Adhesives for Wood and Lignocellulosic Materials. R. N. Kumar. Читать онлайн. Newlib. NEWLIB.NET

Автор: R. N. Kumar
Издательство: John Wiley & Sons Limited
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Жанр произведения: Техническая литература
Год издания: 0
isbn: 9781119605669
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      In recent years, ionic liquids have been used to dissolve carbohydrates, and lignin residue is hoped to be relatively unchanged [21].

      The second method involves the formation of soluble lignin derivatives, namely, lignosulfonate.

       1.4.3.2 Functional Groups in Lignin

      Lignin contains characteristic methoxyl groups, phenolic hydroxyl groups, and some terminal aldehyde groups in the side chains. Only relatively few of the phenolic hydroxyls are free; most of them are occupied by linkages to neighboring phenylpropane units. The syringyl units in hardwood lignin are extensively etherified. Alcoholic hydroxyl groups and carbonyl groups are introduced into the final lignin polymer during the dehydrogenative polymerization process.

       1.4.3.3 Evidences for the Phenylpropane Units as Building Blocks of Lignin

      The following methods based on classical organic chemistry, namely, degradation and synthesis, led to the conclusion, already by 1940, that lignin is built up by phenylpropane units [11].

      1 Permanganate oxidation (methylated spruce lignin) affords veratric acid (3,4-dimethoxybenzoic acid) and minor amounts of isohemipinic (4,5-dimethoxyisophthalic) acid and dehydrodiveratric acid. The formation of isohemipinic acid supports occurrence of condensed structures (e.g., β-5 or γ-5). See structures 1 to 3 in Figure 1.6.

      2 Nitrobenzene oxidation of softwoods in alkali results in the formation of vanillin (4-hydroxy-3-methoxybenzaldehyde). Oxidation of hardwoods and grasses results respectively in syringaldehyde (3,5 dimethoxy-4-hydroxybenzaldehyde) and p-hydroxybenzaldehyde. See structures 4 to 6 in Figure 1.6.

      3 Hydrogenolysis yields propylcyclohexane derivatives. See structure 7 in Figure 1.6.

      4 Ethanolysis yields so-called Hibbert ketones. See structures 8 to 11 of Figure 1.6.

Figure shows the various degradation products of lignin that are Permanganate oxidation (methylated spruce lignin) affords veratric acid (3,4-dimethoxybenzoic acid) and minor amounts of isohemipinic (4,5-dimethoxyisophthalic) acid and dehydrodiveratric acid and the formation of isohemipinic acid supports occurrence of condensed structures, also nitrobenzene oxidation of softwoods in alkali results in the formation of vanillin (4-hydroxy-3-methoxybenzaldehyde), and oxidation of hardwoods and grasses results respectively in syringaldehyde (3,5 dimethoxy-4-hydroxybenzaldehyde) and p-hydroxybenzaldehyde, and hydrogenolysis yields propylcyclohexane derivatives and ethanolysis yields so-called Hibbert ketones.

       1.4.3.4 Dehydrogenation Polymer (DHP)

      The biosynthesis of lignin from the monomeric phenylpropane units can be generally described as a dehydrogenative polymerization. The principal ideas about such a pathway were elaborated by Freudenberg and co-workers [11]. They were the first to produce in vitro lignin called dehydrogenation polymer (DHP) by treating coniferyl alcohol with a fungal laccase from the mushroom Psalliota campestris or with a horseradish peroxidase by hydrogen peroxide.

Figure shows the chemical composition of enzymatic dehydrogenation of coniferyl alcohol yielding phenoxy radicals that helps in the first step of the biochemical pathway for building up lignin macromolecules is the enzymatic dehydrogenation of p-hydroxycinnamyalcohols, yielding mesomeric ring systems with a loosened proton.

      The origin of the hydrogen peroxide was cleared up by discovering cell-wall-bound enzyme systems able to deliver H2O2 [22, 23].

      Only 4-phenoxyradical I to IV are actually involved in lignin biosynthesis. Structure V is sterically hindered or thermodynamically not favored [24].

Figure shows the chemical composition of typical dilignol structures [25] where the polymerization of monomeric precursors by random coupling reactions cannot be studied in vivo, but it is known from numerous in vitro experiments to run without enzymatic control as a spontaneous process and the first step in polymerization is the formation of dimeric structures.

      Further polymerization is called end-wise polymerization involving coupling of monolignols with the phenolic end groups of di- or oligolignols or a coupling of two end group free radicals, yielding a branched polymer via tri-, tetra-, penta-, and oligolignols [11].

Percent of the total linkages
Linkage typeb Dimer structure Softwooda Hardwooda
β-O-4 Arylglycerol-β-aryl ether 50 60d
α-O-4 Noncyclic benzyl aryl ether 2–8c 7
β-5 Phenylcoumaran 9–12

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