Effect of MeOH fractionation of lignin on its molecular weight
Medium ash grade lignin (as-received lignin) from Kraft-processed softwood biomass was put into MeOH, stirred, shaken, and centrifuged to separate MeOH-insoluble lignin and MeOH-soluble lignin (Figure 1). As-received lignin gave Mn=10 000 g mol−1 and PDI=110 (Table 1) from DMF size-exclusion chromatography (SEC) without LiBr. The very broad molecular weight distribution is consistent with that obtained from a hardwood lignin in our previous studies.7, 8 The SEC curve of as-received lignin revealed a HMW peak and a LMW peak (Figures 2 a and 3 a). The presence of HMW and LMW peaks is consistent with other work.7, 8, 21, 22 The HMW peak is assigned as the peak area located before a retention time of 22 min, whereas the LMW peak is assigned as the peak located after a retention time of 22 min. The area of the HMW peak corresponds to 93.6 % of the total area, whereas the area of the LMW peak corresponds to 6.4 % of the total area. The presence of a LMW fraction with an area of only 6.4 % contributed to its very large PDI.
Table 1. Molecular weight of lignin from MeOH fractionation (without LiBr).
|Lignin||Number of washes||Mn[a] [g mol−1]||Mw[a] [g mol−1]||PDI[a]||HMW area [%]||LMW area [%]|
|as-received||0||10 000||1 110 000||110||93.6||6.4|
|MeOH-insoluble||1st||28 200||1 310 000||46.3||97.2||2.8|
|2nd||60 600||1 400 000||23.1||98.8||1.2|
|3rd||63 800||1 530 000||24.0||98.8||1.2|
|4th (last)||443 000||1 550 000||3.5||100||0|
|MeOH-soluble|| ||3800||492 000||129||84.7||15.3|
Figure 2. (a) SEC curves of as-received lignin (top), MeOH-washed lignin [MeOH-insoluble lignin; first, second, and fourth wash (bottom)] and (b) area of LMW fraction from the SEC curve at higher than 22 min retention time and PDI as a function of the number of MeOH washes.
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Figure 3. SEC curves of as-received lignin (—), MeOH-insoluble lignin (– – –), and MeOH-soluble lignin (- - - -) (a) without and (b) with LiBr.
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Sequential MeOH washing removed the LMW fraction of lignin. As the number of MeOH washes increased, the LMW peak area of the MeOH-insoluble lignin decreased significantly (Figure 2 a and b, Table 1), so that MeOH-insoluble lignin contained a minimal LMW peak after the fourth wash. As a result of the removal of the LMW fraction, the PDI value decreased dramatically (Figure 2 b, Table 1). After the fourth wash, the DMF SEC analysis of MeOH-insoluble lignin gave Mn=443 000 g mol−1 and PDI=3.5. The isolated yield after the fourth wash was 50 %, which is much greater than that of hardwood lignin as reported in our previous work.7 In addition, MeOH fractionation for the hardwood lignin never achieved a single-digit PDI, possibly because of the presence of a huge LMW fraction and the lack of using a centrifuge and a Silverson mixer. This Kraft-processed softwood-based as-received lignin appears to contain a much higher content of the HMW fraction than solvent-extracted hardwood-based lignin. As the successful removal of the LMW fraction was achieved after the fourth wash, the rest of this study will focus on this MeOH-insoluble lignin residue obtained after four washes with MeOH. The term MeOH-insoluble lignin will refer to this lignin that was washed four times. MeOH-soluble lignin was characterized by SEC, and the results were compared with those of as-received lignin and MeOH-insoluble lignin. As shown by the chromatograms in Figure 3 a and the data in Table 1, the LMW fraction in MeOH-soluble lignin increased compared to that in as-received lignin. This indicates that MeOH extraction enriches the LMW fraction of lignin by dissolving it in the extraction medium.
As a result of an increase of the LMW area (15.3 %), the molecular weight of the MeOH-soluble lignin decreased (Mn=3800 g mol−1), and it showed a broader PDI (PDI=129) than as-received lignin. This principle of the removal of a LMW fraction of lignin with MeOH should be applicable for all sources of lignin or at least the majority of them. Notably, MeOH-soluble lignin still contains a large quantity of the HMW fraction, although the HMW peak shifted toward a slightly higher retention time in SEC. The HMW fraction of lignin might be partially soluble in MeOH, or the colloids of lignin in MeOH solution might stay in the solution rather than precipitating as a solid chunk. It was impossible to distinguish tiny colloid particles and soluble lignin as the appearance of the MeOH solution was dark brown. This MeOH washing strategy is applicable to separate the HMW fraction of lignin as discussed above, but it might not be possible to obtain only the LMW fraction because of its solubility or its tendency to form a colloidal suspension. If one aims to separate only the LMW fraction from the HMW fraction, a membrane filtration type of separation might be recommended.
Lignin is known to aggregate in solution, so the lignin peak from neat DMF in the SEC most likely represents both single lignin molecules and lignin aggregates in DMF.22, 23 The HMW peak in the SEC might represent aggregates of lignin, whereas the LMW peak might correspond to non-aggregate-forming LMW lignin molecules. LiCl and LiBr have been reported to disrupt lignin aggregation.22, 24 Therefore, for comparison, DMF SEC with LiBr (0.05 M) was performed, and the data are summarized in Table 2 and Figure 3 b. All the SEC curves in Figure 3 b show a monomodal peak, rather than the bimodal peaks seen in SEC curves obtained with DMF SEC without LiBr (Figure 3 a). As shown by their narrower monomodal SEC peaks, the PDI of these samples was between 1.90 and 3.05, which is much smaller than that obtained from DMF SEC without LiBr. As seen in the narrower SEC curves in Figure 3 b, the PDI of both MeOH-insoluble lignin (2.28) and MeOH-soluble lignin (1.90) were smaller than the as-received lignin (3.05), an indication of achieving effective fractionation by MeOH. The obtained molecular weights (MW) followed the same trend as those obtained with DMF SEC without LiBr, in which MeOH-insoluble lignin fractionated HMW components and MeOH-soluble lignin fractionated LMW components. However, the MW values are significantly smaller. The Mw values corresponding to MeOH-insoluble lignin and MeOH-soluble lignin were 14 900 and 3000 g mol−1, respectively, whereas the Mw of as-received lignin was 7190 g mol−1 according to refractive index (RI) detection. If a light scattering detector was used, the Mw of MeOH-insoluble lignin, MeOH-soluble lignin, and as-received lignin was 188 000, 86 600, and 96 100 g mol−1, respectively. Because the MW measured by the RI detector is relative to the poly(2-vinyl pyridine) standard, the absolute MW measured by the light scattering detector most likely represents a more accurate MW. If the actual MW distribution of these lignins is a monomodal distribution as shown in the SEC data obtained from DMF SEC with LiBr (Figure 3 b), the multimodal peaks obtained from DMF SEC without LiBr represent aggregation and heterogeneity of lignin molecules in neat DMF. The correlation of the LMW fraction obtained from DMF SEC without LiBr with various properties will be discussed in the following sections.
Table 2. Molecular weight of lignin with the addition of LiBr to the SEC solvent.
|Lignin||Mn[a] [g mol−1]||Mw[a] [g mol−1]||PDI[a]||Mw[b] [g mol−1]||dn/dc[c] [mL g−1]|
|MeOH-insoluble||6520||14 900||2.28||188 000||0.1837±0.0092|
Effect of MeOH fractionation of lignin on its thermal response properties
MeOH fractionation altered the Tg of as-received lignin (Figure 4, Table 3). MeOH-insoluble lignin had a Tg of 211 °C, which is ≈50 °C higher than the Tg of as-received lignin (153 °C). The increased Tg after MeOH washing is consistent with the trend in solvent-extracted hardwood lignin reported in our previous work.7 However, the Tg of MeOH-soluble lignin was 117 °C, much lower than that of as-received lignin. The trend of Tg alteration can be explained by the content of LMW lignin measured by DMF SEC without LiBr. The LMW fraction should provide lignin with a plasticizing effect. In DMF SEC without LiBr, the LMW fraction with a retention time higher than 22 min corresponds to a peak MW (Mp) of 700–1100 g mol−1, although Mn and Mw could not be determined because of its molecular weight range, which was outside of the calibration curve. The Mp value of 700–1100 g mol−1 suggests that the LMW fraction consists of lignin oligomers. The presence of a LMW fraction is represented as LMW area [%] in Table 1. As the LMW area increases, the Tg of the corresponding lignin decreases; this trend follows a linear regression based on the Fox equation25 [Eq. (1)] with a correlation coefficient R2 of 0.974 (Figure 5):
Table 3. Tg obtained from DSC and thermal degradation properties obtained from TGA.
|Lignin||Tg [°C]||10 wt % weight loss [°C]||Derivative weight peak [°C]||Residual weight at 1000 °C [%]|
The calculated TgA results in −89 °C. Although the LMW fraction could not be isolated in pure form (as the extracted mass contained some agglomerated HMW fraction), the Fox equation suggests that the Tg of LMW oligomeric lignin is −89 °C (Table 3). The Tg value derived from the Fox equation for oligomeric lignin is so low that it justifies the plasticization effect of the LMW lignin fraction. As Tg is one of the most important lignin processing parameters, the ability to alter Tg is critical for the development of lignin-derived products. MeOH fractionation as depicted here provides the ability to tune the Tg of lignin from existing sources.
MeOH fractionation also alters the thermal stability of lignin (Figure 6, Table 3) under N2. MeOH-soluble lignin decomposed the quickest, as seen in the 10 wt % weight loss at 260 °C and in the maximum of the decomposition derivative peak located at 354 °C. The initial degradation of MeOH-insoluble lignin (10 wt % weight loss at 293 °C, the maximum of the decomposition derivative peak located at 362 °C) was slightly faster than that of as-received lignin (10 wt % weight loss at 301 °C, maximum of the decomposition derivative peak located at 384 °C). The alteration of the degradation temperature indicates that not only the molecular weight affects the thermal stability but also the chemical composition as some functional groups have been altered in comparison to as-received lignin, which is addressed in the next section. More importantly, the char yield (residual weight) at 1000 °C increased for MeOH-insoluble lignin (47 wt %) and decreased for MeOH-soluble lignin (32 wt %) compared to that of as-received lignin (41 wt %). If lignin is used as a carbon precursor, such as in the case of carbon fibers or activated carbons, the increase of char yield is highly beneficial for economic and environmental reasons. Thus, MeOH-insoluble lignin, or HMW lignin, could be more valuable as a carbon precursor than LMW lignin. Interestingly, the char yield followed a linear regression with the area of LMW fraction obtained from SEC without LiBr (Figure 7). The coefficient of correlation R2 was 0.997. If we consider that the area of the LMW fraction in SEC by RI intensity corresponds to the mass ratio, such a good fit with a linear regression indicates that the majority of the LMW fraction does not form char. In fact, an increase of 1 % in the LMW area corresponds exactly to a 1 wt % loss in the char yield.
Relationship between the lignin chemical structure and its properties
The chemical structures of as-received lignin, MeOH-insoluble lignin, and MeOH-soluble lignin were investigated by using 13C and 31P NMR spectroscopy to further understand the trends in the lignin properties described previously. 13C NMR spectra (a representative spectrum is shown in the Supporting Information, Figure S1) were quantified by integration. The integration of the 162–102 ppm region was set as the reference, equal to 6.12 carbon atoms, which assumes that it includes six aromatic carbon atoms and 0.12 vinylic carbon atoms.26, 27 The chemical structures are reported as equivalents per aromatic ring [equiv./Ar] in Table 4, and the chemical shift assignments were chosen from data reported in the literature.26–29 These lignin samples did not show any signal between 90–102 ppm (Figure S1), which indicates the absence of sugars.26 The majority of assigned carbon contents in Table 4 were equivalent or similar among the three lignin samples (as-received lignin, MeOH-insoluble lignin, and MeOH-soluble lignin). One noticeable difference is that MeOH-soluble lignin possesses more aliphatic CC (0.4 equiv./Ar) than the others (0.2 and 0.1 equiv./Ar for as-received lignin and MeOH-insoluble lignin, respectively). In general, aliphatic groups, in comparison to aromatic groups, readily allow a free volume expansion and volatilization upon heating. The presence of more aliphatic carbon atoms should decrease the Tg of lignin by a free volume expansion and decrease the char yield by thermal devolatilization.
Table 4. Integration of 13C NMR spectra.
|δ||Assignment||Number of carbon atoms/aromatic rings in lignin|
The quantitative hydroxyl contents were determined by 31P NMR spectroscopy (Figure S2) by using the integrated peak areas of phospholane-reacted lignin and an internal standard (cyclohexanol)26, 30, 31 (Table 5). The content of aliphatic hydroxyl groups was lower in MeOH-soluble lignin than as-received lignin. Less aliphatic hydroxyl groups should led to less hydrogen-bonding interactions, which reduces the Tg. Uraki et al.32 reported that the addition of Cγ-hydroxyl groups in a lignin model compound increased its Tg significantly. A decrease in the content of aliphatic hydroxyl groups, in addition to an increase in the content of aliphatic carbon atoms, possibly contributed to lower its Tg. The most noticeable difference on comparing the different types of hydroxyl groups is the phenolic content, especially guaiacyl groups. The content of guaiacyl groups in MeOH-soluble lignin increased significantly compared to as-received lignin, whereas MeOH-insoluble lignin showed a slight decrease. The number of phenolic end-groups is proportional to the number of chains.33 Namely, the higher content of phenolic end-groups in MeOH-soluble lignin corresponds to its lower MW or it is more highly branched than as-received lignin or MeOH-insoluble lignin.
Table 5. Hydroxyl group content of lignin [mmol g−1] calculated from 31P NMR spectra.[a]
|δ||Assignment||Hydroxyl group content [mmol g−1] in lignin|
|140.0–144.7||C5-substituted “condensed” and syringyl||1.57||1.84||1.83|
|133.6–136.6||carboxylic acid OH||0||0.18||0.25|
| ||total OH||5.15||5.31||5.70|
Ragauskas et al.29, 30 studied the decomposition mechanism of pyrolysis oils obtained from Kraft lignin and the following findings were reported. The amount of guaiacyl groups and aliphatic carbon atoms increases during pyrolysis up to 400 or lit8500 °C. The condensed groups are converted to phenols such as guaiacyl groups, but the content of guaiacyl groups subsequently decreases at higher temperatures. Guaiacyl groups are converted to catechol-type structures by the elimination of methoxy groups at higher temperatures. In this study, the higher content of guaiacyl groups measured by 31P NMR spectroscopy and of aliphatic carbon atoms measured by 13C NMR spectroscopy suggests that MeOH-soluble lignin possesses a chemical structure that is more susceptible to volatilization during pyrolysis. Lignin made of the pure LMW fraction was not characterized because its isolation was not possible. The linear relationship between the char yield and the area of LMW fraction displayed in Figure 7 suggests that the LMW fraction should contain more functional groups to volatilize during pyrolysis. MeOH-soluble lignin also showed a much lower dn/dc value compared to the other lignins (Table 2), indicative of a significant alteration of the chemical composition. In addition, if dominant carbonization and graphitization reactions of lignin occur intramolecularly, the LMW fraction (Mp=700–1100 g mol−1) might be too small to form a char. The majority of hydroxyl group chemical shifts obtained from 31P NMR spectroscopy were similar; however, the presence of carboxylic acid and an increase in the content of C5 condensed units was observed in both MeOH-soluble and insoluble lignin. The presence of caryboxylic acids and the increase in the content of C5 condensed units are probably responsible for the lower initial degradation temperature (observed in the thermogravimetric analysis (TGA) curves) compared to the as-received lignin. The MeOH-based treatment (or the extraction process) might induce ester hydrolysis, which leads to the generation of carboxylic acids or condensation, but the potential alteration of the overall physical characteristics (other than the decomposition behavior) is negligible.
X-ray photoelectron spectroscopy (XPS) was also used to characterize the functional groups in each lignin sample in further detail. The survey spectra of the three samples were very similar (Figure S3, Table S2). Their elemental compositions were overall the same, with an O content of 18–19 %, and contamination with a S-based compound that led to a S content slightly less than 1 %. The lignin used here was a purified version of the Kraft lignin that was partially demineralized by treating it with acid at low pH by following a method similar to those discussed by Kouisni et al.34 Fierro et al. also discussed the decreased S content from 2.2 to 0.5 wt % after acid-washing a softwood Kraft lignin.35 The MeOH-insoluble lignin contains more S than the soluble fraction, likely because of the presence of slightly more mineral S impurities in the MeOH-insoluble fraction (Table S2). The S 2p peak of all the samples could be fitted with two components located at 168.4 and 169.6 eV. The contamination was identified as S-containing groups that remain from the manufacturing process.
Table 6. Relative area of each component peak in the fitting of C 1s and O 1s spectra as a percentage of the total area of the peak from XPS.
|Lignin||C 1s||O 1s|
| ||sp2||sp3||CCOOR||COR||CO||COOR||H2O||PhO, PhCO, OO||CO, COOH||ROH, COC||PhOH, CO||H2Ochem||H2Ophys|