Structure and Characteristics of Lignin

CHAPTE R 2

Structure and Characteristics of Lignin Outline 2.1 Components and Structure of Lignin  25 2.1.1 Components of Lignin  26 2.1.2 Functional Groups  28 2.1.3 Types of Lignin and Dimeric Structures  29 2.1.4 Model Structure of Lignin  34 2.2 Molecular Characteristics of Lignin  37 2.2.1 Molecular Weight and Distribution  37 2.2.2 Molecular and Supramolecular Structures  41 2.2.3 Associated Characteristics of Lignin  41 2.3 Physical Properties of Lignin  44 2.3.1 Apparent Physical Properties  44 2.3.2 Dissolubility of Lignin  45 2.3.3 Thermal Properties of Lignin  46 References  47

The name “lignin” is derived from the Latin word for wood (lignum) and first used by F. Schulze in 1865. Peter Klason in 1897 put forward the idea that lignin was chemically related to coniferyl alcohol and later proposed that lignin is a macromolecular substance by ether linkage between coniferyl alcohol units. The principal structural elements in lignin have been clarified largely as a result of detailed research based on modern analysis equipment and newly developed methods. This chapter will introduce basic knowledge about the structure and physical properties of lignin

2.1  Components and Structure of Lignin Lignin is built up of phenylpropane units by the chemical linkages of alkyl-alkyl, alkyl-aryl, and aryl-aryl groups. The precursors of lignin synthesis in nature has been demonstrated by comprehensive studies by Freudenberg and colleagues from 1940 to 1970. These precursors include p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, shown in Fig. 2.1. Different plant species contain different ratios of these precursors, such as the lignin is mainly constituted of G-unit in softwood, G-S unit in hardwood and G-S-H units in herbaceous plant Lignin Chemistry and Applications. https://doi.org/10.1016/B978-0-12-813941-7.00002-3 Copyright © 2019 Chemical Industry Press. Published by Elsevier Inc.

25

26  Chapter 2 OH

OH

OH

OMe

OMe

I

II

OMe OH

OH

OH

III

Fig. 2.1 Lignin precursors. I: p-coumaryl alcohol (H); II: coniferyl alcohol (G); III: sinapyl alcohol (S).

Natural lignin is not a simple connection of the monomers, but it is formed by the irregular coupling or addition of them (to p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol) [2]. There are significant differences in lignin content in different tissues or cell wall. Some ecological factors, such as plant growth, climate, nutrition, and illumination, also affect the chemical structure of lignin [2]. The chemical structure of lignin is one of the most difficult problems left in the field of natural polymers. Lignin molecules and their degradation products have many asymmetric centers, but there is no optical activity, so it is not constructed as cellulose or protein by a single key-type linkage [1]. There are many carbon-carbon bonds between the structural units, however, and the molecular structure cannot be resolved by the general decomposition method. The ability to extract lignin from plant tissue is still a big problem in lignin chemistry. Miiled wood lignin (MWL) obtained by Björkman method is closest to the original lignin and has been adopted by most wood chemical workers, but its yield accounted for only half of the total lignin and could not reflect the whole structure of lignin [3]. In addition, because the plant has a complex regulatory mechanism, the structure of lignin in some wood is mainly guaiacum lignin, and some is mainly syringa lignin, but also there is the formation of wood quality stress, taking into account different plant growth environments. Therefore, lignin is a class of complex molecular structure of polymers that cannot be described by a structural formula, and the properties of lignin can be expressed only from the elements, functional groups, and the combination of the form of each unit

2.1.1  Components of Lignin Natural lignin mainly consists of three elements: carbon, hydrogen, and oxygen, of which the carbon content is much higher than carbohydrate. The carbon content of softwood and hardwood lignin is 60%–65% and 56%–60%, respectively, and the carbon content of cellulose is only 44.4% [4–6]. The content of methoxy in hardwood lignin is relatively high, so the oxygen content of hardwood is higher than that of softwood [6]. Wood lignin generally is considered to contain no nitrogen elements, but the lignin of grasses contains a

Structure and Characteristics of Lignin  27 small amount of nitrogen, such as the nitrogen content of MWL prepared from wheat straw, straw, and Arundo donax, respectively, 0.17%, 0.26%, and 0.45% [7]. The element contents of different lignin have slight differences with the variety of raw material and separation methods. The elemental analysis results of lignin generally are expressed in terms of the element content of the average C9 unit, in particular, the benzene propane (C6-C3) unit that is used to remove methoxy, plus the hydrogen atoms, oxygen atoms, and methyl oxygen bases corresponding to each C9 [4, 6]. Table 2.1 lists the elements of the average C9 unit of MWL from several different sources. Lignin could introduce the new elements by the specific chemical reactions, for example, sulfur elements could be introduced to the lignin during kraft pulping and sulfite pulping. From Table 2.2, the sulfur content of lignosulfonate is higher, generally >5%. The elemental composition of lignin would vary with the changes of lignin functional groups; the sulfur elements in lignosulfonate are present mainly in the sulfonic acid base, and the sulfur elements in sulfate lignin exist mainly in hydrosulfonyl group.

Table 2.1: Element composition of average C9 unit of MWL [4–8] MWL

Element Composition of Average C9 Unit

Spruce Beech Birch Wheat straw Rice straw Glantreed Bagasse Bamboo Corn stalk

C9H8.83O2.37(OCH3)0.96 C9H7.10O2.41(OCH3)1.36 C9H9.03O2.77(OCH3)1.58 C9H7.39O3.00(OCH3)1.07 C9H7.44O3.38(OCH3)1.03 C9H7.81O3.12(OCH3)1.18 C9H7.34O3.50(OCH3)1.10 C9H7.33O3.81(OCH3)1.24 C9H9.36O4.50(OCH3)1.23

Table 2.2: The elemental composition and content of functional groups in different lignin [9] Elements

Phenolic hydroxyl Aliphatic hydroxyl SO3H SH OCH3 Molecular weight

Lignosulfonate

Sulfate Lignin

Organic Solvent Lignin

C: 53% H: 5.4% S: 6.5% 1.9% 7.5% 16.0% – 12.5% 400–150,000

C: 66% H: 5.8% S: 1.6% 4.0% 9.5% – 3.4% 14.0% 2000

C: 63% H: 5.5% S: – 4.8% 5.0% – – 19.0% 700

28  Chapter 2

2.1.2  Functional Groups Functional groups, including alcohol hydroxyl group, phenolic hydroxyl group, carbonyl group, carboxyl group, methoxyl, and sulfonic acid, are important structural characteristics of lignin. These functional groups directly determine their optical properties, dispersion characteristics, and chemical reactivity, and the qualitative and quantitative determination of functional groups is an important part of the study of lignin structure. The content of functional groups in lignin from different sources is different, and the content of functional groups of softwood lignin and hardwood lignin summarized by Alén is listed in Table 2.3. Generally speaking, the content of functional groups in softwood lignin is different from that of hardwood lignin, and the phenolic hydroxyl, aliphatic hydroxyl, and carbonyl content of softwood lignin is higher than that of hardwood lignin, and the content of methoxy in hardwood lignin is higher [10]. 2.1.2.1  Hydroxyl groups Hydroxyl groups, including aliphatic hydroxyl group and phenolic hydroxyl group, are functional groups with higher lignin content, which plays an important role in the preparation of functional materials by modification of lignin. The preparation method of lignin is different, the hydroxyl content varies. The presence of residual carbohydrates can affect the determination of the aliphatic hydroxyl group, so it is necessary to remove residual sugars when lignin is prepared. The hydroxyl contents of wood lignin and industrial lignin of different raw materials is showed in Table 2.4. The content of the aliphatic hydroxyl group in industrial lignin is lower than that of wood lignin, but the phenolic hydroxyl content is relatively higher [11–14]. There are many quantitative methods to determine the lignin phenol hydroxyl group, such as Aulin-Erdtman Ultraviolet Spectrum Εi Method (ionization difference spectrum, ionization differential spectrometry), conductivity or potential difference titration of aqueous solution or nonaqueous solution, NMR method, and the chemical coloration method [15]. The chemical coloration method mainly uses the color reaction of the lignin phenol hydroxyl group, such as reaction with 1- nitroso -2- naphthol, which will produce a material with characteristic color, and the absorption value of the characteristic peak (505 nm), which can be quantified by the phenol hydroxyl group [16]. The Folin-Ciocalteu method is a quantitative method of identifying phenol hydroxyl, the principle of which is to use phenol to restore the complex under alkaline conditions Table 2.3: The content of functional group of every 100 propane units [10] Functional Groups

Softwood Lignin/100 Units

Hardwood Lignin/100 Units

Phenolic hydroxyl Aliphatic hydroxyl Methoxy Carbonyl

20–30 115–120 90–95 20

10–20 110–115 140–160 15

Structure and Characteristics of Lignin  29 Table 2.4: Hydroxyl content of different lignin [11–14] OH (mol/C9 Unit) Phenolic Hydroxyl

Aliphatic Hydroxyl

Total Hydroxyl

1.18 1.11 1.13

1.46 1.33 1.49

1.39

1.51

0.77 0.56

1.35 1.00

MWL  Spruce  Aspen   Phyllostachys pubescens

0.28 0.22 0.36 Hydrochloric lignin

  Liquidambar styraciflua

0.12 Industrial lignin

  Pine kraft lignin   Bamboo kraft lignin

0.58 0.44

and produce blue reaction products that can be detected at 765 nm wavelength. The method has been applied to the determination of phenolic hydroxyl of lignin and its derivatives [17]. 2.1.2.2 Carbonyl Lignin also has a large number of carbonyl groups, such as aldehyde, ketone, and carboxyl groups. Carboxyl groups do not exist in the original lignin, but it is produced by a degrading modification of lignin. The carbonyl group in original lignin consists of conjugate and nonconjugated carbonyl, and the sum of content of these two kinds of carbonyl is the total content of carbonyl [18]. The carbonyl that is connected to the α-carbon atom is the conjugated carbonyl in the form of ketone, the carbonyl of the γ-carbon atom exists in the form of the conjugated aldehyde, and some of nonconjugated carbonyl might be in the form of aldehyde. Benzene-conjugated carbonyl is one of the reactive groups in lignin structure, the content of which will affect the degradation reaction rate of lignin and cause the phenomenon of yellowing of pulp products after heating or UV irradiation during pulping. The different types of cyclic conjugated carbonyl [18] can be detected quantitatively by reductive differential spectrometry, but the spectral method relies on the lignin model, and its accuracy is influenced by the source of lignin. The NMR method is a more accurate method to determine the lignin carbonyl structure and content, and it can apply to a wide range of raw materials [19]. The near-infrared-fast Fourieres-la-Maine spectroscopy technology can quickly analyze the content of benzene conjugated carbonyl in lignin [20]. Table 2.5 lists the content of carbonyl groups of softwood (spruce) and hardwood (birch, eucalyptus) lignin [18].

2.1.3  Types of Lignin and Dimeric Structures Lignin is a macromolecular substance that is connected by benzene propane units to a threedimensional lignin structure through various chemical bonds. The type and relative quantity

30  Chapter 2 Table 2.5: Functional groups of typical coniferous and hardwood lignin [18] Functional Groups (mol/100 C9) Methoxy group   Total hydroxyl group   Aliphatic hydroxyl group   Primary hydroxyl   Secondary hydroxyl   Benzyl hydroxyl   Phenolic hydroxyl Total carbonyl group  Aldehyde  Ketone   α-CO   Nonconjugated carbonyl   COOH Degree of polycondensation

Source of Lignin Spruce

Birch

Eucalyptus globules

Eucalyptus grandis

92–96

164 186 166 86 80

164 117–121 88–91 68 20 16 29–30 24 9 15 10 10 4 18

160 144 125 70 55 54 19 17 24 8 8 8 5 21

15–20

15–30 20

20

of the bonds between benzene propane units are the main contents of lignin chemical structure. This section provides an overview of the structure of wood lignin, industrial lignin, and pulp residue lignin. 2.1.3.1  Characteristics of the bonding of MWL Early studies on MWL mainly focused on spruce (softwood) and birch (hardwood), and the content of main chemical bond types in spruce and Birch MWL are listed in Table 2.6. The main bond types, such as α-O-4, β-O-4, 5-5, β-β′, 4-O-5, and β-1, are shown in Fig. 2.2. With the extensive use of beech and eucalyptus as papermaking materials, the structural analysis of their lignin is increasingly important. Through the comparison of the main bonds in the lignin structure of beech and eucalyptus (see Table 2.7), Capanema found that the proportions of their lignin-structured units were different, but showed little difference in their bond types [19]. Table 2.6: The content of key bonds in spruce and birch MWL Bond Types

Spruce/100 C9 Unit

Birch/100 C9 Unit

A (aryl glycerol-β-aryl ether, β-O-4) B (glycerol-2-aryl ether) C (nonring benzyl-O-aryl ether, α-O-4) D (coumaran,β-5, α-O-4) E (dibenzodioxocin structure) F (biphenyl,5-5) G (diaryl ether,4-O-5) H (1,2-diaryl propane, β-1) I (β-β linkage) Total

48 2 6–8 9–12 2.5–11

60 2 6–8 6 1.5–4.5

3.5–4 7 2 86–92

6.5 7 8 92–94

Structure and Characteristics of Lignin  31 C C

C

C O

C

C

C

C

O

C

O C

C

C O

O

(A)

(B)

(C)

C

C

C

C

C C

O

C

C

C

C

C

C

O

O

C

O

O

O

O

(D)

(F)

(E) C

C C

C

C

C

C

C

O

C

C

C

C

C

C

C

O

O

O

O O

(H)

(G)

(I)

Fig. 2.2 The key bond types between the phenylpropane units in MWL the linkages in β-O-4 (A), glycerol-2-aryl ether (B), α-O-4 (C), β-5 (D), dibenzodioxocin (E), 5-5 (F), 4-O-5 (G), β-1 (H), β-β (I).

Table 2.7: The characteristic of key bond in beech and eucalyptus MWL β-O-4 α-O-4 γ-O-alkyl 5-5′ 4-O-5′ 6(2)-condensed: G or S S: G: H

Beech

E. globules

E. grandis

65

56 20

61

2 1.5

3 1.5/10 4/10 84:14:2

23 3 3/6 3/3 62:36:2

32  Chapter 2 2.1.3.2  β-O-4 The common β-O-4 structure in lignin is aryl propyl alcohol-β-aryl ether that easily happened during the reaction of acid and alcohol hydrolysis [21]. The content of β-O-4 structure in bamboo, beech, Japanese Platycladus orientalis WML was 0.56, 0.51, and 0.35, respectively [1]. Erickson inferred that the proportion of β-O-4 in birch MWL was 62% by measuring content of free phenolic hydroxyl after acid hydrolysis, sulfate cooking, and decomposition of sodium hydroxide and copper oxidation, but the proportion in spruce lignin was 49%–51% [22]. Nimz et al. [23] inferred that the proportion of β-O-4 in beech MWL was 65%. β-O-4 structure in lignin could produce Hibbert ketone by an alcoholysis reaction. The structure of aryl propyl alcohol-β-aryl ether was quantitatively analyzed by Alder using this method [4]. Freudenberg also found this structure in the dehydrogenation polymers (DHPs). Nimz found that the β-aryl ether dimer, which has a side chain of cinnamic alcohol, aldehyde, and glycerol, is isolated from the hydrolysis products, and these products also are found in hydrogenation decomposition [23]. Olcay found that the content of benzene propane monomer (converted to C6-β-O-C3) was 21.8% when the spruce WML was hydrodecomposized [24]. Pepper found that 52.2% of the Hibbert ketone monomer from the poplar was isolated from Aspen Klason lignin [25]. 2.1.3.3  β-5 The β-5 bond is a bond with the phenyl propane β-carbon atom connected with the fifth carbon atom of the benzene ring of another structural unit, and it was represented by a structure of phenyl coumarin [6]. The β-5 model can be obtained from the dehydrogenation polymerization of coniferyl alcohol and by hydrogenation reduction decomposition [26]. Adler found the content of the β-5 structure in spruce lignin was 0.11/OMe, which has 0.03/OMe as open ring type β-5 structure (β′), by transforming β-5 structure into the phenyl coumaran structure of cyclic 1,2-two styrene and using its ultra-violet absorption characteristics [6]. Larsson and Miksche inferred that the content of β-5 type structure in birch and spruce lignin was 0.05 and 0.09–0.12/C6-C3 respectively, and the open loop β-5 (β′) structure 0.01/C6-C3 and 0–0.03/C6-C3 in birch and spruce lignin by oxidation decomposition β-5 lignin phenol with potassium permanganate to produce isooctyl pinic acid [27]. 2.1.3.4  β-1 The representative of β-1 bond was Daryl propane, which was first separated from the hydrolysis products of beech by Nimz. A variety of β-1 compounds in guaiacum lignin, syringa lignin was isolated from the original lignin of spruce and Fraxinus mandschurica by hydrolysis of 1, 4-Dioxane -water solution [6]. Freudenberg also found the β-1 structure in the dehydrogenation products of coniferyl alcohol. The structure of C6-C3-C6 type was produced by the dehydrogenation of the side chain along the free radical coupling [28]. The nonconjugated carbonyl is derived from the structure of the glycerol aldehyde-2-aryl ether,

Structure and Characteristics of Lignin  33 and if the structure is conjugated to the β-1 type structure, the content of β-1 structure is 0.1/ C6-C3 [23]. Nimz found that the content of β-1 in beech lignin was 0.15/C6-C3. Miksche assumed that the content of β-1 structure in spruce and beech lignin obtained with Lundquist method were 0.02/C6-C3 [23, 27]. 2.1.3.5 5-5′ bond (biphenyl) The 5-5′ structure is separated from nitrobenzene oxides of dehydrogenated vanillin by Pew, and he has proved that the connection structure between the fifth carbon atoms of the two phenyl propane cells was not produced by reaction but existed in the wood lignin [29]. Aulin-Erdtman quantitatively examined the structure by using differential spectrometric method and considered that the content of 5-5′ structure in spruce BNL with at least one free phenol hydroxyl group was about 0.06/C6-C3 [30]. According to the results of ultraviolet spectroscopy, Pew suggested that the content of 5-5′ structures was 0.25/C6-C3 or higher [31]. Researchers suggest that the content of 5-5′ structure in birch lignin is 0.045/C6-C3, spruce lignin 0.095–0.11/C6-C3, and beech wood lignin 0.023/C6-C3 [6], showing considerable differences in the content of the 5-5′ structure obtained by different lignin. 2.1.3.6  β-β′ bond The β-β′ bond is a lignin-phenolic structure represented by pine resin phenol. Pine resin phenol was obtained by dehydrogenation polymerization of coniferyl alcohol, and then the structure was detected when spruce lignin decomposed by methanol at room temperature. The content of this structure is very small in softwood lignin, however, while it is generally larger in hardwood lignin. Syringaresinol was separated from the beech by a mild hydrolysis method by Nimz, and phenolic, guaiacyl, syringyl copolymer, and α-carbonyl two methoxy Larch resin phenolic compounds were isolated [6, 23]. Nimz inferred that the content of β-β′ bond in beech lignin was 0.05/C6-C3 [23]. Miksche inferred that the content of β-β′ bond in birch and spruce lignin was 0.03–0.05/C6-C3 and 0.02/C6-C3, respectively [22, 27]. 2.1.3.7  Other bonds In addition to β-O-4, β-1, β-5, 5-5′, and β-β′, the carbon-carbon bond (β-6, α-6) and ether bonds (α-O-4, 4-O-5) exist in lignin. Miksche suggested that the content of β-6 and 4-O-5 in spruce lignin was 0.025–0.03/C6-C3 and 0.035–0.04/C6-C3, respectively [22]. Evtuguin inferred that the content of β-O-4 and α-O-4 in eucalyptus lignin were 0.56/C6-C3 and 0.23/ C6-C3 by the Py-GC/MS technique combined with NMR analysis [32]. 2.1.3.8  Condensed bond The stable ortho-quinone structure was obtained through oxidation of substituted phenol with nitro potassium sulfate by Adler and Lundquist. When the fifth carbon atom was not replaced, the resulting unit, which they named the guaiacyl unit, had a “noncondensation” bond. The structural unit that when the fifth carbon atom formed the carbon-carbon bond or the

34  Chapter 2 ether bond the structural unit had a “condensation”bond. They proposed that the content of noncondensation element of spruce lignin was 0.15–0.18/C6-C3 [33]. However, the oxidation reaction is limited to the structural units with free phenolic hydroxyl groups. Later researchers called the structure with carbon-carbon binding, in addition to the benzene phenol hydroxyl group to the side chain, as the “condensed “structure, such as β-5, β-6, 5-5 carbon-carbon connections, while the structure that the benzene ring 2, 3, 5, 6-bit carbon atoms are not replaced or only by methoxy-substituted structural units was called “noncondensation” unit [6, 33]. The proportion of condensation type in softwood was about half of total lignin, with a higher proportion of lignin found in noncondensed units in hardwood because there are more syringa units. For example, the proportion of noncondensation structural units of eucalyptus acidic lignin was 78% [32]. In addition, acidic treatment could markedly increase lignin condensation structure. For example, the proportion of condensed units in acidic hydrolysis lignin of softwood was 70%–72%, while its condensation units were only 18%–25% after alkali treatment and sulfate cooking. The condensation degree would increase with the treatment conditions [34]. 2.1.3.9  Structural characteristics of bonds in residual lignin After chemical pulping of plant materials, the pulp still contains a small amount of lignin, known as residual lignin. The chemical bond in residual lignin is different from that of wood, mainly because of the increase of polycondensation structure in lignin [6, 35]. The structure of residual lignin in softwood kraft pulp was quantitatively analyzed with NMR by Froass et al., and the content of main bond is listed in Table 2.8 [35]. Comparing the effect of traditional conventional kraft cooking (CK) and extended modified continue cooking (EMCC) on the residual lignin structure of pulp, they found that the content of β-O-4 in residue lignin of pulp obtained by EMCC was lower, while the content of condensation-type structure was higher [36].

2.1.4  Model Structure of Lignin 2.1.4.1  Typical lignin model Freundberg obtained a polymer of lignin model, DHP, by dehydrogenation polymerization of coniferyl alcohol in 1961. By analyzing the DHP structure, a model for spruce lignin Table 2.8: Characteristics of main bond in residual lignin β-O-4 5–5 β-5 β-1 α-O-4 5-O-4 β-β

Dimer Structure

The Content of Bond (%)

Aryl glycerol-β-aryl ether, β-O-4 Biphenyl,5-5 Phenyl coumaran 1,2-diaryl propane Benzene-propane α-O-aryl ether Diaryl ether β-β linkage

45–48 4–25 9–12 7–10 6–8 4–8 3

Structure and Characteristics of Lignin  35 with 15 basic units was proposed. He also proposed a model for spruce lignin with 18 basic units in 1968 [37]. In 1965, according their research results, Forss and Fremer proposed the hypothesis that lignin in hardwood was made up of a number of repeating units, each of which contained 16 guaiacyl propane benzene and 2 p-hydroxyphenyl propane. Although this hypothesis is not accepted by most scientists, the study of lignin structure still has some implications [38]. In 1974, Nimz proposed the beech lignin fragment model, which contains 25 benzene propane units, and guaiacum and syringa units are the main structural units. Although the arrangement of the units is highly arbitrary, this model still helps to understand the structure of hardwood lignin [5]. In 1977, Adler presented a representative model (see Fig. 2.3), in which 16 benzene-propane structural units formed three-dimensional mesh structures containing more than 10 linkages. The model contains guaiacyl unit, syringyl unit, and pinoresinol, which explains the lignin formed in the cell wall [6]. However, the structure of the model was not quantitative and indicated only the existence of these structures. Adler calculated the frequency of the main connecting structure of spruce and birch lignin in order to make up for this point. The application of electronic computers improves the ability of people to process data information. In 1974, according to their research results and computer simulation, Glasser and Glasser obtained a lignin model of 94 benzene propane units, which had rich structure

CH2OH

CH2OH

O

HC

CH2

CHOH

OH

CHOH

H2COH CH

O

CH

CH

CH

H2COH

O CH2OH C H

HCOH O

CH2OH O

H3CO

CH H3CO

H3CO

OCH

CH O

O

CH

CH2OH

H3CO O

CH

H2COH

H2COH

CH2OH CH

HO

HCOH

CH

H3CO

OCH3

H3CO O

HC

CH CH

O

CH2

CHOH

O

O

H3CO CH2OH O

CH CH

O

CH

OCH3 H3CO

CH

CH

O CH2

CHOH

O

C H

CHOH

OCH3 O

OCH3 CH2OH CH

H3CO CH2OH O

CH

CHOH

C O

OH

OH

H3CO

OCH3

OCH3

Fig. 2.3 Adler proposed wood quality structure model.

36  Chapter 2 Table 2.9: Analysis and comparison of sakakibara lignin structure model and spruce MWL [40] Lignin and Model Complexes

C9-Formulate Without Methoxyl Group

Number of Dehydro

Number of Hydrate

C9H10O2 C9H9.05O2 (H2O)0.37

0.95

0.37

C9H8.07O2 (H2O)0.40

1.93

0.40

C9H8.08O2 (H2O)0.39

1.92

0.39

C9H8.03O2 (H2O)0.43

1.97

0.43

C9H8.06O2 (H2O)0.41

1.94

0.41

C9-Formulate

Coniferyl alcohol MWL (Björkman 1957) MWL (Freudenberg 1968) Model complexes A

C9H9O(OMe) C9H8.83O2.37 (OMe)0.96 C9H7.95O2.40 (OMe)0.92 C9H7.93O2.39 (OMe)0.93 C9H7.96O2.43 (OMe)0.93 C9H7.95O2.41 (OMe)0.93

Model complexes B Average of A and B

Model A: C252H222O67 (OMe)26; MW 5124. Model B: C252H223O68 (OMe)26; MW 5141.

information, included the key bond such as β-O-4, β-5, β-β′, 5-5′ and some controversial structures, and the molecular weight (Mw) of lignin is 17,000 [39]. In 1980, Sakakibara presented a preliminary lignin model of softwood [40]. In Table 2.9, model A and model B are the lignin models proposed by Sakakibara, and the Mw of lignin is >5100. 2.1.4.2  A new model of wood lignin structure Since 1990, a number of new wood lignin structures have been reported. In 1995, Karhunen reported that two hydroxy biphenyl and coniferyl alcohol were oxidized to become dibenzodioxocin substitutes, and inferred a new type of lignin structure [41]. Subsequently, they proved that softwood lignin contains dibenzodioxocin that was characterized by the existence of a ring containing six carbon and two oxygen atoms (as shown in Fig. 2.4A) using the HMQC NMR technique [42].

R

R

OAc

OAc OMe

OMe

5H MeO

O

OMe O OAc

O O

5H

5H

O

OH

OMe

HO

O

OMe

O

O

OAc

O

OH

HO O

OH OMe OH

(A)

S OMe

G

G OMe

OMe

OAc

OAc

(B)

OMe OMe

OAc

OMe O

(C)

Fig. 2.4 The new lignin structures. (A) Dibenzodioxocin; (B) benzodioxane; and (C) spironolactone.

Structure and Characteristics of Lignin  37 In 2001, Ralph and others reported the new benzene-propane oligomer structure (Fig. 2.4B) was discovered by methylation of 5-hydroxyconiferaldehyde catalyzed by enzyme. The results by NMR analysis showed that transgenic plants lacking methyl transferase cannot effectively synthesize syringa lignin but could produce new benzene and oxygen six-ring lignin [43]. Zhang and Gellerstedt also reported the structure of a ring of spironolactone (Fig. 2.4C). When analyzing structure of spruce and birch lignin by NMR, they observed and confirmed that both of the guaiacum unit in spruce lignin and syringa unit in birch lignin could form this structure [44, 45]. Based on these structures, Brunow suggested a spruce lignin model consisting of 25 benzene propane units in 2001 (Fig. 2.5A). Boerjan et al. proposed the poplar lignin model consisting of 20 benzene propane units (Fig. 2.5B) [46]. After a comprehensive study of wood lignin with NMR, Crestini put forward a new view that wood lignin is a series of linear oligomers (Fig. 2.6), rather than cross-linked network structure. Because these oligomers are prone to supramolecular association, the Mw measured by GPC is much higher than NMR data [47]. The Crestini linear lignin oligomer association model is consistent with the lignin module assembly model proposed by Wayman and Obiaga in 1974 [48]. 2.1.4.3  Industrial lignin model Industrial lignin is produced from lignocellulosic materials through chemical pulping (Table 2.10) [50]. The structure of these lignin is very different from that of the original lignin. In the sulfite pulping process, the lignin molecule contains sulfonic acid groups because the sulfonation reaction occurs in the α-position of the side chain of lignin. Condensation between lignin units also forms α-6 linkages. In the kraft pulping process, a nucleophilic substitution reaction could happen at the β-position carbon atom of side chain, and because of sulfur atom attacking at β-position, CK results in a more condensed structure in lignin, such as α-5, β-1, 4-O-5. Because the side chains are degraded, there are also double bonds, ketone groups, and carboxyl groups on the side chains [49].

2.2  Molecular Characteristics of Lignin 2.2.1  Molecular Weight and Distribution Methods for measuring the Mw of lignin include viscosity measurement, gel permeation chromatography (GPC), light scattering, vapor pressure permeation, and ultracentrifugation, with GPC being the most commonly used. Because of the poor solubility of lignin, lignin is usually derivatized (e.g., acetylated) to be dissolved in organic solvents, and then measured for Mw. For MWL, the ball milling and derivatization conditions have a great influence on the Mw. The longer the ball milling time, the more the mass average Mw of lignin decreases. The average Mw does not change much, but the polydispersity significantly decreases.

38  Chapter 2 OH HO OH

OMe

HO OMe

MeO OH

HO

OH

OMe

O

OMe

O

HO

HO

HO

O

OMe

O OMe

MeO

OMe

OH O

HO MeO

O

O

OH OH

HO

O

MeO

HO

OH

OH

OH O

MeO

HO

OH O

MeO

OMe

OMe

OMe

MeO O

OMe

HO

OH OMe

OMe

MeO

O

O

HO

O

HO

MeO

OH

O

HO

MeO

O

OH

HO

O

OMe

OH MeO

OMe

O

O

OH

OH

OMe

HO

OMe

O

HO

O

HO

OMe

HO

OH

MeO

OH

O

O

OH

OH

HO

OMe O

OH

OH

OMe

HO

O

O

OH OH

MeO HO

OMe

HO

OH

O OH

OMe HO

OMe

HO

O HO

O

O

O

O

OH

OMe

O MeO

HO

OH O

OH

OH

OMe

O

OH HO

O OMe MeO HO

OMe

HO

OH

HO

(A)

OMe

O

O

OH

HO

OH

MeO

MeO O

MeO

O

OH

O

OMe

HO

OH

OH

OMe

MeO

O O

O

OMe

O

OMe

OH

OH OMe

HO

OMe OH

OH OMe

(B)

Fig. 2.5 Lignin model proposed by Brunow and Boerjan [46]. (A) Structures with guaiacyl units, (B) structures with guaiacyl and syringyl units.

Structure and Characteristics of Lignin  39

Fig. 2.6 Crestini proposed the coexistence model of various lignin oligomers. Table 2.10: Molecular weights measured at different times of ball milling of corn stalks [50] Mp Sample Milling Time, h

The First Peak

The Second Peak

Mn

Mw

Mw/Mn

5 10 20 30

50,200 46,000 34,200 31,500

2670 2930 4350 3950

3330 3300 3630 3950

191,000 96,500 42,100 24,000

57.36 29.24 11.60 6.08

The higher the temperature of the benzoylation reaction is, the greater the measured Mw of the lignin, which suggests that the increase in temperature will cause a polycondensation reaction of lignin, thus increasing the Mw of the separated lignin [50]. In order to better understand the connection between lignin and carbohydrates, Zoia et al. determined the molecular weights of holocellulose and lignin in Norwegian spruce, oak, and corn stover using the derivatized GPC methods. The data in Table 2.11 show that among spruce lignin, oak lignin and corn stover lignin, the average Mw of eucalyptus lignin is the largest and the spruce is the smallest, while the polydispersity of spruce lignin is the least, and that of eucalyptus lignin is the largest [50].

40  Chapter 2 Table 2.11: The molecular weight and distribution of lignin of several materials Norway spruce

E. grandis

Corn stover

MW HOLO CEL EMAL HOLO+EMAL MW HOLO CEL EMAL HOLO+EMAL MW HOLO CEL EMAL HOLO+EMAL

Mp

Mn

Mw

Mw/Mn

36,300 34,400 3600 3700 34,200 37,000 35,800 11,500 5700 35,200 34,200 31,000 4900 4900 30,500

2850 12,800 1950 2000 1900 12,500 16,300 2500 1830 6800 36,300 15,400 1370 1450 2870

36,800 75,500 13,050 6100 37,600 49,400 64,300 18,900 10,500 39,500 42,100 49,600 10,100 7060 35,600

12.91 5.90 6.69 3.05 14.53 3.95 3.94 7.56 5.74 5.81 11.60 3.22 7.37 4.87 12.40

Note: MW, raw material; CEL, cellulolytic enzyme lignin; EMAL, enzymatic mild acidolysis lignin; HOLO, holocellulose.

Table 2.12: Molecular weight changes after acetylation of lignin [51] Source of Lignin

Derivatization Time/Day

Mw g mol−1

Mn g mol−1

D

Spruce

0 30 0 10 0 10 0 10 0 20 0 10 0 10

83,200 9350 65,200 10,000 49,500 10,100 57,000 7500 57,600 11,400 23,400 8100 10,100 10,090

10,000 3350 7760 3700 7700 3740 77,000 2800 9760 4200 6500 2890 2730 2650

8.3 2.8 8.4 2.7 6.4 2.7 7.4 2.7 5.9 2.7 3.6 2.8 3.7 3.8

Redwood Tochigi White Pelican Southern Pine Eucalyptus Wheat straw

Mw Start/Mw End 8.9 6.5 4.9 7.6 5 2.9 1

The test results of lignin Mw also are affected by the degree of lignin association. Guerra found that the Mw data measured immediately after bromo-acetylation treatment of lignin was several times as large as the data measured at a certain time, see Table 2.12. The longer the reaction time, the higher the degree of acetylation of lignin, the smaller the degree of association, and the smaller the Mw value measured by GPC. This result also showed that lignin has a strong association [51].

Structure and Characteristics of Lignin  41

2.2.2  Molecular and Supramolecular Structures With the development of modern instrumental analysis technology, researchers have made a great process on the lignin structure. Early researchers primarily described lignin as a complex, amorphous, three-dimensional network of macromolecules based on X-ray diffraction data. Later, with the aid of electron microscopy and atomic force microscopy, MWL can be observed as spherical or lumpy. In addition to providing topographical data, the AFM can measure the three-dimensional size of the particles. Goring et al. observed that the lignin sulfonate particles were discoid and the monolayer thickness was about 2 nm by TEM [52]. Houst studied the adsorption layer of lignin sulfonate on magnesium oxide surface by atomic force microscope and found that the thickness was 1.5. At −3 nm [53], Liu et al. measured lignin sulfonate particles with a diameter of 60–90 nm and an average thickness of 2.14 nm on mica [54]. The supramolecular structure of lignin is determined by its chemical structure and environment. Supramolecular structure of different types of lignin are different, such as the properties of industrial lignin are different from that of original lignin because it is degraded to a certain extent. The supramolecular structure in the dry state of the same species of lignin will be different from that in solvent. Small-angle or ultra-small-angle X-ray scattering can be used to study the supramolecular structure of lignin in solid and solution. Vainio et al.'s ultra-small-angle X-scattering data showed that dry kraft lignin (CKL) was an aggregate with a fractal structure with a surface fractal dimension of 2.7 ± 0.1, while solvent lignin below 200 nm in size did not show fractal features. The pore diameter of the dried CKL was about 3.5 nm, which is close to or just on the surface of the aggregate. CKL particles are long ellipsoids in NaCl and NaOH solutions. When the CKL was redissolved, the particles in the solution formed a chain with a thickness of about 1–3 nm thick. The chain length increased as the polymer concentration increased, and the chain width was about 10%–40% of the length. When the pH of the system was adjusted from 12.8 to 7 with acid, no obvious association was observed and there was no associated complex with a size >100 nm, which indicated that the association was not sensitive to pH changes [55]. Lignin can associate in some solutions, and the lignin in the association state is obviously different from the single lignin molecule. Therefore, each research method has its own applicable scope and limitations. It is difficult to measure the exact shape and actual size of lignin with only one method. Table 2.13 lists the different lignin shapes, sizes, and methods of study.

2.2.3  Associated Characteristics of Lignin The super-assembled structure of lignin in the association state exists in the natural woody tissue. Terashima et al. believed that lignin formed an ordered structure during

42  Chapter 2 Table 2.13: Methods and results of study on size and shape of lignin particles in solution [55] Lignin Species

Solvent

Maple lignin (methanolhydrochloric acid method and sodium hydroxide-ethanol method) Milled wood lignin, dioxane lignin, Sulfate lignin Mildly dissolved alkali lignin

Various organic solvents

Spruce Björkman lignin

Pyridine

Dioxane lignin

0.2–4 mol L−1 NaOH solution Solvents: DMSO, DMF, dioxane, pyridine

Pine wood dioxane lignin

Disperse in water

NaHCO3-NaOH Buffer, pH.5

Thioglycolic acid lignin

Pyridine -DMSO-H2O

Organic solvent lignin

Aqueous solution, pH 3–10

Sulfate lignin

0.1 mol L−1 alkaline solution

Sulfate lignin

1.0 mol L−1 NaOD, Buffer

Sulfate lignin

0.1 mol L−1 NaOH/ NaOD, Chloroform

Acetylated dioxane lignin Hardwood Kraft lignin

DMSO, DMF, methyl cellulose, pyridine

Size and Shape Assessment 3 × 16 × 100 au, Oval particle, shape factor 7.5

Analytical Method Viscosity method, spreading method, Langmuir groove method

Film thickness 1.7 nm, Spreading method, each kraft lignin area Langmuir groove method 2.1–2.4 nm2 Microgel particles are Settlement method, surrounded by linear viscosity method molecular chains, between random coils and rigid spheres When Mw = 7150, Settlement method, Rh = 2 nm viscosity method Spherical particle Intrinsic viscosity, effective Rh = 2.2–2.3 nm potential back titration Size 110–157 nm, or Viscosity method, photon 9–23 nm, depending correlation spectroscopy on solvent and relative (PCS) molecular weight Apparent Rh = 0.97– Spin labeling method, 2.09 nm, assuming a viscosity method solid, loose-surfaced Einstein sphere inside the network Size 40 nm at pH = 10, Filtering, PCS Size 150 nm at pH = 3, 70% Particles 2–50 nm Inflation factors 2.5 to Gel chromatography, 3.7, inflated random ultracentrifugation coil conformations, irrespective of long chain branching effects Ordinary Rh = 2.05– Self-diffusion, pulse 2.28 nm, aggregate gradient field spin Rh = 38 nm (in D2O, echo nuclear magnetic pH = 6.5) resonance (PGSE-NMR) Rh = 1.0–2.2 nm Self-diffusion, (Mw = 1600–12,100) PGSE-NMR Rh = 0.5–1.31 nm, flat Self-diffusion, oval, axis ratio ≤ 18 PGSE-NMR Size 2.4–2.7 nm or PCS 120–350 nm, depending on the relative molecular mass

Structure and Characteristics of Lignin  43 deposition [56]. Agarwal believed that the benzene rings of lignin in spruce tended to align with the cell walls [57]. Atalla suggested a strong association between the prepolymer of lignin and the polysaccharide substrate. The association of lignin has internal and external causes. The internal cause is mainly functional groups in lignin, including benzene ring (1/ C9), carboxyl (1/C9), phenolic hydroxyl (0.6/C9), alcoholic hydroxyl (0.48/C9) [58]. The external factors that affect the association of lignin are solvent type, alkalinity, concentration, ion composition, organic additives, time, and temperature. There are four possibilities for the lignin molecule association mechanism: intermolecular hydrogen bonds; stereo regular association; hydrophobic bonds; and electrostatic association. Secondary chemical bonds and long-range van der Waals forces are also important reasons for the association behavior. The Mw distribution of MWL is very wide, and the data obtained by different methods are quite different. These phenomena might be related to the association of lignin, see Table 2.12. Because industrial lignin contains certain acidic groups (such as ArOH, COOH, SO3H, etc.), it is usually in the form of electrolytes, with colloidal properties. Lignosulfonates exhibit the characteristics of anionic polyelectrolytes in aqueous solution. Under acidic conditions, lignin molecules tend to associate to form copolymers and are relatively stable under alkaline conditions. Kraft lignin also can be considered to be a polyelectrolyte in alkaline solution. Norgren studied the agglomeration of kraft lignin in dilute alkaline solutions and found that elevated temperatures led to irreversible agglomeration of kraft lignin at high ionic strength. Kraft lignin solution would be separated at 175°C and pH 12 because of agglomeration, and, when a small amount of CaCl2 was added, kraft lignin could be tempestuously precipitated from the solution at pH of 13–12 [59, 60]. The degree of association of kraft lignin in alkaline solutions is affected by pH and is reversible. In addition, the association of small molecules with macromolecules is also different [61]. Large Mw lignin molecules have strong associations when pH is between 12 and 13.5 and no association at low pH, while small Mw lignin associates only at pH of 10–13 because it is related to the isoelectric point of phenolic oxygen ion protonation of lignin. In the isoelectric point of lignin, lignin tends to associate, and the isoelectric point of phenolic oxygen ion protonation is related to its ka value. The higher the Mw of lignin is, the higher the ka value, so that high Mw lignin tends to associate at high pH [61, 62]. When pH is 13.8, the association of kraft lignin has a significant effect on the Mw distribution. It can be seen from Table 2.14 that when the concentration of lignin is high, the association of lignin is stronger, the ionic strength is increased and lignin association is enhanced [59–62]. The addition of some organic compounds, such as urea, betaine, and sodium dodecyl sulfate (SDS) in the lignin solution, can reduce the mutual exclusion coefficient of lignin and reduce the association of lignin [63].

44  Chapter 2 Table 2.14: Association of high-concentration lignin [63]a

a b

pH

Mother Liquor Concentration g L−1

13.8

13.0

12.0

10.0

10 25 50 100 50 + Ib

– 0.940 0.846 0.744 0.440

1.0 0.848 0.748 – –

0.479 0.604 0.740 – –

0.77 0.364 0.696 – –

Leaching of lignin through the column g L−1. The lignin concentration in the mother liquor is 50 g L−1, and the NaCl concentration is 3 mol L−1.

2.3  Physical Properties of Lignin 2.3.1  Apparent Physical Properties Natural lignin does not have a maximum absorption peak in the visible light spectrum. Mill wood lignin is generally a pale-yellow powder, the color caused by a series of chromophoric groups (Fig. 2.7). For example, milled wood lignin of spruce contains 1% of the structure of o-dihydroxybenzene and 0.7% of ortho quinone structure. The lignin produced from heartwood is darker in color because it contains tannin and flavonoid impurities. Lignin in kraft and sulfite pulping effluents tend to appear brown or brownish red because it contains a variety of chromophoric groups. The color of the residual lignin in the pulp actually is caused by chromophoric group structures. Therefore, the type of pulp is different, and the color is also different [64]. The apparent color of lignin is caused by the absorption of light waves in the ultraviolet range. The ultraviolet spectrum of a typical softwood and hardwood lignin is usually three absorption peaks between 270 and 280 nm and between 200 and 208 nm, and at 230 nm. There is a shoulder peak with a weak absorption between 310 and 350 nm and a very small absorption at 260 nm. In addition to these characteristics, there is an absorption peak or shoulder near 312 to 315 in grass lignin [30, 64, 65]. Lignin from different raw materials or dissolved in different solvents also have large difference in the absorption coefficient of ultraviolet (UV) spectrum. The UV absorption coefficient is 18–20 L g−1 cm−1 for typical softwood lignin, about 12–14 L g−1 cm−1 for CH CH CHO

R2

R1 O

O

R1

R2 O

R1

O

R1

O

Fig. 2.7 Chromogenic groups in lignin structure.

R2 O

OH

Structure and Characteristics of Lignin  45 temperate hardwood lignin, which is lower than that of the softwood. The UV absorption of tropical hardwood and herbaceous lignin are close to that of softwood. The UV absorption coefficient of the reduced lignin sample decreases with the increase of the ratio of OCH3/C9. Because of a large change in structure of industrial lignin, its UV absorption coefficient is much different from that of material MWL. The UV absorption coefficient of kraft lignin is much higher than the same source of lignin sulfonate. The UV absorption coefficients of several lignin preparations are listed in Table 2.15 [65].

2.3.2  Dissolubility of Lignin Because lignin should be dissolved from the raw material after proper chemical treatment, the solubility of lignin is related to the method of separating lignin. There are three types of lignin solubility: dissolved in water, such as lignosulfonate; dissolved in organic solvents, such as ethanol, methanol, phenol, and dioxane, such as solvent lignin; insoluble in water and Table 2.15: UV absorption coefficients of several lignin [65] Lignin Preparations

UV Absorption Coefficient at 280 nm (L g−1 cm−1) Softwood MWL

Solvent

Spruce Korean pine Hemlock Douglas fir Larch

19.6 19.3 17.7 19.7 20.2

Dioxane Dioxane Methyl Fibrin/ethanol Methyl Fibrin/ethanol Methyl Fibrin/ethanol

Temperate hardwood MWL Beech Poplar Birch Maple

13.0 14.2 14.1 12.9

Methyl Fibrin/ethanol Methyl Fibrin/ethanol Methyl Fibrin/ethanol Methyl Fibrin/ethanol

Tropical hardwood MWL Red willow

17.0

Methyl Fibrinolytic/ethanol/water

Gramineous plant MWL Wheat straw Arundo donax Bagasse

20.4 20.1 18.6

Dioxane/water Dioxane/water Dioxane/water

Industrial lignin Spruce quality sulfonate Beech lignosulfonate Pine kraft lignin Kraft pulp residual lignin

11.9 10.4 24.6 18.3

Scots pine kraft Lignin

27.0

Water Water Water Cadmium Ethylenediamine Cadmium Ethylenediamine Cadmium Ethylenediamine Cadmium Ethylenediamine

46  Chapter 2 organic solvents, such as kraft lignin and hydrolyzed lignin. Swelling or dissolution of lignin is determined mainly by its Mw and the polarity of the solvent. In recent years, it has been reported that ionic liquids, such as imidazole base cationic ionic liquids capable of dissolving mill wood lignin and wood flour, can be used to dissolve lignin directly from wood raw materials [66, 67]. The ionic liquids [Mmim][MeSO4] and [Bmim][CF3SO3] are highly soluble in industrial lignin and [Emim][CH3COO] can selectively liberate lignin from wood flour (about 40%) [68]. The apparent solubility of lignin is characterized by the intrinsic viscosity, branching parameters, and polydispersity of lignin. Lignin dissolved in various solvents, such as dioxane lignin, kraft lignin, lignosulfonate, and alkali lignin, have a lower intrinsic viscosity with a Mark-Houwink index of 0–0.5. Among them, the lignin molecular shape is an Einstein sphere. The kraft-lignin Kuhn-Mark-Houwink-Sakurada (KMHS) equation has a small exponential factor of 0.11 in DMF and 10.23 in 0.5 mol L−1 NaOH. The surface lignin macromolecule is a compact spherical structure. The lignin is acetylated and dissolved in tetrahydrogenfuran with a KMSS value of 0.17–0.35 [69].

2.3.3  Thermal Properties of Lignin The glass transition temperature of lignin is wider than that of synthetic polymers because of its complex chemical composition and structure. Therefore, the determination of the Tg value of lignin requires a longer heat treatment time. Goring studied lignin’s thermal properties, such as heat softening, swelling, and glass transition. It was reported that the Tg temperatures of several common lignin were in the range of 127–227°C. Later, Irvine measured found that Tg of the eucalyptus MWL was 137°C [70, 71]. The heat treatment of the model of softwood lignin-indulin (Fig. 2.8) yields a Tg temperature range of 150–160°C [72].

Fig. 2.8 The structure of the softwood kraft lignin model.

Structure and Characteristics of Lignin  47 Table 2.16: Temperatures for different degrees of decomposition [73] Decomposition Temperature, Td Sample Weight Loss Industrial lignin

Before heat treatment After heat treatment ΔT, °C

1%

2%

3%

176

195

235

215

230

256

+29

+35

+19

The thermal stability of lignin can be evaluated by measuring its weight loss in the N2 atmosphere (TGA). The main parameters that reflect the change in mass with temperature are the weight loss rate DTG and the weight loss value. Industrial lignin did not lose weight before 125°C and then began to lose weight. Lignin heat treatment will cause a small amount of structural changes, improving the stability of lignin. When the temperature of the heat treatment of the lignin exceeds the Tg, the heat stability of the lignin is improved, see Table 2.16 [73].

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