Desempeño del sector primario mexicano en el TLCAN

Chapter 1

Introduction

1.1 Biomass components

Cellulose

Cellulose is a linear homopolymer consisting of anhydro-β-D-glucopyranose units (AGU) that are linked together by (1-β-4) glycosidic bonds. Cellobiose is the repeating unit, consisting of two AGUs (Fengel and Wegener, 1989;

Sjöström, 1993). In the solid state, AGU units are rotated by 180° with respect to each other due to the constraints of β-linkage. The number of AGUs determines the chain length, i.e. degree of polymerization (DP), but the DP of cellulose is greatly dependent on the origin and the method of isolation. Native cellulose has a degree of polymerization (DP) between 5000 - 15 000 (Klemm et al., 2005) but the cellulose used in practice has an average DP of between 800-3000 (Krässig, 1996). Every AGU unit has three hydroxyl groups in the positions C2 and C3 (secondary hydroxyl groups) and C6 (primary hydroxyl groups). One end contains an alcoholic OH group at C4 (non-reducing end), while on the other end C1 is part of an aldehyde group with reducing activity (Figure 1-1).

Figure 1-1 Molecular structure of cellulose with repeating cellobiose units showing reducing (right) and nonreducing (left) end-groups (from Olsson and Westman 2013)

The reactivity and chemical character of cellulose is determined by reactivity of the primary and two secondary OH groups in the AGU (Klemm et

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al., 1998). These hydroxyl groups are responsible for forming both intra- and intermolecular hydrogen (H)-bonding network. Intramolecular H-bonds can be formed between the C6 hydroxyl and the C2 hydroxyl and, between the C5 oxygen and C3 hydroxyl within the same molecule, stabilizing the glycosidic bond and making the structure stiff. H-bonds can also be formed between two neighboring cellulose chains which interact via their C3-OH and C6-OH groups.

Those interactions are called intermolecular H-bonds (Figure 1-2).

Figure 1-2 Intermolecular and intramolecular hydrogen bonds in cellulose (from Wang et al. 2012)

The H-bonds are responsible for cellulose structure and for its insolubility in water and common organic solvents. The characteristic rigidity is via co-crystallization of multiple chains into parallel structures forming elementary fibrils, which are further organized into microfibrils (Fengel and Wegener, 1989;

Kontturi et al., 2006; Gandini, 2011; Moon et al., 2011). However, the fundamental reason behind this is still on debate. One hypothesis states that the main reason of cellulose almost non-dissolving properties lies in the fact that the hydroxyl groups are responsible for the extensive hydrogen bonding network which suppresses the solubility (Klemm et al., 1998). Recently, Lindman et al.

(2010) asserts that the solubility characteristics of cellulose are based upon amphiphilic and hydrophobic molecular interactions, which have been extensively discussed by authors such as Glasser et al. (2012). Also Johansson et al. (2011) showed important evidence that the reactivity of the surface hydroxyl groups governs the behavior of cellulose in different media.

As mentioned earlier, the supramolecular model of cellulose is based on the organization of cellulose chains. Parallel synthesis of cellulose in the biosynthesis of wood or other non-wood plants leads to noncovalent association

1-3 between multiple chains, which results in substructures termed microfibrils.

Wood microfibrils consist of elementary fibrils which start formation from the terminal enzyme complexes (TC) that take shape of a six-lobed ―rosette‖, where β-D-glucose chains are held together via van der Waals forces. Those microfibrils are micrometers long and few nanometers thick. They contain both ordered and disordered components (Atalla and VanderHart, 1999; Vietor et al., 2002) but their structure and exact size are still under discussion (Figure 1-3).

Cellulose fibrils are thus ordered 3-dimensional crystals. Thus, it may be interpreted as the crystals have different bonds in each dimension. The first dimension is given by the covalent bonds (enforced by some hydrogen bonds) along the cellulose chains, giving the final length of the fibril. The second dimension constitutes the hydrogen bonds, holding the cellulose chains together in sheets. The Van der Waals bonds and x-interactions bridging the cellulose sheets in the fibril form the third dimension (Ek et al., 2009).

Figure 1-3 Crystalline and non-crystalline regions (from Börjesson and Westman 2015)

There are several polymorphs of crystalline cellulose (I, II, III, IV) (O’sullivan, 1997). Cellulose I (native cellulose) is the form found in nature, produced by trees, tunicates, algae, plants and bacteria. Native cellulose (cellulose I) occurs in two allomorphs, I and Iβ, with different parallel crystalline structures. Cellulose I (triclinic structure) is the dominant crystalline structure for native bacterial cellulose and Valonia (algae) cellulose, whereas cellulose Iβ (monoclinic structure) dominates in the higher plants such as wood and cotton. These two different crystal forms differ in cellulose chain conformation, hydrogen bonding and different arrangement of cellulose molecules in the unit cell (Nishiyama et al., 2003). Cellulose I is

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thermodynamically metastable and can be converted to either cellulose II or III.

Cellulose II has been the most stable structure of technical relevance and can be produced by regeneration (solubilization and precipitation) or by mercerization (aqueous sodium hydroxide treatments) (Klemm et al., 2005). Mercerization is considered to begin at NaOH concentrations above 8-9% (Sixta, 2006) and the conversion occurs rapidly (Crawshaw et al. 2002). Under these concentrations, the most severe changes materialize in the crystallinity (Sixta; Ek et al., 2009).

The changes during the process depend not only on the alkalinity but also on the treatment time (Borysiak and Doczekalska, 2008). Cellulose II has antiparallel chains, i.e. that every second chain has opposite polarity to the next.

Thus, the hydrogen pattern is different, and there is one more hydrogen bond per glucose residue, compared to cellulose I. Cellulose III can be formed from Cellulose I or II through liquid ammonia or organic amine treatments, and subsequent thermal treatments lead to Cellulose IV. The crystalline regions of cellulose are interspersed with less ordered paracrystalline or amorphous areas, arising from imperfect packing and interactions with other non-cellulosic polysaccharides. Those amorphous areas were shown to be distributed on the surface (Larsson et al., 1997) as well as along the cellulose microfibrils (Nishiyama et al., 2003). Depending on the source, the degree of crystallinity in native cellulose is typically 50-70%, but can also be over 94% (Olszewska, 2013).

Hemicelluloses

The monosaccharides that constitute hemicelluloses are pentosans (D-xylose and arabinose), hexosans (D-glucose, D-galactose, galactose, D-mannose, L-rhamnose, L-fucose) and uronic acids (D-glucuronic acid, D-galacturonic acid) (Figure 1-4). Generally, the units of main chain are linked together by β-(14) linkages, although some species combine β-(14) and β-(13) linkages. The composition and relative amount of hemicelluloses in the cell wall depends on the wood species. The main hemicellulose in hardwood is xylan, more specifically a 4-O-methylglucuronoxylan (15 - 30% in wood, 4-O-MeGlcA:Xyl = 1:10) and glucomannan (about 2 - 5% in wood, Glc:Man = 1:1-2). The main softwood hemicelluloses are galactoglucomannan (about 20% in wood, Gal:Glc:Man = 0.1-1:1:3-4) and arabino-4-Omethylglucuronoxylan (5-10% in wood, Ara:4-OMeGlcA:Xyl = 1.3:2:10) (Fengel and Wegener, 1989; Sjöström, 1993; Iakovlev, 2011). Some other polysaccharides present in both softwood and

1-5 hardwood biomass are glucans (starch, callose, located in parenchyma cells), pectins (polyrhamnogalactouronides), arabinans, galactans (mostly located in the compound middle lamella) (Fengel and Wegener, 1989). Hemicelluloses are associated with cellulose microfibrils via hydrogen bonding and with other cell wall constituents via covalent linkage. Hemicelluloses contribute to the strength formation of cell wall in association with cellulose microfibrils. Another function of hemicelluloses is to bring flexibility to the structural assembly of cell wall components. The presence of hemicelluloses also inhibits the coalescence of cellulose microfibrils, which improve the fibrillation of cellulose into nanosized fibrill. Hemicelluloses possess side groups on the chain molecules (amorphous form), therefore are easy to dissolve and degrade in acid and alkaline solutions, and their DP ranges from 200 to 300 (Olszewska, 2013).

Figure 1-4 Monosaccharide constituents of hemicelluloses (from Fengel and Wegener 1989) HEXOSANS

glucose mannose galactose fucose rhamnose

PENTOSANS

arabinose xylose arabinofuranose

URONIC ACIDS

Galacturonic acid Glucuronic acid

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Lignin

Lignin is an aromatic heteropolymer built from 4-phenylpropane structural units (guaiacylpropane, G; syringylpropane, S; and hydroxyphenylpropane, H) bound by ether and carbon-carbon bonds (Figure 1-5).

Figure 1-5 Phenylpropane precursors of lignin (from Sjöstrom 1981)

Lignin composition depends on its botanical origin. Softwood lignin has mainly G units, hardwood lignin G and S units and nonwood lignin H, G and S units, at different proportions. S units are known to be more reactive than G units, hence lignin rich in S units is easier to remove by pulp delignification

o et al., 2001). The S/G ratio has a direct effect on delignification efficiency:

the higher S/G, the higher the delignification rate is. As a consequence, less alkali is needed and thus resulting in a higher pulp yield.

Lignin represents between 25-33% of dry softwood biomass and about 18-34% of hardwood biomass. It is produced by maturing cells and it is placed between fibrous walls mainly in intercellular regions (middle lamella), creating a stiff and cohesive structure. Moreover, lignin is responsible for transportation of water, nutrients and metabolites in the vascular system. Also, it plays an important role in the system of plant defense against degradation by microbial enzymes (Fengel and Wegener, 1989). Because of its hydrophobicity, lignin inhibits water absorption and fiber swelling, which can make fibers less responsive to mechanical refining. However, because it is thermoplastic, lignin possesses a feature that can be used to advantage in mechanical pulping to soften it at high temperature. The covalent union between lignin, cellulose and hemicellulose give the well-known lignin-carbohydrate complexes (LCC). As a result, lignocellulosic materials are considered as a lignocellulosic matrix rather

Sinapyl alcohol Coniferyl alcohol p-coumaryl alcohol

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Extractives

Wood also contains small quantities of extractives, between 2-10%

depending on wood species. These compounds are mainly resin and fatty acids and esters (Gullichsen, 1999). They are partly soluble in water or in organic solvents but may cause major difficulties in wood pulping since extractives are released from fibers and can form colloidal pitch and cause production problems such as deposits in water circuits.