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Industrial Crops and Products 26 (2007) 116124

Analytical methods for determining functionalgroups in various technical lignins

Nour-Eddine El Mansouri 1, Joan Salvado

Rovira i Virgili University, Department of Chemical Engineering, Avinguda dels Pasos Catalans 26,

43007 Tarragona (Catalunya), Spain

Received 8 March 2006; accepted 7 February 2007

Abstract

In this paper we compare various analytical methods for determining functional groupsin technical lignins of five differentorigins:

kraft, sulfite, soda/anthraquinone, organosolv and ethanol process lignins. These lignins were characterized in terms of methoxyl,

phenolic and aliphatic hydroxyl, carbonyl, carboxyl and sulfonate groups. The analytical methods used were: gas chromatography,

aminolysis, UV-spectroscopy, 1Hand 13C NMR spectroscopy, the oximating method, FTIRspectroscopy, acid number determination,

and non-aqueous and aqueous potentiometry.

The statistical comparison of the various analytical methods for hydroxyl groups (phenolic and aliphatic) shows that the results

obtained are not fully comparable. Aminolysis and non-aqueous potentiometry are assumed to be the most reliable for phenolic

hydroxyl. We observed the same trend for the methods for carbonyl groups and selected the oximating method as reliable for

determining total carbonyl. The results for the methods used for carboxylic groups showed correspondence at a significance level of

0.05. We selected aqueous and non-aqueous titration as being reliable for the lignins studied. We also compare all the commercial

lignins in terms of functional groups.

Finally, by completely characterizing the functional groups of various technical lignins, we have established the most complete

representative expanded formula C9for each lignin under study.

2007 Elsevier B.V. All rights reserved.

Keywords: Characterization; Technical lignins; Analytical methods; Functional groups; Expanded molecular formula C9

1. Introduction

With the exception of cellulose, no other renewable

natural resource is more abundant than lignin. Lignin is a

highly-branched, three dimensional polymer with a wide

variety of functional groups providing active centers

for chemical and biological interactions. In wood, the

Corresponding author. Tel.: +34 977 559 641;

fax: +34 977 558 544.

E-mail addresses: [email protected](N.-E. El Mansouri),

[email protected] (J. Salvado).1 Tel.: +34 977 558 656; fax: +34 977 558 544.

lignin content generally ranges from 19 to 35% (Dence

and Lin, 1992).It is extracted by several pulping tech-

niques and ethanol production process as a by-product

available inexpensively in large quantities. Technical

lignins are divided into two categories (Gosselink et

al., 2004b). The first one comprises sulfur-containing

commercial lignins, including lignosulfonates and kraft

lignins, which are produced in huge quantities. The

second one compriseslignins without sulfurin their com-

position, such as organosolv, soda/anthraquinone lignin

and lignin from the ethanol process production.

The potential of lignins is clearly not valued because

almost all are burned to generate energy and recover

0926-6690/$ see front matter 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.indcrop.2007.02.006
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N.-E. El Mansouri, J. Salvad o / Industrial Crops and Products 26 (2007) 116124 117

chemicals. Only a limited quantity has been used for

applications such as biomaterials, fuels, biocides and

biostabilisers, animal feed, health products and crops

cultivations (Lora and Glasser, 2002).However, indus-

trial applications are only possible if lignins added value

is enhanced, which is only possible if industrial and

scientific research can be intensified to find better appli-cations. Current research faces several problems that

could be avoided. These problems are the low purity, het-

erogeneity, odour, colour of lignin-based products and

the absence of reliable analytical methods (Gosselink

et al., 2004a). Thus, the availability of the analytical

methods for chemical and physical properties adopted

by both suppliers and users can allow any laboratory

to reproduce the results and analyze any various exist-

ing types of lignins. Using these methods lignin can be

properly characterized and its behavior with regard to

several potential uses can be determined (Gosselink etal., 2004c).

Several studies have established new methods or

compared existing methods to characterize lignins

(Gosselink et al., 2004a; Milne et al., 1992; Faix et al.,

1998).Much interest has focused on functional groups

analyses. The main chemical functional groups in lignin

are the hydroxyl, methoxyl, carbonyl and carboxylic

groups. The proportion of these groups depends on the

genetic origin and isolation processes applied. Func-

tional group analysis can be used to determine the lignin

structure. However, the increasing interest in using ana-lytical methods to determine the functional groups is

mainly due to the following reasons: (i) the appearance

of new technical lignin generated from new and more

environmentally friendly cellulose-production methods.

To understand the reaction mechanisms during delig-

nification and to predict and develop different uses for

byproducts of the pulping process, we therefore need to

study their functional properties; (ii) lignin is currently

of interest to the specialist in various fields of science

and industry searching for new practical applications.

Functional group analysis is therefore an indispensable

research method. The only way to achieve these aims isto compare the various analytical methods.

In this paper, we review the main analytical methods

in the field of lignin chemistry, especially for functional

groups analysis, and select 11 analytical methods. We

selected five technical lignins for the structural charac-

terization, focusing on different functional groups, with

these analytical methods. These characterization per-

mit a critical comparison between these methods, and

choose the most adequate in each case, and the compar-

ison between these lignins in term of functional groups.

Finally, we established the most representative formula

C9, which contains the important information about the

structure of each lignin.

2. Materials and methods

2.1. Raw materials

Kraft lignin (KL) and lignosulfonate (LS) derived

from softwood were purchased from Ligno-Tech Iber-

ica. Soda/anthraquinone lignin (SAL) from a mixture of

long fiber plants, was supplied by CELESA Celulosa de

Levante S.A. of Tortosa, Catalonia, Spain. Organosolv

lignin (ORS) obtained fromMiscanthus sinensis, was of

the formasolv lignin type, which was supplied by the

University of Santiago de Compostela (Galicia-Spain).

Ethanol process lignin (EPL) was supplied by CIEMAT

(Centro de Investigacion Energeticas, Medioambientales

y Tecnologicas) of Madrid, Spain, from Populus woodpretreated by steam explosion and the simultaneous sac-

charification and fermentation process (SSF).

These lignins were purified and analyzed for chem-

ical composition in a previous study (El Mansouri and

Salvado, 2006).The characteristics of these lignins are:

total lignin content of over 94% (except lignosulfonate)

and a sugar content of close to 2% (except ethanol

process lignin). All lignins were air-dried at room tem-

perature to equilibrium moisture content and stored in

plastic bottles for characterization. The technical lignins

were analyzed in this study by the following methods.

2.2. Analytical methods

2.2.1. Elemental analysis

Carbon, hydrogen, sulfur and nitrogen contents were

determined using a Perkin Elmer 640-C Analyzer. After

correction for ash content, the percentage of oxygen was

calculated by difference.

2.2.2. FTIR spectroscopy for unacetylated lignins

The FTIR spectra of the unacetylated lignin samples

embedded in KBr disk were obtained with a BRUKERspectrometer using a resolution of 4 cm1 and 32 co

addition scans in a frequency range of 4004600 cm1.

The spectra were analyzed by Nicolet software to com-

pare the absorbance corresponding to each functional

group. The absorption bands were assigned as suggested

byFaix (1992).

2.2.3. Methoxyl groups

Methoxyl group was determined as suggested by

Vazquez et al. (1997). The lignin (0.15 g) was treated

with refluxing concentrated sulfuric acid (10 ml) for

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118 N.-E. El Mansouri, J. Salvado / Industrial Crops and Products 26 (2007) 116124

Fig. 1. Types of phenolic structure determined in different lignin samples.

10 min. The reaction mixture was cooled, 70 ml of dis-

tilled water was added, and the methanol produced in the

reaction was distilled off under vacuum and quantified

by gas chromatography.

2.2.4. Acetylation

A weighted amount of each lignin except lignosul-fonate was acetylated for 48 h with a mixture of purified

pyridine-acetic anhydride (1:1, v/v). Methanol was used

to quench the remaining acetic anhydride. Finally, a flow

of nitrogen was applied to evaporate the solvents and the

samples were dried under vacuum (Chum et al., 1985).

2.2.5. Hydroxyl groups: aliphatic and phenolic

Phenolic hydroxyl groups were determined by

three wet chemical methods (aminolysis, ultraviolet-

spectroscopy and non-aqueous potentiometry) and two

spectroscopy methods (

1

H NMR and

13

C NMR). Thetwo spectroscopy methods enabled aliphatic hydroxyl

quantification. These methods are described below.

2.2.5.1. Aminolysis. The procedure described by Lai

was used to determine free phenolic hydroxyl groups

in lignin (Lai, 1992). The acetylated lignin, dis-

solved in 1.0ml of dioxane containing 5 mg of

1-methylnaphtalene, was treated with 1.0 ml of dioxane-

pyrrolidine (1:1, v/v) solution, which initiated the

aminolysis reaction. After the addition of pyrrolidine,

samples were taken from the reaction mixture at different

times (total reaction time was approximately 120 min)and analyzed by gas chromatography. The amount of 1-

acetylpyrrolidine formed (equivalent to the amount of

hydroxyl groups) was recorded as a function of time.

The content of phenolic hydroxyl groups was calculated

by extrapolation of the curve at zero time.

2.2.5.2. Phenolic hydroxyl groups by ultraviolet-

spectroscopy ( method). The content of various

phenolic units in lignin samples was determined by

UV spectroscopy as described by Zakis (1994). This

method is based on the difference in absorption at 300

and 360 nm between phenolic units in neutral and alka-

line solutions. The content of ionizing phenol hydroxyl

groups can be quantitatively evaluated by comparing the

values of substances studied at certain wavelengths

to the values ofof the respective model compounds

(I, II, III, IV types) (Fig. 1).

2.2.5.3. Proton nuclear magnetic resonance spec-

troscopy (1H NMR). We used proton nuclear magnetic

resonance to analyze all acetylated technical lignins

under study.1H NMR spectra of 10 mg acetylated lignin

samples dissolved in 0.5 ml of CDCl3 were recorded

on a VARIAN GEMINI 300 Hz apparatus using tetram-

ethylsilane as internal standard under the same condition

as those described byLundquist (1992).Proton signals

were integrated from the baseline and referred to the

integrated signal of the methoxyl protons for proton

quantification of aliphatic and phenolic hydroxyl.

2.2.5.4. Carbon nuclear magnetic resonance spec-

troscopy 13C NMR. 13C nuclear magnetic resonance

is the most suitable method for determining benzylic

alcohol groups in lignins. For all acetylated lignins, the13C NMR spectra were recorded in acetone-d6 under

the same conditions as those described by Robert and

Brunow (1984). The quantitative estimation of differ-

ent hydroxyl groups (located at 170.8 and 170 ppm

of primary and secondary aliphatic hydroxyl groups,

respectively, and 168.9 ppm for the phenolic hydroxyl

group) were achieved by expanding ten times, beforeintegration, the signal areas corresponding to each func-

tional group and combining these results with those of

elemental analysis and methoxyl groups.

2.2.6. Carbonyl groups

Carbonyl groups for all lignins were determined by

two wet chemical methods: the Modified Oximating

method and differential UV-spectroscopy. The Modified

Oximating method was described byFaix et al. (1998)

that present a correction technique, which is necessary

for lignins containing carboxyl groups. Differential UV-

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Fig. 2. Types of carbonyl structures determined in various lignins.

spectroscopy was developed by Alder and Marton in

1966 and reported by Zakis (1994). It involves differ-ential absorption measurements that take place when

carbonyl groups are reduced at the benzylic alcohol

corresponding with sodium borohydride. This method

determines some carbonyl lignin structures such as alde-

hydes and ketones structures described inFig. 2.

2.2.7. Carboxyl groups

We analyzed carboxyl groups using three methods:

acid number determination and aqueous and non-

aqueous titration methods. These methods are described

below.

2.2.7.1. Acid number determination. Carboxylic

groups were determined as described by Gosselink et

al. (2004a).The pH of 100 ml of 95% ethanol in water

was adjusted to 9.0 using 0.1 mol/l sodium hydroxide in

water. After adding 1 g of dried lignin, the mixture was

stirred for 4 h and subsequently titrated back to pH 9.0

with 0.1 mol/l sodium hydroxide solution.

2.2.7.2. Aqueous titration method. This method was

used by Gosselink et al. (2004a). A weight of lignin

sample (1 g) was suspended in 100 ml of alkaline aque-ous solution. After stirring for 3 h, the pH was adjusted

to 12 with sodium hydroxide. After stirring again, the

solution was potentiometrically titrated with 0.1 mol/l

aqueous hydrochloride acid.

2.2.7.3. Non-aqueous potentiometry method. This pro-

cedure, reported by Dence, involves a non-aqueous

potentiometric titration of lignin with tetra-n-butyl-

ammonium hydroxide in the presence of an internal

standard, which isp-hydroxybenzoic acid (Dence, 1992;

Gosselink et al., 2004a). The advantage of this method is

that it determines not only the carboxyl groups in lignin

butit concurrentlydetermines the weakly acidic phenolichydroxyl groups. When combined with an ion-exchange

treatment, the aforementioned titrimetric procedure was

also used to determine the strongly acidic groups (sul-

fonates groups) in lignosulfonate.

2.2.8. Sulfonate groups

Sulfonate groups were determined by non-aqueous

potentiometry, as described above (Dence, 1992).

2.2.9. Expanded C9 formulae

The expanded formulae C9 contain complete infor-mation about the lignin structure. They are obtained

by combining the results from elementary analysis and

functional groups analysis.

2.2.10. Statistical analysis

We compared the methods for determining the func-

tional groups in lignins by applying paired two-sided

t-tests at a 95% confidence level for mean values and

combining the two methods. The results are presented

as averages and their standard deviation.

3. Results and discussions

3.1. Structural characterization with FTIR

spectroscopy

The IR absorption spectra of the five technical lignins

studied were recorded in the 4004000 cm1 region (see

Fig. 3).These spectra show that there were clear differ-

ences between these lignins. The band at 3400 cm1,

which is attributed to OH groups in lignins, had a lower

absorption intensity for KL and SAL than for ORS, EPL

andLS. This is attributed to thehigh oxidation anddegra-

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Fig. 3. FTIR spectra of unacetylated lignin samples.

dation power of soda during the two pulping processes.

The 30002800 cm1 region of the C H stretch in the

methyl and methylene groups was present in different

quantities. These bands, which were mainly attributed

to methoxyl groups, were substantially higher for SAL,

EPL and ORS and presented relatively lower absorbance

bands for KL and LS. The carbonyl stretching vibra-

tion at 1720 cm1 appeared in the IR spectra of KL,SAL and ORS but was absent in the spectra of EPL and

LS. At 1600 and 1500 cm1, aromatic skeletal vibration

bands were observed for all lignins. Between 1300 and

1000 cm1, the bands and peak ratios were very differ-

ent due to various vibrations modes such as C O, C H

and C O. The distinct band appearing at 620 cm1 in

the spectra of LS was assigned to the sulphonic groups

(S O stretching vibration) formed from the reaction of

sodium sulphite with the secondary OH of the aliphatic

side chain of lignin. FTIR spectroscopy showed that the

lignins studied were clearly structurally different. Thestructural differences between other lignins analyzed

by FTIR spectroscopy were reported by Carmen et al.

(2004). This will be analyzed in further detail in this

study.

3.2. Hydroxyl groups: phenolic and aliphatic

hydroxyl

The phenolic hydroxyl groups of all lignin sampleswere determined by several methods: aminolysis, UV-

spectroscopy, 1H NMR, 13C NMR and non-aqueous

potentiometric titration (Table 1). Aliphatic hydroxyl

groups were determined by 1H NMR and 13C NMR

spectroscopy (Table 2). The amounts of the various

phenolic structures present in lignin as determined by

UV-spectroscopy are shown inTable 3.

Comparison of the methods used for phenolic

hydroxyl quantification by statistical analysis (paired

t-test) as listed in Table 4 shows that aminolysis/13C

NMR, UV-spectroscopy/13

C NMR and non-aqueous-potentiometry/1H NMR show a poor correspondence

Table 1

Phenolic hydroxyl content in various technical lignins determined by different methods (%, w/w)

Aminolysis Non-aqueous potentiometry 1H NMRa 13C NMR UV-spectroscopya

KL 4.60 (0.04) 4.54 (0.15) 4.10 4.99 4.50 (0.32)

SAL 4.90 (0.07) 5.10 (0.23) 4.50 5.31 4.40 (0.30)

ORS 2.80 (0.10) 3.56 (0.12) 3.33 3.23 2.66 (0.32)

EPL 2.55 (0.08) 2.92 (0.18) 2.65 2.70 2.30 (0.36)

LS NA 2.55 (0.31) NA NA 2.00 (0.16)

NA: Not acetylated; ( ) standard deviation.a

Data fromEl Mansouri and Salvado (2006).

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Table 2

Aliphatic hydroxyl content of various technical lignins determined by

NMR spectroscopy methods (%, w/w)

1H NMR 13C NMR

KL 10.09 9.80

SAL 3.10 2.45

ORS 3.50 3.20EPL 4.73 4.55

at a significance level of 0.05. In contrast, the other

paired analyses show a correspondence at a significance

level of 0.05. This variability in results is attributed to

an incomplete acetylation in the case of methods based

on lignin acetylation, such as 1H NMR, 13C NMR and

aminolysis. This incomplete acetylation was confirmed

by Gosselink et al. for sulphur-free lignin and model

compounds that may be attributed to steric hindranceby the methoxyl groups present in lignins (Gosselink et

al., 2004a).Moreover, NMR-spectroscopy is character-

ized by an overlapping signal that lowers the accuracy

of these techniques. Also, UV-spectroscopy determines

only some phenolic structures, so the phenolic groups

might be underestimated. For non-aqueous potentiome-

try it is difficult to observe the inflection point with some

lignins.

From theresults obtained,we cansee that the methods

used are not fully comparable. The standard deviations

for each lignin analysis lead us to assume that aminolysisand non-aqueous potentiometry are the most reliable for

the determination of phenolic hydroxyl. These results

are in agreement with those ofMilne et al. (1992)and

Gosselink et al. (2004a).The two selected methods pro-

vide quantitative data on the frequency with which the

phenolic OH occurs in lignin, but they do not reveal the

structural environment in which it occurs. This informa-

tion about the lignin structure can be obtained by the

spectral techniques. The UV spectroscopy is an easy

method to quickly estimate some phenolic hydroxyl

structures.

The 1H NMR showed a poor correspondence inthe results with 13C NMR at a significance level of

0.05 for aliphatic hydroxyl determination (p-value is

0.04 < 0.05). This is attributed to the overlapping signals

that can easily introduce significance errors and to the

well-known incomplete acetylation of lignin with NMR

spectroscopy. A similar discrepancy was observed by

Gosselinket al. (2004a) whenestimating the ratio of phe-

nolic/aliphatic hydroxyl by methods such as 1H NMR

and 13C NMR spectroscopy. 1H NMRand 13C NMR are

therefore not comparable for aliphatic hydroxyl quan-

tification.These results show the phenolic hydroxyl contents

were highest for kraft and soda/anthraquinone lignins,

high for organosolv lignin and relatively low for ethanol

process lignin and lignosulfonate.The aliphatic hydroxyl

content was highest for the kraft lignin and relatively low

for the other samples.

3.3. Carbonyl groups

Table 5 shows the quantitative determination ofcarbonyl groups by differential UV-spectroscopy and

modified oximating method with and without the cor-

rection technique.Table 6shows the amount of different

carbonyl structures as determined by differential UV-

spectroscopy.

Table 3

Relative abundance of different phenolic structures in lignins determined by UV-spectroscopy (%, w/w)

KL LS SAL ORS EPL

Non-conjugated phenolic

structures (I + III)

[OH]I 2.63 1.34 2.74 0.89 1.43

[OH]III 0.49 0.48 0.57 0.44 0.68Conjugated phenolic

structures (II + IV)

[OH]II 1.30 0.14 1.10 1.31 0.14

[OH]IV 0.08 0.03 0.02 0.02 0.05

Table 4

Comparison of methods for the determination of phenolic hydroxyl content by paired t-test (two-sidedp-values)

Method Aminolysis UV-spectroscopy 1H NMR 13C NMR Non-aqueous potentiometry

Aminolysis 0.07 0.79 0.01 0.16

UV-spectroscopy 0.48 0.013 0.051H NMR 0.20 0.0213C NMR 0.88

Non-aqueous potentiometry

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Table 5

Content of carbonyl groupsin samples from various analytical methods

(%, w/w)

Lignin types Oximating method UV-spectroscopy

Without correction With correction

KL 3.13 (0.05) 2.91 (0.05) 2.35 (0.32)SAL 2.62 (0.10) 2.13 (0.10) 1.94 (0.25)

LS 5.30 (0.10) 4.50 (0.10) 4.70 (0.27)

ORS 4.05 (0.10) 3.94 (0.09) 2.90 (0.19)

EPL 6.48 (0.11) 5.73 (0.11) 5.20 (0.23)

( ) Standard deviation.

Table 7 indicate that the correspondence between

the results for the carbonyl groups determined by

oximating method without correction and for the

oximating method with correction and differential UV-spectroscopy method were poor at a significance level

of 0.05. These differences in the results are attributed

to a correction method introduced in order to sub-

tract CO from carboxylic origin in the oximating

method and to the existence of other forms of carbonyl

groups underestimated by differential UV-spectroscopy

for example quinone forms, which exist in highly oxi-

dized lignins such as those in this study. The results

from UV-spectroscopy and the oximating method with

the correction technique corresponded at a 0.05 signifi-

cance level. These results show that the methods are not

completely comparable. From the standard deviationsof each lignin analysis, we concluded that the oximating

method with the correction technique is reliable for total

carbonyl quantification, which was confirmed byFaix et

al. (1998).Differential UV-spectroscopy enables some

carbonyls, such as aldehydes and ketones structures, to

be determined.

Ethanol process lignin and lignosulfonate showed

higher contents of carbonyl groups than other lignins.

Values for kraft and organosolv lignins were within

the range found by Faix et al. (1998) when analyz-

ing alcell-organosolv from yellow poplar (4.40%) and

kraft indultin AT (3.32%). The higher carbonyl con-

tents of technical lignins than of ball milled enzymelignin 2.2% are plausible because technical lignins

underwent oxidation during the treatment process (Faix

et al., 1998).

3.4. Carboxyl groups

Table 8lists the carboxyl content for lignins deter-

mined by acid number and aqueous and non-aqueous

titration methods, as described above. Statistical com-

parison of these methods shows that there were

no significant differences at a 95% confidence level

(Table 9).However, the carboxylic contents of lignins

were different for the three titration methods. These

differences were due to the solubility of the lignins in

the selected solvents. The same trend was observed by

Gosselink when analyzing soda lignins with the same

methods (Gosselink et al., 2004a).The accessibility of

the carboxylic groups is therefore higher when DMF is

used as solvent for non-aqueous titration and when the

agitation time is longer in the alkaline medium for aque-

oustitration. From the standarddeviations foreach lignin

analysis, we concluded that non-aqueous titration andaqueous titration, in this order, provide reliable results

for the determination of carboxyl groups. The acid num-

ber method cannot be used for lignosulfonate because

this lignin is insoluble in 95% ethanol. With this method

the solubility of the other lignins is also poor, which is

reflected in the low values for the carboxylic groups.

Table 6

Relative abundance of some aldehydes and ketones types in samples obtained by differential UV-spectroscopy (%, w/w)

KL LS SAL ORS EPL

Coniferyl aldehydestructures (I + II)

[CO]I 0.38 0.98 0.31 1.03 1.50[CO]II 1.09 1.80 0.56 1.14 1.53

Ketones structures

(III+IV)

[CO]III 0.51 0.90 0.73 0.66 1.28

[CO]IV 0.37 1.02 0.34 0.07 0.89

Table 7

Comparison of methods for the determination of carbonyl content by paired t-test (two-sidedp-values)

Method Oximating without correction Oximating with correction UV-spectroscopy

Oximating without correction 0.03 0.01

Oximating with correction 0.11

UV-spectroscopy

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Table 8

Contents of carboxylic and sulfonate obtained by the analytical methods (%, w/w)

Lignin type Non-aqueous potent Acid number Aqueous titration Sulfonate

KL 7.06 (0.15) 5.97 (0.50) 7.10 (0.31)

SAL 6.91 (0.22) 5.42 (0.56) 6.90 (0.37)

ORS 3.15 (0.20) 2.79 (0.60) 2.86 (0.45)

EPL 2.02 (0.27) 1.82 (0.69) 2.17 (0.41) LS 4.63 (0.21) a 4.30 (0.42) 12.23 (0.39)

( ) Standard deviation.a Sample not completely dissolved.

Table 9

Comparison of methods for the determination of carboxyl content by

pairedt-test (two-sidedp-values)

Methods Non-aqueous

potent

Acid

number

Aqueous

titration

Non-aqueous potent 0.08 0.40

Acid number 0.10Aqueous titration

The contents of carboxylic groups for kraft and

soda/anthraquinone were higher than for the other

lignins. This indicates that the two lignins were highly

degraded during the kraft and soda/anthraquinone pulp-

ing. Ethanol process lignin seemed to be less degraded.

3.5. Sulfonate groups

Table 8shows the sulfonate group contents of lig-nosulfonate. These groups ensure ready water solubility

in the presence of a suitable counter ion (Na, Ca, Mg,

NH4, etc.). These results are in agreement with those

in the literature. The results from non-aqueous potentio-

metric titration and elementary analysis show that not

all the sulfur content in lignosulfonate is in the form of

sulfonate.

3.6. Expanded molecular formulae

Table 10lists the expanded molecular formulae for

the various technical lignins under study. The expanded

Table 11

Elemental composition of different lignins studied (El Mansouri and

Salvado, 2006)

%C %H %N %S %O

KL 65.00 5.41 0.05 1.25 28.24

SAL 65.00 6.12 0.17 0.00 28.64

LS 44.84 5.15 0.02 5.85 44.14ORS 63.51 5.55 0.02 0.00 30.92

EPL 58.34 6.01 1.26 0.00 34.40

C9 formulae were obtained from elemental analysis

(Table 11)and functional groups analysis, which pro-

vides a number for each functional group per expanded

formula C9. Each expanded formula C9summarizes all

the information about the structure of these technical

lignins.

4. Conclusions

We have conducted a comparative study of the dif-

ferent analytical methods for the functional groups in

various technical lignins. Statistical comparison shows

that the methods used for phenolic OH are not fully

equivalent. Each method has some disadvantage or

other: incomplete acetylation with techniques based on

acetylation, an overlapping signal in nuclear magnetic

resonance, the difficulty of showing the inflection point

in non-aqueous titration, and the underestimation of phe-

nolic hydroxyl content with UV-spectroscopy. Despite

Table 10

Expanded molecular formulae for the technical lignins studied

Lignins Expanded formulae C9

KL C9H6,010O0,269N0,006S0,065(OCH3)0,597(OHAr)0,425(OH

Al)1,046(OCO)0,183(OOHCOOH)0,277SAL C9H6,825O0,560N0,020S0,065(OCH3)1,166(OH

Ar)0,493(OHAl)0,338(OCO)0,141(OOHCOOH)0,286

LS C9H10,360O2,880N0,003S0,070(OCH3)0,730(OHAr)0,260(OCO)0,354(OOHCOOH)0,227(HSO3)0,330

ORS C9H6,705O1,205N0,002(OCH3)0,971(OHAr)0,396(OH

Al)0,380(OCO)0,260(OOHCOOH)0,130EPL C9H9,036O2,270N0,166(OCH3)0,646(OH

Ar)0,289(OHAl)0,515(OCO)0,378(OOHCOOH)0,083

OCH3: Methoxyl groups; OHAr: aromatic phenolic hydroxyl; OHAl: aliphatic phenolic hydroxyl; OCO: carbonyl groups; OOHCOOH: carboxyl

groups; HSO3: sulfonate groups.

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these contradictory results, we selected aminolysis and

non-aqueous titration with TnBAH as reliable methods.

Non-aqueous titration with TnBAH can be used with all

technical lignins and can determine not only phenolic

OH but also carboxylic and sulfonate groups. Also, the

methods used for the aliphatic hydroxyl groups are not

comparable because the spectral technique is based onacetylation, which is incomplete for lignin, and because

an overlapping signal affects reliability. The methods for

quantifying carbonyl are also not comparable because

one determines total carbonyl content and the other deter-

mines only some carbonyl structures. The oximating

method is reliable for determining total carbonyl groups.

The methods used to determine carboxylic groups are

comparable and we selected non-aqueous titration and

aqueous titration as reliable methods for the technical

lignins in this study.

By analyzing the various lignin functional groups,we determined their structural characteristics. Several

analytical methods showed that the highest content of

phenolic hydroxyl were in kraft and soda/anthraquinone

lignins and that there was a high content in organosolv

lignin but a relatively low content in ethanol process

lignin and lignosulfonate. Kraft lignin had the highest

content of aliphatic hydroxyl: the other lignin samples

had low contents. Lignosulfonate and ethanol process

lignin had the highest contents of carbonyl groups than

the other lignins. Carboxyl groups analysis also showed

that Kraft lignin and soda/anthraquinone were morehighly degraded than the other lignins under study. In

conclusion, the technical lignins analyzed in this study

have different functional group contents.

By combining elementary analysis and functional

groups analysis, we can represent the expanded formu-

lae C9, which contains all the information about the

structural environment of the lignins.

Acknowledgements

The authors would like to thank Ligno-Tech Iberica,S.A., Santiago de Compostela University, the Cen-

tro de Investigaciones energeticas, medioambientales

y tecnologicas (CIEMAT) and Celulosa de Levante,

S.A. (CELESA) for supplying the lignins. We would

also like to express our sincere appreciation to the

Rovira i Virgili University for their award of a schol-

arship, the Spanish Ministry of Science and Technology

for providing finance under project number ENE2004-

07624-C03-03, and the autonomous government of

Catalonia also for providing finance under project num-

ber 2005SGR00580.

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