Lignin is a class of complex organic polymers that form important structural materials in the support tissues of vascular plants and some algae. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity and do not rot easily. Chemically, lignins are cross-linked phenolic polymers.
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Lignin was first mentioned in 1813 by the Swiss botanist A. P. de Candolle, who described it as a fibrous, tasteless material, insoluble in water and alcohol but soluble in weak alkaline solutions, and which can be precipitated from solution using acid. He named the substance “lignine”, which is derived from the Latin word lignum, meaning wood. It is one of the most abundant organic polymers on Earth, exceeded only by cellulose. Lignin constitutes 30% of non-fossil organic carbon and 20-35% of the dry mass of wood. The Carboniferous Period (geology) is in part defined by the evolution of lignin.
The composition of lignin varies from species to species. An example of composition from an aspen sample is 63.4% carbon, 5.9% hydrogen, 0.7% ash, and 30% oxygen (by difference), corresponding approximately to the formula (C31H34O11)n. As a biopolymer, lignin is unusual because of its heterogeneity and lack of a defined primary structure. Its most commonly noted function is the support through strengthening of wood (mainly composed of xylem cells and lignified sclerenchyma fibres) in vascular plants.
Global commercial production of lignin is around 1.1 million metric tons per year and is used in a wide range of low volume, niche applications where the form but not the quality is important.
Lignin fills the spaces in the cell wall between cellulose, hemicellulose, and pectin components, especially in vascular and support tissues: xylem tracheids, vessel elements and sclereid cells. It is covalently linked to hemicellulose and therefore cross-links different plant polysaccharides, conferring mechanical strength to the cell wall and by extension the plant as a whole. It is particularly abundant in compression wood but scarce in tension wood, which are types of reaction wood.
Lignin plays a crucial part in conducting water in plant stems. The polysaccharide components of plant cell walls are highly hydrophilic and thus permeable to water, whereas lignin is more hydrophobic. The crosslinking of polysaccharides by lignin is an obstacle for water absorption to the cell wall. Thus, lignin makes it possible for the plant's vascular tissue to conduct water efficiently. Lignin is present in all vascular plants, but not in bryophytes, supporting the idea that the original function of lignin was restricted to water transport. However, it is present in red algae, which seems to suggest that the common ancestor of plants and red algae also synthesised lignin. This would suggest that its original function was structural; it plays this role in the red alga Calliarthron, where it supports joints between calcified segments. Another possibility is that the lignins in red algae and in plants are result of convergent evolution and not of a common origin.
Lignin plays a significant role in the carbon cycle, sequestering atmospheric carbon into the living tissues of woody perennial vegetation. Lignin is one of the most slowly decomposing components of dead vegetation, contributing a major fraction of the material that becomes humus. The resulting soil humus, in general, holds nutrients onto its surface, and hence increases its cation exchange capacity and moisture retention, hence it increases the productivity of soil.
Highly lignified wood is durable and therefore a good raw material for many applications. It is also an excellent fuel, since lignin yields more energy when burned than cellulose. Mechanical, or high-yield pulp used to make newsprint contains most of the lignin originally present in the wood. This lignin is responsible for newsprint's yellowing with age. Lignin must be removed from the pulp before high-quality bleached paper can be manufactured.
In sulfite pulping, lignin is removed from wood pulp as sulfonates. These lignosulfonates have several uses:
The first investigations into commercial use of lignin were reported by Marathon Corporation, a paper company based in Rothschild, Wisconsin, starting in 1927. The first class of products that showed promise were leather tanning agents. The lignin chemical business of Marathon was operated for many years as Marathon Chemicals. It is now known as LignoTech USA, Inc., and is owned by the Norwegian company Borregaard.
Lignin removed via the kraft process is usually burned for its fuel value as part of a concentrated black liquor stream, providing energy to run the mill and its associated processes. Three commercial processes exist to remove lignin from black liquor for higher value uses: LignoBoost (Sweden), LignoForce (Canada), and SLRP (US). Higher quality lignin presents the potential to become the main renewable aromatic resource for the chemical industry in the future, with an addressable market of more than $130bn.
In 1998, a German company, Tecnaro, developed a process for turning lignin into a substance, called Arboform, which behaves identically to plastic for injection molding. Therefore, it can be used in place of plastic for several applications. When the item is discarded, it can be burned just like wood.
In 2007, lignin extracted from shrubby willow was successfully used to produce expanded polyurethane foam.
In 2012, it was shown carbon fiber can be produced from lignin instead of from fossil oil.
In 2013, the Flemish Institute for Biotechnology was supervising a trial of 448 poplar trees genetically engineered to produce less lignin so that they would be more suitable for conversion into bio-fuels.
In 2013, researchers from the University of California, Berkeley, demonstrated that lignin derived from a Miscanthus grass could be catalytically degraded into monophenolic products, which could potentially serve as an aromatic chemical feedstock.
Given that lignin is the most prevalent biopolymer after cellulose and is ubiquitous in the Earth's biosphere, the same economic principles that drive the desire for cellulosic ethanol as a biofuel also call for the investigation of lignin as a feedstock for biofuel production. Lignin can already be burned in furnaces, but interest in the idea of instead chemically converting it to liquid fuel is strong.
Lignin is a cross-linked racemic macromolecule with molecular masses in excess of 10,000 u. It is relatively hydrophobic and aromatic in nature. The degree of polymerisation in nature is difficult to measure, since it is fragmented during extraction and the molecule consists of various types of substructures that appear to repeat in a haphazard manner. Different types of lignin have been described depending on the means of isolation.
There are three monolignol monomers, methoxylated to various degrees: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol (Figure 3). These lignols are incorporated into lignin in the form of the phenylpropanoids p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S), respectively. Gymnosperms have a lignin that consists almost entirely of G with small quantities of H. That of dicotyledonous angiosperms is more often than not a mixture of G and S (with very little H), and monocotyledonous lignin is a mixture of all three. Many grasses have mostly G, while some palms have mainly S. All lignins contain small amounts of incomplete or modified monolignols, and other monomers are prominent in non-woody plants.
Lignin biosynthesis (Figure 4) begins in the cytosol with the synthesis of glycosylated monolignols from the amino acid phenylalanine. These first reactions are shared with the phenylpropanoid pathway. The attached glucose renders them water-soluble and less toxic. Once transported through the cell membrane to the apoplast, the glucose is removed and the polymerisation commences. Much about its anabolism is not understood even after more than a century of study.
The polymerisation step, that is a radical-radical coupling, is catalysed by oxidative enzymes. Both peroxidase and laccase enzymes are present in the plant cell walls, and it is not known whether one or both of these groups participates in the polymerisation. Low molecular weight oxidants might also be involved. The oxidative enzyme catalyses the formation of monolignol radicals. These radicals are often said to undergo uncatalyzed coupling to form the lignin polymer, but this hypothesis has been recently challenged. The alternative theory that involves an unspecified biological control is however not widely accepted.
Biodegradation of lignin by white rot fungi leads to destruction of wood on the forest floor and human-made structures such as fences and wooden buildings. However biodegradation of lignin is a necessary prerequisite for processing biofuel from plant raw materials. Current processing setups show some problematic residuals after processing the digestible or degradable contents. The improving of lignin degradation would drive the output from biofuel processing to better gain or better efficiency factor.
Lignin is indigestible by animals, which lack the enzymes that can degrade this complex polymer. Some fungi (such as the Dryad's saddle) and bacteria do however biodegrade lignin using so-called ligninases (also named lignases). The mechanism of the biodegradation is speculated to involve free radical pathways. Well understood ligninolytic enzymes are manganese peroxidase and lignin peroxidase. Because it is cross-linked with the other cell wall components and has a high molecular weight, lignin minimizes the accessibility of cellulose and hemicellulose to microbial enzymes such as cellobiose dehydrogenase. Hence, in general lignin is associated with reduced digestibility of the overall plant biomass, which helps defend against pathogens and pests. Syringyl (S) lignol is more susceptible to degradation by fungal decay as it has fewer aryl-aryl bonds and a lower redox potential than guaiacyl units. This means that organic matter that is enriched with G lignol (like the bark of woody vascular plants) is more resistant to microbial attack.
Lignin is degraded by micro-organisms including fungi and bacteria. Lignin peroxidase (also "ligninase", EC number 1.14.99) is a hemoprotein firstly isolated from the white-rot fungus Phanerochaete chrysosporium with a variety of lignin-degrading reactions, all utilizing hydrogen peroxide as an oxygen source. Other microbial enzymes may be involved in lignin biodegradation, such as manganese peroxidase and the copper-based laccase.
Pyrolysis of lignin during the combustion of wood or charcoal production yields a range of products, of which the most characteristic ones are methoxy-substituted phenols. Of those, the most important are guaiacol and syringol and their derivatives; their presence can be used to trace a smoke source to a wood fire. In cooking, lignin in the form of hardwood is an important source of these two chemicals, which impart the characteristic aroma and taste to smoked foods such as barbecue. The main flavor compounds of smoked ham are guaiacol, and its 4-, 5-, and 6-methyl derivatives as well as 2,6-dimethylphenol. These compounds are produced by thermal breakdown of lignin in the wood used in the smokehouse.
The conventional method for lignin quantitation in the pulp industry is the Klason lignin and acid-soluble lignin test, which is standardized according to SCAN or NREL procedure. The cellulose is first decrystallized and partially depolymerized into oligomers by keeping the sample in 72% sulfuric acid at 30 C for 1 h. Then, the acid is diluted to 4% by adding water, and the depolymerization is completed by either boiling (100 °C) for 4 h or pressure cooking at 2 bar (124 °C) for 1 h. The acid is washed out and the sample dried. The residue that remains is termed Klason lignin. A part of the lignin, acid-soluble lignin (ASL) dissolves in the acid. ASL is quantified by the intensity of its UV absorption peak at 280 nm. The method is suited for wood lignins, but not equally well for varied lignins from different sources. The carbohydrate composition may be also analyzed from the Klason liquors, although there may be sugar breakdown products (furfural and 5-hydroxymethylfurfural).
A solution of hydrochloric acid and phloroglucinol is used for the detection of lignin (Wiesner test). A brilliant red color develops, owing to the presence of coniferaldehyde groups in the lignin.
Thioglycolysis is an analytical technique for lignin quantitation. Lignin structure can also be studied by computational simulation.
Thermochemolysis (chemical break down of a substance under vacuum and at high temperature) with tetramethylammonium hydroxide (TMAH) has also been used to analyse the ratios of lignols with fungal decay as well the ratio of the carboxylic acid (Ad) to aldehyde (Al) forms of the lignols (Ad/Al). Increases in the (Ad/Al) value indicate an oxidative cleavage reaction has occurred on the alkyl lignin side chain which has been shown to be a step in the decay of wood by many white-rot and some soft rot fungi.
Solid state 13C NMR has been used to look at the concentrations of lignin, as well as other major components in wood e.g. cellulose, and how that changes with microbial decay. Conventional solution-state NMR for lignin is possible. However, many intact lignins have a crosslinked, very high molar-mass fraction that is difficult to dissolve even for functionalization.