Bio-based chemicals from biorefining: lignin conversion and utilisation
A.L. Macfarlane, M. Mai and J.F. Kadla, University of British Columbia, Canada
Abstract:
An understanding of lignin chemical structure and properties is required before connections can be drawn between lignin sources and utilisation. Of particular interest, and one cause for the limited use of lignin, is chemical heterogeneity. The principal applications for lignin are power/fuel (short term), macromolecules (medium term) and aromatic chemicals (long term), with the preferred use ultimately directed by the global price of oil and biomass. The intention of this chapter is to compile and evaluate the properties of lignins from various sources and to identify applications that match the unique properties of each lignin.
Key words
lignin; lignocellulose; kraft process; sulfite process; organosolv; carbon fibres; lignosulfonate
20.1 Introduction
Lignin has traditionally been derived from the kraft and sulfite processes that are ubiquitous in the pulp and paper industry. Over 50 million tonnes are produced annually worldwide (Gosselink, 2004; Dos Santos et al., 2012; Zakzeski et al., 2010). The vast majority of lignin is burned in order to recover chemicals from the pulping process and provide process heat and only 2% of all lignins are utilised for rather low-value applications (Gosselink, 2004). This led to the general perception that lignin is just a waste product of the pulping process and, as a result, this has restrained the research for value-added applications for lignin and lignin-based materials for quite some time. However, higher value products are constantly being sought.
A multitude of lignin applications have been explored at the research level so far. The current industrial products are primarily produced from lignosulfonates and sulfonated kraft lignin. Products include dispersants, emulsifiers, raw material for vanillin and dimethyl sulfoxide (DMSO) production and road-dust suppression agents. There is the opportunity to classify and refine lignin for use in many polymeric products currently being served by synthetic petroleum-derived polymers. Many biorefinery platforms produce lignin that can be incorporated into resins, foams and adhesives with improvement in properties. However, these applications still use lignin as a by-product from the pulping process. The next phase in biorefinery technology will be the design of processes with lignin as the primary product generating the greatest revenue.
20.2 Structure and properties of lignin
Lignin is a highly branched aromatic biopolymer only exceeded by cellulose in its abundance. Lignin is found in the cell walls of woody plants where its content reaches up to 30%. It plays a significant role in trees by giving rigidity to the cell walls and making the wood resistant to compression, impact and bending. It is also involved in the transport of water from the roots to the leaves. Through covalent bonds, lignin is interlaced with hemicellulose and forms a matrix that is reinforced by cellulose. Lignin is produced by enzyme-initiated dehydrogenative polymerisation of phenyl-propanoid groups derived from coniferyl alcohol (G), sinapyl alcohol (S), and p-coumaryl alcohol (H). The structures of these monomers are shown in Fig. 20.1 (Dimmel, 2010).
Softwoods, hardwoods and grasses each have different proportions of S, G and H monomers, and proportions can vary greatly between species and growing conditions (see Table 20.1). Hardwood lignins consist mainly of coniferyl and sinapyl alcohol (guaiacyl-syringyl lignin), while softwood lignins are made primarily from coniferyl alcohol. The lignin in grass and annual plants is composed mainly of p-coumaryl alcohol units. The difference between the three lignin precursors is the number of methoxyl groups attached to the aromatic ring which results in different amounts of certain inter-unit linkages in hardwood, softwood or grasses.
Table 20.1
Approximate composition of some important classes of lignins
| p-Coumaryl alcohol | Coniferyl alcohol | Sinapyl alcohol | |
| Softwood lignin | 4% | 95% | 1% |
| Hardwood lignin | ∼ 2% | ∼ 50% | ∼ 50% |
| Grass lignin | 5% | 70% | 25% |
| Compression wood lignin | 30% | 70% | ∼ 0% |
Source: Dimmel (2010).
The enzymatic dehydrogenation yields several resonance-stabilised phenoxy radicals that can couple randomly with another radical and therefore form a variety of different bonds with exceptional stability (see Fig. 20.2): biphenyl carbon–carbon linkages between aromatic carbons, alkyl–aryl carbon–carbon linkages between an aliphatic and aromatic carbon, and hydrolysis-resistant ether linkages. The only relatively weak and hydrolysable linkage is the α-aryl ether bond (Wool and Sun, 2005). The percentage distribution of lignin units is shown in Table 20.2.
Table 20.2
Percentage distribution of lignin units
| Type of unit | Softwood lignin | Hardwood lignin |
| β-O-4 | 35–50 | 45–60 |
| α-O-4 | 2–8 | 5–8 |
| 5-O-4 | 3.5–4 | 7–15 |
| β-5 | 9–12 | 4–6 |
| 5–5 | 10–23 | 2–9 |
| β-1 | 3–7 | 5–15 |
| β-β | 2–4 | 3–8 |
| Others | 13 | 5 |
Source: Pandey and C. S. Kim 2011; Dimmel 2010; Gratzl and Chen 2000; E. A. Capanema et al. 2004; E. Capanema et al. 2005.
Although it has been intensely studied for many years, the structure of native lignin still remains unclear. However, several structures have been proposed based on the analysis of degradation products and the identification of the dominant linkages between the phenylpropane units and their abundance, as well as the abundance and frequency of certain functional groups (see Figs 20.3 and 20.4). Those functional groups have a great impact on the reactivity of lignin. Lignin mostly contains methoxyl groups, phenolic and aliphatic hydroxyl groups, and a few terminal aldehyde groups. However, most of the phenolic hydroxyl groups are not available since they are occupied in interunit linkages.
Depending on the delignification process employed, the amount and the nature of functional groups change (see Tables 20.3 and 20.4). Some processes hydrolyse the structure and make more phenolic hydroxyl groups available. Others introduce new functional groups to improve solubility such as for sulfonated lignin. One aim of lignin research is to investigate the structure–properties relationship of all kinds of different lignins and to find suitable applications for all of them.
Table 20.3
Properties of lignin from different extraction processes
| Procedure | Wood species | Mw | Mn | Mw/Mn | Phen. OH (per C9) | Aliph. OH (per C9) | OCH3 (per C9) | s:g ratio | Tg (°C) |
| Kraft | SW | 1100–39,000 | 530–2400 | 1.4–28 | 0.57–0.78 | 0.39–0.72 | 0.77 | 1 | 124–169 (DSC) |
| HW | 2900–4200 | 1090–2900 | 2.2–2.8 | 0.7 | 0.35 | 1.15 | |||
| Organosolv | SW | 1750 | 700 | 2.5 | |||||
| ALCELL | HW | 1000–8000 | 600–1700 | 1.8–6.3 | 0.3–0.65 | 0.23–0.6 | 1–1.3 | 1.33 | 97–130 |
| Steam explosion | SW | 113–139 (DSC) | |||||||
| HW | 2100–7100 | 790–1900 | 2.6–4 | 0.5–0.63 | 0.42 | 1.6 | 1–1.5 | ||
| MWL | SW | 20,600–22,700 | 7900–9000 | 2.4–2.6 | 0.15–0.3 | 0.15–0.7 | 0.92–0.95 | 160 (DSC) | |
| HW | 0.19–1.58 | 0.09–0.71 | 0.21–1.6 | 1.34–1.72 | 110–130 (DSC) | ||||
| Ligno-sulfonate | SW | 400–150,000 | 3.1–7.1 |
Source: Moerck et al. 1986; Wool and Sun, 2005; Gratzl and Chen, 2000; Gosselink et al., 2010; El Mansouri and Salvadó, 2006; Tejado et al., 2007; Lora and Glasser, 2002; Hu, 2002; Thring et al., 1996; Capanema et al., 2004, 2005; Sarkanen and Glasser, 1989.
Table 20.4
| Procedure | Wood species | Mw | Mn | Mw/Mn | Phen. OH (per C9) | Aliph. OH (per C9) | OCH3 (per C9) | Tg (°C) |
| Soda | Wheat | 3300–4400 | 1200–1800 | 2.4–3.6 | 0.38–0.9 | 0.26–0.68 | 0.71–1.0 | |
| Hemp | 600 | 0.49–0.6 | 0.57 | 0.86–0.9 | ||||
| Soda-AQ | Wheat | 3270 | 1770 | 1.8 | 0.75 | 0.87 | 1.02 | 160 |
| Organosolv | Wheat | 4430 | 800–2020 | 2.2–5.5 | 0.39–0.65 | 0.62–0.82 | 0.75–1.02 | 142 |
| Bagasse | 2704 | 830–840 | 3.2 | 0.77 | 0.8 | 170 | ||
| Reed | 1410–1480 | 650–680 | 2.2 | 0.5–0.62 | 0.52–0.56 | 0.95–1.01 | 97 (1 bar) | |
| Kenaf | 0.43–0.47 | 0.58–0.76 | 1.01 | 66–70 (1 bar) | ||||
| Kraft | Bagasse | 0.51 | 0.95–1 |
Source: Lora and Glasser, 2002; Hu, 2002, pp.268–273.
20.3 Traditional processes for the production of lignin
The process of extracting lignin from biomass is known as delignification and disintegrates lignocellulosic material into its fibrous components. Two main categories of chemical delignification processes, namely aqueous and organic solvent-based, have been developed and include a wide range of methods. The chemical aqueous pulping processes include alkaline (or kraft pulping) and sulfite pulping, which are the two industrially most widely used methods (Wool and Sun, 2005). The primary goal of chemical pulping is to selectively remove as much lignin as possible without degradation of the carbohydrates. Therefore, the selectivity of delignification is determined by the weight ratio of lignin and carbohydrate removal. The pulping process can be divided into three stages:
1. ‘initial delignification’ occurring at < 140°C; lignin is extracted and hemicellulose degraded
2. ‘bulk delignification’; the delignification rate is accelerated at increased temperature and stays high until 90% of the lignin is removed
3. ‘residual delignification’ is characterized by a slow delignification rate while the degradation of carbohydrates becomes dominant.
The degree of delignification is typically indicated by the ‘kappa number’ of a pulp, where a high kappa number stands for high lignin content. The kappa number of unbleached softwood and hardwood pulps is 30–40 and 18–20, respectively (Kadla and Dai, 2006).
Soda pulping dates back to 1851, when it was invented in England by Burgess and Watts (Smook, 1986). It is one of the simplest pulping processes and a source of sulfur-free lignin. In soda pulping lignocellulose is treated with aqueous NaOH at a temperature between 150 and 170°C. Under these conditions both lignin and carbohydrates are degraded. Therefore the selectivity of soda pulping is low and it is currently limited to easily pulped materials like straw and some hardwoods (Kadla and Dai, 2006). To improve selectivity, additives are typically used, such as in the advanced soda-anthraquinone (AQ) process, where anthraquinone is added as a pulping additive/catalyst to decrease the carbohydrate degradation (Smook, 1986). During the soda-AQ process, the catalyst undergoes a redox cycle between AQ and the reduced form anthrahydroquinone (AHQ), which can effectively cleave β-ether bonds in lignin. The hydrolysed lignin can be further degraded by NaOH. The reducing end groups of the carbohydrates are oxidised to a carboxylic acid group during the redox cycle which is stable under alkaline conditions and prevents ‘peeling’ reactions. Soda lignin has been used as a biocide for wastewater treatment and as a dispersant (Gosselink, 2004). The Swiss holding company GreenValue SA (Lausanne) is a current producer of soda lignin.
The kraft process (or sulfate process) was invented in 1879 by the German chemist Dahl. It is a modified soda process in that it uses NaOH and Na2S (‘white liquor’) to fragment lignin until it is soluble in the alkaline medium. The pulping step is conducted at 140–180°C and lasts approximately 2 hours. The selectivity of kraft pulping is higher than soda pulping because hydrogen sulfite ions (HS−) preferentially react with lignin (Gierer, 1985). As a result, a typical Kraft pulping process removes up to 80% of the lignin, 50% of the hemicellulose and less than 10% of the cellulose. Compared to soda lignin, kraft lignin is far less condensed due to the thiol groups ‘blocking’ the reactive benzylic centres.The thiol groups that are incorporated into the lignin structure result in a sulfur containing technical lignin. Kraft lignins are soluble in alkali (pH > 10.5) and some polar organic solvents (DMF, DMSO, methyl cellusolve, etc.). They are relatively low molecular weight and quite polydispersed.
The technological advantage of kraft pulping over other delignification processes is also one of the largest impediments to the utilisation of kraft lignin: the recovery of chemicals and energy from residual black liquor is at the core of the kraft process. It has been claimed that kraft lignin is not a viable feedstock for chemical production because of its energy value as a heat source in chemical recovery furnaces (Zakzeski et al., 2010). However, the greatest impetus for kraft lignin valorisation is that the majority of kraft mills, particularly in Canada, are recovery furnace limited and debottlenecking by lignin precipitation is considered by many pulp mills (Gosselink, 2004). Diverting lignin from heat to value-added polymers and platform chemicals would allow an increase in plant capacity and greatly improve economics. Commercial kraft lignin is produced by MeadWestvaco Corporation (Richmond, USA) and Metso Corporation (Helsinki, Finland). Up to 70% of MeadWestvaco’s kraft lignin is chemically sulfonated to make it water soluble and better suitable for different applications (Zakzeski et al., 2010). The Structure of kraft pine lignin is shown in Fig. 20.5.
The sulfite process has been practised since 1874, when the first mill using the process was opened in Sweden. The active species are sulfur dioxide, hydrogen sulfite 
, and/or sulfite ions 
, depending on pH (Kadla and Dai, 2006). The most common industrial process is the acid sulfite process, which uses mixtures of sulfurous acid and/or its alkali salts (Na+, NH3+, Mg2 +, K+ or Ca2 +) to solubilise lignin through the formation of sulfonate functionalities and cleavage of lignin bonds (Biermann, 1996). In acid sulfite pulping, α-hydroxyl, and α-ether groups are eliminated to form carbonium ion intermediates and lignin fragmentation. The carbonium ions subsequently react with hydrated SO2 and/or 
, leading to sulfonation, or with other lignin fragments, leading to condensation. Sulfonation leads to increased hydrophilicity and solubility of the lignin, while condensation (which is enhanced at low pH) leads to a decrease in solubility due to a higher molecular weight (Kadla and Dai, 2006).
Worldwide, 1.06 mega tonnes of lignosulfonate are produced annually (Holladay et al., 2007). Sulfite lignin has a relatively high molecular weight ranging from 20,000 to 50,000 g/mol, a polydispersity of 6–8 and it is soluble in water from pH 0–14. One major disadvantage of lignosulfonates is the level of impurities as compared to that of kraft lignin; lignosulfonates typically contain 20–25% carbohydrate, ash and other inorganics. The structure of spruce lignosulfonate is shown in Fig. 20.6.
20.4 Emerging processes for the production of lignin
At the present time, solvent-based or organosolv processes are either in the developmental stage or used in small-scale production (bio-based polymers and composites). As the name implies, these processes use organic solvents to delignify lignocellulosic materials. This kind of delignification is in general more effective for hardwoods than softwoods due to the differences in the chemical structure of their respective lignins. However, they promise higher value by-products, lower capital costs and lower emissions (Pye and Lora, 1991).
The organosolv process uses an organic solvent to dissolve lignin by hydrolytic cleavage at elevated temperatures, typically between 170 and 220°C. Hemicellulose is also hydrolysed and solubilised. Various solvents have been studied including, but not limited to, acetone, methanol, ethanol, butanol, acetic acid and formic acid. Lower alcohols and specifically ethanol are the most promising of solvents due to the low cost and easier recovery by distillation (Zhao et al., 2009). The concentration of solvent is typically between 30 and 70%. An acid catalyst is often used to increase delignification rates or allow lower temperature (Aziz and Sarkanen, 1989). The lignin from organosolv using mild process conditions is reported to be virtually unmodified compared to milled wood lignin (El Hage et al., 2010) and thus allows exploitation of the natural properties of lignin such as resistance to water and biological attack. However, acidic conditions are required for effective delignification rates (Hallberg et al., 2008; Pan et al., 2006a) resulting in some etherification of lignin. The use of organosolv lignin in phenol-formaldehyde resin is identified as a possible high revenue gainer for a biorefinery (Arato et al., 2005).
The best known organosolv process is the Alcell® process (from: alcohol cellulose) using ethanol–water mixtures to delignify wood. It was developed by General Electric Corporation in the early 1970s and commercialised by Repap Enterprises Inc. (Stamford, USA) between 1978 and 1997 in a pulp mill in New Brunswick, Canada. It was later modified by Lignol Energy Corporation (Burnaby, Canada). The principal advantage of the Alcell® process is that it provides lignin, hemicelluloses and cellulose in separate streams of relatively high purity. The lignin is un-sulfonated and has a higher purity than other lignins (Zakzeski et al., 2010). The properties and chemistries of lignin can be manipulated by altering processing conditions such as temperature, solvent concentration and type (Goyal et al., 1992; Tirtowidjojo et al., 1988).
Acid hydrolysis, which entails the use of dilute acid at high temperature or concentrated acid at mild temperatures, is used to hydrolyse the cellulose and hemicelluloses portion of biomass for fermentation to ethanol. The lignin residue that remains is highly condensed, has high contamination with carbohydrates and is recovered in low yields from 50 to 70%. Nevertheless, NREL has developed a one-stage dilute acid biorefinery and acid hydrolysis lignin may be available for product development. In the 1980s, a South African company, C.G. Smith Sugar Limited, developed an acid hydrolysis lignin product with the trade name Sucrolin. It is produced by treating bagasse with superheated steam at 180°C in the presence of small quantities of acetic acid. The lignin is isolated from the lignin–cellulose residue by dissolution in sodium hydroxide, followed by neutralisation of the solution which precipitates the lignin (van der Hage et al. 1993). Sucrolin was available as a technical lignin from Sigma Aldrich (Lora and Glasser, 2002).
Acetic acid has been used as a pulping agent for both hardwoods and softwoods. The acidity increases the rate of hydrolytic cleavage of lignin but also increases rates of subsequent lignin recondensation and the production of furfural, a fermentation inhibitor (Vazquez et al. 1995, 1997; Parajo et al., 1993, 1995a, 1995b).
Formic acid pulping allows lower temperatures to be used (120°C), but results in higher cellulose losses compared to ethanol pulping (Erismann et al., 1994). For example, 99% formic acid at just 95°C results in pulps containing 8% klason lignin (Baeza et al., 1991). Formic acid and acetone in the ratio of 7 : 3 has been used (Baeza et al., 1999), giving high pulp yield and low lignin content (kappa ∼ 25), although it is reported that high pulping pressure is necessary to achieve adequate penetration of solvent.
Lignin can also be recovered from the pyrolysis process, which heats lignocellulosic material to around 450°C in limited oxygen atmosphere to thermally degrade all components into hydrocarbons. A water insoluble product that contains lignin can be isolated from the pyrolysis oil. It is distinct from other forms of lignin because it is derived from oligomers containing up to 8 monolignols (Scholze et al., 2001). Mw ranges from 600 to 1300 g/mol and Mn from 300 to 600. Carbon fibre has been suggested as a high value application for pyrolysis lignin (Holladay et al., 2007; Qin and Kadla, 2012). Structures of pyrolytic lignins are shown in Fig. 20.7.
The physical delignification process of steam explosion uses saturated steam at high pressures (150–230°C) and elevated temperatures (450–500°C) for a period of time to hydrolyse lignin and hemicellulose from lignocellulosic material (Laser et al., 2002; Li et al., 2007). When SO2 gas is used as catalyst, the process times are as short as 1–20 minutes (Bura et al., 2002). The wood is softened by the heat and after the explosive decompression it is widely defibrillated (Kadla and Dai, 2006), although the explosive decompression may not be necessary and refining may be used to separate fibres (Schütt et al., 2012). Steam explosion is an efficient technique to separate cellulose, hemicellulose and lignin with relatively high yield (Heitz et al., 1991; Grethlein and Converse, 1991; Kadla and Dai, 2006). Hardwoods are delignified much more easily than softwoods and, although only a small amount of lignin is dissolved during the process, the water-insoluble lignin can be extracted by 0.1 M alkaline solution or organic solvents. Stake technology uses steam explosion to produce sulfur-free lignin with a Tg of 113–139°C (Heitz et al., 1991; Lora and Glasser, 2002). It has been marketed under the trade name Angiolin.
Recently, novel solvent systems such as ionic liquids (ILs) have attracted a great deal of interest. The ability to process at atmospheric pressure and dissolve any component with a well-tuned solvent has spurred significant research into ILs for lignocellulosic processing (Kim et al., 2011; Lee et al., 2009; Stark, 2011; Tan et al., 2009; Fort et al., 2007; Pu et al., 2007; Pinkert et al., 2011). Defined as salts with melting points below 100°C (Wilkes, 2002), ILs generally feature an organic cation and inorganic anion. The major benefit of ILs for wood component solubilisation is their low vapour pressure coupled with their ability to dissolve part or all of the wood cell wall. Some of the most common IL cation and anion combinations are shown in Fig. 20.8. (Olivier-Bourbigou et al., 2010).
One major benefit of IL processes includes the potential for in-situ derivatisation of wood components. Fort et al. (2007) note that very cleanly fractionated cellulose can be dissolved from wood shavings after precipitation with a suitable anti-solvent. Whole solubilisation after an extended time was not achieved for any sample and recovery of the lignin and hemicelluloses was not detailed. Relatively inexpensive ionic liquids exist that are capable of recovering 93% of the lignin from sugarcane bagasse. A mixture of 1-ethyl-3-methylimidazolium as the cation, a mixture of alkylbenzenesulfonates and xylenesulfonate as the main anion was used by Tan et al. (2009). Recovery of ionic liquid was between 96.1 and 99.4%, although the complexity of the recovery process was noted.
Whole wood solubilisation can be achieved but separation of the lignin from the IL is problematic. Selective dissolution of holocellulose can be achieved allowing filtration of the solid lignin. However, efficient recovery of low molecular weight oligosaccharides has not been achieved and attention must be paid to the corrosive properties of the IL. Lignin can also be preferentially dissolved in a process analogous to the kraft process. In all cases, dissolution only occurs if biomass is thoroughly dried and substantially reduced in size. Many ionic liquids have very high viscosity, 2–3 orders of magnitude greater than solvents. This remains a challenge in ionic liquid commercial application in solubilisation processes that are mass transfer limited. The energy cost of biomass comminution may be restrictive depending on the required size (Olivier-Bourbigou et al., 2010; Stark, 2011).
The cost of manufacturing ILs is limiting their use. Higher IL recovery rates or cheaper production must be achieved. Furthermore, separating solubilised wood components from the IL may require complicated processing (Olivier-Bourbigou et al., 2010).
20.5 Applications of lignin and lignin-based products: an overview
Lignin is an excellent source of energy as it has a higher heating value than the other constituents in wood and is comparable to lignite (Raveendran and Ganesh, 1996). Pulp mills make use of this by combusting spent liquor in the recovery furnace, providing the energy for recovery of pulping chemicals. Spent liquor can also be diverted to the lime kiln or used for electricity generation for plant utilities or sale back to the grid. A great deal of research effort has been devoted to utilising lignin for applications other than heat generation, particularly specialty chemicals. These products include platform chemicals, adhesives and resins, plastics, paints, inks, soaps, coatings, cleaning compounds, lubricants and hydraulic fluids, greases, pesticides, toiletries, fragrances and cosmetics (Wool and Sun, 2005). However, very little has been realised on a commercial scale for a number of reasons. One reason is the uncertainty of lignin supply and the heterogeneity of the material (Lindberg et al., 1989), as the properties of lignin depend highly on the treatment of the lignocellulosic plant and the utilisation of lignin is thus limited by the physical and chemical properties it shows after the pulping process (Wool and Sun, 2005).
Addition of lignin to improve particular products and development of novel products, rather than replace existing products and chemicals, is a promising area of research. The value of such products and additives will not be subject to competition with the oil-derived equivalent. However, in many cases, addition of lignin to a product resulted in a product with strictly worse properties, which is unacceptable. Besides large volume, low value applications, also high value, low volume products are needed to operate pulp mills economically and to make the extraction of lignin practicable. Platform chemicals that are indistinguishable from their petroleum-derived version can be made already. Although aromatics such as benzene, toluene and xylene have a large market and high value, the cost to produce these from lignin is high and spot prices are volatile.
20.5.1 Traditional lignin products
Lignosulfonate
The only lignin products obtained from a pulping process that have found a vast range of applications up to now are lignosulfonates. These are non-hazardous materials with excellent properties that are used as binders, emulsifiers and dispersants for a great variety of materials. As inexpensive components in binders, lignosulfonates are commonly used for commodities like coal briquettes, ceramics, briquetting of mineral dust, and the production of plywood or particle boards. Their ability to retain moisture and suppress dust makes them a useful tool for construction works, gravel roads, airports and sports facilities. As an anti-settling agent that also prevents lumping, lignosulfonates are used in concrete mixtures, ceramics, gypsumboard production and for leather tanning. Lignosulfonates provide flowability and plasticity to cement. This is a replacement for more expensive materials that provide set retardation such as superplasticisers, gluconates and gluconic acid. Wet-process Portland cement mills utilise lignosulfonates to increase the solids content of raw slurries. Lignin-based concrete additives are in demand and can be worth as much as $1.05–$1.32/L as an aqueous solution (Holladay et al., 2007). Sulfur-free lignin such as soda lignin has also been shown to improve flowability of mortar (Nadif et al., 2002). Lignosulfonates can also stabilise emulsions of immiscible fluids like asphalt emulsions, pesticide preparations, pigments and dyes. Due to their low toxicity, they can be used as binders in animal feed and thereby improve the feed properties of pellets. In addition, lignosulfonates show the ability to keep micronutrients in solution which is useful for micronutrient transport or as a cleaning and decontaminating agent in water and soils (Nadif et al., 2002; Calvo-Flores and Dobado, 2010).
Copolymer/resin applications
Copolymers and resins are a large market of medium to high value for lignin, provided lignin can improve material performance. In order to prepare formulations with lignin, it can be either used in an unmodified or chemically modified form. Due to its aromatic structure, lignin can undergo a wide range of chemical modifications that include, but are not limited to, alkylation, acylation, amination, carboxylation, halogenation, oxidation and reduction, nitration and sulfonation (Boye, 1985). Polyblending is a common industrial technique to physically mix two or more polymers in order to produce new high-performance materials from readily available materials and to reduce the production costs.
Lignin has been blended with other polymers or used as simple filler for decades in order to obtain biodegradable materials with improved mechanical properties, thermal and light stability. However, the practical results are still moderate and leave lots of room for further research (Calvo-Flores and Dobado, 2010). Already in 1983 Lyubeshkina could show that the addition of dry sulfate lignin can yield a frost-resistant polypropylene (Lyubeshkina, 1983). Numerous studies have been carried out that used lignosulfonate, kraft or organosolv lignin in blends with polyethylene, polypropylene or poly(ethylene-co-vinyl acetate) with a lignin content up to 70%. The polyblends could show good thermal stability (Lindberg et al., 1989), increased modulus (stiffness) with increased lignin content (Kubat and Stromvall, 1983), improved mechanical properties and electrical resistance (Kharade and Kale, 1998), the ability to absorb UV light (Kosikova et al., 1993), improved modulus and occasionally strength when the co-component to lignin contained polar functions (Ciemniecki and Glasser, 1989), and pronounced matrix reinforcement of blends with increased lignin content (Roesch and Muelhaupt, 1994). Gandini et al. could show that lignin acts as a stabiliser against the photo-oxidative degradation of polymers like polyolefins (Gandini and Belgacem, 2008; Gandini, 2008), which makes it possible to replace costly synthetic additives as long as the brown colour of the resulting blends does not affect the application. Besides polyolefins, also poly(vinyl chloride), poly(vinyl alcohol), hydroxypropyl cellulose and wheat starch have been blended with lignin and showed promising mechanical properties (Feldman, 2002; Rials and Glasser, 1989; Baumberger et al., 1997, 1998; Corradini et al., 1999).
Organosolv lignin has been incorporated into inks, varnishes and paints with positive effects on viscosity and misting with the addition of up to 10% w/w lignin. There were no negative side-effects of the addition apart from a brown discoloration, which was deemed irrelevant for most applications (Belgacem et al., 2003).
Stewart (2008) considers the commercial application of lignins to phenolic resin (PF resins), inevitable given the move towards sustainable practices in the chemical industry and the increasing price of phenol. The size of the phenolics market and the economic attractiveness of utilising lignin for a phenol replacement justify the large volume of research in this area. Phenol and formaldehyde react together by condensation to form a methylene bridge between phenol units at the ortho- and para-positions of the aromatic ring. As phenol can react with three molecules of formaldehyde and formaldehyde can react with two molecules of phenol, resoles are capable of forming a highly cross-linked 3-D network polymer. Because lignin consists of phenyl-propanoid units, it is capable of reacting with formaldehyde in a similar manner to phenol. The similarity of lignin and phenol is shown in Fig. 20.9.
The resin market represents a large potential sink especially for kraft and organosolv lignin, which have been incorporated into PF or epoxy resins in many studies (Cook and Sellers, 1989; Shiraishi, 1989; Ono and Sudo, 1989). In order to incorporate lignin into PF resin, lignin can be methylated or phenolated prior to addition. This ensures the chemical crosslinking of lignin into the resin. However, simple addition of lignin prior to synthesis is possible given a sufficiently reactive form of lignin and represents a significant capital cost saving over pre-synthesis reaction of lignin (Muller et al., 1984). Lignin has been proven to reduce synthesis time (Benar et al., 1999) and ammonia-modified lignin can improve thermal degradation resistance of PF resin (Alonso et al., 2001).
Lignin has also successfully been used in epoxy resins (EP resins) since the 1960s and has been sold for example by Lenox Polymers Ltd (Port Huron, USA) until they liquidated in 2000. Epoxy resin containing 50% lignin can be produced with electrical and mechanical properties equivalent to non-lignin-containing epoxy. It is possible to produce large quantities of lignin-epoxy resin on the order of 500 kL/year using relatively simple purification methods (Simionescu et al., 1993). Feldman (2002) could show in several studies that kraft lignin as a component in EP resins can improve the adhesive joint shear strength with a content up to 30% and the DSC data indicates a monophasic system up to 20% lignin. The production costs of the lignin-based EP resin were estimated to be 81% lower than the commercial EP resin. Studies investigating the influence of lignin type on the adhesive properties of EP–lignin polyblends showed that the addition of hardwood lignins results in a higher adhesivity than the addition of softwood lignin (Indulin). The improvement could also be related to the molecular weight of lignin. Hardwood lignin was shown to crosslink more efficiently than softwood lignin (Feldman, 2002). Methylolation of lignin in PF resins was shown to depend on the number of reactive sites (Peng et al., 1993).
Another application for lignin is its use in the production of polyurethane films or foam. Polyurethane films have been produced incorporating Alcell® lignin. Flexible but weak polyurethanes were produced at low lignin content. However, relatively tough polyurethane could be produced with lignin content from 15 to 25% w/w. Above 30% w/w lignin, the polyurethanes were hard and brittle (Thring et al., 1997). Precured polyurethanes have been synthesised by Gandini et al. from organosolv lignin, polyethylene glycol or polypropylene glycol and methylene diphenyl isocyanate. The resulting products are rigid polyurethane foams or sheet (Gandini et al., 2002).
Some recent research showed that bioplastics can be produced from lignin by mixing a specific low-sulfur lignin with natural fibres and natural additives such as wax. The resulting thermoplastic material is recyclable and due to the additives can survive contact with water or saliva. The bioplastic can be injection-molded and is commercially available under the brand name ARBOFORM® (Calvo-Flores and Dobado, 2010; Nägele et al., 2002).
Metal ion absorption
Quite a few studies have shown that lignin can be used as a good adsorbent for metal ions as well as dyes, bile acids, cholesterol, surfactants, pesticides and phenols. For kraft pine lignin, the binding strength for metals was found to be in the following order Pb(II) > Cu(II) > Zn(II) > Cd(II) > Ca(II) > Sr(II). It was also found that protons or existing metals are released from the lignin during the uptake of other metal ions. At low pH, only the more tightly bound metals (Pb(II), Cu(II), Zn(II), Cd(II)) can compete with the protons for binding sites and therefore the uptake of Ca(II), Sr(II) and Li(I) only occurs at higher pH. Despite the differences between the different studies, the adsorption capacity of unmodified lignin is relatively low and inferior to activated carbon for the removal of metal ions from solutions (Suhas et al., 2007).
High value products
Due to the pulping process under high pressure and temperature, also low molecular weight products are obtained that can be transformed into high value products by oxidation. For instance, vanillin has been produced from lignosulfonates since 1937, and this was the dominant procedure for many years until environmental issues forced these mills to close. However, the Norwegian company Borregaard continued to produce vanillin from lignin and developed an environmentally friendly process, including an oxidation step with a copper catalyst that is economically viable and emits less CO2 than other processes based on petrochemical precursors. Another valuable chemical derived from the pulping process is DMSO. It can be synthesised by oxidation of dimethyl sulfide (DMS) which occurs as a by-product of kraft pulping when lignin is treated with molten sulfur in alkaline media (Calvo-Flores and Dobado, 2010).
20.5.2 Emerging lignin products
Antioxidants
If used directly without blending with other polymers, lignin shows some interesting antioxidant behaviour and acts as a free-radical scavenger. It could be shown that free phenolic hydroxyl groups are involved in that mechanism and that ortho-methoxyl substituents help to stabilise the phenoxyl radicals by resonance (Pan et al. 2006b). Kraft lignin and steam-exploded lignin have shown antioxidant activity in human red blood cells, whereas certain water-soluble lignin derivatives present anti-viral activity in vitro. Some other derivatives have shown antibiotic and anti-carcinogenic activities which make them interesting materials for the health and pharmaceutical industry (Calvo-Flores and Dobado, 2010).
Activated carbon/carbon black/carbon fibre
During wartime, lignin was investigated as a substitute for carbon black and other fillers for rubbers. Since the studies could not be completed before the end of the Second World War, they were pursued afterwards (Keilen and Pollak, 1947) and as far back as 1947 lignin was added to various rubber latexes as a reinforcing agent. In 1964 the first practical test with tyres where half of the carbon black was replaced by kraft lignin was performed. Lignin is compatible with various rubber materials and does not affect the stability of those latexes even in high concentrations. Due to the low specific gravity of lignin, the resulting materials are much lighter than those filled with carbon black and the mechanical properties of lignin-reinforced rubbers can compete in terms of abrasion resistance, modulus, elongation, hardness and tensile strength (Table 20.5). There are basically two ways of introducing lignin into a rubber preparation: by co-precipitation of lignin and rubber from a lignin-rubber latex mixture, or by mixing hydrated lignin into the rubber (Feldman, 2002). However, the tyre safety and testing standards of today present high barriers to the modification of tyre composition and restrain utilisation.
Table 20.5
Physical properties of lignin and carbon black reinforced styrenebutadien copolymer rubber (SBR) at 68 parts of lignin per 100 parts of rubber (oil-extended SBR)
| Property | Lignin | HAFa carbon black | ISAFb carbon black |
| Modulus (MPa) | 3.6 | 4.2 | 5.1 |
| Tensile strength (MPa) | 21.7 | 17.5 | 20.5 |
| Elongation (%) | 720 | 720 | 750 |
| Hardness (Shore) | 54 | 56 | 61 |
| Tear resistance (MPa) | 2.4 | 2.1 | 2.3 |
| Corrected pico abrasion | 86 | 91 | 114 |
Because of their highly porous structure with a large internal surface area, activated carbons have a good adsorption capacity for many substances. They are able to remove organic and inorganic pollutants from liquid as well as gaseous phases. Activated carbons are available as powdered activated carbons (PAC) or granular activated carbons (GAC) and differ in diameter and surface area. Coal, lignite, peat, wood and coconut shells are commonly used to prepare activated carbons but also lots of other materials have been investigated as a source material. Among them, lignin is a very promising material from which activated carbon with a high surface area and pore volume can be produced that is comparable to common activated carbon. The adsorption capacity of activated carbons can be controlled by the parameters of either physical or chemical activation or a post-activation treatment. The results of sorption studies of inorganic or organic substances are promising, but more systematic studies are necessary because some published works are contradictory. Also the relationship between adsorption properties and molecular structure has to be investigated in more detail (Suhas et al., 2007). Although the use of lignin as precursor for activated carbon is more costly than wood dust or coconuts shells, its ability to form fibres could lead to interesting applications such as activated carbon fibres.
Carbon fibres are some of the most important and widely used advanced engineering materials: they are lightweight, fatigue resistant materials that possess high strength/modulus and high stiffness. These unique properties result from their flawless structure and the development of highly anisotropic graphic crystallites orientated along the fibre axis during the production process. Carbon fibres are manufactured by thermally treating fibres at 1,000–2,000°C in an inert atmosphere while maintaining the fibrous structure. This is aided by a stabilisation stage in which the precursor fibres are heated under tension at 200–300°C in the presence of air. This causes crosslinking on the fibre surface, among other reactions, and prevents shrinking, melting and fusing (Kadla et al., 2002b).
Advanced fibres, such as carbon fibres, are routinely used in sports equipment, marine products and the transportation industries. There are primarily two types of precursor materials of commercial significance: pitch (petroleum or coal) and polyacrylonitrile (PAN). However, increasing demand, rising energy costs and concern over the sustainability and environmental impact of fossil fuels is driving the need to find sustainable alternatives. Lignin, as a polyaromatic macromolecule, is thought to be a petroleum progenitor and therefore an ideal biopolymer precursor for the production of carbon fibres.
Although widely studied for the production of carbon fibres (Kadla et al., 2002a; Sudo and Shimizu, 1992; Otani et al., 1969; Uraki et al., 1995), ligninbased carbon fibres still suffer from poor fibre properties and limited lignin availability. As compared to pitch, lignin-based carbon fibres have an advantage of higher carbonisation yields and shorter crosslinking stage, believed to be due to the lignin structure.
Production of functional fibres by adding carbon nanotubes or magnetic particles to the lignin can open the market to applications like magnetic shielding or conductive materials. The mechanical expectations for these materials are not as high as for other applications and fibres with lower tensile strength can readily be used.
The properties of lignin-based carbon fibres depend largely upon the source of lignin. Lignosulfonates and kraft lignin typically have sodium impurities, which can lead to inclusions and microvoids arising from catalytic graphitisation and thereby decrease mechanical performance (Johnson et al., 1975). This can be eliminated via ion exchange and/or fractionation of the feed lignin (Kadla et al., 2002a; Baker, 2007). By contrast, organosolv lignins have fewer inorganic impurities, have better thermal properties, and can readily be spun into fibres. However, their lower softening point can lead to problems in the subsequent thermal processing (Kadla et al., 2002a). As with the utilisation of petroleum pitch, wherein thermal and/or catalytic modification produces mesogens and subsequently high performance carbon fibres (Donnet et al., 1998), thermal and catalytic treatment of lignin (Sudo et al., 1993) enhances fibre properties, but the underlying lignin structure seems to be a limiting factor (Dave et al., 1993).
Platform chemicals
Several thermochemical methods have been studied in the recent past in order to depolymerise lignin and to convert it into value-added chemicals. Pyrolysis, gasification, hydrogenolysis, chemical oxidation and hydrolysis under supercritical conditions are the major methods to produce pyrolytic oil, syngas or phenols. An overview of these methods and the resulting products is shown in Fig. 20.10.
Pyrolysis means the thermal breakdown of an organic substance into smaller units in the absence of air. The limited oxygen supply prevents the further combustion to CO2. The pyrolysis of lignin is a complex process and is affected by the lignin type, heating rate, reaction temperature and additives. The main products of lignin pyrolysis are gaseous hydrocarbons such as CO2 and CO, volatile liquids (methanol, acetone and acetaldehyde), monolignols, monophenols (phenol, guaiacol, syringol and catechol), and other monosubstituted phenols. Depending on the reaction temperature, parts of the lignin are also converted to different amounts of char (Pandey and Kim, 2011). The cleavage of weaker bonds in lignin occurs at lower temperatures while the cracking of aromatic rings happens at fairly high temperatures, thus the pyrolysis is covering a wide range of temperatures (Yang et al., 2007; Ferdous et al., 2002). Ferdous’ studies of Alcell® and lignin show that the conversion at lower temperatures is higher for lower heating rates, but for temperatures above 700°C, the conversion is higher for higher heating rates. However, the fast pyrolysis has the advantage of lower char and coke formation (Windt et al., 2009). Several groups have also studied the influence of lignin source and isolation technique and it was shown that the structure of extracted lignin is (slightly) different from native lignin. Thermoanalysis of various lignin preparations showed that the highly condensed Klason lignin is more heat resistant than lignin from steam explosion, enzymatic hydrolysis or hydrochloric acid treatment, which became evident by the significantly higher solid residue and the lower yield of monophenols after pyrolysis (Gardner et al., 1985). Due to their different H/C ratio, the amount of syngas is higher after pyrolysis of kraft lignin than from Alcell lignin (Ferdous et al., 2002).
When pyrolysis is performed in the presence of hydrogen, it is known as hydrogenolysis or hydrogenation. Different solvents or catalysts can be added in order to speed up the reaction and to increase the yield. While neat pyrolysis is not the best choice for the production of liquid fuels or chemicals because it produces mainly gases and solid coke, hydrogenolysis is a very promising method with a high net conversion, high yield of monophenols and low solid residue. Shabtai et al. (1999a, 1999b) proposed a three-step process that includes base-catalysed depolymerisation, hydrodeoxygenation and hydrocracking to produce reformulated gasoline from lignin. The base-catalysed depolymerisation is usually performed with an alkali hydroxide and a supercritical alcohol such as methanol or ethanol. At temperatures of about 270°C the system generates a pressure of 140 bar. Shabtai modified this initial process by a selective hydrocracking reaction followed by an exhaustive etherification reaction and partial ring hydrogenation. This leads to a reformulated, partial oxygenated/etherified gasoline (Shabtai et al., 2001; Pandey and Kim, 2011).
A new approach for the depolymerisation of lignin has been described by Kleinert and Barth (2008). Lignin is subjected to a reductive treatment with formic acid and ethanol at temperatures of 350–400°C. The lignin is simultaneously depolymerised and deoxygenised. The product separates into a lighter organic phase which consists mainly of small substituted phenols and a heavier phase which consists of C8–C10 aliphatics. In a follow-up study, this procedure could be successfully applied to lignin isolated after enzymatic degradation of wood (Kleinert et al., 2009). Hydrogen-donating solvents such as tetralin as well as reactions under pressurised molecular hydrogen with various catalysts have been described to be advantageous for the depolymerisation of lignin (Pandey and Kim, 2011).
Just like wood-derived and vegetable bio-oils, lignin (Alcell® lignin) can be turned into gasoline range hydrocarbons by pyrolysis using the zeolite catalyst HZSM-5 (Chantal et al., 1985). Compared to typical hydrotreatment processes requiring high operating pressures and hydrogen, this process can be operated at atmospheric pressure and at moderate temperatures of at least 550–650°C. The highest yield of liquid product of 43% w/w consisting mainly of aromatic hydrocarbons such as benzene, toluene and xylene could be obtained at a temperature of 550°C and a mass flow of 5 h− 1. With increasing temperature the yield of the liquid product decreases while the amount of gas increases dramatically due to extensive cleavage of the major bonds in lignin which can be concluded from the increase in CO2 and CO production. Also the amount of char and coke decreases with rising temperature. At 550°C the amount of char and coke is relatively high at 38%. Those results are quite promising for the production of hydrocarbons from lignin, particularly because of the moderate reaction conditions and the high yields. In addition, the zeolite catalyst can be easily regenerated by heating in an air flow for 1 h at 600°C. The only drawback of this procedure is the utilisation of acetone as a solvent for lignin. Although it was the only feasible solvent in this process, acetone is a quite valuable chemical itself and due to the low boiling point less safe at the chosen reaction temperatures. For the future, it would be desirable to find another solvent to minimise or substitute the amount of acetone (Mullen and Boateng, 2010; Guo et al., 2008).
All these procedures seem to be promising for the depolymerisation of lignin and therefore the production of platform chemicals and gasoline. But they require quite harsh reaction conditions (temperature and pressure), expensive catalysts and the separation of a variety of different reaction products. It will be a challenging task for the future to establish an economically viable process that can compete with common petrochemical processes. Even though the platform chemical market is large, the value is not yet high enough to warrant production from isolated lignin. This is expected to change as fossil resources become more costly to extract and prices for specialty chemicals increase.
Enzymatically treated lignins
As a result of governmental requirements and also consumer demand for eco-friendly, biodegradable and non-toxic products, the focus of research and production worldwide has turned towards renewable and natural raw materials and cleaner industrial practices. One emerging trend is the application of biotechnological processes, which includes the use of bacteria and fungi or treatment with isolated enzymes. Also in lignin chemistry, the utilisation of enzymatic treatment is becoming increasingly popular for breakdown of lignin or the modification of the functional groups.
Up to now, lignin degradation has been studied mainly in white-rot and brown-rot fungi, which produce enzymes that can metabolise lignin (Wong, 2009). The main extracellular enzymes active in lignin degradation are the heme-containing lignin peroxidase (ligninase, LiP, EC 1.11.1.14) and manganese peroxidase (MnP, EC 21.11.1.13) and the Cu-containing laccase (benzenediol:oxygen oxidoreductase, EC1.10.3.2) (Sena-Martins et al., 2008). However, these findings have not yet evolved into commercialisation of lignin breakdown, mostly because of the inherent difficulties in fungal genetics and protein expression (Ahmad et al., 2011). Lignin metabolising soil bacteria has been reported (Crawford et al., 1983; Zimmermann, 1990; Ramachandra et al., 1988; Vicuna, 1988) but the enzymatic process of bacterial lignin degradation is not yet well characterised. Ahmad et al. (2011) reported two spectrophotometric assays for lignin breakdown which could identify several bacterial strains with lignin degradation activity. They were also able to identify the existence of two DyP-type peroxidases, DyP B thereof being a lignin peroxidase. On-going research in this area will help to elucidate the enzymatic cleavage of lignin linkages. The obtained degradation product could comprise small molecules as well as lower molecular weight polymers with improved properties due to new/different functionalities than present in native lignin. This could open a large area of promising applications.
The various functional groups present in lignin are highly accessible for modification by chemical, physical as well as enzymatic reactions. Compared to chemical or physical treatments, enzymatic modifications possess the advantages of high selectivity and efficiency, they are performable under mild conditions and a wide range of substrates can be used. However, enzymes are only reusable when sufficiently immobilised and they are quite sensitive to denaturating agents and several sensory or toxicological effects (Sena-Martins et al., 2008). For lignin processing, the oxidative enzymes of certain ligninolytic fungi have been primarily investigated. For several reasons the industrial production of ligninolytic enzymes is still very low, making the research quite expensive and uneconomic. Nevertheless, much research is on-going in this field and the results present high potential for future use. Copolymers of straw pulp lignin with cresol and horseradish peroxidise as catalyst have been produced and can replace common phenolic resins. Also copolymers from different lignins (organosolv, Indulin AT and a synthetic hydroxypropylated lignin) with vanillic acid, diisocyante and acrylamide catalysed by laccase could be produced (Sena-Martins et al., 2008). In order to reduce the amount of harmful formaldehyde in wood binders, Hüttermann et al. (2001) reported some approaches with binders that were derived from laccase catalysed reactions between water-insoluble lignin and common resins. The treatment of organosolv (acetosolv) lignin with polyphenoloxidase was reported to result in a higher amount of hydroxyl and carbonyl groups and therefore enhances the chelating properties of lignin. Those products can be used for the treatment of heavy metals containing wastewater (Gonçalves and Benar, 2001). In addition, environmentally friendly coatings and paintings can be produced by the enzymatic polymerisation of different lignin materials (lignosulfonates, kraft and organosolv lignin) and enzymes such as catechol oxidase, laccase or peroxidase (Bolle and Aehle, 2001).
20.6 Future trends
In the short term, a large market for lignin from the ubiquitous kraft process must be found. This is necessary for the de-bottlenecking of kraft mills around the world. The most promising market for kraft lignin is the resins and adhesives market, which is substantial and represents an increase in value over traditional heat and power generation. Given the push for renewables and the phenolic nature of lignin, penetration into this market is considered inevitable by some (Stewart, 2008). In addition, if lignin improves material properties, this will be reflected in its market price.
In the medium term, more lignin from unsulfonated sources will become available. This lignin, from biorefinery platforms, will be unique in properties compared to other feedstocks and have small-scale availability. Initially, the resin and adhesives market may act as a catch-all for this more reactive lignin. However, a product niche of high value and sufficient market size must be found for each lignin source. For example, hydrophobic organosolv lignins could add value to resins used for water resistant particleboard. Platform chemicals are an attractive medium-term market as there is already an existing market and no product development is needed.
In the long term, biorefineries will be tuned to produce lignin for specific materials of high value, and lignin is expected to yield greater incomes than the other components of wood.