Sustainable Lignin for Carbon Fibers: Principles, Techniques, and Applications [1st ed.] 978-3-030-18791-0;978-3-030-18792-7

Table of contents :
Front Matter ....Pages i-xi
Chemistry and Structure of Lignin (Emmanuel Isaac Akpan)....Pages 1-50
Lignin Conversion to Carbon Fibre (Oluwashina Phillips Gbenebor, Samson Oluropo Adeosun)....Pages 51-64
Bio-sourced Lignin: Recovery Techniques and Principles (Emmanuel Isaac Akpan)....Pages 65-150
Biosourced Lignin: Sources and Properties (Samson Oluropo Adeosun, Oluwashina Phillips Gbenebor, Odili Cletus)....Pages 151-191
Characterization Techniques and Quality Assessment of Lignin and Lignin Carbon Materials (Samson Oluropo Adeosun, Oluwashina Phillips Gbenebor)....Pages 193-279
Melt-Processing of Lignin (Emmanuel Isaac Akpan)....Pages 281-324
Stabilization of Lignin Fibers (Emmanuel Isaac Akpan)....Pages 325-352
Carbonization, Activation and Graphitization of Lignin-Based Materials (Emmanuel Isaac Akpan)....Pages 353-394
Lignin Carbon Fibres: Properties, Applications and Economic Efficiency (Sikiru Oulwarotimi Ismail, Emmanuel Isaac Akpan)....Pages 395-426
Surface Treatment of Lignin Sourced Carbon Fibers: Principles, Processes, and Challenges (Sibel Demiroğlu Mustafov, Mehmet Özgür Seydibeyoğlu)....Pages 427-439
Back Matter ....Pages 441-454

Citation preview

Emmanuel Isaac Akpan  Samson Oluropo Adeosun Editors

Sustainable Lignin for Carbon Fibers: Principles, Techniques, and Applications

Sustainable Lignin for Carbon FibersPrinciples, Techniques, and Applications

Emmanuel Isaac Akpan Samson Oluropo Adeosun Editors

Sustainable Lignin for Carbon Fibers: Principles, Techniques, and Applications

Editors Emmanuel Isaac Akpan Institute for Composite Materials Technical University Kaiserslautern Kaiserslautern, Germany

Samson Oluropo Adeosun Department of Metallurgical and Materials Engineering University of Lagos Lagos, Nigeria

ISBN 978-3-030-18791-0    ISBN 978-3-030-18792-7 (eBook) https://doi.org/10.1007/978-3-030-18792-7 © Springer Nature Switzerland AG 2019 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG. The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

To God almighty, the all-knowing and all-powerful

Preface

The future of automotive and aerospace industries is tied to the development of cost-effective and environmental-friendly carbon fibre source. Recently, lignin has been discovered as an ideal replacement for polyacrylonitrile (PAN) precursors for automotive grade carbon fibre production because it contains high carbon content (≈60% carbon content) and is widely available. It can be obtained from nearly all plants, agricultural wastes and even industrial by-products. However, lignin from these bio-sources is different from PAN and other carbon fibre precursors in terms of isolation, purification, and further processing into carbon fibres. Until now, mechanical performance requirements for automotive applications have not yet been realized from lignin-based carbon fibres. The way forward is a very good understanding of the lignin complex system, the role of lignin chemistry as it relates to carbon fibre production and the evolution of lignin carbon fibre structure during processing. ‘Sustainable lignin for carbon fibres’ is designed to provide a wide understanding of lignin carbon fibre processes, chemistry, mechanisms, and techniques that will help in further development of lignin carbon fibre for automotive applications. Each step in the processing of lignin carbon fibres is presented as a separate chapter so that issues concerning the processes are exhaustively discussed. ‘Sustainable lignin for carbon fibres’ is a comprehensive volume for academia, industrialist and research scientist, who will learn basic principles and techniques in the recovery, processing and application of bio-sourced lignin for carbon fibres. Research challenges and gaps outlined within each chapter and summarized in the conclusion/ outlook are very important to the research scientist. For the industrialist, successful recovery and purification techniques, state-of-the-art melt processing and stabilization techniques, most successful carbonization schedule for improved properties, potential applications, and market value of lignin-based carbon fibres are presented. All the chapters are structured such that they present basic principles governing each stage of lignin carbon fibre processing, current state of research and mechanisms behind the processes. Illustrations (graphs, structures and sketches) and careful explanation of mechanisms are used for better understanding of principles. These make the book relevant to lecturers and university students. vii

viii

Preface

The book is divided into ten chapters with each chapter dealing with a specific step in the processing of lignin carbon fibres. Chapter 1 gives a general understanding of lignin as a polymer, its chemistry and molecular structure. Basic information about lignin as a macromolecule and its potential are presented in this chapter. Chapter 2 provides an overview of the processes involved in the processing of lignin into final carbon fibre. General concepts behind the processing steps are outlined. The different methods used in isolation of lignin from their sources are presented in Chap. 3. Mechanisms behind these extraction methods and their effects on the lignin structure are extensively reviewed. It is an important source of information for any industry who will not want to depend on the pulping industry for lignin. Over 250 plant sources of lignin and their prevalent monomeric units are presented in Chap. 4. In Chap. 5, techniques of the characterization of lignin and lignin carbon fibres are presented. For each characterization technique, basic principles behind the technique are presented with an extensive literature review on their use in lignin characterization. Chapter 6 deals with the methods used in the formation of initial lignin carbon fibres before stabilization. Mechanism and technology of melt, wet, and dry spinning of lignin carbon fibres are explained in details. In Chap. 7, stabilization methods for lignin carbon fibres are presented. A review of the current stabilization methods and the properties of the fibres are outlined. An important aspect of the chapter is the analysis of the effect of stabilization parameters on the properties of the carbon fibres. Chapter 8 provides a good understanding of carbonization, activation and graphitization of stabilized fibres. Mechanism behind these processing methods, their current state of the art and suggestions for improvements are outlined. The economic efficiency, the cost analysis and the properties of lignin-­ derived carbon fibres and activated carbon are presented in Chap. 9. The book closes with a chapter on surface treatment of lignin-based carbon fibres for composite applications. We are grateful to Brinda Megasyamalan and Anita Lekhwani of Springer Nature for their wonderful contributions toward the success of this book. We are also grateful to the contributors who came to the rescue when the book project was about to fail due to withdrawal of authors for various reasons. Kaiserslautern, Germany Lagos, Nigeria 

Emmanuel Isaac Akpan Samson Oluropo Adeosun

Contents

  1 Chemistry and Structure of Lignin��������������������������������������������������������    1 Emmanuel Isaac Akpan   2 Lignin Conversion to Carbon Fibre ������������������������������������������������������   51 Oluwashina Phillips Gbenebor and Samson Oluropo Adeosun   3 Bio-sourced Lignin: Recovery Techniques and Principles ������������������   65 Emmanuel Isaac Akpan   4 Biosourced Lignin: Sources and Properties������������������������������������������  151 Samson Oluropo Adeosun, Oluwashina Phillips Gbenebor, and Odili Cletus   5 Characterization Techniques and Quality Assessment of Lignin and Lignin Carbon Materials������������������������������������������������  193 Samson Oluropo Adeosun and Oluwashina Phillips Gbenebor   6 Melt-Processing of Lignin ����������������������������������������������������������������������  281 Emmanuel Isaac Akpan   7 Stabilization of Lignin Fibers ����������������������������������������������������������������  325 Emmanuel Isaac Akpan   8 Carbonization, Activation and Graphitization of Lignin-Based Materials ��������������������������������������������������������������������������������������������������  353 Emmanuel Isaac Akpan   9 Lignin Carbon Fibres: Properties, Applications and Economic Efficiency��������������������������������������������������������������������������������������������������  395 Sikiru Oulwarotimi Ismail and Emmanuel Isaac Akpan 10 Surface Treatment of Lignin Sourced Carbon Fibers: Principles, Processes, and Challenges����������������������������������������������������  427 Sibel Demiroğlu Mustafov and Mehmet Özgür Seydibeyoğlu Index������������������������������������������������������������������������������������������������������������������  441 ix

Contributors

Samson  Oluropo  Adeosun  Department of Metallurgical and Materials Engineering, University of Lagos, Lagos, Nigeria Emmanuel Isaac Akpan  Institute for Composite Materials, Technical University Kaiserslautern, Kaiserslautern, Germany Odili Cletus  Department of Metallurgical and Materials Engineering, University of Lagos, Lagos, Nigeria Sibel  Demiroğlu  Mustafov  Department of Nanotechnology and Nanoscience, İzmir Katip Çelebi University, İzmir, Turkey Oluwashina  Phillips  Gbenebor  Department of Metallurgical and Materials Engineering, University of Lagos, Lagos, Nigeria Sikiru  Oulwarotimi  Ismail  School of Engineering and Computer Science, University of Hertfordshire, Hatfield, Hertfordshire, England, UK Mehmet Özgür Seydibeyoğlu  Department of Material Science and Engineering, İzmir Katip Çelebi University, İzmir, Turkey

xi

Chapter 1

Chemistry and Structure of Lignin Emmanuel Isaac Akpan

1.1  Lignin in Nature The quest for bio-based and renewable materials coupled with depletion of fossil fuel resources has led to greater focus on lignin and its applications in chemical, material and structural industries. Lignin happens to be the second most abundant lignocellulosic natural polymer after cellulose [1]. It is mainly found in the cell wall of plant and woody species. It is regarded as the main source of renewable aromatic structures on earth and is expected to play a role in sustainable production of fuel, functional polymers, aromatic chemicals, phenols, vanillin, ferulic acid, etc. [2]. The study of lignin dates to the nineteenth century. Payen in 1838 and Schulze in 1865 described and defined lignin as a hydrophilic substance present in plant cell walls [3]. Lignin is a very complex polymer with divergent structure and properties. Processing lignin into useful products (such as carbon fibres) requires an extensive understanding of the chemistry and structure of lignin. In this chapter, the chemistry and structural aspects of lignin are discussed. Lignin macromolecular structure, possible reactions sites, mechanism of reactivity, surface and colloidal chemistry are presented in details.

1.1.1  Occurrence and Abundance Lignocellulose materials (plant and wood) are composed of cellulose, hemicellulose and lignin with small portion of other extractives such as wax and pectin [4]. In lignocellulose materials, cellulose occurs as microfibrils embedded within a pool of

E. I. Akpan (*) Institute for Composite Materials, Technical University Kaiserslautern, Kaiserslautern, Germany e-mail: [email protected] © Springer Nature Switzerland AG 2019 E. I. Akpan, S. O. Adeosun (eds.), Sustainable Lignin for Carbon Fibers: Principles, Techniques, and Applications, https://doi.org/10.1007/978-3-030-18792-7_1

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lignin and hemicellulose is usually regarded as the matrix (Fig.  1.1). Cellulose is formed as ordered polymer chain containing tightly packed crystalline regions and is responsible for the load carrying ability of the lignocellulose material. On the other hand, lignin fills the spaces between hemicellulose and cellulose acting as an adhesive to bind them together [5]. Lignin strengthens the cell walls of plant cells by covering the cellulose fibrils. It is also responsible for barrier against insect and microbe attacks in plants as well as enabling the transportation of water and ions from soil to the plant. Because lignin binds the cellulose and hemicellulose together, it increases impermeability and contributes to mechanical strength of the lignocellulosic material [3]. The binding provided by lignin occurs both physically and chemically. Lignin is estimated to have a worldwide production of approximately 26–50 million tons per year [6, 7]. The amount of lignin in plant varies with species [8]. General reports show that lignin in softwood is usually in the range of 18–25% and that of hardwood in the range of 25–35%. Other authors also reported that woody plants lignin accounts for 27–32% and about 14–25% in herbaceous plants [9]. Fig. 1.1  Occurrence of lignin in plant. Reprinted with permission from ACS [10]

1  Chemistry and Structure of Lignin

3

As mentioned earlier, lignin exists basically in the cell wall of lignocellulosic materials. However, the distribution of lignin is not uniform throughout the cell wall [3]. This can be seen in the imaging diagram in Fig. 1.2b where the bright areas represent lignin. It is generally agreed that lignin concentrates mostly in the middle lamella and the primary wall bust sparsely in the secondary wall (Fig. 1.2a). Because secondary cell wall appears to be the largest proportion of plants, the largest amount of lignin in plants is contained in the secondary cell wall (75–85%). It is evident that lignin is formed in the primary and corner middle lamellar which are afterwards transported to the secondary cell walls [10]. Using field-­emission-­scanningelectron-microscopy, Fromm et al. [11] showed that lignin particles occur in high concentrations in the middle lamella but at low concentrations in secondary cell layers intermingled with the microfibrils. The lignin in this middle layer has been said to be four times that in the secondary cell wall [12]. Xu et al. [13] also reported that lignin in the vessels and middle layer is more than that in the fibres. This distribution has been reported to be affected by cell type, the site of the growth ring, vessel arrangement and the producing area [14]. Wu et al. [14] studied 25 kinds of hardwood and concluded that amount and distribution of lignin in woods are generally affected by cell type, site of growth rings, vessel arrangement and the area where they were produced.

Fig. 1.2 (a) Schematic structure of wood cell wall: W thin warty layer in cell lumen; S3, S2, S1 secondary wall layers; P, mL primary wall and middle lamella, respectively; CC cell corner middle lamella [15]. (b) Photographic imaging showing the distribution of lignin in plant cell wall [10]. Reprinted with permission from [10, 15]

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1.2  General Classification of Lignin Lignin varies in structure and properties with source, extraction, location, environment, etc. Moreover, lignin from different plant species presents different chemical structure, molecular weight and macro-molecular behaviour [5]. This makes it necessary for lignin to be classified. In this book, lignin is classified into two categories including; classification according to extraction method and classification according to the biomass origin.

1.2.1  Classification According to Taxonomy or Biomass Origin Based on biomass species lignin is classified as softwood (gymnosperms), hardwood (angiosperms) and non-woody (graminoids) lignin. To understand the differences in lignin from these sources, it is important to introduce the three basic units in lignin structure. Despite the source, lignin is biologically synthesised from three basic monomeric units including p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) units (Fig. 1.3) [8, 16]. These units form the major building blocks that make up lignin from any source. Classification of lignin according to taxonomy depends a great deal on the relative presence of these units. Softwood lignin is known to contain more of the G unit with low levels of H unit and is therefore called the G-lignin. Hardwood lignin, on the other hand, contains more of the G and S units with traces of H unit and are therefore called the GS lignin, whereas non-woody lignin usually contains a mixture of G, S and H units and are therefore referred to as the HGS-­lignin [8, 17]. Softwood compression wood has also been referred to as GH lignin [18, 19].

γ

5

γ

β

α 6

OH

1 4

3

6

HO

OH

1 4

6

MeO

OH

HO

1 4

OH guaiacyl (S)

Fig. 1.3  Lignin monomeric units

2

Monolignols OMe

OH

MeO

OH p-hydroxyphenyl (H)

β

α

2 3

OH

γ

β

α

2

OH

OMe OH syringyl (S)

Aromatic residues in polymer

1  Chemistry and Structure of Lignin

5

Hardwood lignin displays a linear structure compared to softwood lignin which displays a kind of a network structure. This is because the S unit contains an additional methoxyl group which is not present in the G unit (Fig. 1.3). In hardwood lignin, the five position is blocked by the methoxyl group, preventing condensation with other monomers [20, 21]. The presence of this group results in decreased tendency for crosslinking in the S unit, and therefore the relative linearity in the structure arising from the presence of large polymeric fraction [22, 23]. This linearity can be pictured in Fig. 1.4. Illustrating the difference between softwood and hardwood lignin, Henriksson [24] showed that softwood lignin presents a higher amount of carbon-carbon bonds and a lower amount of ether bonds than the hardwood lignin. This implies that hardwood lignin possesses a more condensed structure than softwood lignin. Softwood lignin is known to be reactive than hardwood lignin [25, 26]. Softwood lignin is also more uniform in different wood ­species due to struc-

Fig. 1.4  Lignin derived from (a) hardwood and (b) softwood. Reprinted with permission from [22]

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tural similarity. They tend to contain only one predominant type of monolignol as building block (guaiacyl-propane units) [27]. A major difference between hardwood and softwood lignin is in the functional groups. Phenolic –OH groups in hardwood lignin is higher than softwood lignin. However, the aliphatic groups are lower in hardwood than in softwood lignin [28]. Because the aliphatic groups form intermolecular hydrogen bonds, it is expected that softwood lignin possesses a higher hydrogen bonding network than hardwood lignin. Kubo and Kadla [29] showed that softwood Kraft lignin possesses higher OH band in FTIR than hardwood lignin. This corresponds to a more rigid structure which also implies higher glass transition temperature for softwood lignin. It is also important to note that G unit possesses unoccupied C5 sites which facilitate crosslinking. This has implications on reactivity and thermal stability of lignin with more of G unit.

1.2.2  Classification According to Extraction During extraction of lignin, parts of the original structure of lignin are destroyed. This results in a situation where lignin from different extraction process possesses different chemical structure, functionalisation, physical and chemical properties and molecular mass. Arising from this, it is safer and simpler to classify lignin according to extraction process. According to extraction methods, lignin can be classified into Kraft lignin, lignin sulphonates, organosolv lignin, steam exploded lignin, soda lignin and hydrolysis lignin [5, 8, 27]. A comprehensive description of these processes and the properties of the lignin arising from them will be dealt with in Chap. 3. In this section, the differences in structure of these lignin types are ­illustrated. 1.2.2.1  Kraft Lignin Kraft lignin is obtained industrially by applying an aqueous solution containing Na2S and NaOH to dissolve lignin from lignocellulosic materials forming a black solution called ‘black liquor’. The Kraft process (alkaline hydrolysis) proceeds by cleaving the 1,4 links in cellulose, thereby allowing the lignin component of the biomass to be extracted. In the process, lignin with aliphatic thiol groups is produced with extensive hydrophobic character [4]. Evaporation and precipitation methods are then used to recover the lignin from the liquor. Kraft lignin (Fig. 1.5) undergoes substantial structural changes during extraction including cleavage of aryl ether bonds arising from depolymerisation during the cooking process. During the pulping, lignin undergoes cleavage of phenolic α-O-4 and β-O-4 at low temperatures accompanied by the formation of enol ether. At high temperatures, cleavage of non-phenolic β-O-4, demethylation and condensation also occur. This makes it very rich in phenolic OH groups with high tendency to condensation reactions. Excess C-C bonds are found alongside the OH groups. Kraft lignin contains small amount

1  Chemistry and Structure of Lignin

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Fig. 1.5  Structure of Kraft lignin (a), and lignosulphonate (b)

of sulphur because of Na2S used in the process. The average molecular weight of Kraft lignin is estimated to be between 1000 and 3000 [3]. This is very low compared to that of lignosulphonate lignin. It also possesses an average polydiversity between 2 and 4 [30]. Kraft or Kraft residual lignin generally possesses less β-O-4 linkages, more phenolic hydroxyl groups, lower molar mass, more chromophores and more condensed lignin structures. 1.2.2.2  Lignosulphonate Lignosulphonated lignin also called sulphite lignin is obtained from waste pulping liquor of the sulphite pulping procedure of the Howard process after the recovery of sulphur [3]. The pulping process utilises several salts of sulphurous acid including sulphites and bisulphites [8]. The different pulping conditions impart different properties to the resulting lignins. Because they contain sulphonate groups [2] (Fig. 1.5) attached to the benzylic carbon of the phenylpropane unit of the lignin chain, they present both hydrophilic and hydrophobic characteristics. These sulphonate groups occur 0.4–0.5 groups in every C9 unit of lignin. They are polyelectrolyte polymers, highly soluble in water. They possess increased weight-average molecular weight with low amount of OH groups. Their water solubility arises from the low pKa of the sulphonate groups [4]. They are highly crosslinked with approximately 5 wt.% sulphur content and two ionising groups: sulphonate and phenolic hydroxyl. Ligninosulphonates physicochemical properties are greatly affected by the metal cations of the sulphite salt used for the pulping. The cations to a large extent are responsible for their reactivity. Experiments have shown that calcium-based lignin

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possesses the lowest reactivity, sodium and magnesium possess the medium reactivity, whereas ammonium-based lignin possesses the highest reactivity [31]. The process also leads to cleavage of the α and β-ether linkages of lignin. Their average molar mass is higher than that of Kraft lignin with a polydispersity index between 6 and 8 [32]. 1.2.2.3  Organosolv Lignin Organosolv lignin is obtained using organic solvents such as ethanol or ethanolwater mixture. The lignin is usually recovered from the liquor by precipitation or solvent removal and recovery processes [3]. There are varieties of organosolv lignin depending on the solvent used in the extraction. However, the Allcel lignin is the most widely known variant of the organosolv lignin. They are sulphur free and have a less modified structure. They have low molecular weight, high chemical purity and are hydrophobic. The organosolv process involves solubilisation making it possible to obtain less modified lignin. Kubo and Kadla [33] showed that Alcell lignin contains flexible alkoxyl chains introduced at the Cα and Cɣ positions of the side chains during the isolation process. Ethylation of the carbocation at Cα of organosolv lignin reduces the formation of condensed linkages. This is the reason for low molecular weight, glass transition and ability to dissolve in organic solvents. They possess higher amount of phenolic hydroxyl groups and more oxidised structures relative to other lignin sources (Fig. 1.6a) [33]. The homogeneity of organosolv lignin is higher than those of lignosulphonates and alkali lignin [31]. Organosolv lignin contains a lot of reactive side chains available for chemical reactions (Fig. 1.6b). 1.2.2.4  Soda Lignin Lignin derived from pulping of lignocellulose materials using soda or soda–anthraquinone solutions is called soda lignin [31]. This method is mainly used to recover lignin from annual crops such as straws, bagasse, flax and to some extent hardwoods. This method is preferred over other methods because of the absence of sulphur aerial emissions and the need for additional desulphurisation process steps. Lignin obtained through this process is the closest to the natural lignin which has not undergone any reaction. The main fragmentation reactions during soda delignification are the cleavages of α-aryl ether bonds in phenolic units, and of β-aryl ether bonds in phenolic and non-phenolic units (Fig. 1.7). The non-phenolic β-O-4 bond cleavage leads to the formation of a phenolic lignin end group and epoxides and because of the presence of NaOH, these products later participate in nucleophile activated reactions. On the other hand, phenolic β-O-4 bond cleavage results in the formation of quinone methide formed which has the tendency for further reaction releasing formaldehyde and an enol ether structure. It may also undergo a condensation- or reduction reaction. Soda lignin is sulphite free and has been found useful

1  Chemistry and Structure of Lignin

O

O

O OCH3

HO– / HS–

OCH3

- CH2O - lignin-O–

O

Lignin

Lignin

Lignin

OH

Lignin

9

H+ Lignin-OH

O

Lignin

Lignin-OH Lignin

OH O

Lignin

OH O

CH3CH2OH

OCH3

O OCH3

H+ Lignin

O

Lignin

O

HO

O

O

OH

O

OH

O O

Acid condition

H O

H O

HO

Alkaline condition

O

O

O

O

O O H

O H O H O O H

O

O O

O O H

O

Fig. 1.6 (a) Proposed ethylation of lignin in organosolv pulping. Reprinted with permission from [33]. (b) Reactive sites in organosolv lignin

in the production of phenolic resins, animal nutrition and dispersants because of their purity and supposed bio-compatibility [31]. They are also suitable for the ­synthesis of polymers and low molecular weight substances [34]. Soda lignin ­possesses molecular weight between 1000 and 3000 g mol−1 and a polydispersity between 2.5 and 3.5.

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HO

CH3O CH2

HC HC

O

HC OR

O

HC –

H3CO

O

O –

O

CH3O CH2

H C O –

O CH

OR

H3CO

O–

–

CH3O CH2

O

H3CO

–

O

CH2

CH2

HCOH

CH –

CH

O

CH

–

O -OH

H3CO

H3CO O

O

H3CO O

Fig. 1.7  Cleavages of α-aryl and β-aryl ether bonds. Reprinted with permission from [35]

1.2.2.5  Steam Explosion Lignin Steam explosion lignin are those lignin obtained by application of steam and pressure to the biomass. In the process, the biomass is subjected to high-pressure steam followed by a rapid decompression, which forces the fibrous material to ‘explode’ into components. It is one of the most attractive methods of lignin recovery because of its potential for significantly reducing the environmental pollution, costs and energy consumption. Moreover, in a purely steam exploded process, no hazardous chemicals are used [36]. During steam explosion, the major reaction is autohydrolysis of the ether bonds present in the biomass polysaccharides and lignin. It is widely believed that at high-temperatures, acetic acid from wood components is released which catalyses hydrolytic reactions of the constituent polymers. In the process, there is a loss in hemicelluloses and amorphous cellulose followed by a decrease in the amount of β-O-4 structure in lignin [37–39]. Steam explosion results in lignin with a molecular weight range from 150 to 7000. Because they are extensively depolymerised by cleavage of the ether bonds, they are partially soluble in alkaline solutions and some organic solvents [40–47]. Steam-exploded wood lignin has been used as extender in asphalts [44], to obtain graft copolymers with styrene [48] and as binders for composites [49, 50].

1  Chemistry and Structure of Lignin

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1.3  The Chemistry of Lignin 1.3.1  Lignin Structural Architecture Lignin is an amorphous aromatic polymer with a very complex chemical structure. It contains three primary precursors which are different in proportion depending on their sources. These precursors (p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) units) are linked together by different functional groups. The frequency of these functional groups is what results in the structural variation of various lignins. Chemically, lignin is a phenolic polymer that differs according to plant. It has also been reported that some lignin are linear oligomers with no defined primary structure. However, they can be viewed as a representative random phenyl-propanoid (C9) polyphenols, usually linked by acylglycerol ether bonds between phenolic para-­coumaryl alcohols, coniferyl alcohol and sinapyl alcohol units (Fig. 1.8) [51–53]. Effective processing of lignin into required components requires a good knowledge of the structural architecture of lignin. Although the structure of lignin differs greatly according to plant, source, extraction and location, the basic structure of lignin is the same and is the key to lignin modification. In simple terms, lignin can be viewed as consisting of phenylpropane units with an oxygen atom at the p-­position (as OH or O–C) and with or without methoxyl groups in the o-positions to the oxygen atom. In some cases, the o-positions is C-substituted or O-substituted with constituents other than methoxyl. Some other possibilities in the lignin structure are substitution of other aromatic units in the ring positions. In some cases, a situation arises where either a few percentage of the building blocks not being phenylpropane units are missing or side chains are shortened or replaced by a quinoid group. The coupling between each of the basic units (H, G and S) includes β–O–4, β–5, β–1 etc. Alternatively, these linkages exist as members of a series of characteristic end groups (e.g. cinnamaldehyde units) [54]. The general structure of lignin usually shows eight different motifs for interunit linkages but not all the lignin monomeric units can take part in all coupling modes. The basic condition that determines the coupling of the units is β-position of the monolignol species. This results in the formation of five motifs, namely aryl glycerolβ-aryl ethers (β-O-4′ motif), phenylcoumarans (β-5′), pinoresinols (β-β′), diphenylethane dimers (β-1′) and spirodienones (SD). On the other hand, dilignols and some higher oligomers prefer to couple at positions 4 and 5, giving rise to two motifs, namely diaryl ethers (4-O-5′ motif) and biphenyls (5–5′ motif). It is only possible to couple these oligomers by the formation of dibenzodioxocine units which also gives results in the eighth motif (5,5′- α, β-O-4′ motif) (Fig. 1.8). β–O–4 (β-aryl ether) linkage is the most frequent inter-unit linkage in lignin. It is an important linkage in lignin because it is easily cleaved chemically and thus providing a basis for lignin related industrial processes, such as chemical pulping. All other linkages (β–5, β–β, 5–5, 5–O–4 and β–1) are very resistant to chemical degradation [17]. These linkages of lignin are what give the plant cell walls many physicochemical properties and biological activities such as their rigidity and pathogen defence. They provide useful information on the production of other biopolymers from lignin. A good example is the use of phenolic groups in the lignin structure to produce phenol formaldehyde resins.

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Fig. 1.8  Lignin structural architecture. (a) Representative structures of lignin biopolymers, (b) specific lignin types, and (c) main linkage motifs found in lignin and lignin extracts. Reprinted with permission from [3]

1  Chemistry and Structure of Lignin

13

1.3.2  Lignin Biosynthesis Lignin is synthesised principally from the three monolignols precursor (G, H and S) monomers. The monolignols are formed from amino acid phenylalanine and lignin is synthesised from the monolignols through successive demineralisation and polymerisation reactions involving several enzymes. The monolignols are synthesised through the phenylpropanoid pathway. It starts with the deamination of phenylalanine involving successive hydroxylation reactions of the aromatic ring. This is followed by conversion of the side-chain carboxyl to alcohol group and phenolic O-methylation [9, 17, 51]. The process starts by the formation of phenylalanine and tyrosine from the shikimate pathway. In the shikimate pathway, an intermediate shikimic acid is formed by conversion of glucose generated from carbon dioxide by photosynthesis. The shikimic acid is then converted to phenylalanine and tyrosine through prephenic acid (Fig.  1.9) [55]. These acids which are widely present in plants are the starting materials for the cinnamic acid pathway. Using various enzymes, the three lignin monomers are finally synthesised after a set of reactions. These reactions include deamination, hydroxylation, methylation and reduction. Several researchers have thought that these reactions occur at the level of cinnamic acids and that the intermediate compounds p–coumaric, ferulic and sinapic acid are subsequently converted to the monolignols by the actions of 4-coumarate:CoA ligase, cinnamoyl coenzymeA (CoA) reductase (CCR) and cinnamyl alcohol dehydrogenase (CAD). It has been shown otherwise through a number of research works that the suggested pathway was not completely accurate, leading to an updated version to be drawn (Fig. 1.10). However, recent studies on gene expression [56–59] of some plants have shown new findings and have led to an update of the pathway as shown in Fig. 1.11.

Fig. 1.9  Formation of phenylalanine and tyrosine from the shikimate pathway [55, 60]

Fig. 1.10  Revised model of the phenylpropanoid pathway leading to lignin biosynthesis. Reactions thought to be key in lignin biosynthesis are indicated with black arrows. Intermediate compounds and enzymes currently considered to form the prominent path to lignin are highlighted in blue. 4CL 4-(hydroxy)cinnamoyl CoA ligase, C3H p-coumarate 3-hydroxylase, C4H cinnamate 4-hydroxylase, CAD cinnamyl alcohol dehydrogenase, CCoAOMT caffeoyl CoA O-methyltransferase, CCR cinnamoyl CoA reductase, COMT caffeic acid/5-hydroxyferulic acid O-methyltransferase, CQT hydroxycinnamoyl CoA:quinate hydroxycinnamoyltransferase, CST hydroxycinnamoyl CoA:shikimate hydroxycinnamoyltransferase, F5H ferulate 5-hydroxylase, PAL phenylalanine ammonia-­lyase, pCCoA3H p-coumaryl CoA 3-hydroxylase, SAD sinapyl alcohol dehydrogenase. Figure printed with copyright from Elsevier [64]

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Fig. 1.11  General steps in lignification via the phenylpropanoid pathway. The names of the enzymes and intermediates are listed based on current knowledge of the biosynthetic pathway. The enzymes above each arrow are responsible for catalysing the corresponding steps. H, G and S lignin monomers are the basic units of lignin polymers. However, the presence of C and 5H lignins has also been reported in some plant species. The relatively small sizes of the chemical structures of the C and 5H lignins compared with the H, G and S lignin monomers indicate that C and 5H lignin monomers are uncommon. Abbreviations: 4CL 4-coumarate:CoA ligase, C3H p-coumaroyl shikimate 3′-hydroxylase, C4H cinnamate 4-hydroxylase, CAD cinnamyl alcohol dehydrogenase, COMT caffeic acid/5-hydroxyconiferaldehyde 3-O-methyltransferase, CSE caffeoyl shikimate esterase, F5H ferulate/coniferaldehyde 5-hydroxylase, HCT hydroxycinnamoyl CoA:shikimate hydroxycinnamoyl transferase, PAL l-phenylalanine ammonia-lyase. Figure reprinted with permission from [56]

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In summary, it can be said that the lignin monolignols biosynthetic pathway is under development. However, in this chapter, a simplified version of the pathway is described to enable a simple understanding of the formation of lignin. The names of phenylpropanoid compounds are as a result of the basic structure of a three-carbon side chain on an aromatic ring, which is derived from l-phenylalanine. The first reaction involves the deamination of phenylalanine by phenylalanine ammonia-­ lyase (PAL) to yield cinnamic acid [16, 61]. The cinnamate is later hydroxylated by cinnamate 4-hydroxylase (C4H) to form p-coumaric acid. This can then be conjugated to form p-coumaroyl-CoA (an activated thioester precursor) by 4-coumarate: CoA ligase (4CL). p-coumaroyl-CoA is the precursor for synthesis of flavonoids, stilbenes as well as the monolignol p-coumaryl alcohol. On the other hand, p-­ Coumaric acid and p-coumaroyl-COA can both be hydroxylated to yield caffeate and caffeoyl-COA, respectively. The new hydroxyl groups in the previous steps can be methylated by an O-methyltransferase (OMT) to produce ferulate or feruloyl-­ COA. There are also possibilities of caffeate and ferulate being activated to COA thioesters by 4CL. Feruloyl-COA is the primary precursor for synthesis of the lignin monolignol coniferyl alcohol. Another important step is the hydroxylation of ferulate by ferulate 5-hydroxylase (F5H) to form 5-hydroxyferulate. It is also possible to have a methylation reaction of 5-hydroxy group of 5-hydroxyferulate by OMT to produce sinapate. A corresponding methylation reaction is that of 5-hydroxyferuloyl-COA to produce sinapoyl-COA probably catalysed by caffeoyl-­ COA 3-O-methyltransferase (CCoA-OMT). The CoA thioesters is reduced to a corresponding aldehyde catalysed by cinnamoyl-CoA reductase (CCR). The aldehyde is further reduced to monolignol sinapyl alcohol by the action of cinnamyl alcohol dehydrogenase (CAD) [9, 17, 52, 62, 63]. The degree of methoxylation of the aromatic ring in the monolignols (Fig. 1.12) increases from p-coumaryl alcohol, to coniferyl alcohol, to sinapyl alcohol. This increase corresponds to the decrease in reactive sites on the aromatic ring. The potential sites for crosslinking in the para-coumaryl alcohol are carbons 3 and 5, the side-chain carbons and the 4-hydroxyl groups. In coniferyl and sinapyl alcohol, methoxylation of carbon 3 and carbons 3 and 5, respectively, blocks those sites and reduces the number of potential crosslinks between monomers. In the lignin residues, the R groups on the 4–0 positions are either hydrogen which creates free hydroxyl groups or are cell wall polymers linked through ester or ether bonds. It is also important to note that these monolignols are relatively toxic and unstable compounds on their own, which cannot accumulate to high levels in the plant cells. To stabilise the compounds and render them non-toxic, glycosylation of the phenolic hydroxyl group occurs producing monolignol glucosides. These glucosides may be the source of storage and transport for the monolignols. After the synthesis, monolignols are transported to the cell wall where they undergo enzyme initiated radical coupling reactions (polymerisation) to form l­ ignin. In the process, the ‘end-wise’ reaction couples a monolignol to a growing polymer, giving rise to structures which are usually β linked. A generalised illustration of the radical polymerisation is shown in Fig.  1.13a. The polymerisation starts with an oxidation reaction by peroxidase and/or laccase (dehydrogenation) [51]. A full

1  Chemistry and Structure of Lignin

17

Fig. 1.12  The monolignols and the resulting lignin residues. Reprinted with permission from [32]

scheme showing how the enzyme mediated reaction takes place is shown in Fig.  1.13c. After the oxidation (Fig.  1.13b) the delocalised radical has unpaired electron density at 1-, 3-, O–4-, 5- and β-positions [65–67]. The radical is relatively stable because of the delocalisation of the unpaired electron in the conjugated system. Because the reaction favours a radical coupling at the β-position, this creates a possibility of covalent coupling with another monolignol radical so that after re-­ aromatisation, a mixture of dehydro (dimers) with β–β, β–5 and β–O–4-linkages are formed (Fig. 1.13d). This coupling takes place in a chemical combinatorial manner, so that the ratio of possible outcomes depends largely on the chemical nature of each of the monomers and the circumstances in the cell wall [54]. Polymerisation continues by coupling the β-position of a monolignol radical to the O–4-position of the dimer’s phenolic end. This addition happens in an end-wise (endwise coupling) manner, that is, the polymer grows one unit at a time. Coupling may also occur in the G-dimer but at a low frequency to the 5-position creating β–5and β–O–4-linkages by chain elongation [68]. Coupling of two lignin oligomers is relatively uncommon in S/G lignin. In G-lignin, the 5-5 coupling accounts for roughly 4% of the linkages [69, 70]. It is important to note that when already formed lignin oligomers couple, they form 5–5 and 5–O–4 units links. On the other hand, when two monolignols couple it results in the formation of resinol (β–β) units or

Fig. 1.13 (a) General representation of lignin polymerisation. Adapted with permission from ACS [51]. (b) Example of radical delocalisation during lignin polymerisation. (c) A Modified scheme for Lignin Polymerisation. (d) Dimerisation reactions of monolignols. Reprinted with permission from [54]

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1  Chemistry and Structure of Lignin

Fig. 13 (continued)

19

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Fig. 13 (continued)

cinnamyl alcohol end groups [71]. In the course of the coupling reaction, two radicals are ‘spent’ (this is called a ‘termination reaction’) as each single electron contributes to the newly formed bond. This type of radical polymerisation is intrinsically different from the radical chain reactions in polymerisation of polyethylene, polypropylene and polystyrene. Reale et al. [72] and Stewart et al. [66] estimated the average length of a linear lignin chain in poplar to be between 13 and 20 units. The structure of lignin depends largely on the availability of the monolignol radicals. This indicates that the range of effectiveness of the enzyme (peroxidase) may partly determine the structure of the final lignin polymer. This creates new possibilities for altering the lignin structure based on altered expression of definite peroxidase isoforms. It is also important to note that the β–O–4 (β-aryl ether) linkage is the most frequent inter-unit linkage. This linkage is most easily chemically cleaved, providing a basis for industrial processes and analytical methods. The other linkages (β–5, β–β, 5–5, 5–O–4 and β–1) are more resistant to chemical degradation [17]. The relative contribution of a monolignol to the polymerisation process is what determines the relative abundance of the different linkages. The G unit lignin contains more resistant linkages than the S unit lignin because of the availability of the C5 position for coupling. Lignin polymerisation is extremely plastic such that it allows the incorporation of any phenolic that by chance is within the vicinity of the lignification site subject to its tendency of chemical cross-coupling. This knowledge might be useful

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Fig. 1.14  Monolignol—oligolignol cross coupling. Printed with permission from Springer Nature [54]

in the production of more easily amendable lignin in the quest for high performing carbon fibres [54, 58, 73–77]. The structure of the lignin macromolecule after the radical coupling is random because at every step there are several possible linkages (Fig.  1.14). The figure exemplifies the various possibilities with sites for further coupling reactions shown as dashed lines. S-S coupling predominantly gives rise to β–O–4 units making it an exception [54]. This explains why S lignins have elevated β-ether levels. It can also be seen from the figure that cross-coupling monolignol coniferyl MG or sinapyl alcohol MS with a G polymer unit PG provides only two main products (β–O–4- and β–5-products). This is the reason for the formation of more β-ethers during lignification than dimerisation. The major structural units of the lignin polymer are summarised in Fig.  1.15. Bonds printed bold (Fig. 1.15c) are those formed in radical coupling and others are formed in dimerisation reactions. Table  1.1 summarises the various linkages and

Fig. 1.15  Major structural units of the lignin polymer. Printed with permission from Springer Nature [54]

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Table 1.1  Proportions of the different linkages connecting the phenylpropane unit Linkage type β-O-4 α-O-4 β-5 5–5 4-O-5 β-1 β–β

Dimer structure Phenylpropane β-aryl ether Phenylpropane α-aryl ether Phenylcoumaran Biphenyl and dibenzodioxocin Diaryl ether 1,2-Diaryl propane β-β-Linked structures

Approximate percentage 45–50 6–8 9–12 18–25 4–8 7–10 3

Reprinted with permission from Elsevier® [35]

their approximate percentages. Linkages are said to be condensed when the unit is linked to another phenylpropane unit from the 3 and 5 aryl ring positions Fig. 1.15a). Another important aspect of lignin chemistry is the linkages between lignin and other polysaccharides in the immediate vicinity. During lignification and dimerisation of monolignols, some reactive intermediates such as quinone methides are created. These structures are capable of absorbing water or reacting with alcohol groups on neighbouring polysaccharides [78] such as glucuronoxylan, galactoglucomannan, pectins and cellulose. Reactions between these polysaccharides and lignin occur by the formation of covalent bonds. These bonds are often called lignin carbohydrate complexes (LCC) with the most common types being ethers to the α-carbon (Fig. 1.16), and phenyl glucosides [79]. Recently reported LCC-linkages in native wood are those of benzyl ester, benzyl ether and phenyl glycosidic linkages. Benzyl ester linkage may not survive hydrolysis during pulping since they are alkali-labile. However, benzyl ether and phenyl glycosidic linkages are alkali-stable and may not be hydrolysed during pulping. The possibility of ether type LCC formation during pulping process has been demonstrated with the use of model compounds [80, 81]. It has been demonstrated that LCCs are common in wood and that nearly all lignin molecules are covalently attached to polysaccharides [82]. Another study on the oxidation of residual lignin from spruce Kraft pulp suggested the prevalence of lignin linkage to galactose and mannose [83]. These linkages occur mainly with hemicellulose. A relationship between lignin-hemicellulose complexes in birch Kraft pulp was proposed by Tenkanen et al. [84]. Lignin has been found to covalently crosslink xylan and glucomannan, the two dominating hemicelluloses in softwoods [82]. In some cases, hemicellulose molecules often contain more than one lignin functionality making it easy for extensive crosslinking leading to the formation of large networks [85]. Oinonen et al. [86] also proposed a hypothetical Organisation of lignin–polysaccharide networks showing that lignin has the ability to form large covalent networks with wood hemicelluloses by laccase-catalysed crosslinking (Fig. 1.16b). LCCs contribute greatly to the integrity of plant cell wall and the difficulty of delignifying residual lignin. A study has shown that [87] approximately 90% of residual lignin in unbleached spruce Kraft pulp is linked to carbohydrate. This illustrates the importance of LCC in delignification processes.

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Fig. 1.16 (a) Covalent bonds between lignin and polysaccharides, (b) Lignin–polysaccharide networks and their hypothetical organisation in wood. Printed with permission from Elsevier [86]

The content of LCC is known to be responsible for the difficulty of delignification during the residual stage of cooking [88]. Delignification methods such as enzymatic hydrolysis [89, 90], preparative acidolysis [87] and combination of enzymatic and acid hydrolysis [87] are based on the knowledge of lignin-carbohydrate linkages. Covalent linkages also exist between cellulose and lignin [85, 91–95]. Karlsson and Westermark [96] identified a connection between lignin and cellulose in pine Kraft pulp. One other important aspect of lignin formation and chemistry is the botanical origin of the plant. It is widely believed that softwoods contain the highest amount of lignin (21–29 wt.%), followed by hardwoods (18–25 wt.%) and lastly herbaceous crops (15–24  wt.%). In addition, the variation also arises from the ­difference in tissues, cell types, regions in the cell wall, and between different life stages of a cell. Moreover, lignification is affected by physical stress. All these factors alter the distribution of the lignin monomer thereby affecting the chemistry and structure of lignin [97–106].

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1.3.3  Reactivity of Lignin The dominant linkages between the units in lignin were presented in the previous section and the relative frequencies of some of the functional groups were given in Table 1.1. These linkages between phenylpropane units and the functional groups are responsible for the unique and complex structure of the lignin polymer. H ­ owever, lignin also contains a couple of functional groups that impact its reactivity. These functional groups include methoxyl groups which are about 92–96% per 100 units, free phenolic groups which also occur about 15–30 per 100 units, carbonyls occur 10–15 per 100 units, benzyl alcohols occur 15–20 per 100 units and a few terminal aldehyde groups [88]. In the lignin structure, phenolic hydroxyl groups are occupied in linkages to neighbouring phenylpropane units resulting in very small ­proportion of free reaction sites. However, during the enzymatic dehydrogenation the carbonyl and alcoholic hydroxyl groups are locked into the lignin structure. These all make lignin one of the most complex polymers in history. A good understanding of lignin chemical reactivity is essential to the development of lignin as a sustainable precursor for carbon fibres. Lignin has several reactive sites but the most reactive sites are ether units in α- or β-positions (Fig. 1.17). The following reactive sites are present in lignin: hydrolysable ether linkages (β-aryl, α-aryl and α-alkyl ether linkages), phenolic hydroxyl groups, aliphatic hydroxyl groups, uncondensed units (C2, C3, C5 and C6 being unsubstituted or substituted only by a methoxyl group), unsaturated units (coniferyl alcohol or coniferyl aldehyde end groups and α-carbonyl groups), ester groups and methoxyl groups. Other linkages that are also reactive are the LCCs. In this section, the reactivity of lignin under several conditions is presented.

1.3.4  Lignin Chemical Reactions Lignin structure is usually altered through a combination of de- and re-­polymerisation reactions during extraction and industrial processing. These reactions differ according to the prevailing conditions such as, base-catalysed, acid-catalysed, reductive, oxidative, or thermal. This section provides an overview of the possible chemical reactions under these conditions focusing basically on the β-O-4 motifs. The cleavage of lignin bonds is one of the most important reactions in lignin that is directly related to industrial and analytical processes [98, 99, 106]. Lignin units are frequently connected to each other through the β-O-4 bonds at the β [107]. Although these ethers are usually relatively inert, they are prone to cleavage because of the presence of lone pair oxygen. The lone pairs of oxygen are a source of reactivity increasing polarity of the -Cβ-O- bond, making the neighbouring β-carbon atom sensitive to nucleophilic attacks. On the other hand, resonance structure makes the neighbouring benzene ring more stable. The readiness of the β-O-4 to enter a reaction is dependent on whether the structure is phenolic or non-phenolic (Fig. 1.18).

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b-O-4-linkage (b-aryl ether HO linkage)

OH HO HO

Aliphatic hydroxyl group

OH

O OMe

MeO

HO OH

O

HO

HO

OMe

HO

OMe O

HO

a-O-4 – linkage (a-aryl ether linkage)

O

HO

Methoxy group (-OMe)

OMe O

OMe

HO

OH

O

OH OH

O

MeO HO

O

OMe

Unsaturated unit

OH

HO

OH

O

HO

OMe

Uncondensed unit

MeO O

MeO O HO

Phenolic hydroxyl group

HO

O

OMe

OH

OMe OH

Fig. 1.17  Some reactive sites in lignin Fig. 1.18  Phenolic and non-phenolic lignin structures (for phenolic structures: R is H but for non-phenolic: R is C)

1.3.4.1  Reactions Under Alkaline Conditions When the reaction condition is alkaline (such as Soda pulping), the phenolic group ionises and increases the reactivity of the structure [108–112]. For phenolic β-O-4 structure, the reaction begins with the formation of a quinone methide (Fig. 1.19). This only happens in the presence of a hydroxyl group (OH) or

Fig. 1.19  Cleavage of phenolic β-O-4 linkages during initial delignification. Reprinted with permission from Royal Chemical Society [107]

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ether bond (OR) in the α-carbon of the lignin structure. The reaction results in the removal of α-OH group or cleavage of the α-OR linkage. In the case where the R is lignin, the formation of the quinone methide leads to the depolymerisation of the lignin structure. Following the formation of the quinone methide is an attack of nucleophilic hydrogen sulphide ion (HS-Kraft pulping) which results in the cleavage of the β-O-4 linkage and the formation of episulphide intermediates and lignin degrades [35, 113–115]. These intermediates may experience successive reactions, yielding compounds such as coniferyl alcohol which are disposed to degradation and repolymerisation. Quinone methide is a pivotal point in alkaline lignin chemistry. It is susceptible to nucleophilic attack, because of its propensity to re-establish aromaticity. β-O-4 linkages degrade when the sulphide in α-carbon is ionised. The rate of degradation is dependent on the HS− and HO− concentrations (sulphidity and pH). It should be noted that if the HS concentration does not satisfy equilibrium conditions a competing reaction results. This reaction proceeds by the elimination of a proton or formaldehyde from the quinone methide giving rise to an enol ether structure. Whereas the cleavage of a proton is caused by a hydroxyl ion through bimolecular elimination reaction, the elimination of the formaldehyde group is caused by ionisation of a hydroxyl group in the γ-carbon. These reactions do not result in bond cleavage and lignin degradation. They are therefore unwanted reactions and should be avoided. To attain a good degree of delignification under alkaline conditions the hydrogen sulphide ion concentration must be adequate. When this not the case the competing reactions will occur resulting in the formation of stable enol ether linkages which delay the degradation of lignin. The concentration of HO− ion disturbs both the cleavage of β-O-4 cleavage and enol ether formation reactions. This means that the concentration of the OH ions must be kept at the accepted level at the beginning of the reaction. Another important reactive site in lignin under alkaline conditions is the carbonyl structure. The native lignin structure is made up of groups such as α-carbonyls (keto) and coniferyl aldehydes. Because the carbon atoms in these structures ­possess low electron density, they are potential reaction sites for nucleophiles. These nucleophiles attack the carbonyl structures forming extra products. The scheme for this reaction is as shown in Fig. 1.20b. The β–aryl ether cleavage occurs in the same manner as the reaction of phenolic structures. The reaction is independent of hydroxyl ion concentration but on HS− concentration and happens very fast. For non-phenolic lignin structures, reactions proceed as shown in Fig.  1.20c. Although non phenolic structures are less reactive sites under alkaline conditions, at high temperatures these structures become highly reactive and can partake in some reactions. Cleavage of β-O-4 linkages in non-phenolic structures occurs if hydroxyl groups in α- or γ-carbon are ionised. Depending on the degree of ionisation these ionised structures act as a nucleophile, attacking β-carbon thereby causing the cleavage of the aryl ether linkage. Another lignin structure that can go into reaction under alkaline conditions is the methoxy group. Because sulphur nucleophiles (HS−) are much more reactive than

1  Chemistry and Structure of Lignin R

OH HO-

29

O–

H O OMe OMe

K5

O

OMe

OMe

OMe O

KQM

R

K5

O

QM

O

R

OMe 5

O–

R

HO O OMe OMe –

O

Fig. 1.20 (a) A competing reaction to the cleavage of phenolic β-O-4 linkages. (b) Cleavage of β-O-4 linkage by reaction of the carbonyl structure. (c) Cleavage of non-phenolic lignin structure. (d) Demethylation of the methoxy group in lignin

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HO HO

HO

O

R

HO O

OMe O

Me

OMe O–

HS-

OR

MeS-

OR

Fig. 20 (continued)

oxygen nucleophiles (HO−), the presence of HS− causes demethylation of the methoxy groups of lignin (Fig.  1.20d). The reaction produces methyl mercaptan which is capable of forming dimethyl sulphide and disulphide with further nucleophilic and oxidative reactions. These compounds are responsible for the typical odour of the Kraft cooking process. There are indications that nucleophilic substitution can occur in the β-aryl ether structure. However, this is less likely than the demethylation because the methyl group is sterically less hindered than the β-carbon. Other reactions of lignin under alkaline conditions are carbon-carbon (examples are β- and γ-carbons of phenolic lignin) cleavage and condensation reactions. The carbon–carbon bonds are stable in alkaline media with only very limited possibilities of cleavage. Bond cleavage in this case usually results in the formation of formaldehyde and enol ether or formaldehyde and chromoric stilbene or stilbene-like structures. Condensation occurs in a variety of ways during pulping. Studies [116] have shown that condensation reactions usually occur at the unoccupied C-5 position of the phenolic unit. 1.3.4.2  Reactions Under Acidic Conditions Acidic conditions affect the lignin structure by promoting both autocatalytic depolymerisation (i.e., acidolysis) and repolymerisation. Cleavage of the β-O-4 acidolysis under acidic conditions starts with the formation of a benzylic carbenium ion by removing the OH-group from the α-position (Fig. 1.21). The intermediate carbenium ion changes into two possible enol-ether structures. This happens with or without cleavage of the Cβ–Cγ bond and simultaneous creation of formaldehyde (Fig.  1.21). The β-O-4 is hydrolysed to form new phenolic lignin (C2-aldehyde-­ substituted phenolics) units and a ketone type substructure (C3-ketone-substituted phenolics) which is also called the Hibbert’s ketones. These ketones, together with the intermediate carbenium ions and C2-aldehyde-substituted phenolics take part in a complex network of repolymerisation reactions, yielding a condensed lignin polymer [68, 99, 117–126]. The cleavage pathway is generally dependent on the type of acid used. When the media is H2SO4 then, 13b intermediate is formed but if HCl or HBr is used 13a is the main intermediate (Fig. 1.21).

Fig. 1.21  Acid-catalysed lignin chemistry. Reprinted with permission from Royal Chemical Society [107]

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In the presence of mild acidic media, the homolytic cleavage of the phenolic acylglycerol β-aryl ether bonds is a significant reaction especially for hardwood because of the presence of syringyl lignin monolignol. The reaction proceeds via homolyses of an intermediate quinine methide under elevated temperature. This pathway is therefore prevalent in processes such as steam hydrolysis and steam explosion. 1.3.4.3  Reactions Under Reductive Conditions In lignin valorisation, depolymerisation can be obtained under reductive conditions. In this case a redox catalyst is used in combination with an H2 or an H-donor. The process is usually targeted at the inter-unit ether bonds (β-O-4, α-O-4) and side-­ chain hydroxyl groups. Some authors have reported that the reductive pathway occur either by hydrogenolysis of ether bonds, or removal of benzylic OH-groups (OHα), or removal of OHγ-groups [127–130]. A reaction pathway for reductive cleavage of β-O-4 has been proposed by Lu et al. [131] using Pd(III) as the catalyst. The reaction proceeds by dehydrogenation of the dilignol reactant on the α-carbon and then on the –OH group to produce a corresponding ketone 2-phenoxy-1-­ phenylethanone. The ketone is further dehydrogenated on the β-carbon. This happens by initial equilibration of keto–enol tautomerisation to its enol form followed by –OH dehydrogenation. This is accompanied by the cleavage of the C–O ether bond giving rise to one-aromatic-ring surface intermediates, then hydrogenation to produce acetophenone and phenol (Fig. 1.22). It is very important to note that these reductive catalytic systems possess the ability to quench reactive functional groups that are disposed to condensation. Functional groups such as alkenyl and carbonyl groups are mostly affected. This means that reductive conditions have the capacity to avoid repolymerisation to a certain extent which contrasts with alkaline or acidic media. Several studies have also shown that most reductive processes are unable to cleave carbon–carbon linkages. 1.3.4.4  Reactions Under Thermal Conditions Thermal cleavage of β-O-4 linkage is one of the most challenging areas in the chemistry of lignin. Holmelid et  al. [132] used representative lignin-like models to examine the mechanisms of lignin hydrodeoxygenation during thermochemical conversion. The model dimers were found to fragment through a homolytic cleavage [133, 134] of the inter unit linkages producing radical pairs. Liu et al. [135] also used model compounds of G-type lignin to investigate the thermal cleavage of β-O-4 linkage and proposed a pathway as shown in Fig. 1.23. Using five β-O-4 and two α-O-4 linked molecules lignin model compounds, Choi et al. [83] investigated the thermal deconstruction pathway of lignin under thermal conditions. They proposed concerted retro–ene fragmentation and homolytic dissociation as the initial reaction step for the β-O-4 compounds and α-O-4 compounds. A concerted retro–ene

Fig. 1.22  A case of reductive cleavage of β-O-4 linkage. Reprinted (adapted) with permission from [131]

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Fig. 1.23  The possible pyrolysis pathways of a G-type lignin model structure. Adapted from [135]

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fragmentation was also reported in the pyrolysis of 2-phenethyl phenyl ether (β-O-4 lignin model compound) at temperatures from 300 to 500 °C in the presence and absence of a radical scavenger (tetralin) [136]. For Jarvis et al. [137] a concerted retro–ene and Maccoll reactions are the dominant reactions for lignin pyrolysis temperature between 500 and 600 °C but at higher temperatures (over 1000 °C), C–O homolytic bond scission reaction is dominant. Kawamoto et  al. [138] in support of the mechanism proposed that β-ether bond homolysis is what initiates the radical reaction. Elder and Beste [139] realised that the activation energy of retro–ene fragmentation for fully substituted β-O-4 compounds was lower than that of bond dissociation energies associated with homolytic cleavage. Britt et  al. [140] also reported homolytic cleavage of the β-O-4 linkage in phenethyl phenyl ether (PPE) model compounds by methoxy substituents to produce styrene plus phenol and minor amounts of toluene, bibenzyl and benzaldehyde. The methoxy-substituted phenoxy radicals undertake a series of reactions subjugated by 1,5-, 1,6- and 1,4-intramolecular hydrogen abstraction, reordering and α-scission reactions. Chu et al. [141] proposed a free radical reaction pathway to explain the results obtained in their study of the pyrolysis behaviour of a β-O-4 type oligomeric lignin. Vuori et al. [142] used a guaiacol monomeric model compound to study the pyrolysis behaviour of lignin and proposed a free radical and a concerted reaction mechanism. Schlosberg [143] and Masuku [144] also proposed free radical mechanism to explain the pyrolysis of monomeric lignin model compound. It is important to note that most of these propositions utilise model compounds, temperature, pressure, apparatus characterisation techniques and theoretical methods which often differ from each other, as such, definitive conclusions cannot be made. Moreover, most of these studies used lignin model compound which have simple structures and product distributions in contrast to native lignin. With different side chains or substituted groups, the mechanism will likely be different. Most studies on the thermal cleavage of the β-O-4 linkage and lignin model compounds present two predominant mechanisms, namely (1) homolytic bond cleavage, and (2) concerted cleavage via retro-ene fragmentation. Based on these mechanisms Huang et al. [145] proposed a simplified reaction pathway for the pyrolysis of a dimeric lignin model compound (Fig. 1.24). 1.3.4.5  Reactions Under Oxidative Conditions Oxidative cleavage of the β-O-4 linkage of lignin is one of the most important ­reactions in the processing of lignin for industrial applications, especially in the production of lignin carbon fibres. Oxidative functionalisation and subsequent valorisation of lignin holds the possibility to yield highly functionalised, monomeric or oligomeric products that may be starting points for valorisation processes in the chemical and pharmaceutical industries [3]. In thermal stabilisation process of lignin carbon fibres, oxidation reactions are the governing factors for the resulting properties of the fibres. Lignin reactions under oxidising conditions have been reported by many researchers [146–164]. Some of these oxidants include transition metals,

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Fig. 1.24  Reaction pathways for a model dimeric phenethyl phenyl ether (PPE) compound. Adapted with permission from Elsevier [145]

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chlorine, hydrogen peroxide, ozone, oxygen, chlorine dioxide etc. A comprehensive review of the various oxidants and their reactive pathways has been presented by Ma et al. [161]. Most prevalent oxidation mechanisms occur by the attack of electrophilic species (initiated by electrophilic reactions), such as Cl+ (from chlorine), OH+ (from peroxyacids), or oxygen, on sites of high electron density, such as ortho, para, or Cβ positions in lignin. In the case of oxygen, the reaction starts by addition of oxygen to the phenoxy radicals in ortho, para, or Cβ position, leading to the formation of peroxy anions. These anions afterwards change through several routes, resulting in four possibilities including cleavage of the Cα–Cβ bond, C4–Cα cleavage, development of oxirane structures and cleavage of the aromatic ring. The case of peroxy acids occur in six main types of reactions including ring hydroxylation, oxidative aromatic ring cleavage, side-chain substitution, demethylation on aromatic ring, cleavage of ether bond and epoxidation of olefin in ring-conjugated structures. Peroxy acid oxidation of monolignol and β-aryl ether model compounds proceeds by the direct production of phenolic monomeric fragments (benzaldehydes and benzoic acids) which occur by dehydration reactions and double bond cleavage. In addition, ether bonds are hydrolysed in the acidic media of peroxy acid solutions [165, 166]. Figure 1.25 shows in summary the prevalent pathways for the oxidative reactions in lignin. Dependent on the oxidant and other conditions, reactions occur usually by side-chain fragmentation (Cα–Cβ or C4–Cα bond cleavage) or disruption of the aromatic ring. Whereas cleavage of the carbon-carbon bonds retains the aromatic structure, the disruption of the aromatic structure yields aliphatic carboxylic acids [116]. Another mechanism is the radical reaction mechanism which occurs mostly in alkaline aerobic oxidation initiated by oxidation of the phenolate ions into phenoxy radicals.

Fig. 1.25  Oxidative lignin chemistry. Reprinted with permission from ACS [107]

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1.4  Lignin Colloid and Surface Chemistry The surface and colloid chemistry of lignin are additional properties that are of interest if lignin is intended for bulk processing into valuable products. Producing fibres from lignin will require that lignin either dissolves or becomes molten to be drawn. Moreover, lignin is often blended with other polymers to improve its processability. How polymers dissolve in solvents and their free energy of dissolution is an important fundamental principle in polymer science. Surface energy and wettability are also important aspects of surface chemistry. For lignin, surface energy and wettability are important for certain industrial applications. Applications such as papers, cardboards and engineered wood products in some cases are made from wood fibres containing lignin at the interface [167, 168]. The surface energy of this lignin rich surface has a direct effect on the capillary forces during the drying and consolidation phases which in turn affect the strength of the material. For medium density fibreboards, it is important to carefully consider the interfacial energy so as to develop a strong adhesion between the fibre and the resin used. In ceramics, lignin has been used as dispersants which means they are localised at the interfacial region. It is evident, that a full understanding of lignin interfacial properties is a necessary requirement [6, 169, 170].

1.4.1  Lignin Colloidal Chemistry The chemical structure of lignin is not clearly defined because of the difference in the structure arising from the difference in sources and at large on the difference in the chemistry of the extraction methods used. The extraction methods usually leave lignin with structural changes, depolymerisation and introduction of external functional groups during bond cleavage. With such a large difference in chemical structure, it is expected that the solubility and interfacial properties of lignin will be severely changed. Lignin molecule in solution can be viewed as consisting of a strongly immobilised network core surrounded by a loose surface region with ­possible local chain mobility [171]. They behave as either lyophobic or lyophilic colloids depending on the interaction with the solute species. Due to the occurrence of irregular particle structure, intermolecular and intramolecular association, charge distribution and ion exchangeability, quantitative interpretation of lignin colloidal behaviour is challenging. Generally, it is accepted that lignin behaves as a spherical, amorphous macromolecule having polydisperse molecular weight distribution [172, 173]. Lignosulphonate molecules have been viewed as a polyelectrolytic microgel. The polyelectrolytic behaviour was also reported for alkali lignin. They possess ability to aggregate, coagulate and swell in various solutions. The aggregation behaviour of Kraft lignin in aqueous solution is linked to the undissociated ­carboxylic groups and long range van der Waal’s forces [174]. The coagulation behaviour of Kraft lignin has also been linked to the long range van der Waal’s forces [171]. Lindström and Westman [175] also reported that Kraft lignin gels swell (macrosyneresis and hysteresis swelling) typical as a polyelectrolytic network

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with the degree of swelling increasing with an increase in the number of ionised groups and decreasing with an increase in salt concentration. They reported that the crosslinking structure of lignin is largely dependent on the state of dissociation of the introduced carboxylic groups and their capability to engage in intermolecular hydrogen bonding in the network structure [176]. Macromolecular Kraft lignin may undergo phase transition and enter the colloidal state dependent on the solution conditions. The formed colloidal particles coagulate under conditions of decreased hydroxide concentration or increased ionic strength or increased temperature. Lignin flocs with fractal structure viewed in Fig. 1.26a are formed after several minor steps during the aggregation. When the solution condi-

Fig. 1.26 (a) Cryo-TEM of unfractionated KL solutions showing fractal clusters, (b) A schematic representation of the modes of aggregation in Kraft lignin systems. Adapted with permission from [180]

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Fig. 1.27  A schematic illustration of de-aggregation and solubilisation of Kraft lignin particle clusters. Reprinted with permission from Elsevier [179]

tions are gradually changed the macromolecular Kraft lignin fragments begin to self-associate forming colloidal particles [177, 178]. The particles grow in number and size with time, and eventually forms cluster structures (Fig. 1.26b). The rate of aggregation controls the compactness of the fractal ­structures. These aggregates can be re-dissolved (Fig. 1.27) if proper additives are introduced to the lignin solution thereby stabilising the lignin structure [179]. The de-aggregation behaviour is attributed to the presence of hydrophobic sites in the lignin structure. The salt used associate at the hydrophobic sites increasing the associate hydrophilicity. The efficiency of the salts stabilisation and re-dissolution of the Kraft lignin structure is determined by the hydrophilic–lipophilic balance in the different salts. Lignosulphonates are rarely used in industrial applications because of the lack of better understanding of their colloidal behaviour. As a polyelectrolyte, lignosulphonates contain both hydrophobic and hydrophilic groups and dissolves easily in water. Several theories have been proposed to explain the solution behaviour of lignosulphonates. They have been described as an oblate shape macromolecule [181], spherical macromolecules [182], non-spherical shaped molecules [183, 184], oblate spheroid-shaped molecules with an axial ratio of 3.5 [185], and a randomly

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branched polyelectrolyte [186] with the molecule coiling in solution to make ball-­ shaped molecules having sulphonic groups enriched surfaces. Lignosulphonates are said to be amphiphilic molecules which may aggregate in aqueous solution when dynamic and thermodynamic conditions are favourable, for example, when the concentration of lignosulphonate is above critical aggregation concentration. Yan and co-workers [187] explained that lignosulphonates are core-structured molecule with a central hydrophobic core. The surface of the lignosulphonate molecules and aggregates are covered by sulphonic groups and a few phenolic hydroxyl groups whereas the core contains the carboxyl groups. Because the core of the lignosulphonate aggregates contains carboxyl and phenolic hydroxyl groups, the degree of ionisation of these weakly ionised groups increase as the pH increases, and the core of the aggregates swell arising from electrostatic repulsion (Fig. 1.28). The distribution of sulphonic groups on the surface of the molecule and aggregate is not uniform (Fig. 1.28a). Through hydrophobic effect, the portion of the aggregate which has fewer amounts of sulphonic groups adsorbs at air/liquid interfaces. This means that the aggregate can decrease the surface tension of water. When the pH value increases (Fig. 1.28b), the core of the aggregate expands because of electrostatic repulsion, and the hydrophobic chain stretches, resulting in reduced viscosity. Due to expansion of the aggregates, more water molecules have access to the loose hydrophobic core, the number of hydrophobic core which absorbs at air/liquid interface increases, thus the surface tension increase. On the other hand, because the core expands the diameter of the aggregate particles also increases. The potential aggregation phenomenon of organosolv lignin in solution was studied by Gilardi and Cass [188]. They identified that organosolv lignin was polydispersed in dioxane water mixture and forms aggregates. However, the aggregates did not grow in size until precipitation occurs. The authors proposed that the tendency to achieve maximum solvation of the polar side chains by water molecules while sustaining positive interactions of the non-polar parts of the molecule as the driving force for the aggregation. This is an important insight into the enzymatic biodegradation of organosolv lignin. Rao et al. [189] reported that colloidal spheres of organosolv lignin (in ethanol water solution) assemble spontaneously through steady-state hydrophilic–lipophilic aggregation. They proposed that the mechanism of aggregation begins with the hydrophobic parts of the lignin aggregating into spherical cores in the solution, and the hydrophilic regions forming shells via the van der Waals interactions of the aliphatic groups and the π–π connections of the aromatic groups.

1.4.2  Lignin Surface Chemistry Several studies on the surface structure of lignin using equipment such TEM, SEM, AFM, ESR, GPC and STM conclude that lignin consists of globular-shaped macromolecular aggregates which eventually become larger forming a semi-ordered superstructure by rational assembly [190–193]. There are affirmative evidences

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Fig. 1.28 (a) The schematic structures of lignosulphonate aggregates (b) Schematic illustration of the effect of pH on the configuration of the aggregate and monomeric states of the lignosulphonate aqueous solution. Reprinted with permission from Elsevier [54]

indicating that individual lignin globules consist of separate regions having divergent motilities [194–198]. Micic et al. [199] studied the interaction of these globules using a model compound and reported that there exist strong attractive intermolecular forces between the lignin macromolecular globules. Further investigation showed that the globules possess onion-like layered structure with much of the gaps between the globules being filled with the functional groups or surrounding solvent.

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They also proposed six orders of magnitude of scale range for the development of lignin globules ranging from nanometer to a fraction of a millimetre. The proposed mechanism for the formation of lignin globules include formation of modules of about 20 monomers, followed by polymerisation of the modules into supermodules containing about 500 monomers. These supermodules subsequently aggregate into globules and finally, form globules clusters and large superstructures [200]. Based on the studies of Micic et al. [200], it can be deduced that self-ordering property of lignin is both intrinsic and substrate facilitated. As the superstructure of lignin is developed, the adjourning forces sustaining the subunits begin to weaken. At the beginning they may be covalent, but as the growth progresses they become intermolecular (hydrogen bonding and Van der Waals interactions) especially for latter supramolecular structures. The electronic-π orbital structure and the hydrophobic/hydrophilic nature of substrate surface are major mediators for of the final shape of lignin globule assembly. Several other studies have also attempted to determine the preferential orientation of lignin molecules using several standard methods [181, 190, 195, 201–205]. Raman images of wood tissues show that lignin molecules are organised such that phenyl rings are oriented preferentially along the cell wall surface [205]. Another study also shows that lignin molecules also form planes [206]. This gives them ability to cover rough surfaces proving its usefulness in applications where smooth surfaces are required [207]. Studies on condensed Langmuir films show that lignin aromatic rings are arranged somewhat vertical to air-water interface [188, 190, 202]. All these studies picture lignin as a spherical disc-like molecules behaving as a sphere in solution with the tendency of self-organisation on substrates depending on the type of substrate and its interaction with the sample [181, 200]. It can be concluded from these that some lignin form very flat surfaces whereas some do not. This difference in surface structure may be due to the difference in the chemical structure of lignin arising from the difference in extraction methods. Pasquini et al. [207] attempted to clarify how the extraction processes affect the surface properties of lignin using Langmuir-Blodgett films. The Langmuir-Blodgett films of ethanol lignin were found to form ellipsoid aggregates being preferentially oriented parallel to the substrate, whereas saccharification lignin form ellipsoid aggregates being preferentially oriented perpendicular to the substrate surface. In both cases, the films were anisotropic which appear rougher for saccharification lignin. Further studies by Pasquini et al. [208] using four extraction methods showed that the presence of strong polar groups has a major effect on molecular arrangement of the films. Higher content of carbonyl groups led to lower surface potentials making the surface rougher. Norgren et al. [209] realised a stable lignin films in air and aqueous solutions with relatively low surface roughness. Notely and Norgren [167] showed that the surface energy of lignin films is dependent on the charged chemical groups introduced during the extraction process. In another study, the same group [168] reported a higher surface potential for lignin films than cellulose owing to the increased number of carboxylic groups on the lignin molecular structure introduced during the pulping process. The interface between cellulose and lignin was found to be controlled by electrostatic forces. At higher pH (>9) short

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range, steric interaction was detected. Constantino et al. [210] also showed that the surface of lignin Langmuir-Blodgett film was found to be smooth, comparable to that of pure cadmium stearate (typical amphiphilic compound). Hollertz et al. [211] attempted a quantitative estimation of the dispersive interactions between, lignin and hemicelluloses. The study showed that hemicellulose films were homogeneous and had lower surface roughness than lignin films. A more recent study on the surface property of lignin in relation to soybean proteins showed that lignin exhibits strong nonspecific interactions with soy proteins. The absorption of lignin on the soybean protein was found to be better than with cellulose [212].

1.5  Conclusion Lignin is a very irregular, randomly crosslinked polymer of phenylpropane units joined by many different linkages. It occurs as an amorphous, polyphenolic material arising from enzyme-mediated dehydrogenative polymerisation of coniferyl, synapsyl and p-coumaryl alcohols monomeric units. Because of the large difference in lignin structure in regard to their source and extraction process, the chemistry of lignin is somewhat diverse and requires specific attention in each case. Important to the development of carbon fibres from lignin is the molecular, colloid and surface chemistry of lignin. However, the extents of knowledge in these areas are somehow limited and deserve attention of the research community.

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Chapter 2

Lignin Conversion to Carbon Fibre Oluwashina Phillips Gbenebor and Samson Oluropo Adeosun

2.1  Introduction Lignin is the most abundant renewable aromatic polymer whose residues are abundantly produced as by-product in pulp and papermaking. Because of its relatively low cost, high abundance and renewable characteristics, many studies are being taken to develop products of higher value from lignin [1–3]. One of such products is carbon fibre (CF) which has been found to possess much higher added value than other products produced from lignin. CF is an important component of composite materials owing to its unique chemical and physical properties [4]. The production of carbon materials from lignin is difficult as it is highly substituted with oxygen functional groups, which makes the material highly prone to crosslinking reactions for the formation of anisotropic carbons. These groups must be removed while retaining the desired aromatic groups required to produce carbon materials. One of the approaches that have been taken to do this was to first pyrolyse the lignins from wood to produce a tar followed by thermal and/or catalytic treatment of the tars to produce a material that could be a substitute for mesophase pitch produced from petroleum. The secondary pyrolysis of tars is known to reduce the amount of functional group substituents as well as the average molecular weight [5–8]. A two-stage reactor system has been developed [8] with two-stage fixed bed for the lignin pyrolysis/tar cracking. In this, there were two separate pyrolysis zones with independent temperature regulation capabilities having the ability to monitor the product tars online with Fourier-transform infrared spectroscopy (FTIR) while helium is used as a gas carrier. The product stream is extracted out through a filter by means of a peristaltic pump through a line that bypasses the multi-pass cell. The tar component of the products is collected on the filter (cotton wool), washed out with acetone, dried,

O. P. Gbenebor · S. O. Adeosun (*) Department of Metallurgical and Materials Engineering, University of Lagos, Lagos, Nigeria © Springer Nature Switzerland AG 2019 E. I. Akpan, S. O. Adeosun (eds.), Sustainable Lignin for Carbon Fibers: Principles, Techniques, and Applications, https://doi.org/10.1007/978-3-030-18792-7_2

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weighed and analysed. This reactor was used primarily for experiments where lignin was pyrolysed in the first stage at a constant heating rate and the tars were passed to an isothermal second stage. Catalysts that have been identified for the successful lignin extraction include Alcoa (Type F-20) chromatographic grade 80–200 mesh activated Alumina (EM Science), calcium oxide. Alumina has a high affinity for polar materials and it selectively removes oxygen functions from lignin tar and can readily be regenerated once coke formation has occurred. Lignin is often left as a low-value by-product or waste as it is either burned to produce heat for running processes and to recover pulping chemicals in paper mills or sold as a natural component of animal feeds in wet or dry corn mills [2, 10, 11]. It is in the light of this that studies are on towards more efficient processes for the conversion of lignin into biofuels, biochemicals and biomaterials [12]. Lignocellulose is a composite material synthesised by plant cells and provides structural rigidity and protection against biological and chemical assault for plant. Its natural degradation potential poses very strong challenges to use it as a viable and cost-­ effective source of biofuels. Lignocellulose consists of cellulose, hemicellulose, lignin and small amounts of pectin, inorganic compounds, proteins and extractives, such as lipids and waxes [13]. Lignocellulose can be divided into three categories, that is, softwood, hardwood and grass. Lignin is processed into CFs through a series of processes, including extraction, purification, melt processing into fibres, stabilisation and carbonisation. In some cases, lignin has to be graphitised to obtain a graphitic structure. In the production of activated carbon, lignin is either carbonised before activation or activated and carbonised at the same time. Figure 2.1 shows the flow chart of lignin processing into final products. These processes are briefly introduced in this chapter for appreciation.

Fig. 2.1  Lignin processing route [9]

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2.2  Lignin Extraction Lignin exists in combination with cellulose and hemicellulose with small amount of pectin, wax and other external substances [14]. These polymeric compounds are connected to each other in some ways (Fig. 2.2). For example, lignin is connected to hemicellulose via the lignin carbohydrate complexes (LCC). To extract lignin from biomass, a pre-treatment step is usually required to break down the linkages that exist between lignin and carbohydrate components in the feedstock. Generally, there exist two groups of pre-treatment where the first group focuses on removal of lignin into solution before hydrolysing the carbohydrate fraction. The second group first converts the carbohydrate fraction before removing the lignin as solid residue. Removal of lignin makes the cellulose more accessible to enzymes [15]. Examples are alkali pre-treatment [16], wet oxidation [17] ozonolysis [18], biological treatment [18] and organosolv processes [15, 16]. However, wet oxidation, ozonolysis and biological treatment processes somewhat degrade the lignin, and this is not good when it is to be used in other applications. Organosolv lignin isolation involves the use of organic acids with or without a catalyst. This treatment could either be ethanol pulping (Alcell process) or pulping with acetic acid in the presence of small amount of sulphuric acid or hydrochloric acid [19, 20]. The organosolv process is a promising chemical pre-treatment approach used for the wide extraction of lignin from biomass via organic solvents or mixtures of organic solvents with water (ethylene glycol, butanol–water, benzene–water and

Fig. 2.2  Connection between lignocellulose components (U.S. Department of Energy Genomic Science program and the website https://genomicscience.energy.gov)

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ethanol–water) [15, 16]. Usually, these mixtures are used along with acid catalysts, such as HCl, H2SO4, oxalic, salicylic, or Lewis acids, to cleave hemicellulose bonds [15, 21, 22]. The most well-known organosolv process is the Alcell process that has been used at a semi-commercial scale [23]. An alternative organosolv process developed by the French company CIMV uses wheat straw treated with formic acid/ acetic acid/water mixture (30/55/15, v/v/v) for 3.5 h at 105 °C under atmospheric pressure [24] to obtain lignin which is recovered from the solvent by precipitation (typically adjusting concentration, pH and temperature), filtration and drying. The organosolv approach is superior to other processes as it recovers relatively pure lignin as a by-product that is devoid of sulphur and ash [25]. Lignins produced through organosolv process are known to be readily depolymerised than other industrial lignins [26]. But this type of lignin also suffers from degradation and re-condensation, often when acid or base catalysts are used. In some cases, these organosolv lignins are even more difficult to valorise than kraft lignin [27]. The Björkman process [28] is another process that isolates lignin with little structural change. It is extracted from finely ball-milled wood (MWL) using a neutral solvent of dioxane/water (9/1, v/v). This type of lignin is comparable to native lignin and is used as basis for the classification of native lignin–lignin and lignin–carbohydrate interlinkage structures. The second group of pre-treatment methods, namely, hydrolysis by acids, enzymes, or a combination of both are commonly used to convert carbohydrates into fermentable sugars. Full hydrolysis of cellulose and hemicellulose needs fully concentrated acid solutions. The Klason process that uses 72% sulphuric acid is utilised for this purpose [26]. The insoluble acid residue obtained is mainly lignin and ash while the ash-free fraction is known as Klason lignin. This process is generally used for composition analysis of the lignocellulosic biomass. However, it is not suitable for biorefining because the lignin structure has been severely altered [26]. Enzymatic hydrolysis is a simpler process to produce lignin with little structural alteration. Diluted acid pre-treatments, namely, phosphoric acid, sulphuric acid, hydrochloric acid, are used to solubilise the hemicellulose fraction of the biomass to improve accessibility of cellulose to enzymes [15]. Studies have also discovered that an improved lignin isolation process using enzymatic hydrolysis with a mild acid hydrolysis [18, 21, 22] has been proposed in which the initial enzymatic hydrolysis removes most of the carbohydrates, while the mild acid hydrolysis cleaved the lignin–carbohydrate bonds. However, the process of enzymatic Kraft (and soda) lignin produced in alkaline conditions shows more pronounced fragmentation of the lignin that prevented the production of CF particularly with softwood Kraft lignin where char on heating is obtained [29, 30]. This is attributed to the absence of lignin fraction with softening properties that could act as a plasticiser. CF has been produced from purified industrial hardwood kraft lignin. The lignin was thermally treated at 145  °C for 60  min in a vacuum atmosphere to remove volatile contents. This resulted in an increase in molecular mass. A small amount of polyethyleneoxide (PEO) was added as a plasticiser to improve lignin spinnability; otherwise, self-fusion of the lignin fibres would occur at large (>10%) addition of PEO. The next process involves thermo-stabilisation of

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the lignin fibres at 250 °C for 60 min in air followed by carbonisation at 1000 °C to produce CF. If the lignin fibre is thermally treated at 250 °C with heating rate of 12  °C/h from 145  °C followed by carbonisation, an improved CF strength characteristic would be achieved. This improvement could also be obtained through 5% poly(ethyleneterephthalate) (PET) addition to the lignin prior to thermo-­ stabilisation and carbonisation [31]. In either technique, lignin is isolated, purified/ modified and subjected to melt spinning. A comprehensive explanation of the extraction processes can be seen in Chap. 3.

2.3  Melt Processing of Lignin The spinning process which generates fibres from precursor materials may be wet-, melt- and dry-spinning. Examples of spun fibres are shown in Figs. 2.3, 2.4, 2.5, and 2.6. The choice of type of spinning depends on the material to be spun. The wet-­ spinning technique uses a solvent which creates a viscous solution of the materials to be spun. This solution spinning dope as it is popularly called is extruded into fibre via a nozzle into a non-solvent containing bath with coagulating effect on the extruded fibres. The washing of the filaments occurs in several stages to remove and recover the solvent, after which the fibres are dried and wind up [32]. The dry-jet wet spinning differs from wet spinning only in the presence of a small air gap between the nozzle and the surface of the coagulation bath [32]. The melt-spinning process is most often the choice as it does not require the use of solvent [2] and is done at temperatures between the melting and decomposition of such material [33]. Lignin precursor with thermoplastic attributes such as pre-fibres from pitch are produced through melt spinning. Cellulose material which decomposes without melting cannot be melt spun. The thermoplastic material to be spun is dried, melted in the extruder, the melt is filtered/homogenised and then passed into the quench chamber via narrow channels where fibres are formed by solidification of the shaped melt. These fibres are drawn to obtain thinner material and surface finish (surface treatment) is applied before the fibres are wind up [34]. Surface treatments are applied as end steps to avoid static behaviour, entanglement, adhesion and fusing of fibres [35]. When PEO is used as a plasticiser, an improved spinnability of the hardwood lignin is obtained. However, this must be followed with thermo-stabilisation in air at low heating rate to prevent self-fusing of the filaments. The spinnability of acetic acid lignin produced using acetic acid pulping of birch wood improved after a thermal treatment under reduced pressure to modify the lignin structure. This resulted in an increase in average molecular mass at fixed methoxyl and acetyl group contents. The use of softwood using this technique does not yield fusible lignin [29, 36]. The generation of lignin fibres with and without addition of polymer through an electrospinning and extrusion processes has been done. Blended lignin fibres produced from soda hardwood lignin and PEO using an extrusion process is found suitable as carbon precursor. The wood pulp was initially modified using acetylation to avoid agglomeration of wood pulp in polymeric solution when

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Fig. 2.3  Wet spun fibres. (a) Continuously spooled fibers. (b) SEM images of fiber surface. (c) SEM images of fiber cross-section. (d) SEM images of fiber cross-section under high magnification. Reprinted with permission from RCS [53]

electrospinning. The extrusion method was used to develop bio-composite fibres (wood pulp/polypropylene). The procedure adopted is given in the following path: crushed wood pulp − solution preparation (wood pulp + polymer + solvent) − electrospinning − alignment of composite fibres. The lignin fibres were produced by blending the lignin and synthetic polymer using heat and the mixed was thermally extruded in a Rheometer equipped with a 1 mm spinneret. Extrusion temperatures were selected depending on the type of polymer blend. A lot of studies have been done on the spinning of lignin fibres including electrospinning [37–45], gel spinning [46], dry spinning [47] and melt spinning [48–56].

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Fig. 2.4  Dry spun fibres. (a) Ace-SKL as-spun fibers, and (b) carbon fibers. Schematics of fiber cross-section displaying: (c) convex/sharp notches and (d) concave/smooth notches. Reprinted with permission from Elsevier [57]

2.4  Stabilisation of Lignin Fibres The stabilisation process, which is carried out at 200–350 °C in an oxidative atmosphere sometimes under tension application the filaments, makes the filaments thermally stable as it prevents fusing and melting of the fibres during the carbonisation [60]. The structural configuration attained in this process depends on the pre-fibre material (e.g. Polyacrylonitrile (PAN), pitch, rayon, lignin), which usually possess different constituents and structures. The purpose of the applied tension is to improve the tensile strength characteristics for the final CFs [61, 62], as has been observed in PAN-based CFs. Prevention of fibres merging/sticking/agglomeration has been achieved through reactions, such as oxygenation, dehydrogenation and cross-linkage [60]. However, mass transfer of oxygen in the fibre structure is observed and this makes the issue of time and temperature important factors to be considered in some of these reactions. The time spent in stabilisation process depends on the nature and characteristics of the pre-fibre used and this determine overall time requirement for CF production [60]. The stabilisation time has been put to range from 30  min to several hours. Lignin from different sources has been

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Fig. 2.5  Electrospun fibres.  (a) ALFPt (bar length: 5 μm), (b) ALSFPt (bar length: 5 μm), (c) ALCFPt-600 (bar length: 1 μm), and (d) ALCFPt-900 (bar length: 1 μm). Reprinted with permission from Elsevier [58]

successfully stabilised by several researchers, including Chatterjee et  al. [63], Brodin et al. [64], Norberg and co-workers [65], Poeppel and Frank [66], Zhang et al. [67], Lin et al. [68], Lai et al. [69], Brodin et al. [70], Maradur et al. [54], Qin and Kadla [71], Luo [72], Otani and co-workers [47], Thunga et  al. [59], Zhang [73], Eckert and Abdullah [74], Li et al. [75] and Olsson et al. [76]. A comprehensive description of the stabilisation process is given in Chap. 7.

2.5  Carbonisation of Lignin Fibres The carbonisation process is done after fibre stabilisation and it involves fibres treatment in an inert atmosphere at temperatures  8. The precipitated calcium sulphite is filtered and removed [57]. The procedure is repeated the second time and the result is the recovery of calcium lignosulphonates at a pH greater than 12. The product is solid and can be washed and filtered afterwards [68]. Alternative methods of lignin recovery from sulphite spent liquor include amine extraction, ion-­ Fig. 3.11  Schematic of the Howard method to separate calcium lignosulphonates from calcium sulphite spent liquor. Reprinted with permission from John Wiley and Sons [67]

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Fig. 3.12  Flow diagram for amine recovery of lignosulphonates

exchange resin, electrolysis, reverse osmosis and the Pekilo process (fermentation and ultrafiltration) [55, 59]. Amine extraction method (Fig. 3.12) [69] involve the conversion of lignosulphonates into water-insoluble lignosulphonic acid–amine adducts [70], which are then subjected to liquid–liquid extraction [55, 59]. Long chain aliphatic amine [71], poly ethyleneimine [72] and tri-n-hexyl-amine [73] has been utilized. However, the process is slow and it is difficult to completely remove the amine. There are also problems of salt, foam and emulsion formation. Moreover, industrial scalability of the process is limited because of the excessive usage of organic solvents. In electrolysis (Fig. 3.13), desalination and demineralization of magnesium sulphite spent liquor are performed to produce lignosulphonates [74]. The process led to an increase in charge density and molar mass with decrease in pH value. The process uses electricity and is therefore expensive. Moreover, fouling of the electrodes also occurs minimizing its usage industrially. Separation via Ion-exchange resin (Fig. 3.14) involves the use of sandstone [75] or limestone or dolomite as an adsorbent [76]. Polyacryl and polyaromatic resins have been used with high absorption capacities [77, 78]. Precipitation of lignosulphonates can be achieved using ethanol, which can later be recovered through filtration and distillation [79]. 3.2.2.2  Chemistry of Delignification In the sulphite process lignin is solubilized through the formation of sulphonate functionalities and the cleavage of lignin bonds. The three main reaction principles that lead to the dissolution (degradation and condensation) of lignin in the sulphite

3  Bio-sourced Lignin: Recovery Techniques and Principles

Fig. 3.13  Lignosulphonate recovery via electrolysis

Fig. 3.14  Flow diagram for ion exchange recovery of lignosulphonates

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Fig. 3.15  Lignin delignification pathways in sulphite process

process are sulphonation, sulphitolysis and hydrolysis (Fig.  3.15). The reactions occur differently under acidic and neutral conditions. Under acidic conditions, quinone methide intermediates are formed through the loss of hydroxyl group or cleavage of the α-ether linkage (hydrolysis). The reaction occurs via a benzylic cation with phenolic and non-phenolic-type substrates. Subsequently, sulphite ions in the cooking solution are added at the α-position of the quinone methide intermediate producing benzyl sulphonic acid units. This increases the solubility of the lignosulphonates (Fig. 3.16a). The β-O-4 ether linkage is stable under acidic conditions with no strong nucleophiles and cannot cleave. Contrary, a condensation reaction may occur between the benzylic carbon and the meta carbon in the sixth position (aromatic ring) of another molecule (i.e. a C-C linkage), provided that the benzylic carbon can form a carbonium ion via protonation-elimination of the oxygen function (Fig. 3.16b). This reaction is the major competing reaction to sulphonation and can limit the sulphonation reaction because it occurs at the α-position [80–82]. As mentioned earlier, sulphite pulping is selective towards biomass source. This is due to the fact that some woods such as pine contains pinosylvin (3,5-­dihydroxystilbene) and pinosylvin methyl ether (3-hydroxy-5-methoxy-stilbene) in the heartwood, which result in a competition between sulphonation of lignin and condensation reaction between lignin and the C-6 position in the extractives. This results in a lower degree of sulphonation of pine and thus less favourable dissolution of lignin.

3  Bio-sourced Lignin: Recovery Techniques and Principles Sulfonation H RO

83

+

+ OCH3 O(OH) – HSO3

- ROH OCH3 O(OH)

SO3H

OCH3 O(OH) OCH3 + O ( + OH)

CH2OH HC L HC OR

L

O

H+/HSO3–

CH2OH HC L + HC O R H

OCH3 L

O

HC - ROH +

OCH3

O

CH2OH CH HC L HC H

OCH3 L

O

OCH3 + O

OCH3 -H+

CH2OH CH HC L HC

OCH3 O

L

O

OCH3

Fig. 3.16  Lignin dissolution reaction during sulphite pulping under acidic conditions

Lignin dissolution under neutral or slightly alkaline conditions proceeds by hydrolysis or cleavage of the α-ether linkage of phenolic-type substrates to form quinone methide intermediates. Subsequently, sulphonation occurs by electron withdrawal effect of an α-sulphonic acid group of the β-O-4 moiety to facilitate nucleophilic attack by the sulphite on the β-carbon atom resulting in depolymerization (sulphitolytic cleavage) of the β-aryl ether bond (Fig. 3.17). This can only occur at higher pH than the acidic conditions. Carbohydrates may also react during sulphite pulping. The major reaction in this case is the acid catalysed hydrolysis of glycosidic linkages resulting in the formation of large amount of monomeric and oligomeric sugars. Further degradation of the dissolved sugars may occur as the cooking progresses to arrive at furfural from pentoses and hydroxymethylfurfural from hexoses dependent on the acidity, temperature and cooking time. There may also be oxidation of monosugars to aldonic acids by redox reaction initiated by HSO3−.

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Fig. 3.17 Lignin dissolution reaction during sulphite pulping under neutral conditions

OH HO HO

HSO3–

O OH

OH

oxidation disproportion ation

OH HO HO

OH COOH O

S2O32–

H+

2–

S3O6

2–

S4O6

2–

S5O6

reaction with lignin 2–

+ SO4 –

3 HSO3

2SO42– + H+ + H2O + S

Fig. 3.18  Side reactions during acid sulphite pulping

During acid sulphite pulping, side reactions such as disproportionation of HSO3− occurs leading to the formation of thiosulphate, sulphate and molecular sulphur. Most of the hydrogen sulphite ions are consumed in other reactions apart from sulphonation of lignin. Decomposition of sulphur dioxide occurs at elevated temperatures in the absence of wood. The reaction results in the formation of intermediate products such as polythionates. The formation of thiosulphates further accelerates the decomposition reaction acting as an autocatalyst. Sulphur is later precipitated when the concentration of the thiosulphate reaches a critical concentration and the acidity increases rapidly. On the other hand, thiosulphates can be formed by reduction of hydrogen sulphite by wood components especially sugars (Fig. 3.18). It is

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expected that thiosulphates also react with lignin in the liquor producing thioether cross-links. These cross-linked sulphur (organic excess sulphur) account for 5–10% of organically bound sulphur [13]. 3.2.2.3  Structural Nature of Lignosulphonate The various production methods employed in the dissolution of lignin in the sulphite process gives rise to variation in the structure of lignosulphonates. Compared to Kraft lignin (1.2% sulphur), lignosulphonates contain a relatively high sulphur content (5%). They possess a degree of sulphonation between 0.4 and 0.7 sulphonate groups per phenyl-propane unit [83]. Unlike Kraft lignin, the presence of hydrophilic sulphite groups together with hydrophobic aromatic structures makes lignosulphonate to be amphiphilic in nature. Glennie [84] reviewed a few literature on lignosulphonates and proposed a speculative structure of the lignosulphonates as shown in Fig. 3.19. Although a vivid structure of lignosulphonates has not yet been realized, it is known that they are cross-linked polyelectrolytes [85–87]. Hagglund [88] first proposed that lignosulphonates consisted of a polydispersed system of sulphonated guaiacyl propane units together with some catechol propane units [89]. Gardon and Mason [86, 87] proposed that the structure of lignosulphonates is such that they

Fig. 3.19  Model structure for lignin sulphonates by Glennie [84]

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behave as a coiled or expanded polyelectrolyte which is likely to associate in solution with the high molecular weight fraction forming branched networks and the low molecular weight fraction forming less branched network. A slight different model of lignosulphonate was proposed by Rezanowich et  al. [90] showing that instead of a coiled structure, lignosulphonates appear as micelle type microgels with a neutral core consisting of cross-linked aromatic structure and sulphonated charged groups near the surface which facilitates interactions with aqueous surroundings. This model was later supported with high-resolution microscopic imagery [91, 92]. Some other models showing that lignosulphonates appear as monolayer, sandwich-like, spherical and flat disc-like structures were later reported [93–95]. In a later study [96], the conformation of lignosulphonates was found to change from a compact sphere in the absence of an external electric field to a non-free unwinding coil with an application of 10 V m−1 electric field strength. Myrvold [85] proposed that lignosulphonates are randomly branched cross-linked polyelectrolytes. The model structure consists of long continuous chains acting as a backbone with extended short side chains. The author suggested that these side chains may be further branched and still connects back to the backbone forming a close loop (Fig. 3.20). The short side chains are assumed to be the position where the sulphonate groups were introduced after the breaking of the ether bonds. This means that the aromatic structure occupies the backbone making them hydrophobic and the sulphonated structures the side chains making them hydrophilic. Based on the ­randomly branched cross-linked polyelectrolytes model for lignosulphonates a simple structural model was proposed by Matsushita [97] (Fig. 3.21). The author also found that when the salt concentrations are high and electrostatic charges are screened, the randomly branched polyelectrolyte model coincides with that of a non-charged randomly branched polymer. A significant proportion of lignosulphonates are made of monomeric compounds, mainly 4-allyl-2,6-dimethoxyphenyl-α-sulphonic and 4-propyl-2,6-­ dimethoxyphenyl-­α, γ-disulphonic acids [98]. Lignosulphonates [99] exhibit higher average molecular weight and higher monomer molecular weights than Kraft lignin

Fig. 3.20 Polyelectrolytic structure of lignosulphonate. Reprinted with permission from [85]

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Fig. 3.21  Structural nature of lignosulphonate based on random cross-linked branched network [97]

because of the presence of sulphonate groups on the arenes. Phenolic hydroxyl units in lignosulphonates is a necessary criteria for its adhesive durability, it is thus an important criteria to its potential reactivity and should be a subject of concern. When examining the structure of lignosulphonates, Buchholz et  al. [89] realized that paucidisperse gymnosperm lignosulphonates contain similar amount of hydrogen, oxygen and methoxyl per C9 but the hydroxyl and sulphonate groups were found to increase with decreasing molecular weight. In each case, the phenolic hydroxyl units were two times lower than the sulphonated and the methoxy units. Others also reported similar results [74, 100–102]. Glasser et al. [103] also found that the phenolic hydroxyl units of lignosulphonates decreased with increasing molecular weight. In their study, the structural units which contain free phenolic hydroxyl units and those without free phenolic hydroxyl increased in a constant proportion of about 1:4 suggesting a significant degree of order in the lignosulphonate structure. Glennie [84] reported 35 phenolic hydroxyl and 100 methoxy units per 100 C9 units of lignosulphonates. In the study, there was no α-O-4 unit detected but the β-O-4 units were found to be 50 units per 100 C9. The non-sulphonate sulphur was found to exist in very small amounts probably as sulphides or thioethers.

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To shade more light on the structure of lignosulphonates, Lutnaes et  al. [104] ­prepared 15 monomeric and 7 dimeric sulphonated model compounds and evaluated them using 1H and 13C NMR.  The study realized linkages in the model ­compounds including β-β linkage, β-5 linkage, 5–5 linkage and β-O-4 linkage. These linkages were also analytically determined to be present in significant proportion by Glennie [84] (Fig. 3.22).

Fig. 3.22  Structures of sulphonated model compounds. Reprinted with permission from John Wiley and Sons [104]

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3.2.3  Soda Process 3.2.3.1  Process Description Soda pulping is an alkaline pulping process that utilizes sodium hydroxide for delignification of mostly non-wood biomass. It is one of the oldest processes of pulping invented in 1851 by Hugh Burgess and Charles Watt in England. The soda process was developed long before the structure of wood was properly understood. The soda process produces sulphur free lignin and nanocrystalline cellulose pulps which makes it a desirable alternative to the Kraft process. Further advantages of the soda process include a decrease in sulphur aerial emissions and the pulp does not require additional desulphurization process steps [105, 106]. The soda process is essentially like the Kraft process in terms of the processing elements. It is environmentally friendly because sulphide is not used in the processing. The major difference between Kraft and soda process is that the later uses a sulphur free cooking liquor [62]. The soda process is suitable for agricultural harvesting residues and non-wood fibre feedstocks such as straw, sugar cane bagasse, flax [10, 44, 105, 107]. In the soda process, fibrous materials are heated in a pressurized reactor at 140–170 °C in the presence of 13–16% sodium hydroxide. The ratio of the liquid to fibrous material is always in the ratio of 5:1. The liquid called black liquor contains lignin, other polysaccharides and sodium hydroxide. Further processing is usually done to recover lignin and soda [108]. The recovery of soda lignin is usually done via precipitation followed by liquid-­ solid separation and drying [105]. The deficiency in the precipitation method is the co-precipitation of silica with lignin when non-woody biomass sources are used. A commercial recovery process called the LPS® precipitation was developed in Europe (Granit S.A. of Lausanne, VD, Switzerland) to solve the problem of silica precipitation [109]. LPS® lignin is sold today in powdery form. Another commercial process called LignoForce® is also used in the recovery of lignin from the soda spent liquor [110]. The process which was developed by FPInnovations and NORAM involve a combination of several process including acidification, precipitation and oxidation (Fig. 3.23). Because non-woody biomass naturally contains lower amount of lignin, a more exposed structure, and a greater percentage of ester linkages that are alkali-labile, the efficiency of delignification is always poor [10]. Soda pulping is known to result in excessive carbohydrate dissolution and degradation [108]. Improving the efficiency of the soda pulping process has been investigated by many researchers over the years [111]. Anthraquinone (AQ) has been used to improve the efficiency of the soda pulping process [111, 112]. The first attempt to use AQ as a catalyst in soda pulping was reported by Holten in 1976 [113]. The AQ process led to increased delignification rates, selectivity, velocity, reduced alkali charges and pulp yield [114]. It has also been reported that that AQ can result in stabilization of carbohydrates against alkaline degradation by decelerating peeling reactions and also accelerates delignification by cleavage of ether bonds. The mechanism of AQ

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Fig. 3.23  LignoForce System for lignin recovery from black liquor. Reprinted with permission from ACS [110]

Fig. 3.24  The redox cycle of anthraquinone

delignification has been described as oxidation and reduction catalysis in the liquor system. The proposed AQ mechanism of action is shown in Fig. 3.24. AQ is proposed to first react with reducing end groups of carbohydrates in the biomass resource stabilizing them against alkaline peeling, producing reduced anthrahydroquinone (AHQ) which is alkali soluble. The AHQ then reduces lignin which becomes more reactive and undergoes fragmentation reactions. The process is self-­ replenishing with AQ being formed again and undergoes further reaction with

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­carbohydrates [115]. Because the process is semi-catalytic, very small amount of the AQ is needed. Studies using model compounds show that AHQ promotes delignification by promotion of lignin fragmentation reactions and retardation of lignin condensation reactions. It is very important to note that AQ does not dissolve completely during the pulping process leading to operational problems. On the other hand, the soda and soda-AQ process is inherently dependent on conditions such as temperature, alkali load, fraction of AQ added, heating rate, cooking time and liquor to wood ratio [114]. The process has been used for non-wood raw materials, such as straw, bagasse, reed canary grass and other agricultural residues [116–121]. The soda-AQ process is adaptable to softwood than hardwood [122]. The process has been said to out-perform Kraft process in terms of pulp yield and quality. However, the process produce pulps which are more difficult to bleach and requires higher amount of sodium hydroxide content for cooking increasing the causticity in the recovery process [120, 123]. Soda-AQ lignin is usually recovered from the liquor using precipitation techniques. Another method of improving the efficiency of the soda process is the use of oxygen (Soda-O2). Soda-O2 delignification was developed in 1961 was used commercially at the beginning of the 1970s [124]. The process improves delignification of biomass and is very favourable because the oxidative conditions of the O2 affect the ultrastructure of the biomass resource in a deconstructive manner, and reduces the chloronome groups in the pulps [124]. The process was established to compensate for the health deficiencies of the soda-AQ process [112]. The process is reported to lead to extensive oxidation degradations of lignin and carbohydrates resulting in black liquors with lower heating value than those from Kraft and soda-AQ pulping. Oxygen delignification is generally carried out under medium-consistency conditions with sodium hydroxide concentrations in the range of 1–4% with oxygen pressure of 0.4–1 MPa. During the process, pulp with 8–12% consistency is heated to about 80–100 °C in a steam mixer, oxygen is injected in a high shear mixer with a retention times of 20–90 min. The soda-O2 has been commercially applied as either single or two-stage technologies [125]. The two-stage systems are currently being practised in order to increase selectivity and treatment efficiency [44]. The process is mostly suitable for low-density raw materials [126] including most non-wood fibres [127]. The process ensures the retention of silica in the pulp, making sure that least amount of silica is left in the spent pulping liquor. This makes the process a potential solution to the silica precipitation problems encountered in non-wood soda pulping [128]. The resulting liquor can be recycled without further treatment making the process environmentally benign [129]. On the other hand, the soad-O2 process uses low cooking temperature, which means low energy consumption. 3.2.3.2  Chemistry of Delignification Soda pulping is purely an alkaline process, meaning that most of the reactions in alkaline delignification will be possible during soda pulping. Multiple delignification reactions take place during the pulping reaction at varying magnitude and

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Fig. 3.25  Illustration of cleavage of non-phenolic β-O-4 structures in lignin [130]

importance. In this section, only the most important reactions are presented. The most frequent and important delignification reaction in soda pulping is the cleavage of the non-phenolic β–O–4 linkage (Fig. 3.25). The cleavage of the β–O–4 linkage results in the formation of phenolic end groups and an epoxide. The epoxide may participate in an additional reaction through nucleophilic activation of NaOH. Cleavage of phenolic ether bonds is also possible with soda pulping. In this situation, a quinone methide is formed which can react further releasing formaldehyde to form enol ether structure. The quinone methide may also undergo a condensation or reduction reaction. The β-5 linkage can also undergo a reaction to form stilbenes in a similar pathway as cleavage of non-phenolic β-O-4 linkages. In addition, soda delignification is accompanied by various other reactions with carbohydrates. These reactions include peeling and alkaline hydrolysis of the carbohydrate chains. Peeling reactions occur at the carbohydrate end-groups when they encounter the alkali solutions. The reactions proceed faster at higher temperatures. The end groups may form aldehyde and keto groups simultaneously. These may then undergo β-elimination reaction with the keto group resulting in a reduced end group by removal of a sugar monomer. For the aldehyde group, the elimination leads to the stabilization of the carbohydrate chain. Important to the pulping process is the fact that the peeling and the stabilization reactions compete for the same reaction sites, meaning that the state of depolymerization is dependent on the speed of each of the reactions [131]. Alkaline hydrolysis of the glucosidic linkages occur by cleaving of the carbohydrate chains making them shorter. This results in higher number of end groups and consequently increased peeling reactions. Alkaline hydrolysis of carbohydrate chains is only possible at temperatures above 170 °C. The major difference between delignification with NaOH and Soda-AQ is the increased cleavage of the phenolic β-aryl ethers [19]. The mechanism via which this increased cleavage occurs has been the subject of research for several years. It was shown in the previous section that during alkaline pulping AQ first reduced to AHQ after reaction with carbohydrates and the AHQ then enters the delignification ­reaction leading to the recovery of the AQ.  Two mechanisms have been used to explain how AHQ takes part in the delignification process. The adduct mechanism theory is based on the fact that at temperatures below 60 °C, AHQ couples with quinone methides produced during delignification with the alkali and yield adducts [132] (Fig.  3.26). The coupling takes place by the C-10 carbon of the

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CH3

CH3

CH2

HC

C4H9

OCH3

OCH3

OH

OH

1 CH2OH

2 CH2OH

HC

O

HC

OR

HC

O

CH H3CO

-ROH

H3CO

OCH3 OH

OCH3 O

3 -CH2O

HC

O

CH H3CO

OCH3 OH

Fig. 3.26  Illustration of the different reaction of phenolic β-O-4 structures in lignin. Reduction reaction (1), condensation reaction (2) and elimination reaction into an enol ether from quinone methide (3) [130]

a­ nthrahydroquinone forming a bond to the C-α carbon of the β-aryl ether dimer. These adducts fragment at temperatures above 60 °C to produce AQ and a substituted anthrone adduct (C-α/C-β olefin) [133]. The adduct mechanism is supported by the fact that C-10 to C-α adduct was isolated in model compound studies and when it was heated in alkali, reaction products similar to those of direct reaction of

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the β-aryl-ether model with AHQ were realized. However, this mechanism was found to be faulty because when reactions were conducted with hindered anthraquinones, they did not result to any change in yield or distribution of products, as would be expected if the formation of adduct was a part of the normal reaction path [134]. Structurally, during soda-AQ pulping, β-O-4 linkages and S-lignin units are preferentially removed. The reaction is purely a depolymerization reaction and not oxidation. Studies on residual lignin after soda-AQ showed that during the delignification, acetates were completely hydrolysed. This was shown by the complete acetylation of the ɣ-carbon side chain in residual lignin [135]. Mechanism of Alkali-O2 delignification has been studied using model compounds [136–138]. It is generally agreed that free hydroxyl groups play a vital role in Alkali-O2 delignification. The delignification reaction proceeds via a resonance stabilized phenoxy radical. In strongly alkaline conditions, the phenolic hydroxyl group reacts to generate a phenolate ion (ionized). This action generates a site with high electron density which is capable of transferring single electron to molecular oxygen or any available radical species. The phenolate ion is the point of initiation for lignin fragmentation. In the presence of oxygen, the phenolate anion reacts to form a reactive intermediate called a hydroperoxide. This resonance stabilized intermediates may then either undergo a condensation reaction (reaction with other intermediates) or with oxygen species such as hydroxyl (HO·), hydroperoxy (HOO·) and superoxide (O2–) radicals to form organic acids, carbon dioxide and other small molecular weight organic products via side chain elimination, ring opening and demethoxylation reactions [139] (Fig. 3.27). Studies have noted that in alkali-O2 delignification, some phenolic groups such stilbene and enol ethers, react very rapidly because of the presence of conjugated side chains. However, diphenylmethane-type condensed structures are found to be resistant to oxygen bleaching. The phydroxylphenyl and 5,5′-biphenolic units in residual lignin are firm and are inclined to amass during oxygen delignification [140, 141]. Because oxygen delignification results in products which are predominantly organic acids and carbon dioxide, the number of phenolic hydroxyl groups in the pulp decrease while the number of carboxylic acid groups increase [142]. A similarity between residual lignin from alkaline pulping and alkaline oxygen pulping noticed by Tammine and Hortling [143] led to the conclusion that lignin carbohydrate linkages play an important role in hindering alkaline-oxygen delignification. Based on this fact Backa et al. [144] proposed a modified oxygen delignification mechanism. Initially the hemicellulose chain contains two end groups with one reducing end (circle in structure I) and another non-reducing end (bar in structure I). The reducing end undergoes endwise peeling and finally a stopping reaction so that both ends then become alkali stable (Structure II). This reaction called endwise peeling delignification leads to loss of lignin. The residual lignin which is in the form of low molecular weight oligolignins contains a phenolic end which ­dissociates to form phenolate structure (Structure III). The phenolate structure now reacts with oxygen according to the previous mechanism (Fig. 3.28).

3  Bio-sourced Lignin: Recovery Techniques and Principles

a

C

C

C

C

+

95

+

OH-

OCH3

H2O

OCH3 O–

OH

Phenolate ion R

R

+

O

O

O

O–

+

OCH3

OCH3

O–

O

b

C

C

C

C

C

C Lignin Condensation

C C

CH3O

C

OCH3

OCH3 O C

C

C

C

C

O

O

OOH

+

C

HOO

OOH OCH3 O

Side Chain Elimination

Lignin Degradation

Ring Opening

OCH3 O

Demethoxylation

Fig. 3.27  Possible reaction pathway for alkali-O2 delignification (a) initial attack of oxygen on phenolic nuclei (b) reaction of an intermediate phenoxy radical from phenolate anion [138]

L

L

L

L

L

I

L OH

OH

OH L

L OH

L OH

L

L

L

L

L

L

II

Fig. 3.28  Modified oxygen delignification mechanism [145]

O H

O H

L OH

OH

L

L

L OH

L OH

L

O

L

O– L

L

O–

O–

OO

–OCH3

L

L

L

L

III

O2–

L O–

L O–

OH

L

H2O2

L

O– L

L

L

L

–

L

O

O–

OH

IV

L O–

L O–

L

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3.2.3.3  Structural Nature of Soda Lignin Studies on the structural characteristics of soda lignin is lacking probably because many researchers believe that there will be no structural difference from other alkaline processes since they follow the same delignification mechanism. However, some studies on soda-AQ lignin show that precipitated lignin are enriched in β–β resinol structures but contain lower or no β–O–4 alkyl-aryl ether and β–5 phenylcoumarans linkages. Soda-AQ lignin also possess a substantial increase in S/G ratio indicating that the S-lignin units are preferentially removed during the pulping and are enriched in the liquor. Soda-AQ lignin contains higher s-lignin units than Kraft lignin. It was also shown that in soda-AQ lignin, β–O–4 alkyl-aryl ether was completely absent. p-coumaric acid units were also found to be present in free or etherified form in soda-AQ lignin [146]. Figure 3.29 shows various low molecular weight compounds extracted from the liquor of alkali-O2 delignification [140]. Most of the

Fig. 3.29  Structure of major compounds extracted from the liquor after oxygen delignification. Reprinted with permission from [140]

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5-carboxyl group compounds are likely formed from the diphenylmethane or β-5 units. High content of condensed phenolic hydroxyl groups are also noticed in the alkali-O2 lignin. Fu and Lucia [140] noted that under oxygen delignification, the diphenylmethane or β-5  units are partly oxidized and changes to low molecular weight compounds.

3.2.4  Aqueous Alkaline Pre-treatment 3.2.4.1  Process Description Aqueous alkaline pre-treatment (e.g. NaOH, Ca(OH)2), is related to soda pulping and has been mainly studied for herbaceous biomass [147–159]. Pre-treatment with aqueous alkali was originally designed to cost effectively break the natural recalcitrance of lignocellulose, improve digestibility of the components and simultaneously obtain a high recovery of hemicellulose and lignin [158]. Sodium hydroxide pre-treatment, has been used since 1900s to increase the digestibility of cellulose by rumen animals [150]. The effectiveness of the alkaline pre-treatment process is measured by the degree of delignification after the pre-treatment. NaOH pre-­ treatment is usually divided into two types including: low-concentration and high-­ concentration pre-treatments [160]. Low concentration pre-treatment involves the use of typically 0.5–4 wt.% NaOH at high temperatures and pressure and longer pre-treatment times. The variables used for processing are dependent on the type of biomass employed. For wheat straw it was found that the optimum parameters for 72% lignin yield are 2 wt.% concentration of NaOH, a temperature of 120 °C and 90 min pre-treatment time [161]. In another study, when coastal Bermuda grass was pre-treated by 0.75  wt.% NaOH at 121  °C for 15  min, 86% lignin was removed [162]. Karp et al. [153] showed that the optimum NaOH loading at 100 °C to be 120 mg NaOH/g dry stover, to yield 38% of lignin in the liquor. In another study, Karp et al. [147] demonstrated for corn stover that 55 wt.% of the original lignin can be extracted into the liquor. The liquor was termed alkaline pre-treatment liquor (APL). They also noted that another 35 wt.% of lignin can be removed by washing the residual solids with water [147, 153]. Pre-treatment of wheat straw with 2 wt.% NaOH, at 105 °C, for 10 min yielded 70.3% delignification rate [163]. For eucaliptus grandis bark, it was found that with 4 wt.% NaOH, temperature of 120 °C and 1  h pre-treatment time, it is possible attain 59% delignification rate [164]. Pre-­ treating asparagus stem with 10 wt.% NaOH at 35 °C, 120 h yields about 46% delignification rate [165]. For corn stover, 10 wt.% NaOH, 140 °C, 30 min can afford 75.1% [166]. Pre-treatment with NaOH is always more effective on agricultural wastes, herbaceous crops and hardwood. This is probably because softwood possesses a high content of lignin (especially guaiacyl lignin) and is denser than hardwood [167]. The process requires a high amount of the alkali to be used for the pre-treatment. However, it is possible for the alkali to be recovered after the process reducing the environmental impact of the process [160, 168]. Although the recovery process is

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expensive, it is possible to integrate the process with alkali based pulping techniques. Another important advantage of the process is the use of lower temperatures and pressures than other pre-treatment methods. However, heat is released during the process which may pose a safety concern. Lignin is recovered from the liquor by normal acidification. Lime (calcium hydroxide, Ca(OH)2) is another alkali that has been used for pre-­ treatment of lignocellulose materials. Lime pre-treatment reacts in a similar manner as NaOH in the pre-treatment process. In this case, lime is used to remove acetyl groups in relatively milder conditions [169]. The lime process is performed at relatively lower temperatures (typically in the range of 50–121 °C), but longer times and more water may be needed for the process. Chang and co-workers showed that lime loading of 0.1 g Ca(OH)2/g dry biomass, low reaction times (1–3 h) and high temperatures (85–135 °C) are required to achieve a 14% dissolution of lignin [156]. This can also be achieved at lower temperatures (50–65  °C) but higher reaction times (24 h). Another example shows that lime pre-treatment operated within the temperature of 25–130  °C, time of 1  h–8  weeks and a loading of 0.05–0.15  g Ca(OH)2 water)/g of biomass can yield 60–80  wt.% delignification [150]. Pre-­ treatment of corn cob with 6 wt.% lime using 70 °C, and 3 h yielded 32.70% delignification rate [169]. For jatropha seed cakes, 38.2% delignification can be achieved with 20 wt.% lime 100 °C and 1 h [170]. In the case of sugarcane bagasse 40 wt.% lime, 90 °C, 82.5 h was found to yield 70% delignification rate [171]. Although the process is simple and low-cost with low energy requirement, it has been shown that under alkaline conditions, Ca2+ ions will cross-link with lignin molecules and decrease lignin solubilization during the pre-treatment process [172]. There is also evidence of precipitation of calcium salts during pre-treatment especially when the dosage of the lime is high. Sodium carbonate has been used in a few cases for pre-treatment. It is readily soluble in water than lime, inexpensive and can be obtained from the chemical recovery process of soda pulping. It generally exhibits a slight alkaline degradation to lignocellulose. Pre-treatment with Na2CO3 can be done in the temperature range of 70–180 °C [173]. The time is usually dependent on the process and the properties of feedstock. It has been reported that at elevated temperature pre-treatment with sodium carbonate can significantly decrease lignin contents [174]. 3.2.4.2  Chemistry of Delignification The chemistry of delignification in alkaline pre-treatment is the same with alkaline hydrolysis of biomass explained in the previous section. During the pre-treatment process, the alkali dissociates into hydroxide ions (OH−) and a cation (Na+, Ca+ etc.). The hydroxide ion goes into reaction with the biomass leading to cleavage of the ether bonds and the swelling of cellulose. The reaction is always dependent on the concentration of the hydroxide ions. For strong base (e.g. NaOH) hemicellulose and lignin are significantly solubilized under favourable circumstances [131]. During the process, the alkali used attacks the linkage between lignin and hemicellulose in lignin-carbohydrate complexes (LCC). The ether and ester bonds in the

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LCC structure are cleaved in the process. Most of the strong alkali (e.g. NaOH) efficiently cleaves the ester and carbon-to-carbon (C–C) bonds in lignin molecules which leads to delignification. 3.2.4.3  Structural Characteristics of APL Pre-Treated Lignin The alkali pre-treated liquor (APL) is rich in monomeric phenols arising from the hydrolysis of ester linkages. The liquor also contains lignin oligomers and carbohydrate-­derived hydroxy acids (e.g. lactic acid, glycolic acid) arising from degradation of dissolved carbohydrates.

3.2.5  Ammonia-Based Pre-treatment 3.2.5.1  Process Description and Chemistry Most alkaline biomass fractionation methods make use of liquid ammonia. It is widely accepted that ammonia is able to solubilize or redistribute lignin without destroying the carbohydrates [175–179]. Ammonia is used mainly because it is easily recoverable, inexpensive, non-corrosive and non-toxic. The recovery process is well established because it is highly volatile and offers versatile processing options [180]. Moreover, it is believed that ammonia pre-treatment can either modify or remove lignin. It causes swelling of the biomass leading to significant morphological changes. Ammonia pre-treatment also opens up the structure of lignocellulose materials [181]. Ammonia pre-treatment of lignocellulosic biomass are done in three different ways including: Ammonia Fibre Explosion (AFEX), Ammonia Recycle Percolation (ARP) and Soaking Aqueous Ammonia (SAA) [159]. Ammonia Fibre Explosion (AFEX) AFEX is an anhydrous ammonia pre-treatment that is usually conducted at ambient temperature [182]. In AFEX, the lignocellulosic biomass is reacted with liquid ammonia in a sealed vessel at temperatures between 60–90 °C, pressure greater than 3 MPa and reaction time of 30–60 min. After a desired holding time and temperature, the pressure valve is opened to release the pressure. In the process, ammonia is evaporated and the temperature of the system drops momentarily [183]. The reaction with ammonia at elevated temperature generates heat resulting in ammonolysis and hydrolysis of LCC and ester linkages leading to partial dissolution of lignin [8, 178]. After the rapid pressure release, the biomass structure is exposed so that subsequent organic and alkaline extraction leads to efficient extraction of the lignin. Figure  3.30 shows a proposed model for AFEX pre-treatment mechanism. Preliminary NMR studies show that AFEX lignin is close in structure to native lignin. It was also noted that cleavage of the esters producing amides is the major

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Fig. 3.30  Proposed mechanism of APEX. Cell corners (CC), cell lumen (CL), outer wall surfaces (OW), compound middle lamella (CML), secondary wall layers (S1, S2, S3). Scale bar is 250 nm. Reprinted with permission from RSC [178]

cell-­wall-­disrupting reaction occurring during the AFEX process. The reaction to liberate lignin proceeds by cleavage of diferulate linkages (polysaccharides crosslinks), lignin-ferulate and lignin diferulate linkages (cross-links of between polysaccharides and lignin) and other ester linkages [184]. Ammonia Recycle Percolation (ARP) ARP is an aqueous ammonia pre-treatment technique that is usually conducted at high temperatures [182]. It is a flow through process where aqueous ammonia (5–15 wt.%) is passed through a reactor containing biomass at a temperature range between 140 and 210 °C, reaction time of 90 min, 2.3 MPa vacuum pressure and percolation rate of 5  mL/min [185, 186]. The reactor is a fixed bed reactor

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(percolation reactor) which can minimize re-polymerization and re-precipitation of soluble lignin because the solubilized lignin is removed from the reactor as soon as it is formed. The major benefit of ARP is efficient delignification. Over 80% delignification has been reported for various lignocellulosic materials. The process has been found favourable for herbaceous plants, agricultural residues and municipal solid wastes [154, 180, 187, 188]. ARP is a high energy process because of the high temperature requirement. The dissolved lignin is extracted by evaporation (and recycling) of ammonia [189]. The precipitate contains a substantial amount of carbohydrates [107] which are easily removed through acid-catalysed hydrolysis with the lignin structural integrity not compromised [190]. Soaking Aqueous Ammonia (SAA) Closely related to AFEX is the SAA process where biomass is treated with aqueous ammonia in a batch reactor at 30–60  °C.  The SAA process decreases the liquid through-put [191] and requires less energy as it is operated at low temperature. The process was developed to solve the problem of hemicellulose loss in ARP. SAA was tested on corn stover by soaking the corn stover biomass in 15–30% ammonia at low temperature (30–80 °C) for extended periods (4–24 h). The study realized a retention of more than 80% of hemicellulose and near 100% of the cellulose [181] and effectively removed about 60–70% of lignin. The process is an expensive multi-­ stage process with long reaction time and high liquid throughput. 3.2.5.2  Structure of Ammonia Pre-Treated Lignin Lignin obtained from APEX contains oligomeric fragments with well-preserved β-O-4 bonds. Poplar ARP and wheat straw AFEX lignin have been reported to contain well preserved β-O-4 bonds (HSQC NMR: 45% and 37% of interunit linkages) [107]. These values are higher than those reported for Kraft and organosolv lignin by the same authors. Small amounts of phenolic monomers, including aldehydes, p-coumaric acids and amides are also found in AFEX lignin [190, 191]. Other authors also demonstrated that β-O-4 linkages are preserved using ARP [190, 192].

3.3  Acidic Extraction 3.3.1  Dilute Acid Pre-treatment 3.3.1.1  Process Description Lignin can also be extracted from biomass sources during dilute acid pre-treatment (DAP). DAP is considered as one of the leading and promising methods of lignocellulose pre-treatment in a reactor [193–195]. Several reactors have been

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designed for the DAP process including batch [196], plug flow [197], countercurrent [198], percolation [199] and shrinking bed reactors [199, 200]. Dilute acid pre-treatment has been used for lignocellulosic feedstocks such as softwoods, hardwoods, herbaceous crops and agricultural residues [201–205]. The process involves a combination of an acidic solution, heat and pressure. A variety of acids has been used in DAP including hydrochloric acid, nitric acid, phosphoric acid, sulphuric acid and per-­acetic acid. However, sulphuric acid is the most used because it is less expensive and effective. The temperature range for the process is usually within the range of 120–210 °C and the acid concentration typically less than 4 wt.% and residence time from a few seconds to an hour [206]. DAP causes fragmentation of lignin usually resulting in minor delignification (i.e., lignin dissolution) depending on the severity of the process [207–209]. Because the lignin from DAP process is partially soluble in acidic media, it gets deposited on the surface of the biomass in a batch process mode (DAP-batch) [210–213]. This problem is overcome in the flow through process because the dissolved lignin is removed from the solution in the heating zone limiting the extent of lignin structural alterations and re-deposition on the biomass [210–212, 214]. Bhagia and co-workers [212] showed that with flow through pre-treatment 63 and 69% lignin removal at both 140 and 180 °C respectively was possible, while with batch pre-treatment only 20–33% lignin removal was possible at the same conditions. The DAP liquor contains hemicellulose carbohydrates, lignin oligomers and small fraction of lignin monomers. Precipitation is used to recover the lignin from the liquor, but because of the low molecular weight and oxygenated compounds it is difficult to recover all the lignin. A reduction in lignin up to ~2–24% was reported by Silverstein et al. [208] for cotton stalk using DAP pretreatment. Ishizawa et al. [215] and Zhang et al. [216] reported near complete lignin removal from corn stover using DAP. 3.3.1.2  Chemistry During the DAP process the predominant reactions in lignin are fragmentation of aryl ether linkages (primarily β-O-4 linkages) by acidolysis and acid-catalysed condensation [217–219]. Under acidic conditions, the β-O-4 linkages are easily broken resulting in depolymerization of the linkages [220, 221]. The reaction proceeds as shown in Fig. 3.31 [206]. In most cases carbonium ion intermediates which possess a high affinity for nucleophiles within the lignin macromolecules are formed. When these intermediates are involved in a reaction with nucleophiles, the result is lignin condensation. Other prominent reactions that occur during acid pre-treatment process are those that lead to the change in monolignol S/G ratio. Because the β-O-4 linkages are mostly cleaved during the process, the S units are more easily removed so that a lower proportion of S units are found in the pre-treated biomass. This consequently leads to increase in the G units in the pre-treated biomass [222–224].

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CH2OH

HC O H

HO CH MeO

+

– H2O

HC O CH + MeO

OCH3 O

HO H3CO

OCH3 O

Fragmentation

O

OCH3 O

+

Condensation

H3CO

CH2OH HC O CH MeO OCH3 O

Fig. 3.31  Mild acid-catalysed lignin depolymerization. Adapted with permission from Springer Nature [206]

3.3.1.3  Structural Effect on Lignin There are no extensive studies on the structure of lignin obtained from dilute acid pre-treatment liquor. However, the study of Zhang et al. [216] showed that soluble lignin from 0.05% (w/w) sulphuric acid pre-treatment liquor contain mainly vanillin, benzaldehyde, hydroxybenzaldehyde, 2-methoxy-4-vinylphenol, 2,6-­dimethoxy-phenol, 4-hydroxy-3,5-dimethoxy-benzaldehyde and benzoic acids. These products show that the depolymerization reaction involved the oxidation of Cα and cleavage of β-O-4 linkages. They also suggest dehydroxylation or demethoxylation reactions during the pre-treatment. NMR studies of the insoluble fraction shows the presence of propenyl end group structures and Cα carbonyls. β–O–4′ linkage, α–O–4′ and resinol structures were greatly reduced as compared to mill wood lignin indicating significant depolymerization. On the other hand, β–5′ was higher indicating occurrence of lignin condensation. Cinnamyl-like structures were also noticed in the insoluble lignin portion which is an indication that β–O–4′ cleavage proceeded by dehydration at the α-position and oxidation at γ-OH. The possible reactions and the final structural units are shown in Fig. 3.32.

3.3.2  Hot Water Pre-treatment (HWP) 3.3.2.1  Process Description Hot water pre-treatment involves the use of water in liquid or vapour phase to pre-­ treat lignocellulosic materials without added chemicals. It is an autocatalytic method version of DAP. It is also called hydrothermal treatment or autohydrolysis. The pre-­ treatment is usually performed at high temperatures (140–270 °C) sometime with

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Fig. 3.32  Plausible lignin β–O–4 cleavage and condensation reaction pathways in DAP liquor. Reprinted with permission of ACS [216]

pressure over reaction times ranging from a few minutes up to several hours. The process is environmentally sustainable, less expensive and economically viable with low-corrosion potential [214, 225–240]. During the process lignin is partially depolymerized and solubilized in the liquor. Processes used in HWP include (1) co-­ current (batch) process, in which both the slurry of biomass are heated up in the same chamber for the reaction to take place and afterwards cooled, (2) counter current process in which the hot water is pumped against the biomass under controlled conditions—the biomass and the water flows in opposite direction, (3) flow through process, in which the biomass is on a stationary bed and the hot water is made to flow through. Batch and flow through reactors are mostly used in the HWP process [159]. The flow-through hot water process (FT-HWP) provides better delignification because it avoids the condensation of lignin on the surface of the biomass as the lignin is removed from the system during the process [235]. Yan et al. [240] showed that with flow rate up to 65 mL/min, the water flow through process can achieve a

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Flow-through reactor Nitrogen gas

Water tank

Heater

Thermometer

HPLC pump

Sand bath Preheating coil

Filter

Cooler

Sample tank

Fig. 3.33  Example of a water pre-treatment process. Adapted with permission from Springer Nature [226]

100% delignification in poplar wood. Zhuang et al. [226] also recorded the removal of a substantial amount of lignin from sweet sorghum bagasse using FT-HWP. Other researchers also recorded 60–85% lignin removal under 190–230 °C with reaction time between 0 and 300  min in flow through reactors [214, 241, 242]. Further increase in temperature to about 270 °C resulted in 90% removal of lignin. An example of hot water flow through process is depicted in Fig. 3.33. 3.3.2.2  Delignification Chemistry of HWP The mechanism of reaction occurring in the HWP process is similar to that of dilute acid pre-treatment. At elevated temperatures there is increased dissociation of water into hydronium ions and release of organic acids from the lignocellulosic material (e.g. acetic acid from acetate groups). At room temperature water is a polar solvent with dense hydrogen bonding network, but the network weakens as the temperature increases leading to the formation of hydronium ions (H3O+) which is acidic and basic hydroxide ions (OH−) [209, 243–247]. The presence of the hydronium ions enhances the cleavage of glycosidic linkages and aryl-ether through processes such as acidolysis and acid-catalysed condensation [206, 217, 230]. The reaction

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Fig. 3.34  Lignin degradation pathway in hot water. Reprinted with permission from John Wiley and Sons [226]

proceeds with the formation of highly reactive nucleophilic carbonium ion intermediates within the lignin structure. These carbonium ions can further react leading to cleavage of the β-O-4 bonds and final dissolution of the lignin structure. The carbonium ions and the nucleophiles can also react simultaneously leading to condensation reaction which can be prevented by using a flow-through reactor (Fig. 3.34). There are also possibilities of alkaline based reactions during the hot water pre-­ treatment process. Because of the acid-base catalytic behaviour of water, it is expected that the base group brings about cleavage of the ester bonds between ferulic acid and polysaccharides resulting in the liberation of ferulic acid and lignin. The acid group is expected to cleave the ether bonds between lignin and ferulic acid. The products can then be subsequently cleaved by oxidative cleavage to produce vanillin, vanillic acid, p-hydroxybenzaldehyde and p-hydroxybenzoic acid. These products have been noted as part of the HWP effluent after pre-treatment. Based on this

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explanation, Zhuang and co-workers [226] propose a mechanism for HWP delignification shown in Fig. 3.35. 3.3.2.3  Structural Effect on Lignin HWP lignin are majorly oligomeric fragments [228] with a small amount of monomeric phenols including p-hydroxybenzoic acid, vanillin, syringaldehyde and sinapyl alcohol [226–228]. As previously shown in the mechanism of reaction vanillin, syringaldehyde and their respective acids are a result of oxidative degradation (i.e. oxidative cleavage of Cα–Cβ bonds) [228, 248]. The presence of vanillin, coniferyl alcohol and syringaldehyde was confirmed by other researchers [249, 250]. HWP lignin contains a high amount of phenolic hydroxyl units and low amount of aliphatic hydroxyl units [246]. High phenolic and low aliphatic hydroxyl units pre-­ suppose the extensive cleavage of aryl-ether bonds. HWP lignin also contains a low amount of the β-O-4 linkages. Figure 3.36 shows the lignin structures isolated from HWP Eucalyptus globulus wood.

3.3.3  Steam Explosion Pre-treatment 3.3.3.1  Process Description Steam explosion pre-treatment (SEP) is a combination of DAP, AFEX and HWP pre-treatment methods [251]. Steam explosion was first introduced in 1926 by Mason et al. [252] as a biomass pre-treatment process. It can be used for various lignocellulosic materials including softwoods, hardwoods and agricultural residues [253–256]. The process involves the treatment of the biomass with high pressure saturated steam at typical temperatures between 160 and 240 °C and pressures of 0.7–4.8 MPa for very short duration (mostly few minutes). The pressure is explosively released to about 7 MPa (70 bar) for about 5 s after pre-treatment causing the biomass material to undergo explosive decompression disrupting the fibrilla structure [206]. The process is generally dependent on residence time and temperature and results in several changes ranging from small cracks in the wood structure, to total defibrillation of the wood fibres [257]. The process is economically viable, environmentally sustainable, less expensive, less corrosive and energy efficient [258]. The rapid decompression also disrupts the hemicellulose − lignin and cellulose networks, causing hemicelluloses to be solubilized leaving lignin and cellulose as solid material. SEP is an autohydrolysis process, which means that the reactions during the process are catalysed by the organic acids released from the biomass components and dissociation of water at high temperatures. The most prominent reaction leading to structural changes in the process should be via hydrolysis of acetyl groups in hemicelluloses. Lignin is also softened during the process and released from the cell wall and distributed on the surface of the raw material. Steam

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OH

O

Carbohydrate fragment

Phenolics fragment

OH–

O

H+ O

O

O

O

Xyl

OH O

Xyl

OH– OH

MeO

OMe

R O

Lignin

Lignin fragment

HO

HO

O

HO

O

OH

OH O

O

Xyl

OH

Xyl OMe

R

OMe

O

Lignin Lignin polymer

OH

OH

OH

Ferulic acid

Coumaric acid

Arabinoxylan

Oxidative cleavage OH

O

O

OMe OH

Vanillic acid

OH

O

O

OMe OH

Vanillin

OH

p-Hydroxybenzonic acid

OH

p-Hydroxybenzaldehyde

Fig. 3.35  Proposed mechanism for HWP involving acid-base catalysed processes. Reprinted with permission from Springer Nature [226]

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Fig. 3.36  Lignin structures found in HWP of Eucalyptus globulus wood [247]

explosion can be run either as continuous or batch process. The batch process is mostly used in a laboratory scale. An example of a batch process is shown in Fig. 3.37a. The continuous or flow through process is sophisticated and are usually employed in an industrial scale. Example of a continuous process is shown in Fig.  3.37b. SEP itself does not result in considerable delignification [206] but it enables succeeding lignin extraction with organic or alkaline solutions [219, 258– 260]. Separation of steam exploded lignin can be effected by precipitation (acidification, water addition or evaporation of the organic solvent) [256, 261, 262]. 3.3.3.2  Chemistry of SEP and Structural Effect on Lignin During the early stage of steam explosion, hydrolysis of acetyl groups in hemicellulose occurs, resulting in acidic pH of the solution. The acidic condition of the medium initiates a reaction on the lignin structure. Carbonium ions are then formed by proton induced elimination of ROH (R can be H or alcohol or acid) from the benzylic position. The carbonium intermediate reacts further resulting in the cleavage of the β–O–4 linkages and the formation of Hibbert ketones with new phenolic end groups. Lignin extracted after steam explosion treatment of eucalyptus chip wood shows higher content of phenolic hydroxyl groups in the steam exploded

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Fig. 3.37 (a) Batch steam explosion reactor. Reprinted with permission from Elsevier [263]. (b) Continuous steam explosion reactor [264]

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Fig. 3.37 (Continued)

wood relative to the untreated wood lignin [219, 262, 265]. On the other hand, acid-­ catalysed condensation may occur between the aromatic C6 or C5 units and the carbonium ion located at the Cα of the side chain. This reaction leads to the formation of stable C–C linkage between two lignin molecules. The β–O–4 linkages in the original wood were found to be more than in the lignin extracted after steam explosion. Lower S/G unit was also recorded for lignin extracted after steam explosion [266]. The increase in phenolic hydroxyl groups and the decrease in the aryl ether linkages is an indication that steam explosion led to fragmentation of lignin by cleavage of the β–O–4 linkages (Fig. 3.38). SEP was also found to lead to a substantial decrease of cinnamic acids, particularly p-coumaric acid. Tricin was also found to be bound to wheat straw lignin which shows that it participates during the SEP treatment [267].

3.3.4  Organosolv Pulping 3.3.4.1  Process Description Organosolv process (ORP) is a method of delignification that utilizes a non-aqueous media. The process involves the use of organic solvent as the agent of delignification during the pulping process. It is one of the most promising alternatives to Kraft and sulphite pulping processes. ORP significantly enhances the degree of

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Fig. 3.38  A proposed mechanism for (A) SEP homolytic cleavage of β-O-4  in G–O–G with formed phenoxy radical tautomers (The illustrations represent spin densities) and β-O-4 to β-5 transformation. Reprinted with permission from ACS [267]

delignification due to increased lignin solubility in organic medium [268, 269]. The process is usually conducted under three solvent conditions including (1) neutral solvent, (2) neutral solvent with a catalyst (3) acidic solvent. Organosolv processes have a long history of development [270]. The concept came from the knowledge of chemical characterization of biomass component (lignin and carbohydrates), where organic chemicals [271] were previously used to dissolve wood components to study their properties [272–279]. This puts the historical development of ORP to 1893 where Klasson used ethanol and hydrochloric acid to delignify wood. Aronovsky and Grotner [273] also did substantial works on ORP in the 1930s. In 1952, Brounstein [280] made an extensive review on the neglect of ORP processes. More reports were seen from Kleinert in the 1970s [281–283]. Organic solvents that have been applied in ORP process include alcohols (methanol, ethanol, butanol)

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[284–292], organic acids (formic acid, acetic acid) [287, 293, 294], cyclic ethers (Tetrahydrofuran (THF), dioxane) [295–297], ketones (acetone, Methyl isobutyl ketone (MIBK)) [270, 298, 299] and polyols (ethylene glycol, glycerol), [300, 301]. It has been noted that primary alcohols result in better pulping than secondary or tertiary alcohols [273]. In the same study the authors found that n-butyl-alcohol-­ water seems to be the most efficient solvent for the removal of lignin. The organosolv process is very diverse because of the different organic solvents and acidic catalyst that has been used. Moreover, in attempt to optimize the process, several methods have emerged over the years. Although the processes are all based on the principle of delignification using organic solvent with or without a catalyst, the difference in the conditions and the severity of the solvents employed usually impart difference in yield and in some cases difference in the final structure of lignin obtained. A short overview of ORP conditions that have been used are presented in the following sections. 3.3.4.2  Neutral Process Neutral ORP involve the use water and alcohol (or other organic solvent) mixture without addition of chemicals [302]. The process is effective in delignification, easy recovery and is environmentally friendly. It enables easy and uniform penetration of the solvent into the wood leading to uniform delignification. Moreover, the process is not limited by diffusional barriers and chip size. Acetone-water (50:50 v/v) delignification of Eucalyptus urograndis yielded 86% delignification at 195  °C in 120 min [303]. Wildschutz et al. [286] also reported 86% yield with 50% ethanol in water at 210 °C. Ethanol cooking of spruce in a water-ethanol mixture 120 min at 200 °C yielded 70% of lignin. The use of 50:50% (v/v) I-butanol–water to delignify sweet gum yielded 88% delignification at 200 °C and, 120 min [304]. Delignification of southern yellow pine with 50% water-ethanol and water-butanol system at 175 °C and 80  min yielded 16% delignification for the ethanol and 28% for the butanol system [305]. It is important to note that delignification with uncatalysed system is dependent on several parameters such as temperature, time, concentration and reactor. The study of McGee et al. [305] showed that the delignification process for ethanol and butanol followed a two stage kinetic reaction with the first stage being faster than the second for butanol, but for ethanol the reaction reached a plateau in the second stage. This is because in a batch process after initial delignification, lignin condensation occurs hindering further delignification. This is not the case for flow-­ through process [306, 307]. In the flow through process, lignin is removed from the system as they are formed so that the tendency for condensation is reduced. Uncatalysed ORP has been successful in delignification of hardwoods and grasses but does not perform well for softwoods. In uncatalysed delignification, methanol and ethanol have been frequently used because of their low boiling point, less expensive, solubility in water, ease of recovery and low energy requirement. A popular uncatalysed process that has been commercialized is the ALCELL process. The ALCELL process which was successfully used by Repap Enterprises Inc. in

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Fig. 3.39  Example of a neutral organosolv process [311]

Newcastle, N.B.  Canada operates at high pressure (2.8  MPa) and temperature (195  °C) using 50% ethanol in water [308]. The process takes it root from the Alcohol pulp recovery (APR) process of Kleinert [309] (Figs. 3.39, 3.40, and 3.41). In 1987, the APR process was renamed the Alcell process [310]. Unfortunately Alcell process does not perform satisfactorily with softwoods [273]. 3.3.4.3  Catalysed Neutral Process The uncatalysed process is limited in delignification as shown in the previous section. The solution to this is the use of a catalyst [280]. Catalyst in the form of acid and alkali has have been used to increase the delignification rate for biomass by several researchers. Most of the commercial processes used today in the industries are based on catalysed ORP. The process was originally suggested by Klasson in 1893 when he realized that addition of small amounts of hydrochloric acid could extract lignin from wood. Later in 1931, Kleinert and Tayenthal patented a similar process using a mixture of alcohol–water (20–75%) with 0.1% of acid or alkali under pressure at high temperatures (above 150  °C). Various catalysts have been developed for pulping of biomass including; HCl, H2SO4, SO2, oxalic acid, salicylic acid, AlCl3, Al2(SO4)3, BF3, ammonium, ammonium sulphide, ammonium chloride, alkali earth metals, magnesium, calcium or barium chloride or nitrate, magnesium sulphate, NaHSO4 and sodium hydroxide [313–317]. Addition of 0.2% HCl in 60–80% aqueous methanol (MeOH) solution led to a 75% delignification at 170 °C for 45 min. Beech wood was 90% delignified when 50% aqueous methanol solution was used with 0.1% HCl at 160 °C for 45 min. Studies show that the delignification rate increased with increasing temperature, time and concentration of the catalyst. The addition of 0.82 M NH4OH to the delignification liquor increased lignin removal and decreased carbohydrate degradation. Neutral alkali earth metal (NAEM) salt catalyst in 80% methanol was found to lead to more than 60% delignification [318]. However, NAEM is performed at very high pressure, high temperature (typically

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Liquid inlet

Effluent

Thermocouple

Cap liner (PTFE) Conical flange closure

Autoclave lid Stainless steel screen

PTFE o-ring

Wood platelets

Reactor vessel

PTFE pieces; Used to reduce free volume of the reactor.

PTFE liner

Fig. 3.40  Flow through reactor for Organosolv delignification. Reprinted with permission from John Wiley and Sons [312]

>180 °C) and high alcohol concentration. Commercial processes such as Alkaline Sulphite Anthraquinone Methanol (ASAM) [319, 320] and ORGANOCELL are based on catalysed ORP. ASAM is an alternative process to Kraft pulping developed by Patt and Kordsachia in 1986. The process uses sodium sulphite (Na2SO3), sodium hydroxide (NaOH), anthraquinone (AQ) and methanol (CH3OH). Sodium hydroxide and sodium sulphite are the Kraft cooking liquor whereas anthraquinone and methanol serve as the catalyst. This process is found to be better than Kraft pulping in terms of delignification and pulp yield. ASAM is also suitable for softwood delignification. The ORGANOCELL process is one of the most advanced commercial ORP.  Delignification is performed in two stages: the first stage uses pure alcohol (methanol or ethanol) in water and the second stage NaOH is added [321].

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Fig. 3.41  Kleinert APR process

The process is operated in the first stage with 50/50 methanol/water at temperatures up to 195 °C for 20–25 min. The second stage uses 18–22% NaOH at 165–175 °C and 60 min operation time. The process was found to have difficulties with softwood pulping. 3.3.4.4  Acidic Process Organic acids have been used to dissolve lignin in biomass. Acids that have been used include acetic acid, formic acid, peroxyformic acid and esters [322–336]. These acids are used with or without a catalyst [325]. Formic and acetic acids have been in use since 1917. They were first applied to delignify cereal straw and wood

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[337, 338]. These acids are less expensive than other organic solvents and the delignification can be performed at low temperatures and pressures [339, 340]. The use of 79–92% formic acid containing 0.22% HCI under reflux and mechanical stirring for 90 mins led to 80% delignification of eucalyptus grandis wood [340]. Delignification with formic acid has also been reported by other researchers [341– 343]. A combination of formic and acetic acid to delignify mustard stems showed above 33% delignification when the ratio of formic:acetic:water was 50:30:20 [327]. Birch chips cooked in acetic acid and phosphinic acid (H3PO2) at 150 °C was delignified up to 88% after 120 min treatment time [326]. The ester pulping process was patented by Young and Young in 1987 [333]. The process was originally designed to use a mixture of water, acetic acid (catalyst) and ethyl acetate (solvent). The inventors showed that efficient delignification was obtained for poplar wood but not for oak and eucalyptus. Further studies showed that ester pulping delignifies both hardwoods and softwoods leading to good pulp quality and yields [324]. Organosolv processes based on acidic solvents include processes such as Acetosolv, Acetocell, Formacell and Milox [318]. The MILOX process is formic acid/performic acid process developed in 1986 by a Finnish group [344]. Hydrogen peroxide is usually added to highly concentrated formic acid cooking liquor. A three stage MILOX process applied to delignify Bamboo (Phyllostachys acuta) yielded 88.5% delignification [339]. MILOX process is suitable for hardwoods, softwood and non-­ woody biomass as demonstrated by various studies [330, 345–350]. ACETOSOLV is acetic acid process catalysed by HCl or H2SO4. The process uses very high concentration of acetic acid (>80%). The process is good for delignification of hardwood and softwoods with delignification above 80% [336, 351]. Despite the viability of the process, it was found that the process is prone to corrosion problems because of the presence of acids [352]. An attempt to operate the process without catalyst requires that the process is undertaken at very high temperatures (>170 °C) [353]. The uncatalysed process is called the ACETOCELL process. A further modification of the process to use a combination of acetic acid, water and formic acid [354] is what is called the FORMACELL process [355, 356]. Recent studies on the use of FORMACELL reveal good delignification for non-wood biomass [357, 358]. 3.3.4.5  Chemistry of Organosolv Pulping Delignification during ORP has been said to proceed via cleavage of ether linkages. It is likely that α-ether β-aryl ether bonds can be broken under the conditions of many ORP processes. These reactions are governed by parameters such as pH, physical and chemical properties of the solvent. Uncatalysed Processes The uncatalysed process is a neutral process, meaning that delignification under neutral conditions should prevail in the process. When biomass samples are cooked in a neutral solvent or water, acetic acid is released from the wood sample so that the

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solvent becomes acidic and the delignification proceeds in acidic manner. The reactions in organosolv process proceeds primary by solvolytic cleavage of the α-aryl ether linkages. When the α-aryl ether linkage occurs in a structural unit containing a free phenolic hydroxyl group in the para position, the linkage is likely to cleave faster than β-aryl ether linkage. The reaction in this case proceeds by the formation of a quinone methide intermediate (Fig. 3.42a). Alternatively, a nucleophilic substitution occurs at the benzylic position (Fig. 3.42b) which then reacts with water or alcohol to depolymerize lignin [207, 269]. A low concentration of the organic solvent results in a high concentration of hydronium ions (H3O+) which may lead to reduction in the degree of polymerization. Evidence of cleavage of β-aryl ether linkage during uncatalysed ORP have been reported [359]. β-aryl ether linkages are cleaved in ORP through a rather complex mechanism. The reaction proceeds by the loss of γ-methyl group leading to the elimination of formaldehyde and the formation of enol ether. The enol ether is further hydrolysed forming guaiacol and 2-(4-hydroxy-3-methoxyphenyl) acetaldehyde. The guaiacol is then demethylated to yield catechol (Fig. 3.43). The β-O-4 bonds are cleaved through quinone methide intermediate leading to the formation of β-1 linkages. These linkages then further degrade to produce stilbenes in acidic conditions. Condensation reactions are also possible under uncatalysed ORP. The acetic acid released from the hydrolysis of hemicellulose may protonate the OH groups in Cα of lignin side chain. This leads to the formation of reactive resonance stabilized benzyl carbocation, which easily bonds with C5 or C6 atoms (Fig. 3.44) of another aromatic ring (re-polymerization) [269, 360, 361]. On the other hand, benzyl carbocation formed during the cleavage of α-aryl ether may also enter into bonding with electron-rich carbon atoms of another aromatic ring leading to further re-polymerization.

Fig. 3.42 (a) Solvolytic cleavage of phenolic α-aryl ether linkage via quinone methide intermediate. (b) Solvolytic cleavage of α-aryl ether linkage by nucleophilic substitution

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Fig. 3.43  Solvolytic cleavage of β-aryl ether linkage by a mechanism that involves the elimination of formaldehyde. Reprinted with permission from Elsevier [360]

Fig. 3.44  Lignin condensation reaction during autocatalysed organosolv. Reprinted with permission from Elsevier [360]

Acid and Acid-Catalysed Processes Delignification in the presence of acid usually involves the cleavage of β-O-4 linkages, condensation of lignin, hydrolysis of α-ether linkages and esterification of OH groups. Under acidic conditions, α-ether linkages are hydrolysed by acid catalysis (Fig. 3.42b). Alternatively, the ether linkage can be broken directly by the formation of a resonance-stabilized benzyl carbocation (Fig.  3.45). This reaction is un-­ desirable as it may lead to condensation reactions. Villaverdi et al. [362] showed that dissolved lignin after acetic acid extraction contains more acetyl groups in the Cα and Cɣ positions of lignin compared to mill wood lignin confirming the fact that ester linkages are cleaved in acidic ORP.

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O

R’

H O + R

R’ R” H

+

+ OCH3

OCH3

OCH3 OR

R’

OR

OR

Fig. 3.45  Formation of benzyl carbocation in lignin under acidic conditions

Fig. 3.46  Cleavage of the β ether linkages under acidic conditions [364]

In cleavage of the β ether linkages under acidic conditions, the hydroxyl group on the alpha carbon becomes protonated leading to production of a carbocation at the alpha carbon (Fig. 3.46). A double bond is formed between the alpha and beta carbon to stabilize the carbocation with the loss of proton on the beta carbon. Subsequently, oxygen atom in the β-O-4 linkage then gets protonated producing a leaving group which forms a carbocation at the β-carbon. This enables water to be added to the β-carbon forming an enol ether structure and finally a ketone at the β-carbon [17, 218, 219, 363, 364]. Moreover, the large increase in phenolic hydroxyl content of lignin shown by Gallagher et  al. [359] during autocatalysed ethanol-­ water ORP indicates a rather extensive β-ether cleavage. This is also supported by the studies of Constant et  al. [365], Lai and Guo [366], Nimz and Robert [367], Shimada et al. [368] and Ede et al. [369]. The study of Ede et al. indicated that formylation of the ɣ-methylol group inhibit β-ether cleavage. The reaction tends to remove the degradation pathways of intermediate benzylic cation thereby hindering cleavage of the β-aryl ether linkage. This is more prevalent when the concentration of the organic acid is high [340]. ORP lignins are also known to contain low sugar impurities suggesting that linkages between carbohydrates and lignin are cleaved

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during ORP [370, 371]. In Simon et al. [372], it is shown that depolymerization of lignin under high severity of formic/acetic acid is supported by oxidized phenolic syringyl unit. Studies have also shown that ORP leads to the hydrolysis of cellulose. Hemicellulose is also degraded during ORP [373] via the hydrolysis of xylan to produce xylose leading to the formation of furfural. Alkali-Catalysed ORP Processes Delignification in alkaline-catalysed ORP likely proceeds in similar manner to delignification with soda pulping except that lignin is dissolved by the alcohol and the occurrence of condensation reactions are greatly hindered. Very limited studies have been done on the mechanism of delignification of these processes, making it hard for emphatic conclusions. Some studies on ASAM process shows that residual lignin from the process possesses high S/G ratio indicating the stability of syringyl units under ASAM pulping conditions. Residual lignin from ASAM process also possesses more aliphatic OH groups than Kraft lignin. This suggest that delignification with ASAM may not occur primarily by cleavage of side chains as in Kraft process [374, 375]. There is evidence that the propane side chain S unit are retained in the ASAM residual lignin during the pulping. Residual lignin from ASAM process also contained lower phenolic OH units than Kraft. This confirms the proposition that the β-O-4 linkages are intact and the syringyl units are in abundance. The authors also tentatively concluded that the loss of CγH2OH could hardly occur during ASAM pulping. Choi et al. [376] also reported no significant reduction of β-1 and β-5 units of residual lignin from ASAM process. The residual lignin was found to contain 40% more C6-C3 form of sinapylaldehyde monomers than milled wood lignin (MWL) [377]. This shows that propane side chains were still intact in the aromatic rings. ASAM residual lignins also present marked decrease in molecular weights but comparable to that of MWL [376]. Structural Nature of ORP Lignin ORP lignins are closer to native lignin than Kraft and lignosulphonates. Most researchers report that ORP lignins contain considerable amount of preserved β-O-4 and β-5 units [378–380]. The β-O-4 units in ethanosolv lignin of miscanthus giganteus was found to be above 80% [380]. A lower amount (54.5%) of β-O-4, β−β and β-5 linkages over total lignin aromatic subunits was reported for Broussonetia papyrifera during ethanol organosolv pretreatment [381]. There is also evidence of partial α-ethoxylation of the of β-O-4 linkages. ORPs also contain resinol, phenylcoumaran and spirodiene structures. Figure 3.47 shows the major substructures of ORP lignin from ethanol process. Similar structures were also reported by Bauer et al. [380], Singh et al. [382] and Bai et al. [379]. ORP lignins contain substantial amount of phenolic OH groups [378]. This high content of phenolic OH units is responsible for low molecular weight and high glass transition temperature

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Fig. 3.47  Main structures in ethanosolv bamboo lignin. Reprinted with permission from Elsevier [360]

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of some ORP lignins. One of the important structural aspects of ORP lignin is the prevalence of S units over G units. Research has shown that lignin G units are preferential removed during ethanol organosolv pre-treatment [381]. ORP lignins show higher solubility in organic solvents than most industrial lignins because of their small molecular size [370, 380, 383]. At the initial stage of delignification, acetosolv ORP lignins are small molecules coming out of small pores but as the delignification proceeds, the molecules grow larger [378]. The presence of α-ethoxylated α-hydroxyl group of the β-O-4 linkages lead to a reduction in polarity of ORP lignins and may contribute to their solubility in organic solvents. Ethanosolv lignin extracted at higher concentration of ethanol was found to possess higher solubility in organic solvents than those with lower solvent concentration. This was attributed to the increase in partial α-ethoxylation of β-O-4 linkages and decrease in carbohydrate impurities. The type of carbohydrate present in lignin affects the solubility of lignin to a greater extent. For example the presence of acetonides in acetone extracted lignin results in reduced polarity of the lignin and consequently higher solubility in organic solvents [380]. ORP lignin is purer and more reactive than Kraft lignin because it contains less or no sulphur and ash but more carbohydrates. ORP possesses low polydispersity and molecular weight than other industrial lignins [292, 360, 378, 380, 381, 384, 385]. Nano-particles of ORP have been found to exhibited spherical morphology (200  nm) with unique core–shell nanostructure [386, 387]. The core shells of the nano-particles are made of phenolic hydroxyl groups. There is need to use model compounds to study delignification using alkaline-­catalysed ORP.

3.4  Ionosolv Lignin 3.4.1  Process Description Delignification using ionic liquids is called ionosolv (IL). Ionic liquids (ILs) are low temperature melting salts. IL applied to delignification function in two ways (1) complete dissolution of biomass and further selective precipitation of the required biomass component usually with addition of an anti-solvent (organic or aqueous– organic solution) to precipitate cellulose after which hemicellulose and lignin can be precipitated in a second stage, (2) selective dissolution of lignin and further precipitation using common methods. The prevailing process in each case is dependent on the IL employed [388–400]. Commonly used ionic liquids are made of organic cations. They are mostly quaternized aromatic or aliphatic ammonium ions, alkylated phosphonium and in some instance sulphonium. Inorganic or organic polyatomic anions are commonly used anions. Figure 3.48 shows the structure of anions and cations which are commonly used in delignification of biomass. The ability of an IL to solubilize lignocellulose components is determined by the nature of its anion. The use of protic ionic liquid solution [N2220][HSO4]80% with a/b = 1.02

Fig. 3.48 (a) Some modern cations used in IL. (b) Some modern anions used in IL

3  Bio-sourced Lignin: Recovery Techniques and Principles 125

Fig. 3.48 (Continued)

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Fig. 3.49  Process flow diagram for ionoSolv [390]

on switchgrass and the grass Miscanthus giganteus recorded 76% delignification after 30 min at 170 °C [389]. 1-ethyl-3-methylimidazolium acetate realized a 63% delignification of wood flour after 90 min at 130 °C [397]. The application of the protic ionic liquid 1-butylimidazolium hydrogen sulphate to Miscanthus at 120 °C yielded 96% delignification after 24 h of treatment [393]. In another study, triethylammonium hydrogen sulphate was used to dissolve about 85% of lignin after 4 h at 120  °C [390]. A typical process flow for delignification using IL is shown in Fig.  3.49. Non-toxic and recyclable 1-ethyl-3-ethylimidazolium acetate has been shown to yield over 75% delignification for wheat straw and Miscanthus at 160 °C for 90 min. The same treatment for pine wood yielded about 40% delignification. The use of 1-ethyl-3-methylimidazolium acetate and [bmim][Cl] [401–403] has also been reported by other researchers. Complete dissolution of biomass before selective precipitation of components have been attempted by various researchers [404–407]. Studies have shown that softwood (southern yellow pine) and hardwood (red oak) can be completely dissolved in ionic liquid (1-ethyl-3-methylimidazolium acetate) [401]. Norway spruce sawdust and Norway spruce and Southern pine fibres were easily dissolved in 1-butyl-3-methylimidazolium chloride and 1-allyl-3-methylimidazolium chloride [403]. 1-n-butyl-3-methylimidazolium chloride ionic liquid was only able to afford partial dissolution of pine, poplar, eucalyptus and oak [408]. Complete dissolution of Japanese beech (Fagus crenata) in 1-ethyl-3-methylimidazolium chloride has also been reported [409]. Factors affecting dissolution include the choice of ionic

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liquid, feedstock type, feedstock particle size, dissolution time, water content [410] and temperature. It is widely acknowledged that hard wood are easily dissolved in IL than softwood. IL dissolution of biomass is generally inclined to conditions such as temperature, time and moisture content of the ionic liquid and/or biomass. General trend is that higher temperature and longer delignification periods result in higher degree of delignification whereas high water content results in less delignification efficiency. Important to note in IL delignification is the propensity for the formation of pseudo-lignin. Because extensive condensation reactions occur at higher temperatures, lignin fragments react between each other to form pseudo-­ lignin fractions.

3.4.2  Chemistry of Ionosolv Delignification The prevailing chemistry during delignification using IL is dependent on the type of cation in the IL. However, the anion also significantly affects the yield of the lignin [411]. Most studies on the mechanism of delignification of biomass using IL are based on dissolving Kraft or alkali or Organosolv lignin in IL to check the reaction progress. Although this method provides insight into the kind of reactions expected when lignin is in contact with IL, it does not show precisely how delignification from biomass proceeds. Studies with model lignin compounds are the closest to reality. The following discussion is based on model compound studies and analysis of residual or precipitated IL lignin fractions. Studies using phenolic and non-­phenolic model compounds with [HMIM]Cl in the presence of water showed that IL delignification proceeds by hydrolytic cleavage of the β-O-4 linkages through intermediate products [412]. Based on various evidences arising from their studies the authors proposed a reaction pathway for delignification using IL as shown in Fig. 3.50. In their case, the IL used has an acidic cation [HMIM]+ implying that H+ exist in the system which means that the mechanism of reaction is possibly an acid-­catalysed mechanism. The reaction starts by acid-catalysed dehydration and coupling occurs forming an intermediate compound. Water may then attack the β-carbon of the intermediate compound leading to the cleavage of the β-O-4 bond. Studies on precipitated IL lignin indicates that the mechanism of delignification differ with duration [391]. At early stages of delignification, reaction proceeds by hydrolysis of LCC linkages and cleavage of glycosidic bonds between arabinosyl substituent and xylan core chain. As the delignification continues, cleavage of the β-O-4 linkages occur through Hibbert’s ketone intermediates confirming the mechanism proposed using model compounds [389]. At longer delignification times the β-β and β-5 linkages are chemically altered, and significant condensation reactions also occur. Early stage lignins were similar to MWL with high S/G ratio but this ratio decreased with time. Subsequent model compound studies by Cox et al. [411] show similar delignification mechanism with possibility of formation of two intermediates including enol ether and vinyl ether intermediates which then undergo subsequent hydrolysis to form guaiacol and a carbonyl compound in the case of vinyl ether but rearrangement to form Hibbert’s ketones in the case of enol ethers. ILs used in the study were

3  Bio-sourced Lignin: Recovery Techniques and Principles

129

Fig. 3.50  Proposed acid-catalysed mechanism for hydrolysis of the β-O-4 bonds. Reprinted with permission from [412]

1-methylimidazolium cation with chloride, bromide, hydrogen sulphate and tetrafluoroborate counterions along with 1-butyl-3-methylimidazolium hydrogen ­sulphate. Vinyl ether was not observed when HMIMCl and HMIMBr were used and enol ether was not observed when HMIMBF4 was used but in the case of [HSO4]− both enol ether and vinyl ether were seen as intermediates. The difference in pathway is attributed to the anion present in the IL as the acidity of the IL did not show significant effect on the reactions. When there is a coordinating anion, the reaction proceeds primarily via enol ether intermediate but when there is no coordinating anion involved, the reaction proceeds primarily via vinyl ether intermediate (Fig.  3.51). In other words, the presence of strong nucleophiles such as HS− or SO32− result in β-O-4 bond cleavage to form a carbon-carbon double bond, whereas weak nucleophiles create vinyl ether [398].

Fig. 3.51  Formation of intermediate compounds during IL delignification

130 E. I. Akpan

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3.4.2.1  Structural Nature of IL Lignin The structure of IL lignin differs with the severity of the treatment employed (temperature, time and acidity of the IL). Irrespective of the IL employed, early delignified IL lignin contain significant amount of β-O-4 units [389, 390, 413]. They are close in structure and composition to MWL [391]. They contain a small amount of carbohydrates especially arabinose which will likely make the lignin polar. They also contain alpha methylene group of the β-aryl ether linkage, p-coumaric acid units and traces of ferulic acids. They possess higher S:G ratio with overall abundance of phenolic hydroxyl units exceeding aliphatic hydroxyl units [414]. Increase delignification time and severity leads to a marked reduction in the amount of β-O-4 units and increase in phenolic hydroxyl units. There is also increase in aromatic C-C bonds as a result of condensation reactions [391]. These IL lignins are carbohydrate free [390] and do not contain p-coumaric and ferulic acid units with very low molecular weight. Generally, IL lignins are more condensed than ORP lignins.

3.5  Milled Wood Lignin 3.5.1  Process Description Mill wood lignin also called Björkman Milled wood lignin or even Björkman lignin is a process of extracting lignin that is close to native lignin [415] using a neutral solvent. The method was established by Björkman around 1950 [416–418] involving the use of extensive mechanical pre-treatment to facilitate extraction. Usually, the wood used must be sapwood, which is first air dried and milled to pass 40 mesh using a Wiley mill. Subsequently, the milled wood is extracted with acetone in water (9:1  v/v) by percolation at room temperature and then with ethanol: benzene (2:1 v/v). The extracted wood is later dried under vacuum in a desiccator over phosphorous pentoxide or any other efficient desiccant. Dried wood chips are later subjected to ball milling in a non-swelling solvent such as toluene or dried under nitrogen or carbon dioxide purge gas. The milled wood is dispersed in dioxane: water (96:4 v/v), mechanically stirred and allowed to stay for 24 h. The suspension is centrifuged, and the residue is re-dispersed in the extracting solution. The last step is repeated as many times as possible. The original procedure has been modified by several researchers to increase lignin yields and minimize structural alterations [284, 419–426]. The major disadvantage of MWL process is the low yield of lignin. It has also been argued that long ball milling duration leads to structural degradation of lignin. Increasing ball-milling intensity was found to lead to a decrease in β-O-4 linkages in extracted lignin [426–428]. In attempt to reduce structural degradation and processing time, some new processes have been suggested in line with the MWL process. These processes include cellulolytic enzyme lignin

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(CEL), enzymatic mild acidolysis lignin (EMAL) and regenerated cellulolytic enzyme lignin (RCEL). CEL, proposed by Chang and co-workers involve subjecting ball-milled wood to enzymatic treatment with cellulolytic enzymes (crude cellulases) to remove most of the polysaccharides before aqueous dioxane treatment [429]. The major advantage of CEL is the increase in lignin yield over MWL [429– 431]. EMAL uses a combination of enzymatic and mild acidolysis proposed by Wu and Argyropoulos [432]. The process involves initial mild enzymatic hydrolysis of milled wood and subsequent mild acid hydrolysis. The idea is the initial removal of carbohydrates using the cellulolytic enzyme stage and further selective cleavage of LCC using mild acid in a second stage. The process yields increased delignification efficiency and shorter extraction periods than MWL and CEL. Moreover, EMAL does not require extensive purification steps since most of the non-lignin contaminants are removed by the mild acidolysis [426]. Although some researchers have found a higher amount of β-O-4 linkages in EMAL than MWL, EMAL still contains some amount of carbohydrates [431]. EMAL also possesses higher molecular weight than CEL and MWL. There have been conflicting research results on the difference between MWL, CEL and EMAL.

3.5.2  Chemistry of MWL MWL process is a two-step process and each case is expected to contribute to the alterations in the structure of lignin. The first step involves the use of ball or vibratory milling under varying conditions to reduce the size of the biomass. Studies of lignin model compounds show that milling generally leads to cleavage of etherified β-O-4 lignin structure producing new phenolic β-O-4 structures [422, 430], increasing the content of carbonyl structures [429, 433]. The cleavage is higher when vibratory mill is used under dry conditions and lower with ball milling. The α-O-4 linkages are also affected by milling meaning that depolymerization involving free radicals occur during milling of lignocellulose materials [433]. Vibratory dry milling has been found to result in a substantial increase in α-aryl ether structures. Studies on a trimer lignin model compounds (containing β-Aryl Ether and β’-Aryl Ether) using vibration ball mill in aqueous media show that homolytic cleavage and conversion by dehydrogenation occurs under induced mechanical energy during the milling [434]. In homolytic cleavage, oxygen containing radicals are generated by mechanical energy leading to homolytic cleavage of the Cα-Cβ, Cα’-Cβ’ and ketones to form pairs of radical intermediates (Fig. 3.52a). Dehydrogenation reaction occurs at Cα’ by *OH and *O2H radicals to convert α’ hydroxyl group to carbonyl groups (Fig. 3.52b). These reactions are strongly governed by the presence of the oxygen containing radicals. Removal of oxygen from the milling system by using nitrogen atmosphere or oxygen scavengers such as 1-octane retards the reaction reducing the amount of carbonyl groups. Milling of a lignin model compound having a biphenyl structure (5–5′) − (β–O–4) showed that the reaction proceeds by simultaneous abstraction of two *H radicals, from phenolic OH and Cα-H [435].

3  Bio-sourced Lignin: Recovery Techniques and Principles OH OMe

HO HO O

OMe

OH (b)

OMe

HO

O

+ • OH HO HO

MeO

OH MeO

OH OMe

HO

MeO

MeO OMe 7

OMe O

OMe OH

OH 11

OH 8

+•H

CH2OH MeO

OMe 5

6

0 MeO

O 9

OMe

O O

OMe

10

M.E.

OMe

OH

O

OMe O (e)

OMe

OMe

O (f) O

OMe

7

OMe

OMe O

+ (b)

OCH2CH2OH

+ • O2H, • OH

OMe

OH

MeO

(d)

OMe

CHO

HO

OMe

OMe

MeO

OH

+ OH. O2H HO

O

(c)

HO

MeO

OH

OH

OMe

OMe

O

OMe 4

HO

OH

HO HO

OMe

HO

(1)

OMe

CHO

OMe

O

O

OMe

O

+•H

O

HO

MeO

HO

MeO

OMe

HO HO

M.E.

OMe

HO

MeO

MeO

HO

OH

(1)

OMe

1 + 2 + others

O

OH OMe

+•H O

OMe OCH2CH2OH 3

2

HO

OMe

OMe

MeO

CHO

1

HO

O

OMe

CH2OH OH

HO

M.E.

OMe

+•H

MeO

OH

+•H

(a)

MeO

OMe

O

133

+ OH HO O MeO

MeO O

OH 12

OMe

OMe

O

9

CHO

OMe MeO

OH

OMe 13

OMe

Fig. 3.52 (a) Possible mechanochemical conversions through homolytic cleavage of lignin model compounds. Reprinted with permission from Taylor and Francis [437]. (b) Possible mechanochemical conversions through the dehydrogenation reactions. Reprinted with permission from Taylor and Francis [437]

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This was also noted for another lignin compound [436]. Another study by Ito and co-workers [437] confirmed that *H, OH and O2H generated by ultrasonic waves in aqueous media during milling act as initiators for the cleavage reaction in lignin β–O–4 model compounds. In the study, they confirmed that the reaction starts with simultaneous abstraction of two *H from a phenolic hydroxyl unit and Cα-H leading to the formation of p-carbonyl phenol which can then be oxidized by addition of *H, and O2H to the p-carbonyl. This is then followed by repeated radical hydrolysis. Secondly, intermediate esters can be formed resulting in vanillic acid and a dihydroxy ethane (Fig. 3.53a). There is also evidence of condensation reactions during the milling process. Abstraction of *H from phenolic OH group may lead to the formation of dienone radical, two of which may undergo a condensation reaction (Fig. 3.53b). This was also noted by Itoh et al. [438]. The generation of *H, OH and O2H by ultrasonic waves during milling in aqueous media was also confirmed by Wu and Sumimoto [439].

3.5.3  Structural Nature of MWL MWL is said to be the closest to native lignin in structure arising from the fact that very minimal alteration in lignin structure is expected during the extraction. Following the fact that lignin structural units are mostly terminal, Crestini et  al. [420] proposed that MWL are linear oligomers composed of supramolecular aggregates and not network structure (Fig. 3.54). They predominantly contain β-O-­4′ aryl ether linkages [419, 440, 441]. They also contain a few condensed structures such as spirodienone and dibenzodioxocin structures [420, 440, 442]. MWL usually contain more aliphatic hydroxyl units than phenolic hydroxyl units [423, 443–446].

3.6  Reductive Delignification Process (RDP) RDP is basically a fractionation process that combines both solvolytic extraction of lignin with simultaneous reductive catalytic depolymerization by a heterogeneous redox catalyst. The process is essentially an organosolv process with a reductive catalyst which produces highly depolymerized lignin oil instead of polymeric lignin precipitates [368, 447–461]. Organic solvents such as ethanol, methanol, iso-­ propanol and acetone are used as fractionation media coupled with redox catalyst [368]. A deep illustration of this method is beyond the scope of this book.

Fig. 3.53 (a) Proposed pathway for cleavage in lignin model compound [437]. (b) Condensation reactions during milling of lignin model compounds

3  Bio-sourced Lignin: Recovery Techniques and Principles 135

136

E. I. Akpan CH2OH CH

O

CH2OH

G

CH

HCOH

CH2OH

HOH2C

G

O

G

O

HCOH

HC

CH

HOCH

HCOH

O

G

–.H ×2

H

MeO

MeO O

O

H OH

[I]

G

CH2OH

O HC

CH O

MeO

G

G

HCOH

–4.H OMe

OH

O

[Ia]

HOH1C

HOCH

MeO

H

OH

8

H

H

OMe O

[Ib]

HOH2C

CH2OH

O HC

CH O

O

C

C

OH

OH

MeO

HOH2C

G

O

OMe

9

+.H + . OH + . O 2H

G

O HC O

C

O

OH

O

MeO

OMe

10

Fig. 3.53 (Continued)

3.7  Conclusion The chapter outlines exhaustively the different delignification mechanisms used to remove lignin from biomass sources. It is apparent that each process results in differing effect on the structure and the properties of the lignin recovered. Prospective use of lignin for carbon fibre applications must be based on adequate knowledge of the resultant lignin structural character. Organosolv process is very attractive in terms of cost effectiveness and environmental sustainability. When the focus is retaining the substantial part of the native lignin structure then MWL, CEL and EMAL may be considered. However, the yield of MWL is not industrially viable.

Fig. 3.54  Schematic representation of softwood MWL. Reprinted with permission from ACS [420]

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Chapter 4

Biosourced Lignin: Sources and Properties Samson Oluropo Adeosun, Oluwashina Phillips Gbenebor, and Odili Cletus

4.1  Introduction Lignin happens to be the second most abundant natural polymer in nature. It is an aromatic polymer found in every vascular plant on earth [1]. Lignin is responsible for a weight of 1011 tons in biosphere and an annual increase of 1010 tons accounting for 30% of total carbon atoms in nature [2, 3]. Because lignin has an aromatic backbone with relatively high carbon content (≈60% carbon content) and it is widely available, it qualifies as a source for carbon materials, for example, carbon fibers [4–6]. Moreover, because of environmental concerns attention has been directed on the production of carbon fibers from bio-based sources. Lignin can be found in all plants including hardwood, softwood, and annual crops. Interestingly, lignin does not have to be obtained by falling trees and destroying the ecosystem, but it can be obtained from agricultural waste sources and wastes from the pulping industries. Pulp and paper industries produce around 50–60  million tons of lignin annually. This is beside the amount of lignin that can be obtained from agricultural waste sources. As at the moment, most of the lignins obtained from the pulp industry are combusted and used to produce synthetic vanillin and dimethylsulfoxide (DMSO). They are also used as dispersants, adhesives, emulsifiers, fillers, and binders. Lignin is a three-dimensional polymer of phenylpropane units linked together in different ways. It is synthesized from three monolignols, namely p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol [7, 8]. Lignin derived from these monolignols is commonly referred to as hydroxyphenyl (H), guaiacyl (G), and syringyl (S) lignin [9, 10]. The amount and composition of lignins in plants vary according to botanical source (taxonomy), cell types, and cell wall layers. Wu et  al. [11] showed that lignin in wood is affected by cell type, site of growth rings, vessel

S. O. Adeosun (*) · O. P. Gbenebor · O. Cletus Department of Metallurgical and Materials Engineering, University of Lagos, Lagos, Nigeria e-mail: [email protected] © Springer Nature Switzerland AG 2019 E. I. Akpan, S. O. Adeosun (eds.), Sustainable Lignin for Carbon Fibers: Principles, Techniques, and Applications, https://doi.org/10.1007/978-3-030-18792-7_4

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arrangement, and the area where they were produced. Generally, dicotyledonous angiosperm (hardwood) consists principally of G and S lignin units and traces of H units [1, 12], gymnosperm (softwood) consists mostly of G lignin units with low levels of H units [13, 14], and grasses (monocots) mostly have G and S units at comparable levels but with H units more than that of hardwood [8, 10]. The different monomers in lignin are linked together by various linkages including β-O-4 (β-aryl ether) linkage, β-5, β-β, 5-5, 5-O-4, and β-1 [15, 16] (Fig. 4.1). Because of the difference in the composition of lignin in the different sources and the linkages present, they exhibit varying structures, molecular weights, syringyl/guaiacyl (S/G) ratios, and macromolecular behaviors. To produce carbon fibers, lignin must be converted into fibers via melt, dry, or wet spinning. In all cases, the processing of lignin into fibers is affected by the properties of the lignin arising from the various units and linkages outlined above.

Fig. 4.1  Monolignol units and popular linkages in lignin. Adapted with permission from RSC [17]

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For melt processing, the lignin must have a considerable glass transition temperature and must not contain too much volatiles. Although lignin has been obtained from different sources, only lignins from hardwood and softwood sources have been widely investigated for conversion to carbon fibers. A few studies have reported the conversion of lignin from non-woody biomass to carbon fibers [18–20]. To motivate research in the conversion of lignin from other sources to carbon fibers and provide up-to-date knowledge of other viable sources of lignin, this chapter outlines several sources where lignin can be obtained and their relative abundance. Where possible, the type of lignin in its crude form (in terms of HGS) and a brief description of the structure are presented.

4.2  Sources of Lignin Lignin has been extracted from various sources. However, a few of them have been processed into carbon fibers. In this chapter, a catalogue of lignin sources have been extracted and characterized. The predominant monolignol unit is used to represent the type of lignin, and the percentage abundance of the aryl ether linkage is also presented. Milled wood lignin (MWL) has been extracted from Paulownia fortunei wood [21]. Relative content of the lignin was found to be 22.7% Klason lignin. Lignin from this wood is a basic G unit lignin having 62% of β-O-4 linkage after extraction. The major subunits are shown in Fig. 4.2. Milled wood lignin has also been extracted from brewer’s spent grain [22]. The relative lignin content was found to be 8.8% Klason lignin with a guaiacyl structure. The lignin was found to possess 77–79% alkyl-aryl ethers units after MWL extraction. The basic lignin units are shown in Fig. 4.3. Milled wood lignin from Eucalyptus grandis [23] was found to possess syringyl structure with about 62% of β-O-4 units. Lignin from Eucalyptus grandis [24] is reported to have a crude relative content of 8–10%. Ethanol organosolv lignin has also been extracted from bamboo [25]. Lignin from bamboo was found to be an HGS-type lignin. The major lignin side chains were β-O-4, β-β, and β-5. Lignin from Norway spruce and pine [26] extracted using ionic liquids shows 26% lignin content with largest unit as β-O-4 structures. In another study on Norway spruce [27] with mild acidolysis, the relative content of lignin was found to be 45% with prevalent β-O-4 structures. Ionic liquid lignin from yellow pine and red oak [28] has been extracted with 31% and 38% relative content, respectively. The lignins were found to be G and GS lignins, respectively. Moniruzzaman et al. showed that lignin can be obtained from hinoki cypress (Chamaecyparis obtusa) using ionic liquids [29]. Lignin has been extracted from oil palm [30] wood using ionic liquids with relative content of 8.5%. Organosolv lignin from rice husk [31] was found to be a G lignin with prevalent structure being β-O-4 units. Lignin has also been extracted from chemical pulp of spruce and beech wood [32] with relative content of 6.32% and 4.30%, respectively. The lignin was found to contain 50–60% of β-O-4 structures. Lignin has also been extracted from loblolly pine and sapwood [33] using cellulolytic enzyme

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Fig. 4.2  Major structural units in Paulownia fortune wood lignin. Reprinted with permission from Elsevier [21]

lignin and milled wood lignin methods. About 75% beta aryl ether linkages were preserved after extraction. Cybulska et  al. extracted lignin from prairie cordgrass, switchgrass, and corn stover [34] using the ethyl acetate–ethanol–water mixture. Lignin relative contents in these plants were 20.96%, 20.45%, and 16.29%, respectively. Quantitative analysis of phenolic hydroxyl groups showed that the only types of phenolic structures present were those of guaiacyl, syringyl-propane, and stilbene. Lignin has also been extracted from wheat straw [35] using organosolv process. Lignin in wheat straw was found to be a GS lignin with relative content of 22.34%. The main inter-unit linkages are shown in Fig. 4.4. Lignin has also been extracted from sweetgum [36] using the MWL method with 25.6% relative content. The extracted lignin contains prevalently the β-O-4 structure. Another study on bamboo using MWL and kraft lignin processes has been reported [37]. The lignins were found to be guaiacyl and syringyl lignins. Several studies have been reported on lignification, delignification, and chemical composition of

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Fig. 4.3  Major structural units in Brewer’s spent grain lignin. Reprinted with permission from ACS [22]

bamboo lignin [38–49]. Lignin has also been extracted from spruce and birch wood chips [50] using organosolv and alkaline processes. The biomass had relative lignin contents of 26.1% and 28.1%, respectively. Results show that these lignins are Gand GS-type lignins, respectively, with prevalent β-O-4 ether bonds after extraction. Lignin has been obtained from alfalfa fibers, pine straw, wheat straw, and flax fibers using formic acid/acetic and peroxyformic acid/peroxyacetic acid [51]. Lignin relative contents in the fibers were determined as 34%, 22.65%, 20.40%, and 14.88%, respectively. Palm oil empty fruit bunches [52] have been shown to contain 25.1% lignin. Lignin extracted with organosolv process was found to be a GS-type lignin. Figure 4.5 shows the chromatogram of lignin from palm oil empty fruit bunch. Lignin has been extracted from sugarcane bagasse [53] using organosolv pretreatment. The bagasse had lignin relative content of 22.69%. α-O-4 and β-O-4 were the prevalent side-chain structures. The OrganoCat process has been used to obtain lignin from beech wood, reed, and spruce [54]. Theoretical lignin content in beech

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Fig. 4.5 GPC chromatogram of EFB acetylated lignin sample, showing the Mn of the different fractions [52]

Detector response

Fig. 4.4  Monomer units in the extracted lignin. Reprinted with permission from Elsevier [35]

7.1.103 2.3.103 1.3.104

18

20

1.1.103

22

24

26

0.5.103

28

30

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wood was 26%, Marte tea had 28%, reed had 17%, and spruce had 27%. Lignin from almond shells [55] was found to be GS-type lignin with maximum relative content of 7.8%. The prevalent inter-unit structure was β-O-4 when extracted with organosolv pretreatment. Lignin has been extracted from wheat straw [56] using acetic and formic acid organosolv. Crude lignin content reported in the study was 21% with 91% delignification using the organosolv process. Huijen et al. reported 16% lignin in wheat straw from Spain and 79% delignification using acetone organosolv [57]. Wheat straw lignin reported by Jimenez et al. [58] had 17% crude lignin. Lignin from corn Stover [59] has a relative content of 15% and possess a GS structure. Vietnam rice straw has been found to contain 19.02% lignin [60]. Lignin extracted from Miscanthus giganteus [61] using ethanol organosolv was found to be HGS-type lignin with 82–84% β-O-4 and 21.4% relative content. Sun et al. [62]

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extracted lignin from oil palm empty fruit bunch using soda anthraquinone process. Lignin fractions had relatively higher content of phenolic hydroxyl groups compared to primary aliphatic hydroxyl groups. Lignin has been extracted from dhaincha (Sesbania aculeata), kash (Saccharum pontaneum), and banana stem (Musa Cavendish) using formic acid pulping [63]. Dhaincha had lignin relative content of 27.4%, Kash had 16.0% lignin, and banana stem had 12.7% lignin. All lignins were SG type with S:G of 1.4 for kash, 1.6 for banana, and 2.0 for dhaincha. Lignin from southwest birch [64] was found to be SG-type lignin with relative content of 23.4%. Fescue (Festuca arundinacea Schreb.) [64] extracted using organosolv process is reported to be an HGS lignin with relative content of 19.4%. Other studies on wheat straw show 17.28% relative content [65] and 17.6% [66]. A study on delignification of Eucalyptus globulus wood [67] shows that the wood contains 23.3% lignin. Willow wood [68] was found to contain 28.5% acid-insoluble lignin. Huijgen et al. showed that lignin in wheat straw is HGS-type lignin [69] with S/G substantially lower than that of Alcell lignin. Triticale straw, wheat straw, corn residue, flax shives, and hemp hurds have been studied by Monteil-Rivera et al. [70] and were found to contain 21.1%, 20.5%, 22.0%, 25.3%, 24.6% lignin, respectively. All the lignins are HGS type and were extracted successfully with organosolv pretreatment. The difference in structure of the lignins is illustrated in Fig. 4.6. Buddleja davidii [71] has a relative lignin content of 30% lignin with GS structure. Structural units of the lignin are shown in Fig.  4.7. B. davidii lignin obtained using the organosolv delignification possesses low oxygen content, molecular weight, and aliphatic OH as well as high phenolic OH, which make it suitable for carbon fiber applications. Moreover, the lignin content is high enough to justify commercial use.

Fig. 4.6  FTIR spectra of ethanol organosolv lignins extracted from (T) triticale straw, (W) wheat straw, (C) corn residues, (F) flax shives, and (H) hemp hurds [70]

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Fig. 4.7  Lignin substructures identified in B. davidii MWL. Reprinted with permission from ACS [71]

Miscanthus lignin is an H/G/S type (4%, 52%, 44%, respectively) with approximately 0.41 β-O-4 linkage per aromatic ring and contains coumarylate linkages (0.1/Ar) [72]. Oil palm empty fruit bunches (OPEFB) [73]. Lignin extracted from oil palm empty fruit bunch [74] can be considered an SGH lignin. The oil palm empty fruit bunch has lignin relative content of 14.2%. Wen et al. [75] investigated the extraction of lignin from birch wood using organosolv process. Birch lignin was found to possess 0.66 β-O-4 linkage per aromatic ring with original relative lignin

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content of 23.4%. Exploded wood lignin has been extracted from aspen (Populus tremuloides) wood using methanol [76]. Lignin has also been extracted from seed coats of Vanilla planifolia using chloroform and methanol [77]. The lignin was found to be a C-lignin (G units lacking the methyl group on the 3-oxygen) or poly(caffeyl alcohol). Electrospun carbon fibers obtained from this lignin have superior hardness modulus to kraft lignin fibers. Lignins have been extracted from stems and foliage of Arundo donax linn [78] using milled wood and alkaline processes (Fig. 4.8). The lignins were found to be HGS-type lignins. The stem has original lignin content of 19.66% and the foliage had lignin relative content of 12.45%.

Fig. 4.8  Main structures of lignin fractions of Arundo donax. Reprinted with permission from ACS [78]

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Fig. 4.9  Total-ion chromatogram (TIC) from GC/MS of the DFRC′ degradation products from the MWL isolated from coconut coir Reprinted with permission from ACS [79]

Lignin from coconut coir fibers has been extracted using mill wood process. The coconut lignin is demonstrated to be an HGS lignin with G unit (S:G ratio of 0.23) predominance. Total lignin content in coconut coir fibers was found to be 33.5% [79]. An important observation in the study of lignin from coconut shell is that the main linkages present in this lignin are β-O-4 alkyl aryl ethers, followed by phenylcoumarans, resinols, with small amounts of dibenzodioxocins. It is also shown that coconut coir is partially acylated (11% of all side chains), and exclusively at the γ-OH of the side chain, with p-hydroxybenzoates. Although this lignin is a grass-­ type lignin, it has a predominance of G units and has a very high relative content (Fig. 4.9). More studies on the isolation and processing of lignin from this source into useful products are required. Lignin from different species of cactus species has been extracted and characterized [80]. Results show that lignin from Melocactus salvadorensis is a catechyl lignin polymer (C lignin), Mammillaria densispina is a GS lignin, Mammillaria lasiacantha is a C lignin, Escobaria albicolumnaria is a GS lignin, and Escobaria dasyacantha is HGS lignin (Fig. 4.10). Nuclear Magnetic Resonance (NMR) analysis studies show that lignin from Escobaria dasyacantha seeds consists almost entirely of 5H units. H lignin is characterized by chains of benzodioxane units. Lignins have been extracted from various poplar genotypes using organosolv fractionation [81]. Lignin has been extracted from [82] oil palm empty fruit bunch using kraft and soda pulping processes. The lignin from the empty fruit bunch was found to be a GS lignin. Report shows a relative lignin content of 18.8%.

Fig. 4.10  Gas chromatogram traces of the thioacidolysis products of lignin from seeds of the cactus species. Reprinted with permission from John Wiley and Sons [80]

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Oliveira et al. [83] extracted and characterized lignin from stalks of banana plant using dioxane. Lignins from banana stalks are HGS lignin. Lignin extraction from sugarcane bagasse shows a crude lignin relative of 21.4%. The lignin was found to be HGS in structure [84]. Lignin has also been obtained from corn stalk using alkali extraction [85]. Studies show that steam-exploded birch wood (Betula platyphylla) lignin can be converted to carbon fibers with good properties [86]. Santos et al. [87] investigated the extraction and structure of lignin from different eucalyptus species. Results showed that the original relative content of bangalay, forest red gum, gray iron bark, and lemon-scented gum were 28%, 32.4%, 36.7%, and 25.3% respectively. All the organosolv lignins extracted were characterized as a GS-type lignin. Lignin has been extracted from olive tree wood by steam explosion and alkaline peroxide delignification [88]. Lignin relative content of the wood was found to be 20.4%. Vignon et  al. [89] investigated the extraction of lignin from hemp chenevotte. The lignin content of the woody core was determined to be 28%. Jeibing et  al. [90] investigated the extraction of lignin from various sources including Norway spruce (Picea abies), Scots pine (Pinus sylvestris), birch (Betula verrucosa), and aspen (Populus tremula). Original lignin contents were 28%, 28%, 23%, and 23%, respectively, and are basically of the GS-type lignin. Triploid of Populus tomentosa Carr [92]. lignin was found to be an HGS-type lignin with original Klason lignin content of 21.5%. Mill wood and CELL extraction processes retained 81.1–84.5% β-O-4 of the aryl ether linkages. Lignin in Pinus radiata [93] is purely a G-type lignin and can be easily extracted with dichloromethane, water, and ethanol. Cortex and pith of elephant grass stems [91] were found to contain 20 and 17.1% lignin (Figs. 4.11 and 4.12). The lignin is typically an HGS lignin and when extraction was done with MWL process about 82% β  −  O  −  4 alkyl aryl ether linkage was retained. Another study on bamboo [94] showed 26.1% relative lignin content of HGS type and retained 57–64% β-O-4 aryl ether when extracted with formic acid organosolv pretreatment (Fig.  4.13). Basswood and loblolly pine [95] lignin are GS-type lignins with relative contents of 16.8 and 27.5% Klason lignin, respectively. Eucalyptus tereticornis [96] lignin are SG lignin with a relative content of 30%. After extraction, the lignin retained 86.2– 90% β-O-4 linkages. Another study on Eucalyptus globulus [97] shows relative lignin content of 27%. The lignin is a GS-type lignin (Fig. 4.14). The lignin was found to contain 77–80% phenolic hydroxyl groups after ethanol organosolv process. Another study determined the relative lignin content of wheat straw [99] to be 15%. The lignin from this study is basically a GS-type lignin and retained 75% β-O-4 ethers after extraction using the MWL method. Lignin from jute fiber [98] is typically an HGS lignin with relative content of 13.3%. After MWL extraction, the lignin was found to contain 72% β-O-4 aryl ether linkages (Figs. 4.15 and 4.16). Another study on Eucalyptus globulus [100] showed that lignin content in the plant increases with the age of the plant. Plants that were 1 month old had 16% lignin but plants that were 9 years old had 25% lignin. MWL treatment resulted in a lignin with 69–72% β-O-4 linkages. Investigation of lignin from loblolly pine [101] showed 29% Klason lignin. Extraction with ethanol organosolv process showed dominance of β-O-4 linkages.

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Fig. 4.11  Py–GC/MS chromatograms of elephant grass (P. purpureum) cortex (a) and pith (b), and the MWLs isolated from cortex (c) and pith (d). Reprinted with permission from ACS [91]

Lignin has been extracted from black liquor of KOH/anthraquinone pulping of oil palm empty fruit bunch fiber [102]. The lignin is an HGS-type lignin with original content of 14.2%. Aspen wood [103] was found to contain 20.1% lignin and after steam explosion, 10–50% β-O-4 linkages was retained. Another study on wheat straw [104] showed 14.1% lignin content. The study confirmed that wheat straw lignin is a GHS-type lignin. Shi et al. investigated bamboo as a good source of lignin for commercial purposes [105]. Original lignin content in bamboo was found to be 27.08%. After ethanol organosolv extraction, the lignin was found to retain 74.3% β-O-4 aryl ether linkages. Figure  4.17 shows the main structures of the bamboo lignin. Lignin has been extracted from leaf, pith, and whole bagasse of yellow poplar wood chips, peanut hulls, and sugarcane bagasse [106]. Eucalyptus globulus [107] wood has been investigated as a source of lignin using the kraft process. The study reported 21.8% lignin content. Lignins obtained from pine by acetosolv process have been characterized [108]. All lignins obtained from the study were classified as G type. Lignin has been obtained from birch sawdust through simultaneous solvolysis and catalytic hydrogenolysis [109]. Birch wood sawdust has a relative lignin

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Fig. 4.12  Main structures present in the lignins of elephant grass (P. purpureum). Reprinted with permission from ACS [91]

content of 19%. Hattalli et  al. [110] extracted and characterized lignin from alfa grass using soda pulping process. The grass has a relative lignin content of 14.9%. The lignin is a typical HGS lignin with relatively high content of H and G structures. Lignin has been recovered from brewery spent grain [111, 112] using soda pulping. The spent grain had original lignin content of 27.8%. Malaysian cultivated kenaf fiber is reported to have up to 21% lignin content and can be a source for lignin recovery [113]. Kraft lignin has been extracted from oil palm empty fruit bunch

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Fig. 4.13  Main structures present in the acid-insoluble lignin fraction of bamboo. Reprinted with permission from Elsevier [94]

pulping residue [114]. Corn stover has been found to contain over 18% lignin and can be used as a source of lignin for carbon fibers [115]. A comparative study on the isolation of lignin from alfa grass has been reported by Nadji et al. [116]. The lignin is a typical HGS lignin with relative content of 21.8%. Miscanthus sinensis pulping with 7.5% soda has been studied [117]. The miscanthus has a lignin content of 18%. Waste from oil palm extraction process has been used as a source of lignin [118]. The structure of the waste lignin was found to be similar to that of commercial lignin as determined by Fourier Transform Infrared Spectroscopy (FTIR). Formacell lignin has been obtained from oil empty fruits bunches by Hidayati et  al. [119]. Lignin in triploid clones of Populus tomentosa [120] can be up to 17% crude content. The structure of lignin from triploid P. tomentosa has similar characteristics to that of hardwood from temperate areas. Ibrahim et  al. [121] isolated and characterized lignin from corn cob, banana plant, cotton stalk, and cotton gin wastes using steam explosion followed by ­alkaline treatment. Lignin from these sources is 15.83, 20.31, 20.56, and 27.58% for corn

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Fig. 4.14 (a) Main substructures identified in ethanol organosolv lignin from Eucalyptus globulus. Reprinted with permission from Elsevier [97]. (b) Ionization difference spectra of organosolv lignins of Eucalyptus globulus. Reprinted with permission from Elsevier [97]

Fig. 4.15  Py-GC/MS chromatogram of jute (C. capsularis) fibers. Reprinted with permission from ACS [98]

Fig. 4.16  Main structures present in jute (C. capsularis) lignin. Reprinted with permission from ACS [98]

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Fig. 4.17  Main substructures presented in alkaline ethanol extractable bamboo lignin. Reprinted with permission from John Wiley and Sons [105]

cob, banana, cotton gin waste, and cotton stalk, respectively. Fang et al. [122] isolated lignin from tobacco stem using ionic liquids. Lignin accounts for 6% of total mass in tobacco stem. Sjöstrom [123] reported lignin from scot pine, spruce, silver birch, and eucalyptus. Lignins in these plants were found to be 27.7%, 27.5%, 22%, and 31.3%. Lignin from corn stalk [124] belonged to G–S lignin with predominantly G-type units. Ammonia-extractable lignin fractions had 75.6% β-O-4 interunit linkages. The relative lignin content of the stalk was 20.99%. Sugar maple (Acer saccharum) has also been identified as a source of lignin for industrial applications [125]. The lignin is an SG lignin. Lignin has been extracted from sugar maple wood [126] using ammonia. Aleurites moluccana (candlenut) nutshells [127] have been suggested as a source of lignin. The lignin is predominantly guaiacyl with high hydroxyl group content. Lignin has also been obtained from Japanese rice straw. It has original lignin content of 25.8% [128]. Huang [129] isolated lignin from corncob residue using enzymatic hydrolysis followed by acid hydrolysis. The lignin is a GS-type lignin with relative content of 13.52%. El Hage et  al. [130] isolated lignin from Miscanthus x giganteus. The relative content of lignin in the

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plant was found to be 26.0%. Lignin from Eucalyptus pellita [131] was reported to be a GS-type lignin. Three species of Eucalyptus (camaldulensis, grandis, and urophydis) [132] have been studied as a source of lignin for industrial applications. The extracted lignin fractions are SG-type lignins with 26.2%, 28.95%, and 25.75% original contents, respectively. Lignin from Eucalyptus grandis [133] is about 10% weight of the biomass. Lignin from bamboo studied by Li et  al. [134] is a HGS-type lignin with initial lignin content of 26.1%. Formic acid organosolv delignification resulted in 57–64% β-O-4 retained in the lignin. Lignin has been isolated from mustard branches, stems, and lentil stalks [135]. These agricultural wastes were characterized by relatively moderate amounts of lignin (20–23%). Alamo switch grass and yellow poplar chips have been investigated as sources of lignin [136]. These lignins are typically HGS and GS lignin with more than 80% β-O-4 aryl ether linkages after organosolv pulping. Lisperguer [137] isolated lignin from Eucalyptus nitens and Pinus radiata. Lignins from these sources are S-type lignin. Barely straw, Chamaecytisus, wheat straw, and Leucaena have been investigated as sources of lignin [138] using soda pulping. Lignins in these sources are 16.30%, 16.80%, 17.70%, and 18.40%, respectively, and belong to the HGS-type lignin. Lignin isolation from four different species of Eucalypts (Eucalyptus camaldulensis, Eucalyptus microtheca, Eucalyptus tereticornis, and Eucalyptus citriodora) from Sudan [139] have been studied using kraft-AQ (anthraquinone), soda-AQ, modified AS/AQ (ASA), ASAM, and kraft methods. Lignin contents are 20%, 22.7%, 20.7%, and 23.5% for Eucalyptus camaldulensis, Eucalyptus citriodora, Eucalyptus microtheca, and Eucalyptus tereticornis, respectively. Mohtar et  al. [140] isolated lignin from oil palm using ionic liquids. Original lignin content was reported to be 23% and belong to the GS-type lignin. Moroccan sugarcane bagasse [141] contains approximately 25% lignin of the GS type. Jeibing et al. [142] investigated the isolation of lignin from aspen wood using autohydrolysis. The study reported that the lignin is of the HGS type and after hydrolysis retained only 54% β-O-4 linkages. Lignin in two species of fresh pear (pyrus) (jingaisu and kousui) [143] has been reported. The study reported relative content of 29.8% and 24.6% for Pyrus jingaisu and Pyrus kousui, respectively. William et al. [144] investigated the isolation of lignin from sugarcane bagasse and curaua using acidolysis. Lignin in sugarcane bagasse is of the HGS type and that of curaua is GS type. Sugarcane bagasse has a relative content of 25.3% and curaua 7.5%. Sawdust of aspen wood (Populus tremula) has been found to contain 21.4% lignin of the GS type [146]. Structural differences of lignins from three nonwoody angiosperms have been analyzed by 2D NMR to reveal the differences in their molecular structures [145]. MWL from Musa textilis had S/G ratio of 9 with near 85% acylation of side chains, MWL from Cannabis sativa showed S/G of 0.8 with no acylation of side chains and MWL of Agave sisalana showed S/G of 4 with 80% acylation of side chains. The main structures of the lignin are shown in Fig. 4.18. Musa textilis, Cannabis sativa, and Agave sisalana are GS-, GS-, and HGS-type lignins and retained 93%, 87%, and 69% β-O-4, respectively, after MWL process. Lignin from kenaf (Hibiscus cannabihus) [147] has been isolated using soda pulp-

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Fig. 4.18  Main classical and acylated substructures of the Agave sisalana, Musa textilis, and Cannabis sativa lignins. Reprinted with permission from Elsevier [145]

ing. The study reported original relative content of kenaf to be 18.3% with S-rich structure. Fagus syvatica [148] was found to contain Klason lignin of 23.7% and acid-­ soluble lignin of 2.7%. Delignification according to the Milox process was found to be suitable for isolating lignin from the plant. Four eucalypt wood clone samples [149] have been investigated. The clones include Eucalyptus urophylla/E. urophylla, E. grandis/E. urophylla/E. globulus, Eucalyptus dunnii/E. grandis/E. urophylla, and E. urophylla/E. grandis with original lignin contents of 30.3%, 28.9%, 29.8%, and 27.2% and S/G ratio of 2.8, 2.9, 2.6, and 2.7, respectively. Mittal et al. [150] showed that the lignin from corn stover is GS-type lignin with relative original content of 16.5%. Lignin extracted using ammonia pretreatment retained predominantly β-O-4 linkages. Lignin from Eucalyptus grandis wood chips and sugarcane bagasse [151] has been isolated using the Milox process. Lignin in E. grandis was 27.5% and

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Fig. 4.19 Qualitative 13C NMR of the acetylated lignins El (top), E2 (middle), and B1 (bottom). Reprinted with permission from Taylor and Francis [151]

that of sugarcane bagasse was 25.1%. Structural comparison between sugarcane bagasse and E. grandis is illustrated in Fig. 4.19. With such high percentage content in sugarcane bagasse and similarity in structure to wood lignin, added to the fact that sugarcane bagasse is a waste material, more studies on this lignin and its conversion to useful products are required. Ruggiero et al. [152] also isolated lignin from E. grandis chips by combining peroxyformic/formic delignification. Rio et al. investigated the occurrence of native acetylated lignin in a large set of vascular plants [153] by modified derivatization followed by reductive cleavage (DFRC) method. The study showed the S/G ratio of 2 for sisal, 0.2 for palm tree, 1.1 for bamboo, 3.0 for abaca, 1.4 for beech, 2.8 for hornbeam, 0.6 for hemp, 3.1 for kenaf, 1.2 for jute, 1 for aspen, 2.3 for E. globus, and none for spruce and pine. Ralph [154] found that lignin isolated from kenaf bast fibers had a high S/G ratio, and was highly etherified with predominantly erythro stereochemistry (Fig. 4.20). Miscanthus giganteus lignin has been isolated after extraction with the protic ionic liquid 1-butylimidazolium hydrogen sulfate followed by precipitation with the antisolvent water [155]. The lignin is an HGS-type lignin. Bai et al. [156] confirmed that bamboo MWL is of HGS type associated with a considerable amount of p-­ coumarate and ferulic esters of lignin. Lignin content of the untreated bamboo was 23.8% Klason lignin. The chemical structure of milled wood lignins from E. globulus, E. nitens, Eucalyptus maidenii, E. grandis, and E. dunnii has been investigated [157]. All the lignins showed a predominance of β-O-4 linkages (66–72% of total side chains). Residual lignins from spruce and beech wood have been isolated by

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Fig. 4.20  13C NMR spectra of the isolated kenaf lignins showing the preponderance of syringyl peaks. Reprinted with permission from ACS [154]

enzymatic hydrolysis from sulfite, kraft, ASAM, and soda/AQ/MeOH pulps [158]. Lignin from spruce is GH type with G predominance (over 90%) but beech is an HGS lignin with S/G ratio in the range from 0.68 to 1.12 depending on the method used in extraction. Prado et al. [159] investigated delignification of willow lignin. Kuroda et al. [160] investigated the lignin present in softwood species of Japanese red pine, Japanese cedar, and Japanese cypress using pyrolysis. Milled wood lignin from loblolly pine stem wood, forest residue, and bark have been isolated and characterized [161]. The stem wood and forest residue had similar functional group

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content. Lignin contents of raw samples are 28.0, 26.8, and 33.7 for stem, residue, and bark, respectively. Lignin in loblolly pine seems to be a GS lignin but with trace amounts of H monolignol unit. Lignin has been extracted from Triploid of Populus tomentosa Carr [162]. The plant had initial lignin content of 27.9% with GS structure. Lignin has also been isolated from salix using ionoSolv pretreatment [163]. Delignification of Broussonetia papyrifera with ethanol organosolv pretreatment has been studied [164]. Extractive-free Broussonetia papyrifera showed 13.28% of Klason lignin with SG structures. Residual lignins from unbleached and oxygen-­ bleached birch kraft pulps have been isolated by acid hydrolysis and characterized [165]. It was shown that the lignin is of the SG type with a syringyl-to-guaiacyl ratio of approximately 1:1. Pinus sylvestris pine and Salix viminalis willow have been depolymerized to obtain lignin [166]. Residual lignins from the chemical pulps of spruce (Picea abies) and beech (Fagus sylvatica) wood have been isolated and characterized [167]. Organosolv lignins have been isolated from different species of rice husks [168]. The lignin is a GS-type lignin with G predominance. Lignin has been isolated from cedar wood chips using solvolysis with polyethylene glycol (PEG) 400 and converted to carbon fibers [169]. The lignin content of the cedar wood chip measured by Klason method was 30.1%. Organosolv lignin has been isolated from Alamo switchgrass (Panicum virgatum) and yellow poplar (Liriodendron tulipifera) and converted to carbon fibers [170]. Yellow poplar is GS-type lignin and switch grass is HGS-type lignin. Lignins from birch, spruce/pine, and Eucalyptus globules have been investigated for carbon fiber applications [171]. Enzyme saccharification lignin obtained from Pinus radiata wood pulp [172] has been modified in lieu of carbon fiber production. Chemical composition and functional properties of lignin from wheat straw, sisal, abaca, hemp, jute, and flax have been examined by Boeriu et al. [173]. Straw lignin from various sources has been characterized [174]. Lignin from Jute has been isolated and characterized [175]. Jute contains 11–12% lignin with β-O-4 structures mainly of the erythro-syringyl ether type. Lignin from Bagasse has been isolated and analyzed structurally by Fernandez [176]. Milled wood lignin from Reed canary grass (Phalaris arundinacea) has been isolated and characterized by Galkin et al. [177]. He and Terashima [178, 179] studied the formation and structure of lignin in of sugarcane and rice plants. The study showed that lignin content differs according to the location in the plant. The study realized that phenolic acid constituent in sugarcane cell wall includes sinapic acid in addition to p-coumaric and ferulic acids. Kondo et al. [180] studied the extraction of lignin from wheat straw after ammonia pretreatment. Lignins from untreated and treated wheat straw were found to contain guaiacyl- and syringyl propane units. Scalbert et al. [181] also studied the isolation of lignin from wheat straw. Results showed that p-coumaric acid in the lignin was mainly ester-linked whereas 35–75% of the recovered ferulic acid was ether-linked to milled straw lignin or enzyme lignin. A comparative study of wheat straw lignin structure using different extraction methods has been reported by Xu et al. [182]. Wheat straw from this study had crude lignin content of 17%. All lignin fractions from the various treatments were HGS-type lignin with G predominance. Another study of wheat straw lignin reported by Papatheofanous et al. [183] showed that the wheat straw had a crude lignin content of 16.4% for the whole straw but the

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internodes had 17.9%. The lignins had S:G ratio of 0.75 and 0.78, respectively. Steam explosion followed by ethanol extraction was applied to isolate lignin from wheat straw [184]. Results showed that the lignin belonged to GSH lignin and its purity rate was about 85.3%. Saha et al. [185] reported a low content (8.17%) of lignin in wheat straw. Billa et al. [186] suggested that wheat straw lignin is a relatively homogeneous copolymer of G, S structure (which compares favorably with standard angiosperm lignins). The structure of acetic and MWL lignin from wheat straw has been investigated [187]. Acetic acid lignin had few p-hydroxyphenyl units and more syringyl units (HGS ratio of 11:44:45), and MWL also had few p-­ hydroxyphenyl units and more syringyl units. Fidalgo et al. [188] reported crude lignin content of wheat straw as 16.6 and an HGS ratio of 8:41:51. Comprehensive studies on structure of wheat straw lignin can be found in Crestini et al. [189] and Sun et al. [190]. Another study on the structural variability of lignin in what straw was presented by Billa et al. [191, 192] Fractionation of flax shives into cellulose, hemicellulose, and lignin has been performed using water and aqueous ammonia [193]. The study reported a crude lignin content of 30.2%. Kim and Mazza [194] reported crude content of lignin in flax fiber as 5% and flax shive as 24%. Pulping of flax shives has been studied by Sain et al. [195] and Sain et al. [196]. For lignin in flax bast fibers, it was noted that the lignin content varied between 1.5% and 4.2% of the dry cell wall residues as compared to 23.7% and 31.4% in flax xylem tissues. Immunological and chemical analyses indicated that both flax xylem- and bast fiber lignins are rich in guaiacyl (G) units with S/G values inferior to 0.5 [197]. Lignin extracted from flax fibers has been characterized by Morrison et al. [198]. The study confirmed a low amount of lignin in flax fibers. Muir and Westcott [199] also studied the presence of ligands in flax seed and flax meal. Analyzing the principal components of flax [200], it was noted that seed flax straw had 23.3% lignin, bast fibers and seed flax had 10.1% lignin, and shives from seed flax had 27.9% lignin. Other studies on lignin from flax include Akin et al. [201] on flax fiber and seeds, Bray and Peterson [202] on chemistry of pulping flax straw, Oomah and Mazza [203] on compositional changes of flax seed and Eliasson et al. [204] on alkaline extraction of flax seed. Corn stover has been identified as a source of lignin. Öhgren et al. [205] reported crude lignin content of 17.2% for American stover and 21.2% for Italian stover. Liu and Wyman [206] reported a crude lignin composition of 17.6% and the possibility of isolation using a water flow through process. Kim et al. [207] reported 20.83% total lignin content in corn stover and the use of aqueous ammonia pretreatment to improve delignification. In another study, the same authors reported lignin in corn stover be 17.2% Klason lignin and the possibility of delignification with low temperature ammonia pretreatment for several days [208]. The method led to about 74% delignification. In another study, [209] they reported delignification using aqueous ammonia recycle percolation of the same corn stover. In another study, they introduced the use of hot water pretreatment to obtain higher delignification [210]. Moxley et al. [211] attempted a structural investigation on corn stover in lieu of enzymatic digestibility. However, lignin from corn stover was not properly analyzed. Structural studies on maize lignin demonstrated that p-coumaric acid is esterified primarily to syringyl units in lignin [212]. Alkaline peroxide delignification of rye

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straw has been investigated [213]. Crude lignin in the rye straw was found to be of the HGS-type lignin with GS prominence with 17.6% relative content. The β-O-4 ether bonds together with β-β and β-5 carbon–carbon linkages were found to be present in the lignin structural units. The monomeric composition of in situ lignins in sorghum, maize, and wheat straw stems has been investigated by two thioacidolysis and alkaline nitrobenzene oxidation [214]. The S:G ratios are 1.3 and 1.10 for maize, 0.97 and 0.74 for sorghum, 1.3 and 0.99 for wheat straw for thioacidolysis and nitrobenzene, respectively. Milled wood lignin from beech, aspen, and bamboo have been investigated [215]. The difference between lignin from the three sources and spruce is shown in Fig. 4.21. Lignin from bast fibers, hemp, flax, jute sisal, and abaca have been isolated and characterized using pyrolysis [216]. Lignin was extracted using alkali method and subjected to pyrolysis analysis to study the composition. The non-woody lignins comprised H, G, and S moieties. The SG ratios are 0.8 for hemp, 0.4 for flax, 1.7 for jute, 3.4 for sisal, and 2.9 for abaca. Comparative pyrograms are shown in Fig. 4.22. Kenaf varieties have been studied for lignin including everglades (Hibiscus cannabinus L.), Aokawa (H. cannabinus L.), and Mesta (Hibiscus sabdariffa L.) [217]. Figure 4.23 shows the variation in the content of lignin in their crude form. The SG ratios are given in Fig. 4.24. Lignins from wheat, rice, rye, and barley straws, maize stems, and fast-growing poplar wood have been isolated and examined [218]. Crude lignin contents are 16.8%, 12.3%, 17.6%, 15.8%, 15.0%, and 23.4%, respectively. Garrote et al. [219] conduct hydrothermal treatment of corn cobs, and rice husk to examine their by-­ products. Corn cob had crude lignin content of 18.8% and rice husks had 21.3%. Composition and isolation of lignin from rice hulls and straw have also been studied [220]. Rice straw had 19% lignin, whereas rice hull had 15% lignin. The content of lignin in corn barley, rice, wheat, sorghum, and sugarcane from different countries has been identified [221]. Efficient means of isolating lignin from corn stalk was studied by Wang et al. [222]. Using poplar wood, Lapierre et al. [223] investigated the molecular architecture of hardwood lignin. Milled wood lignin (MWL) from Carpinus betulus, Eucryphia cordifolia, Picea abies, Pinus sp., and Bambusa sp., kraft lignin from Fagus sylvatica and Eucalyptus globulus, and alkali lignin and hemicellulose-linked lignin from Triticum aestivum, have been studied to compare their HGS content [224]. HGS ratios for the lignins after extraction are; Carpinus betulus MWLc, 1:19:80, Eucryphia cordifolia MWL, 6:35:59, Picea abies MWL, 22:69:9, Pinus sp. MWL, 21:68:11, Bambusa sp. MWL, 20:23:57, Triticum aestivum alkali lignin, 10:48:42, Fagus sylvatica kraft lignin, 3:25:72, Eucalyptus globulus kraft lignin, 6:22:73 and Triticum aestivum LCC, 32:51:17. Figure  4.25 shows the by-products of their pyrolysis. Gellerstedt et  al. [225] isolated and characterized lignin from pine wood using the kraft process. The structure of lignin extracted from Pinus taeda wood after enzymatic treatment has been reported by Guerra et al. [226]. Non-wood plant fibers, kenaf, jute, sisal and abaca, have been analyzed for lignin using pyrolysis-gas chromatography/ mass spectrometry [227]. All lignins were of the HGS type with high S/G ratio between 2.0 and 5.4, the highest corresponding to kenaf. Lignin has been extracted from switchgrass, corn stover, and fescue feedstocks with either 95% ethanol or hot

Fig. 4.21  Pyrograms of various lignins. Reprinted with permission from Elsevier [215]

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Fig. 4.22  Py-GC/MS analysis of lignins isolated from different non-wood cellulosic fibers selected for this study: (a) hemp, (b) flax, (c) jute, (d) sisal, and (e) abaca. Reprinted with permission from Elsevier [216]

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Lignin contents (%)

40

30

Everglades

Bast Inner Bast Core

Aokawa-3

Mesta

20

10

To p pp er Lo w Bo er tto m U

To p pp er Lo w Bo er tto m U

U

To p pp er Lo w Bo er tto m

0

Fig. 4.23  Lignin contents of kenafs. Note: Lignin contents show the 72% sulfuric acid-insoluble plus soluble lignin contents. Reprinted with permission from Elsevier [217] 7 Bast Inner Bast Core

S/G ratio (mol/mol)

6 5

Everglades

Aokawa-3

4 Mesta

3 2 1

To U p pp e Lo r w Bo er tto m

To U p pp e Lo r w Bo er tto m

To U p pp e Lo r w Bo er tto m

0

Fig. 4.24  S/G ratios by thioacidolysis. Note: The S/G ratio means the molar ratio of the yields of S1 + S2/those of G1 + G2 Reprinted with permission from Elsevier [217]

water [228]. Crude lignins in the samples were 24%, 21%, and 21% for switchgrass, corn stover, and fescue feedstocks, respectively. Tapin et  al. [229] investigated lignin from wheat and oilseed flax straws. Cronier et  al. [230] studied lignin in hemp stems, Bunzel et al. [231] studied lignin monomers from whole grains of corn (Zea mays cv. microsperma KOERN.), wheat (Triticum aestivum L.), spelt (Triticum spelta L.), and rice (Oryza sativa L.). The presence of lignin monomers and linkages in grass straws has been studied by Jacquet et al. [232]. Lignin has been isolated from maize stem and characterized [233]. Maize lignin contained high amounts of esterified p-coumaric acid. Lignin in wheat and rye straw was investigated by Salehi

Fig. 4.25  Py-GC/MS analysis of permethylated hardwood, softwood, wheat, and industrial lignins: Pyrograms of Carpinus betulus MWL (a), Pinus sp. MWL (b), Triticum aestivum alkali lignin (c) and Fagus sylvatica kraft lignin (d). Reprinted with permission from John Wiley and Sons [224]

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et al. [234]. Wheat and rye straw had 18.5% and 17.8% crude lignin, respectively. Lignin can be obtained from rapeseed straw (species Brassica napus L. convar. Napus) by soda pulping of [235]. Rapseed stalks had lignin content of 21.35%. Pulping of jute has also been investigated by Sahin [236]. High purity lignin was obtained from poplar wood using ammonia percolation treatment [237]. Phenolic hydroxyl units in lignin from hemp and flax have been investigated [238]. Lignin in cotton stalks from America and Egypt was isolated and characterized [239]. Crude lignin in American stalks was 27% and that from Egypt was 22.50%. Other lignin studies include bamboo culms [240], bamboo [241], eucalyptus wood [242], pulp and paper bagasse [243], rice straw [244], pine wood [245], loblolly pine [246], Egyptian bagasse [247], rice and wheat straw [248], sugarcane [249], Triticale straw [250], and Bambusa tulda [251].

4.3  Lignin Sources Already Exploited as Carbon Material Lignin from annual plants extracted using soda pulping process has been converted to carbon fibers through a series of process [252]. Figure 4.26 shows the solid-state NMR spectra of treated and untreated plant lignin. Eucalyptus kraft lignin has been

Fig. 4.26 Solid-state 13C CP/MAS NMR spectra of the lignin samples during the thermostabilization. (a) Untreated lignin; (b) 170 °C; (c) 200 °C; (d) 230 °C; (e) 260 °C; (f) 290 °C; (g) 320 °C; (h) 350 °C. Reprinted with permission from Elsevier [252]

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converted to carbon fibers [253]. Kadla et  al. [254] converted organosolv lignin, softwood kraft lignin, and hardwood kraft lignin to carbon fibers. Bio-oil from mixed hardwood saw dust has been investigated as carbon fiber raw materials [255]. Lignin carbon fibers have been fabricated from softwood (spruce/pine) and hardwood (birch/aspen) lignin sources by Norberg et  al. [256]. Acetylated softwood kraft lignin has been converted to carbon fibers [257]. Lin et al. [169, 258] used cedar wood chips lignin in combination with PEG to fabricate lignin carbon fibers through a series of processes. Foston et al. [259] reported carbonization of Alcell lignin fibers to carbon fibers. Baker et al. [260] carbonized organic purified commercial lignin from hardwood. Lai et  al. [261] converted commercial lignin to carbon fiber. Ding et al. [19] used lignin from organosolv process of corn stover to produce carbon fibers in combination with PAN.  Maradur et  al. [262] converted commercial hardwood lignin to carbon fibers. Solvent-extracted hardwood using the Alcel extraction process has been converted to carbon fibers [263]. Choi et al. [264] also fabricated carbon fibers using commercial lignin and PAN. Commercial lignosulfonate has been used top form carbon fibers by Xia et al. after purification [265]. Luo [266, 267] investigated carbonization of hardwood lignin obtained from a combination of birch, beech, maple, poplar, and trace amount of softwood into carbon fibers. Lignin from birch wood yielded a general purpose carbon fiber with tensile strength of 660 MPa and modulus of 40 GPa [86]. Cho et al. [268] investigated the carbonization of softwood kraft lignin in combination with polyethylene oxide and crystalline cellulose. Schreiber et al. [269] also investigated carbonization of hardwood-based organosolv lignin from Lignol in combination with cellulose acetate fibers. Zhang [270, 271] reported the carbonization of lignin from wheat straw, grass, poplar, and softwood kraft lignin into carbon fibers. Organosolv switchgrass lignin and hardwood lignin have been converted to carbon fibers [272]. Typical carbon fibers from switchgrass lignin are shown in Fig. 4.27. Ruiz-Rosas et al. used commercial Alcel lignin to produce carbon fibers [273]. Dallmeyer et al. [274] used commercial softwood kraft lignin to synthesize interconnected kraft lignin carbon fibers. Kleinhans et al. [275] also used softwood lignin obtained from the LignoBoost process. Lignin from corn residue has been converted in carbon fibers after treatment with iodine [276]. Beechwood organosolv lignin/cellulose hybrid has been used to form carbon fibers [277]. Jin et al. [278] activated straw lignin into activated carbon. Lin and Zhao [279] used lignin extracted from cedar wood chips to produce activated carbon materials. In another study, kraft lignin obtained from a paper making black liquor from china was used to fabricate activated carbon by Li et al. [280]. Another study reported activated carbon from the same source [281]. Wu et al. [282] used commercial alkali lignin to form activated carbon materials. Correa et al. [283] used lignin from beech wood, pine bark, and oak bark to form activated carbon materials. Lignin derived from Brewery spent grain has been used to form activated carbon by Mussatto et al. [284]. Sun et al. [285] reported activation of lignin from corn stalk. Corn straw lignin used as activated carbon yielded a maximum Brunauer–Emmett–Teller (BET) surface area of 820 m2/g [286]. Khezami et al. [287] conducted a two-step chemical activation of birch wood lignin and obtained good results. Rodríguez-Mirasol et al.

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Fig. 4.27  Typical morphology of lignin-based CFs. (a) Cross-section of a fiber bundle, (b) fracture surface of a single fiber, (c, d) fiber surface. Reprinted with permission from John Wiley and Sons [272]

[288] investigated the activation of eucalyptus kraft lignin. Kuznetsov and Schipko [289] reported the physical activation of hydrolytic lignin. Bagno et al. [290] developed activated carbon directly from kraft black liquor. Activated carbons have been produced from hydrolysis lignin by chemical activation with potassium hydroxide [291]. Granulated carbons from cotton lignin have been used to form activated carbon in combination with vegetable raw materials [292]. In a similar way, hydrolytic lignin and cedar nits were used to make activated carbon as absorbent [293]. Lignol hardwood lignin has been used to form thin films with graphite oxide as a freestanding electrode [294]. Kraft lignin from Eucalyptus grandis has been used to obtain activated carbons with a high BET surface area up to 1800  m2/g [295]. In another study, kraft lignin from Eucalyptus grandis could yield activated carbon with BET surface area up to 1850 m2/g [296]. Klose and co-workers obtained porous carbons as active material for supercapacitors from softwood kraft lignin [297]. Hayashi et  al. [298] used spruce wood lignin to produce activated carbon with BET surface area up to 2000 m2/g. Yan et al. [299] also reported the graphitization of commercial kraft lignin into graphite fibers. Commercial kraft lignin has been graphitized in combination with

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iron nanoparticles [300]. Hardwood acetic acid lignin isolated from birch (Betula platyphylla Sukachev) has been studied for structural changes during catalytic graphitization [301].

4.4  Conclusion This chapter shows that lignin can be obtained from over 250 sources. Some of these sources contain lignin in very high relative abundance (over 30%), which makes them potential candidates for commercial lignin production. It is also clear that lignin from agricultural waste sources is potential candidates for commercial lignin although they may need further treatment. Some lignins from these waste sources are not yet utilized in carbon fiber production.

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Chapter 5

Characterization Techniques and Quality Assessment of Lignin and Lignin Carbon Materials Samson Oluropo Adeosun and Oluwashina Phillips Gbenebor

5.1  Introduction Characterization is an important aspect of material science because it does not only provide quality assessment of materials but also provides understanding of the material in terms of structure, composition and properties. Effect of processing and processing parameters on the structure and consequently the resulting properties are possible with the help of accurate characterization techniques. Material properties are generally governed by its structure, composition, chemistry and processing, all of which are inter-related. The key to understanding their inter-relationship is characterization (C1–C4, and D1–D4 in Fig. 5.1). With characterizations of the C2 category, the effect processing on the structure of the material can be understood and with D3 characterization, the effect of processing on the properties can be understood. This means that a combination of characterizations C2 and D3, properties, processing and structure can be correlated. Characterization of lignin is very difficult because of the diversity in lignin chemistry and structure with respect to origin and method of extraction. This complex heterogeneous nature is linked to the variations in composition, size, cross-­ linking, functional groups and linkage type between the phenyl propane monomers [1]. The amplification of well-defined investigative methods for lignin characterization is an imperative for successful industrial applications of lignin and lignin products. On the other hand, processing of lignin into final products such as carbon fibres and activated carbon has profound effects on the structure, composition and properties of the final product. As at this moment, the performance requirement for lignin carbon fibres in automotive applications is not yet achieved. It is widely believed

S. O. Adeosun (*) · O. P. Gbenebor Department of Metallurgical and Materials Engineering, University of Lagos, Lagos, Nigeria e-mail: [email protected] © Springer Nature Switzerland AG 2019 E. I. Akpan, S. O. Adeosun (eds.), Sustainable Lignin for Carbon Fibers: Principles, Techniques, and Applications, https://doi.org/10.1007/978-3-030-18792-7_5

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Fig. 5.1  Central place of characterization in understanding materials

that improvement can be attained by appropriate understanding of the lignin structure and the structural development during processing. For example, proper understanding of the development of graphitic turbostratic structure in lignin carbon fibres during processing will help to develop experiments that will arrive at a more ordered structure. This will ultimately lead to improvement in the properties. The use of transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray diffraction (XRD), Raman and high-resolution scanning electron microscopy (SEM) will be beneficial in this respect. In this chapter, lignin and lignin products characterization techniques are presented. The chapter begins by illustrating how lignins are determined in the lignocellulosic material and examination of their composition and structure before and after extraction. Several methods of lignin characterization including, Fourier transform infrared (FTIR), nuclear magnetic resonance (NMR), ultra violet (UV), chromatography, SEM, TEM AFM, Raman spectroscopy and mechanical characterization are outlined in details. Basic principles of operation are also presented to guide researchers and industrialist on interpretation of results and further development in this area.

5.2  Determination of Lignin The development of carbon fibres from lignin begins with the extraction of lignin from their sources. Lignin has been extracted from various sources including hardwood, softwoods, grasses, agricultural wastes and industrial wastes sources such as brewery spent grains. Industrialist will want to know how much lignin can be extracted from a proposed source for economic purposes. This makes the quantitative determination of lignin an absolute necessity. Several methods have been used to quantitatively determine the amount of lignin in biomass. These methods are

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classified as chemical or physical methods [2]. The chemical methods are the best, but it is important to note that using chemical methods require a full knowledge of the substrates being analysed and the limitations of the procedure. A more generalized method for lignin quantification is the determination of Klason insoluble lignin fraction followed by UV determination of the soluble lignin fraction. Other methods such as acid detergent fibre method and kappa method are also used to quantitatively determine lignin in biomass. Because the chapter is focused on characterization of lignin and lignin-based materials, only a few methods on determination of lignin are presented.

5.2.1  Klason Method Originally designed by Peter Klason in 1923, the Klason method is based on primary hydrolysis of lignin in 61–68% sulphuric acid [3, 4]. It is the simplest and most reliable method of lignin quantification. The method entails the solubilization of all the polysaccharides in the tested biomass leaving lignin as an insoluble residue using 72% sulphuric acid. The 72% sulphuric acid was optimized in 1932 and published by Ritter et  al. [5] and is now referred to as the Klason lignin or acid insoluble lignin (AIL). The method has major disadvantages including (1) the possibility of condensation of proteins and suberins with lignin giving a faulty reading and (2) the solubilization of some parts of lignin into the solution especially the syringyl residues. This problem is solved by complementing the Klason method with UV analysis of the hydrolysate to determine the soluble lignin fraction. The Klason method gives good reproducibility with both gymnosperm and angiosperm tissues [2]. The Klason procedure has been standardized as TAPPI T 222 om-88. Procedure  A determined mass (oven dried basis) of the biomass sample previously ground to pass at least 20–40 mesh screen (210–420  μm) and freed from extractives [6] by Soxhlet extraction is first mixed with 72% H2SO4 at a solid to liquid ratio of 1:15 in a centrifuge tube (e.g. 1 g of biomass is mixed with 15 mL of 72% H2SO4). The mixture is reacted at 30  °C for 1–2  h with continuous stirring (every 10–15 min) [3, 5]—Primary hydrolysis. The solution is then diluted to 3% by adding a calculated amount of deionized water—Secondary. The diluted solution is placed in an autoclave and reacted at 121 °C for 4–6 h. The reacted sample is cooled and filtered (through a tared Gooch, alundum, or fritted glass crucible) and the residue washed, dried and the weight determined (to the nearest 0.1  mg) [7, 8]. Klason lignin is calculated using Eq. (5.1). Modifications to this method have been proposed by some researchers in attempt to reduce the work load and improve on the reproducibility of the results [8–10]. Klasson Lignin =

Weight of residue ×100% Original weight of sample

(5.1)

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The soluble lignin fraction is determined from the solution by UV spectroscopy. For reproducibility of results the test should be conducted in triplicates or more. The test is usually influenced by, extractives, particle size of the biomass, mass of sample, moisture content, ash content and reagent concentration. The moisture content and ash content should not be above 10% in each case and the particle size should not deviate from the range because of bias to carbohydrate. Removal of extractives should be done with precision to avoid interference with the results. Some solvents used in the removal of extractives also remove some amount of lignin from the biomass; attention must be paid to this fact when choosing the solvent [11]. N/B: If inorganics are suspected in the sample, the lignin should be washed to determine the amount of ash and used as a correction for the lignin content. Secondly, an indication of lignin purity can be obtained by conducting checking the methoxyl content. Methoxyl contents of various biomass samples have been reported by Brauns [12].

5.2.2  Goering-Van Soest Method The method involves pre-treatment of the biomass with acid detergent [13] to remove protein, hemicelluloses and other components and further application of the Klason lignin to determine the lignin content or permanganate oxidation solubilization. It is important to note that pre-treatment with acid detergent can also remove some amount of lignin from the sample. Furthermore, protein may be remaining in the sample and interfere with the results. The lignin from the process is also termed acid detergent lignin [14]. Procedure: Acid-Detergent Fibre Preparation [2, 13, 15, 16]  1 g air dried sample of the biomass sample ground to pass 20- to 30-mesh is weighed into a beaker suitable for refluxing. One-hundred millilitre of acid detergent solution is added, and the mixture heated to boiling in 5–10 min and refluxed for about 60 min from the onset of boiling. Heat is reduced during after boiling to avoid foaming. The sample is filtered in a tared, 50  mL coarse porosity Gooch crucible and washed thoroughly with hot (90–100 °C) water. The sample is washed repeatedly washed with acetone and the acetone is allowed to evaporate. The sample is then freed from solvents by drying it overnight in a vacuum oven at 100–105 °C and weighed. N/B: Acid-detergent solution is prepared by dissolving 20 g of cetyltrimethylammonium bromide (CTAB, technical grade) in 1 L of 0.5 M H2SO4. The method has been modified for optimum results [14, 17]. Lignin Determination  The air dried sample is subjected to lignin determination using either the Klason method given above or the use of selective chemical oxidation and extraction [18]. The selective chemical oxidation method is performed by adding the acid detergent fibre into a crucible and the crucible is placed in a shallow pan containing cold water to a depth of about 1  cm. Saturated potassium

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p­ ermanganate is mixed with lignin buffer solution in a ratio of 2:1 (v:v) to form a solution 25 mL of the solution is added to the crucible and the level of water in the pan increased to reduce flow of the solution out of the crucible. The sample is mixed continuously with a glass rod and maintained at 20–25 °C for 90 min with addition of mixed permanganate solution if needed to keep the sample wet. After the reaction, the solution is removed by suction filtration and the crucibles are placed in a clean pan filled to about half full with demineralizing solution. The demineralizing solution is removed by filtration and the method repeated until the sample is white. Furthermore, the sample is washed thoroughly in the crucible wit 80% ethanol and then with acetone and finally dried at 100 °C and weighed. Subtracting the weight of the residue from the original acid detergent weight gives the weight of lignin in the sample [19]. Correction for ash can be applied according to the method described above. N/B: The saturated permanganate solution is prepared by dissolving 50  g of KMnO4 and 0.05 g Ag2SO4 in 1 L of distilled water, whereas the lignin buffer solution is prepared by dissolving 6 g of Fe(NO3)3.9H2O and 0.15 g of AgNO3 in 100 mL of distilled water combining with 500 mL of acetic acid and 5 g of potassium acetate and adding 400 mL of tert-butanol. Demineralizing solution is a mixture of oxalic acid di-hydrate, ethanol and HCl. The solution is prepared by dissolving 50 g of oxalic acid dihydrate in 700 mL of 95% ethanol, adding 50 mL of concentrated HCl and 250 mL of distilled water. Other useful methods for the quantitative estimation of lignin include the Kappa, chlorine consumption and thioglycolic acid methods. Methods based on chemical degradation products have been reported by various authors [20–22]. The Kappa number involves the oxidation of lignin in a measured amount of pulp with a known quantity of standard permanganate under acidic conditions. The lignin content is measured as Kappa number, which is defined as the volume of 0.1 M potassium permanganate solution consumed by 1 g of moisture-free pulp under specified conditions. The method is used predominantly in the pulp industry but can be applied to determine lignin content with good reproducibility. Thioglycolic acid method involves acid catalysed derivatization of lignin with the acid to arrive at acid-­ insoluble lignin thioglycolate. The isolated lignin from this method is much less modified, soluble and can be analysed easily. The effect of non-lignin components on the result is minimal. However, it is imperative that purity of the lignin is verified by the methoxyl content and correction for thioglycolate moieties be applied based on sulphur analysis.

5.3  Structural Characterization Chemical structure and functional groups present in extracted lignin is very important towards further processing of the lignin into carbon fibres. Lignin is a complex random, three-dimensional network polymer with no regular and repetitive units

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and bonding patterns (Fig. 5.2). It is highly branched with high but irregular molecular weight distribution. Because of lack of suitable analytical techniques, the degree of polymerization, structural and degree of branching of lignin is not yet fully understood. One interesting thing about the structure of lignin is the fact that they all contain three basic subunits (p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S)) which are connected to each other in several ways. The chemistry of lignin and these interunit linkages were introduced in Chap. 1. Lignin is generally characterized by the relative abundance of these subunits (H/G/S) and the distribution of interunit linkages in the polymer. Another important way of characterizing lignin is the determination of the functional groups found on the lignin structure. These can be performed in various ways using various methods and characterization equipment. One serious issue in all the structural characterizations of lignin is that lignin is first destroyed before it is characterized. This pose uncertainty to the results obtained. Even with the most sophisticated equipment the study of the structure of lignin is still evolving. Figure 5.3 shows the repeating units, side chain structure in end-groups and various functional groups present in lignin. Although the structure of lignin differs with botanical source, extraction method and post extraction treatment, these features are always present in lignin. A good understanding of the structural features will help in the design of characterization methods for any step of lignin processing.

5.3.1  FTIR Spectroscopy When a light wave is passed through a material, multiple processes occur including reflection, transmission, scattering, absorption, fluorescence or vibration. These processes result from the changes in the energy level of the material due to the incident light radiation. These processes are the mechanisms behind most of the material characterization techniques. Infrared spectroscopy is a vibrational spectroscopic technique. In vibrational spectroscopy, transitions due to absorption or emission of electromagnetic radiation is measured and used to characterize the material. The transitions appear in the 3.3–13,300  cm−1 range which is usually divided into 3 regions: (1) Near-infrared region which ranges between 13,300 and 4000 cm−1, (2) Mid-infrared region which ranges between 4000 and 200 cm−1 and (3) Far-infrared region which ranges between 200 and 3.3 cm−1. The transition originates from the vibration of nuclei constituting the molecules. When a sample is irradiated with a beam of incident radiation, it absorbs energy at characteristic frequencies specific to that of the vibration of chemical bonds present in the molecules. This energy absorbed by vibrating the chemical bonds results in a spectrum called the Infrared spectrum with signatures specific to the type of bond and the group of atoms involved in the vibration. Chemical bonds in molecules vibrate in several ways identified as vibrational modes. Molecular vibrations occur in two ways: stretching and bending. In stretching vibration there is movement along the bond axis with changing interatomic distances. In bending vibration there is a change in bond

Fig. 5.2  Proposed structure of milled wood lignin showing no repeated units and bonding. Reprinted with permission from ACS [23]

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Fig. 5.3  Illustration of repeating units, end groups and functional groups in lignin Fig. 5.4  Examples of stretching and bending vibration of atoms

angles with respect to the other part of the molecule (Fig. 5.4). In FTIR the wavelengths at which absorption occurs are identified by quantifying the change in intensity of the light after reflection (transmission) as a function of wavelength [24–27]. A representation of the infrared measurement is shown in Fig. 5.5. The goal of the infrared measurement is to measure the ratio between the intensity of the incident beam (Io) and the transmitted beam (It) as a function of the wavelength (Eq. 5.2). A plot of this ratio versus the wavelength is called the infrared spectrum. The infrared spectrum is commonly plotted in either three formats: transmittance, reflectance or absorbance. Equation (5.2) is used to measure the fraction of light transmitted through the sample but in the case of reflectance, Eq. (5.2) is equated to reflectance

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Fig. 5.5 Schematic representation of FTIR measurement

with It replaced by intensity of reflected light. Absorbance (Aw) is related to transmittance by the Beer-Lambert Law (Eq. 5.3). (5.2)



I  Tw =  r   I o w

(5.3)



I  Aw =  t  = ( ε w )( bc )  I o w

where c is the concentration of the chemical bonds, b is the sample thickness, and εw is the frequency-dependent absorptivity. It is experimentally determined at each w by measuring the absorbance of samples with known values of bc. The infrared spectrometers are divided into dispersive infrared (IR) and Fourier Transform Infrared spectrometers (FTIR). In dispersive IR a grating type monochromator is used to disperse the polychromatic radiation into different spectral elements which are then measured one at a time. The procedure is incredibly tedious with very weak signal intensity because a small portion is measured at a time. In FTIR all wavelengths are measure at the same time giving rise to faster sampling, and better signal to noise ratio. Equation (5.3) provides a foundation for quantitative analysis with FTIR. This makes collecting data in terms of absorbance valuable in quantitative analysis better than reflectance. In analysis of IR spectrum, peak positions are most commonly used for qualitative identification because each functional group displays peaks at exclusive set of characteristic frequencies. The positions of these peaks are compared with a table or computer database of standard peak positions providing a finger print that can be used to identify chemical groups. FTIR spectra can be obtained using several techniques including Attenuated Total Reflectance (ATR-FTIR) and Diffuse Reflectance Infrared Fourier Transform (DRIFT). ATR uses the phenomenon of total internal reflection as shown in Fig. 5.6. The process uses a crystal as a waveguide for the incident radiation causing the beam to be back-reflected several times as it propagates down the length of the crystal before it exits to the detector. The configuration allows the beam to penetrate only a fraction of wavelength away from the reflecting crystal surface so that when a material that selectively absorbs radiation is in that close contact, the beam loses energy at the wavelength where the material absorbs [28]. The resulting attenuated radiation is measured and plotted as a function of wavelength to give the spectrum.

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Fig. 5.6  Total internal reflection in ATR

This is very useful in lignin analysis since lignin can be formed into a homogeneous thin film. DRIFT is useful in characterizing polymers and has been found to be more sensitive to surface species than transmission techniques and a very good in situ technique [29]. It is capable of providing chemical and structural information for all types of solid surfaces [30]. The sample usually in a powdered form is mixed with purified salt and pressed to form a translucent pellet which is then subjected to the IR measurement [31]. Characterization of lignin is usually conducted in the mid-IR range using ATR-­ FTIR, IR and DRIFT. A comprehensive assignment of IR bands for hardwood and softwood milled wood lignin has been published by Agarwal and co-workers [32, 33]. Adapting the data from various sources a summary of IR band assignment for various types of lignin is shown in Table 5.1 [32–42]. FTIR is a useful characterization tool at various stages of lignin processing. FTIR can be used to identify the type of lignin, methoxyl groups, carbonyl groups and hydroxyl groups present in the lignin. It can also be used to measure the extent of purity of the extracted lignin. On the other hand, it can be used to study modification reactions such as alkylation, esterification and propargylation. Vibrational bands at 1269–1268 (G ring and C=O stretch), 1140 (C–H in plane deformation), 858–853 and 817 (C-H out of plane vibrations in 2, 5 and 6 of G units) cm−1 are characteristics of G lignin [1]. The 1326  cm−1 band is a characteristic of S unit plus G unit condensed. A band at 1115 cm−1 is characteristics of aromatic C-H in S lignin unit whereas 843–835 cm−1 is characteristic of is also characteristic of S lignin arising from the C-H out of plane vibrations in positions 2 and 6 of the S unit [43]. During methylation the OH bonds are split and the H is replaced by a methyl group (CH3), this will cause a decrease in the OH band and an increase in the CH stretch of the –CH3− group [44]. FTIR is very useful in studying the blending of lignin with other polymers [45, 46]. FTIR is also very useful in the studies of hydrogen bonding characteristics of lignin [47]. FTIR is also very important in studying the progress of stabilization and carbonization of lignin precursors [48]. An example of FTIR spectrum of lignin is shown in Fig. 5.7.

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Table 5.1  Assignment of bands in FTIR spectra of different kinds of biomass [32–42] Softwood 3430 vs 2938 m 2885 sh 2849 sh 1717 sh

Hardwood 3440 vs 2942 m 2882 sh 2848 sh 1737 vs

1667

sh 1670 sh

1645 1600 1513 1466 1458 1428

sh s vs s sh m

1375 1331 1270 1226 1142

w 1367 sh s 1330 m vs 1252 vs m s 1159 sh 1127 vs w 1082 sh s 1050 vs vw 905 w sh w w vw vw

1085 1035 914 878 863 823 748 742

1643 1596 1506 1464 1425 1379

sh s vs s m m

Non-­wood 3411 2964 2929 2863 1716 1659

1603 1514 1462 1427 1371 1331 1265 1217 1160

1036 854 837

Assignment O–H stretch, H-bonded C–H stretch methyl and methylene groups C–H stretch in methyl and methylene groups C–H stretch O–CH3 group C=O stretch, unconjugated ketone, carboxyl and ester groups Ring-conjugated C=O stretch of coniferaldehyde/ sinapaldehyde Ring-conjugated C=C stretch of coniferyl/sinapyl alcohol Aryl ring stretching, symmetric Aryl ring stretch, asymmetric C–H deformation, asymmetric O–CH3 C–H deformation, asymmetric Aromatic skeletal vibration combined with C–H in plane deformation O–CH3 C–H deformation symmetric Aryl ring breathing with C–O stretch Aryl ring breathing with C=O stretch C–C, C–O and C=O stretches Aromatic C–H in plane deformation Aromatic C–H in plane deformation C–O deformation, secondary alcohol and aliphatic ether Aromatic C–H in plane deformation C–H deformation of out of plane, aromatic ring C–H deformation of out of plane, aromatic ring C–H deformation of out of plane, aromatic ring C–H deformation of out of plane, aromatic ring CCH wag Skeletal deformation of aromatic rings, substituent side groups, side chains

Note: Data for softwood and hardwood were obtained from milled wood lignins (MWLs). Data for non-­wood lignin were obtained as upper limit values from different sources and extraction methods including straw, corn stalk and rover, flax, hemp, grass etc. The intensity of the vibrations varies widely with the different sources for the non-wood samples and cannot be included in the table as it will be confusing

5.3.2  Raman Spectroscopy Raman spectroscopy is a vibrational spectroscopic technique but is different from infrared spectroscopy because it involves inelastic scattering with a photon from a laser light source. Raman does not require correlating the incident radiation to the energy difference between the ground and the excited states. In Raman spectroscopy, a sample is irradiated with a strong monochromatic light source—Laser.

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Fig. 5.7  An example of FTIR spectra used to study the effect of heat treatment on the structure of lignin (150-L, 200-L, 250-L and 300-L) represent the temperature of heat treatment. Reprinted with permission from Elsevier [49]

The incident light interacts with the molecule polarizing the cloud of electrons around the nuclei. Most of the radiation will scatter “off” the sample at the same wavelength as that of the incoming radiation (elastic scattering). This scattering process is the dominant scattering process called Rayleigh scattering. When a molecule at rest encounters electric field of the laser radiation, the energy of the system is raised momentarily by through the polarization induced in the chemical specie. This polarization is not a true energy state and it is widely referred to as a “virtual state”. The excitation is short lived as relaxation occurs almost instantaneously so that the molecule returns to its initial energy state (ground state). In this case there is no net energy transfer from photons to molecular vibration meaning that the incident radiation will possess the same frequency as the incident radiation in a similar manner to elastic collision between photons and molecules. In the case that the excited molecular vibration does not return to its initial level, then the scattered photons would possess either lower or higher energy than the incident photons similar to inelastic collision between photons and molecules.

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Fig. 5.8  Elastic and In-elastic scattering of incident light by molecules

Fig. 5.9  Schematic Raman scattering spectrum

If the final energy is higher than the initial energy, it is called Stokes scattering (Fig. 5.8). But if the energy is lower than the initial energy, it is called anti-Stokes scattering. Raman spectrum records the frequency changes caused by Stokes scattering by molecules. The frequency change (difference between laser radiation and scattering frequency) is called Raman shift in the spectrum. The separation of these lines from the Rayleigh line is a direct measure of the vibrational frequencies of the sample [27]. An individual band of Raman shift corresponds to a specific molecular vibration frequency [24] (Fig. 5.9). The vibrational states examined by Raman spectroscopy are the same as those of Infrared. Raman scattering is dependent on the changes of polarizability arising from the molecular vibrations but IR absorption is based on changes in the dipole moments. This means that vibrations involving

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dipole moments (hydroxyl- or amine-stretching vibrations and the vibrations of carbonyl groups) will be strong in Infrared but weak in Raman. On the other hand, non-polar functional group vibrations (stretching vibrations of carbon double or triple bonds and symmetric vibrations of aromatic groups) show very strong Raman bands but weak IR bands [50]. These conclude that IR and Raman spectroscopies are complementary. By observing the intensity of the Raman bands, it is possible to identify the chemical compounds or study intermolecular interactions. Key information on the structure of molecules can be obtained using Raman spectroscopy. Like IR, Raman also uses spectral library searching to identify compounds. Raman operational wavelength range is usually independent of the vibrational modes being studied; this gives it additional advantages over other vibrational spectroscopic methods. Furthermore, Raman allows microscopy with spatial resolution as fine as 1 μm. Any Raman spectrometer uses either one of two technologies namely; Dispersive Raman or Fourier transform Raman. Each of these techniques is peculiar and is ideally suited to specific applications. The recommended practice for Raman spectroscopy is to get the full 100–3100  cm−1 spectrum at all laser wavelengths. Dispersive Raman is very useful in microscopic analysis due to higher sensitivity and FT-Raman works better for bulk material analysis because of their lack of fluorescence, wavelength precision and the cost-effectiveness of the technique. Dispersive Raman is ideal for impurity analysis in solutions, polymers or environmental sampling, semiconductor and microelectronics industries analysis of silicon materials, analysis of single crystals in pharmaceutical and life sciences, analysis of aqueous samples, analysis of very dark or highly coloured samples (carbon black), analysis of polymer laminates, layered paints and samples where depth is of prime importance and inorganic material analysis and identification. On the other hand, FT-Raman is very useful for unknown identification, raw material characterization, final product quality and quantitative analyses, surface and bulk structure in combinatorial chemistry, forensic analysis, pulp and paper, textiles and petrochemicals and analysis of modified polymers (plasticized polymers etc.). Analysis of lignin structure with Raman spectroscopy dates back to 1980 [32]. It provides complementary information which is not possible with IR spectroscopy. Raman microprobe analysis was used to show that cell wall lignin phenyl rings are preferentially oriented by surrounding carbohydrate matrix [51]. However, the limitation of Raman in lignin investigation is that lignin fluoresces intensely when the excitation energy falls within the UV-vis range. To eliminate this problem the use of Kerr gate was proposed and successfully applied to characterize the structure of lignin [52]. A very important use of Raman in lignin carbon fibre processing is the application of Raman to study the structure of the structure of carbon during carbonization. Figure  5.10 shows a typical spectrum of a lignin carbonized material. Raman peaks of carbon materials usually show two overlapped broad peaks with intensity maxima at ∼1358 cm−1 (D band, in valence state of sp3 hybridization) and ∼1589 cm−1 (G band, in valence state of sp2 hybridization). The ratio of the intensity of D band to that of G band is referred to R and it is the measure of the degree of carbon structure ordering. A lower R value indicates good ordering whereas a

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Intensity/a.u

a

h g f e d c b a

800

1000

1200

1400

1600

1800

2000

Raman shift/cm–1 4.0

b 800°C

3.5

1000°C

R

3.0

1200°C

2.5

1400°C 2.0

1.5 0

25

150 180 210 240 270 300 330 360 390 Temperature (°C)

Fig. 5.10  Raman analysis of the carbonized samples. (a) Raman spectra of 1400°C carbonized samples. (a) Untreated lignin; (b) 170  °C; (c) 200  °C; (d) 230  °C; (e) 260  °C; (f) 290  °C; (g) 320 °C; (h) 350 °C. (b) Relationships between the R values (ID/IG) of 800, 1000, 1200, 1400 °C carbonized samples and the thermostabilization temperatures. Reprinted with permission from Elsevier [53]

higher R value indicates poor ordering [53, 54]. The FWHM of the peaks are also used as indication of the crystallinity of the carbon structure. The lower the FWHM the higher the crystallinity [55]. R can also be used to evaluate the in-plane graphitic crystallite size La and the graphitic mole fraction xG. The graphitic crystallite size is calculated from Eq. (5.4) and the graphitic mole fraction from Eq. (5.5) [56–58].

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La ( nm ) = xG =



( 2.4 × 10 ) λ

I (G)

−10

4 L

R

I (D) + I (G)

=

1 1+ R

(5.4)

(5.5)

5.3.3  NMR Spectroscopy Neutron magnetic resonance is a spectroscopic technique that uses magnetic resonance of atomic nuclei to probe structure, composition, conformation and physical properties. All nuclei possess electrical charge, and some possess electronic spin. Spin is the rotation of a nucleus about its own axis. These rotating electrically charged nuclei generate a magnetic field and therefore carry a magnetic moment (a measure of an object’s tendency to align with a magnetic field). If an external magnetic field is applied to the nuclei, energy is transferred at a wavelength corresponding to radio frequencies. The electrons of the atom circulate along the direction of the applied magnetic field causing a small opposing magnetic field at the nucleus. When the spin returns to its base level a certain amount of energy is released at the same frequency. The frequency emitted is measured in many ways and processed to yield a spectrum for the nucleus in question. The resonant frequency of the transition is usually dependent on the effective magnetic field at the nucleus. The magnetic field of the nucleus is dependent on the electron shielding which is in turn dependent on the chemical environment. This means that information about the chemical environment of the nucleus can be obtained from the obtained frequency. Electron density around a nucleus of a molecule differs with the type of nuclei and bonds that it has, making the static and opposing magnetic fields to differ. This phenomenon is called a chemical shift, usually measured in ppm or Hz and is the fundamental information needed to characterize the structure of the material tested (Table 5.2). Chemical shift is a measure of how far the signal produced from the target is from the reference compound signal. The reference compound is set at position zero on the very left of the spectrum, and as it moves toward the left, the ppm values become larger. When two nuclei are experiencing the same chemical shift (chemical environment), they are called equivalent but when they experience different chemical shifts, they are called non-equivalent. When two nuclei are in close proximity, they exert an influence on each other’s effective magnetic field. If the nuclei are non-equivalent, the effect shows up in the NMR spectrum. The effect becomes observable if the distance between non-equivalent nuclei is less than or equal to three bond lengths. This effect is called spin-spin coupling or J coupling. In order to standardize the NMR spectra, the chemical shifts are positioned in relation to a

5  Characterization Techniques and Quality Assessment of Lignin and Lignin Carbon… Table 5.2 Isotopes commonly used in NMR spectroscopy

Isotope 1 H 13 C 19 F 31 P

Spin 1/2 1/2 1/2 1/2

209

Chemical shift range ~10 ppm ~200 ppm ~300 ppm ~500 ppm

C is a much less sensitive nucleus than H for NMR spectroscopy

13 1

Fig. 5.11 Typical 1H NMR chemical shifts ranges

reference proton set at 0.00 ppm. In most cases tetramethylsilane (TMS) is used as the reference compound for 1H NMR. Typical chemical shifts are shown in Figs. 5.11 and 5.12. Lignin is one of the polymers that present a lot of challenges for NMR studies. The highly mixed molecular weight, high viscosity, low solubility of lignin compared to small molecules results in low mobility leading to short relaxation times and broadening of spectral lines and limiting the detailed characterization of lignin. There is no existing method that will allow for a complete characterization of lignin in its native state. Characterization of lignin basically requires fractionation and isolation of each component in order to obtain qualitative and quantitative information about its composition and structure. Fractionation and isolation lead to alterations in the original structure of lignin.

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Fig. 5.12 Typical 13C NMR chemical shifts ranges

Proton NMR  Proton NMR spectra are inherently quantitative, providing the sample is allowed to relax sufficiently between scans affording a considerable higher sensitivity than 13C NMR. Functional groups such as methoxyl and acetate methyl groups in acetylated lignin, obtain improvements due to the multiplicity of identical protons. However, signals of the proton NMR are broad occurring over a narrow frequency range (12–0 ppm) rendering limited quantitative information [59]. Proton NMRs are either performed with derivatized or underivatized samples. Acetylated lignin samples show better resolution than non-acetylated lignin. However, the use of acetylation can cause a change in the lignin structure. Proton NMR of underivatized lignin provides key information of lignin functionality [60]. An example of 1 HNMR chemical shift of lignin is given in Table 5.3. Heteronuclear NMR (31P, 29Si, 15N, 19F NMR)  To solve the limitations associated with the determination of functional groups using conventional 1D 1H and 13C NMR and 2D correlation NMR techniques, a method to “selectively tag” specific functional groups in lignin with active nucleus using derivatization techniques before analysing with NMR was developed. For selecting NMR-active nuclei for NMR of lignin, the nuclei must be sensitive, with readily available derivatization reagents, and it should be easy to obtain quantitative derivatization under mild conditions. The analysis involves using a reagent that will effectively tag the specific functional groups. Heteronuclear NMR cases available are silicon-29 NMR, Nirogen-15 NMR, Mercury-199 NMR, tin-119 NMR, Flourine-19 NMR and phosphorous-31 NMR. 29Si NMR can be twice as sensitive as 13C NMR and can be used to distinguish aromatic, aliphatic and carboxylic acid hydroxyl groups in trimethylsilyated lignin [61, 62]. Other derivatization methods such as subsequent methylation of

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Table 5.3  Typical signals assignment and chemical shifts in the 1HNMR spectrum of acetylated spruce lignin using deuterated chloroform as solvent δ (ppm) 1.26 2.01 2.28 2.62 3.81 4.27 4.39 4.65 4.80 5.49 6.06 6.93 7.41 7.53 9.64 9.84

Assignment Hydrocarbon contaminant Aliphatic acetate Aromatic acetate Benzylic protons in β-β structures Protons in methoxyl groups Hγ in several structures Hγ in, primarily, β-O-4 structures and β-5 structures Hβ in β-O-4 structures Inflection possibly due to Hα in pinoresinol units and Hβ in noncyclic benzyl aryl ethers Hα in β-5 structures Hα in β-O-4 structures (Hα in β-1 structures) Aromatic protons (certain vinyl protons) Aromatic protons in benzaldehyde units and vinyl protons on the carbon atoms adjacent to aromatic rings in cinnamaldehyde units Aromatic protons in benzaldehyde units Formyl protons in cinnamaldehyde units Formyl protons in benzaldehyde units

silylated lignin can eliminate signal overlap with phenolic groups allowing the detection of alcoholic groups [63]. However, the methods suffer from negative gyromagnetic ratio and long spin-­ lattice relaxation times because of signal reduction when proton decoupled spectra are acquired. This means that the concentration of the sample must be very high and longer delay times will be required. To overcome this limitation Intensive Nuclei Enhanced Polarization Transfer sequence (INEPT) was suggested but the sequence pose major challenges in quantitative interpretation of data [64]. 15N NMR on the other hand, is rarely used in lignin characterization because of the low sensitivity, negative Nuclear Overhauser Effect (nOe) and high selectivity of the 15N atom with chemical environment which result in loss of some signals. The 15N NMR may be useful in oxidative conversion of lignin to determine the nitrogen containing products [65]. 199Hg NMR may be useful in the elucidation of aromatic substitution patterns of lignin [66] when coupled with 1H NMR. 19F NMR is useful in the determination of various hydroxyl groups in lignin by series of conversion. The method is a bit clumsy and may introduce complexities during quantification. The method has the inherent problem of not being able to distinguish primary from secondary OH groups [67–71]. 31P NMR is a nucleus that is 100% naturally abundant leading to a 15 times sensitivity over proton NMR. On the other hand, it has a large chemical shift (more than 1000 ppm). Phosphorous reagents are found effective in tagging hydroxyl groups or quinone structures in lignin so that their concentration

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Fig. 5.13 Derivatization scheme of lignin with 2-chloro-4, 4, 5, 5 tetramethyl-1, 3, 2-­dioxaphospholane. R lignin side-chain; R′ lignin unit [60]

can be determined by 31P NMR [72–78]. In some cases 31P NMR is capable of revealing stereo-chemical information [79, 80]. The technique is able to distinguish phenolic hydroxyl groups, primary and secondary aliphatic hydroxyl groups, erythro- and threo-forms of β-O-4 structures [81]. Several reagents can be used in derivatization of lignin before NMR analysis including 2-chloro-1, 3,2-­dioxaphospholane and 2-chloro-4,4,5,5-tetra-methyl-1,3,2-diaoxophospholane. A typical derivatization scheme using 2-chloro-4,4,5,5 tetramethyl-1,3, 2-­dioxaphospholane is shown in Fig. 5.13. Typical chemical shifts and integration regions for lignin in a 31P NMR spectrum. Internal standards are applied for the quantitative 31P NMR analysis of hydroxyl-containing functional groups [74, 82]. The solvent employed in 31P NMR is usually a mixture of anhydrous pyridine and deuterated chloroform (c.1.6:1.0 v/v) with a relaxation agent (i.e., chromium (III) acetylacetonate) and an internal standard. The lignin sample is dissolved in NMR solvent and the derivatization reagent added and mixed for a short time at room temperature [60]. An example of 31P NMR spectrum is shown in Fig. 5.14. Tables 5.4 and 5.5 show the chemical shifts for 31P NMR using different derivatization agents. C NMR in Lignin Characterization  13C NMR is frequently used in lignin analysis to determine the amount of interlinkages, providing a comprehensive overview of the structure of the macromolecule. The development of 13C NMR in lignin analysis was an important advancement in lignin the study of lignin structural chemistry [85]. It benefits from a broader spectral window (240–0 ppm) in comparison, a good resolution and less overlap of signals but with a longer time (24–36 h) than 1H NMR [60]. 13

CH2 –OH

L

H

C O

H

C

O

OH

Primary - OH

L OMe

Syringyl - OH Gualacyl - OH

OMe Threo MeO

OMe OH

OH -COOH

Erythro

Aldehydes Carbohydrates

137

136

135

134

133

132 131 31 P (ppm)

130

129

128

127

126

Fig. 5.14 The 31P NMR spectrum of birch dioxane lignin derivatized with 2-chloro-1,3,2-­ dioxaphospholane. Reprinted with permission from Taylor and Francis [83] Table 5.4  31P NMR chemical shift ranges showing the dependency on concentration of pyridine after derivatization with 2-chloro-1,3,2-dioxaphospholane [83] Chemical shift (ppm) with high pyridine concentration 136.5–135.8 136.8–135.2

Chemical shift (ppm) with low pyridine concentration 136.7–136.2 136.2–135.4

135.2–135.4

135.4–134.7

134.5–133.7

Around 134.7

133.7–133.2

134.3–133.8

133.2–132.7

133.7–132.8

132.7–132.1

132.8–1 32.3

132.1–131.6

132.3–131.6

131.6–131.0

131.6–131.0

130.4–129.7

130.5–129.9

129.7–1 29.3

129.9–129.5

27.1–126.5

27.1–126.5

Assignment Hydroxyl groups in xylan Erythro α-hydroxyls in acylglycerol-­ β-­syringyl units Erythro α-hydroxyls in acylglycerol−/β-guaiacyl units, xylan or LCC Threo alpha-hydroxyls in aryl glycerol-β-syringyl and guaiacyl units, LCC Gamma hydroxyls in α-carbonyl containing units, cinnamyl alcohol, LCC Gamma hydroxyls in aryl glycerolβ-aryl units Primary aliphatic hydroxyls (probably phenylcoumaran structures) Phenolic hydroxyls in syringyl structures Phenolic hydroxyls in biphenyl units, cinnamic aldehydes Phenolic hydroxyls in guaiacyl structures. Phenolic hydroxyls in guaiacyl and catechol structures Carboxylic hydroxyls in aliphatic acids, cynnamic acid

214 Table 5.5  31P NMR chemical shift ranges after derivatization with 2-chloro4,4,5,5-tetra-methyl-1,3,2-­ diaoxophospholane [83, 84]

S. O. Adeosun and O. P. Gbenebor Chemical shift (ppm) Assignment 150.8–146.3 Aliphatic OH groups 144.3–142.8 Condensed phenolic units: diphenylmethane 143.7–142.2 Syringyl phenolic units 142.8–141.7 Condensed phenolic units: 4-O-5 141.7–140.2 Condensed phenolic unit: 5–5’ 140.2–138.4 Guaiacyl and demethylated phenolic units 138.6–136.9 p-hydroxyphenolic units 135.6–133.7 Carboxylic acids

C NMR spectroscopy is relatively advantageous over proton NMR because in 13C NMR spectrum, the spectral data arise from the “backbone” of the molecule, providing information about the nature of all carbons in the molecule. Secondly, 13C NMR spectra are not complicated by spin−spin coupling effects, giving rise to single lines for each carbon environment when proton decoupling is applied during the 13C acquisition. On the other hand, 13C NMR chemical shift range spans a much wider region than its proton counterpart at 500 MHz. 13C NMR are routinely applied to determine certain moieties in lignin but lack the capacity for quantitative analysis. General practice in the use of 13C NMR spectroscopy analysis is to use the aromatic and methoxyl signals as internal standards [59] in expressing the various functional groups per C9 or per methoxyl unit, respectively [86]. This would have been the best method for lignin structural characteristics if technical lignin was not structurally altered and degraded during. This applies studies with structurally altered mutants or genetically modified lignin. Expressing functional groups per 100 phenyl propane units from such samples are particularly very misleading. To avert this, current practice involve the application of distortionless enhancement by polarization transfer (DEPT) and inverse gated proton decoupling to minimize the noise effect [59]. 13C NMR is particularly limited when the lignin is of high molecular weight. The peaks are severely overlapped in 13C spectra so that quantitative information can only be derived from peak clusters representing specific carbon types. Recent works shows that the amount of side chain Cα and Cβ can be determined with quantitative 13C NMR and DEPT using internal standards. However, for quantitative 13C NMR spectrum there are only 6% (for lignin model compounds) and (8% for native lignin) RSD of aliphatic side chain carbons [59, 87]. The poor resolution and sensitivity make it difficult to determine side chain carbons in different substructures by quantitative 13C NMR. The major disadvantage of using ­internal standards in 13C NMR acquisitions is their relatively long spin−lattice relaxation time constants (∼6  s), which would render the total acquisition time very long. To obtain good results with 13 C NMR spectroscopy, the lignin sample must be free from contaminants such as carbohydrates or extractives. The concentration of lignin in the solution must be high enough to maximize signal-to-voice ratio and minimize baseline and phasing distortions. As discussed earlier the use of DEPT and the inverse-gated decoupling sequence are used to reduce the noise effect. Lignin analysis is sometimes performed 13

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with or without acetylation. Standard chemical shifts for 13C NMR are given in Tables 5.6 and 5.7. An example of 13C NMR spectrum is also shown in Fig. 5.15. 2D NMR in Lignin Characterization  Two-dimensional NMR spectroscopy is based on pulse programming concept. The idea is to simulate a second dimension by acquiring a series of spectra by sequentially increasing the delay period in the pulse sequence. For example, carbon chemical shift can be correlated in one dimension with proton chemical shift of its attached proton in a second dimension. Possible correlation with 2D NMR can be detected from the correlational diagram of Ralph et al. [89] shown in Fig. 5.16. In COSY it is evident that Hα correlates with Hβ which also correlates with Hγ protons etc. In TOCSY all side chain protons correlate with each other. TOCSY provides extra correlations to COSY which is Table 5.6  The chemical shift value (δ, ppm) of 13C NMR spectrum of non-acetylated lignin ppm 166.5 160.0 144.7 130.3 125.1 116.0 115.0 152.5 149.7 148.4 148.0 146.8 145.8 145.0 143.3 138.2 34.6 133.4 132.4 131.1 129.3 128.0 128.1 125.9 122.6

Assignment C-9 in p-coumarate (PCE) C-4 in PCE C-7 in PCE C-2/C-6 in PCE C-1 in PCE C-3/C-5 in PCE C8 in PCE C-3/C-5, etherified S units C-3, etherified G units C-3, G units C-3, G units C-4, etherified G C-4, non-etherified G C-4, etherified 5–5 C-4, non-etherified 5–5 C-4, S etherified C-1, S etherified C-1, G etherified C-1, S non-etherified; C-1, G non-etherified C-5, etherified 5–5 C-1, non-etherified 5–5 C-β in Ar-CH=CH–CHO C-α and C-β in Ar–CH=CH–CH2OH C-2/C-6, in H units C-5/C-5′ in non-etherified 5–5 C-1 and C-6 in Ar–C(=O)C–C

125.9 C-5, non-etherified 5–5 Adapted from [88]

ppm 123.0 122.6 119.4 118.4 115.1 114.7 111.1 110.4 106.8 104.3 86.6 84.6 83.8 72.4 71.8 71.2 63.2 62.8

Assignment C-6, FE ester C-1 and C-6 in Ar–C(=O)C–C units C-6 in G units C-6 in G units C-5 in G units C-5 in G units C-2 in G units C-2 in G units C-2/C-6, S with α-CO C-2/C-6, S units C-α in G type β-5 units C-β in G type β-O-4 units (threo) C-β in G type β-O-4 units (erythro) C-γ in β-β; C-γ, β-aryl ether C-α in G type β-O-4 units (erythro) C-α in G type β-O-4 units (threo) C-γ in G type β-O-4 units with α–C=O C-γ in G type β-5, β-1 units

C-γ in G type β-O-4 units C in Ar-OCH3 C-β in β-β units C-β in β-5 units CH3 group, ketones (conj) or in aliphatic CH2 in aliphatic side chain CH3 or CH2 group in saturated side chains 14.0 γ-CH3 in n-propyl side chain 60.2 55.6 53.9 53.4 36.8 29.2 26.7

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Table 5.7  Chemical Shift Assignments for Acetylated lignins in Acetone-d6 ppm 20.2 20.5 20.6 20.8 29.83 50.7 51.1 55.3 56.3 62.8 63.2 63.4 64.2 65.0 65.2 65.9 72.4 72.9 74.4 14.9 75.2 75.7 16.3 16.5 79.9 80.3 80.6 81.0 81.4 83.3 85.3 86.1 86.3 88.3 88.6 103.0 103.4 103.6 104.5 105.2 106.6 111.0

ppm 20.3 20.6 20.7 20.9 – 51.1 51.4 55.5 56.5 63.3 63.6 63.8 64.5 65.2 65.5 66.1 72.7 73.4 74.7 75.4 75.7 75.8 76.1 77.0 80.4 80.7 81.0 81.4 81.8 83.6 85.5 86.3 86.0 88.6 88.9 103.3 103.6 103.9 105.0 105.5 107.3 111.3

Assignment AcMe on phenolic OH of S-rings AcMe on Bß in S-b-S-glycerol AcMe on primary (γ) OH AcMe on phenolic OH of G-rings & on benzylic OH (α) Center signal of acetone-d6 multiplet (solvent and internal standard) ß in EG-bl-GF ß in G-c-G ß in G-r-G, G-r-S, S-r-S OMe γ in G-b-G(e), S-b-G(e),*G-c-G-glycerol γ in G-b-S(e), S-bS(e), G-a-G γ in G-b-G(t), S-b-G(t)* γ in G-b-S(t), S-b-S (t) γ in dibenzodioxocin moiety γ in EG-bl-GF γ in C-alc & S-alc end-groups γ in G-c-G γ in G-r-G, G-r-S, S-r-S Bß in G-c-G-glycerol,S-b-S-glycerol α in G-b-G(e) α in G-b-S(e) α in G-b-G(t), S-b-S ( e ) α in EG-bl-GF α in G-b-S(t) α in S-b-S (t) ß in G-b-G(e), S-b-S(e)* ß in G-b-G(t), a in G-a-G ß in G-b-S(e), S-b-S(e) ß in G-b-S(t), S-b-S(t), G-a-G ß in dibenzodioxocin moiety α in dibenzodioxocin moiety α in G-r-G α in S-r-S α in G-c-G α in S-c-G 2,6F in S-r-S 2,6F in G-c-G 2,6E in S-c-G 2,6E, F in S-b-S(e) 2,6E in S-b-S(t) 2,6 in S-rings with αC=O 2F in G-c-G, G-r-G, G-r-S (continued)

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Table 5.7 (continued) ppm 111.3 111.7 112.2 112.4 112.5 114.2 115.7 116.2 118.2 118.5 118.6 118.5 1189.0 119.3 119.9 120.1

ppm 111.7 112.0 112.4 112.5 113.1 114.4 115.9 116.4 118.4 118.6 118.7 119.1 119.2 119.5 120.1 120.7

121.5 122.1 122.4 122.7 123.0 123.2 123.6 124.0 125.9 127.8 128.2 128.7 128.9 129.3 129.9 131.0 131.5 131.7 132.0 132.2 132.6 133.0 133.3 133.8 134.0

121.7 122.3 122.6 122.9 123.2 123.4 124.2 126.1 126.1 128.0 128.4 128.9 129.3 129.5 130.1 131.2 131.7 132.0 132.2 132.6 133.0 133.3 133.4 134.4 134.2

Assignment 2E in G-c-G, G-r-G,C-alc, vanillin 2F in G-55-G 2F in G-b-S; B2 in G-c-C-alc 2 in EG-b1-GF 2E,F in G-b-G, G-c-G; 2E in G-b-S, cinnamaldehydes; 2F in G-a-G 2F in EG-b1-GF 5E in vanillin B6 in G-c-C-alc 5E in cinnamaldehydes 6F in G-r-G,G-r-S 6F in G-c-G 5E in G-b-G, G-b-S, G-r-G, EG-b1-GF 5E in C-alc; 6E in G-c-G 5E in G-c-G,6E in G-r-G 6F in G-b-S 6E,F in G-b-G;6E in G-b-S,EG-bl-GF,C-alc; 6F in G-a-G, in dibenzodioxocin moiety 6F in G-55-G ßE in C-alc; 6F in EG-bl-GF 5F in G-r-S, in dibenzodioxocin moiety 3,5E in p-OH-benzoate 5F in EG-bl-GF 5F in G-b-G, G-b-S, G-a-G; ßE in C-alc 5F in G-c-G,G-r-G;6E in cinnamaldehydes ßE in S-alc 6E in vanillin 1F in p-OH-benzoate ßE in C-ald 4F in S-r-S B5 in G-c-G 4F in S-b-S 1E in cinnamaldehydes B 1 in G-c-G-glycerol 5F in dibenzodioxocin moiety; B1 in G-c-C-alc;2,6F in p-OH-benzoate 5F in G-55-G 1E in vanillin 1E in C-alc 1E in G-b-G B5 in dibenzodioxocin moiety 1E in S-alc 1E in S-b-S αE in C-alc (continued)

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Table 5.7 (continued) ppm 134.3 134.5 134.7 135.5 135.8 136.0 136.6 137.0 137.5 137.1 138.0 138.3 138.5 140.0 140.5 140.7 141.2 141.7 145.0 147.0 147.8 148.0 149.2 150.9 151.4 151.8

ppm 134.5 134.7 134.9 135.7 136.0 136.6 136.8 137.3 137.7 137.9 138.2 138.5 138.9 140.2 140.7 141.0 141.4 142.0 145.4 147.6 148.0 148.5 149.4 151.0 151.7 152.2

152.3 152.8 153.4 153.7 155.6 165.5 168.2 168.4 168.9 169.2 169.9 170.2 170.6 170.9 180.5

152.7 153.3 153.6 154.1 155.8 166.5 168.3 168.7 169.0 169.4 170.0 170.4 170.8 171.0 180.9

Assignment αE in S-alc Bα in G-c-C-alc 1 in EG-bl-GF 4 in ES-r-S 1 in FG-55-G 1 in FG-b-G; 1F & 4E in S-b-S; 4 in ES-c-G; 1 in EG-r-G 1 in FG-b-S; 1 in EG-c-G 1 in FG-a-G B1 in EG-b1-GF IF in dibenzodioxocin moiety 4 in FG-55-G 1 in ES-r-S 4 in FG-r-G, EG-bl-GF 4F in G-b-S, G-a-G 4F in G-b-G, in dibenzodioxocin moiety; 1,4F in G-c-G 1 in FS-r-S 1 in FG-r-G B3 in G-c-G 4 in EG-r-G 4 in EG-bl-GF 4E in G-b-G,C-alc B4 in G-c-C-alc, S-c-C-alc 4E in C-ald 3E in G-b-G, G-b-S, vanillin, EG-bl-GF 3E in C-alc, cinnamaldehydes, G-r-G, G-c-G; 3F in G-b-G, G-b-S, G-a-G, EG-b1-GF,in dibenzodioxocin moiety; 4E in G-b-S 3F in G-r-G, G-c-G, G-55-G 3,5F in S-b-S, S-r-S; αE in C-ald 4E in vanillin 3,5E in S-b-S, S-r-S 4F in p-OH-benzoate Benzoate C=O in p-OH-benzoate Phenolic AcC=O on S-rings with a C=O Phenolic AcC=O on S-rings Phenolic AcC=O on G-rings Phenolic AcC=O on p-OH-benzoate entities Benzylic (α) AcC=O in G-& S-rings Bß AcC=O in G-c-G-glycerol,S-b-S-glycerol Primary (γ) AcC=O in G-b-G, G-b-S, G-a-G, G-c-G-glycerol Primary (γ) AcC=O in G-c-G p-Quinone C=O in G-rings (continued)

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Table 5.7 (continued) ppm 191.1 193.8

ppm Assignment 191.5 C=O in vanillin 194.2 C=O in cinnamaldehydes

G guaiacyl, S syringyl, C coniferyl, S sinapyl, V vanillyl, E etherified phenolic, F free phenolic, a α-O-4, b ß-O-4, r resinol (ß-ß), c coumaran (ß-5), bl ß-1, 55 5-5 (biphenyl), (e) erythro, (t) threo, alc alcohol, ald aldehyde. Adapted from [89] * All assignments refer to the A-ring of the dimer fragment unless otherwise noted. ‘Shifts are from model compound S-b-Gonly and were not found in lignins or DHPs

Fig. 5.15 Quantitative 31C NMR spectra of milled wood lignin (MWL) and mild acidolysis lignin (MAL). Reprinted with permission from ACS [90]

particularly valuable. They are commonly used in lignin analysis and they possess the capability of identifying lignin minor structures [91–102]. INADEQUATE is a method that correlates [103–105] two neighbouring 13C-resonances and operates through direct 13C-13C coupling. They are low-sensitivity experiments at natural 13 C-abundance (1.1%). The INADEQAUTE-type experiments provide diagnostic connectivity information on the carbon skeleton of lignin but this connectivity information can be easily obtained using long range C-H correlations in a more sensitive procedure such as HMBC.

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Fig. 5.16  Conceptual representation of the correlation contours resulting from various homonuclear (1H–1H) (a) COSY and (b) TOCSY and heteronuclear (13C–1H) (c) HMQC or HSQC, (d) HMQC-TOCSY or HSQC-TOCSY, (e) HMBC 2D NMR experiments. Chemical shifts are not accurate and are for illustrative purposes only [89]

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For HMQC or HSQC each carbon correlates directly with its corresponding proton e.g. Cβ correlates with Hβ. They are by far the most useful experiments with extra apparent resolution which exceeds any of the 1D NMR. In HMQC the correlation goes further to include other protons in the same coupling network e.g. Cα correlates with Hα and Hβ and the two Hγ protons. The side chain correlations of lignin substructures separate well with 2D 1H-13C HSQC spectra such that the use of Q-HSQC approach provides a powerful tool to analyse the composition of wood lignin. An example of HSQC spectrum of lignin is given in Fig. 5.17. HMBC goes further to correlate protons and carbons that are related by long range coupling interactions over 2–3 bonds giving rise to very extensive valuable connectivity information, e.g. Hα correlates with all carbons within three bonds including Cβ, Cγ, C1, C2, C6 and the α-acetate carbonyl (Fig. 5.18). This makes HMBC a versatile technique to remove ambiguity of carbon assignments. HMQC technique does not allow detection of quaternary carbon. Recent advancement in the use of 2D NMR to eliminate the problem of signal assignment is the application of two different 2D NMR in concert. For example, most of the spin systems in lignin have been assigned reliably by applying 1H-detected 1H–13C correlation (HMQC) and homonuclear Hartman–Hahn (HOHAHA) in concert [95, 99, 106]. Another method involves a combination of HOHAHA and HMQC data, providing HMQC correlations together with HOHAHA in the same 2D plot [102]. However, some overlap of resonances remains so the assignments of signals are still guesswork. For quantitative analysis with 2D NMR, the (short-range) 13C–1H correlation experiments (HMQC or HSQC) are the best candidates. 3D HMQC-HOHAHA NMR Spectroscopy in Lignin Characterization  Three-­ dimensional (3D) NMR experiments have been applied in lignin characterization by some researchers [89, 91, 107–109]. The value of 3D NMR hinges on further dispersion realized by correlating 2D spectra over other dimensions. The use of 3D NMR has been found to eliminate most of the doubts in assigning NMR signals to structural units in lignin. Known 3D NMR methods in lignin characterization include 3D HMQC-TOCSY, HMQC-HOHAHA and HSQC-TOCSY NMR [89, 91, 107–109]. Although the value of 3D NMR is appreciated nowadays, it is not so much compelling over 2D because of its increased complexity, data size, and the reduced resolution. Example of a 3D NMR of lignin is shown in Fig.  5.19. The advantages of 3D HMQC-HOHAHA include less overlapping of the spectrum because both carbon and proton chemical shifts in two spin systems seldom c­ oincide and secondly, ambiguous assignments can be cross-checked from different planes of the 3D spectrum. The data can be approached in the proton–proton correlations are extended according to their carbon chemical shifts or in the plane where proton– carbon correlations are spread according to proton chemical shifts. However, the 3D NMR characterization of lignin employs 13C enrichment to obtain reasonable measuring times. Structural Moieties and Linkages  HSQC is a very good way to study the main structural characteristics of the lignin including various units linked by ether and C-C bonds. Zhang et  al. [113] used 1H-13C HSQC NMR and 31P NMR to study

Fig. 5.17  2D NMR HSQC spectrum (in DMSO-d6) of organosolv lignin sample 8; (a) before and (b) after 8 h reaction time; (c) exemplary structures for the β-O-4′ aryl ether linkages (A, A′), resinol linkages (B) and phenylcoumaran linkages (C) [110]

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Fig. 5.18  Partial HMBC spectrum of wheat straw MWL [111]

structural changes in lignin after extraction. Figure 5.20 shows the results obtained from the 2D NMR and 31P NMR respectively. It is evident that NMR is very useful in detecting structural differences in lignin arising from extraction methods. 1H, 13C, 1 H and 2D 1H-COSY HSQC and HMBC NMR were also used to elucidate the structure of hardwood lignin to determine its suitability as a precursor for carbon fibres. The study showed the efficiency of HMBC in the detection of quaternary carbons in

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Fig. 5.19  3D NMR (750 MHz) “isolation” of the major units in COMT-deficient transgenic poplar lignins. (a) A 3D gradient-selected TOCSY-HSQC spectrum (70 ms TOCSY mixing time) of a natural 13C-abundance lignin (acetylated) from the sense-suppressed COMT transgenic; (b) 2D gradient-selected HMQC spectrum; (c) the first F2–F3 plane which is essentially a 2D 13C–1H HSQC spectrum; (d)–(g) 2D F2–F3 slices for the major structural units (A, B, C and novel H) [112]

Fig. 5.20 (a) Quantitative 31P NMR spectra of eucalyptus lignin fractions using 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane as phosphitylating reagent (b) HSQC spectra of the lignin fractions. Reprinted with permission from ACS [113]

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the lignin [114]. 1H NMR exclusively can be used to determine the chemical structure of lignin [115, 116]. 1H NMR has also been used to determine the structural constituents of lignin and the relative hydroxyl groups and condensation reactions during lignin extraction [117–123]. 13C NMR spectroscopy has proven a reliable method to investigate the structure of the carbon skeleton in lignin and has been used at several instances to characterize lignin [86, 90, 102, 124–130]. Inter-units and inter-unit linkages in lignin have also been successfully investigated using 1H– 13 C correlation NMR spectroscopy [87, 111, 125, 131–140]. Evidence of new structural units and side chain structures in lignin are made available by employing 3D NMR spectroscopy to lignin where conflicting results were earlier reported when using 2D NMR [134, 141]. The use of 3D TOCSY-HSQC made possible striking details of lignin units and even goes as far as differentiating the stereoisomers. This can be seen in Fig. 5.13b where the syn-isomer is distinguished particularly by its lower chemical shift γ-proton pair [142]. Hallac et al. [77] used 13C NMR to perform a structural overview of lignin after ethanol Organosolv delignification and employed DEPT-135 13C NMR to confirm the chemical shifts for CH2, and CH regions. The technique enabled the confirmation that Cα-H, Cβ-H and Cγ-H are cleaved during the treatment. 13C NMR and 31P NMR are also used as complementary experiments for quantitative analysis of lignin molecular structure [143]. 13C NMR and 31P NMR are also used to study the mechanism of cleavage during extraction of lignin [144]. The use of one-bond 13C–1H NMR correlation spectroscopy was found useful in revealing the mechanism of delignification during hydrothermal pre-treatment. Deducing the type of lignin present in a plant has also been made simple with a combination of 13C NMR and HSQC [145]. Another important use of 31 P NMR is in the quantitative study of reactions between lignin and other polymers [146–148]. A method that will allow plant cell wall to be examined without derivatization was developed for 2D NMR experiments, using gradient-HSQC 1-bond 13C–1H correlation spectroscopy [149]. To aid in the analysis of lignin inter-­ unit linkages the method of quick quantitative HSQC (QQ-HSQC) NMR experiment has been applied with guaiacyl C2 and syringyl C2–C6 signals as internal standards [150]. The method reduced the error and time needed for 2D NMR analysis of lignin and allowed it to be used as a sole analytical technique for lignin analysis eliminating the need for 1D complementary methods. For easy study of lignin structure, Rencoret et  al. [151] proposed a method of analysing wood with 2D NMR. The wood is first swollen in deuterated dimethylsulphoxide and further sonication to obtain a gel for NMR analysis. Using the gel in HSQC experiment gave the same results as conventional methods but with minimal sample preparation. Structural characterization with 13C NMR has been used to show the major ­differences between the coniferyl alcohol-based lignin and the coniferaldehyde/ ferulic acid lignin [152]. Lignin Carbohydrate Linkages  A direct evidence of alpha ether linkages between lignin and carbohydrates in plant cell walls has made possible by the use of 3D 1 H–13C TOCSY-HSQC NMR spectra (Figs. 5.21 and 5.22). 2D 1H-13C HMQC and HMBC have been used to directly detect phenyl glycoside, ester and benzyl ether

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Fig. 5.21  Slices from a 3D NMR spectrum of acetylated pine ball-milled cell walls, illustrating the spectacular lignin sub-structural editing possible in this complex sample. The 3D spectrum was acquired with the acquisition restricted to the lignin sidechain region. (a–d) F2–F3 (HSQC) slices at a given 1H shift; (e–h) F1–F2 (HSQC-TOCSY) slices at a given 1H shift; (i–l) F1–F2 (TOCSY) slices at a given 13C shift. Contours in d are artificially enlarged to enhance their visibility. Partial substructures of β-ether units A and phenylcoumaran units B are given with conventional sidechain labelling [142]

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Fig. 5.22 3D 1H–13C TOCSY-HSQC NMR spectra

lignin-carbohydrates linkages in lignin samples [153]. Yuan et al. [154] investigated lignin carbohydrate complex (LCC) linkages using a combination of quantitative 13 C and 2D HSQC NMR techniques. The mechanism of formation of p-Coumaric acid in maize lignin was investigated using one-bond 13C–1H NMR and long range C-H correlation (HMBC) [155]. The use of 31P NMR in analysis of LCC showed that hemicelluloses–lignin complexes are formed during the early stages of lignification through radical enzymatic cross-linking catalysed by laccase [156]. Wen et al. [157] used Q-HSQC NMR spectroscopy to study the LCCs in bamboo. The combination of 13C NMR and HSQC 2D NMR techniques was used to quantify the lignin–carbohydrate linkages in plant cell wall [158]. Modification of Lignin Structure  Two-dimensional heteronuclear single-­ quantum coherence (1H–13C HSQC) NMR has been used to study structural modifications in lignin arising from their extraction methods [159]. 1H NMR has also been used to study grafting of other molecules on to lignin [160]. Grabber et al. [161] used 1H NMR to study the incorporation of coniferyl ferulate into lignin. Yelle et al. [162] used 1H–13C HSQC to study the cleavage of lignin by brown rot basidiomycete. 1H–13C correlations (HSQC) have been used to study structural changes in lignin due to acylation [163]. 2D NMR is also useful in investigating the acylation of lignin at the γ-carbon of the side chain [164]. Gilca et al. [165] used 31P NMR to confirm the fact that ultra-sonication of lignin leads to increase in the amount of OH groups probably arising from oxidative processes which are related to side-chain cleavage. In the same study QQHSQC analysis was employed to quantitatively study modifications in inter-unit linkages arising from the sonication process. 2D NMR experiments (COSY, HSQC and CIGARAD) have been used to show that β-O-4 linkage in lignin model compounds could easily undergo Cα–Cβ bond fragmentation in the presence of acidic catalyst [166]. 2D-HSQC structural changes in the lignin of archaeological wood compared lignin in reference wood samples [167,

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168]. Quantitative 13C NMR, 2D NMR and 31P NMR have been used to explore structural changes in lignin due to steam explosion treatment [169]. Using 2D-HSQC NMR analysis Wen et al. [170] was able to show that the use of neutral solvent in the isolation of leads to acetylation, whereas the acetylation is destroyed when lignin is isolated with alkali solvents. Leschinsky et al. [171] used a combination of quantitative 13C Q-DEPT NMR spectra, 1H NMR and 2D 1H/13C Q-HSQC NMR to study the changes in lignin structure during auto-hydrolysis. Li et  al. [172] also examined the effect of auto-hydrolysis on lignin structure. The effect of genetic transformation on the structure of lignin has been successfully examined using 2D 13 C-1H correlation experiment [173]. Using HSQC experiment, Samuel et al. [174] showed the similarity in the structures of steam pre-treated and dilute acid pre-­ treated lignin samples. The authors also used the same method to study structural changes in switchgrass lignin during pre-treatment [175]. Runcang et al. [176] used 13 C NMR spectroscopy to study the structural differences in lignin with difference in alkali type and alkali concentration. 2D NMR is also useful in the study of lignin structural evolution during degradative method of delignification [177]. The Case of Solid-State NMR  Solid state NMR analysis is a non-destructive method of NMR spectroscopic analysis which is not limited by solubility. For lignin analysis, there is no need for degradative treatments of lignin or derivatization. The most prevalent solid-state NMR method in lignin analysis is the cross-polarization/ magic angle spinning (CP/MAS) method. Hatfield et  al. [178] demonstrated the ability of solid state C/P MAS NMR to identify many of the key functionalities in lignin. Although it is a good method for the elucidation of lignin structure and does not require long experimental times, solid state NMR does not present a satisfactory quantitative analysis [179]. The method is very useful for structural investigation at atomic level but suffers from severe spectral crowding of the responding signals [180]. An example of the solid-state NMR spectra of lignin is presented in Fig. 5.23. 1 H–13C Cross-polarization solid-state NMR has been used to examine the interactions between two polymers at the molecular scale [181]. However, the resolution is very poor and does not meet the requirement for detailed structural examination of lignin [182]. It is also capable of providing information concerning two or more allomorphic forms of a crystalline material. However, because the carbon line widths are larger than crystallite size, it is difficult to acquire an informative 2D spectrum. To improve on the scope of solid state 13C CP/MAS NMR technique, the use of an internal standard was proposed [183]. In the study, sodium-3-­ trimethylsilylpropionate was used as an internal standard. It was mixed homogeneously with lignin at different proportions before measurement (Fig. 5.24). This method was found to be effective for estimating lignin content in certain samples of unknown lignin composition. An improvement in structural analysis of biomass was obtained using dynamic nuclear polarization (DNP)-enhanced solid-state NMR. This technique enabled the measurement of 2D homonuclear 13C–13C correlation solid state NMR spectra under natural abundance, producing a picture of the structure at atomic level [184]. Recently Kang et al. [185] used solid-state NMR spectroscopy supported by dynamic nuclear polarization to reveal that lignin in plants

Fig. 5.23  1H 13C CPMAS spectra, of chitosan film, 10 wt% lignin film 30 wt% lignin films and pure lignin. Reprinted with permission from ACS [181]

Fig. 5.24  Solid state 13C CP/MAS NMR spectra using an internal standard. Reprinted with permission from ACS [183]

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Fig. 5.25  DNP enhanced solid state NMR analysis of LCC plants. (a) DNP enhances the NMR sensitivity by 23-fold on maize. The inset shows a representative DNP sample. (b) Lignin-edited (top) and control (bottom) 13C–13C correlation spectra measured using DNP [185]

possess an abundant electrostatic interactions with polar motifs of xylan (Fig. 5.25). Plants used for the study were isotope labelled with 13C and the samples were measured as 40 wt% hydrated samples. Other studies involving the use of solid state NMR of lignin has also been reported [186–191]. A very interesting study that is of interest is the use of solid-state NMR to study the formation of char in biomass. The chemistry of transformation of lignin into carbon fibres can be made clearer by the application of solid-state NMR at the various stages of lignin carbon fibre processing. Braun et  al. [192] used solid state NMR to study the reactions taking place during thermal treatment of lignin based carbon fibres. Using solid state NMR Foston et al. [193] showed that during stabilization and carbonization of lignin carbon fibres there is increased relative amount of carbon–carbon bonds on aryl structures and a relative decrease of aryl ethers.

5.3.4  X-Ray Photoelectron Spectroscopy X-ray photoelectron spectroscopy (XPS) is a type of electron emission spectroscopy (EES). Energy emission spectroscopy is a technique in which the energy distribution of electrons ejected from a material as a result of incident radiation is measured and used to identify the chemical and atomic state of the material. Other methods in EES include ultraviolet photoelectron spectroscopy, Auger electron spectroscopy and reflected electron energy-loss spectroscopy. When an incoming electromagnetic radiation such as visible light, ultra-violet light, X-rays and gamma rays is incident upon a material surface, atoms can be ionized producing an ejected

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Fig. 5.26  Schematic of hoe electrons are ejected from the surface of a material

electron (Fig.  5.26). The ejected electron is called a photoelectron. This ejected electron possesses kinetic energy which depends on the energy of the incident photon. The kinetic energy can be measured using the Einstein photoelectric law (Eq. 5.5).

KE = hv − BE

(5.5)

where hv is the photon and BE is the binding energy of the particular electron to the atom in question. Because hv is known, the kinetic energy is dependent on the binding energy. From basic science, it can be recalled that a material is made of atoms which compose of protons, neutrons and electrons. In a neutral atom the number of protons (positive) in the nucleus equals the number of electrons (negative). The electrons are arranged in orbitals around the nucleus bound by electrostatic forces (attractive). An orbital contains only two electrons of opposite spin. The energy levels of each orbital are discrete and are distinct from the same orbital in a different atom owing to the difference in number of protons in the nucleus which is never the same for two atoms. One can quickly measure the binding energy for a particular electron by measuring the energy required to remove it from the orbital. Because this energy is specific to the atom in question, a measure of this energy would be a way to identify the atom. This would be true if the only thing that happens when an electron is ejected is the removal of this one electron and the other electrons remain unchanged. The photoelectric process comprises (1) photon absorption, (2) electron emission, (3) displacement of the electron to fill the emitted electron and (4) escape of the electron completely from the material into a vacuum where it can be detected. In basic terms, photoelectron spectroscopy detects and measures the kinetic energy of the electron removed from the surface of the material. For example, let us assume that an XPS X-ray source beams a ray of energy on a carbon atom (a carbon atom consists of 6 electrons with 2 electrons in each orbital—1s, 2s and 2p). Electrons can be ejected from each of them in an ideal situation meaning that three groups of photoelectrons with three different KEs will be ejected and detected. The KE can be plotted against the number of electrons as shown in Fig. 5.27. Important to note from the figure is the intensity of the various peaks which is dependent on the probability for ejecting an electron from each orbital which is different for different orbitals. The probability also varies for a given orbital in different atoms and depends on the energy of the x-radiation used. From Eq. (5.5) the KE scale can be replaced by BE so that a direct experimental

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Fig. 5.27  Schematic of the KE energy distribution of photoelectrons ejected from an ensemble of C atoms subjected to XPS analysis

determination of the electronic energy levels in the carbon atom can be obtained. The number of peaks in Fig. 5.25 corresponds to number of occupied energy levels in the atoms and the positions of the peaks represent a measure of the BEs of the electrons which can be used to identify the atom. These are mere approximations based on the assumption that all electrons behave independently of each other. However, this is not the case in all situations. XPS uses soft X-rays (1486.6  eV, from an Al anode) to eject photoelectrons from the surface of the material tested. Generally, in XPS, electrons from the core levels are used to identify the elements present using the BEs, and the chemical shifts in the BEs are used to provide additional chemical state information. The relative peak intensities are used to measure the relative concentrations of the different elements. XPS can be used for elemental analysis (except H and He), empirical formula, electronic state, chemical state analysis and surface sensitivity of materials. However, XPS shows moderate absolute sensitivity and lacks good spatial resolution. Practically, XPS is useful in the analysis of corrosion rates, catalytic activity, adhesive properties, bioactivity of surfaces, thin film electronic surfaces, wettability, contact potential and failure mechanisms of materials. It is the standard tool for surface material characterization. XPS is relatively easy to use, interpret and quantify. It is extensively popular and poses practically absent irradiation damage [194]. XPS characterization of lignin materials is usually the measurement of surface sensitivity and chemical activity of the surface. Surface analysis of XPS is based on the fact that electrons from XPS can only travel short distances (1–10 nm) through solids before losing energy in collision with atoms. The photoelectrons ejected from atoms on the near surface will escape un-scattered and appear in the XPS peaks. Theoretically, if the flux of electrons originating at depth d of a material is Io, the flux emerging without being scattered, Id, can be given by Eq. (5.6).

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I d = I o λe sin θ

(5.6)

where θ is the angle of electron emission and d/sin θ is the distance travelled through the solid at that angle. λe is called the inelastic mean free path length. It determines quantitatively the measurement of the surface sensitivity and is dependent on the KE of the electron and the material through which the electron travels. In the study of lignin, different scan resolutions can be used to obtain different information. Using a low-resolution scan one can obtain the percentage of each element present and the atomic concentrations are then calculated from the peak intensities. Elements important in the study of lignin include carbon (C1s), oxygen (O1s), nitrogen (N1s), sodium (Na1s) and sulphur (S2s and S2p). Using a high-resolution scan, the types of bonds present and their concentrations can be determined using peak shifts depending on which atoms the analysed atom is bound to. Consider the case of carbon (C1s), a high-resolution spectra may consist of four component peaks around 285.0, 286.9, 288.7 and 289.3 eV. These subpeaks can be assigned to C1 (C-C or C-H), C2 (C-OH or C-O-C), C3 (O-C-O or C=O) and C4 (O-C=O), respectively. Applications of XPS in Lignin Characterization  XPS has been used to study the effect of modification on the surface properties of lignin [195]. The functional groups present in the near surface of the modified lignin was measured and used to measure the degree of condensation after the modification. XPS was also used to confirm the successful incorporation of lignosulphonates into the lignin [196]. The study concluded that lignin can be modified through by condensation reactions. Zhou et al. [197] used XPS to determine distinct chain scission and introduction of oxygen based functional groups on the surface of plasma treated lignin. Saarineen et al. [198] examined the adsorption of laccases on the surface of lignin substrates. Surface reactivity of lignin when blended with PVA has been analysed using XPS [199]. The use of XPS was used to confirm the existence of phase separation phenomena in the fibres. An important application of XPS in lignin is the study of the oxidative stabilization of lignin-based carbon fibres. Using XPS, Ruiz-Rosas et al. [200] showed that oxidative stabilization leads to the increase in oxygen on the surface of lignin fibres. The progress of cross-linking reactions during stabilization of lignin fibres was examined using XPS [201]. In the study of lignin based activated carbon, XPS has been used to study the effect of activation time of the chemical structure of the activated carbon. Using XPS Lin et al. [202] showed that the longer the activation time the higher the tendency to form oxygenated compounds (Fig. 5.28). Rosas et al. [203] also reported the use of XPS in the analysis of lignin activated carbon. XPS can be used to measure the number of aliphatic carbon-­ carbon bonds in stabilized and carbonized carbon fibres. A high percentage of carbon-­carbon is an indication of high mechanical properties [204]. Evidence of oxidation and condensation reactions occurring during stabilization, carbonization and graphitization are usually visible in the C(1s) and O(1s) region of XPS spectra [192]. Cross-sectional studies of lignin carbon fibres using XPS was reported for the first time by Brodin et al. [205]. The study illustrated the changes in carbon across

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Fig. 5.28  X-ray photoelectron spectroscopy spectra of carbon fibres and activated carbon (a) and of C1s region for activated carbon (b) [202]

Pt1 Pt2

surface C1 = 49 C2 = 37 C3 = 5 C4 = 8

surface

10µm

/air

Pt3 40µm

C1 = 69 C2 = 15 C3 = 6 C4 = 10 C1 = 56 C2 = 32 C3 = 6 C4 = 7 C1 = 57 C2 = 34 C3 = 4 C4 = 5

Fig. 5.29  Cross-sectional analysis of lignin carbon fibres using XPS [207]

the cross section of the fibres (Fig. 5.29). With this technique the authors were able to detect inhomogeneity in the structure of birch wood fibres and skin-core structure exhibited by pitch-based fibres. Nar et  al. [206] also used XPS to determine the chemical bonding in electrospun carbon fibres. Carbon peaks in XPS are usually assigned as shown in Table 5.8. Table 5.9 shows the typical chemical shifts for bond assignment in XPS.

5.3.5  X-Ray Diffraction X-ray diffraction (XRD) is a characterization technique based on the diffraction of electromagnetic radiations when incident on material. Diffraction methods identify chemical compounds using their crystalline structure as opposed to spectroscopic

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Table 5.8  Different types of carbon obtained with XPS [207]

Carbon C1 C2 C3 C4

Number of bonds to oxygen No bond to oxygen One bond to oxygen Two bonds to oxygen Three bonds to oxygen

Functional groups C-C, C=C-H C-O, C-O-C C=O, O-C-O O-C=O, C(=O)-OH

The positions of these carbons are exemplified in Fig. 5.30 Table 5.9  Typical XPS chemical shift in lignin [121, 192, 201, 202, 204, 205, 207–209] Binding energy (eV) 284.6–284.7 285.8–286.1 286.9–287.1 287.1–287.4 288.5–288.8 290.1–294.0 290.7 398.3 399.0–400.0 400.1 401.6 531.1–531.4 532.9–533.2 533.7 535.1–535.4

Assignment C-C/C-H of graphitic carbon C–O (ether /hydroxyl), and/or C–N Carbonyl or quinine groups C=O (carbonyl) COOH (carboxyl) functionalities Satellite peaks due to π–π* transitions Carbonate groups Pyridinic Amide functionality Pyrrolic Amine moieties and quaternary C=O groups (ketone, lactone, carbonyl). C–O in esters, amides, carboxylic anhydrides and oxygen atoms in hydroxyls, ethers Oxygen in O-C, C-O-C, C=O, O-C-O, O-C=O Chemisorbed oxygen and/or water

methods which uses compositions of chemical elements. XRD can identify crystalline phases present in materials and measure their structural properties. It can also determine the thickness of thin films and multilayers, atomic arrangements in amorphous materials and interfaces. X-ray diffraction uses X-radiations as incident radiations. X-radiations are electromagnetic waves with wavelength much shorter than visible light (in the order of 0.1 nm). The atoms in a crystal are arranged in a periodic manner and thus can diffract incident light. When X-rays are incident upon a material, the X-rays are scattered and produce a diffraction pattern, which contains information about the atomic arrangement within the crystal. This is only possible in materials with short-range crystal arrangement. Materials with long-range crystal arrangement (glass and other amorphous materials) cannot diffract X-rays. The XRD uses the information acquired from the diffraction patterns (Fig. 5.31) to determine material properties. Using Eqs. (5.7)–(5.9) one can calculate amorphous carbon fraction (Xa), aromaticity (fa), interlayer spacing (d002), crystallite lateral size (La) and thickness (Lc).

λ = 2d sin θ

(5.7)

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Fig. 5.30  High-resolution spectra of hardwood Kraft lignin showing the positions of C1, C2, C3 and C4 [207]

Fig. 5.31  Bragg diffraction by crystal planes. Reproduced with permission from Callister WJ Jr. Materials science and engineering: an introduction, 10th ed. Wiley, New York. © 2018 John Wiley & Sons Inc.

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fa =

A002 A002 + Aγ

La / c =

(5.8)

Kλ B cos θ

(5.9)

where A002 and Aγ are the areas underneath of (002) and γ peaks, respectively. Equation (5.7) is the popular Bragg’s equation. Equation (5.8) is the Scherrer equation. λ is the wavelength (nm); B and θ correspond to the full width at half maximum (FWHM) and the Bragg angle of the corresponding peak, respectively; K is equal to 0.89 and 1.84 for (002) and (101) peaks, respectively. Xa value is resolved by adjusting the (002) peak to the most symmetrical peak. fa describes the ratio of carbon atoms in edge aliphatic chains versus aromatic rings [210–212]. In lignin characterization XRD is used mostly in the analysis of the crystalline structure of lignin carbon fibres and activated carbon. It is very important in the analysis of stabilization and carbonization stages. The crystal structure of electrospun carbon fibre mats deposited on silver particles, intended for fuel cell applications has been studied using XRD [213]. With the help of XRD, the authors were able to monitor the development of crystalline structure in the carbon fibre mat in relation to thermal treatment. Lai et al. [214] also used XRD to monitor the development of graphitic structure in electrospun fibre mat during carbonization. Berenguer et al. [215] also studied the development of graphitic structure during activation of lignin activated carbon material for energy storage applications. By Using the Scherrer equation, the authors were able to calculate the mean crystal dimensions and realized that lignin activated carbon from the study possess the largest crystal dimensions for porous carbons prepared at the investigated temperature range (700– 1000 °C). Another important application of XRD in lignin material analysis is the determination of internal defects arising from carbonization and graphitization reactions. In the study of lignin as free standing carbon electrodes Tenhaeff et al. [54] realized high density of lattice defects or distortions between the graphitic basal planes which can cause anisotropy in the carbon material, translational stacking faults, interlayer spacing or distortion of the graphitic layers in relation to carbonization temperatures of the carbon material. These results indicate that XRD can be an effective tool in study of structural defects during carbonization of lignin carbon fibres which can be a step towards increasing the performance of lignin carbon fibres. In another study, Rios et al. [216] used XRD to confirm the absence of unwanted phases when silicon particles were embedded in lignin to form carbon fibres for Lithium-Ion batteries. The effect of blending of PAN and lignin on the structural characteristics and generation of defects has been studied using XRD. Results showed that addition of lignin to PAN led to the introduction of large amount of topological defects and structural disorder to the carbon fibres [217] (Fig. 5.32). The formation of graphene from sodium lignosulphonate by Fe-catalysed thermal conversion has been studied using XRD [218]. The use of XRD in analysis of thermal treatment of Kraft lignin shows that increasing temperature reduced amorphous carbon but increased aromaticity in thermally treated materials [219].

5  Characterization Techniques and Quality Assessment of Lignin and Lignin Carbon… Fig. 5.32  XRD patterns of different PAN/lignin-based carbon fibres. Reprinted with permission from Elsevier [217]

239

PL9/1 PL7/3 PL5/5

(800°C)

Intensity (a.u.)

(1000°C)

(1300°C)

10

20

30

40

50

60

70

80

90

2-Theta (degree)

Studying the effect of catalyst on the thermal stability of lignin carbon fibre–CNT hybrid structure, Xu et al. [220] using XRD was able to propose the mechanism for superior thermal stability of palladium catalysed hybrid. XRD proves to be a useful tool in the study of activation reactions in lignin activated carbon [221]. Effects of activation temperature on the formation of ordered carbon and increase in crystallite size are possible with the use of powdered XRD [222]. The development of turbostratic graphitic structure during carbonization

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Fig. 5.33  Evolution of the (002) X-ray diffraction. Reprinted with permission from Elsevier [225]

(Fig. 5.33) has been studied using XRD [203, 223–231]. The presence of (002) diffraction peak is always attributed to the presence of both graphitic or highly ordered carbon and turbostratic or less ordered carbon. The width at half maximum of the peak is also an indication of average thickness of the graphite-like crystallites. A higher distance between the interfacial crystallite layer (dhkl) than that of ideal graphite (0.335 nm), is an indication of the existence of pre-graphitic turbostratic carbon structure. dhkl can be calculated from the Bragg’s law in Eq. (5.7). Structural changes in gel-structure after aging of lignin/PVA blended fibres have been studied using Wide Angle XRD (WAXRD) [232]. Maldhure et al. [233] also studied the compatibility of lignin with PP using WAXRD. The formation β crystalline form shows the presence of partial solubility. XRD has also been used in confirming the purity of lignin after extraction and precipitation [234–237]. Evidence of lignin intercalation into the silicate layers of clay nanofillers was observed using X-ray diffraction [238]. Mottweiler et al. [239] studied the behaviour of catalyst in the cleavage of lignin aryl ether linkages. Guo et al. [240] also examined the role of catalyst in the selective cleavage of lignin C-O bonds. Nar et al. [206] used XRD to examine the difference in degree of graphitization of poly-(caffeyl alcohol) lignin and Kraft lignin. Small angle X-ray scattering allowed for estimation of microporosity in lignin carbon fibres [241]. A comprehensive analysis of Kraft softwood and hardwood lignins has been reported by Goudarzi et al. [242]. XRD thin films catheterization was applied to study lignin films. The (101) peak was used to monitor the conversion of lignin to graphitic structure in relation to temperature.

5.3.6  UV Spectroscopy UV spectroscopy is a spectroscopic technique that uses ultraviolet and visible radiation (200–400 nm) as the sources of incident radiation. Generally, the interaction of radiation with matter usually produces a number of processes including; reflection,

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scattering, absorbance, fluorescence/phosphorescence (absorption and reemission) and photochemical reactions (absorbance and bond breaking). In UV-visible spectroscopy, absorbance is the process under consideration. The energy content of molecules (or atoms) increases when the material absorbs light. In some cases, the absorption of UV photons by molecules causes transitions between different electronic energy levels. The energy required to move an electron from one energy level (low level) to another (high energy level) will be equal to the wavelength of light absorbed. The transitions result in narrow absorbance bands at wavelengths which is characteristic of the difference in energy levels of the absorbing material. This would be a rough measure to determine the characteristics of the absorbing material. However, in the case of molecules, vibrational and rotational energy levels are superimposed on the electronic energy levels leading to broadened bands because of many transitions occurring with different energies. The broadening is greater in solutions because of the interaction between solvent and solution. Although the bands are broadened and does not allow for in situ identification of substances, the identity of substances can be done through comparison of the measured spectrum with a reference spectrum. On the other hand, derivative spectra can be used when the spectra are highly similar. UV can be used for quantitative analysis based on the proposition that the amount of UV light absorbed by a material is proportional to the number of absorbing molecules through which the light passes. This can be deduced from the Beer-Bouguer-Lambert law (Eq. 5.10) T=

I = e − kbc Io

(5.10)

where T is transmittance, c is the concentration of the absorbing species, Io is the incident intensity, I is the transmitted intensity, e is the base of natural logarithms, k is a constant, and b is the path length (usually in centimetres). When Eq. (5.10) is transformed to a linear form we arrive at Eq. (5.11) which is generally known as the Beer’s law.



 I A = − log T = − log   Io

  Io  = log   I 

  = ε bc 

(5.11)

where ε is the extinction coefficient which is characteristics of a given substance under defined conditions including wavelength, solvent and temperature. It is also thought to be dependent on the instrument used. It is a safe practice to always calibrated or construct a working curve for the substance to be analysed using one or more standard solutions with known concentration of the analyte [243]. UV characterization is usually performed in solution. Solvents generally used include water, dimethylformamide, ethanol, 2-methoxyethanol, dioxane, dimethylsulphoxide (DMSO), pyridine, dichloroethane, cellosolve and hexafluoropropanol. For lignin characterization, hexafluoropropanol has been identified as a suitable solvent. UV spectroscopy has been used to characterize lignin in various ways. Lignin is aromatic in nature and strongly absorbs UV light showing characteristic maxima

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in the UV light region. The position and the intensity of the maxima are characteristic of the type of lignin, chemical modifications and the solvent used. UV has been used to measure the phenolic hydroxyl group content in lignin [1, 37, 234, 244]. Measurement is based on the UV difference method where the difference in adsorption at 300 and 350 nm between the free phenolic units in neutral and alkaline solutions. The absorbance in the 300–350  nm range provides information about unconjugated and conjugated phenolic structures [143]. UV spectroscopy has been used to examine structural and purity difference between two types of lignin [118]. UV spectroscopy is widely used in the determination of the amount of lignin in a solution [245–247]. UV was used to study lignin content in wet spinning coagulant [248]. The Klason method only measures acid insoluble lignin with the remaining lignin remaining in the acid solution. This amount of lignin in the solution is usually measured. UV has been used in several other studies for the analysis of lignin [130, 143, 249–251].

5.4  Chromatographic Methods In the characterization of polymeric materials, the separation of liquid media into different components is very important. Chromatography is the process of separating individual components of a mixture based on their relative affinities towards stationary and mobile phases. Chromatography started with the independent studies of David Day and Mikhal Tsvet back in the nineteenth and twentieth centuries. The work of Day featured development procedures for the fractionation of crude petroleum by passing it through Fuller’s earth. Using a column packed with chalk, Tsvet realized that leaf pigments can be separated into coloured bands. The invention of chromatography is credited to Tsvet because he was able to interpret his data and named the phenomenon chromatography. This characterization method did not evolve immediately but after some years (1940s), column partition and paper chromatography were developed followed by gas chromatography (GC) in 1952. Following the success of GC in industrial applications and the development of sophisticated instrumental techniques, advanced methods such as high-performance liquid chromatography (HPLC) and size exclusion chromatography (SEC) were developed. Today chromatography exists in various forms for various purposes with peculiar capabilities. Figure 5.34 shows the major classification of chromatography. Generally, chromatography can be considered as a mass exchange process involving adsorption. The setup is usually made of a stationary phase or adsorbent (a substance that stays fixed inside the separation column), a mobile phase or carrier (a solvent or gas moving through the separation column) and the analyte (mixture whose individual components must be separated and analysed). In the process, the mobile phase flows through the stationary phase carrying the components of the mixture with it (Fig.  5.35). The separation is based on the type of interaction between the stationary phase, mobile media and the substances contained in the mixture. Regardless of the scientific technique applied, mechanisms of chromatog-

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Fig. 5.34  Subdividing chromatography according to various applied techniques

Fig. 5.35  Basic principle of chromatography

raphy employed in the separation or fractionation of various compounds are based on several physicochemical techniques shown in Fig. 5.36. Commonly used chromatographic methods in lignin analysis are GC, SEC and HPLC.

5.4.1  High-Performance Liquid Chromatography High-performance liquid chromatography (HPLC) is a form of liquid column chromatography that works by pushing the analyte under high pressure through a column with chromatographic stationary phase. The analyte is transported through the stationary phase by a gas stream of helium or nitrogen. Segments of the sample (usually in microliters) travel through the segment at various speeds depending on

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Cross section of open tubular column

Solute adsorbed on surface of stationary phase

Solute dissolved in liquid phase bonded to the surface of column

Partition chromatography

Adsorption chromatography

–

–

+ – +

–

+ + – + + – – + –+

–

–

– –

+ –

– +

+

–

–

+ – + –

Mobile anions held near cations that are covalently attached to stationary phase

–

–

–

+ –+ + – + – – – + – + – – – – + + – + –

Small molecules penetrate pores of particles

–

–

Large molecules are excluded

Anion-exchange resin; only anions can be attracted to it

Ion-exchange chromatography

Molecular exclusion chromatography

One kind of molecule in complex mixture becomes attached to molecule that is covalently bound to stationary phase

All other molecules simply wash through Affinity chromatography

Fig. 5.36  Mechanisms of separation in chromatography

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the interaction of the molecules with the stationary phase. The velocity of the different components of the analyte is dependent on the compound nature and composition of the mobile phase. HPLC measures the time at which a particular analyte travels through the column under specific conditions called the retention time. The retention time is the distinguishing normal for a given analyte. The HPLC system consists of a solvent delivery system, a sample injection valve, a high-pressure column, a detector and a computer to control the system and display results. For details of operation and mechanisms of HPL, the reader is referred to Haris [252]. Carmona et al. [253] used HPLC to identify monoacetylated and diacetylated monolignols released from asparagus lignin. HPLC was also used to study the difference between soda lignin and Soda anthraquinone lignin [254]. With HPLC the authors were able to identify and quantify the various oxidation products in both lignins. HPLC was also used to determine the ratio of the lignin monomers obtained by alkaline nitrobenzene oxidation [255]. Paraj et  al. [256] used HPLC to assess the presence of carbohydrate impurities in acetosolv lignin. Gierer and Lindeberg [251] used HPLC to quantitatively estimate condensation products during alkaline treatment of lignin. In another study, the same authors also used HPLC to quantify dimeric products from sulphate spent liquor [257]. Itoh et al. [258] also used HPLC to quantify the products of mechano-chemical conversion products of lignin model compounds. Wu and Arygyropoulos [259] used HPLC to purify lignin. Liquid phase sugars in catalytic converted lignin have been quantified by HPLC analysis [260]. Insoluble residues of ionic liquid extracted lignin have been analysed using HPLC [261]. The presence of sugars in hot water fractionated lignin was examined using HPLC [262]. HPLC coupled with MS [263] has been used to study cleavage in lignin. Hydrophobic interaction chromatography (HIC) [264] has been employed to understand the hydrophobic/hydrophilic nature of lignin with very good reproducibility.

5.4.2  Gas Chromatography Gas chromatography (GC) is based on the principle that a substance of several components will separate into individual substances when heated. In GC, gaseous analyte is transported through the column by a gaseous mobile phase. The stationary phase is either a solid or an immobilized polymeric liquid. The stationary phase is typically made of polydimethylsiloxane (PDMS) applied to the inner surface/walls of a very narrow capillary tube. The mobile phase is usually helium gas. During the process, volatile samples or samples volatilized by heating are introduced into the gas phase and flow through column during which the components of the substance interact with the stationary phase through physical interactions and adsorption. The molecular components are separated by their degree of affinity for the stationary phase. Components exiting through the column are detected using flame ionization detector or a mass spectrometer. In lignin analysis, GC is basically used to identify dimers and oligomers [265]. Monomeric products from DFRC treatments of lignin have been quantitatively determined using gas chromatography [266]. Products

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from catalytic depolymerization of lignin have been analysed using GC [267]. Bosch et al. [268] employed GC to analyse dimers obtained from reductive fractionation of lignocellulose materials. The solution was fist derivatized with trimethylsilylation with N-methyl-N-(trimethylsilyl) trifluoroacetamide before GC analysis. One of the most important advances in lignin characterization is the advent of 2D GC analysis techniques. 2D GC affords the possibility of identifying volatile components of lignin with very high resolution [269]. GC coupled with FID has been found useful in the study of [263] cleavage in lignin.

5.4.3  G  el Permeation Chromatography/Size-Exclusion Chromatography GPC is a type of HPLC. It employs a stagnant liquid present in the pores of beads as stationary phase and a flowing liquid as the mobile phase. Molecules are separated according to their size. This makes it a very good candidate for the determination of molecular weight distribution of substances. GPC is one of the most important characterization methods of lignin especially in the melt processing stage. The molecular weight distribution of Kraft and soda lignin has been analysed using GPC [244]. Results from the study were used to infer the severity of both treatments on the structure of lignin. Bosch et al. [268] combined GPC and NMR to quantitatively analyse the products of reductive fractionation of lignocellulose materials (Fig. 5.37). In another study Bosch et al. [270] used GPC to determine the molecular weight distribution of lignin oil derived from reductive fractionation. The study showed that the MW distribution of the di- and oligomers were less dependent on the type of catalyst. Choi and Faix [271] used GPC with refractive index and UV detectors to analyse the difference between lignins from Kraft, ASAM, sulphite and MWL processes. The effectiveness of redox catalyst in depolymerization of lignin has been demonstrated using GPC [265]. Results also show that addition of H3PO4 to the depolymerization imparts a good degree of depolymerization than the use of NaOH (Fig.  5.38). Schutyser et  al. [272] used GPC to determine lignin product yield (phenolic mono-, di- and oligomers), carbohydrate retention (C5 and C6 sugars) and delignification efficiency of birch wood. MW of lignin obtained from soda delignification of almond shell using GPC has been reported by Erdocia et al. [273]. Other studies on the use of GPC have been reported [38, 206, 234, 274–284]. GPC was also found useful in the analysis of lignin/polymer blending [285]. The use of GPC to study catalytic delignification has shown that in the process mainly oligomers are converted to monomers, highlighting that the catalyst foster hydrogenolysis reactions leading to a reduction in MW but does not release lignin itself [286]. Structural and molecular properties of residual birch Kraft lignin have been studied using high performance GPC [287]. The difference between milled wood lignin (MWL) and cellulolytic enzyme lignin (CEL) has been studied using GPC [193]. The study reported no significant difference in the MW between both

Fig. 5.37  A typical gas chromatogram [268]

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Fig. 5.38  Gel permeation chromatography of reactions with varying H3PO4 and NaOH concentrations [265]

lignins. Another study investigated the effect of milling time on the MW of lignin using GPC [288]. Effect of milling conditions and milling media on the MW in lignin model compounds has been studied using GPC [289]. GPC has been used to provide an overall assessment of the reduction in MW of Organosolv lignin resulting from hydrogen-transfer reductive processes [290]. GPC has been used to confirm the essential role of catalyst during reductive catalytic fractionation of lignin [291]. Structural characteristics of swelled enzyme lignin has been investigated using GPC [292]. Huang et al. used GPC to determine the MW distribution of MWL from loblolly pine stem [293]. GPC has also been used to study MW distribution of depolymerized lignin isolated by catalytic hydrogen transfer reactions [269]. GPC is also very useful in the analysis of reactions during lignin extraction [294]. The degree of depolymerization has also been studied using GPC [295]. Influence of

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catalyst molar ratio on the MW distribution of birch wood fractionation product yield is also possible with GPC [296]. Average MW of MWL oligomers has also been studied using GPC [297]. Average MW for milled wood lignin and ethanol organosolv lignin from miscanthus has also been studied using GPC [143]. The study showed that Organosolv lignin possessed a higher structural degradation compared to MWL. Effect of treatment time on the MW of lignin obtained from ionic liquid delignification process has been analysed using GPC [298]. Combining GPC and NMR, Wen and co-workers [299] were able to study the chemical transformations taking place during ionic liquid delignification process. Another study on the severity of ionic liquid treatment on lignin using GPC has been reported by Weigand et al. [300]. Another important use of GPC in lignin characterization is the study of lignin modifications. Difference in MW distributions after oxypropylation and methylation treatments has been reported [301]. Effect of depolymerization/repolymerization reactions occurring during Organosolv pre-­ treatment on the MW of lignin from Broussonetia papyrifera has been examined using GPC [302]. Guerra et al. [303] used GPC to understand the mechanism of lignin isolation using enzymatic mild acidolysis process. In another study the authors used GPC to understand the difference in the mechanism of isolation between enzymatic mild acidolysis and MWL and CEL [304]. Depolymerization reactions during ionic liquid pre-treatment of biomass has also been studied using GPC [305]. The study concludes that lignin of different molecular masses can be isolated using different process conditions. Using GPC analysis, it has been determined that lignin molar mass changes with the yield during MWL delignification [306]. Molecular weights of lignin bio-oil components obtained after catalytic fractionation have been determined using GPC [307]. Molecular weight distribution of acetosolv lignin as a function of the pulping time and acetic acid concentration used during pulping was investigated using GPC [308]. Other research on the use of GPC in lignin characterization has been reported [309–312]. Although GPC is very versatile in lignin characterization, the use of UV/Vis detectors poses a certain disadvantage as they are not capable of producing universal response to the eluates. Braun et al. [192] used GPC to investigate oxidative stabilization of lignin.

5.4.4  Gas Chromatography–Mass Spectrometry Gas chromatography–mass spectrometry (GC-MS) is simply the combination of gas-chromatography and mass spectrometry to identify different substances within a test sample. This is done by attaching a mass spectrometry detector to the GC system. Mass spectrometry is used in studying the masses of atoms or molecules. In mass spectrometry, gaseous species desorbed from a condensed phase are ionized and the ions are accelerated by an electric field and then separated based on their mass-to-charge ratio. In the GCMS process, the GC separates volatile and semi-­ volatile compounds with good resolution, and the MS provides detailed structural information on the separated compounds. In lignin analysis GC-MS is very useful

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in the analysis of lignin reaction mixture [294]. It has been widely used in quantitative and qualitative analysis of lignin and lignin fractions [295, 296, 306, 313]. GC-MS has been used to show different chemical structures of dimers and oligomers of reductive fractionated lignin and their dependence on the catalyst used [270]. Singh and Dhepe [314] demonstrated the effect of structural properties of organosolv lignin on their liquefaction using ionic liquids. Using a two dimensional GC with time of flight mass spectrometry (GCxGC-TOFMS) procedure, Karp et al. [315] to identify and quantify acidic compounds in corn stover extracted using alkaline pre-treatment liquor in relation to treatment severity. Figure 5.39 shows a representative chromatogram. The high-resolution GC scan was able to show dominant low molecular weight components and their concentrations as a function of pre-­ treatment severity. GCMS has also been used to determine the extent of ammonolysis and hydrolysis reactions during ammonia fibre expansion of biomass [316]. The analysis showed that the treatment in near theoretical cleavage of acetyl linkages to acetic acid during dilute acid pre-treatment. Bosch et al. [268] used a combination of GC, GC-MS and GPC to identify structural features of lignin monomers, dimers and oligomers after reductive fractionation of lignocellulose materials. GC/MS was applied to identify the phenolic mono- and dimers. A similar study was conducted on birch wood lignin by Schutyser et al. [272]. Gregorio et al. used GCMS to analyse oxidation products of lignin using POMs in ionic liquids [317]. Quantitative analysis of depolymerization products from soda delignification of almond shell using GCMS has been reported by Erdocia et al. [273]. The formation of monomers during mechanomechanical treatment of lignin model compounds has been studied using GC-MS [289]. The distribution of volatile products present in lignin has been analysed using 2D GCxGC-FID/MS [290]. An example of the 2D GC is shown in Fig. 5.40.

Fig. 5.39  Representative chromatogram of a derivatized corn stover alkaline pre-treatment sample. Reprinted with permission from RSC [315]

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Fig. 5.40 (a) GC-FID, (b) 2D GC × GC-FID and (c) 2D GC × GC-MS [290]

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5.4.5  Pyrolysis GC-MS Pyrolysis GC-MS is one of the most advanced destructive methods of polymer analysis which requires little or no sample preparation. In the process, a sample is introduced into a heated environment under defined conditions. The sample is pyrolysed and broken down into smaller fragments/components depending on their structure. The pyrolysed products are then separated in the same manner as traditional GC-MS. The fragments are separated using the mechanism of intrinsic affinity for a “stationary phase”. Py-GC/MS can be a very powerful tool for characterization of lignin. When lignin is pyrolysed, it releases a mixture of relatively simple phenols, arising from the cleavage of ether and carbon-carbon linkages. These phenols retain their substitution patterns from the lignin polymer, making it possible to identify the compounds from the H, G and S lignin units. This is particularly of interest in the study of lignin oxidation reactions and lignin purity [318]. Py-GC/MS procedure is very versatile because it can distinguish between carbohydrate derived components and the different lignin subunits [311]. Py–GC/MS has been used to analyse the composition of lignin [319, 320]. It has been widely applied in the determination of H, G and S units of lignin and Biomass [321–327]. Rencoret et al. [328] used Py-GC/MS to identify and quantify the relative molar abundances of compounds in MWL lignin from Paulownia fortunei. The study was able to determine the H:G:S ratio of the lignin. Using a model compound, Wang et al. [329] investigated the pyrolysis product of lignin using Py-GC/MS in 150–850 °C temperature range. Liu et al. also investigated the pyrolysis product of H and G type lignin model compounds [312] and proposed a possible pyrolysis pathway for the G-type lignin. The effect of fungal attack on the phenolic content of lignin has been investigated using Py-GC/MS [330]. The mechanism of lignin pyrolysis has also been investigated with the help of Py-GC/MS [331]. Prado et al. [332] determined the MW and phenolic proportion of lignin by Py-GC/MS. Choi et al. [333] used Py-GC/MS to analyse seven lignin isolates with the aim of determining the lignin compositions in terms of H, G and S units. Results showed a systematic difference between the various residual lignins. The effect of ethanol Organosolv pre-treatment on the structure of lignin has been examined using Py-GC/MS [145]. Chu et al. [334] investigated the chemistry of pyrolysis of beta aryl ether type oligomeric lignin model compounds using Py-GC/MS. The study resulted in the proposition of free radical reaction pathway to explain the chemistry of lignin pyrolysis. Py-GC/MS has been applied to examine changes in lignin subunit composition arising from ionic acid pre-treatment [311]. An example of Py-GC/ MS pyrogram is shown in Fig. 5.41. One of the soft points of lignin analysis with Py-GC/MS is that some lignin may remain as non-volatile components - char. This should be taken into consideration when interpreting Py-GC-MS data. On the other hand, chemical processes occurring during pyrolysis may give rise to misleading results. For example, in py-GC/MS PCA and H, and FA and G units cannot be distinguished. Py-GC/MS is also useful in examination of treatments of lignin for further applications. Other studies have been reported on the use of Py-GCMS for characterizing lignin [335–337]. Podschun et  al. [148] investigated the effect of phenolation on the structure of lignin using Py-GC/MS.

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Fig. 5.41  Py-GC-MS (left) and Py-TMAH-GC-MS (right) pyrograms of Miscanthus giganteus and two Ionosolv lignins (5 and 12 h) [311]

5.5  Thermal Characterization The responses of lignin as temperature is changed with time are very important characteristics of lignin that must not be neglected during processing of lignin to carbon fibres. These responses occur either as mass change, temperature transport, heat difference, deformations, dimensional change (shrinkage or swelling), volume change and decomposition. In thermal analysis of methods, several methods are employed which distinguished from one another by the property which is measured.

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Data Processing

Detector

Sample temperature

Sample Tem. control

Computer Unit

Furnace Control signal

Detection Unit

Fig. 5.42  Schematic of general thermal analysis or calorimetry apparatus

When the change is mass with change in temperature is the desired quantity; then thermo-gravimetric analysis is used. When the difference in heat evolved or absorbed as the temperature is changed is the required quantity; then differential scanning calorimetry is used etc. As applied to other characterization methods, two methods can be combined to obtain a desired quantity. These methods are based on the fact that when a material is heated certain changes occur which may or may not be visible to the eye. In any case, whatever change occurs when a material is heated tells a lot about the chemical composition, the bonding structure and strength, the capacity to absorb heat energy etc. Figure 5.42 shows the schematic of the general set up for a thermal analysis.

5.5.1  Differential Scanning Calorimetry The DSC is a technique which measures the difference in heat flow rate between a sample and a reference (inert) sample against time when the samples are exposed to a temperature programme. Figure  5.43 shows a typical setup of a DSC.  Several DSC techniques can be used for material analysis including highly sophisticated temperature modulated DSC (TOPEM), alternating DSC and isothermal DSC.  Explanations of these techniques are beyond the scope of this book. The reader is referred to Laye [338] and Haines et al. [339]. In lignin characterization DSC is used basically in the determination of the glass transition temperature. DSC has been used to study thermolysis of lignin materials for activated carbon materials [340]. The effect of propargylation on the thermal behaviour of lignin was studied using DSC. The DSC thermogram show the presence of exotherms which are attributed to Claisen rearrangement reaction and subsequent thermal polymerization reactions in the treated samples [341]. García et al. [342] studied the effect of selective precipitation on the Tg of lignin using DSC. There was no marked effect

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Fig. 5.43  Schematic of DSC

of treatment on the Tg of lignin. The effect of lignin extraction method on the Tg of lignin carbon fibres was demonstrated using DSC [343]. DSC has also been used to investigate the cross-linking index of lignin fibres after thermal treatment [344]. Cross-linking index is measured as the ratio of melting enthalpy change (due to treatment) to that of initial fibre. Baker et al. [345] examined the effect of organic purification on Tg of hardwood lignin precursors using DSC. Effect of heating rates on the Tg of lignin fibres under oxidative conditions was also studied using DSC [345]. Effect of addition of organoclay on glass transition of lignin has been examined with DSC [346]. Thermal properties of the PAN–lignin copolymer for carbon fibre have been examined using DSC [347]. A similar study was also conducted on blending of lignin with PP and PET [117]. Using Organosolv lignin and PAN, the effect blending on the glass transition have been examined with the help of DSC [348] (Fig.  5.44). DSC is very important in the study of thermo-stabilization of lignin. The expectation during thermo-stabilization of lignin is that the glass transition increases. This can be monitored using a DSC measurement. Several authors used DSC to study the effect of thermo-stabilization on glass transition temperature of lignin fibre precursors [192, 205]. The best blending fraction to obtain a good lignin precursor can be determined with the use of a DSC. Kadla et al. used DSC to investigate the best blend ratio for PEO/lignin blend [121]. Hossenaei et al. [349] also used DSC to determine the optimum blending ratio lignin/lignin blends. Mousavioun et al. [350] investigated the optimum PHB/lignin blend ratio to obtain a high glass transition temperature using DSC.  Li et  al. applied different DSC techniques to determine the glass transition temperature of fractionated lignin. Conventional DSC

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Fig. 5.44  Glass transition temperature (Tg) of lignin and lignin/PAN blends. Reprinted with permission from Elsevier [348]

with and without annealing, modulated temperature DSC (MTDSC), TMA and dynamic rheology were applied in the study. MTDSC and TMA were the only methods that cold determine Tg of the samples in this study [336]. With the help of DSC, it was shown that fractionation affords the possibility of obtaining lignin with combined low MW and Tg. DSC has been used in different ways to study lignin as illustrated in several literature [48, 146, 351–363].

5.5.2  Thermogravimetric Analysis Thermogravimetric analysis (TGA) is a thermal technique that measures the change in mass of a material as a function of temperature whilst the substance is subjected to a controlled temperature programme. The process can also be conducted by keeping the temperature constant and the mass loss recorded against time. Although some processes other than mass loss may take place during heating TGA basically measures the overall change in weight. Information on the thermal stability of a material is usually obtained by a plot of the mass loss against temperature recorded directly from the TGA equipment. For more information, a derivative of the weight loss is also plotted against temperature. The plot is referred to as the DTG curve and can be used to determine the maximum temperature of each degradation product. It is very useful in lignin characterization as it can be used to show the exact temperature at which a component of the lignin fibre degrades.

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TGA has been used to study the oxidation resistance was confirmed by thermogravimetric analysis [215]. Fu et al. used TGA analysis of lignin to determine the temperature range for carbonization of lignin [340]. The effect of propargylation modification on thermal stability of lignin has been studied using TGA [341]. García et al. [342] examined the effect of selective precipitation on the thermal stability of lignin using TGA. Selective precipitation at alkaline pH improved purity of the lignin and led to improved thermal stability. Using TGA experiment Shi et al. [343] showed that increase of lignin molecular weight results in improvement of thermal stability of lignin. The effect of sodium on the thermal stability of electrospun PEO/treated lignin activated carbon fibres has been examined by TGA [364]. TGA thermogram was used to confirm the miscibility of PEO in the treated lignin. Thermal stability of lignin-based carbon fibres decorated with carbon nanotubes has been examined using TGA [220]. TGA was used to study the carbonization behaviour of the hybrid material and the thermal stability of the final carbon fibres. With TGA it was possible to fix the parameters for the carbonization of the hybrid material. TGA is also used as an alternative method of pyrolyzing lignin precursor fibres [365]. Kinetic parameters of lignin pyrolysis are possible when TGA is used for pyrolyzing lignin. On another hand, TGA can be used to calculate the carbon yield of lignin carbon fibre precursors. Moreover, it is easy to study the effect of different stabilization medium and temperature on the stabilization behaviour and carbon yield of lignin precursor fibres using TGA (Fig. 5.45) [366]. Effect of thermal treatment on thermal stability of pyrolytic lignin carbon fibres has been examined using TGA [367]. Kim et al. [49] also studied the effect of heat treatment on the thermal stability of lignin. The study showed that thermal treatment leads to structural rearrangement of lignin to reduction of β-O-4 linkage as well as accumulation of C-C bonds resulting in improved thermal stability. Mousavioun et al. [350] investigated the optimum PHB/lignin for the best thermal stability using TGA. Effect of lignin fractionation using methanol on the thermal stability of lignin was examined by Li et al. [336]. Other studies on TGA of lignin can be found in literature [146, 233, 235, 281, 356–360, 368–371].

5.6  Morphological Characterization Surface and morphological examination of materials is a very important aspect of material analysis. For lignin characterization, surface and morphological examination is very important during melt processing, stabilization, carbonization, activation and graphitization processes. Surface defects in carbon fibres are undesirable as they constitute a starting point for mechanical failure. This underscores the importance of surface examination in the study of lignin carbon fibres. Surface and morphological examination are performed with microscopy methods (Fig.  5.46). Microscopy is a technique used to view or reveal objects and details that are not visible to the naked eyes. General, microscopy methods are either light, optical or electron microscopy. In lignin characterization, electron microscopy is the most used method.

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Fig. 5.45 (a) Comparing air-stabilized with HCl-stabilized HWL fibres using TGA, (b) Effect of maximum stabilization temperatures on carbon yield and form of the carbonized lignin powder samples in TGA. Reprinted with permission from John Wiley and Sons [366]

5.6.1  Scanning Electron Microscopy The scanning electron microscopy (SEM) examines microscopic structure of a material by scanning the surface with focused electron beam. Characteristic information from SEM include topography, morphology, composition and crystallography.

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Fig. 5.46  Detection limit of microscopy methods

SEM has been used in lignin carbon fibre analysis for the measurement of fibre diameter [54, 200, 213, 215, 224, 345, 349, 364, 372]. Hu and Hsieh [373] used SEM to examine the surface structure of lignin activated carbon materials. The difference in morphological structure of the activated carbon with difference in activation medium was visible in the SEM [374]. Surface defects on carbon fibre surfaces are always easy to see with SEM. An example of surface defects in lignin carbon fibres are shown in Fig. 5.47. Kadla et al. [121] also studied surface defects and surface nature of lignin carbon fibres using SEM. Effect of carbonization on surface morphology of carbon fibres can be examined using SEM [375]. Effect of UV treatment on the surface nature of lignin carbon fibres has been examined by Zhang et al. [55]. The study showed the presence of crenulated structures on the fibre surface after treatment (Fig. 5.48). Particle size distribution of lignin precipitates has been examined using SEM [376]. Surface nature and morphological features of lignin carbon fibres and activated carbon can be analysed with SEM [56, 201, 222, 225, 340, 344, 347, 366, 377–383]. Microdomain structure of lignin activated carbon can be studied with the combination of SEM and scanning transmission microscopy [384]. The mechanism of thermal decomposition of lignin can also be monitored by SEM [385].

5.6.2  Transmission Electron Microscopy In transmission electron microscopy (TEM), a beam of high energy electrons is transmitted through a specimen to form an image. The specimen is most often an ultra-thin sample of a few nanometers thick. As the incident high energy beams

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Fig. 5.47  SEM morphology of thermostabilized PEG/lignin blend at various treatments [374]

Fig. 5.48  SEM of carbonized fibres from (a) UV/thermal dual stabilization (b) slow thermal stabilization [55]

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travels through the sample, it creates a sort of shadow of the material. Depending on the structure of the material, some parts of the material stop or deflect electrons more than other parts. The transmitted electrons are collected under the sample on a phosphorescent screen or through a camera. Regions where electrons did not transmit will appear dark on the image and regions where the electrons were transmitted will appear bright. The image therefore contains differing grey areas depending on the way the electrons interact with the sample. TEM has been used to characterize lignin materials in several ways. TEM has been used to study the morphology of electrospun lignin nanofibers with deposited silver nanoparticles for fuel cell applications (Fig. 5.49). The TEM images was able to capture the internal reduction process in the material during processing and the crystallographic structure of the hybrid fibres [213]. Difference in morphology of activated carbon fibres due to difference in chemical used was also studied using TEM [364]. The presence of mesopores, micropores and macropores was confirmed by TEM images. Effect of activation temperature on the disordered structure of lignin activated temperature has been studied using TEM. The images showed disordered distribution of crystallographic orientations and controllable degrees of

Fig. 5.49  TEM images of the lignin-derived ECNF mats. Reprinted with permission from Elsevier [213]

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Fig. 5.50  TEM images (scale bars are 0.1  μm (a) 1  μm (b) and 1  μm (c)) of LCF-IA fibres. Reprinted with permission from RSC [215]

graphitization dependent on carbonization temperature [54]. Porous structure of lignin carbon fibres has been examined using TEM (Fig. 5.50) [215]. Xu et al. [220] studied the morphology of lignin-based carbon fibres decorated with carbon nanotubes using TEM. The study reveals that the fibre consists of regions with different levels of carbon layer ordering with the graphene layers lying not parallel to each other but have bends and branches suggesting turbostratic nature. TEM has been used to study the aggregation behaviour of lignin [165, 386] and their usefulness in forming thin films that have UV resistance characteristics [387]. TEM has been used to study the location of lignin in the ultra-structure of plants [388, 389]. Surface characteristics of carbon fibres from Kraft and poly-(caffeyl alcohol) lignin has been examined using TEM [206]. The study shows difference in crystal dimensions between the two fibres (Fig. 5.51). TEM has been used to characterize lignin colloidal spheres [390]. The use of TEM in analysis of lignin carbon fibres is very limited. The cost and availability of a TEM microscope may be the reason for the limited studies. However, it should be noted that TEM will be very useful in the study of the development of turbostratic structure in lignin carbon fibres [219]. Fibre structures and defects arising from processing of lignin can also be studied using TEM [200]. Mun et al. [218] used TEM and SEM to investigate the formation of graphene layers using lignosulphonates. With high-resolution TEM the development and arrangement of graphite layers could be studied.

5.6.3  Atomic Force Microscopy The atomic force microscopy (AFM) is a type of scanning probe microscope that generates topographical image of the sample surface by an interaction between a tip and a sample surface. It measures the forces acting between a fine tip and a sample. The tip is connected to a free end of a cantilever. When the tip is brought near a

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Fig. 5.51  TEM and electronic diffraction image of (a) Kraft (b) poly-(caffeyl alcohol) lignin carbon fibre. Reprinted with permission from Elsevier [206]

surface, attractive or repulsive forces resulting from the interactions between the tip and the surface causes a positive or negative bending of the cantilever. The bending is detected by means of a laser beam. AFM can measure surface characteristics of materials with very high resolution in a range of 100 to 1 μm. A few lignin studies have reported the use of AFM. AFM has been used to study the surface profile of lignin deposited on gold interdigitated electrode surface [391] (Fig. 5.52). Pasquini et  al. [392] also used AFM to stud the surface profile of lignin Langmuir films deposited on mica. Constantino et  al. [393] also examined Langmuir films from sugar cane Bagasse lignin using AFM. Gordobil et al. studied lignin/PLA blends using AFM [357]. The images show lignin nanoparticles homogeneously distributed in the surface of the PLA matrix. Holmgren et al. [152] studied surfaces of spin coated dehydrogenation polymers (synthetic lignin). A similar study was conducted on wood lignin coated on silicon wafers [394]. AFM has also been used to study lignin as a filler in polymer composites [359]. Aadil et  al. [395] studied surface characteristics of lignin gelatin films with glycerol as plasticizer (Fig. 5.53). Lignin– lignin interactions have also been examined using AFM [396]. AFM has not been widely applied in the characterization of lignin carbon fibres.

5.7  Mechanical Characterization Mechanical properties of carbon fibres are the major properties of interest when they are intended for structural applications in areas such as automobiles, aerospace and sport. Mechanical properties of attention are tensile strength, elastic modulus and elongation. Measurement of tensile strength, modulus and elongation of individual carbon fibres are usually performed using standard single-filament testing, according to BS ISO 11566 or ASTM D3379-75 standards.

Fig. 5.52 (a) AFM images for a bare Au IDE, (b) surface profile and (c) 3D plot of the AFM image shown in part a. (d) AFM image for a 120 nm lignin PVD film deposited onto the Au IDE, (e) surface profile and (f) 3D plot of the AFM image shown in part d. Reprinted with permission from ACS [391]

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Fig. 5.53  AFM topography images of lignin films [395]

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5.7.1  Single Filament Tensile Testing The single filament test is conducted by first making random selection of 10–20 single filaments from the fibres to be tested. The selected filaments are centre-line mounted on special slotted tabs. The filaments are bonded to avoid slippage during test. The taps can be graphite plate with the graphite bonding agent. For these tabs are gripped so that the test specimen is aligned axially to the test machine. The test is conducted by stressing the filaments to failure at a constant strain rate. The filament cross-sectional areas are determined by microscopic measurements of the randomly selected number of filament. High-precision SEM software is available for this purpose. The cross-sectional areas can also be determined with optical gages, image-splitting microscope, linear weight-density method. Tensile strength and Young’s modulus are calculated from the load-elongation curves and the cross-­ sectional area measurements. Statistical analysis of the results is necessary because of the variability in the test results. On the other hand, consistency analysis is used to determine the consistency of the machine. Several successful mechanical testing of lignin carbon fibres has been reported in literature [55, 117, 226, 279, 346, 367, 379, 397–402]. Mechanical testing of electrospun fibre mats is done differently [403].

5.8  S  urface Area, Pore Size and Pore Size Distribution Characterization Quality assessment of activated carbon materials is based on their pore size, surface area and pore size distribution. These parameters are tested by simple test methods but with applied models. Models for testing pore size, surface area and pore volumes are extensively reviewed in Chap. 8 of this book. Experimentally, an absorption experiment is conducted to obtain an absorption isotherm which is then subjected to the models to determine the various parameters. Several methods have been used for determination of absorption isotherm including N2, methylene blue and iodine adsorption experiments. In the study of lignin activated carbon Nitrogen absorption experiment is the most popular adsorption experiment. The test principle hinges on the determination of the amount of N2 adsorbed by the material versus relative pressure at constant temperature. The test is conducted at 77 K in an automated absorption machine. The machine records the amount absorbed against the partial pressure (the graph is called the adsorption isotherm). The isotherms are then used to determine pore size, pore size distribution and pore volume as discussed in Chap. 8. Several studies on lignin activated carbon have been reported on the use of nitrogen absorption [202, 222, 373, 384, 404, 405]. An example of nitrogen isotherm is shown in Fig. 5.54. A few studies have reported the use of methylene blue [340].

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Fig. 5.54  Nitrogen adsorption isotherms of AC

5.9  Conclusion Meaningful progress in the performance of lignin carbon fibres is hinged on a very clear understanding of the lignin structure, chemistry and changes in the structure as lignin is processed into final products. The key to understanding the structure and chemistry of lignin is modern characterization methods. Although there have been

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numerous advancements in the characterization of lignin, the gaps are still enormous. The use of solid-state NMR on lignin carbon fibres after stabilization will give insight into further processing for improved properties. Attention should be paid to the combined use of TEM and AFM to analyse the structural evolution of lignin carbon structure during carbonization.

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Chapter 6

Melt-Processing of Lignin Emmanuel Isaac Akpan

6.1  Introduction Production of carbon fibers from lignin involves multiple processing steps, including melt-processing, oxidative thermo-stabilization, and carbonization. In the melt-­ processing step, the lignin polymer is either thermally or solution processed into fibers which are subsequently cross-linked to form carbon fibers. In thermal meltprocessing, the lignin must be fusible (able to melt and flow), without suffering thermal-induced depolymerization. Because of the nature of lignin and its structural components, thermal mobility and melt flow characteristics are poor. At elevated temperatures, chemical and structural transformations also occur leading to the release of volatiles. Moreover, varying botanical sources and the extraction methods leads to the emergence of lignins with varying structures and characteristics. Impurities are also found in lignin arising from both the botanical source and the extraction methods [1]. For example, lignosulphonates contain a substantial amount of bound sulphur because of the sulphite used in the extraction [2]. Switch grass was found to contain residual hemicelluloses, ash, and protein-based components arising from the botanical source [3]. Other authors also confirm the presence of impurities in lignin arising from both extraction methods and botanical source [4–7]. In the case of solvent processing into fibers, the lignin has to be completely soluble in the solvent (e.g., Dimethylacetamide (DMAc), Dimethylformamide (DMF), Tetrahydrofuran (THF), and Dimethyl sulfoxide (DMSO)). The solubility of lignin is dependent on the lignin structure and the level of purity of the lignin. In this chapter, detailed descriptions of the methods used in melt-processing of lignin into fibers and their principles are presented. Factors affecting processability and modifications to overcome the challenges in lignin processing are also covered in details. E. I. Akpan (*) Institute for Composite Materials, Technical University Kaiserslautern, Kaiserslautern, Germany e-mail: [email protected] © Springer Nature Switzerland AG 2019 E. I. Akpan, S. O. Adeosun (eds.), Sustainable Lignin for Carbon Fibers: Principles, Techniques, and Applications, https://doi.org/10.1007/978-3-030-18792-7_6

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6.2  Melt-Processing Methods 6.2.1  Melt-Spinning Melt-spinning is the most versatile, convenient, and economic method of polymer fiber production because of its high productivity, simplicity, and the absence of need for auxiliary materials. The process does not use solvents, and more homogeneous lignin fibers are achieved from the liquid state. Conventional melt-spinning processes involve heating a polymer to a suitable viscosity and ejecting the molten polymer stream into a cooling gas at ambient temperature. The gas cools the filament as it is being ejected and exerts a drag force on the fast moving filament. Basically melt-spinning setup (Fig. 6.1) usually have two sections: (1) the capillary

Fig. 6.1  Example of melt-spinning process

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die and (2) the spun jet. In the capillary die, shear flow is applied to the polymer melt, while in the jet section, the flow changes to uniaxial elongation flow. As the polymer flows through the spinneret, it undergoes a die swell phenomenon. Resulting from the winder drag force, the fibers attenuate giving rise to chain orientation and crystallinity before melt solidification. Resultant properties of the fibers, including percent crystallinity, molecular orientation, and diameter, depend mostly on the melt-spinning parameters, such as extrusion temperature, cooling conditions, mass flow rate, length of the line, velocity, size, and the shape of spinneret hole [8, 9]. Some studies have been focused on the effect of these parameters on the properties of the fibers [10, 11]. A lot of modifications to the process have been proposed over the years in attempt to improve on the resultant properties of the fibers. The incorporation of laser thinning led to a substantial improvement in the process [12– 15]. Fibers with extremely high amorphous orientation and low crystallinity were also obtained by incorporating a modified isothermal liquid bath [16, 17]. A combination of enhanced and/or retarding air quenches have also been used as a modification mechanism for melt-spinning of polymers [18]. Because lignin is an amorphous thermoplastic polymer, its melt-spinning characteristics is dependent on the glass transition temperature, softening temperature, and molten viscosity. Melt-spinning of lignin is generally done at lower temperatures than the glass transition temperature to avoid early cross-linking. Moreover, the lignin must have a low softening temperature to allow the flow of the melt. Difference in the structure of lignin in relation to source and extraction methods results in difference in spinning temperature. The advantages of melt-spinning over other processes include simplicity and ease of processing. However, melt-spinning suffers from fiber breakdown, difference in filament thickness, the fineness of fiber is limited, and it sometimes suffers spinneret clogging. Very important to melt-spinning is the thermal characteristics of the lignin since melt-spinning is conducted at temperatures between melting and decomposition temperatures or cross-linking temperature. This makes the spinning temperatures for lignin to differ between types of lignin and may somewhat be dependent on the source and extraction of the lignin. Melt-spinning of lignin has been observed to fail when there is an increasing high-­ molecular weight proportion in the lignin. This is possibly caused by the presence of non-melted regions in the bulk melt resulting in inhomogeneities. When low molecular weight components are present in the melt, improved spinnability is ensured. However, this results in a lowering of the glass transition temperature of the lignin making it difficult to stabilize the fibers [19]. Melt-spinning of lignin has been studied by different authors [20–32]. In most cases lignin require chemical and heat treatments before they are suitable for melt-spinning (Fig. 6.2 and 6.3) [21–23, 31–34]. Figure 6.2 shows the use of laser heating to modify melt-spinning while Fig.  6.3 shows the use of heating (Fig.  6.3b)  and cooling (Fig.  6.3a)  systems to modify melt-spinning. Furthermore, precursors from melt-spinning always require significant chemical treatment making it neither an economically or an environmentally favorable process [35].

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Fig. 6.2  Laser modified melt-spinning

a

b Spinneret

Spinneret

Heated sleeve

Heated sleeve Hot liquid jet

Cooling L

L

LIB

LIB

Liquid collector

Spinline

Take-up godet

Fig. 6.3  Heating and cooling modification of melt-spinning

Liquid collector

Spinline

Take-up godet

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6.2.2  Wet Spinning In a situation where melt-spinning of lignin will cause degradation, wet spinning is applied. Generally, wet spinning is used when the polymer requires to be dissolved in a solvent before it can be spun. The name “wet spinning” arise from the fact that fibers are extruded directly into a liquid bath. In the process, the spinning dope (a solution of lignin in a non-volatile solvent) is extruded through a spinneret into a liquid or coagulation bath (Fig.  6.4). As soon as the solution emerges from the spinneret hole into the bath it coagulates instantly. Nascent fibers are then pulled from the bath with the aid of a godet and linked to the stretching zone and washing baths. The fibers are finally collected either in continuous filament form or in staple form after which they are dried. Because the fibers are extruded directly into a liquid, a drag force greater than those of melt-spinning and dry spinning is exerted on the filament meaning that the processing speed is reduced compared to melt and dry spinning [36–43]. Like the other spinning methods wet spinning is controlled by some key parameters, including the type of lignin used, temperature of the bath, the coagulating solvent, line size, and feed rate [40]. Studies on wet spinning of lignin/polymer blends have been reported by some researchers [44–52]. Wet spinning usually results in fibers with voids that cause deterioration of properties [41]. This defective fiber structure is attributed to the rapid solvent removal from the extruded fiber during coagulation/precipitation phase. Research has shown that draw ratio is an important parameter that must be controlled for efficient wet

Dope tank

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Fig. 6.4  Schematic of wet spinning process

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Fig. 6.5  Schematic of dry-jet wet-spinning. Reprinted with permission from [55]

spinning of fibers [51]. It is also known that higher bath temperature during spinning increases the number of voids in filaments resulting in poor fiber properties. A modified wet spinning technique called dry-jet wet-spun was developed to spin lignin and cellulose dissolved in ionic solution into carbon fibers [53]. The process takes place at low temperatures. It differs from the conventional wet spinning method by the use of a small air gap between the nozzle and the surface of the coagulation bath. A wet-or air-gap spinning method for spinning lignin containing carbon fibers was also patented by Lehmann et al. [54] (Fig. 6.5). Although melt-spinning is less expensive in relation to wet spinning, superior mechanical properties are always obtained with wet spinning if the parameters are carefully controlled to obtain the level of molecular entanglement that can lead to highly drawn fibers [56]. The key to high stiffness and strength of spun fibers is the molecular alignment. Regardless of the method applied, a separate step of fiber drawing is usually required to arrive at required molecular entanglement. An important aspect of wet spinning is the fact that the lignin does not have to be thermally stable unlike the other processes [57].

6.2.3  Dry Spinning Dry spinning involves dissolution of the polymer in a volatile solvent or form polymer solutions in high-boiling solvents and the solution pumped through a spinneret  (Fig.  6.6). The fibers exit the spinneret and the solution evaporates under atmospheric conditions allowing the fibers to solidify and is collected on a take-up wheel. The method was originally used to produce fibers from industrial polymers, such as polyacrylonitrile, polyurethane, cellulose acetate, and some other thermo-­ stable polymers. It has superior economic advantages over wet spinning for several reasons, including better physico-mechanical properties of the fibers, the use of high concentration solutions, higher spinning speeds, easy scalability, and the possibility of complete solvent recovery. Dry spinning results in wholly amorphous

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Spinneret

Air flow

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fibers and is readily oriented and toughened. The physical processes in dry spinning process are complex than those of melt-spinning. The fiber in this case can be considered a two-component system which is continuously enriched with the polymer system as the solvent evaporates until it finally solidifies [58]. Efficiency of the process is significantly dependent on the concentration, viscosity, temperature of the solution, the temperature in the spinning cabinet, the spinning speed, the velocity and direction of the gas flow, and the spinneret design [59–64]. One of the deficiencies of the process is environmental concerns over the solvent used. It is particularly interesting in lignin because most types of lignin are soluble in volatile organic solvents. The patent application of Otani [65] included the use of dry spinning for different types of lignin precursors. Lignin was dissolved in water and NaOH solution and dry spun into fibers. Solution based dry spinning has the potential to reduce surface fusion of fibers during heat treatment [66]. Moreover, the use of dry spinning on partially acetylated lignin resulted in fibers with tensile strength higher than those in literature [66]. In another study, the possibility of dry spinning of acetylated softwood Kraft lignin into precursor fibers and subsequent successful stabilization and carbonization was reported [67]. These fibers had crenulated surface patterns resulting from out-diffusion of the solvent during solidification (Fig. 6.7). Commercial carbon fiber made from dry spinning of alkali lignin solution with poly (vinyl alcohol) by Kayacarbonlignin was available in the market for a short time. Dry spinning is probably the most suitable method for melt-processing of lignoSulfonic acid or lignoSulfonic acid salts lignin solutions [19]. However, dry spun lignin usually requires long thermo-oxidative stabilization step due to a slow heating rate needed

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Fig. 6.7  Crenulated surfaces of dry-spun: (a) Ace-SKL as-spun fibers and (b) carbon fibers

to prevent the fibers from sticking to each other [67]. In the study of Otani [65], residual NaOH in the fibers led to defect during the carbonization step.

6.2.4  Gel Spinning Gel spinning (also known as semi-melt-spinning) is an advanced spinning method used to create polymer fibers from their gel state. The process produces increase in fiber strength and modulus by enhancing polymer molecular weight, degree of orientation, crystallinity, and fiber density. The process was developed at DSM in Netherlands around 1970 [69–73] (Fig.  6.8). During the process a semi-diluted polymer solution or plasticized gel of low polymer concentration is squeezed through a spinneret and after quenching in a bath of solvent or water, a gel-like filament is obtained [74]. It is an attractive method that has been applied to manufacture high strength fibers from blends of lignin with vinyl polymer that are capable of trans–trans, extended-chain conformations. The process has also been applied to produce Polyacrylonitrile (PAN) carbon fibers and carbon nanotubes [75–78]. Like other spinning processes, effectiveness of gel spinning is dependent on factors such as, solvent mixture for dissolution, gelation temperature, drawing temperature, and draw ratio. Lu et al. demonstrated the possibility of gel spinning of lignin fibers [79]

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Continuous extrusion/solutions metering pump spinneret

quenching/extraction bath

oven

fibre

Fig. 6.8  Schematic of gel spinning. Printed with permission from [68]

Fig. 6.9  Gel spinning apparatus used in Lu et al. [79]

(Fig.  6.9). Using PVA as the polymer blend and 15/85 methanol/acetone as the coagulating solvent, the authors were able to produce lignin fibers with improved mechanical performance. It was realized that lignin existed in semicrystalline polymer-­rich domains (Fig. 6.10) inside the Polyvinyl Alcohol (PVA) contributing to enhanced thermal stability and increased gel melting point of the blend. The same authors also reported that aging the gel spun fibers in a chilling solvent led to a substantial increase in mechanical properties of the fibers [80].

6.2.5  Electrospinning Electrospinning is a low-cost spinning process that uses electrostatic fields and shear forces to produce submicron diameter fibers from polymer solutions at room temperature [81]. An electrospinning setup consists of three major components: a

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Fig. 6.10  Illustration of fiber microstructures at (a) low and (b) high lignin contents in (1) as-spun gel fibers and (2) gel-drawn fibers. Reprinted with permission from [79]

high voltage power supply, a spinneret (e.g., a pipette tip), and a grounded collecting plate (Fig. 6.11). The polymer is dissolved in a volatile solution such as Dimethyl Sulfoxide (DMSO) and filled in the syringe and high voltage is applied across the tip of the syringe and the collecting plate. The polymer solution is held by its surface tension at the end of syringe but at sufficient electric field the repulsive electrical forces overcome the surface tension forces. This makes the charged jet of the polymer solution to be ejected from the tip of the spinneret generating a Taylor cone with a rapid jet occurring between the spinneret and the plate leading to the evaporation of the solvent leaving the polymer fibers to be deposited on the collecting plate [82, 83]. It is conceptually simple, versatile and offers several potential advantages relative to other fiber producing methods [56]. Because of the large electrostatic field and shear force applied to the solution, the resulting fiber shows very high alignment of polymer chains. Moreover, the extreme confinement of the fiber because of small aperture size contributes to high chain alignment. Another advantage of the process is the fast solvent removal process which has the potential to rapidly lock in the polymer chain alignment avoiding relaxation in the center of the fiber diameter. The process is controlled by the several parameters, including processing voltage, feed rate and distance between needle tip and collector, temperature and humidity, concentration, viscosity, surface tension, conductivity/surface charge density, vapor pressure, and molecular weight [84–87]. These parameters can be controlled to result in fibers of acceptable characteristics. Several successes have been achieved in electrospinning pure lignin without prior modifications or added polymers [88, 89]. Modifications of the electrospinning methods (coaxial and triaxial configurations; Fig. 6.12) enabled the formation of very smooth filled and hollow fibers (Fig. 6.13) [88, 90, 91]. Although lignin has been successfully electrospun into fibers they mostly display poor electro-spinnability with substantial bead

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a Syringe Polymer solution

Spinneret

High Voltage

Fibers

Collector

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Syringe

Collector

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Fig. 6.11  Typical setup of electrospinning: (a) typical vertical setup and (b) horizontal setup of electrospinning apparatus. Reprinted with permission from [107]

formation. Several studies have been conducted on electrospinning of lignin into carbon fibers for several applications [92–100]. To achieve good fiber spinnability, plasticizers such as Polyethylene Oxide (PEO) and Polyvinyl Alcohol (PVA) have been introduced and smooth fibers obtained [92, 97]. Submicron fiber mats were also produced by introducing Polyacrylonitrile (PAN) [101]. Higher concentration

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Fig. 6.12 (a) Coaxial needle for electrospinning with sheath. (b) General setup used in triaxial electrospinning to produce lignin nanotubes. Reprinted with permission from [88]

Fig. 6.13  Alcell lignin fibers: (a) electrospun fibers (b) lignin carbon fibers

of lignin led to beaded morphology. A novel method of electrospinning of lignin fibers is emulsion electrospinning [100]. Most fibers produced from electrospinning of lignin usually display low mechanical properties [102]. However, the process possesses the advantage of being able to produce micro-nano fiber mats which cannot be obtained by any other melt-processing technique. Only fiber mats can be produced with current electrospinning procedures. However, some studies have shown that continuous aligned fibers can be obtained with modified electrospinning methods [103–106].

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6.3  Factors Affecting Melt-Processing of Lignin 6.3.1  Structural Features Lignin structure varies widely with source and extraction methods. The physical, chemical, and thermal properties of lignin depend to a large extent on the syringyl, guaiacyl and p-hydroxyphenol monomer units, functional groups, molecular weight, degree of branching, and purity of the extracted lignin. The reader is referred to Chap. 3 for a detailed explanation of the effect of extraction methods on the eventual structure of lignin. For a better understanding of the effect of lignin structural features on melt-properties, an illustration of the various structural units and functional groups in lignin is given in Fig. 6.14. 6.3.1.1  Repolymerization and Depolymerization A series of depolymerization and repolymerization reactions occur in lignin when the temperature is increased above the glass transition temperature [108]. The prevailing reaction is strongly dependent on the availability of syringyl (S) and guaicyl (G) units and the temperature. The G units are known to be prone to cross-linking than the S units which result in higher tendency of condensation than the S unit. Lignin samples with higher amount of G (lower S/G ratio) units than S units will possess higher tendency for condensation reactions [30, 109–112]. The tendency to cross-linking in G unit is attributed to the active phenyl C5 site that easily forms condensed structures [113–115]. These reactions have been shown to occur at low temperatures and result in higher viscosity and molecular weight [116]. Repolymerization reaction starts with initial homolysis of Cα and/or Cβ–ether linkages generating a phenoxyl and 1-phenyl-2-propyl radical. Further reactions leads to the formation of new 4-O-5, 5–5′, 3–3′, 3–5′, and 3-O-5 linkages through a radical coupling of the phenoxyl radical with C5 and/or C3 centered radicals [117–119]. Because repolymerization yields higher viscosity and molecular weights, it restricts thermal mobility and melt flow characteristics leading to poor spinning performance [3, 116]. Condensation can also occur during cooling in the presence of oxygen affecting the macromolecular structure (Fig. 6.15). At elevated temperatures lignin depolymerization occurs leading to a considerable reduction of β-O-4 ether linkages, aliphatic, and carboxylic hydroxyls resulting in lignin with relatively low molecular weight and viscosity [119, 120]. Cleavage of the aryl ether bonds is expected to lead to a decrease in aliphatic hydroxyl groups and an increase in phenolic hydroxyl units [114]. A reduction in aliphatic hydroxyl units results in a reduction in hydrogen bonding strength and consequently high thermal mobility. Although the decrease in molecular weight is beneficial to meltspinning, cleavage of ether bonds during melt-processing results in excess volatiles that lead to the formation of pores on the surface of fibers [3]. On the other hand, cleavage of β-O-4 linkages may result in the formation of new C–C linkages

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Fig. 6.14  Structural units, linkages, and functional groups in lignin relevant to melt-processing

between C5 and Cβ which are relatively stable [121, 122]. Figure  6.16 shows a proposed mechanism for the high temperature cleavage of the ether bonds. The reaction begins with a radical abstraction occurring at C-H to form a benzyl radical which possess a lower unpaired spin density at the alpha-position with extensive

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Fig. 6.15  Repolymerization reactions in lignin during low temperature rheology

Fig. 6.16  Depolymerization reactions during high temperature rheology

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electron delocalization analogous to the formed phenoxyl radical. The reaction then proceeds in two pathways (1) resulting in low molecular weight units and (2) formation of C–C linkage from reactive C–O bonds [117, 120]. 6.3.1.2  Inter-Units and Linkages The inter-units and linkages in extracted lignin have a profound effect in the melt processability of the lignin sample. The presence of higher concentration of aryl ether linkages increases the thermal mobility of lignin and decrease the glass transition temperature [114, 123, 124]. This can be attributed to cleavage of these linkages at low melt temperatures releasing a higher fraction of phenolic hydroxyl groups and decreases aliphatic hydroxyl groups which are directly responsible for thermal mobility of lignin. Increased amount of S units available in the lignin structure improves the thermal processing of lignin. The methoxyl groups in S units prevent cross-linking and increase the free volume, inhibiting chain entanglement giving rise to higher chain mobility and lower Tg [125]. As stated earlier G units are detrimental to thermal chain mobility in lignin. The prevalence of aliphatic hydrogen bonds (γ-OH and α-OH) strongly reduces the thermal mobility of lignin [116, 123, 126]. Aliphatic hydroxyl groups at the Cγ position have the tendency to form strong hydrogen bonds that reduce thermal mobility and increases Tg [123, 126–128]. Hosseinaei et al. [114] and Kubo et al. [127] showed that stronger intermolecular hydrogen bonds are formed between aliphatic hydroxyl groups which results in higher glass transition temperatures affecting fusibility of lignin. It is also reported that if the presence of aliphatic OH groups is combined with some phenolic acid groups such as p-coumarates, melt-spinning becomes very difficult because of combined increase in melt viscosity and reduced thermal stability [3]. The presence of a higher amount of phenolic hydroxyl is beneficial to the fusibility of lignin. This is because intramolecular hydrogen bonding can be formed between neighboring phenolic hydroxyl units and between phenolic hydroxyl units and methoxyl groups. This hydrogen bonding network hinders the formation of intermolecular hydrogen bonds [127]. However, it is noted that aliphatic hydroxyl units have higher tendency to form hydrogen bonding than phenolic hydroxyl units. A high amount of phenolic units are needed to overcome the hydrogen bonding formed by the aliphatic hydroxyl units. Another functional group that directly affects the melt processability of lignin is the carbonyl groups. Carbonyl concentration favors thermal fusibility of lignin [114]. Carbonyl groups are generally formed during the extraction process of lignin [130]. The carbonyl groups occur mostly because of the phenolic acid groups in lignin (p-coumaric acid and ferulic acid). These acid units are naturally esterified (Fig. 6.17) [131]. It has been suggested by various researchers that p-coumaric acid is linked to lignin via ester or ether bonds while ferulic acid is a bridge binding to polysaccharides through ester bonds and to lignin through ether bonds [129, 132]. The ester units can plasticize lignin leading to increase in thermal mobility [20, 133].

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Fig. 6.17 (a) Intra- and/or intermolecular ester bonds of wheat straw lignin side chain. (b) Phenolic groups in C-5 condensed structures selectively esterified in wheat straw lignin. (c) Ester bonds on wheat straw lignin terminal units. Reprinted with permission from [129]

6.3.1.3  Polydispersity Another lignin structural parameter that strongly affects melt-processing behavior is the polydispersity index (PDI). PDI is the ratio of weight averaged molecular mass to number average molecular mass of lignin. It is a measure of the relative size distribution of the molecules. A low PDI means, the substance has about the same size of the molecules. This means the substance will have a narrow melting interval [134]. This translates to high homogeneity and shows that all the molecules are having approximately the same glass transition temperature [135]. A low PDI increases the possibility of softening and therefore favors melt-processing [136, 137]. A high PDI leads to increase in softening temperature [135]. Polydispersity is dependent on the molecular weight, hemicellulose, and carbohydrates in the lignin [33, 134]. The effect of polydispersity on melt-processing of lignin is a subject that should be given much research attention because it was observed that low molecular weight lignin cannot be spun into fibers due to  inadequate viscosity. On the other hand, high molecular weight lignin cannot melt easily [133]. The normalizing factor between high and low molecular weight is the polydispersity index. Investigations have shown that there exist a particular range of PDI that favors melt-processing [137]. A high polydispersity contributes to non-meltable lignin and a broad range of softening temperature. Fractionation and removal of hemicellulose has been found to lower PDI and consequently melt-processing. A good combination of lower molecular weight and lower polydispersity has been obtained using a fractionation protocol involving organic solvent of varying polarity and dielectric constant [138–141].

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6.3.1.4  Thermal Character Glass transition temperature and thermal decomposition temperature are material inherent properties which affect melt-processing of lignin. Glass transition temperature which is the temperature at which lignin undergo a transition from a glassy to a rubbery state is a very important parameter in melt-processing of lignin. It determines the processing temperature for melt-spinning of lignin. Thermal softening and the melt flow temperature of lignin are dependent on the Tg [3]. The temperature should be low enough to allow for melt-processing without decomposition. High Tg signifies less thermal mobility and restriction in molecular motion which are contrary to fusibility. Tg is dependent on several factors, such as hydroxyl groups (phenolic and aliphatic), impurities (proteins, carbohydrates, nitrogen, etc.), polymerization and depolymerization, free volume and interactions among polymer chains, molecular chain length, and stiffness [3, 128, 142–144]. High hydrogen bonding network results in high Tg, which is detrimental to melt-processing. Hydrogen bonding strength in lignin is dependent on the amount of aliphatic hydroxyl groups which is dependent on cleavage reactions during extraction and melt-processing [126]. Glass transition should not be below the gelation temperature as it drastically affects melt-processing. In some cases (e.g., Kraft lignin) gelation occurs too close or above Tg, limiting the thermoplastic behavior of lignin. On the other hand, thermal decomposition temperature is the temperature at which lignin loses 5 wt% of its weight during thermal processing. When thermal decomposition occurs at low temperatures, melt-spinning is greatly affected. The presence of low temperature volatiles reduces the thermal decomposition temperature and results in the formation of pores during melt-spinning. Volatiles in lignin may result from carbohydrate impurities, aliphatic hydroxyl unit which releases water and formaldehyde at relatively lower temperatures, and lower molecular weight phenols. Secondly, the presence of high amount of hydroxyl groups may result in lower thermal decomposition as they are prone to water absorption and loose the water at low temperatures during thermal processing.

6.3.2  Measure of Purity Lignin exists alongside hemicellulose, pectin, wax, cellulose, and other materials in biomass. During extraction lignin is obtained with some of these substances as impurities [145]. Impurities in lignin may include carbohydrates, hemicellulose, sulphur, proteins, particulates, ash, inorganic salts, extractives, lignin-derived phenolics, metal-containing salts, and water [146]. Some of these impurities affect melt-processing of lignin in several ways [134, 147–153]. It has been shown that impurities can lead to higher glass transition temperature of lignin. Some of these impurities are bonded and cannot be removed by washing.

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6.3.2.1  Carbohydrates Carbohydrates exist with lignin in biomass through lignin carbohydrate complexes (LCC) which are very recalcitrant to extraction. During extraction short length polysaccharides are attached to the lignin samples and are considered as impurities. The presence of carbohydrates may influence the resulting particle size and morphology of lignin [151]. However, the influence of carbohydrate on particle size of lignin has not been fully investigated. It is also reported that carbohydrates can make lignin hard to dissolve [154]. Carbohydrates are also reported to reduce the possibility of obtaining lignin capable of softening [26]. On the other hand, Tg varies depending on carbohydrate impurities. They can also decrease the thermal stability of lignin and result in the formation of volatiles during melt-spinning giving rise to voids and defect on the fibers [114]. The presence of carbohydrate impurities can contribute to rise in the amount of aliphatic OH groups in lignin. This implies a rise in the hydrogen bonding strength and consequently difficulty in melt-processing. It is reported that carbohydrate impurities in lignin can cause plugging of spinning dies during extended melt-processing. They also increase the amount of bound water in lignin which is not beneficial to melt-processing [155]. 6.3.2.2  Proteins Protein impurities in lignin are linked to the presence of certain carbohydrates [156, 157] which means they originate from the raw materials. They are the major sources of nitrogen in technical lignin [158]. The presence of proteins may affect lignin valorization. During melt-spinning they can form undesirable by-products leading to low thermal decomposition temperature [159]. 6.3.2.3  Volatiles Volatiles exist in lignin in several forms, including phenols, moisture, and guaiacol. Although the dominating source of volatiles is water, phenolic acids also produce volatiles at low temperatures [160]. Volatiles can cause bubble formation during melt-spinning [155]. Even after pre-spinning, most volatiles present in lignin remain in the form of residual micro-voids in the fibers. Aliphatic hydroxyl unit can also release water, lower molecular weight phenols and formaldehyde at relatively low temperatures. Among various volatiles released during melt-processing of lignin are guaiacol derivatives [161]. They are formed at lower temperatures (150–190  °C) [162] and are detrimental to surface properties of the fibers.

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6.3.2.4  Water Lignin precipitated from aqueous solution usually contains a substantial amount of physically adsorbed water [160]. The presence of water can cause the bridging of individual lignin molecules through hydrogen linkages. They also result in the formation of char when the lignin is treated at high temperatures because water acts as reducing agent under such conditions. Water may also hinder chemical modification reactions of lignin [154, 163]. Water can also behave as volatiles causing voids on the surface of fibers during melt-spinning operations [164]. 6.3.2.5  Ash Ash in lignin is related to inorganic impurities. They occur because of the extraction method employed or the presence of silicates. High ash content is known to hinder thermal mobility and therefore affects thermal processing [26]. High ash content also leads to early thermal degradation of lignin [67] and may be responsible for creating defects on the surface of resulting fibers.

6.4  Improving Melt-Processing Melt-processing of lignin is somewhat a very difficult task because of the complex nature of the lignin polymeric structure. This difficulty is basically because of the reactive nature of lignin. To improve on the melt-processing of lignin, several modifications have been proposed, including blending with thermoplastic polymers, esterification, alkylation, etherification, phenolation, fractionation, etc. These modifications involve altering the structure of lignin or addition of a structural unit to replace an existing unit (Fig. 6.14). Phenolic and aliphatic hydroxyl units are the most reactive functional groups in lignin and determine the chemical reactivity of the lignin polymer. Most of the modifications involve reactions directed at these groups.

6.4.1  Melt Blending One way of improving the melt-processing ability of lignin is to blend it with thermoplastic polymers to enhance its flow ability. Blending is a convenient and inexpensive means of producing fibers with desired surface characteristics. In most cases, the thermoplastic polymers used in the blend only serves as a medium to allow the formation of fibers and are afterwards removed in another process. Over the years, several polymers have been blended with lignin so that they can be melt processed into fibers. Due to the presence of excessive reactive groups in lignin, compatibility with other polymers is very difficult, therefore treatment methods  (Fig. 6.18) to

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Fig. 6.18  Different routes of lignin modification

Fig. 6.19  Porous hollow-core fibers from the thermal treatment of immiscible polymer blended [166]

make blending with other polymers possible has been developed and used alongside. These methods function by disruption of the extensive cross-­linking and strong intra-molecular interactions in lignin which make them hard to be melt-spun so that viscoelastic properties are altered. Blending of lignin with polypropylene (PP) was studied by Maldhure and Ekhe [165]. To enhance the miscibility with PP, lignin was modified by alkylation and arylation. It was realized that modified lignin can work as effective processing stabilizers for polypropylene in melt extrusion. The blends also showed improved thermal stability with increasing modified lignin proportion. Kadla et al. [166] used PP and lignin to manufacture core shell structured fibers by manipulating the processing conditions and blend composition. Utilizing the immiscible behavior of PP/lignin, core shell structured fibers were formed based on the scheme in Fig. 6.19. Kubo and Kadla [167] also reported thermal blending of thermally purified lignin and PP of differing molecular weights. Spinning properties of lignin/ medium molecular wright PP blends was found to improve remarkably. This is because the PP in this case possesses a molten viscosity comparable to the spinning temperature allowing for continuous spinning. Comparing spinnability of the blend with PET blend

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Fig. 6.20  PLA-rich phases self-assembly in PLA–lignin blend. Reprinted with permission from [170]

showed that PET is better than PP. Core shell structured carbon fibers have also been reported by blending pyrolysis fuel oil (PFO) with THF treated lignin [168]. Results show that the use of THF led to effective removal of the low molecular weight volatiles and highly cross linked high molecular weight fraction. This in addition to the plasticizing effect of PFO contributed largely to the improved flow ability of the lignin. Zhong and co-workers [169] also showed that alkylating lignin with bromododecane results in compatibility with polypropylene enabling up to 70 wt% lignin incorporation into PP to form a thermally stable thermoplastic. Another polymer that has been investigated as a blend for lignin is PLA because of its renewable nature. Thunga and co-workers [170] investigated blending butyrated lignin with PLA and reported successful melt spinning into fibers. The improved compatibility of lignin with PLA was attributed to the removal of reactive –OH groups by the esterification reaction which in turn reduced the cross-linking ability and enhance melt processability. The fiber structure appeared to be homogeneous, but TEM revealed micro-phase separation in all blends. Distinctively, PLA microfibers were found to self-assemble inside the bulk fibers (Fig. 6.20). In another study, acetylated lignin was melt blended with PLA to improve on thermal stability [171]. The addition of acetylated lignin increased thermal stability of PLA and prevented hydrolytic degradation of PLA. This improvement in thermal behavior and compatibility of PLA with lignin is attributed to esterification of the lignin which

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also acts like internal plasticizers. Wang et al. [172] also reported blending of PLA with fractionated lignin with improved spinnability. Carbon fibers formed from the blends possessed moderate properties with surface pores. Mousavioun et  al. [173] showed that soda lignin is miscible with Polyhydroxybutyrate (PHB) at lignin contents less than 40 wt%. The study noted higher Tg for blends obtained from the lignin fraction containing higher xylan and phenolic hydroxyl content, but lower methoxyl and carboxylic acid content and confirmed that the miscibility was due to hydrogen bond formation between the carbonyl group of PHB and the phenol hydroxyl group of lignin. The study also noted improvement in thermal stability, but the study did not examine melt-processing into fibers. Studies have shown that thermally treatment of hardwood Kraft lignin renders it miscible with PEO and Polyethylene Terephthalate (PET) but not with PP and PVA [174]. The miscibility with PEO is attributed to the hydrogen-bonding interactions between the hydroxyl hydrogen of lignin and the hydrogen accepting sites of PEO. In a similar study, the authors examined the miscibility of long and short chain PVA with thermally treated lignin. Thermal treatment was conducted for 30 min at 160 °C. Results show good fiber spinning for the lignin/short-chain PVA blends but lignin/long chain PVA blends was difficult to blend. It is apparent that a strong hydrogen bond was formed between hydroxyl groups of the short-chain PVA and lignin. This indicates that lignin can form hydrogen bonds with semicrystalline polymers even when they are not miscible. Blending of PEO and lignin after desalting and thermal treatment of the lignin was found to produce fusible lignin with excellent spinnability [26]. Thermal treatment was conducted under vacuum and the resulting fusible lignin was able to form fine filament. The blend presented the problem of fusion of individual fibers during stabilization. Another study on blending PEO and Kraft lignin was reported by Kubo and Kadla [175]. The softwood lignins used in the study were esterified by acetylation before blending. The blends were found to be miscible at the fractions studied and were thermally stable for melt-processing into fibers. Awal and Sain [176] also used PEO as plasticizer in soda lignin to enable spinnability. A detailed study on the structure of PEO-lignin blend shows that hardwood and softwood lignin aggregate into larger global structures made of building blocks that have a cylindrical shape. The blend was processed via solvent casting in DMSO [177] without modification of the lignin. Softwood lignin–PEO blends showed larger aggregates than hardwood lignin–PEO blends. The difference in aggregated structure is attributed to the less methoxy functional groups in softwood lignin than hardwood lignin. The less methoxy groups in softwood lignin make it to display a more branched structure, leading to larger interaction with the PEO. This is not the case with hardwood lignin because it contains a high amount of methoxy functional groups leading to less branching and cross-linking. The PEO chains act as a shield for the lignin chains limiting aggregate growth (Fig. 6.21). A blend of Polyethylene Glycol (PEG) and lignin was successfully produced by applying PEG-400 during the extraction process [178]. The lignin was found to be fusible because of the addition of PEG moiety into lignin which acts as an internal plasti-

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Fig. 6.21  PEO constrained aggregation of softwood lignin. Reprinted with permission from [177]

cizer giving rise to high thermal mobility. Melt-spinning of the lignin led to the formation of infusible fibers resulting from partial cleavage of the PEG moiety from the lignin fibers. The modified lignin had a polydispersity index of 4.2 and Tg of 128 °C. Homogeneous blends containing 85% (w/w) of unmodified Kraft lignin and poly(vinyl acetate) with two plasticizers has been reported by Li et al. [179]. A series of thermoplastics were fabricated with Tg close to room temperature and good mechanical properties using diethyleneglycol dibenzoate and indene as plasticizers. Successful blending of lignin and PAN has been reported by Dong et al. [49]. The blending was done by dissolving lignin in DMSO and the solution was added to PAN polymer solution. The dope was wet spun into fibers. The authors reported that there was no evidence of cross liking between PAN and lignin which is attributed to the method used—wet spinning. The blend did not show evidence of macrophase separation but a sea-island structure in the fibers indicating a good compatibility between PAN and lignin. Another attempt to blend lignin copolymer with acrylonitrile (AN) has also been reported [50]. The lignin was copolymerized with AN in DMSO by radical copolymerization and wet spun into fibers and the

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305

Fig. 6.22  Illustration of possible abstraction that facilitates radical polymerization of lignin. Reprinted with permission from [180]

Fig. 6.23  HKLP fiber (a), and a SKL fiber with 5% HKLP (b), cross-section and fiber surface of a carbonized SKL fiber with 10% HKLP (c, d) [31]

fibers were successfully carbonized into high quality carbon fibers. The authors cited the benzylic sites in lignin as a potential site for grafting of AN through radical hydrogen abstraction (Fig. 6.22). Lignin–lignin blends has been reported as a means of producing thermally spinnable lignin for carbon fiber applications [31]. The study involved the addition of fractionated hardwood lignin as a softening agent to unfractionated soft and hardwood Kraft lignin. The addition of fractionated lignin to unfractionated lignin resulted in lignin that can be spun into fibers. The fibers were subsequently carbonized into high quality smooth carbon fibers (Fig. 6.23) with carbon content ranging from 93 to 97 atomic %. Blending lignins from different sources to enable melt

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Fig. 6.24  SEM images of carbon fibers made from switchgrass and yellow poplar lignin blends: (a, b) 50% YP:50% SG thermostabilized at heating rate of 0.05 °C min−1, (c, d) 50% YP:50% SG thermostabilized at heating rate of 0.5 °C min−1, (e) 75% YP:25% SG thermostabilized at heating rate of 0.05  °C  min−1, and (f) 85% YP:15% SG thermostabilized at heating rate of 0.05 °C min−1 [3]

processing has also been attempted [3]. Organosolv lignin from Alamo switchgrass and yellow poplar were used as blends without further treatment. Results indicate that the spinnability of switchgrass lignin was markedly improved by blending with yellow poplar lignin. The absence of fusion of individual fibers indicated that switchgrass lignin significantly improved thermo-stabilization of poplar lignin. The surface quality of the resulting carbon fibers is shown in Fig. 6.24.

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6.4.2  Fractionation Lignin is made up of molecules having different molecular weights (MW), various functional groups, and diverse inter-unit linkages. These cause intrinsic heterogeneity of the lignin structure which is responsible for poor melt-properties and poor miscibility with other polymers. A good way to improve on melt-properties will be make the structure of lignin homogeneous. The process of deriving a uniform lignin structure by removing some components is called fractionation. Fractionation processes used in modification of lignin are usually based on the manipulation or modification of the functional groups and side chains. In some cases, fractions that constitute a wide difference in the molecular weight are removed to arrive at a uniform molecular weight. Several fractionation processes have been developed based on several parameters. A commonly practiced method of fractionation is pH fractionation [26, 181, 182]. The process involves the washing of lignin at a certain pH and adjusting the pH continuously until the desired purity is reached. Washing Alcell lignin with HCl was found to reduce the amount of salt to about 1000 ppm resulting in melt spinnable lignin [26]. Another method of fractionation is solvent extraction to remove high-molecular-­ mass fractions. The method is based on the removal of excessive aryl-ether structures which are responsible for the high molecular weight fraction in lignin. Cooking lignin in 70% acetic acid solvent resulted in large reduction in the aryl-ether ­structures giving rise to high thermal mobility of softwood lignin which could be converted to corresponding carbon fibers by direct carbonization without thermostabilization [136, 183]. A mixture of methyl isobutyl ketone, ethanol, and water with sulfuric acid as catalyst was also used as a solvent for fraction of lignin. The fractionation process resulted in lignin with low aliphatic OH groups and increased phenolic OH groups with increase in treatment time [184]. At longer periods of fractionation, lower ether bonds lignin with condensed structure was formed leading to increased PDI and reduction in MW. Sequential fractionation of lignin with ether followed by methanol to obtain low molecular weight lignin with low PDI has also been reported [140]. The use of combined ethanol, acetone, and propyleneglycolmonomethyl ether has been reported to successfully separate lignin into homogeneous fractions [185]. The study reported an increase in free phenolic guaiacyl units and carboxylic acid groups steadily as the fractionation proceeded. Continuous washing (Fig. 6.25) of softwood and hardwood lignin with acetic acid introduced certain amounts of acetyl groups into the lignin structure making the softwood lignin fusible [133]. The washing separated the initial infusible lignin to fusible (low molecular weight) and infusible (high molecular weight) fractions. The low molecular weight fraction was fusion spun into fibers and used to form carbon fibers without prior thermo-stabilization. Fractionation of organosolv and soda lignin using acetone has been reported by Domínguez-Robles et al. [186]. The low concentration acetone soluble fraction was found to be the most pure and homogeneous lignin fraction with low molecular

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Fig. 6.25  The acetic acid fractionation process

weight (Fig.  6.25) and polydispersity. The fraction also possesses low aliphatic hydroxyl groups, low carboxylic groups, and high phenolic OH groups. Methanol soluble fractions of lignin were also reported to contain fewer condensed structures, lower ratio of aromatic: aliphatic hydroxyl groups, high content of aryl ether linkages resulting in lower MW and glass transition temperatures with improved thermal stability [187]. Fractionation with acetone to low and high molecular weight fractions was also reported by Sadeghifar and Argyropoulos [142]. Gosselink et  al. [139] also designed a fractionation procedure for successive fractionation of lignin according to molar mass. Some fractions of lignin showed low MW but thermal analysis was not reported by the authors. Polarity fractionation was also used for ammonium lignosulphonates. Isopropanol and Isopropanol-water mixture were used as solvents [188]. Solvent fraction according to the scheme in Fig.  6.26 was reported for hardwood and softwood fibers. Glass transition was found to increase with increase in molecular weight. Phenolic hydroxyl groups and S/G ratio decreased with increase in MW [144, 189, 190]. Solvent fractionation was also used to separate Kraft lignin into low, medium, and high MW fractions [191].

309

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BIRCH KRAFT LIGNIN

CH2Cl2

undissolved

dissolved FRACTION 1

CH3OH

undissolved

dissolved FRACTION 2

FRACTION 3

Fig. 6.26  Solvent fractionation process

Aliphatic OH groups were found to decrease with decreasing MW but phenolic and carboxylic OH-groups were found to increase resulting in a lower aliphatic to phenolic OH-group ratio which supports melt-processing. Apart from organic solvents, ionic liquids has also been investigated as fractionation solvents for soda lignin [192]. The study recorded a drastic reduction in PDI with the use of ionic liquids. Fractionation by ultrafiltration and selective precipitation is also widely practiced [172, 193–196]. The method utilizes the membrane cutoffs of different sizes to separate dissolved lignin into different MW fractions [191]. Lignin fractionated by dissolution in sodium hydroxide solution (pH  12) and passed through ceramic membrane with molecular mass cutoff was found to be compatible with PLA and could be spun into high quality fibers [172]. Enzymatic fractionation has also been proposed to reduce the amount of low-­ molecular phenolic VOCs [162]. Laccases in mild acidic and slightly alkaline conditions and laccase-catalyzed oxidation were examined. A marked reduction in guaiacol was observed with oxygen-pressurized alkali treatment. Laccase enzymes were also used to oxidize lignosulphonates without the use of mediators [197]. Results show an increase in molecular weight and polydispersity as well as a decrease in phenolic content. The mechanism involves the initiation of oxidation reaction on the phenolic groups by the enzymes forming stabilized radicals. These radicals subsequently undergo radical–radical coupling leading to the formation of phenyl ether–carbon and carbon–carbon bonds. The enzymes can only affect the phenolic end groups located on the surface of the lignosulphonates. Another study on the use of enzymatic fractionation using laccase reported a reduction in –OH functional group from lignin side chains giving rise to low MW lignin [198]. In a further study the laccase treatment was modified by adding a mediator—HBT (1-hydroxybenzotriazole hydrate). Using this on Kraft lignin led to fractionation of lignin into water soluble (high MW) and insoluble fractions (low MW). The treatment results in lignin with lower PDI, reduced aliphatic –OH and phenolic –OH groups and increased β-O-4 inter-unit linkages, leading to enhanced melt-processing and miscibility with PAN [199]. Although thermal properties and the performance of the modified lignin have not been investigated, environmental friendly modification of lignin to improve melt-processing will be an appreciated area of research.

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6.4.3  Esterification Phenolic and aliphatic hydroxyl groups are the major functional groups in lignin. The presence of these hydroxyl units contributes to very high polarity of lignin molecules resulting in strong self-interactions which hamper melt processability and hinder melt blending with thermoplastics. Utilizing the OH groups as a source of reaction to form ester containing lignin fractions is the basis behind several of the esterification techniques. Lewis and co-workers [200] patented an esterification process for the formation of ester lignin with predetermined characteristics of melting point solubility and compatibility with thermoplastic materials. The method presented by the patent involved subjecting the lignin material to a condensation reaction with a derivative of an organic acid, under certain controlled conditions to form a corresponding ester of the acid. The ester lignin is thermoplastic in nature and may be used to make thermoplastic products. The patent also claims that the ester lignin is compatible with thermosetting resins. They demonstrated the use of acid halide of organic acid (Fig. 6.27a) and acid anhydride in the presence of a suitable organic base such as pyridine (Fig. 6.27b). In a subsequent patent, Adrianowycz and others [201] reported the formation of lignin acetate using acetic anhydride, lignin butyrate using butyric acid and lignin hexanoate using hexanoic anhydride all of which possess at least 50% reduction in glass transition temperature against unmodified lignin and were found to melt without the release of volatiles. This proves that esterification of lignin led to the elimination of the tendency to water absorption because of the hydroxyl unit and the formation of volatiles by the degradation of phenolic acids during melts treatment. The patent also claimed the compatibility of the formulated lignin with pitch. The blends of which were used to produce melt fibers with less brittleness that can form carbon fibers. The chemistry of lignin esterification with acid anhydrides under neutral conditions is primarily an acyl substitution reaction. The reaction starts with the addition of a nucleophile to the carbonyl carbon (Fig. 6.28). The alkoxy (–O–R) part of the

Fig. 6.27  Some esterification reactions

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Fig. 6.28  Mechanism of esterification under neutral conditions

Fig. 6.29  Locations of aliphatic and phenolic OH in lignin monomeric units

acid anhydride then departs rapidly to form a nucleophilic acyl substitution product. As depicted in Fig. 6.29 the three lignin monomeric units have both aliphatic and phenolic OH groups at the C-ɣ on the side chain and the C-4 of the aromatic ring respectively. It is possible that esterification under neutral conditions can lead to both hydroxyl groups being esterified. There is also a possibility of lignin cross-­ linking taking place when cyclic anhydrides are used at elevated temperatures. Aliphatic hydroxyl groups in the presence of acid anhydrides are mostly esterified at room temperatures but it is apparent that with long reaction times both aliphatic and phenolic hydroxyl units can be esterified so that all the OH groups are eliminated. In another study, esterification using carboxylic acid anhydrides with varying chain lengths led to a linear decrease in glass transition as the number of carbon atoms in the substituent increases. The MW was higher for lignin esters with longer chain lengths but the polarity tended to be lower [202]. The study reported subsequent compatibility with poly(3-hydroxybutyrate-co-3-hydroxyvalerate) ­ (PHBV) or polypropylene (PP) at certain percentages. Esterification of lignin by reacting lignin with butyric anhydrate in the presence of ionic liquid catalyst has been reported [203]. The ester lignin formed was found to possess increased solubility in styrene. When different acid anhydrates were used to investigate the effect of varying carbon chain length of the lignin ester, it was revealed that increasing the carbon chain lengths results in increased solubility in organic solvents. The ester lignin in this case has been found to possess good miscibility with PLA with good melt characteristics and can be used to form carbon fibers [204]. Other studies on esterification of lignin include the use of stearoyl chloride, acetic [115, 171, 205, 206], phthalic [207], and maleic anhydride [165]. Attempt

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Fig. 6.30  SEM images of the carbons from (a) unmodified lignin, (b) phthalic anhydride modified lignin, and (c) microscale porosity of phthalic anhydride modified lignin [208]

was not made to form fibers with these modified lignins but they were found to be compatible with thermoplastic polymers to certain extents. Chatterjee et al. [208] modified organosolv lignin by esterification reactions using dicarboxylic acid (phthalic, acetic, succinic or maleic) anhydrides. Ester lignins were formed, and melt extruded into carbon fibers. Analysis of the ester lignin showed a moderate increase in the aromatic character and higher thermal stability. The carbon fibers showed a tunable microstructure with microscale porosities when cyclic acid anhydride was used (Fig. 6.30). Acetylation is another esterification process for lignin that uses acetyl chloride, acetic anhydride, or acetic acid. The method was patented by Eckert et al. [209] and involves reaction at temperatures between 70 and 100 °C with up to 16 wt% of acetyl content to produce acetyl lignin that can be melted and thermally extruded. Subsequent study showed that acetylation of softwood Kraft lignin using acetic acid with higher extent of substitution resulted in lignin that could be melt-spun into fibers but were tacky during thermo-oxidation. However, acetylation with lower degree of substitution could be solution spun and performed well during thermo-­ oxidation. Softening point and glass transition temperature were found to increase with decreasing extent of acetylation which gave positive result during stabilization [66]. Esterification by acylation using fast solvent-free catalyst-free microwave-­ assisted method was also examined on softwood Kraft lignin. Acetic, propionic,

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H

O

H

H

C

OH

H

C

O

H

C

O

H

C

O

H

C

R

H

C

R

C

CH2CH2COO

(OCCH2CH2CO)O H2O H3CO

OCH3

H3CO

OR’

OCH3 OR’

(R= OH or H, R’ = H or C9unit) C9 Unit of parent lignin

Succinylated lignin

Fig. 6.31  Esterification with succinic anhydride

butyric, methacrylic, and maleic anhydrides were used. A degree of substitution >90% was achieved in 10  min. Results indicated that non-cyclic anhydrides ­eliminated both aliphatic and phenolic OH groups, whereas maleic anhydride exclusively removed aliphatic OH groups. All modified lignins were thermally stable than the original lignin [210] with a marked reduction in glass transition, and decomposition temperature increased except maleated lignin which showed very poor decomposition temperature. Xiao et  al. [211] recorded the possibility of formation of a monoester adduct bonded to both aliphatic and phenolic hydroxyl groups in lignin using succinic anhydride (Fig. 6.31). The method was previously used for esterification of wood for improvement in mechanical properties [212]. The reaction led to increase in thermal stability of the lignin but the reaction had a low degree of substitution.

6.4.4  Alkylation Alkylation of lignin involves the addition of an alkyl group to the lignin structural chain by the removal of hydroxyl groups. Methylation of lignin with dimethyl sulfate in aqueous NaOH and methyl iodide in DMF in the presence of K2CO3 and oxypropylation using propylene oxide in aqueous NaOH were found to selectively remove phenolic hydroxyl units with no change in MW. The mechanism of reaction is purely a stoichiometric and non-polymerized grafting of propylene glycol onto the lignin [213] (Fig. 6.32). The authors proceeded to examine the thermal properties and molecular structure of the modified lignin [214]. They reported a reduction in glass transition temperatures and improved thermal stability of all modified lignin attributing it to the reduction in hydrogen bonding strength of the lignin. Molecular weight of methylated samples was found to remain constant over time

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L

Me2SO4, NaOH, H2O 1, R.T. / 0.5 hr MeO

2, 80 °C / 2 hr

(I) MeO MeO

OH

L

L

Mel, K2CO3, DMF (II) R.T. / 18 hrs

MeO

MeO MeO

OH

L

O

L

NaOH, H2O

(III)

40 °C / 18 hrs MeO MeO

O

CH2

CH

OH

O

n

H

CH3

Fig. 6.32  Formation of methylated and oxypropylated lignins. Reprinted with permission from [213]

when heated at 148 °C. This can be attributed to the action of the methylation reaction. Low temperature (less than 120 °C) thermal degradation of Kraft lignin has been reported to occur by the formation of phenoxy radicals via H abstraction process when interacting with thermally activated radicals. The phenoxy radical can be transformed into various resonance mesomeric forms (Fig. 6.33). However, when the sample is fully methylated, the phenoxy radical formation is avoided by the methylation-preventing radical coupling reactions. This was not the case for oxypropylated lignin as the MW increased with time when it was heated to 148 °C. This observed behavior is possibly due to the dehydration of secondary hydroxyl groups of the oxypropyl substituent. Phenoxonium intermediates can be formed by dehydration of oxypropylated phenolic OH groups through alpha-hydride transfer. New phenolic groups can then be formed by hydrolysis of the intermediate and they can engage in fresh radical coupling reactions leading to increase in MW similar to untreated lignin (Fig. 6.34). Methylation of acetone soluble lignin fraction has been found to offer unexpected plasticizing action to PE blend attributable to the low molecular weight and

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Fig. 6.33  Thermally induced radical coupling in Kraft lignin [214]

Fig. 6.34  Formation of new phenolic OH groups during thermal processing of oxypropylated lignin. Reprinted with permission from [214]

spherical configuration of the modified lignin [142]. The selective methylation of the phenolic OH groups offered relative thermally stable melts. Dichloroethane has also been used in alkylation of lignin [215] in attempt to make lignin compatible with polyproylene. The authors reported poor adhesion between the ethylated lignin and PP matrix. In another study [165] based on the same method the authors reported increase in thermal stability of PP/alkylated lignin blend attributing the thermal stability to the alkylation of the lignin.

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6.4.5  Phenolation Phenolation of lignin involves the addition of phenolic compounds to lignin to give it melting properties. Sudo et  al. [35] reported the formation of pitch-like-lignin material by phenolysis of steam exploded lignin. Gas permeation chromatography (GPC) results confirmed that phenol groups were attached to the lignin structure. Phenolation generally leads to a reduction in MW, PDI, and improved solubility of lignin which is important to melt-spinning [216]. Phenolation affects almost all lignin resulting in substantial decrease in aliphatic–OH but increase of phenolic– OH groups [217]. The reaction leads a marked reduction in formaldehyde structures. The fact that MW and PDI are greatly reduced means that the resulting lignin may possess good thermal processability. On the other hand, a decrease in aliphatic OH groups may mean a reduction in Tg due to the reduction in intermolecular OH bonding. Increased study on thermal properties of phenolated lignin and conversion to carbon fibers is required (Fig. 6.35).

6.4.6  Other Methods Hydrogenation of lignin has been attempted as a modification technique to render it melt processable into carbon fibers [21]. Lignin was first hydrogenated in the presence of Raney-Ni as a catalyst in 2% NaOH aqueous solution. The resulting lignin was then fractionated with chloroform to obtain chloroform soluble fraction which was then subjected to carbon disulphide treatment after which it was heated to temperatures above 300  °C under vacuum to obtain a pitch-like lignin fraction. The lignin was then melt-spun into fibers, thermostabilized, and carbonized into carbon fibers. It was shown that hydrogenation led to an extensive cleavage of alkylaryl ether bonds and the formation of ethylene bridges between aromatic nuclei. These results from the decrease of OH groups and cleavage of hydroxyl methyl parts on the propanoid side chain in the lignin structural unit. The carbon fibers produced from the process had moderate tensile properties. Further studies on hydrogenation of lignin for the production of carbon fibers are needed. Another important modification reaction for improving thermal processability of lignin is the addition of carbon atoms to the lignin chain patented by Argyropuolus [219]. The process involves propargylation of lignin and subsequent rearrangement of the structure to form a completely thermoplastic lignin that can be melt-spun into fibers (Fig. 6.36). The group reported the systematic propargylation of acetone soluble softwood lignin using propargyl bromide in a basic media [218]. The study realized thermoplastic lignin with different degrees of substitution. The obtained lignin was somewhat intractable and was not suitable for thermoplastic processing because of its highly branched nature. To solve this problem the authors attempted a reduction in the degree of substitution and masking the remaining hydroxyl units

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Fig. 6.35  Phenolation reaction with lignin substructures. Reprinted with permission from [217]

by methylation. In a subsequent study, the authors attempted to elucidate the mechanism behind the reaction [141]. Addition of carbon chains to lignin is a promising means of improving on the final properties of lignin carbon fibers. Research in this area will be of interest toward the realization of high performing lignin carbon fibers.

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Fig. 6.36  Mechanism of propargylation of lignin and subsequent rearrangement [218]

6.5  Conclusion Lignin contains several reactive end groups contributing to difficulty in meltprocessing of lignin into fibers. Phenolic/aliphatic hydroxyl units, G units, and aryl ether linkages amongst others undergo reactions during thermal and solvent processing to limit the melt processability. During thermal processing, repolymerization and depolymerization reactions occur coupled with the emission of volatile substances. However, utilizing the reactive functional groups, lignin can be modified to improve melt processability. Despite various studies on melt-processing of lignin, certain difficulties exist that require research attention.

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196. A. García, A. Toledano, L. Serrano, I. Egüés, M. González, F. Marín, J. Labidi, Sep. Purif. Technol. 68, 193–198 (2009) 197. D. Areskogh, J. Li, G. Gellerstedt, G. Henriksson, Biomacromolecules 11, 904–910 (2010) 198. C. Zhao, S. Xie, Y. Pu, R. Zhang, F. Huang, A.J. Ragauskas, J.S. Yuan, Green Chem. 18, 1306–1312 (2016) 199. Q. Li, S. Xie, W.K. Serem, M.T. Naik, L. Liu, J.S. Yuan, Green Chem. 19, 1628–1634 (2017) 200. H.F. Lewis, F.E. Brauns, Esters of Lignin Material, U.S. Patent 2,429,102, 1947 201. O.  Adrianowycz, B.K.  Al-Nasleh, C.-F.  Chang, A.  Hausner, M.  Robert, Carbon Fibers Derived from Lignin/Carbon Residue, WO 2014/078120 AI, 2014 202. S. Luo, J. Cao, A.G. McDonald, Ind. Crop. Prod. 97, 281–291 (2017) 203. W. Thielemans, R.P. Wool, Compos. Part A Appl. Sci. Manuf. 35, 327–338 (2004) 204. K. Chen, Bio-Renewable Fibers Extracted from Lignin/Polylactide (PLA) Blend (Iowa State University, Ames, IA, 2012) 205. H. Jeong, J. Park, S. Kim, J. Lee, J.W. Cho, Fibers Polym. 13, 1310–1318 (2012) 206. O. Gordobil, R. Delucis, I. Egüés, J. Labidi, Ind. Crop. Prod. 72, 46–53 (2015) 207. R.R.N. Sailaja, M.V. Deepthi, Polym. Compos. 32, 199–209 (2011) 208. S. Chatterjee, A. Clingenpeel, A. McKenna, O. Rios, A. Johs, RSC Adv. 4, 4743–4753 (2014) 209. R.C. Eckert, Z Abdullah, Carbon Fibers from Kraft Softwood Lignin, U.S. Patent 7,678,358 B2, 2010 210. F. Monteil-Rivera, L. Paquet, Ind. Crop. Prod. 65, 446–453 (2015) 211. B. Xiao, X.F. Sun, R. Sun, Polym. Degrad. Stab. 71, 223–231 (2001) 212. C.A.S. Hill, S. Mallon, Holzforschung 52, 427–433 (1998) 213. H. Sadeghifar, C. Cui, D.S. Argyropoulos, Ind. Eng. Chem. Res. 51, 16713–16720 (2012) 214. C. Cui, H. Sadeghifar, S. Sen, D.S. Argyropoulos, Bioresources 8, 864–886 (2013) 215. A.V. Maldhure, J.D. Ekhe, E. Deenadayalan, J. Appl. Polym. Sci. 125, 1701–1712 (2012) 216. J. Podschun, B. Saake, R. Lehnen, Eur. Polym. J. 67, 1–11 (2015) 217. X. Jiang, J. Liu, X. Du, Z. Hu, H.M. Chang, H. Jameel, ACS Sustain. Chem. Eng. 6, 5504– 5512 (2018) 218. S. Sen, H. Sadeghifar, D.S. Argyropoulos, Biomacromolecules 14, 3399–3408 (2013) 219. D.S.  Argyropoulos, High Value Lignin Derivatives, Polymers, and Copolymers and Use Thereof in Thermoplastic, Thermoset Composite, and Carbon Fiber Applications, U.S. Patent 2013/0255216 A1, 2013

Chapter 7

Stabilization of Lignin Fibers Emmanuel Isaac Akpan

7.1  Introduction The production of carbon fibers from lignin involves several steps including lignin extraction, purification, melt processing, thermo-stabilization, and carbonization. Subsequent steps, such as graphitization and activation, may be employed after carbonization. Extraction, purification, and melt processing have been explained in the previous chapters. In melt processing, lignin is converted to thermoplastic fibers, which should thereafter be carbonized to form carbon fibers. However, to prevent sticking, shrinking, or melting of the fibers during carbonization, stabilization is conducted [1–3]. In the process, the thermoplastic material is converted into a thermosetting material and made infusible through a series of reactions including cross-­ linking, oxidation, cyclization reactions, and so on [4]. In some processes, oxidative gases and liquids, such as oxygen, nitrogen dioxide, sulfur dioxide, sulfur trioxide, nitric acid, sulfuric acid, potassium permanganate, and hydrogen peroxide, are used. Methods that have been studied include oxidative, electron-beam irradiation, UV radiation, and plasma treatment. However, oxidative stabilization is the most studied lignin stabilization method. In this chapter, a comprehensive overview of the stabilization step, mechanism involved, and the processing parameters are explained.

E. I. Akpan (*) Institute for Composite Materials, Technical University Kaiserslautern, Kaiserslautern, Germany e-mail: [email protected]

© Springer Nature Switzerland AG 2019 E. I. Akpan, S. O. Adeosun (eds.), Sustainable Lignin for Carbon Fibers: Principles, Techniques, and Applications, https://doi.org/10.1007/978-3-030-18792-7_7

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7.2  Thermo-stabilization 7.2.1  Process Description Carbonization of fibers requires that the glass transition temperature (Tg) be greater than the carbonization temperature (T). For lignin Tg is far less than T [2] making it necessary to conduct pre-treatment that will prevent softening during the carbonization. During continuous heating of polymer materials at very slow heating rates, the Tg of the polymer can increase fast enough to maintain the material in a glassy state as the temperature increases. In thermosetting materials, continuous heating transformation (CHT) curves show that there exists a particular heating rate where the material vitrifies creating a situation where the Tg is ahead of temperature (Fig. 7.1) [5]. Applying this to lignin creates a situation where lignin transforms from thermoplastic to thermosetting behavior. Thermo-stabilization of lignin is based on this concept. In thermo-stabilization, lignin fibers formed during the melt-processing step are subjected to stepwise increase in temperature at a slow rate. The process involves heat treatment of the fibers at lower temperatures under tension in gaseous or liquid medium to produce changes in the chemical structure of the fibers such that they become thermally stable to subsequent temperature processing (carbonization, graphitization, and activation). The process involves heating the fibers in an airtight oven to a certain temperature at a specific heating rate and time depending on the nature of the fibers (Fig. 7.2). Because of the variability in the structure of lignin,

Fig. 7.1  CHT diagram for thermosetting polymer. Reprinted with permission from Wiley [5]

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Fig. 7.2  Schematic of fiber stabilization

there is no standard method for stabilization of lignin fibers. The effectiveness of the process is dependent on such factors as heating rate, temperature, and stabilizing media. One primary goal of stabilization is to cross-link the as-spun fiber structure, ensuring that there is no loss of the molecular and the fibrillar orientation during final heat treatment. Thermo-stabilization in the presence of air (oxidative) is the prevalent thermo-stabilization method for lignin, but other methods including ultraviolet (UV) irradiation [6], electron-beam irradiation [7], chemical thermo-­ stabilization [8, 9], and iodine treatment [10] has been applied. Stabilization of fibers is considered the most decisive step in carbon fiber production because it governs the evolution of the final structure of the fibers. Different schedules have been used in thermo-stabilization of lignin over the years. Li et al. investigated the reactions in thermo-stabilization of soda pulp lignin in air using a muffle furnace at 30 °C/min, heated to target temperatures between 170 and 350 °C [11]. Using a convection oven Chatterjee et al. [3] investigated stabilization of lignin using heating rate of 3 °C/min at 249 °C in air. The fibers were arranged in a cross-hatched pattern to ensure fiber–fiber fusion. The study reported successful stabilization of esterified softwood Kraft lignin. Lignin blended with polyethylene oxide (PEO) was also successfully stabilized under tension using heating rates between 0.2 and 3 °C/min at 250 °C for 1 h in air. The success of stabilization was dependent on the type of lignin and the fraction of the thermoplastic polymer in the blend [12]. Brodin et al. [13] stabilized birch lignin in N2 atmosphere in a tube furnace (closed system) at a heating rate of 0.2 °C/min to 280 °C isothermal treatment with a holding time of 2 h. In the same study lignin powder and fibers were stabilized in air under the same conditions. Norberg et  al. [4] investigated stabilization of modified soft and hardwood kraft lignin fibers using a conventional GC oven using heating rates between 0.2 and 15  °C/min under air and nitrogen atmospheres. For nitrogen stabilization, a closed vial containing nitrogen was used. In a patent application, Poeppel and Frank [14] proposed a stabilization method for all kinds of lignin involving heating of the lignin in the presence of a cross-linker at temperatures below cross-linking temperatures and subsequent thermal treatment to allow cross-linking. The cross-linker can be any formulation (Fig. 7.3) which can

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R1

N

N

N

R2

Fig. 7.3  Poeppel type cross-linker. R1 and R2 should be halogen, or from the group consisting of Cl, Fl, and Br; and R3 renders the molecule water soluble. R3 is from the group consisting of Cl, Fl, Br, I, Na, OH, H(CH2O)n, H (C2H4O)n, with n from 1 to 10, preferably 1. R1 and R2 are preferably Cl. R3 can be Cl or H(CH2O)n, H(C2H4O)n, with n from 1 to 10, preferably 1. R3 can be Cl or methoxy [14]

Fig. 7.4  Lignin stabilization with cross-linkers [14]

suitably form cross-links between two or more hydroxyl groups of lignin (e.g., cyanuric chloride or its derivative). The schematic of the stabilization process is shown in Fig.  7.4. The inventors reported successful stabilization of hardwood organosolv lignin, softwood kraft lignin, and hardwood kraft lignin using water solution of natrium cyanuric chloride at varying temperatures (40, 50, and 60 °C) and treatment times (30, 60, and 180 min). The final treatment was conducted by heating the resulting lignin up to 250 °C. Zhang et al. [6] utilized a two-step stabilization process to achieve stabilization of dry-spun lignin fibers resulting in excellent mechanical properties. The first step involved UV stabilization under tension using a Nordson 4.5 kW UV curing lamp

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Fig. 7.5  Lai et al. stabilization scheme for electro-spun fibers

having a mercury bulb radiation source at 50 °C. This was followed by stabilization in air under tension up to 250 °C. Lin et al. [9] proposed a chemical stabilization of polyethylene glycol (PEG)-lignin involving immersing the fibers in aqueous hydrochloric acid with a concentration of 1–6 M at 100 °C for 2–6 h, washed in water and dried in an oven at 105 °C for 2 h. Lai et al. [15] investigated the stabilization of electro-spun lignin-poly(vinyl alcohol) (PVA) blend fiber mat using a six step ­process. Electro-spun mat was tied to stainless steel rod and stabilization was conducted following the scheme in Fig. 7.5 under constant flow of air through the tube. A two-step stabilization process was also proposed for kraft lignin by Brodin et al. [16]. The fibers were stabilized in a tube furnace by first increasing the temperature to 250  °C at 0.5  °C/min and left for 1  h in air or nitrogen. Second, the temperature was decreased to 90 °C at 3 °C/min after which the sample was cooled to room temperature. Fiber mats electro-spun from organosolv lignin/polyacrylonitrile blends were stabilized in a one-step process [17]. Fiber mats were fixed along short edges and stabilized in a tube furnace in an oxidizing atmosphere (air flow rate of 150 cm3/min) and increasing the temperature to 200 °C at 0.2 °C/min, held at 200 °C for 12 h before cooling to ambient temperature. Softwood acetic acid lignin was also stabilized by heating to 250 °C for 1 h at a heating rate of 2 °C/min [18]. In thermo-stabilizing a lignin copolymer blend PAN, Maradur et  al. [19] first raised the temperature of the fibers to 105 °C at 1 °C/min and held for 1 h in air to remove moisture, followed by heating at the same hearting rate to 280 °C in nitrogen and held for 1 h. Heating was done in a tube furnace with a 10 cm heating zone (Fig.  7.6) and the fibers were kept in constant tension of 21.2  Pa throughout the heating time. The report showed that the fibers were stabilized and could be carbonized without problems. Qin and Kadla [20] stabilized organoclay modified pyrolytic lignin in a gas chromatography oven by heating the lignin fibers in air at a rate of 0.5 °C/min to 250 °C and then held for an hour under tension. Alcell lignin melt blown fiber mats were successfully stabilized in a convection furnace by heating the materials to 250 °C at heating rates between 0.1 °C and 1 °C/min in air.

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Fig. 7.6  Maradur et al. thermo-stabilizing setup [19]

Lignosulfonates-based AN copolymeric fibers were successfully stabilized by heating the fibers in an air furnace from room temperature to 250 °C with a heating rate of 10 °C/min and held at 250 °C for 1 h [21]. Luo [22] stabilized hardwood lignin by heating melt-spun fibers in an air stream at a heating rate of 0.25 °C/min to 200 °C and then held for 24 h at 200 °C using a tube furnace. In another report the same author included a prior stage of moisture removal by heating the fibers to a temperature of 105 °C at 1 °C/min [23]. Baker et al. [24] determined the correct thermo-stabilizing scheme for organic purified lignin to be heating to 250 °C at rates below 0.05 °C/min in a box furnace using air flow rate of 20 L/min. Lignin pitch was thermo-stabilized by Sudo and Shimaza [25] by heating the lignin to temperatures between 65 and 210 °C at heating rates of 1–2 °C/min in air atmosphere. The patent of Otani and co-workers showed successful stabilization of thiolignin at 150 °C in air after 10 h [26]. In the same report chemically treated short length alkali lignin were stabilized by first keeping the fibers at 80 °C for 7 h in ozone atmosphere in a porcelain tube, after which the fibers (still in the tube) were heated to 100 °C at a heating rate of 2 °C/min in an electric furnace and then to 150 °C at a rate of 0.5 °C/min, maintaining the temperature for 10 min. For long fiber alkali lignin, a thermo-stabilization at 200  °C for 10  h was able to produce accurately stabilized lignin. In another example, thiolignin was successfully stabilized at 270 °C for 10 h in air. A mixture of thiolignin from chemically treated hardwood and softwood lignins were stabilized at 150 °C for 10 h in air. Glycerin-modified thiolignin long melt-spun fibers from chemical treatment of softwood was stabilized by holding at 80 °C for 10 h in ozone atmosphere and then heated at 250 °C and allowed to stand for 5  h in air. Calcium lignosulfonate-PVA blended melt-spun fibers were stabilized at 200 °C for 10 h in a closed vessel. The same procedure was applied to alkali lignin-PVA blended but with a temperature of 300  °C for 5  h. Treatment of the same fibers at 200 °C for 10 h yielded good stabilization. Similar procedure was used to successfully stabilize thiolignin-PVA blend fibers, sodium lignosulfonate-PVA blend fibers, calcium lignosulfonate-PVA blended fibers,

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alkali-cellulose-thiolignin blend, alkali-cellulose/alkali-lignin blend, and PAN-­ thiolignin blend. PAN/lignin fiber tows were successfully stabilized under tension in an oxidation oven using a heating rate of 1 °C/min up to 300 °C, and held for 1 h [27]. Lignin/ lignin blends [28] were stabilized by heating the samples to 250  °C under air at heating rates (0.05, 0.1, 0.2, and 0.5 °C/min) and then holding for 30 min. Qu et al. [29] followed the same procedure but used a heating rate of 0.1 °C/min and a holding time of 1 h to stabilize pyrolytic lignin. Thunga et al. [30] also stabilized lignin/ polylactic acid (PLA) blends by heating the blend to 105 °C at 1 °C/min to remove moisture and then to 250 °C at a rate of 0.25 °C/min and held for 5 h in a stream of oxygen [31]. Electro-spun Alcell lignin was successfully stabilized by heating the fibers from room temperature to 200 °C at a heating rate of 0.05 °C/min and holding for 36 h in a tubular furnace with an air flow rate of 150 cm3/min [32]. Alcell lignin hollow fibers were stabilized by heating the hollow nanofibers at 0.05 °C/min from room temperature to 200 °C in a horizontal furnace with air flow rate of 200 mL/ min. Zhang [33] used the profile in Fig. 7.7 to successfully stabilize ECN-organosolv lignin. Acetylated softwood kraft lignin was stabilized by heating from 70 to 140 °C, at a rate of 0.4 °C/min and holding for 10 h then further heated to 220 °C, at a rate of 0.1 °C/min with holding time of 3 h at intermediate temperatures of 150, 160, 170, 180, and 220 °C. Stabilization was done under tension in air. Electro-spun kraft lignin/PEO fibers were stabilized by heating in a gas chromatography oven at a rate of 5 °C/min to 250 °C and held for 1 h [34]. Choi et al. [35] also stabilized electro-spun polyacrylonitrile/lignin fibers by heating from room temperature to 250 °C at a rate of 1 °C/min and held for 3 h. Eckert and Abdullah [36] also thermo-stabilized acetylated lignin fibers by heating in a furnace at a rate of 0.2 °C/min to 240 °C under air and held for 2 h. Kadla and others [37] stabilized

Fig. 7.7  Thermo-stabilization profile of ECN-organosolv lignin by Zhang [33]

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hollow core fibers of PP/lignin blend by heating to 250 °C at 1.5 °C/min. Pyrolysis fuel oil-lignin blend was stabilized by heating to 280 °C at a heating rate of 1 °C/ min in air and held for an hour in a stream of air flow [38]. Wohlmann and others [39] in their patent proposed two-step oxidation process for softwood lignin in a furnace free of tension. The oxidation involved heating the lignin from 25 to 170 °C at 2 °C/min, then further from 170 to 250 (or 300 °C) °C at 0.2 °C/min and held for 4 h at 250 °C (or 300 °C for 2 h). In another study, lignin/PLA fibers were stabilized by heating from 60 to 280 °C at a rate of 0.25 °C/min and held for 1 h [40]. Li et al. [41] stabilized fractionated lignin in a split tube furnace with vacuum system by heating in air from room temperature to 250 °C at a heating rate of 1 °C/min and holding for 1  h and the furnace was allowed to cool automatically to room temperature. Andersson stabilized lignin fibers impregnated with metal ions [42] by heating under tension in a muffle oven with an air flow in excess of oxygen. The heating was done from room temperature to 200  °C at a rate of 0.2  °C/min and from 200 to 250 °C at a rate of 1 °C/min and held for 1 h before ramping down at 0.2 °C/min to room temperature. This procedure was originally designed for LightFibre project reported by Olsson et al. [43]. From the forgone discussion, it is obvious that stabilization method varies widely dependent on the type of lignin and the purification method employed. Generally, it is important to make sure that the glass transition is not below the ramp temperature during stabilization to avoid fusing of the fibers. This implies that the heating be conducted at slow rates enough to keep the Tg ahead of the ramp temperature at every point. Reactions during lignin stabilization are strongly dependent on the mass transfer of oxygen in the fiber structure meaning that time, temperature, and rate of heating are important factors.

7.2.2  Effect of Process Parameters 7.2.2.1  Effect of Heating Rate The rate of heating of lignin fibers during thermo-stabilization has a deep effect on the stability and fusion of the fibers. During stabilization process, three major phenomena are in action including heat transfer, mass transfer, and shrinkage. Mass transfer involves diffusion of oxygen into the fibers and evolution of gases, such as hydrogen. Because stabilization in lignin is oxygen controlled, heating rates that will favor the increase in oxygen content will be the most appropriate heating rate. Insufficient oxygen diffusion causes a skin-core structure during stabilization, a situation where the oxygen on the surface is higher causing it to be more compact than the inner part. Because the surface is more compact oxygen diffusion into the core is difficult. Reaction rates that will avoid the formation of a core shell structure are desired during stabilization. Slower heating rates favor increase in oxygen content at low temperatures but at high temperatures the reverse is the case. It has been established that oxidation of lignin dominates at slow heating rates and

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Fig. 7.8  Effect of temperature and heating rate on mass loss and elemental composition of lignin. Reprinted with permission from Elsevier [2]

temperatures below 250 °C. Low heating rates favor the formation of oxygen-containing compounds [44]. On the other hand, slower heating rates are found to favor oxygen gain leading to increased formation of cross-links thereby increasing glass transition temperatures [2]. Oxygen content in lignin during stabilization is strongly dependent on the rate of heating (Fig. 7.8). The figure shows that slower heating rates favor reactions that increase oxygen content [45] at low temperatures probably by auto-oxidation [46]. Heat transfer during stabilization occurs as a result of the chemical reactions occurring during the process  (Scheme 7.1). Overheating can cause destruction of the fiber. It is important to use slow heating rate.

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Scheme 7.1  Example of reactions that increase oxygen content in lignin during stabilization

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Fig. 7.9  Continuous heating transformation diagram for lignin oxidation. Reprinted with permission from Elsevier [2]

A concise study of the effect of heating rate on stabilization of lignin fibers has been reported by Braun et al. [2]. The authors constructed a CHT curve (Fig. 7.9) for hardwood kraft lignin using powdered samples. The figure shows that the fastest allowable heating rate that will avoid thermal softening is 0.06  °C/min. Another important aspect of this figure is the fact that there exists a range of heating rates where glass transition completely disappears. Within this range lignin can be stabilized without the fibers fusing together. It should be noted that this value is only for the sample tested, when using fibrous samples from other sources, the results will likely be different. For instance, softwood lignin is known to be amendable to thermo-oxidation than hardwood probably because they originally possess a more

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cross-linked structure than hardwood lignin [4]. On the other hand, softwood lignin possesses a more branched molecular structure than hardwood lignin. Since stabilization occurs by oxidation and cross-linking of the structure, it will be faster to cross-link softwood lignin fibers than hardwood lignin fibers [8, 47]. Moreover, the initial content of oxygen in softwood lignin structure is high compared to those of petroleum pitch and PAN [4]. Prevalent stabilization rates for melt-spun lignin fibers in literature are in the range from 0.05 to 2.0 °C/min depending on the botanical source, pulping method, blend, and modifications. Important also is the initial glass transition of the lignin fibers. If the glass transition temperature before stabilization is high then a high rate of heating will be appropriate but if the starting Tg is low then a lower heating rate will be required which also means longer reaction time (Fig. 7.10) [48]. Baker et al. [48] showed that fibers with initial high glass transition can be stabilized at heating rates as high as 20 °C/min but the resulting fibers after carbonization had very poor mechanical properties probably resulting from the melt-processing stage. Baker et al. [24] in a different study exemplified the dependence of stabilization on the rate of heating as shown in Fig. 7.11. All fibers stabilized above 0.01 °C/min did not show good results upon carbonization. Only fibers stabilized at 0.01 °C/min were separable after carbonization. The discussion in Sect. 7.2.1 shows that various heating rates have been used to achieve stabilization for various lignin samples owing to the various factors that contribute to the cross-linking ability of lignin fibers. Heating rates also affect shrinkage of the fibers. It was recorded that axial shrinkage increases with increasing heating rate [49]. Large shrinkage is not Fig. 7.10 Illustrative relationship between initial glass transition and required heating rate. Adapted with permission from Wiley [48]

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Fig. 7.11  Effect of heating rate on stabilization of lignin fibers

desirable as they cause problems during carbonization. From the forgone discussion, it is noted that stabilization requires the use of very low heating rates. This is not economically viable as it increases the cost of the carbon fibers. Although studies have been conducted on methods that can lead to the reduction in stabilization periods, such as the use of UV stabilization [6], chemical stabilization [9], and treatment of the fibers prior to stabilization, reports show that these treatments affect adversely the resulting mechanical properties of the carbon fibers [13]. Finding an optimum point for stabilization of fibers should not only be about reducing the time of ­stabilization but also about resulting properties of the fibers. Some authors have used stepwise stabilization procedure for the stabilization of lignin fibers, but studies to measure the efficiency of these methods and the determination of an optimum profile are lacking. 7.2.2.2  Effect of Temperature Range The temperature of stabilization is an important factor in the stabilization of lignin fibers. Suitable choice of stabilization temperature is vital to gaining appropriate value carbon fibers [50]. Oxygen content in lignin during stabilization has been

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Fig. 7.12  Evolution of the content of C, H, O in the lignin with the thermo-stabilization temperature. Reprinted with permission from Elsevier [11]

found to be temperature dependent (Fig. 7.12) [11]. Oxidation reaction results in increase of oxygen content, and the higher the temperature the higher the oxygen content [51]. The oxygen content in the fiber increases to a maximum at a certain temperature and decreases afterwards. This shows that thermo-stabilization should be conducted within the range where the oxygen content is at maximum. At lower temperatures, incomplete stabilization occurs but higher temperatures results in decomposition which creates defects that can lead to poorly carbonized fibers. Brodin et al. [13] noticed that there exist a threshold stabilization temperature above which radical reactions occur leading to the creation of new reaction sites. 7.2.2.3  Effect of Tension To reduce chain relaxation and maintain a highly oriented structure of the fibers during stabilization, tension needs to be applied [1, 52–55]. Stretching of polymeric fibers gives the molecular chains additional energy to readjust and pack into ordered phases. Evidence of tension promoted reactions has been found during the stabilization of PAN [54]. Preservation of molecular orientation is a good reason for the application of tension [6]. Resultant properties of carbon fibers are also dependent on the application of tension. It has been reported that the mechanical properties of stabilized fibers increase with increasing applied tension [50]. Tension is also important because of shrinkage [6]. Without tension, shrinkage occurs during

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stabilization. Two types of shrinkage occur during stabilization including chemical and physical shrinkage. Physical shrinkage occurs as a result of entropy recovery. This type of shrinkage occurs above the glass transition temperature and as the temperature increases the fibers become plasticized. On the other hand, chemical shrinkage occurs as result of chemical reactions during the stabilization process. Both types of shrinkages lower significantly with increasing applied tension [56]. It has been reported that increasing tension leads to increase in the tensile strength and modulus of final carbon fibers in many cases [57–59]. However, excessive tension will break the fibers or lead to deterioration in the mechanical properties of the fibers as it may disturb preferred orientation in the supra-molecular structure [60]. Application of adequate tension can lead to improvement in the mechanical properties of resulting carbon fibers [33] but inadequate tension results in shrinkage of the fibers [6]. 7.2.2.4  Effect of Additives, Oxidizing Medium, and Medium Flow Rate Stabilization reactions are conducted in mediums, such as air, oxygen, nitrogen dioxide, sulfur dioxide, sulfur trioxide, nitric acid, sulfuric acid, potassium permanganate, and hydrogen peroxide. Studies on the stabilization of PAN have shown that the use of SO2, HCl, or Br2 as oxidation agents create more cross-links leading to improved quality [55, 61]. Some additives to serve as oxidizing agents have been used to improve on the stabilization process of carbon fiber precusors. Gump et al. [62] embedded an organic anti-oxidant in acrylic fibers that serves to moderate the oxidative portion of the stabilization reaction. This resulted in a fiber with substantially low polymer chain degradation when it was stabilized in an oxygen containing atmosphere at temperatures of about 200–300 °C. Kishimoto and Okazaki [63] also impregnated AN fiber with a primary amine and/or quaternary ammonium salt to assist the stabilization reactions. Other methods involving the use of hydroxylamine solution, aminophenoquinones, aminosiloxanes have been used impregnators to control stabilization exotherm and improve stabilization. It is also reported that cuprous chloride widens the exothermic peak of PAN fibers, potassium permanganate leads to increase in density, crystallinity index, preferred orientation index and decrease the crystal size. Potassium permanganate also reduced stabilization time. The use of hydrogen peroxide on the other hand decreased fiber shrinkage during stabilization, whereas cobaltous chloride increased crystal size, crystallinity, and preferred orientation of the fiber, and also improved the tensile strength (15–40%) and modulus (10–20%) of the resulting carbon fibers. Functional polysiloxanes bearing epoxy or amino group coatings on PAN promoted cross-linking during stabilization [64–70]. Intermittent laser-beam irradiation (produced from CO2 infrared laser) during stabilization of acrylic fibers was also found to expedite thermal stabilization of the fibers [71]. They propose that CO lasers and, HF lasers can also be used. Riggs [72] proposed a stabilization process for PAN fiber, involving treatment of the fibers first in an inert atmosphere until the reaction is essentially completed and subsequently

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treatment in an oxygen atmosphere. Schimpf [73] proposed a stabilization process for PAN involving initial heating of the fiber in an atmosphere substantially free of oxygen and or vacuum (nitrogen or any other inert gas atmosphere) until the residual rate of reaction is between 10 and 35% before transferring it to an oxygen containing atmosphere to oxidize the fibers and create a stabilized fiber. The application of persulfate, cobalt salt, hydrogen peroxide, potassium permanganate, ­dibutyltindimethoxide, aqueous guanidine carbonate, and hydrazine hydrate also shows the ability to modify the structure of the stabilized precursor, acts as healing agents for surface defects, and increases stabilization rate of PAN-based fibers [74– 79]. The use of electron-beam, thermochemical, and ultraviolet treatments to improve stabilization of PAN fibers has also been reported. It is believed that plasma processing affects both oxygen diffusion and oxidation reactions [80, 81]. Stabilization of lignin is also an oxidation-based reaction meaning that the stabilizing media and flow rate will affect the outcome. The study of Brodin et al. [13] on oxidative stabilization of lignin showed that the reaction is diffusion dependent. This is the reason for the core-shell structure which has been reported by other researchers during stabilization of lignin. They also reported a difference in mechanism of stabilization under air and nitrogen atmosphere. Mainka et  al. [82] also suggested that optimization of oxygen concentration in the stabilization oven will lead to optimized quality of the fibers and stabilization time. Goulis et al. [83] also investigated the effect of difference in oxidizing media on the stabilization of lignin fibers blended with high-density polyethylene (HDPE). Their report shows that the oxidizing media significantly affects the stabilization process. They only succeeded in stabilizing fibers using thermochemical treatment (sulfonation) of the fibers.

7.3  Mechanism of Thermo-stabilization To design a better stabilization schedule for lignin fibers, it is important to understand the mechanism of stabilization of the fibers. Lignin is a three-dimensional network polymer of phenolic units. The linkages in lignin are rotatable, aliphatic, and connected with ether. To make lignin fibers thermosetting, these linkages must be transformed into stiff and conjugated cyclic structures [84]. This transformation can be facilitated by oxygen through the formation of heterocyclic units. Lignin thermo-stabilization follows a complex multi-step mechanism involving oxidation and high temperature cleavage of bonds. These reactions include homolysis of C–O bonds, demethylation, cleavage of C–C bonds, oxygenation, radical formation, and rearrangement of radicals. It is assumed that the first step in thermal decomposition of lignin is the homolytic cleavage of the C–O bonds (Fig. 7.13a, b) in the β-O-4 linkage [2]. At high temperatures, the C–O bond cleaves to form phenethyl and phenoxy radical. This homolytic cleavage is determined by the amount of methoxy groups on the radical ring. The presence of methoxy groups in the β-O-4 unit lowers the bond dissociation energy of the phenoxy O–C bonds leading to increase in the rate of homolysis [85]. Beste [84] also reported cleavage of the C–O bonds in α-O-4 unit (Fig. 7.13c).

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Fig. 7.13  Homolytic cleavage of C–O bonds [2, 86]

Demethoxylation is a major reaction occurring during stabilization of lignin fibers. NMR studies of stabilized Alcell lignin show a significant decrease in methoxy groups after stabilization [87, 88]. This will occur as shown in reaction 2 of Fig. 7.13 at higher temperatures and is expected to be slower than homolysis of the β-O-4 ether bonds [2]. Since research has shown that restricted mass transport does not eliminate the possibility of free radical chain reactions [89], it is expected that the intermediates formed in Fig. 7.13 undergo a series of reactions depending on the heating rate, temperature, oxygen levels, and diffusion conditions. A reaction pathway for demethoxylation reaction proposed by Mainka et al. [88] is shown in Fig. 7.14. Possible further rearrangement reactions are shown in Fig. 7.15. Consistent with oxidation studies of lignin model compounds [84, 90, 91], bond homolysis is followed by hydrogen abstraction and rearrangement reactions as shown in Fig. 7.15a–d. This supports the autocatalytic reaction mechanism involving oxygen proposed by Braun et al. [2]. It is expected that the phenoxy radical intermediate 3 in Fig. 7.15a may possibly experience a 1,5-hydrogen shift leading to the formation of o-hydroxyphenoxymethyl radical intermediate 6, which can then undergo a β-scission and formaldehyde loss as shown in reaction 4 (Fig. 7.15b) or a shift in 1,2-phenyl which can later experience a loss in hydrogen as shown in reaction 5 (Fig. 7.15b). The formation of the radical intermediate may be possible as a result of the extensive intra-molecular hydrogen bonding present in the lignin macro-­

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Fig. 7.14  Demythylation in lignin [88]

molecular structure [92] and the fact that the intermediates are limited in mobility. Free radical chain reactions can also occur arising from intermediates 4 and 5 as shown in Fig. 7.15c, d. Intermediate radical 4 can undergo β-scission leading to the formation of styrene analogue. On the other hand, from intermediate 6, the occurrence of 1,2-phenyl migration from oxygen to carbon followed by β-scission is expected as shown in Fig. 7.15d. These reactions are shown to be faster than hydrogen abstraction at temperatures close to 375  °C [93]. During stabilization, auto-­ oxidation occurs by the introduction of carbonyl and carboxylic groups into the lignin structure [94–96]. Oxygen is very reactive and efficient radical scavenger which will definitely compete with other radical couplings and rearrangements stated earlier. The auto-oxidation will also lead to the formation of highly reactive oxygen-based radicals. These radicals may further oxidize the lignin structure by generating carbonyl and carboxylic functionalities all over the lignin molecule [97]. Auto-oxidation was also noticed in the study of Mainka et al. [88] (Fig. 7.16). Other reactions that have been reported during oxidation of lignin is the cleavage of C–C bonds in the β-1 unit (Fig. 7.17). The reaction produces two radicals that are stabilized by electron delocalization. Formation of ether linkages during stabilization has also been noted by Mainka et al. [88] (Fig. 7.18). This occurs by initial hydrogen abstraction and further rearrangements and the release of formaldehyde giving rise to the formation of a reactive site capable of linkage formation. Thermal degradation of lignin during stabilization leads to the formation of products, such as formaldehyde, water, carbon dioxide, and methanol. Beste [86] computational models of lignin oxidation showed the release of formaldehyde by β-β, α-O-4, β-5, β-O-­ 4, β-1, 5-5, and 4-O-5 models (Fig.  7.19). For β-β model, formaldehyde is first formed after ring opening occurs, after which the second is formed yielding stable conjugated products. For the α-O-4, β-O-4, and β-1 units, formaldehyde is formed after hydrogen abstraction, and for β-5, internal rearrangement of hydrogen has to occur. The release of formaldehyde was also presented by Braun et  al. [2] and Mainka et al. [88]. In these cases formaldehyde release leads to the formation of an

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Fig. 7.15  Further reactions following homolysis of C–O and –OCH3

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Fig. 7.15 (continued)

Fig. 7.16  Auto-oxidation of aldehydes

Fig. 7.17  Cleavage of C–C bonds in β-1 unit

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Fig. 7.18  Formation of ether linkages during stabilization [88]

Fig. 7.19  Mechanism for initial CH2O formation at 2100 K: (a) β-β, (b) α-O-4, (c) β-5, (d) β-O-4, (e) β-1, (f) 5–5, and (g) 4-O-5 model [86]

unstable radical which subsequently undergoes hydrogen abstraction to form a stable product. Very important to the stabilization of lignin is the formation of stiff aromatic chains. In PAN, this occurs by cyclization involving cyano groups and dehydrogenation. However, lignin does not contain nitrogen, but oxygen containing functional

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Fig. 7.20  Formation of condensed structures [86]

groups that can facilitate the transformation of the ether bonds and aliphatic linkages in lignin to stable and stiff aromatic units. Beste [84] attempted a molecular dynamics simulation to detect chemical transformations that can transform flexible linkages in lignin to rigid connections. They noted that when lignin was heated at higher temperatures carboxylic structures condense leading to the formation of esters or anhydride groups (e.g., dibenzo-1,4-dioxin and dibenzofuran structures). Condensation of aromatic rings occurs alongside oxygenation giving rise to stable aromatic C–C linkages or conjugated bonds [2, 84, 86, 88]. At longer durations or high temperatures, thermostable oxygenated and hydrogenated structures are removed allowing for continuous increase in the relative amounts of sinlge or ­conjugated C–C bonds (Fig. 7.20).

7.4  UV-Assisted Thermo-stabilization The use of ultra violet (UV) radiation to improve on the stabilization of lignin fibers has been reported by Zhang et al. [6]. Dry-spun fibers based on acetylated softwood kraft lignin were used in the study. Fiber tows were mounted so that tension was applied to the fibers. The tensioned tows were put into a UV chamber and irradiated with a 4.5 kW UV lamp with cooling air blown into the chamber at temperatures below 50 °C. The UV-treated tows were then removed and stabilized in air under

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Fig. 7.21  SEM images of stabilized fibers (a) without prior UV treatment stabilized at 0.2 °C/min and (b) UV irradiated before stabilization at 1.2 °C/min [6]

tension at 250 °C. UV irradiation led to the formation of carbonyl groups and degradation of aromatic rings. The fibers were successfully stabilized at heating rate of 1.2 °C/min without fiber fusing against 0.2 °C/min with several dwell steps earlier reported for the same type of fibers. This amounted to about 900% reduction in stabilization time. After carbonization, the fibers retained the crenulated surface with a tensile strength of 0.9 GPa (Fig. 7.21). Further studies on the use of UV irradiation for lignin fiber stabilization are needed.

7.5  Chemo-thermo-stabilization Some chemical methods have been proposed to improve the stabilization of lignin fibers. Yue et al. [49] showed that stabilization of organosolv lignin from softwood and hardwood can be accelerated by modification with HCl. Lignin samples were initially pre-treated in a tube furnace with mixed HCl gas and air. HCl gas was produced by bubbling air into 37% HCl solution. A one-step stabilization process was conducted by rapidly heating the fibers to 110 °C at a heating rate of 20 °C/min and then slowly heated to 210  °C under an air/HCl atmosphere. Results showed that stabilization time reduced considerably but the final carbon fibers possessed poor mechanical and surface properties (Fig. 7.22).

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Fig. 7.22  Typical morphology of the carbon fibers: (a) Cross-section of a fiber bundle, (b) fracture surface of a single fiber, (c and d) fiber surface [49]

Lin et al. [8, 9] studied the modification of lignin fibers with acid before stabilization. Lignin fibers were immersed in aqueous mixed solution of hexamethylenetetramine and hydrochloric acid at 85  °C for 1  h. The treated fibers were thermo-stabilized by heating up to 250 °C at a rate of 2 °C/min in air. The chemical treatment decreased the stabilization time from 2 days to 2 h. The carbonized fibers showed a tensile strength of 0.7 GPa, about 150% improvement over the untreated fibers [98]. The chemical thermo-stabilized fibers showed the presence of splits and pores on the surface (Fig. 7.23). The presence of the defects was attributed to the promotion of ether bonding with PEG by protonizing action of HCl followed by the cleavage of ether bonding (Fig. 7.24) resulting in a significantly more flabby structure in the interior of the fibers [98]. Another chemical treatment that has been reported is the use of iodine [10]. Electro-spun fibers from organosolv lignin and lignin/cellulose acetate blend were placed into jars with iodine and the jars were placed into a 100 °C oven to vaporize the iodine for a certain period. The jar was removed from the oven and allowed to cool and the samples were then removed and subjected to thermo-stabilization by heating the samples at 2 °C/min to 300 °C in air and holding for 2 h. Blended fibers showed the best results after stabilization and carbonization (Fig. 7.25).

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Fig. 7.23  SEM morphology of chemically thermo-stabilized PEG-lignin fibers treated with 6 M HCl solution at 100 °C for 2 h (a) and (b), 4 h (c) and (d), 6 h (e) and (f) [98]

7.6  Electron-Beam Irradiation The use of electron-beam irradiation for stabilizing lignin has been investigated by Seo et al. [7]. Electro-spun fiber mat made from lignin/PAN blend was subjected to electron beam generated using a 1.14 MeV acceleration voltage, a 4 mA of current, and a 2000  kGy absorbed dose. The mats were subsequently carbonized under nitrogen atmosphere for 1 h. Mats with tensile strength of 0.8 GPa were obtained when the dose was increased to 2000 kGy. The use of electron beam is a prospective method that has been applied in PAN stabilization. It has not been fully integrated in the processing of carbon fibers from lignin.

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Fig. 7.24  The cleavage behavior of ether bonding between lignin and polyethylene glycol moiety [98]

Fig. 7.25  SEM micrographs of the 2:1 ratio lignin/cellulose acetate blend fibers after being thermo-stabilized at 300 °C for 2 h in air: (a) untreated and (b) 20 min iodinated lignin/cellulose acetate blend fibers [98]

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7.7  Conclusion To successfully convert melt-processed fibers into carbon fibers, the fibers have to be made infusible by stabilization. Inadequate stabilization will lead to destruction of the fibers and poor mechanical properties after carbonization. However, if adequate stabilization method is applied, an infusible lignin is obtained and subsequently carbon fibers with good properties. The design of appropriate stabilization regime depends on a good knowledge of the mechanism of stabilization and the evolution of the fiber structure during stabilization. Although many methods have been applied to stabilize lignin, carbon fibers from lignin still possess poor properties compared to fibers from PAN and pitch. Alternative methods of stabilizing lignin fibers to obtain improved mechanical properties are required. The use of electron-beam and UV irradiation is promising environmental benign methodology that requires extensive research. Treatment methods, such as propargylation, that lead to C–C chain extension and aromatization of lignin prior to stabilization may improve thermal stabilization of lignin.

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60. E. Hartford, 15 (1971) 1709–1715 61. V. Rašković, S. Marinković, Carbon N. Y. 16, 351–357 (1978) 62. K.H.  Gump, D.E.  Stuetz, Production of Stabilized Acrylic Fibers and Films, US Patent 3,923,950, 1975 63. S. Kishimoto, S. Okazaki, Process for Producing Carbon Fiber Shaving Excellent Properties, US Patent 4,024,227, 1977 64. T.H. Ko, L.C. Huang, J. Mater. Sci. 27, 2429–2436 (1992) 65. D.P. Bahl, R.B. Mathur, T.L. Dhami, Mater. Sci. Eng. 73, 105–112 (1985) 66. L. Jie, Z. Wangxi, J. Appl. Polym. Sci. 97, 2047–2053 (2005) 67. D. Jin, Y. Huang, X. Liu, Y. Yu, J. Mater. Sci. 39, 3365–3368 (2004) 68. T.H. Ko, C.H. Lin, J. Mater. Sci. Lett. 7, 628–630 (1988) 69. M.G. Dilkes, I. Cameron, S.J. Quinn, G.S. Kenyon, Lasers Med. Sci. 9, 261–264 (1994) 70. L. Liu, H. Chen, D. Pan, Fibers Polym. 13, 587–592 (2012) 71. S.M. Lee, N.J. Madison, M. Gezovich, Process for the Thermal Stabilization of Acrylec Fbers, US 4,370,141, 1983 72. J.P. Riggs, Acrylic Fiber Conversion Utilizing a Stabilization Treatment Conducted Initially in an Essentially Inert Atmosphere, US 3,961,888, 1976 73. W.C.  Schimpf, Thermally Stabilized Polyacrylonitrile Polymer Fibers for Carbon Fiber Manufacture Thermisch, EP 0 384 299 B1 5, 1997 74. M.V. Mccabe, Pretreatment of PAN Fiber (1987) 75. A. Shiedlin, G. Marom, A. Zilkha, Polymer (Guildf). 26, 447–451 (1985) 76. C.H. Ko, T.H. Yieting, H. Lin, J. Appl. Polym. Sci. 35, 631–640 (1988) 77. J.P. Riggs, Process for the Stabilization of Acrylic Fibers, US Pat. 3656883, 1972 78. J.P.  Riggs, Acrylic Fiber Stabilization Catalyzed by Co(II) and Ce(III) Cations, US Pat. 3656882, 1972 79. J.P.  Riggs, Thermally Stabilized Acrylic Fibers Produced by Sulfation and Heating in an Oxygencontaining Atmosphere, US Pat. 3650668, 1972 80. C.D. Warren, F.L. Paulauskas, F.S. Baker, C.C. Eberle, A. Naskar, in 40th, SAMPE Fall Tech. Conf. Multifunct. Mater. 2008 (Memphis, TN, 2008), p. 36 81. S.A. White, J.E. Spruiell, F.L. Paulauskas, in Int. SAMPE Symp. Exhib. (2006), p. 51 82. H. Mainka, O. Täger, E. Körner, L. Hilfert, S. Busse, F.T. Edelmann, A.S. Herrmann, J. Mater. Res. Technol. 4, 283–296 (2015) 83. P.  Goulis, I.A.K.  Giorgos Konstantopoulos, K.  Mpalias, S.  Anagnou, D.Dragatogiannis, C. Charitidis, J. Carbon Res. 3, 35 (2017) 84. A. Beste, Energy Fuels 28, 7007–7013 (2014) 85. M.J. Cooney, P.F. Britt, A.C. Buchanan, ACS Div. Fuel Chem. Prepr. 42, 89–95 (1997) 86. A. Beste, J. Phys. Chem. A 118, 803–814 (2014) 87. M. Foston, G.A. Nunnery, X. Meng, Q. Sun, F.S. Baker, A. Ragauskas, Carbon N. Y. 52, 65–73 (2013) 88. H. Mainka, L. Hilfert, S. Busse, F. Edelmann, E. Haak, A.S. Herrmann, J. Mater. Res. Technol. 4, 377–391 (2015) 89. P.F.  Britt, A.C.  Buchanan, V.M.  Hitsman, in B.T.-1991 I.C. on C.S.P. ed. by International Energy Agency Coal Research Ltd (Butterworth-Heinemann, 1991), pp. 207–210 90. A. Beste, A.C. Buchanan, J. Org. Chem. 76, 2195–2203 (2011) 91. R. Gao, Y. Li, H. Kim, J.K. Mobley, J. Ralph, ChemSusChem, 11, 2045–2050 (2018) 92. J.F. Kadla, S. Kubo, Macromolecules 36, 7803–7811 (2003) 93. P.F. Britt, A.C. Buchanan III, E.A. Malcolm, J. Org. Chem. 60, 6523–6536 (1995) 94. J.A. Schmidt, C.S. Rye, N. Gurnagul, Polym. Degrad. Stab. 49, 291–297 (1995) 95. G. Gellerstedt, B. Pettersson, Sven. Papperstidn. 83, 314–318 (1980) 96. R.A. Fenner, J.O. Lephardt, J. Agric. Food Chem. 29, 846–849 (1981) 97. J.F. Kadla, H. Chang, ACS Symp. Ser. 785, 108–129 (2001) 98. J. Lin, Preparation and Characterization of Softwood Lignin-Based Carbon Fibers (Hokkaido University, 2013)

Chapter 8

Carbonization, Activation and Graphitization of Lignin-Based Materials Emmanuel Isaac Akpan

8.1  Introduction Lignin derived products such as carbon fibres, activated carbon and graphitized ­carbon materials are converted to their final form from lignin precursors using a series of reactive processes including carbonization, activation and graphitization. Carbonization is the process of converting stabilized lignin fibres into carbon fibres by heat treatment at high temperatures mostly in an inert atmosphere [1] with or without tension. In the process, elements other than carbon are removed in the form of by-products and a graphite-like structure is formed [2]. The by-products could be in the form of water, methane, hydrogen, ethanol, ammonia, carbon dioxide, hydrogen cyanide, carbon monoxide and other gases [3]. Carbonization is the process for which the properties of the fibres are developed [1]. The process is thought to proceed with dehydration, decarboxylation, cross-linking and aromatization reactions [4]. The major aim of carbonization is the efficient removal of all other elements except carbon and graphite from the fibre structure. During carbonization, large weight loss occurs (55–60 wt.%) leading to shrinkage in fibre diameter [2, 5, 6], and as the temperature increases, oxygen-containing functional groups (carbonyl, carboxyl and methoxy groups) decreases leading to a significant increase in aryl and condensed acyl carbon atoms [7]. In the process, an inert atmosphere is necessary in order to avoid oxidation at such high temperatures [8–12]. Inert gases that have been used include nitrogen and argon. Other non-oxidizing media such as HCl, BBr3 and ZnO have been used [13]. The process is dependent on the final heat treatment temperature, heating rate and the carbonizing medium [14]. After carbonization, the fibres are hydrophobic with large changes to the surface structure and porosity of

E. I. Akpan (*) Institute for Composite Materials, Technical University Kaiserslautern, Kaiserslautern, Germany e-mail: [email protected] © Springer Nature Switzerland AG 2019 E. I. Akpan, S. O. Adeosun (eds.), Sustainable Lignin for Carbon Fibers: Principles, Techniques, and Applications, https://doi.org/10.1007/978-3-030-18792-7_8

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the carbon fibres. Carbonization leads to increase in the degree of structural ordering leading to improved mechanical properties as well as electrical and thermal properties of the fibres. Activation is a thermo-chemical process where lignin precursors are transformed into activated carbon that can be used in applications such as supercapacitors, adsorption and energy storage. On the other hand, graphitization is a process of obtaining highly ordered 3D carbon structure from lignin. In this chapter, comprehensive descriptions of these processes are presented to facilitate the development of lignin based structural materials. 

8.2  Carbonization: Process Description The process of carbonization involves heating of the fibres under an inert atmosphere in an oven at a specific heating rate and to a specific temperature. Several studies have been conducted on the carbonization of lignin fibres. In these studies, different methods are used by different researchers probably dependent on the nature of the stabilized fibres. Carbonization can be performed in more than one stage depending on the type of fibres and the desired result. The process entails basically the heating of the fibres with or without tension in a gas flow through situation to a desired temperature at a desired heating rate. Presently, various studies on carbonization use a tube furnace, but recently the use of microwave-assisted plasma process has been suggested [15]. Li et al. [16] used a single step to carbonize soda lignin-based fibres after stabilization under nitrogen atmosphere using a vacuum tube high temperature furnace at 800, 1000, 1200 and 1400 °C using a heating rate of 5 °C/min. The study reported a significant effect of carbonizing temperature on the structure of the carbonized fibres. Esterified lignin samples stabilized at 249 °C at heating rates less than 3 °C/ min were later carbonized at 1000 °C for 30 min in an atmosphere of 96% argon and 4% hydrogen [4]. The carbon fibres were found to contain graphitic domains embedded in an amorphous carbon matrix. Kadla et al. [17] carbonized lignin/polyethylene oxide (PEO) stabilized fibres under nitrogen at a heating rate of 3 °C/min and temperature of 1000 °C using a modified muffle furnace. The study realized tensile strengths in the range 339–448 MPa and modulus in the range 33–59 GPa. They also applied the same schedule for carbonizing lignin/polyethylene terephthalate (PET) and lignin/polypropylene (PP) fibres, realizing tensile strength as high as 703 MPa for lignin/PET (75/25) blend [18]. In another study using pyrolytic lignin, the authors realized a tensile strength of 370  MPa with the same carbonization schedule [19]. Applying the same carbonizing schedule to organoclay-filled pyrolytic lignin, the authors realized tensile strength up to 438 MPa with 1 wt.% Cloisite 30B [20]. A two-step process was utilized for the carbonization of Kraft lignin fibres by Norberg et al. [21]. The process involved heating under nitrogen flow of 15–20 mL/ min at a heating rate of 1 °C/min to 600 °C followed by a heating rate of 3 °C/min to 1000 °C. Results showed that reducing the two-step process to a one-step process

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yielded similar results. UV-irradiated stabilized lignin fibres were carbonized under tension using a high-temperature furnace at 1000 °C [22]. Lin et al. [23, 24] carbonized chemical-stabilized polyethylene glycol (PEG)-lignin fibres in an electronic muffle furnace from room temperature to 1000 °C at a heating rate of 3 °C/min and held for 1  h using nitrogen at a flow rate of 0.15  L/min. Foston et  al. [25] also reported carbonization of Alcell lignin fibres at temperatures ranging from 400 to 1000 °C in a tube furnace under nitrogen with a flow rate of 1 L/min after purging at 10  L/min for 15  min. Other information about the carbonization process was restricted due to export controls. The same method was employed by Baker et al. [26] to carbonize organic purified hardwood lignin realizing a tensile strength of 0.51 GPa. A three-step process was used to carbonize electrospun PVA/lignin fibres in a tube using constant flow of argon gas [27]. In another study, stabilized fibre mat was heated from room temperature to 1200 °C at 5 °C/min heating rate and held for 1 h after which the furnace was cooled to room temperature. The obtained fibres could be used as supercapacitor electrodes without addition of any binder. Ding et al. [28] carbonized lignin/polyacrylonitrile (PAN) at 1000 °C for 30 min at a heating rate of 5  °C/min using argon gas. Maradur et  al. [29] carbonized thermo-­ stabilized lignin copolymer with polyacrylonitrile fibres under a tension of 7.86 Pa at 800 °C with a heating rate of 5 °C/min under nitrogen atmosphere. Robust lithium ion battery anodes have been fabricated from lignin by carbonization of stabilized electrospun Alcel lignin fibre mat in flowing Argon gas at temperatures of 1000, 1500, or 2000 °C and heating rate of 3 °C/min and subsequent cooling at the same rate to room temperature. The structure of the carbonized mat was a perfectly disordered distribution of matrix crystallographic orientations with controllable degrees of graphitization dependent on carbonization temperature. Matrix graphitization level was found to increase with increasing temperature [30]. Choi et al. [6] also fabricated carbon nano-fibres for lithium ion battery anodes by carbonizing electrospun PAN/lignin fibres 1000 °C for 1 h in a nitrogen atmosphere with a heating rate of 10 °C/min. Esterified lignosulphonate and lignosulphonate/ acrylonitrile copolymer fibres were carbonized at 1400 °C using a heating rate of 10 °C/min under nitrogen atmosphere [31]. The copolymer showed tensile strength up to 1.1 GPa. Luo [32, 33] investigated carbonization of Alcel lignin by heating the fibres to 1000 °C at a rate of 5 °C/min in a flow of argon gas. The fibres were held at 1000 °C for 20 min before cooling. This schedule was earlier used to successfully carbonize steam-exploded lignin from birch wood yielding a general purpose carbon fibre with tensile strength of 660 MPa and modulus of 40 GPa [34]. Cho et al. [35] investigated the carbonization of electrospun lignin/nano-crystalline cellulose nano-fibres using a tube furnace under flowing nitrogen at a flow rate of 100 mL/ min. The study investigated different temperatures (800, 900 and 1000 °C) and heating rates (1, 5 and 10 °C/min). Heating rate and holding times were found to significantly affect the structure of the carbon fibres. Schreiber et al. [36] also investigated carbonization of iodine electrospun lignin/ cellulose acetate fibres. Fibres were heated at 2  °C/min to 600, 800, 1000, or 1200 °C and holding for 1 h. Flexible fibre mats with large in-plane graphitic crystals were produced at 1000 °C but at 1200 °C the fibre degraded. Zhang [37] reported

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the carbonization of acetone soluble Kraft lignin at 1000 and 2400 °C under tension with heating rate of 4.5 °C/min in a flowing nitrogen. Fibres carbonized at 2400 °C possess very poor mechanical properties due to the presence of excessive defects whereas those carbonized at 1000 °C showed tensile strength of 1.05 GPa and modulus of 52 GPa. Subsequent report by the author showed that application of tension during the carbonization process resulted in improved properties [38]. Lignin stabilized in HCl atmosphere at 1000 °C and 20 °C/min heating rate using argon gas [39] did not yield interesting properties. Meek et al. [40] carbonized organosolv lignin from switchgrass at 1000 °C holding for 15 min with a heating rate of 3 °C/min under nitrogen atmosphere resulting in a tensile strength of 600 MPa and modulus of 36  GPa. Electrospun Alcell lignin fibres filled with and without platinum (Pt) (Alcell lignin with Pt (ALFPt) and Alcell without Pt (ALF)) have been carbonized at temperatures from 600 to 1000 °C at a heating rate of 10 °C/min [41]. It was shown that increase in carbonization temperature led to a decrease in the oxygen content of the fibres, and an increase in the carbon and surface platinum proportion. This results in fibres with a higher structural order. Lavelle and co-workers [5] produced hollow carbon fibres by carbonization of stabilized Alcell lignin at a heating rate of 10  °C/min from room temperature to 900  °C using flowing air and N2 at 200 mL/min. Dallmeyer et al. [42] synthesized interconnected Kraft lignin-based carbon fibrous materials by carbonizing stabilized electrospun lignin/polyethylene Oxide (PEO) fibres. Carbonization was done by heating the fibres at 20 °C/min to 250 °C, followed by heating from 250 to 600, 800, or 1000 °C at 10 °C/min. The study identified a strong effect of temperature on the carbonized structure of the lignin fibres. Lignin was found to become amorphous carbon at 600 °C and changed to nano-crystalline graphite at temperatures of 800–1000 °C.

8.3  Effect of Process Parameters 8.3.1  Temperature The final temperature during carbonization has a substantial impact on the properties of the resulting carbon fibres. Reactions during carbonization are typically temperature controlled. Kleinhans et al. [1] showed that the strength and the stiffness of lignin carbon fibres increased with increasing temperature of carbonization. The elastic modulus was greatly affected when carbonization was conducted between 600 and 900 °C, but above 900 °C the modulus seems to be independent of carbonization temperature (Fig. 8.1c). Similar behaviour was noted for tensile strength, but the tensile strength reaches a maximum at 1000  °C before levelling out (Fig. 8.1b). Strain to failure also increased but showed maximum increase in the range of 400–600 °C indicating that cross-linking may occur within this range. This trend was also reported for PAN and pitch fibres but with a maximum at 1600 °C for PAN [2, 43–45].

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Fig. 8.1  Effect of carbonization temperature on the mechanical properties of the fibres. Reprinted with permission from John Wiley and Sons [1]

The importance of final carbonization temperature on the final properties of the carbon fibres was first reported by Watt and co-workers in a patent in 1965. It is construed that the reactions during the carbonization is completed at the said temperatures so that there is no further change in the structure. There is also evidence that the degree of preferred orientation can be influenced by the final carbonization temperature [46]. It has been reported that increasing the final carbonization tem-

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perature increases the preferred orientation of pitch carbon fibres [2]. The specific temperatures and temperature profile for carbonization of lignin fibres is highly dependent on the precursor material [38, 47]. Kleinhans et al. [1] also showed that shrinkage of fibres during carbonization is dependent on the carbonization temperature. It has also been reported that at higher carbonization temperatures there is a lower contribution of temperature to structural disorder of the carbon fibres [48]. Schreiber et al. [36] also showed that the final carbonization temperature significantly influenced the graphitization using Raman analysis. This was also supported by the study of Dallmeyer et al. [42]. Evidence of difference in surface properties with difference in carbonization temperature has also been reported [49]. Conducting a statistical analysis, the authors showed that temperature and heating rate interacted to significantly influence the fibre diameter. Increase in temperature also increases the crystalline size of graphite sections in the carbonized fibres [49]. At higher temperatures more organized graphitic structure is formed [41, 42].

8.3.2  Heating Rate Heating rate of carbonization is an important parameter in the production of carbon fibres because it greatly affects the performance of the resulting carbon fibres [50]. The problems of mass transport during carbonization are basically from the evolution of volatile products meaning that an increase in carbonization rate will introduce defects in the fibres which can create a ripple effect on crack propagation. The presence of defects when high heating rate is used was confirmed by Huang [47] for PAN fibres. However, Huang also noted that very low rate causes too much gasification reactions at the early stages of carbonization which is also an undesirable event. Baker and co-workers showed that high heating rates for lignin resulted in fibres with surface defects explaining that relatively slow rates expose the fibres to gasification conditions (H2O, CO2 evolution) (Fig. 8.2). Kleinhans et al. [1] showed that faster heating rates could be preferably used in carbonization of lignin fibres because shorter times are required. They showed that the heating rate affects the evolution of elastic modulus and the final modulus after carbonization (Fig. 8.3). However, they

Fig. 8.2  Effect of heating rate on carbonized fibre structure [51]

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Fig. 8.3  Effect of heating rate on change in length. Reprinted with permission from John Wiley and Sons [1]

Fig. 8.4  Effect of heating rate on length shrinkage. Reprinted with permission from John Wiley and Sons [1]

reported that heating rates in the interval from 1 to 40 °C/min had no effect on the strength development. Using statistical analysis, Pousorkhabi et al. [49] showed that heating rate has a significant effect on shrinkage depending on temperature of carbonization. For Kleinhans et  al. [1], the effects of heating rate on shrinkage are obvious at higher temperatures, but they all follow the same trend (Fig. 8.4). Heating rate was also found to affect the degree of order depending on the temperature of carbonization. At higher temperatures, increasing the heating rate decreases the degree of order but at lower temperatures, increasing the heating rate leads to the formation of more graphitic structure. This suggests that the use of stepwise heating with varying heating rate is worth investigating.

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8.3.3  Effect of Tension Application of tension during carbonization is an important parameter that affects the mechanical properties of the carbon fibres [43]. Fitzer [45] claimed that application of tension during carbonization of PAN fibres can prevent decrease in tensile strength as a result of increase in carbonization temperature. Increase in tension was also reported to increase elastic modulus in PAN fibres [52]. Zhang et al. [38] showed that application of tension during carbonization led to molecular orientation in the fibres which was not present in fibres carbonized without tension. Results from the study showed that carbon fibres with larger extension during carbonization showed better tensile properties, resulting from higher molecular orientation (Fig. 8.5.). Similarly, Kleinhans et al. [1] showed that the use of restrain to shrinkage by application of load has a significant effect on the mechanical properties of the carbon fibres. They posited that increased orientation of the molecules can be obtained by application of adequate restrain and can lead to the improved strength properties. This was also noted by some authors for fibres other than lignin [47, 53]. Following this, Kleinhans et al. [1] designed a method of applying load to restrain fibre shrinkage at temperatures where shrinkage occurs most (Fig.  8.6) enabling them to realize that appropriate increase in load during carbonization produces increase in elastic modulus against mere application of a constant negligible load. Although there have been limited studies on the application of restrain to shrinkage during carbonization, it is a general thought that application of controlled strain is a promising method of improving the strength performance of ligninderived carbon fibres.

Fig. 8.5  Effect of tension on the carbonization. Reprinted with permission from Elsevier [38]

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Fig. 8.6  Effect of tension on shrinkage of fibres. Reprinted with permission from Wiley [1]. Three loading schemes are compared: green: no load applied; blue and red: load ramp applied at 300 and 350 °C, respectively. For the samples loaded during carbonization, the force at the end of the process, when the load was at its maximum, was estimated to be about 100 MPa

8.4  Evolution of Fibre Structure During Carbonization Obviously, the structure of the resulting carbon fibre controls its properties. Apart from the chemistry of the precursor, controlling the development of the structure of the precursor fibre as it is being formed is the most important aspect of carbon fibre production. Structural development of carbon fibres is a complex process that begins from lignin extraction and botanical source (Fig. 8.7). The botanical source of the lignin raw material determines the type of lignin and influences the final structure after extraction. The extraction methods also influence the lignin structure because each process used in lignin extraction alters the original structure in a peculiar manner. Studies have shown that the structure of fibres can be controlled by controlling the melt spinning process [44, 54, 55]. For example, it was realized that the transverse structure of meso-pitch can be changed by disrupting the flow profile prior to extrusion [54]. On the other hand, the use of extrusion capillaries was also shown to have an effect on the fibre structure [44, 55]. The resulting properties are dependent on the type of structure realized. For meso-pitch, radial- and random-folded structures exhibit higher tensile and compressive properties, but linear transverse texture develops lattice-dependent properties such as thermal conductivity [56–58]. During lignin carbonization, a graphitic structure is formed at the end of the carbonization. This carbon structure is called turbostratic graphite structure which is thought to be a phenyl plane structure [59–61] or highly condensed aromatic plane structure. The structure consists of carbon nano-crystallites, amorphous carbon and nano-pores [62]. The nano-crystallites contain a series of turbostratic stacked nano-graphene layers having aliphatic side chains attached to their edges (Fig. 8.8). There is need for a concise study of the structural development of lignin carbon fibres using modern structural equipment such as 3D micro-computed thermography.

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Fig. 8.7  Governing parameters for the development of lignin carbon fibre properties

Although there has been no definite study on the evolution of the turbostratic graphitic structure in lignin-derived carbon fibres, several studies on pyrolysis of lignin show that the structure is formed by a series of decomposition, dehydrogenation, re-arrangement and growth actions. Side chains are decomposed as the temperature increases and aromatic bonded oxygen is removed at somewhat lower temperatures leading to the formation of a disordered structure. This is followed by the re-arrangement of aromatic rings leading to the formation of nano-crystals and amorphous carbon structure. Nano-pores are formed and the graphene layer structure grows as the temperature increases (Fig. 8.9). The final structure is a combina-

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Fig. 8.8  Model image of carbonized Kraft lignin [62]

Fig. 8.9  Evolution of structure of lignin precursors during carbonization

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tion of nano-crystallites (nano-sized graphene structures) cross-linked with amorphous carbon [62]. There has been no study to ascertain which part of the structure is responsible for the mechanical properties. Scientifically, the presence of an amorphous structure may be detrimental to mechanical properties. On the other hand, the cross-links between the graphene structure and the amorphous carbon may be a weak link which is detrimental to mechanical properties. A concise study on this subject may shed light on subsequent processing that can improve the properties of lignin carbon fibres.

8.5  Mechanism of Carbonization After stabilization, lignin fibres contain condensed structures with methoxy and hydroxyl side groups connected to aromatic rings (Fig. 8.10). It is proposed that during carbonization, the methoxy groups fracture from the aromatic rings as the temperature increases via a free radical chain reaction [61, 63] (Fig. 8.11a, b). The hydroxyl groups on another hand also broke out of the aromatic rings (Fig. 8.11c, d). The radicals produced from these reactions combine to facilitate the release of CH4, CO and CO2 (Fig. 8.11e) [63–65]. Further increase in temperature leads to an increase in the ratio of aromatic/aliphatic carbon forming highly condensed aromatic rings (Fig.  8.11f) and finally resulting in a carbon structure called turbostratic structure. Foston et  al. [25] showed that as lignin fibre is carbonized, methoxy groups remaining in the stabilized material are reduced and the amount of carbonyl carbons drops, increasing the proportion of aryl and condensed aryl carbons (Fig. 8.12).

Fig. 8.10  Examples of condensed structures after stabilization

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Fig. 8.11  Proposed chemical mechanism of lignin carbonization. Reprinted with permission from Elsevier [61, 63]

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1000°C 800°C 600°C 400°C Stabilized Lignin Fiber Extruded Lignin Fiber 180 160 140 120 100 80 12

60

C (ppm)

Fig. 8.12  13C Direct polarisation (DP)/MAS spectra of lignin and carbon fibre with a MAS. Reprinted with permission from Elsevier [25]

8.6  Activation Activated carbon materials have been found useful in several applications, including adsorption and energy storage. Carbonized materials are activated by creating a porous structure. The process is similar to the carbonization of fibres with an activation step employed instead of a graphitization step [8, 66]. Activation is usually conducted in the temperature range of 800–1200 °C [67]. Activation process can be divided into chemical and physical activation. When activated carbon is proposed for absorption applications, the quality of the material is usually determined by its absorption capacity [68]. The absorption capacity of activated carbon is governed by the micropores on the surface of the activated material [69–71]. Materials that have been widely used for activated carbon in large scale include wood, coals, lignite, coconut shell, peat, fruit stones etc.

8.6.1  Important Considerations in Activated Carbon Commercial activated carbons are generally made from wood, petroleum resources and mineral ores such as coal and lignite. Activated carbons are characterized by a vast system of pores within the carbon particle. These pores are generally of molecular sizes and constitute the major features of the material. They consist of non-­ polar layers of carbon making up the entire carbon material which describes the presence of porosity and its make-up, over the entire carbon material. The pore surface area is a limiting condition for the proximity of the absorption channels and

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it’s therefore an important aspect of activated carbon materials. The shapes of the pores are curved because of the limited electron density of the carbon material. 8.6.1.1  Classification of Activated Carbon Classifications of activated carbon are usually based on method of preparation, physical characteristics, surface characteristics, such as effective surface area, pore diameter and volume. The following are examples of these classifications: (a) Granulated activated carbon:Activated carbons (ACs): Granulated ACs are granulated or extruded carbon materials that possess relatively large particle sizes and small external surface areas. They are suitable for gases and vapours with high diffusion rate. They are generally used for water treatment to adsorb benzene, toluene and xylene, odour reduction and separation of components of a flow system [72, 73]. (b) Impregnated carbon: Impregnated ACs are porous carbon materials containing several impregnated cations, including Al, Fe, Zn, Mn, Ca and Li. They are used widely in several gas and liquid phase applications such as drinking water treatment and air pollution control [74]. (c) Powdered activated carbon:Activated carbons (ACs): Powdered ACs are usually in the form of powders or fine granules with particle sizes less than 1 mm and pore diameter between 15 and 25 μm. They have small diffusion distance and a large surface area/volume ratio. They are used for colour removal in municipal wastewater treatment [75, 76]. (d) Extruded activated carbons: Extruded ACs are produced by combining powdered AC with a binder fusing them together to form a cylindrical-shaped carbon material of 0.8–130 mm diameter. They possess high mechanical strength, low dust content and pressure drop. They are preferably used for gas phase processes [77]. (e) Polymer-coated activated carbon:Activated carbons (ACs): In polymer-coated AC, the porous carbon material is coated with a biocompatible polymer to enable it to be used in the human body. The coating is done in such a way that it gives the material a smooth coat without blocking the pores of the carbon material. They are used for homoperfusion medical device used to remove toxic substances from human blood [78, 79]. (f) Spherical activated carbon:Activated carbons (ACs): These are spherical balls containing pitch melted in the presence of tetralin or naphthalene. The naphtha solution extracts the naphthalene creating a porous structure. Subsequently, the spheres are oxidized in the presence of an oxidizing agent containing about 30% of oxygen by volume at temperatures between 100 and 400  °C, then heated in the presence of ammonia to introduce nitrogen into the spheres. Activation is further conducted in a stream of CO2 or steam. They possess high mechanical strength, high temperature resistance and are used to absorb SO2 and NO2 [80, 81].

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(g) Bead-activated carbon:Activated carbons (ACs): Bead ACs are produced from petroleum pitch with a size of 0.35–0.80 mm. They are spherical in shape and they possess high mechanical strength, low pressure drop and low dust content, and are suitable for fluidized bed water filtration process [82–84]. 8.6.1.2  Important Concepts Definition of Porous Materials Activated carbon is a porous material with small pore diameters and increased surface area which is later useful in adsorption or chemical reactions. According to IUPAC (international Union of Pure and Applied Chemistry), porous materials can either be macroporous, mesoporous, or microporous. The difference between each type is illustrated in Fig. 8.13. The pore shape on the other hand can be approximated by the three basic pore models: (a) cylindrical (b) ink-bottled and (c) slit-­ shaped pores [85]. Pore size is usually quantified as the pore breadth which is the distance between the two opposite walls of the pore. As illustrated in Fig. 8.13, pore size will only have a precise meaning when the geometrical shape is defined in a definite manner. Material porosity is defined as the ratio of the volume of pores and voids to the volume occupied by the solid [85–88]. Surface area  This is the amount of surface available for adsorption for a given mass of carbon. It is measured using isotherm models and iodine number.

Fig. 8.13  Schematic illustrating pore size distribution of some porous materials [90]

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Iodine number  This is a standard method of determination of the surface area of activated carbon by measuring iodine absorption using defined standard testing conditions. Pore volume  Quantitative analysis of micro-, meso- and total pore volumes of activated carbon is usually performed by numerous analytical methods applied to N2 adsorption data. Some of these methods are presented in the next section for appreciation. Detailed description is not presented as it is beyond the scope of this book. Pressure drop  Pressure drop gives information about the resistance of flow of a gas through activated carbon layer. It is adequately defined by a modified form of the Ergun equation as a function of the form and size of the particles, temperature, pressure and super-critical velocity of the gas. Mechanical strength  Mechanical strength of activated carbon is very important especially in the case of granular ACs. When there is high pressure drop and carbon loses in the system, it is required that the AC has high attrition resistance and mechanical strength. The mechanical strength simulates the resistance to abrasion under practical conditions. Different test methods have been proposed for determination of mechanical strength of ACs. They include ball mill hardness, abrasion strength, impact hardness, ball pan hardness, T-bar hardness, crushing strength and impact hardness (fluidized bed). These test methods use vibration, impact and rotary motion or motion as in fluidized bed. Real and helium density  This is measured as the mass of the solid carbon skeleton excluding the volume of the voids and the pore volume. It is used in some of the empirical isotherm models and it is therefore a very important parameter. Burn off  This refers to the weight difference between the original char and the activated carbon divided by the weight of original char on dry basis. It is very important in the processing of activated carbon as it generally affects the other properties of activated carbon. Measurement of Pore Size Distribution and Internal Surface Area Adsorption isotherms are commonly used to determine the internal surface area of activated carbon. Adsorption isotherm is a plot of the amount of molecules adsorbed by the carbon material against the relative equilibrium pressure of adsorption determined at constant temperature. Several models have been formulated for the determination of internal surface area of AC including Gibbs equation, which is more accurate for multicomponent systems, Langmuir, Freundlich, Brunauer–Emmett– Teller (BET), Dubinin–Radushkevich (DR) and Barrett, Joyner and Halenda (BJH) equations. These models are used to describe pore size distribution and estimate the internal surface area of activated carbon. For characterization of the porosity of

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ACs, Langmuir, BET, DR and BJH are the most prominent models. A brief description of each of these is presented. A full description is beyond the scope of this book. For a complete description of the models, the reader is referred to Marsh et al. [89]. Langmuir Model The Langmuir model describes the phenomena of chemical adsorption involving monolayer surface coverage. It is the most common model used to quantify the amount of adsorbate adsorbed on an adsorbent as a function of partial pressure at a given temperature. The model equation is derived from the equilibrium between particles, empty surface sites and the sites filled by particles: p / po p/ p 1 = a + a o, na bnm nm



(8.1)

where p = equilibrium vapour pressure of molecules in adsorptive (Pa) po = saturation vapour pressure (Pa) na = amount of molecules adsorbed on the adsorbent surface site (mmol/g) nma = amount of adsorbate which forms a monolayer as a measure of monolayer coverage (mmol/g) b = a constant which describes the average energy adsorption Plotting na against p/po in Eq. (8.1) will yield the monolayer coverage nma . The Langmuir isotherm is based on the following assumptions: 1. The adsorbent surface consists of a certain number of active sites and only one molecule may be adsorbed on each site. 2. There is no lateral interaction between the adsorbed molecules meaning that the heat of adsorption is constant and independent of coverage. 3. The adsorbed molecule remains at the site of adsorption until it is desorbed. 4. There is only one monolayer formed during the adsorption. Molecules of adsorbate do not stay on already adsorbed molecules but only on the free surfaces. Dubinin–Radushkevich Model The DR isotherm model is based on the postulation that adsorption in micropores is that of pore filling rather than a layer-by-layer formation of a film on the walls of the pores. It has been used over the years to effectively describe the adsorption mechanism of vapours and gases by microporous solids (Eqs. 8.2 and 8.3) [91]:



2  T p  W = Wo exp  − B  log   . p0     β 

(8.2)

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B can be calculated from

E = 0.001915 1 / B ,

(8.3)

where W = volume of the adsorbate molecules filling micropores excluding the empty spaces between the molecules (ml/g). Wo= total volume of the micropores (ml/g) usually assumed to be equivalent to the value of the capacity of the monolayer coverage. Eo is the characteristic energy of adsorption for a reference vapour, and β is termed the similarity constant. Barrett, Joyner, Halenda Model  The BJH model determines the pore size distributions of carbon materials from experimental isotherms using the Kelvin model. It can only be applied to the mesoporous and microporous materials with small size range. The Kevin equation predicts the pressure at which condensation and evaporation of the absorptive will occur in a cylindrical pore of a given size. The model (Eqs. 8.4 and 8.5) which was proposed in 1951 assumes that pores have a cylindrical shape (pore volume and capillary volume are related to each other as the square of some adequate measure of their cross-sections) and pore radius is equal to the sum of the Kelvin radius and the thickness of the film adsorbed on the pore wall [92]. The model also assumes that the amount of adsorbate in equilibrium with the gas phase is retained by the adsorbent by two mechanisms: (a) physical adsorption on the pore walls and (b) capillary condensation in the inner capillary volume: ln

p 2γ vav = cos θ , p0 rRT

rk ( ¯

)=

(8.4)

4.15 cos θ , p log p0

rp = rk + t ,



(8.5) (8.6)

where rk = Kelvin radius of the pore, t = thickness of the adsorbed film and rp = actual radius of the pore.  BJH underestimates the pore size below 7.5 nm because the model separates the absorbed and the capillary condensate which is not a realistic picture. The fluid wall interactions are neglected. On the other hand, the Kevin equation may not be a viable means for determination of very narrow pores because of the tendency for increase in surface tension with increase in curvature. It is ideal to speak of BJH value rather than pore diameter. This method has been recently modified [93, 94].

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Brunauer–Emmett–Teller Model The BET model is a multimolecular adsorption process used to determine surface area of porous materials and it is the most frequently recognized method of characterizing surface area of porous materials. The method is named after Stephen Brunauer, Paul Hugh Emmett and Edward Teller, the authors of the 1938 paper where the theory originated. The model extends the concept of Langmuir model to multiple molecular layers, allowing measurements of adsorption phenomenon to be correlated to physically significant properties of a porous material such as total surface area, pore-size distribution, micropore analysis and porosity [95]. The following assumptions are applicable to the model: (a) The BET model was developed under the assumption that the surface of the material is completely homogeneous and that adsorption occurs equally across the entire surface with no preferential sorption sites. In addition, each site is either unoccupied or occupied with a single adsorbate molecule (i.e. a maximum one molecule per sorption site). The total adsorption is expressed as a fractional coverage of the surface. (b) The model also assumes that there is limited molecular interaction on the absorption area. Once a molecule is absorbed, it can then act as a single sorption site for another molecule. No other intermolecular interactions are considered including lateral interactions between molecules, molecule to molecule interaction and non-sorption interactions between gas and absorbed phase molecules. (c) The model assumes a local equilibrium in the adsorption system. The uppermost layer is always in equilibrium with the gas/vapour phase molecules. The rate of adsorption is equal to the rate of desorption, with no net change in the number of absorbed molecules at a given vapour pressure. All surface sites have same adsorption energy for adsorbate. (d) The reaction kinetics is the rate limiting factor and not the diffusion constraints. Energy in the form of heat must be provided for the reaction to proceed. The amount of energy required to form the surface layer is equal to the heat of adsorption. Subsequent layers are treated as condensed liquid so that the energy required is equivalent to the heat of condensation. (e) The model also assumes infinite adsorption at saturation. When the saturation pressure (po) is attained, the quantity of adsorbed layers is big enough that the material is presumed to be entirely enclosed by condensed liquid-phase adsorbent. The generalized BET equation is given as Eq. (8.7) [95]: 1



 p   v  o  − 1  p  

=

c −1  p  1 ,  + vm c  p0  vm c

(8.7)

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where v is the adsorbed volume of gas, vm is the adsorbed monolayer volume, p is the equilibrium gas pressure, p0 is the saturation pressure and c is the BET constant. p The equation is arranged such that when it is plotted as a linear function of the p0 y-intercept and the slope can be solved for the constants (Eqs. 8.8 and 8.9): c= vm =

slope +1 intercept

(8.8)

1 . slope + intercept

(8.9)

The specific surface area (S, surface area per unit mass) can then be calculated from Eq. (8.10) [96]: S=

vm NA 22, 400 × m

(8.10)

where N is Avogadro’s number, A is the cross-sectional surface area of a single adsorbed gas molecule, m is the mass of the material used in the measurement and 22,400 is the STP (standard temperature and pressure) volume of one mole of gas. S is expressed in m2/g and can be converted to volume-specific surface area (m3/g) by multiplying by the material density. Other methods that are of relevance include density functional theory and its modifications [97–101], the Broekhoff and de Boer (BdB) theory [102] and its modifications [103–106]. Sorption Isotherms Pore volume and the internal surface area of activated carbon are usually determined using adsorption isotherms. There are six types of isotherms classified by IUPAC (International Union of Pure and Applied Chemistry). The shapes of the isotherms are very important in the characterization of activated carbon and are shown in Fig. 8.14a. The reversible Type I isotherm also referred to as Langmuir isotherm is concave  p to the relative pressure   axis and the apparent absorption approaches a limit p0  ing value as the relative pressure tends to 1. The isotherm reaches a maximum without inflection. The isotherm is characteristic of carbon materials that contain only micropores and having relatively small external surfaces. In this case, the limiting uptake is governed by the accessible micropore volume rather than by the internal surface area [107, 108]. The gradient of the first part (relative pressure (p/p0) from 0 to 0.05) of the isotherm shows the dimensions of the micropores. The steep initial region shows very strong adsorption and a steeper gradient indicate narrower micropores.

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Fig. 8.14 (a) The IUPAC classification of adsorption isotherms showing both the adsorption and desorption pathways. (b) The relationship between the pore shape and the adsorption–desorption isotherm

Type II isotherm is the form of isotherm obtained for macroporous materials. It represents the unrestricted monolayer–multilayer adsorption. The isotherm has a knee in the first section of the curve that indicates the approximate location of the monolayer formation. That is the stage at which the monolayer coverage is complete and the multilayer is about to begin. The absence of hysteresis shows adsorption on and desorption from a non-porous or open surface with multilayer formation, which is assisted predominantly by condensation of the adsorptive molecules and not by volume filling. The reversible Type III isotherm is convex to the reversible pressure axis throughout the entire range and does not exhibit a point of inflexion. It occurs mostly in cases where the adsorbent–adsorbate interaction is weak as compared to the adsorbate–adsorbate interactions. BET model is not applicable to this isotherm. The Type IV isotherm exhibits hysteresis loop, which is associated with capillary condensation taking place in mesopores. The initial part of the isotherm is attributed to monolayer–multilayer adsorption sequence which is the same as that of Type II isotherm. The middle region possesses a low slope illustrating the development of the first few multilayers. The isotherm can be applied to BET and BJH models. Type V represents adsorption isotherms for a low energy, homogeneous solid surface possessing mesopores. It is relatively uncommon and shows extremely weak adsorbate–adsorbent interaction.

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The Type VI isotherm characterizes stepwise multilayer adsorption on an even non-porous surface. The step height represents the monolayer capacity for each adsorbed layer. In simple cases, the step height remains nearly constant for two or three adsorbed layers. Hysteresis in an isotherm is an indication of the presence of mesopores. The pore size distribution can be calculated from the adsorption isotherm, as well as desorption isotherm branches. Figure 8.14b shows the IUPAC classification of adsorption hysteresis. When the hysteresis appears in the multilayer range of the isotherm (e.g. H1 and H4), it is usually associated with the presence of capillary condensation in a mesopore structure. Such loops exhibit varying shapes as can be seen for H1 and H4. In H1, the two arms of the loop are almost vertical to the relative pressure axis and parallel to each other over a wide range of the axis whereas for H4; the two arms of the loop are nearly horizontal to the relative pressure axis and parallel to each other over a wide range. H1 is often associated with porous materials containing welldefined cylindrical-like pore channels or agglomerates of roughly uniform spheres. H2 are characteristic of materials that are often disordered having a distribution of pore size and shape that are not well defined. It indicates the presence of bottleneck constrictions. H3 hysteresis is indicative of materials with slit-shaped pores. The isotherm does not show any limiting adsorption at high P/Po, which is characteristic of non-rigid aggregates of plate-like particles. On the contrary, H4 hysteresis is often associated with narrow slit pores. The presence of dotted lines in the hysteresis reflects low-pressure hysteresis, which may be related to the change in volume of the adsorbent, for example, swelling of non-rigid pores or the irreversible absorption of molecules in pores of similar width as that of the adsorptive molecule.

8.6.2  Physical Activation Physical activation also called thermal activation is conducted by introducing oxidizing gases (normally carbon dioxide, steam, or mixture of both gases) into the carbonized material at elevated temperatures to create a porous structure [67, 109, 110]. The process involves the use of these gasifying agents to extract carbon atoms from the structure of the porous carbon according to reactions (8.11) and (8.12). The process is usually performed at temperatures between 600 and 1200 °C resulting in the removal of more disorganized carbon and the formation of a micropores structure [111]. Physical activation is widely used in commercialized activated carbon due to its lower cost:

C + CO2 = 2CO,



C + H 2 O = CO + H 2 .



(8.11) (8.12)

A few studies have been conducted on the physical activation of lignin. Bagno et al. [112] developed an on-site production of activated carbon from Kraft black liquor using the St. Regis hydropyrolysis Kraft chemical recovery process [113, 114].

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Chars produced from the Regis process were converted to activated carbon using a fluidized bed reactor and CO2, water vapour and nitrogen at temperatures between 750 and 850 °C. The process produced high quality granular carbon with surface area above 1200  m2/g which is higher than those of commercial carbon (550– 000 m2/g). Khezami et al. [115] investigated the activation of Kraft lignin powder using simple pyrolysis under nitrogen. Heating rate was varied between 1 and 10 °C/min but the temperature and time were fixed at 700 °C and 1 h, respectively. The treatment resulted in surface area between 392 and 262 m2/g decreasing as the heating rate decreases. Optimum parameters for activation of hydrolytic lignin have been determined by Baklanova et al. [116]. Plasticized lignin paste was carbonized by heating at a constant rate in a flow of argon up to required temperature (300– 1300 °C) using heating rate in the range of 0.1–4 °C/min for 2 h. Activation was conducted by first heating the sample under a flow of argon to 800 °C at a heating rate of 10  °C/min and then activation in a stream of steam during 5–120  min. Specific surface area of the activated carbon was found to decrease with increasing carbonization temperature and increase with increasing percentage burn off. The highest specific area (825 m2/g) was obtained for materials carbonized at 4 °C/min to 600 °C and then treated by steam at 800 °C and an 82% burn off. Kuznetsov and Schipko [117] reported the physical activation of hydrolytic lignin in a fluidized reactor with AI-Cu-Cr oxide catalyst. Hydrolytic lignin was impregnated with the catalyst and pyrolysed at 780 °C and later activated in steam at 780 °C for 6 h. The highest surface area obtained was 769 m2/g. Hydrolytic lignin was also activated with superheated steam containing 2% oxygen at 800 °C after carbonization. The concentration of surface acidic centres in the activated carbon was found to decrease with increase in carbonization temperature. Rodríguez-Mirasol et al. [118] investigated the activation of Eucalyptus Kraft lignin using CO2. The activation was performed at 850  °C for 20  h producing activated carbon with a surface area of 1853 m2/g and micropore volume of 0.57 cm3/g. Fu et al. [119] also investigated the activation of lignin obtained from black liquor of pulp industry. The study investigated the effect of carbonization, activation temperature and activation time on the quality of the activated carbon. Results show that the carbon material obtained at carbonization temperature of 450 °C and time of 60 min, and at activation temperature of 725 °C and activation time of 40 min had the best properties. Lin et al. [120] realized a surface area of 3100  m2/g by activating softwood lignin isolated with PEG-400 using solvolysis. The electrospun fibres were stabilized in HCl and carbonized at 1000 °C with a heating rate of 3 °C/min held for 1 h under nitrogen and later activated by heating under nitrogen to 900 °C at a heating rate of 10 °C/min and subsequent introduction of a mixture of steam/nitrogen gas.

8.6.3  Chemical Activation Chemical activation involves the use of activation agents such as hydroxides (NaOH and KOH) and acids (phosphoric, nitric and sulphuric acid). In most cases during chemical activation the precursor is first treated with a chemical activation agent

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and then carbonized and activated in a single step. The raw material is then thermally decomposed. The impregnating agents serve as dehydrating and oxidizing (or reducing) agents, so that carbonization and activation take place concurrently. Several studies have shown that chemical activation is considered as a reaction between a solid precursor and the chemical so that concentrations, intimacy of mixing, temperature and activation time are major determinants of the extent of the reaction. Chemical activations are in some cases better than physical activation in terms of yields and amount of micropores [121]. Khezami et  al. [115] conducted a two-step chemical activation of Kraft lignin using KOH as the activation agent. In the first step, the precursor was heated under nitrogen to 300  °C to produce char which was then impregnated with KOH.  The impregnated char was subjected to temperature ramp at 3 °C/min up to 700 °C and held for 1 h. HCl was used to wash the activated carbon to neutrality. The study realized activated carbon with surface area of 514 m2/g. Hayashi et al. [122] investigated chemical activation of Kraft lignin using K2CO3, Na2CO3, KOH, NaOH, ZnCl2 and H3PO4. Results show that different activation agents require different activation parameters to attain their best surface properties. Surface area comparable to those of commercial activated carbon was obtained at the carbonization temperature of 600 °C in both ZnCl2 and H3PO4 activation. Jin et  al. [123] investigated the activation of straw lignin with K2CO3 and KOH. A BET surface area of 1104 m2/g was obtained when the straw lignin was activated at 800 °C for 50 min with K2CO3 and 917 m2/g when the lignin was activated using KOH at 700 °C for 50 min. Impregnation ratio, activation temperature and activation time were found to affect the surface properties of the carbon material. Electrospun lignin impregnated with Fe3O4 particles has been used to produce activated carbon fibres. The electrospun fibres impregnated with the iron ore particles were produced by dispersing in 15% polyvinyl alchohol (PVA) and thermostablized by heating from room temperature to 200  °C at a heating rate of 0.5 °C/min and 2 h holding time. Activation was done under a nitrogen atmosphere at 600 °C for 90 min and a heating rate of 10 °C/min. The surface area of the activated carbon was found to increase from 117 to 1466 m2/g when Fe3O4 was added [124]. Alkali lignin particulates have also been activated using NaOH and KOH [125]. Aqueous mixture of the hydroxide and lignin were dried, simultaneously carbonized and activated by heating at 10 °C/min to 105 °C and held for 30 min to drive off moisture, then to 850, 900 and 950 °C under a flow of nitrogen at different holding times. Surface area of 1100 m2/g was obtained from both hydroxides using impregnation ratio of 1 when activated at 900  °C for 30  min. Li et  al. [126] used KOH activation on Kraft lignin and obtained a BET surface area of 2762 m2/g at 750 °C for 2  h with nitrogen gas. The exceptional property was attributed to the starting materials, pre-treatment methods and the activation methods. Another study on alkali lignin realized BET surface area of 1585 m2/g with K2CO3 [127]. The AC was examined as a double layer capacitor with excellent specific capacitance. Further studies on the activation of Kraft lignin using K2CO3 showed a surface area of 1816 m2/g when activation was done at 900 °C for 2 h. The impregnation ratio K2CO3: Kraft lignin was kept at 3:1 [128]. The study concluded that activation temperature and impregnation ratio have the most effects on the yield and surface area of the activated carbon. Rodríguez Correa et al. [129] investigated the effect of carbonization and

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activation parameters on the quality of activated carbon obtained from lignin and lignin rich biomass. Lignin samples were subjected to carbonization by pyrolysis and hydrothermal carbonization (HTC) and activated by heating to 600 °C at 10 °C/ min using KOH with an impregnation ratio of KOH: Char of 4:1. After the reaction, the furnace was maintained at 600 °C for 2 h. Hydrothermal carbonized AC showed BET surface areas of 2122, 2548, 2183 and 2435 m2/g for lignosulphonate, alkali Kraft, Indulin and organosolv lignin, respectively. Pyrolysis carbonization, on the other hand, yielded AC with BET surface area of 2123, 2031, 2314 and 2538 m2/g for lignosulphonate, alkali Kraft, Indulin and organosolv lignin, respectively. The use of H3PO4 as a chemical activation agent for one-step activation of lignin derived from Brewery spent grain was investigated by Mussatto et  al. [130]. The highest BET surface area was obtained for samples with impregnation ratio of 1:1 activated at 300 °C for 2 h. Using the same activation agent on corn straw lignin yielded a maximum BET surface area of 820 m2/g at 500 °C activation temperature [131].

8.6.4  Effect of Activation Parameters on Activation Quality 8.6.4.1  Activation Temperature Some chemical activation processes are performed as a one-step process (carbonization and activation occur concurrently). However, in some cases activation is done as a two-step process with carbonization performed as a separate step and activation as a subsequent step. It has been shown that when carbonization and activation are used as a single step process, the temperature and time of activation are the major factors determining the quality of the activated carbon [122]. Hayashi et al. [122] noted that the effective activation temperature is dependent on the activation agent used. At temperatures above 600  °C, alkali metal compounds were found to work effectively but ZnCl2 and H3PO4 did not. Jin et al. [123] showed that the effective temperature for carbonization of lignin with K2CO3 is between 500 and 800  °C above or below which the adsorption capacity is low. Sun et  al. [132] reported that the optimum activation temperature for lignin using K2CO3 is 800 °C. Similar results were also reported by Li et al. [128] with optimum temperature of 900  °C.  Hu et  al. [125] also showed that for NaOH and KOH increasing activation temperature leads to broadening of the micropores with an optimum at 900 °C. Li et al. [126] reported increase in surface area with increase in activation temperature up to 750 °C for KOH activated Kraft lignin but decline with further increase. The effect of temperature on the surface quality is evident in Fig. 8.15. 8.6.4.2  Activation Time Activation time has been found to have a profound effect on the surface area and pore volume of ACs. For KOH activated Kraft lignin, Li et al. [126] reported an increase in surface area to a maximum after 2 h but a decrease with further increase

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Fig. 8.15  Effect of temperature on the quality of ACs prepared by one-step mixing activation at 550 °C (a), 650 °C (b), 750 °C (c) and 850 °C (d) [126]

Fig. 8.16  Effect of activation time on surface structure of steam activated PEG-Lignin fibres [120]

in activation time. This was attributed to the enlargement in micropores to mesopores due to over burn off arising from the prolonged activation. Hu and Hsieh [125] showed that holding time has profound effect on both surface area and pore volume of KOH and NaOH activated alkali lignin particulates. Similar to the report by Li et al. [126], surface area increased with increase in holding time to a maximum but decreased with further increase. This behaviour was also noted by Jin et al. [123] for KOH and K2CO3 activated straw lignin. Lin and Zhao [120] show an increase in surface area and total pore volume with increase in activation time for mixed steam and nitrogen activated PEG-400/lignin electrospun fibres. This effect can be pictured in Fig. 8.16. In another study on K2CO3 activation of Kraft lignin by Li et al. [128], it was reported that BET surface area and pore volume increased with increase in activation time up to 2  h but decreased with further increase in activation time.

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Fig. 8.17  Effect of impregnation ratio on the quality of ACs prepared by one-step mixing activation at various KOH to KL mass ratio of 1 (a), 2 (b), 3 (c), 4 (d) and 5 (e) (T = 750 °C; t = 2 h; v = 160 cm3/min). Images are SEM images (5000× and insets 100,000×) of the ACs [126]

8.6.4.3  Impregnation Ratio Impregnation ratio is the ratio of the activation agent to the raw material. It is expressed either as mass or weight ratio. Li et al. [126] showed that increasing the mass ratio of KOH to Kraft lignin (KL) from 1 to 4 increases the surface area to a maximum but further increase led to a decrease in surface area. Figure 8.17 shows the effect of impregnation ratio on the surface morphology of the KL activated carbon. Hu et al. [125] also showed a distinct effect of impregnation ratio on the structural features of NaOH and KOH activated alkali lignin. Increase in impregnation ratio was found to produce a decrease in the size of hysteresis loop indicating that the mesopores become inaccessible or are no longer interconnected to the microporous arrays. Jin et al. [123] showed that with a ratio of K2CO3 and KOH/straw lignin below 2:1, insufficient activation is achieved but higher values resulted in widening of the pores. The optimum ratio was found to be 2:1 for both K2CO3 and KOH. Li et al. [128] showed that the surface area and pore volume of Kraft lignin activated by K2CO3 increases with increase in impregnation ratio from 1 to 3 but decreased as the ratio was increased to 4. The initial increase is attributed to the increase in contact area between KL and the activating agent leading to increase in the release of volatiles and the pore surface area. However, at higher impregnation ratio the network structure collapse so that the material shrinks leading to a decrease in the surface area.

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Fig. 8.18  Effect of impregnation agent on the surface structure of activated alkali lignin. (a) No activation agent; (b) ZnCl2; (c) KOH; (d) K2CO3 [127]

8.6.4.4  Activation Agent Several activation agents have been used for activating lignin into AC. The type of chemical compound used has a profound effect on properties of the AC because of the difference in activation mechanism. Wu et al. [127] investigated the effect of three activation agents (ZnCl2, KOH and K2CO3) on the surface area and pore volume of alkali lignin activated under the same conditions (one-step activation, impregnation ratio 1:1, temperature 700 °C, time 60 min, in nitrogen). Surface area was found to increase in the pattern K2CO3 > KOH > ZnCl2 and total pore volume increase in the pattern KOH  >  K2CO3  >  ZnCl2. The effect can be pictured in Fig.  8.18. Hayashi et  al. [122] showed that above 600  °C alkali metal activation agents work effectively but ZnCl2 and H3PO4 do not. It is also noted that ZnCl2 has catalytic effect on dehydroxylation and dehydration but KOH and K2CO3 act as dehydrating agent and oxidant [133, 134]. Studies have also shown that gas flow rate and pre-treatment conditions have substantial effect on the quality of the resulting AC. Li et al. [126] reported increase in surface area and pore volume of KOH activated KL with increase in nitrogen flow rate to a peak at 160 cm3/min but decrease with further increase. Rodríguez Correa et al. [129] showed that pre-treatment (carbonization) of lignin before activation has a substantial effect on the resulting properties of the activated carbon.

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8.6.5  Mechanism of Activation 8.6.5.1  Pore Development Generally, the development of pores during activation of lignin-based carbon materials is assumed to proceed by initial removal of volatiles from the surface of the material and subsequent reaction of the activation agent to remove atomic carbon from the surface following a series of reduction and oxidation reactions (Fig. 8.19). It is important to note that the degree of activation of produced ACs is largely dependent on the raw material and the carbonization conditions [135–138]. However, it is expected that pore development follows a similar mechanism. For effective description of the development of pore in carbon materials, Shiratori et al. [139] proposed a pore structure model called the micro-domain model. The model pictures AC fibres to compose of micro-domains having a few nanometres of diameter (Fig. 8.20a). The size of which reduces as the degree of activation increases. Conventional description of the development of pores during activation in earlier studies shows that mesopores are formed from micropores through a series of processes, including pore widening, fusing and wall collapsing [140, 141]. However, from the micro-domain model, it is difficult to have a mesopore within the micro-­domain because the micro-domain size is smaller than the size of a mesopore. It is therefore considered that the mesopore structure exist between the micro-domains and the gap between adjacent micro-domains which increase as the activation process progresses. The earlier model assumes the formation of pores to follow a hierarchical structural development from the surface to the core but could not explain the presence of micro-domains observed in a high-resolution scanning transmission electron microscope. The micro-domain theory proposes an activation scheme shown in Fig. 8.20b. At the beginning of activation, the uppermost domains are activated and the core is protected by skin parts such that only relatively small amount of pores are formed in the core. As the activation time and temperature increase, the micropores widen and the size of the micro-domains reduces simultaneously. The mesopores are thus formed because the diameter of the micro-domain is decreased leading to increase in inter-micro-domain distance (the mesopores). Furthermore, as the activation process progresses the inner part of the micro-

Fig. 8.19  Schematic of pore development during activation

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Fig. 8.20  Micro-domain model (a) STM image of micro-domain, (b) Schematic of activattion process in the microdomain, (c) Mild activation condition (d) Severe activation condition [139]

domain is exposed through the inter-micro-domain pores [139]. The difference in pore structure between mild and severe activation conditions is shown in Fig. 8.20c, d. Under mild conditions the uppermost part of the carbon material is activated leading to a narrow region rich in slit-shaped pores. At this point, the core maintains an inactive state so that the surface area is small. However, with increase in severity of activation, the size of the micro-domain in the uppermost region becomes small and the pore width becomes wider and micropores are formed concurrently. Therefore, AC with high severity of activation possesses larger surface area and broad pore sizes. Kim et al. [142] investigated the difference in pore development in physical and chemical activation. The study showed that KOH activation did not have a substantial effect on the size and shape of the micro-domain but produced a sharp increase in the surface area of the carbon material. This indicates that pore development in KOH activation occurs in the intra-micro-domain region. However, steam activation led to a substantial reduction in the micro-domain size indicating that steam activation involves preferential and violent gasification of the outer surface and micro-­ domains without substantial production of micropores. For KOH activation, the gasification takes place uniformly in the core and outer surface of the carbon material whereas in steam activation the gasification is only intense in the outer region and inhomogeneously in the core (Fig. 8.21).

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Fig. 8.21  Difference in pore development between physical and chemical activation. Adapted with permission from Elsevier [142]

8.6.5.2  Mechanism of Activation Activation is somewhat a chemical reaction process and proceeds through a set of reactions, such as dehydration, gasification and reduction reactions. As noted in Sect. 8.6.4, the chemical agent used in the activation has a major influence on the productivity of the process. This difference is attributed to the chemical reactions occurring during the activation process. The mechanism of reactions for some of the activation agents are presented in the following section. It should be noted that some of these reaction mechanisms are applicable only in the stated conditions. For example, the use of any chemical agent in a one-step process will not proceed in the same manner when a two-step process is used especially when the precursor is lignin. The mechanisms presented here are generally for porous solid carbon material and can be applied to lignin in specific cases.

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Steam Activation For activation using steam (physical activation), the principal reactions are shown in Eqs. (8.13–8.18) (Fig. 8.22).

H 2 O = OH ( H ) = ( O )( H 2 ) ,



C + H 2 ( O ) = CO + H 2 ,



CO + H 2 O = CO2 + H 2 ,

(8.15)



C + CO2 = 2CO,

(8.16)



CO + H 2 O = CO2 + H 2 ,

(8.17)



C + 2H 2 = CH 4 .

(8.18)





(8.13) (8.14)

The reaction begins with one ˙OH radical generated by the dissociation of H2O at high temperature followed by the transfer of a hydrogen atom from the hydroxyl to the carbon [143, 144]. At the regeneration temperatures, reactions (8.14) and (8.15) are the most significant. Since the overall reaction is endothermic, it is difficult to maintain isothermal conditions within the reaction zone. The production of hydrogen (Eq. 8.17) during the activation inhibits the rate of reactions (8.14) and (8.15) by occupying the active sites of carbon. Although hydrogen evaporates rapidly at the beginning of the reaction, a steady state is reached in which a considerable amount of it remains on the surface [144]. The amount of hydrogen absorbed seems to be independent of temperature and occurs at a similar rate as steam. These

Fig. 8.22  Mechanism of activation with steam

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actions do not reduce the amount of CO produced during the reaction. Oxygen leaves the surface of the material slower than hydrogen and at the end is almost completely converted into gaseous carbon monoxide but does not retard the steam reaction because it has very little tendency to return to the surface of the carbon material [144]. On the other hand, the steam reaction is controlled by mass transfer because the raw material is a porous solid. Mass transfer during activation is assumed to take place in three ways, including across the stagnant gas film around the particle, pore diffusion within the particle and surface reaction within the pore. It is assumed that when the activation is done at low temperatures, the reaction rate is controlled by chemical reactivity of the solid, but at high and medium temperatures the effectiveness reduces because steam concentration gradient exists within the activated carbon [145]. Although the chemistry of steam activation is well established for other carbon materials, the application of the chemistry in the explanation and development of lignin activated carbon materials is lacking. 8.6.5.3  KOH Activation Activation with KOH is one of the most prevalent activation methods for activated carbon materials. It was invented in 1978 by Wennerberg and O’Grady [146]. KOH produces optimal texture and chemical properties of the resulting activated carbon [147–149]. Research has shown that activation with alkali earth metal compounds is independent of the hydroxides used but depends mostly on the overlapping redox processes [135, 150]. The first step in activation with KOH is a redox reaction which occurs at about 400 °C given by the general reaction (8.19) [135, 150, 151]. At temperatures above 700 °C, other reactions (8.20)–(8.23) occur. Equations (8.23) and (8.24) are important activation reactions that occur alongside the formation of metallic potassium with a negative free energy. They are directly related to the formation of sub-nanometre pores and the penetration of free potassium metal into the lattice of the carbon. The lattice of the carbon is expanded by the intercalated potassium and there is a rapid removal of the intercalate from the carbon matrix (simultaneous intercalation and de-intercalation) [148, 149, 152–154]. The expansion corresponds to an increase in surface area and porosity. In summary, activation with KOH follows the catalytic gasification (Fig. 8.23) of C into CO and CO2 [129], followed by intercalation of the potassium metal between the graphitic layers and finally by rapid removal of the intercalated layers [142] (Fig. 8.24):

6KOH + C = 2K + 3H 2 + 2K 2 CO3 ,

(8.19)



K 2 CO3 + C = 2K 2 O + 2CO,

(8.20)



K 2 CO3 = K 2 O + CO2 ,

(8.21)



2K + CO2 = K 2 O + CO,

(8.22)



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K 2 O + C = 2K + CO,

(8.23)



K 2 O + H 2 = 2K + CO.

(8.24)

8.6.5.4  NaOH Activation Like other activation reactions using hydroxides, activation with NaOH follows a redox process (Eqs. 8.25–8.27) [135]. The sodium hydroxide is reduced by the carbon producing metallic sodium during the activation process. Equation (8.25) starts

Fig. 8.23  Catalytic gasification of carbon by KOH. Adapted with permission from ACS [129]

Mild treatment

K

Severe treatment

Intercalation

Selective Consumption of Graphenes and Local Broadening

Destruction and Rearrangement of Graphenes

Oxidation

O

Fig. 8.24  Intercalation and de-intercalation procedure during KOH activation of carbon. Reprinted with permission from Elsevier [153]

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at temperatures above 600  °C.  The reaction produces carbon dioxide that could react with the remaining NaOH producing sodium carbonate (Eq.  8.26). At temperatures above 700 °C the transformation of NaOH into Na2CO3 does not lead to porosity development [135]. Similar to KOH, NaOH also results in the production of sodium metal that deposits on the carbon causing intercalation at temperatures below 500 °C but at higher temperatures intercalation is absent [151]. Na possesses low intercalation ability in ordered material structure but is very effective in intercalating highly defective materials.

4 NaOH + C = 4 Na + CO2 + H 2 O,

(8.25)



4 NaOH + CO2 = Na 2 CO3 + 2H 2 O,

(8.26)



2 NaOH + CO2 = Na 2 O + H 2 O.

(8.27)

8.6.5.5  ZnCl2 Activation The activation mechanism of ZnCl2 is different from those of alkali earth metal compounds especially at temperatures below 500 °C. Although various successful activations have been done with ZnCl2 as the activation agent, there has been no study to clearly show the mechanism of activation. Some studies allude that ZnCl2 acts as a dehydration agent [155–157] during activation giving rise to charring, aromatization reactions and development of the pore structure [158, 159]. It has also been identified as having a catalytic effect on dehydroxylation [127]. They are also found to inhibit the production of tar which leads to an increase in the yield of AC [122].

8.7  Graphitization In carbonized materials the carbon atoms are in disordered arrangement, limiting the properties of the material. Raising the temperature under controlled conditions allows the atoms to move to suitable positions forming an ordered 3D graphitic structure (Fig.  8.25). The process of changing the highly disordered or defective carbon atom structures to a perfect 3D crystal of graphite is called graphitization. Generally, graphitization is carried out by heating at temperatures higher than 2500– 3500 °C [160, 161]. Graphitization is either done with or without a catalyst. When a catalyst (transition metals or inorganic compounds, such as iron, cobalt and nickel) is used, the process is called catalytic graphitization. The idea of adding a catalyst to achieve graphitization arose from the need to lower the temperature needed to create the required degree of ordering of the carbon atoms [161]. Catalytic graphitization is also used when changes in the physical conditions of the system is used to attain the required degree of graphitization. Studies have shown that both graphitizing and

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Fig. 8.25  3D graphitic structure

non-graphitizing carbon materials can be transformed into highly crystalline materials even at relatively low temperatures when catalytic graphitization is used [162– 164]. A limiting factor in the use of catalytic graphitization is that most of the catalyst remains in the product making it not environmentally sustainable. A few studies on graphitization of lignin showed that it is difficult to develop high graphitic texture in lignin even at extreme high temperatures (3000 °C) [48]. However, the few attempts also reported increase in structural order of the lignin carbon material which culminates in improved properties. Kubo et al. [160] reported catalytic graphitization of acetic acid hardwood lignin using nickel acetate. The study achieved a T-component graphitic structure at 850 °C graphitization temperature with 0.3% of the catalyst. Yan et al. [165] also reported the graphitization of Kraft lignin using Ni, Cu, Fe and Mo transition metals as catalyst at 1000 °C. Results show that graphitization activity of the four metals is in the order of Cu