Relating Dicarboxylic Acid Yield to Residual Lignin Structural Features

Research Article pubs.acs.org/journal/ascecg

Relating Dicarboxylic Acid Yield to Residual Lignin Structural Features Dylan J. Cronin,*,†,‡ Kameron Dunn,† Xiao Zhang,‡ and William O. S. Doherty† †

Centre for Tropical Crops and Biocommodities, Queensland University of Technology, 2 George St., Brisbane, Queensland, Australia ‡ Bioproducts, Science & Engineering Laboratory, Washington State University, 2710 Crimson Way, Richland, Washington, United States S Supporting Information *

ABSTRACT: This study focused on understanding the relationship between dicarboxylic acid (DCA) yields derived from lignin and the structural attributes of their solid residues and corresponding acid-soluble lignin fractions. It is a continuation of the study by the authors on DCA production from bagasse lignin via hydrothermal liquefaction. It characterized the residues derived from the use of H2O2/chalcopyrite and sodium percarbonate for DCA formation, at temperatures between 60 and 300 °C with a reaction time of 3 h. FTIR, GPC, and 2D NMR were used to unravel the functional group changes, molecular weight distribution, and lignin substructures and linkages. The DCA yield correlated well, in a linear fashion, to the aromatic to aliphatic functional group ratio (AAFGR) and the degree of aromatic condensation (DAC). Supporting evidence of lignin depolymerization and repolymerization to explain the DCA yields was provided by GPC. Interestingly, the proportion of the α-oxidized substructure S′ was found to be an indicator of the extent of lignin depolymerization, and its ratio to the S substructure was found to correlate with DCA yield at reaction temperatures up to 200 °C. KEYWORDS: Lignin depolymerization, Aromatics, Dicarboxylic acids, Sodium percarbonate, Lignin repolymerization, Fenton reaction

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proach.9,17 The approach used in these studies was to mimic the biological decomposition of lignocellulosic material, which is known to proceed via a Fenton oxidation mechanism.18 However, the results were generally poor in terms of the yield of products on dry lignin mass, and the significant quantity of residual solid lignin. The previous work by the authors19 demonstrated that DCA yield could be increased appreciably by performing the depolymerization under alkaline conditions, using sodium percarbonate (Na2CO3·1.5H2O2) as a source of hydroxyl radicals. This approach allows for the solubilization of the lignin, which facilitates the depolymerization process. It was also determined that the target DCA compounds are significantly more stable toward thermal degradation under alkaline conditions, which allowed for the depolymerization process to be performed at higher temperatures than when working under acidic conditions. Further increasing the alkaline content by substituting water with 1 M NaOH as the solvent was shown to improve DCA yield by approximately 85 wt % when treated at 200 °C (from 7.4 to 13.7 wt %). The current work is an extension of the authors’ previous study19 on the formation of DCAs from bagasse lignin, by studying the structural properties of the residual lignins and

INTRODUCTION The depletion and negative environmental influences of fossil fuels have created the need to utilize sustainable lignocellulose biomass resources for the production of energy, plastics, and industrial chemicals. The emerging biorefinery industry is set to produce large quantities of lignin, which is currently underutilized as a low-heating-value combustion fuel.1 As such, the sustainability of the industry is dependent on the successful valorization of lignin. Given the highly aromatic nature of lignin, there has been much work devoted toward its depolymerization to low molecular weight aromatic compounds (LMWACs),2−5 chemicals, and fuel additives.6,7 More recently, however, there has been significant interest in the production of DCAs from lignin.8−11 Four carbon (C4) DCAs were reported by the U.S. Department of Energy in 200412 as the first of 12 key product classes considered to be top value-added products derivable from biomass. It was observed that this family of compounds could act as building blocks for a wide range of products that satisfy numerous high volume commercial industries, including synthetic fibers (e.g., lycra) and green solvents. Work has also been conducted utilizing DCA mixtures as green alternatives to commercial insecticides, disinfectants, and antifungal fertilizer products.13,14 Research in the production of C4 DCAs has involved either a biological 15,16 or thermochemical depolymerization ap© 2017 American Chemical Society

Received: September 7, 2017 Revised: October 3, 2017 Published: October 24, 2017 11695

DOI: 10.1021/acssuschemeng.7b03164 ACS Sustainable Chem. Eng. 2017, 5, 11695−11705

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ACS Sustainable Chemistry & Engineering

Table 1. Depolymerization Data of Bagasse Lignin under Various Reaction Temperatures with 3 h Reaction Timea

a

catalyst

reaction temp. (°C)

H2O2/chalcopyrite H2O2/chalcopyrite H2O2/chalcopyrite sodium percarbonate sodium percarbonate sodium percarbonate sodium percarbonate sodium percarbonate and 1 M NaOH sodium percarbonate sodium percarbonate

60 120 150 60 120 150 200 200 250 300

residual solids (wt %) 72 69 55 79 59 53 24 28 12 42

± ± ± ± ± ± ± ± ± ±

0.4 3.0 1.7 1.6 2.9 0.3 1.1 1.3 2.0 3.0

acid-soluble lignin (wt %) 2.3 2.8 1.3 3.1 5.0 3.4 10.9 13.8 5.6 9.0

± ± ± ± ± ± ± ± ± ±

0.1 0.1 0.3 0.1 0.5 0.4 0.3 1 0.3 0.6

DCA yield (wt %) 2.3 1.5 0.1 3.8 4.4 5.5 7.4 13.7 4.8 1.6

± ± ± ± ± ± ± ± ± ±

0.6 0.4 0.1 0.7 0.8 0.7 1.1 0.8 0.7 0.3

other products (wt %) 23 27 43 14 31 38 61 45 76 47

± ± ± ± ± ± ± ± ± ±

1 4 2 2 4 1 3 3 3 4

Data reproduced from Cronin et al.19

Figure 1. Absorbance ATR-FTIR spectrum of bagasse lignin subtracted from the residual solids obtained after treatment with (A) H2O2/ chalcopyrite at 60, 120, and 150 °C; (B) sodium percarbonate at 60, 120, and 150 °C; and (C) sodium percarbonate at 200, 250, and 300 °C.

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Scheme 1. Depolymerization Pathway of a Monolignol to a Terminal Phenyl Ethanone Structure (Adapted with permission from ref 27. Copyright 2014 Queensland University of Technology.)

multiple-bond correlation (HMBC) NMR spectra were recorded using a 400 MHz spectrometer (Agilent, US) at room temperature. Spectral widths of 5 kHz and 20 kHz were used for the 1H and 13C nuclei, respectively. A recycle delay of 1.5 s was used, and 128 scans were recorded in the 13C dimension. Samples were prepared by dissolving 20−30 mg of material in 0.75 mL of DMSO-d6 and filtering through a 0.45 um Teflon syringe filter. The solvent contained a small quantity of tetramethylsilane, which was used as an internal chemical shift reference point (δC/δH 0.0/0.00).

acid soluble lignins derived from the various acid (i.e., H2O2/ chalcopyrite) and alkaline (i.e., sodium percarbonate) treatment conditions. This is aimed at obtaining a better understanding of the relationship between DCA yield and the lignin substructures in the residual solids and acid-soluble lignin, in order to open up reaction pathways for increased DCA yield from lignin.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION Fourier Transform Infrared (FTIR) Spectroscopy. Differential Spectra Analysis. The ATR-FTIR spectra of the residual solids were obtained and compared to that of the original bagasse lignin material. A full listing of lignin’s significant FTIR absorption peaks is provided in Table S1.21−26 By normalizing all FTIR spectra to the aromatic skeletal vibration absorbance at 1590 cm−1 and presenting each residual solid’s spectrum as a subtraction spectrum of the original lignin, an indication of the structural changes, if any, the lignin has undergone as a consequence of treatment becomes apparent (see Figure 1). With the acid treatments, changes in the functional groups of the residual solids relative to the original lignin are minor (see Figure 1A). However, with increasing temperature, a slight increase in the peak intensity is observed at 1705 cm−1, which suggests a greater proportion of the monolignols with aromatic α-carbonyl functionality are present as a result of the oxidation of the more prevalent αhydroxyl. There are decreased absorptions at 1030, 1120, 1220, and 1360 cm−1, indicating a slight decrease in the methoxy, ether, and aromatic content in the residual solids. There is also a slight reduction in the aromatic C−H peak at 830 cm−1, which suggests that some degree of aromatic condensation has occurred in the residue as a consequence of the acid treatment with increasing temperature. Under alkaline conditions, there are noticeable changes in the proportion of the functional groups (see Figure 1B). Increasing the temperature from 60 to 120 °C once again results in an increase in the aromatic carbonyl/carboxyl peak at 1705 cm−1; however, the peaks at 1030 and 1120 cm−1, which were observed to reduce under acid treatment, increased markedly in intensity. The peak at 1120 cm−1 is considered indicative of the S monomer unit of lignin,22 suggesting that this monomer is present in higher proportions in these residues than the starting lignin material. Given the apparent higher proportions of the S monomer in the residues compared to the original lignin, and that the aromatic 1510 cm−1 band decreased, it is likely that the susceptible G and H monomers were either opened or cleaved from the polymer matrix, thus decreasing the relative aromatic content of the residues. Under the conditions used, the ring cleavage of the S unit may be sterically hindered by the presence of two methoxyl

Materials. All chemical reagents were purchased in HPLC grade from commercial suppliers and used without further purification. The following reagents were obtained from Sigma-Aldrich Co. LLC (U.S.): tetrahydrofuran (THF), acetic anhydride, pyridine, and dimethyl sulfoxide-d6 (DMSO- d6). For a detailed description of the extraction procedure applied to prepare the lignin used in this study, as well as the elemental and compositional analysis performed via standard NREL methods,20 refer to Cronin et al.19 Lignin Depolymerization and DCA Analysis. For a complete description of the experimental methods used in the bagasse lignin depolymerization experiments, as well as the qualitative and quantitative analysis of the DCA products formed, refer to Cronin et al.19 Experimental Conditions and Yield. The yields of the major product fractions for each of the experimental conditions investigated are presented in Table 1. All of the reactions studied in this work were conducted using a reaction time of 3 h, as the bulk of the work conducted in the previous publication applied this time.19 Further information regarding the reagents, treatment and workup process, and residue analysis procedures for each of the three reaction conditions used in this work is provided in Figure S1. The DCA yield is defined as the cumulative yield of the three most prevalent DCAs obtained: oxalic, malonic, and succinic acid. Other products are defined as the quantity of the starting lignin not accounted for by residual solids, acid-soluble lignin, or DCA yield. Fourier Transform Infrared (FTIR) Spectroscopy. FTIR spectra of the lignin and residual solids were collected using a Nicolet 870 FTIR spectrometer equipped with a Smart Endurance single bounce diamond ATR accessory (Nicolet Instrument Corp., US). Spectra were collected in the spectral range 4000−525 cm−1, using 64 scans and 4 cm−1 resolution. Spectra were baseline corrected and normalized on the common aromatic skeletal vibration at 1590 cm−1. Gel Permeation Chromatography (GPC). The instrument used for this analysis incorporated a GPC Water Breeze system model 151 with an isocratic HPLC pump. Eluted fractions were detected with UV light (250 nm) and a refractive index detector (Water model 2414). Three Phenomenex phenogel columns (500, 104, and 106 Å porosity; 5 μm bead size) were used for size exclusive separation. The mobile phase was THF, applied at a 1 mL min−1 flow rate at 30 °C. Acetylated lignin samples were prepared at a concentration of 1−2 mg mL−1 in THF and filtered through a 0.45 μm Teflon syringe filter. A 100 μL injection volume was used. For information on the lignin acetylation and molecular weight calibration methods used, see the Supporting Information. Nuclear Magnetic Resonance (NMR) Spectroscopy. The 2D heteronuclear single quantum coherence (HSQC) and heteronuclear 11697


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ACS Sustainable Chemistry & Engineering substituents, making the formation of the quinone intermediary more difficult. Given that the absorption at 1120 cm−1 is also associated with carbonyl stretching, its increase as a result of alkaline treatment may be influenced by the partial breakdown of β-O-4 linkages, to yield both vinylic and α-oxidized ketone structures. This effect has been previously suggested by Dunn27 as a result of hydroxyl (OH−) attack and would explain the relative increase in the vinylic absorption at 985 cm−1, as well as the reduction in the alkyl aryl ether absorption at 1220 cm−1 (see Scheme 1). It appears that with increasing reaction temperature, the lignin undergoes demethoxylation and therefore significant reduction in the 1120 cm−1 S unit peak, as well as further reduction in the 1030 cm−1 alkyl aryl ether peak. Increasing the reaction temperature to 200 and 250 °C does in fact significantly reduce these peaks, along with a simultaneous reduction in the relative aromatic content. The only peaks observed to increase in intensity over this range are the aromatic carbonyl/carboxyl peak at 1705 cm−1 and the aliphatic chain peaks at 2850 and 2920 cm−1. The former is likely caused by a relative increase in the proportion of aromatic units which possess the oxidized α-carbonyl. The latter may be a result of demethoxylation, causing an increase in aryl alkyl chains via methyl migration. Aromatic Content and Condensation. Previous lignin studies have utilized FTIR analysis as a means of comparing the aromatic to aliphatic functional group ratio (AAFGR), and the degree of aromatic condensation (DAC).23,28,29 This is performed by comparing the ratio of the integrated regions from 1650 to 1500 cm−1 (aromatic CC stretching frequency) and 3000 to 2800 cm−1 (aliphatic C−H stretching frequency), in order to approximate the AAFGR. The DAC is then quantified by comparing the integration from 1650−1500 cm−1, to 900−700 cm−1 (aromatic C−H out-of-plane deformation frequency). These values were determined for the residual solids obtained across the range of experimental conditions performed and are presented in Table 2. The data indicate that each of the H2O2/chalcopyrite experiments performed had little influence on the AAFGR of the lignin and that the DAC experienced only a slight increase. However, when bagasse lignin is treated with sodium percarbonate at increasingly higher temperatures (from 60− 250 °C), not only does the yield of residual solids significantly decrease (from 79−17 wt %; see Table 1) but this material has an increasingly greater aliphatic content, and the aromatic portion that remains is more highly condensed. The latter effects are most evident when the reaction temperature is further increased to 300 °C. At this temperature, the residual solid content was high at 42 wt %, the majority of which was a recalcitrant char-like material. The material was isolated from the residual lignin using 1 M NaOH and analyzed by FTIR. It was determined to have a very high DAC value of 11.1, whereas the residual lignin component has a much lower value of 3.9. Figure 2A shows an inverse relationship between DCA yield and AAFGR. DCA yield is observed to increase as the AAFGR decreases, up to the point at which both DCA degradation and aromatic condensation becomes predominant. This is more apparent for the treatment conducted at 300 °C with sodium percarbonate, and to a lesser extent at 250 °C (as “apparent” outliers, see Figure 2). The addition of NaOH to increase the pH of the sodium percarbonate solution (working at 200 °C) had limited

Table 2. Aromatic to Aliphatic Functional Group Ratio (AAFGR) and Degree of Aromatic Condensation (DAC; As Determined by FTIR) for Bagasse Lignin and the Residual Solids Obtained after Various Treatments catalytic environment N/A (original lignin) H2O2/ chalcopyrite H2O2/ chalcopyrite H2O2/ chalcopyrite sodium percarbonate sodium percarbonate sodium percarbonate sodium percarbonate sodium percarbonate and 1 M NaOH sodium percarbonate sodium percarbonate

reaction temp. (°C)

aromatic to aliphatic functional group ratio (AAFGR)

degree of aromatic condensation (DAC)

N/A

1.8

2.3

60

1.6

2.7

120

1.9

2.7

150

1.9

2.8

60

1.5

2.8

120

1.1

2.9

150

1.1

3.5

200

0.4

3.6

200

0.3

3.8

250

1.1

6.0

300

0.7

6.2

influence on both the AAFGR and DAC of the residues obtained (see Figure 2). The yield of residual solids for each test were also similar (see Table 1). These observations suggest that the inclusion of NaOH suppressed DCA breakdown, rather than significantly affecting the lignin depolymerization mechanisms involved. This explanation is supported by the results of Yin et al.,30 who observed that working in alkaline medium limited the breakdown of DCAs derived from catechol via HTL. An alternative explanation is that the presence of NaOH enhanced the production of monomers early during the reaction, allowing for the subsequent cleavage to DCAs while a greater proportion of H2O2 derived hydroxyl radicals remain. Figure 2B indicates that for alkaline treatments at temperatures up to 200 °C there is a correlation between DCA yield and the DAC of the residues. This relationship is effected at higher temperatures, which is most likely due to the thermal stability of the DCAs. Under acidic conditions, the DCAs appear to undergo greater degradation at significantly lower reaction temperatures than under alkaline conditions. The increase in reaction temperature for acidic treatments is observed to have limited influence on both the AAFGR and DAC of the residual solids. Gel Permeation Chromatography (GPC). Acetylated Solid Residue. It was observed that when bagasse lignin was treated with H2O2/chalcopyrite at 60 to 150 °C, the high molecular weight fraction of the material was not significantly affected, but the very low molecular weight material was removed. This fraction is assumed to be present in the aqueous phase as acid-soluble lignin and/or low molecular weight products. No general shift toward material of lower molecular weight than the original lignin sample was observed. Given the very limited variation in the FTIR spectra of these residual solids and the starting lignin, it is not surprising that their molecular weight distributions were not significantly altered. 11698


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Figure 2. Plot of (A) DCA yield (wt %) against AFFGR and (B) DCA yield (wt %) against DAC, for the three different catalytic environments (H2O2/chalcopyrite, sodium percarbonate, and sodium percarbonate with 1 M NaOH).

Figure 3. Gel-permeation chromatogram of acetylated bagasse lignin and the acetylated residual solids obtained after treatment with sodium percarbonate at 60, 120, 150, 200, 250, and 300 °C. 11699


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Figure 4. 2D HSQC NMR spectra of NaOH bagasse lignin (blue) and the residual solids obtained after treatment with sodium percarbonate at 200 °C. (A) Side chain region (δC/δH 50.0−90.0/3.00−5.00) and (B) aromatic region (δC/δH 100.0−150.0/6.10−7.90).

material. It is also common, however, to observe varying degrees of increase in the maximum molecular weight of lignin after treatment at temperatures below 250 °C.32,33 Acid-Soluble Residue. The molecular weight distributions of the aqueous phases obtained via alkaline treatment at 60−250 °C were analyzed and compared to the acid-soluble fraction of the bagasse lignin. As expected, the acid-soluble lignin fraction and aqueous phase samples exhibit a molecular weight range far lower than that of the residual solids (see Figure S2). The acidsoluble lignin fraction contains products of molecular weight < ∼2000 g·mol−1, whereas the lignin material contained a significant proportion >10 000 g·mol−1. After treatment at 60 °C with sodium percarbonate, the aqueous phase obtained did not differ significantly from that of the acid-soluble lignin isolated from the original material. Increasing the reaction temperature to 120 and 150 °C caused a reduction in the products’ polydispersity, with a peak molecular weight of ∼300 g·mol−1. This appears to be indicative of the larger acid-soluble material being selectively broken down to lignin dimer/trimer species. It is at the reaction temperature of 200 °C that a significant reduction is observed in both the molecular weight range of the residual solids (see Figure 3) and their respective yield (from 53 wt % at 150 °C, to 24 wt % at

Figure 3 illustrates the chromatograms of the residual solids obtained after alkaline treatment at 60−300 °C compared to the starting lignin material. Like the acid treatment (chromatograms not shown), the results show that working at a reaction temperature of up to 150 °C was not sufficient to significantly shift the chromatogram to the right through lignin depolymerization. However, the presence of higher proportions of low molecular weight fractions becomes evident at 200 and 250 °C. The molecular weight of the majority of the residue obtained at 250 °C is in the range of only ∼100−1000 g·mol−1, significantly lower than that of the original lignin. Further increasing the reaction temperature to 300 °C reverses the observed trend and results in a residual solid material of very similar molecular weight distribution to that of the starting lignin. This occurs with a substantial increase in the quantity of residual solids, rising from 12 wt % at 250 °C to 42 wt % (see Table 1). This result confirms previous work conducted in the area by Demirbaş,31 who demonstrated that reaction temperatures above 250 °C tend to result in repolymerization of lignin breakdown products. This process is understood to occur via the production of free phenoxyl radicals, which cause the random condensation/repolymerization reactions that result in the formation of recalcitrant solid 11700


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ACS Sustainable Chemistry & Engineering 200 °C, see Table 1). These effects are observed to occur in conjunction with a broadening of the molecular weight range of the aqueous phase products. This indicates that a greater proportion of the high molecular weight lignin is being broken down, thus increasing in acid solubility and leading to its presence in the aqueous phase. When the reaction temperature is further increased to 250 °C, these effects were further increased, with the most prevalent fraction having the approximate size of lignin monomer/dimer species. Nuclear Magnetic Resonance (NMR) Spectroscopy. For a detailed description of the assignments of NMR peaks, including their associated functional groups and lignin substructures, see the Supporting Information. Analysis of Residual Solids. When the results obtained for the acid treatment performed at 120 °C were compared to the starting lignin, the intensity and range of the aromatic and methoxyl peaks were observed to decrease (see Figure S4). However, the only peaks which were completely removed are those associated with hemicellulose. The specific H/G/S monomer peaks of H2,6, G2,6, and S2,6 were found to be susceptible to reduction. This is most likely due to condensation reactions occurring at these sites, resulting in new carbon−carbon intermonomer linkages. The FTIR analysis of this residual material also indicated a slight increase in aromatic condensation when compared to the starting lignin (see Table 2). The results of the alkaline treatment at 120 °C were similar to those obtained for the acid treatment trials. However, the residual hemicellulose impurity still remained, and the S2,6′ peak appears less affected (see Figure S5). The H2,6 region appeared less intense, which suggests that a greater degree of aromatic condensation may have occurred at this site. This agrees with the results of the DAC obtained through FTIR analysis (see Table 2). Considering that the yield of residual solids for both the acid and alkaline treatments at 120 °C were quite high (69 and 59 wt %, respectively; see Table 1), it is not surprising that the bulk of the lignin material has undergone limited structural changes. At 200 °C, the lignin material underwent more structural changes, as reflected in the HSQC spectra (see Figure 4). The more easily cleaved β−O−4 linkages have been completely removed from the residual solid, as have the D, PCA, and PCB substructures. The signals relating to the hydrogens on the 2 and 6 positions of each of the three different monomer units have either decreased in intensity or are completely absent. Given the absence of these peaks and the high DAC of this material (as determined by FTIR, see Table 2), it appears that many of the 2 and 6 G/H/S monomer positions have undergone condensation to give new intermonomer carbon−carbon linkages. The G5/H3,5 peak region appears the least affected; however, it too experiences an apparent reduction in intensity. Given the significant variation in the remainder of the aromatic region, it is possible that these peaks may not be related to true G/H substructures. In an attempt to further characterize the aromatic content of the residual solids, the HMBC spectra of the original lignin and the residual solids of the 200 °C alkaline treatment were compared. The spectrum of the original lignin clearly exhibits many correlation peaks across the aromatic, methoxyl, and aliphatic chain region; however, the majority of these are absent from the residual solids spectrum. The aliphatic chain region is essentially unaffected, with all original peaks remaining in the residual solids. However, the methoxyl to aromatic correlation region (δC/δH 150/3.7) is almost completely absent from the

residual solids, as are the entirety of the intra-aromatic correlation peaks (δC/δH 100.0−200.0/6.00−8.00). This result therefore suggests that the aromatic hydrogen content of the residual solids is not only of limited quantity but is also so sparsely concentrated (with regard to structural position) that no specific HMBC signals were observable. Given the appearance of the aromatic region of the HSQC spectrum for this material (see Figure 4), this is not surprising. Quantitation of Aromatic and Methoxyl Hydrogen in Residual Solids. The HSQC spectra for all the residual solids indicated that the low-field aliphatic region experiences little change. Even after alkali treatment at 300 °C, the fatty acid and the saturated/unsaturated chain substructures are still present in the residual solids (see Figure S6), despite the fact that no significant aromatic or methoxyl peaks remain. In an attempt to quantify depletion of aromatic and methoxyl peaks with treatment conditions, the three major regions of the HSQC spectra were integrated and their ratios compared (see Table 3). As expected, the results indicate that the proportion of Table 3. HSQC Integration Ratios of Key Regions of Bagasse Lignin and Residual Solids Obtained after 3 h Treatment under Various Conditions treatment conditions n/a (original bagasse lignin) H2O2/chalcopyrite, 120 °C sodium percarbonate, 60 °C sodium percarbonate, 120 °C sodium percarbonate, 200 °C sodium percarbonate and 1 M NaOH, 200 °C sodium percarbonate, 250 °C sodium percarbonate, 300 °C

aromatic H:alkyl H

methoxyl H:alkyl H

2.3 1.4 1.6 1.0 0.5 0.4

2.2 1.5 1.3 1.1 0.2 0.1

0.4 0.4

0.02 0.02

aromatic and methoxyl hydrogen decreases significantly with increasing reaction temperature, and more so under alkaline rather than acidic conditions. At 200 °C with alkali treatment, the residual solids appear to contain ∼20% of the aromatic hydrogen present in the starting lignin, and 10% of the starting methoxyl side chain content. Increasing the reaction temperature to 300 °C reduces the aromatic hydrogen and methoxyl content a further 20 and 80%, respectively. These observations are in agreement with those made based on the FTIR analysis, in that as the severity of the treatment conditions increases, the hydrogen and methoxy contents of the aromatic structures decrease. Analysis of Acid-Soluble Lignin in Aqueous Phase. The HSQC spectra of both the acid-soluble fraction of the bagasse lignin as well as the remaining acid-insoluble fraction were independently obtained and compared (see Figure S7). The spectra show that the A and A′ lignin substructures are largely absent from the acid-soluble material, as are the hemicellulose impurities. The aromatic region of these spectra suggests that the acid-soluble lignin fraction contains a lower proportion of the G monomer than the acid-insoluble majority. The spectra also indicate that the α-oxidized S′ and G′ substructures are more prevalent than their α-hydroxy variants. Comparing the spectrum of the aqueous phase products obtained after acid treatment at 120 °C with that of the acidsoluble lignin fraction indicated that the only key peak significantly affected was the H2,6 (see Figure S8). As was observed in the residual solids analysis, this may indicate that 11701


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Figure 5. Chemical structure and key HSQC peaks (δC/δH) for example substructures. (A) S type α-hydroxy (nonterminal), (B) S′ type α-carbonyl (nonterminal), and (C) S′ type α-carbonyl (terminal).

(see Figure 5) of the HSQC spectra were integrated. These peaks are well resolved and are unlikely to contain any overlapping peaks, thus ensuring the validity of the analysis. The HSQC spectrum of each sample also reveals the presence of a terminal methyl group for S′ substructures (δC/δH 2.50/ 26; see Figure 5C). However, there is no evidence of a β ether group (δC/δH 80−84/5.2−5.6;36 see Figure 5B). This indicates that the S substructures that have undergone oxidation to form the oxidized S′ variant have also undergone cleavage of the β-O-4 linkage. This is in agreement with the mechanism previously proposed by Dunn27 and supported by the FTIR analysis (see Scheme 1), wherein the β-O-4 bond is broken in order to oxidize the Cα. As the reaction conditions become more severe there is a clear trend of increasing S′ substructure content (see Table 4).

the H type monomer underwent significant condensation at these positions. It may also reflect a relative reduction in the H monomer ratio, given that this unit is generally considered to be a terminal group and hence more easily cleaved from the polymer matrix. For the 120 °C alkaline treatment, its influence on the type of substructures in the acid-soluble lignin fraction obtained was more pronounced (see Figure S9). The pPB and pCA structures normally present in the acid-soluble fraction were completely removed. There were also additional new peaks in the ether-linkage region in addition to the lignin methoxyl content, which are very likely caused by LMWACs. The alkaline treatment appears to have less of an influence on the hydrogen content of the G/H/S monomer units, indicating that the acid treatment showed a greater tendency toward aromatic condensation at similar reaction temperatures. Increasing the reaction temperature to 200 °C resulted in the complete loss of the A and A′ substructures, as well as a reduction in the H monomer and methoxy content (see Figure S10). The S and G monomer units that remained have undergone significant oxidation and are present predominantly in the α-oxidized carbonyl form (S′ and G′). Analysis of α-Oxidized Content of Lignin Fractions. A recurring result of this study is the apparent prevalence of the α-oxidized forms of the G′ and S′ monomers in both the residual solids and acid-soluble lignins. This is an important observation given that previous studies have proven the significance of the Cα oxidation (from an alcohol to a ketone), for lignin extraction and depolymerization. Gierer and Norén first demonstrated in 1982 that oxidative pretreatment of wood shavings facilitates much higher degrees of delignification during pulping.34 They suggested that the enhanced lignin breakdown observed was due to the oxidation of the Cα, and the resultant ease of breaking the neighboring β-O-4 linkage. This process has since been used in the paper pulp industry, but only recently has it been applied to the study of lignin depolymerization to LMWACs. Rahimi et al.35 studied the breakdown of several β-O-4 containing lignin model compounds, as well as native aspen lignin. Their results concluded that model compounds containing an α-carbonyl group were cleaved far more easily via oxidative thermochemical treatment than those containing an α-hydroxy group. Applying a preoxidative treatment to lignin, intended to oxidize the Cα, was shown to increase the yield of LMWACs obtained from the subsequent depolymerization process from 7.2 to 52 wt %. To quantitatively analyze the amount of α-oxidized substructures in both the residual solids and acid-soluble lignin fractions, the S2,6 (α-hydroxy, δC/δH 102.5−108.7/6.10−6.95) and S2,6′ (α-carbonyl, δC/δH 104.5−108.7/7.15−7.35) peaks

Table 4. HSQC Integration Ratios of S2,6 and S2,6′ Peaks sample

S′2,6: S2,6 (α-carbonyl: α-hydroxy)

bagasse lignin (whole) bagasse lignin (acid-soluble) treatment

0.06 0.53 residual solids

H2O2/chalcopyrite, 120 °C sodium percarbonate, 60 °C sodium percarbonate, 120 °C sodium percarbonate, 200 °C sodium percarbonate and 1 M NaOH, 200 °C sodium percarbonate, 250 °C sodium percarbonate, 300 °C

0.06 0.09 0.10 0.28 0.20 0.33 0.45

aqueous phase 0.2 0.2 1.3 2.6

This trend correlates with both the decreasing yield of residual solids obtained (see Table 1) and the reduced molecular weight distributions. The only exemption to this is the sodium percarbonate treatment at 300 °C, which contains the highest ratio of S′:S substructures but also produces the highest proportion of (high molecular weight) residual solids (see Figure 3). What this implies is that the S′ content is an indicator of the degree of depolymerization, but not necessarily an indication of the yield of the residual lignin solids. The latter observation confirms the work of Zhang et al.,11 who proposed that the depolymerization of lignin via the Fenton process proceeds via the cleavage of β-ether bonds and that recondensation of the breakdown products increases the amount of residual lignin. Figure 6 is the plot of DCA yield against S′:S. It shows that the DCA yield increases with increasing S′:S to 0.28, corresponding to the sodium percarbonate treatment at 200 °C, and then significantly drops with increasing S′:S. The drop 11702


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Figure 6. Plot of DCA yield (wt %) against S2,6′:S2,6 (HSQC volume integration) for various residual solids.

methoxyl content by half. It appears therefore that while the addition of NaOH significantly reduces DCA degradation, it is also effective in enhancing lignin demethoxylation. Also, as the S′:S substructure is doubled (from 1.3 to 2.6) with the addition of NaOH, it is implicit that working under alkaline conditions, as expected, improves lignin depolymerization to LMWACs, an intermediary for DCA formation.

in DCA yield with increasing S′:S is due in part to repolymerization reactions, as lignin has been reported to undergo significant repolymerization at temperatures in excess of 250 °C.31 Therefore, in order to maximize the yield of LMWACs and subsequently DCAs, the reaction conditions should be controlled so as to cause complete oxidation of the αhydroxy group (and therefore cleavage of the β-O-4 linkages), but also minimize secondary repolymerization reactions. This can be achieved by the inclusion of additive reagents or solvents which act as capping agents, scavenging the phenoxyl radicals and preventing polymerization. Previous lignin studies have employed reagents such as boric acid and phenol and solvents such as ethanol for this task.37−39 Table 4 also shows that the higher the S′:S of the acid soluble lignins in the aqueous phase, the higher the DCA yield, and so S unit oxidation positively influences DCA yield. There are however other factors which appear to have greater effect on DCA yield, such as the susceptibility of these products toward thermal degradation and the repolymerization of breakdown products. Influence of NaOH Addition. The influence of the inclusion of 1 M NaOH to the 200 °C sodium percarbonate treatment was analyzed by GPC, FTIR, and NMR. Despite the fact that the DCA yield was observed to increase by ∼85%, the residual solids and acid-soluble lignin yields were not significantly affected (see Table 1). The molecular weight distribution obtained for the solid residue derived with NaOH did not vary from the solid residue obtained without NaOH addition (see Figure S11). Also, as shown in Table 2, the AAFGR and DAC values for the NaOH treatment are similar to that without its addition. The only variation observed in the FTIR spectra is that the NaOH derived residue had a lower intensity peak at 1120 cm−1 but higher intensity at 2850/2920 cm−1, when compared to the FTIR of the solid without its addition (see Figure S12). This spectral variation can be explained by the S′ to S substructure ratio of the residual solids, which was determined by HSQC NMR to decrease from 0.28 to 0.20 with the addition of NaOH (see Table 4). Further integration of the spectra indicates that the aromatic to alkyl hydrogen ratio is 0.4 and 0.5, for treatment with and without NaOH, respectively (see Table 3). Also the methoxyl to alkyl hydrogen ratios are 0.1 and 0.2, with and without NaOH, indicating that the addition of NaOH reduces the relative

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CONCLUSIONS Through the combination of FTIR, NMR, and GPC analyses, relationships between the DCA yield and the residual lignin structural features were derived. An inverse linear relationship was obtained by relating the DCA yield to the aromatic to aliphatic functional groups ratio, with exceptions at relatively high temperatures where the degree of repolymerization plays a predominant role. Evidence is further provided by the existence of a linear trend between the DCA yield and the degree of aromatic condensation. The GPC data further provide evidence of repolymerization reactions with increasing reaction temperatures. The lignin substructure proportion of the S′ substructure in either the solid residue or the acid soluble lignin was found to be a useful marker in predicting lignin depolymerization. On the basis of the present study, conditions that minimize repolymerization reactions, working under alkaline conditions to reduce DCA degradation and increase LMWACs formation, optimizing the concentration of H2O2 and reaction time, and working at mild temperatures will increase DCA yield.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b03164. Flowchart of reagents, treatment and workup process, and residue analysis procedures for each of the three reaction conditions used in this work (Figure S1); lignin acetylation and molecular weight determination, FTIR absorption frequencies of lignin (Table S1); gelpermeation chromatogram of acetylated bagasse lignin and the aqueous phase products obtained after treatment with sodium percarbonate at 60, 120, 150, 200, and 250 °C (Figure S2); key NMR peaks and their structural 11703


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sources, NMR peak assignments of lignin (Table S2); main substructures and aromatic units identified in lignins (Figure S3); 2D HSQC NMR spectra of bagasse lignin compared to that of various residual solids and aqueous phase products (Figures S4−S10); absorbance ATR-FTIR spectrum of bagasse lignin subtracted from the residual solids obtained after treatment with sodium percarbonate at 200 °C in both the absence and presence of 1 M NaOH (Figure S11); gel-permeation chromatogram of (A) acetylated bagasse lignin and the acetylated residual solids obtained after treatment with sodium percarbonate at 200 °C, (B) without and (C) with the inclusion of 1 M NaOH (Figure S12) (PDF)

AUTHOR INFORMATION

Corresponding Author

*Phone: +61448879956. E-mail: [email protected]. ORCID

Dylan J. Cronin: 0000-0001-6832-7702 William O. S. Doherty: 0000-0002-5975-8401 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support provided by the Australia India Strategic Research Fund (AISRF) for this work and the support of the Queensland University of Technology (Australia) and Washington State University (US). The authors also gratefully acknowledge the financial support from the National Science Foundation (Award no: 1454575). D.J.C. would also like to acknowledge the Australian-American Fulbright Commission for their financial support in conducting the research work completed at the Bioproducts, Science & Engineering Laboratory (BSEL), Washington State University (U.S.). The authors would also like to thank Dr. Ruoshui Ma for his kind assistance in facilitating the experimental work conducted at BSEL.

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Relating Dicarboxylic Acid Yield to Residual Lignin Structural Features

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