Lignin Conversion to Low-Molecular-Weight Aromatics via an Aerobic

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Lignin Conversion to Low-Molecular-Weight Aromatics via an Aerobic Oxidation-Hydrolysis Sequence: Comparison of Different Lignin Sources Amit Das, Alireza Rahimi, Arne Ulbrich, Manar Alherech, Ali Hussain Motagamwala, Aditya Bhalla, Leonardo da Costa Sousa, Venkatesh Balan, James A. Dumesic, Eric L. Hegg, Bruce E. Dale, John Ralph, Joshua J. Coon, and Shannon S. Stahl ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03541 • Publication Date (Web): 19 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018

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ACS Sustainable Chemistry & Engineering

Lignin Conversion to Low-Molecular-Weight Aromatics via an Aerobic Oxidation-Hydrolysis Sequence: Comparison of Different Lignin Sources Amit Das,#,† Alireza Rahimi,,#† Arne Ulbrich,† Manar Alherech,† Ali Hussain Motagamwala,‡ Aditya Bhalla,┴ , Leonardo da Costa Sousa,▲ Venkatesh Balan,▲ ,+ James A. Dumesic,‡ Eric L. Hegg,┴ Bruce E. Dale,▲ John Ralph,║ Joshua J. Coon,†,♦,* and Shannon S. Stahl†,* †

Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin 53706, USA ‡ Department of Chemical and Biological Engineering, University of Wisconsin-Madison, 1415 Engineering Drive, Madison, Wisconsin 53706, USA ┴ Department of Biochemistry & Molecular Biology, Michigan State University, 603 Wilson Road, East Lansing, Michigan 48824, USA ▲ Department of Chemical Engineering and Material Science, Michigan State University, 3815 Technology Boulevard, Suite 1045, East Lansing, Michigan 48824, USA ║ Department of Biochemistry and DOE Great Lakes Bioenergy Research Center, Wisconsin Energy Institute, 1552 University Avenue, Madison, Madison, Wisconsin 53726, USA ♦ Department of Biomolecular Chemistry, University of Wisconsin-Madison, 420 Henry Mall, Madison, Wisconsin 53706, USA #

These authors contributed equally to this work. + Present address: Department of Engineering Technology, Biotechnology Division, School of Technology, University of Houston, 4800 Calhoun Road, Houston, Texas 77004, USA. Email: [email protected] (J.J.C.), [email protected] (S.S.S.) ABSTRACT: Diverse lignin samples have been subjected to a catalytic aerobic oxidation process, followed by formic-acid-induced hydrolytic depolymerization. The yield of monomeric aromatic compounds varies depending on the lignin plant source and pretreatment method. The best results are obtained from poplar lignin isolated via a acidolysis pretreatment method, which gives 42 wt% yield of low-molecular-weight aromatics. Use of other pretreatment methods and/or use of maple and maize lignins afford yields of aromatics ranging from 3 to 31 wt%. These

results

establish

useful

references

for

the

development

oxidation/depolymerization protocols. KEYWORDS: biomass, pretreatment, depolymerization, catalysis, poplar 1 Environment ACS Paragon Plus

of

improved

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INTRODUCTION Lignin is an abundant heterogeneous aromatic polymer biosynthesized from phenylpropanoid building blocks in the cell walls of vascular plants.1,2 It has significant potential to serve as a renewable source of aromatic chemicals;3-5 however, the inherent recalcitrance6,7 of this energydense polymer presents major obstacles to the production of well-defined aromatic products via chemical8-31 or biochemical32,33 depolymerization. Consequently, lignin is often simply burned as low-cost fuel.34 Future biorefineries will benefit from the availability of this low-cost energy source; however, only a fraction of the lignin is needed to sustain the refinery.3-5 Moreover, the economic viability of these facilities would improve if lignin could be converted to valuable products, such as commodity or specialty chemicals. The structure of lignin is diverse, and the specific chemical units vary with the source plant material.8-16 Nevertheless, nearly all native lignins have a large fraction of β-aryl ether fragments linked by so-called “β-O-4” units (Scheme 1), 35 which contain a secondary benzylic and a primary aliphatic alcohol in addition to the characteristic alkyl aryl ether. The prevalence of these β-ether units, and the relative ease of their cleavage, makes it a common target for chemical degradative studies in the context of lignin valorization. 36 However, lignin feedstocks are typically obtained from biomass pretreatment methods that have been optimized for production of cellulose and other polysaccharide or sugar-based products.37,38 The pretreatment protocols can significantly modify the native lignin structure, in some cases destroying the β-ether units,39,40 and thereby complicate efforts to convert lignin into well-defined aromatic products. Recently, we developed a method for chemoselective aerobic oxidation of the secondary benzylic alcohol in the β-ether unit of lignin, converting it to the corresponding ketone.41-53 We further showed that this oxidized material undergoes efficient hydrolytic depolymerization in aqueous formic acid under mild conditions to give high yields of aromatic monomers (Scheme.

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1).54 A >60 wt% yield of low molecular weight (MW) aromatic compounds was obtained from aspen lignin when it was subjected to this two-step protocol. Scheme 1. Chemoselective aerobic oxidation of a poplar lignin and its depolymerization under redox-neutral conditions OH

OMe OH

O

O HO HO α

O

HO

AcNH-TEMPO (2.5 wt%) HCl (cat.) OMe HNO3 (cat.)

O

HO

O α

OH

β O 4 OMe

CH3CN:H2O (99:1) 2 atm O2, 65 °C, 28 h

OH

γ

Ligninox

γ

MeO O

HO

O

O Activation by oxidation Ligninox

β O 4

HCO2H/HCO2Na

OH

110 °C, 24h

OMe CH3

MeO

OH O

O

OMe

O

HO

O

O

O MeO

HO

OMe

OMe OH

O

Me

O

O

H/OH

O

OMe

OMe OH

OH

HO

CO2H

OMe

MeO

OH OMe

OH O2N

OH

NO2

Redox-neutral cleavage

The lignin source material, in this case, was obtained from a labor-intensive pretreatment method involving ball-milling of the poplar (aspen) wood chips and digestion of the resulting material with crude cellulolytic enzymes, among other steps.55 This process removes most, but not all, of the polysaccharides, while retaining native-like lignin on the basis of 2D NMR characterization.41 Nevertheless, this purification protocol is not suitable for large-scale use. In the present study, we assess the effectiveness of the oxidation-hydrolysis sequence with a range of more-scalable lignins derived from pretreatment methods that have been developed recently within the Great Lakes Bioenergy Research Center. 56 These samples include lignins derived from different plant source materials. The results presented below show that the outcome of the lignin depolymerization process depends on both the pretreatment method and the lignin source. The implications of these results are discussed.



RESULTS AND DISCUSSION Eight different lignin samples were evaluated in this study (Table 1). The plant sources included poplar, maize and maple; the lignin purification methods included a mild acidolysis pretreatment employing HCl/dioxane,57 extraction with γ-valerolactone containing dilute sulfuric acid, 58 an extractive ammonia process, 59 , 60 and an alkaline hydrogen peroxide pretreatment promoted by soluble copper salts.61 Hereafter, these lignin purification methods will be referred to as the “acidolysis”, “GVL”, “EA”, and “Cu-AHP” processes, respectively, and the original lignin purification method is designated “enzymatic”. Table 1. List of lignin samples used in the present study Plant

Lignin purification method

ref

poplar

enzymatic

41

mild acidolysis – HCl/dioxane (acidolysis)

57

γ-valerolactone/dilute sulfuric acid (GVL)

58

copper - alkaline hydrogen peroxide (CuAHP)

61

mild acidolysis–HCl/dioxane (acidolysis)

57


58

extractive ammonia (EA) (two different fractions)

59,60

mild acidolysis–HCl/dioxane (acidolysis)

57


58

maize

maple

Our initial studies used poplar acidolysis lignin in an effort to determine whether the moreconvenient and scalable acidolysis pretreatment method could serve as a suitable replacement for the cumbersome enzymatic method for separation of the lignin from the sugars. To this end, we isolated the poplar lignin from 2–5 mm wood chips by treating it with a 0.2 M solution of HCl in dioxane:water (9:1, v/v) at moderate temperature (see Supporting Information for details for details).57 The purified lignin sample was characterized by 2D heteronuclear single-quantum coherence (2D HSQC) NMR spectroscopy (Figure 1). The lignin from this acidolysis


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pretreatment method exhibited structural features similar to those from the lignin sample we previously isolated from an enzymatic pretreatment method.41

Native Lignin Bβ

OMe

Cβ

Aβ(G)

Cγ Aβ(S)

Cα

50

OMe

60

Aα Bα

Oxidized Lignin

50

60

70

Aα

70

80

A’β(G) A’β(S)

80

90

90

C

13

6

5.5

5

4.5

4

Py H2/6

H

1

2.5

6

5.5

5

100

S2/6

4.5

H3/5

120

G5/6

4

10

9

3.5

H3/5

G’6

140

Py

Py

Py

C

13

H

9

HO

O

O 4 1

8

OMe

S/G

1 S/G

6

2

S

MeO

1

OMe O

O

S/G

A

C

-aryl ether ( –O4)

resinol ( – )

6 5

R

S Syringyl

1

7

HO

HO

HO MeO S/G

C H ppm 13

1

7

8

120

Py

H2/6 140

9

100

S2/6

G’2

200

H

1

3

S’2/6

180 X

G2

200

9

3

S’2/6

180 X

10

3.5

C

13

O 2

G

6 5

OMe O

O 2 3

H

OH

R

G Guaiacyl

H p-Hydroxybenzoate CHO

O

O

HO MeO 6

S'/G'

O

O 4 1

OMe

S'/G'

A'

-aryl ether ( –O4)

2

S'

MeO

OMe O

R

S' Syringyl

6 5

HO

2

G'

O

OMe O

R

G' Guaiacyl

R' S/G

B

phenyl coumaran

R'' O

R

X Aldehyde

Figure 1. Partial 2D HSQC NMR spectra of an isolated poplar lignin (acidolysis) before and after oxidation. Solvent: 4:1 DMSO-d6/pyridine-d5. Subjecting to the acidolysis poplar lignin sample to the previously developed aerobic oxidation method led to reactivity and selectivity similar to that observed with enzymatic lignin.41 As shown in Figure 1 (bottom), the 2D HSQC NMR spectrum of the aromatics region reveals that most of the benzylic alcohol groups in the syringyl (S) units and all of the benzylic alcohol groups in the guaiacyl (G) units underwent oxidation to the corresponding benzylic ketones (S' and G'). This result is further supported by analysis of the aliphatic region of the 2D



NMR spectrum of the native and the oxidized lignin, which reveals that most of the β-ether units A in the native sample were oxidized to their corresponding A' units (Figure 1, top). The oxidized lignin was then hydrolyzed by treatment of the sample with aqueous formic acid in the presence of sodium formate under refluxing conditions. Upon evaporation of the formic acid, the residue was extracted with ethyl acetate, and the soluble aromatics (i.e., the material soluble in ethyl acetate) corresponded to 53% of the original mass of the lignin sample. This fraction was analyzed by high-resolution mass spectrometry and liquid chromatography (LC– MS) to identify and quantify the specific products of the reaction. The majority (43.2 wt% relative to the original sample) corresponded to known aromatic compounds, the identities of which were confirmed by comparison with authentic samples obtained commercially or independently synthesized (see Figure 2, Tables S1, S2 and see Supporting Information for details). Product distributions show that S- and G-derived diketones (1 and 4, respectively) (Figure 2) are two of the major depolymerization products (16.9 wt%) formed by a hydrolysis mechanism that was elucidated in our previous study.54 The other major products obtained from depolymerization of the acidolysis poplar lignin sample include syringaldehyde (2) (6.0%) and syringic acid (3) (6.3%) from the S unit; vanillin (5) (2.8%) and vanillic acid (6) (2.5%) from the G unit and p-hydroxybenzoic acid (5.7%) from the pendant p-hydroxybenzoate esters on poplar lignin (Figure 2). (Note: These have a different origin than traditional lignin H-units that derive from lignification using p-coumaryl alcohol as a monomer.62) Additionally, a small amount of 4hydroxy-3-nitrobenzoic acid (8) (1.5%) and 3,5-dinitroguaiacol (1.6%) were obtained, presumably derived from nitration of p-hydroxybenzoates and guaiacol (formed from C-C bond cleavage) during the oxidation protocol (Figure 2). The ~2:1 ratio of S:G-derived products obtained from the depolymerization reaction reflects the similar distribution of S:G monomer


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units present within the poplar lignin sample. This outcome is similar to the product distribution obtained from poplar lignin derived from the enzymatic pretreatment method (Figure 2). enzymatic pretreatment

Pretreatment: 14.4 wt%

mild acidolysis 13.0 wt%

29.5 wt%

22.7 wt%

Syringyl Guaiacyl p-Hydroxyphenyl Dimers and trimers

29.7 wt% 8.8 wt%

Mass balance

MeO

R

O

R

R= C(O)Me (1) (10.3%) H (2) (6.0%) OH (3) (6.4%)

O

OH

9.4 wt%

OH

OMe

OMe OH

7.2 wt% 36.0 wt%

4.4 wt%

O

11.7 wt%

Polymeric material

13.2 wt%

R OH

R= C(O)Me (4) (6.6%) R= H (7) (5.7%) H (5) (2.7%) R= NO2 (8) (1.5%) OH (6) (2.4%)

Figure 2. Major products of poplar lignin depolymerization derived from the previously reported enzymatic lignin (dashed red box) and from mild acidolysis pretreatment, together with the full product distribution observed in the latter protocol. Encouraged by this result, the sequential oxidation-depolymerization method was then applied to lignin samples extracted from maize stover and maple wood chips, using the same mild acidolysis pretreatment method (see Supporting Information for details). In comparison to poplar lignin, these lignin samples afforded significantly lower yields of low-MW aromatics, including only 16.9 wt% from maize and 8.1 wt% from maple (Figure 3, Table S1). The basis for these lower yields is not yet known; however, maize lignin has a high level of coumarate and ferulate units63 that could trap free-radical NOx species (such as NO2 and NO) that are important catalytic mediators in the aerobic oxidation method. No analogous explanation exists for the low yields with maple lignin, but the 2D HSQC NMR analysis of both maize and maple lignin samples suggests that these materials are not oxidized as effectively as the poplar sample (cf. Figures S1 to S6). Subsequent hydrolysis by formic acid and formate would, therefore, be less efficient and lead to lower yields of the aromatic monomers.



O

O

R

MeO

R

OMe

OMe OH

OH

Maize:

R= C(O)Me (1) (2.0%) H (2) (1.7%) OH (3) (2.2%)

R= C(O)Me (4) (1.9%) H (5) (2.2%) OH (6) (2.4%)

Maple:

R= C(O)Me (1) (2.1%) H (2) (1.1%) OH (3) (0.7%)

R= C(O)Me (4) (1.5%) H (5) (1.2%) OH (6) (0.7%)

Figure 3. Major products obtained from the depolymerization of maize and maple lignin obtained from mild acidolysis pretreatment. H-lignin-derived products account for ≤ 1 % of the monomers with these lignin sources. Detailed analyses of the ethyl-acetate-soluble portion of the maize lignin shows that welldefined syringyl- and guaiacyl- (S- and G-) derived aromatics are the major depolymerization products. In addition, ferulic acid was obtained in 2.0 wt% from maize lignin depolymerization (Figure 4, Table S2). This fragment is abundant in maize-based cell walls, deriving mainly from (glucurono)arabinoxylans that are specifically acylated by ferulate.64,65 As expected for maize and maple lignins, no 4-hydroxybenzoic acid was obtained from these samples. After evaluating the viability of two-step oxidation-hydrolysis sequences on different isolated lignin samples from the mild acidolysis pretreatment method, we examined lignin samples obtained from other pretreatment protocols. These processes are similar to the mild acidolysis method in that they are more amenable to larger scale application, relative to the original enzymatic pretreatment method. Specifically, we tested poplar, maize and maple lignins obtained from the GVL process,58 poplar lignin obtained from the Cu-AHP process,61 and two maize lignin fractions (F3 and F5) obtained from the EA process.59,60


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O

R

MeO

O

R

OMe OH

O

OH

R

OMe OH

OH

Poplar (GVL)

R= C(O)Me (1) (4.5%) H (2) (3.8%) OH (3) (3.2%)

R= C(O)Me (4) (2.7%) R= H (7) (4.0%) H (5) (1.3%) R= NO2 (8) (1.7%) OH (6) (1.1%)

Maize (GVL)

R= C(O)Me (1) (2.9%) H (2) (2.4%) OH (3) (1.6%)

R= C(O)Me (4) (1.6%) H (5) (1.7%) OH (6) (2.0%)

Maple (GVL)

R= C(O)Me (1) (1.4%) H (2) (0.6%) OH (3) (0.5%)

R= C(O)Me (4) (1.0%) H (5) (0.9%) OH (6) (0.3%)

Poplar (Cu-AHP)

R= C(O)Me (1) (7.9%) H (2) (4.8%) OH (3) (5.1%)

R= C(O)Me (4) (5.1%) R= H (7) (2.3%) H (5) (2.0%) R= NO2 (8) (1.5%) OH (6) (2.0%)

Maize (EA-F3)

R= C(O)Me (1) (0.7%) H (2) (0.7%) OH (3) (0.4%)

R= C(O)Me (4) (0.4%) H (5) (0.8%) OH (6) (0.3%)

Maize (EA-F5)

R= C(O)Me (1) (0.6%) H (2) (0.3%) OH (3) (0.2%)

R= C(O)Me (4) (0.5%) H (5) (0.5%) OH (6) (0.3%)

Figure 4. Major products of lignin depolymerization from GVL, Cu-AHP and EA pretreatments.

Both mild-acidolysis and GVL pretreatment methods employ dilute mineral acids (HCl and H2SO4, respectively) to separate the lignin from cellulose. Use of the GVL-based poplar lignin sample in the oxidation-depolymerization process resulted in an ethyl-acetate-soluble fraction containing 28.9 wt% low-MW aromatics, from which 23.6 wt% was characterized as known



compounds by LC–MS (Figure 4, Tables S1 and S2).66 This yield of aromatic compounds is lower than that obtained from the mild acidolysis poplar lignin. In accord with the trends observed with mild acidolysis lignins (cf. Figure 3), maize and maple lignins obtained from GVL pretreatment afforded lower yields of aromatic monomers relative to the poplar lignin (Figure 4, Tables S1 and S2). The maize lignins obtained from mild acidolysis and GVL pretreatments, however, afforded similar yields of identified low-MW aromatics (16.9 and 15.1 wt%, respectively). Application of the oxidation-hydrolysis sequence to Cu-AHP poplar lignin,61 led to a 30.6 wt% yield of identified low-MW aromatics, which is the second-highest yield among the lignin samples tested in this study (Figure 4, Tables S1 and S2). Partial oxidation of the lignin during the pretreatment process67,68 could account for the higher yields of aromatics, relative to the poplar lignin obtained from the GVL process. The extractive-ammonia (EA) process has been predominantly used with maize-based sources of biomass.59,60 Two lignin fractions obtained from this process were examined: EA-F3 is the ethanol-soluble fraction obtained after initial treatment of the biomass with liquid ammonia, and EA-F5 is the solid lignin fraction remaining after enzymatic hydrolysis and sugar separation. This pretreatment method has been shown to afford high sugar yields (90% or more) following a subsequent enzyme treatment step. Comparatively low yields of well-defined aromatic compounds were obtained from these lignin samples (Figure 4 and Tables S1 and S2), suggesting that the present oxidation-depolymerization protocol is not tolerating some aspect of these lignins. For example, preliminary characterization studies of the lignin suggest that the ammonia treatment incorporates some nitrogen into the lignin fraction, even while much of the β-ether units remain (see below).59,60


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The yields of low molecular weight aromatics were expected to correlate with the amount of β-O-4 content in the isolated lignin samples as the oxidation method reacts with the 2° alcohols present in these units. Each of the lignin samples was analyzed using 2D HSQC NMR spectroscopy to estimate the percentage of β-O-4 content in each sample. This value was estimated from peak-volume integrations associated with the β-O-4 units in the HSQC spectra relative to those of S- and G-derived aromatic rings. This approach is rather imprecise, however, because integration of HSQC spectra has substantial uncertainty due to variability in T2 relaxation rates and 1JC-H constants for each observed nucleus. Even with this uncertainty, no clear correlation is evident between monomer yields and the β-O-4 content present in the different lignin samples (Table 2). The origin of this result is not yet clear, but it could reflect the ability of the oxidation/hydrolysis sequence to operate on units other than β-O-4. Indirect support for this possibility is implicated by the formation of aromatics beyond the diketones that have been shown from model studies to derive from oxidation/hydrolysis of β-O-4 units (Table S2). Future studies will be directed toward exploring this hypothesis.



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Table 2. Estimated percentages of β-O-4 linkages in the lignin samples, derived from 2D HSQC analysis, and wt% of ethyl acetate soluble low molecular weight aromatics after depolymerization Plant

Lignin purification method (cf. Table 1)

poplar

maize

maple

% β-O-4 contenta

Low molecular weight S and G aromatics (wt%)b

acidolysis

45-46

52.6

GVL

26-37

29.1

Cu-AHP

58-62

39.1

acidolysis GVL

34-38 28-49

24.2 21.9

EA-F3 EA-F5

29-40 38-47

11.3 5.7

acidolysis GVL

44-48 23-35

11.2 9.1

a

Percent β-O-4 content with respect to all linkages in lignin on the basis of peak integrations associated with the β-O-4 unit relative to integrations associated with the S and G aromatic rings. Specifically, values were obtained from the ratio of integrals for the following protons: (Aa + A'b)/(G2 + ½ S2/6). Integrations obtained from 2D HSQC spectra are not expected to be rigorously quantitative owing to the dependence of polarization transfer on the 1JC–H value and variability in T2 relaxation rates for each observed nucleus. Thus, these percentage should be treated only as estimates. b wt% of S and G derived compounds from the depolymerized material soluble in ethyl acetate.

CONCLUSIONS Overall, these results highlight both positive and negative features of the recently developed oxidation-depolymerization

method.

Traditional

process

lignins

(e.g.,

kraft

lignins,

lignosulphonates, and lignins resulting from strong acid hydrolysis) often have significantly altered structures relative to native lignin and are not well suited for conversion into low-MW aromatics in high yields. In contrast, many promising pretreatment methods have been identified in recent years that afford native-like lignin, including by the GVL, Cu-AHP, and EA-related processes. The results described herein suggest that depolymerization methods that cleave the relatively labile β-O-4 linkages could be quite successful in generating monomeric aromatic chemicals in high yields (i.e., >30–40%). The variable results observed here with the different lignin samples could have several possible origins. In some cases, some loss of the β-ether subunit during the pretreatment method (cf. Table 2) would logically hinder efficient depolymerization. In other cases, specific lignin types may be poorly matched to the aerobic 12 Environment ACS Paragon Plus

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oxidation method used here41 for reasons that have not yet been identified. For example, maizebased lignin preparations contain hydroxycinnamate (ferulate and p-coumarate) components with alkenes that could interfere with or inhibit the radical NOx-based co-catalysts. The possibility underlies current efforts focused on developing new, more-general oxidation methods that show broader tolerance of chemical structures present in diverse lignin sources. If successful, such methods could pair with any of the numerous emerging pretreatment methods that generate lignins with native-like structures. The results described herein provide valuable benchmarks for the assessment of new oxidation-depolymerization protocols.

ASSOCIATED CONTENT Supporting Information. Experimental description; 2D NMR characterization of the mild acidolysis lignin from poplar, maize and maple; full listing of lignin depolymerization products are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors Email: [email protected] (J.J.C.), [email protected] (S.S.S.)

Author Contributions SSS designed the study. AD and AR performed the oxidation/hydrolysis experiments for the different lignin samples. AU performed LC-MS analysis of the lignin depolymerization samples. AD, AR, AU, JC and SSS analyzed the data. AD and AR performed lignin isolations using the mild acidolysis method. AHM performed lignin isolations using the GVL method. AB performed lignin isolations using the Cu-AHP method. LdCS and VB performed lignin isolations using the EA method. AR, MA, and JR collected and analyzed 2D NMR data of the mild acidolysis lignin



samples, and MA analyzed 2D NMR data for other lignin samples. AD, AR and SSS wrote the paper. JAD, ELH, BED and JR provided technical knowledge and oversight of lignin isolation methods and analyses. All the authors provided comments on paper. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We thank Dr. Charlie Fry for generous assistance with NMR spectroscopy. Use of the Bruker AVANCE III 500 MHz spectrometer with DCH cryoprobe capabilities was made possible by the UW-Madison Department of Chemistry Bender Fund. Financial support for this project was provided by the Great Lakes Bioenergy Research Center (Department of Energy Biological and Environmental Research Office of Science DE-FC02-07ER64494). REFERENCES

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For Table of Contents Use Only Synopsis The success of an oxidation-hydrolysis process for lignin conversion to low-MW aromatics varies with the lignin source material and pretreatment method, with yields of >50% possible. OH

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Lignin Conversion to Low-Molecular-Weight Aromatics via an Aerobic

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