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Keywords:

  • biomass;
  • carbon;
  • polymers;
  • renewable resources;
  • waste prevention

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

The development of technologies to tune lignin properties for high-performance lignin-based materials is crucial for the utilization of lignin in various applications. Here, the effect of methanol (MeOH) fractionation on the molecular weight, molecular weight distribution, glass transition temperature (Tg), thermal decomposition, and chemical structure of lignin were investigated. Repeated MeOH fractionation of softwood Kraft lignin successfully removed the low-molecular-weight fraction. The separated high-molecular-weight lignin showed a Tg of 211 °C and a char yield of 47 %, much higher than those of as-received lignin (Tg 153 °C, char yield 41 %). The MeOH-soluble fraction of lignin showed an increased low-molecular-weight fraction and a lower Tg (117 °C) and char yield (32 %). The amount of low-molecular-weight fraction showed a quantitative correlation with both 1/Tg and char yield in a linear regression. This study demonstrated the efficient purification or fractionation technology for lignin; it also established a theoretical and empirical correlation between the physical characteristics of fractionated lignins.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Lignin, the most abundant natural aromatic polymer, exists in the cell wall of plants. For instance, it represents 18–35 % of wood by weight.1 The annual natural production of lignin on Earth is estimated as 5–36×108 tons.2 Lignin is currently produced on an industrial scale in paper and pulping factories as well as in biorefineries. The paper and pulping industry alone produces lignin in quantities that exceed 50 million tons annually,3 but the majority of lignin produced is currently utilized as a low-cost fuel to balance energy needs. Only approximately 1 million tons per year of lignin is currently used for commercial applications, which include concrete additives, dyestuff dispersants, binders or surfactants for animal feed, dust control, and pesticides.4, 5 However, the market demand for all of these nonfuel lignin usages represents a negligible fraction of current lignin production. One of the biggest hindrances for the commercialization of high-performance lignin products is that lignin has heterogeneous properties, which include molecular weight, functionality, and thermal properties, from different sources and processing methods. Therefore, the isolation of high-purity lignin for high-performance lignin-derived materials should be urgently pursued for the development of new approaches for lignin in various applications. It is critical to develop a wide utilization of the renewable and abundant lignin resource for a sustainable society.

Some of the high-performance lignin derivatives utilize lignin as a macromonomer in a component of lignin copolymers as well as carbon fiber precursors.6 We have recently reported the successful synthesis of new thermoplastic copolymers from lignin, which are rubbery polyester copolymers7 and robust polyurethane copolymers.8 These thermoplastic copolymers from lignin exhibited a two-phase behavior in dynamic mechanical analysis, characteristic of multiphase thermoplastic copolymers.9 Our previous work demonstrated that the lignin content and glass transition temperature (Tg) can be tuned to achieve the desired properties. The capability to tailor the Tg of lignin is extremely important for its processing in applications such as carbon fiber precursors10 because it directly correlates to the melt-processing temperature or the stabilization conditions prior to carbonization. Key to the successful synthesis of lignin-based thermoplastic copolymers was the utilization of a high-molecular-weight (HMW) lignin fraction to produce an efficient connection between the hard segment (i.e., lignin) and the soft segment. The HMW lignin fraction was prepared by formaldehyde crosslinking, whereas as-received lignin contained a significant amount of a low-molecular-weight (LMW) fraction. In both studies, the crosslinked HMW fraction of lignin showed a higher Tg than as-received lignin. The removal of the LMW fraction also reduced its polydispersity index (PDI), which enabled the successful generation of two distinctive phases in the thermoplastic copolymers.

Our previous work7 not only reported the formaldehyde crosslinking but also the increase of the lignin molecular weight and Tg by methanol (MeOH) fractionation. Solvent fractionation of lignin has been used in industry and investigated for several different sources of lignin and different solvents.1120 Solvent fractionation to increase the molecular weight and Tg is a more facile approach than performing a formaldehyde crosslinking reaction; however, our work showed that the isolated yield from MeOH fractionation of lignin was very small (17 %) and that its molecular weight distribution was broad. The previous work investigated hardwood-based solvent-extracted lignin, and the as-received lignin had a low molecular weight (Mn=1840 g mol−1, PDI=122) with a low Tg (108 °C). In this study, we have investigated MeOH fractionation of softwood Kraft lignin, which has a higher molecular weight and the Tg of as-received lignin was higher than hardwood lignin in our previous work. The effect of MeOH fractionation process on the molecular weight, molecular weight distribution, Tg, thermal decomposition, and chemical structure of lignin was investigated. Furthermore, these properties were empirically correlated to fit into a physical theory. The elucidation of these lignin properties through a lignin purification processes is crucial to foster technology for advanced lignin-based materials.

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

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.

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Figure 1. Schematic diagram of the MeOH fractionation of lignin.

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Table 1. Molecular weight of lignin from MeOH fractionation (without LiBr).
LigninNumber of washesMn[a] [g mol−1]Mw[a] [g mol−1]PDI[a]HMW area [%]LMW area [%]
  1. [a] Determined by RI detection.

as-received010 0001 110 00011093.66.4
MeOH-insoluble1st28 2001 310 00046.397.22.8
2nd60 6001 400 00023.198.81.2
3rd63 8001 530 00024.098.81.2
4th (last)443 0001 550 0003.51000
MeOH-soluble 3800492 00012984.715.3
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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.
LigninMn[a] [g mol−1]Mw[a] [g mol−1]PDI[a]Mw[b] [g mol−1]dn/dc[c] [mL g−1]
  1. [a] Determined by RI detection. [b] Determined by light scattering detection. [c] Determined off-line.

as-received236071903.0596 1000.1867±0.0011
MeOH-insoluble652014 9002.28188 0000.1837±0.0092
MeOH-soluble159030001.9086 6000.1636±0.0028

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):

  • equation image(1)
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Figure 4. DSC curves of as-received lignin (middle), MeOH-insoluble lignin (top), and MeOH-soluble lignin (bottom).

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Table 3. Tg obtained from DSC and thermal degradation properties obtained from TGA.
LigninTg [°C]10 wt % weight loss [°C]Derivative weight peak [°C]Residual weight at 1000 °C [%]
  1. [a] Estimated from the Fox equation.

as-received15330138441
MeOH-insoluble21129336247
MeOH-soluble11726035432
LMW fraction−89[a]
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Figure 5. Tg as a function of LMW content in the lignin.

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where wA and wB correspond to the weight fractions of A and B and TgA and TgB correspond to the Tg of components A and B, respectively. In this study, TgA represents the Tg of the LMW fraction and TgB corresponds to the Tg of MeOH-insoluble lignin (211 °C). The area from the RI peaks in SEC represents the weight fraction of the sample in the solution, and thus the LMW area [%] in Table 1 is converted to wA in Figure 5. The linear regression follows the equation y=0.00336 x+0.00207. Equation (1) derives Equation (2), and TgA can be determined from the slope of Equation (2) according to Equation (3).(2), (3), (4)

  • equation image(2)
  • equation image(3)
  • equation image(4)

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.

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Figure 6. TGA curves of as-received lignin (—), MeOH-insoluble lignin (– – –), and MeOH-soluble lignin (- - - -).

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Figure 7. Char yield at 1000 °C as a function of LMW area obtained from SEC.

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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.2629 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 C[BOND]C (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.
δAssignmentNumber of carbon atoms/aromatic rings in lignin
[ppm] as-receivedMeOH-insolubleMeOH-soluble
175–168aliphatic COOR0.10.10.1
162–140CAr[BOND]O2.01.92.0
140–123CAr[BOND]C2.02.02.0
123–102CAr[BOND]H2.12.22.1
98–58aliphatic C[BOND]O0.10.20.2
58–54methoxy OCH30.90.90.9
49–0aliphatic C[BOND]C0.20.10.4

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]
δAssignmentHydroxyl group content [mmol g−1] in lignin
[ppm] as-receivedMeOH-insolubleMeOH-soluble
  1. [a] The peak assignments followed the assignments reported by Ragauskas et al.26

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150.0–145.5aliphatic OH1.711.681.24
144.7–145.5cyclohexanol
136.6–144.7phenols3.433.454.21
140.0–144.7C5-substituted “condensed” and syringyl1.571.841.83
139.0–140.0guaiacyl1.451.232.04
138.2–139.0catechol0.280.250.22
137.3–138.2p-hydroxyphenol0.170.110.10
133.6–136.6carboxylic acid OH00.180.25
 total OH5.155.315.70

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.

The C 1s and O 1s core-level spectra were fitted to obtain more information about the functional groups in the three samples (Table 6, Figure S4). The significant differences that appear in the fitting of the C 1s and O 1s peaks are indicated by an arrow in Figure S4. The majority of the chemical groups determined from the fitting of the core spectra (Table 6) correspond well with the chemical structures obtained from 13C (Table 4) and 31P NMR spectroscopy (Table 5). The increase in the sp2 carbon content and the decrease in amount of C[BOND]O[BOND]R groups in MeOH-insoluble lignin correlate to its higher char yield observed in TGA and its higher Tg value, whereas the decrease in sp2 carbon content and the increase in sp3 carbon content in MeOH-soluble lignin correspond to its lower char yield. An increase in the sp3 carbon content and a decrease in the amount of R[BOND]OH functionalities in MeOH-soluble lignin, which was also seen in the NMR spectra, contribute to its lower Tg. A significant amount of O in XPS analysis comes from some inorganic contaminants such as SO42−, which has a binding energy of approximately 532.0 eV.36 As shown in Table 6, the XPS data for the O 1s peak analysis is consistent with the NMR spectra (Table 5). For example, in both MeOH-soluble and insoluble lignins the >C[DOUBLE BOND]O content from carboxylic acid groups was increased compared to the as-received lignin; the aliphatic [BOND]OH group content in MeOH-soluble lignin is lower than that in MeOH-insoluble lignin.

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.
LigninC 1sO 1s
 sp2sp3C[BOND]COORC[BOND]O[BOND]RC[DOUBLE BOND]OCOORH2OPh[DOUBLE BOND]O, Ph[BOND]C[DOUBLE BOND]O, O[BOND]OC[DOUBLE BOND]O, COOHR[BOND]OH, C[BOND]O[BOND]CPh[BOND]OH, C[BOND]OH2OchemH2Ophys
as-received51272116211535281750
MeOH-insoluble5727274211345301020
MeOH-soluble46321127111339261840

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Methanol (MeOH) fractionation of softwood Kraft lignin enabled the separation of high glass transition temperature (Tg), high char-forming, and high-molecular-weight lignin from its counterparts. The MeOH-insoluble lignin possesses much higher Tg than as-received lignin because of the removal of a low-molecular-weight (LMW) fraction, which is shown in DMF size-exclusion chromatography (SEC) without LiBr. MeOH-soluble lignin showed an increased LMW fraction and a lower Tg compared to the as-received lignin. The trend of Tg values agreed very well with the Fox equation [Eq. (1)], which confirms that the LMW lignin oligomer fraction (estimated Tg −89 °C) contributes to its decreased Tg. The amount of LMW fraction also correlated well with the char yield through a linear regression, that is, an increase in the LMW fraction decreased its char yield. A high char yield is advantageous for the use of lignin as a carbon precursor. This MeOH fractionation study, which includes the characterization of the evolution of Tg and char yield, is the first report to empirically correlate the content of the LMW fraction to its Tg and char yield. This statistical analysis suggests that DMF SEC without LiBr might enable its use as one of the diagnostic tool to identify heterogeneities that exist in lignin and correlate to its Tg and char yield. Single, monomodal peaks of these lignin samples obtained in DMF SEC with LiBr indicate that the size of the lignin corresponds to a single distribution, but the combined results of DMF SEC both with and without LiBr suggest that the chemical structure and compositions between as-received, MeOH-insoluble, and MeOH-soluble lignins are significantly altered. The LMW-fraction-containing MeOH-soluble lignin revealed higher contents of aliphatic carbon atoms and guaiacyl groups and a lower content of aliphatic hydroxyl groups. Such alteration of the functional groups as well as a higher LMW content resulted in its decreased Tg and char yield. The identification of the molecular structure, which includes its functionalities and molecular weight, is crucial to control the char yield and other physical properties. These findings could be utilized as design parameters for various lignin-derived products.

Experimental Section

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Fractionation of lignin

Medium ash grade lignin (as-received lignin) from Kraft-processed softwood biomass was obtained from Kruger Wayagamack, Inc. As-received lignin (100 g) was put into a 1 L centrifuge bottle that contained MeOH (700 mL; BDH Chemicals). The centrifuge bottle was stirred with a spatula and shaken several times. Then, the bottle was placed into a CRU-5000 Centrifuge (DAMON/IEC Division) and centrifuged at 2000 rpm for 15 min at 10 °C. A lignin cake was formed at the bottom of the centrifuge bottle and a small portion of the lignin cake was collected for analysis. The supernatant (MeOH lignin solution) was poured into a beaker, and MeOH was evaporated to collect MeOH-soluble lignin. The residual lignin cake was mixed with MeOH again (700 mL) and the washing step was repeated (second washing). The washing step was repeated three times. A Silverson mixer (Silverson L5M-A) was used only during the fourth wash to homogenize the mixture. The final lignin cake (MeOH-insoluble lignin) was dried in a convection oven followed by drying in a vacuum oven, both at ∼50 °C. The isolated yield of MeOH-insoluble lignin was 50 % (50 g recovery), therefore, the yield of MeOH-soluble lignin was 50 %.

Lignin characterization

High-temperature size-exclusion chromatography (HTSEC) was performed on a Waters GPCV2000 instrument equipped with three Polymer Labs PLgel Mixed C columns in series, a differential refractometer (λ=880 nm), a Precision Detectors two-angle light scattering detector (λ=682 nm), and a Waters viscometer all thermally regulated at 60 °C. Waters Alliance GPC 2000 software was used to collect the data, which were subsequently analyzed using Waters Empower software in which a conventional calibration curve based on low-PDI poly(2-vinylpyridine) standards (1620–256 000 g mol−1) was used to evaluate molecular weight characteristics. DMF with and without LiBr (0.05 M) was used as the eluent at a flow rate of 1.0 mL min−1. A Wyatt Technology Optilab rEX RI detector (λ=658 nm) was used in conjunction with a Harvard ApparatusPHD2000 Infusion syringe pump to determine dn/dc values off-line. Sets of DMF with LiBr (0.05 M) solutions with concentrations that ranged from 0.5–5 mg mL−1 were prepared for the measurement. The dn/dc values were calculated by using Astra V software. Differential scanning calorimetry (DSC, Q-2000 TA Instruments) was used to determine Tg values at a heating rate of 10 °C min−1 under nitrogen. The Tg values are reported as the transition midpoints of the heat capacity change during the second heating cycle. TGA (Q-500 TA Instruments) was performed under N2 at a heating rate of 10 °C min−1.

The hydroxyl groups of lignin were derivatized with 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (TMDP) and 31P NMR spectroscopy was performed to determine the hydroxyl contents of lignin by using a Varian 500 MHz spectrometer. Lignin (40 mg) was dissolved in a pyridine/CDCl3 (1.6/1 v/v) solution (0.6 mL) that contained chromium(III) acetylacetonate (3 mg) and cyclohexanol (3.6 mg) as an internal standard and derivatized with TMDP (100 μL).26, 30, 31 Quantitative 31P NMR spectra were acquired for 256 scans with a 45° pulse angle and a 25 s pulse delay. 13C NMR was performed by using a Varian 500 MHz instrument using [D6]DMSO as the solvent. For the quantitative 13C NMR spectra, lignin (200 mg) dissolved in [D6]DMSO (0.5 mL) with chromium(III) acetylacetonate (2.5 mg, 0.01 M) was placed in an NMR tube and a total of 20 000 scans were collected with a 90° pulse width, a 1.4 s acquisition time, and a 1.7 s relaxation delay.28

XPS analysis of the different lignin fractions was performed by using a Thermo Scientific K-Alpha X-ray photoelectron spectrometer equipped with a hemispherical electron energy analyzer, which was operated in fixed transmission mode at constant pass energy of 200 eV for the survey spectra and 50 eV for the core-level spectra. A monochromatic AlKα source (1486.6 eV) operated at 420 W (14 kV, 30 mA) was used as the incident radiation. Photoemitted electrons were collected at a take-off angle of 90° from the sample, and the pressure was ≈10−7 Pa. The spectrometer energy scale was calibrated with respect to the Ag 3d5/2, Au 4f7/2, and Cu 2p3/2 core-level peaks at binding energies of 368.3, 84.0, and 932.7 eV, respectively. C 1s and O 1s core-level spectra were fitted to GL functions (product of a Lorentzian by a Gaussian) by using Avantage 4.44 software. The background was a variation of an S-shaped (i.e., Shirley) background. The component peaks used to fit the C 1s and O 1s peaks are presented in Table S1. Compared to the fitting of the C 1s peak, the fitting of the O 1s peak is more controversial, and the method followed was based on a recent study that addressed the fitting of O 1s peaks.3739 The peak components suggested by Brender et al.37 and Vautard et al.39 were used in this study. The C 1s peak fitting was performed so that the total area of all the oxy-carbonated components could not be higher than the total area of the O 1s peak.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

This research was sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy. J.H.P. acknowledges support by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office, under contract DE-AC05-00O30725 with UT-Battelle, LLC. Part of the polymer characterization work was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, U.S. Department of Energy. A part of this research was done through the Oak Ridge National Laboratory’s High Temperature Materials Laboratory User Program, sponsored by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Program.

Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

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