Biomimetic Fenton-catalyzed lignin depolymerization to high-value aromatics and dicarboxylic acids.

DOI: 10.1002/cssc.201403128

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Biomimetic Fenton-Catalyzed Lignin Depolymerization to High-Value Aromatics and Dicarboxylic Acids Jijiao Zeng,[a] Chang Geun Yoo,[b] Fei Wang,[a] Xuejun Pan,[b] Wilfred Vermerris,[c] and Zhaohui Tong*[a] tography–mass spectrometry, 31P NMR spectroscopy, and X-ray photoelectron spectroscopy. The results suggest that the Fenton catalyst facilitates lignin depolymerization through cleavage of b-ether bonds between lignin residues. The formation of a lignin–iron chelating complex effectively depresses lignin recondensation; thus minimizing charcoal formation and enhancing the yield of liquid products.

By mimicking natural lignin degradation systems, the Fenton catalyst (Fe3 + , H2O2) can effectively facilitate lignin depolymerization in supercritical ethanol (7 MPa, 250 8C) to give organic oils that consist of mono- and oligomeric aromatics, phenols, dicarboxylic acids, and their derivatives in yields up to (66.0 8.5) %. The thermal properties, functional groups, and surface chemistry of lignin before and after Fenton treatment were examined by thermogravimetric analysis, pyrolysis–gas chroma-

Introduction Environmental concerns and the desire to become less dependent on imported fossil fuels have stimulated the use of lignocellulosic biomass as an alternative and sustainable resource for the production of liquid fuels and chemicals.[1] Lignocellulosic biomass is mainly composed of cell wall polysaccharides (cellulose and hemicellulose) and the aromatic polymer lignin.[2] Although cellulose and hemicellulose have been extensively investigated for the production of fuels and chemicals, lignin, which represents a significant portion (roughly 15– 35 % on a weight basis and 40 % on an energy basis) of lignocellulosic biomass,[3] has received less attention. For example, the paper industry in the USA alone produces more than 50 million tons of lignin per year; only 2 % is used for commercial products and the rest is burned for heat and power.[4] Therefore, the conversion of lignin into value-added products, such as chemicals and liquid fuels, rather than heat and power offers the potential to make biorefineries considerably more profitable.[5] Native lignin is a cross-linked aromatic polymer formed from the oxidative polymerization of hydroxycinnamyl alcohols and related compounds with a carbon number of 800–900.[6] Lignin residues derived from p-coumaryl, coniferyl, and sinapyl alcohol are referred to as p-hydroxyphenyl (H),

guaiacyl (G), and syringyl (S) units, respectively; these can be cross-linked through a variety of ether and carbon–carbon bonds.[7] The exact lignin subunit composition varies as a function of biomass (softwood, hardwood, and grasses), tissue, and developmental stage.[7] Lignin depolymerization can produce aromatic compounds that can be used directly in the chemical industry or blended with conventional transportation fuels. For example, jet fuel is a type of aviation fuel with mixtures of hydrocarbons in the range C9–C16 (kerosene-type) and C5–C15 (naphtha-type). Monoand dimeric lignin products meet the requirement of the carbon chain length of jet fuel. In addition, lignin-derived compounds can be used in industry as solvents and chemical reagents. For example, adipic acids from oxidized lignin depolymerization products can be used to produce nylon 6,6 for carpeting, automobile tire cord, and clothing.[5] Various routes have been explored to depolymerize lignin into aromatic compounds or small-molecule chemicals, including thermochemical cracking, catalytic reduction (hydrogenolysis), and catalytic oxidation.[8] Pyrolysis is a common thermal approach to depolymerize lignin. Mixed phenolics were produced through pyrolysis of lignin at elevated temperature (400–600 8C).[9] The addition of catalysts (e.g., zeolite and transition metals) during pyrolysis facilitated the cleavage of b-O-4 and C C bonds of lignin to improve the yield of aromatics. However, a significant amount of char (more than 40 % mass yield) was formed in this process, resulting from recondensation of depolymerization products.[10] Sub-/supercritical fluid is an effective medium for lignin depolymerization.[11] For example, lignin depolymerization in supercritical carbon dioxide/ acetone/water (300 8C, 100 bar) produced a phenolic oil that consisted of oligomeric fragments and monomeric aromatic compounds with a total yield of 10–12 % based on lignin. Miller et al.[11c] reported a two-step base-catalyzed depolymeri-

[a] Dr. J. Zeng, Dr. F. Wang, Dr. Z. Tong Department of Agricultural and Biological Engineering University of Florida, PO Box 110570 Gainesville, FL 32611 (USA) E-mail: [email protected] [b] Dr. C. G. Yoo, Dr. X. Pan Department of Biological Systems Engineering University of Wisconsin-Madison 460 Henry Mall, Madison, WI 53706 (USA) [c] Dr. W. Vermerris Department of Microbiology and Cell Science & UF Genetics Institute, University of Florida PO Box 103610, Gainesville, FL 32611 (USA)

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Full Papers zation (BCD) process to convert lignin into hydrocarbons in supercritical fluid with a yield of 22 %.[12] Roberts et al. further improved the BCD process by adding boric acid as the stabilizer and achieved a conversion yield of 35 %.[11a] In general, the thermochemical processes described above had low yields of liquid oil with high viscosity and reaction intermediates were usually easy to repolymerize. Catalytic reduction (hydrogenation) is another promising method to depolymerize lignin. The catalysts for hydrogenation of lignin and lignin model compounds include metals such as nickel, platinum, palladium, and ruthenium supported on active carbon or silicon or aluminum in the presence of hydrogen and CuMgAlOx in supercritical ethanol.[13] For example, Zakzeski and Weckhuysen reported that liquid-phase reforming (LPR) of the solubilized lignin at 498 K and 58 bar yielded up to 17 % monomeric aromatic oxygenates with the aid of a Pt/Al2O3 catalyst.[14] Song et al. reported that lignin was depolymerized in alcoholic solvents with a nickel-based catalyst to give monomeric phenols in a yield of 50 %.[15] In another study, approximately 21 wt % of switchgrass lignin was converted into phenolic products over a platinum catalyst with formic acid.[16] Catalytic oxidation with oxygen, ozone, and peroxide is traditionally used for delignification in pulping and bleaching in the paper industry. Oxidative catalysts such as inorganic metal based catalysts (Fe, Mn, and Cubased oxidizer), metallo-based complexes (e.g., vanadium complexes), and organocatalysts [e.g., 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)] have been applied to lignin oxidation to produce high-value chemicals, including phenolic compounds, dicarboxylic acids, and quinones.[17] For example, a metal-assisted catalytic oxidation approach was applied to depolymerize lignin to give benzoic acids and related compounds, but the yield was relatively low and the catalyst had a complicated structure.[18] Rahimi et al. reported that formic acid induced oxidative depolymerization of lignin resulted in more than 60 % yield of low-molecular-weight aromatics.[19] Although the depolymerization of lignin is a promising approach to produce liquid aromatic hydrocarbons and other low-molecular-weight chemicals, the current total yield of the compounds is relatively low. The use of expensive and complicated catalysts (e.g., Pt, Pb) and a high operating temperature (250–600 8C) contributes to a high cost of operation. In addition, the diversity in functional groups in lignin, including phenolic and aliphatic hydroxyl groups, as well as methoxy, carbonyl, and carboxylic groups, provide reactive sites for crosslinking/condensation to form charcoal,[8f, 20] which reduces the rate of depolymerization, blocks active site of the catalysts, and inhibits mass transfer during lignin depolymerization. Herein, we used a low-cost Fenton catalyst (Fe3 + and H2O2) to depolymerize lignin. The rationale for selecting this catalyst was inspired by a natural process in which efficient fungal catalyst systems depolymerize lignin. In this process, a reduced transition metal (manganese) catalyzes the formation of reactive hydroxyl radicals from H2O2 to hydroxylate and demethoxylate lignin.[21] Fenton catalyst (transition-metal ions such as Fe2 + , Mn2 + , and Cu2 + ) and hydrogen peroxide have been widely used for the mineralization of organic carbon.[22] A few researchers have reported that the Fenton reaction was able

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to reduce the molecular weight of lignin and enhance enzymatic saccharification of lignocellulosic biomass.[23] From these studies, however, it is unknown whether the Fenton catalyst can directly depolymerize lignin into aromatic monomers or low-molecular-weight aliphatic chemicals, and how the demethoxylation of lignin affects charcoal formation. To answer these questions, we performed a two-step lignin depolymerization reaction. First, organosolv hardwood lignin was modified by using the Fenton reagent at varying H2O2/ Fe3 + ratios. Pyrolysis–gas chromatography–mass spectrometry (Py–GC–MS) was used to thermally degrade the modified lignin and determine the effect of different Fenton reaction conditions on lignin subunit composition. Detailed structural features, metal-chelating properties, and thermal degradation behavior of unmodified and modified lignin were investigated by 31P NMR spectroscopy, X-ray photoelectron spectroscopy (XPS), FTIR spectroscopy, and thermogravimetric analysis (TGA), respectively. The Fenton-modified lignin was subsequently subjected to depolymerization in supercritical ethanol. GC–MS was used to identify and quantify the resulting degradation products. We show that the Fenton catalyst can effectively enhance demethoxylation of lignin and the cleavage of b-ether bonds, which is the main interunit linkage in lignin, and contribute to high yields of monomeric aromatic compounds and dicarboxylic acids.

Results and Discussion A schematic presentation of the Fenton-catalyzed lignin depolymerization is shown in Figure 1. Organosolv hardwood lignin was used as the feedstock. Ethanol, which is an inexpensive

Figure 1. Overview of the process to depolymerize lignin to aromatic hydrocarbon and dicarboxylic acids with the use of the Fenton catalyst.

solvent that is readily obtained from the fermentation of polysaccharide-derived sugars in the biorefinery, was used as the solvent for a two-step lignin depolymerization process. In this process, lignin was first modified by the Fenton catalyst (Fe3 + and H2O2) in ethanol at room temperature followed by depoly2


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Full Papers merization in supercritical ethanol to produce aromatics and other chemicals (carboxylic acids and derivatives). Hardwood lignin contains methoxyl groups on the C3 (G residues) and C3 and C5 positions (S residues) of the phenolic ring, which, following depolymerization, are prone to form quinone methide intermediates that recondense to charcoal.[8f, 20d] For example, cresols/xylenols ( OCH3 substitution) produced more coke and tar than catechols/pyrogallols ( OH substitution) in the early stages of pyrolysis.[20a, d] Therefore, to reduce interference from methoxyl groups on catalyzed depolymerization, the Fenton reaction was performed first to demethoxylate lignin. The Fenton reaction is the reaction of hydrogen peroxide in the presence of transition metals or ions to generate hydroxyl radicals as highly reactive oxidants.[24] Several researchers demonstrated that hydroxyl radicals led to hydroxylation and demethoxylation of lignin model compounds.[21, 25] Herein, we used the Fe3 + to reduce H2O2 into free COOH radicals under mild conditions to demethoxylate the lignin. During the subsequent depolymerization process, b-ether bonds between lignin residues were cleaved in supercritical ethanol to produce lowmolecular-weight compounds.

Figure 2. The DTG curves of lignin, lignin with Fe3 + , FLa, FLb, and FLc at a heating rate of 10 8C min 1. FLa, FLb, and FLc represent Fenton-modified lignins with molar ratios of H2O2/Fe3 + of 1.98, 3.96, and 7.92, respectively.

shown in Figure 3. The identified compounds and their relative peak area percentages after pyrolysis are summarized in Table S1 in the Supporting Information. As expected with the use of hardwood lignin, the pyrolytic products from both modified and unmodified lignin included breakdown products of both G and S residues; the largest peaks represent 4-ethylguaiacol and 2,6-dimethoxyphenol. The relative yields of the main pyrolytic products from lignin, lignin with Fe3 + , and Fenton-modified lignin are summarized in Figure 4. The results revealed that the main representatives of the S residues in the

Thermal behavior of lignin modified by the Fenton catalyst

TGA was carried out to investigate lignin decomposition behavior in the temperature range between 0 and 800 8C. The derivative thermogravimetry (DTG) curves of lignin at a heating rate of 10 8C min 1 are shown in Figure 2. The strong peak of organosolv lignin around 400 8C is attributed to primary pyrolysis; this is in agreement with previous research.[26] Lignin mixed with Fe3 + showed a similar degradation pattern to that of native organosolv lignin, but the maximum reaction rate was slightly decreased from 18 to 13 % min 1, which suggested that the presence of Fe3 + hindered the thermal decomposition of lignin. The DTG curves of Fenton-modified lignin with all three H2O2/Fe3 + molar ratios (designated as FLa, FLb, and FLc in increasing order) had broader shoulders and the decomposition temperature ranges shifted to a lower temperature (200 to 300 8C), while the strong peak at 400 8C had disappeared, implying that the Fenton-modified lignin had a lower thermal stability than untreated lignin and lignin with Fe3 + . The thermal degradation products of lignin, lignin with Fe3 + , and Fenton-modified lignin at different H2O2/Fe3 + ratios were analyzed by Py–GC–MS Figure 3. The total ion chromatograms (pyrograms) of lignin; lignin with Fe3 + ; and FLa, FLb, and FLc following pyand the resulting pyrograms are rolysis at 450 8C. ChemSusChem 0000, 00, 0 – 0

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Figure 4. Relative yield of the main pyrolytic products. The percentages were the relative ratios of the peak areas of lignin pyrolytic products over the total areas of all identified peaks, which were normalized as 100 %.

thoxylation because it is usually the intermediate of either demethoxylation of S units or hydroxylation of G units during the pyrolysis of lignin.[27] Therefore, the increased content of the H unit-derived compounds can be attributed to the demethoxylation of S and G units of lignin by the Fenton reaction.

pyrograms, such as 2,6-dimethoxyphenol (5) and 2,6-dimethoxy-4-methylphenol (8), decreased in intensity. The relative content of 2,6-dimethoxyphenol (5) decreased from 19.7 (lignin) and 16.6 % (lignin with Fe3 + ) to 9.3 (FLa), 8.6 (FLb), and 7.7 % (FLc). The relative content of 2,6-dimethoxyl-4-methylphenol (8) decreased from 17.5 (lignin) and 20.4 % (lignin with Fe3 + ) to 11.1 (FLa), 9.9 (FLb), and 6.5 % (FLc). The relative content of 4-methylguaiacol (1), which was the main representative of guaiacyl units, decreased from lignin, lignin with Fe3 + , to the Fenton catalyst with increasing H2O2/Fe3 + ratios. Concomitantly, a peak corresponding to a breakdown product from H units, 4-hydroxybenzoic acid methyl ester (9), significantly increased from 3.5 (lignin) and 4.7 % (lignin with Fe3 + ) to 21.9 (FLa), 30.9 (FLb), and 36.7 % (FLc). In addition, a significant reduction in charcoal after pyrolysis of Fenton-catalyzed lignin (less than 20 %) was observed relative to the pyrolysis of native lignin (58 %) and lignin with Fe3 + (56 %; Figure S2 in the Supporting Information). The results from Py–GC–MS and DTG agreed well with each other and indicated that the addition of Fenton catalyst facilitated the thermal decomposition of organosolv lignin. Given that the main effect of the Fenton catalyst is demethoxylation of the lignin, it is reasonable to conclude that this is the cause for the significant reduction in charcoal formation. The presence of 4-methoxybenzene-1,2-diol (2) in the pyrogram of the Fenton-modified lignin (Table S1 in the Supporting Information) provides further evidence for deme-

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Structural features of Fenton-modified lignin To further elucidate the Fenton-catalyzed lignin depolymerization process, 31P NMR spectroscopy and elemental analysis were employed for lignin, lignin with Fe3 + , and lignin with Fenton catalyst at different H2O2/Fe3 + ratios. 31P NMR spectroscopy was performed to quantify the different types of hydroxyl groups in lignin (Figure 5) following derivatization with phosphitidyl groups. The signals in the range of d = 136.5– 133.7 ppm were assigned to erythro- and threo- a-OH in the bO-4 structures. The g-OH moiety in a-carbonyl-containing units, cinnamyl alcohols, b-O-4 units, and primary OH were found in the range of d = 133.7–132.1 ppm. The signals at d = 132.2, 129.8, and 128.3 ppm were assigned to phenolic OH in the syringyl, guaiacyl, and p-hydroxyphenyl units, respectively. The carboxylic group appeared at d = 127.0–126.5 ppm. Quantitative analysis of the proportion of the different hydroxyl groups (Table 1 and Figure 5) indicated that the content of phenolic OH in the S units decreased from 0.86 mmol g 1 4



Full Papers sorption in the Fenton-modified lignin is caused by the increase in phenolic hydroxyl groups following the Fenton reaction. However, due to the formation of a lignin–metal complex through catechyl or galloyl structures, the phenolic OH groups were protected from phosphorylation during sample preparation for 31P NMR spectroscopy analysis, resulting in the failure to detect phenolic OH in G and S subunits, even though these groups could be detected in H units.

Surface chemistry of the Fenton-modified lignin The surface chemistry of lignin, lignin with Fe3 + , and Fentonmodified lignins was examined by XPS (Figure 6). Among all

Figure 5. Comparison of 31P NMR spectra of lignin; lignin with iron; and lignin with FLa, FLb, and FLc phosphorylated with 2-chloro-1,3,2-dioxaphospholane. N-Hydroxynaphthalimide was added as an internal standard.

Table 1. Characterization of organosolv lignin, organosolv lignin with Fe3 + , and Fenton-treated lignin with various H2O2/Fe3 + ratios by quantitative 31P NMR spectroscopy. Functional group a-OH in b-O-4 primary, g-OH S units OH G units OH H units carboxylic OH

lignin 0.84 1.36 0.86 0.67 0.28 0.13

Amount [mmol g 1] lignin with Fe FLa 0.81 1.71 0.86 0.63 0.30 0.08

0.61 1.36 0.29 0.28 0.24 0.11

FLb

FLc

0.47 1.10 0.15 0.10 0.20 0.08

0.37 1.13 0.08 0.10 0.19 0.13 Figure 6. XPS wide scan spectra of organosolv lignin, lignin with Fe3 + , and Fenton-treated lignin.

(lignin and lignin with Fe3 + ) to 0.29 (FLa), 0.15 (FLb), and 0.045 mmol g 1 (FLc). Significant changes to phenolic OH in the G units were also observed. Compared with the S and G units, phenolic OH in the H units changed little after Fenton modification. This seems to conflict with observations on the S and G units described above. To explain this apparent discrepancy, elemental analysis was conducted to quantify the absorbed Fe content in the lignin, lignin with Fe3 + , and Fenton-modified lignin, and the results are shown in Table S2 in the Supporting Information. It was observed that simply mixing the lignin with a solution of iron nitrate nonahydrate increased the iron content of organosolv lignin from 0.029 to 2.2 g kg 1. With increasing H2O2 loading, the iron content in the modified lignin further increased to 7.6 g kg 1 in FLa, 11.7 g kg 1 in FLb, and 14.7 g kg 1 in FLc. Previous research showed that phenolic compounds had the ability to scavenge free radicals and to chelate metal ions.[28] Specifically, catechol (1,2-dihydroxybenzene) and galloyl (1,2,3-trihydroxybenzyl)-containing compounds were shown to be effective at chelating transition metals. It was also reported that a single OH group in the C4-position, or the presence of OCH3 at the C3- and/or C5-position, lowered the chelation capability.[29] Therefore, we propose that the increased iron abChemSusChem 0000, 00, 0 – 0

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samples, carbon (C 1 s) and oxygen (O 1 s) were the major peaks observed at 287 and 534 eV, respectively. The region from 725 to 710 eV represents the iron element (Fe 2 p1/2 and Fe 2 p3/2). Due to the detection limit of XPS as a surface-detection methodology,[30] the peak of iron was only visible in the XPS spectra of Fenton-modified lignins. The Gaussian peaks obtained through deconvolution of the C 1 s peaks are shown in Figure 7. These peaks are carbon-related functional groups and can be differentiated by distinct binding energies. The 285.5 eV peak was assigned to C C bonds; the chemical shifts relative to C C were 2.0 eV for C O bonds and 15.0 eV for C=O bonds.[31] The estimations of the amount of these three functional groups based on relative peak areas are summarized in Table 2. The results indicated a progressive decrease in C O bonds and an increase in C=O bonds from lignin, lignin with Fe3 + , to Fenton-modified lignin; this confirmed that the degree of oxidation of lignin increased as the amount of oxidizer increased. The area of C=O was 12.8 % in FLc, which was 6.4-fold higher than that in organosolv lignin. The oxidation of C O can be partially ascribed to cleavage of the b-O-4 linkages. In addition, the formation of C=O was independently confirmed by FTIR spectroscopy (Figure S1 in the Supporting Information). The intensity at n˜ = 1730 cm 1 of FLc was clearly en5


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Figure 7. Deconvolution of the carbon peak into C C, C O, and C=O peaks.

Table 2. Semiquantitative analysis of the chemical composition and functional groups in lignins by XPS. Lignin type C lignin lignin with Fe3 + FLa FLb FLc

64.9 65.3 62.7 61.1 61.2

Amount [%] Fe C C

O 35.1 34.7 37.0 38.6 38.4

[a]

ND ND[a] 0.3 0.3 0.3

45.8 45.8 48.7 49.4 55.8

C O

C=O

52.2 52.2 41.5 38.0 31.4

2.0 2.0 9.8 12.6 12.8

[a] ND = not determined.

Figure 8. The yields of char, liquid oil, and gas after the depolymerization of lignin, lignin with Fe3 + , and FLc under supercritical ethanol conditions (7 MPa, 250 8C).

3+

hanced relative to that of lignin and lignin with Fe , which indicated that the side chain of lignin was oxidized to unconjugated ketone or carbonyl structures.[32]

depolymerization in supercritical ethanol under nitrogen. This process resulted in the formation of a liquid phase, solid char, and trace amounts of gases. The yield of the products in liquid (Yliquid), char yield (Ychar), and gas yield (Ygas) are summarized in Figure 8; the results indicated that the depolymerization of the organosolv lignin without catalyst resulted in Ychar and Yliquid of (29.0 1.4) and (50 5.7) %, respectively. The addition of trace amounts of metal in lignin did not significantly change Yliquid,

Depolymerization of the Fenton-modified lignin in supercritical ethanol The effects of the Fenton catalyst on the depolymerization of lignin into aromatic monomers and other low-molecularweight compounds were investigated by comparing the reaction products from lignin, lignin with Fe3 + , and FLc following

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Full Papers distribution of lignin liquid oil analyzed by gel permeation chromatography (GPC) indicated that the liquid oil was a mixture of aromatic mono- and oligomers with an average molecular weight of less than 700 (Table S4 in the Supporting Information). The GC–MS spectra of the depolymerization products in the liquid phase of lignin, lignin with Fe3 + , and Fenton-modified lignin are shown in Figure 9 and the relative molar yields of identified main depolymerization products are shown in Figure 10. The identified compounds and their relative peak area percentages are summarized in Table S3 in the Supporting Information. The depolymerization products from FLc included muconic acid derivatives, including levulinic acid Figure 9. Gas chromatograms of organic oil from lignin, lignin with Fe3 + , and lignin with FLc. ethyl ester (1), adipic acid monoethyl ester (3), 3-hydroxypentabut increased Ychar to (36.0 2.8) %. However, the addition of nedionic acid (7), and ethyl hydrogen glutarate (8). These dicarFenton catalyst (FLc) resulted in a lower Ychar of (20.0 4.4) % boxylic acid derivatives have potential as precursors of nylon, lubricants, coatings, plastics, and plasticizers.[7] The relative and drastically increased Yliquid to (66.0 8.5) %. The molar mass

Figure 10. Relative yield of the main products.


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Full Papers content of adipic acid monoethyl ester (3) significantly increased from 2.5 (lignin) and 2.3 % (lignin with Fe3 + ) to 10.2 % (FLc; Figure 10). Commercial adipic acid monoethyl ester was used as a reference for GC–MS analysis to confirm the identification of peak 3. In addition to matches in retention time and mass spectra, when the depolymerization products were spiked with this compound, the peak area increased. The relative contents of 3-hydroxypentanedionic acid (7) and ethyl hydrogen glutarate (8) were increased when the Fenton catalyst was used. The levulinic acid ethyl ester was only observed in the depolymerization products from Fenton-modified lignin (Figure 10). The quantitative analysis of the products in the liquid phase suggested that the increase in the yield of dicarboxylic acids was consistent with the reduction of lignin-derived compounds (Table S3 in the Supporting Information). The relative content of 2,6-dimethoxy-4-(2-propenyl)phenol (26), derived from S units, decreased from 15.6 (lignin) and 17.5 % (lignin with Fe3 + ) to 8.3 % (FLc). 2-Methoxy-4-vinylphenol (10) and 3,5-dimethoxyacetophenone (20) all but disappeared from the depolymerization products of the Fenton-modified lignin.

Fenton-catalyzed lignin depolymerization mechanism The Fenton-catalyzed lignin depolymerization mechanism is proposed in Schemes 1 and 2. As shown in Scheme 1, a series of reactions occur during lignin depolymerization, which include 1) the Fenton reaction to form oxidant hydroxyl radicals to remove methoxy groups from G and S residues in lignin, 2) lignin–metal complex formation through spontaneous chelation, 3) depolymerization of lignin through b-ether cleavage, and 4) lignin ortho-quinone intermediate formation and ringopening reaction. First, H2O2 is decomposed into hydroxyl or perhydroxyl radicals in the presence of transition metals (Fe3 + /Fe2 + ). Then the radicals attack aromatic rings with methoxy groups; this results in the demethoxylation or hydroxylation of the ring, resulting in phenolic hydroxyl groups at the C3- and/ or C5-positions. A free electron is transferred through the ring to the side chain, leading to the cleavage of b-O-4 ether bonds and lignin depolymerization. The formed catechol moieties in lignin (with a phenolic hydroxyl group on the C4- and C3/C5positions) can chelate metal ions (e.g., Fe3 + ) to form a lignin– metal complex. The Py–GC–MS results verified that the methoxyl groups in the lignin modified by the Fenton catalyst were greatly reduced. The reduction of detectable phenolic hy-

Scheme 1. Demethoxylation, lignin–metal complex formation, b-ether cleavage, and the formation of the ortho-quinone structure.

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Full Papers droxyl groups (Figure 5) and the increase in iron content (Table S2 in the Supporting Information) in the Fenton-modified lignin provided strong evidence of the formation of the lignin–metal complex. The lignin–metal complex can act as a capping agent to prohibit intramolecular reactions, and thereby, reduce charcoal formation during subsequent thermal conversion. Previous studies indicated that the lignin depolymerization processes with the aid of capping agents, such as phenols,[33] boric acids,[33a] and formic acid,[12] which could either block the functional groups to reduce charcoal or stabilize aromatic radicals to suppress lignin recondensation, had an improved yield of phenolic compounds. Finally, the chelated monomers can also undergo one-electron transfer and reduce iron(III) to iron(II). The ortho-hydroxyphenoxyl radical will either form an ortho-quinone structure that undergoes a ring-opening reaction to form muconic acid derivatives (detected among the depolymerization products in supercritical ethanol conditions) or will be reduced to phenolic compounds (e.g., 4-hydroxybenzoic acid methyl ester), detected as the primary lignin degradation product by Py–GC–MS. Moreover, the catalytic role of absorbed iron in Fenton-treated lignin cannot be ignored. Iron is the most widely used metal catalyst in the Fischer–Tropsch synthesis of long-chain hydrocarbons.[34] Xu and Etcheverry applied an iron-based catalyst in hydroliquefaction of woody biomass and improved liquid oil yield from 17 to 63 %.[35] Iron-coated zeolite increased the yield of methylsubstituted phenols in bio-oil instead of methoxyl-substituted phenols.[36] In addition, the conversion pathway from lignin quinone monomers to dicarboxylic acids is proposed to occur via obenzoquinone (Scheme 2). A few studies indicated that ethanolysis of lignin led to the production of C8–C10 esters,[13a, 37] but no small molecular dicarboxylic acids (e.g., muconic acid or adipic acid) were detected in the degradation products. In this study, depolymerization of Fenton-treated lignin selectively produced the monoethyl ester of adipic acid (10.2 %; Table S3 in the Supporting Information), which was the product from the esterification reaction between adipic acid and ethanol. However, the reason for the formation of monoethyl ester through the esterification of adipic acid on only one end was not clear. Ethanol under supercritical conditions was reported to produce a potential hydrogen donor either in supercritical water (400 to 450 8C) or in the presence of a copper-based catalyst.[13a, 38] In the presence of a copper-based oxidizer, ethanol could suppress repolymerization through stabilizing lignin in-

termediates by alkylation to prevent the formation of quinone methide. In addition, ethanol itself could be converted into ethyl esters through hydrogenation under supercritical conditions. However, in this study, these alkylation or hydrogenation products have not been detected by GC–MS. This indicated that esterification between dicarboxylic acids with ethanol was a dominant reaction in comparison with ethanol hydrogenation and alkylation reactions when dicarboxylic acids from the lignin ring-opening reaction existed in the system. Adipic acid has been selected as a key high-value product from lignin by the National Renewable Energy Laboratory (NREL) to improve the overall economic feasibility of the biofuel industry.[5, 39] The generation of muconic acid derivatives was observed in ozone oxidation[40] and peroxyacetic acid oxidation of lignin.[41] In addition, Zuckerman, et al. reported a chalcopyrite/H2O2 system that could selectively produce dicarboxylic acids derived from muconic acid under mild conditions.[42] Levulinic acid ethyl ester (3.1 %) is proposed to be obtained from lignin quinone structures with carboxylic acid side groups through a series of reactions, including ring opening, intramolecular Michael addition, decarboxylation, and then the reaction with ethanol (Scheme 2). The esterification reaction of dicarboxylic acid with ethanol only occurred in one end, potentially because reaction at the other end was blocked by metal ions through chelation. The detailed mechanism will be further verified in the future by using lignin model compounds. Although the production of dicarboxylic acids (e.g., adipic acid) has been reported,[39] the separation of these products from aromatic mixtures will be a technical challenge that requires further investigation.

Conclusions Lignin modification with the biomimetic Fenton catalyst followed by lignin depolymerization with supercritical ethanol (7 MPa, 250 8C) could effectively depolymerize organosolv hardwood lignin into organic liquid oil at a conversion yield of (66.0 8.5) %, which was significantly higher than typical yields reported in the literature. The organic liquid contained mainly phenols, aromatic hydrocarbons, dicarboxylic acids, and their esters. Based on the combined results from this study, we propose that the Fenton catalyst demethoxylates lignin, and that the formation of a metal chelating complex stabilizes quinone methide intermediates and prevents recondensation of these breakdown products to char; thus enhancing the yield of liquid products.

Scheme 2. The proposed reaction to form the main ring-opening chemicals (adipic acid monoethyl ester and levulinic acid ethyl ester).


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Full Papers 320 8C (at 20 8C min 1), with a 1.5 min final hold. The transfer line was set at 280 8C. The mass spectrometer was operated in EI mode with 70 eV electrons, with the detector voltage set at 900 V. The mass range included m/z 45–450 and was scanned every 0.2 s. Identification of compounds was based on prior reports,[44] All experiments were performed in duplicate.

Experimental Section Materials and chemicals The organosolv lignin used for this experiment was provided by Lignol (Vancouver, Canada). It was prepared from a mixture of the hardwood species beech (Fagus grandifolia) and birch (Betula spp.) and was free of polysaccharides. Fe(NO3)3·9 H2O, hydrogen peroxide solution (30 %), anhydrous pyridine, deuterated chloroform, and chromium acetylacetonate were purchased from Sigma–Aldrich (St. Louis, MO, USA). N-Hydroxynaphthalimide was obtained from Fisher Scientific (Pittsburgh, PA, USA).

TGA: TGA was carried out with a TGA system from Mettler (Toledo, OH, USA) to compare the thermal decomposition behavior of lignin before and after modification by Fe3 + and Fenton catalyst. Thermal analyses of original and modified lignin samples were conducted from 0 to 800 8C at a heating rate of 10 8C min 1. Analyses were performed under a flow of nitrogen (20 mL min 1). XPS: XPS analysis was performed on a PHI 5100 XPS instrument (PerkinElmer, Waltham, MA) to investigate the surface chemistry of lignin.

Fenton-catalyzed lignin depolymerization Fenton modification: Lignin modification by the Fenton catalyst was performed in a flask at room temperature with stirring at 120 rpm. First, the hardwood organosolv lignin was suspended in pure ethanol to a concentration of 10 g L 1. Next, the Fenton reagent [Fe(NO3)3·9 H2O and hydrogen peroxide] at different H2O2/ Fe3 + ratios was added to the lignin suspension at a Fe(NO3)3·9 H2O loading of 1 g L 1 to initiate the reaction. The molar ratios of H2O2/ Fe3 + investigated were 0 (Fe(NO3)3·9 H2O and lignin without hydrogen peroxide), 1.98 (FLa), 3.96 (FLb), and 7.92 (FLc). The reaction was terminated after 30 min by adding an excess of water. After the reaction, modified lignin samples were collected by filtration on a filter paper, freeze dried, stored at room temperature in a desiccator, and used for Py–GC–MS analysis and depolymerization in supercritical ethanol. All experiments were performed in duplicate.

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P NMR spectroscopy: Quantitative 31P NMR spectroscopy analysis was performed as previously reported.[45] Lignin (40 mg) was dissolved in anhydrous pyridine (500 mL) and deuterated chloroform (1.6:1, v/v), along with N-hydroxynaphthalimide (200 mL) as an internal standard (11.4 mg mL 1 in pyridine and deuterated chloroform) and chromium acetylacetonate (0.28 mg). Next, phosphitylating reagent (2-chloro-1, 3, 2-dioxaphospholane; 100 mL) was added. The mixture was stirred for 10 min and subjected to NMR spectroscopy analysis. The NMR spectra were collected on a Bruker Avance III 500 MHz spectrometer equipped with a 5 mm SMART probe at 27 8C with the following parameters: 1 s acquisition time, 5 s relaxation delay, and 218 scans.

Depolymerization of lignin in supercritical ethanol: Depolymerization of lignin, lignin with Fe3 + , and Fenton-modified lignin in supercritical ethanol was conducted in a 100 mL Parr reactor (Parr Instruments Co., model 4593). Lignin (500 mg) was placed in the reactor along with ethanol (50 mL) under N2. The reaction was carried out at 7 MPa and 250 8C with stirring at maximum speed for 3 h. The liquid phase was separated from solid particles by filtration, evaporated to dryness, and redissolved in dichloromethane. The solids were washed with distilled water, dried in an oven (120 8C), and weighed. The products recovered from the liquid phase were analyzed by GC–MS on a QP 2010S system (Shimadzu Co., Addison, IL) equipped with a SHRXI-5MS column (30 m 0.25 mm id., 0.25 mm film). Helium was used as the carrier gas at a flow rate of 1 mL min 1 with a 1:20 split ratio. The GC oven was programmed with an initial temperature of 100 8C (held for 1 min), followed by heating to 310 8C at 5 8C min 1, and then held at the temperature for 7 min. The transfer line was set at 250 8C. The mass spectrometer was operated in electron ionization (EI) mode with 20 eV electrons. The detector gain was 0.93 kV. All experiments were performed in duplicate.

Acknowledgements We acknowledge assistance from the National High Magnetic Field Laboratory and McKnight Brain Institute of the University of Florida for the NMR spectroscopy measurements. This project was supported by the Biomass Research & Development Initiative Competitive Grant no. 2001-10006-3058 from the USDA National Institute of Food and Agriculture (Z.T., W.V.); by the U.S. Department of Energy’s International Affairs under award number DEPI0000031 from the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy, Bioenergy Technologies Office (Z.T., W.V.); and by an Early Career Award from the University of Florida Institute of Agricultural and Food Science (IFAS),(Z.T., W.V., X.P.). Keywords: biomass · depolymerization · carboxylic acids · iron · lignin

Characterization Py–GC–MS: The effect of Fenton modification on the lignin subunit composition was determined by Py–GC–MS according to the method described by Saballos et al.[43] Duplicate samples consisting of approximately 1–2 mg of native or modified lignin were pyrolyzed by using a Varian 1079 PTV injector mounted on a Varian 3800 GC connected to a Varian 1200 MS. The pyrolysis temperature was 450 8C. The pyrolysate was led onto a SGE BPX5 column (25 m, 0.32 mm i.d.) with helium as the carrier gas by using a 1:10 split ratio and a column flow rate of 1.2 mL min 1. The GC oven was programmed with an initial temperature of 70 8C (held for 4 min), followed by increasing the temperature to 250 (at 5 8C min 1) and

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Received: October 15, 2014 Revised: November 24, 2014 Published online on && &&, 0000

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FULL PAPERS Stripped back: Fenton catalyst (Fe3 + , H2O2) effectively facilitates lignin depolymerization in supercritical ethanol to give liquid products in high yield. The results also support the formation of a lignin–iron chelating complex as an intermediate stabilizer that enhances the yield of the liquid products.

J. Zeng, C. G. Yoo, F. Wang, X. Pan, W. Vermerris, Z. Tong* && – && Biomimetic Fenton-Catalyzed Lignin Depolymerization to High-Value Aromatics and Dicarboxylic Acids

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Metallo-deuteroporphyrin as a biomimetic catalyst for the catalytic oxidation of lignin to aromatics.

Selective conversion of biorefinery lignin into dicarboxylic acids.

Tandem Catalytic Depolymerization of Lignin by Water-Tolerant Lewis Acids and Rhodium Complexes.

Catalytic depolymerization of lignin in supercritical ethanol.

Hydrolytic depolymerization of hydrolysis lignin: Effects of catalysts and solvents.

Metabolism of dicarboxylic acids in rat hepatocytes.

C.

Aromatic monomers by in situ conversion of reactive intermediates in the acid-catalyzed depolymerization of lignin.

Formation and degradation of dicarboxylic acids in relation to alterations in fatty acid oxidation in rats.

In vitro depolymerization of lignin by manganese peroxidase of Phanerochaete chrysosporium.

Effects of mesostructured silica catalysts on the depolymerization of organosolv lignin fractionated from woody eucalyptus.

Depolymerization of organosolv lignin using doped porous metal oxides in supercritical methanol.

Study on dicarboxylic acids in aerosol samples with capillary electrophoresis.

Formation of C-4 dicarboxylic acids by intact spinach chloroplasts.

Interaction of C-nitroso aromatics with polyunsaturated fatty acids: route to lipid peroxidation.

Structural alterations of amino acids at the level of aminoacyl-tRNAs: transformation of dicarboxylic amino acids.

Lignin cross-links with cysteine- and tyrosine-containing peptides under biomimetic conditions.

Heptagons in Aromatics: From Monocyclic to Polycyclic.

Malathion exposure studies. Determination of mono- and dicarboxylic acids and alkyl phosphates in urine.

High-performance liquid chromatographic analyses of hydroxymonocarboxylic acids and dicarboxylic acids in urine as their 2-nitrophenylhydrazides.

Amides of antibiotic streptonigrin and amino dicarboxylic acids or aminosugars. Synthesis and biological evaluation.

Isoquercitrin Esters with Mono- or Dicarboxylic Acids: Enzymatic Preparation and Properties.

Transport of malic acid and other dicarboxylic acids in the yeast Hansenula anomala.

Synthesis, properties and applications of biodegradable polymers derived from diols and dicarboxylic acids: from polyesters to poly(ester amide)s.