b-(1 / 3, 1 / 4)-glucans (1314Gs) consist of a linear chain of b-D-glucopyranosyl units linked by (1 / 3) and (1 / 4) bonds (Table 17.1). 1314Gs are present in Poaceae (grasses and cereals) as well as in eq — uisetum, liveworts and Charophytes. The mixed-linkage glucans are dominated by cellotriosyl and cellotetrasyl units linked by b-(1 / 3) linkages, but longer b-(1 / 4)-linked segments also occur. Cellulose is also b-D-glucan, which is linked by (1 / 4)-glycosidic bonds, and thus cellulose has high stiffness (crystal­linity) and is insoluble in most solvents. Contrary to cellulose the b-(1 / 3) linkages existing in 1314Gs make glucans flexible and soluble (Peng et al., 2012).

Complex Heteroxylans

Complex heteroxylans are present in cereals, seeds, gum exudates and mucilages and they are structurally more complex. In this case the b-(1,4)-D-xylopyranose backbone is decorated with single uronic acid and arabi — nosyl residues and also various mono — and oligoglycosyl side chains (Girio et al., 2010).

Conclusions on Carbohydrate Feedstocks

Storage carbohydrates are uniform in composition and relatively easily to isolate and purify. Therefore many fermentative and catalytic processes have identi­fied these feedstocks as their initial feedstock of choice. Because of costs and societal debates (food versus fuel and indirect land use debates) many researchers both from industry and academia are investigating the use of lignocellulose as feedstock. Pure cellulose has the same advantages as starch in that it is only build up from glucose and relatively easy to hydrolyze (although much more difficult than starch) when pure (and amor­phous). However, to make use of lignocellulose econom­ically also the hemicellulose needs to be used. This overview clearly shows that due to the heterogeneity of the monosaccharides incorporated and large diversity in linkages and side groups, both enzymatic hydrolysis system as well as a catalytic/fermentative conversion system needs to be quite robust to make optimal use of the cellulose and hemicellulose fractions.

LIGNIN

In addition to carbohydrates the major component of lignocelullulosic biomass is lignin. Lignins are major structural components of higher plants, and confer to woody biomass its mechanical structure and resistance to environmental stress and microbial decay. Lignin, the name of which is derived from the Latin word for "wood", accounts for 15—30 wt% of woody biomass and it is also available from agricultural residues such as straw, grass and bagasse.

Lignins are built in plants starting from three basic monolignols via oxidative phenolic coupling reactions to generate the three-dimensional lignin polymer (Ralph et al., 2007). The heterogeneity of lignin polymers exists in molecular composition and linkage types between
the phenylpropane monomers, p-hydroxyphenyl — (H), guaiacyl — (G), and syringyl- (S) units. These are derived from the monolignols sinapyl-, coniferyl-, and coumaryl alcohol, respectively (Table 17.1). Lignin composition will be different not only between species, but also between different tissues of an individual plant. In soft­wood lignin coniferyl alcohol is the predominant build­ing unit (over 95% guaiacyl structural elements), while in hardwoods (and dicotyl fiber crops) the ratio coniferyl/synapyl shows considerable variation. In lignins of cereal straws and grasses the presence of cou — maryl alcohol leading to p-hydroxyphenylpropane struc­tures is typical. The lignin content ranges (Table 17.1) and chemical structures of the three primary building blocks in lignocellulosic biomass are given in Table 17.3 and the occurrence and type of different interunit linkages in Table 17.4.

For the production of aromatic chemicals from bio­refinery lignin selection of suitable resources can be made on the occurrence of building blocks and interunit linkages next to the choice of pretreatment and isolation procedure. The presence of one or two methoxyl groups at the ring may for the production of some chemicals (e. g. guaiacol, syringol) be a requirement. For the con­version of lignin into benzene, toluene, xylene (BTX) or phenol the presence of coumaryl units (H-units) may be an advantage due to the lack of side groups next to the aromatic phenolic group (Table 17.3).

The complex structure of (isolated) lignins needs suit­able characterization methods and an ongoing effort for improvement of these methods have been performed during the last decades. However, results of these analytical procedures are not always consistent.

Methods for lignin characterization can be found in the literature (Gosselink et al., 2004; Baumberger et al., 2007; Tejado et al., 2007; Monteil-Rivera et al.,

2013) and via the International Lignin Institute (www. ili-lignin. com). Two-dimensional nuclear magnetic resonance and pyrolysis gas chromatography mass spectrometry are now established analytical techniques for detailed structural lignin analysis (Table 17.4).

From a chemical perspective, lignins are highly com­plex polyphenolic biopolymers with aromatic units in different configurations. Lignins are traditionally produced in the pulp and paper mills by extracting lignin upon liberation of cellulosic fibers used for paper making. Most kraft lignins are burnt within paper mills to generate heat and power, thus providing energy autonomy and lowered operating costs. The majority of lignosulfonates are used as additives in the building sector, where they provide plasticity and flowability to concrete. Lignosulfonates are also used as binders in animal feed, in road building, oil well drilling and as dispersants and coatings in pesticides used for agricul­ture applications. Sulfur-free lignins derived from soda pulping of annual plants such as grass and wheat straw are produced commercially and used among others in wood adhesives and in animal feed. More recently, biorefinery lignins are produced in so-called biorefinery or fractionation processes, for example for the manufacturing of cellulosic bioethanol. This side stream is for the short term used primarily as energy source, but for the medium to longer term utilization of these lignins for the production of biofuels, aromatic chemicals and materials are expected. So far limited industrial use of technical lignins is seen mainly due to the easy use as

TABLE 17.3 Lignin Content and Chemical Structures of Lignocellulosic Biomass

Source: Azadi et al., 2013.

TABLE 17.4 Frequencies of Different Interunit Linkage Types in Native Softwood and Hardwood Lignin per 100 C9 units

Name Structure Softwood Hardwood

Source: Henriksson et at, 2010.

energy source, the impurities in technical lignin sources, tendency to form condensed structures, inferior perfor­mance compared to synthetic compounds, unique reac­tivity, lack of availability of high-purity lignins and a large variety of different types of lignins (Vishtal and Kraslawski, 2011). Additionally, traditional (heteroge­neous) catalysts work inferior to biorefinery lignin and need to be redesigned (Zakzeski et al., 2010).

Upon depletion of fossil resources, the production of aromatic chemicals from these resources will come under stress. As lignin is by far the most abundant aro­matic renewable resource on earth, lignin is the only resource that could fulfill the quantities needed for the substitution of the main aromatic compounds used in in­dustry (Holladay et al., 2007). These are phenol, BTX, and terephthalic acid (Van Haveren et al., 2008). The annual global production of these largest aromatic chemicals is estimated at 103 million metric tons total, with benzene at 44, toluene at 22, and xylenes at 37 million metric tons (Nexant Chem. Systems, 2012).

With an estimated global biomass production of about 150 billion tons annually about 30 billion tons of lignin is generated each year globally (Balat and Ayar, 2005). This amount of lignin is far exceeding the need for the conver­sion to aromatic chemicals even at low conversion de­grees of about 10%. Currently there is a strong desire from major brand owners (e. g. Coca Cola, Pepsi, Heinz) to "green" their product portfolio by using biobased poly­mer building blocks. Lignin could play in the future an important role as a biobased feedstock. However, there are quite some challenges to overcome for the develop­ment of an economically viable process for the production of aromatic chemicals from lignin (Gosselink, 2011).