Ethylene production from ethanol dehydration over mesoporous SBA-15 catalyst derived from palm oil clinker waste

Journal Pre-proof Ethylene production from ethanol dehydration over mesoporous SBA-15 catalyst derived from palm oil clinker waste Yoke Wang Cheng, Chi Cheng Chong, Chin Kui Cheng, Kim Hoong Ng, Thongthai Witoon, Joon Ching Juan PII:

S0959-6526(19)34193-9

DOI:

https://doi.org/10.1016/j.jclepro.2019.119323

Reference:

JCLP 119323

To appear in:

Journal of Cleaner Production

Received Date: 22 June 2019 Revised Date:

8 October 2019

Accepted Date: 13 November 2019

Please cite this article as: Cheng YW, Chong CC, Cheng CK, Ng KH, Witoon T, Juan JC, Ethylene production from ethanol dehydration over mesoporous SBA-15 catalyst derived from palm oil clinker waste, Journal of Cleaner Production (2019), doi: https://doi.org/10.1016/j.jclepro.2019.119323. 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.

Word count: 6931 words

Ethylene production from ethanol dehydration over mesoporous SBA-15 catalyst derived from palm oil clinker waste

Yoke Wang Chenga,b, Chi Cheng Chonga,b*, Chin Kui Chenga, Kim Hoong Ngc, Thongthai Witoond, Joon Ching Juane

a

Faculty of Chemical and Process Engineering Technology, Universiti Malaysia Pahang,

26300 Gambang, Kuantan, Pahang, Malaysia. b

Centre for Biofuel and Biochemical Research, Institute of Self-Sustainable Building,

Universiti Teknologi PETRONAS, 32610, Seri Iskandar, Perak, Malaysia. c

School of Energy and Chemical Engineering, Xiamen University Malaysia, Jalan Sunsuria,

43900 Sepang, Selangor, Malaysia. d

Center of Excellence on Petrochemical and Materials Technology, Department of Chemical

Engineering, Faculty of Engineering, Kasetsart University, Bangkok 10900, Thailand. e

Nanotechnology & Catalysis Research Centre, Institute of Postgraduate Studies, University

Malaya, 50603 Kuala Lumpur, Malaysia.

*Corresponding author Address: Faculty of Chemical and Process Engineering Technology, Universiti Malaysia Pahang, 26300 Gambang, Kuantan, Pahang, Malaysia. Tel: +60-9-5492896; Fax: +60-9-5492889 E-mail address: [email protected]; [email protected]

Word count: 6931 words ABSTRACT The silica-rich palm oil clinker (POC) from oil palm agroindustry is often dumped in landfill. This work investigated the valorisation of POC into Santa Barbara Amorphous-15 (SBA-15) catalyst, the modulation of its surface acidity, and its application in dehydration of ethanol to ethylene. With commercial SBA-15 [SBA-15(Comm.)] as reference, the successful fabrication of POC-derived SBA-15 [POC-SBA-15(pH = 3, 5, and 7)] were validated by spectroscopic and microscopic characterisation. From the results of temperature-programmed desorption of ammonia, the SBA-15(Comm.) have high strong acidity while POC-SBA-15 exhibit enriched weak-moderate acidity. For ethanol dehydration over SBA-15 at 200 – 400 °C, the ethanol conversion (

) and ethylene selectivity (

) rise with temperature.

The catalytic activity was ranked as SBA-15(Comm.) < POC-SBA-15(3) < POC-SBA-15(7) < POC-SBA-15(5). Spent catalysts characterisation unanimously confirms the least carbon deposition on POC-SBA-15(5), which subsequently used to study the effect of initial ethanol concentration and liquid hourly space velocity (LHSV). When 99.5 wt.% ethanol diluted to 50 wt.%, the competitive adsorption between ethanol and water reduces enhances

. Further ethanol dilution (≤ 30 wt.%) deteriorates

but

following remarkable

ethanol steam reforming at elevated temperature (≥ 350 °C). For 50 wt.% ethanol dehydration over POC-SBA-15(5) at 400 °C, a greater LHSV furnishes a higher ethanol partial pressure that increases

but decreases

. When LHSV > 16 mL/g·h, the saturation of finite

active sites with adsorbates renders the drastic declination of

and

. For ethanol

dehydration over POC-SBA-15(5), the optimal conditions are temperature of 400 °C, initial ethanol concentration of 50 wt.%, and LHSV of 16 mL/g·h. Fresh POC-SBA-15(5) steadily catalyses the optimal process (73.33 %

and 84.70 %

) up to 105 h. Meanwhile,

regenerated POC-SBA-15(5) achieves a lower catalytic activity (71.95 % %

). 1

and 81.96

Word count: 6931 words

Keywords: Palm oil clinker; mesoporous SBA-15; ethanol dehydration; ethylene; surface acidity; operating conditions.

2

GRAPHICAL ABSTRACT

Word count: 6931 words 1

1. Introduction

2

Ethylene (C2H4) is one of the most highly sought raw materials in the downstream

3

petrochemical industries. C2H4 is a primary precursor in plastic manufacturing, polyethylene

4

production and even surfactant fabrication (i.e. ethylene glycol or ethylene oxide) (Soh et al.,

5

2017). Globally, circa 99 % of C2H4 is produced by thermal cracking or steam cracking of

6

hydrocarbons that derived from non-renewable fossil fuels (petroleum and natural gas)

7

(Zhang et al., 2008). The gradual depletion of fossil fuels prompts the seeking of alternative

8

pathway. For this reason, ethanol dehydration has been touted as an attractive route to yield

9

C2H4 due to its sustainability. Bioethanol represents a replenishable feedstock that easily

10

obtained from the fermentation of renewable biomass hydrolysate (carbohydrate-rich

11

solution) (Mohsenzadeh et al., 2017).

12

Fundamentally, there are two plausible reaction mechanisms for ethanol dehydration,

13

viz. unimolecular/intramolecular and bimolecular/intermolecular pathways (Zhang and Yu,

14

2013). The former is an endothermic reaction that favours C2H4 formation (Eq. (1)) while the

15

latter represents an exothermic reaction that renders diethyl ether (DEE) synthesis (Eq. (2)).

16

From a thermodynamic standpoint, low temperature (150 – 300 °C) facilitates DEE formation

17

whereas high temperature (300 – 500 °C) promotes C2H4 production (Chen et al., 2010). At

18

elevated temperature (> 500 °C), ethanol dehydrogenation (Eq. (3)) to acetaldehyde (C2H4O)

19

occurs (Chen et al., 2010). Since water potentially stimulates ethanol steam reforming (Eq.

20

(4)), the initial ethanol concentration is another crucial parameter that determines the

21

tolerance towards the water content of bioethanol (Resini et al., 2009). →

22 23

2

→(

24

→

25

+

+ )

(1) +

(2)

+ → 4

(3) + 2

(4)

3

Word count: 6931 words 26

Acid-catalysed ethanol dehydration is performed over concentrated sulphuric acid

27

(H2SO4) and phosphoric acid (H3PO4) to boost C2H4 selectivity, whereby the latter grants

28

higher C2H4 selectivity at lower temperature. In catalytic ethanol dehydration, the hydroxyl

29

group (-OH) of ethanol (C2H5OH) is protonated by acid (HA) to form ethyloxonium ion

30

(C2H5OH2+), which later deprotonated by the conjugate base (A-) to produce C2H4 via H2O

31

removal (Smith, 2016). Homogeneous catalysis is advantageous for insignificant mass

32

transfer limitation between catalyst and reactant; however, its by-products formation (via side

33

reactions like oxidation and polymerisation) and difficult solvent recovery prompted the

34

recent emphasis on heterogeneous catalysis (Takahara et al., 2005). To date, numerous solid

35

catalysts had been tested for ethanol dehydration, viz. oxides (Al2O3), protonated molecular

36

sieves (HZSM-5), and silica-supported heteropoly acids (HPA/SiO2) (Zhang and Yu, 2013).

37

Oxides resist coking deactivation but require higher dehydration temperature. Protonated

38

ZSM-5 (HZSM-5) with large specific surface area exhibit high catalytic activity; withal, it is

39

more prone to coking deactivation. The ethanol dehydration to C2H4 at low temperature is

40

feasible over HPA/SiO2; howbeit, its application is restricted by low ethanol conversion.

41

Mesoporous silica Santa Barbara Amorphous-15 (SBA-15) is one of the promising

42

nanomaterials for catalysis by virtue of its excellent thermal stability, uniform pore size, and

43

high specific surface area (Nandi et al., 2011). Since silica (SiO2) is an acidic oxide

44

(Richardson, 1989), it is envisaged that the well-ordered, mesoporous SBA-15 would have

45

uniform acid sites that beneficial for catalysing ethanol dehydration and impeding carbon

46

deposition. Today, the growing environmental alertness urges the social preference for eco-

47

friendly catalyst synthesis. For instance, Alvarez et al. (2014) successfully extracted

48

amorphous silica from rice husk char after sequential events of HCl leaching, Na2CO3 reflux,

49

and carbonation. Tropical nations particularly Indonesia, Malaysia, and Thailand represent

50

the top three palm oil producers (Cheng et al., 2019a) that overwhelmed with oil palm wastes.

4

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For energy recovery, tons of palm oil clinker (POC) waste was generated from the

52

incineration of palm kernel shells and empty fruit bunches at 800 – 1000 °C (Chong et al.,

53

2018).

54

Rather than landfilling, the current work attempted the valorisation of silica-rich POC

55

(Altwair et al., 2012) into POC derived SBA-15 (POC-SBA-15) catalysts for C2H4

56

production via ethanol dehydration. For the first time, the surface acidity of POC-SBA-15

57

was modulated by using different preparation pH (3, 5, and 7) to elucidate the effect of

58

acidity on ethanol dehydration. Additionally, commercial SBA-15 (SBA-15(Comm.)) was

59

synthesised at pH 7 to serve as a comparative reference. Apart from reaction temperature

60

(200 – 400 °C), the influence of initial ethanol concentration (10 – 99.5 wt.%) and liquid

61

hourly space velocity (10 – 20 mL/g·h) on ethanol dehydration was investigated with the best

62

POC-SBA-15 catalyst. Lastly, the stability and regeneration studies of catalytic ethanol

63

dehydration were executed at optimal conditions using the best POC-SBA-15 catalyst.

64 65 66 67

2. Materials and methods This section details the materials and methods for catalysts preparation, catalysts characterisation, and catalytic evaluation.

68 69

2.1 Catalysts preparation

70

From a local mill in Pahang, raw palm oil clinker (POC) was collected as blackish

71

powder, which later calcined at 600 °C for 6 h to obtain grey powder by burning off its

72

residual biomass. Since POC exhibits myriad acid-soluble minerals (Altwair et al., 2012), the

73

POC was pretreated by acid-leaching to enrich its silica (SiO2) content (Chong et al., 2018).

74

The POC was stirred with phosphoric acid (H3PO4, 85 wt.%) at 110 °C for 12 h.

75

Subsequently, the acid-leached POC (A-POC) was retrieved by vacuum filtration and washed

5

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with deionised water until neutral pH. After oven-drying at 110 °C for 12 h, the A-POC was

77

refluxed with 2.5 N NaOH solution (mass ratio of A-POC:NaOH = 1.5:1) at 80 °C for 3 h to

78

prepare POC-derived sodium silicate (POC-Na2SiO3) powder.

79

The SBA-15 synthesis pioneered by Zhao et al. (1998) was adapted to produce POC-

80

SBA-15, with POC-Na2SiO3 solution in lieu of commercial Na2SiO3 solution. Chong et al.

81

(2018) synthesised POC-SBA-15 from H2SO4-leached POC and purposed the optimal mass

82

ratio of Pluronic® P123 surfactant (EO20PO70EO20):POC-Na2SiO3:H2O as 1:2.9:36. At first,

83

P123 was dissolved in 2 M hydrochloric acid (HCl, 37 wt.%) for 1 h before the addition of

84

POC-Na2SiO3 solution. After 24 h stirring at 40 °C, the mixture underwent hydrothermal

85

reflux treatment at 80 °C for 6 h. Thereafter, the milky slurry was filtered for its white

86

precipitate. The white precipitate was rinsed with deionised water to a pH of 3, 5, or 7,

87

filtered, oven-dried at 110 °C for 12 h, and calcined at 550 °C for 3 h. Hence, three POC-

88

SBA-15 catalysts were obtained, viz. POC-SBA-15(3), POC-SBA-15(5), and POC-SBA-

89

15(7). For comparison, SBA-15(Comm.) was prepared similarly at pH 7 by using commercial

90

Na2SiO3 solution as its silica precursor.

91 92

2.2 Catalysts characterisation

93

The crystalline structure of fresh and spent catalysts was identified by X-ray

94

diffraction (XRD) analysis. Through Philips X’ Pert MPD instrument (3 kW, 15 mA), the

95

XRD analysis was performed using Cu Kα radiation to acquire the low-angle (2θ = 0.5 – 3°)

96

and wide-angle (2θ = 10 – 80°) XRD diffractograms. The XRD patterns were analysed based

97

on JCPDS (Joint Committee on Powder Diffraction Standards) data files. By N2

98

physisorption, the surface textural properties of fresh and spent catalysts were examined at -

99

196 °C with Micromeritics ASAP-2010 instrument. Before the analysis, the catalysts were

100

evacuated at 300 °C under N2 flow for 3 h. From N2 adsorption isotherm, the specific surface

6

Word count: 6931 words 101

area was calculated via Brunauer–Emmett–Teller (BET) model, while the pore volume and

102

pore size were acquired with Barret-Joyner-Halenda (BJH) model.

103

The functional groups of fresh catalysts were revealed by Fourier transform infrared

104

spectroscopy (FTIR) using attenuated total reflectance (ATR) technique. During FTIR

105

spectra acquisition, the catalysts were irradiated by polychromatic infrared (1/λ = 400 – 1,400

106

cm-1) within Nicolet™ iS5 spectrometer. The 2D morphology of fresh and spent catalysts

107

was captured by transmission electron microscopy (TEM) via JEOL JEM-2100 microscope.

108

For TEM preparation, the catalysts were ultrasonically dispersed in ethanol for 2 h to

109

minimise sample agglomeration. Thereafter, few droplets of mixture were added to the

110

carbon film-coated copper grid, which allowed to be air-dried. The 3D morphology of fresh

111

catalysts was scrutinised with field emission scanning electron microscopy (FESEM) through

112

JEOL JSM-7800F microscope. Prior to the FESEM, the catalysts were sputter-coated with

113

platinum and immobilised on the copper holder with carbon tape.

114

The surface Lewis acidity of fresh catalysts was probed by temperature-programmed

115

desorption of ammonia (NH3-TPD) using Thermo Scientific TPDRO 1100 instrument.

116

Before NH3-TPD, the catalysts were pretreated at 40 °C under He flow for 2 h, cooled to

117

room temperature, adsorbed with CO2 at 120 °C for 0.5 h, and purged with He at ambient

118

temperature for 0.5 h. The NH3-TPD was conducted by heating the catalysts to 800 °C under

119

He flow with a ramping rate of 10 °C/min. The carbon deposition of spent catalysts was

120

quantified by temperature-programmed oxidation (TPO) using TGA Q500 instrument. For

121

TPO, the spent catalysts were heated from ambient temperature to 700 °C under compressed

122

air flow with a ramping rate of 10 °C/min.

123 124

2.3 Catalytic ethanol dehydration

7

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Fig. 1 provides the experimental setup of the catalytic ethanol dehydration. All the

126

reaction studies were performed continuously for 5 h via a stainless-steel packed bed reactor

127

(ID = 11 mm and length = 417 mm). The catalyst bed was quartz wool that loaded with 0.3 g

128

of SBA-15 catalyst, which resided in the middle of reactor. The reaction parameters involved

129

were (i) reaction temperature (200, 250, 300, 350, and 400 °C), (ii) initial ethanol

130

concentration (10, 30, 50, and 99.5 wt.%), and (iii) liquid hourly space velocity, LHSV (4, 8,

131

12, 16, 20 mL/g·h). Before reaction, the entire setup was purged with 100 mL/min of N2 for

132

0.5 h while the reactor was preheated to the desired temperature. The ethanol solution was

133

supplied by a syringe pump to the reactor for its in-situ vaporisation and dehydration, with

134

the co-feeding of N2 (carrier gas). For this study, the total feed rate of vaporised ethanol and

135

N2 was fixed as 150 mL (STP)/min for a weight hourly space velocity (WHSV) of 30,000

136

mL/g·h.

137

From the reactor outlet, the wet gaseous stream was channelled through a series of

138

three condensers that immersed in 60 °C hot water bath to condense the water while keeping

139

diethyl ether (DEE) in the vapour state. Through preliminary HPLC analysis, the liquid

140

condensate solely comprised of water and unreacted ethanol. The gaseous products were

141

further desiccated by a drierite bed, measured for its flow rate with bubbling meter, and

142

hourly analysed in-situ with gas chromatography (GC). The GC instrument was Shimadzu

143

GC-2014 that equipped with three GC columns (Rtx®-1, Rt®-Q-BOND, and Rt®-Msieve 5A),

144

a thermal conductivity detector (TCD: to detect H2, CO, CO2, CH4), and a flame ionization

145

detector (FID: to detect C2H4, DEE, and other hydrocarbons). The temperature of GC

146

columns, TCD, and FID was fixed as 60 °C, 170 °C, and 200 °C, respectively. The carrier gas

147

He was supplied at a rate of 15 mL/min. The catalytic performance was reported in terms of

148

average ethanol conversion (

149

Eqs. (5) and (6).

) and ethylene selectivity (

8

), which computable via

Word count: 6931 words (%) =

150

(%) =

151

152 153 154

where ' *

(

∑

!

∑

×

×

! "! #$

× 100

(5)

× 100

(6)

is the number of carbon of gas species ), * is the outlet flow rate of gas species ),

# is the inlet flow rate of ethanol vapour, and * +

is the outlet flow rate of

ethylene.

155

Since C2H4 cracking could provoke coke formation, a stability study was conducted

156

for ethanol dehydration over the best SBA-15 catalyst at optimal conditions by extending the

157

time-on-stream duration to 150 h. For reusability evaluation, the spent catalyst from the

158

stability test was regenerated in-situ via coke oxidation (C + O2 → CO2) at 700 °C by

159

sparging 100 mL/min compressed air for 2 h. The regeneration study was assessed by using

160

the regenerated catalyst to catalyse the ethanol dehydration at optimal conditions for another

161

150 h.

162 163 164 165

3. Results and discussion This section presents the crucial findings for fresh catalysts characterisation, catalytic evaluation, spent catalysts characterisation, and stability and regeneration studies.

166 167

3.1 Fresh catalysts characterisation

168

Fig. 2(A) presents the low-angle XRD patterns of fresh catalysts. For all the catalysts,

169

three characteristic diffraction peaks of SBA-15, viz. (100), (110), and (200) reflections could

170

be noticed in their low-angle XRD patterns (Zhao et al., 1998). From wide-angle XRD

171

patterns in Fig. 2(B), all the catalysts display a wide hump at circa 23°, which corroborates

172

the existence of amorphous siliceous framework (Jozwiak et al., 2004). XRD results evince

173

the successful SBA-15 formation regardless of the variation on sodium silicate source and

9

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preparation pH. Henceforth, all the catalysts were identified as SBA-15 with well-ordered

175

hexagonal (p6mm) structure and uniform mesoporous packing (Zhao and Wang, 2007). In

176

relative to SBA-15(Comm.), POC-SBA-15 catalysts have a less intense diffraction peak at

177

23°, implying their lower silica contents. As a natural silica source, the palm oil clinker

178

(POC) contains trace impurities like Al2O3 and Fe2O3 (Sanawung et al., 2017). The extracted

179

POC-Na2SiO3 solution reasonably has a lesser silica content than the commercial Na2SiO3

180

solution. The impurities of POC-Na2SiO3 might also deter the perfect formation of the

181

siliceous framework in POC-SBA-15 (Abdullah et al., 2018).

182

The N2 physisorption isotherms of fresh catalysts are illustrated in Fig. 2(C). All the

183

SBA-15 catalysts display prominent type IV adsorption isotherm with type H1 hysteresis

184

loop. These observations reveal the mesoporous structure of SBA-15 catalysts, in addition to

185

their cylindrical pore channels and narrow pore size distribution (Li et al., 2015). For all the

186

SBA-15 catalysts, their N2 uptake increased sharply over the relative pressure (P/P0) range of

187

0.4 – 0.9, due to the capillary condensation of N2 within the uniform mesopores (Liu et al.,

188

2009). In aforesaid P/P0 region, the N2 uptake of SBA-15(Comm.) > POC-SBA-15(7) > POC-

189

SBA-15(5) > POC-SBA-15(3). With commercial Na2SiO3 as silica precursor, SBA-

190

15(Comm.) certainly possesses ideal amorphous siliceous framework, so its greater number

191

of vacant pores renders higher N2 adsorption. In contrast, the lower N2 adsorption of POC-

192

SBA-15 possibly hints at some pore blockage by impurities of POC-Na2SiO3

193

(Bhagiyalakshmi et al., 2009).

194

The specific surface area, pore volume, and pore size of fresh catalysts are tabulated

195

in Table 1. Among fresh catalysts, SBA-15(Comm.) has the highest surface textural

196

properties. The POC-SBA-15 catalysts exhibit lower surface textural properties than the

197

SBA-15(Comm.) due to the utilisation of impure POC-Na2SiO3. The mesoporous structure of

198

POC-SBA-15 catalysts was partially covered by impurities of POC-Na2SiO3, thereby an

10

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inferior surface textural properties (Chong et al., 2018). For POC-SBA-15, the surface

200

textural properties increase with the preparation pH as washing gradually leaches out the

201

impurities from white precipitate (uncalcined SBA-15). The least washed POC-SBA-15(3)

202

shows lowest surface textural properties owing to its highest amount of impurities. With

203

identical preparation pH, SBA-15(Comm.) and POC-SBA-15(7) share a similar pore size

204

distribution as shown in Fig. 2(D). No disparity of surface textural properties exists between

205

SBA-15(Comm.) and POC-SBA-15 catalysts, so the POC-SBA-15 catalysts have

206

satisfactorily high catalytic area.

207

Fig. 2(E) depicts the FTIR spectra of fresh catalysts to identify their functional

208

groups. For all SBA-15 catalysts, a total of six absorption peaks were identified within the

209

wavenumber range of 1,400 – 400 cm-1, viz. 1,225, 1,060, 961, 801, 510, and 450 cm-1. These

210

absorption peaks are caused by different Si interactions. The main peaks at 1,225 cm−1 and

211

1,060 cm-1 were assigned as the longitudinal-optic (LO) and transverse-optic (TO)

212

asymmetric stretching of Si-O-Si bonds, respectively (Wang et al., 1999). The shoulder peak

213

at 961 cm-1 was ascribed to the asymmetric Si-OH vibration (Chong et al., 2018). The 801

214

cm-1 and 450 cm-1 peaks were alluded to the symmetric and asymmetric stretching of Si-O

215

vibrations, individually (Setiabudi et al., 2018). The 510 cm-1 band denotes the bending

216

vibration of tetrahedrally Si-O-Si bonds (Chong et al., 2019). FTIR spectra of POC-SBA-15

217

were less intense than SBA-15(Comm.), conceivably due to their poorer Si- bonding with O

218

atoms and -OH molecules. As asserted by FTIR and XRD findings, POC-SBA-15 catalysts

219

have a poorer siliceous framework than SBA-15(Comm.). The FTIR spectra of POC-SBA-15

220

catalysts were almost indistinguishable from SBA-15(Comm.), substantiating the successful

221

synthesis of SBA-15 from POC-Na2SiO3. The absence of other functional groups in POC-

222

SBA-15 catalysts also informs the high efficiency of phosphoric acid to leach out impurities

223

from POC.

11

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The TEM and FESEM images of fresh catalysts are displayed in Fig. 3. Based on

225

TEM images, all the SBA-15 catalysts show well-defined, highly ordered mesoporous

226

structures with parallel channels and hexagonal symmetry, in parallel with the work of

227

Bukhari et al. (2019). Despite the utilisation of alternative sodium silicate (POC-Na2SiO3),

228

TEM reveals the POC-SBA-15 catalysts well preserved the structure of SBA-15 (Yin et al.,

229

2017). For POC-SBA-15(3) and POC-SBA-15(5), trace impurities are visible in their TEM

230

images because less frequent washing fails to get rid of these impurities thoroughly.

231

However, these impurities could be disregarded since their existence did not remarkably

232

impede SBA-15 formation. The FESEM images disclose all the SBA-15 catalysts composed

233

of rod-like shaped particles with relatively uniform sizes. The practicality of POC as an

234

alternative silica source for SBA-15 preparation was affirmed by the similar morphology

235

between SBA-15(Comm.) and POC-SBA-15 catalysts. Besides, wheat-like shaped impurities

236

are found in the POC-SBA-15 catalysts, with most perceptible appearance on the least

237

washed POC-SBA-15(3).

238

Fig. 4 furnishes the NH3-TPD profiles of fresh catalysts to probe their surface Lewis

239

acidity (acidity is used for simplicity) since NH3 is a Lewis base. For ethanol dehydration,

240

Chen et al. (2010) reported that the increment of weak and moderate acidity assisted in C2H4

241

formation; however, excess strong acidity provoked undesirable C2H4 cracking (coking

242

deactivation) and C2H4 oligomerisation (formation of higher hydrocarbon by-products).

243

Chong et al. (2017) modified the acidity of cerium oxide (CeO2) with H3PO4 for C2H4

244

production via ethanol dehydration. Analogously, the acidity of POC-SBA-15 catalysts could

245

be tailored for ethanol dehydration to generate C2H4. Based on NH3 desorption temperature,

246

the Lewis acid sites of catalysts could be classified into different strengths, viz. weak (100 –

247

250 °C), moderate (250 – 400 °C), and strong (> 400 °C) (Soh et al., 2017). Table 2 compiles

248

the surface Lewis acidity of fresh SBA-15 catalysts, whereby all catalysts exhibit weak,

12

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moderate, and strong acid sites in different proportions. SBA-15(Comm.) has higher

250

proportion of strong acidity while POC-SBA-15 catalysts possess greater proportion of weak

251

and moderate acidity.

252

When commercial Na2SiO3 substituted by POC-Na2SiO3, weak and moderate acidity

253

are generated at the expense of strong acidity, probably imputed to the H3PO4-leaching of

254

POC. This inference is reasonable as the synergy between weak acid (H3PO4) and strong acid

255

(HCl) favourably shifts the acidity to a lower strength region (Soh et al., 2017). Among POC-

256

SBA-15 catalysts, the least washed POC-SBA-15(3) has the highest moderate acid sites

257

(651.81 µmol/g), suggesting the overlay of weak acid sites by abundant protons from HCl.

258

The POC-SBA-15(5) exhibits greatest weak acid sites (824.41 µmol/g) because washing

259

gradually removes excess protons from HCl (Chong et al., 2017). As compared to POC-SBA-

260

15(5), the POC-SBA-15(7) accommodates lesser weak acid sites (444.16 µmol/g), hinting

261

intensive washing conceivably inflicts concomitant removal of protons from H3PO4. In

262

overall, the POC-SBA-15 catalysts possess higher surface acidity than SBA-15(Comm.),

263

attributed to additional H3PO4-leaching treatment of POC. It is envisaged that the protons

264

from H3PO4 infiltrate the POC during acid leaching whereas the protons from HCl invade the

265

POC-SBA-15 during SBA-15 synthesis.

266 267

3.2 Catalytic evaluation

268

The synthesised SBA-15 catalysts were used to catalyse ethanol dehydration for C2H4

269

production. The process was investigated with respect to reaction temperature (200 – 400

270

°C), initial ethanol concentration (10 – 99.5 wt.%), and liquid hourly space velocity (10 – 20

271

mL/g·h).

272 273

3.2.1 Effect of reaction temperature

13

Word count: 6931 words 274

The gas products distribution of ethanol dehydration is primarily governed by the

275

reaction temperature. Zhang et al. (2008) claimed that the formation of diethyl ether (DEE)

276

and ethylene (C2H4) is thermodynamically favoured at 150 – 300 °C and 300 – 500 °C,

277

respectively. The intramolecular dehydration of ethanol to C2H4 is an endothermic process

278

that necessitates a higher energy requirement. Catalysts could endow alternative reaction

279

pathways with lower activation energy for the occurrence of reaction at milder temperature.

280

Hence, the catalytic performance of fresh SBA-15 catalysts was compared at different

281

reaction temperature.

282

Fig. 5 shows the ethanol conversion and ethylene selectivity over different SBA-15

283

catalysts with respect to reaction temperature (initial ethanol concentration = 99.5 wt.% and

284

LHSV = 12 mL/g·h). Under electronic supplementary data, Table A.1 details the gas products

285

distributions obtained with different catalysts and reaction temperatures. Ethanol dehydration

286

over SBA-15 catalysts chiefly produces C2H4 and DEE, with trace amounts of H2, CH4, CO2,

287

CO, and higher hydrocarbon by-products (C3H8, C3H6, C4H10, C4H8, and C5H12). It is

288

believed that the thermal decomposition of ethanol (Eq. (7)) forms the H2, CH4, and CO

289

while the water gas shift (Eq. (8)) generates CO2 and additional H2 by consuming CO

290

(Sharma et al., 2017). C2H4 oligomerisation yields higher hydrocarbon by-products (C3, C4,

291

and C5), viz. propane, propene, butane, butene, and pentane (Phung et al., 2015). In the work

292

of Gayubo et al. (2010), the dehydration of 50 wt.% ethanol over HZM-5 was associated with

293

C2H4 oligomerisation from 280 °C onwards. Here, the dehydration of 99.5 wt.% ethanol over

294

SBA-15 catalysts was accompanied by C2H4 oligomerisation beyond 300 °C. →

295 296

∆

+

↔

+

+

(7)

+

(8)

297

As delineated in Fig. 5(A), the ethanol conversion of POC-SBA-15 catalysts

298

increased sharply with the rising temperature from 200 – 400 °C. At greater temperature, the

14

Word count: 6931 words 299

ethanol reactants possess higher kinetic energy to overcome the energy barrier of dehydration

300

reaction. The SBA-15(Comm.) attains its highest ethanol conversion of 28.96 % at 350 °C

301

that slightly declined to 25.61 % at 400 °C. For SBA-15(Comm.), its deteriorated ethanol

302

conversion at 400 °C could be related to remarkable pore blockage by deposited carbon,

303

which restricted the ethanol adsorption on active sites. The carbon laydown issue of SBA-

304

15(Comm.) was possibly caused by its higher strong acidity, which effects the C-C bond

305

scission of nucleophilic C2H4 (Soh et al., 2017). The POC-SBA-15 catalysts achieve higher

306

ethanol conversion than the SBA-15(Comm.) owing to their greater surface acidity (cf. Table

307

2).

308

For POC-SBA-15 catalysts, their ethylene selectivity gradually rises with increasing

309

reaction temperature (cf. Fig. 5(B)), concurs with the endothermic nature of intramolecular

310

dehydration. In contrast, SBA-15(Comm.) gives a maximum ethylene selectivity of 62.61 %

311

at 350 °C, which marginally dropped to 60.54 % at 400 °C due to remarkable cracking and

312

oligomerisation of C2H4. Evidently, the total selectivity of C3 – C5 by-products for SBA-

313

15(Comm.) was soared from 2.85 % (350 °C) to 7.81 % (400 °C). The POC-SBA-15 catalysts

314

have enriched weak and medium acidity (refers Table 2), which conceivably renders their

315

superior ethylene selectivity over SBA-15(Comm.). The beneficial effect of weak and

316

medium acidity on C2H4 production has been similarly evinced in the work of Xin et al.

317

(2014). A lower strong acidity plausibly discourages C2H4 oligomerisation (Tarach et al.,

318

2016), so the POC-SBA-15 catalysts give a relatively lower total selectivity of C3 – C5 by-

319

products than SBA-15(Comm.).

320

Besides surface acidity, the surface textural properties of SBA-15 catalysts could also

321

affect the ethanol conversion and ethylene selectivity. For SBA-15(Comm.), its greater

322

specific surface area and pore volume engenders the deep travelling of ethanol reactants,

323

lengthens the residence time, and increases the likelihood of side reactions (Soh et al., 2017).

15

Word count: 6931 words 324

Meanwhile, POC-SBA-15 catalysts with relatively lower specific surface area and pore

325

volume have enhanced ethylene selectivity. POC-SBA-15 catalysts have lower surface

326

textural properties that conceivably impede the multiple adsorption of ethanol molecules,

327

thereby minimise the tendency of intermolecular ethanol dehydration and C2H4

328

oligomerisation (Ramesh et al., 2009).

329

For ethanol dehydration at 250 °C, SBA-15 catalysts offer lower ethanol conversion

330

(14.77 – 33.09 %) but higher ethylene selectivity (55.92 – 80.44 %) in relative to industrially

331

used commercial γ-Al2O3 (ethanol conversion = 85 % and ethylene selectivity = 16 %)

332

(Masih et al., 2019). The catalytic performance of SBA-15 in ethanol dehydration at 200 –

333

400 °C could be ranked by ethanol conversion and ethylene selectivity in the descending

334

order: POC-SBA-15(5) > POC-SBA-15(7) > POC-SBA-15(3) > SBA-15(Comm.). POC-

335

SBA-15(5) displays the best catalytic performance in ethanol dehydration, credited to its

336

highest weak acidity, least strong acidity, and reasonably lower surface textural properties.

337

For subsequent investigations, the POC-SBA-15(5) catalyst was adopted by virtue of its

338

exceptional performance among the synthesised SBA-15 catalysts.

339 340

3.2.2 Effect of initial ethanol concentration

341

The initial ethanol concentration was manipulated by diluting the ethanol with

342

deionised water to evaluate the potentiality of catalytic bioethanol dehydration over POC-

343

SBA-15(5). To date, renewable bioethanol could be produced by submerged fermentation of

344

sucrose, starch, and lignocellulosic based feedstocks (Vohra et al., 2014). Howbeit, the

345

bioethanol often exists in diluted form (circa 10 wt.%) because of abundant water content in

346

the fermentation media (Krutpijit and Jongsomjit, 2017). Despite technically feasible

347

bioethanol purification, the concentration of bioethanol is often executed via azeotropic

348

distillation (Mohsenzadeh et al., 2017), which is an energy-intensive process. Direct

16

Word count: 6931 words 349

dehydration of bioethanol to ethylene is highly sought since it associates with lower process

350

expenses as bypassing the additional purification step (Chen et al., 2010).

351

Fig. 6 depicts the ethanol conversion and ethylene selectivity over POC-SBA-15(5)

352

catalyst in variation with initial ethanol concentration and reaction temperature (LHSV = 12

353

mL/g·h). From Fig. 6(A), it is conspicuous that the ethanol conversion progressively dropped

354

with declining initial ethanol concentration from 99.5 wt.% to 10 wt.%, in parallel with the

355

work of Wu et al. (2013). For adsorption on the acid sites of SBA-15, the water molecules

356

prevail over ethanol molecules as they are Lewis base (Cheng et al., 2019b) with smaller

357

kinetic diameter (Wu et al., 2013). As the initial ethanol concentration decreases, greater

358

availability of water molecules denotes higher competitive adsorption between water and

359

ethanol, subsequently provokes a lower ethanol conversion.

360

Unlike the ethanol conversion, the influence of initial ethanol concentration on the

361

ethylene selectivity is more complicated, as presented in Fig. 6(B). For the temperature range

362

of 200 – 300 °C, the ethylene selectivity gradually increased with the reduction of initial

363

ethanol concentration from 99.5 wt.% to 10 wt.%. The boosting effect of ethanol dilution on

364

ethylene selectivity concurs with the result of Chen et al. (2007) but against the work of Wu

365

et al. (2013). Based on these precedent ethanol dehydration studies, the opposing trend of

366

ethylene selectivity was likely caused by different surface acidity of catalyst. With ethanol

367

dilution, the selectivity of C3 – C5 by-products slightly decreased over TiO2/γ-Al2O3 (without

368

strong acidity) (Chen et al., 2007) but drastically increased over SAPO-34 (with high strong

369

acidity) (Wu et al., 2013). During ethanol dehydration, strong acidity eventually plays a vital

370

role in promoting C2H4 oligomerisation.

371

The POC-SBA-15(5) catalyst exhibits 63.21 % weak, 23.79 % moderate, and 12.99 %

372

strong acidity. Since POC-SBA-15(5) has low strong acidity, the enhancement of ethylene

373

selectivity with ethanol dilution could be linked with the competitive adsorption between

17

Word count: 6931 words 374

ethanol and water (Chen et al., 2007). Aforesaid competitive adsorption certainly hinders the

375

multiple adsorption of ethanol molecules and indirectly hampers the intermolecular ethanol

376

dehydration and ethylene oligomerisation (Chen et al., 2010). Table A.2 (supplementary data)

377

reveals the selectivity of DEE and C3 – C5 by-products declined with increasing water content

378

of ethanol feedstock. At higher reaction temperature (350 – 400 °C), the positive effect of

379

ethanol dilution on ethylene selectivity was only observed if the initial ethanol concentration

380

≥ 50 wt.%. If the initial ethanol concentration < 50 wt.%, considerable amount of water

381

probably instigates undesirable ethanol steam reforming and water gas shift that

382

thermodynamically feasible (./0 > 1) at 350 – 400 °C. For 10 wt.% and 30 wt.% ethanol

383

feedstocks, the selectivity of H2, CO, and CO2 abruptly increased from 300 – 400 °C (cf.

384

Table A.2) at the expense of ethylene selectivity. From Table A.2, it is confirmed that

385

simultaneous ethanol steam reforming and water gas shift become more discernible at higher

386

temperature (≥ 350 °C) and water content (≥ 70 wt.%).

387

Apart from notable steam reforming, the influence of ethanol dilution on the

388

decrement of ethanol conversion and the increment of ethylene selectivity gradually

389

diminished at a higher temperature, similar with the work of Chen et al. (2007). It is believed

390

that a greater temperature hastens the endothermic intramolecular ethanol dehydration, which

391

progressively subdues the impact of competitive adsorption between ethanol and water.

392

Dehydration of 50 wt.% ethanol over POC-SBA-15(5) at 400 °C grants a higher ethylene

393

selectivity (88.32 %) than that of 99.5 wt.% ethanol with a comparable ethanol conversion

394

(71.28 %). It is envisaged that 50 wt.% ethanol could serve as an alluring feedstock for C2H4

395

production in lieu of high purity ethanol (≥ 99.5 wt.%). The influence of liquid hourly space

396

velocity (LHSV) was investigated for the dehydration of 50 wt.% ethanol over POC-SBA-

397

15(5) at 400 °C.

398

18

Word count: 6931 words 399

3.2.3 Effect of liquid hourly space velocity

400

In this study, the weight hourly space velocity (WHSV) was fixed as 30,000 mL/g·h

401

to provide a constant residence time for the ethanol reactants to react before leaving the

402

reactor. Since the contact time of ethanol reactants with POC-SBA-15(5) was essentially the

403

same, the liquid hourly space velocity (LHSV) could be varied to reveal the influence of

404

ethanol partial pressure on ethanol dehydration. Although LHSV is a well-studied process

405

parameter, it is often manipulated by other scholars to achieve a different purpose. For

406

instance, Chen et al. (2007) and Wu et al. (2013) studied the influence of residence time on

407

catalytic ethanol dehydration by varying LHSV without a constant WHSV. Lately, Soh et al.

408

(2017) discovered the impact of ethanol partial pressure on catalytic ethanol dehydration by

409

adjusting LHSV at a constant WHSV. Howbeit, none of the previous works address the effect

410

of ethanol partial pressure on catalytic dehydration of diluted ethanol.

411

The catalytic dehydration of 50 wt.% ethanol over POC-SBA-15(5) at 400 °C was

412

examined with different LHSV (4 – 20 mL/g·h) to scrutinise the impact of ethanol partial

413

pressure. For the above process, Fig. 7 furnishes the data of ethanol conversion and ethylene

414

selectivity at various LHSV. As the LHSV augmented from 4 – 16 mL/g·h, a sharp ascent of

415

ethanol conversion was observed with a gradual descent of ethylene selectivity. Under a

416

constant WHSV, a greater LHSV corresponds to a higher ethanol partial pressure, credited to

417

the declined N2 feeding rate. At low LHSV, low ethanol partial pressure retards the

418

adsorption of ethanol on acid sites of POC-SBA-15(5) due to a small concentration gradient,

419

results in poor ethanol conversion. However, low ethanol partial pressure enhances ethylene

420

selectivity with lesser formation of DEE and C3 – C5 by-products (refers Table A.3 under

421

supplementary data) by hindering the multiple adsorption of ethanol.

422

A higher LHSV facilitates the adsorption of ethanol on acid sites of POC-SBA-15(5)

423

by providing a greater ethanol partial pressure. If LHSV ≤ 16 mL/g·h, expedited adsorption-

19

Word count: 6931 words 424

dehydration-desorption enhances ethanol conversion while more pronounced multiple ethanol

425

adsorption deteriorates ethylene selectivity with higher availability of DEE and C3 – C5 by-

426

products. The rise of LHSV from 16 – 20 mL/g·h inflicts a surprise drop of ethanol

427

conversion from 73.56 % to 67.98 %. Since higher LHSV corresponds to greater ethanol

428

partial pressure, the downturn of ethanol conversion at 20 mL/g·h was likely related to the

429

saturation of active sites with ethanol and water molecules. With a finite amount of catalyst,

430

the limited number of active sites plausibly failed to accommodate enormous amounts of

431

ethanol and water molecules at such high LHSV. The accumulation of unreacted ethanol

432

stimulates DEE production (Chen et al., 2010), so the ethylene selectivity drastically

433

decreased from 86.59 % (16 mL/g·h) to 79.64 % (20 mL/g·h).

434

From the industrial standpoint, it is always desired to employ a high LHSV for a

435

greater production rate of C2H4. For catalytic dehydration of 50 wt.% ethanol at 400 °C, the

436

highest LHSV (20 mL/g·h) is inappropriate for POC-SBA-15(5), with appreciable reduction

437

of ethanol conversion and ethylene selectivity. Thus, the optimal LHSV is 16 mL/g·h that

438

renders greatest ethanol conversion (73.56 %) with satisfactorily high ethylene selectivity

439

(86.59 %).

440 441

3.3 Spent catalysts characterisation

442

Spent SBA-15 catalysts were selectively retrieved to elucidate their physicochemical

443

changes after ethanol dehydration via TEM, XRD, N2 physisorption, and TPO analysis. For a

444

fair comparison, all the characterised spent SBA-15 catalysts were sourced from dehydration

445

of 99.5 wt.% ethanol at 400 °C with an LHSV of 12 mL/g·h. Fig. 8 provides TEM images of

446

spent SBA-15 catalysts to observe any morphology changes after ethanol dehydration. All the

447

spent SBA-15 catalysts still retain the morphology of their respective fresh catalysts, judging

448

from their mesoporous structure with parallel channels. The clarity of mesoporous SBA-15

20

Word count: 6931 words 449

structures decreased in the order of POC-SBA-15(5) > POC-SBA-15(7) > POC-SBA-15(3) >

450

SBA-15(Comm.), probably imputed to increasing carbon deposition. For all the spent SBA-

451

15 catalysts, some discernible dark spots were spotted, alluded to deposited carbon from

452

ethanol dehydration via ethylene cracking. From the clarity of mesoporous structure and the

453

denseness of dark spots, it is proposed that the degree of carbon deposition increased in the

454

sequence of POC-SBA-15(5) < POC-SBA-15(7) < POC-SBA-15(3) < SBA-15(Comm.).

455

Wide-angle XRD patterns in Fig. 9(A) reveal the existence of a less intense broad

456

hump at 23° in spent SBA-15 catalysts as compared to fresh catalysts (refers Fig. 2(B)). The

457

discovery of broad hump at 23° symbolises the good preservation of the amorphous siliceous

458

framework in spent SBA-15 catalysts, authenticating the structural stability of SBA-15

459

catalysts at elevated temperature. The XRD patterns of spent SBA-15 catalysts are less

460

intense than fresh catalysts plausibly because of carbon deposition. The above postulation

461

was bolstered by the detection of graphite (JCDPS 26-1080) phase on spent SBA-15(Comm.)

462

and POC-SBA-15(3) from the additional shoulder peak at 26.5° (Dai et al., 2016). It is

463

cogitable that spent POC-SBA-15(5) and POC-SBA-15(7) suffered a milder carbon

464

deposition, on account of undetectable graphitic peak on their XRD patterns.

465

Through N2 physisorption, the surface textural properties of spent SBA-15 catalysts

466

were determined before summarised in Table 1 for the comparison with fresh catalysts. Spent

467

SBA-15 catalysts have poorer surface textural properties than their respective fresh catalysts

468

because deposited carbon ultimately provokes pore occlusion. The deterioration effect of pore

469

occlusion on the surface textural properties of SBA-15 catalysts increased in the order of

470

POC-SBA-15(5) < POC-SBA-15(7) < POC-SBA-15(3) < SBA-15(Comm.). The severity of

471

carbon deposition on spent SBA-15 catalysts could be ranked contrariwise. The TPO profiles

472

of spent SBA-15 catalysts are depicted in Fig. 9(B). The TPO profiles could be divided into

473

two distinct weight loss zones, which involves (i) desorption of physisorbed water and

21

Word count: 6931 words 474

ethanol (Masih et al., 2019) from 25 – 100 °C and (ii) coke oxidation (Siew et al., 2014) from

475

100 – 650 °C. For spent SBA-15 catalysts, a higher weight loss due to coke oxidation

476

corresponds to a greater extent of carbon deposition. The extent of carbon deposition

477

decreased with the order of SBA-15(Comm.) > POC-SBA-15(3) > POC-SBA-15(7) > POC-

478

SBA-15(5).

479

To conclude, all the spent catalysts characterisation unanimously reveal that the

480

degree of carbon deposition increased with the order of POC-SBA-15(5) < POC-SBA-15(7)

481

< POC-SBA-15(3) < SBA-15(Comm.). From Table 2, it is discovered that the strong acidity

482

of SBA-15(Comm.) > POC-SBA-15(3) > POC-SBA-15(7) > POC-SBA-15(5). This finding

483

eventually corroborates the role of strong acidity in engendering carbon deposition of SBA-

484

15 catalysts during ethanol dehydration. Among synthesised SBA-15 catalysts, the POC-

485

SBA-15(5) and SBA-15(Comm.) are the catalysts that least and most susceptible to carbon

486

deposition during ethanol dehydration, respectively.

487 488

3.4 Stability and regeneration studies

489

The stability and regeneration studies were conducted for catalytic dehydration of 50

490

wt.% ethanol over POC-SBA-15(5) at 400 °C with an LHSV of 16 mL/g·h, viz. the optimal

491

reaction for C2H4 production in this work. Figs. 10(A) – (B) depict the transient profiles of

492

ethanol conversion, ethylene selectivity, and by-products (DEE and others) selectivity for the

493

stability study. From the transient profiles, it is surmised that the reaction attains steady state

494

within 0.5 h. Fresh POC-SBA-15(5) catalyst exhibits excellent stability up to 105 h with non-

495

fluctuated catalytic performance (ethanol conversion ≈ 73.33 % and ethylene selectivity ≈

496

84.70 %). This finding conveys that the optimal process grants a daily C2H4 production rate

497

of about 2.43 mol/g·d. After 105 h, the catalytic activity of fresh POC-SBA-15(5) catalyst

498

slightly declined because of coking deactivation. Evidently, the selectivity of by-products

22

Word count: 6931 words 499

(DEE, H2, CH4, CO2, CO, C3H8, C3H6, C4H10, C4H8, and C5H12) progressively increased with

500

time-on-stream at the expense of ethanol conversion and ethylene selectivity. As time goes

501

by, coking deactivation of POC-SBA-15(5) gradually manifested via C2H4 cracking, so its

502

catalytic specificity towards intramolecular ethanol dehydration deteriorated with time.

503

Consequently, coking deactivation triggers more perceptible by-products formation via side

504

reactions like intermolecular ethanol dehydration, thermal decomposition of ethanol, water

505

gas shift, and C2H4 oligomerisation.

506

Fig. 10(C) presents the transient profiles of ethanol conversion and ethylene

507

selectivity for the regeneration study. From Fig. 9(B), TPO profile of spent POC-SBA-15(5)

508

remains flat beyond 650 °C; therefore, the regeneration temperature (700 °C) sufficed for its

509

carbon removal via coke oxidation. As compared to fresh POC-SBA-15(5), regenerated

510

POC-SBA-15(5) catalyst demonstrated comparable stability up to 115 h with poorer catalytic

511

performance (ethanol conversion ≈ 71.95 % and ethylene selectivity ≈ 81.96 %). The delay of

512

conspicuous coking deactivation from 105 h (stability study) to 115 h (regeneration study)

513

could be explained by lower likelihood of C2H4 cracking in response to poorer catalytic

514

activity.

515

Wu and Wu (2017) claimed that the dehydration of 60 wt.% ethanol over P/ZSM-5

516

and La/ZSM-5 (Si/Al ratio = 280) achieved > 99 % ethanol conversion and ethylene

517

selectivity up to 100 h. As researched by Masih et al. (2019), the dehydration of 99.8 wt.%

518

ethanol over Rho zeolite (Si/Al ratio = 3.5) at 350 °C granted 100 % ethanol conversion and

519

99 % ethylene selectivity throughout 65 h. Ouyang et al. (2009) reported the dehydration of

520

50 wt.% ethanol over HZSM-5 (Si/Al ratio = 100) at 260 °C attained > 98 % ethanol

521

conversion and ethylene selectivity up to 400 h. After La impregnation, Ouyang et al. (2009)

522

assured that La/HZSM-5 could accomplish the identical catalytic performance up to 950 h

523

(stability study) and 830 h (regeneration study). For dehydration of ethanol to ethylene, these

23

Word count: 6931 words 524

previously reported zeolite catalysts outperformed the best SBA-15 catalyst (POC-SBA-15(5)

525

in this work) by having a higher catalytic activity at a lower operating temperature. However,

526

the POC-SBA-15(5) could steadily catalyse the dehydration of 50 wt.% ethanol with lower

527

catalytic activity up to 105 h. By valorising POC into SBA-15, this work offers a potential

528

scheme for reutilization of silica-rich agroindustry waste as catalyst.

529 530

4. Conclusion

531

Palm oil clinker (POC) waste was calcined, acid-leached with phosphoric acid, and

532

refluxed with sodium hydroxide to prepare sodium silicate (Na2SiO3). Commercial Na2SiO3

533

and POC-Na2SiO3 were used to prepare mesoporous SBA-15 catalysts, viz. SBA-15(Comm.)

534

and POC-SBA-15(pH = 3, 5, 7). With SBA-15(Comm.) as a reference, the successful

535

synthesis of SBA-15 from POC was validated by characterisation like XRD, FTIR, TEM, and

536

FESEM. Characterisation (inclusive of N2 physisorption) reveals that POC-SBA-15 catalysts

537

have relatively poorer siliceous framework than SBA-15(Comm.) owing to impure POC-

538

Na2SiO3 precursor. NH3-TPD discovers the high strong acidity of SBA-15(Comm.) and the

539

enriched weak-moderate acidity of POC-SBA-15 catalysts. For dehydration of ethanol to

540

ethylene, the catalytic performance was measured as ethanol conversion (

541

ethylene selectivity (

542

reaction temperature from 200 – 400 °C. Despite a similar trend for SBA-15(Comm.),

543

remarkable cracking and oligomerisation of ethylene at 400°C deteriorate its

). The

and

) and

of POC-SBA-15 catalysts increase with

and

544

. The best catalyst for ethanol dehydration was POC-SBA-15(5) with highest weak

545

acidity and lowest strong acidity. As 99.5 wt.% ethanol diluted to 50 wt.%, the competitive

546

adsorption between ethanol and water lowers

but enhances

547

15(5). Further ethanol dilution (≤ 30 wt.%) reduces

at elevated temperature (≥ 350 °C)

548

due to undesirable ethanol steam reforming. Dehydration of 50 wt.% ethanol over POC-SBA24

of POC-SBA-

Word count: 6931 words 549 550

15(5) at 400 °C grants higher

than that of 99.5 wt.% ethanol without compromising

. For the aforesaid process, a higher LHSV at constant WHSV increases

but

551

decreases

by offering a greater ethanol partial pressure. If LHSV > 16 mL/g·h, the

552

saturation of finite active sites with adsorbates appreciably reduces

553

ethanol dehydration over POC-SBA-15(5), optimal conditions are temperature = 400 °C,

554

initial ethanol concentration = 50 wt.%, and LHSV = 16 mL/g·h. Fresh POC-SBA-15(5)

555

steadily catalyses the optimal process (73.33 %

556

Regenerated POC-SBA-15(5) gives a slightly poorer catalytic performance (71.95 %

557

and 81.96 %

558

ethanol dehydration could inspire more research and development on sustainable catalysts

559

from agroindustry wastes.

and 84.70 %

and

. For

) up to 105 h.

). The successful valorisation of POC waste into SBA-15 catalysts for

560 561

Acknowledgement

562

This work was financially supported by the Ministry of Higher Education (MOHE) Malaysia

563

with a grant number of RDU170116. Chi Cheng Chong would like to express her gratitude to

564

Universiti Malaysia Pahang (UMP) for the Doctoral Research Scheme.

565 566

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567 568 569 570 571 572 573 574 575 576 577 578

Abdullah, N., Chong, C.C., Razak, H.A., Ainirazali, N., Chin, S.Y., Setiabudi, H.D., 2018. Synthesis of Ni/SBA-15 for CO2 reforming of CH4: Utilization of palm oil fuel ash as silica source. Mater. Today-Proc. 5(10, Part 2), 21594-21603. Altwair, N.M., Megat Johari, M.A., Saiyid Hashim, S.F., 2012. Flexural performance of green engineered cementitious composites containing high volume of palm oil fuel ash. Constr. Build. Mater. 37, 518-525. Alvarez, J., Lopez, G., Amutio, M., Bilbao, J., Olazar, M., 2014. Upgrading the rice husk char obtained by flash pyrolysis for the production of amorphous silica and high quality activated carbon. Bioresource Technol. 170, 132-137. Bhagiyalakshmi, M., Ji Yun, L., Anuradha, R., Jang, H., 2009. Utilization of rice husk ash as silica source for the synthesis of mesoporous silicas and their application to CO2 adsorption through TREN/TEPA grafting. J. Hazard. Mater. 175(1-3), 928-938.

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691 692

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Table Captions: Table 1: Surface textural properties of fresh and spent SBA-15 catalysts Table 2: Surface Lewis acidity of fresh SBA-15 catalysts

Table 1: Surface textural properties of fresh and spent SBA-15 catalysts Surface textural properties Specific surface area Pore volume Pore size (m2/g)a (cm3/g)b (nm)b

Catalysts Fresh catalysts SBA-15(Comm.) POC-SBA-15(3) POC-SBA-15(5) POC-SBA-15(7)

642 486 508 537

0.83 0.59 0.63 0.71

8.02 6.47 7.31 7.84

Spent catalystsc SBA-15(Comm.) 483 0.66 7.21 POC-SBA-15(3) 451 0.48 5.98 POC-SBA-15(5) 493 0.58 7.18 POC-SBA-15(7) 519 0.63 7.53 a By Brunauer–Emmett–Teller (BET) model. b By Barret-Joyner-Halenda (BJH) model. c Retrieved from dehydration of 99.5 wt.% ethanol at 400 °C with a liquid hourly space velocity (LHSV) of 12 mL/g·h.

Table 2: Surface Lewis acidity of fresh SBA-15 catalysts

Catalysts SBA-15(Comm.) POC-SBA-15(3) POC-SBA-15(5) POC-SBA-15(7)

Weak sites (100 – 250 °C) 121.04 232.44 824.41 444.16

Surface Lewis acidity (µmol/g) Moderate sites Strong sites (250 – 400 °C) (> 400 °C) 328.56 623.14 651.81 437.24 310.19 169.45 525.53 302.86

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Total 1072.71 1321.49 1304.05 1272.55

Figure Captions: Fig. 1: Experimental setup of the catalytic ethanol dehydration over SBA-15 catalysts Fig. 2: Characterisation of fresh SBA-15 catalysts – (A) low-angle XRD patterns, (B) wideangle XRD patterns, (C) nitrogen physisorption isotherms, (D) pore size distributions, and (E) FTIR spectra Fig. 3: TEM (with label 1) and FESEM (with label 2) images of fresh SBA-15 catalysts – (A) SBA-15(Comm.), (B) POC-SBA-15(3), (C) POC-SBA-15(5), and (D) POC-SBA-15(7) Fig. 4: NH3-TPD profiles of fresh SBA-15 catalysts Fig. 5: Effect of reaction temperature on (A) ethanol conversion and (B) ethylene selectivity over different SBA-15 catalysts (initial ethanol concentration = 99.5 wt.% and LHSV = 12 mL/g·h) Fig. 6: Effect of initial ethanol concentration on (A) ethanol conversion and (B) ethylene selectivity over POC-SBA-15(5) at different reaction temperature (LHSV = 12 mL/g·h) Fig. 7: Effect of LHSV on ethanol conversion and ethylene selectivity over POC-SBA-15(5) (reaction temperature = 400 °C and initial ethanol concentration = 50 wt.%) Fig. 8: TEM images of spent SBA-15 catalysts (reaction temperature = 400 °C, initial ethanol concentration = 99.5 wt.%, and LHSV = 12 mL/g·h) – (A) SBA-15(Comm.), (B) POC-SBA15(3), (C) POC-SBA-15(5), and (D) POC-SBA-15(7) Fig. 9: (A) Wide-angle XRD patterns and (B) TPO profiles of spent SBA-15 catalysts (reaction temperature = 400 °C, initial ethanol concentration = 99.5 wt.%, and LHSV = 12 mL/g·h) Fig. 10: Transient profiles of ethanol dehydration over POC-SBA-15(5) at optimal conditions (reaction temperature = 400 °C, initial ethanol concentration = 50 wt.%, and LHSV = 16 mL/g·h) – (A) ethanol conversion and ethylene selectivity for stability study, (B) by-products (DEE and others) selectivity for stability study, and (C) ethanol conversion and ethylene selectivity for regeneration study

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Fig. 1: Experimental setup of the catalytic ethanol dehydration over SBA-15 catalysts

29

Fig. 2: Characterisation of fresh SBA-15 catalysts – (A) low-angle XRD patterns, (B) wideangle XRD patterns, (C) nitrogen physisorption isotherms, (D) pore size distributions, and (E) FTIR spectra

30

Fig. 3: TEM (with label 1) and FESEM (with label 2) images of fresh SBA-15 catalysts – (A) SBA-15(Comm.), (B) POC-SBA-15(3), (C) POC-SBA-15(5), and (D) POC-SBA-15(7)

31

Fig. 4: NH3-TPD profiles of fresh SBA-15 catalysts

32

Fig. 5: Effect of reaction temperature on (A) ethanol conversion and (B) ethylene selectivity over different SBA-15 catalysts (initial ethanol concentration = 99.5 wt.% and LHSV = 12 mL/g·h)

33

Fig. 6: Effect of initial ethanol concentration on (A) ethanol conversion and (B) ethylene selectivity over POC-SBA-15(5) at different reaction temperature (LHSV = 12 mL/g·h)

34

Fig. 7: Effect of LHSV on ethanol conversion and ethylene selectivity over POC-SBA-15(5) (reaction temperature = 400 °C and initial ethanol concentration = 50 wt.%)

Fig. 8: TEM images of spent SBA-15 catalysts (reaction temperature = 400 °C, initial ethanol concentration = 99.5 wt.%, and LHSV = 12 mL/g·h) – (A) SBA-15(Comm.), (B) POC-SBA15(3), (C) POC-SBA-15(5), and (D) POC-SBA-15(7)

35

Fig. 9: (A) Wide-angle XRD patterns and (B) TPO profiles of spent SBA-15 catalysts (reaction temperature = 400 °C, initial ethanol concentration = 99.5 wt.%, and LHSV = 12 mL/g·h)

36

Fig. 10: Transient profiles of ethanol dehydration over POC-SBA-15(5) at optimal conditions (reaction temperature = 400 °C, initial ethanol concentration = 50 wt.%, and LHSV = 16 mL/g·h) – (A) ethanol conversion and ethylene selectivity for stability study, (B) by-products (DEE and others) selectivity for stability study, and (C) ethanol conversion and ethylene selectivity for regeneration study

37

RESEARCH HIGHLIGHTS • • • • •

Palm oil clinker derived SBA-15 (POC-SBA-15) catalysts were synthesized. Surface acidity of POC-SBA-15 was modulated by varying preparation pH (3, 5, or 7). POC-SBA-15 catalysts has enriched weak-moderate acidity with lower strong acidity. POC-SBA-15(5) with highest weak acidity best fitted for ethanol dehydration. 50 wt.% ethanol dehydration at 400 °C renders 73.33 % and 84.70 % .

Date: 8 October 2019 Prof. Dr. Sharifah Rafidah Wan Alwi Associate Editor Journal of Cleaner Production Dear Editor, Declaration of Interest Statement - Revised Original Article (JCLEPRO-D-19-09182R1) We declare that this manuscript is original, has not been published before and is not currently being considered for publication elsewhere. We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome. We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us. We understand that the Corresponding Author (Dr. Chi Cheng Chong) is the sole contact for the Editorial process (including Editorial Manager and direct communications with the office). She is responsible for communicating with the other authors about progress, submissions of revisions and final approval of proofs. Yours sincerely, Authors 1) Dr. Yoke Wang Cheng, [email protected] - Faculty of Chemical and Process Engineering Technology, Universiti Malaysia Pahang. 2) Dr. Chi Cheng Chong, [email protected] - Faculty of Chemical and Process Engineering Technology, Universiti Malaysia Pahang. 3) Assoc. Prof. Dr. Chin Kui Cheng, [email protected] - Faculty of Chemical and Process Engineering Technology, Universiti Malaysia Pahang. 4) Dr. Kim Hoong Ng, [email protected] - School of Energy and Chemical Engineering, Xiamen University Malaysia. 5) Assoc. Prof. Thongthai Witoon, [email protected] - Faculty of Engineering, Kasetsart University. 6) Assoc. Prof. Joon Ching Juan, [email protected] - Nanotechnology & Catalysis Research Centre, University Malaya.