Dissolution of kraft lignin using Protic Ionic Liquids ...

Industrial Crops and Products 84 (2016) 284–293

Contents lists available at ScienceDirect

Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop

Dissolution of kraft lignin using Protic Ionic Liquids and characterization Tazien Rashid a , Chong Fai Kait b , Iyyasamy Regupathi c , Thanabalan Murugesan a,∗ a b c

Department of Chemical Engineering, Universiti Teknologi Petronas, Bandar Seri Iskandar, Tronoh32610, Perak, Malaysia Fundamental and Applied Sciences Department, Universiti Teknologi Petronas, Bandar Seri Iskandar, Tronoh 32610, Perak, Malaysia Department of Chemical Engineering, National Institute of Technology, Surathkal P. O. Srinivasnagar, Mangalore 575 025, Karnataka, India

a r t i c l e

i n f o

Article history: Received 24 August 2015 Received in revised form 5 February 2016 Accepted 6 February 2016 Keywords: Lignocellulose Kraft lignin Protic Ionic Liquids Dissolution Regeneration

a b s t r a c t In the present research three Protic Ionic Liquids (pyridinium formate, pyridinium acetate and pyridinium propionate) were synthesized and tested for the dissolution and subsequent regeneration of kraft lignin. Among the investigated solvents, pyridinium formate showed a higher dissolution capacity (70% w/w) i.e. (710 g/L) at 75 ◦ C within 1 h. The results indicated that the introduced solvent is thermally stable, noncorrosive, possesses low viscosity and is easy to recycle. The dissolution process is purely physical and the physicochemical analysis of the regenerated lignin showed high thermal stability, with reduction in polydispersity and the average molecular weight was reduced from 4119 g/mol to 1249 g/mol. FTIR spectroscopy and 1 H NMR results proved that the regenerated lignin is less degraded. Moreover the O H vibrations of regenerated lignin showed a weak inter and intramolecular interaction in regenerated lignin, which could positively help in reducing its chemical resistance towards processing for further commercial applications. Due to the higher solubility of lignin and its stability towards recyclability, the pyridinium formate proved that present selective dissolution and regeneration of lignin could significantly enhance the pretreatment techniques for lignocellulosic biomass. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Lignin is considered as the second most abundant organic, natural compound in the world and is also considered as a renewable source of aromatic and chemical feedstock. The concept of bio refinery comprises the selective separation of the three main polymeric wood components namely, cellulose, hemicellulose and lignin, and their subsequent utilization for the production of fuels and high value added chemicals. Cellulose and hemicellulose fractions can be readily used as a starting material for the production of biofuels and biochemicals, whereas the efficient utilization of lignin presents an ongoing challenge (Zhang, 2008). The separation techniques employed for lignocellulosic biomass components (i.e., cellulose, hemicellulose, and lignin) are mostly conducted in destructive ways to obtain comparatively pure cellulose, which results in degraded lignin that can only be used as a low value by products or burnt as a low-grade fuel (Zhang, 2008; Zakrzewska et al., 2010). Therefore a key challenge remains on the achievement of an effi-

∗ Corresponding author. Fax: +60 53656176. E-mail addresses: [email protected], tmgesan [email protected] (T. Murugesan). http://dx.doi.org/10.1016/j.indcrop.2016.02.017 0926-6690/© 2016 Elsevier B.V. All rights reserved.

cient, cost effective technology and selective separation of native lignin. Kraft and sulfite processes are the oldest and most common technologies for commercial delignification of wood. However, the high energy inputs and potential pollutants involved (sulfur containing reagents) in the process make it uneconomical. Moreover high temperature and pressure conditions degrade the cellulose structure and lead to the destruction of the fermentable sugar i.e., glucose (Baptista et al. 2008; Gellerstedt and Henriksson, 2008; Vila et al., 2003). Organosolv pulping process has several advantages over sulphite and kraft process through which some of the abovementioned drawbacks were eliminated (Alriols et al., 2009; Li et al., 2012; Vila et al., 2003). Though it is possible to obtain a sulfur free lignin with minimum degradation, however the major drawbacks of this technique are thermal instability of solvent and high cost for solvent regeneration (Sundquist, 1988). da Costa Lopes et al. (2013a) introduced a fractionation methodology by using 1-ethyl3-methylimidazolium acetate for wheat straw cooking followed by alkaline extraction at 120 ◦ C for 6 h and the reported lignin contents were very low (i.e., 41.8% cellulose, 25.4% hemicellulose and 8.0% lignin). Later a multiple linear regression model was developed by da Costa Lopes et al. (2013b) based on their experimental results at various operating conditions.

T. Rashid et al. / Industrial Crops and Products 84 (2016) 284–293

Lignin is the second most important constituent of lignocellulosic biomass, although far less attention has been given to its recovery. The lignin content in normal hardwood and softwood varies from 20% to 30% and 26% to 32% respectively (Maki Arvela et al., 2010; Sun and Cheng, 2002). Lignin is a highly branched aromatic polymer that binds cellulose and hemicellulose via strong hydrogen bonding and ester linkages, which act as “glue binding” in the whole lignocellulosic biomass. These linkages and molecular interactions cause rigidity and microbial resistance in the lignocellulosic biomass. Furthermore, lignin behaves quite differently in solutions compared to cellulose, as the dissolution mechanisms for lignin and cellulose are different. The solubility of cellulose increases almost linearly with hydrogen bonding strength, but in case of lignin it is reverse (Hart et al., 2015; Horvath, 2006; Lee et al., 2009; Vitz et al., 2009). Recently Immidazolium based Aprotic Ionic Liquids (AIL’s) with short side alkyl chains have been extensively used for the dissolution and delignification of biomass (King et al., 2009; Pu et al., 2007). A comprehensive studies regarding lignocellulose (i.e., cellulose, lignin, and monosaccharides) solubility in IL’s was performed by Zakrzewska et al. (2010). Among the IL’s investigated, Immidazolium chloride was found to be more suitable for cellulose dissolution, whereas Immidazolium acetate was considered as the best IL for lignin dissolution. It was also concluded that acetate as anion reduces the thermal stability of the solvent. Despite their usefulness, AIL’s do have certain drawbacks namely high viscosity and high operating temperature and recoverability etc. Apart from these issues, extended dissolution times are required for processing, (generally >12 h). High viscosity and cost of IL’s were reduced by using polar organic solvents as a co-solvent and a variety of co-solvents, including dimethylsulfoxide (DMSO) and N,N-dimethylformamide (DMF), have been tested for delignification (Mai et al., 2014; Pinkert et al., 2011; Rinaldi, 2011). However a comprehensive study on the effect of co-solvents is still lacking. Protic Ionic liquids (PIL’s) possess excellent chemical and thermal stability and negligible vapor pressure and they are capable of hydrogen bonding, including proton acceptance and proton donation. Protic IL’s are less expensive than traditional Immidazolium-based AIL’s. Protic acetate PIL’s were studied for delignification of biomass and the lignin extraction was found to be ≥50% w/w. But due to a lower degree of protonation (e.g., less ionicity) of amines by acetic acid protic acetates were found to be thermally unstable (Achinivu et al., 2014). Recently a techno-economic investigation study along with thermal stability comparison was reported for protic ammonium hydrogen sulfate using AIL (i.e., 1-ethyl-3-methylimidazolium acetate) as benchmark and the results are comparable (George et al., 2015). Due to their numerous desirable properties they offer an attractive alternative to conventional nonionic solvents. Based on the disadvantages of the available techniques for complete delignification and regeneration of lignin, in the present research an attempt has been made to use cost effective and easy to synthesize pyridinium based Protic Ionic Liquids with different anionic compounds and hydrogen bond basicity for the selective dissolution of lignin. For the present research pyridine is selected due to its classic tailored property, through which pyridine can be easily attached to different anions (Scriven and Murugan, 2000) also it has a high mobile proton with a tendency of both electron pair donor and proton acceptor (Shimizu et al., 2000). Pyridine is used in drug formulation, insecticides, plant growth regulator, and other agricultural products etc. (Chaubey and Pandeya, 2011; Haviv et al., 1983; Maga, 1981). In this study carboxylic acid anions are preferably selected due to their advantages; namely, low melting points, low viscosities, and high hydrogen bonding acceptor abilities, all of which should facilitate the dissolution of lignin. Moreover, weak organic acids are less toxic and less corrosive compared to the

285

Fig. 1. General scheme of the reaction.

Fig. 2.

1

H NMR spectra of pyridinium formate and its precursors in DMSO-d6.

mineral acids commonly used for the delignification of the lignocellulose (Li et al., 2012; Zhang et al., 2010; Zhou et al., 2012). 2. Material and methods 2.1. Materials The chemicals used for the present research namely, Pyridine, Formic acid, Glacial Acetic acid, Propionic acid, Chemicals for Karl Fischer titration, Dimethyl Sulphoxide (DMSO-d6 ) (used as a solvent for NMR samples), Lignin (kraft lignin-indulin AT), Microcrystalline Cellulose (MCC) (with a particle size of 20–150 ␮m), Sodium hydroxide, Methanol, Acetone (99% purity), 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. Synthesis of pyridinium based Protic Ionic Liquids Pyridinium based Protic Ionic Liquids are produced when a proton is transferred from a carboxylic acid to pyridine and the general scheme of the reaction is shown in Fig. 1. For the present study three anions are selected based on increasing alkyl chain length i.e. [HCOO− ], [CH3 COO− ] and [CH3 CH2 COO− ], and the PIL’s namely pyridinium formate [PyFor], pyridinium acetate [PyAce] and pyridinium propionate [PyPro] respectively were prepared according to the established procedures (Belieres and Angell, 2007). The PIL’s thus prepared were kept in the vacuum oven at 80 ◦ C for 48 h, in order to remove the excess moisture, which was formed during the reaction. The oven dried solvent was sealed with laboratory parafilm to prevent any moisture contamination. The characterization analysis using 1 H NMR (DMSO-d6 , 500 MHz) (Fig. 2.), TGA (Fig. 3.) and Karl Fischer water content analysis performed on the [PyFor] showed the following results respectively; [ı = 7.37(2H, d),

286


of 1:200, in an agate mortar and the resulting mixture was pressed at 7 tons for 30 s to form pellets. These pellets were analyzed using 32 scans at a resolution of 2 cm−1 . Four scans were recorded for each FTIR spectra and blank KBr pellets were used as the standard background. 2.5.2. 1 H Nuclear magnetic resonance spectroscopy (NMR) The one-dimensional (1D) NMR spectra was measured using a Bruker AVANCE 400 NMR spectrometer at an operating frequency of 500 MHz and room temperature. The samples were dissolved in DMSO-d6 . The 1 H chemical shifts were determined in parts per million.

Fig. 3. TGA thermograms of [PyFor] and its precursors.

7.77(1H, t), 8.16 (1H, s), 8.57 (2H, t), 9.75(1H, s); Thermal decomposition temperature Td = 115 ◦ C; Water contents = 451 ppm]. 2.3. Solubility test for kraft lignin Lignin solubility tests were conducted by dissolving 10% (w/w) of the lignin in glass vials containing pyridinium solvent with different anions i.e., formate, acetate and propionate to determine ideal combination for highest lignin solubility. Choice of anions was based on increasing alkyl chain length and hydrogen bond basicity (Hart et al., 2015). The vials were sealed with parafilm M (laboratory film) and the mixture was then placed into silicon oil bath and heated with vigorous magnetic stirring (700 rpm). The desired temperatures for the dissolution experiments were monitored and measured (±1 ◦ C). The solutions were then visually checked for the complete solubility. If a given solution was transparent with no undissolved solids, the amount of the lignin was then incrementally added to increase the mass fraction until the saturation point was achieved. Apart from visual observations the saturation point was also observed and confirmed using Meiji optical microscope MT 4000 at 100× and 4× (Glas et al., 2015). The concentrations of lignin in the solution were measured by UV–vis spectrophotometer using the stock solutions of known concentrations (150–1350 ppm). The concentration of unknown samples were determined by using the standardized calibration curves made for each of the solvents namely [PyFor], [PyAce] and [PyPro] (Wang et al., 2011). 2.4. Solubility test for cellulose Cellulose solubility tests were conducted by preparing 15% (w/w) sample of cellulose in [PyFor] and its precursors. The vials were placed on silicon oil bath and stirred for different time intervals. The desired temperatures were monitored and measured (±1 ◦ C). The sample was vacuum filtered and then the weight of cellulose dissolved was determined gravimetrically. The similar procedure was extended for both pyridinium acetate [PyAce] and pyridinium propionate [PyPro] solvents. 2.5. Characterizations 2.5.1. Fourier transform infrared analysis (FTIR) The FTIR spectra were taken using PerkinElmer spectrometer, applying the wave number in the domain of 5000 cm−1 to 1000 cm−1 . Recovered lignin was mixed with KBr in a weight ratio

2.5.3. Gel permeation chromatography (GPC) Agilent 1200 Series gel permeation chromatography (GPC) system (dual UV/vis detector with wavelength, 235 nm and 254 nm) was used for all experiments. Agilent 1200 Series refractive index detector with an automatic recycle valve was used to analyze the samples. A calibration curve was created based on 12 polystyrene standards (PSS Ready Cal-Kit Poly) from molecular weight of 266 g/mol to 2.52 × 106 g/mol. Data collection and processing were done by Agilent ChemStation with GPC-SEC data analysis software. 2.5.4. Thermogravimetric analysis (TGA) A PerkinElmer, Pyris V-3.81 thermal gravimetric analyzer was used to measure the onset and thermal decomposition temperature “Td ” of the samples. The equipment was heated at a rate of 10 ◦ C min−1 with a temperature range of 50–800 ◦ C. Approximately 4 mg of the samples were placed in an aluminium pan and the samples were analyzed under N2 gas blanket using a flow rate of 20–25 ml/min. 2.5.5. Differential scanning calorimetry (DSC) A Waters DSC Q2000 V24.11 Build 124A instrument calibrated with Indium was used to measure the glass transition temperature “Tg ” of the samples. All the samples were dried at 150 ◦ C at a rate of 20 ◦ C min−1 and then cooled and equilibrated at 20 ◦ C before measurements. Approximately 7 grams of samples were carefully placed in an aluminium crucible. The heating rate during the measurement was 5 ◦ C min−1 . All reported data are the average of duplicates. 3. Results and discussions 3.1. Effect of anion and alkyl chain length on solubility of kraft lignin Dissolution of commercial kraft lignin in pyridinium based solvents with different anions and hydrogen bond basicity is studied. The investigated anions were formate, acetate and propionate. The solubility of lignin in pyridine and the respective precursors at different temperatures is shown in Fig. 4. It was observed that the lignin solubility was maximum i.e., 26% (w/w) in case of formic acid (at ≈ 50 ◦ C) after that it became approximately constant. Acetic acid showed a very little solubility i.e., ≤1% (w/w) and for propionic acid it was almost negligible at all temperatures. Based on these observations it can be concluded that the lignin solubility decreases with increasing alkyl chain length of the precursors for pyridinium based compounds, whereas the maximum lignin solubility in pyridine was 47% (w/w). The calculated solubility using UV–vis spectrophotometer is reproducible within a deviation of 300 14 460–390

Lee et al. (2009) Wang et al. (2011) Glas et al. (2015)

Kraft lignin

75 ◦ C, 1 h

710

Present work

Ionic liquid

Ionic liquid Organic solvent Ionic liquid Protic Ionic Liquid

1-ethyl-3-methylimidazolium acetate 1,4-Butanediole/water system Ammonium, phosphonium and pyrrolidinium based ionic liquids Pyridinium formate

Kraft lignin

Fig. 4. Solubility of kraft lignin in precursors.

to acetate [Ace] and propionate [Pro] anions with comparatively higher pka values i.e., 4.76 and 4.88 respectively. This could also indicate that lignin is more soluble in the solvent with a higher degree of protonation (e.g., more ionicity). The low hydrogen bonding basicity of anion and the shorter alkyl chain length is favorable for lignin dissolution as reported by (Hart et al., 2015; Glas et al., 2015). The lignocellulosic components i.e., cellulose, hemicellulose and lignin are interlinked through hydrogen and covalent bonding (Remsing et al., 2008). The anionic part of the solvent plays an important role during dissolution process which breaks the intramolecular hydrogen bonding network in lignocellulose, thus allowing the dissolution of lignin. It can be easily realized that cationic part is same for the three reported solvents while the difference is only in the anionic part. In this case the anionic part with low hydrogen bond basicity and smaller alkyl chain length would be a choice for selective dissolution of lignin as shorter the alkyl chain length, higher is the lignin solubility. It is evident that the high hydrogen bonding strength that has been reported to be important for cellulose dissolution is not as influential for the case of lignin (Muhammad et al., 2013). Based on the recent literature (Glas et al., 2015; Lateef et al., 2009; Lee et al., 2009; Pu et al., 2007), the reported solubilities of kraft lignin in different ionic liquids were less than 50% (w/w) in all the cases whereas the present solvent, pyridinium formate [PyFor] showed a very high percentage of lignin solubility (70% w/w) at 75 ◦ C within 1 h (Table 1). The reported solubilities in IL’s (Table 1) were measured at relatively higher temperatures (≈90 ◦ C). Based on the above results, since the observed lignin solubility is higher in [PyFor] solvent (70% w/w), further experiments were conducted using [PyFor] only. 3.2. Effect of water contents on kraft lignin solubility

Fig. 5. Solubilities of kraft lignin in pyridinium formate [PyFor], pyridinium acetate [PyAce] and pyridinium propionate [PyPro].

much higher than [PyAce] and [PyPro]. The lignin solubility follows the order: [PyFor] (70% (w/w) ≥ [PyAce] (64% w/w) ≥ [PyPro] (55% w/w). [PyFor] dissolves the maximum amount of lignin among the other solvents studied in the present research (Fig. 5). Among the anions tested, the formate anion has the lowest pka value of 3.77 and showed the maximum dissolution for lignin as compared

The preparation of pyridinium based solvents involves an acid base neutralization reaction where water is produced as one of the product and hence the effect of water contents is an important parameter of interest. The effect of water content on the solubility of lignin was conducted by deliberately adding triple distilled water (10–95% (v/v)) to the pure [PyFor] solvent. The solubilities of lignin in aqueous solutions of [PyFor] are shown in Fig. 6. It can be seen that the solubility of lignin is highly dependent on water content and showed a decreasing trend with an increase in the water content. Lignin solubility was very low and almost negligible at higher concentrations of water. For the cases where the water content is above 50% (v/v) the solubility becomes negligible. Swatloski et al. (2002) reported that increased water contents (i.e. >1% (10,000 ppm)) would impair the dissolution capacity of the solvent as water would compete with anionic part of the solvent and crowd the hydrogen bond accepting sites of anion, which leads to difficult interaction of anion to lignin and hence the lignin solubility gets reduced (Swatloski et al., 2002). Few researchers have reported that the water contents at the level of parts per million

288


Fig. 6. Effect of water content in [PyFor] on solubility of kraft lignin.

does not have major impact on lignin solubility (Ni and Hu, 1995; Pinkert et al., 2011; Wang et al., 2011). 3.3. Regeneration of kraft lignin Regeneration of dissolved kraft lignin from the solution was made using water as an antisolvent (Glas et al., 2015). The dissolved lignin in [PyFor] was taken in a 100 ml flask with 40 ml of water. The flask was sealed and stirred for complete mixing of the contents. Lignin which is insoluble in water precipitates out and separated by vacuum filtration by applying minimum vacuum. Repeated washing of the precipitates was carried out with triple distilled water and acetone to ensure that all the solvent has been washed out with water and collected in the beaker. The collected lignin was then oven dried at 70 ◦ C overnight and the amount of lignin recovered was estimated gravimetrically. On the other hand the collected solvent and the wash water mixture were saved for recycling purpose. It was observed that lignin recovery was nearly same (≥95%) for all temperatures (i.e., 25 ◦ C, 50 ◦ C, 75 ◦ C, and 100 ◦ C) studied in the present research. Even though a very slight decrease in recovery was observed at high temperature conditions i.e., at 125 ◦ C, the recovered lignin however remains chemically unchanged. 3.4. Analysis of regenerated lignin 3.4.1. FTIR analysis FTIR analysis was performed for the [PyFor] treated and recovered lignin samples using commercially available kraft lignin as a standard. The FTIR spectra for the presence of hydroxyl, carbonyl, methoxyl, and carboxyl functional groups respectively were analyzed according to literature that have been reported for the identification of lignin (Table 2). FTIR analysis of the present [PyFor] treated and regenerated lignin is shown in Table 3. Lignin molecule contains various functional groups such as O H, C H, and C O etc. These functional groups produce specific transmittance at specific wave numbers. FTIR spectra of kraft lignin and [PyFor] treated lignin are compared in Fig. 7a and it was observed that at all specific intensities reported for [PyFor] treated lignin produced low intensity of aromatic and guaiacyl (G) rings (Table 3). For [PyFor] treated lignin the observed intensities of the bands corresponding to the aromatic skeletal vibration and guaiacyl (G) ring breathing are 1510 cm−1 and 1266 cm−1 respectively. These values are lower than those reported for kraft lignin, which could be attributed to the reason that during dissolution and regeneration processes the aro-

Fig. 7. (a) Comparison of FTIR spectrum of kraft lignin and PyFor] regenerated lignin. (b) Comparison of O–H vibrations of kraft lignin and [PyFor] regenerated lignin.

matic rings are transformed into quinonoid structures. In addition, the intensities of the bands corresponding to the phenolic hydroxyl content are slightly lower in [PyFor] treated lignin i.e., 1382 cm−1 compared to kraft lignin. The lower amount of phenolic hydroxyl contents in lignin indicates a less degraded structure as reported by Gellerstedt and Henriksson (2008) which can be useful for further potential lignin applications such as phenolic compounds and carbon fibers. For the O H vibrations of lignin, an extensive vibration band lies in the range associated with 3200–3500 cm−1 as reported by Ji et al. (2012). The absorption band of the O H vibration of the kraft lignin is 3375 cm−1 , while that of the regenerated lignin shifts to a higher wavenumber 3384 cm−1 (Fig. 7b), which could be attributed to the reason that during dissolution and regeneration process the intramolecular interactions of lignin are weakened. This week inter and intramolecular interaction in recovered lignin could positively affect its heterogeneity and can help in reducing its chemical resistance towards processing of this feedstock for further commercial applications. The results of FTIR spectra of kraft lignin and regenerated lignin are comparable and there are no new peaks appeared in the spectrum, which indicates that there is no chemical reaction during the dissolution and regeneration process of lignin and [PyFor] can safely considered as a physical solvent for lignin. 3.4.2. Nuclear magnetic resonance spectroscopy The 1 H NMR analysis was performed for the [PyFor] treated and recovered lignin samples using commercially available kraft lignin as a standard. The 1 H NMR spectra for the presence of aromatic protons in guaiacyl and syringyl units respectively were analyzed according to literature for the identification of lignin (Table 4. The major peaks of aromatic protons in guaiacyl propane (G) appear between 7.5 and 6.7 ppm and similarly that of the aromatic protons in syringyl propane (S) appear between 6.7 and 6.1 ppm (Mainka et al., 2015; Zhang et al., 2010; Zhou et al., 2012). The shift of signals at 2.7 ppm corresponds to DMSO-d6. The chemical shift of methoxyl protons ( OCH3 ) shows a sharp signal at 3.7 ppm in kraft lignin and [PyFor] regenerated lignin. Protons in aliphatic and hydrocarbon region of lignin produce peaks between 2.0 and 0.8 ppm. It was observed that peak assignments in case of [PyFor] regenerated lignin are comparable to that of kraft lignin which indicated that there are no chemical reactions between [PyFor] and


289

Table 2 Band assignments in kraft lignin, hardwood and softwood lignin for FTIR spectra. Band (cm−1 ) Kraft lignin

Assignment Softwood lignin

Hardwood lignin

1598 1513 1463 1426 1384

1725 1660 1596 1510 1463 1423 1375

1269 1218 1139

1269 1221 1140

1735 1658 1603 1510 1462 1425 1375 1328 1269 1220 1140

3200–3500

1126 1086 1032 a b

1086 1032

1116 1086 1033

O H vibrations C O stretching (unconjugated) C O stretching (Conjugated) Aromatic skeletal vibration breathing with C O stretching Aromatic skeletal vibration, Ga > Sb C H deformations asymmetric Aromatic skeletal vibrations combined with C H in-plane deformation Phenolic OH and aliphatic C H in methyl groups S unit breathing with C O stretching and condensed G rings G ring breathing with carbonyl stretching C C plus C O plus C O stretch; G condensed > G etherified C H in-plane deformation of G ring plus secondary alcohols plus C O stretch Ether–O– Aromatic C H deformation in S ring C O deformation in secondary alcohols and aliphatic esters Aromatic C H in-plane deformation (G > S) plus C O deformation in primary alcohols plus C O stretch (unconjugated)

Guaiacyl unit. Syringyl unit.

Fig. 8. Comparison of 1 H NMR spectra of kraft lignin and [PyFor] regenerated lignin.

bonds in lignin molecules indicating the dissolution process as pure physical process. 3.4.3. Molar mass distribution by gel permeation chromatography (GPC) Gel permeation chromatography (GPC) is a technique that provides insight into the molecular weight distribution and fragmentation of lignin during dissolution and regeneration process. Low polydispersity is preferred which increases the possibility of softening for making carbon fiber composites from lignin (Kubo and Kadla, 2005). Changes in the average molecular weight of the lignin were determined and compared for the [PyFor] treated lignin and kraft lignin. Gel Permeation Chromatography (GPC), a common method was used to determine molar mass. The lignin samples were fully dried under vacuum at 40 ◦ C and dissolved in tetrahydrofuran (THF). The molecular weight was then characterized by GPC using size exclusion chromatography (SEC). Three characteristics of lignin namely (i) the number average molecular weight

Mn , (ii) the weight average molecular weight Mw and (iii) polydispersity index (PDI) are of common interest. Ratio of (Mw /Mn ) is the polydispersity index (PDI), which describes the overall distribution of the molar masses in the lignin (Erdocia et al., 2014; Pinkert et al., 2011; Santos et al., 2014). The results show that the [PyFor] treated lignin has lower molecular weights than that for the pure kraft lignin (Table 5). The reduction in molecular weight indicates that the lignin molecule has been fragmented into smaller units, during dissolution process. The molecular weights and the polydispersity of the lignin recovered from [PyFor] treatment are smaller than that for the kraft lignin samples indicating that the regenerated lignin has a more homogenous composition. The PDI is low for [PyFor] treated lignin which is an added advantage for further processing of lignin. This finding is further supported by the SEC chromatographs from which the molecular weights of samples were calculated, Fig. 9 shows that for the [PyFor] treated lignin the intensity of the peak at >8 min is heightened. This high intensity of [PyFor] treated lignin confirms that the dissolution and

290


Fig. 9. Size exclusion chromatograph of kraft lignin and [PyFor] regenerated lignin. Fig. 10. Thermogravimetric analysis of kraft lignin and [PyFor] regenerated lignin. Table 3 FTIR analysis of [PyFor] regenerated lignin. Kraft lignin −1

Band (cm 3375 1600 1598 1513 1463 1426 1384 1269 1218 1139 1086 1032

Regenerated lignin from [Py][For] Band (cm−1 ) 3384 1600 1596 1510 1461 1425 1382 1266 1215 1138 1086 1031

)

Table 4 1 H NMR assignments of kraft lignin and [PyFor] regenerated lignin. Range number

Peak, ppm

Main assignment

1 2 3 4 5 6 7 8 9 10 11

7.5–6.7 6.7–6.1 6.1–5.8 5.8–5.2 5.2–4.9 4.9–4.3 4.3–4.0 4.0–2.5 2.5–2.2 2.2–1.6 1.6–0.8

Aromatic protons in guaiacyl units Aromatic protons in syringyl units H␣ of ␤-O-4 structures H␣ of ␤-5 structures H of xylan residue H␣ and H␤ of ␤-O-4 structures H␣ of ␤-␤ structures and H of xylan residue H of methoxyl protons ( OCH3 ) H of aromatic acetates H of aliphatic acetates Hydrocarbon protons

Table 5 Weight average (Mw ) and number-average (Mn ) molecular weights (gmol−1 ) and polydispersity (Mw /Mn ) of the treated and untreated lignin. Solvent

Mn (g/mol)

Pure kraft lignin 1249 Pyridinium formate—treated lignin 902

Mw (g/mol)

PDI (Mw/Mn)

4119 1738

3.29 1.92

regeneration with [PyFor] can produce more uniform lignin fragmentation with low degree of polydispersity. These results are in agreement with the standard methods for the determination of the molar mass distribution of lignin reported by Baumberger et al. (2007).). The 1 H NMR spectra of kraft lignin and [PyFor] treated lignin are compared in Fig. 8

Fig. 11. DTGA thermograms of kraft lignin and [PyFor] regenerated lignin.

3.4.4. Thermal decomposition behavior Thermal Gravimetric Analysis (TGA) is a convenient method to measure the weight loss with increasing temperature over a definite period of time. The thermal stability of the kraft lignin and the regenerated lignin was examined by Thermal Gravimetric Analysis (TGA) and first derivative thermogravimetric analysis (DTGA) thermograms. The TGA and DTGA curves of kraft lignin and the regenerated lignin from [PyFor] solution are shown in Figs. 10 and 11 respectively. The results indicated low onset temperature for kraft lignin (curve A) at 353 ◦ C, and the onset was increased to 390 ◦ C for the regenerated lignin. It can be due to the development of the condensed linkages of regenerated lignin, which is extremely stable. However the overall thermal decomposition behavior of kraft lignin and [PyFor] treated lignin was found to be similar. For the DTGA the thermal decomposition behavior of lignin can be divided into three major steps for the weight loss in the present studied temperature range of 25–800 ◦ C. Step I (25–150 ◦ C) involves the dehydration of lignin which ensures the complete removal of water and other low molecular weight volatiles, whereas step II (151–565 ◦ C) involves the fragmentation of inter-unit linkages of lignin, the primary products of this step are coke, organic and


291

Fig. 12. DSC thermograms of kraft lignin and [PyFor] regenerated lignin. Fig. 14. Effect of lignin contents in the presence of MCC on lignin recovery.

Fig. 13. Solubility of microcrystalline cellulose (MCC) in pyridinium formate [PyFor] and its precursors. Fig. 15. Lignin solubility in pure and recycled pyridinium formate [PyFor].

phenolic compounds, and gaseous products. In step III (566–800 ◦ C) the thermal decomposition of the aromatic rings of lignin takes place and char or coke are formed in this range (Sun et al., 2013; Tejado et al., 2007; Vasile et al., 2006). The maximum weight loss rates appeared at step II at 280 ◦ C at a rate of 1.49% min−1 for kraft lignin and at 340 ◦ C at a rate of 1.60% min−1 for [PyFor] regenerated lignin. This could be due to the differences in the C C linkages as reported by Tejado et al. (2007). The results of the TGA and DTGA curves, indicates that the lignin obtained after dissolution and regeneration process with [PyFor] is thermally more stable than that of the kraft lignin. 3.4.5. Differential scanning calorimetry Recently lignin based carbon fibre composites are attaining much attention, and understanding the behavior of lignin during heat treatment is extremely important for making carbon fibre (Kadla and Kubo, 2004). Differential scanning calorimetry (DSC) indicates the possible material softening phenomenon of the lignin to be used as a polymer blend. The glass transition temperatures “Tg ” of kraft lignin and regenerated lignin were determined using the DSC thermograms (Fig. 12)), and the glass transition temperatures are 93.72 ◦ C and 98.44 ◦ C for [PyFor] regenerated lignin and

kraft lignin respectively. These results are comparable to those reported in literature, where the reported Tg was in the range of 80 and 180 ◦ C (Tejado et al., 2007; Vasile et al., 2006). No significant changes in the glass transition temperatures of kraft lignin and regenerated lignin are found. The small difference in Tg of kraft lignin and [PyFor] regenerated lignin might be due to the change in the decomposition temperature, intermolecular hydrogen bonding interactions and molecular weight distribution of the regenerated lignin samples (Vasile et al., 2006).

3.5. Effect of solvent on cellulose solubility The solubility of cellulose in [PyFor] and its precursors are compared in Fig. 13, which shows that the solubility of cellulose is very low and almost considered as negligible in [PyFor] (i.e,