Depolymerization of Lignin Using a Solid Base Catalyst | Energy & Fuels

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Depolymerization of lignin using solid base catalyst Richa Chaudhary, and Paresh Laxmikant Dhepe Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00621 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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Energy & Fuels

Depolymerization of lignin using solid base catalyst Richa Chaudhary, Paresh L. Dhepe* Catalysis and Inorganic Chemistry Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India. Academy of Scientific and Innovative Research (AcSIR), New Delhi 110 025, India. *E-mail: [email protected]

1

ABSTRACT: Lignin extraction from lignocellulosic biomass has attracted considerable

2

attention for an alternative production of sustainable fuels and chemicals. We report the lignin

3

isolation from coconut coir using klason, organosolv and soda methods, and the

4

depolymerization of isolated lignin to value added chemicals using solid base catalyst. The

5

yield of isolated lignin by klason method was found to be about 4 to 6 times higher than other

6

methods. The structure of isolated klason lignin (CC-KL), organosolv lignin (CC-ORGL) and

7

soda lignin (CC-SL) were studied using ATR, NMR, microanalysis, etc. The monomer

8

molecular formula derived from microanalysis suggested that coir lignin is rich in guaiacyl

9

units. ATR and

13C

NMR clearly indicate that the CC-ORGL contains more C-C bonds

10

compare to CC-KL and CC-SL. Subsequently these isolated lignins were depolymerized over

11

solid base catalyst (NaX) under atmospheric pressure. CC-SL shows high yield of aromatic

12

products (28%) compared to CC-ORGL and CC-KL. In order to develop a sustainable future

13

technology, one-pot depolymerization of coconut coir was performed which resulted in a high

14

yield (64 %) of aromatic products.

ACS Paragon Plus Environment

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KEYWORDS: Coconut coir, Depolymerization, Lignin isolation, Lignocellulose, Solid base

16

catalyst.

17 18

1. INTRODUCTION

19

Cost-effective, efficient conversion of lignocellulosic biomass, mainly carbohydrates and

20

lignin, is fundamental to realising the full potential of the lignocellulose feedstock. Lignin is

21

especially hard to degrade and represents the major hurdle for the efficient utilization of the

22

lignocellulosic biomass. In bio-refinery concept, the foremost footstep is to isolate lignin from

23

lignocellulosic biomass. For the isolation of lignin, different methods are known in the

24

literature e.g. Klason, Kraft, soda, lignosulfonate, hydrolysis, ionic liquids, enzymatic,

25

microwave isolation etc.1-10 Out of these known isolation processes mainly four processes are

26

currently used on industrial scale for isolation of lignin; the sulfite (lignosulfonate), soda, Kraft,

27

and organosolv process.11 Since coconut is one of the oldest crops grown worldwide and

28

specially in India where it presently covers ca. 1.5 million hectares of land12 it can be a potential

29

source of lignocellulosic material. The approximate quantity of coconut production per annum

30

in world is 62.4 x 106 tonnes.13 According to official website of International Year for Natural

31

Fibres 2009, approximately, 500,000 tonnes of coconut fibres (coir) are produced annually

32

worldwide, mainly in India and Sri Lanka.14 Its total value is estimated at $100 million. India

33

and Sri Lanka are the main exporters, followed by Thailand, Vietnam, Philippines and

34

Indonesia. Around half of the coconut fibres produced are exported in the form of raw fibre.15

35

Moreover, it is most readily available material that farmers has access to. So, this abundantly

36

available raw material was chosen as a viable source for the lignin isolation and production of

37

value-added chemicals. A typical cross section of coconut is shown in Figure 1 and as seen

38

coir is a middle part of coconut.


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Energy & Fuels

Coir (middle fibrous coat of fruit) Coir(middlefibrouscoat of fruit)

Husk (outer coat of fruit)

Husk(outercoat of fruit) Kernel

Kernel

Shell (innerhard coat of thefruit)

Shell (inner hard coat of fruit)

39 40

Figure 1. Cross section of Coconut.

41

Coir is a biomass residue which is generated during the extraction of coir fibre from coconut

42

husk. The available literature for the chemical composition of coconut coir is presented in

43

Table 1.

44

Table 1. Chemical composition of coconut coir. Water

Pectin

soluble related (%)

andHemi-

Cellulose Lignin

cellulose (%)

Ash

(%)

(%)

Refs.

compounds (%) (%)

45

5.25

3.30

0.25

43.44

45.84

2.22

16

nd

nd

31.1

33.2

20.5

nd

17

nd

nd

15-28

35-60

20-48

nd

18

nd

nd

16.8

68.9

32.1

nd

19

nd

nd

-

43.0

45.0

nd

20

nd

nd

0.15-0.25 36-43

41-45

nd

21

nd

3.0

0.25

45.84

5.6

22

43.44

(nd): not determined


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As seen from the Table 1, the main contents of coconut coir are cellulose, hemicellulose

47

and lignin. Cellulose and hemicellulose are polysaccharide compounds while lignin is

48

a macromolecular polyphenolic compound.23 Compared to other lignocellulosic

49

materials (wheat straw, rice husk, bagasse, etc.) wherein lignin content is in the range

50

of 10-35%, coir contains higher amount of lignin (20-48%).23-25 Its accumulation on the

51

ground during rainy seasons can pollute the soil and water through leaching of

52

polyphenols which makes the coir pith unfit for the normal landfill practices. Therefore,

53

suitable waste management strategies have to be employed to solve the pollution risks

54

arising due to this particular lignocellulosic biomass. Recent research programs are

55

focusing to convert waste coir pith in to useful products such as biomanure,26 biochar27

56

etc. Thus, the lignocellulosic composition of the coconut coir has a huge potential to be

57

explored for its utilization as a substrate for the production of value-added chemicals.

58

Looking at the presence of high lignin content in coconut coir, it was believed that coir

59

can be converted into value-added chemicals and thus an alternate source of income can be

60

generated for the rural population. Several literature reports suggest that depending upon the

61

isolation procedures used, lignin properties vary.28 In order to avoid the use of complex lignin

62

as a substrate, most of the reports in the literature worked with lignin model compounds and

63

tried to develop systems for its valorization. Moreover, few of the researchers have used real

64

lignin as well as isolated lignins for the depolymerization with different catalytic systems like

65

solid acids, solid bases, ionic liquids, etc.28-33 In the present work, the initial phase of the

66

research was to try to understand the lignin content in the coconut coir. Further, focus was on

67

the isolation of lignin from the coconut coir by three methods namely, Klason, organosolv and

68

soda

69

Depolymerization of isolated lignin was studied at the best optimized reaction condition (250

process

followed

by

its

complete

characterization


and

depolymerization.

4

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Energy & Fuels

70

oC,

71

coconut coir as a substrate for the production of value-added chemicals was also studied.

1h) described in our previous work.30, 34 Furthermore, possibility of direct utilizing the

72 73

2. EXPERIMENTAL SECTION

74

2.1. Chemicals & Materials

75

Coconut coir samples (free of edible part) were first dried in sunlight for two days and further

76

oven dried at 55 oC for 16 h. The dried coconut coir was crushed to powder form and again

77

kept in oven at 55 oC for 16 h followed by vacuum drying at 120 oC for 4 h and stored in air-

78

tight lid container. Basic zeolite, NaX (Si/Al = 1.2) was synthesized using known procedure.35

79

Various aromatic monomers like Guaiacol (99%), 2-methoxy-4-methylphenol (98%),

80

pyrocatechol (99%), resorcinol (99%), 2,6-dimethoxyphenol (99%), 4-hydroxy benzyl alcohol

81

(99%), Diethylterephthalate (99%), 2,4-ditert-butylphenol (99%), Acetoguaiacone (98%), 4-

82

hydroxy-3-methoxybenzyl alcohol (98%), 1,2,4-trimethoxybenzene (97%), vanillin (99%),

83

eugenol (99%), 3,4-dimethoxyphenol (99%) used for GC calibration were purchased from

84

Sigma Aldrich, Alfa Aesar and TCI chemicals. All the chemicals were used as received.

85

Solvents like ethanol (99%, Changshu Yangyuan Chemical Co., Ltd, China), diethyl ether

86

(99.9%, LOBA Chemie) and ethyl acetate (99.9%, LOBA Chemie) were purchased and used

87

as received. NaOH (98%, Loba Chemie), H2SO4 (98%, Loba Chemie), HCl (37%, Merck) and

88

HF (48%, Merck) were also purchased and used as received.

89 90

2.2. Isolation of lignin from Coconut Coir

91

As known, depending upon the isolation methods, the structural and chemical properties of

92

isolated lignin vary. It is well known from the literature that linkages and functional groups

93

present in lignin varies from plant to plant. Due to these reasons, determination of the exact

94

structure, bonding and functional groups present in lignin is still a big challenge for the


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researchers. Among various procedures known for the isolation of lignin, in this work, Klason,

96

organosolv and soda processes for the delignification of coconut coir (CC) were employed (for

97

more details on isolation procedures please refer supporting information). The brief on the

98

isolation methods is illustrated in Figure 2.

99

From Klason method 48% of the lignin yield was obtained. Since this procedure is

100

known for the quantification of lignin present in the lignocellulosic biomass, it is estimated that

101

the coir used in this study has 48% lignin content. In order to avoid any experimental error, all

102

the isolation experiments were performed at least three times to check the reproducibility.

103

Based on the lignin present in coconut coir (considering Klason method), 16% and 20%

104

isolation of lignin was possible using organosolv and soda processes, respectively.


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Energy & Fuels

Coconut Coir

Lignin Isolation

Organosolv method

Soda method

Coir (8.0 g) + H2SO4 (1.024 mmol) + EtOH:H2O (60 mL, 1:1 v/v)

Coir (3.0 g) + 2 wt.% NaOH Solution (60 mL)

Klason method

Coir (1.0 g) in 100 mL RB + 72% H2SO4 (15 mL) Stirred vigorously @ 30 oC, 2 h Another 1000 mL RB + 150 mL H2O + Transferred H2SO4 digested mass slowly

Cooled reaction mixture

Kept RB @ 30 oC, 16 h Filtered with G2 crucible & washed with H2O Liquid (Acid soluble lignin, Polysaccharides)


Filter

Wash 100 mL RB with 195 mL H2O & transfer it into 1000 mL RB 1000 mL RB placed in preheated oil bath @ 100 oC, 4 h, stirring

160 oC, 5 h

180 oC, 1 h

Solid (Pulp; Cellulose, Hemicellulose, Ash, etc)

Filtered & washed with H2O

Liquid (Lignin & Soluble Sugars)

Oven Dried @ 55 oC, 16 h Oven Dried @ 55 oC, 16 h

Vacuum Dried @ 90

Oven dried @ 55 oC, 16 h Vacuum Dried @ 110 oC, 1 h

Liquid Acidified to pH 1 with conc. H2SO4

180 mL H2O

oC,

Boiled, 1 h Kept for 12 h

Precipitate (Hydrophobic lignin)

Solid

Solid (Cellulosic residue)

4h Wash with 100 mL H2O

Liquid

Vacuum Dried @ 90

Soluble Sugars

oC,

4h

Precipitate obtained

Filtered & washed with H2O until pH comes 7

Oven Dried @ 55 oC, 16 h


Vacuum Dried @ 90 oC, 4 h


Organosolv Lignin (CC-ORGL)

Soda Lignin (CC-SL)

Uncorrected Lignin Heat @ 620 oC, 2 h for ash correction

105 106

Klason Lignin (CC-KL)

Figure 2. Isolation of lignin by Klason, organosolv and soda processes.

107


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108

2.3. Catalytic runs of Coconut Coir and Isolated Lignin

109

Prior to the catalytic runs, the best active alkali metal substituted zeolite catalyst (NaX)

110

explored in our previous work was thoroughly characterized (Table S1, ESI).30 In a typical

111

reaction, lignin : NaX or coir : NaX mass ratio was maintained at 1: 1 (lignin : NaX molar ratio

112

= 13.3) and lignin : solvent (EtOH: H2O, 1: 2 v/v) mass ratio was kept 1 : 60. Initially, the

113

reactor was flushed with nitrogen and the heating of the reactor was started under slow stirring

114

(100 rpm). After attaining the desired reaction temperature (200 °C/250 °C), the stirring rate

115

was increased to 1000 rpm and this was considered as the starting time of the reaction. After

116

the reaction, the reactor was cooled to room temperature. The catalyst was separated from

117

reaction mixture by centrifugation and washed thoroughly with EtOH : H2O (1 : 2 v/v) in order

118

to remove any adsorbed lignin or products on the catalyst. After separation of catalyst from the

119

reaction mixture, acidification of the liquid layer was done with 2N HCl solution until the pH

120

reached to 2. Acidification process helped precipitate out the high molecular weight products

121

(filter cake). Further the centrifugation and filtration. The liquid and solid fractions were

122

subjected to the extraction process for the isolation of products. Organic solvents diethyl ether

123

(DEE) and ethyl acetate (EtOAc) were used for products isolation. Further the organic solvent

124

(DEE and EtOAc) soluble products were analyzed using GC and GC-MS. The methodology

125

for the extraction of products is represented in Figure 3.


8

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Energy & Fuels

Reaction Charge Depolymerization Reaction Mixture Centrifugation

Solid (SR) (Catalyst + Solid) Extraction with DEE and EtOAc

Solution (EtOH + H2O soluble) HCl (pH 1-2) Acidified mixture

Soluble*

Insoluble

Evaporation Solid (Filter cake) Extraction with DEE Soluble* Evaporation Aromatic products

Insoluble Extraction with EtOAc

Soluble*

Soluble* Evaporation Aromatic products

Insoluble

Evaporation

127

Insoluble Extraction with EtOAc

Soluble*

Insoluble

Evaporation

Aromatic products

126

Aromatic products

Liquid Extraction with DEE

Aromatic products

Figure 3. Methodology for products extraction (* Analyzed by GC, GC-MS & HPLC).

128

3. RESULTS AND DISCUSSIONS

129

From the isolation studies, it was realized that in the CC sample used in this study

130

contains 48% of lignin.

131 132

3.1. XRD

133

To understand the crystalline and amorphous nature of the coconut coir (CC) and isolated lignin

134

(Klason, organosolv, soda), XRD analysis was done (Figure 4). In CC sample, peak at 21.7 o

135

is observed for the crystalline phase of cellulose and 16.7

136

cellulose. These very low intensity peaks which are in contrast to other samples wherein peaks

137

for cellulose have higher intensity is because CC sample contains very high concentration of

138

lignin (48%) and thus concentration of cellulose is very low compared to other lignocellulosic

139

material samples. After the isolation of lignin (Klason, organosolv, soda) from the coconut coir


o

for the amorphous phase of

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140

(CC), no peaks are observed for the crystalline and amorphous phase of cellulose. This

141

confirms that isolated samples do not contain cellulose impurities. A broad peak pattern for the

142

isolated samples confirms the amorphous nature of lignin.

143 144

Figure 4. (A) XRD patterns of Coconut Coir (CC) and Isolated Lignin {Organosolv (CC-

145

ORGL), Soda (CC-SL) and Klason (CC-KL) lignin}, (B) XRD patterns of different samples

146

of Coconut Coir (CC-1-4).

147 148

3.2. Microanalysis

149

To derive the monomer molecular formula for the samples, elemental analysis (C, H and O)

150

for the coconut coir and isolated lignin was performed (Table 2 and Table S2, ESI). As it is


10

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Energy & Fuels

151

well known that the composition of polysaccharides and lignin varies as the origin and type of

152

plant varies. Even for the same species collected from different places shows the variation in

153

its composition as it is affected by the type of soil, climate, etc. Here, four different coir samples

154

were used and the results are compared with the isolated lignin samples (Table 2 and Table S2,

155

ESI). It was observed that coconut coir has higher O/C ratio (1.09) compared to isolated lignin

156

(0.46-0.56). This is very obvious as the oxygen content decreases after the removal of

157

polysaccharides (C5 and C6 sugars) from the coconut coir. Higher heating value (HHV)

158

calculated using Dulongs formula shows that coir having less HHV (13.4 MJ/kg) compared to

159

isolated lignin (22.3-24.9) because of the presence of higher oxygen content and presence of

160

polysaccharide in the coconut coir. This can be simply understood based on the molecular

161

formula of polysaccharide (cellulose and hemicellulose) and lignin monomer like guaiacol.

162

The molecular formula of cellulose (C6 sugar) and hemicellulose (C5 sugars) are C6H12O6 and

163

C5H10O5, respectively. These will give the O/C ratio of 1 which is well resembles with the O/C

164

ratio of coconut coir of approximate molecular formula C7H9-11O6. Coniferyl alcohol, a lignin

165

building block unit and guaiacol (lignin monomer) have the molecular formula C10H12O3 and

166

C7H8O2 respectively, shows the less oxygen content with low O/C ratio. Similarly, H/C ratio

167

was also calculated and observed to be ca. 0.1. The low H/C and O/C ratios are desirable for

168

the use of coir for energy generation while in another case low O/C and high H/C ratio is

169

suitable for using those as fuels. High HHV and lower O/C ratio of isolated lignin shows that

170

it is a good source for the production of fuels and chemicals. Moreover, double bond

171

equivalence (DBE) of all the samples (coir and isolated lignin) were also calculated and the

172

data shows the DBE between 5.6-5.8 for all the lignin samples. This shows a good correlation

173

with monomer molecular formula and building blocks of lignin i.e. sinapyl, coniferyl and

174

coumaryl alcohol. DBE 4 is considered for one benzene ring and one for exo carbon-carbon

175

double bond. Furthermore, higher value of DBE for CC-ORGL sample is observed that reflects


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176

the fact that CC-ORGL has a highly condensed more aromatic structure. During organosolv

177

pulping treatment, condensation reactions besides from the fragmentation of lignin might also

178

occur which gives rise to new carbon-carbon bonds in ORGL. According to a former study,

179

these carbon-carbon inter-unit bonds were more easily formed by the guaiacyl (G) type lignin

180

due to the presence of the free C-5 position.36, 37 It can be stated that CC-ORGL is rich is 5-5

181

biphenyl type of linkages. Monomer molecular formula was also derived using elemental

182

analysis and based on that it is suggested that lignin is rich in guaiacyl units which is very well

183

matches with the literature.38

184 185 186

Table 2. Microanalysis of Coconut Coir and Isolated Lignin. Microanalysis

CC

CC-ORGL

CC-SL

CC-KL

C (%)

45.35

60.78

63.58

63.59

H (%)

4.86

5.41

6.08

6.12

O (%)

49.79

33.81

29.34

29.39

O/C

1.09

0.56

0.46

0.46

H/C

0.11

0.09

0.10

0.10

HHV (MJ/kg)a

13.4

22.3

24.8

24.9

DBEb

2.18

5.8

5.6

5.6

MMFc pHd

C7.6H8.84O6.22 6.4

C10.1H10.7O4.2 6.3

C10.6H12.1O3.8 5.9

(a)

C10.6H12.1O3.8 2.95

187

Higher heat value (HHV) = [0.3383 x C + 1.442 x [H-(O/8)] + 9.248 x S] where C, H, O and S

188

are wt.% of carbon, hydrogen, oxygen and sulphur; (b) Double bond equivalence (DBE) = [C –

189

(H/2) + (N/2) + 1] where C, H and N are number of carbon, hydrogen and nitrogen atoms found

190

from monomer molecular formula (c) Monomer molecular formula (MMF) = 100 - (‘C’ wt.% +

191

‘H’ wt.% + ‘O’ wt.%). (d) pH was measured by dissolving 0.08 g sample in 5 mL water.

192


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Energy & Fuels

193

3.3. Thermo Gravimetric Analysis - Differential Thermal Analysis (TGA - DTA)

194

Thermal analysis of coconut coir (CC) and isolated lignins was performed under

195

nitrogen atmosphere upto 1000 oC (Figure S2-S5, Table S3, ESI). It was observed that

196

coconut coir and lignin start losing weight at around 150 oC, may be due to the removal

197

of moisture. Cleavage of α and β-aryl-alkyl-ether linkages was observed in the range of

198

200-450 oC. Complete decomposition of organic moieties was observed at ~600 oC.

199

Klason lignin shows ca. 35% of weight loss from 200-500 oC. It might be due to the

200

cleavage of side chain or aryl alkyl ether linkages followed by the cleavage of C-C

201

linkages present in lignin structural units. It can be stated from the TGA-DTA analysis

202

of Klason lignin that it contains more number of aryl-alkyl-ether linkages as it takes

203

long time to decompose at ~200-300 oC. It is well known from the literature that

204

organosolv lignin is rich in carbon-carbon linkages between lignin subunits. It is well

205

matched from the TGA-DTA analysis of organosolv lignin. Similar observation was

206

made for the soda lignin also as it shows weight loss in the range of 200-360 oC for ether

207

linkages and until 400 oC for the cleavage of C-C linkages. A careful observation

208

suggests that for all the lignin samples having DTA peak maxima at different positions.

209

This is because each lignin has different-different linkages with various functional

210

groups (shown in ATR and

211

polymerization also reflect in TGA-DTA graphs. Klason lignin shows peak maxima at

212

390 oC, having highest molecular weight and Soda lignin (low molecular weight

213

compared to Klason lignin) shows at 375 oC, which indicates the complete cleavage of

214

carbon-carbon linkages present in lignin structure. However, organosolv lignin has least

215

molecular weight among them, and it shows the peak maxima at 335 oC, which indicates

216

the splitting off the aliphatic side chains and C-C bond.

13C

NMR). Moreover, molecular weight and degree of


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217

It was also observed that in extracted lignin, 22-25% of unburnt residue is

218

obtained while in coir sample only 6% residue is observed. These studies suggest that

219

carbon in the sample will remain unburnt. This can be explained on the basis of

220

monomer molecular formula derived from elemental analysis. Coir is having 7.6 ‘C’,

221

8.8 ‘H’ and 6 ‘O’. Considering this, almost 95% of carbon is possible to burn in the

222

form of CO and CH4. It is possible that 6 ‘C’ will be consumed in the form of 6 CO and

223

another 1.2 carbon can be consumed as 1.2 CH4 molecules. Still 0.4 ‘C’ is remaining

224

which will remain as unburnt residue. A quick calculation discloses that this remaining

225

0.4 ‘C’ out of 7.6 ‘C’ gives rise to ca. 5 % of residue. This percentage of unburnt residue

226

matches well with the experimental data. Moreover, it is well correlation with the ash

227

analysis which is also 3-4%. Similarly, for isolated lignin it contains 10 ‘C’, 11 ‘H’ and

228

4 ‘O’. After the complete use of ‘H’ and ‘O’ present in lignin in the form of 4 CO + 3

229

CH4, there will be remaining 3 ‘C’ as unburnt residue. This will give ca. 30% of residue.

230

Likewise, moisture present in the sample can be correlated with the dryness analysis.

231

For e.g. From dryness analysis (Section 2.1, ESI), it was calculated that coir shows

232

94.4% dryness in the sample which shows the remaining is a moisture (ca. 6-7%). This

233

result also matches well with the experimental data (8% moisture) obtained from TGA-

234

DTA.

235

Although all the lignins were isolated from same coconut coir (CC) still slight

236

quantitative variation in the loss of weight percentage can be derived from TGA data.

237

The decreasing order for the percentage of weight loss of aryl-ether linkages (200-350

238

oC)

239

weight loss for the cleavage of C-C linkages (350-400 oC) is 10% in CC-ORGL while

240

it was 8% for both, CC-KL and CC-SL. At the temperature range of 400-600 oC, ca.

241

20% weight loss was observed for CC-ORGL while for CC-KL and CC-SL, 18% weight

was as follows: CC-KL (17%) > CC-SL (18%) > CC-ORGL (20%). The percent


14

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242

loss was seen. Moreover, different DTA peak positions also confirms the variation in

243

the molecular weight of lignins. Hence, it can be clearly stated that depending upon the

244

selection of isolation procedure, lignins with different properties can be obtained.

245 246

3.4. UV-Visible Analysis

247

Contribution of colour in the lignin is identified by using UV-Vis spectroscopy. Samples were

248

prepared in EtOH : H2O (1 : 2 v/v) and were analysed in the range of λ = 200-800 nm (Section

249

2.6, Figure S6, ESI). UV-Vis spectra show an adsorption band at 280 nm and a shoulder at 230

250

nm with a gradual decrease in absorption extending towards visible region representing

251

presence of several different chromophores. These bands are common in all the samples which

252

corresponds to π-π* electronic transition in aromatic ring of unconjugated phenolic units. Peak

253

appearing at 280 nm can be assigned for the presence of non-conjugated phenolic compounds,

254

which confirms the presence of hydroxyl groups. Moreover, occurrence of a shoulder band at

255

230 nm confirms the presence of mono and di-substituted aromatic rings.

256 257

3.5. Attenuated Total Reflection (ATR) Spectroscopy

258

The Attenuated Total Reflection (ATR) spectroscopic analysis of the coir and isolated

259

lignin (from CC) (Figure 5) confirms the presence of various functional groups. In the

260

ATR spectrum of the coir absorption band at the region near 1720 cm-1 which may be

261

due to a carboxyl group of acetyl ester in cellulose and carboxyl aldehyde in lignin is

262

observed.39 Lignin present in the coir gives characteristics peak at 1220, 1608 and 1720

263

cm-1 for the aromatic skeletal vibrations and C=O stretching in ketone, carbonyl and

264

ester groups. Isolation of lignin reduces hydrogen bonding due to the removal of the

265

hydroxyl groups. This results in the decrease of –OH group concentration, as can be

266

seen from the decreased intensity of the peak between 3650-3200 cm-1 compared to the


15

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

Page 16 of 43

267

coir. Absorption band present at 2900-2800 cm-1 can be assigned for the asymmetric

268

stretching and symmetric stretching of C-H bond in methyl and methylene groups.

269

Significant functional groups in the range of 1700-1550 cm-1 present in all the isolated

270

lignin samples. Peaks in the range of 1600-1400 cm-1 confirms the presence of

271

aromaticity or benzene ring. Moreover, this band can be assigned for the presence of

272

C=C attached to the aromatic rings. The ATR spectrum of isolated lignin clearly

273

indicates the presence of the characteristic band of the C-O stretching of alkoxy groups

274

or presence of ether linkages in the region of 1300-1000 cm-1. Peaks in the range of 850-

275

820 cm-1 corresponds to substituted phenolics and alkene groups. The observance of a

276

peak at 780 cm-1 is due to deformation vibrations of C-H (oop) bonds associated to

277

aromatic rings. All the absorption band observed are summarized in Table 3.

278

Furthermore, a careful observation at the ATR spectra of isolated lignins shows

279

the variation in the presence and intensities of peaks. CC-ORGL and CC-SL shows the

280

presence of more intense peak for C=C group attached to aromatic rings at 1600-1400

281

cm-1

282

cm-1. Again, a more intense peak for the deformation vibrations of C-H bonds in

283

aromatic rings was observed in CC-ORGL at 1110 cm-1. A peak at 1690 cm-1 was

284

observed in CC-ORGL and CC-SL for the presence of C=O stretching in unconjugated

285

ketone, carbonyl and ester groups which was not observed in CC-KL.

and

for

C-C

stretching

in

guaiacyl


units

at

1300-1200

16

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286 287

Figure 5. ATR analysis of Coir (CC) and Isolated lignin (CC-KL, CC-ORGL, CC-SL).

288 289

Table 3. Summary of ATR bands present in Coir (CC) and Isolated Lignin (CC-SL,

290

CC-ORGL, CC-KL).

Wavenumber (cm-1) Band Type of vibration (cm-1)

CC-

CC-

CC-

SL

ORGL

KL

CC


17

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

3650-3200

Alcoholic & phenolic O-

Page 18 of 43

3298

3280

3378

3386

2916

2922

2949

2889

1720

1690

1690

1706

-

-

-

1677

H stretching (free and involved in hydrogen bonding) 2960-2910

C-H asymmetric stretching in methyl and methylene group

1740-1680

C=O stretching in unconjugated ketone, carbonyl and ester groups

1670-1620

C=O stretching in conjugated substituted

1645

aryl group 1615-1595

C=O stretching with

1608

1598

1605

-

1520

1508

1510

1518

1440

1450

1442

-

1355

1366

-

1208

1228

1290

aromatic skeleton vibrations 1520-1505

Aromatic skeleton vibrations

1470-1440

Deformation vibrations of C-H bond

1370-1350

Aliphatic C-H stretching in methyl and phenolic OH

1300-1200

C-C, C-O, C=O stretching 1200 in guaiacyl units


1283

18

Page 19 of 43 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

291

Energy & Fuels

1120-1105

Deformation vibrations of

292

C-H bonds in aromatic

293

rings

294

1195-1124

295

1110

-

-

1172

-

1145

1027

1010

1020

1010

-

812

804

873

rings

297 1027-1010

C-O stretching in alcohol, ether/ in-plane

299 300

deformation vibrations of

301

C–H bonds in aromatic

302

rings.

303

1105

C-H bonds in syringyl

296

298


1113

875-700

Substitution on aromatic

304

ring or substituted

305

phenolics

780

306 307

(-) Not observed

308 309

3.6. Solid State 13C NMR

310

13C

311

reveals mainly the presence of monolignols and end group distributions. The 13C NMR

312

spectra of coir and isolated lignin (Klason, organosolv and soda lignin) are shown in

313

Figure S7-S10, ESI. Peaks appearing in the range of 160-180 ppm represents the

314

presence of ester group. High intense peak as observed in the range of 110-150 ppm

315

which corresponds to the presence of sp2 carbon in aromatics and alkenes, which again

316

confirms the aromatic nature of the samples. The chemical shift for the aromatic regions

317

for all the samples were recorded in the range of 90-140 ppm. Presence of sp3 carbon

NMR characterizations of coir (CC) and isolated lignin was performed which


19

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Page 20 of 43

318

attached to the oxygen atom is also observed in the range of 55-90 ppm. An intense peak

319

at 55-56 ppm common in all the lignin samples shows the presence of methoxyl group

320

attached to the aromatic ring. The appearance of methoxy groups in the range of 20-50

321

ppm was observed in all the samples. Detailed summary for the assigned peaks are

322

tabulated in Table 4.

323

The CC-KL shows the presence of C3 and C4 in the etherified guaiacyl units at

324

147.13 ppm while it was not observed in CC-ORGL and CC-SL. Moreover, the intensity

325

of peak at 130.85 ppm for C2,6 in p-hydroxyphenyl units is more in CC-ORGL. A

326

comparatively more intense peak for the presence of C3,5 in p-hydroxyphenyl units is

327

observed in CC-ORGL at 116.12 ppm while the presence of C2 in guaiacyl unit is only

328

present in CC-SL at 111.11 ppm. The presence of C2,6 in tricin was observed in CC-

329

ORGL & CC-SL at 106.82 ppm & 106.65 ppm, respectively while C8 tricin is present

330

in CC-KL & CC-SL at 92.52 ppm & 92.51 ppm respectively. Similarly, C in -O-4

331

substructures was observed in CC-ORGL & CC-SL at 82.28 ppm & 84.76 ppm while

332

C in -O-4 substructures is present in CC-ORGL at 67.06 ppm. Among all lignins a

333

highest intense peak at 56.05 ppm was observed in CC-KL for the presence of large

334

amount of methoxyl groups attached to the aromatic rings. It was clearly understood

335

from the ATR and 13C NMR that CC-ORGL contains more C-C bonds compare to CC-

336

KL and CC-SL which corelates well with the literature also. Moreover, although same

337

coir is used

338

for the isolation of lignin using different isolation procedures, different structures

339

of lignins are observed.

340 341

Further on the basis of 13C NMR analysis peaks were correlated to the structures present in isolated lignin samples (Figure S11, ESI).

342


20

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Energy & Fuels

343

Table 4. Summary on 13C NMR of Coconut Coir (CC) and Isolated Lignin (CC-KL,

344

CC-ORGL, CC-SL). Chemical Shift (ppm) Functional group CC

CC-KL

CC-ORGL

CC-SL

-

-

177.91

-

177.98

C=O group in Hibbert’s

175.53

175.04

175.87

-

-

-

173.39

174.82

C4 in p-hydroxyphenyl unit -

160.14

161.95

-

C in -O-4 substructures

155.55

151.89

153.20

152.37

-

147.13

-

-

-

-

130.85

129.19

116.22

115.61

116.12

115.94

C2 in guaiacyl units

-

-

-

111.11

C2,6 in tricin

105.47

-

106.82

106.65

-

99.18

-

100.86

-

C8 in tricin

93.49

92.52

-

92.51

-

89.04

89.28

-

88.31

C in -O-4 substructures

83.66

-

82.28

84.76

C in -O-4 substructures

74.74

72.97

72.41

73.35

ketone -

linked with C=O C3 and C4 in etherified guaiacyl unit C2,6 in p-hydroxyphenyl units C3,5 in p-hydroxyphenyl units


21

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

345 346 347 348 349 350 351 352

Page 22 of 43

72.60 C in -O-4 substructures

-

-

67.06

-

-

64.48

-

63.14

65.18

C-H in methoxyl group

55.44

56.05

55.88

56.10

-

-

-

48.93

49.75

-

-

46.64

-

46.52

 carbon CH2 with

-

43.47

42.86

42.92

353

aliphatic substituted group

354

-

43.47

-

-

39.59

355

 carbon in phenyl

36.87

-

36.23

36.50

356

propanol group 32.69

32.90

-

-

-CH2 alkyl group

30.48

30.15

29.71

30.31

-

21.17

23.62

23.18

-

-

-

-

-

19.15

Terminal -CH3 group

-

14.58

14.08

14.91

357 358 359 360 361 362 363

(-) Not assigned

364 365

3.7. Depolymerization of isolated lignin using solid base catalyst (NaX)

366

Depolymerization studies were carried out for isolated Klason lignin (CC-KL),

367

organosolv lignin (CC-ORGL), soda lignin (CC-SL) and coconut coir (CC) using NaX

368

as the basic catalyst.30 All the reactions were conducted in a batch mode autoclave (Parr,

369

USA). EtOH : H2O (1 : 2 v/v) was used as the solvent system in all the reactions. After

370

the reaction, the extraction of products from the reaction mixture was carried out as

371

shown in Figure 3. When reactions with different lignins were carried out at 250 oC for


22

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Energy & Fuels

372

1 h, variation in the depolymerisation product yield was observed from 5-28 % (Figure

373

6A). It is known from the literature that, order of molecular weight (Da) of isolated

374

lignin is in the following order: Klason lignin (CC-KL) > soda lignin (CC-SL) >

375

organosolv lignin (CC-ORGL).40, 41 As seen from Figure 6, CC-KL showed the lowest

376

depolymerisation product yield mostly due to higher molecular weight41 Although, In

377

case of CC-ORGL and CC-SL, lower molecular weight of lignin is present but still not

378

much improvement in the yield was seen. The probable reason for lower yield is that

379

CC-ORGL is known to have more C-C linkages compared to C-O-C linkages (as proven

380

by TGA, ATR and NMR studies) and very few C-C linkages can be broken at milder

381

reaction conditions and thus it may give lower yield. It can be seen from the ATR spectra

382

of CC-ORGL and CC-SL, more intense peak at 1600-1400 cm-1 was observed which

383

belongs to C=C groups attached to aromatic groups and 1300-1200 cm-1 for C-C

384

stretching in guaiacyl units. Moreover, 13C NMR also validate this fact as the intensity

385

of the peak at 150-110 ppm for the presence of sp2 carbon is more in CC-ORGL compare

386

to CC-SL. Considering this, it was obvious to see the maximum depolymerization

387

product yield (28%) with CC-SL sample. Further the formation of aromatic products

388

was confirmed using GC-MS (Figure S12-S15, ESI). Products formed with isolated

389

lignin samples were observed similar with the commercial lignin studies.30 A careful

390

look at the GC-MS chromatographs of the organic solvent soluble products reveals that

391

DEE contains higher concentration of low molecular weight products than EtOAc. This

392

might be because the products were extracted consecutively in DEE and EtOAc.

393

However, the weight of products extracted in EtOAc is higher than in DEE. Similar

394

phenomenon was also observed in our previous work.30 Further, it was assumed that the

395

variation in the properties of lignin, yields different types of products for e.g. In the case

396

of CC-ORGL it was expected to observe higher C-C type of products or dimeric/trimeric


23

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

Page 24 of 43

397

products since C-C bond is difficult to break under this reaction condition. Looking at

398

the GC-MS chromatographs of products it is clear that under this reaction condition

399

ether bonds are breaking. More to this, the catalytic results (product yields and products

400

formed) also show that the reaction is governed as per the molecular weight of the lignin

401

which is explained above in this section. The identified products are tabulated in Table

402

S4, ESI.

403

404 405

Figure 6. (A) Depolymerization of isolated lignin and coir using NaX. Reaction

406

Condition: Lignin/Coir (0.5 g), NaX (0.5 g), EtOH : H2O (1 : 2 v/v, 30 mL),

407

250 oC, 1 h, (B) Direct hydrolysis of CC using NaX. Reaction Condition: Coir (0.5 g),

408

NaX (0.5 g), EtOH : H2O (1 : 2 v/v) 30 mL, 200 oC, 1 h.

409 410

As it is always preferable to use abundant and inexpensive lignocellulose material

411

(coconut coir, CC) directly for the synthesis of value-added aromatic chemicals in order

412

to develop a sustainable future technology, here we have also used it as a

413

substrate. Hydrolysis of CC was also carried out using NaX as a catalyst and it was

414

compared with non-catalytic reaction (Figure 6B). When reactions were done at 200 oC

415

for 1 h, it was observed that direct hydrolysis of CC shows a good product yield of 64%


24

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Energy & Fuels

416

compared to non-catalytic reaction (27%). The product yield was calculated by

417

considering 0.5 g of substrate charged. Although, CC contains 48% of lignin but under

418

this reaction condition we observed peak for HMF from GC-MS and small peaks for

419

glucose & oligomers from HPLC. Hence, yield was calculated by considering the weight

420

of products obtained after the reaction/substrate charged. Further the formation of aromatic

421

products was confirmed by using GC, GC-MS and HPLC techniques. The GC-MS

422

chromatogram of the products confirms the formation of aromatic products (Figure S16,

423

ESI). The quantification of aromatic products was done by procured standard compounds

424

and product distribution is given based on total detected products from GC-MS and HPLC (Table

425

S5, ESI). As cellulose and hemicellulose are also present in the CC, so in order to check

426

the formation of any sugar products, HPLC analysis was also performed and very low

427

intensity peaks for glucose and HMF were observed and quantified (Figure S17, Table S5,

428

ESI). A meticulous look to the formation of more aromatic products (mostly lignin

429

depolymerized products) in case of coir can be understood as lignin present in CC : catalyst

430

wt./wt. ratio is almost double (0.24 g lignin present in coir). Hence, the product yield

431

increases. Further the variation observed in the CC and isolated lignin products is possible

432

due to the presence of cellulose and hemicellulose in the coir.

433 434

ASSOCIATED CONTENT

435

Supporting Information

436

The following file is available free of charge on the ACS Publications website:

437

Isolation of lignin from coconut coir by Klason method, organosolv method and soda method;

438

Characterization of Coconut Coir and Isolated Lignins via Dryness Analysis, Analysis of Ash,

439

X-Ray Diffraction (XRD) analysis, Elemental analysis, Thermo Gravimetric Analysis-

440

Differential Thermal Analysis (TGA-DTA), Ultraviolet-Visible (UV-Vis) Spectroscopy,


25

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

Page 26 of 43

441

Attenuated Total Reflection (ATR) Spectroscopy, Solid state 13C NMR; Analysis of reaction

442

mixture by Gas Chromatography (GC-FID), Gas Chromatography-Mass Spectrometry (GC-

443

MS) and High Performance Liquid Chromatography (HPLC),

444 445

AUTHOR INFORMATION

446

Corresponding Author

447

*P.L.D.: tel, +91-20 2590-2024; fax, +91-20 2590-2633; e-mail, [email protected].

448

Author Contributions

449

All authors contributed equally to this work. All authors discussed the results and implications

450

and commented on the manuscript at all stages.

451

Notes

452

The authors declare no competing financial interest

453 454

ACKNOWLEDGMENTS

455

Richa Chaudhary thanks, University Grants Commission (UGC), India for Research

456

Fellowship.

457 458

ABBREVIATIONS

459

GC-FID, gas chromatography-flame ionization detector; GC- MS, gas chromatography mass

460

spectrometry; HPLC, high performance liquid chromatography; ATR, Attenuated Total

461

Reflection spectroscopy; NMR, nuclear magnetic resonance spectroscopy; TGA-DTA,

462

thermogravimetric analysis-differential thermal analysis; XRD, X-ray diffraction, UV-vis,

463

ultraviolet-visible.

464 465


26

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Energy & Fuels

466 467

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468

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469

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2. Long, J.; Xuehui Li; Bin Guo; Jurong Wang; Yinghao Yu; Wang, L., Simultaneous

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delignification and selective catalytic transformation of agricultural lignocellulose in

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cooperative ionic liquid pairs. Green Chemistry 2012, 7, (14), 1935-1941.

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3. Zhang, J.; Haibo Deng; Lu Lin; Yong Sun; Chunsheng Pan; Liu., S., Isolation and

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characterization of wheat straw lignin with a formic acid process. Bioresource technology

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2010, 7, (101), 2311-2316.

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4. Perez-Pimienta; Jose A.; Monica G. Lopez-Ortega; Patanjali Varanasi; Vitalie Stavila; Gang

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Cheng; Seema Singh; Simmons., B. A., Comparison of the impact of ionic liquid pretreatment

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on recalcitrance of agave bagasse and switchgrass. Bioresource technology 2013, 127, 18-24.

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5. Bharadwaj, R.; April Wong; Bernhard Knierim; Seema Singh; Bradley M. Holmes; Manfred

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Auer; Blake A. Simmons; Paul D. Adams; Singh., A. K., High-throughput enzymatic

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hydrolysis of lignocellulosic biomass via in-situ regeneration. Bioresource technology 2011,

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6. Dereca Watkins; Md Nuruddin; Mahesh Hosur; Alfred Tcherbi-Narteh; Jeelani., S.,

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Extraction and characterization of lignin from different biomass resources. Journal of

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7. Zakzeski J; Bruijnincx PCA; Jongerius AL; BM., W., The catalytic valorization of lignin for

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10. Francisco Avelino; Kássia Teixeira da Silva; Men de Sá Moreira de Souza; Selma Elaine

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11. Zea Strassberger; Tanase, S.; Rothenberg, G., The pros and cons of lignin valorisation in

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12. NIIR Board of Consultants & Engineers, The Complete Book on Coconut & Coconut

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13. Food and Agriculture Organization of the United Nations: Statistical Division (FAOSTAT).

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16. Coir Board, Ministry of MSME, Govt. of India, (http://coirboard.gov.in/?page_id=62).

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of corroded fibres on the strength of mortar. Cement and Concrete Composites 2005a, 27, (5),

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575-582.

509

18. Agopyan, V.; Savastano Jr, H.; John, V. M.; Cincotto, M. A., Developments on vegetable

510

fibre-cement based materials in Sao Paulo, Brazil: An overview. Cement and Concrete

511

Composites 2005, 2, (5), 527-536. .

512

19. Asasutjarit, C.; Hirunlabh, J.; Khedari, J.; Charoenvai, S.; Zeghmati, B.; Shin, U. C.,

513

Development of coconut coir-based lightweight cement board. Construction and Building

514

Materials 2007, 21, (2), 277-288. .


28

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20. Satyanarayana, K. G.; Sukumaran, K.; Mukherjee, P. S.; Pavithran, C.; Pillai, S. G. K.,

516

Natural fibre-polymer composites. Cement and Concrete Composites 1990, 12, (2), 117-136.

517

21. Corradini, E.; De Morais, L. C.; De Rosa, M. F.; Mazzetto, S. E.; Mattoso, L. H. C.; Agnelli,

518

J. A. M., A preliminary study for the use of natural fibers as reinforcement in starch-gluten-

519

glycerol matrix. Macromolecular Symposia 2006, 245-246, 558-564. .

520

22. Shukala, S. R.; Roshan, S. P., Comparison of Pb(II) Uptake by Coir and Dye Loaded Coir

521

Fibers in a Fixed Bed Column. Journal of Hazardous Materials 2005, 125, 147−153.

522

23. Muensri, P.; Kunanopparat, T.; Menut, P.; Siriwattanayotin, S.,Effect of lignin removal on

523

the properties of coconut coir fiber/wheat gluten composite. Composites 2011, 42, 173-179.

524

24. Waifielate, A. A.; Abiola, B. O., Mechanical property evaluation of coconut fibre. Master's

525

Degree Thesis, Department of Mechanical Engineering, Blekinge Institute of Technology,

526

Sweden 2008.

527

25. Khalil, H.S.A., Alwani, M.S. and Omar, A.K.M., Chemical composition, anatomy, lignin

528

distribution, and cell wall structure of Malaysian plant waste fibers. BioResources 2007, 1(2),

529

220-232.

530

26. S.P. Sebastian; C.Udayasoorian; R.M. Jayabalakrishnan; E. Parameswari, Effect of

531

amendments and varieties on sugarcane yield and quality with poor quality irrigation water.

532

Journal of Environmental Research And Development 2009, 3, (3), 817-829.

533

27. Bright Singh, I.S. and Rojith, G., Lignin recovery, Biochar Production and Decolourisation

534

of Coir pith Black Liquor. Research Journal of Recent Sciences 2012, 1, (International Science

535

Congress Association-2011), 270-274.

536

28. Sandip K. Singh; Dhepe., P. L., Isolation of lignin by organosolv process from different

537

varieties of rice husk: understanding their physical and chemical properties. Bioresource

538

technology 2016, 221, 310-317.


29

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

Page 30 of 43

539

29. Deepa, A. K.; Dhepe, P. L., Lignin Depolymerization into Aromatic Monomers over Solid

540

Acid Catalysts. ACS Catalysis 2015, 5, (1), 365-379.

541

30. Chaudhary, R.; Dhepe, P. L., Solid base catalyzed depolymerization of lignin into low

542

molecular weight products. Green Chemistry 2017, 19, (3), 778-788.

543

31. Singh, S. K.; Dhepe, P. L., Ionic liquids catalyzed lignin liquefaction: mechanistic studies

544

using TPO-MS, FT-IR, RAMAN and 1D, 2D-HSQC/NOSEY NMR. Green Chemistry 2016,

545

18, (14), 4098-4108.

546

32. Sturgeon, M. R.; O'Brien, M. H.; Ciesielski, P. N.; Katahira, R.; Kruger, J. S.; Chmely, S.

547

C.; Hamlin, J.; Lawrence, K.; Hunsinger, G. B.; Foust, T. D.; Baldwin, R. M.; Biddy, M. J.;

548

Beckham, G. T., Lignin depolymerisation by nickel supported layered-double hydroxide

549

catalysts. Green Chemistry 2014, 16, (2), 824-835.

550

33. Singh, S.K.; Dhepe, P. L., Effect of structural properties of organosolv lignins isolated from

551

different rice husks on their liquefaction using acidic ionic liquids. Clean Technologies and

552

Environmental Policy 2017, 20, (4), 739-750.

553

34. P.L. Dhepe; Richa, K., An improved heterogeneous base catalyzed process for

554

depolymerization of lignin. Patent No. 201611007650 (IN) 2016.

555

35. Joshi, U. D.; Joshi, P. N.; Tamhankar, S. S.; Joshi, V. V.; Shiralkar, V. P., Adsorption

556

Behavior of N(2), Water, C(6) Hydrocarbons, and Bulkier Benzene Derivative (TMB) on NaX

557

Zeolite and Its K(+)-, Rb(+)-, and Cs(+)-Exchanged Analogues. Journal of Colloid and

558

Interface Science 2001, 235, (1), 135-143.

559

36. Jose, C., Gutiérrez, A., Rodríguez, I.M., Ibarra, D. and Martinez, A.T., Composition of

560

non-woody plant lignins and cinnamic acids by Py-GC/MS, Py/TMAH and FT-IR. Journal of

561

Analytical and Applied Pyrolysis 2007, 79, 39-46.


30

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562

37. Guerra, A.; Elissetche, J. P.; Norambuena, M.; Freer, J.; Valenzuela, S.; Rodríguez, J.;

563

Balocchi, C., Influence of lignin structural features on Eucalyptus globulus kraft pulping.

564

Industrial & Engineering Chemistry Research 2008, 47, 8542-8549.

565

38. Rencoret, J., Ralph, J., Marques, G., Gutiérrez, A., Martínez, A.T. and del Río, J.C.,

566

Structural characterization of lignin isolated from coconut (Cocos nucifera) coir fibers. Journal

567

of agricultural and food chemistry 2013, 10, (61), 2434-2445.

568

39. Ismail, H.; Edyhan, M.; Wirjosentono, B., Bamboo Fiber Filled Natural Rubber

569

Composites: the Effects of Filler Loading and Bonding Agent. Polymer Testing 2002, 21, (2),

570

139-144.

571

40. Vishtal; Alexey Grigorievich; Kraslawski., A., Challenges in industrial applications of

572

technical lignins. BioResources 2011, 3, (6), 3547-3568.

573

41. Koshijima, T., and Takashi Watanabe., Association between lignin and carbohydrates in

574

wood and other plant tissues. Springer Science & Business Media, 2013., 131-140.


31

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

Page 32 of 43

Coir (middle fibrous coat of fruit) Coir(middlefibrouscoat of fruit)

Husk (outer coat of fruit)

Husk(outercoat of fruit) Kernel

Kernel

Shell (innerhard coat of thefruit)

Shell (inner hard coat of fruit)

575 576

Figure 1. Cross section of Coconut.

577 578 Coconut Coir

Lignin Isolation

Organosolv method

Soda method

Coir (8.0 g) + H2SO4 (1.024 mmol) + EtOH:H2O (60 mL, 1:1 v/v)

Coir (3.0 g) + 2 wt.% NaOH Solution (60 mL)

Klason method

Coir (1.0 g) in 100 mL RB + 72% H2SO4 (15 mL) Stirred vigorously @ 30 oC, 2 h Another 1000 mL RB + 150 mL H2O + Transferred H2SO4 digested mass slowly


oC,

Kept RB @ 30 16 h Filtered with G2 crucible & washed with H2O Liquid (Acid soluble lignin, Polysaccharides)

Solid Oven dried @ 55 oC, 16 h Vacuum Dried @ 110 oC, 1 h


Filter

Wash 100 mL RB with 195 mL H2O & transfer it into 1000 mL RB 1000 mL RB placed in preheated oil bath @ 100 oC, 4 h, stirring

160 oC, 5 h

180 oC, 1 h

Solid (Pulp; Cellulose, Hemicellulose, Ash, etc)

Filtered & washed with H2O

Liquid (Lignin & Soluble Sugars)

Solid (Cellulosic residue)

Liquid Acidified to pH 1 with conc. H2SO4

180 mL H2O

Boiled, 1 h

Oven Dried @ 55 oC, 16 h Oven Dried @ 55 oC, 16 h

Kept for 12 h Precipitate (Hydrophobic lignin)

Vacuum Dried @ 90 oC, 4 h Wash with 100 mL H2O

Liquid Soluble Sugars


Precipitate obtained

Filtered & washed with H2O until pH comes 7





Organosolv Lignin (CC-ORGL)

Soda Lignin (CC-SL)

Uncorrected Lignin Heat @ 620 oC, 2 h for ash correction

579 580

Klason Lignin (CC-KL)

Figure 2. Isolation of lignin by Klason, organosolv and soda processes.

581 582


32

Page 33 of 43 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

583 Reaction Charge Depolymerization Reaction Mixture Centrifugation

Solid (SR) (Catalyst + Solid) Extraction with DEE and EtOAc

Solution (EtOH + H2O soluble) HCl (pH 1-2) Acidified mixture

Soluble*

Insoluble

Evaporation Solid (Filter cake) Extraction with DEE

Aromatic products

Liquid Extraction with DEE

Soluble*

Insoluble Soluble* Insoluble Extraction Extraction Evaporation Evaporation with EtOAc with EtOAc Aromatic products Aromatic products

Soluble* Evaporation

584 585

Aromatic products

Insoluble

Soluble*

Insoluble

Evaporation Aromatic products

Figure 3. Methodology for products extraction (* Analyzed by GC, GC-MS & HPLC).

586


33

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

Page 34 of 43

588 589

Figure 4. (A) XRD patterns of Coconut Coir (CC) and Isolated Lignin {Organosolv (CC-

590

ORGL), Soda (CC-SL) and Klason (CC-KL) lignin}, (B) XRD patterns of different samples

591

of Coconut Coir (CC-1-4).


34

Page 35 of 43 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

592 593

Figure 5. ATR analysis of Coir (CC) and Isolated lignin (CC-KL, CC-ORGL, CC-SL).


35

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

Page 36 of 43

594

595 596

Figure 6. (A) Depolymerization of isolated lignin and coir using NaX. Reaction

597

Condition: Lignin/Coir (0.5 g), NaX (0.5 g), EtOH : H2O (1 : 2 v/v, 30 mL),

598

250 oC, 1 h, (B) Direct hydrolysis of CC using NaX. Reaction Condition: Coir (0.5 g),

599

NaX (0.5 g), EtOH : H2O (1 : 2 v/v) 30 mL, 200 oC, 1 h.


36

Page 37 of 43 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

600

Energy & Fuels

Table 1. Chemical composition of coconut coir. Water

Pectin

soluble related (%)

andHemi-

Cellulose Lignin

cellulose (%)

Ash

(%)

(%)

Refs.

compounds (%) (%)

601

5.25

3.30

0.25

43.44

45.84

2.22

16

nd

nd

31.1

33.2

20.5

nd

17

nd

nd

15-28

35-60

20-48

nd

18

nd

nd

16.8

68.9

32.1

nd

19

nd

nd

-

43.0

45.0

nd

20

nd

nd

0.15-0.25 36-43

41-45

nd

21

nd

3.0

0.25

45.84

5.6

22

43.44

(nd): not determined


37

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

602 603

Page 38 of 43

Table 2. Microanalysis of Coconut Coir and Isolated Lignin. Microanalysis

CC

CC-ORGL

CC-SL

CC-KL

C (%)

45.35

60.78

63.58

63.59

H (%)

4.86

5.41

6.08

6.12

O (%)

49.79

33.81

29.34

29.39

O/C

1.09

0.56

0.46

0.46

H/C

0.11

0.09

0.10

0.10

HHV (MJ/kg)a

13.4

22.3

24.8

24.9

DBEb

2.18

5.8

5.6

5.6

MMFc pHd

C7.6H8.84O6.22 6.4

C10.1H10.7O4.2 6.3

C10.6H12.1O3.8 5.9

(a)

C10.6H12.1O3.8 2.95

604

Higher heat value (HHV) = [0.3383 x C + 1.442 x [H-(O/8)] + 9.248 x S] where C, H, O and S

605

are wt.% of carbon, hydrogen, oxygen and sulphur; (b) Double bond equivalence (DBE) = [C –

606

(H/2) + (N/2) + 1] where C, H and N are number of carbon, hydrogen and nitrogen atoms found

607

from monomer molecular formula (c) Monomer molecular formula (MMF) = 100 - (‘C’ wt.% +

608

‘H’ wt.% + ‘O’ wt.%). (d) pH was measured by dissolving 0.08 g sample in 5 mL water.


38

Page 39 of 43 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

610

Table 3. Summary of ATR bands present in Coir (CC) and Isolated Lignin (CC-SL,

611

CC-ORGL, CC-KL).

Wavenumber (cm-1) Band Type of vibration (cm-1) 3650-3200

CC-

CC-

CC-

SL

ORGL

KL

3298

3280

3378

3386

2916

2922

2949

2889

1720

1690

1690

1706

-

-

-

1677

CC Alcoholic & phenolic OH stretching (free and involved in hydrogen bonding)

2960-2910

C-H asymmetric stretching in methyl and methylene group

1740-1680

C=O stretching in unconjugated ketone, carbonyl and ester groups

1670-1620

C=O stretching in conjugated substituted

1645

aryl group 1615-1595

C=O stretching with

1608

1598

1605

-

1520

1508

1510

1518

1440

1450

1442

-

aromatic skeleton vibrations 1520-1505

Aromatic skeleton vibrations

1470-1440



39

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

612 613

C-H bond 1370-1350

614

617

1300-1200

C-C, C-O, C=O stretching 1200

1120-1105

Deformation vibrations of C-H bonds in aromatic

621

rings 1195-1124


623

C-H bonds in syringyl

624

rings

625

1027-1010

626

-

1208

1228

1290

C-O stretching in alcohol,

1283 1113

1105

1110

-

-

1172

-

1145

1027

1010

1020

1010

-

812

804

873

ether/ in-plane

627

deformation vibrations of

628

C–H bonds in aromatic

629

rings.

630 631

1366

in guaiacyl units

620

622

1355

OH

618 619

Aliphatic C-H stretching in methyl and phenolic

615 616

Page 40 of 43

875-700

Substitution on aromatic

632

ring or substituted

633

phenolics

780

634 635

(-) Not observed


40

Page 41 of 43 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

636

Table 4. Summary on 13C NMR of Coconut Coir (CC) and Isolated Lignin (CC-KL,

637

CC-ORGL, CC-SL). Chemical Shift (ppm) Functional group CC

CC-KL

CC-ORGL

CC-SL

-

-

177.91

-

177.98

C=O group in Hibbert’s

175.53

175.04

175.87

-

-

-

173.39

174.82

C4 in p-hydroxyphenyl unit -

160.14

161.95

-

C in -O-4 substructures

155.55

151.89

153.20

152.37

-

147.13

-

-

-

-

130.85

129.19

116.22

115.61

116.12

115.94

C2 in guaiacyl units

-

-

-

111.11

C2,6 in tricin

105.47

-

106.82

106.65

-

99.18

-

100.86

-

C8 in tricin

93.49

92.52

-

92.51

-

89.04

89.28

-

88.31

C in -O-4 substructures

83.66

-

82.28

84.76

C in -O-4 substructures

74.74

72.97

72.41

73.35

ketone -

linked with C=O C3 and C4 in etherified guaiacyl unit C2,6 in p-hydroxyphenyl units C3,5 in p-hydroxyphenyl units


41

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

638 639 640 641 642 643 644 645

Page 42 of 43

72.60 C in -O-4 substructures

-

-

67.06

-

-

64.48

-

63.14

65.18

C-H in methoxyl group

55.44

56.05

55.88

56.10

-

-

-

48.93

49.75

-

-

46.64

-

46.52

 carbon CH2 with

-

43.47

42.86

42.92

646

aliphatic substituted group

647

-

43.47

-

-

39.59

648

 carbon in phenyl

36.87

-

36.23

36.50

649

propanol group 32.69

32.90

-

-

-CH2 alkyl group

30.48

30.15

29.71

30.31

-

21.17

23.62

23.18

-

-

-

-

-

19.15

Terminal -CH3 group

-

14.58

14.08

14.91

650 651 652 653 654 655 656

(-) Not assigned


42

Page 43 of 43 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

657

Energy & Fuels

Table of Content:

658

659


43

Depolymerization of Lignin Using a Solid Base Catalyst | Energy & Fuels

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