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 CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 39, 2014 

A publication of 

 
The Italian Association 

of Chemical Engineering 

www.aidic.it/cet 
Guest Editors: Petar Sabev Varbanov, Jiří Jaromír Klemeš, Peng Yen Liew, Jun Yow Yong  

Copyright © 2014, AIDIC Servizi S.r.l., 

ISBN 978-88-95608-30-3; ISSN 2283-9216 DOI: 10.3303/CET1439271 

 

Please cite this article as: Prado R., Erdocia X., Labidi J., 2014, Study of the influence of reutilization of ionic liquid for lignin 

extraction, Chemical Engineering Transactions, 39, 1621-1626  DOI:10.3303/CET1439271 

1621 

Study of the Influence of Reutilization 

of Ionic Liquid for Lignin Extraction 

Raquel Prado, Xabier Erdocia, Jalel Labidi*
 

Chemical and Environmental Engineering Department, University of the Basque Country Plaza Europa, 1, 20018, 

Donostia-San Sebastián, Spain 

Jalel.labidi@ehu.es 

The development of green techniques for biomass processing and fractionation is crucial from the point of 

view of sustainability and environmental protection. Lignin is the second most abundant bio-renewable 

material on Earth. Ionic liquids are considered as green solvents due to their low vapour pressure and their 

reuse possibility. In this work, the recyclability of Methyl sulphate Butylmethylimidazolium ionic liquid on 

the obtaining of lignin from biomass was studied. The experimental results showed that the obtained 

lignins were similar until the third cycle and the extraction process showed good performances, whereas in 

the fourth cycle, obtained lignin was contaminated by ionic liquid and the performance of ionic liquid 

decreased dramatically. 

1. Introduction 

The efficient utilization of biomass is becoming increasingly important due to diminishing resources of 

fossil fuels as well as global warnings caused by greenhouse gas emissions (Kilpeläinen et al., 2007). 

Lignocelluloses is the most abundant renewable material produced from biomass photosynthesis, has a 

yearly supply of approximately 200 billion metric tons worldwide (Zhang et al., 2007). Lignocellulose 

consists mainly of plant cell wall materials; it is a complex natural composite with three main biopolymers: 

cellulose (50 %), hemicelluloses (25 %) and lignin (25 %) (Erdocia et al., 2012)  

Lignin is a phenolic polymer built up by oxidative coupling of three major C6-C3 (phenylpropanoid) units, 

namely, syringyl alcohol, guaiacyl alcohol and p-coumaryl alcohol, which form a randomized structure in a 

three dimensional network inside the cell wall (García et al., 2009). In chemical pulping processes, around 

100 million tons per year lignin are produced as byproduct, most of which is burned to generate the energy 

required by the process. Only a very small fraction of the lignin (ca. 1.2 %) is used as material in industrial 

processes. The revalorisation of lignin obtained by traditional methods requires several treatments prior to 

its use for materials application or for building blocks obtaining. Therefore, there is an increasing demand 

for new processes that could provide new ways to use this resource in a more efficient manner, not only as 

fuel but also as starting material for chemical industry with the aim of producing commodity and fine 

chemicals (Kilpeläinen
 
et al., 2007).  

Room-temperature ionic liquids (IL) are receiving much interest owing to their characteristics as 

environmentally friendly solvents for a range of chemical processes both for catalysed and uncatalysed 

reactions, or as possible constituents in electrochemical applications. Thus molecular designs, synthesis 

and characterization of ILs have been the focus of many recent scientific investigations. Their unique 

properties like a large liquid range, very low vapour pressure and high thermal, chemical and 

electrochemical stability promise a wide applicability. Their properties can be adjusted by choosing specific 

combinations of cations and anions (Nockemann et al., 2005).   

One of the main advantages of the ionic liquids is that they are easily recovered and reutilized, so it is 

reduced the amount of wastes generated on a process. The combination of the reutilization with their low 

volatility is the reason why ionic liquids are considered as green solvents (Anastas et al., 2010).  



 

 

1622 

 
The application of ionic liquid in polymer material is based on their good solubility on water, so ionic liquid 

is used as solvent and water as coagulation agent (Liu et al., 2008). After coagulation of the polymer, the 

ionic liquid can be used as solvent again, for several cycles until saturation, and then further purification 

techniques are needed, as nanofiltration or ion exchange (Anastas et al., 2010). There are in the literature 

several examples about ionic liquids reutilization and the behaviour; Formentín et al. (2004) used butyl 

methyl imidazolium hexafluoro phosphate (bmim)PF6 as solvent for Knoevenagel reaction with good yields 

until cycle 3, (bmim)PF6 was also used as solvent in Claisen Schmidt condensation with good yields until 

cycle 3 (Fomentín et al., (2004), in addition, Wong et al. (2006) used ionic liquids in the Suzuki reaction 

with different results depending on the used co-solvent until cycle 14.  

It has been shown that some IL (such as 1-butyl-3methyl- and 1-allyl-3-methylimidazolium chloride, 

[bmim]Cl and [amim]Cl, respectively) can effectively dissolve biopolymers (Sun et al., 2009). [bmim]Cl 

(Kilpeläinen et al., 2007); and [amim]Cl are also able to dissolve different types of lignin samples (Pu et al., 

2007). In addition, lignin is dissolved by 1,3-dimethylimidazolium methylsulphate, 1-butyl-3-

methylimidazolium methylsulphate (Bmim[MeSO4]) and 1-hexyl-3-methylimidazolium 

trifluoromethanesulphonate (Tan and Macfarlane, 2009), the solubility of the lignin on the ionic liquid is 

based in the sulphate anion more than in the cation.  

In this work, the lignin was obtained by Bmim[MeSO4] directly from Malus domestica. The influence of the 

reutilization of ionic liquid on the obtained lignin structure was evaluated. In addition, the number of cycles 

of reutilization of ionic liquid without further purification was studied.  

2. Material and methods 

Apple tree pruning (Malus domestica), provided by a local farmer in the area of Guipuzcoa (Spain) was 

used as raw material, Bmim[MeSO4] ionic liquid was provided by Sigma Aldrich, and H2SO4 was provided 

by Scharlab. 

Lignin was isolated from raw material by ionosolv process with Bmim[MeSO4] enhanced by microwave 

radiation. The IL was reused several times, in order to evaluate the efficiency of the reutilization of the 

ionic liquid on the process yield.  

2.1 Analysis of the raw material 
Characterization of apple tree pruning (Malus domestica) fibres was done according to standard methods 

(TAPPI, 2008). Moisture content (8.80 ± 0.03 % wt) was determined after drying the samples at 105 ºC for 

24 h (TAPPI T264 om-97). Chemical composition, given on an oven dry weight basis, was the following: 

3.2 ± 0.2 % ash (TAPPI T211 om-93), 16.7 ± 0.2 % hot water soluble matter (TAPPI T207 om-93), 32.0 ± 

0.5 % aqueous NaOH soluble matter (TAPPI T212 om-98), 10.7 ± 0.5 % ethanol-benzene extractives 

(TAPPI T204 om-97), 26.15 ± 0.09 % lignin (TAPPI T222 om-98), 57 ± 1 % holocellulose (Wise et al., 

1946) and 27.3 ± 0.2 % α-cellulose (Rowell, 1983). 

2.2 Ionosolv process 
Ionosolv process was carried out following the conditions studied before (Prado et al., 2013).

 
Raw material 

was mixed with Bmim[MeSO4], in a solid:liquid mass ratio 1:10, under microwave radiation, maximum 

power of 30 W, for 3 min at 180 ºC on a CEM microwave Discover system model. The lignin was 

precipitated from the black liquor by adding acidified water at pH 2 achieved by adding H2SO4 solution. 

Then the liquor was centrifuged at 5,000 rpm for 15 min. Precipitated lignin was separated, washed with 

acidified water and dried at 50 ºC in an oven.  

To recover the ionic liquid, the liquid phase was vacuum distilled in order to eliminate the water and dried 

at 105 ºC in the oven for 16 h in order to achieve moisture < 2.5 %. 

The whole process was repeated reutilizing ionic liquid until saturation. 

2.3 Lignin characterization 
All lignin samples were characterized by attenuated-total reflectance infrared spectroscopy (ATR-IR) by 

direct transmittance in a single-reflection ATR System (ATR top plate fixed to an optical beam condensing 

unit with ZnSe lens) with an MKII Golden Gate SPECAC instrument. Spectrums data was 30 scans in a 

range of 4,000-700 cm
-1

 and resolution of 4 cm
-1

. 

Thermogravimetric analysis (TGA) of lignin was carried out under nitrogen atmosphere using a Mettler 

Toledo TGA/SDTA RSI analyser with a dynamic scan from 25 to 800 ºC at 10 ºC min
-1

. 

The purity of lignin samples was determined based on modified TAPPI standards (T222 om-83 and T249 

cm-85). Each dry lignin sample was pre-hydrolysed for 1 h with 72 % v/v sulphuric acid in a thermostatic 

bath at 30 ºC. Then deionised water was added in order to dilute samples up to 4 % sulphuric acid. 

Samples were then hydrolysed for 3 hours at 100 ºC, and afterwards ice cooled. The acid insoluble 

fraction of lignin samples (Klason lignin) was separated by filtration (glass microfiber filters MFV3, Filter-



 

 

1623 

Lab Inc.), washed with deionised water until neutral pH and oven-dried at 105 ± 3 ºC. The Klason lignin 

content of each sample corresponded to the acid insoluble fraction gravimetrically determined. From each 

experiment, the resulting hydrolysates were reserved for the subsequent monosaccharide content and 

acid soluble lignin (ASL) determination. In order to determine the content of sugars in lignin samples, the 

filtered solutions were characterized by High Performance Liquid Chromatography (HPLC) Jasco LC-Net II 

/ADC equipped with a photodiode array detector, refractive index detector and Rezex ROA_Organic Acid 

H
+
 (8 %) column. As mobile phase, dissolution of 0.005 N H2SO4 prepared with 100 % deionised and 

degassed water was used (0.35 mL min
-1

 flow, 40 ºC and injection volume 20 µL). High purity xylose, 

glucose, galactose, mannose and arabinose purchased from Sigma–Aldrich were used for calibration. A 

linear calibration (R
2
 > 0.999) was obtained for all sugars. 

Lignins were subjected to High Performance Size Exclusion Chromatography (HPSEC) to evaluate lignin 

molecular weight (MW) and molecular weight distribution (MWD) using a JASCO instrument equipped with 

an interface (LC-NetII/ADC) and a reflex index detector (RI-2031Plus). Two PolarGel-M columns (300 x 

7.5 mm) and PolarGel-M guard (50 x 7.5 mm) were employed. Dimethylformamide + 0.1 % lithium bromide 

was the eluent. The flow rate was 0.7 mL min
-1

 and the analyses were carried out at 40 ºC. Calibration was 

made using polystyrene standards (Sigma-Aldrich) ranging from 266 to 70,000 g mol
-1

.  

2.4 Ionic liquid characterization  
In order to determine the influence of the process on the nature of the ionic liquid and elucidate how the 

chemical structure of the Bmim[MeSO4] was affected ATR-IR spectra and NMR spectra were recorded at 

30 ºC on a Bruker Avance 500 MHz equipped with a z-gradient BBI probe. Typically, 40 mg of sample 

were dissolved in DMSO-d6. The spectral widths were 5,000 and 25,000 Hz for the 
1
H and 

13
C dimensions, 

respectively. 

3. Results and discussion 

In Table 1 is shown raw material dissolution yield (RMY) and extracted lignin yield (LIGY). It was observed 

that the raw material was not totally dissolved, and the RMY was a bit lower in the cycle 4. It was observed 

that the recovery of IL was about 90 % in all cycles, due to the viscosity of the ionic liquid was very hard to 

recover the 100 % of it because it remains stitched to the biomass and the walls of the reactor. The LIGY 

increased until cycle 3 and then decreased considerably. The mechanism of dissolution of lignin with ionic 

liquid was done with a link which was broken with the addition of water that acted as antisolvent because 

the affinity of the Bmim[MeSO4] with water is higher than with lignin. However, this process was not 

quantitative, part of the lignin remained linked to the Bmim[MeSO4], so the Bmim[MeSO4] used in the 

second cycle was not totally lignin free, this fact leaded to an increment in the recovery of lignin in the 

following cycles, until cycle 4 where the behaviour changed completely. 

It was observed that the lignin purity was very similar in the 3 first cycles and total sugar content was 

negligible (Table 1). This fact could mean that the obtained lignin in the 3 first cycles was similar, in any 

case, to confirm this fact more extensive characterization was needed. For the cycle 4, it was not possible 

to measure the lignin purity because of the characteristics of the sample. 

Table 1: Reaction yields and lignin purity 

%  RMY LIGY Lignin purity Sugar content 

Cycle 1 53±6 18.84±0.02 92±2 <DL 

Cycle 2 48±5 25.1±0.1 93±2 <DL 

Cycle 3 53±8 32.8±0.3 93±1 0.7±0.1 

Cycle 4 38±4 15.24±0.01 --- --- 

 

ATR-IR spectra of the lignins isolated in the fourth cycles were recorded (Figure 1). In these spectra 

following typical lignin structures could be identified: aromatic skeletal vibrations (1,597, 1,513 and 1,456 

cm
-1

), syringyl ring breathing with C-O stretching (1,327 cm
-1

), C-H in plane deformation in guaiacyl ring 

and syringyl ring (1,113 cm
-1

), C-H in plane deformation in guaiacyl ring and C-O deformation in primary 

alcohol (1,033 cm
-1

) and aromatic C-H out of plane deformation (823 cm
-1

). Other structures were also 

determined: OH- stretching (3,450 cm
-1

), C-H stretching in methyl and methylene groups (2,927 and 2,852 

cm
-1

), carbonyl stretching in un-conjugated ketones and conjugated carboxylic groups (1,704 cm
-1

), -OH 

deformation (1,201 cm
-1

), C-O-C asymmetric vibration (1,165 cm
-1

) and =C-H out of plane deformation 

(1,002 cm
-1

) (Garcia et al., 2011) it was observed that for the cycles 1 to 3, the lignins showed the same 

characteristic bands, while the intensity and definition of the bands decreased considerably for the lignin in 

cycle 4; even the bands at 1,002 and 1,033 cm
-1

 were coupled to a single band at 1,025 cm
-1

. 



 

 

1624 

 
ATR-IR spectra of the Bmim[MeSO4] ionic liquid utilized in the fourth cycles was compared with pure ionic 

liquid (Figure 1). In these spectra following typical Bmim
+
 structures were identified: stretch vibrational 

mode of CH in the imidazolium ring (3,147 cm
-1

), stretch vibrational mode of CH in the imidazolium ring 

(3,103 cm
-1

), stretching of the CH3 of the butyl chain attached to the imidazolium ring (2,960 cm
-1

), 

stretching of the CH2 of the butyl chain attached to the imidazolium ring (2,875 cm
-1

), in plane C-C and C-N 

stretching vibrations of the imidazolium ring (1,574 cm
-1

), symmetrical CH vibration of imidazolium ring 

(1,221 cm
-1

), in plane –C-H deformation vibration of imidazolium ring (1,169 cm
-1

), in plane wagging 

vibrations of alkyl chain (1,059 cm
-1

) and C-H in plane vibration of imidazolium ring (856 cm
-1

) (Jeon et al., 

2008). Other structures were also determined: SO3-OMe stretching (1,338 and 1,385 cm
-1

) (Pretsch et al., 

2000) and C=C out of plane deformation (1,009 cm
-1

). The intensity of the bands at 856 and 1,221 cm
-1

 

increased and it were displaced to lower wave-number. This could be due to lignin contamination since it 

presented characteristic bands at 823 and 1,201 cm
-1

 as it was observed in the Figure 1. The intensity of 

the band at 1,059 cm
-1

 increased and finally in the cycle 4 it was coupled with the band at 1,009 cm
-1

.  

 

Figure 1: ATR-IR spectra of a) lignins and b) Bmim[MeSO4] 

The obtained lignins of consecutive processes RM* were subjected to HPSEC analysis in order to obtain 

the MW and MWD of different lignins obtained by Bmim[MeSO4] pulping. The results of MW average 

(MW), Mn average (Mn) and polydispersity are shown in Table 2. This behaviour indicated that the MW 

were similar for the 3 first cycles as it is shown in the Table 2, whereas in the cycle, 4 decreased 

considerably. Otherwise, in Table 2 it was also observed that the Mn and polydispersity were similar into 

the three first cycles. However, in the fourth cycle decreased considerably (Hage et al., 2009). It indicated 

that lignins obtained in first three cycles were similar, but the 4 cycle lignin was completely different. 

Table 2: Molecular weight results 

 

 

The thermogravimetric analysis is widely used to study how the organic polymers decompose. It was 

observed in the Table 3 that all samples showed a weight loss behind 100 ºC, which was corresponded to 

moisture of the samples. The weight loss around 150-260 ºC corresponded to hemicelluloses fraction 

degradation, and the weight loss between 260 and 400 ºC corresponded to lignin fraction degradation 

(Sun et al., 2000). The lignin degradation occurs in two steps due to its complex structure, it was observed 

in the Table 3 that the lignins of the cycles 1, 2 and 3 showed a narrow peak and a shoulder between 300 

and 400 ºC, whereas the cycle 4 lignin only showed a peak, it indicated that LIG cycle 4 had simpler 

structure than other lignins. These results were in agreement with HPSEC results because more complex 

structures with higher MW tend to degrade at higher temperatures. In the cycles 1 and 2 there were not 

hemicelluloses degradation peak which confirmed the results obtained by HPLC. The final inorganic 

residue was similar in all samples, and it was around 45 %, which was in agreement with the results 

obtained by other authors (Toledano et al., 2010). 

 

Sample MW Mn Polydispersity 

LIG cycle 1 15,503  1,756  8.83 

LIG cycle 2 13,186  1,384  9.53 

LIG cycle 3 15,549  1,631  9.54 

LIG cycle 4 4,382  879  4.99 



 

 

1625 

Table 3: Temperature degradation results 

Sample Temperature ºC Weight loss (%) Residue (%) 

LIG Cycle 1 

85.0 2.0 

44.6 310.9 29.5 

387.3 23.9 

LIG Cycle 2 

84.3 1.4 

42.3 316.0 30.6 

394.5 25.1 

LIG Cycle 3 

78.2 1.8 

46.3 
189.8 1.1 

313.2 27.5 

389.5 23.3 

LIG Cycle 4 

97.0 0.9 

46.9 146.8 0.7 

308.0 51.5 

 
13

C-NMR spectra of Bmim[MeSO4] of different cycles are shown in the Figure 2. The bands were assigned 

as follow; 13.42 ppm to -CH3 (10), 18.80 ppm to -CH2- (9), 31.54 ppm to -CH2- (8), 35.75 to -CH3 (6), 39.54 

ppm to DMSO, 48.49 ppm to –CH3 (11), 52.92 ppm to CH2 (7), 122.12 to =CH (4), 123.71 ppm to =CH (5) 

and 136.62 to =CH (2) (Pretsch et al., 2000). The same bands appeared in all the spectra, so it seemed 

that C linkage structure had no change over the cycles.  
1
H-NMR spectra of Bmim[MeSO4] of different cycles are shown in the Figure 2. The following bands 

appeared in all the spectra, and were assigned as follow: 0.89 ppm (t) was assigned to -CH3 (10), 1.26 

ppm (q) to -CH2- (9), 1.77 ppm (m) to -CH2- (8), 2.51 ppm (s) to DMSO, 3.40 ppm (s) to –CH3 (11), 3.86 (s) 

to -CH3 (6), 4.17 ppm (t) to CH2 (7), 7.71 ppm (s) to =CH- (4), 7.78 ppm (s) to =CH- (5) and 9.13 ppm (s) to 

=CH- (2) (Jeon et al., 2008;). The differences started to occur in the cycle 3 where a new band appeared 

at 5.61 ppm(s) (Shi and Deng, 2005). This displacement could be caused by a 
1
H from phenolic OH, the 

main structure present in the lignin. In the cycle 4 this band was more intense, which indicated more 

contamination on the ionic liquid by the lignin.  

 

Figure 1: NMR analysis of Bmim[MeSO4] samples, a) 
13

C-NMR spectra and b)  
1
H spectra  

4. Conclusion 

The extraction of lignin from biomass by ionic liquids was studied reutilizing the solvent several times 

without further purification. The Bmim[MeSO4] can be reutilized without being purified during three cycles 

to extract lignin from the biomass. The extraction yield and the lignin structure were not severely affected 

by the reutilization of Bmim[MeSO4] until the cycle 4. The Bmim[MeSO4] also underwent significant 

changes in its structure in the cycle 4 and contamination with hydroxyl groups was observed. 

Acknowledgements 

Authors would like to thank the Departments of Agriculture, Fishing and Food and Department of 

Education, Universities and Investigation of the Basque Government (scholarship of young researchers 

training and project IT672-13). 



 

 

1626 

 
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