Free-Radical Analysis on Thermochemical Transformation of Lignin to

Dec 10, 2012 - MWL of flax shives(14), 8825, 2337, 3.77. organosolv pulping lignin (alfa grass)(30), 3230, 1300, 2.48. MWL of elephant grass (pith)(31...
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Free-Radical Analysis on Thermochemical Transformation of Lignin to Phenolic Compounds Jun Hu, Dekui Shen, Rui Xiao,* Shiliang Wu, and Huiyan Zhang Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, Southeast University, Nanjing 210096, People’s Republic of China ABSTRACT: The chemical characteristics of lignin isolated from industrial black liquor were identified by gel permeation chromatography (GPC), Fourier transform infrared (FTIR) spectroscopy, and two-dimensional (2D) heteronuclear singlequantum coherence (HSQC) nuclear magnetic resonance (NMR), concerning its average molecular weight, distribution of typical interunit linkages, and functional groups. The frequency of β−O−4 linkage was determined to be 17−28/100 C9 units by 2D NMR, while the content of unit [guaiacol (G), syringol (S), and p-hydroxyphenyl (H)] presents a ratio of 7:2:1.5 for G/S/H. The mass-average molecular weight of lignin was characterized to be 2238 g/mol by GPC analysis. The low polymerization degree of the units in lignin leads to the substantial extent of interunit linkage cleavage at low temperatures. The guaiacol-, syringol-, and phenol-type compounds from fast pyrolysis of lignin in a pyroprobe at 500 °C were notably identified by gas chromatography−mass spectrometry (GC−MS) and presented a ratio of the peak area as 7:2:1. More fragments were observed at higher temperatures from pyrolysis (Py)−GC−MS analysis, because of the commencement of demethoxylation and cracking of side chains. The scheme concerning the cleavage of characterized interunit linkages in lignin was proposed to produce the free radicals. The side chains on the free radicals were preferably to crack on β-site bonds to produce a number of methyl phenolic compounds. The methoxyl group was intensively cracked with the increased temperature because of its high bond dissociation energy (BDE), giving rise to the notable increase of cresol-, phenol-, and catechol-type compounds under high temperatures. pyrolysis by Py−GC−MS.15 Jiang et al. examined the effect of the temperature on fast pyrolysis of lignin (Alcell lignin and Asian lignin), finding that the maximum yield of phenolic compounds was obtained at 600 °C. Moreover, it was reported that the demethylation, demethoxylation, decarboxylation, and alkylation were all promoted at higher temperatures.16 Patwardhan et al. investigated the pyrolysis mechanism of corn stover lignin assisted with a micropyrolyzer coupled with a GC−MS/flame ionization detector (FID), reporting monomeric compounds as the primary pyrolysis products, which can further combine to produce oligomeric compounds.17 It should be admitted that the thermal performance of lignin was tightly related to its chemical structure.18 Therefore, the chemical pathways for lignin pyrolysis should be correlated to its chemical structural, including the interunit linkages and typical functional groups, to gain the inherent mechanism for thermochemical transformation of lignin to valuable chemicals. A large amount of black liquor was emitted annually from the industrial pulping process, being harmful for the underwater and soil systems. Lignin, as a substantial byproduct from black liquor, contains various active functional groups, which makes it a potential kind of feedstock for the production of energy, fuels, and chemicals through thermochemical methods.1,2,19 In this work, the black-liquor lignin was prepared through condensation, extraction, and purification processes, the structure of which was characterized by gel permeation chromatography (GPC), FTIR spectroscopy, and 2D HSQC NMR, giving

1. INTRODUCTION Lignin is a polymer with a complex three-dimensional (3D) structure of polymerized phenyl propane (C9) units (phydroxyphenyl unit, guaiacyl unit, and syringyl unit) through ether bonds or carbon−carbon linkages.1−3 The monomeric units and interunit linkages in lignin were significantly varied with its origins and preparation methods.4,5 A great number of previous studies were conducted to specify the chemical information of different kinds of lignins through wet chemistry methods (such as oxidative degradation, thioacidolysis, and titration) and/or spectroscopic techniques [such as Fourier transform infrared (FTIR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy].6−8 During recent years, 1H and 13C NMR spectroscopies were widely used to characterize the chemical structure of lignin.9,10 However, the overlap of several resonance signals because of its one-dimensional property limited its precision and further application. The challenge was gradually overcome by 1H NMR coupled with the higher resolution of 13C NMR and two-dimensional (2D) 1 H−13C NMR [such as heteronuclear single-quantum coherence (HSQC)].8 Fast pyrolysis is estimated as a promising technique to convert lignin to gas, liquid, solid fuels, and chemicals (especially phenolic compounds).11 Pyrolysis−gas chromatography−mass spectrometry (Py−GC−MS) was widely used to investigate the structural characterization and fast pyrolysis of lignin. Residue lignin from eucalyptus kraft pulps and milled wood lignin (MWL) of elephant grass were tested in Py−GC− MS by Rió et al., presenting the lignin/carbohydrate and syringol/guaiacol ratios.12−14 Fahmi et al. predicted the lignin content in grass through the variation of markers from lignin © 2012 American Chemical Society

Received: October 10, 2012 Revised: December 8, 2012 Published: December 10, 2012 285

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39.5/2.49 was used as the internal standard. Pulse widths of 8.35 and 16.00 μs were used for protons and carbons, respectively. 2.2.4. Py−GC−MS. The fast pyrolysis analyzer (CDS5250) was coupled with GC−MS (trace DSQII) to investigate the distrubition of products from lignin. About 0.5 mg of lignin sample was loaded in the pyrolysis tube, and the pyrolysis temperature was set to be 500, 700, and 900 °C with the flash heating rate of 20 000 °C/s. Because of the poor thermal conductivity of lignin and the high heating rate, the actual pyrolysis temperature for lignin was about 100 °C lower than the fixed value.22 The residence time for the sample was 30 s, to ensure that most of the solid sample was pyrolyzed. The evolved volatiles were identified by GC−MS, with the conditions as follows: the injector temperature was kept at 300 °C; the chromatographic separation was performed with a TR-35MS capillary column; the oven temperature was programmed from 40 °C (3 min) to 200 °C (1 min) with a 4 °C/min heating rate and then to 280 °C (1 min) with a 20 °C/min heating rate; and the mass spectra were operated in electron ionization (EI) mode at 70 eV. The mass spectra were obtained from m/z 50 to 650. The chromatographic peaks were identified according to the National Institute of Standards and Technology (NIST) MS library and previously published work.12−16

guidance for the following pyrolysis analysis. The fast pyrolysis behaviors of the lignin sample were examined by Py−GC−MS, concerning the distribution of prominent products and their variation against temperature. On the basis of the information provided by structure characterization and a fast pyrolysis experiment, a series of free-radical reactions were proposed to present the cleavage of characterized interunit linkages and the formation of typical phenolic compounds during fast pyrolysis of lignin. The kinetic relationship for the free-radical reactions was estimated through the bond dissociation energy (BDE) analysis, specifying the preference of the formation of the phenolic compound.

2. MATERIALS AND METHODS 2.1. Materials. The solvents used in the study were purchased from Nanjing Reagent Co. and used as received. The black liquor used in our work was obtained from a pulping company directly. Black liquor is a condensed byproduct from an alkali pulping company in Hunan Province, China, where Chinese polar and reed are used as the main feedstock. Lignin was precipitated from black liquor by 50 wt % H2SO4. Black liquor was acidified to a pH value of about 2 with H2SO4 and stirred for 1 h at 55 °C. Then, the acidified black liquor was filtrated under vacuum. The filtrated cake were collected as isolated lignin and washed with deionized water to neutral. After drying at 80 °C for 12 h, the isolated lignin was extracted with benzene/ethanol (1:2, v/v) for 12 h in a Soxhlet extractor at 90 °C to remove some low-molecular-weight sugars. The extracted liquid was heated at 40 °C under 0.4 MPa to remove the benzene/ethanol solvent, and the remaining solid lignin was dried at 80 °C for 12 h. Further purification for benzene/ethanolextracted lignin was carried out according to the method by Lundquist.19 Lignin extracted was dissolved in pyridine/acetic acid/ water (9:1:4, v/v/v) and then extracted with chloroform. After shaking and stewing, the organic layer were separated and heated at vacuum to remove the solvents. The solid lignin obtained was dried at 80 °C for 12 h. The structure characteristic and fast pyrolysis behavior of other lignins carried out by previous researchers were compared in this work, showing the effects of different preparing methods. Specific preparing processes were introduced in published works.20,21 MWL was mostly prepared according to the classical procedure proposed by Björkman.20 In brief, biomass was extracted with dioxane/water (9:1, v/v) to isolate the crude lignin, and this crude lignin were further purified by redissolving in acetic acid/water (9:1, v/v) and 1,2-dichloroethane/ ethanol (2:1, v/v) to obtain the purified MWL. As for ethanol organosolv lignin (EOL), the ethanol cooking pulp was washed with ethanol/water (8:1, v/v) and then precipitated with water added to obtain the EOL.21 2.2. Methods. All of the characterizations were carried out 2 or 3 times, and the statistics were normally presented as the average value of the runs. 2.2.1. GPC. The weight-average molecular weight (Mw), numberaverage molecular weight (Mn), and polydisperity degree (Mw/Mn) of lignin were determined by GPC analysis. The GPC analysis was carried out on 1 mg of lignin dissolved in 1 mL of tetrahydrofuran (THF) using Agilent 1100 high-performance liquid chromatography (HPLC) with an ultraviolet detector set at 280 nm. The sample (50 μL) was injected into a system of columns connected in series (HR5E and HR1). The analysis was carried out using THF as the eluent at a flow rate of 1 mL/min. Polystyrene was used as standards for calibration. 2.2.2. FTIR Spectroscopy. The FTIR spectra of the lignin were recorded on a FTIR spectrophotometer (Bruker Vector 22) using a KBr disc containing 1% samples. The scan was conducted in the range from 4000 to 400 cm−1. 2.2.3. 2D HSQC NMR. The 2D HSQC NMR was performed on 80 mg of lignin dissolved in 600 μL of dimethylsulfoxide-d6 (DMSO-d6) using a Bruker AV400 spectrometer. The central peak of DMSO-d6 at

3. RESULTS AND DISCUSSION 3.1. Structure Characterization. 3.1.1. GPC. The massaverage molecular weight (Mw), number-average molecular weight (Mn), and polydisperity degree (Mw/Mn) for the blackliquor lignin were shown in Table 1, together with the data Table 1. Summary of Mass-Average Molar Mass (Mw), Number-Average Molar Mass (Mn), and Molar Mass Dispersities (Mw/Mn) of Lignins sample

Mw

Mn

Mw/Mn

black-liquor lignin (present work) MWL of poplar wood24 MAL of poplar wood24 MWL of Miscanthus21 ethanol organosolv lignin of Miscanthus21 MWL of stem wood of loblolly pine17 black liquor from oil palm29 MWL of flax fibers14 MWL of flax shives14 organosolv pulping lignin (alfa grass)30 MWL of elephant grass (pith)31

2548

1254

2.03

4710 3670 13700 7060

2730 2060 8300 4690

1.73 1.78 1.65 1.51

7310

3800

1.92

1980−2160

1390−1530

1.40−1.43

7620 8825 3230

2123 2337 1300

3.59 3.77 2.48

6720

2490

2.7

reported in previous studies. The Mw and Mn are 2548 and 1254 g/mol for black-liquor lignin, respectively, and the polydispersity value is 2.03. It could be found that lignin obtained from black liquor has a relative lower molecular weight (Mw around 2000 g/mol) than those lignins isolated from other methods (Mw from 3600 to 11000 g/mol; Table 1). It indicates that the depolymerization of lignin, such as the cleavage of interunit linkages, occurred to a considerable extent during the pulping process. The polydisperity degree seems to be independent with the origin of lignin, despite isolation methods. 3.1.2. FTIR Analysis. The FTIR spectra of the black-liquor lignin are shown in Figure 1, and the notable peak assignment is referenced from previous works.18,23 Typical signals for the O−H stretching vibration (3425 cm−1, peak 1 in Figure 1), C− H stretching vibration (2920 cm−1, peak 2 in Figure 1), CO 286

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CO stretching between 1720 and 1770 cm−1 is also absent, ascribed to the overlapped signals of unconjugated ketones, carbonyls, and esters. These phenomena indicate the possible carbonyl oxidation as well as the ester and ether cracking of light catalytic cracking (LCC) during the pulping process. 3.1.3. 2D HSQC NMR. The side chain (δC/δH 50−90/2.5− 6.0) and the aromatic region (δC/δH 100−135/5.5−8.5) in the 2D HSQC NMR spectrum of the black-liquor lignin are shown in Figure 2. The little correlation observed at the anomeric region of associated carbohydrate (δC/δH 90−105/3.9−5.4) and much lower intensity in the polysaccharide/lignin sidechain domain, especially in the δC/δH 65−80/3−4 range (Figure 2), indicate a little amount of polysaccharide residue.24 The absence of those correlation signals clearly confirms a good purification effect to eliminate the polysaccharides. 3.1.3.1. Interunit Linkages. Three typical linkage bonds in lignin, termed β−O−4′, β−β′, and β−5′ bonds, were marked in 2D HSQC NMR spectra (Figure 2), the frequencies of which per 100 C9 units are listed in Table 2. The β−O−4′ bond is determined to be the predominant interunit linkage, corresponding to the intense correlation identified at δC/δH of 72/ 4.8 (Cα−Hα of β−O−4′ linkage, Aα and Aα′), 84/4.3 [Cβ− Hβ of β−O−4′ linkage of G units, Aβ(G)], 87/4.1 [Cβ−Hβ of β−O−4 linkage of S units, Aβ(S)], 61/3.5 (Cγ−Hγ of β−O−4 linkage, Aγ), and 62/4.2 (Cγ−Hγ of β−O−4 linkage, Aγ′). The acylation at the γ carbon in β−O−4′ lingkages of A forms the structure of A′. The β−β′ linkage in resinol was identified with the strong correlation at δC/δH of 85.5/4.6 (Cα−Hα of β−β′

Figure 1. FTIR spectra of black-liquor lignin.

stretching vibration in unconjugated carbonyls (1715−1705 m−1, peak 3 in Figure 1), aromatic skeletal vibrations (1510− 1505 cm−1, peak 4 in Figure 1), and C−O/CO stretching vibrations in guaiacyl rings (1270 cm−1, peak 5 in Figure 1; 1220 cm−1, peak 6 in Figure 1) can be observed from the spectra. The conjugated carbonyls at 1660 cm−1 ascribed to the coumaryl ether or aldehyde groups in FTIR spectra of MWL21,23 cannot be observed from that of black-liquor lignin.

Figure 2. 2D HSQC NMR spectra of black-liquor lignin and three typical interunit linkages: (a) aliphatic region and (b) aromatic region. 287

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Table 2. Frequency of Typical Interunit Linkages in Lignin Per 100 C9 Units

a c

linkage

β−O−4′

β−5

β−β

black-liquor lignin (present work)a MWL from pine6 b MWL from Norway spruce8 a MWL from beech8 a MWL from beech25 b,c kraft lignin from spruce8 a

16 41 44.7 60.3 65 22.7

5 9 10.6 1 6

3 2 3.19 8 5.5 3.9

Determined by 2D HSQC NMR. Determined by degradation.

b

Determined by

13

Table 3. Relative Molar Distribution of the Main Products for Black-Liquor Lignin in Py−GC−MS under Different Temperatures relative content

C NMR

linkage, Bα), 52/2.9 (Cβ−Hβ of β−β′ linkage, Bβ), and 72/ 3.7−4.2 (Cγ−Hγ of β−β′ linkage, Bγ). Signals at δC/δH 87/5.5 (Cα−Hα of β−5′ linkage, Cα), 53/3.5 (Cβ−Hβ of β−5′ linkage, Cβ), and 62/3.6 (Cγ−Hγ of β−5′ linkage, Cγ) can indicate the existence of the β−5′ linkage. The distribution of three main interlinkages per 100 C9 units of lignin is described in Table 3. The frequency of β−O−4′ is identified to be 16 per 100 units, which is much less than that on MWL (40−50 per 100 C9 units).6,8,25 It might be attributed to the hydrolysis reaction of ether bonds during the pulping process.7,21 The frequency of β−β′ and β−5′ linkage presents the amount of 3 and 5−6 per 100 C9 units, being close to those in MWL (2−4 per 100 C9 units).6 The lower frequency of interunit linkages per 100 C9 units in black-liquor lignin contributes to the low molecular weight, as characterized by GPC analysis. 3.1.3.2. Aromatic Region and Functional Groups. The prominent spectra correlation in the aromatic region is observed at δC/δH of 112/6.8, 116/6.6, and 120/6.7 for C2− H2, C5−H5, and C6−H6 in G units. Signals at δC/δH of 129/ 6.95 can be assigned to C2−H2 in H units, while C5−H5 in the H unit is overlapped by that in G. Notable signals at 105/6.7 can be identified as C2,6−H2,6 in S units. The ratio of the content of G/S/H is estimated to be about 7:2:1.5 through integration of the peak area of the corresponding NMR spectra. 3.2. Py−GC−MS Analysis. The GC spectrum of products from fast pyrolysis of black-liquor lignin at the temperature of 500, 700, and 900 °C was presented in Figure 3. Notable peaks were identified by the NIST library and previous studies.12−16 The identified compounds can be categorized into six groups (Table 4): guaiacol type, syringol type, phenol type, cresol type, catechol type, and others.26 The guaiacol-type compounds are dominant in the list, accounting for 49.1% (black-liquor lignin) of total GC spectrum peak areas at 500 °C. The peak area ratio of guaiacol-type/syringol-type/phenol-type compounds is determined to be about 7:2:1, which is in close accordance with the content ratio of three basic C9 phenylpropanoid units (G/S/H) of 7:2:1.5 in black-liquor lignin. Comparatively, cresol type, catechol type, and others were presented in a small amount at this temperature (total of 8.08%). This implies that the pyrolysis of lignins under 500 °C is mainly attributed to the cleavage of interunit linkages and partly to the cracking of side chains, while the methoxy group and phenolic hydroxyl groups are rarely fragmented. A variation of product distribution against temperature can be found in Figure 4 and Table 4. The production of guaiacoltype compounds was significantly decreased from 49.1% at 500 °C to 27.73% at 900 °C, while that of syringol-type compounds was also decrease from 5.83 to 2.75%. Comparatively, the amount of phenol, cresol, and catechol types was notably

number

RT (min)

G1 G2

18.86 22.01

G3 G4 G5 G6 G7 G8

22.67 25.53 26.94 27.76 28.63 29.85

G9 G10

30.21 31.34

G11

32.82

G12

33.93

G13

34.8

S1

28.3

S2

33.43

S3

35.69

S4 S5

37.91 38.66

S6

39.75

P1 P2

15.4 22.26

Cr1 Cr2 Cr3

17.98 21.4 26.09

Cr4

29.04

Ca1 Ca2

24.1 27.17

O1 O2

24.39 35.54

product compounds guaiacol 2-methoxy-6methyphenol(5methyphenol) 2-methoxy-4-methylphenol 2-methoxy-4-ethylphenol 2-methoxy-4-vinylphenol 3-methyl-5-methoxyphenol 2,4-dimethoxyphenol phenol,2-methoxy-4-(1propenyl) vanillin phenol,2-methoxy-4-(1propenyl) ethanone,1-(4-hydroxy-3methoxyphenyl) 2-propanone,1-(4-hydroxy3-methoxyphenyl) 4-((1E)-3-hydroxy-1propenyl)-2methoxyphenol guaiacol type (sum) syringol 2,4,6-trimethoxytoluene 3,4,5-trimethoxytoluene phenol,2,6-dimethoxy-4-(2propenyl) syringaldehyde phenol,2,6-dimethoxy-4-(2propenyl) ethanone-1-(4-hydroxy-3,5dimethoxyphenol) syringol type (sum) phenol 4-ethylphenol phenol type (sum) o-cresol (m-cresol, p-cresol) phenol, 26-dimethyl (2,3) 2-methyl-1,3-benzendiol 1,4-benzenediol, 2,6dimeththyl (1,4-benzenodiol, 2,5dimethyl) cresol type (sum) catechol 4-methyl-1,2-benzendiol catechol type (sum) benzofuran 1,2-dimethoxy-4-npropylbenzene others (sum)

500 °C 700 °C 900 °C 7.22 0.78

5.3 0.44

4.6 0.15

9.98 6.21 3.01 0.54 0.57 0.4

6.68 3.78 2.9 0.46 0.51 0.39

4.44 2.33 2.36 0.41 0.36

1.05 4.05

1.38 2.81

1.5 1.96

0.85

0.72

0.42

0.83

0.68

0.55

0.43

0.48

0.36

49.1 3.47

35.97 1.36

27.73 1.45

0.92

0.46

0.27

0.44

0.41

0.63

0.23 0.59

0.39 0.57

0.4

0.18

0.3

5.83 1.23 1.86 3.09 0.34 0.4 0.86

3.49 1.79 1.56 3.35 0.92 1.08 1.2

2.75 2.18 2.14 4.32 1.1 1.45 1.45

0.23

0.46

0.65

1.83 2.22 1.68 3.9 1.97 0.38

3.66 2.57 2.92 5.49 2.25 0.32

4.65 3.13 3.85 6.98 2.69

2.35

2.57

2.69

increased with the temperature, presenting from 3.09 to 4.32%, from 2.65 to 7.05%, and from 3.08 to 4.58%, respectively (Table 4). It indicates that the demethoxylation reaction was significantly favored with the increased temperature.26 The substitution of the methyl group on the aromatic ring was enhanced to produce the cresol-type compounds, most of which originated from the methoxyl group.18 The dealkylation reaction of the side chain was also promoted under higher 288

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OCH3 groups would be discussed by means of BDE analysis in the following parts. 3.3. Possible Chemical Pathways for Fast Pyrolysis of Lignin. Understanding the inherent chemical mechanism may enable us to control the pathways to produce specific products. According to the Py−GC−MS analysis of black-liquor lignin at different temperatures, a two-step reaction scheme was proposed: the cleavage of characterized interunit linkages and subsequent free-radical reactions for the formation of the phenolic compounds. The standard BDE of the typical bonds (mainly C−H, C−C, C−O, and O−H bonds) in lignin and possible free radicals formed during pyrolysis was summarized in Table 4, analysis of which helps to understand and verify the proposed chemical pathways in Figures 4 and 5. 3.3.1. Cleavage of Interunit Linkages. The depolymerization of lignin under low temperatures was considerable through the cleavage of interunit linkages (such as α−O−4 and β−O−4 bonds) because of their relative low BDE. Some monomeric compounds and free radicals were produced through the homolytic cleavage of interunit linkages (Figure 5). The Cβ−O bond is first cleaved for the β−O−4′ linkage because of its lowest bond dissociation energy (268 kJ/mol for C6H5O− C2H5), producing C9 monomeric free radicals (FR 1 and FR 2 in Figure 4). For β−5′ and α−O−4′ linkages, the homolytic cleavage of the Cα−O bond (268 kJ/mol for C6H5O−C2H5 in Table 4) gives a free radical (FR 3), following by the

Figure 3. Py−GC−MS spectra of black-liquor lignin at 500, 700, and 900 °C.

temperatures, contributing to the formation of small fragments. The competitive and/or consecutive kinetic relationship among the products was intensively discussed in the previous study,18 while the detailed chemical pathways involving cleavage of interunit linkages, cracking of side chains, and transformation of

Table 4. Summary of BDE of Typical Bonds in Lignin and Its Relevant Free Radicals32,33 a

a

Dissociation atoms or atomic groups are indicated in bold font. 289

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Figure 4. Cleavage mechanism for β−O−4, β−5, α−O−4, and β−β bonds in lignin.

subsequent Cβ−CAr′ cracking and H abstraction to produce a monomeric compound and the monomeric free radical (FR 4 in Figure 4). Disruption of the β−β′ linkage is initiated by the homolytic cleavage of the Cα−O bond followed by the Cβ−Cβ′ cracking to produce two monomeric free radicals (FR 6). 3.3.2. Formation of the Prominent Phenolic Compounds. The C9 monomeric radicals formed through cleavage of typical interunit linkages act as the important precursor for the production of phenolic compounds. It is notable that the BDE of the β site bonds in a free radical on α-position atom is much lower than that in the corresponding compound (such as 99.2

presenting the formation of the prominent phenolic compounds (Table 4). Guaiacol (G1) and (E)-2-methoxy-4-(prop-1-enyl) phenol (G10) were produced from the FR 1 formed by the cleavage of β−O−4 through chemical pathway 1-1 in Figure 5. The Car− •

Cα bond related to the free radical C6H5−CH2CH 2 CH3 with the BDE of 38.5 kJ/mol was favorably cracked to produce the free radical FR 7, followed by a H abstraction to produce guaiacol (G1). The cracking on the Cα−O bond followed by cracking on O−Cγ and a H abstraction explain the formation pathway of (E)-2-methoxy-4-(prop-1-enyl) (G10). Because of the relative low BDE of C−C compared to that of C−O, pathway 1-1 is preferable over pathway 1-2 to produce more guaiacol (7.22%) than (E)-2-methoxy-4-(prop-1-enyl) (4.05%) at 500 °C. The FR 2, another free radical from β−O−4 cleavage, can produce 4-(1,3-dihydroxypropyl)-2-methoxyphenol through H abstraction. It might be the important precursor for the formation of vanillin (G9) and 2-methoxy-4-vinylphenol (G5) through O−Cγ cracking (pathway 2-1) and Cβ−Cγ cracking (pathway 2-2). Cβ−Cγ has a much lower BDE of 308 kJ/mol (C6H5CH2−CH3) than that of O−C (396 kJ/mol for HO−CH2C6H5), giving rise to the greater amount of 2methoxy-4-vinylphenol (3.01%) than that of vanillin (1.05%) at

•

kJ/mol for CH 2 CH2−CH3 and 370.3 kJ/mol for CH3CH2− CH3), leading to the priority of cleavage during pyrolysis. The BDEs of C6H5−OCH3 and C6H5O−CH3 are 416.7 and 273 kJ/mol, while C6H5−OH and C6H5O−H have higher BDEs of 463 and 378 kJ/mol (Table 4). There, the cleavage of C−C bonds on side chains seems preferable over those on −OMe and −OH groups, because of the lower BDE of the free radicals on side chains. This speculation was also confirmed by the distribution of relevant compounds characterized by Py−GC− MS. The cleavage of C−C bonds on the side chain in free radicals from the cleavage of interunit linkages of lignin is specified in forms of a set of free-radical reactions (Figure 5), 290

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Figure 5. Formation pathway of the prominent products of lignin by the fragmentation reaction on side chains.

Figure 6. Possible H abstraction mechanisms of lignin radicals through disproportionation, displacement, and coupling reactions.

500 °C. 4-(3-Hydroxypropyl)-2-methoxyphenol was formed from FR 4 by H abstraction and then cracked to 2-methoxy-4methylphenol (G3) (pathway 3-1) or 2-methoxy-4-ethylphenol (G4) (pathway 3-2). Although Cβ−Cγ (308 kJ/mol for

C6H5CH2−CH3) presents a lower BDE than that of O−Cγ (similar to 396 kJ/mol for HO−CH2C6H5), the product content of 2-methoxy-4-ethylphenol (G4) (6.21% at 500 °C) is less than 2-methoxy-4-methylphenol (9.98% at 500 °C). This 291

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Figure 7. Possible cleavages on the methoxyl group during lignin pyrolysis.

•

CH2CH 2 CH3). The mentioned H abstraction reaction was summarized in Figure 6, regarding the disproportionation, displacement, and coupling reactions. At 500 °C, the Car−OH and Car−OCH3 bonds are slightly cleaved as a result of the analysis of distribution of compounds identified from Py−GC−MS. The proportion of guaiacol- and syringol-type compounds was significantly decreased with the increased temperature, implying that the demethoxylation and demethylation reactions on methoxyl groups were promoted to produce more phenol-, cresol-, and catechol-type compounds. The formation of phenol- and cresol-type compounds can be attributed to the demethoxylation of guaiacol- and syringol-type compounds, while that for catechol-type compounds was mainly produced by the demethylation reaction. A radical reaction mechanism for the formation of catechol and cresol is depicted in Figure 7.27,28 The formation of catechol can be initiated by the homolytic cracking of ArO−CH3, followed by hydrogen stabilization, while releasing of cresol can be the result of Ar−OCH3 cracking, followed by coupling with methyl free radical. Because of the higher BDE of Ar−OCH3 (416.7 kJ/mol in Table 4) than ArO−CH3 (273 kJ/mol in Table 3), cracking of ArO−CH3 is preferable than that of the former bond, giving the greater amount of catechol (3.9% at 500 °C, 5.49% at 700 °C, and 6.98% at 900 °C) than that of cresol (1.83% at 500 °C, 3.66% at 700 °C, and 4.65% at 900 °C).

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4. CONCLUSION Black-liquor lignin presents different structure characterization from other lignins, with lower molecular weight and less interunit bonds, and the content ratio of three kinds of C9 units (guaiacyl/syringyl/p-hydroxyphenyl) was identified to be 7:2:1.5. The thermal performance of black-liquor lignin in the fast pyrolysis system presented a two-step mechanism ascribed to the cleavage of interunit linkages under relative low temperatures and intensive cracking of side chains and methoxyl groups with the increased temperature. A possible pyrolysis mechanism was proposed mainly consisting of the interunit linkage cleavage to produce monomeric free radicals and the side chains cracking of those radicals to produce methyl phenolic compounds. This investigation consolidates the understanding of lignin fast pyrolysis and propels the thermochemical use of black-liquor lignin.

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ACKNOWLEDGMENTS The acknowledge was updated as “The authors greatly acknowledge the funding support from the projects supported by the National Natural Science Foundation of China (51106030), the National Basic Research Program of China (973 Program, Grant 2012CB215306, and 2010CB732206), and the Key University Science Research Project of Jiangsu Province (12KJB480005). The authors also acknowledge Prof. Zhu Xifeng and Zheng Yan from the University of Science and Technology of China for their technical support on Py-GC-MS.

phenomenon can be attributed to the fact that a great number of FR 16 was cracked on Car−Cα to produce guaiacol because of the low BDE of Car−Cα (38.5 kJ/mol for C6H5−

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