Phenol-Enhanced Depolymerization and Activation of Kraft Lignin in

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Applied Chemistry

Phenol-enhanced depolymerization and activation of kraft lignin in alkaline medium Linhuo Gan, and Xuejun Pan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b01147 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 22, 2019

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Industrial & Engineering Chemistry Research

Phenol-enhanced depolymerization and activation of kraft lignin in alkaline medium

Linhuo Gan1,2 and Xuejun Pan2,* 1College

of Chemical Engineering, Huaqiao University, Xiamen, Fujian, 361021, China

2 Department

of Biological Systems Engineering, University of Wisconsin-Madison, 460

Henry Mall, Madison, WI 53726, USA

*Corresponding author: Email: [email protected], Phone: 608-262-4951, Fax: 608-262-1228

GRAPHICAL ABSTRACT

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Phenolation is an effective method to improve the reactivity of lignin in the synthesis of ligninphenol-formaldehyde (LFP) resin. Previous lignin phenolation was almost exclusively catalyzed by acid. This study investigated the phenolation of kraft lignin (KL) in alkaline medium under different conditions of mass ratio of phenol to lignin, reaction temperature, and reaction time. Results indicated that under the condition of mass ratio of phenol to KL 3:1, 160 °C and 2 h, KL was depolymerized to phenolated lignin segments (51.2%), monomeric and dimeric phenolics (12.5%), and organic acids (36.3%). The lignin segments had low molecular weight, narrow dispersity, and high phenolic hydroxyl group (4.5 mmol/g), which could be used as precursors of LFP resin/adhesive. The phenolics and organic acids would have a potential as platform chemicals.

INTRODUCTION With the depletion of fossil fuel resources and deterioration of ecological environment, more and more attention has been paid to the exploration of renewable and environmentally-friendly natural resources such as lignocellulose.1 Lignin is one of the three major components in plant biomass and the most abundant natural aromatic polymer and accounts for approximately 1535wt% of plant cell walls. However, due to its structural complexity and poor chemical reactivity, the depolymerization and high-value utilization of lignin remains a challenge. Over the last few decades, many processes, such as pyrolysis, liquefaction, gasification, hydrogenolysis, and oxidative cracking, have been explored to depolymerize lignin to lowmolecular-weight fragments, aromatic precursors, bio-oils, and platform chemicals.2-4 Among


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them, phenol-enhanced depolymerization and activation is an approach to convert lignin into low-molecular-weight

and

high-reactivity

polyphenols

to

synthesize

lignin-phenol-

formaldehyde (LPF) resin adhesive.5,6 For example, Funaoka and Abe7 reported the phenolation of lignin with excessive phenol and boron trifluoride as catalyst via phenyl nucleus-exchange method (NE-method), which led to the depolymerization of lignin through selectively cleaving C-C linkages between the side chains and the phenyl nuclei. The phenolation not only engrafted phenol onto lignin backbone structure but also reduced the molecular weight, thus favoring the incorporation of phenolated lignin to the LPF resin. It is worth noting that the previous phenolation studies exclusively used acid (such as H2SO4) as catalyst.5-10 In the acid-catalyzed phenolation, phenol was crosslinked to the aromatic ring and the sidechain of lignin, which introduced more reactive sites into lignin for crosslinking by formaldehyde during LPF synthesis but also resulted in lignin repolymerization (increased molecular weight); and meanwhile the β-O-4 ether bond of lignin was partially cleaved during the phenolation, which led to the depolymerization of lignin.10 However, the acid-catalyzed lignin phenolation could not be directly incorporated into the synthesis of LPF resin adhesive as an one-pot process because the LPE synthesis is usually an alkali-catalyzed process.11 More recently, to avoid excessive repolymerization of lignin during the phenolation process, Nanayakkara et al12 depolymerized lignin with 4-tert-butyl-2,6-dimethyl-phenol (TBDMP) in the presence of Cu as a catalyst under oxygen at 180 °C. TBDMP was selected because its unique structure (the methyl and tertbutyl groups on ortho and para positions, respectively), which could restrain repolymerization of lignin under the reaction conditions. However, TBDMP treatment was able to depolymerize lignin but unable to improve the reactivity of lignin, which seemed not very



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useful to activate lignin for preparing lignin-based adhesives. Kraft lignin (KL) separated from spent liquor of kraft pulping in paper industry is the most available and abundant industrial lignin. KL has been used to substitute for phenol to synthesize LPF resin adhesive for wood products.13-15 The KL-based LPF resin was also used as a carbon precursor to prepare mesoporous carbon via soft-template method.16,17 However, the reactivity of raw KL toward formaldehyde is not satisfactory for high-quality LPF, partially because the lignin condensed during kraft pulping.18 Phenolation has been explored to activate KL, and previous studies almost exclusively used acids as catalyst.19-21 It was reported that alkali could catalyze lignin phenolation as well, but these studies focused on biorefinery lignins from steam explosion and autohydrolysis processes, and the phenolation conditions were mild (99%) was also purchased from Sigma-Aldrich Corporation (USA). Sodium hydroxide (NaOH) solution (50%, w/w) was purchased from VWR International (USA). Ethyl ether (anhydrous, purity≥99%) was purchased from Thermo Fisher Scientific (USA). All chemicals in this study were used as obtained without further purification. Phenolation of Kraft Lignin: In a typical phenolation reaction, 3.0 g of KL, 9.0 g of phenol and 0.287 mL 50% of NaOH aqueous solution (5.0 wt% on the total mass of KL and phenol to maintain the basicity required for the reaction17,22) were added into a glass reactor and heated at 160 °C in an oil bath with magnetic stirring for 1 h. The resulting mixture was poured into 30 mL water; the solution pH value was adjusted to 1 using concentrated hydrochloric acid (HCl); and then 30 mL ethyl ether was added into the above mixture to extract the free phenol unreacted. The precipitated phenolated KL was separated by centrifugation, washed twice with ethyl ether to remove residual phenol, and dried in a vacuum oven at 40 °C. In the meantime, all of supernatants were combined, and the mixture was divided into an aqueous phase and an organic phase that contains unreacted phenol and depolymerization products from KL. The overall process of the kraft lignin phenolation and product fractionation is summarized in Figure



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1. In this study, the phenolated KL products are labeled as PKL-X-Y-Z, where X (X=1, 2, 3) is the mass ratio of phenol to KL; Y (Y=120 °C, 140 °C, 160 °C) is the reaction temperature; and Z (Z=1 h, 2 h, 3 h) is the reaction time. For example, PKL-3-160-1 represents the phenolated KL sample prepared under the conditions of ratio of phenol to KL 3, temperature 160 ºC, and 1 h.

Kraft Lignin

Wash with dilute HCl

Purified Kraft Lignin for Characterization Phenolation with phenol in alkaline medium

Acidification and ethyl ether extraction Solid

Liquid

Phenolated Kraft Lignin (PKL) Organic phase

Aqueous phase

Lignophenols Organic Acids Figure 1. Flow diagram of phenolation treatment of kraft lignin.

Acetylation of Kraft Lignin and Phenolated Kraft Lignin: 0.1 g of lignin sample was dissolved in 2 mL of pyridine-acetic anhydride (1:1, v/v) in a glass vial and kept in a dark cabinet at room temperature for 72 h. The above solution was added dropwise to 120 mL of ice-cold water containing 1 mL concentrated HCl with magnetic stirring. The precipitated 6 ACS Paragon Plus Environment

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acetylated lignin was collected by filtration, thoroughly washed with water, and then dried under vacuum. Characterizations: The Mw and Mn of lignin samples before and after phenolation treatment were estimated using GPC on an ICS-3000 chromatographic system (Dionex, Sunnyvale, CA) with three 300 mm × 7.8 mm (i.d.) Phenogel 5U columns (10 000, 500, and 50 Å) and a 50 mm × 7.8 mm (i.d.) Phenogel 5U guard column (Phenomenex, Torrance, CA). Acetylated lignin sample (5 mg) was dissolved in 1 mL HPLC-grade tetrahydrofuran (THF) without stabilizer, and 50 μL of the solution was injected into the GPC columns. THF was used as eluent at a flow rate of 1 mL/min. The column temperature was 30 °C. Polystyrene standards were used for calibration. Lignin samples and polystyrene standards were detected using a variable wavelength detector at 280 and 254 nm, respectively. GPC profiles of all lignin samples are given in Figure S1 in Supporting Information. NMR spectra were recorded on a Bruker AV 500 MHz spectrometer (Billerica, MA). In brief, lignin sample (50 mg) was dissolved in 0.5 mL of dimethyl sulfoxide-d6 (DMSO-d6). 2D HSQC spectrum was recorded at 25 °C using the hsqcetgpsisp 2.2 pulse program. The contents of ArOH and AlkOH groups in the lignin samples were estimated using 1H NMR according to the method previously reported.23,24 In brief, 50 mg of acetylated lignin sample and 5 mg of pnitrobenzaldehyde (internal standard) were dissolved in 0.5 mL of CDCl3-d. The contents of ArOH and AlkOH groups were calculated from the integration ratio of the protons of functional group to those of the internal standard. Quantification of the major organic acids in aqueous phase generated in the phenolation of kraft lignin was conducted using a HPLC (ICS 3000, Dionex, Sunnyvale, CA) system equipped



with ultraviolet detector and SupelcogelTM C-610H column at 210 nm. Calibration curves were created using pure organic acids including oxalic acid, acetic acid, and formic acid. Phosphoric acid (1 kg/m3) was used as eluent at a flow rate of 0.7 cm3/min at 20 °C.25 GC-MS qualitative analyses of the products in organic phases were performed on a Shimadzu GCMS-QP 2010S with SHRXI-5MS column (30 m × 0.25 mm (i.d.), 0.25 μm film). Firstly, the organic solvent extractable products were silylated by N,O-bis(trimethylsilyl) trifluoroacetamide (BSTFA) with trimethyl-chlorosilane (TMCS) (99:1) prior to GC-MS analysis. Then, the silylated sample was analyzed with GC-MS as described by Lyu et al.25 The NIST 08 library was used for compound identification.

RESUTLS AND DISCUSSION Effect of phenolation conditions on the yield, molecular weight and functional group of phenolated kraft lignin (KL) As listed in Table 1, increasing the mass ratio of phenol to KL (Entry 1, Entry 2, and Entry 7), the yield of phenolated KL (precipitable by acid) decreased significantly from 50.3 wt% to 31.0 wt% and then increased slightly to 33.7 wt%. Compared to original lignin (KL), the phenolation at a low mass ratio of phenol to lignin (w/w, 1:1) reduced Mw and DPI dramatically from 14400 g/mol to 6000 g/mol and from 7.74 to 2.90, respectively, indicating that kraft lignin was depolymerized. Meanwhile, part of phenol was grafted onto lignin structure, which was one of the reasons why the content of ArOH group increased significantly from 2.93 mmol/g to 4.22 mmol/g. However, the majority of the phenolated lignin (PKL-1-160-3) was still precipitable by acid, and the yield of the acid-precipitable fraction was 50.3/55.3 = 91.0 wt% (based on raw


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KL). With the increased dosage of phenol in Entry 2, more lignin was depolymerized and became soluble segments, which was supported by the decreased yield (31.0 wt%) of the acidprecipitable fraction (PKL-2-160-3). The precipitated lignin had higher Mw (8600 g/mol) and DPI (3.24) and lower content of ArOH group (3.73 mmol/g), which suggested that the acidprecipitable lignin represented the hard-to-depolymerize fraction of the lignin or the newly condensed one. However, further increasing phenol dosage (Entry 7) did not lead to more lignin depolymerization but slightly increased Mw and DPI (PKL-3-160-3 vs. PKL-2-160-3). In other words, higher dosage of phenol was not necessarily more beneficial to the depolymerization of lignin into small molecular segments in alkaline medium, and excessive phenol might stimulate the repolymerization of lignin, as evidenced by the result that PKL-3-160-3 had higher Mw and DPI (10500 g/mol and 3.66, respectively) than PKL-2-160-3. When the dosage of phenol was sufficient (for example at a mass ratio of phenol to kraft lignin 3:1), the yield of acid-precipitable fraction decreased with reaction temperature, 45.2 wt% (Entry 3), 37.4 wt% (Entry 4), and 28.3 wt% (Entry 6) at 120, 140, and 160 °C, respectively, indicating that high temperature enhanced the depolymerization of lignin into low-molecularweight segments. Meanwhile, the MW and DPI of the acid-precipitable fraction of lignin decreased with increased temperature, and the content of ArOH group increased significantly (3.03 mmol/g of Entry 3, 3.98 mmol/g wt% of Entry 4, and 4.52 mmol/g of Entry 6). Both observations above implied that the lignin depolymerization was enhanced by elevating temperature. The quantity of ArOH groups in the phenolated KL (acid-precipitable fraction) (Table 1) was comparable to or slightly higher than those reported previously.11,22 The effect of reaction time on the phenol-enhanced depolymerization of KL using NaOH



as catalyst was also investigated. As seen in Table 1 (Entry 5, Entry 6, and Entry 7), the effect of reaction time was similar to that of phenol dosage. Sufficient reaction time was necessary to get desired depolymerization, but prolonged reaction would cause repolymerization of lignin. Based on the results above, it seems that the optimal conditions for phenol-enhanced depolymerization of the KL in alkaline medium are of mass ratio of phenol to KL 3:1, 160 °C, and 2 h. Under the conditions, the resultant acid-precipitable fraction (PKL-3-160-2) had relatively low Mw (9300 g/mol) and DPI (3.24) and high content of ArOH group (4.52 mmol/g), and more low-molecular-weight segments (unprecipitable fraction by acid) were produced (55.3 wt% − 28.3 wt% = 27.0 wt%) from the depolymerization of KL. All these are desirable for downstream LPF preparation. However, considering both the properties of phenolated lignin (molecular weight and ArOH of acid-precipitable fraction) and preparation conditions, PKL-1160-3 seemed to be more suitable and attractive for LPF resin adhesive preparation because of low phenol dosage, high yield of acid-precipitable fraction, low Mw and DPI, as well as relative high content of ArOH group as reactive sites of lignin. On the other hand, to prepare mesoporous carbon material reported in our previous work17, the LPF carbon precursor and the soft template of amphiphilic surfactant of Pluronic F127 should be miscible in acidic medium. Thus, PKL-3-160-2 might be more favorable to the synthesis of mesoporous carbon using softtemplate method owing to its low yield of acid-precipitable fraction, high content of ArOH group for crosslinking with formaldehyde and high content of AlkOH group for forming hydrogen-bonding interactions between LPF and F127.16 Sun et al22 reported the similar effects of phenol dosage and reaction temperature on the phenolation process of lignin in alkaline medium. Under the milder conditions used (mass ratio


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of phenol to lignin 1:1 at 95 ºC for 1 h), no significant cleavage of β-O-4 linkages were observed. However, when reaction temperature was raised to 160 °C in this study, the Mw of lignin decreased by 58% from 14400 g/mmol of KL to 6000 g/mmol of PKL-1-160-3, which might be attributed to the cleavage of linkage or other bonds in lignin, as further discussed below.

Table 1. Characterizations of lignin samples before and acid-insoluble lignin residues after phenolation in alkaline medium under different reaction conditions Reaction condition Entry

Sample

Wphenol/WKL

Characteristics

Temp. (ºC)

Tim

Yield

e

a

(h)

(%)

Mn

Mw

g/mol

g/mol

DPIb

ArOH

AlkOH

mmol/g

mmol/g

-

-

-

55.3c

1900

14400

7.74

2.93

4.28

PKL-1-160-3

1:1

160

3

50.3

2100

6000

2.90

4.22

4.06

PKL-2-160-3

2:1

160

3

31.0

2600

8600

3.24

3.73

4.11

3

PKL-3-120-2

3:1

120

2

45.2

2800

12700

4.55

3.03

3.66

4

PKL-3-140-2

3:1

140

2

37.4

2900

10700

3.66

3.98

5.02

5

PKL-3-160-1

3:1

160

1

38.0

2800

10700

3.74

3.94

4.41

6

PKL-3-160-2

3:1

160

2

28.3

2900

9300

3.24

4.52

5.24

7

PKL-3-160-3

3:1

160

33.7

2900

10500

3.66

4.08

4.81

0

KL

1 2

aYield

= (mass of PKL) / (mass of raw KL).

3 bDPI=M

w/Mn.

cMass

percentage of acid-precipitable fraction of raw

KL.

Insights into the alkali-catalyzed phenolation of kraft lignin The kraft lignin before and after the alkali-catalyzed phenolation was investigated using NMR. The side-chain (δC/δH 50-90/2.5-6.0) and the aromatic (δC/δH 100-135/5.5-8.5) regions of the 2D HSQC spectra of KL and PKL-3-160-2 are shown in Figure 2, and the main substructures are assigned based on the previous literature reports.10,21,25,26 The main cross-signals in the aromatic regions of 2D HSQC spectra were attributed to the different aromatic lignin units in the form of the phenylpropanoids including p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S), respectively. 11 ACS Paragon Plus Environment


Figure 2. 2D HSQC NMR spectra of KL in DMSO-d6 before and after phenolation treatment.

As shown in Figure 2, only the signals of G2, G5 and G6 were observed in the aromatic region on the spectra of kraft lignin before phenolation treatment, indicating that the kraft lignin used in this study was a G-type lignin. Moreover, as most β-O-4 bonds had been cleaved during the kraft pulping, no β-O-4 but only β-β substructure was observed in the side-chain region. Compared to KL, the intensity of the β-β signals decreased slightly, indicating that the cleavage of sidechain C-C bond might have occurred. The cleavage of sidechains was not observed in


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the phenolation under the mild conditions reported by Sun et al.22 It was apparent that raising phenolation temperature or increasing phenol dosage promoted the depolymerization of lignin. Meanwhile, there was a clear structural change in the aromatic region of the 2D HSQC spectra after the phenolation. The appearances of cross-signals attributed to o/p phenol confirmed the introduction of phenol into lignin backbone during the phenolation, which provided more reactive sites of unsubstituted C3 and C5 positions for crosslinking in LPF preparation. The phenolation of KL (Entry 6 in Table 1) generated some water-soluble organic acids from extensive lignin decomposition, which retained in aqueous phase after organic solvent extraction, as shown in Figure 1. The three organic acids identified and quantified using HPLC are oxalic acid, acetic acid, and formic acid, respectively. Among them, oxalic acid and acetic acid were the primary organic acids with the yields of 5.7 wt% and 14.3 wt% (on starting KL), respectively, while only trace amount of formic acid (yield of 0.1 wt%) was detected by HPLC. The third fraction of KL phenolation products is the low-molecular-weight compounds extracted by organic solvent. Based on the data listed in Table 1 (Entry 6) and the quantity of the organic acids above, the KL phenolation generated 51.2% phenolated lignin (PKL-3-1602) and 36.3% organic acids (both based on starting KL). Therefore, the estimated yield of the low-molecular-weight compounds extracted by organic solvent was approximately 12.5% (based on starting KL). The low-molecular-weight compounds (the products in the organic phase of Figure 1) were analyzed using GC-MS. The products were extracted using ethyl ether and ethyl acetate separately to compare the difference of the two solvents in extracting the compounds. The products from Entry 6 (Table 1) were analyzed, and the results are presented in Figure 3 and



Scheme 1, respectively. Phenolics and organic acids were identified in the products in the organic phase (Scheme 1). The phenolics included both monomeric phenolics such as phenol, catechol, guaiacol, vanillin, acetovanillone, 3-vanilpropanol and dimeric phenolic compounds including 2,2'diphenylmethane, 2,4’-diphenylmethane and 4,4'-diphenylmethane in the organic phases of both solvents. The organic acids are those from extensive lignin degradation. Comparing the compounds extracted by ethyl ether and ethyl acetate in Scheme 1, it was found that ethyl ether was able to extract the phenolics, while ethyl acetate extracted both organic acids and the phenolics. Furthermore, only small organic acids were detected by HPLC in the aqueous phase above, while additional large organic acids were identified in the organic phase. These results suggested that the phenolics and the organic acids from the lignin decomposition could be roughly separated by sequential extractions with different solvents, which favored the downstream applications of the two groups of the compounds.27 For example, ethyl ether could be employed to extract the phenolics, and then ethyl acetate was used to recover the large organic acids, while the small organic acids would be primarily in the aqueous phase. In the compounds, catechol, phenol and different organic acids were probably from the further conversion of the phenolics from lignin degradation, such as guaiacol and vanillin. It was reported that guaiacol could be transformed to catechol and phenol under hydrothermal condition,28 and catechol could be further converted to phenol.29 The organic acids were probably originated from either the side chains of lignin or the further decomposition of mono aromatics such as vanillin and p-coumarate.25 It was also reported that the phenyl nucleus-exchange was the mechanism for the


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depolymerization of lignin, which was able to selectively cleave the C-C linkage between side chain and phenyl nuclei during the phenolation process with acid as catalyst.7 One of the unique features of the phenyl nucleus-exchange method was the ability to dealkylate the diphenylmethane structure of lignin, which is stable in many processes. The results (identified phenolics and organic acids above) from the present study suggested that similar reactions might occur in the alkali-catalyzed lignin phenolation treatment, since the primary decomposition products were similar to those from the acid-catalyzed phenolation.7,10 The sidechain cleavage was probably the approach how the kraft lignin was depolymerized in the alkali-catalyzed phenolation, since there was no β-O-4 structure detected in the starting kraft lignin, and thereby the cleavage of β-O-4 was not the reason of lignin depolymerization during the phenolation. In summary, the results above suggested that cleavage of sidechain C-C bond in β-β substructure might have occurred to the kraft lignin during the phenolation in alkaline medium, which led to the depolymerization of kraft lignin, and further degradation of the small lignin fragments and cleaved side chains resulted in the phenolic compounds and small organic acids, respectively. Exact reaction mechanisms could be elucidated and verified using lignin model compounds via carefully designed experiments in further research.



Figure 3. GC chromatograms of the primary compounds extracted by ethyl ether (A) and ethyl acetate (B) from lignin phenolation products. Phenolation conditions: mass ratio of phenol to KL of 3:1, reaction temperature and time of 160 °C and 2 h.


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Scheme 1. Compounds identified by GC-MS in the organic phase (Figure 1) from the extraction of lignin phenolation products with ethyl ether and ethyl acetate.



Conclusion In conclusion, phenol-enhanced depolymerization of kraft lignin was studied with alkali (NaOH) as catalyst under different mass ratio of phenol to kraft lignin, reaction temperature, and time. The results indicated that the NaOH-catalyzed phenolation was an effective method to depolymerize kraft lignin. The phenolation depolymerized the lignin probably via sidechain CC bond cleavage. For example, under the conditions of mass ratio of phenol to kraft lignin of 3:1, 160 ºC, and 2 h, the kraft lignin was depolymerized to approximately 51.2% phenolated lignin, 36.3% small organic acids, and 12.5% phenolics and large organic acids. The resultant lignin segments had high phenolic hydroxyl group (4.52 mmol/g), low molecular weight and dispersity, which would be a good feedstock for the synthesis of LPF adhesive/resin. The monomeric and dimeric phenolics and organic acids would have potential as reactive precursors and platform chemicals for downstream high-value applications.

ACKNOWLEDGMENTS This research was partially funded by the USDA NIFA McIntire Stennis grant (WIS01861) to Pan and the National Science Foundation of China grant (No. 21706085) to Gan. The China Scholarship Council (No. 201708350043) supported Gan for her visiting research at the University of Wisconsin-Madison.

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Phenol-Enhanced Depolymerization and Activation of Kraft Lignin in

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