Impact of catalytic oil palm fronds (OPF) pulping on organosolv lignin properties

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Polymer Degradation and Stability 109 (2014) 33e39

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Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate /polydegstab

Impact of catalytic oil palm fronds (OPF) pulping on organosolv ligninproperties

M. Hazwan Hussin a, b, Afidah Abdul Rahim a, Mohamad Nasir Mohamad Ibrahim a,Dominique Perrin b, Mehdi Yemloul c, Nicolas Brosse b, *

a Lignocellulosic Research Group, School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysiab Laboratoire d'Etude et de Recherche sur le MAteriau Bois (LERMAB), Faculte des Sciences et Techniques, Universite de Lorraine, Bld des Aiguillettes,F-54500 Vandoeuvre-les-Nancy, Francec Institut de Sciences Moleculaire de Marseille, Aix-Marseille Universite, Service 512, Campus Scientifique de St J�erome, F-13397 Marseille, France

a r t i c l e i n f o

Article history:Received 19 May 2014Received in revised form19 June 2014Accepted 22 June 2014Available online 5 July 2014

Keywords:Organosolv ligninOil palm fronds1,8-DihydroxyanthraquinonePhenolic eOHAntioxidant

* Corresponding author. Tel.: þ33 3 83 68 48 62; faE-mail address: [email protected]

http://dx.doi.org/10.1016/j.polymdegradstab.2014.06.00141-3910/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

This article sheds light on the structural characteristic and antioxidant activity of the ethanol organosolvlignin extracted from oil palm fronds (OPF) via incorporation of 1,8-dihydroxyanthraquinone during thedelignification process. The resulting modified organosolv lignin (DEOL) was studied by 31P NMR, HSQC,HMBC and GPC. It was proposed that addition of a catalytic amount of 1,8-dihydroxyanthraquinoneduring pulping process; (1) enhanced the dissolution of lignin and the delignification rate, (2)improved the solubility of the resulting modified lignin (DEOL) by reducing its hydrophobicity propertiesand (3) improved its antioxidant activity compared to untreated organosolv lignin (EOL) (DEOL: 78% andEOL: 53% of Oxygen Uptake Inhibition (OUI) respectively). It was shown that antioxidant activity wasclosely related to its average molecular weight and phenolic hydroxyl content.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The most crucial issues faced by the world today are to ensurethe sustainability of consumption for energy and natural resources.As the fossil fuel is creating problematic issues (such as globalwarming, increase in price and running out), the use of renewableresources to shift the oil-based economy into bio-based economyleads to a new discovery as an alternative. With a goal of reducingnet greenhouse gas emission, this marks an important turningpoint in effort to promote the use of renewable energy to fulfill thecommitments of the Kyoto Protocol [1,2]. Lignocellulosic biomass isbest-suited for energy and chemical applications due to its suffi-cient availability, inexpensive and environmentally safe. Recentwork in this area has mainly focused on the delignification oflignocellulosic biomass separating lignin, cellulose and hemi-celluloses to be used in both physical and chemical applications.

It has been acknowledged that organosolv delignification allowsa clean fractionation of lignocellulosic feedstocks and the recoveryof high-quality lignins (relatively pure, less condensed than otherindustrial lignins, sulfur free, soluble in organic solvent) are of great

x: þ33 3 83 68 44 98..fr (N. Brosse).

16

interest and are currently a focus of attention [3]. Thus, availabilityof such organosolv lignin fractions in large quantities shouldstimulate development in new lignin utilizations. However, thevaluable utilization of the lignin produced at the industrial scalerequires a good control of its variability which is a function of thenature of the raw material and also of the processes used for thelignin extraction [4].

Lignin can act as a neutralizer or inhibitor in oxidation pro-cesses, via stabilizing reactions induced by oxygen radicals andtheir respected species due to high content of diverse functionalgroups (phenolic and aliphaticeOH, carbonyls, carboxyls, etc.) andits phenylpropanoid structure. The applicability of lignins fromdifferent sources as potential antioxidants has been also success-fully tested [5e7]. Moreover, it was revealed that the extractionprocesses of lignin may give major effect on its antioxidant capacity[8]. Consequently, the antioxidant properties exhibited by lignincan give broader applications as anti-microbial, anti-aging agentsand corrosion inhibitors.

Nevertheless, the complexity of lignin structures might jeopar-dize the fate of these organic compounds. In addition, high hy-drophobicity of organosolv lignin can limit its capability to beemployed in other possible applications. Therefore, the modulationof suitable lignin structures (by considering its solubility, molecularweight, phenolic content) is important so that it can overcome such

M.H. Hussin et al. / Polymer Degradation and Stability 109 (2014) 33e3934

implications. Obviously, the properties of lignin can be improved bymodifying the structure into a more suitable structure type. Elec-tropolymerization [9] and graft polymerization [10] of lignin aresome effective ways to convert insoluble lignin into a water-solublelignin. It was also demonstrated that the presence of carbonium ionscavengers in combinative treatment (prehydrolysis followed bydelignification) could substantially improve the lignin extractionyields and its properties through inhibition of recondensation re-actions [7,11e13]. In this approach, the lignin deconstruction duringthe prehydrolysis treatment increases its extractability on orga-nosolv treatment through the breaking of ligninecarbohydratebonds, resulting in smaller lignin fragments. Smaller and compactstructure (low molecular weight, high phenolic eOH content withbetter solubility) of lignin is indeed beneficial for later usageespecially in antioxidant applications.

Addition of anthraquinone in soda pulping is described toaccelerate wood delignification. Anthraquinone has been shown tooperate in a redox cycle: the reducing end groups of carbohydratesdissolved in the pulping liquor reduce the anthraquinone toanthrahydroquinone (Fig. 1). In the soda pulping conditions,anthrahydroxyquinone then reacts with lignin quinone methidesthrough an additioneelimination reaction and causes the b-arylether linkage in the lignin molecule to cleave [14,15].

In the previous work, we have successfully revealed the effect of2-naphthol as an organic scavenger during combinative treatmentof oil palm fronds (OPF) biomass [7]. Even though incorporation oflignin with 2-naphthol improves the lignin extraction yield and itsantioxidant activity (Oxygen Uptake Index (OUI) %: ~70%), it wasfound that the solubility inwater (~2.07% dissolution) of this type oflignin was very low. Higher solubility of lignin is somewhatimportant for subsequent applications. Therefore, in order toimprove the properties of lignin, the aim of the present workwas toinvestigate the impact of the incorporation of another suitable ar-omatic moieties derived from anthraquinone (1,8-dihydroxyanthraquinone) during the delignification process onthe lignin physicochemical properties. The influence of ligninstructure by its (syringyl) S, (guaiacyl) G and (p-hydroxyphenyl) Hbasic units, phenolic/aliphatic hydroxyl content and molecular sizewere also identified via FTIR, 31P NMR and GPC. Besides, the effecton the lignin structure in the presence of 1,8-dihydroxyanthraquinone as a carbonium ion scavenger was alsostudied with additional information obtained from two dimen-sional 13Ce1H HSQC, HMBC NMR. The improvement of lignin sol-ubility and antioxidant properties were also investigated viadissolution test, oxygen uptake inhibition and reducing powerassay.

2. Material and methods

2.1. Material

The oil palm fronds (OPF) were obtained from Valdor Palm OilMill near Sungai Bakap plantation (Seberang Prai, Malaysia) in mid2012. The OPF leaves were removed and the strands were chipped

O

O

reducing

sugars

OH

OHanthraquinone anthrahydroxyquinone

Fig. 1. Anthraquinone reduction into anthrahydroquinone by reducing sugars.

into small pieces. After sun dried for 3 days, the chips were thenground to a 1e3 mm size usingWiley mill and the fiber was furtherdried in an oven at 50 �C for 24 h. The OPF biomass was first sub-jected to Soxhlet extraction with ethanol/toluene (2:1, v/v) for 6 hbefore use. All chemical reagents used in this study were purchasedfrom Sigma Aldrich, Merck, QRec (Malaysia) and VWR (France) andused as received. Dried matter contents were determined using amoisture balance, KERN MRS 120-3 Infra-red moisture analyzer(drying at 105 �C to constant weight). The effective dry mattercontent of raw OPF biomass was ~89%.

2.2. Pretreatment with organic scavengers and autohydrolysispretreatment

About 20 g (oven driedmatter) of OPFwere immersed in 100mLof acetone containing 0.8 g of 1,8-dihydroxyanthraquinone (Merck)at room temperature. After thorough mixing, biomass sampleswere air-blown to dryness at room temperature followed byautohydrolysis. The treated OPF sample was loaded into a 0.6 Lstainless steel pressure Parr reactor with a Parr 4842 temperaturecontroller (Parr Instrument Company, Moline, IL) and was supple-mented with an appropriate amount of deionized water to obtain afinal solid to liquid ratio of 1:9, taking into account the moisturecontent of the sample. The mixture was heated at 150 �C withcontinuous stirring for 8 h (time zero was set when the presettemperature was reached with the heating rate of 5 �C min�1,severity factor: S0 ¼ ~4.1). At the end of each reaction, the reactorwas cooled and the liquid phasewas recovered by filtration throughWhatman No. 4 filter paper.

2.3. Lignin extraction through organosolv pulping

OPF (200 mm particle size, 25 g dry weight of autohydrolyzedtreated biomass) was mixed with water:ethanol (35:65, v/v) and0.5% w/w sulfuric acid as a catalyst at 190 �C for 60 min (severityfactor: S0 ¼ ~2.5), following the method outlined by El Hage et al.[16]. The solid to liquid ratio used was 1:8. Treatments were carriedout in a 0.6 L stainless steel pressure Parr reactor with a Parr 4842temperature controller (Parr Instrument Company, Moline, IL). Thereactionmixturewas heated at a rate of 5 �Cmin�1 with continuousstirring. At the end of the treatment, the free liquid was removedand then the fibrous residue was washed with 89% v/v aqueousethanol (3 � 50 mL) at 60 �C and air dried overnight. The washedliquid samples were combined, and three volumes of water wereadded to precipitate the ethanol organosolv lignin (EOL), whichwas collected by centrifugation at 4000 rpm for 10min and then airdried. The purification of lignin was conducted by extracting it inthe Soxhlet apparatus for 6 h with n-pentane to remove lipophilicnon-lignin matters such as wax, lipids and anthraquinone impu-rities. The purified organosolv lignin was then dried in an oven at40 �C under atmospheric pressure for another 24 h.

2.4. Characterization of lignin

The FTIR spectrophotometry was carried out in a direct trans-mittance mode using Perkin Elmer model System 2000 instrument.The region between 4000 and 400 cm�1 with a resolution of 4 cm�1

and 20 scans was recorded. The samples were prepared accordingto the potassium bromide technique, in a proportion of 1:100(200 mg of KBr approximately). Interpretation of the IR spectra wasdone using Perkin Elmer software.

All nuclear magnetic resonance (NMR) spectroscopy experi-ments were performed on a Bruker Avance-400 spectrometer.Quantitative NMR spectra were acquired using an inverse-gateddecoupling (Waltz-16) pulse sequence to avoid Nuclear

M.H. Hussin et al. / Polymer Degradation and Stability 109 (2014) 33e39 35

Overhauser Effect (NOE). The phosphitilating reagent employedhere was 2-chloro-4,4,5,5-tetramethyl-1,1,3,2-dioxaphospholane(TMDP) as previously described by Granata and Argyropoulos[17]. Approximately 25 mg of lignin was added into 400 mL solventmixture of pyridine:CDCl3 (1.6/1, v/v) (Aldrich). Then each 150 mL ofchromium (III) acetylacetonate (3.6 mg mL�1) and cyclohexanol(4.0 mg mL�1) solution in pyridine/CDCl3 was added to the ligninsolution. The chromium (III) acetylacetonate served as a relaxationreagent and cyclohexanol as the internal standard. The lignin so-lution was then vigorously stirred until completely dissolved. Mi-nutes before starting the NMR experiment, approximately 50 mL ofTMDP was added to the vial and the solution transferred into a5 mm NMR tube. The 31P NMR spectra were recorded with thefollowing acquisition parameters; inverse-gated pulse sequence,25 s pulse delay, 200 acquisitions, 61.7 ppm sweep width and 30�

pulse width (p1 ¼ 6.5 usec, pl 1 ¼ �2.0 db). 13Ce1H 2D Hetero-nuclear Single Quantum Correlation (HSQC) and HeteronuclearMultiple Bond Correlation (HMBC) NMR spectra of autohydrolyzedlignin treated and untreated with 1,8-dihydroxyanthraquinonewere recorded on a Bruker 500 MHz instrument using standardpulse program.

Lignin samples were subjected to acetylation in order toenhance their solubility in organic solvents, used in gel permeationchromatography (GPC). The number average molecular weights(Mn) and weight (Mw) of lignin were determined after derivatiza-tion by GPC with a Dionex Ultimate-3000 HPLC system consistingof an autosampler and a UV detector and using tetrahydrofuran aseluent. Standard polystyrene samples were used to construct acalibration curve. Data were collected and analyzed with Chrome-leon software Version 6.8 (Dionex Corp., USA).

The dissolution test was carried out as follows. Approximately200 mg sample was added to 500 mL distilled water at 25 �C andagitated constantly at 100 rpm for 6 h. 2 mL solution was with-drawn at predetermined intervals and filtered through a 0.45 mmsyringe filter. An equal volume of distilled water was replaced aftereach withdrawal [18,19]. After diluted with distilled water, the so-lution absorbance was measured at 280 nm (Shimadzu UV-2550Japan). According to the UV linear regression equation, ligninconcentration and the percentage of dissolution (D %) are calcu-lated as follows:

A� A0 ¼ K � C � L (1)

D% ¼ ðCmax � VtotalÞ=minitial � 100 (2)

where A0 is the absorption of distilled water, A is the absorption ofsample, K is the absorption constant (21 L g�1 cm�1) [18], C is theconcentration of lignin, Cmax is the maximum concentration at600 min, L is the thickness of quartz cell (cm), Vtotal is the totalvolume and minitial is the initial mass of lignin.

2.5. Measurement of lignin antioxidant activity

2.5.1. Oxygen uptake inhibitionAntioxidant properties of organosolv lignins were investigated

by evaluating oxygen uptake inhibition during oxidation of methyllinoleate. The induced oxidation by molecular oxygen was per-formed in a gas-tight borosilicate glass apparatus. Butan-1-ol wasused as solvent for lignin dissolution. Temperature was set to 60 �C,initial conditions inside the vessel were as follows; methyl linoleate(Fluka, 99%) concentration: 0.32 mol L�1; 2,20-azobisisobutyroni-trile (AIBN) (Fluka, 98%) concentration: 7.2 � 10�3 mol L�1; ligninconcentration: 0.2 g L�1; oxygen pressure: 150 Torr. Oxygen uptakewas monitored continuously by a pressure transducer (Viatronmodel 104). Without any additive, oxygen uptake is roughly linear

and constitutes the control. In the presence of an antioxidant, ox-ygen consumption is slower, and the antioxidative capacity (OUI) oforganosolv lignin was estimated by comparing oxygen uptake at achosen time (3 h), in the presence of this compound (pressurevariation DPsample) and in the absence of the compound (DPcontrol)according to:

OUIð%Þ ¼�DPcontrol � DPsample

�.DPcontrol � 100 (3)

This ratio defines antioxidative capacity as an oxygen uptakeinhibition index (OUI); it should spread from 0 to 100%, for poorand strong antioxidants, respectively, and may be negative forproxidants.

2.5.2. Reducing power assayThe reducing power of samples was determined by the method

proposed by Gulcin et al. [20] after a slight modification. Standardsyringaldehyde and vanillin (Aldrich) solution of concentrations2.0 � 10�2 mg mL�1 e 0.1 mg mL�1 were prepared. To 1.0 mL of thestandard solution, 2.5 mL of 0.2 M phosphate buffer at pH 6.6(prepared from the addition of 0.2 M Na2HPO4 and 0.2 M NaH2PO4(QRec) and 2.5 mL of 1% (w/v) potassium ferricynide, K3Fe(CN)6(Aldrich) solution were added. The mixture was incubated at 50 �Cfor 20 min after which 2.5 mL 10% (w/v) trichloroacetic acid(Merck) was added. The resultant mixture was centrifuged for20 min at 2500 rpm. The upper layer (2.5 mL) was dispensed and2.5 mL distilled water and 0.5 mL of 0.1% (w/v) ferric chloridehexahydrate, FeCl36H2O (Merck) solution were added. The absor-bance was measured at 700 nm. The procedure was repeated forlignin samples.

3. Results and discussion

3.1. Characterization of lignin

Organosolv lignins extracted from oil palm fronds with (DEOL)and without (EOL) 1,8-dihydroxyanthraquinone as scavenger dur-ing the pulping process were characterized. Fig. 2A shows the infrared spectra of the unmodified and modified organosolv lignin.Absorption signal at 1160 cm�1 which appears in all spectra wasassigned for typical HGS lignin. The presence of absorption bands at1328 cm�1 (syringyl), 1219 cm�1 (guaiacyl), 1120 cm�1 (syringyl)and 1033 cm�1 (guaiacyl) revealed that OPF lignin were mainlycomposed of S and G basic units. In addition the absorbance signalof S unit (1328 and 1120 cm�1) was higher compared to G units(1272 and 1033 cm�1) which suggested the higher syringyl content[21]. Based on Fig. 2A, when 1,8-dihydroxyanthraquinone (DEOL)was introduced during the autohydrolysis process, decrease in theabsorption of 1700e1720 cm�1 can be observed after the organo-solv treatment. These bands may explain the fact that the organicscavenger reduced significantly the number of b-keto (Hilbert ke-tone) groups by condensing with either their precursor hydroxylgroups or the already formed b-carbonyl groups [22]. Besides, thereduction was also caused by limitation of the deconstruction onthe lateral chain because of the scavenging effect.

Data from the quantitative 31P NMR of the two organosolvlignin samples obtained following their derivatization with TMDPare presented in Fig. 2B. The concentration of each hydroxylfunctional group (in mmol g�1) was calculated on the basis of thehydroxyl content (Table 1) of the internal standard cyclohexanoland its integrated peak area [17]. From this data, it can beobserved that lignin grafted with 1,8-dihydroxyanthraquinone(DEOL) gave higher concentrations of HGS phenolic eOH groups(phenolic eOH ¼ 1.49 mmol g�1) and lower concentrations ofaliphatic eOH content compared to unmodified organosolv lignin

Fig. 2. (A) The infra red and (B) 31P NMR spectra of; unmodified organosolv lignin (EOL) and lignin with 1,8-dihydroxyanthraquinone (DEOL). (H: p-hydroxyphenyl unit; G: guaiacylunit; S: syringyl unit).

M.H. Hussin et al. / Polymer Degradation and Stability 109 (2014) 33e3936

(EOL) (phenolic eOH ¼ 1.35 mmol g�1). Very severe conditions(used during autohydrolysis pretreatment) have enhanced thecleavage of a- and b-ether linkages between lignin structural unitswhich later led to the formation of new phenolic eOH whendelignification process takes place [12,13,22]. It was believed thatduring the autohydrolysis step, 1,8-dihydroxy anthraquinone was

Table 1Organosolv lignins characterized by 31P NMR.

ppm Assignments (31P NMR) mmol g�1

EOL DEOL

150e145 Aliphatic eOH 1.49 1.08144.8 Cyclohexanol (internal standard)144e140 Syringyl eOH 0.69 0.74140e138 Guaiacyl eOH 0.45 0.46138e136 p-Hydroxyphenyl eOH 0.21 0.29135e133 Carboxylic acid 0.07 0.01

reduced in presence of reducing sugars into the highly nucleo-philic 1,8-dihydroxyanthrahydroxyquinone (according Fig. 1). Theincorporation of 1,8-dihydroxyanthrahydroxyquinone scavengeron the lignin matrixes according the well known mechanism [13]should retard the condensation process (through an electrophilicsubstitution reaction at the Ca position) and leads to the produc-tion of higher phenolic OH groups. Higher signals for S unit in alllignin may suggest that OPF lignin contains more syringal basicunit than guaiacyl unit, as similarly shown in the FTIR spectra.

To acquire further information on the structural characteriza-tion of modified organosolv lignin, the autohydrolyzed organosolvlignin in the presence of 1,8-dihydroxyanthraquinone was sub-jected to HSQC and HMBC NMR analyses. The main signals in thearomatic region (dC/dH 100e150/6.0e8.0 ppm) of the HSQC NMRspectrum are shown in Fig. 3. As previously reported by She et al.[23], the correlation at dC/dH 103.8/6.69 ppm corresponds to 2/6position of S units, whereas the correlations for C2eH2 (dC/dH 110.8/6.96 ppm) and C5eH5 (dC/dH 115.5/6.84 ppm) are assigned to G

Fig. 3. 2D-HSQC NMR spectrum of (A) unmodified organosolv lignin (EOL); (B) ligninwith 1,8-dihydroxyanthraquinone (DEOL).

Fig. 4. 2D-HMBC NMR spectrum at side chain region of lignin with 1,8-dihydroxyanthraquinone. (Inlet: Structure of 1,8-dihydroxyanthraquinone).

Fig. 5. Dissolution profiles of unmodified organosolv lignin (EOL) and lignin with 1,8-dihydroxyanthraquinone (DEOL).

M.H. Hussin et al. / Polymer Degradation and Stability 109 (2014) 33e39 37

units. Signals for the p-hydroxyphenyl units (H) were identified atdC/dH 129.7/7.02 ppm. This supports our earlier prediction that theorganosolv lignin obtained from different pretreatments could beverified as typical HGS lignin. p-coumarates (pCE 2/6) correlationswere also observed in all organosolv lignin samples appearing at dC/dH 131.2/7.6 ppm. Besides, the presence of anthaquinone ring can bedetected at around dC/dH 120e140/7.0e8.0 ppm respectively.

The modification of the lignin structure in the presence of 1,8-dihydroxyanthraquinone also can be supported by the HMBCanalysis (Fig. 4). In this study, a strong interaction at dC/dH 162.0/4.5 ppm was detected that indicated a strong correlation betweenCa of lignin matrix to C3 and/or C6 position of 1,8-dihydroxyanthraquinone. This correlation demonstrates the pres-ence of the anthraquinone structures on the external ring. In gen-eral, the acidic conditions during the autohydrolysis andorganosolv treatment lead to the formation of carbonium ion by theproton-induced elimination of water from benzylic position. Thecondensation reactions may occur in the presence of electron-richcarbon atom such as the C2/C6 presence in guaiacyl and syringylring leading to repolymerization of the lignin. By introducingorganic scavengers like 1,8-dihydroxyanthraquinone, it would bondto a lignin quinine methide thus forming a covalently-bondedadduct. The role of anthraquinone here is therefore to induce/scavenge the carbonium ion ligninelignin condensation reactionthrough an electrophilic substitution reaction at the Ca position[11e13]. Therefore, it is proposed that the modification of lignin in

the presence of 1,8-dihydroxyanthraquinone will form an interac-tion possibly at C3 and/or C6 of the anthraquinone moiety (Fig. 4).

The weight average (Mw) and number average (Mn) molecularweight of all lignin were computed from their chromatograms. Itwas revealed that the weight average (Mw) of lignin incorporatedwith 1,8-dihydroxyanthraquinone ( DEOL: 3624 gmol�1) was lowerthan the Mw of unmodified organosolv lignin (EOL: 5151 g mol�1).This indicates that the incorporation of organic scavenger (such as1,8-dihydroxyanthraquinone) with assessment of autohydrolysispretreatment facilitates the production of smaller lignin fragmentswith low molecular weight. Severe conditions autohydrolysis pre-treatment led to more extensive depolymerization (ether linkagescleavage) of the lignin and decrease the molecular weight. Ac-cording to Alriols et al. [24], low molecular weight of lignin willincrease its solubility in some organic solvents. In addition, verylow polydispersity (PD) values of lignin grafted with 1,8-dihydroxyanthraquinone ( DEOL: Mn 1888 g mol�1, PD 1.92)compared to the unmodified organosolv lignin (EOL: Mn2373 g mol�1, PD 2.17) were observed. The low polydispersity oflignin contributes to the uniformity of overall packing structures oflignin and may cause higher solubility [7].

In Fig. 5, we can see that the modified lignin and the unmodifiedorganosolv lignin exhibit different dissolution curves. The results

M.H. Hussin et al. / Polymer Degradation and Stability 109 (2014) 33e3938

showed that there was about 8.86% of DEOL lignin dissolved,compared to only 1.15% of unmodified organosolv lignin (EOL). Thesolubility of DEOL lignin was 10.67 times higher than the solubilityof EOL. Better solubility of lignin can be obtained when 1,8-dihydroxyanthraquinone was incorporated onto lignin structure.This is due to the fact that 1,8-dihydroxyanthraquinone containsmore polar hydroxyl functional groups thus increase the hydro-philicity of lignin and thereby promotes dissolution in water.

3.2. Lignin antioxidant activity

3.2.1. Oxygen uptake inhibitionThe oxygen uptake profile during autoxidation of methyl lino-

leate is shown in Fig. 6A. By comparing the linear curve of methyllinoleate alone (control), it appears that all lignin samples exhibitedantioxidant activity by slowing down the oxidation of methyllinoleate. The antioxidant efficiency of lignin has been related withtheir structure, phenolic-aliphatic eOH content, purity and poly-dispersity [25,26]. The assessment of autohydrolysis pretreatmentduring the incorporation of organic scavengers has greatlyincreased the antioxidant activity of lignin. By referring to the 31PNMR analysis, it was revealed that the phenolic eOH (syringyleOH þ guaiacyl eOH þ p-hydroxyphenyl eOH) of lignin graftedwith 1,8-dihydroxyanthraquinone was greater than its aliphaticeOH. Very severe conditions (used during autohydrolysis pre-treatment) have enhanced the cleavage of a- and b-ether linkagesbetween lignin structural units which later led to the formation ofnew phenolic eOH when delignification process takes place [26].Thus the process generatesmore phenoliceOH than aliphaticeOH.

Fig. 6. Antioxidant profile of unmodified organosolv lignin (EOL) and lignin with 1,8-dihydroxyanthraquinone (DEOL); (A) oxygen uptake inhibition and (B) reducing powermethod.

In addition, ortho-methoxyl substitution (which was exhibitedin S and G units) might provide resonance stabilization to theincipient phenoxyl radical that can increase the antioxidant activity[27,28]. Therefore the antioxidant activity of lignin should increasewith the free phenolic eOH and ortho-methoxyl S and G groupcontent. Close inhibition values of DEOL (phenoliceOH ¼ 1.49 mmol g�1) lignin was observed with OUI ¼ ~78% whileunmodified organosolv lignin (phenolic eOH ¼ 1.35 mmol g�1)with OUI ¼ ~53%. It was also observed that modified lignin thatposses low molecular weight tends to experience high antioxidantactivity than unmodified organosolv lignin (EOL). This observationis consistent with the previous report regarding the effect of mo-lecular weight of lignin on antioxidant activity [6,26]. The lowmolecular weight lignin resulted from the inter-unit bonds cleav-age in lignin structure is also accompanied by an increment ofphenolic eOH and decrease of aliphatic eOH.

3.2.2. Reducing power assayFig. 6B shows the reducing power of modified and unmodified

organosolv lignin as a function of their concentration. In this assay,the yellow color of the test solution changes to various shades ofgreen and blue, depending on the reducing power of each lignin.The presence of reducers (i.e. antioxidants) causes the reduction ofthe Fe3þ (ferricyanide) complex to the ferrous complex that has anabsorption maximum at 700 nm [29]. As a comparison, reducingpower of syringaldehyde and vanillin standard was employed tomimic the reduction ability of syringyl (S) and guaiacyl (G) unit.Increased absorbance of the reaction mixture indicates increase inreducing power. Higher reducing power of syringaldehyde thanvanillin standard revealed that the S unit could facilitate betterreduction ability. In this study, the reducing power of ligninincreased with concentration. The reducing power of lignin incor-porated with 1,8-dihydroxyanthraquinone (DEOL) was better thanthe unmodified organosolv lignin (EOL). It was reported that thereducing power of some chemical compoundsmight be due to theirhydrogen-donating ability [30]. Hence higher reducing power ofDEOL was perhaps due to the presence of more electronegativeorganic scavenger of 1,8-dihydroxyantharaquinone on the ligninmoiety. Therefore DEOL lignin might contain higher amounts ofreductant, which could react with free radicals to stabilize andblock radical chain reactions. Better antioxidant propertiesexhibited by the lignin grafted with 1,8-dihydroxyanthraquinone isindeed beneficial for the various aqueous applications such as mildsteel corrosion inhibition.

3.3. Improved lignin properties for green applications

Structural alteration of lignin via incorporation of organicscavenger (1,8-dihydroxyanthraquinone) during the pretreatmentprocess has tremendously altered both physical and chemicalproperties of the lignin. Lignin grafted with 1,8-dihydroxyanthraquinone gave higher S-unit with almost nochange to its G-unit. In fact, the presences of G-type unit in theDEOL lignin structure (with a free C5 position), could give highlypotential active sites for polymerization process such as in phenol-formaldehyde condensation reactions.While higher S-type unit (C3and C5 positions are linked to methoxy group) tends to show a lowreactivity towards formaldehyde [31], but still it offers as a goodantioxidant due to its ortho-methoxyl substitution which mightprovide resonance stabilization to the incipient phenoxyl radicalthat can increase the antioxidant activity. Higher phenoliceOH andortho-methoxyl groups content with smaller fragments of ligninstructures (low average molecular weight) was produced in whichlater gave a good antioxidant activity as observed from oxygenuptake inhibition and reducing power assay.

M.H. Hussin et al. / Polymer Degradation and Stability 109 (2014) 33e39 39

Furthermore, based from its Mw and PD values, DEOL lignincould also be used to improve the properties of viscous media usedfor inks and paints, extender or phenoleformaldehyde resinscomponent and for favoring blend capability in polymer formula-tions. An improvement toward its solubility right after beingmodified with 1,8-dihydroxyanthraquinone was observed. Thehydrophobicity of lignin was then reduced with this technique andtherefore this could widen the area of research for lignin to beapplied in aqueous systems. Our current research is concerning theutilization of modified lignin to be employed as green corrosioninhibitor.

4. Conclusion

In this work, it was successfully proven that the modification ofthe lignin structures by the incorporation of organic scavengers(1,8-dihydroxyanthraquinone) during the autohydrolysis pre-treatment has greatly improved the properties of lignin. Addi-tionally, lignin fractions with higher phenolic hydroxyl content butlower molecular weight, polydispersity as well as aliphatic hy-droxyl content gave higher values of antioxidant activity. Bettersolubility of lignin can be obtained when 1,8-dihydroxyanthraquinone was incorporated onto lignin structuredue to the fact that 1,8-dihydroxyanthraquinone contains morepolar hydroxyl functional groups thus increase the hydrophilicity oflignin and thereby promotes dissolution in water. An improvedproperty of lignin is indeed beneficial for its subsequentapplications.

Acknowledgments

The authors are grateful for the financial support of this researchfrom CPER 2007e2013 “Structuration du Pole de Comp�etitivit�e Fi-bres Grand'Est” and from Universiti Sains Malaysia through USMResearch University Grant e 1001/PKIMIA/854002. M. HazwanHussin would like to express his gratitude to the Ministry of HighEducation (MOHE) of Malaysia andMinistere Affaires Etrangeres deFrance for the MyPhD scholarship and Boursier du GovernementFrancais (CampusFrance). The EA 4370 LERMAB is supported by theFrench National Research Agency through the Laboratory ofExcellence ARBRE (ANR-12-LABXARBRE-01).

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