scopus (56) Google Scholar, 47.Lan W. Luterbacher J.S. Preventing lignin condensation to facilitate aromatic monomer production.Chimia. 2019; 73: 591-598Crossref PubMed Scopus (31) Google Scholar]. Substituting aromatic materials derived from the synthesis of individual fossil-based building blocks with extracted lignin poses several challenges, most of which come from the fact that most of the briefly reviewed extraction techniques, especially those used at an industrial scale, have little to no control over the lignin’s final structure. As characte