Relating Dicarboxylic Acid Yield to Residual Lignin Structural Features

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Relating Dicarboxylic Acid Yield to Residual Lignin Structural Features Dylan John Cronin, Kameron Dunn, Xiao Zhang, and William O. S. Doherty ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03164 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 31, 2017

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ACS Sustainable Chemistry & Engineering

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

Centre for Tropical Crops and Biocommodities, Queensland University of Technology, 2

George St, Brisbane, QLD ‡

Bioproducts, Science & Engineering Laboratory, Washington State University, 2710 Crimson

Way, Richland, WA

KEYWORDS: lignin depolymerization • aromatics • dicarboxylic acids • sodium percarbonate • lignin repolymerization • Fenton reaction

ACS Paragon Plus Environment

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ABSTRACT: This study focused on understanding the relationship between dicarboxylic acids (DCAs) 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 characterised 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-oxidised substructure S′, was found to be an indicator of the extent of lignin depolymerisation, and its ratio to the S substructure was found to correlate with DCA yield at reaction temperatures up to 200 °C.

INTRODUCTION The depletion and negative environmental influences of fossil-fuels have created the need to utilise 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 underutilised 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 towards its depolymerization to low molecular weight aromatic compounds (LMWACs)

2-5

, chemicals and


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fuel additives 6-7. More recently however, there has been significant interest in the production of DCAs from lignin in 2004

12

8-11

. Four carbon (C4) DCAs were reported by the US Department of energy

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 fibres (e.g., lycra) and green solvents.

Work has also been conducted

utilising 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 thermochemical depolymerization approach

9, 17

15-16

, or

. 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 authors

19

demonstrated that DCA yield could be increased appreciably by

performing the depolymerization under alkaline conditions, using sodium percarbonate (Na2CO3·1.5 H2O2) 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 towards 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%).


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The current work is an extension of the authors’ previous study 19 on the formation of DCAs from bagasse lignin, by studying the structural properties of the residual lignins and 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 acidsoluble lignin, in order to open up reaction pathways for increased DCA yield from lignin.

EXPERIMENTAL SECTION 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 (US); 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 depolymerisation 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


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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 3 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.

Table 1 - Depolymerization data of bagasse lignin under various reaction temperatures with 3 h reaction time Acidsoluble lignin

Reaction temp.

Residual solids

(°C)

(wt%)

H2O2/chalcopyrite

60

72 ± 0.4

2.3 ± 0.1

2.3 ± 0.6

23 ± 1

H2O2/chalcopyrite

120

69 ± 3.0

2.8 ± 0.1

1.5 ± 0.4

27 ± 4

H2O2/chalcopyrite

150

55 ± 1.7

1.3 ± 0.3

0.1 ± 0.1

43 ± 2

Sodium percarbonate

60

79 ± 1.6

3.1 ± 0.1

3.8 ± 0.7

14 ± 2

Sodium percarbonate

120

59 ± 2.9

5.0 ± 0.5

4.4 ± 0.8

31 ± 4

Sodium percarbonate

150

53 ± 0.3

3.4 ± 0.4

5.5 ± 0.7

38 ± 1

Sodium percarbonate

200

24 ± 1.1

10.9 ± 0.3

7.4 ± 1.1

61 ± 3

Sodium percarbonate & 1 M NaOH

200

28 ± 1.3

13.8 ± 1

13.7 ± 0.8

45 ± 3

Sodium percarbonate

250

12 ± 2.0

5.6 ± 0.3

4.8 ± 0.7

76 ± 3

Sodium percarbonate

300

42 ± 3.0

9.0 ± 0.6

1.6 ± 0.3

47 ± 4

Catalyst

DCA yield (wt%)

(wt%)

Other products (wt%)

Data reproduced from Cronin et al. 19


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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 normalised 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 a UV (250 nm) and 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.

2D heteronuclear single quantum

coherence (HSQC) and heteronuclear 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


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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).

RESULTS AND DISCUSSION Fourier transform infrared (FTIR) spectroscopy. Differential spectra analysis. The ATRFTIR 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 is 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 α-hydroxyl. 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


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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 substituents, making the formation of the quinone intermediary more difficult.


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Figure 1 - Absorbance ATR-FTIR spectrum of bagasse lignin subtracted from the residual solids obtained after treatment with; A) H2O2/chalcopyrite at 60, 120 & 150 °C, B) sodium percarbonate at 60, 120 & 150 °C, and C) sodium percarbonate at 200, 250 & 300 °C 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-oxidised ketone structures. This effect has been previously suggested by Dunn

27

as a result of hydroxyl (OH-) attack, and would explain the relative increase in the


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vinylic absorption at 985 cm-1, as well as the reduction in the alkyl aryl ether absorption at 1220 cm-1 (see Scheme 1).

Scheme 1 – Depolymerisation pathway of a monolignol to a terminal phenyl ethanone structure (adapted from 27) 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 oxidised α-carbonyl. The latter may be as 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 - 1500 cm-1 (aromatic C=C stretching frequency) and 3000 - 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


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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.

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 Reaction temp. (°C)

Aromatic to aliphatic functional group ratio (AAFGR)

Degree of aromatic condensation (DAC)

N/A

1.8

2.3

H2O2/chalcopyrite

60

1.6

2.7

H2O2/chalcopyrite

120

1.9

2.7

H2O2/chalcopyrite

150

1.9

2.8

Sodium percarbonate

60

1.5

2.8

Sodium percarbonate

120

1.1

2.9

Sodium percarbonate

150

1.1

3.5

Sodium percarbonate

200

0.4

3.6

Sodium percarbonate & 1 M NaOH

200

0.3

3.8

Sodium percarbonate

250

1.1

6.0

Sodium percarbonate

300

0.7

6.2

Catalytic environment N/A (Original lignin)

The data indicates 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


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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 analysed 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 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 affect the lignin depolymerisation 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 whilst 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


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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.

Figure 2 – Plot of; A) DCA yield (wt%) against AFFGR, and B) DCA yield (wt%) against DAC, for the 3 different catalytic environments (H2O2/chalcopyrite, sodium percarbonate, and sodium percarbonate with 1 M NaOH) 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


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weight fraction of the material was not significantly affected, but that 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 towards 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. 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 which result in the formation of recalcitrant solid 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.


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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

Acid-soluble residue. The molecular weight distributions of the aqueous phases obtained via alkaline treatment at 60 - 250 °C were analysed 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 >10000 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,


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with a peak molecular weight of ~300 g.mol-1. This appears to be indicative of the larger acidsoluble 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 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 inter-monomer 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).


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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 S′2,6 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 inter-monomer carbon-carbon linkages. The G5/H3,5 peak region appears the least effected, 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.


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Figure 4 - 2D HSQC NMR spectra of NaOH bagasse lignin (blue), & 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), & (B) aromatic region (δC/δH 100.0 - 150.0/6.10 – 7.90)

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


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(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 remains. In an attempt to quantify depletion of aromatic and methoxyl peaks with treatment condition, 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 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 increase, the hydrogen and methoxy contents of the aromatic structures decrease.


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Table 3 – HSQC integration ratios of key regions of bagasse lignin and residual solids obtained after 3 h treatment under various conditions Treatment conditions

Aromatic H:alkyl H

Methoxyl H:alkyl H

n/a (original bagasse lignin)

2.3

2.2

H2O2/chalcopyrite, 120 °C

1.4

1.5

Sodium percarbonate, 60 °C

1.6

1.3


1.0

1.1


0.5

0.2

Sodium percarbonate & 1 M NaOH, 200 °C

0.4

0.1


0.4

0.02


0.4

0.02

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-oxidised 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 acid-soluble 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 the H type monomer underwent significant condensation at these positions. It may also


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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 towards 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-oxidised carbonyl form (S′ and G′).

Analysis of α-oxidised content of lignin fractions. A recurring result of this study is the apparent prevalence of the α-oxidised 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 pre-treatment 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 neighbouring 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


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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 pre-oxidative treatment to lignin, intended to oxidise the Cα, was shown to increase the yield of LMWACs obtained from the subsequent depolymerization process from 7.2 to 52 wt%. To quantitatively analyse the amount of α-oxidised 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 S′2,6 (αcarbonyl, δC/δH 104.5 – 108.7/7.15 – 7.35) peaks (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 oxidised S′ variant have also undergone cleavage of the β-O-4 linkage. This is in agreement with the mechanism previously proposed by Dunn 27 and supported by the FTIR analysis (see Scheme 1), wherein the β-O-4 bond is broken in order to oxidise the Cα.

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


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As the reaction conditions become more severe there is a clear trend of increasing S′ substructure content (see Table 4). 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 produced 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 Zeng 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.

Table 4 - HSQC integration ratios of S2,6 and S′2,6 peaks Sample

S′2,6 : S2,6 (α-carbonyl : α-hydroxy)

Bagasse lignin (whole)

0.06

Bagasse lignin (acid-soluble)

0.53

Treatment

Residual solids

Aqueous phase

H2O2/chalcopyrite, 120 °C

0.06

0.2


0.09

-


0.10

0.2


0.28

1.3

Sodium percarbonate & 1 M NaOH, 200 °C

0.20

2.6


0.33

-


0.45

-


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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 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 α-hydroxy group (and therefore cleavage of the β-O-4 linkages), but also minimise 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 polymerisation. 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 towards thermal degradation, and the repolymerization of breakdown products.


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Figure 6 - Plot of DCA yield (wt%) against S′2,6:S2,6 (HSQC volume integration) for various residual solids

Influence of NaOH addition. The influence of the inclusion of 1 M NaOH to the 200 °C sodium percarbonate treatment was analysed 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


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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 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 condition, as expected, improves lignin depolymerisation to LMWACs, an intermediary for DCA formation.

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 provides 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 depolymerisation. 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.


26

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ASSOCIATED CONTENT Supporting Information 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); Gel-permeation 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 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 (FigureS4 – 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 1M NaOH (Figure S12). AUTHOR INFORMATION Corresponding Author email: [email protected], ph: +61448879956 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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ACKNOWLEDGMENT 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). Dylan Cronin 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 (US). 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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23. Guo, Y.; Bustin, R. M. FTIR spectroscopy and reflectance of modern charcoals and fungal decayed woods: implications for studies of inertinite in coals. International Journal of Coal Geology 1998, 37, 29-53. 24. Jahan, M. S.; Chowdhury, D. N.; Islam, M. K.; Moeiz, S. I. Characterization of lignin isolated from some nonwood available in Bangladesh. Bioresource technology 2007, 98, 465469. 25. Malutan, T.; Nicu, R.; Popa, V. I. Lignin modification by epoxidation. BioResources 2008, 3, 1371-13767. 26. Mao, J.; Zhang, L.; Xu, F. Fractional and structural characterization of alkaline lignins from Carex meyeriana Kunth. Cellulose Chemistry and Technology 2012, 46, 193. 27. Dunn, K. Conversion of sugar cane lignin into aromatic products and fractionation of products for industrial use. Doctor of Philosophy, Queensland University of Technology 2014. 28. Guo, Y.; Renton, J. J.; Penn, J. H. FTIR microspectroscopy of particular liptinite(lopinite-) rich, Late Permian coals from Southern China. International Journal of Coal Geology 1996, 29, 187-197. 29. Ganz, H.; Kalkreuth, W. Application of infrared spectroscopy to the classification of kerogentypes and the evaluation of source rock and oil shale potentials. Fuel 1987, 66, 708-711. 30. Yin, G.; Jin, F.; Yao, G.; Jing, Z. Hydrothermal Conversion of Catechol into FourCarbon Dicarboxylic Acids. Industrial & Engineering Chemistry Research 2015, 54, 68-75. 31. Demirbaş, A. Mechanisms of liquefaction and pyrolysis reactions of biomass. Energy Conversion and Management 2000, 41, 633-646. 32. Li, J.; Henriksson, G.; Gellerstedt, G.; Träkemi och, m.; Kth; Fiber- och, p.; Skolan för, k. Lignin depolymerization/repolymerization and its critical role for delignification of aspen wood by steam explosion. Bioresource Technology 2007, 98, 3061-3068. 33. Kringstad, K.; Månsson, P.; Mörck, R. In Changes in the molecular weight distribution of kraft lignins resulting from various chemical treatments. The Ekman Days 1981, Proc Int Symp Wood Pulp Chem, SPCI, Stockholm, 1981; pp 91-93. 34. Gierer, J.; Norén, I. Oxidative pretreatment of pine wood to facilitate delignification during kraft pulping. Holzforschung-International Journal of the Biology, Chemistry, Physics and Technology of Wood 1982, 36, 123-130. 35. Rahimi, A.; Ulbrich, A.; Coon, J. J.; Stahl, S. S. Formic-acid-induced depolymerization of oxidized lignin to aromatics. Nature 2014, 515, 249. 36. Ralph, S. A.; Ralph, J.; Landucci, L.; Landucci, L. NMR database of lignin and cell wall model compounds. US Forest Prod. Lab., Madison, WI (http://ars.usda.gov/Services/docs.html) 2004. 37. Roberts, V. M. Homogeneous and Heterogeneous Catalyzed Hydrolysis of Lignin. 2008. 38. Toledano, A.; Serrano, L.; Labidi, J. Improving base catalyzed lignin depolymerization by avoiding lignin repolymerization. Fuel 2014, 116, 617-624. 39. Huang, X.; Korányi, T. I.; Boot, M. D.; Hensen, E. J. Ethanol as capping agent and formaldehyde scavenger for efficient depolymerization of lignin to aromatics. Green Chemistry 2015, 17, 4941-4950.


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SYNOPSIS The structural features of residual lignin are used to produce information for predicting and improving dicarboxylic acid yield.


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Not applicable (ToC graphic) 85x47mm (300 x 300 DPI)


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Absorbance ATR-FTIR spectrum of bagasse lignin subtracted from the residual solids obtained after treatment with; A) H2O2/chalcopyrite at 60, 120 & 150 °C, B) sodium percarbonate at 60, 120 & 150 °C, and C) sodium percarbonate at 200, 250 & 300 °C 100x117mm (300 x 300 DPI)



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Plot of; A) DCA yield (wt%) against AFFGR, and B) DCA yield (wt%) against DAC, for the 3 different catalytic environments (H2O2/chalcopyrite, sodium percarbonate, and sodium percarbonate with 1 M NaOH) 318x381mm (96 x 96 DPI)


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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 99x65mm (300 x 300 DPI)



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Plot of DCA yield (wt%) against S′2,6:S2,6 (HSQC volume integration) for various residual solids 315x190mm (96 x 96 DPI)


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2D HSQC NMR spectra of NaOH bagasse lignin (blue), & 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), & (B) aromatic region (δC/δH 100.0 - 150.0/6.10 – 7.90) 108x98mm (300 x 300 DPI)



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Chemical structure and key HSQC peaks (δC/δH) for example substructures; A) S type α-hydroxy (nonterminal), B) S′ type α-carbonyl (non-terminal), and C) S′ type α-carbonyl (terminal) 75x27mm (300 x 300 DPI)


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Depolymerisation pathway of a monolignol to a terminal phenyl ethanone structure 101x29mm (300 x 300 DPI)


Relating Dicarboxylic Acid Yield to Residual Lignin Structural Features

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