Lignin: Historical, Biological, and Materials Perspectives - American

Chapter 9

Classification of Lignin According to Chemical and Molecular Structure

Downloaded by UCSF LIB CKM RSCS MGMT on September 14, 2014 | http://pubs.acs.org Publication Date: November 30, 1999 | doi: 10.1021/bk-2000-0742.ch009

Wolfgang G. Glasser Biobased Materials and Recycling Center, Department of Wood Science and Forest Products, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061

The commercial trade in isolated lignins generated in conjunction with the fractionation of biomass into constitutive components, including pulp and paper and saccharification/fermentation processes, requires the adoption of standard analytical techniques by both suppliers and users of such products. Simple, reliable, quantitative, and widely accepted techniques include elemental analysis, functional group analysis, analytical degradations, various spectroscopic techniques, molecular weight analysis, and thermal analysis. These techniques are capable of elucidating the chemical composition of repeat units; the nature of intermonomer bonds; molecular weights and weight distributions; and the potential for inter- and intramolecular interactions. Several aspects of this classification methodology are reviewed.

The huge potential of lignin as an underutilized (or wasted) raw material has been pointed out repeatedly (1-3). Although lignin's potential is not entirely unexploited (lignin sulfonates, for example, constitute a major resource in markets for ionic surfactants, drilling mud additives, dye stuff dispersants, etc., see refs. 4,5) the acceptance of non-sulfonated lignins in markets for structural polymers and materials has been slow at best. Despite the fact that numerous studies have pointed repeatedly to lignin's potential contribution to the physical properties of structural thermosets and thermoplastics, only a very small fraction of the lignin separated from wood (or biomass) finds its way today into engineered structural materials. One of the reasons for this apparent reluctance results from the absence of well-defined, standard analytical techniques that might be adopted by both suppliers and users of lignins in markets for structural polymers. Since most lignins are cogenerated in the process of pulp and paper making, and since this separation is optimized for pulp production and paper properties, lignin is usually viewed as a material of significant nonuniformity and variability in both chemical and molecular structure. It is this

216

© 2000 American Chemical Society In Lignin: Historical, Biological, and Materials Perspectives; Glasser, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.


217 perceived variability in combination with the absence of widely accepted and understood, quantitative, fast and simple analytical techniques suitable for general adoption in quality control procedures which are widely practiced in relatively nonspecialized laboratories, that present the most significant obstacles to lignin utilization in engineered structural materials. It is the objective of this review to draw attention to analytical methodologies qualified to overcome this obstacle. Lignins are separated from lignified ("woody") biomass by technologies that involve partial depolymerization and/or partial derivatization, and combinations thereof (6-9). These depolymerization and modification chemistries and their underlying reactions are well understood. However, this understanding has often resulted from work with lignin-like model substances, and it has ignored macromolecular properties and molecular interaction parameters. It is the extent to which each individual depolymerization and/or modification reaction has taken place within a certain native lignin, the structure of which is inherently variable with tree age, tree location within the ecology of a forest, cell type, etc. (10), that presents the lignin analyst with the challenge of quantitation. One can distinguish between four somewhat interrelated tasks for lignin analysis: (a) chemistry of the basic repeat unit(s); (b) chemistry of intermonomer bonds; (c) molecular size/weight considerations; and (d) inter/intramolecular interactions. The following is to summarize widely accepted and understood, fast and simple, quantitative, and (hopefully) universally available analysis techniques in nonspecialized laboratories that are qualified to shed light on these four points. Excellent reviews dealing with several details of the structure analysis of lignin have recently appeared elsewhere (11-13). The Chemistry of Monomeric Repeat Units The chemistry of lignin's monomeric repeat units, its "monolignols," normally relies on the determination of elemental composition in conjunction with the determination of methoxy groups. Standard C, H , N , (S?), and OCH3-analysis allows the formulation of basic Co-unit structures in accordance with the suggestion of Freudenberg (14) (Figure 1). Co-structures provide useful information on the overall composition of monomeric repeat units (15,16). They also provide information about the overall composition of lignin in terms of the specific mixture of p-OH cinnamyl alcohols, /?-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol (Fig. 1). Additional information on the chemical composition of the average, basic repeat unit can be obtained by optional functional group analysis (17,18). This is particularly relevant in relation to phenolic and total hydroxyl groups, and carbonyl groups (18). Widely accepted methods of phenolic OH-determination involve aminolysis (19), and H - N M R spectroscopy of acetylated lignin derivatives (20-25), and, to a lesser extent and with less reliability, UV-spectroscopy (17,18).

In Lignin: Historical, Biological, and Materials Perspectives; Glasser, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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Elemental Analysis Results Downloaded by UCSF LIB CKM RSCS MGMT on September 14, 2014 | http://pubs.acs.org Publication Date: November 30, 1999 | doi: 10.1021/bk-2000-0742.ch009

C , H,N, S,OCH

i

3

Interpretation (Freudenberg,1968)

Mixture of p-OH cinnamyl alcohols

C9H0.6) are rich in carbon to carbon bonding (Table I). Conversely, high ΒΑ/PA ratios typically signify lignins with a high proportion of alkyl aryl ether bonds compared to C-C bonds; and those lignins that have low BA/PA-ratios are mostly C-C-linked and have few (remaining) alkyl aryl ether linkages (Table I). Not surprisingly, lignins with intermonomer bonding closely resembling their native structure reflect a high degree of alkyl-aryl ether bonding; and lignins isolated following significant depolymerization catalyzed by alkali or acid reveal lesser or greater degrees of ether-cleaving depolymerization and



220

(À)

LIGNIN Ethylotion

|

DES0 /0H~ 4

ETHYLATED LIGNIN Oxidative Depolymerization

j

CuO/OH"

HYDROLYZEO LIGNIN Methylotion

DMS0 /0H* 4

METHYLATED HYDROLYZEO UGNIN Oxidation

j

KMn0 /OH~ond H 0 4

2

2

DEGRADATION PRODUCT MIX Methylotion

j

CH N 2

2

METHYLESTERS OF AROMATIC CARBOXYLIC ACIDS Quantitative Separation

GC Fig. 2 (A) Reaction pathway for lignin analysis by degradative permanganate oxidation; and (B) Typical gas chromatogram of a mixture of monomeric and dimeric permanganate oxidation products using the degradation protocol illustrated in Fig. 2A. The sample represents a hardwood lignin. (Reproduced with permission from ref. 27. Copyright 1983 by American Chemical Society.)



ÇOjCH, f

CH

8

X

RETENTION TIME (minutes) Figure 2. Continued


222

ι

ι R,*CH orC H


3

Monomeric Unit

5-5

2

5

ΑΙ - 0 - 4

4-0-5

Fig. 3 Types of (softwood) intermonomer bonds determined quantitatively by permanganate oxidation on the basis of the distribution of monomeric and dimeric fragments found in the gas chromatogram of Fig. 2B. (Reproduced with permission from ref. 27. Copyright 1983 by American Chemical Society.)


223 Table I. Intermonomer Bond Types of Isolated Lignins (Adopted from ref. 16,27,28). Lignin Types

BA/PA

2)

M W L - softwood 3.7-4.8 0.28-0.32 - hardwood 0.22 13-16 Kraft Lignin (pine) 2.1 0.83 Acid Hydrolysis Lignin (2% H S 0 , > 220°C, 10-60s) - Softwood 0.72 0.91 - Hardwood 0.39 0.79 Steam Explosion Lignin - Straw 2.0 0.81 5.2-8.1 - Hardwood 0.5-0.8 Organosolv Lignin 2.1 - Softwood 0.87 1.6-15 - Hardwood 0.2-0.9 'Molar ratio of uncondensed Cç-units originally present with free phenolic O H groups to the sum total of all monobasic, monomeric-benzoic acid units. This ratio expresses alkyl-aryl ether content (i.e., inverse relationship). Molar ratio of benzoic to phthalic and isophthalic acid units. This ratio decreases with increasing degree of condensation. 2


1

Hydrolysis Ratio '

4

2)

condensation, respectively (Table I). These prevalent structural features can be expected to have a significant impact on the utilization potential of isolated lignins in structural polymers. Thioacetolysis involves the sequential treatment of lignin (isolated or in wood) with thioacetic acid followed by treatment with alkali and Raney Nickel (for desulfurization) (35,36). While thioacetolysis of hardwood lignin was found to generate as much as two-third (by weight) monomeric and dimeric degradation products (35), and this was substantially in excess of all previously available degradation methods, this method failed to find the universal acceptance required of a standard protocol. The emergence of an alternative degradation technique, with fewer steps and equal quantifiability, may have been responsible for the low acceptance rate. Thioacidolysis, first advocated in the early 1980's, provided for the treatment of lignin with ethanethiol/BF3 (37). The desulfiirized (with Raney Nickel) reaction product mixture typically consists of between 1/3 and 2/3 of monomeric and dimeric degradation products per average phenylpropane repeat unit (i.e., >1,000 to

Lignin: Historical, Biological, and Materials Perspectives - American

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