Investigations of novel nitrile-based ionic liquids as pre-treatment solvent for extraction of lignin from bamboo biomass

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Investigations of novel nitrile-based ionic liquids as pre-treatment solvent forextraction of lignin from bamboo biomass

Nawshad Muhammad *, Zakaria Man, M. Azmi Bustam, M.I. Abdul Mutalib, Sikander Rafiq

PETRONAS Ionic Liquid Center, Department of Chemical Engineering, Universiti Teknologi PETRONAS (UTP), 31750 Tronoh, Perak, Malaysia

1. Introduction

Wood is a renewable source of valuable compounds such ascellulose, lignin, hemicelluloses and small amount of extractives[1]. In acquiring these valuable compounds for specific usage, thewood biomass has to undergo a treatment and separation processto recover them and this has to be done in a cost-effective mannerusing preferably green technology in line with the principles ofgreen chemistry. Among the main requirements for the processinvolved as recommended by the green chemistry principlesconsist of evasion in the use of toxic chemicals, reduction in theenergy consumption and minimizing the release of toxic chemicalsto the environment [2]. Different types of conventional methodshave been used for the pretreatment and separation of the woodbiomass to recover the valuable compounds stated earlier but theyall have some main drawbacks. For example the process involvingbiological treatments, despite of not requiring any chemicals andenergy, takes significantly longer time and also causes uncontrol-lable reaction at times [3]. On the other hand, the milling process

which is a physical method produces relatively poor yield and alsorequiring high energy consumption during its pretreatmentprocess [4,5]. The steam explosion method, although simple inoperation but due to its high temperature operation causes thedegradation of the biomass thus reducing the yield of cellulose andxylan significantly [6]. The hot water treatment is capable ofremoving 50% of lignin from the biomass and produces highlydigestible cellulose, but it needs high energy for operation [7]. Thechemical pre-treatments method such as dilute acid hydrolysis [8],ammonia fiber explosion [9], ammonia recycle percolation [10],lime process [11] and organosolv process [12] are also consideredas potential options but they require severe reaction conditions i.e.,high pressure and temperature and expensive to operate. As aconsequence, the search for new methods that is less toxic withlower energy consumption while at the same time meeting therecommended green chemistry principles are still being activelypursued.

ILs are organic salts with a melting point less than 100 8C andare considered as relatively green solvents due to their attractiveproperties such as negligible vapor pressure, nonflammable andnonexplosive, high electrochemical and thermal stability, highconductivity and easily recyclable [13,14]. The polar nature of ionicliquid has the ability for dissolving and fractionating the wood

Journal of Industrial and Engineering Chemistry 19 (2013) 207–214

A R T I C L E I N F O

Article history:

Received 19 June 2012

Accepted 4 August 2012

Available online 10 August 2012

Keywords:

Nitrile-based ionic liquids

Lignin extraction

Crystallinity

Characterization

A B S T R A C T

In this work, nitrile-based ionic liquids (ILs) i.e., 1-propyronitrile-3-butylimidazolium chloride

[C2CNBim]Cl, 1-propyronitrile-3-allylimidazolium chloride [C2CNAim]Cl, 1-propyronitrile-3-(2-hydro-

xyethyl)imidazolium chloride [C2CN HEim]Cl and 1-propyronitrile-3-benzyllimidazolium chloride

[C2CN Bzim]Cl were used as pre-treatment solvent for the extraction of lignin from bamboo biomass. The

pre-treatment process was investigated with respect to several factors such as the types of ionic liquid

cation used, the effect of pretreatment temperature and time, sample loading and particle size, the effect

of recycling the ionic liquid on lignin extraction and the effect of multi-extraction to enhance the recovery

of lignin which were collectively found to have an impact on the lignin extraction as a whole. The

crystallinity of the cellulose-rich material obtained from the extraction was analyzed using XRD while

the extracted lignin was characterized using FTIR, NMR, TGA and elemental analysis. From the XRD

analysis, the crystallinity of the cellulose-rich material obtained after treatment with the synthesized

nitrile-based ILs was found to remain the same. Among the nitrile-based ILs used, [C2CN Bzim]Cl

demonstrate the best performance for the extraction process in a predetermined condition (T = 120 8C,

t = 24 h) where 53% of the lignin from the bamboo was successfully extracted. This was confirmed from

the FTIR and NMR analysis showing the characteristic peaks indicating the presence of lignin in the

spectra of the respective samples tested.

� 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

* Corresponding author. Tel.: +60 125436650; fax: +60 53688151.

E-mail address: [email protected] (N. Muhammad).

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http://dx.doi.org/10.1016/j.jiec.2012.08.003


biomass for various applications. Dissolved wood biomassfractions can be regenerated through the addition of non-solventssuch as water, acetonitrile, methanol and ethanol mixture, andacetone/water mixture [15–18].

To date, very few ILs have been studied for the extraction oflignin from lignocellulosic materials [18–21]. Even though lignin isthe main constituent of lignocellulosic materials, but it did not gainimportance as cellulose. The aromatic structure of lignin could bebenefited for production of commercially valuable chemicals(phenols, etc.) and high energy fuels (bio-oil, etc.) [22]. It is obviousthat lignin, which could be considered as the largest renewablesource of aromatic compounds [23], will gain importance in thenear future for production of valuable products.

The present study is commissioned for extraction of lignin usingdifferent types of ILs namely 1-propyronitrile-3-butylimidazoliumchloride [C2CNBim]Cl, 1-propyronitrile-3-allylimidazolium chlo-ride [C2CNAim]Cl, 1-propyronitrile-3-(2-hydroxyethyl)imidazo-lium chloride [C2CN HEim]Cl and 1-propyronitrile-3-benzyllimidazolium chloride [C2CN Bzim]Cl. The lignin extractionperformance from the bamboo biomass is studied with respect tothe type of ionic liquids cations used, extraction process time andtemperature, loading amount and particle size of bamboo sampleused and the recycling effect of the ionic liquid. The extractedlignin is characterized using FTIR, NMR, TGA and CHNS analysis.

2. Experimental

2.1. Materials

1-Butyl-3methylimidazolium chloride ([Bmim]Cl) is purchasedfrom Merck while the other ILs[C2CNBim]Cl, [C2CNAim]Cl, [C2CNHEim]Cl, and [C2CN Bzim]Cl are synthesized in our laboratory.Acetone and indulin AT (lignin standard) are obtained from Sigma–Aldrich. Bamboo (cellulose, xylan and acid insoluble lignincontents are 47.08%, 16.9% and 24%, respectively), locally knownas buluh Semantan, is acquired from the local market and it wasgrinded into different particle sizes ranging between 125 mm and1 mm. It undergoes a drying process in an oven set at 70 8C for 24 h.

2.2. Synthesis of nitrile-based ionic liquids

The nitrile-based ILs were synthesized using the methodreported earlier [24] with the exception that the description aregiven in more detail in this paper.

2.2.1. 1-Propyronitrile-3-butylimidazolium chloride, [C2CN Bim]Cl

The synthesis of [C2CN Bim]Cl involves two steps. In the firststep, 0.2 moles of imidazole is charged together with methanol(solvent) into a three-necked flask, and the mixture is stirred untilthe imidazole has been completely dissolved. 0.23 moles ofacrylonitrile is then added gradually (drop wise) and the systemis heated and maintained at 55 8C for 16 h under nitrogen flow withcontinuous stirring. The methanol solvent and unreacted acrylo-nitrile are removed using a rotary vacuum pump, operating at70 8C. In the second step, the resultant 1-propyronitrile imidazoleis again charged into a three necked flask, and 0.4 moles of 1-chlorobutane is added. The mixture in the flask is cooled usingexternal cooling water flow while keeping it under continuousstirring. The mixture is then heated at 70 8C for 48 h under nitrogenblanket. The resultant viscous product is washed three times withdiethyl ether, and the remaining solvent is removed by rotaryvacuum pump at 80 8C for 6 h. It is then dried in a vacuum oven at80 8C for 48 h. The characterization analysis performed on theproduct shows the following results; [1H NMR (CDCl3): d = 0.90(3H, t), 1.38 (2H, m), 1.85 (2H, m), 3.20 (2H, t,), 4.25 (2H, t), 4.59(2H,t), 4.70 (1H, s), 7.75 (1H, s), 9.25 (1H, s). Elemental Analysis (%).

Calculated: C, 56.20; H, 7.54; N, 19.66. Found: C, 56.05; H, 7.71; andN, 19.54. TGA analysis; Td 514 K].

2.2.2. 1-Propyronitrile-3-allylimidazolium chloride, [C2CN Aim] Cl

1-Propyronitrile imidazole is synthesized according to the sameprocedure described above by reacting the imidazole withacrylonitrile in methanol solvent. The mixture is stirred for 16 hat 55 8C under continuous flow of nitrogen. The desired 1-propyronitrile imidazole (0.4 moles) is charged into a three neckedflask, and a slightly excess amount of allyl chloride (0.42 moles) isadded drop wisely under continuous flow of nitrogen. The mixtureis heated and maintained at 55 8C for 12 h. The resultant brownviscous liquid, after removing the excess allyl chloride undervacuum, is washed three times with diethyl ether. The remainingsolvent is then removed under vacuum and the product is dried in avacuum oven at 70 8C for 48 h. The characterization analysisperformed on the product shows the following results; [1H NMR(DMSO-d6): d = 3.22 (2H, t), 4.53 (2H, t), 4.89 (2H, d), 5.33 (2H, d),6.05 (1H, m), 7.78 (1H, s), 7.87 (1H, s), 9.30 (1H, s). ElementalAnalysis (%). Calculated: C, 54.68; H, 6.11; N, 21.25. Found: C,54.49; H, 6.34; and N, 21.12. TGA analysis; Td 526 K].

2.2.3. 1-Propyronitrile-3-(2-hydroxyethyl)imidazolium chloride,

[C2CN Heim]Cl

1-Propyronitrile imidazole is also synthesized according to thesame procedure described above where imidazole is initiallydissolved in methanol solvent, followed by its reaction withacrylonitrile at 55 8C for a period of 16 h under continuous flow ofnitrogen. After the synthesis, 1-propyronitrile imidazole is chargedinto a three necked flask and an equal moles of 2-chloroethanol isadded drop wisely under continuous flow of nitrogen. The mixtureis heated at 70 8C for 48 h. The resultant white viscous liquid, afterremoving the excess 2-chloroethanol under vacuum, is washedwith diethyl ether for three times. The remaining solvent isremoved under vacuum and the product is then dried in a vacuumoven at 70 8C for 48 h. The characterization analysis performed onthe product shows the following results; [1H NMR (DMSO-d6):d = 3.28 (2H, t), 3.73 (2H, t), 4.28 (2H, t), 5.33 (2H, d), 4.55 (2H, t),5.43 (1H,t), 7.85 (1H, s), 7.91 (1H, s), 9.40 (1H, s). ElementalAnalysis (%). Calculated: C, 47.64; C, 5.99; N, 20.83. Found: C,47.39; H, 6.18; and N, 20.71. TGA analysis; Td 560 K].

2.2.4. 1-Propyronitrile-3-benzyllimidazolium chloride, [C2CN Bzim] Cl

The same synthesis procedure is again adopted for 1-propyronitrile imidazole. After the synthesis, 1-propyronitrileimidazole is charged into a three necked flask, and excess moles ofbenzyl chloride is added drop wisely under continuous flow ofnitrogen. The mixture is heated and maintained at 55 8C for 24 h.The resultant brown viscous liquid, after removing the excessbenzyl chloride under vacuum, is washed three times with diethylether. The remaining solvent is removed under vacuum and thenthe product is dried in vacuum oven at 70 8C for 48 h. Thecharacterization analysis performed on the product shows thefollowing results; [1H NMR (DMSO-d6): d = 3.29 (2H, t), 4.55 (2H,t), 5.47 (2H, s), 7.44 (5H, m), 7.89 (1H, s), 7.93 (1H, s), 9.62 (1H, s).Elemental Analysis (%), Calculated: C, 63.02, H, 5.69; N, 16.83.Found: C, 62.95; H, 5.83; and N, 16.74. TGA analysis; Td 529 K].

2.3. Pre-treatment process

For the study on the pretreatment process, 5 g of each ionicliquid are charged in different reagent bottles which are kept in oilbath heated by a hotplate. Prior to mixing the ionic liquids with thebamboo biomass for the pretreatment process study, the ionicliquids are all heated and maintained at a temperature of 120 8C for30 min to dry them further in order to eliminate any effect from the

N. Muhammad et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 207–214208


presence of water. A predetermined amount of bamboo powder isthen added to the respective ionic liquids which are then subjectedto various pretreatment temperature and time in order to conductthe study. In addition, the loading amount of the bamboo biomass isalso varied and for different particle sizes. This enables a systematicstudy to be conducted on determining the effect of these parameterson the performance of the pretreatment process. Besides the above,the recycling effect of the ionic liquids is also investigated. Duringthe pretreatment process, magnetic stirring device is used as ameans of blending the mixture and the speed is kept constant at400 rpm across all samples. After the pretreatment process, thesample is mixed with a mixture containing acetone and water at aratio of 7:3, respectively and the stirring is continued at 400 rpm for1 h at atmospheric condition (T, P). The lignin was found to dissolvein the acetone/water mixture while the cellulose-rich materialprecipitates out. The cellulose rich material is filtered using avacuum filtration system and the produced filtrate is subjected torotary evaporator for recovering the acetone. As the acetone contentreduces in the filtrate, the lignin begins to precipitate out. Theprecipitated lignin is then filtered using a vacuum filtration system.To recover the ionic liquid, the resultant filtrate is made to saturateby evaporating the water followed by addition of ethanol (95%)(three times the quantity of the filtrate) for removal of remainingresidual polysaccharide components. After that, the filtrate issubjected to a rotary evaporator for 6 h at 80 8C followed by avacuum oven drying at 80 8C for 24 h for removing the ethanol andwater. The filtrate left represents the recovered ionic liquid which isthen reused for another round or pretreatment process. Therecovered lignin (%) is measured with respect to the acid insolublelignin content related to the bamboo (i.e., 24%, measured byNREL method). The calculations are made on an oven-dried(105 8C, 4 h) basis.

2.4. Characterizations

2.4.1. X-rays diffraction analysis (XRD)

The crystalline structure of the untreated bamboo and theregenerated cellulose-rich materials are analyzed using powder X-ray diffraction (PXRD), model Bruker D8 Advance horizontal X-raydiffractometer equipped with Cu anode, at room temperature. Thesamples are scanned within the range of 5.00–40.008 2u in a stepmode with a change of 0.018 at a rate of 18 min�1.

2.4.2. Fourier transform infrared analysis (FTIR)

The FTIR spectra for the samples are taken using SHIMADZU8400S spectrometer within the wave number range of 4000–400 cm�1 with 20 scans at 4 cm�1 resolution. The samples aremixed with KBr in a weight ratio of about 1:100 to form a pellet.Each FTIR spectrum is recorded with the blank KBr pellet as thebackground.

2.4.3. Nuclear magnetic resonance analysis (NMR)

Indulin AT and the ionic liquid regenerated lignin arecharacterized using Bruker Avance 400 spectrometer. For theNMR analysis, indulin AT and extracted lignin are dissolved inDMSO-d6 at 100 8C for 24 h. The 13C NMR spectra are taken at roomtemperature with 2000 scans.

2.4.4. Thermogravimetric analysis (TGA)

Perkin-Elmer, Pyris V-3.81 thermal gravimetric analyzer is usedto measure the onset and thermal decomposition temperature ofthe samples. The samples are placed in an aluminum pan beforebeing subjected to heating with temperature range 40–550 8Cand increasing rate of 10 8C min�1 under nitrogen blanket (flowrate 20 mL/min). The temperature accuracy is found to be betterthan �3 C.

2.4.5. Elemental analysis (CHNS)

The amount of carbon, hydrogen, nitrogen and sulfur in thesamples are analyzed according to ASTM D-5291 method usingLeco-CHNS-932 analyzer. The amount of each sample used is lessthan 2 mg and is covered in a silver capsule prior to analysis.

3. Result and discussion

3.1. Dissolution ability of bamboo

The dissolving capability of the ionic liquids on bamboo iselucidated by studying the crystallinity of the cellulose obtained inthe cellulose-rich materials after the extraction of lignin from thebamboo biomass. In the past, XRD analysis has been used toobserve the change in crystallinity for cellulose during ionic liquidpre-treatment [1].

XRD diffractograms of the untreated bamboo and the onetreated with nitrile-based ILs are shown in Fig. 1. The XRD patternsfor both are found to be similar i.e., the locations of the peak are at22.2 (22.4 for treated sample) and 15.8 which have been reportedfor type I cellulose before [25]. Normally, one would expect thecrystalline form of the native cellulose will change from cellulose Ito cellulose II type during dissolution and regeneration from ILs[26,27]. Therefore it can be concluded that no proper dissolution ofbamboo biomass has taken place using the nitrile-based ILs.However, except [C2CNHEim]Cl treated samples (for which thecrystallinity was increased), the crystallinity of cellulose in thetreated bamboo biomass decreased to a certain extent duringdissolution as indicated by the decrease in peak intensity as shownin Fig. 1.

The SEM analysis (results have not shown) of the untreatedbamboo and the one treated with nitrile-based ILs also revealedthat the bamboo has not been completely dissolved in the nitrile-based ionic liquids.

3.2. Extraction ability of lignin

3.2.1. Type of ionic liquid cations

The results obtained for the present synthesized nitrile-basedILs were compared among each other as well to [Bmim]Cl in orderto study the effect of structures of cations on lignin extraction as

4035302520151050

20

40

60

80

100

120

140

E

D

C

A

Inte

nsity

2-theta

B

Fig. 1. Diffractograms of (A) untreated bamboo, (B) treated material of

[C2CNAim]Cl, (C) treated material of [C2CN Bim]Cl, (D) treated material of

[C2CN Bzim]Cl and (E) treated material of [C2CNHEim]Cl.

N. Muhammad et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 207–214 209


the anion type used in all the ILs is the same i.e., Cl. The nitrile-based ionic liquids, especially [C2CNBzim]Cl and [C2CNAim]Cl, dueto the presence of more unsaturated bonds are found to berelatively more effective for extraction of lignin compared to[Bmim]Cl (53%, 47% and 38% of lignin were extracted by[C2CNBzim]Cl, [C2CNAim]Cl and [Bmim]Cl, respectively). Irre-spective of bamboo dissolution in the studied ILs the increased inlignin extraction by nitrile-based ILs is possibly due to more p–pinteraction between ionic liquids and the lignin p system.Comparatively, [C2CNHEim]Cl which extract 44%, was moreefficient than [C2CNBim]Cl having efficiency of 41% for ligninextraction under similar conditions. The high extraction capabilitymight be due to more hydrogen bonding interactions betweenhydroxyl group of lignin and [C2CNHEim]Cl cation. The molecularstructures of the ILs and the lignin are as shown in Fig. 2.

From the experiment conducted, the order of increasing ligninextraction for all the ILs was found to be [Bmim]Cl < [C2CN-CNBim]Cl < [C2CNHEim]Cl < [C2CNAim]Cl < [C2CNBzim]Cl asdemonstrated in Fig. 3. As the anion type used in all the ILs isthe same i.e., Cl, the difference in the extraction efficiency isobviously due to the molecular structure of the cation. The cationswhich possess more attraction sites for the interactions can be saidto be more efficient for lignin extraction. The experimentalconditions (temperature – 120 8C, particle size – 125 mm,extraction time – 24 h and loading amount – 5% of ionic liquid)are kept constant for all ionic liquids.

3.2.2. Effect of extraction temperature and time

Figs. 4 and 5 show the recovery of lignin (wt%) with respect totemperature and time. The figures indicate that more lignin was

[Bmim]Cl [C 2CNHEim] Cl

[C2CNB zim] Cl [C2CNB im]Cl

[C2CNAim] Cl

Lignin Subuni t

N+N

N

Cl-

OCH3

OH

OH

N+

N

N

OHCl-

N+

N

N

CH3

Cl-

N+

N

N

CH2Cl-

N+

NCH3CH3 Cl

-

Fig. 2. Structure of lignin subunit (Coniferaldehyde), and [Bmim]Cl and nitrile based ionic liquids.

[Bmim]Cl[C2CNBim]Cl

[C2CNHEim]Cl[C2CNAim]Cl

[C2CNBzim]Cl 0

10

20

30

40

50

B

Wei

ght (

%) o

f rec

over

dlig

nin

Ionic liquids

Fig. 3. Effect of cations on lignin extraction from bamboo biomass (temperature 120 8C, and extraction time 24 h).



recovered at higher temperatures and longer pre-treatment time.For example the lignin extraction by [C2CNBzim]Cl was 66% at160 8C and 53% for 24 h as compared to 13% at 100 8C and 34% for4 h. This is in line with the earlier reported findings fromexperiments conducted using different type of biomass, ionicliquid and experimental conditions [28,29]. Temperature has amore profound effect on lignin extraction compares to pre-treatment time especially between 140 and 160 8C which might beassociated with the glass transition temperature of the lignin asreported elsewhere [30]. Below 120 8C, the temperature effect issubstantially reduced. As for the extraction time, the increased in

the extraction performance with time is more gradual andconsistent.

3.2.3. Effect of sample loading

Individual bamboo samples of 5, 10 and 20% are loaded in[C2CNBzim]Cl ionic liquid to study the effect of sample loading onextraction efficiency of lignin. The sample used in the study has aparticle size of 125 mm. The mixtures of ionic liquid and bambooare heated at 120 8C for 24 h. The plot showing the amount of ligninrecovered against sample loading is shown in Fig. 6. The resultshows that increasing the loading amount results in decreased

90100110120130140150160170 0

10

20

30

40

50

60

70

BmimCl

C2CNBimCl

C2CNHEimCl

C2CNAimCl

C2CNBzimCl

Wei

ght (

%) o

f rec

over

dlig

nin

Temperature / °C

Fig. 4. Recovered lignin vs. temperature, after 4 h.

510

1520

25 0

10

20

30

40

50

C2CNBzimCl

C2CNAimCl

C2CNHEimCl

C2CNBimCl

BmimCl

Wei

ght (

%) o

f rec

over

dlig

nin

Time /hours

Fig. 5. Recovered lignin vs. time at 120 8C.



extraction efficiency of the ionic liquid. As the sample load wasincreased from 5 to 10%, the extraction efficiency reduces by 9.05%and was further reduced by 24.5% as the sample loading increasesto 20%. The reduction of the extraction efficiency can be attributedto the fact that higher sample loading causes the viscosity of themixture to increase which in turn reduces the mobility of ionicliquid ions [31]. This causes less interaction between the ionicliquids and the biomass, thus reducing the extraction efficiency ofionic liquids.

3.2.4. Effect of particle size

Particle size of 125 mm and 1 mm are selected for theinvestigation on lignin extraction efficiency using [C2CNBzim]Clionic liquid. The temperature and the pre-treatment time aremaintained at 120 8C and 24 h, respectively. It was found that theextraction efficiency of the ionic liquids decreases when biggersized particles are used as shown in Fig. 7, showing the drop from53% for particle size of 125 mm to 32.8% for particle size of 1 mm. Apossible reason to explain the reduced efficiency is the inability ofthe ionic liquid to penetrate deeper into the biomass structurewhen the particle size is larger [31]. Moreover, this ionic liquid has

a low solvency capacity to dissolve bamboo biomass as discussedin the previous section thus hindering the interaction of the ionicliquids with lignin entrapped by a three dimensional network oflignocelluloses biomass.

3.2.5. Recycling of ionic liquid and its effect on lignin extraction

The recovered mass of the ionic liquids after recycling wasfound to be more than 97%, which indicated the ionic liquids as apotentially good recyclable solvent. The ionic liquid [C2CNBzim]Clis recycled for three times, and the recycling effect is evaluated forthe extraction efficiency. The results are shown for ligninextraction in Fig. 8 which are 53%, 54.2% and 52.5% for repeatedrecycling cycles 1, 2 and 3, respectively. For the study, thetemperature, pretreatment time, and particle size are keptconstant at 120 8C, 24 h and 125 mm, respectively. Fig. 8 showsthat the recycled ionic liquid demonstrated relatively similarefficiency as the fresh ionic liquid (similar findings for other typesof ionic liquids were reported by Pinkert et al. [31]). This suggeststhat the interaction between the ionic liquids and bamboo biomassis mainly physical instead of chemical reaction and thereforemaking the ionic liquids to be easily regenerated.

0

10

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30

40

50

40%

48.2%

20%10%5%

Wt %

of r

ecov

ered

lign

in

Sample loading

53%

Fig. 6. Weight % of recovered lignin with varying sample load as compared to ionic

liquids using [C2CNBzim]Cl at 120 8C for 24 h.

0

10

20

30

40

50

32.8%

1mm125 µm

Wt %

of r

ecov

ered

lign

in

Particle size diameter

53%

Fig. 7. Weight % of recovered lignin with varying particle size using [C2CNBzim]Cl at

120 8C for 24 h.

3210

10

20

30

40

50

6054.2%

52.5%

Wt %

of r

ecov

ered

lign

in

Recycling cycle

53%

Fig. 8. Effect of recycling cycle of ionic liquid on the recovery of lignin using

[C2CNBzim]Cl at 120 8C for 24 h.

210

10

20

30

40

50

10.5%Wt %

of r

ecov

ered

lign

in

extraction numbers

53%

Fig. 9. Effect of multi-extraction on recovery of lignin using [C2CNBzim]Cl at 120 8Cfor 24 h.



3.2.6. Recovery of lignin by multi-extraction

The recovery of lignin was also investigated by performingmulti-extraction experiment using the same experimental condi-tions as the pretreatment process. The same bamboo sample istreated twice with fresh ionic liquid ([C2CNBzim]Cl) and therecovery of lignin after each extraction cycle was determined. Theresults are shown in Fig. 9. The first stage extraction efficiency wasfound to be 53% while the second stage only showed 10.5%. Thereduction in the extraction efficiency on the second stage is mostlikely due to the increasing difficulties in extracting the remaininglignin which is then situated deeper in the bamboo texture causingmore resistance [31]. Although the efficiency of the subsequentstage has dropped to 10.5% but by combining the two stages, thetotal recovery of lignin has been enhanced to 63.5% by using themulti-extraction method.

3.3. Characterizing of the extracted lignin

3.3.1. FTIR analysis

For confirmation, the FTIR spectra of the recovered lignin andthe commercial standard indulin AT (Sigma–Aldrich) are taken andcompared as shown in Fig. 10. The FTIR spectra are analyzed for thepresence of hydroxyl, carbonyl, methoxyl and carboxyl functionalgroups that have been reported for identification of lignin [32]. Abroad band of hydroxyl and methoxyl groups were observedaround 3400 cm�1 and 2960–2930 cm�1, respectively. The band at1703 cm�1 is due to the C55O of nonconjugated carboxyl groupfrom hemicelluloses contamination while the band at 1652 cm�1 isdue to the carbonyl stretching conjugated with aromatic ringskeleton. The strong intensity bands from 1600 to 1507 cm�1 areassociated with the aromatic skeleton of lignin. The bands at1457 cm�1, 1400 cm�1 and low intensity band at 740 cm�1 are dueto the vibration of C–H deformations and aromatic ring group.Bamboo lignin contains high proportion of syringyl residues whichcan be observed by an intense single band at 832 cm�1, and1264 cm�1 in spectrum B [33]. The low intensity band at1213 cm�1 is attributed to the aromatic phenyl C–O stretching.C–H in-plan deformation of syringyl units is noted from the bandsat 1119 and 1028 cm�1 [34]. Most of the bands observed in the FTIRspectra of the extracted lignin correspond well with the bandsobtained for the commercial lignin standard i.e., indulin AT,reflecting the similarities in the structure of the two.

3.3.2. NMR analysis

The 13C NMR spectrum obtained for the lignin standard i.e.,indulin AT and the extracted lignin as shown in Fig. 11 shows the

presence of the aromatic OMe 13C resonance at 56.3 ppm whichcorresponds to lignin [35]. The peak at 39.8 ppm belongs to DMSOsolvent, and the peak at 52.6 ppm in spectra (B) corresponds tosome impurity either from carbohydrates or ionic liquid.

3.3.3. TGA analysis

The results on thermogravimetric analyses conducted onindulin AT and the extracted lignin from bamboo, as shown inFig. 12, showed the onset temperature to be around 244 8C and238 8C and the maximum decomposition temperature to bearound 370 8C and 276 8C, respectively. The behavior of thethermal decomposition for extracted lignin was found to havesimilar pattern as reported by [30] for lignin extracted frombagasse. The decomposition behavior can be divided into threestages i.e., stage I, stage II, and stage III, corresponding to therespective temperature ranges of 50–150 8C, 151–470 8C and471 8C onward. In stage I, no weight losses was observed as thesample was dried isothermally in the furnace pan in the TGA undernitrogen flow to ensure removal of water and other volatiles [36].In stage II, maximum decompositions were observed for bothsamples giving the products comprising of coke, organic andphenolic compounds, and gaseous products. In stage III, thearomatic rings of the lignin finally decomposed [37]. DTG showsthat the lignin commercial standard i.e., indulin AT, is thermallymore stable than the extracted lignin which might be due to the

50010001500200025003000350040000

10

20

30

40

50

60

1/cm

% tr

ansm

ittan

ce

B

A

Fig. 10. Infrared spectra of (A) indulin AT, and (B) extracted lignin using


3540455055606570

0

200

400

600

Inte

nsity

ppm

B

A

Fig. 11. 13C NMR spectrums of (A) indulin AT, and (B) extracted lignin using


50040030020010040

50

60

70

80

90

100

Temperature /°C

% w

eigh

t los

s A

B

Fig. 12. Thermal decomposition profile and derivatives of (A) indulin AT and (B)

extracted lignin using [C2CNBzim]Cl at 120 8C for 24 h.



difference in the content of C–C linkages [36,37]. The residual charyield from the regenerated sample at 550 8C is slightly lower thanthe indulin AT and was observed at between 45 and 50 wt%.

3.3.4. CHNS analysis

The CHNS analysis conducted on the extracted lignin samplesshowed that it has slightly lower carbon content compared tothe indulin AT which indicates the presence of polysaccharidecomponents in the extracted lignin (Table 1). Higher nitrogencontent is observed in the extracted materials, most likely due tothe slight contaminations from the nitrile based ionic liquids. Onthe contrary, the sulfur content of the indulin AT is observed tobe higher than the extracted lignin which might be due to thedifferent procedures adopted for extracting the lignin.

4. Conclusion

The study showed that the incorporation of nitrile and otherunsaturated functionality to the cation of imidazolium has helpedto increase the lignin extraction efficiency of ionic liquids.Temperature is found to have significant effect on the extractionefficiency of the pre-treatment particularly within the range of120–160 8C as the glass transition temperature for lignin resideswithin this temperature range. Higher sample loading andincreased in particle sizes are observed to have inverse effect onthe extraction efficiency of the lignin. Although lower ligninextraction efficiency is observed for the subsequent treatment in amulti stage extraction, higher overall extraction efficiency wasobtained. The recyclability of the ionic liquid was also demon-strated from the experimental work conducted on three cycles,signifying its potential for the use as a solvent for lignin extractionfrom bamboo. A comparative study on the molecular structurebetween the extracted lignin and the commercial standard lignini.e., indulin AT, showed almost perfect match signifying thesimilarities between the two although slightly lower thermalstability and carbon content were measured for the former againstthe later.

Acknowledgements

The financial assistance provided by FRGS, MOHE andPETRONAS Ionic Liquid Center, Universiti Teknologi PETRONAS(UTP) is gratefully acknowledged.

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Table 1CHNS analysis of indulin AT, and extracted lignin using [C2CNBzim]Cl at 120 8C for

24 h.

Sample Carbon

(%)

Hydrogen

(%)

Nitrogen

(%)

Sulfur

(%)

Indulin AT 58.72 4.64 1.12 1.77

Extracted lignin 57.21 4.52 1.50 0.36


Investigations of novel nitrile-based ionic liquids as pre-treatment solvent for extraction of lignin from bamboo biomass

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