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The effect of temperature on the molecular weight distribution of kraft lignin during its ..... [16] M. Y. Balakshin, E. A. Capanema, C.-L. Chen, and H. S. Gracz.

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ScienceDirect Procedia Engineering 148 (2016) 1363 – 1368

4th International Conference on Process Engineering and Advanced Materials

Effect of Temperature on Molecular Weight Distribution of Pyridinium Acetate Treated Kraft Lignin Tazien Rashida, Chong Fai Kaitb, Thanabalan Murugesana, * b

a Department of Chemical Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, Tronoh, 32610, Perak Malaysia. Fundamental and Applied Sciences Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, Tronoh, 32610, Perak Malays ia.


The effect of temperature on the molecular weight distribution of kraft lignin during its dissolution in Pyridinium Acetate [PyAce] a protic ionic liquid was examined in detail by gel permeation chromatography. Lignin solubility increased with increasing temperature from 50 oC to 100 oC.The average molecular weight (Mw) of [PyAce] treated and recovered kraft lignin after thermal treatment with [PyAce] decreased from 4119 g mol Ѹ1 to 1995 g molѸ1 within the temperature range of 50 oC-75 oC. A significant increase in molecular weight for [PyAce] treated lignin was observed as the process temperature was further increased from 75 oC to 100 °C. Results indicated that the condensation reactions occurred at elevated temperatures resulting in increased molecular weight during the dissolution process. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the committee of ICPEAM 2016 2016. Peer-review under responsibility of organizing the organizing committee of ICPEAM Keywords: Protic Ionic Liquid; Kraft Lignin;Dissolution; Gel Permeation Chromatography.

1. Introduction Lignin is the second most abundant organic, natural compound on earth after cellulose [1]. The lignin content in lignocellulosic biomass varies from 16% - 33 % [2]. Development of new materials from renewable bio-based products has attained much interest in the recent years. However, efficient utilization of lignin presents an ongoing

* Corresponding author. Tel: +6053687620 ; fax: +6053656176 E-mail address: [email protected]

1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ICPEAM 2016



Tazien Rashid et al. / Procedia Engineering 148 (2016) 1363 – 1368

challenge as compared to cellulose [3]. Lignin is the major byproduct of pulp and paper industry and is still underused and mostly burned as a low-grade fuel, only 1-2 % of lignin is used for the production of aromatics and chemical feedstock. The potential application of lignin to produce aromatics has attained a major interest in the scientific community. Lignin acts as a “glue binding” in the whole lignocellulosic biomass and therefore is structurally recalcitrant andheterogeneous in nature. Recently, the research on the mechanistic studies on lignin depolymerization is attaining much interest. Extensive characterization techniques are used toevaluate the treated and recovered lignin behavior during kraft pulping process. Some of the earliest research in this area was conducted by Gellerstedt and coworkers[4]. The lignin structure mainly consists of three phenylpropane monomer units i.e. i) p-hydroxylphenylpropane, ii) guaiacylpropane, and iii) syringylpropane units (Fig.1). These phenylpropane units are interlinked through variety of irregular linkages [1, 5]. About one half of the bonds interlinking these phenylpropanemonemers are β-O-4 linkages [6]. The lignin depolymerization strategy is basically to target these β-O-4 linkages while successfully maintaining the aromaticity of the lignin fragments. Therefore, the effect of process parameters is of much interest in depolymerization studies [5]. It is well known that the molecular weight of lignin decreases as the pulping process proceeds, which is primarily because of the cleavage of β-O-4 linkages present in lignin. Raising the process temperature leads to fragmentation of the intermediate linkages present in lignin [7]. This intermolecular cleavage results in an increase in phenolic O-H contents of recovered lignin. Recently Protic Ionic Liquids (PIL’s) have proven to be more effective solvents for kraft lignin dissolution than the conventional immidazolium based IL’s [810]. More interestingly, they have strong abilities to dissolve lignin and the characteristics of the recovered lignin are comparable indicating that they may offer promising new opportunities for efficient conversion of lignin into valuable products. The focus of this study was to investigate the effect of processing temperatures (i.e., 50 oC-100 °C) on the molecular weight distribution of lignin. GPC analysis was performed to investigate the molecular weight distribution of the kraft lignin before and after treatment with [PyAce].

Fig.1Major linkages present in lignin structure

2. Experimental 2.1 Materials and Methods The chemicals used for the present research namely, Pyridine, Glacial Acetic acid, Lignin (Kraft lignin-Indulin AT), Acetone (99 % purity), 12 polystyrene standards (PSS Ready Cal-Kit Poly) and Tetrahydrofuran THF (used as a

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solvent for GPC samples) were of analytical grade and purchased from Sigma Aldrich. All the chemicals were used as received. Triple distilled water was used for the preparation of all aqueous solutions. 2.2 Dissolution of kraft lignin Kraft lignin dissolution in [PyAce] was conducted as described recently in our work [8]. Briefly 1% w/w of kraft lignin was dissolved in glass vials containing 5 g of [PyAce]. The resulting mixture was subjected to vigorous stirring at 50 oC, lignin was added incrementally until a clear solution was achieved. This clear solution was examined using Meiji optical microscope MT 4000 at 100X apart from visual observations to detect any undissolved particles [11]. The solubility of lignin in [PyAce] at different temperatures was investigated. It was observed that the lignin solubility increased with increasing temperature followed by a reduction in solubility at 100 oC, after that it became approximately constant. Based on these observations the dissolution experiments were carried out from 50 o C to 100 oC for present study. The experimental solubility of lignin in [PyAce] was also calibrated by plotting calibration curves using Perkin Elmer UV-Vis Spectrophotometer [12]. 2.3 Lignin Regeneration The dissolved lignin was precipitated by addition of acetone/water mixture (1:2) in the resulting saturated solution. Water was used as an antisolvent as lignin is insoluble in water, lignin was precipitated and was recovered and washed with acetone/water mixture several times[11]. The recovered lignin was vacuum filtered and further washed with acetone/water mixture to ensure complete removal of [PyAce] contaminations. The filtered lignin was then oven dried at 50 oC overnight and the amount of lignin recovered was calculated gravimetrically. 3. Characterization of Recovered Lignin 3.1 Gel Permeation Chromatography The relative molecular weights of recovered lignin were determined using Agilent 1200 Series gel permeation chromatography (GPC) system (dual UV/Vis detector with wavelength, 235 nm and 254 nm). Lignin samples (4 mg) were dissolved in THF (2 ml), calibration of GPC system was performed using 12 polystyrene standards (PSS Ready Cal-Kit Poly) and the recovered lignin was compared with commercial kraft lignin. 4. Results and Discussions The GPC Chromatograms of treated and untreated kraft lignin are shown in Fig.1, from which the molecular weights were calculated. It is evident that for the [PyAce] treated lignin at 75 oC the intensity of the peak at > 8 min is at maximum height. This high intensity of [PyAce] treated lignin at 75 oC confirms that the dissolution and regeneration with [PyAce] can produce more uniform lignin fragmentation with low degree of polydispersity. Whereas, further increase in temperature from 75 oC to 100 oC resulted in a reduction in the peak height. This reason could be attributed to the condensation of degraded intermediates at higher temperature (i.e. 100 oC) [5].



Tazien Rashid et al. / Procedia Engineering 148 (2016) 1363 – 1368

Fig.2 GPC elution profiles for lignin recovered from [PyAce] at 50 oC, 75 oC and 100 oC

These intermediates eluted at smaller intensity during GPC analysis. These results demonstrate that the fragmentation reactions are more dominant at 75 oC and below, whereas condensation reactions take place at 100 oC for [PyAce] treated lignin. In the present study the lignin fragmentation is achieved as a function of thermal treatment without using a catalyst. The possible explanation of this reason could be that the protic ionic liquids are capable of hydrogen bonding and can interact easily with hydroxyl groups of lignin, which results in cleavage of these bonds and reduced polydispersity. At high process temperature the cleavage of ether linkages present in lignin further leads to condensation and presence of other compounds resulting in the degradation of lignin as well [13,14]. Based on these GPC chromatograms the molecular weights of the [PyAce] treated lignin were estimated. The number-average molecular weight (Mn) and the weight-average molecular weight (Mw) of [PyAce] recovered lignin at various temperatures are given in Table 1. Polydispersity Index PDI (the ratio of Mw/Mn) was calculated at various investigated temperatures. The Mw of the untreated kraft lignin was calculated to be 4119 g mol −1, which is within the Mw range (600−180000 g mol−1) of kraft lignin obtained from black liquor [7]. It was observed that after dissolution and regeneration at 50 oC the Mw of kraft lignin decreased from 4119 g mol−1 to 3119 g mol−1. Diop et al., [5] also reported that the molecular weights of the recovered kraft lignin are reduced after treatment with ionic liquid. In addition, the polydispersity Index (Mw/Mn) of kraft lignin decreased from 3.29 to 2.50 and 2.18 after treatment and regeneration from [PyAce] solution at 50 oC and 75 oC respectively. It can be concluded that the recovered lignin has structure that is more homogeneous than that of untreated kraft lignin. Furthermore, it was observed that at higher temperature i.e. 100 oC the molecular weight of the recovered lignin is increased from 1995 g mol−1 to 2988 g mol−1.


Tazien Rashid et al. / Procedia Engineering 148 (2016) 1363 – 1368 Table 1. Weight average (Mw) and number-average (Mn) molecular weights (gmol-1) and polydispersity (Mw/Mn) of the [Py][Ace]treated and untreated lignin. Temperature (oC)

Mn (g mol-1)

Mw (g mol-1)

PDI (Mw/Mn)

Pure kraft lignin





[PyAce] treated lignin

50 oC




[PyAce] treated lignin

75 oC




[PyAce] treated lignin

100 oC




Based on the present results, it was observed that the Weight average (Mw) of [PyAce] treated and recovered lignin decreased within the temperature range of 50 oC-75 oC followed by anincrease in Mw at 100 oC (Table.1). Beyond 75 oC, the Mw did not show any significant decrease. The most likely cause for the observed increase in molecular weight of [PyAce] treated lignin at 100 oC might be due to the condensation reactions which normally occurs at high temperature, similar to those that occur during kraft pulping process. The term condensed lignin structures refers to lignin groups that contain alkyl or aryl substituents at the C-5 and/or C-6 position of the aromatic ring. These substituted alkyl or aryl contents result in increased molecular weight of lignin [15]. Argyropoulos (1994) [16], investigated the content of alkyl or aryl substituents on condensed lignin structures at C-5 position in kraft lignin. This observation that is the increase in molecular weight of treated lignin due to the condensation reactions satisfactorily agrees with the reported literature [5, 15, 17]. Conclusions In conclusion, this study demonstrates that significant changes in the molecular weight of [PyAce] treated lignin occurred during its thermal treatment. The molecular weight was lowered at an optimum temperature of 75 oC. The GPC results showed that high temperature is not suitable for depolymerization of lignin. The recovered lignin showed much lower polydispersity and the average molecular weight was reduced from 4119 g mol-1 to 1995 gmol-1. Acknowledgements The financial support in the form of GA (Tazien Rashid) by UniversitiTeknologi PETRONAS Malaysia is highly acknowledged. References [1] [2] [3] [4] [5] [6]

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