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The effects of #-valerolactone/H2O solvent on the degradation of pubescens for its fullest utilization Yiping Luo, Zheng Li, Yini Zuo, Zhishan Su, and Changwei Hu J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b01563 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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Journal of Agricultural and Food Chemistry

The effects of γ-valerolactone/H2O solvent on the degradation of pubescens for its fullest utilization Yiping Luoa,b, Zheng Lia, Yini Zuoa, Zhishan Sua and Changwei Hu*a a

Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry,

Sichuan University, Chengdu, Sichuan 610064, P. R. China. b

Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu, Sichuan 610041, P. R. China.

*Email: [email protected] KEYWORDS: Lignocellulosic biomass; Inter- and intramolecular bonds; γ-valerolactone; H2O; Solvent effects. 1

ABSTRACT Solvent-thermal conversion of biomass was promising for obtaining

2

value-added chemicals. However, little was known about the interactions between solvents

3

and biomass in the process, which hindered the effective utilization of biomass. The effects of

4

γ-valerolactone(GVL) and H2O on enhancing pubescens degradation via the cleavage of

5

inter- and intramolecular linkages were studied. At 160 oC, H2O selectively promoted the

6

cleavage of the intermolecular linkages by forming hydrogen bonds, making mainly

7

contributions to hemicellulose dissolution, while GVL and H2O promoted lignin dissolution

8

by forming hydrogen bonds with -OCH3 group of lignin. H2O promoted the cleavage of

9

β-(1,4)-glycosidic bonds in hemicellulose derived oligomers to xylose, while the oxygen in

10

the ring of GVL might interact with hydroxyl groups of xylose unit to enhance the

11

dehydration of xylose to furfural. Whereas GVL with H2O promoted the depolymerization of 1

ACS Paragon Plus Environment


12

lignin, to oligomers mainly including β-O-4’ and β-β’ linkages connecting to G and S units.

13

Introduction

14

As results of the excessive consumption of non-renewable fossil fuels and the increasing

15

global environmental problems, biomass as a renewable and unique carbon-containing

16

resource was considered as a promising alternative to fossil resources for the production of

17

value-added chemicals and fuels in the future.1-5 Lignocellulosic biomass largely comprised

18

of three main components, hemicellulose, cellulose and lignin, with complicated structure.6

19

To achieve the complete utilization of the three main components in raw biomass, keeping

20

the naturally formed chemical bonds and units, hence preserving a high atom efficiency, was

21

an encouraging strategy to develop lignocellulosic biorefinery, because this could on the one

22

hand provide economic benefits, and on the other hand avoid the waste of resource and

23

prevent new pollution.7 However, lignocellulosic biomass was extremely stable against

24

chemical and biochemical processing due to the rigid structure of polymeric composite and

25

complicated interaction connecting the three main components.8-9 The effective utilization of

26

biomass to produce valuable chemicals therefore remained challenging.

27

Solvent-thermal conversion of raw biomass has received considerable attentions in

28

recent years for high efficient conversion of lignocellulose.10-13 Various co-solvent treatments

29

such as THF/H2O14-16, ethanol/H2O17 and GVL/H2O

30

relative to aqueous-only methods by enhancing the degradation of biomass. Among these

31

co-solvent systems, GVL/H2O co-solvent system was highly desired for a sustainable

32

bio-industry, because GVL was a biomass-derived green solvent and could be easily

33

recovered due to its high boiling point

3,18-21

22-24

had demonstrated advantages

. Luterbacher et al. achieved the complete

2


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solubilization of biomass (hardwood, softwood and corn stover) with high yields of 70% to

35

90% in a solvent mixture of biomass-derived GVL, water, and 0.05 wt% H2SO4. 18 Wu et al.

36

used one-pot methods to convert cotton stalk using GVL/H2O mixture containing a low

37

concentration of H2SO4.25 The water contained H2SO4 at high temperature could degrade

38

hemicellulose and cellulose into the aqueous solution, and simultaneously, GVL would

39

extract lignin fraction. The simultaneous degradation of the three components in biomass in

40

solvent-thermal process was efficient to improve the conversion of biomass. However, a

41

complex product mixture, containing many kinds of carboxylic acids, furans, phenols and

42

some oligomers was obtained, which caused the difficulty in product separation and in the

43

further use.13 Fang and Sixta found that GVL/H2O co-solvent system was effective for the

44

fractionation of wood in the presence of H2SO4 to recover pure cellulose (90.5 wt%) as solid

45

residue, and obtained dissolved uniform sugar components from hemicellulose, and dissolved

46

lignin fraction.3 Lê et al. studied the fractionation of woody biomass in a GVL/H2O mixture

47

to obtain high purity cellulose without the addition of any catalysts or additives.20 The above

48

investigations achieved selective co-dissolution of hemicellulose and lignin without

49

significant dissolution of cellulose, however, the selectivity to target products and the

50

subsequent separation and purification needed to be improved.

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Li and Luterbacher reported that solvents had significant effects on reaction rate, reaction

52

pathways, product distribution, and yields in biomass conversion.26 In our previous work，the

53

simultaneous dissolution of hemicellulose and lignin in pubescens in a GVL/H2O co-solvent

54

was achieved, while a high purity cellulose was obtained.27 In the research of Lê et al.28, GVL

55

was coupled with water to form binary mixture in which water hydrolyzed the hemicellulose 3



56

whereas GVL dissolved the lignin fraction, leaving cellulose intact in biomass fractionation

57

process. However, little was known about the solvent-biomass interactions and the

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mechanism of solvent treatment, especially for the cleavage of the strong intermolecular

59

chemical bonds, such as ether bonds or hydrogen bonds, linking hemicellulose, cellulose and

60

lignin. Besides, the selective cleavage of intramolecular chemical bonds (such as β-(1,4)-

61

glycosidic bonds, C-O bonds, C-C bonds) was needed to obtain target products with high

62

selectivity. Thus, the investigation of solvent effects on the cleavage of inter- and intra-

63

molecular linkages was highly required, which could not only make the effective separation

64

of the three main components from biomass, but also provide a guidance to produce target

65

products with high yield and selectivity from the selective dissolution and depolymerization

66

of biomass. With the aim to obtain high yield and selectivity of products in GVL/H2O system,

67

the mechanism of the effects of GVL and H2O on the selective solubilization and

68

depolymerization of biomass needed to be further investigated.

69

Herein, we studied the solvent effects of GVL/H2O on the dissolution and conversion of

70

pubescens via the cleavage of inter- and intramolecular linkages. GVL and H2O facilitated

71

the degradation of pubescens, allowing the selective dissolution and conversion of

72

hemicellulose and lignin in pubescens to form small molecular products and oligomers at

73

optimal co-solvent concentration and reaction temperature, while cellulose was kept with the

74

naturally formed structure of C6 sugars intact. It provided a guidance for the choice of solvent

75

and design of reaction process for the effective dissolution and conversion of raw biomass as

76

starting materials to value-added chemicals or biofuels.

77 4


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Materials and Methods

79

Materials

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Pubescens sample (80 meshes, Anji county of Zhejiang Province in China), including

81

46.5 wt% cellulose, 17.9 wt% hemicellulose and 25.4 wt% lignin, 13 was dried at 110 oC in an

82

oven overnight before use. GVL (γ-valerolactone, 99%, J&K SCIENTIFIC LTD) used was

83

from a commercial source without further purification.

84

Typical solvothermal process

85

Solvent-thermal conversion of pubescens was conducted in a 25 mL stainless steel

86

sealed autoclave. In a typical run, 0.5 g pubescens with 10 mL solvent was placed in the

87

reactor. The air in the reactor was replaced by nitrogen and the initial pressure was added to

88

1.0 MPa with nitrogen. The reactor was heated from ambient temperature to the designated

89

temperature and the reaction time started to be recorded at this point. When the reaction was

90

completed, the reactor was fetched out from the electric furnace, and cooled down to room

91

temperature in air. The mixture including reaction solvent, liquid products and solid residue

92

was collected, and the reactor was washed three times with the reaction solvent. The reaction

93

mixture collected was filtered through a pre-weighed filter paper to separate the solid residue

94

from the liquid phase, and the solid residue obtained was then dried at 110 oC in an oven

95

overnight before weighing for conversion calculation and characterization.

96

Analysis of liquid products

97

Liquid products mainly from hemicellulose were quantitatively measured by

98

Dionex U-3000 High Performance Liquid Chromatography (HPLC) equipped with a

99

shodex 101 Refractive Index Detector (RID). A dionex PG-3000 pump and an aminex

100

HPX-87 column (Bio-Rad) were used. The temperature of the column oven and

101

detector were respectively 50 oC and 35 oC, and the mobile phase was 0.005 M H2SO4 5



102

solution at a flow rate of 0.6 mL/min. The content of liquid products was quantified by

103

external standard method. The liquid products mainly from lignin were quantitative

104

analyzed by gas chromatograph using flame ionization detector (GC-FID, Fuli 9750).

105

The GC-FID was equipped with an HP-innowax column (30 m × 0.25 mm × 0.25 µm),

106

and the parameters of the instrument were recorded according to the literature.29

107

The molecular weight distribution of liquid products was analyzed using gel

108

permeation chromatography (GPC, Agilent 1260) with an Agilent PL aquagel-OH 20

109

column and a refractive index detector. The injection volume was 5 µL, and pure water

110

was used as the mobile phase at a flow rate of 1.0 mL/min. A molecular weight

111

calibration curve was obtained using polysaccharide standards with molecular weight

112

from 150 to 642000 Da.

113

The 2D HSQC NMR spectra of liquid fraction were qualitatively determined on a

114

BRUKER ADVANCE 400 MHz spectrometer. Before NMR analysis, liquid fraction

115

obtained after the treatment of H2O was treated by rotary evaporator to remove H2O,

116

then viscous samples could be obtained. The liquid fraction obtained after the

117

treatment of 25% GVL/H2O was treated through precipitation with addition of H2O,

118

then powder samples could be obtained. About 50 mg viscous samples and the powder

119

samples were fully dissolved in 0.55 mL deuterated dimethyl sulfoxide (DMSO-d6)

120

separately to prepare samples for 2D HSQC NMR analysis. The parameters of the

121

instrument were recorded according to the literature.30

122

The liquid products obtained after pubescens being treated by GVL/H2O

123

co-solvent system with different ratios were also detected by ESI-MS (Shimadzu).

124

During analysis process, a N2 flow rate of 1.5 L/min and detector voltage of 1.60 KV

125

were used.

126

Characterisation of solid residues 6


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The

13

C CPMAS solid-state NMR experiments were carried out at room temperature

128

with a BRUKER ADVANCE III 500 MHz instrument. The FTIR spectra of solid samples

129

were recorded on a Nicolet 6700 Fourier transform infrared spectrometer in the range of

130

4000-400 cm-1 with a resolution of 4 cm-1. The crystalline structures of pubescens feedstock

131

and residues after solvent-thermal treatment were characterized by X-Ray Diffraction (XRD)

132

on DANDONG FANGYUAN DX-1000 instrument with monochromatic Cu Kα radiation

133

(λ=1.542 Å) operated at 40 kV and 25 mA. SEM (FEIINSPECT F) was used to investigate

134

the surface morphology of pubescens feedstock and the reaction residues, and the instrument

135

was run at an acceleration voltage of 20 kV.

136

Quantum chemical calculation

137

To investigate if solvation of the dissolved oligomers could help the dissolution, the

138

stability of several typical oligomers observed by ESI-MS was probed by quantum chemical

139

calculation. Proton or Na+ and water were added or subtracted to get the calculated species.

140

All the geometry optimizations and frequency analysis at 433 K were performed at the

141

M06-2X/6-31g(d) level in the Gaussian 09 program.31 Polarizable continuous model (PCM)

142

[2, 3]

143

systems, that is, pure H2O, 25% GVL/H2O, 50% GVL/H2O, 75% GVL/H2O and pure GVL.

144

The oligomers chosen were the typical ones observed derived from hemicellulose and lignin.

145

Results and Discussion

146

The cleavage of intermolecular linkages connecting hemicellulose, cellulose and lignin

147 148

32-33

was employed to evaluate the effects of solvents in the five different solvent

The understanding of the interaction between biomass and GVL-H2O solvent was critical for the effective utilization of the three main components. The FTIR spectra of pubescens

7



Figure 1 FTIR spectra of solid samples in GVL/H2O co-solvent system with different ratios. (A) The whole spectra, ranging from 500-4000 cm-1; (B) Partial spectra ranging from 800-2000 cm-1: (a) Pubescens; (b) 100% H2O; (c) 25% GVL; (d) 50% GVL; (e) 75% GVL; (f) 100% GVL.

149

feedstock and solid residues obtained after being treated in GVL/H2O co-solvent system with

150

different ratios at 160 °C for 4 h were given in Figure 1. The dominant peaks at 3450-3300,

151

2922 and 2840 cm-1, corresponded respectively to the stretching of –OH, -CH3 and -CH2

152

groups34, existed in the three main components of pubescens (Figure 1(A)). It was shown that

153

the characteristic peaks of –OH groups at 3450-3300 cm-1 were broadened after all the

154

solvent treatments, and the peaks were wider after being treated by 75% GVL/H2O system.

155

Xu et al. reported that the peaks around 3356 cm-1 were related to hydrogen bonding linking

156

the three main components in pubescens.35 It was suggested that the interaction of solvent

157

and -OH groups in pubescens might be one reason for the disruption of the intermolecular

158

hydrogen bonds linking hemicellulose, cellulose and lignin. Furthermore, the results showed

159

that GVL had significant effects on -OH groups in pubescens. Therefore, it could be

160

speculated that GVL might affect the cleavage of intermolecular hydrogen bonds. To further

161

investigate the biomass-solvent interaction, the differences in the region of 2000-800 cm-1 8


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representing the interactions of the three main components in pubescens were discussed in

163

detail (Figure 1(B)).

164

The breaking of hydrogen bonds by water

165

In Figure 1(B), after being treated by all the solvent systems, it was found that new

166

peaks at 1209 and 1030 cm-1, respectively assigned to C-H deformation of G units, and C–

167

O(H) and C–O(C) stretching of first order aliphatic OH and ether groups in lignin appeared.36

168

This might indicate that the chemical bonds linking hemicellulose, cellulose and lignin were

169

disrupted after solvent treatments, which resulted in the deconvolution of new peaks

170

representing partial lignin structure from the previously overlapping peaks.37 When the

171

pubescens was treated by GVL/H2O co-solvent systems with higher or equal H2O

172

concentration, it was observed that the intensities of the two new peaks at 1209 and 1030

173

cm-1 were higher, compared with those obtained with other ratios of GVL/H2O. Besides, the

174

peak at 1209 cm-1 shifted to 1201 cm-1 with increasing H2O concentration in GVL/H2O

175

system. The intermolecular linkages connecting the three main components in pubescens

176

were disrupted more strongly, and the intensities of the new peaks appeared would become

177

higher, whereas the shift of peaks would be increased. Therefore, these results indicated that

178

H2O made mainly contributions for the cleavage of intermolecular linkages connecting

179

hemicellulose, cellulose and lignin. Ferulic acids and p-coumaric were considered responsible

180

for the cross-linkages through ester and ether bonds between lignin and hemicellulose.38 The

181

peak at 976 cm-1 (out-of-plane deformation of C=C in p-coumaric and ferulic acids) shifted in

182

FT-IR spectra after solvent treatments, and gradually shifted to 986 cm-1 with increasing H2O

183

concentration.39 The results implied that solvents affected the interactions between 9



184

hemicellulose and lignin, and further supported the important roles of H2O in the cleavage of

185

intermolecular linkages connecting the three main compounds in pubescens.

Figure 2 2D HSQC NMR spectra of the liquid fraction obtained in H2O single system (a) and 25% GVL/H2O co-solvent system (b) at 160 oC for 4 h, and the relevant unit structures of lignin (c). 186 187

Two typical 2D HSQC NMR spectra of liquid fraction after pubescens being treated by pure

188

H2O and 25% GVL/H2O system were analyzed (Figure 2). From 2D HSQC NMR spectra,

189

p-coumaric acid and ferulic acid structures were observed in 25% GVL/H2O, while these

190

structures were not existed in pure H2O. Because H2O played important roles in the cleavage

191

of intermolecular linkages connecting the three main components in pubescens, it could be

192

speculated that GVL helped break down the intermolecular linkages and promoted the

193

dissolution of these sub-structures. This further supported the above results that GVL affected

194

the cleavage of intermolecular linkages, as also indicated by the existence of the interaction

195

between GVL and -OH groups in FT-IR results. After pubescens samples being treated in 10


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GVL/H2O co-solvent system with higher or equal H2O concentration (Figure 1(B)), the

197

disappearance of the peak at 1643 cm-1 (attributing to hydrogen bonds between hemicellulose

198

and lignin) was also observed,37 which indicated the damage of hydrogen bonds between

199

hemicellulose and lignin. It was also found that new peaks at 1698 and 1265 cm-1 assigned to

200

the stretching vibration of the aromatic skeleton C-C bonds and the C–O stretching vibration

201

of the G unit, respectively appeared.

202

connecting hemicellulose and lignin might cause the exposure of the representative partial

203

lignin structure from the previously overlapped peaks. Moreover, higher concentration of

204

H2O promoted the appearance of the new peaks representative of partial lignin structure.

205

Therefore, the results confirmed that H2O was mainly responsible for the cleavage of

206

hemicellulose-lignin chemical bonds and GVL promoted this breaking.

36,40

The disruption of intermolecular linkages

207

The peaks at 1115 and 1055 cm-1 representing glycosidic structures increased after all

208

solvent treatments.34,37 Jiang et al. reported that the changes in the two peaks were related to

209

the interactions of hemicellulose with lignin and cellulose.37 The hemicellulose-lignin

210

linkages were removed from the surface of cellulose, which disrupted the interactions of

211

hemicellulose-lignin and cellulose, thus increasing the intensity of cellulose. The more

212

hemicellulose-lignin linkages were removed, the higher the intensity of cellulose was. It was

213

observed that the intensities of the two peaks at 1115 and 1055 cm-1 were higher after

214

pubescens being treated in GVL/H2O co-solvent system with higher or equal H2O

215

concentration. This suggested that H2O made mainly contributions for the cleavage of these

216

intermolecular linkages linking hemicellulose-lignin and cellulose. The characteristic peaks

217

of cellulose occurred at 1374, 1327, and 1165 cm-1 showed only a slight change compared to 11



218

pubescens feedstock after solvent

Figure 3 (A) 13C CPMAS solid-state NMR spectra of solid samples after different treatments at 160 °C for 4 h: (a) Pubescens; (b) 100% H2O; (c) 25% GVL; (d) 50% GVL; (e) 75% GVL; (f) 100% GVL. (B) The effect of reaction temperature on FTIR spectra of solid samples obtained from 50% GVL/H2O system: (a) Pubescens; (b) 100 °C, 4 h; (c) 140 °C, 4 h; (d) 160 °C, 4 h; (e) 180 °C, 4 h.

219

treatments.34,37 Noticeably, the peak at 898 cm-1 assigned to crystalline cellulose was

220

invariant,41 which implied the dominant crystalline structure of cellulose was kept in solid

221

residues after solvent treatments. Therefore, the results supported that H2O played important

222

roles in the disruption of hemicellulose-lignin and cellulose interactions, while the dominant

223

structure of cellulose was not significantly affected and kept in the solid residue after solvent

224

treatments.

225

The XRD analysis of the reaction residues obtained after solvent treatments showed that

226

the degree of crystallinity was much higher than that of pubescens feedstork (Figure S1). The

227

results suggested that the components in amorphous region, like hemicelluloses, lignin and

228

part amorphous cellulose, were removed. SEM analyses of pubescens before and after

229

different solvent treatments were carried out (Figure S3). Compared to pubescens feedstock 12


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(Figure S3(A)), the cellulose bundle was almost intact after all the solvent treatments. These

231

results further supported the FT-IR results. The signals between 60 and 110 ppm in

232

CPMAS NMR spectra (Figure 3(A)) were assigned to the carbon of cellulose.30, 42 These

233

signals showed a slight change after solvent thermal treatments, which was similar to the

234

changes of characteristic peaks assigned to cellulose in FT-IR spectra, indicating the

235

remaining of cellulose in the solid state after the cleavage of intermolecular chemical bonds

236

linking hemicellulose, cellulose and lignin.

13

C

237

H2O, acting as a nucleophile agent with higher hydrogen bond accepting ability

238

compared with GVL, could preferentially form hydrogen bonds with inter-molecular linkages

239

connecting hemicellulose, cellulose and lignin, which contributed to the cleavage of these

240

linkages. Because of the lower nucleophilic ability of GVL than H2O, these inter-molecular

241

linkages were hard to be broken down in pure GVL system. In GVL/H2O co-solvent system,

242

higher concentration of GVL in GVL/H2O might weak the interaction of H2O-intermolecular

243

linkages and inhibit the performance of H2O for the cleavage of intermolecular linkages.

244

Therefore, the cleavage of inter-molecular linkages could be explained by the formation of

245

hydrogen bonds between H2O and intermolecular linkages, while GVL helped breaking down

246

the intermolecular linkages.

247 248

Dissolution of acetyl, uronic ester or ferulic and p-coumaric units by H2O

249

The absorption peak at 1737 cm-1 corresponded to acetyl and uronic ester groups of

250

hemicelluloses or the ester linkage of carboxylic group in ferulic and p-coumaric acid

251

presented in hemicellulose.42 While the absorption peak at 1246 cm-1 was assigned to the 13



252

stretching vibration of C=O in acetyl, uronic and ferulic ester groups in hemicellulose.37,43

253

After the pubescens sample being treated in pure H2O system, the two peaks disappeared,

254

while they only showed a slight change after being treated in pure GVL system. It revealed

255

that H2O completely dissolved the acetyl, uronic ester or ferulic and p-coumaric units in

256

hemicellulose, while GVL alone could not. After the pubescens sample being treated by

257

GVL/H2O co-solvent system with different ratios, it was observed that the two peaks

258

appeared when the concentration of GVL increased to 75%. The higher concentration of

259

GVL in GVL/H2O co-solvent system might reduce the binding of H2O to hemicellulose and

260

then weaken their interactions, so the performance of H2O for the dissolution of

261

hemicellulose was weakened. In addition, according to

262

the signal at 21 ppm assigned to the methyl carbon of the acetyl group30 in hemicellulose

263

disappeared after the pubescens sample being treated in pure H2O system, while it only

264

showed a slight change with the treatment of pure GVL. After being treated by GVL/H2O

265

co-solvent system with different ratios (Figure 3A (c, d, e)), the signal was only observed

266

when the concentration of GVL increased to 75%. These results were consistent with the

267

changes of the peaks at 1737 and 1246 cm-1 in the FT-IR spectra. Therefore, the results

268

showed that H2O dissolved acetyl, uronic ester or ferulic and p-coumaric units in

269

hemicellulose.

13

C CPMAS spectra (Figure 3(A)),

270 271

Dissolution of S and G units in lignin

272

The characteristic FT-IR peaks at 1605 and 1513 cm-1 assigned to aromatic skeletal

273

vibration (C=C) of lignin2, 36 gradually decreased with increasing GVL concentration to 75% 14


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in GVL/H2O co-solvent system. However, no obvious changes in these peaks assigned to

275

lignin were observed after pubescens sample being treated in pure GVL or pure H2O system.

276

The results suggested that the synergetic effect of GVL and H2O promoted the extraction of

277

lignin. If the samples were only treated by pure water or pure GVL (Figure 1(B)-b and 1(B)-f,

278

it was hard to realize the collapse of lignin skeletal structure. Besides, high concentration of

279

GVL in GVL/H2O co-solvent system greatly affected the lignin skeletal structure, then helped

280

the dissolution of lignin. Xue et al. reported that the property of the GVL/H2O co-solvent

281

system was changed compared with pure GVL and pure water, and this change in the

282

property of GVL/H2O co-solvent was beneficial for the breakage of the strong hydrogen

283

bonding in lignin, which resulted in much higher lignin solubility.21 The results obtained in

284

present work confirmed that GVL/H2O co-solvent played important roles in the dissolution of

285

lignin, which was in accordance with Xue’s work.

286

In 13C CPMAS NMR spectra(Figure 3(A)), the signal at 136 pm was assigned to C-1 in

287

S units of lignin, while the signal at 133 pm was assigned to C-1 in G units or C-4 in S units

288

of lignin, whereas the signals at 127 and 116 pm were assigned to C-1/2 in phenolic acids and

289

C-5 in G units of lignin, respectively.41 It was observed that all these peaks disappeared in all

290

the solvent treated systems even with the treatment of pure H2O or pure GVL. In FT-IR

291

spectra (Figure 1B), it was observed that the peaks at 855 and 833 cm-1, respectively assigned

292

to out-of-plane deformation vibration of C-H in the S unit and C-H in the G unit of lignin,40

293

decreased after solvent treatments. Furthermore, the intensity of the two peaks was the

294

weakest in 75% GVL/H2O compared with the other ratios of GVL/H2O systems. Xu et al.

295

reported that there existed some easily degradable lignin in pubescens.44 Therefore, these 15



296

results suggested that these S and G units in lignin might be easily degraded in both pure

297

water and pure GVL, and high concentration of GVL in GVL/H2O co-solvent system might

298

be more benefit for the extraction of G and S units in lignin. Xue et al. studied the

299

interactions of lignin and GVL/H2O solvent by the calculation of Kamlet-Taft hydrogen bond

300

basicity parameter, and confirmed that the mechanism of lignin dissolution was attributed to

301

the formation of strong hydrogen bond interactions between lignin and the solvent system.21

302

Because G and S units in lignin contained more -OCH3 groups, so it was speculated that the

303

hydrogen bonds could be formed with GVL or H2O, and then promoted the dissolution of

304

lignin.

305

Smith et al. used molecular dynamics simulations to examine the structure of lignin in

306

co-solvent enhanced lignocellulosic fractionation process, and demonstrated that organic

307

solvent was more likely to be found near lignin compared with water, thus organic solvent

308

preferentially solvated lignin.45 The trends of organic solvent concentration near the lignin

309

surface increased with the water concentration decreased.45 That was why 75% GVL/H2O

310

co-solvent system was benefit for the dissolution of lignin with S and G units. Therefore, we

311

speculated that H2O exhibited advantages on breaking down the intermolecular linkages

312

linking the three main components in pubescens, while GVL preferentially dissolve lignin

313

with S and G units after the cleavage of intermolecular linkages by H2O. This could also be

314

further proved by 13C CPMAS NMR spectra (Figure 3(A)), where the gradually disappearing

315

of the signal at 56 pm assigned to methoxy groups of lignin42 was observed with increasing

316

concentration of GVL to 75% in GVL/H2O co-solvent system.

317

We also studied the effects of reaction temperature via FTIR spectra of solid samples in 16


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318

50% GVL/H2O (1:1) co-solvent system (Figure 3(B) and Figure S5). It was found that the

319

optimal temperature for the co-dissolution of hemicellulose and lignin via cleavage of

320

intermolecular linkages connecting the three main components was 160 oC. XRD (Figure S2)

321

and SEM (Figure S4) also confirmed that cellulose was kept almost intact after the removal

322

of hemicellulose and lignin at 160 oC. The results indicated that reaction temperature was

323

also responsible for the cleavage of intermolecular linkages connecting hemicellulose,

324

cellulose and lignin.

325

Therefore, H2O might form hydrogen bonds with linkages connecting hemicellulose,

326

cellulose and lignin and promote the cleavage of intermolecular chemical bonds, while the

327

synergetic effect of GVL and H2O promoted the dissolution of lignin via the formation of

328

hydrogen bonds with -OCH3 groups in lignin. H2O was responsible for the dissolution of

329

hemicellulose, while GVL affected lignin skeletal structure and helped the dissolution of

330

lignin after the cleavage of intermolecular chemical bonds by H2O.

331 332

The cleavage of intramolecular linkages of hemicellulose and lignin

333

The distribution of liquid products obtained from GVL/H2O co-solvent system with

334

different ratios by GPC analysis was used to study the cleavage of intramolecular linkages of

335

hemicellulose and lignin (Figure 4). After the pubescens samples being treated in pure H2O

336

system at 160 oC for 4 h, although the distribution of molecular weight in liquid products was

337

mainly in the range of 150-5000 Da (74%), 5% liquid products with molecular weight greater

338

than 50000 Da still existed (Figure 4(A)). Because most hemicellulose and partial lignin were

339

dissolved with the treatment of H2O, so oligomers derived from hemicellulose and lignin with 17



340

high molecular weight existed after being treated by H2O. After the pubescens samples being

341

treated by pure GVL, the liquid products with molecular weight greater than 50000 Da were

342

almost not observed. Because the three main components in pubescens were hardly dissolved

343

in pure GVL, it was difficult to say if GVL exhibited better performance on the

Figure 4 GPC analysis of liquid products obtained from GVL/H2O co-solvent system with different ratios at 160 oC for 4 h in the first step reaction: (A) 100% H2O; (B) 25% GVL/H2O; (C) 50% GVL/H2O; (D) 75% GVL/H2O; (E) 100% GVL

344

depolymerization of oligomers than H2O. After the pubescens samples being treated in

345

GVL/H2O co-solvent system, it was observed that oligomers with molecular weight greater

346

than 50000 Da were completely degraded to oligomers with relatively lower molecular

347

weight (Figure 4 (B), 4(C) and 4(D)). The results suggested that the coexistence of GVL with

348

H2O significantly promoted the depolymerization of oligomers derived from hemicellulose

349

and lignin to lower molecular weight oligomers. ESI-MS results showed that the molecular

350

weight in liquid products obtained by being treated in GVL/H2O co-solvent system with

351

different ratios was mainly in the range of 200-800 (m/z, Figure S6). GPC results showed that 18


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352

about 33% to 43% liquid products with molecular weight in the range of 200-800 Da were

353

obtained after being treated in GVL/H2O co-solvent system with different ratios (Figure S7).

354

Therefore, ESI-MS results matched well with GPC results.

355

In our previous work27, HPLC results showed that a much lower yield of small

356

molecular products derived from hemicellulose was obtained in pure GVL system, because

357

little hemicellulose was dissolved. Because high concentration of GVL in GVL/H2O

358

co-solvent system inhibited the performance of H2O for the dissolution of hemicellulose,

359

lower yield of small molecular weight products was obtained in 75% GVL/H2O system.

360

When the pubescens sample was treated in GVL/H2O co-solvent system with higher or equal

361

H2O concentration, the complete dissolution of hemicellulose was achieved. In order to study

362

the effects of GVL and H2O on the cleavage of intramolecular linkages in hemicellulose, the

363

yields of small molecular products obtained after the pubescens sample being treated in pure

364

H2O, 25% GVL/H2O and 50% GVL/H2O were compared. The main small molecular product

365

derived from hemicellulose in pure H2O system was xylose (30.2 wt%) accompanied with a

366

small amount of formic acid, acetic acid, furfural, HMF and glucose, and the total yield of

367

small molecular products was only 59.1 wt%. The low total yield of small molecular products

368

derived from hemicellulose obtained suggested the formation of oligomers derived from

369

hemicellulose with the treatment of H2O. The liquid products obtained from pure H2O system

370

were characterized by 2D HSQC NMR analysis (Figure 2 (A)) and the assignments of the

371

main hemicellulose cross-signals were done according to literature5 (Table S2). The results

372

suggested that β-D-xylp was the backbone of the dissolved hemicellulose. So the formed

373

oligomers derived from hemicellulose in pure H2O system were mainly β-D-xylp oligomers. 19



374

The total yield of small molecular products increased after being treated in 25% GVL/H2O

375

(82.9 wt%) or 50% GVL/H2O (74.7 wt%) system compared with pure H2O system. Therefore,

376

the contribution of GVL and H2O promoted the further depolymerization of dissolved

377

hemicellulose via the

378

Figure 5 Proposed reaction network for GVL and H2O promoted the cleavage of intramolecular linkages in hemicellulose. 379 380

cleavage of intramolecular linkages in hemicellulose. Mostofian et al. emphasized that when

381

THF was added, water molecules could form stronger H-bonds with the glycosidic linkages

382

of cellulose by molecular dynamics simulation.14 Similarly, in GVL/H2O co-solvent system,

383

H2O might form hydrogen bonds with β-(1,4)-glucosidic bonds in hemicellulose derived

384

oligomers, then promote the depolymerization and hydrolysis reaction to form xylose by the

385

cleavage of β-(1,4)-glucosidic bonds, while GVL might promote the binding of H2O to

386

glucosidic bonds to form xylose (Figure 5). It was also observed that the yield of furfural

387

significantly increased from 2.3 wt% in pure H2O system to 33.9 wt% in 25% GVL/H2O and

388

21.7 wt% in 50% GVL/H2O co-solvent system, respectively. Mellmer et al. studied the effect 20


Page 20 of 35

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389

of GVL on the acid-catalyzed conversion of xylose into furfural by reaction kinetics.46 Their

390

work showed that the use of GVL as a solvent changed the energetics of the reaction network

391

for the conversion of xylose to favor the formation of the desired furfural product. The

392

activation energy barrier for xylose dehydration was decreased in GVL, whereas the barrier

393

for furfural degradation was higher in GVL. In our previous work27, we have confirmed that

394

THF could interact with the hydroxyl groups of xylose by forming hydrogen bonds, which

395

promoted the cleavage of C1-O6 to form furfural. FT-IR spectra showed that the -OH peaks at

396

3300-3450 cm-1 became broader with the addition of GVL, suggesting GVL had significant

397

effects on -OH peaks (Figure 1(A)). Based on the above results, we speculated that GVL

398

might had similar function as THF in the conversion of xylose to furfural. The oxygen in the

399

ring of GVL might form hydrogen bonds with hydroxyl groups of xylose monomers, which

400

might promote the dehydration reaction and the cleavage of C1-O6 bonds in xylose to form

401

furfural (Figure 5). Therefore, significantly increase of furfural formation was observed in

402

GVL/H2O co-solvent system compared with pure H2O system. The significant promotion of

403

dehydration reaction might shift the equilibrium of the reaction in GVL/H2O co-solvent

404

system, which further promoted the depolymerization of oligomers derived from

405

hemicellulose and inhibited repolymerization. However, increasing GVL concentration in

406

GVL/H2O co-solvent system might inhibit the performance of H2O on the cleavage of

407

β-(1,4)-glucosidic bonds in hemicellulose derived oligomers to form xylose, so a lower yield

408

of furfural was obtained in 50 % GVL/H2O system compared with 25% GVL/H2O.

409

Because GVL was a high boiling solvent, so it was hard to be removed before NMR

410

analysis. The dissolved lignin could be obtained by precipitation with the addition of 21



411

water.3,19 Luterbacher et al. confirmed that lignin isolated by precipitation in GVL/H2O

412

solvent with the addition of water showed the similar structure to native lignin by 2D HSQC

413

NMR analysis.19 In the present work, we found that lignin was hard to precipitate with higher

414

GVL concentration in GVL/H2O co-solvent system. So the liquid products obtained from

415

single H2O system and 25% GVL/H2O co-solvent were further characterized by 2D HSQC

416

NMR analysis (Figure 2). The assignments of the main lignin cross-signals were done

417

according to literature (Table S3)2,5,30,46. As shown in Figure 2 (a), in the side-chain region

418

(δC/δH 50-103/2.6-6.0 ppm), cross-signals of various substructures linked by carbon-carbon

419

and ether bonds can be observed. β-O-4’ structures (A linkages) were the major inter-unit

420

structures in lignin fractions, other signals of linkages in lignin, such as β-β’ structures (B

421

linkages) and β-5’(C linkages), were also presented in the spectra. It was observed that the

422

signals of Aβ(G) and Aβ(S) respectively assigned to Cβ-Hβ in β-O-4’ structures of A linkages

423

connecting to G unit and Cβ-Hβ in β-O-4’ structures of A linkages connecting to S unit

424

appeared in the spectra after the pubescens samples being treated in 25% GVL/H2O

425

co-solvent system. In addition to the abundant β-O-4’ linkages in 25% GVL/H2O co-solvent

426

system, signals for β-β’ structures of B linkages also presented in the spectra with Cα-Hα,

427

Cβ-Hβ and Cγ-Hγ (Figure 2(b)). We also observed that the signal of -OCH3 become stronger

428

after being treated in 25%GVL/H2O co-solvent system compared with pure H2O system. The

429

phenomenon was probably due to the fact that more hydrogen bonds could be formed with

430

-OCH3 group in 25%GVL/H2O co-solvent compared with pure H2O system, then promoted

431

the dissolution of lignin especially A and B linkages in lignin. This further confirmed the 13C

432

CPMAS NMR and FT-IR results. In the aromatic region (δC/δH 103-145/6.0-8.0 ppm), 22


Page 22 of 35

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433

syringyl (S), guaiacyl(G) and small amounts of ρ-hydroxyphenyl (H) units were observed in

434

liquid fraction after H2O treatment. After being treated in 25% GVL/H2O co-solvent system,

435

more amount of S and G units were dissolved. This suggested that GVL and H2O were

436

benefit for the dissolution of S and G units in lignin. Notably, the signals of HB2,6 assigned to

437

C2,6-H2,6 in oxidized (Cα=O) p-hydroxyphenyl units were observed, indicating that the

438

co-contribution of GVL and H2O disrupted the Cα-OH bonds and promoted

Figure 6 Proposed degradative routes for breaking down the intramolecular linkages of lignin in pubescens. 439 440

the formation of Cα=O bonds. In all the GVL/H2O co-solvent systems at 160 oC for 4 h,

441

monophenols were hardly detected by GC-FID analysis, indicating that oligomers were the

442

main products via the cleavage of intramolecular linkages in dissolved lignin. Based on the

443

above 2D HSQC NMR results of 25% GVL/H2O, it was noticed that the oligomers obtained

444

were mainly including β-O-4’ and β-β’ linkages connecting to G and S units in lignin (Figure 23



445 446

6). Therefore, at lower temperature of 160

o

C, H2O promoted the cleavage of

447

β-(1,4)-glycosidic bonds in hemicellulose derived oligomers to xylose, while GVL enhanced

448

the dehydration of xylose to furfural. The synergetic effect of GVL with H2O promoted the

449

depolymerization of lignin to produce oligomers, which mainly included β-O-4’ and β-β’

450

linkages connected to G and S units in lignin. The understanding of the breaking of the

451

intramolecular linkages in hemicellulose and lignin would guide the tuning of reaction

452

conditions and process for effective conversion of biomass into value-added chemicals or

453

biofuels.

454

Theoretical investigation on the solvation effect of the dissolved oligomers

455

To investigate if solvation of the dissolved oligomers could help dissolution, the stability

456

of several typical oligomers observed by ESI-MS was probed by quantum chemical

457

calculation. Based on oligomers derived from hemicellulose obtained in ESI-MS spectrum,

458

the stabilities of xylose and typical xylose derived oligomers in GVL/H2O solvent systems

459

with different ratios were evaluated by quantum chemical calculation (Table S4 and Figure

460

S6). The results showed that these products mainly from C5 sugars in hemicellulose were the

461

most stable in the pure H2O system compared with the other solvent systems, which further

462

confirmed that H2O was benefit for the hemicellulose dissolution. The stability of some

463

typical lignin derived oligomers in ESI-MS spectra obtained in different ratios of GVL/H2O

464

systems, were also evaluated by quantum chemical calculation (Table S5). The results

465

showed that the products mainly included β-O-4’ of A linkages and β-5’ of C linkages

466

(m=298 and m=438) were more stable in pure H2O, suggesting that H2O promoted the 24


Page 24 of 35

Page 25 of 35


467

dissolution of part of lignin with these substructures. The product (m=304) with β-O-4’ of A

468

linkages was more stable in pure GVL, which suggested that GVL promoted the dissolution

469

of the part of lignin with this sub-structure. The product (m=480) mainly included β-O-4’ of

470

A linkages and β-β’ of B linkages was more stable in 75% GVL/H2O system, indicating GVL

471

with H2O might promote the dissolution of this type of lignin structure. It could be seen that

472

H2O and GVL respectively exhibited different performance on the dissolution of lignin

473

derived oligomers with different structures. Based on the above FT-IR results, GVL and H2O

474

exhibited synergistic effects on the dissolution of lignin, compared with single H2O or GVL

475

solvent. Therefore, we speculated that the solvation effects of GVL/H2O co-solvent with

476

suitable proportion were important for the complete dissolution of lignin.

477

The mechanism for degradation of pubescens in GVL/H2O system seemed like

478

“packing candies”, where these candies needed to be cut into small size and then “wrapping

479

paper” was used to packing them. Similarly, during the degradation of pubescens in

480

GVL/H2O system, acting as “knife”, H2O broke down the intermolecular bonds linking the

481

three main components by forming hydrogen bonds, and then promoted the dissolution of

482

hemicellulose. H2O might form hydrogen bonds with β-(1,4) glucosidic bonds in

483

hemicellulose derived oligomers and promote them to be converted to xylose. Besides, H2O

484

also promoted the dissolution of some easily degradable lignin with β-O-4’ of A linkages and

485

β-5’ of C linkages. while GVL acting like “wrapping paper” (solvation) helped H2O break

486

down the intermolecular linkages to dissolve hemicellulose. For intramolecular linkages of

487

hemicellulose, the interactions between oxygen of GVL and hydroxyl groups of xylose unit

488

could be formed, which promoted the dehydration reaction and ring open of xylose to furfural. 25



489

For intramolecular linkages of lignin, GVL preferentially dissolved lignin with β-O-4’ of A

490

linkages by forming hydrogen bonds with -OCH3 group of lignin. In GVL/H2O co-solvent,

491

the synergistic effects of GVL and H2O promoted the co-dissolution and conversion of

492

hemicellulose and lignin, avoiding the significant dissolution of cellulose and keeping the

493

naturally formed structure of C6 sugars almost intact. GVL and H2O enhanced the conversion

494

of hemicellulose to furfural, and promoted the depolymerization of lignin to oligomers

495

mainly including β-O-4’ and β-β’ linkages connecting to G and S units. Furthermore, the

496

other roles of GVL and H2O on the degradation of pubescens were unknown and needed to

497

be further investigated.

498

The present work studied the solvent effects of GVL and H2O on the dissolution and

499

conversion of hemicellulose and lignin in pubescens via the cleavage of inter- and intra-

500

molecular linkages. The understanding of the cleavage of inter- and intramolecular linkages

501

in raw biomass by solvent treatment was important, which would provide more information

502

on the effects of GVL and H2O on more complex biomass materials. This also can aid in

503

improving reaction conditions for effective conversion of biomass into value-added

504

chemicals or biofuels.

505

ASSOCIATED CONTENT

506

Supporting Information.

507

The supporting information is available free of charge on the ACS Publications website.

508

XRD, SEM, FT-IR results, ESI-MS, GPC, the details for the assignment in 13C CPMAS

509

spectra and 2D HSQC spectra and quantum chemical calculation details and results are

510

provided in the supporting information. 26


Page 26 of 35

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511

AUTHOR INFORMATION

512

Corresponding Author

513 514

*[email protected]. Author contributions

515

Y. P. Luo carried out the majority of the experiment and wrote the paper. Z. Li helped

516

with the analysis of ESI-MS, GPC and FT-IR. Y. N. Zuo and Z. S. Su carried out the quantum

517

chemical calculation. C. W. Hu supervised the project and revised the paper. All authors have

518

given approval to the final version of the manuscript and contributed to the scientific

519

discussion.

520

Notes

521 522 523

The authors declare no competing financial interests. ACKNOWLEDGMENT The authors acknowledge National Natural Science Foundation of China (No. 21536007)

524

111 Project (No. B17030) and Sichuan Science and Technology Program(No.2018JY0207)

525

for financially supported. The characterization of solid residues and liquid products from

526

Analytical and Testing Center of Sichuan University and comprehensive training platform of

527

specialized laboratory, college of chemistry, Sichuan University are greatly appreciated.

528

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Table of Contents

The solvent effects of GVL/H2O on the degradation of pubescens gave a guidance for the effective full utilization of biomass.

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H2O solvent on the ... - ACS Publications

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