Lignin Complex Polymer - Research Paper Example

Lignin Introduction Lignin is a complex polymer derived from aromatic alcohols that are called monolignols (Vanholme, Demedts, Morreel, Ralph, & Boerjan, 2010). It is a generic term used for classifying together a large group of aromatic polymers that have been formed due to the oxidative combinatorial coupling of 4-hydroxyphenylpropanoids (Ralph, et al., 2004) (Bédué, 2013). Lignin is normally found in wood and is a very important element of the secondary walls of plants and of certain algae (Lebo, Gargulak, & McNally, 2001). It was in 1813 that lignin was first talked about by A.P. de Candolle. His explanation about lignin was that it is a fibrous material that does not have any taste, it cannot be dissolved in water or alcohol but is soluble in weak alkaline solutions, and in order extract it from the solution an acid could be added to it which would cause its precipitation. Lignin is among the most commonly found organic polymers, and only cellulose and hemicelluloses is ahead of it regarding abundance (Shi, Xiao, Deng, & Sun, 2013). Thus, lignin forms the majority of the natural resources that man has. 30% of non-fossil organic carbon is made up of lignin while making up almost one-fourth to third of the dry mass of wood. Every species contains a unique type of lignin, the difference lying in its composition. Since lignin is a biopolymer, it attains its uniqueness owing to its heterogeneity and because it does not really have a properly defined primary structure. The most important and common function of lignin is to strengthen wood in trees, which is made from xylem cells, by providing it with support. Classification The natural state of lignin as present within a plant is known as protolignin (Kutscha & Gray, 1970). The classification of lignins depends on their structural elements. Softwoods, hardwoods and grasses contain different lignins and the reason behind the difference between the three is the different content of guaiacyl (G), syringyl (S) and p-hydroxyphenyl (H) units. Guaiacyl lignin is present in the majority of softwoods and is mainly a polymerization product of coniferyl alcohol. Hardwoods typically contain the guaiacyl-syrinngyl lignin which is a copolymer of coniferyl and sinapyl alcohols, the ration varying from 4:1 to 1:2 for the two monomeric units (Pereira, Portugal-Nunes, Evtuguin, Serafim, & Xavier, 2013). Another type is that of compression wood that is largely made up of phenylpropane units of the p-hydroxyphenyl type along with the usual guaiacyl units. At times the term syringyl lignin and p-hydroxyphenyl lignin are used for denoting the respective structural elements even if these units do not contain any natural lignins (Sjostrom, 2013). There is lot of lignin present in the middle lamella and the least in the secondary wall. Owing to its thickness, around 70% of the lignin of softwoods is present in the secondary wall and this was determined through quantitative UV microscopy. Similar is the case with hardwoods, the difference being that analytical uncertainties are present here due to the extreme heterogeneity of wood and because the lignin contains both guaiacyl and syringyl units. From the measurements done it is concluded that the lignin present in the secondary wall of hardwood fibers contains a large percentage of syringyl units while the middle lamella lignin has huge quantities of guaiacyl units. Apparently only guaiacyl lignin is present in the vessels in birch while parenchyma cells are predominantly made up of syringyl lignin (Sjostrom, 2013). Methods The usual procedure for isolating lignin involves dissolving it out of the cell wall or dissolving the non-lignin parts present around the lignin. There are many techniques for isolating lignin using each of these two methods. Still, on separating lignin from the other cell wall materials that are usually related to it, primarily cellulose and hemicellulose, there is an invariable change seen in it. Lignin can be isolated for identification through chemical degradation. One method of this category is by oxidizing it through nitrobenzene which leads to various quantities of the corresponding aldehydes being formed (vanillin, syringaldehyde, p-hydroxybenzaldehyde) (Fengel & Wegener, 1983). While nitrobenzene has been commonly and extensively used for oxidation for this purpose its possible alternative is cupric oxide oxidation in alkaline solution (Chen, 1992a). Other chemical methods include acidolysis, permanganate oxidation and methoxyl determination (Higuchi, Tanahashi, & Sato, 1972). The disadvantage of using chemical degradation methods is that each of these methods suffers, to an extent, from the idea that just the non-condensed C9-units can allow for the degradation methods, which means that the mainly condensed p-hyrdroxyphenyl units are very less in quantity (Nimz, Robert, Faix, & Nemr, 1981). Other methods include the physical methods through which lignins can be classified. UV and IR spectroscopy come under this category and through these methods the G-, S- and H-units are identified within lignin (Hergert, 1971). 13C-NMR spectroscopy is a more recent method that has started being used for the same purpose (Robert & Gagnaire, 1981). Besides chemical degradation and physical methods, there are also histochemical methods for differentiating lignins (Meshitsuko & Nakano, 1978). Due to the fact that classifying lignins into softwood, hardwood and grass is not sufficient enough to group the so many lignins present, another classificiation mechanism was come up with wherein a division took place of all the lignins into two important categories: guaiacyl lignins (G-lignins) and guaiacyl-syringyl lignins (GS-lignins) (Gibbs, 1958). Later on (Kawamura & Higuchi, 1964) refined this classification mechanism and for this purpose they differentiated the major groups into a number of subgroups related to botanical plant groups. Yet further refinement took place under (Nimz, Robert, Faix, & Nemr, 1981) who based the new mechanism upon extensive 13C-NMR spectroscopic investigations on softwood, hardwood and grass lignins. Through this refinement all the monocotyledon lignins or at least the grass lignins were distinguished as GSH-lignins from the lignins of dicotyledons (GS-lignins). Reaction Different reactions of lignin have been examined and a lot of time has been spent on their understanding. The reactions of lignin can be classified into two groups: pulping and bleaching. Pulping involves nucleophilic reactions which are also of two types: addition and displacement. Bleaching involves electrophilic reactions whose types include additional and displacement (Parasuraman, 2007). The pulping reaction is conducted in two ways – neutral and acidic sulfite. During sulfite pulping lignin remains insoluble for the most part. Chemical modification of lignin takes place via the introduction of anionic group via sulfonation and because of that the lignin is able to dissolve at a wide pH range. Condensation also takes place during sulfite pulping and these cause the production of brown color in the lignin. After pulping the color of lignin turns brown therefore it requires bleaching. This process is possible through oxidation due to which the “sticky” aromatic groups in lignin are removed and converted into hydrophilic structures - for instance, carboxylic acid. The process of bleaching is pretty complicated and the three most important reactions taking place during bleaching include oxygen delignification, alkaline peroxide bleaching and chlorine dioxide bleaching. The resultant products are able to dissolve. These undergo oxidation and thus the energy they contain is less than what is left after pulping. Even the molecular weight of the leftover lignin is less and there is a partial loss of the aromatic structure of the lignin (Crocker, 2010). Synthesis The synthesis of glycosylated monolignols from the amino acid phenylalanine leads to the biosynthesis of lignin that starts in the cytosol. The phenylpropanoid pathway is a part of these reactions. Because of the glucose molecule present, these dissolve in water and also decrease in toxicity. After it has been moved to the apoplast via the cell membranes the glucose molecule leaves them and thus begins the process of polymerization. There is not a lot of information available regarding the way it anabolizes (Boerjan, Ralph, & Baucher, 2003). The presence of oxidative enzymes helps with the polymerization process. These oxidative enzymes could be either or peroxidase or laccase, which can both be found in the plant cell walls but it has not yet been determined whether both or one, and if one which of them, is involved with the process. There is a possibility of participation by oxidants with a small molecule. The oxidative enzyme(s) help with the production of monolignol radicals. With these radicals happen uncatalyzed coupling for the formation of the lignin polymer; however, this view has now been confronted with (Davin & Lewis, 2005). The biosynthesis of lignin happens to be among the most commendable achievements of wood chemistry. It was Freudenberg who contributed the most to the industry by actually preparing synthetic lignin. In order to determine the biosynthesis of lignin it was necessary that its structure be first determined and this is what (Freudenberg & Neish, 1969) have written about in their book about what makes up lignin and how it can be biosynthesized. In spite of the exact structure of protolignin being unknown there was enough information available that allowed for a basis on which could be determined the biosynthesis pathways. These pathways were determined via three steps. First was identified the organic compounds and the precursors that could be possibly present in lignin of wood cambial tissue. The second step was introducing different organic compounds and ratioactive labeled compounds into living wood tissues. The third included preparing ligninlike polymers, basically artificial lignins, using different pure organic compounds (Libby, 1962). Achievements of organic, bio- and physical chemists along with those of botanists, plant physiologists and plant pathologists have lead to this possibility. Properties Protolignin is completely amorphous. It does not have any colour (Timell, 1968)although this property is still being investigated (Goring, Pulp and paper research institute of Canada, 1969b). Certain researchers have indicated that the hygroscopicity of protolognin is much less than that of cellulose (Panshin, deZeeuw, & Brown, 1964) while others say that it can absorb almost two-thirds as much water as can cellulose (Christensen & Kelsey, 1968). Protolignin has thermoplastic properties required for the bond present between fiber and particle board. There is disagreement regarding whether these thermoplastic properties are lost or not when lignin has been separated from wood. Because of this certain property wood is bendable and can be formed by steaming (Harkin, 1969). The other commonly known properties are the ones that isolated lignins contain and the following paragraph will describe these. Lignin is an insoluble compound following the treatment of the plant with 72% sulphuric acid after which it is diluted and boiled (Panshin, deZeeuw, & Brown, 1964). There are two common reactions that occur in lignins; the first concerns the functional groups but does not result in any change in the molecular size, and the second is where the opposite happens, that is, the functional groups remain unchanged while the molecular size is altered. The reactions would proceed depending on the way the lignins had been isolated. Other properties of lignins are that it cannot be dissolved in water, majority of the organic solvents and strong sulfuric acid. They can be unhydrolyzed by acids, can be oxidized easily, can dissolve in hot alkali and bisulfate solutions, and easily and quickly condense in the presence of alcoholic, phenolic and thio compounds. Quick and easy decomposition of lignin is possible with chlorine. Molecular weight is another significant characteristic of lignin. The exact molecular weight of protolignin has still not been determined; however, Bjorkman lignin is 11,000 and that obtained from isolated lignosulfonic acids is anywhere between 260 to 50 million (Goring, The Physical Chemistry of Lignin, 1962). The maximum ultra violet absorption by lignin is 282 millimicrons, therefore, it is possible to quantitatively follow the lignifications procedure of the cell in the ultra violet microscope using the 280 millimicron line of a mercury lamp (Frey-Wyssling & Muhlethaler, 1965). Lignins do not have a specific melting point because, when isolated, they are normally amorphous rather than in a crystalline form. However, certain lignins have been found to show specific softening points at higher temperatures. The mechanical properties of wood have a relation with its chemical composition. The structure of a lignified cell wall is similar to the one displayed by reinforced concrete, the similarity lying between the cellulose microfibrils and the iron rods (Frey-Wyssling & Muhlethaler, 1965). There is also the possibility that lignin affects the rheological properties of wood. Applications and Uses The pulp and paper industry produces a great deal of waste and utilizing that as a by-product has been a growing concern (Pearl, 1969b). The pulp manufacturers end up with a great deal of pulping waste that has to be disposed of, and they wish to utilize it into by-products that would also prove to be lucrative. Lignin-derived compounds make up a huge percentage of such wastes and a huge number of patents have been presented that detail into how lignin can be used (Harkin, 1969). Being an element of finished paper, lignin is used most profitably by the paper industry. Lignin is being used more and more because new and high yielding pulping procedures are being utilized in combination with novel refining techniques. Further utilizations for lignin-derived compounds include the ones attained by means of sulfite pulping processes and those resulting through alkaline processes. Sulfite liquor and lingo-sulfonates are produced through the sulfite pulping processes. The sulfite liquor has adhesive, dispersing and surface active properties and these affect its usage for linoleum pastes, foundry sand casting forms, emulsions, Portland cement and ceramic mixes, road binders, animal feed pellet binders and the processes of soil stabilization, dust control and dye dispersion (Pearl, 1969b). Lignosulfonates are utilized for controlling the viscosity of oil well drilling muds, as concrete additives, as dispersing agents of dyes and pesticides, and for potash extraction. These applications are possible because lignosulfonates have dispersant properties along with being able to assemble together metallic ions. Other usages of lignosulfonates include the production of vanilla flavoring, and the extra vanillin is used for production of vanillic acid, ethyl vanillate and other similar compounds that may be utilized in the preservation of food, for sunburn preparations and for treating specific diseases and skin fungi. Structure Talking about lignin organically it is a complex polymer with a very complex structure. Its structure was difficult to determine due to the compound’s insolubility and because acids cannot hydrolyze it. Besides, lignin does not really have a regular structure or a simple repeating unit and removing it from the cell wall to study it usually modifies it (Kratzl, 1965). Organic chemists, biochemists, botanists and plant physiologists have together worked on several different methods to come up with a theoretical structure of lignin (Kratzl, 1965). Plant pathologists have also worked for this by using the selective enzymatic action of particular fungi on model compounds that are more or less similar to lignin (Kirk, 1968). Lignin molecules are made up of only carbon, hydrogen and oxygen, although each of them is present in variable percentages in different kinds of lignin. However, the phenyl propane constituent forms the basic building unit of all lignins. C-O-C and C-C bonds link these units such that it results in a three-dimensional polymer through cross linking. The resultant cross linked polymer makes a molecular network which extends throughout the wood. Therefore, it can be assumed that all the lignin within a tree is one huge macromolecule (Goring, 1964). Describing the lignin macromolecule is not possible using a simple combination of one or few monomeric units by one of few kinds of linkages (Reale, Tullio, Spreti, & Angelis, 2004). It is through random coupling of various units having different regiochemistry that the macromolecule can be described, and therefore it is determined that there are no repetitive units present, even at an oligomeric level. Different monomeric units and their interunit linkages can be used for describing the structure of lignin. Composition of lignin has already been described earlier in the paper. The average molecular weight of lignin ranges anywhere between 7,000 and 20,000 though this value depends upon the way that lignin was isolated and the methods used for determining the molecular weight (Smith, Kadish, & Guilard, 2003). Many models of the lignin polymer have been developed for representing the general structure on the basis of the frequency of building constituents and interunit linkages. (Brunow, et al., 1998) developed a schematic representation of the lignin structure in which he included all the important linkages that could possibly be present in the polymer. The following diagram shows one possible structure of lignin. Bibliography Bédué, O. (2013). Lignocellulosic Biorefineries. EFPL Press. Boerjan, W., Ralph, J., & Baucher, M. (2003). Lignin biosynthesis. Annual Review of Plant Biology , 54, 519–549. Brunow, G., Kilpeläinen, I., Sipilä, J., Syrjänen, K., Karhunen, P., Setälä, H., et al. (1998). Oxidative Coupling of Phenols and the Biosynthesis of Lignin. In N. G. Lewis, & S. Sarkanen (Eds.), Lignin and Lignan Biosynthesis (Vol. 697, pp. 131-147). Chen, C. (1992a). Nitrobenzene and cupric oxide oxidations. (S. Lin, & C. Dence, Eds.) Methods in Lignin Chemistry , pp. 301-319. Christensen, & Kelsey. (1968). Crocker, M. (Ed.). (2010). Thermochemical Conversion of Biomass to Liquid Fuels and Chemicals. Royal Society of Chemistry. Davin, L., & Lewis, N. (2005). 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