Highly efficient renewable heterogeneous base catalyst derived from waste Sesamum indicum plant for synthesis of biodiesel

Journal Pre-proof Highly efficient renewable heterogeneous base catalyst derived from waste Sesamum indicum plant for synthesis of biodiesel Biswajit Nath, Pranjal Kalita, Bipul Das, Sanjay Basumatary PII:

S0960-1481(19)31710-0

DOI:

https://doi.org/10.1016/j.renene.2019.11.029

Reference:

RENE 12576

To appear in:

Renewable Energy

Received Date: 24 November 2018 Revised Date:

23 August 2019

Accepted Date: 8 November 2019

Please cite this article as: Biswajit Nath, Pranjal Kalita, Bipul Das, Sanjay Basumatary, Highly efficient renewable heterogeneous base catalyst derived from waste Sesamum indicum plant for synthesis of biodiesel, Renewable Energy (2019), doi: 10.1016/j.renene.2019.11.029 This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.

Graphical abstract:

Highlights:

•

Renewable heterogeneous catalyst derived from waste Sesamum indicum plant was employed in biodiesel synthesis.

•

High amounts of K and Ca in the catalyst were confirmed by AAS, EDX and XPS techniques.

•

Yield of 98.9% biodiesel was achieved at optimum conditions in a short reaction time of 40 min.

•

The catalyst showed promising catalytic activity up to 3rd cycle of reaction without any significant loss of activity.

•

Properties of produced biodiesel are compatible with international standard.

1

Highly efficient renewable heterogeneous base catalyst derived from waste

2

Sesamum indicum plant for synthesis of biodiesel

3

Biswajit Natha, Pranjal Kalitab,*, Bipul Dasc, Sanjay Basumatarya,*

4 5

a

6

b

7

c

8

Jorhat-785006, Assam, India

Department of Chemistry, Bodoland University, Kokrajhar-783370, Assam, India Department of Chemistry, Central Institute of Technology, Kokrajhar-783370, Assam, India

Chemical Engineering Division, CSIR-North East Institute of Science and Technology,

9 10

*

11

*

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E-mail addresses: [email protected] (S. Basumatary), [email protected] (P. Kalita).

These authors contributed equally to this work. Corresponding authors.

13 14

Abstract

15

Waste Sesamum indicum plant derived heterogeneous catalyst was utilized for the first time

16

for biodiesel synthesis from sunflower oil. The derived catalyst was characterized by using

17

Powder XRD, FT-IR, BET, TGA, XRF, AAS, XPS, SEM-EDX and TEM, and the

18

characterization revealed the presence of Na, K, Ca, Mg, Fe, Mn, Zn, Si, Sr and Cl with high

19

percentage of K (29.64 wt. %) and Ca (33.80 wt. %) as oxides and carbonates. The catalyst

20

with a moderate surface area of 3.66 m2 g-1 exhibited excellent catalytic activity producing a

21

yield of 98.9 % biodiesel under the optimized conditions of 12:1 methanol to oil molar ratio

22

and catalyst loading of 7 wt. % at the reaction temperature of 65 oC in a short reaction time of

23

only 40 min. The catalyst could be reused up to the 3rd cycle of reaction with the yield of 94.2

24

% biodiesel. The characterization of biodiesel composition was done by using FT-IR, NMR,

25

and GC-MS techniques. The fuel property of produced biodiesel meets the prescribed limits

26

of international standard. The prepared catalyst is easy to handle, reusable, and found to be

27

highly efficient green catalyst that could help in reduction of biodiesel cost. Thus, the catalyst

28

can be recommended as a potential candidate for cost-effective biodiesel production at a large

29

scale.

30 31

Keywords: Biodiesel; heterogeneous catalyst; sunflower oil; transesterification; waste

32

Sesamum indicum.

33 1

34

1. Introduction

35

Fossil fuels are the major sources to meet the present requirement of energies worldwide

36

for transport and other allied industries [1]. Fossil fuels are obtained from non-renewable

37

sources and these fuels may be exhausted by 2050 due to limited resources and current

38

depletion of their reserves at a high rate [2]. Currently, increasing usage of fossil fuels in

39

vehicles leads to more environmental pollutions due to the emission of greenhouse gases [3].

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Hence, researchers are searching for viable alternate sources of energy to cope with the

41

present dwindling situation. In this context, biodiesel obtained from various oil resources is

42

gaining worldwide attention as an alternate source of energy as biodiesel is beneficial over

43

conventional fossil fuels due to less emission of greenhouse gases, low viscosity, high cetane

44

number, high flash point, non-toxic, biodegradability and lubricity [4, 5]. The properties of

45

biodiesel are comparable with conventional diesel fuels and its blend with diesel fuel up to 40

46

% can be used in any diesel-engine [6]. Moreover, the usage of biodiesel as the alternative

47

fuel reduces the effects on the environment [1]. In search of biodiesel feedstocks, researchers

48

reported various edible and non-edible oil sources for biodiesel synthesis such as soybean oil

49

[4], palm oil [7, 8], Jatropha curcas oil [9, 10], Mesua ferrea oil [11], Thevetia peruviana oil

50

[5, 12], Bauhinia monandra oil [13], Azadirachta indica oil [14, 15], waste cooking oil [16],

51

animal fats [17], and many others.

52

Biodiesel is the mixture of fatty acid methyl esters (FAME) obtained by catalytic

53

transesterification of triglycerides (oils or fats) with alcohol preferably methanol.

54

Homogeneous, heterogeneous and enzyme catalysts are utilized in biodiesel synthesis.

55

Homogeneous catalyzed transesterification uses acids or bases as the catalyst. Acid catalyzed

56

transesterification reaction requires high molar ratio of alcohol to oil, high pressure and

57

temperature with longer reaction times as well as the catalysts are corrosive and not

58

recyclable [18]. Homogeneous base catalyzed transesterification takes shorter reaction times

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for conversion of oil to FAME with a high yield but possesses the difficulty of soap

60

formation during conversion and the catalyst cannot be separated easily [1, 4]. Enzyme

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catalysts are not feasible for wider application in biodiesel production because of higher cost

62

and very slow reaction rate [4, 19]. In search of other convenient methods for

63

transesterification, heterogeneous catalysts are reported with promising results in conversion

64

of oil to biodiesel which is gaining much more attention worldwide. Heterogeneous base

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catalysts are more advantageous and have more potential over heterogeneous acid and

66

enzyme catalysts as they can achieve complete conversion in shorter reaction times at

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ambient conditions, catalysts can be separated easily by filtration and can be reused for 2

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several cycles of reaction [20]. Moreover, heterogeneous catalysts do not form soaps during

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the reaction which can enhance the yield of biodiesel. Both chemical and natural resources

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for the generation of heterogeneous catalysts are reported by the researchers. Heterogeneous

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catalysts derived from chemical sources showed good catalytic activity with 100 %

72

conversion of oil to biodiesel but these catalysts are quite expensive as the preparation steps

73

are longer and more complex which utilized intensive energy [1]. Moreover, these catalysts

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are generated from non-renewable resources and they are not biodegradable, not environment

75

friendly and they are not cost-effective for the production of biodiesel [1]. Hence, prime

76

importance should be given to obtain the ideal heterogeneous catalyst that is eco-friendly,

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low-cost and recyclable apart from demonstrating the high catalytic activity in biodiesel

78

production.

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Considering the various facts of homogeneous, enzyme and heterogeneous catalyst from

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chemical sources, nowadays the researchers are attracted more towards the heterogeneous

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catalysts derived from renewable natural resources for biodiesel synthesis. The solid catalysts

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obtained from natural resources exhibit excellent catalytic activity and after use, these can be

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thrown out to the environment without any potential harm as they are biodegradable. In

84

recent years, heterogeneous base catalysts derived from agricultural wastes are being used as

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the efficient green heterogeneous catalysts for biodiesel synthesis without any further

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chemical modification [1]. Lemna perpusilla ash [21], Musa balbisiana peel ash [16], Musa

87

balbisiana underground stem ash [9, 11], coconut husk ash [22], banana peel (Musa Gross

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Michel) ash [13], Musa paradisiacal peel ash [12], Musa acuminata peel ash [23], plantain

89

fruit peel ash [14], cocoa pod husk ash [7, 15], Astrocaryum aculeatum ash [4], Birch bark

90

ash [8], Acacia nilotica wood ash [24] etc. are some of the agricultural wastes derived

91

heterogeneous catalysts reported with remarkable catalytic activities for synthesis of

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biodiesel. These catalysts are highly basic in nature which can be utilized for several cycles

93

of reaction with a high yield of biodiesel [1] and further such catalysts could reduce the

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overall biodiesel production cost.

95

In the present study, an efficient heterogeneous catalyst is introduced from the waste

96

Sesamum indicum (sesame) plant for biodiesel synthesis. Sesamum indicum plant belonging

97

to the family Pedaliaceae is cultivated in the tropical region of the world for the nutritional

98

values of the edible seed as well as for its high quality oil. Sesame is used as a medicinal and

99

religious crop in Iran and India [25, 26]. Gouveia et al. [27] and Prasad et al. [28] studied the

100

medicinal properties of sesame and reported anti-oxidative, anti-hypersensitive, anticancer,

101

and anti-diabetic properties. In the state of Assam of Northeast (NE) India, the Bodo people 3

102

and other tribal communities widely cultivate Sesamum indicum plant for its seed and

103

traditionally use the grinded seeds in making of the rice cake. The plant materials after

104

untying seeds are usually thrown out as wastes which have no economic value. These dry

105

post-harvest plants are ignited thoroughly in the open air and the ashes obtained are used as

106

an insect repellent in cultivating crops by the rural people of NE India. It is noteworthy to

107

mention that the water extract of ash obtained from Sesamum indicum plant being highly

108

basic (alkaline) in nature is traditionally used by the people of this region as a food-additive

109

to make several traditional dishes which replaces the commercially available “SODA” whose

110

chemical composition is K2CO3. This water extract is locally known as Khar or Khardwi and

111

the people also use it as a medicine for recovery from gas related problems in the stomach.

112

Hence, we are motivated to utilize highly basic Sesamum indicum ash as an efficient

113

heterogeneous catalyst for production of biodiesel through transesterification reaction. The

114

literature study also showed that this waste plant ash has yet not been reported as a catalyst

115

for the production of biodiesel as well as in the area of organic chemistry. Moreover, the raw

116

materials could be obtained from easily available renewable sources and will provide an easy

117

process for the preparation of catalyst at minimum cost. As a result, it will significantly

118

reduce the cost of biodiesel. Since the agro-waste being renewable is taken as raw materials

119

for preparation of catalyst, the derived heterogeneous catalyst is considered as non-corrosive,

120

green and environmental friendly. In this study, for the first time, we are reporting the

121

catalytic activity of calcined Sesamum indicum ash as a green heterogeneous base catalyst for

122

biodiesel synthesis which is found to be highly efficient, reusable and has a very strong

123

potential as a candidate for reducing the cost for biodiesel production. The prepared catalyst

124

was characterized by various sophisticated analytical techniques and chemical composition

125

reported.

126 127

2. Materials and methods

128

2.1. Materials

129

In this study, the waste Sesamum indicum plant was collected after harvesting the seeds

130

from Dotma village of Kokrajhar, Assam, India for the preparation of the catalyst. Sunflower

131

oil was purchased from the Kokrajhar market, Assam, India to investigate the catalytic

132

activity in the synthesis of biodiesel. Methanol (≥ 99.0 %), ethyl acetate (≥ 99.5 %), silica gel

133

G (particle size through BSS 350 ≥ 95 %), and anhydrous sodium sulphate (≥ 99.0 %) were

134

obtained from Merck, Mumbai, India. Petroleum ether (40–60 oC) of analytical reagent grade

4

135

and acetone (99 %) were purchased from Rankem, Maharashtra, India. All the chemicals

136

were used as received.

137 138

2.2. Catalyst preparation

139

The waste Sesamum indicum plants obtained after untying seeds from the plant were dried

140

under the sun for 15 days. The dry materials were completely burnt into ash in the open air

141

and the ash obtained was further calcined at 550 oC for 2 h in a programmable muffle furnace

142

(Havells, B 25, DHMCBSPF025). The calcined Sesamum indicum ash was milled to obtain

143

the powder with the help of pestle and mortar, and stored in sealed plastic container for

144

further analysis and use.

145 146

2.3. Catalyst characterization

147

The Powder X-ray diffraction (XRD) technique was applied to characterize the calcined

148

Sesamum indicum catalyst using the X-ray diffractometer (Bruker AXS, D8 FOUS,

149

Germany). The credible chemical components present in the catalyst were identified

150

comparing the peaks of XRD spectrum located at different angles (2θ) against the Joint

151

Committee on Powder Diffraction Standards (JCPDS). Fourier transform infrared (FT-IR)

152

spectrophotometer (Perkin Elmer, C-107727, USA) was used for recording the FT-IR spectra

153

in the range of 4000–400 cm-1. The surface area of the catalyst was examined by Brunauer–

154

Emmett–Teller (BET) method using Quantachrome ASiQwin™ version 3.0 instrument. The

155

analysis was performed using N2 gas at a bath temperature of 77.4 K and an outgas

156

temperature of 373 K for 1 h. Thermogravimetric analysis (TGA) of the catalyst was

157

performed using a thermogravimetric analyzer (Make: TA, Model: SDTQ600, USA) at the

158

heating rate of 10 oC/min up to 800 oC at the constant flow of N2 gas. The surface

159

morphology was investigated using scanning electron microscopy (SEM) instrument (JEOL,

160

JSM 6390LV, Japan). The elements present in the catalyst were investigated by using X-ray

161

fluorescence (XRF) spectrometer (PANalytical, AXIOS DY824, Netherland). Quantitative

162

analysis of the elements present in the catalyst was done using the atomic absorption

163

spectrophotometer (AAS) (AAS-ICE 3500, Thermo Scientific, UK). The surface chemical

164

composition of the catalyst was determined employing X-ray photoelectron spectrometer

165

(XPS) (Make: Thermo Fischer Scientific, Model: ESCALAB Xi+, USA). Energy dispersive

166

X-ray (EDX) spectroscopic technique was used for analysis of the elemental composition of

167

the catalyst by using SEM (JEOL, JSM 6390LV, Japan). The structural information of the

168

catalyst was studied from transmission electron microscopy (TEM) image and selected area 5

169

electron diffraction (SAED) pattern recorded from JEM-2100, 200 kV, JEOL instrument. The

170

pH value of the catalyst was determined by using a digital pH meter (Labtronics Auto deluxe

171

pH meter, LT10). Separately, 1 g of calcined catalyst was dissolved in the volume of 10, 20,

172

30, 40 and 60 mL of distilled water. Each solution was well-stirred for few min and filtered

173

through Whatman no. 1 filter paper and the pH value of the filtrate was measured.

174 175

2.4. Biodiesel synthesis and characterization

176

The biodiesel synthesis was carried out by reacting sunflower oil with methanol using the

177

prepared catalyst in a round bottomed flask fitted with a magnetic stirrer (650 rpm) at a

178

temperature of 65 oC. The desired amount of oil was taken in a 100 mL round bottomed flask

179

and required methanol was added in a particular molar ratio to oil followed by the addition of

180

the catalyst for transesterification reaction. The optimum reaction conditions were

181

investigated and the reactions were performed by varying the methanol to oil ratio such as

182

3:1, 6:1, 9:1, 12:1, 15:1, and 18:1 with different catalyst load of 3 wt. %, 5 wt. %, 7 wt. %

183

and 9 wt. % of oil. Under the above optimized conditions, the reaction was also investigated

184

at different temperatures of 32, 45, and 55 oC. The conversion of oil to biodiesel was

185

monitored by using thin layer chromatography. The biodiesel synthesized was extracted with

186

petroleum ether (40–60 oC) using a separating funnel, dried over anhydrous sodium sulphate

187

followed by removal of the solvent in a rotary vacuum evaporator (Almicro, QuickVap) at 45

188

o

189

C. 1

H NMR and

13

C NMR spectra of oil and biodiesel were recorded in CDCl3 at 400 MHz

190

and 100 MHz, respectively using FT-NMR Spectrometer (Bruker Avance II, 400 MHz) for

191

comparison of structural changes in the conversion of oil to biodiesel. Biodiesel composition

192

was determined by gas chromatography-mass spectrometer (GC-MS) using Perkin Elmer,

193

Claurus 680 Gas Chromatograph/Claurus 600C Mass spectrometer. During analysis, initially

194

the oven temperature was held at 60 oC for 6 min, increased at the rate 5 oC/min to 180 oC

195

followed by 10 oC/min to 280 oC. After 20 min when the temperature dropped to 250 oC, the

196

sample was injected and analysed with Turbomass Ver5.4.2 software. The source and transfer

197

temperatures were 160 and 180 °C. The carrier gas used is helium at a flow rate of 1 mL/min,

198

split ratio is 20:1, GC column used is Elite 35 MS with the dimension of 60.0 m × 250 µm,

199

and mass scan is from 50 to 500 Da.

200 201 202 6

203

2.5. Determination of biodiesel fuel properties

204

The fuel properties of the produced biodiesel such as density, kinematic viscosity, pour point,

205

cold filter plugging point were tested following IS 1448 method, and cetane number (ASTM

206

D4737 method) and total sulphur (ASTM D2622 method) were also determined at the quality

207

control laboratory of Indian Oil Corporation, Bongaigaon Refinery, Assam, India. Other

208

parameters such as higher heating value (HHV), American petroleum index (API), aniline

209

point, and diesel index were determined following ASTM D2015 standard equations [29],

210

and the saponification number was also determined following standard equation [30].

211 212

3. Results and discussion

213

3.1. Characterization of catalyst derived from waste plant

214

3.1.1. Powder XRD analysis

215

The crystalline components of metals present in calcined Sesamum indicum catalyst were

216

identified from the XRD pattern shown in Fig. 1 by comparing with JCPDS data and reported

217

literatures. This analysis of the catalyst indicated the complex mixture of oxides of metals

218

such Na, K, Ca, Mg, Sr, Si and Fe, carbonates of metals like Na, K and Ca, and hydroxide of

219

Ca. The XRD analysis also revealed that the various components of potassium are present in

220

the form of KCl, K2CO3 and K2O. The JCPDS study shows that the peaks at 2θ values of

221

28.27, 40.47, 49.53, 58.74 and 73.74 are due to KCl and the presence of K2CO3 is represented

222

by the peaks at 2θ values of 26.71, 29.67, 30.93, 31.72, 34.06 and 41.87. The peaks at 2θ

223

values of 27.64, 39.84, 46.70, 48.12, 52.18, 56.71, 63.27 and 65.16 are due to the presence of

224

K2O in the catalyst. The presence of KCl in the calcined Sesamum indicum catalyst is

225

supported with the results of calcined coconut husk catalyst reported by Vadery et al. [22]

226

and calcined banana peel catalyst reported by Betiku et al. [13]. Gohain et al. [16] also

227

characterized the catalyst derived from Musa balbisiana peel and reported that 2θ values at

228

26, 31 and 41 are due to the presence of K2CO3 and the 2θ value at 46 is due to K2O which

229

are supporting the results of current study. The existence of CaO in the catalyst is revealed by

230

the 2θ values of 32.33, 37.18, 43.00, 54.21, 64.22, 66.55 and 69.39 which are also

231

comparable with the recently reported 2θ values of CaO in the works of Uprety et al. [8],

232

Gohain et al. [16], Kumar et al. [31] and Jin et al. [32]. The peak at 2θ value of 48.70 is due

233

to CaCO3 present in the current catalyst which is in accordance with the result reported by Ho

234

et al. [33]. A minor peak of Ca(OH)2 is also observed in the XRD pattern of the catalyst at 2θ

235

value of 51.55 which conforms the results reported by Kumar et al. [31, 34]. The JCPDS data

236

predicted the existence of SiO2 in the catalyst at 2θ values of 21.39, 25.78 and 72.78 (Fig. 1). 7

237

The presence of SiO2 was also found in the calcined wood ash catalyst [24] and M. balbisiana

238

underground stem catalyst [9] reported for biodiesel synthesis. According to JCPDS data, the

239

presence of SrO in the catalyst is represented by the peaks at 2θ values of 30.46, 50.62 and

240

62.33 and the Fe2O3 is indicated by the peaks at 2θ values of 23.74 and 32.96. The presence

241

of Na in the form of Na2O is identified from the peaks at 2θ values of 32.19 and 54.98 and

242

Na2CO3 is indicated by the peak at 37.95. The peak at 2θ value of 43.50 showed the presence

243

of MgO. The study of XRD pattern reveals that the catalyst is a mixture mainly composed of

244

oxides and carbonates of metals and also predicts that K and Ca are the dominant elements

245

which are responsible for their catalytic activity.

246

247 248

Fig. 1. Powder XRD pattern of calcined Sesamum indicum catalyst.

249 250

3.1.2. FT-IR analysis

251

The FT-IR spectrum of calcined Sesamum indicum catalyst is shown in Fig. 2. In the

252

spectrum, the broad peak at 3442 cm-1 indicates the -OH group which is due to the water

253

molecules adsorbed on the surface of the catalyst. The peak at 2143 cm-1 is attributed to M-O-

254

K stretching vibration which shows the presence of potassium in the catalyst and this is in

255

accordance with the IR peak (2178 cm-1) of Musa acuminata peel catalyst reported by Pathak

256

et al. [23]. The existence of peaks at 1645 cm -1, 1445 cm -1 and 1404 cm-1 are indicative of C8

257

O stretching vibrations due to the presence of metal carbonates in the catalyst which is in

258

compliance with the FT-IR study results of catalysts reported by Betiku et al. [15], Gohain et

259

al. [16] and Pathak et al. [23]. The peaks at 1106 cm-1, 1030 cm-1 and 957 cm-1 may be due to

260

Si-O-Si bond vibration which is supporting the SiO2 component in the XRD pattern (Fig. 1).

261

The peak at 622 cm-1 corresponds to the K-O and Ca-O stretching vibrations which may be

262

due to the presence of K2O and CaO in the catalyst which was also predicted in the XRD

263

analysis (Fig. 1). Similarly, Pathak et al. [23] also reported FT-IR peak at 687 cm-1 for K-O

264

and Ca-O stretching vibrations in the catalyst derived from Musa acuminata peel. In this

265

study, the FT-IR analysis is in well agreement with the XRD pattern which confirmed the

266

presence of oxides and carbonates of metals in the calcined Sesamum indicum catalyst.

267 60 3 41 2

Transmittance (%)

55 50

3 29 2

5 4 6 1

45 40

2 44 3

35

3 78

4 04 1

54 4 1

7 5 9

30 6 0 11

25 4000

3500

3000

2500

2000

1500

0 74 0 3 0 1

1000

2 2 6

500

0

-1

268 269

Wavenumber (cm ) Fig. 2. FT-IR spectrum of calcined Sesamum indicum catalyst.

270 271

3.1.3. Surface area, pore size and pore volume determination

272

The BET surface area analysis of the calcined catalyst resulted in a surface area of 3.66 m²

273

g-1. The BJH (Barrett-Joyner-Halenda) average pore volume of the catalyst is found to be

274

0.012 cm3 g-1. The pore diameter of the catalyst is 1.677 nm and its adsorption pore size

275

distribution (1.47–2.25 nm) is clearly shown in Fig. 3a. In a study reported by Pathak et al.

276

[23], the surface area of Musa acuminata peel ash catalyst is lower (1.45 m² g-1) compared to

277

the current study. Sharma et al. [24] also prepared a solid heterogeneous catalyst from Acacia

9

278

nilotica wood by calcining at different temperatures for 3 h and they reported their surface

279

areas in the range of 1.33–3.72 m² g-1. It is reported that calcination temperature and time are

280

the main factors that affect the surface area of the heterogeneous catalyst that is directly

281

linked to the catalytic activity providing superior results of oil transesterification reaction

282

with higher surface area [4, 24]. The N2 adsorption‐desorption isotherm shown in Fig. 3b is

283

the characteristic of a type‐IV isotherm which indicated that the calcined Sesamum indicum

284

catalyst is mesoporous materials. Similarly, other waste biomass derived catalysts with type-

285

IV isotherm showing mesoporous materials were also reported in the works of Pathak et al.

286

[23] and Laskar et al. [35] for biodiesel synthesis.

287 288

0.0040 1.67 0.0035

0.0025 0.0020

3

-1 -1

dV(d) (cm nm g )

0.0030

0.0015 0.0010 0.0005 0.0000 -0.0005 -2

0

2

4

6

8

10

12

14

16

18

20

Pore diameter (nm)

289 290

Fig. 3a. Adsorption pore size distribution of calcined Sesamum indicum catalyst.

291 292

10

12

Desorption Adsorption

3 -1

Volume (cm g )

10

8

6

4

2

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure, P/Po 293 294

Fig. 3b. N2 adsorption‐desorption isotherm of calcined Sesamum indicum catalyst.

295 296 297

3.1.4. Thermogravimetric analysis

298

The thermal stability of the uncalcined catalyst was checked by TGA and the thermogram

299

is shown in Fig. 4. It is observed that on heating the catalyst till about 150 °C the weight loss

300

takes place which is due to the loss of water molecules adsorbed on the surface of the

301

catalyst. The catalyst was found to be stable from about 150 °C to about 350 °C as indicated

302

by the almost constant weight at this range. The weight loss at a slow rate from about 150 to

303

500 oC was observed and this might be due to the slow loss of water of crystallization as no

304

decomposition of chemicals is expected in this range. However, the carbonaceous materials

305

present in the catalyst are expected to decompose in between 500 and 800 °C, and the loss of

306

weight at this range may be linked to the release of CO and CO2 [4, 23].

307

11

102 100

Weight loss (%)

98 96 94 92 90 88 86 84 0

100

200

300

400

500

600

700

800

o

Temperature ( C)

308

Fig. 4. TGA thermogram of uncalcined catalyst.

309 310 311 312

3.1.5. SEM analysis

313

The morphological characteristic of the surface of the catalyst was studied from the SEM

314

image of calcined Sesamum indicum catalyst. The SEM images (Fig. 5A–C) showed the

315

formation of agglomeration of particles and layers like sheets of different sizes. The SEM

316

image also reveals that the catalyst contains oxygenated matters represented by the bright

317

particles which may be the oxides of metals [36].

318 319 320

12

321 322

Fig. 5. SEM (A–C) and EDX (D) images of calcined Sesamum indicum catalyst.

323 324 325

3.1.6. Elemental analysis

326

The XRF study for the elemental composition of the catalyst was performed and the

327

spectra are shown in Fig. 6. The various peaks in XRF spectra reveal the presence of Na, K,

328

Ca, Mg, Si, Mn, Fe, Zn and Sr as the metallic constituents, and the oxides and carbonates of

329

these are confirmed from the XRD pattern (Fig. 1). The XRF spectra showed a high intense

330

peak for K and Ca and nearly equal intense peak for Si and Mg. From the peak area and

331

intensity of XRF spectra, the higher percentage of K and Ca in the calcined catalyst can be

332

predicted. The SEM-EDX (Fig. 5D) study provides the quantitative composition of the

333

elements and the elemental composition is depicted in Table 1. As that of XRF spectra, the

334

EDX analysis shows that K (29.64 wt. %) and Ca (33.80 wt. %) are the dominant metals

335

followed by Si, Sr and Mg in comparison to other elements present in the catalyst. The AAS

336

analysis (Table 1) of the catalyst also indicated the existence of K (289.26 ppm) at the highest

337

concentration in the calcined Sesamum indicum catalyst followed by Ca (265.32 ppm) and

338

Mg (231.08 ppm). Low levels of Na, Mn, Fe and Zn were observed in the AAS study which 13

339

is in agreement with EDX report. However, some values of concentration of the elements as

340

determined by SEM-EDX and AAS techniques are quite different and not in the similar

341

trend. These differences may be because of employing different analytical tools and

342

procedures. EDX is a qualitative and semi-quantitative technique for detection of elemental

343

distributions. However, AAS is a quantitative tool. It is to be noted that EDX analysis is

344

usually done at different areas or locations of the sample where the elements may not be

345

distributed uniformly for which the concentration of elements detected at a particular area

346

may not be same at different areas of the sample, and this might be the reason for not being in

347

the line with AAS results. Although, the concentrations of K and Ca are not so significantly

348

different in both EDX and AAS analyses, it is not disturbing in obtaining the satisfactory

349

yields of biodiesel. However, the concentrations of some elements determined by the AAS

350

and XPS techniques are almost in the similar trend in the order of K > Ca > Mg > Mn > Fe >

351

Na > Zn (AAS, Table 1) and K > Ca > Mg > Na > Mn > Fe > Zn (XPS, Table 2). The main

352

aim of using these analytical techniques is to determine the alkali elements from the agro-

353

waste ash as expected. Thus, the characterization confirmed the presence of high

354

concentration of K and Ca for which we could successfully utilized the agro-waste ash as a

355

base catalyst in biodiesel synthesis. The higher concentration of K and Ca depicted by XRF,

356

EDX and AAS analyses is accrediting the high basicity of the calcined catalyst which in turn

357

showed higher catalytic activity in biodiesel synthesis. Mendonça et al. [4] reported that

358

higher percentage of alkaline elements in the heterogeneous catalyst derived from waste

359

biomass contribute better catalytic activity in the transesterification reaction of oil to

360

biodiesel. Gohain et al. [16] studied EDX for determination of elemental composition in their

361

catalyst prepared from Musa balbisiana peel and reported higher levels of K (41.37 %)

362

followed by Ca (36.08 %) and Mg (12.02 %) compared to other elements. Chouhan et al. [21]

363

studied a heterogeneous catalyst derived from Lemna perpusilla for biodiesel production and

364

reported higher amount of silica (82.51 %) followed by K (11.32 %) compared to other

365

elements. In most of the heterogeneous catalysts derived from waste biomasses which are

366

reported for biodiesel synthesis, higher concentration of K and Ca in the form of oxides and

367

carbonates are reported resulting in good catalytic activity [1, 5, 12, 23].

14

368 369

Fig. 6. XRF spectra of calcined Sesamum indicum catalyst.

370 371 372 373 374 375 376 15

377

Table 1

378

Elemental composition of calcined Sesamum indicum catalyst. Element

SEM-EDX Analysis

AAS Analysis

Weight (%)

Atomic (%)

Na

1.42

2.33

Concentration (ppm) 6.51

Mg

9.68

15.06

231.08

Si

11.32

15.25

–

K

29.64

28.66

289.26

Ca

33.80

31.89

265.32

Mn

0.80

0.55

21.27

Fe

1.70

1.15

19.74

Zn

0.54

0.31

5.84

Sr

11.09

4.79

–

379 380 381

The surface property of Sesamum indicum catalyst and the 3rd recycled catalyst was

382

determined using XPS study and their full scan and the high-resolution spectra are depicted in

383

Fig. 7. The elemental compositions are presented in Table 2. The XPS analysis confirmed the

384

presence of C, O, Na, K, Ca, Mg, Mn, Zn, Fe, Si, Sr and Cl which agrees with the results of

385

XRD, XRF and EDX studies. The high resolution spectrum of O 1s (Fig. 7B) shows a peak at

386

531.11 eV representing the binding energy of elemental oxygen of oxides present in the

387

catalyst. The spectrum of C 1s (Fig. 7C) shows two peaks at 284.01 eV and 288.05 eV

388

showing the binding energies of sp2 hybridized carbon (C=O) of carbonates [23]. The two

389

peaks in the spectrum of K 2p (Fig. 7D) at the binding energies of 292.22 eV and 294.96 eV

390

may be due to potassium in the form of oxide or carbonate [23]. Two peaks are observed in

391

the spectrum of Ca 2p (Fig. 7E) with binding energies 346.05 eV and 349.52 eV representing

392

the calcium oxide or carbonate present in the catalyst [37]. Furthermore, the Si 2p spectrum

393

(Fig. 7F) with a single peak at the binding energy of 101.6 eV may be due to the oxide of

394

silicone. Comparison of the elemental composition of the ash catalyst reported from various

395

agro-wastes for biodiesel synthesis is summarized in Table 3. In the present work, it is

396

observed that a high amount of K (29.64 %) and Ca (33.8 %) are found in Sesamum indicum

397

ash catalyst followed by Si (11.32 %), Sr (11.09 %) and Mg (9.68 %) along with other

398

elements as minor components. However, it is observed from Table 3 that different ashes

399

obtained from different agro-wastes have different chemical compositions as the quality and 16

400

composition of plant derived ash is mainly dependent on the soil, climatic condition,

401

geographical locations and age of the plants. Further, Table 3 indicates that Sesamum indicum

402

ash catalyst contained higher amounts of K and Ca compared to the ash catalysts of Musa

403

balbisiana underground stem [9], Lemna perpusilla Torrey [21], Acacia nilotica tree stem

404

[24], Camphor tree leaf [38], and uncalcined red banana peduncle [39], and as a result, the

405

present ash catalyst is showing superior catalytic activity for the synthesis of biodiesel.

406 407 408 409 410

17

411 412

Fig. 7. (A) XPS survey spectra of calcined Sesamum indicum catalyst and 3rd recycled

413

catalyst; XPS spectra of (B) O 1s, (C) C 1s, (D) K 2p, (E) Ca 2p and (F) Si 2p.

414 415 416 417 418 18

419

Table 2

420

XPS analysis of surface composition of the catalyst. Elements

Calcined catalyst

O1s K2p Na1s Si2p C1s Ca2p Mg1s Mn2p1 Mn2p3 Fe2p1 Fe2p3 Zn2p1 Zn2p3 Sr3d Cl2p

Atomic (%) 52.53 6.13 0.48 8.33 22.78 5.27 3.08 0.25 – 0.15 0.11 0.1 0.1 0.27 0.43

Recycled catalyst (3rd) Atomic (%) 47.46 1.14 0.14 5.29 33.97 6.21 4.9 0.11 0.04 0.13 0.04 0.06 0.1 0.4 0.01

421 422

19

423

Table 3

424

Comparison of elemental composition of the ash catalyst derived from various agro-wastes. Ash catalyst Present study (EDX) Lemna perpusilla Torrey [21] Acacia nilotica tree stem [24] Acacia nilotica tree stem [24] Musa balbisiana underground stem [9] Musa balbisiana peels [16] Musa paradisiacal peels [14] Camphor tree leaf [38] Red banana peduncle [39] Red banana peduncle [39]

Calcination conditions

Composition (%)

o

550 C, 2 h

Na 1.42

K 29.64

Ca 33.80

Mg 9.68

Al –

Si 11.32

P –

Cl –

Fe 1.70

Mn 0.80

Zn 0.54

Sr 11.09

550 oC, 2 h

0.53

11.32

–

–

–

82.51

–

–

–

–

–

–

500 oC

0.6

6.7

13.3

2.7

8.3

15.7

0.80

–

–

–

–

–

800 oC

5.7

5.7

17.8

4.5

1.2

21.5

0.5

–

–

–

–

–

550 oC, 2 h

0.61 (Na2O)

25.09 (K2O)

10.44 (CaO)

10.04 (MgO)

4.07 (Al2O3)

35.92 (SiO2)

4.47 (P2O5)

–

1.88 (Fe2O3)

–

–

1.89 (SrO)

700 oC, 4 h

10.41

41.37

36.08

12.02

–

–

–

–

–

–

–

–

700 oC, 4 h

–

51.02

–

1.15

0.29

2.51

1.84

6.27

–

–

–

–

800 oC, 2 h

0.23

1.22

12.05

1.82

2.70

–

–

–

–

–

–

–

Uncalcined

–

25.63

–

0.97

–

0.56

0.78

–

–

–

–

–

700 oC, 4 h

–

42.23

1.70

1.39

–

1.54

1.91

–

–

–

–

–

20

425

3.1.7. TEM analysis

426

The structural information of the catalyst was studied by TEM. From the TEM images,

427

various informations about the structure of the catalyst are obtained. The low resolution (100

428

nm) TEM image (Fig. 8A) shows that the particles are in irregular shapes with cylindrical and

429

hexagonal of which the large particles may be the crystallites and smaller particles are metal

430

oxides [40]. The smaller clusters of particles give effective surface area for the catalytic

431

activity of the catalyst. The high resolution TEM images of the catalyst showed the well-

432

ordered porous materials which are shown in Fig. 8B–E. The SAED (Fig. 8F) pattern shows

433

the presence of mixed polycrystalline components in the catalyst. Similarly, a heterogeneous

434

catalyst with polycrystalline nature which is prepared from Eichhornia crassipes plant was

435

also reported in the works of Talukdar et al. [41].

436 437

438 439

Fig. 8. TEM images (A–E) and SAED pattern (F) of calcined Sesamum indicum catalyst.

440 441 442 21

443

3.1.8. Measurement of pH value of the catalyst

444

The variation of pH value with the volume of water dissolving 1 g of calcined Sesamum

445

indicum catalyst is shown in Fig. 9. The pH value was found to be 11.4 when 1 g catalyst is

446

dissolved in 10 mL of distilled water indicating the highly basic nature of the catalyst. With

447

dilution, the pH value of the catalyst changes (Fig. 9) and it is observed that the pH value

448

slightly decreases with dilution up to 60 mL (pH = 10.9). Higher amounts of metals like K

449

and Ca detected through AAS, EDX and XPS analyses are the major factors for the high

450

basic nature of the catalyst that could be potentially utilized for base catalytic

451

transesterification in biodiesel synthesis. Talukdar and Deka [41] successfully utilized a

452

heterogeneous catalyst derived from Eichhornia crassipes in organic transformation reaction

453

and reported the pH value of the catalyst to be 9.6 (1 g in 20 mL water) which is relatively

454

lower compared to the catalyst of this study (pH = 11.3; 1 g in 20 mL).

455 11.4

11.4

11.3

pH value

11.3

11.2

11.2

11.1

11.1

11.0

10.9

10.9 10

456 457 458

20

30

40

50

60

Volume of water (mL) Fig. 9. Variation of pH value with volume of water dissolving 1 g of calcined catalyst.

459 460

3.2. Catalytic activity for transesterification of sunflower oil

461

3.2.1. Effect of catalyst loading

462

The rate of transesterification reaction of triglyceride to biodiesel is greatly influenced by

463

the load of catalyst applied. For the investigation of the effect of catalyst loading on the yield

464

of biodiesel and reaction time, four different catalyst concentrations such as 3, 5, 7 and 9 wt.

465

% of the oil were tested in transesterification of sunflower oil using 12:1 methanol to oil ratio 22

466

at 65 oC and the results are depicted in Fig. 10. It was observed that 7 wt. % of catalyst

467

loading showed considerably better results giving 98.9 % of biodiesel yield in a minimum

468

time period of 40 min which is reasonably shorter reaction time compared to other

469

concentration (loading) of catalyst. On increasing the catalyst concentration from 3 wt. % to 7

470

wt. %, the reaction time decreases from 80 min to 40 min, and the yield of reaction slightly

471

increases. Further, if catalyst loading is increased to 9 wt. %, the time taken for reaction

472

completion is 40 min and no significant increase in biodiesel yield was noticed. Therefore,

473

the catalyst loading of 7 wt. % of the oil was considered as the optimum catalyst

474

concentration for this transesterification reaction. Gohain et al. [16] also reported that an

475

increase of catalyst concentration increases the biodiesel yield to a certain level; however,

476

catalyst loading beyond the optimum level does not increase the biodiesel yield which almost

477

remains unchanged.

478 479 480 Reaction time (min) Yield (%) 98.2

Reaction time (min) and yield (%)

100

98.9

98.6

98.9

90 80

80 70

70 60 50

40

40

40

30 20 10 0 3

481

5

7

9

Catalyst load (wt.%)

482

Fig. 10. Effect of catalyst loading on reaction time and yield of biodiesel. Reaction

483

conditions: reaction temperature = 65 oC and methanol to oil molar ratio = 12:1.

484 485 486 23

487

3.2.2. Effect of methanol to oil molar ratio

488

Like catalyst concentration, the molar ratio of methanol to oil also plays an important role

489

in the rate of transesterification reaction. By varying the methanol to oil ratio such as 3:1, 6:1,

490

9:1, 12:1, 15:1 and 18:1, the reaction time and yield of biodiesel were investigated using

491

optimum catalyst concentration (7 wt. %) at 65 oC and the results are depicted in Fig. 11. It

492

was noticed that the reaction time decreases from 70 min (3:1 molar ratio) to 40 min (12:1

493

molar ratio) and then it remains constant when the methanol ratio is increased beyond 12:1.

494

Similarly, the biodiesel yield increases to a maximum level of 98.9 % in 12:1 molar ratio and

495

no significant increase in yield of biodiesel could be achieved beyond 12:1 molar ratio. It was

496

reported that the rate of reaction decreases at higher methanol concentration due to dilution of

497

oil and flooding of active sites of catalyst leading to reduced interaction in the reaction

498

mixture which leads to a decrease in rate and biodiesel yield [40, 42]. Hence, 12:1 molar ratio

499

of methanol to oil was taken as the optimum condition for the transesterification of sunflower

500

oil to biodiesel in the present study. The biodiesel synthesis using the heterogeneous catalysts

501

investigated by Uprety et al. [8], Vadery et al. [22], Sharma et al. [24] and Wang et al. [40]

502

also reported 12:1 as the optimum molar ratio of methanol to oil.

503 Reaction time (min) Yield (%)

Reaction time (min) and yield (%)

100

97.9

97.2

98.9

98.9

98.2

98.8

90 80 70

70

70

60

55

50 40

40

40

40

30 20 10 0 3:1

504

6:1

9:1

12:1

15:1

18:1

Molar ratio of methanol to oil

505

Fig. 11. Effect of methanol to oil molar ratio on reaction time and yield of biodiesel. Reaction

506

conditions: reaction temperature = 65 oC and catalyst loading = 7 wt. %.

507 24

508

3.2.3. Effect of reaction temperature on biodiesel synthesis

509

To know the effect of temperature, we have also investigated the reaction at different

510

temperatures of 32, 45, 55, 75 and 85 oC using the identical reaction conditions of 12:1

511

methanol to oil ratio and 7 wt. % of catalyst loading, and the results are depicted in Fig. 12. It

512

is observed that there is a significant decrease in the reaction time for completion of the

513

reaction from 140 to 40 min while the temperature is increased from room temperature (32

514

o

515

mention that there is no significant decrease in reaction time while increasing the temperature

516

from 65 to 75 oC and then to 85 oC. Moreover, it is noticed that there is no further increase in

517

biodiesel yield beyond 65 oC. Hence, among the investigated temperatures, better result was

518

found to be at 65 oC with promising catalytic activity and satisfactory yield of biodiesel.

519

Thus, 65 °C is chosen as the optimum reaction temperature for our study.

C) to 65 oC with the biodiesel yields of 97.2 and 98.9 %, respectively. It is noteworthy to

520

Reaction time (min) and yield (%)

150

Reaction time (min)

140

105 100

97.2

98.2

Yield (%)

98.9

98.0

98.8

98.7

75

50

40

35

35

0 32

45

55

65

75

85

o

Temperature ( C)

521 522

Fig. 12. Effect of temperature on the reaction of biodiesel synthesis. Reaction conditions:

523

catalyst loading = 7 wt. % and methanol to oil ratio = 12:1.

524 525

3.3. Comparative study of the catalyst with other reported heterogeneous catalysts in

526

biodiesel synthesis

527

In current study, investigation on the effects of catalyst loading, molar ratio of methanol to

528

oil, and reaction temperature on biodiesel synthesis showed that the optimum reaction

25

529

conditions were 12:1 methanol to oil ratio and 7 wt. % of catalyst load which significantly

530

produced good biodiesel yield (98.9 %) at 65 oC in a reaction time of 40 min only. A

531

comparison of the present catalyst with previously reported heterogeneous base catalysts for

532

the synthesis of biodiesel is briefly summarized in Table 4. It is observed that in the works of

533

Sarma et al. [9], Kumar et al. [10], Aslam et al. [11] and Wang et al. [40], heterogeneous

534

catalysts derived from agricultural wastes were tested for biodiesel synthesis and

535

transesterification reactions were performed at higher temperatures and longer reaction times

536

were reported, but the product yields are comparable to that of the calcined Sesamum indicum

537

catalyst reported in this study (Table 4). On the contrary, Mendonça et al. [4], Deka et al. [5],

538

Uprety et al. [8], Chouhan et al. [21], Gohain et al. [16] and Laskar et al. [35] reported longer

539

reaction times in biodiesel synthesis employing heterogeneous base catalysts derived from

540

waste biomasses, however the product yields are comparable to that of the present catalyst

541

reported herein. It is also noticed that the surface areas reported by Mendonça et al. [4], Deka

542

et al. [5], Kumar et al. [10] and Wang et al. [40] are comparatively smaller to that of

543

Sesamum indicum catalyst which showed a surface area of 3.66 m2 g-1 and this might be the

544

reason for the higher catalytic activity of the present catalyst. In addition to this, the better

545

performance of Sesamum indicum catalyst may be due to its high basic nature (Fig. 7),

546

presence of alkali and alkaline earth metal oxides and carbonates depicted by XRD (Fig. 1)

547

and FT-IR (Fig. 2) spectra and due to higher levels of K and Ca as revealed from XRF, AAS,

548

EDX and XPS analyses which facilitate the active species formation. However, it is observed

549

from Table 3 that the ash catalysts derived from Musa balbisiana underground stem [9],

550

Lemna perpusilla Torrey [21], Acacia nilotica tree stem [24], Camphor tree leaf [38], and

551

uncalcined red banana peduncle ash [39] are exhibiting lower amounts of K and Ca compared

552

to the Sesamum indicum catalyst, and this is the reason for lower catalytic activities in the

553

biodiesel synthesis reported using these catalysts (Table 4). In summary, it is noticeable that

554

under the optimized reaction conditions, the calcined Sesamum indicum catalyst is excellent

555

and superior in terms of reaction time, temperature and biodiesel yield amongst the other

556

catalysts listed in Table 4. The post-harvest Sesamum indicum plants which are normally

557

thrown out as wastes have no economic value. The raw materials for the preparation of

558

catalyst are readily available and these can be obtained with nominal cost. The process for

559

preparation of catalyst from waste material is found to be easy with minimum cost which is

560

carrying the major advantage for reducing the cost of biodiesel production. It is noteworthy to

561

mention that the prepared catalyst is easy to handle, non-corrosive, renewable, green and

562

environmental friendly. 26

563 564

Table 4

565

Comparison of activity of catalyst derived from waste Sesamum indicum plant with other

566

reported heterogeneous catalyst in biodiesel synthesis.

Biodiesel feed stock

Catalyst source

Surface area

Met/Oil ratio

(m2 g-1)

Parameter Catalyst (wt. %)

Temp (oC)

Time (min)

References Y or C (%)

Sunflower oil

Sesamum indicum

3.66

12:1

7

65

40

98.9 (Y)

This work

Soybean oil

Astrocaryum aculeatum

1.0

15:1

1

80

240

97.3 (C)

[4]

Thevetia peruviana oil

Musa balbisiana

1.487

10:1

20

32

180

96 (Y)

[5]

Palm oil

Birch bark

–

12:1

3

60

180

69.7 (C)

[8]

Jatropha curcas oil

Musa balbisiana underground stem

39

9:1

5

275

60

98 (Y)

[9]

Jatropha curcas oil

SrO-Musa Balbisiana underground stem

0.043

9:1

5

200

60

96 (Y)

[10]

Mesua ferrea oil

Musa balbisiana underground stem

38.71

9:1

5

275

60

95 (C)

[11]

Waste cooking oil

Musa balbisiana peel

10.176

6:1

2

60

180

100 (C)

[16]

Jatropha curcas oil

Lemna perpusilla

9.622

9:1

5

65

300

89.43 (Y)

[21]

Jatropha curcas oil

Acacia nilotica tree

3.72

12:1

5

65

180

98 (C)

[24]

27

stem Soybean oil

Snail shell

7

6:1

3

28

420

98 (Y)

[35]

Soybean oil

K2CO3Camphor tree ash

–

14:1

5

65

210

92 (Y)

[38]

Jatropha curcas oil

K2CO3Wood ash

12

12:1

5

65

180

99 (C)

[24]

Jatropha curcas oil

CaCO3Wood ash

14

12:1

5

65

180

91.7 (C)

[24]

Rapeseed oil

Gasified straw slag

1.266

12:1

20

200

480

95 (C)

[40]

Met–Methanol; Temp–Temperature; min–minute; Y–Yield; C–Conversion. 567 568 569

3.4. Study of the catalyst reusability

570

The heterogeneous catalysts have the major advantages and important property that they

571

can be reused for the subsequent cycle of reactions [3, 4]. Accordingly, in this study, the

572

reusability test was investigated under the optimum reaction conditions. The reaction mixture

573

after completion of the first transesterification reaction was filtered using vacuum pump to

574

separate the catalyst. The filtered catalyst was washed several times with petroleum ether

575

followed by acetone for residual removal of the product and glycerol. The washed catalyst

576

taken in a silica crucible was kept in a hot air oven at 110 oC for 4 h. The dried catalyst was

577

cooled in a desiccator, and in the subsequent reaction, the reactants are taken with respect to

578

the catalyst obtained after each reaction cycle. From the study, it was found that the catalyst

579

could be reusable up to the 3rd cycle of reaction with a good catalytic activity and the yields

580

of biodiesel achieved with the fresh catalyst, 1st, 2nd, and 3rd recycled catalysts were 98.9,

581

97.1, 96.1, and 94.2 % in the reaction times of 40, 85, 125, and 165 min, respectively. These

582

results specify that the catalytic activity of the Sesamum indicum catalyst is decreased

583

gradually after every subsequent reaction cycle and this is due to loss of catalyst during its

584

recovery process. This is in line with the results reported by Deka et al. [5], Sarma et al. [9]

585

and Gohain et al. [16]. Further, XPS analysis of the calcined catalyst and 3rd recycled catalyst

586

showed the substantial leaching of K and oxygen concentration from 6.13 % to 1.14 %, and

587

52.53 % to 47.46 %, respectively and also confirmed the little loss of Na and Si in the 28

588

recycled catalyst (Fig. 7, Table 2). The XPS study clearly indicated that potassium oxide due

589

to its high basicity played the main role in the transesterification of sunflower oil to biodiesel.

590

Other metal oxides and carbonates, to some extent, are also contributing to the catalytic

591

activity which is confirmed with the leaching of metals (K, Na and Si). The loss and leaching

592

of these components in the recycled catalyst results in reduced active sites of the catalyst

593

which in turn decreases the catalytic activity [16]. Further, during repeated use of the catalyst

594

in the reaction, there is a potential chance of agglomeration of ester and glycerol molecules

595

over the surface of catalyst in which the active sites of catalyst would be blocked resulting to

596

a reduced conversion of biodiesel [23].

597 598

3.5. FT-IR and NMR analyses of biodiesel

599

The C=O stretching vibration at 1745 cm-1 of sunflower oil (Fig. 13) changes to 1741 cm-1

600

in biodiesel (Fig. 14) and this predicts the conversion of oil to biodiesel due to change from

601

glyceride linkage in triglyceride to methyl esters. The peaks at 3006 cm-1 (Fig. 13) and 3007

602

cm-1 (Fig. 14) are due to =C-H (–CH=CH–) stretching frequencies present in triglyceride and

603

methyl ester molecules. The weak peak at 1657 cm-1 in both the IR spectra indicates the

604

stretching frequency of C=C present in both the molecules. The signals at 2855 cm-1 and 2927

605

cm-1 (Fig. 13), and signals at 2850 cm-1 and 2923 cm-1 (Fig. 14) are because of C-H stretching

606

vibrations. The IR peaks at 1463 and 1370 cm-1 (Fig. 13) and peaks at 1462, 1435 and 1361

607

cm-1 (Fig. 14) are due to CH3 bending vibrations. The C-O stretching bands of triglycerides

608

are observed at 1242, 1167, and 1106 cm-1 and that of methyl esters are seen at 1167 and

609

1013 cm-1. The IR band at 725 cm-1 in both the spectra is due to –CH2– rocking of the long

610

fatty acid chain.

611 612

29

80

Transmittance (%)

75 70 65

7 56 1

87 43

36 4 1 2 4 7 21 61 1

60 60 0 3

55

52 7

0 73 1 6 01 1

50 45 40 4000

72 9 2

3500

3000

54 71

5 58 2

2500

2000

1500

1000

500

0

500

0

-1

Wavenumber (cm )

613

Fig. 13. FT-IR spectrum of sunflower oil.

614 615 616

Transmittance (%)

80

60

7 5 61

6 4 43

0 78

40

1 63 1

20 70 0 3 32 92

0

4000

3 10 1

3500

3000

05 82

14 7 1

2500

2000

26 4 1

1500

52 7

5 3 7 41 6 11

1000

-1

617 618

Wavenumber (cm ) Fig. 14. FT-IR spectrum of biodiesel obtained from sunflower oil.

619 620

30

621

The 1H NMR (Fig. 15 and Fig. 16) and 13C NMR (Fig. 17 and Fig. 18) spectral analyses of

622

the sunflower oil and biodiesel clearly showed the major differences in the signals which

623

indicated the conversion of oil to its biodiesel. It is observed that the signals at δ 4.11–4.15

624

(dd, 3J = 6, 12 Hz) ppm and δ 4.28–4.32 (dd, 3J = 4, 12 Hz) ppm in the 1H NMR spectrum of

625

oil (Fig. 15) representing the protons of the glycerol moiety of the glyceride is disappearing

626

in the 1H NMR spectrum of biodiesel (Fig. 16) and new a singlet signal at δ 3.65 ppm is seen

627

due to methoxy protons (-CO-OCH3) which indicated the formation of methyl esters

628

(biodiesel). Similarly, the signals at δ 61.96 ppm and δ 68.83 ppm in the 13C NMR spectrum

629

of oil (Fig. 17) due to the methylene and methine carbons of the glycerine moiety are not

630

observed in the

631

signal at δ 51.28 ppm due to methoxy carbon (OCH3) of the methyl esters which confirmed

632

the conversion of oil to biodiesel. The signals at δ 5.25–5.38 ppm in 1H NMR spectrum of oil

633

and the signals at δ 5.31–5.38 ppm in the 1H NMR spectrum of biodiesel showed the olefinic

634

protons (-CH=CH-). On the other hand, the signals at δ 127.81–129.97 ppm in

635

spectrum of oil and signals at δ 127.80–129.99 ppm in the

636

indicated the olefinic carbons present in the oil and biodiesel. The signals at δ 2.74–2.77 ppm

637

(t, 3J=6.2) in 1H NMR spectrum of oil and at δ 2.75–2.78 ppm (t, 3J= 6.2) in 1H NMR

638

spectrum of biodiesel are due to bis-allylic protons (-C=C-CH2-C=C-) of the unsaturated

639

fatty acid chains present in oil and biodiesel. The α-methylene protons to ester (-CH2-CO2R)

640

of the oil and biodiesel are represented by the signals at δ 2.28–2.31 ppm (t, 3J = 7.4) and

641

signals at δ 2.27–2.31 ppm (t, 3J = 7.6), respectively. The α-methylene protons to double

642

bond (-CH2-C=C-) is seen at signals δ 2.001–2.07 ppm (Fig. 15) and at signals δ 1.985–2.071

643

ppm (Fig. 16). The signals at δ 1.608 ppm (Fig. 15) and δ 1.59–1.63 ppm (Fig. 16) represent

644

the β-methylene protons to ester (-CH2-C-CO2R) of oil and biodiesel, respectively. The

645

protons of backbone methylenes of the long fatty acid chain of oil and biodiesel are seen at

646

signals δ 1.25–1.38 ppm (Fig. 15) and at δ 1.26–1.39 ppm (Fig. 16), respectively. The signals

647

at δ 0.861–0.905 ppm (Fig. 15) and δ 0.862–0.905 ppm (Fig. 16) are due to the terminal

648

methyl protons. In 13C NMR study, the signals at δ 172.51, 172.89 and 172.92 ppm (Fig. 17),

649

and δ 174.19 ppm (Fig. 18) represent the carbonyl carbon of the triglyceride and biodiesel,

650

respectively. The signals at δ 14.003–34.05 ppm and δ 13.911–33.94 ppm are indicating the

651

methylene and methyl carbons of fatty acid moiety of the oil and biodiesel, respectively.

13

C NMR spectrum of biodiesel (Fig. 18) and there is appearance of a new

652

31

13

13

C NMR

C NMR spectrum of biodiesel

5.379 5.370 5.355 5.352 5.343 5.338 5.328 5.313 5.301 5.297 5.285 5.279 5.264 5.253 5.250 4.323 4.313 4.293 4.283 4.157 4.142 4.127 4.112 2.778 2.762 2.747 2.318 2.300 2.281 2.070 2.054 2.037 2.019 2.001 1.608 1.389 1.369 1.305 1.303 1.283 1.268 1.259 0.905 0.888 0.879 0.871 0.861 NAME EXPNO PROCNO Date_ Time INSTRUM PROBHD PULPROG TD SOLVENT NS DS SWH FIDRES AQ RG DW DE TE D1 TD0

1H FO-1 1 1 20180508 14.39 spect 5 mm BBO BB-1H zg 131072 CDCl3 8 0 20000.000 0.152588 3.2768500 4.5 25.000 6.00 300.0 1.00000000 1

Hz Hz sec usec usec K sec

653 654

12

11

10

9

8

7

6

5

4

1

3

2

1.39

1.01 8.22

1.00 1.73

0.59

0.66

1.53

======== CHANNEL f1 ======== NUC1 1H P1 11.50 usec PL1 -1.00 dB SFO1 400.1310305 MHz SI 65536 SF 400.1299795 MHz WDW EM SSB 0 LB 0.50 Hz GB 0 PC 1.00

1

Fig. 15. H NMR spectrum of sunflower oil.

32

0 ppm

5.376 5.361 5.359 5.349 5.343 5.333 5.318 3.653 2.782 2.766 2.751 2.316 2.297 2.278 2.071 2.056 2.039 2.018 2.002 1.985 1.632 1.616 1.599 1.390 1.371 1.304 1.269 NAME EXPNO PROCNO Date_ Time INSTRUM PROBHD PULPROG TD SOLVENT NS DS SWH FIDRES AQ RG DW DE TE D1 TD0

1H FP-1 1 1 20180508 14.36 spect 5 mm BBO BB-1H zg 131072 CDCl3 8 0 20000.000 0.152588 3.2768500 4.5 25.000 6.00 300.0 1.00000000 1

Hz Hz sec usec usec K sec

655 656

12

11

10

9

8

7

6

5

4

1

3

2

1.07

6.59

1.01

0.99

0.87

0.28

1.00

0.68

======== CHANNEL f1 ======== NUC1 1H P1 11.50 usec PL1 -1.00 dB SFO1 400.1310305 MHz SI 65536 SF 400.1299712 MHz WDW EM SSB 0 LB 0.50 Hz GB 0 PC 1.00

1

0 ppm

Fig. 16. H NMR spectrum of biodiesel obtained from sunflower oil.

657 658

33

172.928 172.895 172.513 129.976 129.833 129.782 129.759 129.559 129.536 127.986 127.970 127.813 77.472 77.151 76.832 68.831 61.964 34.050 33.882 31.898 31.876 31.479 30.374 29.723 29.684 29.651 29.600 29.554 29.502 29.453 29.352 29.305 29.280 29.134 29.059 29.022 28.983 27.150 27.123 27.004 25.556 24.792 24.767 22.650 22.540 14.046 14.003 NAME EXPNO PROCNO Date_ Time INSTRUM PROBHD PULPROG TD SOLVENT NS DS SWH FIDRES AQ RG DW DE TE D1 D11 TD0

13C FO-1 1 1 20180508 15.04 spect 5 mm BBO BB-1H zgpg 66560 CDCl3 252 0 40760.871 0.612393 0.8165193 362 12.267 6.00 300.0 5.00000000 0.03000000 1

Hz Hz sec usec usec K sec sec

======== CHANNEL f1 ======== NUC1 13C P1 7.00 usec PL1 0.00 dB SFO1 100.6208180 MHz ======== CHANNEL f2 ======== CPDPRG2 waltz16 NUC2 1H PCPD2 80.00 usec PL2 -1.00 dB PL12 16.00 dB PL13 20.00 dB SFO2 400.1320007 MHz SI 32768 SF 100.6127690 MHz WDW EM SSB 0 LB 0.50 Hz GB 0 PC 1.40

659 660

200

180

160

140

120

100

80

60

13

40

20

Fig. 17. C NMR spectrum of sunflower oil.

661 662 663

34

0

ppm

174.196 129.991 129.830 129.584 127.927 127.802 86.299 77.442 77.124 76.804 51.286 33.940 33.906 32.544 31.839 31.714 31.442 31.364 29.684 29.632 29.590 29.528 29.501 29.456 29.391 29.303 29.265 29.245 29.195 29.081 29.034 29.005 28.920 28.859 28.837 27.103 27.079 27.054 25.519 24.831 24.770 22.605 22.491 22.452 13.997 13.949 13.911 NAME EXPNO PROCNO Date_ Time INSTRUM PROBHD PULPROG TD SOLVENT NS DS SWH FIDRES AQ RG DW DE TE D1 D11 TD0

13C FP-1 1 1 20180508 14.35 spect 5 mm BBO BB-1H zgpg 66560 CDCl3 310 0 40760.871 0.612393 0.8165193 362 12.267 6.00 300.0 5.00000000 0.03000000 1

Hz Hz sec usec usec K sec sec

======== CHANNEL f1 ======== NUC1 13C P1 7.00 usec PL1 0.00 dB SFO1 100.6208180 MHz ======== CHANNEL f2 ======== CPDPRG2 waltz16 NUC2 1H PCPD2 80.00 usec PL2 -1.00 dB PL12 16.00 dB PL13 20.00 dB SFO2 400.1320007 MHz SI 32768 SF 100.6127690 MHz WDW EM SSB 0 LB 0.50 Hz GB 0 PC 1.40

664 665

200

180

160

140

120

100

80

60

40

20

0

ppm

Fig. 18. 13C NMR spectrum of biodiesel obtained from sunflower oil.

666 667 668

3.6. GC-MS analysis of biodiesel

669

The chemical composition of biodiesel was analyzed by GC-MS study. The library search

670

with S/W Turbomass NIST 2008 software revealed the presence of ten different fatty acid

671

methyl esters (FAMEs) in the sunflower biodiesel (Table 5). The mixture is composed of

672

both saturated and unsaturated FAME. The predominant methyl ester present is found to be

673

methyl linoleate (68.72 %), an unsaturated fatty acid ester followed by methyl palmitate

674

(11.25 %) which is a saturated fatty acid ester. The other saturated FAMEs are methyl

675

myristate, methyl stearate, methyl arachidate, methyl behenate, and methyl lignocerate. Other

676

unsaturated fatty acid esters such as methyl oleate, methyl gondoate and methyl erucate were

677

also detected in the study.

678 679 680 681 35

682

Table 5

683

GC-MS analysis of chemical composition of biodiesel. FAME

Composition (%)

Methyl myristate (C14:0)

0.29

Methyl palmitate (C16:0)

11.25

Methyl linoleate (C18:2)

68.72

Methyl oleate (C18:1)

4.40

Methyl stearate (C18:0)

1.59

Methyl gondoate (C20:1)

3.24

Methyl arachidate (C20:0)

1.47

Methyl erucate (C22:1)

3.08

Methyl behenate (C22:0)

4.93

Methyl lignocerate (C24:0)

0.95

684 685 686

3.7. Study of properties of produced biodiesel

687

The fuel properties of sunflower biodiesel and standard values for biodiesel prescribed by

688

ASTM D6751 along with fuel properties of other reported biodiesel prepared from agro-

689

waste base catalysts are listed in Table 6 for comparison. The density at 15 oC of sunflower

690

biodiesel is found to be 0.859 g cm-3 which is comparable to the values of other reported

691

biodiesel properties (Table 6), and the specific gravity of biodiesel is 0.854. The viscosity of

692

the biodiesel must be low for good combustibility and non-deposition in the engine [5].

693

Accordingly, the kinematic viscosity of biodiesel was found to be 3.11, which is well within

694

the standard limit as prescribed by ASTM D6751, and is lower compared to the values 4.3,

695

4.33 and 4.75 reported by Odude et al. [7], Deka et al. [5] and Kumar et al. [10], respectively.

696

The cetane number of the biodiesel is found to be 53.95, which is higher than the minimum

697

value prescribed in ASTM D6751, and also higher than the reported biodiesel results of

698

Jatropha oil [9], and soybean oil [43] indicating the good ignition and combustion quality of

699

the biodiesel. There is no specification found about the saponification number in the ASTM

700

D6751 standard. The saponification number (SN) indicates the number of saponifiable unit

701

present per unit weight of the biodiesel [44], and the SN (mg KOH/g) found in this study is

702

188.57. The pour point and cold filter plugging point were found to be -6 oC and -3 oC,

703

respectively. The API, diesel index and aniline point of the biodiesel are 34.277, 50.82, and

36

704

148.27, respectively. The total sulphur detected was 3 ppm which is well below the

705

maximum limits given in the international standards. The higher heating value (HHV) of the

706

biodiesel was found to be 39.79 MJ/kg which is comparable with 39.25, 40.20, 44.986, 43.19,

707

and 40.37 MJ/kg as reported by Sarma et al. [9], Gohain et al. [16], Deka et al. [5], Betiku et

708

al. [13], and Nath et al. [45].

709 710 711

Table 6

712

Comparison of properties of produced biodiesel with ASTM D6751 and reported biodiesel

713

properties.

Properties

0.875

Waste cooking oil [16] 0.89

Reported biodiesel Thevetia Palm oil peruviana [7] oil [5] 0.875 0.86

Bauhinia monandra oil [13] 0.876

0.885

0.86–0.90 1.9–6.0

0.875 5.7

– 3.12

– 4.33

0.86 4.3

0.876 4.90

0.885 4.9

53.95 188.57

47 (min) NS

48.6 –

55 –

61.5 –

76.93 –

59.83 –

50 –

-6

NS

3

-9

+3

-6

0

0

CFPP ( C)

-3

NS

–

–

+6

–

–

–

API

34.277

36.95

0.875

–

–

32.42

30.03

–

Diesel index

50.82

50.4

–

–

–

92.95

69.21

–

Aniline point

148.27

331

–

–

–

–

–

–

Total sulphur (ppm) HHV (MJ/kg)

3

15 (max)

9

–

–

–

–

–

39.79

NS

39.25

40.20

44.986

–

43.19

–

Density (15 oC, g/cm3) Specific gravity Kinematic viscosity (40 oC, mm2/s) Cetane number Saponification number (mg KOH/g) Pour point (oC) o

Biodiesel (This work)

ASTM D6751

0.859

NS

0.854 3.11

Jatropha oil [9]

Soybean oil [43]

NS–Not specified; min– minimum; max– maximum; CFPP–Cold filter plugging point; API–American petroleum index; HHV–Higher heating value.

714 715 716

4. Conclusions

717

In this study, we have reported a renewable heterogeneous base catalyst derived from

718

waste Sesamum indicum plant for efficient biodiesel synthesis. Characterization of catalyst

719

confirms the well-ordered porous materials polycrystalline in nature. Moreover, the presence

720

of high amounts of K and Ca in the form of oxides and carbonates in the catalyst are 37

721

confirmed that plays a key role in the high catalytic activity for transesterification of

722

sunflower oil into biodiesel. The catalyst yielded 98.9 % of biodiesel under the optimized

723

conditions of 12:1 methanol to oil ratio and 7 wt. % of catalyst loading at 65 oC in a reaction

724

time of 40 min, and was found to be reusable with good catalytic activity. The raw materials

725

for the preparation of this catalyst are readily available, and the cost of catalyst depends

726

mainly on the energy spent for its production which may be considered very nominal. Thus,

727

this catalyst has the advantage for reducing the cost of biodiesel production. Since the

728

catalyst is derived from biodegradable raw materials, it is non-corrosive, easy to handle, and

729

green catalyst. In addition, large scale disposal of this catalyst after use would not produce

730

any potential harm to the environment and society as well. Moreover, utilization of the wastes

731

in a convenient way to the benefit of mankind as a part of waste management is highly

732

encouraged and thus the present catalyst which is a highly efficient green catalyst can be

733

recommended as a potential candidate for biodiesel production at industrial scale in a cost-

734

effective manner.

735 736

Acknowledgments

737

The authors are thankful to SAIF, Gauhati University, India for XRF analysis, SAIC, Tezpur

738

University, India for XRD, SEM, and AAS analyses, CSIR-NEIST, Jorhat, India for TGA

739

and XPS analyses, SAIF, NEHU, India for NMR and TEM analyses, Bholanath College,

740

Dhubri, India for FT-IR analysis, Biotech Park, Guwahati, India for GC-MS analysis and

741

IOCL, Bongaigaon refinery, India for testing fuel properties of biodiesel.

742 743

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744

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745

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