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Review
. 2016 Jul 11;55(29):8164-215.
doi: 10.1002/anie.201510351. Epub 2016 Jun 17.

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

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Free PMC article
Review

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

Roberto Rinaldi et al. Angew Chem Int Ed Engl. .
Free PMC article

Abstract

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.

Keywords: bioengineering; biorefining; catalysis; lignin; lignocellulose.

Figures

Scheme 1
Scheme 1
Summary of the phenylpropanoid pathway focusing on the formation of native lignin monomer precursors.11, 66 Asterisks (*) represent points at which the intermediate itself may be introduced as a monomer or used to create a new monomer that is used by the plant in its lignification following genetic modification (see Scheme 2). Enzymes are abbreviated as follows: phenylalanine ammonia‐lyase (PAL); cinnamate 4‐hydroxylase (C4H); tyrosine ammonia‐lyase (TAL); 4‐coumarate: CoA ligase (4CL); p‐coumarate 3‐hydroxylase (C3H); shikimate/quinate hydroxycinnamoyltransferase (HCT); cinnamoyl‐CoA reductase (CCR); cinnamyl alcohol dehydrogenase (CAD); caffeoyl shikimate esterase (CSE); caffeic acid O‐methyltransferase (COMT); caffeoyl‐CoA O‐methyltransferase (CCoAOMT); ferulate 5‐hydroxylase (F5H); hydroxycinnamaldehyde dehydrogenase (HCALDH). These enzymes were often named according to the assumed substrates at the time of discovery. They therefore do not necessarily reflect the true preferred substrate. For example, the preferred substrate for F5H is actually coniferaldehyde;95, 96 some have suggested renaming it CAld5H, but the name has not been well adopted. Similarly, the preferred substrates for COMT are 5‐hydroxyconiferaldehyde or 5‐hydroxyconiferyl alcohol;95, 97 AldOMT has been suggested as an alternative name,98 although COMT persists as the preferred name among lignin researchers.
Scheme 2
Scheme 2
Incorporation of non‐native monomers in the lignification process, via genetic modification to the phenylpropanoid biosynthetic pathway (Scheme 1, see asterisked intermediates that are either the new monomers themselves, or give rise to a new monomer via the subsequent transformations shown here). The incorporation of new monomers (highlighted in red) by the same radical coupling modes gives rise to new structures in the lignin polymer (Table 3), mainly as a result of new opportunities for rearomatisation of the intermediate quinone methides: a) In CCoAOMT‐deficient softwoods, caffeyl alcohol is incorporated into the lignins producing benzodioxane structures. Some plants, such as vanilla, make lignin in their seed coats entirely from caffeyl alcohol, producing a polymer that is almost entirely composed of long chains of benzodioxane units. b) Employing a novel monolignol transferase enzyme, FMT, Feruloyl‐CoA can be conjugated to a monolignol, coniferyl alcohol (CA) or sinapyl alcohol (SA), to produce monolignol ferulate conjugates. Formation of labile ester linkages in the lignin polymer backbone can be achieved via lignification with a proportion of its monomer pool as these monolignol ferulate conjugates. Conjugates of this type are incorporated into the lignin structure in the same manner as conventional monomers.1 c) Ferulic acid itself can be formed either via the action of the enzyme HCALDH or by downregulation of CCR (Scheme 1). During lignification, double‐β‐O‐4‐coupling is undergone, to yield acid‐labile acetal bonds in the lignin. d) The intermediate, coniferaldehyde, accumulates in CAD‐deficient plants and can be incorporated into lignins via β‐O‐4‐coupling (shown) as well as other modes (not shown); the β‐O‐4‐coupling product is unsaturated due to the acidity of the β‐proton in the quinone methide intermediate that allows rearomatisation via H‐abstraction. e) In COMT‐deficient angiosperms, the intermediate 5‐hydroxyconiferaldehyde that accumulates can still be reduced by CAD, producing 5‐hydroxyconiferyl alcohol that, like caffeyl alcohol, produces benzodioxane analogues in the lignin. f) Sinapaldehyde also incorporates into lignins in CAD‐deficient angiosperms.
Scheme 3
Scheme 3
High‐throughput multi‐gene engineering scheme.131 Final field trial stages are of critical importance to identify potential environmental impacts, (e.g., toxicity to insects, impact on soil chemistry). For a thorough review of environmental risk assessments for genetically‐engineered trees, the reader is directed to other literature.154 Adapted from Plant Sci. 2013, 212, 72–101.
Figure 1
Figure 1
Topochemical distribution of likely G (magenta) and S (green) enriched lignin domains of lignocellulose feedstocks: monochrome fluorescence images of Pine (A) and Poplar (B) woods, and; the corresponding images coloured according to the fluorescence emissions (C and D, respectively); spectral image of Salix chilensis demonstrating the difference in guaiacyl and syringyl distributions in vessels and middle lamella from fibre walls (E); uniform distribution of lignin units in Acacia melanoxylon (F) and Eucalyptus nitens (G). The scale bars represent 60 μm. Reproduced with permission from IAWA Journal 2013, 34, 3–19, copyright 2013 Brill.160
Scheme 4
Scheme 4
Degradation of (native) spirodienone (F) structures under mildly acidic conditions.164
Figure 2
Figure 2
Calculated ΔH r ranges for homo‐coupling and cross‐coupling reactions, yielding a variety of lignin linkages and bonding motifs (M06‐2X/6‐311++G(d,p) level of theory). Adapted with permission from J. Phys. Chem. B 2012, 116, 4760–4768.181 Copyright 2012 American Chemical Society.
Scheme 5
Scheme 5
Diagram highlighting the bonding motifs and potential linkages as targets for depolymerisation (in green), % occurrence values from the literature, and bond dissociation energies for a range of commonly encountered linkages/bonding motifs in native lignins.177, 178, 179, 180, 186, 187 It is important to point out that many of these % occurrence values, although reported in the literature, are unlikely or even untenable: β‐1 moieties likely do not exceed 1–2 % of all structures (substantially lower than the 9 % reported); it is impossible to encounter 19–27 % of 5‐5‐units in any lignin (it is probably restricted to a maximum of approximately 9 % in softwoods), and; the abundance of 4‐O‐5‐linkages in softwoods is almost certainly much lower than the 4–7 % claimed.
Figure 3
Figure 3
Influence of pretreatment severity on the nature of a processed lignin as revealed by HSQC NMR spectroscopy. Knowing the method by which a lignin sample is prepared does not yield sufficient information about structural properties—it must be characterised. Here, a comparison is drawn between A) a maize Enzyme Lignin (EL), isolated in the lab, with B–D) lignins that are precipitated from an acetosolv process,195 in which acetic acid is the organic solvent. As can be seen by examining especially the β‐ether A correlations (cyan) but also the general nature of the aromatics, the mild process in (B) produces a rather native‐like lignin, with β‐ethers largely intact, and only a little “distortion” of the aromatics; the “lignin” does however contain significant levels of polysaccharide‐derived material (as seen by the additional grey peaks). With the medium‐severity treatment (C), which uses added mineral acid and a lower level of AcOH, β‐syringyl ethers have disappeared, the β‐ether level in general is lower, more tricin has detached from the polymer, and the aromatics are decidedly more complex. Under the highest severity conditions, in (D), no recognizable structural features (other than methoxyl and general aromatic signals) are evident—it has no β‐ethers. Any or all of these could be marketed as “acetosolv” lignin yet, clearly, those processes that rely on β‐ether cleavage may be wholly effective with the low‐severity material but would be completely worthless against the high‐severity material; material D is not valueless, nevertheless the method for valorisation will be highly different from material B. At some point, when effective processes for lignin utilisation evolve, techno‐economic analysis must be used to optimise the production of not just the sugars or pulp, but also the best lignin component, to fully optimise the profitability of the biorefinery. L:W=liquor to wood ratio. Contours in the NMR spectra, where they are sufficiently well resolved, are color‐coded to match the structures below; overlapping peaks are simply colored gray along with peaks from polysaccharides or other unidentified materials.
Figure 4
Figure 4
Reaction profiles for Kraft delignification of two native softwood biomass feedstocks: a) evolution of Spruce wood lignin into the liquor (green) as a function of cooking time and temperature, and; b) the quantification of β‐ethers in Pine wood lignin (quantified via analytical acidolysis), isolated from the liquor (light red) and residual lignin in the pulp (dark red), as a function of cooking time and temperature. For each graph, the programmed temperature (blue) increases steadily up to a fixed maximum of 170 °C.188, 190
Scheme 6
Scheme 6
Reaction pathways for the conversion of β‐O‐4‐rich native lignins to recalcitrant and highly‐condensed/cross‐linked Kraft lignins via a quinone methide (QM) intermediate (shaded green). The Kraft lignins are characterised by C‐C linkages with high bond dissociation energies (86–118 kcal mol−1, shaded red).203 For clarity, the Scheme depicts only G‐units.
Figure 5
Figure 5
Superimposed gel permeation chromatography (GPC) traces of the black liquor obtained following varying extents of delignification from Pine wood.190
Scheme 7
Scheme 7
a) Reaction channels (based on the acidolysis method) for the acid‐catalysed depolymerisation/degradation of lignin under Organosolv conditions, via “Hibbert's ketone” intermediates.205, 228 b) Inset highlighting measured concentrations of sum and individual species along the pathway of acidolysis of milled wood lignin (Pine), as a function of reaction time. For clarity, the scheme depicts only G‐units.
Figure 6
Figure 6
Graph highlighting relative rates of β‐O‐4 ether cleavage for phenolic and non‐phenolic model compounds, under 0.2 m aqueous H2SO4/150 °C conditions. Adapted with permission from ACS Sustain. Chem. Eng. 2014, 2, 472–485.230 Copyright 2014 American Chemical Society.
Scheme 8
Scheme 8
Process chains for valorisation of lignin isolated from conventional fractionation processes and from the emerging catalytic upstream biorefining processes based on Early‐stage Catalytic Conversion of Lignin (ECCL).
Figure 7
Figure 7
Holocellulosic fractions derived from two different Ni‐catalysed up‐stream processes: A) Raney‐Ni catalysed removal of lignin from Poplar wood chips (pulp conducive to further downstream processing198), and B) Ni/C catalysed removal of lignin from Birch wood saw dust (pulp not conducive to enzymatic hydrolysis281). Image (A) reproduced with permission from Angew. Chem. Int. Ed. 2014, 53, 8634–8639; Angew. Chem. 2014, 126, 8778–8783. Copyright 2014 John Wiley and Sons. Image (B) reproduced with permission from Energy Environ. Sci. 2013, 6, 994–1007. Copyright 2013 Royal Society of Chemistry.
Scheme 9
Scheme 9
Two alternative approaches to depolymerisation of low‐value lignin streams: a) A funneling scheme for the stepwise, convergent generation of a limited number of end products, exemplified here for benzene, via depolymerisation‐dealkylation‐hydrodeoxygenation (HDO); b) A stepwise approach whereby lignin is first depolymerised under mild conditions affording high‐value fine chemicals, and residual technical lignin is then treated by more harsh depolymerisation conditions to afford fine/bulk chemicals and transportation fuels (and remnant material is used as fuel to heat/power the process). For the funneling scheme, downstream products are not necessarily of higher value, although costs associated with separation are reduced.
Figure 8
Figure 8
Graph representing Equation (1) for various P values. For lignin with a low percentage of cleavable bonds, it is close to impossible to obtain high individual yields of products under low severity conditions. Lignin varieties with a high proportion of reactive linkages (β‐O‐4, or esters in genetically modified plants) are required for high‐yield depolymerisation to be possible.
Scheme 10
Scheme 10
A variety of strategies for selective cleavage of the β‐O‐4 lignin linkages (represented by a simple dimer model compound), via C−O cleavage (green), C−C cleavage (red) and benzylic oxidation (blue).
Scheme 11
Scheme 11
Mild oxidative pathways for cleavage of β‐ether lignin model compounds, via selective oxidation of the secondary alcohol functional group. The “Ar” groups represent simple aryl functionalities. For methods expanded to actual lignins, the results are indicated below the reaction scheme.
Scheme 12
Scheme 12
Mild oxidative pathways catalysed by vanadium‐oxo complexes, CuCl/TEMPO or Cu(OTf)/TEMPO, for cleavage of β‐O‐4‐linked lignin model species: (upper) 1‐(3,5‐dimethoxyphenyl)‐2‐(2‐methoxyphenoxy)propane‐1,3‐diol, and: (lower) 1‐(4‐hydroxy‐3,5‐dimethoxyphenyl)‐2‐(2‐methoxyphenoxy)propane‐1,3‐diol.
Scheme 13
Scheme 13
Mild reductive cleavage of a simple β‐ether model compound, using excess triethylsilane and catalytic tris(perfluorophenyl)borane.326 For experiments with actual lignins, the results are indicated below the reaction scheme.
Scheme 14
Scheme 14
Select mild redox‐neutral cleavage pathways, a–g), for β‐ether lignin model substrates. For experiments performed using actual lignins, the results are indicated below the reaction scheme.
Figure 9
Figure 9
Comparison of lignin monomer yields via catalytic depolymerisation (green) vs. thioacidolysis (orange), of differing lignin samples, as a measure of degree of condensation.194
Figure 10
Figure 10
Images for dissolution of an a) Organosolv lignin and b) Kraft lignin (in water from 20 to 225 °C), taken in a high‐pressure autoclave equipped with an optical window. Adapted with permission from ChemSusChem 2011, 4, 369–378.375 Copyright 2011 John Wiley and Sons.
Figure 11
Figure 11
HSQC NMR spectra highlighting differences in reactivity of phenol in the presence of a) methanol or b) ethanol as solvent: with methanol, cross peaks corresponding to methylene bridges between phenol units are observed; with ethanol, C2‐alkylated phenols are instead visible. Adapted with permission from Green Chem. 2015, 17, 4941–4950.288 Copyright 2015 Royal Society of Chemistry.

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