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Hydrothermal degradation of cellulose at temperature from 200 °C to 300 °C Tanja Gagic, Amra Perva-Uzunali#, Zeljko Knez, and Mojca Skerget Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00332 • Publication Date (Web): 12 Apr 2018 Downloaded from http://pubs.acs.org on April 12, 2018

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Industrial & Engineering Chemistry Research

1

Hydrothermal

degradation

of

cellulose

at

2

temperature from 200 ˚C to 300 ˚C

3

Tanja Gagić†, Amra Perva-Uzunalić†, Željko Knez†, ‡, Mojca Škerget*, †

4

†

5

Chemical Engineering, University of Maribor, Smetanova ulica 17, 2000 Maribor, Slovenia

6

‡

7

*

8

Abstract

9

Cellulose was treated with subcritical water in a batch reactor within a temperature range of

10

200-300 ˚C and reaction time of 5-60 min. The main phases, such as water-soluble fraction,

11

acetone-soluble fraction and solid residue (remaining cellulose or char), were separated and

12

analysed. The analysis of water-soluble phase was done by HPLC equipped with UV and RI

13

detector, while acetone-soluble phase was analysed by GC-MS. Total sugar content was

14

determined by the phenol-sulphuric acid colorimetric method. The properties of char such as:

15

specific surface area, pore volume and pore diameter were determined by gas adsorption

16

method. A water-soluble phase mainly consists of sugar monomers and monomer degradation

17

products, while acetone-soluble phase, referred to also as bio-oil, consists of furans, phenols,

18

carboxylic acids, aldehydes, ketones and high molecular compounds. The reaction mechanism

19

of cellulose in subcritical water has been proposed based on the obtained results.

20

Keywords: Subcritical water; Cellulose degradation; Sugars; 5-hydroxymethylfurfural;

21

Levulinic acid; Bio-oil

Laboratory for Separation Processes and Product Design, Faculty of Chemistry and

Faculty of Medicine, University of Maribor, Taborska ulica 8, 2000 Maribor, Slovenia

Corresponding author. fax: +386 2 2527 774, E-mail address: [email protected]

1 ACS Paragon Plus Environment

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Introduction

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1

23

The green chemical process engineering represents an attractive topic in past few decades,

24

especially green treatments of biomass as renewable raw materials. One of environmentally

25

friendly technologies is subcritical water technology with many advantages compared to

26

conventional techniques 1.

27

Subcritical water is water at temperatures above its normal boiling point and below its critical

28

point at pressure at which it remains in liquid state. It represents a cheap, safe and non-toxic

29

processing fluid, which makes treatments inexpensive, ecofriendly, more selective and less

30

time consuming. Therefore, it is necessary to explain the behaviour of water under subcritical

31

conditions to better understand biomass treatment and the reasons for which subcritical water

32

is a suitable replacement for organic solvents. At ambient conditions, water is a polar solvent

33

with the dialectic constant of 79.9 Fm-1 and density of 1000 kgm-3 and thus represents a good

34

solvent for polar materials 2. The increase of the temperature leads to significant decrease in

35

the number of hydrogen bonds between molecules, causing the decrease of dialectic constant

36

values and density of water. Thus, at 250 ˚C and 50 bar the dialectic constant has a value of

37

32.5 Fm-1, which is similar to the value of the dialectic constant for methanol at ambient

38

conditions 3. As a result of these changes subcritical water can be used as an organic solvent

39

and becomes better miscible with less polar compounds 1. Moreover, compared to the organic

40

solvents, water is readily available, reusable, green, safe and low in cost. Another important

41

property is the ionic product, which increases with increasing temperature from 10-14 mol2L-2

42

at 25 ˚C to ≈ 10-11 mol2L-2 at 300 ˚C 4. Above this temperature, the ionic product decreases

43

again. These changes make subcritical water an important player in acid- and base-catalysed

44

reactions. Furthermore, lower viscosity and higher values of diffusion coefficient and thermal

45

conductivity at subcritical temperatures improve mass and heat transfer rates 5. All these


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46

properties make subcritical water a promising medium for many processes and also

47

satisfactory replacement for organic solvents.

48

Thus, subcritical water and biomass make an excellent combination for a sustainable chemical

49

industry, which comprises novel opportunities for production of chemicals, fuels and energy.

50

Furthermore, the utilization of waste biomass and by-products as a new feedstock increases

51

the profitability of the process and also avoids the problems related with their disposal.

52

Biomass has a complex structure, which includes three main structural components: cellulose,

53

hemicellulose and lignin 6. As one of the main biomass resource, cellulose represents a linear

54

glucose block structured polymer, which can easily undergo various chemical transformations

55

in order to obtain desired biomass products. Different articles studying the hydrolysis of

56

cellulose, as a biomass model compound by acid

57

have been published. Further on, there are many studies focused on the properties and

58

behaviour of cellulose in water under subcritical conditions with 12-14 or without 9, 15-21 using a

59

catalyst. Hydrothermal biomass treatments can be carried out using different reactor systems:

60

batch, semi-continuous and continuous. Different studies have shown that the yield and

61

composition of biomass products strongly depends on the reactor system used

62

batch reactor isn’t a great option for production of sugars, but an excellent method for

63

favouring the secondary reactions of the primary hydrolysis products to produce a high yield

64

of oil and char. Oppositely, semi-continuous and continuous reactor systems, which allow to

65

achieve reaction time of milliseconds, represent better option for production of oligomers and

66

monomers 22.

67

Subcritical water treatment of cellulose strongly depends on the reaction temperature,

68

residence time, catalyst used and, at near critical conditions, on the pressure 23. By changing

69

reaction temperature and pressure the properties of water are changed, which may inhibit or

7-9

or enzymatic

10, 11

catalysed reactions

22

. Thus, the



Page 4 of 30

24

70

accelerate reactions

. Cellulose begins to decompose at temperature of 200 ˚C and the

71

reaction rate is faster as the temperature increases

72

products doesn’t require long residence time. It was proved that pressurized water hydrolysis

73

of cellulose performed in residence times between 0.02–0.03 s can result in high selectivity of

74

sugars (up to 98 %)

75

treatment of cellulose in batch reactor within temperature range 220- 300 °C and reaction

76

times higher than 3 min were distributed in a water-soluble phase, acetone-soluble phase,

77

solid residue (remaining cellulose or char) and gases 6. The primary products of cellulose

78

were water-soluble sugars (oligomers and monomers) obtained by dissolution and hydrolysis

79

of cellulose and carboxylic acids, ketones and aldehydes obtained by dehydration of sugars.

80

Further reactions, such as re-polymerization, condensation and pyrolysis, which start at 250

81

˚C cause the formation of char, gases and bio-oil (acetone-soluble phase). These conclusions

82

were represented by Minowa et al. 25. The gas products from cellulose are CO2, CO, CH4 and

83

trace amounts of H2

84

non-sugar materials, is not easily gasified and doesn’t readily react to produce gas

85

gases are directly obtained from water-soluble products

86

derived from the oil 25, 26, 29, 31-33. Tolonen et al. 34 investigated the kinetics of subcritical water

87

treatment of microcrystalline cellulose at temperatures between 245 ˚C and 319 ˚C at constant

88

pressure of 25 MPa and proved that the conversion kinetics follow a pseudo-first-order

89

reaction kinetics.

90

The first report on the hydrothermal treatment of sucrose to produce carbonaceous

91

microspheres was by Wang et al.

92

hydrothermal conditions at temperatures between 220-250 ˚C was proposed by Sevilla et al.

93

36

94

of the carbonaceous microspheres. However, the oxygen in the core probably consists of less

27

25, 26

. Cellulose conversion into primary

. The main products, which were obtained from subcritical water

28

. However, it was proved that the oil phase, produced from sugars and 29

. The

30

. On the other hand, the char is

35

. The formation mechanism of

hydrochar under

. It was proved that there is a high concentration of oxygen both in the core and in the shell


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95

reactive groups (i.e. ether, quinone, pyrone), while the oxygen in the shell mainly contains

96

more reactive (hydrophilic) oxygen functionalities (i.e. hydroxyl, carbonyl, carboxylic, ester)

97

36

98

Therefore, many different reactions may occur during the treatment of cellulose by subcritical

99

water

.

37

. Consequently, the topic of this work is the investigation of the behaviour of

100

cellulose under subcritical water conditions and analysis of degradation products (i.e. water-

101

soluble and acetone-soluble products and char). This paper shows that it is possible to obtain

102

relatively high yield of important hydrolysis products of cellulose (sugars, 5-HMF, furfural

103

and organic acids) by hydrothermal process using a batch reactor. The reaction parameters,

104

temperature and reaction time, show a high influence on concentration and type of

105

degradation products. Compared to the previous investigations published in the literature,

106

some new degradation compounds were observed (e.g. lactose in water-soluble phase and

107

many compounds in acetone-soluble phase). Based on these results, the new reaction

108

mechanisms of cellulose degradation were also proposed. In addition, morphological

109

characteristics of char were determined. Furthermore, it is an introduction for future work

110

with real biomass material, such as waste from paper industry.

111

2

112

2.1 Materials

113

Microcrystalline cellulose powder was purchased from Merck (Germany). Sugar and other

114

standards, also high-purity reagents for HPLC analysis, were obtained from Sigma-Aldrich

115

(Germany).

116

2.2 Cellulose treatment with subcritical water

117

The subcritical water treatment of microcrystalline cellulose was carried out in a 75 mL

118

stirred batch reactor (series 4740 stainless steel, Parr instruments, Moline, IL, USA), which is

Materials and experimental methods



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119

designed for maximum operating temperature of 540 ˚C and pressure of 580 bar (Figure 1).

120

The experiments were performed within temperature range of 200-300 ˚C and reaction time of

121

5-60 min. A suspension of 3 g cellulose in 30 mL of deionized water was prepared and poured

122

into the reactor. The reactor was purged several times with nitrogen (Messer, Slovenia) in

123

order to remove oxygen, and thus avoid unwanted oxidation reactions as well as to control the

124

pressure in the reactor. The reactor was heated by an electrical wire. The heating rate was

125

around 10 ˚C per minute. The mixture was stirred at 600 rpm. The reaction time was

126

measured from the moment the set temperature was reached. After the reaction, the reactor

127

was subjected to rapid cooling in an ice bath.

128

The reactor content was filtrated by vacuum filtration using a pre-weighted standard filter

129

paper. The solid phase was primarily washed with water and afterwards with acetone.

130

Therefore, the water-soluble phase, the acetone-soluble phase and solid residue (remaining

131

cellulose or char) were obtained. The solid residue was dried in oven at 110 ˚C. The water and

132

acetone from phases were both evaporated at 40 ˚C under reduced pressure in a rotary

133

evaporator, after which water-soluble fraction was analysed by HPLC, while the acetone-

134

soluble fraction was analysed by GC-MS.

135

The total conversion and products yield were calculated by following equations: = 100 − =

∙ 100 %

∙ 100 %

136

Where i represents char (CH), water-soluble phase (WS) or acetone soluble phase (AS), while

137

RC represents remaining cellulose.

138

The yield of gases and losses was calculated according to following equations:


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/ = 100 −

+ !" + #" ∙ 100 %

139

2.3 Analysis of water-soluble products

140

The water-soluble phase was analysed by Shimadzu Nexera HPLC system equipped with

141

DGU-20A SR degasser, LC-20AD XR pump, SIL-20AC XR autosampler, CTO-20AC

142

column heater, RI and UV detector. The sugars and sugar derivatives were detected by RI

143

detector, while the other compounds were detected at 210 nm and 280 nm by UV detector.

144

The separation was achieved as isocratic method on chromatography column Rezex RHM-

145

Monosaccharide H+ (300 × 7.8 mm) at 80 ˚C with flow rate of 0.6 mL/min and using water

146

as mobile phase. The quantification of obtained products was performed using calibration

147

curves of standards. Results were expressed in weight percentage (% w/w).

148

Content of total carbohydrates in water-soluble phase was determined with the phenol-

149

sulphuric acid colorimetric method. A 2 mL aliquot of a sample was mixed with 1 mL of 5 %

150

aqueous solution of phenol, which was rapidly followed by addition of 5 mL of concentrated

151

sulfuric acid. The mixture was placed in an ultrasonic bath for 10 min and left to stand at

152

room temperature for 20 min for colour development. In hot acidic medium glucose is

153

dehydrated to hydroxylmethylfurfural which forms a green colour product with phenol and

154

has an absorption maximum at 490 nm. The calibration curve was obtained with glucose

155

standard 38. Results were expressed in weight percentage (% w/w).

156

2.4 Analysis of acetone-soluble products

157

The acetone-soluble fraction was analysed by Shimadzu GC-MS QP 2010 ULTRA system

158

with Headspace analyser. The system was equipped with Phenomenex ZB-5MS capillary

159

column with dimensions 30m x 0.25 mm ID and 0.25 µm film thickness. The carrier gas was

160

helium with a flow rate of 1.37 mL/min. Using headspace technique, the sample was heated at

161

temperature 140˚C for 10 min and a 1000 µL of headspace sample was injected using split 7 ACS Paragon Plus Environment


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162

injection (1:20) at 250 °C. The oven temperature program was as follows: 120 ˚C for 2 min

163

increased to 250 ˚C at 5 ˚C/min and kept for 5 min. The ion source temperature and interface

164

temperature were 200 ˚C and 250 ˚C, respectively. Mass spectra was recorded from m/z 35 to

165

500. Solvent cut time was 0.5 min, while the total run time was 33 min. Peak identification

166

was done using NIST library and thus the peak areas were calculated.

167

2.5 Analysis of char

168

The char characteristics (specific surface area, pore volume and pore diameter) were analysed

169

by gas adsorption method using Micrometrics ASAP 2020MP instrument and nitrogen as gas.

170

Char samples were degassed under vacuum at 70 ˚C for 660 min until a stable 10 µm Hg

171

pressure was established. Further, the adsorption measurements were performed at -196 ˚C 39.

172

The specific surface area was calculated by the BET (Brunauer- Emmett- Teller) equation,

173

while the overall pore volume and the average pore diameter were determined by BJH

174

(Barrett-Joyner-Helenda) method using the desorption isotherms 39.

175

3

176

3.1 Overall conversion of cellulose

177

Figure 2 shows the effect of temperature and reaction time on the overall conversion of

178

cellulose. It is obvious that at lower temperatures of 200 ˚ and 220 ˚C, the conversion

179

increased almost linearly with the increasing reaction time and reaches the values of 20.8 %

180

and 55.7 % in 60 min, respectively. Therefore, the rate and degree of conversion increased

181

with increasing temperature. At 250 ˚C all cellulose was converted into products: water-

182

soluble phase, acetone-soluble phase, gases and char.

183

3.2 Water-soluble phase

184

In Figure 3 the yield of water-soluble fraction as a function of residence time at different

185

temperatures is presented. At 200 ˚C and 220 ˚C the water-soluble fraction yield increased

Results and discussion


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186

almost linearly with increasing reaction time. The maximal yield was obtained after 60 min of

187

reaction and at 220 ˚C was 37.8 %, while at 200 ˚C it was much lower and amounted to 8.50

188

%. The yield of water-soluble products increased significantly with increasing temperature to

189

250 ˚C with maximum value of 68.6 % obtained in only 5 min, but further increase in time

190

caused a drastic yield decrease. With further increase of temperature to 300 ˚C, the yield of

191

water-soluble fraction was much lower than the yield obtained at 250 ˚C. The maximal yield

192

of water-soluble fraction at 300 ˚C reached after 5 min was only 22.0 %. These results are in

193

good agreement with the previous published data 6, 13.

194

The total carbohydrate content determined in the water-soluble phases is shown in Figure 4.

195

At temperature of 200 ˚C, the total carbohydrate content increased with reaction time,

196

reaching the maximum of 42.4 % in 1h. At 220 ˚C, the maximum yield was 69.0 % in 30 min,

197

after which it started to decrease. Oppositely, at 250 ˚C and 300 ˚C, the maximal carbohydrate

198

contents of 41.4 % and 23.4 %, respectively were obtained in 5 min and afterwards it

199

decreased with increasing reaction time.

200

Main sugars detected by HPLC were cellobiose, glucose, fructose and lactose. The

201

concentration of sugars is represented in Figures 5-6-7 as a function of reaction time for

202

different temperatures.

203

The cellobiose was detected already at 200 ˚C and 5 min and the concentration reached the

204

maximum of 11.0 % (w/w) at 15 min, after which it started to decrease. Cellobiose was still

205

present at 250 ˚C although only in small amount at 5 min, after which it finally disappeared.

206

The cellobiose conversion into glucose was maximal at 250 ˚C and 5 min, where the glucose

207

concentration was 42.6 % (w/w). The glucose isomerisation leads to the formation of fructose.

208

The maximal concentration of fructose of 15.8 % (w/w) was obtained at temperature of 220

209

˚C and reaction time of 30 min. The lactose was detected in low concentrations at 200 °C and



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210

reaction time up to 30 min and at 220 and 250 °C at reaction time of 5 min. The lactose was

211

probably formed from cellobiose due to the isomerisation of one of glucose ring to galactose.

212

At 300 °C no sugars were detected in water-soluble phase.

213

The sugar decomposition products, which were also detected by HPLC, were 5-

214

hydroxymethylfurfural (5-HMF), furfural, glycolaldehyde, erythrose, glyceraldehyde, 1,3-

215

dihydroxyacetone, 1,6-anhydroglucose, levulinic acid and sorbitol. Figure 8 shows

216

decomposition path of cellulose under subcritical conditions and possible products obtained

217

under these conditions proposed based on publications 40-42 and our results.

218

The concentrations of sugar degradation products are shown in Table 1. 5-HMF can be

219

produced directly from glucose, or from fructose formed by glucose isomerisation

220

concentration of 5-HMF increased with the increasing temperature and reaction time,

221

achieving the maximum of 19.1 % (w/w) at 250 ˚C and 15 min, where the fructose was not

222

present in the sample anymore. In samples obtained at 300 ˚C and 5 min only trace amount of

223

5-HMF were detected and as the reaction time was increased, it finally disappeared.

224

The furfural formation can be explained by isomerisation of glucose to fructose to produce 5-

225

HMF, which then losses –CH2O group to form furfural

226

increased with increasing temperature, achieving the maximum of 10.0 % (w/w) at 300 ˚C in

227

5 min, after which it was not present in the sample anymore.

228

Glucose can be degraded by retro-aldol condensation to produce glycolaldehyde and erythrose

229

31

230

% (w/w). The maximal glycolaldehyde concentration was 1.54 % (w/w) obtained at 200 ˚C

231

and 1h, after which it started to decrease. The maximal concentration of erythrose was 0.651

232

% (w/w) obtained at temperature of 250 ˚C and reaction time of 5 min. The dehydration of

233

glucose leads to the formation of 1,6-anhydroglucose

43

. The

44

. Furfural concentration was also

. Glycolaldehyde occurred already at 200 ˚C and 15 min where the concentration was 0.510

45

. The 1,6-anhydroglucose was first


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234

detected at 200 ˚C after 1 h of reaction and it achieved the maximal concentration of 3.89 %

235

(w/w) at 250 ˚C and 5 min. Furthermore, the fructose can be degraded by retro-aldol

236

condensation producing the glyceraldehyde, which further isomerizes to dihydroxyacetone 46.

237

The glyceraldehyde and 1,3-dihydroxyacetone were formed at 220 ˚C after 15 min with

238

concentrations of 7.63 % (w/w) and 0.352 % (w/w), respectively. The maximal concentration

239

of glyceraldehyde was 12.6 % (w/w) obtained at temperature of 220 ˚C and reaction time of

240

30 min, after which the concentration decreased. The both, glyceraldehyde and 1,3-

241

dihydroxyacetone, were not detected in samples obtained at higher temperatures (250 ˚C and

242

300 ˚C). Levulinic acid, as the main product of 5-HMF degradation was first detected at 250

243

˚C and 15 min. As the temperature and reaction time increased, the amount of levulinic acid

244

was also increased and achieved the maximum of 22.7 % (w/w) at temperature of 300 ˚C and

245

reaction time of 1h.

246

It can be concluded that the temperature and reaction time are important factors influencing

247

product distribution. However, it was proved that the yield of water-soluble fraction mainly

248

depends on the reaction time, due to the fast cellulose decomposition. Therefore, at

249

temperatures of 250 ˚C or higher the reaction time needs to be as short as possible in order to

250

obtain sugars as the main product. This is the main reason why batch reactors are not entirely

251

suitable for production of sugars. In batch reactors the reaction time is measured from the

252

point when the set temperature is achieved while the mixture of cellulose and water is

253

meanwhile in the reactor during the heating time. Thus, secondary reactions are enabled

254

which decrease the water-soluble fraction, but increase the yield of acetone-soluble phase,

255

char and gases 6.

256

3.3 Acetone-soluble phase

257

In Figure 9 the influence of temperature and reaction time on the conversion of cellulose into

258

acetone-soluble phase is presented. At lower temperatures of at 200 ˚C and 220 ˚C, the yield 11 ACS Paragon Plus Environment


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259

of acetone-soluble fraction was quite low and increased with the reaction time. At both

260

temperatures after 30 min of reaction, the yield of acetone-soluble phase reached 2 % and

261

with further increase of reaction time it stayed constant at 200 ˚C, while at 220 ˚C it slowly

262

increased to 5.44 % at 60 min of reaction. The yield of acetone-soluble phase significantly

263

increased with increasing the temperature to 250 ˚C. The highest yield of acetone-soluble

264

phase (15.5 %) was obtained at conditions of 250 ˚C and 30 min. After the maximum was

265

reached, the yield of acetone-soluble fraction decreased with increasing reaction time at same

266

temperature. By further increase of temperature to 300 ˚C, the yield of acetone-soluble phase

267

reached the maximal value of 14.2 % in 15 min and afterwards it decreased.

268

According to the literature the characteristic compounds of acetone-soluble phase are: furans,

269

organic acids and their derivatives, aldehydes, ketones, alcohols, esters, phenols, alkanes,

270

alkenes and high molecular compounds 47, 48.

271

The reaction mechanism of cellulose conversion to acetone-soluble compounds which is

272

proposed based on publications

273

mentioned, that the 5-HMF can be obtained directly from glucose or from fructose produced

274

by glucose isomerisation

275

various compounds and chemicals. As the main compound formed already at lower

276

temperatures, 5-HMF easily losses its acetol group forming a furfural 44. At the same time, the

277

different furan derivatives were detected, such as: 3-(2-furanyl)-2-propenal (1), methyl-2-

278

furoate (2), 2,2´-methylenebis(5-methylfuran) (3), etc. Furthermore, the loss of –OH group,

279

decarbonylation and hydroxylation of 5-HMF probably lead to the formation of 4-hydroxy-4-

280

methyl-2-pentanone, occurred at 200 ˚C in 15 min, which with increasing reaction time also

281

losses the –OH group forming the 4-methyl-3-penten-2-one. At longer reaction times, the 1,3-

282

butadiene carboxylic acid was detected, which represents the suitable basis for production of

283

levulinic acid detected at higher temperatures. Levulinic acid can be further converted into

40, 49

and our results is shown in Figure 10. It was already

43

. 5-HMF represents the crucial building block for production of


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284

cyclopentanones and aliphatic alkanes and alkenes. The detected benzene derivatives, such as

285

4-hydroxy-3-methylacetophenone (4), methyl benzoate (5), 1,1-dimethylethoxybenzene (6)

286

and others, were possible products of cycloaddition of furans. As the temperature and reaction

287

time were increased the benzene derivatives lead to the formation of phenols. Afterwards, the

288

Diels-Alder reactions between dienes and furans would lead to the production of benzofurans,

289

such as: 4,7-dimethylbenzofuran (7), 2-acetylbenzofuran (8), 2-methyl-5-hydroxybenzofuran

290

(9), etc. Likewise, acetic acid, propenoic acid, maleic acid, succinic acid derivatives were also

291

detected.

292

3.4 Gases, losses and char

293

Figure 11 shows the changes in the yield of char with reaction time at different temperatures.

294

At lower temperatures a high amount of solid residue was obtained due to poor conversion of

295

cellulose. As the reaction time increased the yield of solid residue slowly decreased.

296

However, at temperature of 250 ˚C and reaction time of 5 min, the whole cellulose was

297

converted; the char was formed and the yield of char reached the value of 20.8 % at 250 ˚C

298

and 5 min. Afterwards, it significantly decreased with increasing reaction time to 15 min,

299

after which again linearly increased with the reaction time. At 300 ˚C the yield of char was

300

21.2 % in 5 min and with further increase of reaction time to 15 min it increased to 30.5 %.

301

The characteristic properties of char were determined by gas adsorption. The results for each

302

sample were almost the same. The surface area was around 11 m2/g determined by the BET

303

(Brunauer-Emmet-Teller) method. The pore volume and the average pore diameter, which

304

were determined by BJH (Barrett-Joyner-Helenda) method, were around 0.014 cm3/g and

305

12.6 nm, respectively. Therefore, we can conclude that the obtained char consists of

306

mesopores.



Page 14 of 30

307

Figure 12 represents the dependence of the yield of gases and losses on temperature and

308

reaction time. It is obvious that at lower temperatures the yield of gases was quite low, while

309

with increasing the temperature and reaction time the yield of gases significantly increased.

310

Therefore, the maximal yield of gases and losses was 57.4 % at temperature of 300 ˚C and

311

reaction time of 60 min. Thus, it can be assumed that the yield of gases would continue to

312

increase with further increase of temperature and reaction time.

313

4

314

The hydrothermal reactions were performed in a batch reactor within temperature range of

315

200-300 ˚C and reaction time of 5-60 min. It was proved that the maximal yield of overall

316

cellulose conversion was 87.9 % at temperature of 250 ˚C and reaction time of 5 min. At

317

these conditions, the yield of water-soluble phase was the highest (68.6 %). The main

318

products of water-soluble fractions were sugars, such as glucose, fructose, cellobiose and

319

lactose. The main product was definitely glucose with maximal concentration of 42.6 %

320

obtained by treating cellulose with subcritical water at 250 ˚C and 5 min. The detected sugar

321

degradation products were 5-HMF, furfural, glycolaldehyde, erythrose, 1,6-anhydroglucose,

322

1,3-dihydroxyacetone, levulinic acid, glyceraldehyde, sorbitol. The maximal yield of acetone-

323

soluble fraction was 15.5 %, obtained at temperature of 250 ˚C in 30 min. The characteristic

324

compounds of acetone-soluble phase were furans, carboxylic acids, aldehydes, ketones,

325

alcohols, esters, phenols and high molecular compounds. 5-HMF, as the main product of

326

carbohydrates found in both water-soluble and acetone-soluble phase, represents a chemical

327

building block, important for production of various organic acids and biofuels

328

degradation product of 5-HMF is definitely levulinic acid, which is used industrially as a

329

solvent, as well as for production of polymers, plasticizers, fuels, antifreeze ingredients, food

330

additives and also in pharmaceuticals 50. Valuable product is also sorbitol, which is important

331

in medical, food and cosmetic uses 51. The highest yield of char was obtained at temperature

Conclusion

50

. Important


Page 15 of 30 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


332

of 300 ˚C in 30 min (30.5 %), while the maximal yield of gases was obtained at 300 ˚C in 60

333

min (57.4 %). It was proved that char structure consists of mesopores. All four main types of

334

products possess great potential for further synthesis of commodity chemicals, but it is mainly

335

necessary to choose proper reaction conditions in order to optimize the selectivity to desired

336

products and minimize undesirable products 6.

337

Supporting Information

338

The table with the main compounds of acetone-soluble phase detected by GC-MS (Area %).

339

Acknowledgments

340

The authors would like to acknowledge the Slovenian Research Agency (ARRS) for financing

341

this research in the frame of Program P2-0046 (Separation processes and production design).

342 343 344

Abbreviations and Symbols CO

Carbon monoxide

345

CO2

Carbon dioxide

346

CH4

Methane

347

GC-MS

Gas-chromatography-mass spectrometry

348

H2

Hydrogen

349

HPLC

High-performance liquid chromatography

350

mAS

Mass of acetone-soluble phase

351

mRC

Mass of remaining cellulose

352

mCH

Mass of char

353

mWS

Mass of water-soluble phase

354

m0

Mass of initial material

355

nd

Non-detected

356

RI

Refractive index

357

UV

Ultraviolet

358

!"

The yield of water-soluble phase

359

#"

The yield of acetone-soluble phase 15 ACS Paragon Plus Environment


360 361

Page 16 of 30

The yield of char

/

The yield of gases and losses

362 363

References

364

(1) Shitu, A.; Izhar, S.; Tahir, T. M., Sub-critical water as a green solvent for production of valuable

365

materials from agricultural waste biomass: A review of recent work. Global J. Environ. Sci. Manag.

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367

(2) Cheigh, C.-I.; Yoo, S.-Y.; Ko, M.-J.; Chang, P.-S.; Chung, M.-S., Extraction characteristics of

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26.

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(3) Singh, P. P.; Saldaña, M. D. A., Subcritical water extraction of phenolic compounds from potato

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Chem. Rev. 2002, 102, (8), 2725-2750.

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(6) Pavlovič, I.; Knez, Z. e.; Škerget, M., Subcritical water - a perspective reaction media for biomass

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nanopores. Carbon 2001, 39, (14), 2211-2214.

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cellulose. Carbon 2009, 47, (9), 2281-2289.

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food processing residues and model compounds by hydrothermal liquefaction. Renew. Sust. Energ.

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lignocellulosic biomass in the biorefinery: sugar reaction mechanisms. Carbohyd. Res. 2014, 385, 45-

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Furaldehyde from Glucose in Hydrothermal Reaction. In 14th International Conference on the

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(45) Cantero, D. A.; Vaquerizo, L.; Martinez, C.; Bermejo, M. D.; Cocero, M. J., Selective

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transformation of fructose and high fructose content biomass into lactic acid in supercritical water.

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Catal. Today 2015, 255, 80-86.

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Decomposition in Subcritical and Supercritical Water: Detailed Reaction Pathway, Mechanisms, and

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Kinetics. Ind. Eng. Chem. Res. 1999, 38, (8), 2888-2895.

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hydrothermal treatments of cellulose. Energy 2012, 42, (1), 457-465.

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(48) Jakab, E.; Liu, K.; Meuzelaar, H. L. C., Thermal Decomposition of Wood and Cellulose in the

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Presence of Solvent Vapors. Ind. Eng. Chem. Res. 1997, 36, (6), 2087-2095.

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(49) Williams, P. T.; Onwudili, J., Composition of Products from the Supercritical Water Gasification

481

of Glucose: A Model Biomass Compound. Ind. Eng. Chem. Res. 2005, 44, (23), 8739-8749.

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(50) Möller , M.; Nilges , P.; Harnisch, F.; Schröder, U., Subcritical Water as Reaction Environment:

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Fundamentals of Hydrothermal Biomass Transformation. ChemSusChem 2011, 4, (5), 566-579.

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(51) Zhang, L.; Etienne, M.; Vilà, N.; Walcarius, A., Functional Electrodes for Enzymatic

485

Electrosynthesis. In Functional Electrodes for Enzymatic and Microbial Electrochemical Systems,

486

WORLD SCIENTIFIC (EUROPE): 2017; pp 215-271.

487

Figure Captions

488

Figure 1. The batch reactor system.

489 490

Figure 2. The influence of reaction time on overall conversion of cellulose at different temperatures.

491 492

Figure 3. The influence of reaction time on water-soluble fraction yield at different temperatures.

Kabyemela, B. M.; Adschiri, T.; Malaluan, R. M.; Arai, K., Glucose and Fructose

Gao, Y.; Wang, X.-H.; Yang, H.-P.; Chen, H.-P., Characterization of products from


Page 21 of 30 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

493 494


Figure 4. The influence of reaction time on total carbohydrate content at different temperatures.

495

Figure 5. The influence of reaction time on sugar yield at temperature of 200˚C.

496


497


498

Figure 8. The decomposition pathway of cellulose.

499 500

Figure 9. The influence of reaction time on acetone -soluble fraction yield at different temperatures.

501 502

Figure 10. The proposed reaction mechanism of cellulose conversion to acetone-soluble compounds.

503

Figure 11. The influence of reaction time on char yield at different temperatures.

504

Figure 12. The influence of reaction time on gases and losses yield at different temperature.



505

Figure 1. The batch reactor system.

506

60 overall conversion, %

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

Page 22 of 30

40 200˚C 220˚C

20

0 0

10

20

30 40 time, min

50

60

70

507 508

Figure 2. The influence of reaction time on overall conversion of cellulose at different

509

temperatures.


Page 23 of 30

water-soluble fraction yield, %

80 60 200˚C 220˚C

40

250˚C 300˚C

20 0 0

10

20

30 40 time, min

50

60

70

510 511

Figure 3. The influence of reaction time on water-soluble fraction yield at different

512

temperatures.

total carbohydrates content, %

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


80 70 60 50 40 30 20 10 0 200˚C

220˚C 5 min

15 min

250˚C 30 min

300˚C

60 min

513 514

Figure 4. The influence of reaction time on total carbohydrate content at different

515

temperatures.


20 15 10 5 0

glucose

516 517

Page 24 of 30

25

5

15 30 reaction time, min cellobiose fructose

60 lactose


concentration of sugars, % (w/w)

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



30 25 20 15 10 5 0 5 glucose

15 30 reaction time, min cellobiose fructose

60 lactose

518 519



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



Page 25 of 30

45 40 35 30 25 20 15 10 5 0 5

15 reaction time, min

glucose

cellobiose

fructose

lactose

520 521




Page 26 of 30

522 523

Figure 8. The decomposition pathway of cellulose.


Page 27 of 30

18 acetone-soluble fraction yield, %

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


16 14 12 10

200˚C

8

220˚C

6

250˚C

4

300˚C

2 0 0

10

20

30 40 time, min

50

60

70

524 525

Figure 9. The influence of reaction time on acetone -soluble fraction yield at different

526

temperatures.

527 528

Figure 10. The proposed reaction mechanism of cellulose conversion to acetone-soluble

529

compounds.



35

yield of solid residue, %

30 25 20 250˚C

15

300˚C 10 5 0 0

10

20

30 40 time, min

50

60

70

530

Figure 11. The influence of reaction time on char yield at different temperatures.

531

70 60 yield of gases and losses, %

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

Page 28 of 30

50 200˚C

40

220˚C 30

250˚C

20

300˚C

10 0 0

10

20

30 40 time, min

50

60

70

532 533

Figure 12. The influence of reaction time on gases and losses yield at different temperatures.

534 535 536 28 ACS Paragon Plus Environment

Page 29 of 30 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


Table 1. The influence of reaction time and temperature on concentration (in % (w/w)) of degradation products of glucose and fructose detected in water-soluble phase.

200˚C Components

220˚C

250˚C

300˚C

5 min

15 min

30 min

60 min

5 min

15 min

30 min

60 min

5 min

15 min

30 min

60 min

5 min

15 min

30 min

60 min

Erythrose

nd1

nd

nd

nd

nd

nd

nd

0.382

0.651

nd

nd

nd

nd

nd

nd

nd

1,6-anhydroglucose

nd

nd

nd

1.24

nd

2.21

2.33

1.73

3.89

nd

nd

nd

nd

nd

nd

nd

5-HMF

1.80

6.24

4.00

9.41

5.22

9.28

12.3

5.70

4.01

19.1

nd

nd

0.406

nd

nd

nd

Furfural

1.19

2.40

1.87

3.32

2.54

3.10

3.24

2.90

2.93

nd

nd

nd

10.0

nd

nd

nd

1,3-dihydroxyacetone

nd

nd

nd

nd

nd

0.352

1.14

0.881

nd

nd

nd

nd

nd

nd

nd

nd

Glyceraldehyde

nd

nd

nd

nd

nd

7.63

12.6

2.14

nd

nd

nd

nd

nd

nd

nd

nd

Glycolaldehyde

nd

0.510

1.21

1.54

0.871

0.475

0.345

0.245

0.435

nd

nd

nd

nd

nd

nd

nd

Levulinic acid

nd

nd

nd

nd

nd

nd

nd

nd

nd

6.01

15.1

17.7

7.16

9.50

18.7

22.7

Sorbitol

nd

nd

nd

nd

1.56

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

nd

1

Non-detected

29

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