Characterization of Aqueous Products Obtained from Hydrothermal

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The characterization of aqueous products obtained from hydrothermal liquefaction of rice straw: focus on products comparison via microwave assisted and conventional heating Chong Liu, Qing Zhao, Yechun Lin, Yihuai Hu, Haiyan Wang, and Guichen Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03007 • Publication Date (Web): 08 Dec 2017 Downloaded from http://pubs.acs.org on December 9, 2017

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Energy & Fuels

1

The characterization of aqueous products obtained from hydrothermal

2

liquefaction of rice straw: focus on products comparison via microwave assisted

3

and conventional heating

4

Chong Liu†,‡,*, Qing Zhao‡, Yechun Lin†, Yihuai Hu†, Haiyan Wang† ,Guichen Zhang†

5

†

Merchant Marine College, Shanghai Maritime University, Shanghai 201306, China

6

‡

School of Physical Electronics, University of Electronic Science and Technology of

7

China, Chengdu, Sichuan 611731, China

8

Abstract: This paper focuses on the comparison of aqueous products obtained from

9

hydrothermal liquefaction (HTL) of rice straw via microwave (MW) assisted and

10

conventional treatment. A systematic investigation of HTL experiments of rice straw

11

via MW assisted and conventional heating treatment have been carried out, covering a

12

broad but mild temperature range from 150 to 230 °C at 20 °C intervals. In addition,

13

different reaction times were studied when the HTL rection temperature was 210 °C.

14

Comparing with conventional HTL, considerable aqueous products could be obtained

15

for MW assisted HTL while consumes less time and what’s more, repolymerization

16

behavior could be efficiently decreased and high saccharide yields could be obtained

17

with MW assisted heating. Besides, HTL temperature appeared to be the dominant

18

factor, while the increased residence time slightly changed the content of Total

19

Organic Carbon (TOC), typical sugars and acids both for conventional and MW

20

assisted HTL heating.

21

Key words: aqueous products; hydrothermal liquefaction; microwave assisted heating;

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conventional heating; rice straw

23 24

1.

Introduction

ACS Paragon Plus Environment

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

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Due to the increasement of world population and rapid evolvement of industries,

26

energy demand is constantly increasing in recent decades1. As a complementary

27

resource to fossil fuel, biomass has some advantages versus fossil fuel as it is

28

renewable and CO2 neutral energy source2. However, an economic way of converting

29

biomass to bio-energy has not yet been devised despite the fact that lingocellulosic

30

biomass costs significantly less than crude oil. HTL is an attractive process to produce

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bio-oil from wet feedstock as it reduces the need for feedstock drying comparing

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pyrolysis3, 4. HTL utilizes water as the only solvent and reaction medium. The

33

omission of expensive or hazardous chemicals during HTL process make it simple,

34

cost effective and environmentally friendly5. These characteristics are in compliance

35

with the principles of Green Chemistry6. The HTL is usually performed in water at

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temperature range of 150-374 °C under pressures of 4-22 MPa7, 8. In a typical HTL

37

process, feedstock is converted into bio-crude oil, aqueous products, gaseous

38

products9,

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composition strongly depends on parameters (temperature and residence time) of HTL

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and the great variety of biomass feedstock such as grasses and trees, and other sources

41

of lingocellulosic biomass12.

10

and solid residue while water as the solvent and reactant11. Products

42

Besides, MW radiation has recently been shown to be energy efficient heating

43

method comparing with conventional heating and it has become widely accepted as a

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mild processing. MW assisted hydrothermal treatment at the same temperature range

45

could get considerable amound of products with a less residence time as MW

46

irradiation is rapid and volumetric with the whole material heated simultaneously13.

47

Besides, there is also a good evidence to suggest that it can cause specific molecular

48

activations14, 15.

49

HTL degradation takes place in water and degrade the feedstock into small


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Energy & Fuels

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components into the aqueous phase firstly. Therefore, aqueous products analysis at a

51

mild HTL condition could help understand the initial HTL reaction mechanism.

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Despite the growing interest in the composition of HTL related process waters, so far

53

no attempt was published that characterize the key substances in the aqueous phase to

54

assess the stage of the HTL process heated by MW assist and conventional method.

55

Meanwhile, rice straw composed of cellulose, hemicellulose and lignin, is one of

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typical agricultural residues biomass and has a high utilization potential16, 17. Annually,

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around 731 million tons of rice straw is produced by Asia alone10.

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The aqueous products of HTL are intrinsically related to the HTL conditions such

59

as reaction temperature, residence time, particle size and the feedstock to water ratio18.

60

Among these factors, rection temperature and residence time are generally viewed as

61

two decisive parameters19, 20. Therefore, the present work aims to compare the HTL

62

aqueous products of rice straw via MW assist and conventional heating at mild

63

conditions. A systematic investigation of rice straw HTL processing has been carried

64

out, covering a broad temperature range from 150 to 230 °C. In addition, when the

65

temperature was 210 °C, different reaction times were studied through this two

66

different heating methods. TOC, pH and typical sugars and acids were quantified for

67

the analysis of the obtained aqueous products.

68

2.

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2.1 Feedstock

Materials and methods

70

Rice straw was milled and grounded to pass through a #20 mesh sieve and the

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particle size ranging between 0.2 and 1.0 mm. Rice straw was obtained from Shanghai

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Pudong district. The three main fractions of cellulose, hemicelluloses and lignin of the

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rice straw feedstock is shown in Table 1.

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Table 1 Chemical composition of rice straw (wt % dry matter)


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Cellulose

Hemicellulose

Lignin

32.18

18.88

24.20

2.2 HTL treatment

76

As Fig. 1 depicts, HTL converted rice straw into a carbon-rich aqueous phase via

77

MW assisted or conventional treatment. HTL samples were prepared by mixing

78

milled 5 g rice straw feedstock with 100 ml deionized water. A range of MW assisted

79

and conventional HTL experiments were carried out between 150 and 230 °C at 20 °C

80

intervals within 10 min. The HTL conditions were shown in Table 2. The residence

81

time is the time of HTL reaction under the preset temperature, excluding the time of

82

heating and cooling. This slurry was heated without any catalysts and additives in a

83

MW tube or conventional rector to designed temperatures.

84 85 86

Fig. 1. Schematic of MW assisted and conventional HTL of rice straw 2.2.1 Conventional HTL

87

HTL of rice straw was performed in a 250 mL completely mixed stainless steel

88

(316L) reactor containing a stirrer. The reactor was heated by a standard resistance

89

heater with power 1500 W. The temperature of the reactor was controlled by a

90

programmable temperature controller with a temperature detector in the reactor. The

91

reactor was sealed and heated to the desired temperature after loading the rice straw

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slurry. After reaching the desired residence time, the reactor was removed from the

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heater and cooled rapidly by a fan. The solid and liquid products were collected after


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depressurization, and HTL aqueous phase were separated from solid products by a

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vacuum buchner funnel through 0.45 µm membranes. Aqueous phase was stored in

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refrigerator at 4 °C for further utilization. Longer residence time, as Table 2 shows,

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was designed considering thermal gradient of conventional heating.

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2.2.2 MW assisted HTL

99

MW assisted HTL was performed in a 250 mL MW tube containing a magnetic

100

coil and a stirrer. The power of MW magnetic coil is 1200 W. Temperature of the

101

slurry during HTL was measured by a thermocouple. The temperature was then kept

102

constant for designed residence time before a fan started to cool the samples. Products

103

were treated using same steps as conventional HTL. Table 2 HTL conditions via MW assisted and conventional treatment

104

Tempe rature (oC)

Rice straw (g)

Water (mL)

150

5

170

Heating time (min)

Residence time (min)

Cooling time (min)

MW

Conventi onal

MW

Conventi onal

MW

Convent ional

100

10

10

5

30

10

10

5

100

10

10

5

30

10

10

190

5

100

10

10

5

30

10

10

210

5

100

10

10

5

30

10

10

230

5

100

10

10

5

30

10

10

210

5

100

10

10

5

30

10

10

210

5

100

10

10

10

60

10

10

210

5

100

10

10

15

120

10

10

210

5

100

10

10

20

180

10

10

210

5

100

10

10

30

240

10

10

105 106

2.3 Analytical methods

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TOC of aqueous products obtained from HTL was analyzed by a TOC analyzer

108

(TOC-L CPH, Shimadzu, Japan) and pH value was measured using a pH meter (FE20,

109

Mettler Toledo, Switzerland). The concentration of typical sugars and acids in the


Energy & Fuels

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aqueous products were measured by high performance liquid chromatography

111

(HPLC). The aqueous phase was filtered by a membrane of 0.22 µm before the test.

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Sugars and organic acids in aqueous phase product were measured by the differential

113

refraction detector of HPLC at 50 °C with a Hi-Plex H column. The mobile phase

114

contained 0.005 mol/L of an aqueous sulfuric acid solution. The flow rate was 0.4

115

mL/min and the temperature of the column was 55 °C. Calibration range was 0.5-2.5

116

mg/ml. Sulfuric acid applied during the mobile phase were chromatographically pure.

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3.

118

3.1 Characterization of conventional HTL aqueous products

Results

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It is not surprising that the HTL process was governed by rection temperature and

120

residence time to a large extent, as shown in Fig. 2. As a primary indicator of the

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production of soluble organic compounds, the values of TOC showed versatile

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tendencies with the increasing of temperature and residence time. The rice straw

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leaded to a TOC between 2.01 and 5.71 g/L during HTL under the conditions applied.

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Meanwhile, a significant increase of the TOC concentrations between 150 and 190 °C,

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while a decreasing tendency was observed between 190 and 230 °C or residence time

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from 30 min to 120 min. The pH of conventional HTL aqueous decreased from 6.32

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to 3.95 and 4.51 to 3.86, respectively, for the increasing of both reaction temperature

128

from 150 to 230 °C and residence time from 30 min to 240 min.

7

7

6

6

TOC (g/L) and pH

TOC (g/L) and pH

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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5 4 3

TOC pH

2

3

TOC pH

2

0 150

130

4

1

1

129

5

165

180

195

Temperature (

210

225

240

0

50

100

150

Time (min)

)

(A)

(B)


200

250

Page 7 of 17

131

Fig. 2. TOC and pH of the aqueous products obtained from conventional HTL of

132

rice straw (A) different reaction temperature when residence time was 30 min; (B)

133

different residence time when temperature was 210 °C

134

Fig. 3 (A) (B) presents the contents of typical sugars formed during conventional

135

HTL. As the temperature elevated from 150 to 210 °C, the concentrations of total

136

sugars increased from 0.06 to 0.35 g/L and then decreased to 0.25 g/L at 230 °C for 30

137

min. For all aqueous samples obtained from conventional HTL, levoglucosan, xylose

138

and fructose were the most prevalent sugars especially when reaction temperature

139

increased appropriately to 210 °C. Aqueous phase contained significant amounts of

140

levoglucosan, xylose and fructose with value of 0.16 g/L and 0.14 g/L at 210 °C for

141

30 min. However, all of the typical sugars concentrations dropped rapidly below 0.05

142

g/L when residence time was increased to 240 min at 210 °C.

0.15

0.20 Cellobiose Glucose Xylose & fructose Rhamnose Levoglucosan

Typical sugars content (g/L)

Typical sugars content (g/L)

0.20

0.10

0.05

0.00 140

Cellobiose Glucose Xylose & fructose Rhamnose Levoglucosan

0.15

0.10

0.05

0.00 160

180

200

220

240

0

50

o

143 144

(A)

146 147

250

200

250

Lactic acid Formic acid Acetic acid Levulinic acid

2.0

1.5

1.0

0.5

0.0 140

200

2.5 Lactic acid Formic acid Acetic acid Levulinic acid

Typical acids content(g/L)

2.0

150

(B)

2.5

145

100

Time (min)

Temperature ( 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

Energy & Fuels

1.5

1.0

0.5

0.0 160

180

200

220

240

0

50

Temperature (oC)

100

150

Time (min)

(C)

(D)

Fig. 3. Typical sugars and acids contents of aqueous products obtained from ACS Paragon Plus Environment

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148

conventional HTL of rice straw (A) different reaction temperature when residence

149

time was 30 min; (B) different residence time when temperature was 210 °C; (C)

150

different reaction temperature when residence time was 30 min; (D) different

151

residence time when temperature was 210 °C

152

As illustrated of acids contents in the aqueous products in Fig. 3 (C) (D), the total

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acid concentrations of aqueous phase which led to an increase of acidity have showed

154

a stable increase with the increasing of HTL temperature from 150 to 230 °C or

155

residence time from 30 min to 240 min and this was consistent with pH values

156

variation. The major acids of the aqueous products identified by HPLC were lactic

157

acid, acetic acid, formic acid and levulinic acid. As temperature elevated to 230 °C,

158

the contents of dominate acids of lactic acid and acetic acid were induced a significant

159

increasement to 1.86 and 1.66 g/L, respectively. Meanwhile, the total acids achieved

160

to 6.05 g/L at 210 °C when residence time increased to 240 min.

161

3.2 Characterization of MW assisted HTL aqueous products

162

The aqueous characterizations of TOC and pH values for MW assisted HTL

163

samples are shown in Fig. 4. The pH values decreased from 6.52 to 3.96 and from

164

4.15 to 3.68 at the designed HTL temperatures and residence time ranges. It is noticed

165

that TOC concentration of aqueous samples increased from 2.24 to 4.18 g/L when

166

HTL temperature increased from 150 to 230 °C. Meanwhile, TOC concentration

167

increased from 3.71 to 4.28 g/L with the increasement of residence time from 5 to 10

168

min and then decreased to 3.89 g/L when residence time was 30 min.

169


Page 8 of 17

7

6

6

5

5

TOC (g/L) and pH

7

4 3 2

TOC pH

0 140

4 3 TOC pH

2 1

1

0 150

160

170

180

190

200

Temperature (

170 171

210

220

230

240

5

10

15

20

25

30

Time (min)

)

(A)

(B)

172

Fig. 4. TOC and pH of the aqueous products obtained from MW assisted HTL of rice

173

straw (A) different reaction temperature when residence time was 5 min; (B) different

174

residence time when temperature was 210 °C

175 176

Fig. 5 presents the typical six sugars and four acids identified in the aqueous

177

products of MW assisted HTL. The concentrations of six sugars and four acids ranged

178

from 0.06 to 0.43 g/L and 0.56 to 4.73 g/L, respectively, when HTL temperature

179

varied from 150 to 210 °C for 5 min. Meanwhile, increased yields of sugars from 0.43

180

to 0.84 g/L and acids from 3.22 to 5.45 g/L were observed when residence time

181

increased from 5 to 30 min at 210 °C.

0.25

0.20

0.5 Cellobiose Glucose Xylose & fructose Rhamnose Levoglucosan

Typical sugars content(g/L)

Typical sugars content(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

Energy & Fuels

TOC (g/L) and pH

Page 9 of 17

0.15

0.10

0.05

0.00 140

Cellobiose Glucose Xylose & fructose Rhamnose Levoglucosan

0.4

0.3

0.2

0.1

0.0 160

180

200

220

240

0

5

o

182 183

10

15

Time (min))

Temperature ( C)

(A)

(B)


20

25

30

Energy & Fuels

4.0

3.0

4.0


2.5 2.0 1.5 1.0 0.5 0.0 140


3.5


3.5


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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3.0 2.5 2.0 1.5 1.0 0.5 0.0

160

180

200

220

240

0

5

o

184 185

10

15

20

25

30

Time (min)

Temperature ( C)

(C)

(D)

186

Fig. 5. Typical sugars and acids contents of the aqueous products obtained from MW

187

assisted HTL of rice straw (A) different reaction temperature when residence time was

188

5 min; (B) different residence time when temperature was 210 °C; (C) different

189

reaction temperature when residence time was 5 min; (D) different residence time

190

when temperature was 210 °C

191

As Fig. 5 depicts, sugars increased significantly when MW temperature at around

192

210 °C. Xylose, fructose and levoglucosan were the most prevalent sugars especially

193

when increased the residence time appropriately (30 min). Particularly, glucose

194

concentration was highly increased to 0.16 g/L at 210 °C for 30 min. Meanwhile,

195

significant upwards trends especially acetic acid were observed at temperature range

196

of 150-230 °C or residence time range of 5-30 min. Formic acid concentrations have

197

displayed about a third to half of the acetic acid load which was consistent with

198

Becker’s results21. Other chemicals with low concentrations such as HMF,

199

levoglucosenone, furfural and phenyl ethanol were also detected by HPLC, which

200

were shown in Table S6 and Table S7. Solid yields at different HTL conditions for

201

both heating methods were shown in Table S8.

202

4 Discussion

203

4.1 Typical prodcuts and HTL conditions


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Energy & Fuels

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As a primary indicator of the soluble organic compounds, the values of TOC

205

showed versatile tendencies with the increasing of temperature and residence time

206

both for conventional and for MW assisted HTL as as Fig. 2 and Fig. 4 show. There

207

was a maximum TOC concentration at 190 °C for conventional HTL or at 230 °C for

208

MW assisted HTL, which suggests that the soluble organics revealed a further

209

cleavage at appropriate higher temperature. TOC concentrations of aqueous phase

210

obtained from conventional HTL decreased with the increase of reaction temperature

211

from 190 to 230 °C or residence time from 30 to 120 min. It was also the same

212

tendency for MW assisted HTL when residence time increased from 10 to 30 min.

213

Akhtar et al. reviewed that repolymerization reactions lead to the decrease of TOC

214

value when increased rection temperature or residence time22. Therefore, the TOC

215

negative tendencies of aqueous products obtained from both heating methods were

216

possibly due to repolymerization reactions during HTL.

217

Significantly, xylose, fructose and levoglucosan had a relatively higher selectivity

218

for this mild HTL conditions at 210 °C for 20 min using MW assisted HTL and at

219

210 °C for 30 min using conventional HTL with maximum total sugurs of 0.75 g/L

220

and 0.35 g/L, respectively. The repolymerization behavior of the dissolved oligomers

221

or further decomposition to produce furfuals is the main challenge for efficient

222

monosaccharide yield at exorbitant temperature or residence time23, 24. As Fig. 3 (A)

223

(B) and Fig. 6 show, the total sugars expecially xylose, fructose and levoglucosan

224

decreased significantly when HTL temperature increased up to 230 °C or residence

225

time lasted for more than 60 min for convernational HTL. Longer residence time and

226

higher rection temperature promote acids production during HTL via conventional

227

heating method, which is consistent with Chen’s results10. Meanwhile, it is also the

228

same positive correlation for total acids content with residence time and rection


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229

temperature when HTL via MW assisted. Acids variation consistent with pH values,

230

as Fig. 2 reveals, the more acids produced, the lower of the pH values were.

231

There were intricate chemical rections during HTL of biomass including

232

hydrolysis, deoxygenation, cracking and repolymerization25. These reactions may

233

occur selectively depending on the reaction conditions and significantly promote the

234

formation of target product, while inhibiting other products. Typical chemicals such as

235

levoglucosan, glucose, xylose, fructose, acetic acid and formic acid were formed

236

during the mild HTL of rice straw, as Fig. 6 shows. Xylose and fructose which come

237

from hemicellulose24 had prominent concentrations in both conventional and MW

238

assisted HTL samples.

239

cellulose which releases glucose monomers occurred when HT temperature higher

240

than 150 °C, however, the yields of glucose were low as shown in Fig. 3 (A) (B) and

241

Fig. 5 (A) (B). This might well be the nature structure of rice straw which composed

242

of cellulose, hemicellulose and lignin. Cellulose is organized into microfibrils

243

surrounded by hemicellulose and encased inside a lignin matrix and even though

244

cellulose and hemi-cellulose have similar chemical compositions, cellulose is more

245

stable to be hydrolyzed to monosaccharide than hemicellulose27. Instead of glucose,

246

levoglucosan was also one of the main saccharides for both of the heating methods

247

present in Fig. 3 (A) (B) and Fig. 5 (A) (B). However, the destruction of monomer

248

sugars contributed to the formation of organic acids and furans when HT temperature

249

elevated to 210 °C as Fig. 6, Table S2 and Table S4 show.

According to the report by Savage et al.26, hydrolysis of


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Page 13 of 17

250

Fig. 6. The hydrolysis reactions of cellulose and hemicellulose leading to the formation of

251

sugars and acids

252 253

4.2 Comparision of conventional and MW assisted HTL

254

It was found that the HTL products including acids and sugars were more

255

sensitive to rection temperature than residence time, but different heating methods

256

like conventional or MW assisted heating still played an important role. For example,

257

when HTL temperature increased from 150 to 230 °C, both the primary parameters

258

like pH, TOC and the specific products like acids and sugars had tremendous changes

259

comparing with influence of residence time in spite of the multiply increasement of

260

reaction time as Fig. 3 (C) (D) and Fig. 5 (C) (D) present.

0.8 9

Total sugars concents (g/L)

Conventional MW 6

TOC (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

Energy & Fuels

3

0

140

261 262 263

160

180

200

220

240

0.6

Conventional MW

0.4

0.2

0.0

-0.2 140

160

180

200

220

240

Temperature (oC)

Temperature (oC)

(A)

(B)

Fig. 7. Total sugars and TOC of the aqueous products obtained from conventional


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264

HTL for 30 min and MW assisted HTL for 5 min (A) TOC of aqueous products (B)

265

total sugars contents of aqueous products

266

As Fig. 7 presents, it need only 5 min for MW heating to get considerable or even

267

higher amount of sugars and it has taken 30 min for conventional heating HTL at

268

210 °C although the TOC values were lower. MW has the potential to provide rapid

269

method for HTL28 as it interacts directly with the materials by changing

270

electromagnetic energy into heat transfer inside the dielectric materials29 instead of

271

conducting heat from an external heat source. MW heating can overcome the

272

problems of conventional heating method of thermal gradient and has a slight

273

temperature difference between the surface and interior of the slurry. The

274

repolymerization behavior or secondary reaction might be decreased and high

275

saccharide yields were obtained at a even higher HTL temperature but short residence

276

time which counteracted low TOC of the aqueous for MW assisted HTL. Therefore,

277

we can conclude that MW assisted HTL is propitious to sugars production comparing

278

with conventional heating at this mild HTL condition.

279

4.

Conclusion

280

The aqueous products obtained from MW assisted and conventional HTL of rice

281

straw were compared in this study. Levoglucosan, glucose, xylose, fructose, acetic

282

acid and formic acid were main chemicals formed during the mild HTL of rice straw.

283

HTL temperature appeared to be the dominant factor, while the increased residence

284

time slightly changed the content of TOC, typical sugars and acids both for

285

conventional and MW assisted HTL heating. The results also indicate that MW

286

radiation is an efficient method to decrease residence time to get considerable or even

287

higher amount of saccharide yields as it could decrease repolymerization behavior or

288

further decomposion.


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ASSOCIATED CONTENT

290

Supporting Information

291

Table S1-S8 can be found in supplementary information.

292

AUTHOR INFORMATION

293

Corresponding Author

294

† Chong Liu

295

Present Addresses

296

† Merchant Marine College, Shanghai Maritime University, Shanghai 201306, China,

297

E-mail: [email protected].

298

Author Contributions

299

The manuscript was written through contributions of all authors. All authors have

300

given approval to the final version of the manuscript.

301

ACKNOWLEDGMENT

302

The authors thank anonymous reviewers for fruitful suggestions.

303

ABBREVIATIONS

304

HTL, hydrothermal liquefaction; MW microwave; HPLC high performance liquid

305

chromatography; TOC , Total Organic Carbon.

306

REFERENCES

307 308 309 310 311 312 313 314

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Characterization of Aqueous Products Obtained from Hydrothermal

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