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6.2d: Lignin

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  • 6.2d Lignin

    So, we’ve identified the chemical structures of starch, cellulose, and hemicellulose (see Figure 6.3 to see how cellulose and hemicellulose are related.) Now we’re going to take a look at what lignin is, chemically.

    Vascular land plants make lignin in order to solve problems due to terrestrial lifestyles. Lignin helps to keep water from permeating the cell wall, which helps water conduction in the plant. Lignin adds support – it may help to “weld” cells together and provides stiffness for resistance against forces that cause bending, such as wind. Lignin also acts to prevent pathogens and why it is recalcitrant to degradation; it protects against fungal and bacterial pathogens (there is a discussion in Lesson 5 about recalcitrance). Lignin is comprised of crosslinked, branched aromatic monomers: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol; their structures are shown in Figure 6.10a-c. Figures 6.10d and 6.10e show how these building blocks fit into the lignin structure. p-Coumaryl alcohol is a minor component of grass and forage type lignins. Coniferyl alcohol is the predominant lignin monomer found in softwoods (hence the name). Both coniferyl and sinapyl alcohols are the building blocks of hardwood lignin. Table 6.2 shows the differing amounts of lignin building blocks in the three types of lignocellulosic biomass sources.

    Table 6.2: The amount of different building blocks in grasses, softwood, and hardwood.
    Lignin Sources Grasses Softwood Hardwood
    p-coumaryl alcohol 10-25% 0.5-3.5% Trace
    coniferyl alcohol 25-50% 90-95% 25-50%
    sinapyl alcohol 25-50% 0-1% 50-75%
    Chemical structure for p-coumaryl alcohol
    Figure 6.10a: Chemical structure for p-coumaryl alcohol.

    Credit: P-coumaryl alcohol: from The Board of Regents of the University of Wisconsin System

    Chemical structure for coniferyl alcohol
    Figure 6.10b: Chemical structure for coniferyl alcohol.

    Credit: Coniferyl alcohol: from The Board of Regents of the University of Wisconsin System

    Chemical structure for sinapyl alcohol
    Figure 6.10c: Chemical structure for sinapyl alcohol.

    Credit: Sinapyl alcohol: from The Board of Regents of the University of Wisconsin System

    Chemical structures for varieties of lignin
    Figure 6.10d: Chemical structures for varieties of lignin.

    Credit: https://en.Wikipedia.org/wiki/Lignin

    Chemical structures for varieties of lignin
    Figure 6.10e: Chemical structures for varieties of lignin.

    Credit: Lignin: from Wikimedia Commons

    There are several different materials that can be made from lignin, but most are not on a commercial scale. Table 6.3 shows the class of compounds that can be made from lignin and the types of products that come from that class of compounds. If an economic method can be developed for lignin depolymerzation and chemical production, it would benefit biorefining of lignocellulosic biomass.

    Table 6.3: Low molecular chemicals and the products made from these types of chemicals.
    Class of Compound Product Examples
    Simple aromatics Biphenyls, Benzene, Xylenes
    Hydroxylated aromatics Phenol, Catechol, Propylphenol, etc.
    Aromatic Aldehydes Vanillin, Syringaldehyde
    Aromatic Acids and Diacids Vanillic Acid
    Aliphatic Acids Polyesters
    Alkanes Cyclohexane

    There are also high molecular weight compounds. These include carbon fibers, thermoplastic polymers, fillers for polymers, polyelectrolytes, and resins, which can be made into wood adhesives and wood preservatives.

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