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2?Main Lignin Types: Origin, Producers, End Users and Characteristics

2?Main Lignin Types: Origin, Producers, End Users and Characteristics

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384



P. C. Rodrigues Pinto et al.



the balance between the two main groups of reactions having an opposite effect: (i)

cleavage reactions and introduction/liberation of hydrophilic groups, leading to

dissolution of lignin fragments and (ii) condensation reactions which increase the

molecular weight of lignin.



12.2.1 Kraft Lignins

12.2.1.1 Origin and Isolation

Kraft pulping is the dominant process for production of pulp for paper [21]. The

delignification is carried out in a strong alkaline solution composed mainly of OHand HS- ions, removing around 90% of the initial lignin [22, 23]. In this process

lignin undergoes reactions involving sulfidolytic cleavage of a- and b-aryl ether

bonds in both phenolic and non-phenolic lignin units. Reactions of conjugate

addition of carbanions to quinone methide intermediates lead to the increase of

condensed structures in lignin [22]. Additionally, some other complex reactions

between lignin and other wood components can also occur [8, 24].

Unmodified kraft lignin renders insoluble in aqueous solution at pH bellow the

pKa values of the phenol groups of lignin, which are in the range 10.0–11.5 [25].

The pH currently used for precipitation and further filtration of lignin is near 7.

Sulfuric acid or carbon dioxide have been used for this propose. Recently, the

isolation of high purity kraft lignin (both softwood and hardwood lignins) has met

a new advancement with Lignoboost process [26–28]. With this process, the

washing at controlled conditions of precipitated lignin was improved leading to a

final product with rather low content of ashes and carbohydrates, and then opening

the perspectives to improve the existent applications or upgrading lignin in new

valuable applications [29, 30]. Significant work concerning the recovery [31, 32]

and fractionation [33, 34] of lignin from kraft liquors has been published in recent

years, many of them via membrane processes.



12.2.1.2 Producers and End Users

Kraft lignin is presently available from MeadWestvaco Corp. (USA) and Borregaard Lignotech (about 10 thousand t/year at Bäckhammar plant), accounting for a

total annual production around 1 million t [5]. This is a rather low production value

considering that a mill with capacity for 500 thousand t of kraft pulp can produce

about 200 thousand t of lignin in black liquor. The potential for lignin production in

the existing pulp and paper industry is more than 50 million t/year [35].

Currently, few kraft pulp mills recover the lignin. The main utilization of black

liquor is energy production in the recovery boiler, allowing the simultaneous

recovery of cooking chemicals to reintroduce in the digesters. This is economically

advantageous, unless the recovery boiler becomes the bottleneck of the process.



12



Lignin as Source of Fine Chemicals



385



In this case, separation of lignin could be one solution to increase the pulp

production capacity. Alternatively, the deviation of a fraction of lignin could

become sustainable by the upgrading lignin for materials and specialty chemicals

with high added-value [36].

Commercial kraft lignins are usually modified to their increase it solubility in

aqueous solutions by means of oxidative sulfonation, carboxylation, and sulfomethylation. The major final application of these lignins is as dispersant: the use

of sulfomethylated kraft lignin was patented in 1954 [37]. MeadWestvaco Corp.

and LignoTech Sweden produce lignosulfonates by sulfonation of kraft lignin, but

the product has a much lower molecular weight than the lignosulfonates produced

from sulfite pulping [38]. Other end uses are asphalt emulsions, lead-acid storage

battery industry and products for cement and concrete industries [39]. This lignin,

after chemical modification, competes with the lignosulfonates coming from the

sulfite pulping industry.



12.2.1.3 Characteristics

In general, hardwood kraft lignin presents lower weight-average molecular weight

(around 1 kDa) than the respective wood lignin (2–3 kDa) [10, 20, 40]. Comparatively to hardwood, softwood kraft lignin Mw, in general, is higher (2–3 kDa)

[23]. Other characteristics of kraft lignin are the higher contents of phenolic

hydroxyl groups and condensed structures than the respective wood lignin [20,

41]. The predominant inter-unit linkage is still the b-O-4, although in lower

absolute amount than in wood lignin. The extension of lignin reactions depends

fundamentally on kraft pulping conditions and wood species [10]. Some information about the composition and chemical structure of lignin recovered from

kraft pulping streams can be found in literature [10, 20, 42] and is presented in

Table 12.2.



12.2.2 Lignosulfonates

12.2.2.1 Origin and Isolation

Lignosulfonate is the resulting lignin from acid sulfite pulping of wood, which was

the dominant process for cellulose production until it was surpassed by the kraft

process in the 1940s. Sulfite pulps account now for less than 10% of the total

chemical pulp production. In this process, sulfites (SO32-), or bisulfites (HSO3-)

are the pulping agents depending on the pH [1, 18]. The counter ion can be

sodium, calcium, potassium, magnesium, or ammonium, which could change the

behavior of the lignin product. Rather than splitting of b-O-4 structures and liberation of hydroxyl phenolic groups, the main reaction during sulfite pulping is the

introduction of sulfonic groups in CA and Cc of C3-alquil lateral chain of ppus



LKWest

Indulina AT

Curan 27 11P

LKBoostS

LSBor

LOrgsB

Alcell

LKEg

LKBoostH

LSEgd

LOrgsMxG

Sarkanda



2.6

16.2

17.0

0.78

16.5

1.2

0.05

7.5

0.71

8.2, 9.0



3.3



2.3

5.6

2.0

2.3

8.8

2.3

0.2

5.0

3.4

7.3, 12.8



5.0



2,350

2,480



2,630



1,745

3,300

1,150

1,065

1,250, 2,400

4,690





1,430

1,490



1,440



1,180

900

900

825



7,060





0.94

0.88

0.83

0.91

0.82

1.56

1.11

1.62

1.69

1.51, 1.59



0.98



0.63

0.60

0.69

0.72

0.55

0.52

0.70

0.85

0.76

0.40

0.49

0.39-0.48



OHph/ppu



b



S:G:H













0:97:3

0:96:4



0:98:2

0:95:5

72:28:0



82:18:0

69:30:1



c



[20]

[20]

[83]

[20]

[20]

[20]

[83, 84]

[20]

[20]

[20, 63]

[67]

[83]



Ref.



b



Values are reported to oven-dry weight of lignin material

Values corrected for ash and sugar content

c

Values reported to the non-condensed moiety of lignin

d

Two distinct fractions of lignosulfonate [63]; the phenolic hydroxyl groups (OHph) per ppu were calculated based on data of the Ref. [63]

LKWest kraft lignin supplied by MeadWestvaco Corp. Indulin AT kraft lignin supplied by MeadWestvaco Corp., Curan 27 11P kraft lignin supplied by

Borregaard Lignoteck, LKBoostS Kraft lignin from softwood isolated by Lignoboost process, supplied by Innventia AB, LSBor Lignotech DP257 (high

molecular fraction of a calcium softwood lignosulfonate supplied by Borregaard Lignotech, Norway, LOrgsB organosolv beech wood lignin supplied by

Fraunhofer, Germany, Alcell Organosolv lignin from mixed hardwoods (maple, birch, and poplar) produced by Repap Enterprises, Inc, LKEg Eucalyptus

globulus kraft lignin obtained from laboratorial kraft pulping (at industrial operating conditions) and isolated by acidification, LKBoostH kraft lignin isolated

by Lignoboost process supplied by Innventia AB, LSEg Magnesium lignosulfonate of Eucalyptus globulus; sulfite liquor provided by Caima, S.A. Portugal

and isolated as described in literature [63], LOrgsMxG organosolv lignin from Miscanthus 9 Giganteus (perennial grass), Sarkanda nonwood lignin

obtained from a soda pulping–precipitation process supplied by Granit SA



a



Non-wood



Hardwoods



Softwoods



Table 12.2 Characteristics of commercial and emergent lignins obtained from different process

a

a

b

Ash,

Sugars

Mw

Mn

OCH3/ppu

Lignin

% wt.

% wt.

(g/mol)

(g/mol)



386

P. C. Rodrigues Pinto et al.



12



Lignin as Source of Fine Chemicals



Fig. 12.3 Representation of

a lignosulfonate ppu



387

HO

HO3S



MeO



HO3S

HO3S



O

OMe



MeO



OMe

O



MeO



OMe

O



(Fig. 12.3), the sulfonation, and the cleavage of a-aryl ether linkages (a-O-4)

[1, 18]. Sulfonic groups increase the hydrophilicity of the lignin fragments, conferring them water solubility, allowing their removal from the polysaccharide

matrix. In phenolic b-aryl ether structures, the initial sulfonation in the a-position

may be followed by sulfidolytic cleavage of the b-aryl ether bond, but the

extension of the reaction is lower than in kraft pulping [18].

Different methods have been developed for the separation of lignosulfonates

from the sulfite liquor (namely from the dissolved carbohydrates) and for separating various molecular weight fractions. The traditional industrial process for

recovery lignosulfonates is the Howard process, where lignosulfonate is precipitated in different stages from sulfite liquor by addition of lime [23]. Fermentation

and yeast growth have been used to remove main sugars, allowing further utilization of lignosulfonates for several proposes. For lignin separation and fractionation, ultrafiltration [43–46], chromatographic processes [47] as adsorption in

chitin [48], extraction with amines [49], liquid membranes [50] and combination

of processes [51, 52] have been attempted.



12.2.2.2 Producers and End Users

The main world producer (and processor) of lignosulfonates is Borregaard

LignoTechn (Norway) [53] with an annual production of 160 thousand t/year,

followed by Tempec (Canada). Nowadays, Borregard is the only producer of

vanillin from lignin; other current similar products are acetovanillone and veratric

acid [54]. A joint venture between Borregaard and a South African pulp company—the major producer of high-grade chemical cellulose from an hardwood,

Eucalyptus [55], was initiated in 1998. Roughly, about 50% of spent liquor volume

is deviated for Lignotech plant close to the pulp mill [56], leading to the recovery

of about 200 thousand ton/year of lignosulfonates.

Recently, Borregaard patented an integrated process for the production of

second generation biofuels and/or sugar-based chemicals and sulfonated lignin

from annual plants, agricultural waste, and wood by a modified sulfite pretreatment

process [57]. Based on this process, Borregaard has invested in pilot plant running

the BALI process, a pretreatment of bagasse based on sulfite delignification [54].

These investments confirm the economic importance of lignosulfonates in the

actual scenario as one of the well-established branches of the integration of

biorefinery concept in the existent mill plants.



388



P. C. Rodrigues Pinto et al.



The lignosulfonates, the most important existent commercial lignins, are widely

used, for example, as plasticizer in cement and concrete additives, emulsifies/

stabilizers, binders/adhesives, phenolic resins, and even as tanning agents [58].

Some products based on lignosulfonates chemical reactions and processing are

polymers, fine chemicals, and flavors. The production of vanillin, a widespread

flavoring agent, is one of the most well-known applications of softwood lignosulfonates, recently claimed as an environmental friendly process compared to the

process of vanillin production from guaiacol (mineral oil derivative) [59].



12.2.2.3 Characteristics

The weight-average molecular weight (Mw) reported for softwood lignosulfonates

are in the range of 10–60 kDa with high polidispersity [23, 60, 61]. These lignins

present, in general, higher molecular weight and have fewer hydroxylphenolic

(OHph) groups than kraft lignin [20, 23] (as depicted in Table 12.2) which is in

accordance with low rate of cleavage of ether linkages of lignin reported for sulfite

pulping. The condensation reactions in sulfite pulping and kraft pulping follow the

same pattern, leading mainly to CA-aryl linkages (diphenylmethane structures)

[18]. The increase of condensed lignin in spent liquor results from these reactions,

and also from the dissolution of condensed lignin moieties present in wood lignin.

The frequency of sulfonation is about 0.4–0.5/ppu [62].

Published studies concern mostly softwood lignosulfonates. However, more

recently, hardwood lignosulfonates have been the subject of increasing research

and interest [55, 61, 63–65] and substantial differences on characteristics, particularly concerning Mw, have been found. Recent results concerning Eucalyptus

globulus lignosulfonates from Caima, Indústria de Celulose S.A., in Portugal

showed an high content of partially sulfonated fragments with rather low molecular weight (around 1 kDa) formed via cleavage of b-O-4 bonds [63]. The Mw of

Eucalyptus globulus lignosulfonates (1–2.4 kDa) is even lower than the already

low value reported for lignosulfonates from other hardwoods (5.7–12 kDa) as

compared to typical Mw of softwood lignosulfonates [60, 61]. Some of lignins

characteristics are summarized in Table 12.2.



12.2.3 Organosolv Lignins

12.2.3.1 Origin and Isolation

Organosolv lignins are those derived from delignification processes using an

organic solvent, frequently ethanol or methanol, and an acid catalyst (mineral or

organic), leading to liberation of lignin from cellulosic fibers. High temperatures

(approximately 195°C) and pressures (about 28 bars) lead to the cleavage of A- and

b-ether linkages of lignin structure [66] and some linkages between lignin and



12



Lignin as Source of Fine Chemicals



389



other cell wall components. As for other pulping processes, hardwoods are more

readily delignified than softwoods. At lab scale, the isolation is usually performed

by acidification of resulting lignin solution and precipitation with water. The solids

are recovered by centrifugation or filtration and dried [67]. At industrial scale, the

liquor lignin is recovered by precipitation with an aqueous process stream, followed by filtration, washing and drying [68]. The yields reported are considerably

high [67, 69, 70].



12.2.3.2 Producers and End Users

Organosolv is a concept dating back to 1970, developed by the General Electric

Company to produce clean biofuels. The process for pulp production was patented

in 1971 [71] and further developed into the AlcellTM process in 1980s. The process

was subsequently commercialized by Repap Enterprises, Inc. to produce bleachable chemical pulps, lignin, furfural, acetic acid, and xylose [72]. The Alcell

processTM was operated in a demonstration plant in Canada during the operation

period had produced 3.5 thousand t of organosolv lignin [68]. In 2001, Lignol

Innovation Corporation, (Canada) acquired the technology and since then has been

developing biorefining technology for the production of fuel-grade ethanol from

polysaccharide fraction of lignocellulosics, lignin, and renewable chemicals [73].

The development of high-value co-products from lignin (and hemicelluloses) is

one of the keys to successful commercialization of the ethanol organosolv process.

The process is claimed to produce a particularly high-quality lignin fraction [70,

74]. A similar approach has been developed in Europe aiming the generation of

added-value products by biotechnological and/or chemical processes and component separation [75, 76]. Also in this case, sulfur-free lignin is generated.

High challenging end uses are expected from organosolv lignins: as source of

aromatic compounds [20, 69], phenolic resins synthesis and thermoplastic applications [75, 77, 78], polyurethanes [77, 79, 80], carbon fibers [81]; the use of

lignosulfonates in polymers was recently reviewed [82].



12.2.3.3 Characteristics

In organosolv pulping, lignin undergoes less transformation as compared to kraft

and sulfite pulping. In general, organosolv lignin has lower content of hydroxyl

groups, higher molecular weight, and lower condensation degree comparatively to

lignin coming from more drastic delignification processes. Additionally, the

absence of organic sulfur (either as tiol groups, as in kraft lignin, or as sulfonic

groups, as in lignosulfonates) is an advantage from the point of view of lignin

valorization. The main characteristics of some organosolv lignins are presented on

Table 12.2.



390



P. C. Rodrigues Pinto et al.



12.2.4 Other Lignins

Other lignin preparations based on high temperature extraction with organic solvent and additives have been developed with the designations of Organocell,

Formacell, Acetosolv, Alcetocell, ASAM, among others. Additional processes are

referred in literature [38]. The common fact is the total absence of sulfur. The

description of each process was recently reviewed by Sridach [85] for non-wood

plants with a complete reference section on sulfur-free delignification processes.



12.3 Lignin as Source of Monomeric Compounds

12.3.1 General Overview

The production of high added-value chemicals from biomass process streams, as

lignin, is crucial in the integrated approach of multiple processes and multiple

major products in the concept biorefinery [86]. Consequently, reaction and separation processes for the production of compounds from biomass, namely lignocellulosic, have been continuously the subject of applied research. Due to its

structure and somewhat complex chemistry, lignin is one of the most fascinating

targets of research in three essential modes [38]: one aiming to breakdown the

tridimensional network for conversion to aromatic (or non-aromatic) chemicals

(thermochemical processes, Figs. 12.4, 12.5); the other one intending to use the

lignin functionalities to integrate it in more complex matrices or construct

renewable polymers; and the third one, dealing with lignin as source of power

(green fuels and syngas [87] Fig. 12.4).



12.3.2 Industrial Vanillin Production

12.3.2.1 Vanillin Market

Vanillin (4-hydroxy-3-methoxybenzaldehyde) is widely used as flavoring and

fragrance ingredient in food, cosmetic and as intermediate for the synthesis of

several second generation fine chemicals (as veratraldehyde, protocatechualdehyde, and respective acids) and pharmaceuticals (as papaverine, levodopa and

cyclovalone) [91, 92].

The global market for vanillin and ethyl vanillin is estimated as high as 16

thousand t/year, with 2 thousand t coming from lignin-based vanillin. Production

of pure natural vanillin is estimated around 40 t/year [93].Vanillin market is

mainly constituted by large multinational holders in the field of flavor and fragrance, chocolate and ice cream production, and synthesis of pharmaceuticals.



12



Lignin as Source of Fine Chemicals

Lignin



391



Process



MeO

H

HOH 2C C O



Products



End-use



Char



Combustion



Electricity



Syngas



HOHC MeO



Heat

HOH2 C



OMe

O



OH



Thermolysis



MeO



OMe

OH



O



Fuel



Aliphatic compounds:

methane, ethane, formic

acid, substituted

ciclohexanes



Chemicals



Pyrolysis



CHOH



MeO



Simple phenols: phenol,

resorcinols, cresols,

guaiacol, seringol



Gasification



CH2 OH

CH



Hydrocraking



OMe

H

HOH2 C C CH2



MeO



OMe



Oxidized lignin

monomers (vanillin,

syringaldehyde, vanillic

acid, acetovanillone)

and phenolic oligomers



Oxidation



O



Fig. 12.4 Thermochemical processes for lignin conversion, main products and end uses [38, 84,

88–90]



OH



OH



OH



O



O

O



OH



OH

O



Mass selective detector

Abundance



OO



O



O



OH

HO



O



O

O



OH



O



OH



O



OH



O



OH



OH

O



7.5e+06



O



O



OH



O



OH



O

O



O



O



O



O



O

O



OH

O



O



O

O



OH



OH

OH



1e+07



OH



O



O



OH



OH

O



O OH



O



OH

O



O

OH



OH



OH



OH

O



O



O



OH



O



OH



OH



OH



O



OH



OH

OH



OH



OH

O



5.0e+06



O



O



OH



2.5e+06



internal standard

1.0e+6

4.00



6.00



8.00



10.00



12.00



14.00



16.00



18.00



20.00



22.00



24.00



26.00



28.00



30.00



32.00



Time (min)



Fig. 12.5 Gas chromatogram with mass selective detector of monomeric products obtained from

catalyzed hydrothermal degradation of organosolv beech lignin [69]. Courtesy of Dr. Detlef

Schmiedl, Fraunhofer Institute for Chemical Technology, Germany



Today there are two commercial types of vanillin: (1) synthetic vanillin, derived

from petrochemical guaiacol and glyoxylic acid or lignosulfonates and (2) vanilla

extract obtained from the cured beans, or pods, of tropical Vanilla orchids [94, 95].

The raw material costs turn the natural vanillin more expensive than the synthetic

counterpart [94]. Hence, synthetic vanillin became competitive and widely used.



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