Catalytic activity of heterogeneous acid catalysts derived from corncob in the esterification of oleic acid with methanol

Journal Pre-proof Catalytic activity of heterogeneous acid catalysts derived from corncob in the esterification of oleic acid with methanol Suppasate Dechakhumwat, Plaifa Hongmanorom, Chachchaya Thunyaratchatanon, Siwaporn Meejoo Smith, Supakorn Boonyuen, Apanee Luengnaruemitchai PII:

S0960-1481(19)31671-4

DOI:

https://doi.org/10.1016/j.renene.2019.10.174

Reference:

RENE 12543

To appear in:

Renewable Energy

Received Date: 25 September 2018 Revised Date:

16 August 2019

Accepted Date: 31 October 2019

Please cite this article as: Dechakhumwat S, Hongmanorom P, Thunyaratchatanon C, Smith SM, Boonyuen S, Luengnaruemitchai A, Catalytic activity of heterogeneous acid catalysts derived from corncob in the esterification of oleic acid with methanol, Renewable Energy (2019), doi: https:// doi.org/10.1016/j.renene.2019.10.174. 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 Published by Elsevier Ltd.

1 1

Catalytic activity of heterogeneous acid catalysts derived from

2

corncob in the esterification of oleic acid with methanol

3 4

Suppasate Dechakhumwata, Plaifa Hongmanoroma, Chachchaya Thunyaratchatanona,

5

Siwaporn Meejoo Smithc, Supakorn Boonyuend, Apanee Luengnaruemitchaia,b,*

6 7

a

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10330, Thailand b

10 11

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Center of Excellence on Catalysis for Bioenergy and Renewable Chemicals (CBRC), Chulalongkorn University, Phayathai Rd., Pathumwan, Bangkok 10330, Thailand

c

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The Petroleum and Petrochemical College, Chulalongkorn University, Soi Chula 12, Phyathai Road, Bangkok

Center of Alternative Energy, Faculty of Science and Department of Chemistry, Faculty of Science, Mahidol University, 999 Phuttamonthon Sai 4 Rd., Salaya, Nakhon Pathom 73170, Thailand

d

Department of Chemistry, Faculty of Science and Technology, Thammasat University, Rangsit Centre, Khlong Luang, Pathum Thani 12120, Thailand

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ABSTRACT

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Sulfuric acid (H2SO4) pretreated corncob-derived residue was used as a starting material to prepare

18

solid acid catalysts using different sulfonation chemicals (H2SO4, p-toluenesulfonic acid (TsOH)

19

and H2SO4/TsOH mixtures) for biodiesel production from the esterification of oleic acid with

20

methanol. Effects of the different sulfonation agents on the properties of the derived carbon-based

21

materials were investigated using various characterization techniques. Lignin residues were

22

obtained after the H2SO4 pretreatment step, and high lignin-containing carbon-based catalysts of ca.

23

69% (w/w) lignin were derived after sulfonation with H2SO4. Employing TsOH or H2SO4/TsOH

24

mixtures for sulfonation gave materials with a higher carbon/hydrogen (C/H) ratio, indicating a

25

relatively effective carbonization compared to that with H2SO4 sulfonation. The catalytic activity of

26

the sulfonated corncob in the esterification of oleic acid with methanol was influenced by the acid

27

density, acid strength and porous structure of the sulfonated materials. High methyl oleate yields (>

2 28

80% after 8 h at 60 °C) were achieved using the acid catalyst obtained from either H2SO4 or TsOH

29

sulfonation, whereas those from H2SO4/TsOH sulfonation gave slightly lower yields. Thus, the use

30

of the non-volatile TsOH solid as a ‘greener’ sulfonating agent for the production of carbon-based

31

solid acid catalysts with a high catalytic activity in the esterification reaction is supported.

32 33

Keywords: FAME; carbon based acid catalyst; sulfonation; p-Toluenesulfonic acid; esterification;

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oleic acid

35 36

∗

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Tel.: 662-218-4148; Fax: 662-611-7220; e-mail address: [email protected]

Address author correspondence:

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1.

Introduction

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Green economy, an initiative that aims to lead to a low-carbon industrial revolution, should

41

primarily focus on “improved human well-being and social equity, while significantly reducing

42

environmental risks and ecological scarcities”, according to the UNEP [1]. Being aligned well with

43

this sustainable initiative, bioeconomy has attracted increasing interest as it involves the use of

44

renewable natural resources as raw materials in several manufacturing industries, such as the

45

production of food, feed, bio-based products and biofuels [2]. Although biodiesel had a small

46

production level from 2001–2005 (< 100 million gallons), a significant (more than 15-fold) increase

47

in the annual biodiesel production has occurred from 2013 onwards, with a recorded 1,556 million

48

gallons being produced in 2016 [3]. Based on current projections (2010–2025) of the European

49

Union and the US EIA [4, 5], demand for biodiesel will continue to escalate each year.

50

Interestingly, the US biodiesel production in 2016 surpassed the forecasted value by 1.7-fold,

51

indicating the outstanding growth of the US biodiesel sector. This significant growth in biodiesel

52

production in the US could be attributed to the acceptable operational performance and

3 53

environmentally friendly perception of bio-based fuels, as well as an important tax incentive

54

scheme [6].

55

Fatty acid methyl esters (FAMEs), or biodiesel, can be produced via either transesterification of

56

triglycerides or esterification of free fatty acids (FFAs) with a short-chain alcohol in the presence of

57

a catalyst. Esterification of FFAs and alcohol generally requires an acid catalyst to achieve a high

58

FAME yield. On the other hand, basic catalysts are not ideal as they often react with FFAs to form

59

soap and so give a low FAME yield and purity. Previous works have reported the utilization of

60

homogeneous acid catalysts in the production of FAMEs from poor quality feedstocks, particularly

61

for triglycerides with low FFA contents [7-9]. However, using a homogeneous catalyst requires large

62

volume reactors, a large amount of catalyst. In addition, catalyst recyclability is a formidable

63

problem since homogeneous catalysts are miscible with the reaction mixture, causing a large

64

amount of waste water from the production of FAMEs. Thus, the use of heterogeneous catalysts has

65

gained considerable attention, owing to the reduced occurrence of corrosion, an improved

66

environmentally friendly processes and the possibility to reuse or recycle the solid catalyst.

67

Generally, homogeneous Bronsted acids, such as sulfuric acid (H2SO4) and hydrogen fluoride,

68

have been applied onto a solid support to increase the acid sites, stability and reusability of

69

heterogeneous catalysts in many acid-catalyzed reactions. However, most heterogeneous catalysts

70

developed so far are expensive and require complex synthetic procedures. Recently, sulfonated

71

carbon-based materials derived from H2SO4 treatment have shown great promise as effective

72

catalysts in the esterification of FFAs [10,11]. Nevertheless, to produce sulfonated materials from

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sulfonating agents like H2SO4 requires special safety guidelines, as well as a high pressure-

74

withstanding reactor that resists damaging corrosion. Alternative sulfonating agents with a low

75

vapor pressure would be preferable to establish a ‘greener’ sulfonation process, which is more

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environmentally benign in numerous industrial processes. In this vein, the sulfonation of sugar and

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extracted lignin was performed using the non-volatile p-toluenesulfonic acid (TsOH), which

78

resulted in a material with a high acid site content and strong acidity [12-14]. Furthermore, a

4 79

carbon-silica composite bearing -SO3H functional groups was obtained through TsOH treatment

80

followed by H2SO4 sulfonation [15], where the TsOH treatment reduced the amount of corrosive

81

H2SO4 reagent required in the sulfonation process.

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Corn (or maize) is one of the most common biomass used to produce ethanol in the US (ranked

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1st in the world for ethanol production), as well as in China and Canada. Note that, in 2016, the US

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retained its position as the top ethanol producer at around 60% of the world’s ethanol production

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[16]. In the ethanol production industry, machines are used to separate corn kernels from corncobs

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during harvesting, and the kernels are then stored for further liquefaction, saccharification,

87

fermentation, distillation and dehydration [17]. The post-harvested corn leaves, stems, husks and

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cobs are typically left in the fields, for boosting up the soil’s nutrients, or in some countries like

89

Thailand, are often burnt. Corn plantation is a major source of grains supplied in the animal feed

90

sector in Thailand [18]. Thus, an appropriate management of the large amount of agricultural waste

91

is necessary to prevent air pollution caused by the burning of the waste prior to planting a new crop.

92

In this work, corncob residue was used as a starting material for the preparation of solid acid

93

catalysts. The simple treatment of corncobs with H2SO4 can produce a series of porous solid

94

Brønsted acids. Several attempts have been made towards the synthesis of solid acid catalysts using

95

organosulfonic acid-functionalized catalysts on corncob residues. The sulfonating agent TsOH is an

96

aromatic ring linked to a long alkyl chain that endows the acid catalyst with an amphiphilic nature.

97

The main objective of this present work was to compare the catalytic performance in the oleic acid-

98

methanol esterification reaction of the corncob materials functionalized with sulfonic acid by

99

H2SO4, TsOH or H2SO4/TsOH mixtures. The optimum conditions of the catalyst preparation, in

100

terms of the type and amount of reagent and ratio of acid, were determined and benchmarked with

101

those in the previous reported works. As a comparison, commercial lignin was also treated in a

102

similar way with H2SO4 or TsOH. The catalytic activity of the carbon based materials in the

103

esterification of oleic acid with methanol was evaluated in terms of the methyl oleate (FAME)

104

yield. Note that oleic acid was selected as an unsaturated FFA model as it is the major FFA in

5 105

commonly available vegetable oils, such as 37% by weight (wt.%) in palm oil, the world’s most

106

widely used oil. The characteristics of the obtained sulfonated materials, in terms of their

107

morphology, functional group, surface area, pore size, internal structure, bulk composition, types of

108

acid site, acid site concentration and sulfur content, were examined. The reusability of the

109

sulfonated corncob-derived materials was also investigated.

110

The physical and chemical properties of the sulfonated corncob-derived materials were

111

characterized using thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron

112

microscopy (SEM), Brunauer-Emmett-Teller (BET) specific surface area, elemental analysis,

113

Fourier transform-infrared spectrophotometry (FT-IR) and ammonia-temperature-programmed

114

desorption (NH3-TPD).

115 116

2. Materials and methods

117

Corncobs were supplied as complimentary samples by the Betagro Company, Thailand. The

118

average particle size (homogenized in a single lot) of the corncob was 1.6 mm. They were dried at

119

105 °C in an oven overnight and stored at room temperature for further use. Previously, when

120

corncobs were used as a raw material to produce biobutanol, they were first pretreated by dilute

121

acid treatment followed by enzymatic hydrolysis and acetone-butanol-ethanol fermentation [19].

122

The pretreated corncobs were then filtered from the supernatant and were further treated by

123

enzymatic hydrolysis, which had a synergetic effect on the overall yields of reducing sugars. Solid

124

acid catalysts in the present work were, accordingly, prepared by the same H2SO4 acid pretreatment of

125

the corncobs but then used for the subsequent sulfonation. All supplied chemicals were AR grade

126

except as described otherwise.

127 128

2.1. Solid acid catalyst preparation

129

Following the reported optimum conditions [19], 10 g of dried ground corncob was suspended

130

in 100 mL of 2 wt.% H2SO4 in an 80-mL Teflon-lined stainless-steel autoclave with stirring for 15

6 131

min. The suspension was heated in a temperature-controlled oven at 120 ºC for 5 min, cooled and

132

then washed with de-ionized water (DW) to eliminate excess ions. The solid sample was separated,

133

dried in an oven at 110 ºC for 12 h, and kept in a desiccator and refereed to hereafter as pretreated

134

corncob (or C).

135

The pre-treated corncob was acidified by sulfonation using H2SO4 (98 wt.%, Sigma-Aldrich) or

136

TsOH (98 wt.%, Sigma-Aldrich) at varying concentrations, or H2SO4/TsOH mixtures at varying

137

weight ratios as follows. The H2SO4 sulfonation was performed in a closed-system reactor at 110 °C

138

for 5 h, using 5 g of pretreated corncob and 50 mL of H2SO4, with the obtained product denoted as

139

C-H2SO4. The TsOH sulfonation was performed by mixing 5 g of pretreated corncob and TsOH

140

powder (5, 10, 15 or 20 g) in an acid digestion bomb reactor at 180 °C. The obtained sulfonated

141

samples were denoted as C-TsOH-x, where x represents the weight of TsOH. Finally, the

142

H2SO4/TsOH sulfonation of 5 g of pretreated corncob was performed by 15 min mixing on a

143

magnetic stirrer, followed by heating in an acid digestion bomb reactor at 180 °C for 24 h. The

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sulfonated samples derived from the H2SO4/TsOH treatments at H2SO4: TsOH (w/w) ratios of 1:3

145

and 3:2 were denoted as C-M-3-10 and C-M-15-10, respectively.

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In addition, the sulfonation of commercial lignin (kraft and low sulfonate alkali, Sigma Aldrich)

147

was performed in a similar procedure to that used to prepare C-H2SO4 and C-TsOH-10, with the

148

sulfonated lignin samples denoted as L-H2SO4 and L-TsOH-10, respectively. At the end of the

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respective sulfonation process, each sample was washed by warm DW until the filtrate had a neutral

150

pH, and the conductivity of supernatant was about 0–10 mS. Then, the separated solid sample was

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dried in an oven at 110 °C for 12 h, and subsequently stored in a silica gel desiccator until use.

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2.2. FAME production

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Esterification of oleic acid with methanol catalyzed by the sulfonated materials was performed

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using a 1-L Parr reactor under 0.3 MPa of nitrogen (N2) gas with stirring at 300 rpm. For each

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reaction, a certain amount of oleic acid was added in a suspension of solid catalyst (of specific

7 157

weight) in methanol. The effects of the three operating parameters of the reaction time (up to 8 h),

158

reaction temperature (60, 80 and 100 ºC) and catalyst loading level, on the FAME yield were

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investigated by univariate analysis. Unless stated otherwise stated, the reaction was performed at 60

160

ºC with a methanol: oil molar ratio of 15:1 and a catalyst loading of 5 wt.% compared to vegetable

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oil.

162

After the reaction, the samples drawn from the reaction mixture were centrifuged and excess

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methanol was removed at 90 °C in a water bath. The solid catalyst was filtered from the reaction

164

solution. The methyl oleate (FAME product) content was kept in a vial prior to analysis by gas

165

chromatography using a Hewlett Packard GC model 5890 equipped with a flame ionization

166

detector, as previously reported [20]. The methyl oleate yields were determined using Eq. (1); C=

167

(∑ A) − A

EI

AEI

×

C EI × VEI × 100 m

(1),

∑ A is the overall area of methyl ester from C

168

where C is the methyl ester content,

169

the peak area of that which is aligned with the methyl heptadecanoate solution, C EI and VEI are the

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concentration (mg/mL) and volume, respectively, of the methyl heptadecanoate solution (mL) and

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m is the weight (mg) of the sample.

14

to C 24 , AEI is

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All experiments were repeated at least twice. To demonstrate the reusability of the prepared

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catalyst, it was separated by filtration after completion of the esterification reaction and reused in a

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2nd and 3rd successive run under the same conditions without regeneration.

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2.3. Catalyst characterization

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All samples were characterized as follows. The crystallinity of the acid catalysts was

178

characterized by XRD analysis using a Rigaku Smartlab powder X-ray diffractometer. Each

179

powdered sample was placed on a glass sample holder to record the XRD profile range from 10–

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90° at a scanning speed of 2°/min, step of 0.01° and acceleration voltage of 40 kV.

8 181 182

The morphological structure of the synthesized catalysts was characterized by SEM using a Hitachi TM 3000 scanning electron microscope operated at an acceleration voltage of 30 kV.

183

The BET specific surface area, pore volume and Barrett-Joyner-Halenda (BJH) pore diameter

184

were evaluated from the results of N2-desorption analysis using a Thermo Finnigan Sorptomatic

185

1990 Series analyzer. Before analysis, the volatile species adsorbed on the catalyst surface were

186

eliminated by heating 1 g of the catalyst under a vacuum atmosphere at 300 °C for 18 h. Helium gas

187

was used as an adsorbate for the blank analysis, and N2 gas was used as the adsorbate.

188

The atomic carbon, hydrogen, nitrogen and sulfur composition of each sample was evaluated by

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CHNS and X-ray fluorescence (XRF; Panalytical Axios PW 4400) analyses, while FT-IR analysis,

190

using a Nicolet Nexus 670 spectrometer, was employed to identify the chemical functional groups

191

in the samples using the KBr pellet method and recording the FT-IR spectra over the range of

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4,000–600 cm−1.

193

The weight change of a material as a function of temperature and time in a controlled

194

atmosphere was measured by TGA using a Perkin: Pyris Diamond instrument. It is ideally used to

195

assess the volatile content, thermal stability, degradation characteristics, aging/lifetime breakdown,

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sintering behavior and reaction kinetics of sample. The respective sample (10 mg) was heated from

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50 °C up to 700 °C at a 20 °C/min. The TGA was performed at atmospheric pressure in a N2 flow

198

of 20 mL/min.

199

The acidity of the solid acid catalysts was evaluated by NH3-TPD / reduction / oxidation analysis

200

(NH3-TPD/R/O) using a Thermo Finnigan TPDRO 1100 instrument. The acidity was calculated

201

from the derived temperature profile by integration of the peak area compared with standard

202

samples.

203

In addition, an acid-base titration method was utilized to quantify the total acid density of each

204

spent catalysts. In brief, 0.05 g sample was suspended in 15 mL of 2 M NaCl solution and kept in

205

an ultrasonic bath for 30 min. The ultrasonic treatment should allow an exchange between H+ ions

206

existing in sulfonated (–SO3H) catalyst and Na+ ions. Next, the ultrasonically treated sample was

9 207

titrated with 0.02 M NaOH, using phenolphthalein as an indicator. The total acidity of each

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sulfonated sample was quantified from the concentration of the NaOH solution multiplied by the

209

volume of the NaOH solution consumed and divided by the weight of catalyst used [11].

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3. Results and Discussion

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3.1.Catalyst Characterization

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TGA analysis. Three decomposition curves were observed in the TGA plots (Fig. 1) and

214

utilized to determine the amount of cellulose, hemicelluloses, lignin and residue components in

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each sample. The compositions of dried biomass based samples were estimated (wt.%) by

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optimization of the reported decomposition model [21]. Fresh corncob had cellulose,

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hemicelluloses, lignin and residue compositions of 32.15, 27.31, 39.41 and 1.21 wt.%, respectively.

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An initial weight loss was observed up to 100 °C due to water evaporation. Hemicellulose

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depolymerization occurs at 200–300 °C and cellulose degradation occurs at 300–400 °C [22]. The

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weight loss observed for fresh corncob was observed at 320 °C whereas at this temperature no

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weight loss was observed for C-TsOH-10, probably due to its low content of cellulose. Lignin

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degradation occurs at 200–500 °C, which covers a wide range of the cellulose and hemicellulose

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degradation temperature [23-26]. Sulfonation of the corncob led to an increased residual lignin

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(except for C-TsOH-10) and residue materials. The samples with a high lignin content (> 60 wt.%),

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C-H2SO4 and L-H2SO4, were comparable to that in the commercial lignin. The sulfonation

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treatments generally led to complete decomposition of hemicellulose as well as cellulose removal.

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Only the C-H2SO4 material had some residual cellulose, at around 37% of the initial cellulose

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content in the fresh corncob. Therefore, it can be concluded that the sulfonation using TsOH

229

effectively removed both cellulose and hemicellulose in the biomass material.

230

XRD analysis. Figure 2 shows representative XRD profiles of the powdered fresh and

231

sulfonated corncob samples. The three diffraction peaks at a 2θ of around 16.0°, 22.6° and

232

35.0°attributed to the crystalline regions of cellulose type I were observed in the fresh sample [27].

10 233

Sharper XRD peaks corresponding to cellulose were observed from the pretreated corncob, possibly

234

due to the surface damage of the biomass that allowed effective X-ray exposure [28]. This indicates

235

that acid pretreatment disrupted the native cellulose crystalline structure and increased the porosity,

236

surface area and crystallinity level compared to the untreated samples [29]. Poorly crystalline

237

materials were obtained after sulfonation by either H2SO4 or TsOH. After sulfonation, the

238

amorphous carbon structure (at 2θ = 15−35°) was observed, assigned to the (002) plane of the

239

carbon [30], and the weak diffraction band located at 2θ = 40−50° becomes more visible. Note that

240

the characteristics of the amorphous carbon in the sulfonated corncob are in good agreement with

241

previous work that reported on sulfonated carbon base materials derived from glucose [12]. In

242

addition, the XRD results agree well with those obtained from the TGA analyses.

243

Morphology and surface properties. The morphology of fresh corncob, pretreated corncob and

244

the C-H2SO4 catalyst are shown in Fig. 3. The fresh corncob had a less porous structure and

245

smoother surface compared with the others. Generated pores can be observed in the pretreated

246

corncob, and were more evident after the sulfonation with H2SO4. Small pores appeared on the

247

surface of the pretreated corncob, while larger pores were visible on the H2SO4 sulfonated sample.

248

Nevertheless, TsOH sulfonation of the corncob using varies amount of TsOH (5, 10, 15 or 20 g)

249

resulted in highly porous materials, as shown in Fig. 3(d-g). Similar results were obtained from the

250

sulfonation when using H2SO4/TsOH mixtures, as seen in Fig. 3(h-i). The surface characteristics of

251

the obtained C-H2SO4 material corresponded well to the porous sulfonated coffee residue reported

252

previously [28]. Notably, sulfonation with H2SO4/TsOH mixtures changed the structure of the

253

catalysts less than with pure TsOH. The SEM images of commercial lignin after sulfonation with

254

H2SO4/TsOH mixtures also showed a porous structure (Fig. 3(j-l)).

255

The BET specific surface area, specific pore volume and pore diameter of the fresh corncob and

256

sulfonated carbonaceous materials derived from corncob are summarized in Table 1. The specific

257

surface areas of the sulfonated samples were much higher than that of the fresh corncob, in

258

excellent agreement with the porous structure observed in the SEM results. The H2SO4 sulfonation

11 259

was less effective at production of a porous material compared with the TsOH processes, and

260

increasing the TsOH content increased the surface area and pore volume of the obtained C-TsOH-X

261

materials in a dose-dependent manner. Utilizing the H2SO4/TsOH mixtures for sulfonation further

262

developed pores in the sulfonated materials, as suggested by the significantly higher values of the

263

surface area (> 360 m2/g) and pore volume (ca. 0.29 cm3/g) observed in the sulfonated materials

264

prepared with the H2SO4/TsOH mixtures. Overall, the pore diameters of the obtained sulfonated

265

materials were ranked with respect to the acid system used as H2SO4 >>TsOH > H2SO4/TsOH.

266

Notably, a low surface area and small pore volume were observed in the sulfonated carbonaceous

267

material derived from lignin, indicating the advantages of using corncob biomass as the starting

268

material for sulfonation.

269

Composition. The composition of light chemical elements (C, H, N and including S) in the

270

samples is summarized in Table 1. The elemental composition of fresh corncob and commercial

271

lignin were around 50% C and 5–6% H (C/H ratio of 8–9), but corncob had more N and less S than

272

lignin. The increased C/H values (15.6–17.6) found in the sulfonated samples, with C levels

273

increased to 56-72% and H levels decreased to 3.44–4.59%, are evidence that the carbonization

274

process occurred during sulfonation. The higher (14.3- to 23-fold) sulfur content found in the

275

sulfonated samples may represent the increased number of –SO3H active sites on the sulfonated

276

samples [12,31]. The S contents in the sulfonated samples were in the range of 3.22–5.98 wt.%,

277

where a 23-fold higher sulfur content was found in C-TsOH-10 (5.98 wt.%) than that in the fresh

278

corncob (0.26 wt.%). Note that commercial lignin contained quite a high sulfur content (4.81 wt.%)

279

and subsequent H2SO4 or TsOH sulfonation slightly enhanced its sulfur content to 6.44 and 7.90

280

wt.%, respectively.

281

Furthermore, the XRF analysis data (not shown) also confirmed the higher sulfur content in the

282

sulfonated samples than that in the fresh corncob (5.85 wt.% of sulfur oxides), supporting the

283

reported CHNS analysis results. Other chemical elements in the fresh corncob included Al, Si, P,

284

Cl, K and Ca at 6.97, 18.16, 1.69, 5.85, 19.01 and 44.09 wt.%, respectively. The very high sulfur

12 285

content detected in the C-H2SO4 and C-TsOH-10 samples (78.56 and 95.81 wt.%, respectively)

286

suggested the effective sulfonation processes used in this work.

287

From the FT-IR analysis (Fig. 4), the absorption peaks at 1420 cm-1 (CH2 bending vibration

288

mode, representing crystallinity band [32]) and at 1040 cm-1 (C-O stretching vibration mode) were

289

detected in the fresh and pretreated corncob samples. However, a greatly reduced intensity of these

290

peaks was observed in the sulfonated materials, C-H2SO4 and C-TsOH-10, implying the

291

modification and/or degradation of lignin units. After sulfonation, the appearance of absorption

292

bands at 1030 and 1155 cm-1 were attributed to the O=S=O symmetric stretching and SO3 stretching

293

[32], indicating that -SO3H groups were successfully incorporated into the framework in the form of

294

C-SO3H. The stronger absorption peaks around 1030 and 1155 cm-1 in C-TsOH-10 compared to C-

295

H2SO4 suggested a higher amount of sulfonic acid functional groups was obtained in C-TsOH-10

296

than in C-H2SO4.

297

Acidity properties. The acidity property of a solid catalyst is comprised of the number, strength

298

and type (Brønsted or Lewis) of acid sites. However, no single method can provide these properties.

299

The total acidity of the catalysts was obtained using acid-base titration [33], while the number and

300

strength of acid sites of the carbonaceous samples was examined based on the peak position and

301

peak area obtained from the NH3-TPD profiles. The disadvantages of this method are that NH3 can

302

be adsorbed on the non-acidic part of a surface, and the peak temperature does not show the acid

303

strength directly [34].

304

The total acidity obtained from titration ranged from 0.32–1.93 mmol/g, where C-H2SO4 gave

305

the highest acidity (1.93 ± 0.01 mmol/g) and L-TsOH-10 gave the lowest acidity (0.315 ± 0.01

306

mmol/g). Whereas the amount of acid sites for C-H2SO4 was estimated from desorption peak area of

307

NH3 released through TPD at 1.17 mmol/g. As illustrated in Fig. 5, the desorption peaks occurring

308

between 100–300 °C (-OH, -COOH), 300–550 °C (-SO3H) and higher than 550 °C were assigned to

309

the weak, medium and strong acid sites on the surface, respectively [35]. The peak at the higher

310

desorption temperature (620–700 °C) was clearly observed for C-H2SO4 and L-H2SO4 (Fig. 5 (a)

13 311

and (h)) corresponding to the stronger acid sites, which agrees well with the two largest acid

312

quantity obtained from titration method (Table 1). For the C-TsOH samples (Fig. 5 (b)-(e)), the total

313

acidity determined by NH3-TPD revealed that increasing the TsOH amount decreased the amount of

314

strong acid sites and the Tmax of the strong acid sites slightly shifted toward a higher temperature.

315

The material derived from corncob sulfonation with the H2SO4/TsOH, especially C-M-15-10,

316

showed the highest amount of acid sites. This result is consistent with previous work, which

317

reported that carbon based materials with a higher acid density were obtained when using an acid

318

mixture (ClSO3H/H2SO4) as the sulfonating agent rather than when using H2SO4 alone [36].

319

Surprisingly, the TPD profile of L-TsOH-10 showed the smallest amount of acid sites and the Tmax

320

of the strong acid sites was shifted slightly to the lowest temperature compared to the other

321

catalysts.

322 323

3.2. Effect of the sulfonation agent

324

The solid carbonaceous materials of this study were then employed as acid catalysts in the

325

esterification of oleic acid with methanol to produce methyl oleate (FAME), with the corresponding

326

yields reported in Table 1. The total acidity of the catalysts obtained from the titration ranged from

327

0.32–1.93 mmol/g, depending on the synthesis conditions, and gave a FAME yield in the range of

328

39.5 ± 2.0% to 86.5 ± 0.6%. The FAME yield was related to the total acid site obtained from

329

titration, where C-H2SO4 gave the highest FAME yield (86.5%) and L-TsOH gave the lowest

330

FAME yield (39.5%), suggesting that more acid sites and stronger acid strength (from NH3-TPD)

331

could activate the protonation of the carbonyl oxygen. However, the obtained FAME yields

332

suggested that the catalytic activity of the sulfonated carbon-based materials depended on not only

333

their acid property but also on their porous structure. It should be noted that the acidity present in

334

the catalysts in this work is low compared to the total acid density of the commercial Amberlyst-15

335

(4.2 mmol/g) [11], where a FAME yield of 85% was obtained with 7 wt.% Amberlyst-15 at a

336

7:1 molar ratio of methanol: oleic acid and 60 °C. Whereas H2SO4-Zr2O (total acidity by NH3-TPD

14 337

of 1.66 mmol/g) gave a 98% yield at 100 °C at a 10 h reaction time and HCl-SO3-ZrO2

338

(6.29 mmol/g) under the same condition gave a nearly 100% yield [35].

339

Catalysts prepared from TsOH sulfonation resulted in high FAME yields (≥ 73%) with C-

340

TsOH-10 achieving the highest FAME yield (80.4%) of the C-TsOH-X catalysts, but these FAME

341

yields were still lower than that with C-H2SO4. Note that the higher catalytic activity of the acid

342

catalysts derived from TsOH sulfonation were obtained from the catalyst having a higher amount of

343

strong acid sites, but, as already stated, C-H2SO4 gave a higher FAME yield despite having a lower

344

level of strong acid sites. Using the H2SO4/TsOH acid mixture as the sulfonating agent led to an

345

acidic carbonaceous catalyst with a markdly larger amount of strong acid sites, in agreement with

346

previous work [36], but with FAME yields of only around 75%. Thus, additional explanation is

347

required for the highest FAME yield being obtained with the C-H2SO4 catalyst. Catalysts with a

348

better performance have previously been reported to have larger pore diameters that allow the

349

reactants (methanol, fatty acid) to efficiently diffuse into the catalytically active sites [37]. On this

350

basis, it was reasonable that the catalysts with the smaller pore diameters gave lower FAME yields

351

despite having a high number of strong acid sites. This could be the reason why the highest FAME

352

yield was obtained with C-H2SO4, as it had the largest pore diameter. The surface acidity obtained

353

in this work showed the same trend as that in other studies. The sulfonated solid acid catalyst

354

obtained from algae increased from 0.6 mmol/g to 1.46 mmol/g [38] and the density of –SO3H sites

355

in the rice husk char catalyst of 0.70 mmol/g [39] gave the best catalytic performance for

356

esterification at 90 °C, although this reaction temperature was higher than that used in the present

357

work.

358

The presence of S in the C-H2SO4 and C-TsOH-10 samples was also confirmed by X-ray

359

photoelectron spectroscopy (XPS) analysis, as shown in Fig. 6. The main C 1s bands at 285 eV

360

(Fig. 6 (a) and (b)) were attributed to C–C bonding and the bands at 286.3, 287.7 and 289.1 eV are

361

attributed to C–O bonds, carbonyl and carboxylic acid groups, respectively, [40]. The relative

362

amount of C–O bonding in the C-H2SO4 sample was higher than that in C-TsOH-10. The band at

15 363

169 eV attributed to the S in –SO3H groups was clearly seen in both catalysts (Fig. 6 (c) and (d)).

364

On the other hand, the band at 164 eV attributed to S in the SH groups [41] was only observed in

365

the C-TsOH-10 sample.

366

More specifically, in the case of C-TsOH-X catalysts, it could be noticed that the catalytic

367

performance might directly correlated with acidity properties. For C-TsOH-10, it gave the quite

368

high FAME yield, albeit the acid quantity was merely 0.58 mmol/g, which was lower than that of

369

the other C-TsOH-X catalysts (Table 1). Nevertheless, it provided the highest level of S contents

370

which could refer to both SO3H and SH groups, and as a result, having more acidic sites to proceed

371

the esterification reaction. As presented in Figure 5, the NH3-TPD profile of C-TsOH-10 catalyst

372

also reveals the more distinct desorption peak centered around 150 °C than other catalysts,

373

attributed to the more existence of –OH and/or –COOH. These weak acid groups could enhance the

374

catalytic performance by acting as active sites, facilitating the adsorption of reactants on the catalyst

375

surface, and consequently, increasing the reaction possibility [42].

376 377

3.3. Effect of the reaction time and temperature

378

To determine the optimum condition for FAME production, the C-H2SO4 and C-TsOH-10

379

catalysts were chosen as they exhibited the highest catalytic activity. The esterification of methanol

380

and oleic acid was performed for 2, 4 and 8 h at 60 °C to evaluate the effect of the reaction time

381

(Fig. 7). Interestingly, after 2 h of reaction, C-TsOH-10 gave a higher (about 2.75-fold) FAME

382

yield than C-H2SO4, which was probably due to the higher amount of strong acid sites (Fig. 5 (b)).

383

At a longer reaction time of 4 h, a greater FAME yield was obtained with both catalysts than at 2 h,

384

but they both now gave a broadly similar FAME yield, while after 8 h C-H2SO4 gave a higher

385

FAME yield than C-TsOH-10. The large diameter pores in the C-H2SO4 catalyst, which will both

386

allow larger substrates to diffuse in and also become blocked with smaller by-product molecules

387

less easily, may facilitate the relatively high FAME yields at longer reaction times.

16 388

Considering the faster initial reaction kinetics of C-TsOH-10 than C-H2SO4, then C-TsOH-10

389

was selected to study the effect of the reaction temperature. The temperature-dependent FAME

390

yields as a function of reaction time in the esterification of oleic acid with methanol catalyzed by C-

391

TsOH-10 is shown in Fig. 8. At 60 °C, a 4 h esterification gave a FAME yield of ∼72%, being

392

comparable to the highest FAME yield obtained using sulfonated coffee residue as catalyst in the

393

esterification of caprylic acid, a shorter chain fatty acid, at a similar temperature and reaction time

394

[43]. In addition, the C-TsOH-10 catalyst gave higher FAME yields and required a lower reaction

395

temperature, compared with that reported previously for various acid catalysts, such as Amberlyst

396

15, Nafion NR50 and sulfonated polydivinylbenzene [44].

397

For comparison, the sulfonated carbon synthesized by the hydrothermal carbonization of a

398

mixture of furfural/sodium dodecylbenzene sulfonate (SDBS)/H2SO4 at 180 °C, with a surface area

399

of 40 m2/g and total acid level of 2.3 mmol/g, gave a 87.3% FAME yield for the esterification of

400

oleic acid and methanol at 65 °C within 4 h [45], whereas the chitosan sulfonate bead with 0.34

401

mmol/g gave 70% methyl oleate at 50°C within 4 h [46]. As expected, higher temperatures

402

enhanced the oleic acid conversion rate [47]. Here, the C-TsOH-10 catalyst was relatively active at

403

80 °C and 100 °C giving FAME yields of ca. 70% and 83% after 1 and 4 h, respectively.

404

Temperature was the important variable with the greatest effect on FAMEs yield, which was

405

increased by about 1.6-fold when increasing the reaction temperature from 60 °C to 80 °C after 1 h.

406

Since it is economically desirable to conduct high-yield reactions at the lowest temperature, the

407

catalyst reusability was further examined at 80 °C.

408 409

3.4. Reusability of the C-TsOH-10 catalyst

410

The reusability of the C-TsOH-10 catalyst was investigated in the esterification of oleic acid and

411

methanol. The reaction was performed under the same reaction conditions for four cycles. After

412

each cycle, the catalyst was separated from the mixture of reactant and products and retested in a

413

fresh reaction without regeneration.

17 414

Unlike synthetic catalysts, sulfonated mesoporous polymer [36] showed a great promise as an

415

effective and reusable catalyst in esterification of oleic acid (> 90%) for up to four cycles, though

416

with the requirement of a high temperature (100 °C) and high methanol: oil molar ratio (30:1).

417

However, this polymeric catalyst required several chemical reagents and well-controlled synthesis

418

steps to ensure sufficient yields of the catalyst end product. In contrast, the potential benefits of

419

corncob as the starting material for the production of solid acid catalysts is the natural and

420

renewable abundance of this non-toxic waste biomass.

421

The FAME yields obtained with C-TsOH-10 dropped substantially from 77.51% to 44.91 % and

422

28.76% in the 2nd and 3rd cycle, respectively. This was probably due to some leaching of acid sites

423

from the catalyst surface, with a reduction in the acid quantity obtained from titration of ca. 24% in

424

the 3rd cycle (from 0.58 to 0.50 and 0.44 mmol/g in the 2nd and 3rd cycle, respectively). Catalyst

425

deactivation may also be caused by poisoned active sites covered by the water by-product [43,48].

426

The deactivation of catalysts after the first cycle was also observed in the case of other sulfonated

427

biomass catalysts [48,49], where a washing step could be added to regenerate the catalyst.

428 429

4. Conclusions

430

This work systematically investigated the relationship between the properties and the catalytic

431

activity of carbonaceous acid catalysts derived from the sulfonation of corncob using various

432

sulfonating agent systems. Both solid TsOH and liquid H2SO4/TsOH mixtures can be used as

433

effective sulfonating agents in the preparation of biomass-based solid acid catalysts with a high

434

amount of strong acid sites. However, in terms of catalytic activity in the esterification of oleic acid

435

with methanol, only the H2SO4 and TsOH sulfonated materials gave methyl oleate (FAME) yields

436

of 80% or above. A faster initial reaction rate was observed in the esterification catalyzed by C-

437

TsOH-10 than C-H2SO4. The catalytic activity of the studied sulfonated corncob materials

438

depended not only on the quantity of strong acid sites, but also on their specific surface area and

439

pore sizes. The C-TsOH-10 catalyst had a high surface area of 240 m2/g, and suitable pore size of

18 440

3.5 nm in diameter, and that allowed long chain oleic acid to easily diffuse into the catalyst pore and

441

react with methanol at the catalytically active sites. Utilizing TsOH as a sulfonating agent could

442

dismiss the requirement for a large-volume reactor and lower the risk of the sulfonation process

443

compared to the use of H2SO4.

444 445

Acknowledgements

446

The authors thank: the Government Research Budget (Grant No. GRB_APS_49_59_63_08);

447

Chulalongkorn University (CU-GES-60-04-63-03); the Thammasat University Research Fund

448

under the Research University Network (RUN) Initiative (No.8/2560) for research funding.

449 450

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24 583

TABLE CAPTIONS

584

Table 1 Physical and chemical properties of the materials and FAME yield.

585 586

FIGURE CAPTIONS

587

Fig. 1 Representative TGA profiles of the fresh corncob, commercial lignin and sulfonated

588 589 590

materials.

Fig. 2 Representative XRD patterns of the (a) fresh corncob, (b) pretreated corncob, (c) C-H2SO4 catalyst and (d) C-TsOH-10 catalyst.

591

Fig. 3 Representative SEM images at 3,000 x magnification of the (a) fresh corncob, (b) pretreated

592

corncob, (c) C-H2SO4, (d) C-TsOH-5, (e) C-TsOH-10, (f) C-TsOH-15, (g) C-TsOH-20, (h)

593

C-M-3-10, (i) C-M-15-10, (j) commercial lignin, (k) L-H2SO4 and (l) L-TsOH-10.

594 595 596 597 598 599 600 601 602 603

Fig. 4 Representative FT-IR spectra of the (a) fresh corncob, (b) pretreated corncob, (c) C-H2SO4 and (d) C-TsOH-10

Fig. 5 Representative NH3-TPD profiles of the (a) C-H2SO4, (b) C-TsOH-5, (c) C-TsOH-10, (d) CTsOH-15, (e) C-TsOH-20, (f) C-M-3-10, (g) C-M-15-10, (h) L-H2SO4 and (i) L-TsOH-10.

Fig. 6 Representative XPS spectra of C 1s of the (A) C-H2SO4 and (B) C-TsOH-10 catalysts and S 2p of the (C) C-H2SO4 and (D) C-TsOH-10 catalysts.

Fig. 7 Effect of the reaction time on the FAME yield in the presence of either the C-TsOH-10 or the C-H2SO4 catalyst at 60 °C.

Fig. 8 Effect of the reaction temperature on the FAME yield in the presence of the C-TsOH-10 catalyst.

Table 1 Physical and chemical properties of the materials and FAME yield Material

Carbon

Hydrogen

Nitrogen

Sulfur

C/Ha

Surface

Pore volume

Avg. pore

Acid quantityb

FAME yieldc

area (m2/g)

(cm3/g)

diameter (nm)

(mmol/g)

(%)

Fresh corncob

50.50

6.03

0.68

0.26

8.4

n/a

n/a

n/a

n/a

n/a

C-H2SO4

56.30

3.44

0.24

3.72

16.4

14.1

0.03

154.5

1.93±0.01

86.5 ± 0.6

C-TsOH-5

67.90

4.34

0.27

5.69

15.6

71.7

0.14

58.8

0.35±0.03

73.2 ± 1.1

C-TsOH-10

68.20

3.88

0.18

5.98

17.6

241.0

0.14

3.5

0.58±0.03

80.4 ± 0.9

C-TsOH-15

72.50

4.50

0.13

4.62

16.1

290.4

0.26

3.7

0.60±0.02

77.6 ± 1.1

C-TsOH-20

71.80

4.59

0.14

4.41

15.6

297.5

0.28

3.7

0.62±0.03

76.4 ± 2.1

C-M-3-10

71.00

4.37

0.24

3.22

16.2

371.1

0.29

2.1

0.63±0.03

74.2 ± 1.2

C-M-15-10

69.90

4.13

0.15

3.93

16.9

360.5

0.29

2.4

0.60±0.02

75.4 ± 1.4

Commercial Lignin

49.90

5.41

0.11

4.81

9.2

n/a

n/a

n/a

n/a

n/a

L-H2SO4

53.40

3.32

0.13

6.44

16.1

62.4

0.06

3.3

1.20 ± 0.03

78.2 ± 0.7

L-TsOH-10

67.30

3.88

0.10

7.90

17.3

10.0

0.01

6.3

0.32 ± 0.01

39.5 ± 2.0

a

measured by CHNS analyzer,

b

c

determined by titration method, measured by GC

Figure 1

Figure 2

a)

c)

b)

d)

e)

f)

g)

h)

i)

j)

k)

l)

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Research highlights

•

Corncob-derived residue can be as a starting material for the production for the esterification.

•

Both TsOH and H2SO4/TsOH mixtures can be used as effective sulfonating agents.

•

TSsOH sulfonated sample presented a considerable increase in surface area.

•

H2SO4 and TsOH sulfonated materials gave high methyl oleate (FAME) yields.