Lignin extraction and purification with ionic liquids

Research ArticleReceived: 19 May 2012 Revised: 10 September 2012 Accepted: 13 September 2012 Published online in Wiley Online Library:

(wileyonlinelibrary.com) DOI 10.1002/jctb.3965

Lignin extraction and purification with ionicliquidsRaquel Prado, Xabier Erdocia and Jalel Labidi∗

Abstract

BACKGROUND: In new biorefineries the use of new green solvents is essential in order to minimize the employment of volatileorganic solvents. Lignin is the most abundant residue in paper industries, however, it is necessary to purify it in order to obtainrevalorized products.

RESULTS: In this study, soda and organosolv lignins obtained from apple tree prunings (Malus domestica) were purifiedusing [Bmim][MeSO4], and the lignin extracted from raw material was tested comparing different conditions. [Bmim][MeSO4],EtmimAc (1-Ethyl-3-methylimidazolium acetate) and BmimCl, were the ionic liquids chosen for the experimentation. The ligninsobtained were characterized by FTIR, TGA, RMN and HPLC, in order to determinate the influence of the different treatments ontheir structures. Lignin of 91.2% purity was obtained directly from raw material using ionic liquids, whereas organosolv ligninpurity ranges from 85.7% to 90.9%, and soda lignin from 12.9% to 89.6% after treatment with [Bmim][MeSO4].

CONCLUSION: Results showed that the use of [Bmim][MeSO4] is the best option to purify and extract lignin from raw material.In addition, microwave radiation enhanced the energy consumption of the process.c© 2012 Society of Chemical Industry

Supporting information may be found in the online version of this article.

Keywords: ionic liquid; lignin; microwave; purification

INTRODUCTIONLignocellulosic biomass is the most important renewable sourceof materials. It is constituted principally by cellulose, lignin andhemicelluloses, in order of abundance, although the quantitativecomposition varies considerably depending on the specie thatis treated. In any case, this means that lignin is the secondmost abundant renewable material on earth.1 In the pulp andpaper industry a large amount of lignin is obtained in thedelignification process. Normally it is treated as a residue, andit has historically been utilized as a low value heating fuel,binder, dispersant, emulsifier, and sequestrant, but currently,because of the aromatic structure of lignin, is gaining importanceas a potential and renewable source for aromatic chemicals2.The structure of lignin is very complex and cannot yet bedescribed completely; basically it is a polymer composed of threephenol derived compounds, guaiacylpropane, syringylpropaneand p-hydroxyphenylpropane.3,4 Depending on the source ofthe lignin, the structure can change, and it is also influencedby other factors such as the growing, harvesting and drying ofthe lignocellulosic material. On the other hand, transformationand extraction procedures also influence the structure. Thereare several processes that are used in industry to obtain ligninsuch as the Kraft process, alkaline treatment, steam explosionand organosolv processes. It is very difficult to obtain highpurity lignin, because with these industrial processes cellulose,hemicelluloses and other products are obtained as impurities.In order to transform lignin into added value products, furtherpurification is needed.5,6

Nowadays, to solve purity problem other processes areinvestigated, such as microwave radiation, which is basedon two different mechanisms for transferring energy to thesubstance: dipole rotation and ionic conduction. It consists inthe instantaneous superheating of the organic substance dueto the ionic motion generated by an electric field, when thetemperature increases, the transfer of energy is more efficient.Microwave radiation is used to reduce reaction time, avoid sideproducts, increase the yield and simplify the reaction.7

Currently, the use of volatile organic compounds (VOC)-freesolvents is gaining in importance to avoid the increase ofatmosphere contamination; most of the investigations in thisarea are focused on minimization of the greenhouse effect. Forthis purpose green solvents are used and studied. Ionic liquids arethe most widely investigated green solvents, particularly in thearea of biomass. Ionic liquids are liquid salts in which at least one ofthe ions is large, and has very low symmetry. Generally the cation isorganic, whereas the anion can be organic or inorganic. One of themost important characteristic is their low melting points <100◦C,which is the main difference from the classical molten salts.8

Ionic liquids have many applications in very diverse areasdue to their special combination of properties. Because of

∗ Correspondence to: Jalel Labidi, Chemical and Environmental EngineeringDepartment, University of the Basque Country Plaza Europa, E-mail:[email protected]

Chemical and Environmental Engineering Department, University of the BasqueCountry Plaza Europa, 1, 20018, Donostia-San Sebastian, Spain

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Figure 1. Scheme of the experimental procedure.

Table 1. Performances of pulping methods

Sample

Raw material

dissolved (%)

Lignin (%)

(respect to raw material)

Organosolv pulping 43.2 ± 0.7 13.6 ± 0.5

Soda pulping 45.7 ± 0.6 30.4 ± 0.2

IL pulping [Bmim][MeSO4] 48.1 ± 0.9 18.8 ± 0.3

Table 2. Lignin solubility and recovery in [Bmim][MeSO4]

50◦C Microwave radiation

Sample

Solublity

(%)

Recovery

(%)

Solubility

(%)

Recovery

(%)

Organosolv lignin 90 ± 2 72.0 ± 1.0 97 ± 1 75 ± 2

Soda lignin 27.2 ± 0.4 3.9 ± 0.1 73.1 ± 0.7 64 ± 1

Figure 2. ART-IR spectra of IL lignin, organosolv lignin and soda lignin.

their specific physicochemical properties, particularly non-flammability and high thermal stability and due to theirvery low vapour pressure, they do not emit volatile organiccompounds and are good option for substitution of organicsolvents in many processes in order to make the processgreener.9 Ionic liquids have been described as ‘designer solvents’,combining appropriate anions and cations, their properties

Figure 3. ATR-IR spectra of soda lignin purification with [Bmim][MeSO4].

Figure 4. ATR-IR organosolv purification by [Bmim][MeSO4].

such as melting point, viscosity, density, and hydrophobicitycan be adjusted to the specific application.9 Because of theircombination of properties and versatility of synthesis, theyare gaining interest as solvent for biomass, and have beenstudied by several authors. Ionic liquids such as BmimCl (butylmethylimidazolium chloride) have been used for dissolvingcellulose and further depolymerisation or re-precipitation.10

Lignin is also solubilised by some ionic liquids, such as,

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Figure 5. 13C-NMR of soda lignin.

BmimCl, AmimCl (1-Alyl-3-methylimidazolium chloride),11 1,3-dimethylimidazolium methylsulfate,3 and [Bmim][MeSO4] (1-butyl-3-methylimidazolium methylsulfate).1

Another advantage of ionic liquids is that they are easilyrecovered and reutilized, thus reducing the amount of wastegenerated in a process. The combination of reutilization with theirlow volatility is the reason why ionic liquids are considered asgreen solvents.

In this work, lignin obtained from Malus domestica by alkaline(NaOH 7.5%, 90 min 90◦C) and organosolv (60% ethanol, 90min 180◦C) processes was purified by different methods. Thepurification methods used combine treatment with ionic liquid[Bmim][MeSO4] (1-butyl-3-methylimidazolium methylsulfate) asa new green solvent, and microwave radiation. The lignin wasisolated from malus domestica using three different ionic liquids,[Bmim][MeSO4], BmimCl (Buthyl methylimidazolium chloride) andEtmimAc (1-ethyl methylimidazolium acetate). Lignins obtainedby organosolv and soda were compared with those obtainedby ionic liquid. In addition, the effect of microwave radiationon the solubilisation of raw material and lignin was studied.The apple tree, Malus domestica, was chosen for this studybecause of its relative high content of lignin. The aim was todetermine the best option to extract quantitatively pure ligninfor further applications and synthesis of added value products.The lignin purity obtained using different pulping methods andpurification processes was compared. The influence of ionicliquids and microwave radiation on the process of extractionand purification of lignin was also determined. The chemical

structure of the lignins obtained and their thermal behaviourwere characterized by several different techniques (ATR-IR, HPLC,13C-NMR and TGA).

MATERIALS AND METHODSRaw material (Malus domestica) was provided by a local farmerin Guipuzkoa (Spain; [Bmim][MeSO4], EtmimAc, and BmimCl ionicliquids were provided by Sigma Aldrich, and ethanol was providedby Scharlab.

Lignin was isolated from raw material by organosolv pulping,soda pulping and IL pulping ([Bmim][MeSO4],. EtmimAc, andBmimCl). Then soda and organosolv lignins were purified with[Bmim][MeSO4] enhanced by heating and microwave radiation.The scheme of the experimental procedure is shown in Fig. 1.

Analysis of the raw materialCharacterization of apple tree pruning fibres was done accordingto standard methods.12 Moisture content (8.80 ± 0.03 wt%) wasdetermined after drying the samples at 105◦C for 24 h (TAPPIT264 cm-97). Chemical composition, given on an oven dry weightbasis, was as follows: 3.25 ± 0.24% ash (TAPPI T211 om-93), 16.73± 0.17% hot water soluble matter (TAPPI T207 om-93), 32.00 ±0.57% aqueous NaOH soluble matter (TAPPI T212 om-98), 10.71 ±0.47% ethanol–benzene extractives (TAPPI T204 cm-97), 26.15 ±0.09% lignin (TAPPI T222 om-98), 57.44 ± 1.66% holocellulose13

and 27.32 ± 0.23% α-cellulose.14

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Figure 6. 13C-NMR of soda lignin treated with [Bmim][MeSO4].

Organosolv pulpingThe Organosolv pulping was carried out under the followingconditions: 60% (v/v) ethanol solution, solid:liquid mass ratio of1:4 at 180◦C for 90 min. The lignin was precipitated from the blackliquor by adding acidified water at pH 2 [15]. Then the liquorwas centrifuged at 5000 rpm for 15 min. Precipitated lignin wasseparated, washed with acidified water and dried at 50◦C in anoven.

Soda pulpingSoda pulping was carried out using 7.5% (w/v) NaOH solution ina solid:liquid mass ratio of 1:10 at 90◦C for 90 min. The lignin wasrecovered from the liquor by adding sulphuric acid until the blackliquor reached pH 2.15 The liquor was centrifuged at 5000 rpm for15 min. Precipitated lignin was separated, washed with acidifiedwater and dried at 50◦C in an oven.

IL pulpingIonic liquid pulping was carried out using [Bmim][MeSO4], BmimCland EtmimAc, in a solid:liquid mass ratio 1:20 at 150◦C for 6 h,under inert atmosphere.

IL pulping was also carried out using [Bmim][MeSO4], in asolid:liquid mass ratio 1:10, under microwave radiation, maximumpower 30 W, 3 min at 200◦C using a CEM microwave Discoversystem model.

In all cases the lignin was precipitated by the addition ofacidificied water with sulphuric acid at pH 2. Then was centrifuged

at 5000 rpm for 15 min and the solid part was separated, washedand dried at 50◦C in an oven. To recover the ionic liquid, theliquid phase was vacuum distilled in order to eliminate thewater.

Lignin purification with [Bmim][MeSO4]Lignin obtained from soda and organosolv pulping was dried inthe oven before treating it with the ionic liquid. Dried lignin wasintroduced in a flask along with [Bmim][MeSO4] in a solid:liquidratio of 1:25 at 50◦C for 6 h in an inert atmosphere.

Soda lignin was also treated under microwave radiationwith [Bmim][MeSO4], using the following optimized conditionsdetermined in preliminary experiments: 30 W, 3 min at 200◦C.

In both cases the solution was filtered, the filter residue wasdried at 50◦C and characterized. Acidified water at pH 2 wasadded to the liquid fraction to recover the lignin from the ionicliquid. The solution was centrifuged at 5000 rpm for 15 min.Recovered lignin was separated, washed and dried at 50◦C in anoven.

Lignin characterizationAll lignin samples were characterized by attenuated-totalreflectance infrared spectroscopy (ATR-IR) by direct transmittancein a single-reflection ATR System (ATR top plate fixed to an opticalbeam condensing unit with ZnSe lens) with a MKII Golden GateSPECAC instrument. Spectral data were 30 scans in the range4000–700 cm-1 and resolution of 4 cm-1.

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Figure 7. 13C-NMR of [Bmim][MeSO4] lignin.

Thermogravimetric analysis (TGA) of lignin was carried outunder nitrogen atmosphere using a Mettler Toledo TGA/SDTA RSIanalyzer with a dynamic scan from 25 to 800◦C at 10◦C min-1.

The purity of lignin samples was determined based onmodified TAPPI standards (T222 om-83 and T249 cm-85).12 Eachdry lignin sample was pre-hydrolyzed for 1 h with 72% v/vsulphuric acid in a thermostatic bath at 30◦C. Then deionisedwater was added in order to dilute samples to 4% sulphuricacid. Samples were then hydrolyzed for 3 h at 100◦C, andafterwards ice cooled. The acid insoluble fraction of lignin samples(Klason lignin) was separated by filtration (glass microfiber filtersMFV3, Filter-Lab Inc.), washed with deionised water until neutralpH and oven-dried at 105 ± 3◦C. The Klason lignin contentof each sample corresponded to the acid insoluble fractiondetermined gravimetrically. From each experiment, the resultinghydrolizates were reserved for subsequent monosaccharidecontent and acid soluble lignin (ASL) determination. In orderto determine the content of sugars in lignin samples, thefiltered solutions were characterized by high performance liquidchromatography (HPLC) Jasco LC-Net II /ADC equipped witha photodiode array detector, refractive index detector andRezex ROA_Organic Acid H+ (8%) column. As mobile phase,dissolution of 0.005 N H2SO4 prepared with 100% deionisedand degassed water was used (0.35 mL min-1 flow, 40◦C andinjection volume 20 µL). High purity xylose, glucose, galactose,

mannose and arabinose purchased from Sigma-Aldrich were usedfor calibration. A linear calibration (R2 > 0.999) was obtained forall sugars.

Lignins were subjected to high performance size exclusionchromatography (HPSEC) to evaluate lignin molecular weight(MW) and molecular weight distribution (MWD) using a JASCOinstrument equipped with an interface (LC-NetII/ADC) and areflex index detector (RI-2031Plus). Two PolarGel-M columns(300 × 7.5 mm) and PolarGel-M guard (50 × 7.5 mm)were employed. Dimethylformamide + 0.1% lithium bromidewas the eluent. The flow rate was 0.7 mL min-1 and theanalyses were carried out at 40◦C. Calibration was made usingpolystyrene standards (Sigma-Aldrich) ranging from 70000 to 266g mol-1.

NMR spectra were recorded at 30◦C on a Bruker Avance 500 MHzequipped with a z-gradient BBI probe. Typically, 40 mg of samplewere dissolved in DMSO-d6. The spectral widths were 25000 Hzfor the 13C dimensions.

Ionic liquid characterizationIn order to determine the influence of the process on the natureof the ionic liquid, the moisture of the sample was measured. Inaddition, 1H- NMR spectra were recorded at 30◦C. For this purpose20 mg of sample were dissolved in DMSO-d6. The spectral widthswere 5000 Hz for the 1H dimensions.

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Figure 8. 13C-NMR of organosolv lignin.

RESULTS AND DISCUSSIONIonic liquid pulpingThe raw material was treated with three different ionic liquids.Under the same conditions [Bmim][MeSO4] showed betterperformance. BmimCl was not able to dissolve the raw material at150◦C under nitrogen atmosphere. On the other hand EtmimAcwas able to dissolve the raw material with very good yield; however,it was not possible to recover pure lignin without ionic liquidcontamination, because at that temperature the ionic liquid wasslightly degraded. When microwave radiation was used it was notpossible to have an inert atmosphere, so BmimCl did not workproperly because during the reaction water was produced. In thecase of EtmimAc the temperature achieved was too high so ligninwas completely degraded. Other authors had used these ionicliquids in other conditions with good performance.16,17 For thepurpose of this study, only the treatment with [Bmim][MeSO4] wasselected for the rest of the experiments.

The performances of different pulping methods are shown inTable 1, the percentage of dissolved raw material was more orless the same for the three methods, but the percentage of ligninobtained was higher in the soda lignin. However, the soda ligninhad a lot of impurities, as was described in the spectroscopicand thermal characterization. Nevertheless, the amount of ligninobtained with [Bmim][MeSO4] pulping was higher than withorganolv pulping.

Lignin solubility on [Bmim][MeSO4]The measure of solubility into [Bmim][MeSO4] is important todeterminate in order to recover all the pure lignin of the sample.

The results are shown in the Table 2. The solubility of NaOH ligninwas lower than organosolv lignin. Organosolv lignin was dissolvedinto [Bmim][MeSO4] at 50◦C and with microwave radiation at 50◦C,and in both cases the same performance was achieved. However, todissolve soda lignin microwave radiation and higher temperaturewere necessary. Even though the microwave radiation improvedthe solubility of soda lignin considerably, high amounts of solidresidue were obtained. The amount of impurities had an importanteffect on the solubility of lignin in [Bmim][MeSO4].

Moreover, the recovery yield is lower than the solubility. Themain reason may be that the hemicelluloses are soluble in the ionicliquid and remain in solution during the lignin recovery process.

Lignin characterizationLignin spectroscopic characterizationThe ATR-IR spectra of the different lignin samples are shown inFigs 2, 3 and 4. First of all, the common bands of all spectra arediscussed. All spectra showed a vibration band around 3370 cm-1,caused by stretching of the hydroxyl group, and between 2990cm-1 and 2790 cm-1 two bands that correspond to the stretchingvibration in methyl and methylene groups. Around 1700 cm-1 ashoulder appeared which is caused by the carbonyl stretching inun-conjugated ketones. The bands observed at 1595 and 1515cm-1 correspond to the aromatic skeletal vibrations on ligninsamples.

Second, the main differences between spectra were analyzed.The chemical structure of the lignin obtained by the distinctpulping methods was different, as observed in Fig. 2. In thesoda lignins a vibration band appeared at 1631 cm-1 which

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Figure 9. 13C-NMR of organosolv lignin treated with [Bmim][MeSO4].

corresponds to the carbonyl stretching in γ -lactone. In general,the bands which are more characteristic of lignin, are those thatcorrespond to guaiacyl or syringyl units; bands correspondingto these units appeared in all spectra of the different pulpingmethods but not at the same wave number. For ionic liquid ligninand organosolv lignin two bands appeared at 1330 cm-1 and1324 cm-1 corresponding to syringyl ring breathing with carbonylstretching. On soda lignin and organosolv lignin spectra a bandappeared around 1030 cm-1, which is indicative of C–H in planedeformation in guaiacyl.18,19 Therefore in Fig. 2, it can be observedthat the chemical structure of the lignin changed depending onthe production process; the aromatic ring vibration bands werepresent in all spectra but in different forms. This showed that thetreatment with [Bmim][MeSO4] was suitable to obtain lignin.

On the other hand, the [Bmim][MeSO4] treatments to purifyorganosolv and soda lignin also caused changes on their chemicalstructure (Figs 3, 4), and it had special relevance for the soda ligninsample. It can be observed in the spectra of the soda lignin treatedsample (Fig. 3) that vibration bands corresponding to aromaticskeletal vibrations were intensified in the treated sample; thisbehaviour can be observed especially around 1595 cm-1 and 1514cm-1. In addition, a vibration band corresponding to syringyl ringbreathing with C–O stretching appeared at 1330 cm-1, and theOH stretching band around 3450 cm-1 is reduced considerably,probably due to the raised concentration of hemicelluloses inthe sample. The waste spectrum corresponded to the sodiumsulphate spectrum which showed a very intensive vibration band

Table 3. Results of sugar concentration in lignin samples

Lignin purity (%)

Sample Sugar (%) insoluble soluble

Organosolv lignin 6.70 ± 0.08 83 ± 1 2.7 ± 0.1

Organosolv lignin +[Bmim][MeSO4]

7.1 ± 0.2 87 ± 2 3.9 ± 0.1

Soda lignin 22 ± 1 9 ± 1 3.6 ± 0.1

Soda lignin +[Bmim][MeSO4]+MW*

<DL** 85 ± 1 4.6 ± 0.9

IL lignin ([Bmim][MeSO4])+MW* <DL 86.1 ± 0.4 5.1 ± 0.9

*MW: microwave radiation.**DL: detection limit.

around 1100 cm-1.20 This sodium sulphate was formed when thesoda lignin was precipitated with sulphuric acid. [Bmim][MeSO4]cannot dissolve the inorganic impurity even when microwaveradiation is used, so the treatment eliminated the main impurityof the soda lignin.

The organosolv lignin treated sample (Fig. 4), did not showlarge differences with the starting sample; the only differenceobserved was around 3400 cm-1, which correspond to the hydroxylstretching band that had a significant increase in intensity. Thisbehaviour could be assigned to the decrease of hemicellulosesconcentration in the sample.

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Table 4. Thermogravimetric data of the samples

Sample

Temperature

(◦C)

Weight loss

(%)

Residue

(%)

Organosolv 76.2 3.33 40.08

258.4 16.00

362.8 40.59

Organosolv +[Bmim][MeSO4]

69.0 1.64 41.57181.8 8.61

318.3 24.14

399.4 24.04

Soda 82.7 10.04 43.91

234.6 16.29

359.4 16.68

784 13.08

Soda lignin +[Bmim][MeSO4] + MW*

62.5 5.00 39.58265.4 31.84

367.5 23.58

Soda waste 238.1 1.84 92.69

693.5 5.47

IL lignin [Bmim] [MeSO4]+ MW*

70.9 6.18 41.96183.6 4.57

318.6 21.95

390.3 25.34

*MW: microwave radiation.

In order to study more deeply the chemical structure changes ofthe [Bmim][MeSO4] treated lignin in comparison with organosolvand soda lignins, 13C NMR analysis was performed. Most of theobserved bands were previously elucidated in lignin spectra byother authors. In the 13C NMR spectra the signal at 40 ppmarises from the DMSO-d6, the signals between 60 and 100 ppmare typical from polysaccharides, specifically, signals from C5 onxyloses appears between 60 and 70 ppm, the C2, C3 and C4 onxyloses between 70 and 75 ppm and C1 of xyloses from 85 to100 ppm.21,22 The aromatic part of the lignin was observed in theregion between 100 and 155 ppm. Between 100 and 120 ppmguaiacyl units appeared, between 130 and 150 ppm a mixture ofnon-etherified and etherified syringyl and guaiacyl units, from 150to 175 generally p-coumarate units and on 180 ppm glucuronicacid.23,24

In Fig. 5, which shows the 13C-NMR spectra of soda lignin, agreat number of signals are observed in the region between 60and 80 ppm, which corresponded to the hemicelluloses region.Other signals in the aromatic region from 100 to 180 ppm canalso be observed. The number of signals in the 60–80 ppm regiondecreased considerably in the 13C-NMR spectra of soda ligninpurified with [Bmim][MeSO4] (Fig. 6) when compared with Fig. 5.This indicates that the treatment with [Bmim][MeSO4] reduced thehemicelluloses concentration.

There were many differences between the 13C-NMR spectraof organosolv lignin (Fig. 7) and organosolv lignin puridied with[Bmin][MeSO4] (Fig. 8). In the first one, many signals over thewhole spectra appeared while in the second one signals weremore defined. In the aromatic region, between 120 and 140 ppmguaiacyl and syringyl units have higher intensity in the case ofpurified lignin, and the signals between 60 and 100 ppm, whichcorrespond to hemicelluloses fraction, disappeared completely.

The [Bmim][MeSO4] lignin 13C-NMR spectra (Fig. 9) shows onlytwo signals in the hemicelluloses region between 60 and 90 ppm;

Table 5. Lignin molecular weight distribution

Sample Mn Mw Polidispersity

IL lignin ([Bmim][MeSO4])+MW* 5918 42505 7.18

Organosolv lignin + [Bmim][MeSO4] 6299 37135 5.90

Soda lignin + [Bmim][MeSO4] + MW* 7958 36968 4.65

Organosolv lignin 3548 8568 2.41

Soda lignin 4227 8529 2.02

*MW: microwave radiation.

Table 6. Ionic liquid moisture (%)

New Used Dried

[Bmim][MeSO4] 0.01 9.65 0.48

on the other hand on the aromatic region a considerable quantityof signals were observed. Compared with Figs 5 and 7, the ligninobtained directly with [Bmim][MeSO4] showed less hemicellulosesconcentration.

Purity of ligninThe treatment with [Bmim][MeSO4] increased the purity of thelignin sample. In addition, when microwave radiation was appliednot only the sugar concentration decreased significantly, butalso the soluble lignin increased (Table 3), which might indicatefractionation of the lignin because of the aggressive conditionsachieved.

The treatment of the raw material directly with [Bmim][MeSO4]and microwave radiation produced lignin with the same purity asthe other pulping lignins after purification with [Bmim][MeSO4].

The use of [Bmim][MeSO4] improved the purity of the lignin inall cases, and the use of microwave radiation degraded the sugarspresent in the sample, and also slightly degraded the lignin itself.

Lignin thermal propertiesThe results of thermogravimetric analysis of the different samplesare shown in Table 4. All of them showed a weight loss above100◦C, which corresponded to moisture in the samples. Theweight loss around 150–260◦C corresponded to hemicellulosesfraction degradation, and the weight loss between 260 and 400◦Ccorresponded to lignin fraction degradation.25 It can be observedin Table 4, that some samples showed two degradation peaksbetween 260 and 400◦C because of the slow lignin degradation,due to its complex structure. The different molecules formingthe lignin structure are degraded at different temperatures andthe aromatic ones are the latest to degrade.26 It is also observedin Table 4, comparing the data of degradation peaks amongthe lignins obtained by the three pulping methods, that therewas no hemicelluloses degradation peak for [Bmim][MeSO4]pulping, whereas soda lignin and organosolv lignin showed a peakin the range of temperatures corresponding to hemicellulosesdegradation. A slight decrease in the thermal stability of[Bmim][MeSO4] pulping lignin can be observed; the degradationpeak appeared at lower temperature in this case. The samebehaviour occurred when soda lignin and organosolv ligninwere treated with [Bmim][MeSO4]; the thermal stability of thelignin decreased slightly and the hemicelluloses degradation

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Figure 10. 1H-NMR spectra of [Bmim][MeSO4].

peak disappeared, as is shown on Table 4. The waste from thesoda lignin sample treated with [Bmim][MeSO4] showed only apeak corresponding to moisture in the sample and a very lowweight loss. In general, the thermal analysis showed that sodawaste was an inorganic compound. In addition the treatmentswith [Bmim][MeSO4] and microwave radiation decreased theconcentration of hemicelluloses in the lignin samples, which waspositive, however, it could also cause slight degradation of thelignin itself.

Molecular weight distributionMolecular weight (w), number-average (mn) and polydispersity ofthe samples are shown in Table 5. The lignin obtained directly by[Bmim][MeSO4] had higher weight average than lignin obtained bythe other pulping methods. In addition, the organosolv and sodalignin treated by [Bmim][MeSO4] also showed higher molecularweight average than the original samples. Therefore, treatmentwith [Bmim][MeSO4] caused an increase in the weight average oflignin samples.

Evaluation of the ionic liquid recoverabilityThe [Bmim][MeSO4] moisture results are shown in Table 6. It canbe observed that the [Bmim][MeSO4] moisture is negligible. Themoisture content increased during the lignin extraction process,although it can easily be removed by drying treatment (12 h at105◦C).

The 1H-NMR spectra of new, used and cleaned [Bmim][MeSO4]are shown in Fig. 10. The three spectra present almost the same

signals. Most of the signals appear at the same displacement forall samples; 9.12 (s), 7.78 (s), 7.71 (s), 4.17 (m), 3.86 (s), 3.40 (s),2.51 (s), 1.77 (m), 1.29 (m) and 0.90 (m) ppm. On the other hand,there were a few differences: in the [Bmim][MeSO4] used sampletwo new signals appeared at 3.98 (s) and 1.91 (s) ppm, which, aftercleaning treatment, disappeared. These results suggested that the[Bmim][MeSO4] can be reutilized after the extraction process.

CONCLUSIONSThe [Bmim][MeSO4] is a suitable solvent to dissolve lignin and forits posterior recovery, as has been observed in the solubility andperformance results; it is the best option, when compared withorganosolv and soda pulping, to extract lignin from the studiedraw material.

The characterisation results showed that it is possible to obtainpure lignin using [Bmim][MeSO4], requiring no further treatment,and producing lignin with a higher molecular weight.

The use of microwave radiation improves the yield and thereaction time of the process, however, the lignin treated isdegraded and its thermal stability slightly decreased.

Contrary to expectations, the higher molecular weight ligninsshowed the worst thermal degradation peaks (Tables 4, 5). Thisfact could be explained by the polydispersity data. It was observedthat with higher molecular weight the polydispersity increasedvery much. In the thermograms (Supporting Material: Appendix 1)it can be observed that the degradation peak is wider, due to theheterogeneity of lignin, and despite the fact that the main peak

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www.soci.org R Prado, X Erdocia, J Labidi

is present at lower temperatures than for smaller lignins, clearlyshows a marked shoulder at higher temperatures, which would bedue these larger fragments.

The best option to obtain pure lignin is to extract directlyfrom the raw material employing [Bmim][MeSO4] enhanced withmicrowave radiation. On the one hand, the purity of lignin obtainedby this method is similar to the purity of that purified fromorganosolv and soda pulping. On the other hand, it is a moreefficient process, both economically and environmentally, becausepure lignin is obtained in one step without further purification,and the use of volatile solvents is not necessary. The recoveredionic liquid can easily be cleaned and reused for further extractionprocesses.

ACKNOWLEDGEMENTSThe authors would like to thank the Departments of Agriculture,Fishing and Food and Department of Education, Universitiesand Investigation of the Basque Government (scholarship ofyoung researchers training) the Saiotek program for supportingfinancially this work.

Supporting InformationSupporting information may be found in the online version of thisarticle.

REFERENCES1 Yungiao P, Nan J and Ragauskas AJ, Ionic liquids as a green solvent for

lignin. J Wood Chem Technol 27:23–33 (2007).2 Binder JB, Gray MJ, White JF, Zhang ZC and Holladay JE, Reactions

of lignin model compounds in ionic liquids, Biomass Bioenergy33:1122–1130 (2009).

3 Tan SSY and Macfarlane DR, Ionic liquids in biomass processing. TopCurr Chem 290:311–339 (2009).

4 Ewellyn A, Capanema M, Balakshin Y and Kadla JF, A comprehensiveapproach for quantitative lignin characterization by NMRspectroscopy. J Agric Food Chem 52:1850–1860 (2004).

5 Toledano A, Serrano L, Garcia A, Mondragon I and Labidi J,Comparative study of lignin fractionation by ultrafiltration andselective precipitation. Chem Eng J 157:93–99 (2010).

6 Toledano A, Serrano L, Garcia A, Mondragon I and Labidi J, Ligninseparation and fractionation by ultrafiltration. Sep Purif Technol71:38–43 (2010).

7 Martınez-Palou R, Ionic liquid and microwave-assisted organicsynthesis: a ‘green’ and synergic couple. J Mex Chem Soc 51:252–264(2007).

8 Gordon CM, New developments in catalysis using ionic liquids. ApplCatal A 222:101–111 (2001).

9 Wasserscheid P and Joni J, Green organic synthesis in ionic liquids,in Green Solvents. Volume 6: Ionic Liquids. Handbook of Green

Chemistry, 1st edn, ed by Anastas PT.Wiley-VCH, Germany, 41–91(2010).

10 Lee SH, Doherty TV, Linhardt RJ and Dordick JS, Ionic liquid-mediatedselective extraction of lignin from wood leading to enhancedenzymatic cellulose hydrolysis. Biotechnol Bioeng 102:1368–1376(2009).

11 Kilpelainen I, Xie H, King A, Granstrom M, Heikkinen S and ArgyropoulosDS, Dissolution of wood in ionic liquids. J Agric Food Chem55:9142–9148 (2007).

12 TAPPI Standards, TAPPI Test Methods, Atlanta (2007).13 Wise LE, Murphy M and D’Adieco AA, A chlorite holocellulose, its

fractionation and bearing on summative wood analysis and studieson the hemicelluloses. Paper Trade J 122:35–43 (1946).

14 Rowell R, The chemistry of solid wood: based on short course andsymposium sponsored by the division of cellulose, paper, andtextile chemistry at the 185th meeting of the American ChemicalSociety, Seattle, Washington, 70–72 (1983).

15 Garcıa A, Toledano A, Serrano L, Egues I, Gonzalez M, Martın Fand Labidi J, Characterization of lignins obtained by selectiveprecipitation. Sep Purif Technol 68:193–198 (2009).

16 Fu D, Mazza G and Tamaki Y, Lignin extraction from straw by ionicliquids and enzymatic hydrolysis of the cellulosic residues. J AgricFood Chem 58:2915–2922 (2010).

17 Muhammad N, Man Mohamad Z and Khalil AB, Ionic liquid - a futuresolvent for the enhanced uses of wood biomass. J Wood Prod70:125–133 (2012).

18 Tejado A, Pena C, Labidi J, Echeverria JM and Mondragon I,Physicochemical characterization of lignins from different sourcesfor use in phenol-formaldehyde resin synthesis. Bioresource Technol98:1655–1663 (2007).

19 Sun RC and Tomkinson J, Comparative study of lignins isolated byalkali and ultrasound assisted alkali extractions from wheat straw.Ultrason Sonochem 9:85–93 (2002).

20 Nakamoto K, Infrared and Raman Spectra of Inorganic and CoordenationCompounds: Theory and Applications in Inorganic Chemistry, 6th edn.Wiley Interscience (2009).

21 Sun XF, Sun R, Fowler P and Baird MS, Extraction and characterizationof original lignin and hemicelluloses from wheat straw. J Agric FoodChem 53:860–870 (2005).

22 Sun XF, Jing Z, Fowler P, Wua Y and Rajaratnamc M, Structuralcharacterization and isolation of lignin and hemicelluloses frombarley straw. Ind Crops Prod 33:588–598 (2011).

23 El Hage R, Brosse N, Chrusciel L, Sanchez C, Sannigrahi P and RagauskasA, Characterization of milled wood lignin and ethanol organosolvlignin from miscanthus. Polym Degrad Stab 94:1632–1638(2009).

24 Kim H, Ralph J, Lu F, Ralph SA, Boudet AM, MacKay JJ, SederoffRR, Ito T, Kawai S, Ohashi H and Higuchi T, NMR analysisof lignins in CAD-deficient plants. Part 1. Incorporation ofhydroxycinnamaldehydes and hydroxybenzaldehydes into lignins.Org Biomol Chem 1:268–281 (2003).

25 Sun RC, Tomkinson J and Jones GL, Fractional characterization ofash-AQ lignin by successive extraction with organic solvents fromoil palm EFB fibre. Polym Degrad Stab 68:111–119 (2000).

26 Sun RC, Lu Q and Sun XF, Psycho-chemical and thermalcharacterization of lignins from Caligonum monogoliacum andTamarix spp. Polym Degrad Stab 72:229–238 (2001).

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