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Valorization of Lignocellulosic Biomass Towards Multi-Purpose Fractionation: Furfural, Phenolic Compounds and Ethanol Hairui Ji, Cuihua Dong, Guihua Yang, and Zhiqiang Pang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03766 • Publication Date (Web): 12 Sep 2018 Downloaded from http://pubs.acs.org on September 15, 2018

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ACS Sustainable Chemistry & Engineering

Valorization of Lignocellulosic Biomass Towards Multi-Purpose Fractionation: Furfural, Phenolic Compounds and Ethanol Hairui Jia, Cuihua Dongab, Guihua Yanga*, Zhiqiang Panga* a

State Key Laboratory of Biobased material and Green papermaking, Qilu University of Technology (Shandong Academy of Sciences), 3501 Daxue road, Jinan 250353, China b State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, 381 Wushan road, Guangzhou 510640, China; *Corresponding author: Phone: (+86) 531 89631623; Email: [email protected], [email protected]

ABSTRACT Lignocellulose is typically considered as one of the most promising feedstocks to produce a variety of renewable fuels and value-added chemicals. In this study, we developed an alternative valorization of lignocellulosic biomass towards multi-purpose fractionation. Since the adopted pretreatment with a fully recyclable acid, p-TsOH, can significantly facilitate the degradation of hemicellulose, we initially converted hydrolysate into furfural with an average yield of 63.38% under the selected pretreatment condition. Next, a biomass-derived solvent, γ-valerolactone (GVL), was used to extract lignin from pretreated substrate, and a suggested pretreatment temperature 100 o C was proposed according to the characterization results from FTIR, 31P-NMR, TGA, GPC, and 2D HSQC NMR. The obtained lignin, which exhibited excellent properties such as low molecular weight (Mw, 16053 g/mol), narrow polydispersities (Mw/Mn, 2.48), and high content of aliphatic OH (3.72 mmol/g), was easily upgraded to phenolic compounds by hydrogenation with a yield of 76.41%. Finally, the leftover cellulose-rich solid was directly degraded to glucose after enzymatic saccharification (92.57% theoretical yield). Following fermentation of glucose led to high yield of ethanol production (41.08 g/L). Therefore, the described synergy exhibited a practical significance for the comprehensive use of lignocellulose resource.

KEY WORDS：：Lignocellulose valorization, Furfural, Phenolic monomers, Ethanol

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INTRODUCTION Due to depleting fossil fuel reserves and climate change linked to fossil resource use, the global community has as always been seeking renewable alternatives 1. Lignocellulosic biomass, a renewable resource mainly consisting of three primary cell wall components: cellulose, hemicellulose and lignin, is considered as a potential source for biofuels and chemicals production to satisfy future energy demands 2, 3. Specifically, cellulose has been recognized the most economically useful, as it is firstly depolymerized into mono-saccharide, glucose, and subsequently converted into alcohols or other chemicals by chemical or biochemical strategies 4. Of all the derivatives, ethanol is already produced on a fair scale (about 25 Billion gallons worldwide in 2015) 5. However, the current low market price and the high production cost of ethanol have left it little or no profit margin, which has made cellulosic ethanol industry more cost-uncompetitive

6

. One attractive option of improving the economic returns from

lignocellulosic ethanol production is to valorize the other two components, hemicellulose and lignin. In fact, the conversion of hemicelluloses into fuels and high-value chemicals has exhibited excellent economic feasibility. Lignin, a three-dimensional compound composed of methoxylated phenylpropanoid subunits and various functional groups including phenyl rings, hydroxyl and carboxyl, has received broad attention in the production of various polymers, biojet fuels and so on

7, 8

. Therefore, a feasible approach towards capitalizing on all the

lignocellulosic biomass components should be developed. In order to remove biomass recalcitrance from hemicellulose and lignin making cellulose amenable to enzymatic hydrolysis, many pretreatment methods such as using dilute acid, liquid hot water decomposition, SPORL method, and steam explosion have been applied in pretreatment of various lignocellulosic materials for removal of hemicellulose and to obtain more digestible substrates for enzymatic saccharification 9. These strategies have contributed to the obvious degradation of hemicellulose and significantly improved accessibility and chemical reactivity of cellulose. However, high pretreatment severity is usually adopted to lignocellulosic fractionation in the process pretreatment. As a result, lignin structure is easily altered due to the formation of carbon-carbon bonds mainly from the most predominant β-O-4 bonds under high severity condition. Such alterations make it increasingly difficult for its depolymerization and further upgrading during traditional processing methods

10, 11

. Lignin obtained from processes

using these pretreatments has been typically burned for energy recuperation or used in low-value 2 ACS Paragon Plus Environment

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material applications

11

. Therefore, it is essential to develop an economical pretreatment

technique to efficiently degrade hemicellulose while lignin suffers relatively minor in structural damage. Fortunately, we have found a promising pretreatment method using a recyclable acid hydrotrope, p-TsOH, with 90% above xylan dissolution under mild temperatures in our previous research, while 90% of the above original cellulose retained and slight changes of the lignin structure occurred in the process of pretreatment 12. Meanwhile, other pretreatment methods, which mainly depend on the solubility performance of lignin in the solutions, such as organic solvent, alkali, ionic liquid and ammonia, have significantly improved enzymatic saccharification 9. For example, Sousa et al. developed an Extractive Ammonia (EA) pretreatment methodology which obviously enhanced enzyme accessibility to cellulose by selectively extracting 45% of the lignin from lignocellulosic biomass and converting recalcitrant crystalline cellulose Iβ (CI) to a highly digestible cellulose IIII (CIII) allomorph 13. Currently introduced was a novel biorefinery concept based on the fractionation of woody biomass in a binary mixture system composed of γ-valerolactone (GVL) and water was introduced, , which makes it possible to facilitate, with a mild pretreatment as such, the complete removal of lignin from hardwood with negligible degradation of cellulose, and consequently, allow pretreated substrate to easily undergo high-solids enzymatic hydrolysis for ethanol fermentation

14-16

. These processes preserved extracted lignin functionalities offering the

potential to co-produce lignin-derived fuels, materials and chemicals13, 17

18 19 20

. The efficient

use of hemicellulose and its hydrolysate, however, was largely ignored. For a biorefinery to be sustainable and economically viable, both hemicellulose and lignin are still needed to be sequentially or simultaneously used to the maximum of their potentials. In this study, we reported an alternative valorization of lignocellulosic biomass towards multi-purpose fractionation in view of superior performance of a recyclable acid catalyst, pTsOH, on removing hemicellulose and excellent capability of GVL on delignification. As shown in Fig.1, hemicellulose was firstly hydrolyzed into xylose using p-TsOH under mild condition and further conversion of xylose generated furfural product with a batch reactive distillation (BRD) approach. Lignin was extracted from pretreated substrate using GVL and subsequently upgraded to phenolic monomers by hydrogenation. The leftover cellulose was directly degraded to glucose after enzymatic saccharification and fermentation of glucose led to high yield of ethanol production. The described synergy exhibited a practical significance for the 3 ACS Paragon Plus Environment


comprehensive utilization of lignocellulose resources.

Fig. 1. A schematic valorization of lignocellulosic biomass towards multi-purpose fractionation

MATERIALS AND METHODS Materials Wood powder with a size of 20-40 mesh consists of glucan 41.61, xylan 16.15%, lignin 21.07% and other 11.17%; p-TsOH was purchased from Yuanye biotechnology Company (Shanghai, China); H2SO4 with a purity of 98% was provided by Beijing Chemical works (Beijing, China); GVL, Ru/C catalyst, 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane, dioxane and other solvents were supplied by Sigma-Aldrich; DMSO-d6 was purchased from Cambridge Isotope Laboratories Inc.; Liquid cellulase enzyme with activity of 215 FPU/mL was purchased from Novozymes (Beijing, China).

Pretreatment and Batch Reactive Distillation for Furfural Production A mixture of 50 g wood powder and 500 ml p-TsOH solution (1.18mol/L) was added into 1L stainless reactor and. The reaction mediums were heated by electrical heating to target temperatures of 90, 100, 110, 120, and130 o C for 60 min with a stir speed of 200 rpm. The solidliquid separation for the reaction mixture was conducted using filter paper (slow) at the end of each reaction. The components in the collected liquid was analyzed using HPLC as described previously

21, 22

. The solid was washed to neutral pH and freeze-dried for further components

analysis and lignin extraction. Batch reactive distillation experiments for furfural production were performed in the 1L stainless reactor equipped with a furfural collection set as described in a previous publication 23. 4 ACS Paragon Plus Environment

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The reactor was filled with 400 mL of collected liquid and heated electrically for 40 min. Once the temperature reached at a target temperature of 150 o C, the valve of furfural collection set was slightly opened, and furfural-water azeotrope was extracted from the reactor through a copper tube that was immersed in an ice bath to condensate the extracted vapor from the reactor headspace. A total of 150 mL distilled furfural solution was extracted from the reactor and analyzed by HPLC, the residual liquid was continually reused for the next pretreatment.

Lignin Extraction, Characterization and Depolymerization 25g pretreated solid and 125 ml GVL solution (4:1, v/v, GVL:water) were loaded into 250 ml stainless reactor. The mixture was rapidly heated to 120 o C with a stir speed of 200 rpm and the reaction time was maintained for 60 min. After reaction, the reactor was promptly cooled down using water. The cellulose-riched solid and GVL-lignin solution were separated by filtration. Lignin in the spent liquor was precipitated by addition of water (water: GVL solution, v/v, 6:1) and centrifuged, freeze-dried for further characterization. The solid was washed to neutral pH and freeze-dried for further components analysis and enzymatic saccharification. The lignin structural properties were detected by Fourier transform infrared spectroscopy (FT-IR), Themogravimetric analysis (TGA), Gel permeation chromatography (GPC),

31

P NMR

and 2D-HSQC. FT-IR measurements were conducted on a spectrophotometer (VERTEX 70, Bruker, Germany). Each sample was scanned with a wavenumber range from 4000 to 800cm-1 at 4 cm-1 resolution. GPC equipped with an ultraviolet detector (UV) and a Waters Styragel columns was used to measure lignin molecular weight at 280 nm. Each sample consists of 25mg lignin and 3ml DMF.

31

P NMR analysis was performed on a Bruker 400 MHz spectrometer

(AVANCE III) according to a previous publication

24

. 2D-HSQC measurements for a solution

containing 40 mg lignin and 0.6 mL DMSO-d6 were conducted on a Bruker 600 MHz spectrometer (AVANCE III) according to a previous publication. For the 1H-dimension, the number of collected complex points and the spectral widths were 1024 with a recycle delay of 1.5 s and 2200 Hz, respectively. For the

13

C-dimension, the number of sans and the spectral

widths were 128 with 256-time increments and 15,400 Hz, respectively. The 1JCH was set to 146 Hz 12. For the depolymerization of lignin, a mixture of 0.3g lignin, 0.2g Ru/C catalyst, and 20 ml methanol was added into 50 ml stainless reactor. After checking the sealing performance, the 5 ACS Paragon Plus Environment


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reactor was subsequently purged with H2 for five times and finally charged with 1 MPa H2. The mixture was electrically heated to 230 o C for 15h with a varied pressure from 8 to 1.5 MPa. Crude phenolic monomers products were analyzed using a GC-MS (Agilent 7890A-5975C) equipped with an HP-5 MS column as described in a publication 25.

Enzymatic Hydrolysis and Ethanol Fermentation Enzymatic hydrolysis was performed according to a previous publication

26

. The mixture

of 10 g cellulose-rich solid and 50 ml citrate buffer (0.05 mol/L-1, pH 4.8) with the cellulase loading of 15 FPU/g glucan was incubated on a shaker (ZWY-240, Shanghai Zhicheng Analytical Instrument Manufacturing Co. ltd.) at 180 rpm and 50

o

C. After sterilization, the

liquefied sample was cooled down to 30 o C and inoculated with 0.2 % dry yeast (Angel Yeast Company, Hubei province, China) based on solid, w/w. Fermentation was conducted on the same shaker above at 180 rpm and 30 o C.

RESULTS AND DISCUSSION Pretreatment and Furfural Production Wood powder was subjected to p-TsOH solution (1.18 mol/L) at varied temperatures from 90 to 130 o C for 60 min. The effects of pretreatment temperatures on residual xylan in solids and chemical components in the spent liquors were exhibited in Fig.2. As shown in Fig.2 (a), the remained xylan in the pretreated solids rapidly decreased with pretreatment temperature increasing. 95% xylan dissolution (5% remained xylan) was observed at 110 o C. Approximately 99% of the xylan was dissolved at 130

o

C. The concentrations of theoretical xylose from

dissolved xylan was higher than that of measured xylose in the spent liquors, suggesting that a certain amount of the dissolved xylan remained as oligomers, which was coincided with a previous publication 12. Measured xylose concentrations initially increased and followed a trend of decrease as pretreatment temperature raised due to its degradation to furfural as revealed in Fig. 1b. Elevating the pretreatment temperature not only improved xylan degradation to xylose following by the formation of furfural, but also enhanced the generation of acetic acid (Fig. 2 (b)). The concentrations of acetic acid plateaued at 120

o

C then acetyl groups mainly from

acetylated hemicelluloses. The accumulation of acetic acid in turn improved the degradation of hemicellulose. As shown in Fig. 2 (c), this mild pretreatment facilitated removal of hemicellulose 6 ACS Paragon Plus Environment

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while caused negligible release of glucose with a concentration of approaching 2.31 g/L at 110 o C. Meanwhile, no HMF and LA in the pretreatment spent liquors were detected within the

20 Theoretical maximal xylose from xylan

25

15

20 15

10 5

10

(a)

5 0

0 4

Furfural (g/L)

3 Acetic acid

Furfural

3

2

(b)

2

1

1

0

0

4

Acetic Acid (g/L)

Xylose (g/L)

Measued xylose Remained xylan Dissolved xylan as xylose

Remained xylan (%)

reaction conditions in this study.

Glucose (g/L)

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


Glucose

3 2

(c)

1 0 90

100

110

120

130

o

Temperature ( C) Fig.2. The effects of pretreatment temperature on (a) remained xylan, the concentrations of xylose, (b) furfural, acetic acid, (c) glucose in the spent liquor.

The spent liquors produced by fractionation of wood powder were selected for furfural production by BRD dehydration. Traditional batch conversion usually leads to side reactions of furfural by resinification and condensation, the yield of furfural was significantly limited as a result. BRD dehydration was an effective strategy to improved furfural yield by immediately removing furfural from conversion system soon after it formed

23

. The dehydration reactions

o

were conducted with define parameters at 150 C for 40 min. The formation of furfural in distillates and residual liquors under different pretreatment temperatures was shown in Tab.1. A low furfural yield of 48.78% was observed under mild pretreatment condition (90 o C) due to relatively less xylan dissolution. Specifically, the pretreatment at 110

o

C produced a high

furfural yield of 65.53% theoretical value with concentrations of 20.34 and 3.19 g/L in distillate 7 ACS Paragon Plus Environment


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and residual liquor, respectively. The results indicated that the BRD dehydration process resulted in upward 65% of furfural yield. Therefore, extraction of hemicellulose and subsequent furfural conversion before integrating utilization of lignin and cellulose was a feasible approach.

Tab.1. The formation of furfural in distillate and residual liquor under different pretreatment temperatures Pretreatment o

Furfural in residual liquor

Furfural in distillate

Total yield

T ( C)

Cfurfural (g/L)

Yield (%)

Cfurfural (g/L)

Yield (%)

(%)

90

15.73

40.18

2.42

10.30

50.48

100

18.65

47.64

3.43

14.60

62.24

110

20.34

51.95

3.19

13.58

65.53

120

17.86

45.62

2.83

12.05

57.66

130

21.21

54.17

3.27

13.92

68.09

Extraction, Characterization and Depolymerization Of Lignin Lignin Extraction The substrates after pretreatment by p-TsOH were subsequently used for lignin extraction with GVL. The components analysis for obtained samples are shown in Fig.3. The results indicated that p-TsOH caused near complete degradation of hemicellulose and a negligible impact on the content of lignin in substrates in the process of pretreatment, which was attributed in part to the low activation energy of hemicellulose (about 112 kJ/mol) among three components. As pretreatment temperature raised, GVL exhibited an improved capacity towards dissolving lignin indicating that pretreatment significantly facilitated lignin dissolution in GVL. The delignification yields ranged from 69.27% (6.47% retained lignin at 90

o

C pretreatment

condition) to 81.64% (3.87% retained lignin at 130 o C pretreatment condition), while an average high yield of 72.51% original cellulose remained in residual solids. Lignin was easily precipitated by adding water into the spent liquor. In order to identify optimal pretreatment temperature and evaluate potential value of extracted lignin according to its structural properties and enzymatic saccharification, the characteristics of obtained lignin was subsequently measured by FTIR, GPC, 31P NMR and 2D HSQC NMR while cellulose-rich slurries were used to produce


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ethanol via high-solid enzymatic hydrolysis and fermentation. Lignin Xylan Glucan

80

Content (%)

60

40

20

0 Or ig in al 90 -T sO H 10 0Ts O H 11 0Ts OH 12 0Ts O H 13 0T 90 sO -T H sO H 10 -G 0VL Ts O H11 G 0VL Ts OH 12 -G 0V Ts L O H13 GV 0Ts L O HG VL

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


Fig.3. Composition analysis of pretreated substrates by p-TsOH and GVL

Lignin Characterization The FT-IR spectra of the extracted lignin by GVL and the MWL are showed in Fig. 4. Signals were identified according to a previous publication 27. All the curves showed the typical structural characteristics of lignin FT-IR spectra. A wide absorption band at 3440 cm-1 was attributed to the stretching vibration of hydroxyls in aromatic and aliphatic regions. The peaks at 2938 and 2843 cm-1 were belonged to the C-H asymmetric and symmetrical vibrations for the methyl (CH3) and methylene (CH2) groups, respectively. The appearance of peak at 1695 cm-1 for the absorption of C=O stretching vibrations indicated the existence of conjugated carboxylic acid and ketone groups in lignin. Moreover, aromatic skeletal vibrations and the C-H deformation combined with aromatic ring vibration were shown at 1595, 1508, 1458, and 1419 cm-1, respectively. Breathing vibration of syringyl and condensed guaiacyl, guaiacyl units and CC plus C=O stretch were detected at 1326 cm-1, 1326 cm-1, and 1217 cm-1, respectively. Aromatic C-H in-plane deformation vibrations and C-H out-of-plane stretching was also found at 1028 and 834 cm-1, respectively. The spectra of the extracted lignin by GVL exhibited a similar stretching vibration compared with that of MWL while the intensities of absorption peaks were gradually weakened with increase of the pretreatment temperature, suggesting that the extracted 9 ACS Paragon Plus Environment


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lignin maintained the “core” of its structure as that of milled wood lignin (MWL) but only with slight changes in the process of pretreatment and extraction. MWL

L100

L110

L120

MWL

L130

L100

L110

L120

L130

916 834

2843 1028 1166

1695

2938

1419 1595

1326

3440 1508 1458

3600

3400

3200

3000

2800

1800

1600

1400

1267 1219

1123

1200

1000

-1

-1

Wavenumbers(cm )

Wavenumbers(cm )

Fig. 4. FT-IR spectra of extracted lignin with GVL Tab.2 Weight-average (Mw), number-average (Mn) molecular weights and polydispersity index (PDI) of the lignin samples.

a

Samples

Mw (g⋅mol−1)

Mn (g⋅mol−1)

PDIa

MWL

15005

4896

3.07

L100

16053

6467

2.48

L110

13223

5845

2.26

L120

10759

5576

1.92

L130

11671

10347

1.13

PDI : polydispersity index, PDI = Mw/Mn.

The molecular weights of lignin, a fundamental property that influenced the valorization of lignin, were measured with GPC. Fig. S1 shows the molar mass distribution of extracted lignin with GVL. The results of weight-average (Mw), number-average (Mn) molecular weights and polydispersity index (Mw/Mn) are listed in Tab.2. Most noteworthy, numbers of Mw for the isolated lignin (16053 g⋅mol−1) via mild pretreatment (100

o

C) and MWL (15005 g⋅mol−1)

appeared surprisingly similar. As the pretreatment temperature elevated from 100 to 120 o C, the Mn firstly exhibited a decreased trend changing from 16053 g⋅mol−1 to 10759 g⋅mol−1. This fact suggested that high pretreatment temperature resulted in a comprehensive depolymerization / 10 ACS Paragon Plus Environment

800

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fragmentization of lignin. By contrast, a higher molecular weight was detected for the lignin from pretreated substitute via 130 o C pretreatment, which suggested that lignin recondensation occurred under harsh pretreatment condition. In fact, the Mw variance of lignin was closely tied to its depolymerization at low pretreatment severity and recondensation at high severity in the pretreatment process. When the depolymerization played a leading reaction, Mw of lignin thus decreased under such conditions. As the pretreatment severity raised, recondensation reaction of lignin probably occurred leading to a higher Mw. Similar result was found in the publication reported by Shen et al

27

. In addition, lignin obtained from pretreated substitute via mild

pretreatment (100 o C) showed a narrow polydispersity (polydispersity index, PDI) of 2.48 with low recondensation. Therefore, 100 o C was a suggesting pretreatment temperature, which agreed with the analysis of the following measurement by 31P-NMR, TGA and 2D- HSQC NMR. The thermal stability of lignin, an important characteristic for its future applications in preparation of composites and phenolic compounds, was measured by TGA. The rate of lignin thermo-decomposition was very closely related to its inherent chemical structure and various functional groups. The TGA curves, a sign that indicated the relationship between structural characteristics and thermal stability, for the MWL and extracted lignin are displayed in Fig. 5.

Obviously, the decomposition rate of the MWL was higher than those of extracted lignin. This occurrence of this phenomenon could be attributed to higher content of β-O-4 linkages in MWL. The weak C-O bond in the β-O-4 linkages was cleaved quickly at the beginning of decomposition (≤ 200

o

C). The aryl ether bond linkages began to be broken apart at a

temperature range of 200-350

o

C. As the temperature increased, side chain oxidation

decomposition, mainly the carbonylation / carboxylation of aliphatic hydroxyl group and side chain dehydrogenation, started to be activated 27. Afterward, aromatic ring and C-C bonds such as 5-5 (biphenyl) linkages were subsequently cleaved. H2O, CO2 and CO began to release as temperature raised to above 400 o C. Finally, when all the oxygen element was totally consumed, the formation of biochar from carbon element resulted into a typically flatten TGA curves at above 500

o

C. As the pretreatment temperature elevated from 100 to 130

o

C, the thermo-

decomposition rate of extracted lignin has become slower and slower. The mainly possible reason was the decrease of oxygen content due to the changing of ether bonds into stable carboncarbon bonds (lignin recondensation). The results were in accordance with that of 31P-NMR and GPC analysis. As a result, a mild pretreatment temperature was suggested to produce high valued 11 ACS Paragon Plus Environment


lignin. MWL L100 L110 L120 L130

Weight loss (%)

100 80 60 40 20 0

o

Deriv. weight X 10-4 (%/ C)

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

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-15

-30

-45 100

200

300 400 Temperature (oC)

500

600

Fig. 5. The TGA curves of extracted lignin samples with GVL Quantitative 31P NMR was used to further investigate structural changes and the relative e content of the hydroxyl groups at different structural locations for MWL and the extracted

lignin by GVL. The spectra and corresponding results are presented in Fig. S2 and Tab.3, respectively. The detailed assignments of lignin were obtained according to a previous literature

12

. The results in Tab.2 indicated that the content of aliphatic hydroxyls as

predominant groups in MWL were determined to be 7.85 mmol/g, while the extracted lignin by

GVL showed a decreased aliphatic hydroxyl content from 3.72 to 2.34 mmol/g as the pretreatment temperature raised. The content of phenolic OH groups were significantly increased with a range from 3.68 to 4.32 mmol/g as pretreatment temperature elevation (100 to 130 o C), which was attributed to the cleavage of the β-O-4 aryl ether linkages 27. Of which, the S-OH and G5,5-OH are the OHs in condensed lignin dimeric units. A larger amount of G5, 5-OH, an indication for condensed units, were observed at high pretreatment severity (130 o C), indicating


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that further increase in harshness of pretreatment conditions directly resulted into lignin recondensation. Therefore, the pretreatment should occur under the mild condition of 100 o C as reveled from FT-IR analysis. Under such pretreatment condition, a competitive mount of pCoumaric acid-OHs (1.02 mmol/g) were observed comparing with that of MWL (0.60 mmol/g). Moreover, a considerable number of carboxyl groups (0.60 mmol/g) were measured which was very close to that of MWL (0.750 mmol/g). Tab.3 Hydroxyl contents of the lignin samples as determined by 31P NMR. Types of OH

α-OH (erythro)

Primary OH

Aliphatic OH (mmol/g)

S-OH

G5,5OH

G-OH

p-Coumaric acid-OH

Phenolic OH (mmol/g)

COOH (mmol/g)

MWL

7.79

0.06

7.85

0.2

0.1

0.07

0.6

0.97

0.75

L100

3.36

0.36

3.72

1.97

0.41

0.28

1.02

3.68

0.6

L110

2.31

0.41

2.72

2.07

0.54

0.35

0.96

3.92

0.37

L120

1.74

0.62

2.36

2.1

0.59

0.46

1.06

4.21

0.34

L130

1.82

0.52

2.34

1.9

0.77

0.54

1.11

4.32

0.38

The MWL and extracted lignin were finally characterized with 2D-HSQC NMR to identify their all the structural characteristics and changes. The spectra were exhibited with two independent regions, side-chain (δC/δH 50.0-90.0/2.50-6.00) and aromatic regions (δC/δH 100.0150.0/5.50-8.50). Fig. 6 presents the main signals and substructures of the lignin. The assignments of cross-signals were assigned according to the previous publications

28-30

. In the

side-chain region, the signals were mainly a predominant with interunit linkages such as β-arylether (β-O-4’, A), resinol (β–β’, B) and phenylcoumaran (β-5’, C). Specifically, the appearance of characteristic signals of Cα-Hα, (δC/δH 71.9/4.85), Cβ-Hβ (δC/δH 84.4/4.4) and Cγ-Hγ at δC/δH (δC/δH both 85.6/4.2




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Page 15 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


R

O

MeO O

O

HO

γ

γ β

O α

4'

β

O α

O

4'

O

O

γ

OMe

OMe

β α

α' β'

5' β

γ'

α

O

OMe O

OMe

OMe

O

O

O

A'

A OH

MeO

O

OMe O

S

O

OMe

MeO

B

O

OH

O

O

OMe

OMe O

O

C

OMe

O

S'

G

O

PCA

O R

PB

Fig. 6. HSQC spectra of the extracted lignin by GVL and their main structures: (A) β-aryl-ether units (β-O-4’); (A’) β-O-4’ alkyl–aryl ethers with acylated γ-OH with p-coumaric acid; (B) resinol substructures (β–β’); (C) phenylcoumaran substructures (β-5’); (S) syringyl units; (S′) oxidized syringyl units; (G) guaiacyl units; (PCA) p-coumarate moieties; (PB) p-benzoate.

and 59.4/3.7) indicated the existence of β-O-4’ substructure in A unit, whereas the responses at δC/δH 64.6/4.21 belonged to the Cγ-Hγ correlations of β-O-4’ substructure in A’ units. What’s more, resinol (β–β’) substructures were detected revealing with the signals of their Cα-Hα at δC/δH 87.7/5.5 and double Cγ-Hγ at δC/δH 63.4/3.6. Appearance of signals at δC/δH 85.5/4.6 and 71.2/4.2 and 3.8 were definitely proved to coincide with the Cα-Hα and Cγ-Hγ correlations in phenylcoumaran (β-5’) substructures. It's clear that the pretreatment dramatically promoted the depolymerization of lignin with the increase of temperature. As shown in spectra of L110(a), only faint signals Aα and Cγ can be observed. For a comparison, the extracted lignin from pretreated substitute via 100 o C pretreatment maintained a relatively complete chemical structure 15 ACS Paragon Plus Environment


Page 16 of 24

according to its clear signals. To be brief, the appearance of these signals suggested that the obtained lignin sample under mild pretreatment condition (100 o C) contained natural structural characteristics as that of the MWL. In the aromatic regions/olefinic regions, cross-signals from syringyl (S) and guaiacyl (G) were easily observed in the spectra. a prominent signal for C2,6 - H2,6 (S2,6) in S units was found at δC/δH 103.8 /6.69, while the Cα-oxidized (Cα=O) structure in oxidized S unit (S’) was observed at δC/δH 106.1/7.32 for C2,6 - H2,6. The correlation for C2-H2 in G units was showed at δC/δH 110.9/7.00. The cross-signals at δC/δH 130.0/7.46 and 15.4/6.84 belonged to the characteristic signals of C2,6 - H2,6 and C3,5 - H3,5 in PCA, respectively. Appearance of signals at δC/δH 131.6/7.7 revealed the existence of PB units with the correlations of C2,6 - H2,6. It was similar with the characteristics in side-chain region that some of signals such as G2 gradually disappeared from aromatic regions/olefinic regions with the increasing pretreatment severity, specifically for L120 (b), which was due to the occurrence of condensation between guaiacyl units in lignin result was in accordance with that of

31

30

. This

P-NMR, TGA, and GPC analysis. The signals were

particularly evident in the HSQC spectrum L100(b) indicating that this obtained lignin sample preserved its natural structural characteristics as that of the MWL. According to results from FTIR,

31

P-NMR, TGA, GPC and 2D HSQC NMR analysis, we

suggested an optimum pretreatment temperature of 100 o C which significantly enhanced furfural yield and facilitated collection of high valued lignin. The extracted lignin from this condition was subsequently depolymerized by hydrogenation to produce phenolic compounds.

Lignin Depolymerization


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Fig. 7. GC-MS analysis of the bio-oil obtained from lignin depolymerization

The lignin L100 was depolymerized by the hydrogenation with Ru/C catalyst. The obtained bio-oil was analyzed by GC-MS. Fig.7 showed the chemical components of its degradation product at the corresponding retain time. The main chemical compounds in the bio-oils were phenols, guaiacols and syringols including p-Xylene, Phenol, 4-ethyl-2-methoxy- Phenol, 2methoxy-4-propyl-Phenol, Methylparaben, 5-tert-Butylpyrogallol, et. The aromatic aldehyde, a major product in depolymerization compounds, was produced indicating that the ether bonds concluding α-O-4, 5-O-4 and β-O-4 and C–C bonds such as β-5 were cleaved, which in turn verified a natural lignin structure of L100 close to MWL. The results demonstrated that a competitive yield of 76.41% for total aromatic aldehyde was obtained, which was comparable to that reported in the previous publications under similar conditions

25

. Therefore, the extracted

lignin by GVL from pretreated substitute via mild pretreatment (100

o

C) would have an

enormous potential in production of bio-jet fuels and high valued chemicals such as phenolic monomers.

Enzymatic Saccharification and Ethanol Production The leftover cellulose-rich solid after lignin extraction containing 72.79% of cellulose was directly hydrolyzed by enzymatic saccharification, the obtained glucose was subsequently converted to ethanol via anaerobic fermentation. The corresponding results are shown in Fig.8. 17 ACS Paragon Plus Environment


The degradation of hemicellulose during the pretreatment and the barriers removal of lignin for significantly enhanced the accessibility of enzyme to cellulose, therefore, it led to an improved enzymatic digestibility because of more porous cell wall. A high saccharification yield of 92.57% was reached at 72 h. The released monomeric sugars were subsequently subjected to fermentation to produce ethanol. As shown in Fig.8, no significant inhibitory effect was observed due to less formed inhibitory compounds such as acetic acid, furfural (hemicellulose hydrolysate) and aromatic compounds (lignin hydrolysate) during two step pretreatments. The fermentation was completed within 48 h. The maximum concentration of ethanol was 41.08 g/L with a yield of 91.53% theoretical conversion. Considering our results, this strategy would have enormous potential to higher ethanol titer production. Consequently, compared with the ethanol production efficiency obtained via single pretreatment by the acid, alkali, steam explosion treatment and so on, the one obtained currently by two step pretreated profiles would be a quite promising approach, particularly contributing to the comprehensive use of hemicellulose and lignin under mild conditions, excellent enzymatic digestibility of cellulose-rich solid and high

120

75

90

50

60

25 0 100

0

20

Saccharification

Glucose

Glucose

Ethanol

40

60

80

30 0 45

75

30

50 15

25

0

0 0

10

20

30

40

Glucose (g/L)

100

Ethanol (g/L)

Saccharification (%)

yield of ethanol production.

Glucose (g/L)

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

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50

Time (h) Fig. 8. Enzymatic hydrolysis of cellulose-rich solid and fermentation for ethanol production

CONCLUSION In this study, we developed an alternative valorization of lignocellulosic biomass towards


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multi-purpose fractionation. Hemicellulose was firstly removed and subsequently converted into furfural with an average yield of 63.38% by p-TsOH. Secondly, lignin was extracted from pretreated substrate using GVL and a suggested pretreatment temperature 100 o C was proposed according to the characterization results from FTIR, 31P-NMR, TGA, GPC, and 2D HSQC NMR. The obtained lignin was subsequently upgraded to phenolic monomers by hydrogenation with a yield of 76.41%. The leftover cellulose-rich solid was enzymatically hydrolyzed to glucose with yield of theoretical 92.57%, the subsequent fermentation lead to a high yield of ethanol production (41.08 g/L). The described synergy exhibited a practical significance for the comprehensive utilization of lignocellulose resource.

ACKNOWLEDGEMENT The authors are grateful for financial support from the Natural Science Foundation of Shandong Province (ZR2017MC007, ZR2018LB29), the R&D Focus of Shandong Province (2017GGX80102), the Taishan Scholars Program, and the State Key Laboratory of Pulp and Paper Engineering (Project number 201720). Supporting Information The curves of molar mass distribution and

31

P NMR spectra of extracted lignin samples with

GVL. AUTHOR INFORMATION Corresponding Author Phone: (+86) 531 89631623. E-mail: [email protected], [email protected]. Notes The authors declare no competing financial interest. REFERENCES (1) Luterbacher, J. S.; Martin Alonso, D.; Dumesic, J. A., Targeted chemical upgrading of lignocellulosic biomass to platform molecules. Green Chem. 2014, 16 (12), 4816-4838, DOI 10.1039/C4GC01160K. 19 ACS Paragon Plus Environment


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Synopsis Valorization of lignocellulose towards multi-purpose fractionation: furfural, phenolic compounds and ethanol, which exhibited an approach for comprehensive use of lignocellulose.


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Furfural, Phenolic Compounds and Ethanol - ACS Publications

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