Auto-Catalytic Depolymerization of Alkali Lignin by Organic Bound

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Auto-Catalytic Depolymerization of Alkali Lignin by Organic Bound Sodium in Supercritical Ethanol Daliang Guo, Bei Liu, Yanjun Tang, Junhua Zhang, and Xinxing Xia Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01343 • Publication Date (Web): 12 Sep 2017 Downloaded from http://pubs.acs.org on September 17, 2017

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Auto-Catalytic Depolymerization of Alkali Lignin by Organic Bound Sodium in Supercritical Ethanol Daliang Guo,*,a,b Bei Liu,c Yanjun Tang,a Junhua Zhang,a Xinxing Xia,a a Engineering Research Center for Eco-Dyeing & Finishing of Textiles, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China b Key Laboratory of Biomass Energy and Material, Nanjing 210000, China c Key Laboratory of Pulp and Paper Science & Technology of Ministry of Education of China, Qilu University of Technology, Jinan 250353, China

ABSTRACT: In order to study the catalytic depolymerization process of alkali lignin by organic bound sodium in supercritical ethanol, the solid chars and liquid oils products were firstly prepared by using a high pressure autoclave at different temperatures and times. Then the properties of solid chars were analyzed by elemental analysis, FT-IR and SEM, and the composition of liquid oils was characterized by GC-MS. FT-IR and SEM results showed that the depolymerization process of alkali lignin catalyzed by organic bound sodium in sub- and supercritical ethanol formed different micron-sized spheres products, and the sphere size of the char products was obviously affected by the depolymerized temperature. The C/O and C/H ratios of the chars products also increased with depolymerized temperature increasing. GC-MS results indicated that the components of liquid oils obtained from sub- and super-critical ethanol mainly were ester and phenolic compounds, respectively. 1. INTRODUCTION

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The depleting stocks of crude have forced our world to find renewable resources as an alternative to petrochemical products.1,2 Lignin is the only renewable resource that can direct provide aromatic ring in nature.3,4 In recent years, more and more researches have been trying to use lignin replace fossil fuels for producing bio-based mono-phenol chemicals and other value-added chemicals. 5-8 Alkali lignin as a by-product of industrial pulping process is burned to produce heat and steam.9 With the increasing demand for renewable energy, the efficient utilization of alkali lignin has attracted worldwide interest. However, alkali lignin is a degraded and condensed lignin owing to delignification and coupling reactions during the soda pulping process, leading to that the selective depolymerization of alkali lignin into smaller phenolic molecular units is difficult.10 In addition, chemical action and solubilization in soda pulping process make sodium ions connected with lignin phenolic hydroxyl and carboxyl groups by chemical bonds, forming phenolic sodium and carboxylate sodium groups (organic bound sodium).

11

Our previous

researches indicated that the organic bound sodium in the chemical structure of alkali lignin can be considered as a key factor for converting it into value-added chemicals.12,13 Many previous studies have also suggested that thermochemical method (mainly pyrolysis) is considered as a potential method for converting lignin to phenolic chemicals.14-16 However, the total phenols yields obtained from alkali lignin pyrolysis process were about 60% of liquid oils, while the pyrolysis solid product yield were also reached 30% of lignin sample.11,12 This phenomenon mainly caused by that the pyrolysis reaction is difficult to inhibit the condensation reaction. Therefore, the inhibition of condensation reaction during lignin depolymerization process is an effective way to enhance the yield of liquid oil products. In recent years, supercritical solvent is considered to be a reaction system with the capability which can prevent


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the condensation reaction.17 Based on the related researches,18,19 the abundant hydrogen radicals released from supercritical solvent can couple with the intermediate products generated from lignin depolymerized process, thereby preventing the condensation reaction among the intermediate products. Specially, a number of studies have illustrated the strong effects of the reaction conditions on the product properties of lignin depolymerized in sub- and super-critical ethanol. Cheng et al.20 studied the degradation of alkali lignin for the production of bio-phenolic compounds, and found 89% yield of degraded alkali lignin was achieved in 50/50 (v/v) waterethanol at 300 oC for 2h under 5 MPa H2. Riaz et al.21 explored process parameters (pressure, time, formic acid) for producing high-yield and high-calorific bio-oil from sulfuric acid hydrolysis lignin in supercritical ethanol, and proposed that the effective liquefaction associated with supercritical ethanol resulted in high conversion of 92% and high bio-oil yield of 85 wt% at 350 oC and 30 min. The objective of the present study was to investigate the process and product properties of alkali lignin catalytic depolymerized by organic bound sodium in supercritical ethanol. For this purpose, alkali lignin with organic bound sodium was firstly prepared and analyzed by 1H NMR, 13

C NMR, SEM and TG. Secondly, alkali lignin was depolymerized in sub- and super-critical

ethanol at different temperatures (180, 210, 240, 270 and 300 oC) and times (1, 2, 4 and 8h) by using a laboratory autoclave to obtain the depolymerization products (solid chars and liquid oils). Thirdly, the morphology of the solid chars was characterized by SEM, and the component of the liquid oils was analyzed by GC-MS. Finally, the possible formation patterns of liquid oils compounds was proposed. 2. EXPERIMENTAL SECTION


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2.1 Material Preparation. Alkali lignin was separated from wheat straw soda-AQ pulping black liquor according to the previous researches.11, 22 The organic bound sodium loaded alkali lignin was prepared by impregnating method. A known amount of pure lignin was first added into a beaker. 0.1 mol/L NaOH solution was then added into the beaker, until it was completely dissolved, and the loading amount of NaOH was 10% at this time. The process of addition needed to be as gradual and slow as possible in order to avoid loading excessive NaOH solution. Finally, the lignin solution was dried, and then alkali lignin with organic bound sodium was obtained. The elemental and proximate analysis results of alkali lignin are provided in Table S1. 2.2 Depolymerization Experiment. The depolymerization experiment of alkali lignin was conducted in a 100 mL SLP high pressure autoclave (Beijing Century Senlong experimental apparatus CO., Ltd). In a typical run, 0.6 g alkali lignin was firstly loaded into the reactor with 30 mL anhydrous ethanol. Secondly, the reactor was sealed and allowed to run a pre-specified temperature i.e. 180~300 oC and the reaction time i.e.1~8 h. As the depolymerization reaction finished, the reactor was shock cooled down with water. After the reactor was cooled down to room temperature, the depolymerization products were poured into a beaker, and the reactor was rinsed with anhydrous ethanol. Thirdly, the mixture of depolymerization products and ethanol washing liquid was filtered through 0.2 um microporous filtering film under vacuum. The retentate was dried at 50 oC for 8 h, and then the solid char product was obtained. The permeating liquid product was diluted to 500 mL with anhydrous ethanol, and then 100 mL of the diluted liquid product was dried at 78 oC for 24 h. The dried liquid product was designated as liquid oils products. Three replicates were conducted for each condition and the average values were reported. The liquid oil yield and the lignin conversion yield were calculated using the following equations:


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Liquid oil yield (wt%) =

Weight of liquid oil Weight of dry ash free lignin

Lignin conversion yield (wt%) =

× 100

(1)

Weight of dry ash free lignin - Weight of solid char

× 100

(2)

weight of dry ash free lignin

2.3 Analysis Methods. 2.3.1 Lignin analysis. The 1H NMR spectra and 13C NMR of alkali lignin were obtained using a Bruker Avance Digital 400 MHz NMR spectroscopy. 20 and 200 mg of alkali lignin was firstly dissolved in 20 and 180 mL DMSO-d6 (99.9% D), respectively. Then the 1

H NMR and

13

C NMR spectra were tested at 74.5 and 75.5 MHz at room temperature,

respectively. The thermogravimetric characteristic of alkali lignin was analyzed on a Jupiter Thermo Gravimetric Analyzer STA 449 F3. In experiment, the weight of alkali lignin was less than 10 mg to avoid possible temperature gradient and to ensure kinetic control of the process. The heating rate of TG experiment was 20 oC/min and the temperature range was from 50 to 1000 oC. High purity nitrogen was used as carrier gas with a flow rate of 20 mL/min. 2.3.2 Liquid oils analysis. The liquid oils was dehydrated by anhydrous sodium sulfate, filtrated by Millipore filtration, and then detected on a GC-MS (Agilent 6890N GC equipped with a 5973I MSD using a 30 m×0.25 mm×0.25 µm DB-5ms column). The GC-MS programming as follows: a 5 min hold at an initial oven temperature of the GC was 40 oC followed by an increase of 5 oC min-1 up to 200 oC, and then the temperature was raised to 280 o

C for 5 min. The solvent delay was set at 5 min. Carrier gas was the highest purity helium with a

flow rate of 0.8 mL/min. The separated components were determined by using NIST08, and the relative concentrations of specific compounds identified by the methods of areas of peak normalization. 2.3.3 Solid char analysis. The surface morphology of alkali lignin and solid char samples were performed by using a scanning electron microscope (VLTRA55, Germany). The chemical bonds


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of alkali lignin and solid chars were analyzed by FT-IR (NICOLET5700, America). The measurement was performed by KBr method for their functionality changes in the range of 400~4000 cm-1 (scans=32). 3. RESULTS AND DISCUSSION 3.1 Alkali Lignin Characteristics. The 1H NMR and

13

C NMR spectra of alkali lignin are

shown in Figure 1. δH 3.36~3.72 ppm and δC 55.9 ppm in 1H NMR and

13

C NMR spectra

originated from the chemical shift of hydrogen and carbon atom in methoxy group of lignin, while δH 6.98~7.42 ppm and δC 34.4~37.7 ppm arose from the chemical shift of hydrogen and carbon atom in guaiacol and syringyl type unit, respectively.

23-26

The H and C proton signal of

methoxy group, guaiacol and syringyl type unit was strong indicating that the alkali lignin contained a large number of guaiacol and syringyl type unit although the alkali lignin has been degraded by the pulping process. 1

Methoxy group H

a) H-NMR

DMSO-d6 Alcohol hydroxyl H

ppm 10

Guaiacyl and Syringyl H

8

6

13

b) C-NMR

4 Methoxy group C

2

0 DMSO-d6 Guaiacyl phenyl propane Ca and Cß

Aryl ether Aromatic C and C a ß bond C4 ring C ether bond C ppm 200

160

120

80

40

0

Figure 1. 1H NMR and 13C NMR spectra of alkali lignin: a) 1H NMR; b) 13C NMR. As shown in Figure 2, the remarkable weight loss stage of alkali lignin started at 180 oC, finished at 580 oC, accounting for a weight loss of 53.2 wt% with the maximum loss rate at 380 o

C. It indicated that the depolymerization reaction of alkali lignin may be difficult to occur as the


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reaction temperature below 180 oC. This inference was also confirmed in the depolymerization experiment of alkali lignin. Specially, lignin conversion yield obtained from alkali lignin depolymerized in subcritical ethanol (150 oC, 2.1 MPa) is only about 58%. 100

0.0

-0.1

80 70

-0.2 60

DTG/%oC-1

90

TG/%

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-0.3

50 Main mass loss stage

40 0

200

400

600

800

-0.4 1000

o

Temperature/ C

Figure 2. TG and DTG Curves of alkali lignin. 3.2 Product Yields. The reaction condition state in the subcritical or supercritical was determined by the reaction temperature and pressure together. The experiment equipment using in this paper is a self-boosting system, so the reaction system state was mainly affected by the reaction temperature. Lignin conversion yield and liquid oil yield varied with reaction temperature are shown in Figure 3. The critical temperature (pressure) of ethanol was 243.1 oC (6.7 MPa), so the ethanol reaction system is seen to in subcritical state as the reaction temperature below 240 oC, while the system is supposed to in supercritical ethanol state when the temperature above 240 oC. The values of lignin conversion yield and liquid oil yield were increased with reaction temperature (pressure) increasing from 150 (2.1) to 240 oC (7.2 MPa) during alkali lignin depolymerized in subcritical ethanol, while these two yields values were decreased with temperature (pressure) increasing from 240 (7.2) to 300 oC (10.3 MPa) as depolymerized in


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supercritical ethanol (see Figure 3). Meanwhile, the maximum values of lignin conversion yield and liquid oil yield were 74.88% and 67.58%, respectively, and the two maximum yields values were appeared at the critical temperature and pressure of ethanol. This phenomenon indicated that alkali lignin could efficiently auto-catalytic depolymerized by organic bound sodium at the critical temperature and pressure of ethanol. The reason of this phenomenon may be that the depolymerization

process

of

lignin

is

endothermic

reactions,

thermodynamically

disadvantageous at a lower temperature16 and lead to depolymerization reaction inadequate. The supercritical ethanol system has high heat transfer efficiency and high content of hydrogen free radical, which can ensure the energy needed for lignin depolymerization reaction and inhibit the repolymerization reaction of the intermediate free radical.21 Oppositely, as the depolymerization temperature is too high (300 oC), the depolymerization reaction rate too fast, and lead to a large amount of depolymerization intermediate free radicals generate, then the occurrence probability of the repolymerization reaction increase, leading to the solid char yield increase but the liquid oil yield decrease.8, 15 This reason has been also confirmed in the properties analysis of the chars obtained from alkali lignin depolymerized in sub- and super-critical ethanol. Lignin conversion yield Liquid oil yield

80

7.2 MPa

6.5 MPa

70

Yields (%)

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10.0 MPa 10.3 MPa

3.2 MPa 60

2.1 MPa

50

40

120

150

180

210

240

270

300

330

o

Temperature ( C)

Figure 3. The depolymerization product yields of alkali lignin varied with reaction temperature.


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Lignin conversion yield and liquid oil yield obtained from alkali lignin depolymerized for various reaction times (1, 2, 4 and 8 h) at 180, 240 and 300 oC are shown in Figure 4. Lignin conversion yield and liquid oil yield for 240 oC depolymerization process remained constant as the reaction time increasing from 1 to 8 h. Meanwhile, the values of lignin conversion yield and liquid oil yield were larger than these for 180 and 300 oC (see Figure 4). Therefore, alkali lignin could efficiently and stably auto-catalytic depolymerized by organic bound sodium in the reaction time ranges of 1 to 8 h. 80

Lignin conversion yield

a)

Liquid oil yield

2.6 MPa

3.0 MPa

3.2 MPa

3.0 MPa

80 b) 6.9 MPa

7.0 MPa

7.2 MPa

7.0 MPa

10.2 MPa

10.3 MPa

10.2 MPa

60 40

Yields (%)

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60 40 80 60

c)

10.2 MPa

40 1h

2h

4h

8h

Time (h)

Figure 4. The depolymerization product yields of alkali lignin varied with reaction time: a) 180 o

C; b) 240 oC/; c) 300 oC.

3.3 Chars Properties. An understandable dissolution and degradation process is important to improve the depolymerization efficiency of alkali lignin in supercritical ethanol system. Therefore, lignin chars obtained from the depolymerization process were analyzed by elemental analysis, FTIR and SEM, then the dissolution and depolymerization process of alkali lignin in sub- and super-critical ethanol was inferred. The element compositions of alkali lignin and solid chars obtained from alkali lignin depolymerized at different temperature for 4 h in sub- and super-critical ethanol were presented


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in Table 1, respectively. The content of C increased but the content of O and H declined as the temperature increased from 180 to 300 oC (see Table 1) due to the dehydration and fracture of ether linkages as alkali lignin depolymerized at high temperature.27 The C/O and C/H ratios of solid chars gradually increased with reaction temperature increasing from 180 to 300 oC, indicated a mass loss of O and H and a gradual enrichment of C .28 Meanwhile, the increased amount of C/O and C/H ratios have been a marked increase as the depolymerized temperature exceeded 240 oC. It indicated that the supercritical ethanol was more favorable for the cleavage of ether linkages than subcritical ethanol. This is also the reason that the yields of lignin conversion and liquid oil were higher for supercritical ethanol than these for subcritical state (Figure 3 and 4). Table 1 Elemental compositions of alkali lignin and solid chars

Samplea

Elemental composition (wt%)

Atomic ratio

C/%

H/%

N/%

S/%

Ob/%

C/O

C/H

H/O

Material

53.10

5.38

0.62

1.52

39.37

1.35

9.87

0.14

Char-180

54.06

4.80

0.67

1.31

39.16

1.38

11.26

0.12

Char-210

55.18

4.63

0.65

1.07

38.47

1.43

11.92

0.12

Char-240

58.15

4.53

0.82

1.21

35.28

1.65

12.84

0.13

Char-270

62.95

4.12

0.78

0.95

31.21

2.02

15.28

0.13

Char-300

66.68

4.20

0.88

1.04

27.20

2.45

15.88

0.15

a

Sample: Char-180, -210, -240, -270 and -300 obtained from alkali lignin depolymerized at 180,

210, 240, 270 and 300 oC, respectively. b

O: Stands for subtraction method. Based on the previous research29, the amount and distribution of functional groups on the

surface of lignin chars were obviously affect by the reaction conditions. So the chars obtained from alkali lignin depolymerized in sub- and super-critical ethanol system have the respective


10

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distribution of functional groups. In this study, the inherent properties of char functional group were used to infer the depolymerization process. FT-IR spectra of chars obtained from alkali lignin depolymerized at different temperature and time in sub- and super-critical ethanol are presented in Figure 5. As shown in Figure 5a, the chars prepared at different temperatures had similar IR absorptions but with different intensity. Specially, the chars obtained from alkali lignin depolymerized at 180 and 210 oC contained more methyl and methylene groups (alkyl groups) as the intensity of absorptions near 2930 and 1460 cm-1 was relatively stronger. By contrast, the lignin chars prepared at 240, 270 and 300 oC had much weaker methyl group vibration at 2930 cm-1. It indicated that the alkyl groups in the side chain of the phenylpropane unit were hardly removed during alkali lignin depolymerized in subcritical ethanol, while the demethylation reactions easily occurred for the supercritical ethanol. This conclusion was also deduced from the composition characteristics of liquid oil products (More details are provided in Table 2.). The bands at 1610 and 1110 cm-1 originates from the aromatic skeleton and C-O linkage stretching, respectively.30 The absorptions intensity of aromatic skeleton and C-O linkage for the char-180 and char-210 were stronger than these for the char- 240, char-270 and char-300. This phenomenon was mainly caused by that the higher temperature and pressure not only helped to the breakage of ether linkages leading to a decrease of the C-O bond in IR spectra, but also promoted the carbonization reaction resulting in a decrease of the aromatic ring structure.27 In a conclusion, in order to efficiently break the ether linkages of lignin and selectively depolymerize lignin, it is necessary to raise the reaction conditions from subcritical to supercritical state, but at the same time the carbonation reaction of lignin char enhanced. From the IR spectra of the chars prepared at 240 oC (see Figure 5b), as the reaction time decreased from 4 h (Char-4) to 1 h


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(Char-1), the absorption intensity of C-O bond increased, while the absorption intensity of aromatic skeleton remained essentially constant. It indicated that the reduction of reaction time resulted in the breakage of the ether bond decreasing, but could not inhibit the carbonation reaction of lignin char. This is also the reason that the maximum yield of liquid oil occurred at 4 h during alkali lignin auto-catalytic depolymerized by organic bound sodium in the supercritical ethanol. Material Char-180

Transmittance (%)

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Char-210

Char-240 Char-270 Char-300 3430 2930 4000

3000

1460 1610 2000

1110 1000

-1

Wavenumber(cm )

Figure 5. FT-IR spectra of alkali lignin and lignin chars. SEM micrographs of chars obtained from alkali lignin depolymerized at 180, 240 and 300 oC for different times are shown in Figure 6. The morphology of chars obtained from alkali lignin depolymerized at 180 oC for 1h appeared plenty of micron-sized spheres (see Figure 6a). The accumulation intensity of micron-sized spheres increased with the depolymerized temperature increasing to 240 oC, while this value of micron-sized spheres decreased with the temperature increasing to 300 oC (see Figure 6d and g). The trend of char morphology may illustrated that the depolymerization temperature has an obviously effect on the sphere size of the char products. The different size char spheres underwent different depolymerization or repolymerization reactions, and the incidence of the depolymerization or repolymerization reaction was


12

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determined by the reaction conditions. Specially, when alkali lignin depolymerized in subcritical ethanol conditions (180 oC, 2.6 MPa, 1 h), the accumulation intensity of micron-sized spheres on the surface of chars was the lowest (see Figure 6a). As the reaction conditions was close to the supercritical ethanol state (240 oC, 1 h, 6.9 MPa), the number of the micron-sized spheres increased, but the particle size of micron-sized spheres decreased (see Figure 6b). As the depolymerization temperature increased to 300 oC (10.2 MPa, 1 h), the gap spacing of the matrix on the char surface increased, but the number value of micron-sized spheres decreased (see Figure 6c). (a)

(d)

(b)

(e)

(c)

(f)

Figure 6. SEM micrographs of chars obtained from alkali lignin depolymerized in sub- and super-critical ethanol: (a) 180 oC/1 h; (b) 240 oC/1 h; (c) 300 oC/1 h; (d)180 oC/4 h; (e) 240 oC/4


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h; (f) 300 oC/4 h. As shown in Figure 6b, e and h, as the reaction time increased from 1 to 4 h, the surface of char prepared at 180 oC appeared a number of larger size spheres, but the 240 and 300 oC depolymerization process did not occur. The reason may be that low depolymerization temperature (180 oC) was more conducive to the formation of lignin microspheres, but the further depolymerization of the microspheres31, 32. The microspheres, which did not undergo the depolymerization reaction, were fused together to form larger size spheres. However, the reaction temperature of 240 and 300 oC may be beneficial to the depolymerization of the microspheres, and leading to the number of microspheres decreasing. In addition, the surface morphology of the char obtained from alkali lignin depolymerized at 240 °C for 4 h was similar to that prepared at 240 °C for 1 h (see Figure 6d and e), while the matrix and micron-sized spheres on the surface of char formed at 300 °C for 4 h were melted together (see Figure 6f). 3.4 Oils Properties. GC/MS analysis was employed to analyze the chemical composition, and the detected compounds and their relative area% are summarized in Table 2. Based on the molecular formula properties, the liquid oils obtained from alkali lignin depolymerized in subcritical ethanol (180 oC) were mainly ester compounds, while the components of liquid oils prepared in supercritical ethanol (240-300 oC) were mainly phenolic compounds. Specially, the total phenols yields of alkali lignin depolymerized at 180, 210, 240, 270 and 300 oC were 9.66%, 52.73%, 53.72%, 56.94% and 68.21%, respectively. Meanwhile, the liquid oils obtained from alkali lignin depolymerized at 180 oC mainly including Benzoic acid, 4-hydroxy-3-methoxy-, ethyl ester; Phenylacetic acid, 4-hydroxy-3-methoxy-, ethyl ester; Phenylacetic acid, 3-methoxy4-hydroxy; Phenylacetic acid, 3,5-dimethoxy. The liquid oils products prepared at 300 oC mainly


14

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containing Phenol; Phenol, 2-methoxy; Phenol, 4-ethyl-2-methoxy-; Phenol, 2-methoxy-4methyl; and Phenol, 2, 6-dimethoxy. Phenolic compounds were the primary products of liquid oils as alkali lignin depolymerized in supercritical ethanol, which can be naturally expected since phenol is the basic entity of the lignin structure.21 Meanwhile, the content of phenolic compounds containing methyl, methoxy, and ethyl groups bonded to the aromatic ring were obviously high, and similar phenolic compounds substituted with methoxy and alkyl groups were observed in the relevant researches. 33-36

This results implied that the ether linkages of the methoxy groups and the C-C linkages of

alkyl groups in the lignin structural unit were difficult to break compared with the ether linkages among the lignin structural units. This reason led to the demethoxylation and demethylation reactions did not occur easily during alkali lignin depolymerized in subcritical ethanol. The common features of ether compounds obtained from alkali lignin depolymerized in suband super-critical ethanol were all ethyl ether. It indicated that the ester products were generated by the esterification reaction between the ethanol solvent and the depolymerized reaction intermediates. Furthermore, the molecular formula of No. 10 ether, No. 7 acid and No. 12 ketone all have the same aromatics structure and methoxy substituent groups, and the molecular formula of No. 11 ether, No. 8 acid and No. 15 ketone also have the same matrix structure. Meanwhile, the similar relationship was found in No. 14 ether and No. 9 ketone. This phenomenon indicated that the compounds of No. 7, 10 and 12, the compounds of No. 8, 11 and 15, and the compounds of No. 9 and 14 should have the same formation reaction pattern, except that the difference reaction initiators result in the diversity of the products. Based on the composition properties of the main oils products, the depolymerized reaction patterns of alkali lignin in sub- and supercritical ethanol were proposed, and the possible cleavage patterns are presented in Figure 7.


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Table 2 GC-MS detectable compounds in the liquid oil products. Ret. time (min)

Oil products name

11.68

Phenol

OH

Relative composition by area (%)a 210 240 270 180 300 oC o o o o C C C C 1.23 3.74 3.84 7.04 12.80

OH

-b

11.07

13.92

16.53

16.98

-

-

-

-

6.13

-

4.88

4.83

13.23

14.86

1.57

7.42

8.36

8.99

10.43

6.86

25.62

22.77

11.15

7.01

3.23

7.90

8.05

7.14

5.95

6.46

1.94

1.76

2.11

1.63

6.86

8.85

9.02

11.23

13.59

OH

10.02

2.79

2.89

3.43

3.55

O

9.44

3.17

2.86

-

-

10.23

-

-

-

-

14.79

7.49

7.43

6.54

5.72

-

6.18

6.40

5.53

-

10.21

1.88

1.36

1.58

1.35

19.10

7.06

6.53

5.51

-

9.66

52.73

53.72

56.94

68.21

Molecular formula

O

14.95

Phenol, 2-methoxy-

17.64

Phenol, 4-ethyl-

OH O

18.18

Phenol, 2-methoxy-4-methyl

OH O

20.66

Phenol, 4-ethyl-2-methoxy-

22.70

Phenol, 2, 6-dimethoxy

OH

O

O OH

25.19

O

Benzoic acid, 4-hydroxy-3-methoxy-

OH HO

26.23

Acetophenone, 4-hydroxy-3-methoxy

27.25

2-Propanone,1-(4-hydroxy-3-methoxyphenyl)-

O

O

O OH O O OH

O

28.69

O

Benzoic acid, 4-hydroxy-3-methoxy-, ethyl ester

O

Phenylacetic acid , 4-hydroxy-3-methoxy-, ethyl ester

30.51

O O

OH

O

30.69

Benzaldehyde, 4-hydroxy-3-methoxyHO O

O

31.95

Ethanone, 1-(4-hydroxy-3,5-dimethoxyphenyl) O

O OH O

Ethyl-β-(4-hydroxy-3-methoxy-phenyl)propionate

32.31

O O OH

HO

33.05

Phenylacetic acid, 3-methoxy-4-hydroxy

O O OH

HO

34.41

phenylacetic acid, 3,5-dimethoxy

O O OH O

Total Phenols Yields a

: Total area was obtained based on the integration of major peaks, without including the small

peaks with an area% < 1.0%. b

: Not detectable.


16

Page 17 of 24

As shown in Figure 7, the α and β type ether bonds of lignin units occurred the homolytic cleavage reaction as alkali lignin depolymerized at 180 oC, and generated phenyl methyl and phenyl ethyl radicals, respectively. Then part of phenyl methyl and phenyl ethyl radical was coupled with propionyl radicals releasing from the homolytic cleavage reaction of ethanol, mainly forming compounds 10 and 11, respectively. The other of phenyl methyl and phenyl ethyl radical was coupled with hydrogen radicals, forming compounds12, 8 and 13, respectively. So the propionyl and hydrogen radicals formed by the homolysis of ethanol are involved in the coupled reactions among the lignin depolymerization intermediates, and inhibiting the repolymerization reaction of intermediates. In another word, these esterification reactions between the depolymerization intermediates, propionyl and hydrogen radicals reduced

OCH3

Hydrogen radicals

R H HO C O

lignin

Ethanol

O

R: H or -OCH3

β-O-4 type lignin

β-lignin

lation thoxy Deme

O C CH2

Homolysis

OH

Compound 1

Ethanol

O

CH3 C

H3CO

Hydroxyl radical OCH3

R OH

180 oC

Phenyl ethyl radicals

CH3CH2O

Propionyl radicals

O

Hydrogenation Demethylation

H3CO

Compound 6

OH

Demethoxylation OCH3 OH

OH

H3CO OH

Compound 8

300 oC

Demethylation H3C

OHH 2 C C O

OH

Compound 1

Compound 2

Compound 11

Hydroxyl radical

OCH3 OCH3

Compound 16

OCH3 CH3 C

H3CO

OCH3

H2 O C O C CH3 CH2

Hydrogen radicals

R: H or -OCH3

O C OH CH2

OH

Compound 13 R

oC

Compound 10

lignin

OH

Compound 2

OH

OCH3

Hydrogen radicals

R

OH

Compound 4

OH

Phenyl methyl radicals

OCH3

210-270 oC

300

Propionyl radicals

R OH

R

OCH3

CH3CH2O R

α-lignin

R

Demethylation

210-270 oC

180 oC

OH

R

OH

CH3

Hydrogenation

H2 O C O C CH3

Homolysis

R

H HO C O CH2

OCH3

C

R R

OH

Ethanol

O C H

COOH

Hydroxyl radical

Hydrolysis

α-O-4 type lignin

Compound 7

repolymerization rates, in this way, and prevent char formation. Compound 12

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

Energy & Fuels

OCH3 OH

Compound 15

H3C

CH2

CH2

Demethoxylation

Hydrogenation 210-270 oC

OCH3 OH

Compound 5


300 oC OH

Compound 3

17

Energy & Fuels

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

Page 18 of 24

Figure 7. The possible depolymerized patterns of alkali lignin in sub- and super-critical ethanol In addition, the ether product corresponding to compound 13 was not detected in the liquid oil. The reason may be that the phenyl ethyl radical corresponding to compound 13 has two methoxy groups resulting in the steric hindrance increasing, which are only suitable for the attack of the smaller hydrogen radicals to form compound 13. Then the compound 13 was coupled with the hydroxyl radicals to form the acid compound 16. Meanwhile, the acid compounds 7 and 15 were generated from the hydrolysis reaction of the esters 10 and 11, respectively. As the depolymerization temperature increased to 210-300 oC, the phenolic compounds 4, 5 and 6 were generated from the hydrogenation and deoxygenation reaction of the acid compounds 7, 15 and 16, respectively. The phenolic compound 2, 1 and 3 was formed by the demethylation and demethoxylation reactions of the compounds 4 and 5. Based on this mechanism, we speculated that benzoic acid, 4-hydroxy-3-methoxy-, ethyl ester; benzoic acid, 4-hydroxy-3-methoxy- and phenol, 2-methoxy-4-methyl were generated from the depolymerization reaction of α-O-4 type alkali lignin, while Ethanone, 1-(4-hydroxy-3, 5-dimethoxyphenyl)- and 4-Hydroxy-3methoxyphenylacetic acid, ethyl ester; phenol, 2, 6-dimethoxy and phenol, 4-ethyl-2-methoxywere released from the depolymerization reaction of β-O-4 type alkali lignin. 4. CONCLUSIONS Alkali lignin can efficient and auto-catalytic depolymerized by organic bound sodium in the supercritical ethanol. The dissolution and degradation process of alkali lignin in sub- and supercritical ethanol formed different micron-sized spheres products, and the sphere size of the char products was obviously affected by the depolymerized temperature. The C/O and C/H ratios of the chars products also increased with depolymerized temperature increasing. In addition, the


18

Page 19 of 24

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

Energy & Fuels

components of liquid oil obtained from sub- and super-critical ethanol were mainly ester and phenolic compounds, respectively. ASSOCIATED CONTENT Supporting Information. Proximate and ultimate analysis results, FT-IR spectra of alkali lignin chars. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 31500492), China Postdoctoral Science Foundation (Grant 2017M612035), Zhejiang Provincial Natural Science Foundation of China (Grant LY16C160005), the open fund of Key Laboratory of Biomass Energy and Material of China (Grant JSBEM201504), the open fund of Key Laboratory of Pulp and Paper Science and Technology of Ministry of Education of China, Qilu Univer-sity of Technology (Grant KF2015015), the open fund of State Key Laboratory of Pulp and Paper Engineering (Grant No. 201605) and the Science Foundation of Zhejiang Sci-Tech University (Grant No. 14012079-Y). ABBREVIATIONS GC-MS = gas chromatography coupled with mass spectrometry SEM = scanning electron microscope FT-IR = fourier transform-infrared spectroscopy


19

Energy & Fuels

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

Page 20 of 24

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Auto-Catalytic Depolymerization of Alkali Lignin by Organic Bound

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