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.

The structure of the lignin in wheat straw has been investigated by a combination of analytical pyrolysis, 2D-NMR, and derivatization followed by reductive cleavage (DFRC). It is a p-hydroxyphenyl-guaiacyl-syringyl lignin (with an H:G:S ratio of 6:64:30) associated with p-coumarates and ferulates. 2D-NMR indicated that the main substructures present are β-O-4′-ethers (∼75%), followed by phenylcoumarans (∼11%), with lower amounts of other typical units. A major new finding is that the flavone tricin is apparently incorporated into the lignins. NMR and DFRC indicated that the lignin is partially acylated (∼10%) at the γ-carbon, predominantly with acetates that preferentially acylate guaiacyl (12%) rather than syringyl (1%) units; in dicots, acetylation is predominantly on syringyl units. p-Coumarate esters were barely detectable (<1%) on monomer conjugates released by selectively cleaving β-ethers in DFRC, indicating that they might be preferentially involved in condensed or terminal structures.

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Structural Characterization of Wheat Straw Lignin as Revealed by Analytical Pyrolysis, 2D-NMR, and Reductive Cleavage Methods

NMR fingerprinting of the components of finely divided plant cell walls swelled in DMSO has been recently described. Cell wall gels, produced directly in the NMR tube with perdeutero-dimethylsulfoxide, allowed the acquisition of well resolved/dispersed 2D 13C–1H correlated solution-state NMR spectra of the entire array of wall polymers, without the need for component fractionation. That is, without actual solubilization, and without apparent structural modification beyond that inflicted by the ball milling and ultrasonication steps, satisfactorily interpretable spectra can be acquired that reveal compositional and structural details regarding the polysaccharide and lignin components in the wall. Here, the profiling method has been improved by using a mixture of perdeuterated DMSO and pyridine (4 : 1, v/v). Adding pyridine provided not only easier sample handling because of the better mobility compared to the DMSO-d6-only system but also considerably elevated intensities and improved resolution of the NMR spectra due to the enhanced swelling of the cell walls. This modification therefore provides a more rapid method for comparative structural evaluation of plant cell walls than is currently available. We examined loblolly pine (Pinus taeda, a gymnosperm), aspen (Populus tremuloides, an angiosperm), kenaf (Hibiscus cannabinus, an herbaceous plant), and corn (Zea mays L., a grass, i.e., from the Poaceae family). In principle, lignin composition (notably, the syringyl : guaiacyl : p-hydroxyphenyl ratio) can be quantified without the need for lignin isolation. Correlations for p-coumarate units in the corn sample are readily seen, and a variety of the ferulate correlations are also well resolved; ferulates are important components responsible for cell wall cross-linking in grasses. Polysaccharide anomeric correlations were tentatively assigned for each plant sample based on standard samples and various literature data. With the new potential for chemometric analysis using the 2D NMR fingerprint, this gel-state method may provide the basis for an attractive approach to providing a secondary screen for selecting biomass lines and for optimizing biomass processing and conversion efficiencies.

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Solution-state 2D NMR of ball-milled plant cell wall gels in DMSO-d6/pyridine-d5

Lignin is the most abundant aromatic polymer in nature. Due to its high amount of phenolic compounds storage, lignin is considered as an alternative source for various polymers and biomaterials production. Except for the native lignin in lignocellulose, a massive amount of technical lignin is being produced daily all over the world. However, the complex structure and low reactivity of lignin limit its further applications and currently, most of the lignin is burned for generating energy. Therefore, the depolymerization of lignin is considered one of the important challenges in lignin utilization. Methods for lignin depolymerization can be divided into thermochemical treatment, mechanical treatment, chemical catalysis, and biological treatment. Different methods for lignin depolymerization, their characteristics and products are extensively discussed in this review.

Lignin utilization: A review of lignin depolymerization from various aspects

To characterize the lignin structures and lignin-carbohydrate complex (LCC) linkages, milled wood lignin (MWL) and mild acidolysis lignin (MAL) with a high content of associated carbohydrates were sequentially isolated from ball-milled poplar wood. Quantification of their structural features has been achieved by using a combination of quantitative (13)C and 2D HSQC NMR techniques. The results showed that acetylated 4-O-methylgluconoxylan is the main carbohydrate associated with lignins, and acetyl groups frequently acylate the C2 and C3 positions. MWL and MAL exhibited similar structural features. The main substructures were β-O-4' aryl ether, resinol, and phenylcoumaran, and their abundances per 100 Ar units changed from 41.5 to 43.3, from 14.6 to 12.7, and from 3.7 to 4.0, respectively. The S/G ratios were estimated to be 1.57 and 1.62 for MWL and MAL, respectively. Phenyl glycoside and benzyl ether LCC linkages were clearly quantified, whereas the amount of γ-ester LCC linkages was ambiguous for quantification.

Characterization of Lignin Structures and Lignin–Carbohydrate Complex (LCC) Linkages by Quantitative 13C and 2D HSQC NMR Spectroscopy

This study has been supported by the Spanish MEC (project AGL2005-01748), the EU contract NMP2-CT-2006-26456 (BIORENEW) and the CSIC project 2006-4-OI-39. JR thanks the Spanish CSIC for an I3P fellowship; GM thanks the Spanish Ministry of Education for a FPI fellowship.

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Occurrence and structural characteristics of highly acylated (acetylated and/or p-coumaroylated) native lignins from diverse herbaceous plants

Enzymatic bleaching of flax alkaline pulp is intended here by using laccase-mediator systems (LMS) based on Pleurotus eryngii, Trametes versicolor and Pycnoporus cinnabarinus laccases and two mediators. The efficiency of the different LMS treatments was compared in terms of brightness, kappa number and viscosity modifications. Furthermore, changes in the molecular structure of residual lignin and lignin/carbohydrate ratio were analyzed by pyrolysis-gas chromatography-mass spectrometry of the treated pulps. Pycnoporus cinnabarinus and T. versicolor laccases in the presence of 1-hydroxybenzotriazole (HBT), were the most effective for flax pulp delignification. Up to 20% ISO brightness increase with respect to the initial pulp and a decrease of kappa number from 9 to 3 were attained after alkaline extraction of the pulps treated with those LMS. Pycnoporus cinnabarinus laccase plus HBT gave also the best selectivity in lignin removal. Analytical pyrolysis of the LMStreated pulps showed a selective removal of lignin against cellulose and a strong alteration of the residual lignin with preferential removal of the syringyl units. Pycnoporus cinnabarinus LMSHBT gave the best results in terms of decrease of lignin/cellulose and syringyl/guaiacyl ratios, in agreement with data from pulp parameters. For the above reasons laccase from P. cinnabarinus and HBT were selected for further assays including a peroxide step. Up to 89% relative delignification rate (attaining 1.3 kappa number) and an increase of 44% ISO brightness were attained after LMSHBT-peroxide bleaching. Ongoing studies focus on improvement of a LMSHBT-based totally chlorine free sequence for flax pulp bleaching.

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Flax pulp bleaching and residual lignin modification by laccase-mediator systems

Abstract Olive tree pruning (OTP) is an abundant and inexpensive agricultural lignocellulosic residue that is an interesting feedstock for producing bioethanol and other bio-products in the context of lignocellulosic biorefineries. However, the presence of lignin in OTP hinders the transformation processes as it limits the access to cell wall polysaccharides. On the other hand, the aromatic/phenolic structure of the lignin polymer makes it an interesting raw material for producing chemicals, fuels and other commodities that are nowadays produced from fossil fuels. Thus, the knowledge of the OTP lignin structure is crucial to develop tailor-made pretreatments for their removal as well as for additional valorization of the lignin polymer. In this work, the OTP lignin was isolated as milled wood lignin (MWL), a lignin preparation that is considered representative of the native lignin, and characterized by two-dimensional nuclear magnetic resonance (2D-NMR) and thioacidolysis. The results demonstrated that the lignin is mainly composed of guaiacyl (G) and syringyl (S) lignin units in similar abundances (S/G ratio of ~1), with minor amounts of p-hydroxyphenyl (H) units. The most abundant lignin inter-unit linkages are β-O-4′ alkyl-aryl ethers (75% of all linkages), followed by the condensed phenylcoumarans (12%) and resinols (8%), and with lower amounts of dibenzodioxocins (2%) and spirodienones (3%). The analysis of the thioacidolysis dimers gave additional information regarding the distribution of the lignin units involved in condensed interunit linkages, including 5-5′, 4-O-5′, β-5′, β-1′ and β-β′. The high lignin content (25%), together with the relatively low S/G ratio and the abundance of condensed (carbon-carbon linked) structures, points to a low reactivity of OTP lignin during delignification pretreatments.

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Structural characteristics of lignin in pruning residues of olive tree (Olea europaea L.)

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Corrections to: Lipids from flax fibers and their fate after alkaline pulping

This work has been supported by the European contract QLK5-99-1357, and the Spanish project 1FD97-742. The first author acknowledges a fellowship from the Spanish Agency for Iberoamerican Cooperation.

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José Carlos del Río Andrade

Papers

Occurrence and structural characteristics of highly acylated (acetylated and/or p-coumaroylated) native lignins from diverse herbaceous plants

Flax pulp bleaching and residual lignin modification by laccase-mediator systems

Structural characteristics of lignin in pruning residues of olive tree (Olea europaea L.)

Corrections to: Lipids from flax fibers and their fate after alkaline pulping

Eucalyptus globulus wood and pulp localization pof sterols involved in pitch deposition using filipin fluorescent staining