Molybdenum hybrid – Nanocrystals supported on modified Laponite composite as superior catalyst for vapour phase hydrodeoxygenation of clove oil

Journal Pre-proof Molybdenum hybrid – Nanocrystals supported on modified Laponite composite as superior catalyst for vapour phase hydrodeoxygenation of clove oil P. Santhana Krishnan, P. Tamizhdurai, G. Sonia Theres, K. Shanthi PII:

S0960-1481(19)31543-5

DOI:

https://doi.org/10.1016/j.renene.2019.10.052

Reference:

RENE 12421

To appear in:

Renewable Energy

Received Date: 24 January 2019 Revised Date:

2 August 2019

Accepted Date: 10 October 2019

Please cite this article as: Krishnan PS, Tamizhdurai P, Theres GS, Shanthi K, Molybdenum hybrid – Nanocrystals supported on modified Laponite composite as superior catalyst for vapour phase hydrodeoxygenation of clove oil, Renewable Energy (2019), doi: https://doi.org/10.1016/ j.renene.2019.10.052. 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.

Molybdenum hybrid – nanocrystals supported on modified Laponite composite as superior catalyst for vapour phase hydrodeoxygenation of clove oil P. Santhana Krishnan, P. Tamizhdurai, G. Sonia Theres and K. Shanthi* Department of Chemistry, Anna University, Chennai 600 025, Tamil Nadu, India

*Author for correspondence: Dr. K. Shanthi Professor of Chemistry INDIA Phone: +91-44- 22358654 E-mail: [email protected] [email protected]

Molybdenum hybrid – nanocrystals supported on modified Laponite as superior catalyst for vapour phase hydrodeoxygenation of clove oil

Graphical abstract

1

Molybdenum hybrid – nanocrystals supported on modified Laponite composite as

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superior catalyst for vapour phase hydrodeoxygenation of clove oil

3

P. Santhana Krishnan, P. Tamizhdurai, G. Sonia Theres and K. Shanthi*

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Department of Chemistry, Anna University, Chennai 600 025, Tamil Nadu, India

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*Author for correspondence:

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Dr. K. Shanthi

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Professor of Chemistry

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INDIA

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Phone: +91-44- 22358654

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E-mail: [email protected]

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[email protected]

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Abstract

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Valorisation of bio-oil to sustainable energy through hydrodeoxygenation process has

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been attracted much attention in the development of bio-refineries. The present work

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emphasizes on the development of a highly active heterogeneous catalyst for

32

hydrodeoxygenation reaction with modified Laponite-γ-alumina and SBA-15 – γ-alumina

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composites as support for Mo and NiMo catalysts using Mo–inorganic–organic hybrid

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nanocrystals (HNCs) prepared under hydrothermal condition. The synthesized catalysts were

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characterised using N2–physisorption, XRD, FT–IR, Raman, HRTEM, TPD/TPR, H2 pulse

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chemisorption and XPS techniques. The catalysts were evaluated for HDO of eugenol

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(Lignin model compound, Clove oil) at 400 ˚C under atmospheric pressure. The

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physicochemical characterization of the support composites revealed that Mo-HNC supported

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on modified Laponite exhibited extraordinary stability, metal- support interaction and metal

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dispersion over the support. The catalytic activity results revealed that the Laponite

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composite supported NiMo-HNC catalyst performed complete conversion of eugenol with

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enhanced % selectivity to 3- and 4-propyl phenol (mono-deoxygenated products) and to

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benzene and 4-propyl cyclohexene (complete deoxygenated products). Thereby, a new Mo-

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HNC supported modified Laponite composite catalyst has been successfully developed. The

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role of the HNC-Mo preparation method, textural properties, morphology of Laponite

46

composite catalysts have been demonstrated for efficient HDO of clove oil.

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Keywords: Laponite composite, HNC-Mo, Eugenol, HDO, Synergetic factor

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2

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1. Introduction

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Lignocellulosic biomass has been considered as a sustainable alternative replacement

58

for the depleting oil resources. Lignin is a valuable, aromatic– rich component of biomass

59

and hence an excellent renewable source for the production of chemicals and fuel additives

60

[1]. Coumaryl, coniferyl and sinapyl alcohol monomers are the building blocks of lignin,

61

which are connected through a different alkyl, aryl and ether linkages [2]. Lignin is found

62

mainly in the waste streams of paper and pulp industries, where 50 million tonnes of crude

63

lignin is produced every year [3]. Sludge of nitrification unit has been demonstrated as a

64

green source for the generation of biomass-associated products [4]. The thermo chemical

65

conversion of lignin produces condensed heavy products due to its massive structure. Also,

66

the ease of use as a solid fuel is the main barrier to the development of lignin– based Bio

67

refineries. Fast pyrolysis reaction of lignin produces bio–oil which contains a mixture of

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oxygenated compounds. Eugenol is a common product of lignin pyrolysis, which is a

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prototypical aromatic compound of coniferyl alcohol. This is industrially produced by the

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steam distillation of Zanzibar cloves that contains about 85–90% eugenol. The extraction

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process is simple and economically feasible [5, 6]. In addition to that, clove stem oil (CSO) is

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generally non-edible renewable oil; CSO can be produced profusely in certain countries of

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East Africa and South Asia. Recent results revealed that 50% CSO can be blended with diesel

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for specific energy consumption savings [7]. Hydrodeoxygenation has been proven to be one

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of the effective methods in bio–oil processing to remove oxygenated groups for the

76

production of liquid transportation fuels.

77

Development of potentially active catalyst for hydrodeoxygenation (HDO) is

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important and essential for bio-oil up gradation to avoid the loss of substantial amount of

79

gases and char from it. Also achieving a high dispersion of active metal species over the

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support is challenging. To attain this, various methods of preparation such as incorporation,

81

impregnation, co-impregnation, reverse order impregnation and post grafting have been

82

adopted. The conventional impregnation of Mo precursor on the alumina support leads to the

83

formation of Anderson-type heteropolyanions Al(OH)6Mo6O183− [8]. On calcination, this

84

heteropolyanions results in the formation of bulk MoO3 and Al2(MoO4)3 species which in

85

turn result in poor dispersion and unfavourable morphology of the supported Mo catalyst [9].

86

This method of Mo catalyst preparation over γ-Al2O3 leads to weak promoting effect and low

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catalytic activity to the resulting bimetallic catalyst. Most of the times, the metal species

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migrate out of the surface of the support due to instability even though the metals are well 3

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dispersed on the support. In order to improve the extent of dispersion and stability of metal, it

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is ultimately essential to obtain the metal in its nanoparticle dimension. The functional

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organic molecular intercalation route has been adopted for the synthesis of mono-dispersed

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hybrid nano composites (HNCs) of molybdenum by self-assembly method in the mesoscale

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and macroscale range [10]. This method can result in redistribution of small particles, on the

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support thereby the catalytic activity and stability can be enhanced in addition to a high metal

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loading on the support.

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The choice of support for hydrodeoxygenation reactions has been changed from γ–

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Al2O3 to unitary and binary supports such as carbon, SiO2, TiO2, TiO2–Al2O3, SiO2–Al2O3

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and V2O5–Al2O3, due to poor thermal stability and severe carbon deposition [11]. Hence, for

99

efficient hydrotreating activity, the resulting catalyst needs suitable modification of support

100

and dispersion of molybdenum oxide over the support. The nature of support plays an

101

important role in impacting catalytic performance. Recently, mesoporous silica has been

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widely used as a catalyst support because of its very high specific surface area with narrow

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pore size distribution and large pore volume, high thermal stability and tunable acidity when

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compared to zeolites and commercial γ–Al2O3 [12, 13]. In particular, SBA–15 is of great

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interest because of its high ordered porous structure, high wall thickness, high thermal

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stability, low cost and non–toxic characteristics [14]. The addition of basic support such as

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MgO to acidic/neutral support has been proved to increase the production of liquid product

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by suppressing the gaseous product formation [15]. However, MgO as catalyst support for

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MoO3 has been least favoured because of its inconsistency in the material characterization

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studies [16, 17]. The addition of beta zeolite materials to the alumina increased the HDN

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activity of the resulting Ni- Mo catalyst [18]. Huang et al. reported that thermal stability of

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MCM-41/Al2O3 composite support was found to be a beneficial in catalytic applications [19].

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Further, it has been reported that addition of B2O3, ZrO2 and P2O5 to Al2O3 increased the

114

acidity of the supported catalyst towards hydrodenitrogenation process [20]. In the present

115

investigation, in order to create acid sites in the catalytic system, commercial γ–Al2O3 has

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been impregnated to the Laponite and SBA–15 supports to make the composites.

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As an alternative, Laponite clay material has been used as catalyst support due to its

118

great abundance, low cost with their particular properties as well as high thermal stability.

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Laponite RD clay, a versatile synthetic layered silicate has been applied in various fields such

120

as surface coatings, household products, polymer films and building products. By

4

121

intercalating the parallel silicate layers of the swellable clay with particles of various metal

122

oxides, large porosity has been created [21, 22].

123

Additionally, we have adopted hydrothermal method to synthesize modified Laponite

124

with high surface area by delaminating the Laponite sheets using a co–surfactant. In order to

125

create acid sites; tune the surface area, pore size and pore volume in the supported catalytic

126

system, commercial γ–Al2O3 has been introduced to the Laponite and SBA–15 framework to

127

make the composites. To the best of our knowledge, for the first time Laponite derived

128

composite as support in the Hydrodeoxygenation reaction has been reported. NiMo based

129

hybrid nanocrystals have been successfully synthesised and uniformly deposited over

130

Laponite:γ–Al2O3 and SBA–15:γ–Al2O3 support composites under hydrothermal conditions.

131

The structural features of uniformly dispersed MoO3 species over the supports and the

132

composite support effect on the catalytic activities of HDO of eugenol has been investigated

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by comparing the activity of the composite support catalyst with the catalyst supported on

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Al2O3. The outcome of these results focuses on the comparison of hydrothermal aided

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synthesis of SBA–15 with Laponite material, and both modified with γ–Al2O3 helps to

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generate fundamental information related to an alternative use of this material as a support for

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SBA–15 and γ–Al2O3 in the development of support for biofuel production.

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2. Experimental

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2.1 Materials

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Pluronic P123 (EO20PPO70EO20) (Aldrich), tetraethyl orthosilicate (TEOS) (Merck,

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98%), sodium molybdate dihydrate (Merck, 99%), nickel chloride hexahydrate (Ranbaxy,

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98%), Laponite RD (Rockwood additives), Eugenol (Aldrich, 99%), Dodecyl trimethyl

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ammonium bromide (Alfa Aesar, 99%), hydrochloric acid (Merck, 36%), Cetyl trimethyl

144

ammonium bromide (CTAB) (99%), dodecylamine (DDA) (Alfa Aesar, 98%) and γ–Al2O3

145

(Alfa Aesar, 99.97 %) were purchased and used for the experimental study. All the chemicals

146

were used without any further purification.

147 148

2.1 Catalyst synthesis

149

2.2.1 Synthesis of support materials (modified Laponite, SBA–15) and support

150

composites (SBA–15 with γ–Al2O3, Laponite with γ–Al2O3)

5

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Siliceous SBA–15 with high surface area was synthesised by the method mentioned

152

elsewhere [23]. In a typical synthesis, 20 g of P123 as a structure directing agent was added

153

to 400 ml of 1M HCl. The solution was heated at 50 ˚C until complete dissolution and stirred

154

overnight at 30 ˚C to obtain a homogeneous mixture. The temperature was then increased to

155

40 ˚C followed by the dropwise addition of 40 g TEOS under stirring to obtain a white

156

precipitate. The mixture was then maintained at 40 ˚C for 24 h and then transferred to

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Teflon–lined autoclave and treated at 100 ˚C for 72 h, followed by filtration and washing

158

with distilled water and dried at 90 ˚C overnight. The dried material was calcined in the

159

tubular furnace at 500 ˚C for 6 h in the air flow.

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Modified Laponite was synthesised by exchanging the interlayer sodium cations by

161

organic cetyltrimethylammonium cations (CTA+) [24]. An appropriate amount of aqueous

162

solution of CTA+ was added to 4g of an aqueous suspension of Laponite and stirred for 16 h

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at 60 ˚C. The excess alkylammonium salt was removed by repetitive washing with hot

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distilled water until complete absence of bromide anions was confirmed by the AgNO3 test.

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In the next step neutral amine co–surfactant (dodecylamine – DDA) and TEOS were added to

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1 g of CTA–Laponite in the molar ratio corresponding to organoclay/amine/TEOS = 1/10/75

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[24]. This ratio lead to the delamination of Laponite sheet that resulted in high surface area,

168

pore volume and pore size. After 4 h of interaction at room temperature, the solid component

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was recovered by filtration and air–dried overnight, followed by calcination at 650 ˚C for 10

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h.

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SBA–15: γ–Al2O3 and Laponite: γ–Al2O3composites with a wt.% ratio of 3:1 were

172

prepared by incipient wetness impregnation method. In this method, 3 g of SBA–15 and

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Laponite were taken separately in a china dish. Then, 1 g of γ–Al2O3 powder was mixed with

174

an appropriate amount of 2% acetic acid as a peptizing agent and mixed well with SBA–15

175

and Laponite materials separately using a glass rod. The composites were then dried at 90 ˚C

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overnight. The SBA–15: γ–Al2O3composite was calcined at 550 ˚C for 5h whereas Laponite:

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γ–Al2O3composite was calcined at 650 ˚C for 10 h. The resulting materials were denoted as

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SiAl for SBA–15:γ–Al2O3 and LapAl for modified Laponite:γ–Al2O3.

179 180

2.2.2 Synthesis of Mo–HNC

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In a typical synthesis, 2.0 mL of 2.40 M HCl was added to 18 ml of 0.15 M aqueous

182

solution of sodium molybdate to yield a translucent solution containing polyoxymolybdate

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anions (POMs). The pH was analysed to be 4.5. Then 10 ml of 0.15 M DTAB aqueous 6

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solution was added to the above solution under vigorous stirring. The obtained mixture was

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acidified to pH = 3 by dropwise addition of 2.40 M HCl solution with continuous stirring for

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2 h to obtain a Mo– HNC suspension [25].

187 188

2.2.3 Synthesis of Ni promoted Mo–HNC supported on γ–Al2O3, SiAl and LapAl

189

materials

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The as–synthesized Mo–HNC suspension was stirred with 4g of the support material

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(γ–Al2O3/SiAl & /LapAl) at room temperature for 2 h before transferring to the

192

polypropylene bottle and subjected to hydrothermal treatment at 120 ˚C for 12 h. The

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resulting composite mixture was filtered, washed with distilled water and dried at 120 ˚C for

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3 h, calcined at 500 ˚C for 4 h in theN2flow. The materials were named as Mo–HNC/Al, Mo–

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HNC/SiAl and Mo–HNC/LapAl for the Mo–HNC supported over Al, SiAl and LapAl

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materials respectively.

197

The hydrothermal method of synthesis offers uniform dispersion of the Mo–HNC

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over the supports. As seen in Figure S1, Mo–HNC/Al is white in colour which is similar to

199

the alumina support alone. This is due to the deposition of Mo–HNC nanoparticles on the

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inner pore of γ–Al2O3 support due to high pore size (∼10 nm) of the support. In comparison

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with SiAl and LapAl support composites, the nanoparticles were deposited predominantly on

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the outer surface of the support that can be demonstrated by the increase in the pale yellow

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colour intensity for the composite supported catalyst. This increase in colour intensity

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indicates the core–shell (DTA)4Mo8O26 phase formation over the surface of the support

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composite.

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The Ni as a promoter is introduced to the supported Mo–HNC catalyst by incipient

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wet impregnation method. An appropriate amount of aqueous solution of nickel nitrate

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hexahydrate precursor was impregnated over 1g of respective Mo–HNC catalyst. The

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material was dried and calcined at 500 ˚C for 4 h in the N2 atmosphere. The respective

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catalysts were designated as NiMo–HNC/Al, NiMo–HNC/SiAl and NiMo–HNC/LapAl.

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2.3 Characterization

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BRUCKER D8 diffractometer was used to record the obtained diffraction patterns of

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the supports and catalysts at low (0.5˚–5˚) and high angle (10˚–80˚) at a step scan rate of 0.02

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seconds using Cu K radiation (λ=1.548 Å). The QUADRASORB SI automatic analyser was

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used to measure N2 adsorption/desorption isotherms, pore volume and pore size at liquid N2 7

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temperature. Prior to the analysis, 0.05g of the sample was degassed for 4 h at 300˚C under

217

N2 purging. Specific surface areas were calculated by the BET method, the pore volume (Vp)

218

was determined by nitrogen adsorption at a relative pressure of 0.98 and pore size

219

distributions from the desorption branch of isotherms by the BJH method. The temperature

220

programmed reduction and desorption studies were carried out with QUADRASORB

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ChemBET TPD/TPR using the gaseous mixtures of 5%H2/95%Ar and 10%NH3/90%He,

222

respectively. Prior to the analysis in both TPD/TPR, 0.05g of the sample was degassed under

223

N2atmosphere for 2 h at 200 ˚C and 3 h at 450˚C under He atmosphere to clean the surface of

224

the material. TPD/TPR was performed at a heating rate of 15 ˚C/min with a flow rate of 80

225

cm3/min. The quantity of NH3 and H2 consumed was determined by TCD detector. H2 pulse

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titration technique was carried out to measure H2 consumption, metal surface area and %

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dispersion, the crystal size of Mo using TPR with the help of TPRWin software. For this

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analysis, 0.05g of the sample was loaded in the quartz U–shaped tube and degassed first at

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200 ˚C under N2 purging for 2 h. The degassed sample was then reduced in the flow of H2/Ar

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gas mixture at the rate of 15 ˚C per minute to 450˚C and maintained for 2 h. After reduction,

231

H2 pulse injection was performed in helium gas atmosphere (in an automatic mode of 16

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pulse of pure H2, 50 µl per pulse). The chemical environment of HNC–Mo and Ni over the

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composite catalysts was analysed by XPS spectra acquired using M/s. Omicron

234

Nanotechnology (GmBH, Germany) with XM1000 monochromatic AlKα source (hν =

235

1486.6 eV) operated at 300 W (20 mA and 15 kV) and a hemispherical electron energy

236

analyser. Spectral fitting of Si, Al, O, C and N species were adopted using CasaXPS

237

software, the number of Si, Al, O, C and N components were observed in association with Ni

238

and HNC–Mo. The effect of support, Ni and HNC–Mo species interaction has been well

239

discussed. The chemical environment of Mo oxide in the supported catalyst was analysed by

240

UV–Vis electronic spectra. The samples were recorded in the wavelength range 200–800 nm

241

using a Shimadzu UV–2450 spectrophotometer equipped with a diffuse reflectance

242

attachment. For this study, BaSO4 was used as a reference. The high–resolution transmission

243

electron microscopy (HRTEM) images were recorded using TECNAI–G2 (model T–30) S–

244

twin HRTEM with a field emission gun operating at 300 kV. The high–resolution scanning

245

electron microscopy (HRSEM) studies were performed using a Quanta 200 FEG microscope

246

with magnification, from minimum of 12x to greater than 1,00,000 X and resolution. The

247

solids were ultrasonically dispersed in ethanol, and the suspension was collected on carbon–

248

coated grids. FTIR studies were carried out using a PerkinElmer FTIR spectrophotometer.

8

249

The solid samples were pelletized using KBr technique, the pellet was scanned at 4

250

cm−1resolution in the range of 4000–400 cm−1 and FT–Raman analysis was carried out using

251

a multi RAM, BRUKER RFS 27: Stand–alone model. The spectral range is 4000 – 50 cm–1

252

with a scanning rate of 2 cm–1. The laser source is Nd: YAG 1064 nm. % carbon formation

253

over the spent catalyst was analysed using thermogravimetric analyser (Shimadzu–50) in

254

pure oxygen with a flow rate of 20 ml/min. up to 800 °C with a heating rate of 10 °C/min.

255

2.4 Catalytic tests

256

The catalytic activity of composite supported HNC-Mo and NiHNC-Mo are evaluated

257

for the HDO of eugenol. The reaction parameters such as effect of temperature, reactant feed

258

rate and time on stream were optimized for high conversion and product selectivity. HDO of

259

eugenol was carried out for 6-8 h at different temperature (573 K -673 K) in a stream of 50

260

cm3/min ultrapure hydrogen using fixed bed reactor operating at atmospheric pressure of H2.

261

3.5wt %.of eugenol was dissolved in decalin and used for the reaction. The catalyst was

262

activated prior to performing reaction by passing 10% H2/He at 400 ˚C for 3 h. The liquid

263

product formed during the course of reaction was collected every hour and analyzed with a

264

GC–17A Shimadzu gas chromatograph using a RTX–5 column and a flame ionization

265

detector. The distribution of products was analysed with a JEOL GCMATE II GC–MS. The first order rate constant can be calculated by the following expression

266 267

= −

268



( − τ)

(1)

269 270

The specific reaction rate can be expressed by the following expression (Equation 2)

271

=

272

= −

( − τ)

(2)

273 274

Where, F is total molar flow of reactant (mol s–1), τ is total conversion, C is the initial

275

concentration of reactant (mol L−1) and W is the weight of the catalyst . The intrinsic reaction rate ri can be expressed by the following expression (Equation

276 277

3).

278

9

= ∗

279

(3)

280 281

Where, n is the number of Mo atoms per gram of the catalyst and N is Avogadro number.

282

3. Results & Discussions

283

3.1 Catalyst characterisation

284

3.1.1

Low and High Angle X-ray Diffraction (XRD)

285

The low angle XRD patterns of as–synthesized SBA–15 and modified Laponite

286

support composites along with the supported catalyst materials are shown in Figure 1 (A).

287

SBA–15 exhibits three well–resolved peaks at 2θ = 0.9°, 1.5° and 1.8° associated with p6mm

288

symmetry in the planes reflection of (100), (110) and (200) respectively that corresponds to

289

the mesoporosity and crystallinity of the material. However, for the SiAl composite and

290

NiMo–HNC/SiAl, the usual trend of reduction in peak intensity along the (100) plane was

291

observed. Furthermore, the 2θ value shifted from 0.9° to 1.05° for the SiAl composite, which

292

further shifted to 1.16° for the NiMo–HNC/SiAl catalyst. Regarding the modified Laponite

293

support and the supported catalyst, no considerable peaks were observed in the low angle

294

XRD region (0.5° - 5.0°).

295

High angle XRD pattern of commercial γ–Al2O3 (Figure 1 (B)) exhibits 2θ peak

296

values at 19.52°, 32°, 37.28°, 39.5°, 45.9° and 66.8° which is consistent with the literature

297

(JCPDS: 00–046–1215) [26]. SBA–15 material exhibits amorphous silica peak at 2θ =23.5˚,

298

which represents the ordered array of channels that are present between the silica walls [27].

299

Laponite RD clay shows diffraction peaks at 2θ values of 20.35°, 29.06°, 35.38°, 53.14°,

300

60.91°, and 72.83° which has been reported in the literature [28]. On modifying Laponite

301

with CTAB/TEOS/DDA, the characteristic peaks due to Laponite disappeared thereby

302

indicating that the Laponite clay framework was delaminated and coated with porous silica

303

similar to the observation made with SBA–15 in which a 2θ peak value at 23.3° is closer to

304

silica as noted (Figure 1 (B)). By examining the high angle XRD pattern of SiAl and LapAl,

305

it is clear that γ–Al2O3 is interacted with the silica matrix of modified Laponite and SBA-15

306

as evident from the Figure 1 (B).

10

307 308 309 310 311 312 313 314 315 316 317 318

Figure 1 Low angle XRD patterns of (A) SBA–15, modified Laponite, SiAl, LapAl,

319

NiMo–HNC/SiAl & NiMo–HNC/LapAl, High angle XRD patterns of (B) γ-alumina,

320

SBA–15, Laponite RD, modified Laponite, SiAl, LapAl and (C) NiMo–HNC/Al, NiMo–

321

HNC/SiAl & NiMo–HNC/LapAl

322

NiMo-HNC supported on the composites and pure γ–Al2O3 was subjected to X-ray

323

diffraction. The characteristic peaks of MoO3 or NiO were not observed on NiMo–HNC/Al

324

catalyst confirming that MoO3 and NiO are highly dispersed over γ–Al2O3. However, the

325

trend observed with NiMo–HNC/SiAl and NiMo–HNC/LapAl is different. Distinct peaks at

326

2θ = 26.12°, 37.08°, 53.68° and 60.64° were observed for NiMo–HNC/LapAl catalyst with a

327

small shift in the same peaks for NiMo-HNC/SiAl which are different from that of bulk

328

MoO3 whose 2θ peak values are observed at 12.7°, 23.5°, 25.7°, 27.4° and 33.9° [29]. This is

329

clearly due to the formation of hybrid nanocrystals of Mo over the support as reported by Han

330

et al. [10]. This also further indicates the dispersion of NiMo as thin layer of nanoparticles

331

over the outer surface of the composites. Hence, it can be concluded from Figure 1 (C) that a

332

thin layer of HNC–Mo formation is more likely on NiMo-HNC/LapAl catalyst than NiMo-

333

HNC/SiAl catalyst.

11

334

3.1.2

Nitrogen Sorption Analysis

335

The N2 adsorption–desorption isotherms along with the pore size distribution curves

336

of the as–synthesized composite support and catalysts are shown in Figure 2 (A & B) with

337

their textural properties and isotherm type indicated in Table 1. NiMo-HNC/γ–Al2O3 (Figure

338

2B) possesses type IV sorption isotherms with a typical H2 hysteresis loop. NiMo-HNC/SiAl

339

materials possess type IV and H2 type hysteresis loop which represents typical mesoporous

340

ordered nature of materials [23]. The synthesized NiMo–HNC/LapAl possess type II

341

isotherm with H3 hysteresis loop (Figure 2B) due to aggregates of plate–like particles along

342

with slit–shaped pores in contrast to the parent Laponite material that possesses type IV

343

isotherm with H2 hysteresis loop encountered in materials possessing complex pore structure

344

made up of interconnected networks of pores of different size and shape [24]. Thus, the

345

change in isotherm of modified Laponite is due to the new type of linkage that is formed

346

between Laponite particles after the removal of DTA template. The porous silica coating over

347

the disconnected (enlarged) Laponite particles is responsible for the high surface area and

348

pore volume and pore size.

349 350

Figure 2 N2 sorption studies and pore size distribution of supports (A) SBA-15 and

351

modified Laponite (B) composites (SiAl, LapAl) and catalysts (NiMo-HNC/Al, NiMo-

352

HNC/SiAl, NiMo-HNC/LapAl)

353

The average pore diameter of SiAl and LapAl support composite increased by 0.1 nm

354

and 0.2 nm, respectively when compared to the parent supports which is due to the 12

355

development of new type of pores in the support composite after modification with γ–Al2O3.

356

When compared to SiAl, LapAl possesses a wide range of pore size which could be due to γ–

357

Al2O3 that interacts with the distorted Laponite particle surface. At the same time, the pore

358

volume was lesser than that of modified Laponite. The average pore diameter of modified

359

Laponite derived support and catalyst are higher than SBA–15 derived supports and catalysts.

360

From these results, it can be concluded that suitable modification of the Laponite material

361

would offer a wide pore size and pore volume (Table 1). The pore size distribution curves of

362

all the calcined supports and catalysts are shown in Figure 2A and B (inset). The peak

363

intensity of SBA–15 pore size distribution decreased after γ–Al2O3 impregnated into the

364

SBA–15 framework with a new aperture in the range of 3.4 to 4.6 nm, thus peak shifting was

365

observed. In the NiMo-HNC/SiAl catalyst, the two peaks were well saturated with Mo-HNC

366

(Figure 2). However, LapAl support composite shows newly developed well–broadened

367

aperture in the range of 4 to 16 nm after interaction of γ–Al2O3 into the modified Laponite

368

material. The decrease in BET values was observed in the NiMo–HNC/LapAl catalyst, It is

369

due to HNC–Mo saturation over the interconnected pores of LapAl composite (Figure 2B).

370

Table 1 Textural property of supports, fresh and spent catalysts

Sample

Isotherm/hysteresisa

SBET (m2/g)b

VP (cm3/g)c

DP ( nm )d

SBA-15

IV/H2

1001

1.28

5.1

Modified Laponite

II/H3

974

0.70

5.5

SiAl

IV/H2

633

0.82

5.2

LapAl

II/H3

409

0.58

5.7

NiMo-HNC/Al

IV/H3

170

0.57

9.1

NiMo-HNC/SiAl

IV/H2

460

0.69

4

Spent NiMo-

IV/H2

410

0.45

4.9

NiMo-HNC/LapAl

II/H3

382

0.47

4.9

Spent NiMo-

II/H3

410

0.65

4.1

HNC/SiAl

HNC/LapAl 371

a&b

obtained from BET method, cPore volume, dPore diameter

13

372

3.1.4

Fourier Transform-Infrared (FT-IR), Fourier Transform-Raman (FT-Raman)

373

& Diffuse Reflectance Ultraviolet Visible (DRS-UV-vis) Spectroscopy

374

FT–IR spectra of supported Mo-HNC (uncalcined) are depicted in Figure 3 (A). The

375

peaks appeared at 1473 cm–1, 2850 cm–1 and 2917 cm–1 in the Mo-HNC/SiAl, Mo-

376

HNC/LapAl and Mo-HNC/Al are attributed to the symmetric and asymmetric stretching

377

vibrations of –CH2– and asymmetric scissoring vibration of C–H group, respectively [30].

378

The peaks between 960 to 500 cm–1 are ascribed to the vibrations of Mo=O and O–Mo–O

379

[31]. It can also be observed that the characteristic peak intensity of Mo-HNC increased for

380

Mo-HNC/SiAl and Mo-HNC/LapAl when compared to the Mo-HNC/Al. This can be

381

attributed to the formation of core-shell form (DTA)4Mo8O26 over the surface of the

382

composite support whereas, in the case of γ–Al2O3, the deposition of (DTA)4Mo8O26 was

383

inside the support as reported in literature [10].

384

Figure 3 (B) presents the Raman spectra of calcined Mo-HNC/Al, Mo-HNC/SiAl and

385

Mo-HNC/LapAl materials. The vibration bands at 970 cm–1, 944 cm–1, 862 cm–1, 750 cm–1,

386

630 cm–1 and 245 cm–1 are ascribed to the vibrational bands of Mo–O–Mo deformation,

387

Mo=O bending vibrations, symmetric Mo–O–Mo stretches, asymmetric Mo–O–Mo stretches,

388

and symmetric and asymmetric Mo=O terminal stretches of HNC over Mo-HNC/LapAl

389

support. These values are shifted from the unsupported counter part of HNC–Mo [10]. The

390

Mo=O bending vibration of Mo-HNC on Mo-HNC/Al is shifted from 952 cm–1 to 970 cm–1 in

391

both Mo-HNC/SiAl and Mo-HNC/LapAl. When compared to Mo-HNC/LapAl support, the

392

HNC–Mo is dispersed more in Mo-HNC/SiAl as the peak intensities are reduced. The degree

393

of dispersion of the support is in good agreement with the high angle XRD result (Figure 1

394

(B)). From FT–IR and FT–Raman spectra results, the formation and successful deposition of

395

Mo-HNC over the composites has been confirmed.

396

DRS UV–Vis spectra of Mo-HNC/Al, Mo-HNC/SiAl and Mo-HNC/LapAl were

397

recorded to understand the chemical coordination of Mo in different supports (Figure 3 (C)).

398

Generally, the absorption bands of Mo oxide are observed around 200–280 nm and 280−350

399

nm that are assigned to tetrahedral and octahedral charge transfer of O–Mon+ [31].

400

Additionally, the position of the bands is a clear indication of the degree of agglomeration of

401

Mo species over the support [10]. A broad band was observed around 300 nm in the Mo-

402

HNC/γ-Al2O3 catalyst that implies the presence of Mo species in the polymolybdate

403

octahedral structure. However, the peak intensities and position varied with the type of 14

404

support. When compared to γ-Al2O3 supported HNC-Mo, a shift in wavelength towards

405

shorter wavelength was observed in the case of Mo-HNC/SiAl and Mo-HNC/LapAl. This

406

demonstrates that the Mo species are well–dispersed over the composites.

407 408 409

Figure 3 (A) FT-IR spectra (B) DRS-UV-Vis spectra & (C) Raman spectra of Mo–HNC/Al, Mo–HNC/SiAl and Mo–HNC/LapAl catalyst

410 411 412 413 414

15

415

3.1.5

Hydrogen Temperature Programmed Reduction (H2-TPR) & Hydrogen Pulse

416

Chemisorption

417

The TPR characterization was carried out to understand the type, reducibility and metal-

418

support interaction in HNC–Mo and nickel promoted HNC–Mo supported over three

419

different supports. The typical profiles and quantitative data of the catalysts are displayed in

420

Figure 4 (A & B) and Table 2 respectively. The TPR profiles were examined and assigned to

421

the reduction of Mo species based on the literature [32]. In the TPR profiles of Mo-HNC/Al,

422

Mo-HNC/SiAl and Mo-HNC/LapAl catalysts, the reduction peak maxima were observed at

423

543 °C, 566 °C and 556 °C respectively corresponding to reduction of Mo6+ to Mo4+ species.

424

In addition to this peak, second and third reduction peak maxima were observed at 628 °C

425

and 906˚C for Mo-HNC/Al catalyst while for Mo-HNC/SiAl and Mo-HNC/Al catalyst, the

426

second and third reduction peaks observed at about 556 and 820 °C corresponding to

427

complete reduction of Mo4+ to Mo0.

428

Figure 4 H2-TPR profiles of (a) Mo-HNC/Al, Mo-HNC/SiAl and Mo-HNC/LapAl (b)

429

NiMo-HNC/Al, NiMo-HNC/SiAl and NiMo-HNC/LapAl

430

The extent of reduction at different temperatures is highly useful to compare the

431

interaction of the metallic species with the support [33]. The reduction temperature at low

432

temperature region indicates the weak metal–support interaction on the Mo-HNC/Al catalyst.

433

This result confirms that interaction of Mo metal and γ–Al2O3 support is weak as compared

434

with SiAl and LapAl composites. To understand the reduction of Mo on Ni-promoted

435

catalysts, the TPR experiment was extended to Ni containing HNC-Mo catalysts (Figure 4

16

436

(B)). The TPR peaks are not well resolved. However, the incorporation of Ni, appears to shift

437

the first reduction peak to high temperature. This may be due to increase in the interaction of

438

Mo with support. In contrast to the HNC-Mo supported catalysts, the catalyst NiMo-HNC/Al

439

has two well resolved peak maxima at 673 °C and 755 °C along with one other small

440

reduction peak maximum at 824 °C. However, the presence of Ni in the Mo-HNC/SiAl and

441

Mo-HNC/LapAl catalyst, the two peaks i.e one at low and other at high reduction

442

temperature merged together resulting in a broad band centred at 683 °C and 723 °C

443

respectively. Further, the reduction peak of Mo-HNC/LapAl catalyst decreased by 40 ˚C and

444

72 °C as compared with Mo-HNC/SiAl and Mo-HNC/Al catalysts respectively. This

445

decrease in reduction temperature can be attributed to weakly bound Ni and Mo over the

446

LapAl support composite. Thus, LapAl composite facilitates reduction of bimetallic NiMo-

447

HNC species with much ease compared to other supports due to weaker metal–support

448

interaction.

449

From hydrogen consumption values measured using chemisorption (Table 2), it is

450

clear that addition of promoter Ni increases Mo surface area and metal dispersion of Mo. On

451

comparing H2 consumption values of LapAl & SiAl supported catalysts, it is observed that

452

Mo-HNC/LapAl has the highest capacity for hydrogen dissociation further this also

453

emphasizes the fact that the synergism between Ni and Mo is highest over LapAl composite

454

compared with SiAl and Al support. It becomes plausible to conclude that the dissociation of

455

hydrogen (split over hydrogen) on the NiMo-HNC/LapAl catalyst have promisingly showed

456

promotional effect for the formation of large number of CUS.

457

3.1.6

Temperature Programmed Desorption of Ammonia (NH3-TPD)

458

The results of NH3 TPD measurement of Al, LapAl and SiAl supported Mo-HNC and

459

Ni promoted Mo-HNC catalysts are presented in Figure 5 (A & B) and Table 2. This study

460

was made to understand the changes in the acid characteristics of catalysts due to the

461

contribution of acidity of the support. The strength of the acid sites of the material can be

462

classified as weak (< 340 °C) and strong (> 340 °C) from the temperature programmed

463

desorption of ammonia.

464 465

17

466

Table 2 H2 Chemisorption values, acid site distribution of Mo-HNC/Al, Mo-HNC/SiAl

467

and Mo-HNC/LapAl (b) NiMo-HNC/Al, NiMo-HNC/SiAl and NiMo-HNC/LapAl Average Mo Mo Catalysts

surface area dispersion temperature (m2/g)a

Mo-HNC/Al

(Ni) Reduction

18.5

(%)b 45 (− −)

(°°C)c 543, 628 &

Acid sites (mmol NH3/g catalyst) Weak

Total

Acidity

(mmol

NH3/

Strong g cat)d

0.29

0.06

0.35

0.71

0.12

0.83

0.4

0.13

0.53

0.25

0.37

0.62

0.71

0.31

1.02

0.49

0.25

0.74

906 Mo-

21.2

48 (− −)

HNC/SiAl Mo-

820 23.6

55 (− −)

HNC/LapAl NiMo-

26.3

60 (15)

HNC/LapAl 468

a, & b

469

methods

673, 755 & 824

28.3

62 (20)

HNC/SiAl NiMo-

556, 646 & 822

HNC/Al NiMo-

566, 692 &

723, 862 & 1025

31.6

68 (22)

683, 794 & 970

Obtained from H2 pulse chemisorption method,

c & d

Obtained from TPD/TPR

470

TPD experiments were performed to measure the acid strength of the HNC-Mo and

471

Ni promoted, supported HNC-Mo catalysts. TPD profiles, weak, strong and total acidity

472

values are shown in the Figure 5 (A & B) and Table 2, respectively. The catalyst acidity was

473

determined by temperature programmed desorption of NH3 molecule. All the catalysts

474

showed well resolved peaks (Figure 5 (A & B)). The Gaussian fittings were applied to

475

calculate the total acidity of catalysts. There are three NH3 desorption peak maxima, one in

476

the temperature range from 147 °C to 171 °C and there are two others in the range from 343

477

°C to 629 °C due to weak and strong acid sites respectively [34]. The first peak maxima

478

observed at 171 °C is due to weak acid site, and the other peaks observed at 293 °C and 629

479

°C are attributed to strong acidity. It is also observed from the total acidity values (Table 2)

480

that the acidity of Mo-HNC/SiAl is the highest. It has been reported that incorporation of Al

481

atoms in the silica matrix creates acidity to some extent [35]. In this particular case the total

18

482

acidity SiAl is greater than LapAl composite and the acidity follows the trend Mo-HNC/SiAl

483

> Mo-HNC/LapAl > Mo-HNC/Al. It is obvious from TPD profile that number of weak acid

484

sites has increased after the loading of Ni on all the Mo-HNC supported catalysts, due to the

485

formation of more number of coordinatively unsaturated sites on Mo (Table 2). This is

486

probably due to spill over hydrogen species formed from Ni (electron donors) and these acid

487

sites are CUS of Mo (electron acceptors) which is expected to facilitate electron transfer from

488

the metal sites by hydrogen transport [36, 37]. It is obvious that the addition of Ni to Mo-

489

HNC is responsible for the creation of more CUS on the LapAl and SiAl supported catalysts.

490

It has been reported that adsorption of phenol and 4-methyl phenol molecule on Mo takes

491

place via weak acid sites (Bronsted acid sites) and coordinatively unsaturated sites (CUS) for

492

high HDO conversion [38, 39]. The present study reveals that the SiAl and LapAl composites

493

are responsible for the creation of large number of weak sites and its strength significantly on

494

all the composited catalysts (Figure 5 (B)).

495

Figure 5 NH3-TPD profiles of (A) Mo-HNC/Al, Mo-HNC/SiAl and Mo-HNC/LapAl (B)

496

NiMo-HNC/Al, NiMo-HNC/SiAl and NiMo-HNC/LapAl

497

3.1.7 High-Resolution Transmission Electron Microscopy (HR-TEM)

498

HR-TEM images of the catalysts NiMo-HNC/LapAl and NiMo-HNC/SiAl were

499

recorded as shown in Figure 6. Figure 6 (C) clearly shows the interaction between SBA–15

500

with γ–Al2O3. It is seen from the images that hexagonal pore structure of SBA–15 collapses

19

501

on loading γ–Al2O3 and Mo-HNC. This observation is further supported by the disappearance

502

of the characteristic peaks of SBA-15 in XRD pattern of NiMo-HNC/SiAl catalyst (Figure 1

503

(A)). The average size of NiMo-HNC crystals in the supported catalyst materials was

504

calculated to be around 7–8 nm. γ-Al2O3 particles are clearly visible on both the SBA–15 and

505

Laponite (Figure 6 (A)). From the TEM images, it is clear that the average crystal size of

506

HNC-Mo on NiMo-HNC/LapAl catalyst is smaller than that of NiMo-HNC/SiAl catalyst. 507 508 509 510 511 512 513 514 515 516 517 518 519 520

521

Figure 6 HRTEM images of NiMo-HNC/LapAl (A & B) and NiMo-HNC/SiAl (C & D)

522

In NiMo-HNC/LapAl material, calculation of the size of HNC–Mo particles was relatively

523

difficult. This may be due to formation of thin layer of HNC–Mo particles over the surface of

524

the support. It can be understood that growth of HNC–Mo is very much dependent on nature

525

of the support. It is reported [40] that Laponite is stacked disc-shaped crystallites, with the

526

primary particle size of 25–30 nm in the range of height and length approximately. From the 20

527

images recorded in the present study (Figure 6 (A) and (B)), the primary delaminated particle

528

size is measured and the value is 30.5 nm along with MoO3 fringes. However, the aggregated

529

particles are having a size of more than 100 nm. Laponite plates are delaminated by CTAB

530

and silica coating over Laponite which is clearly visible in the HRTEM as it indicates that the

531

delaminated Laponite plate particles acts like a core and porous aluminosilicate acts like a

532

shell [Figure 6 (A)]. From HRTEM images of NiMo-HNC/Al, it is concluded that the

533

synthesis method leads to the delamination of the Laponite disc particles (clay layers) with

534

interconnected porous silica. NiMo-HNC/LapAl exhibits nanoporous rod-like morphology

535

with randomly arranged aluminosilicate coated shells (Figure 6 (A)).

536

3.1.8 X-ray Photoelectron Spectroscopy (XPS)

537 538 539

Figure 7 XPS analysis of Mo 3d and Ni 2p region in NiMo-HNC/SiAl and NiMoHNC/LapAl catalysts

21

540

XP spectra and survey scans were recorded for NiMo-HNC/LapAl and NiMo-

541

HNC/SiAl catalysts for surface elemental analysis and the results are shown in Figure 7 &

542

Table 3. Four peaks were observed in the Mo 3d spectra for both the catalyst, For NiMo-

543

HNC/SiAl catalyst, 233.3 eV and 236.3 eV were assigned to the major molybdenum species

544

(Mo6+ 5/2 and 3/2) on the surface of the catalyst while 231.8 eV and 234.7 eV were assigned

545

to Mo4+ 5/2 and 3/2 respectively and 229.4 eV corresponding to Moδ+ oxidation state [41]. In

546

the case of NiMo-HNC/LapAl catalyst, the major molybdenum species Mo6+on the surface

547

are 235 eV and 238.6 eV, whereas 231.8 eV and 237.0 eV corresponding to Mo4+5/2 and 3/2

548

respectively and Moδ+ species was observed at 228.9 eV. This lower valence of Mo can be

549

regarded as active species for the adsorption of reactant molecule. This kind of Moδ+ species

550

was also observed and proved as active reactive species for CO to form alcohol [42]. Mo4+

551

and Moδ+ species concentration of NiMo-HNC/LapAl is higher than that of NiMo-HNC/SiAl

552

catalyst. This type of higher concentration of species responsible for the high catalytic

553

activity of the sulfided catalyst of molybdenum oxide Ni 2p peaks were recorded for both the

554

catalysts confirming the presence of impregnated promoter Ni over the catalysts. For NiMo-

555

HNC/SiAl catalyst, Ni2+ 2p3/2 and Ni2+ 2p1/2 peaks were centered at 856.3 eV and 874 eV

556

respectively. However, in the case of NiMo-HNC/LapAl catalyst, Ni2+ 2p3/2 and Ni2+ 2p1/2

557

peaks correspond to 858.4 eV and 876.5 eV respectively.

558

The surface characteristic of all the elements was calculated by XPS analysis and

559

tabulated in Table 3. Si/Al atomic ratio of NiMo-HNC/SiAl catalyst is higher than NiMo-

560

HNC/LapAl catalyst. That is the reason, for the higher surface acidity of NiMo-HNC/SiAl

561

than NiMo-HNC/LapAl catalyst. The higher Mo and Mo/Si atomic ratio of NiMo-

562

HNC/LapAl than NiMo-HNC/SiAl catalyst can be regarded as a higher surface dispersion of

563

Mo species during catalyst preparation over the support [10] in contrast to the reduction of

564

peak intensity observed in wide-angle XRD pattern for good dispersion of active species over

565

NiMo-HNC/SiAl catalyst. Even though Mo/Al (5.32) of NiMo-HNC/SiAl is higher than

566

Mo/Al (3.39) of NiMo-HNC/LapAl catalyst, the surface availability of Mo/Ni and Ni/Al of

567

NiMo-HNC/LapAl are 5.23 and 1.01 respectively, higher than the same counterpart of NiMo-

568

HNC/SiAl catalysts accounts for higher accessibility of the active sites to the reactants [33],

569

because these values are reflected in the reducibility of molybdenum over the support i.e. the

570

higher surface Mo/Ni and Ni/Al determine the low-temperature reduction of Mo (weak Mo

571

and support interaction) over NiMo-HNC/LapAl catalyst. It is clear that the amount of H2

572

chemisorbed to NiMo-HNC/LapAl is higher than that to NiMo-HNC/SiAl even though both 22

573

the catalysts contain nearly same amount of Mo contents as determined by ICP. In addition to

574

that, XPS results also support the same surface concentrations of Mo between two catalysts.

575

Consequently, high Mo/Ni value of NiMo-HNC/LapAl catalyst signifies smaller size

576

formation of Mo on the support [33] and high dispersion of Mo particles could result in large

577

number of Mo surface area and adsorb large quantity of hydrogen which is consistent with

578

the results of our previous studies [37, 43].

579

Table 3 Distribution of elements (%), its atomic surface concentration and

580

corresponding percent of peaks of Mo species in NiMo-HNC/LapAland NiMo-

581

HNC/SiAl

Catalysts

Al C Mo Ni O Si N Si/ Mo/ Ni/ Mo/ Mo/ Mo6+ Mo4+ Moδ+ (%) (%) (%) (%) (%) (%) (%) Al Al Al Ni Si (%) (%)

NiMo18.9 10.2 59.7 3.1 1.96 6.12 1.02 5.32 1.01 5.23 1.67 62.07 30.17 7.76 HNC/Lap 1.93 4 7 7 7 Al 20.8 59.9 2.4 NiMo8.08 1.88 5.84 1.03 3.39 0.78 4.29 1.38 63.34 29.00 7.66 2.38 6 3 5 HNC/SiAl 582 Obtained from XPS 583

Table 4 H2 uptake values, Specific reaction rate and Intrinsic rate of all the catalysts for

584

Eugenol HDO Volume

of

H2 Catalysts

r(Eug)c

r(Eug-

consumption 10−7mol

HDO)d10−8

(µ µmol/g)a&

mol g−1 s−1

g−1 s−1

% Mob Mo-HNC/Al

ri(Eug)(ri(EugHDO))e 10−5molec. at−1 s−1

Mo

Synergetic of

Factor Eugenol

transformation (hydrocarbon products)f

160 & 10.1

1.5

1.2

14 (1.2)

N.d

Mo-HNC/SiAl 201 & 10.8

3.0

1.8

27 (1.6)

N.d

220 & 10.4

4.3

2.9

40 (2.7)

N.d

NiMo-HNC/Al 250 & 10.6

2.3

2.1

20 (1.9)

1.4 (1.5)

MoHNC/LapAl

23

NiMo-

301 & 10.5

5.8

3.2

53 (2.9)

1.9 (1.8)

359 & 10.3

17.5

5.0

162 (4.7)

4.0 (1.8)

HNC/SiAl NiMoHNC/LapAl 585

a

Obtained from H2pulse chemisorption method, b(% Mo obtained from ICP-OES spectra),

586

c

587

oxygen free hydrocarbon, eIntrinsic rate of eugenol conversion &HDO, fSynergetic factor for

588

eugenol transformation = ri(Eug) (NiHNC-Mo)/ri(Eug) (HNC-Mo) and for hydrocarbon

589

formation = ri(Eug-HDO) (NiHNC-Mo)/ri(Eug-HDO) (HNC-Mo).

590

4. Catalytic activity test

591

4.1 Influence of Temperature & Time on Stream (TOS)

Specific rates :r(Eug), total transformation rate of eugenol, dr(Eug-HDO, rate of formation of

592

The HDO of eugenol was carried out on NiMo-HNC/LapAl and NiMo-HNC/SiAl

593

catalysts at different reaction temperatures (573 K -673 K) under steady state condition with

594

H2 flow of 50 ml/min and reactant feed rate of 4.27 h-1at atmospheric pressure of H2.

595

As shown in Figure 8 (A & B), only 48 − 52 % conversion was observed on both the

596

NiMo-HNC/LapAl and NiMo-HNC/SiAl catalysts at 573 K. As the temperature increased

597

from 573 K to 673 K, % conversion of eugenol increased from 48 % to 90 % on NiMo-

598

HNC/SiAl and 52 to 100 % on NiMo-HNC/LapAl respectively. A similar trend was also

599

observed in case of product selectivity. Particularly, the % yield of 3 & 4-propyl phenol and

600

hydrocarbons increased from 20, 11 and 5 to 35, 17 and 18 on NiMo-HNC/LapAl catalyst.

601

Comparatively, the % conversion and % yield of eugenol over NiMo-HNC/LapAl was found

602

to be higher than that of NiMo-HNC/SiAl catalyst. It is to be mentioned that temperature

603

lower than 673 K may not be sufficient to activate the reactant molecule, The decrease in

604

conversion of eugenol beyond 673 K, can be indicative for the occurrence of possible

605

cracking reaction instead of deoxygenation. To optimise other experimental conditions,

606

experiments were carried out at 673 K. It is clearly demonstrated that the temperature has a

607

marked effect on eugenol conversion and product yield.

608

24

609 610

Figure 8 Influence of reaction temperature on eugenol conversion (%) and

611

product yield (%) over (A) NiMo-HNC/LapAl and (B) NiMo-HNC/SiAl catalysts

612

(Reaction Conditions: Reactant: 3.5 wt% of eugenol in decalin, Reactant Feed: 2.2

613

mL/h; Reaction time: 6-8 h; Temperature: 573 K – 673 K; H2 flow rate = 50 cm3/min.

614

Influence of Time on Stream (TOS) on conversion (%) and product yield (%) over (C)

615

NiMo-HNC/LapAl and (D) NiMo-HNC/SiAl catalyst (Reaction Conditions: Reactant:

616

3.5 wt% of eugenol in decalin; WHSV: 4.27 h-1; Reaction time: 1-8 h; Temperature: 673

617

K; H2 flow rate = 50 cm3/min., Hydrocarbons: benzene and propyl cylcohexene, Others:

618

phenol, 4-propylcyclohexanol, 2-methyl-4-propylphenol and catechol)

619

The study of eugenol conversion (%) and product yield (%) as a function of time-on-

620

stream was performed on NiMo-HNC/LapAl and NiMo-HNC/SiAl catalysts at WHSV: 4.27

621

h-1and 673 K (Figure 8 (C & D)). It was observed that % conversion increased from 40 to 98

622

on NiMo-HNC/LapAl and 32 to 84 on NiMo-HNC/SiAl with an increase in time from 1 h to

623

4 h and remained steady after 4 h on both the catalysts. At the same time, the % product

624

yields of mono-oxygenated hydrocarbons and hydrocarbons. The selectivity of the products 25

625

(mono-oxygenated hydrocarbons and hydrocarbons) also has the same trend. Hence, the

626

activity of catalysts and performance of the various supports for Ni-MoHNC were compared

627

under steady state reaction conditions.

628

4.2 Catalytic Activity of HNC-Mo and Ni Promoted HNC-Mo Supported Catalysts for

629

HDO of Eugenol

630

Catalytic activity of HNC-Mo and Ni promoted Mo-HNC composite supported

631

catalysts was evaluated for the conversion(%) of eugenol to hydrocarbon and oxygenated

632

hydrocarbon (%) in the presence of H2 at 673 K (Figure 9 (A & B)) under steady state

633

reaction condition (6-8 h). The liquid products identified by GC-MS were benzene, phenol,

634

propyl cyclohexene, 4-propylcyclohexanol, guaiacol, 3-propylphenol, 4-propylphenol, 2-

635

methyl-4-propylphenol and catechol (GC-MS profile (Figure S2)). Among all, 4-

636

propylphenol and 3-propylphenol are the major mono oxygenated products.

637

On NiMo-HNC/LapAl catalyst, 100 % of eugenol was converted completely to

638

products whereas only 90 % and 60 % eugenol was converted on NiMo-HNC/SiAl and

639

NiMo-HNC/Al catalysts respectively. Overall, the total conversion from HNC-Mo supported

640

catalysts were less than that of Ni promoted HNC-Mo catalysts, which is a clear indication

641

for the promotional effect offered by Ni. However, on the Ni promoted HNC-Mo catalyst, the

642

% hydrocarbon and oxygenated hydrocarbons formed were more than on HNC-Mo catalyst.

643

This was due to the hydrogenation ability of Nickel as evident by high H2 consumption values

644

of nickel promoted catalysts (Table 4). Benzene and propylcyclohexene were formed by the

645

complete deoxygenation whereas 4-propyl phenol and other mono oxygenated hydrocarbon

646

were formed by the partial deoxygenation of eugenol. The highest percentage yield of

647

benzene, propylcyclohexene (hydrocarbons) and 4-propyl phenol were 6, 12 and 35 was

648

observed on NiMo-HNC/LapAl catalyst. 4, 8 and 33 on NiMo-HNC/SiAl catalyst while 2, 4

649

and 22 on NiMo-HNC/Al catalyst. It is obvious that Ni promoted catalysts show significant

650

improvement in hydrocarbon yield in HDO of eugenol. Among three different supports, it is

651

the Laponite that facilitate HNC-Mo and Ni HNC-Mo to be a good catalyst for the highest

652

conversion of eugenol via HDO than γ-Al2O3 and SBA-15-γ-Al2O3 as support.

26

653

Figure 9 Eugenol conversion (%) and product yield (%) over HNC-Mo (A) and Ni

654

promoted (B) Mo-HNC/Al, NiMo-HNC/SiAl, Mo-HNC/LapAl catalyst (Reaction

655

Conditions: Reactant: 3.5 wt% of eugenol in decalin; WHSV: 4.27 h-1; Reaction time: 6

656

h; Temperature: 673 K; H2 flow rate = 50 cm3/min., Hydrocarbons: benzene and propyl

657

cylcohexene, Others: phenol, 4-propylcyclohexanol, 2-methyl-4-propylphenol and

658

catechol).

659

4.3

660

Physicochemical Properties and Synergetic Factor

Correlation of Catalytic Activity of Supported NiHNC-Mo Catalysts with

661

It is worth expressing the efficiency of the catalysts in terms of the specific reaction

662

rates than in terms of % yield and conversion. The specific rate, HDO rate of eugenol

663

conversion and HDO product and synergetic factor of all the catalysts were calculated and is

664

presented in Table 4. Moreover, the addition of the promoter Ni to HNC-Mo has significantly

665

enhanced the specific reaction rate and HDO rate of all the catalysts. It also evidences the fact

666

that there is a strong influence of support for the NiHNC-Mo catalyst. The specific rate is 8

667

times higher on LapAl supported catalyst than γ-Al2O3 support and 3 times higher than SiAl

668

support. Hence, the result shows that there is a significant change in the activity due to

669

change in the support from γ-Al2O3 support to composites (LapAl, SiAl).

670

Among all the supports, LapAl expressed a maximum r(Eug), r(Eug-HDO)and

671

ri(Eug)(ri(Eug-HDO)) rate (Table 4). It may be due to the textural property and morphology

672

(N2 sorption studies and HR-TEM) which influenced the % dispersion of active metal species

673

and H2 uptake as calculated from volumetric chemisorption technique as shown in Table 4.

674

The delaminated sheets of Laponite are responsible for improved textural properties, fine

27

675

dispersion of HNC-Mo and formation of lower oxidation state of Moδ+ (1 <δ+ < 4) species on

676

the support (Table 3). The lower valent Moδ+ can be regarded as active species for the

677

adsorption of reactant molecule as reported in literature [42]. Mo4+ and Moδ+ (1 <δ+ < 4)

678

species concentration in NiMo-HNC/LapAl is higher than that in NiMo-HNC/SiAl catalyst

679

accounting for the high catalytic activity.

680

To understand the promotional effect of the Ni on Mo, the synergism between Ni and

681

Mo is measured in terms of HDO activity and synergetic factor for the eugenol conversion

682

and hydrocarbon formation using the values of intrinsic rate as shown in the foot note of

683

Table 4. Table 4 shows HDO activity and synergetic effect for eugenol conversion which

684

appears to be more on NiMo-HNC/LapAl catalyst than that on NiMo-HNC/SiAl and NiMo-

685

HNC/Al catalyst. The intrinsic HDO activity and synergetic factor of NiMo-HNC/LapAl is

686

two times higher than that of NiMo-HNC/SiAl catalyst indicating high synergism between Ni

687

and Mo on LapAl composite for eugenol transformation and having no impact on

688

hydrocarbon formation. However, such synergism was much less on NiMo-HNC/Al catalyst.

689

These results conclude the excellent nature of the support LapAl for Ni-Mo HNC catalyst.

690

The high HDO activity of NiMo-HNC/LapAl catalyst can be accounted due to its

691

hydrogenation ability. H2 pulse Chemisorption study revealed that NiMo-HNC/LapAl

692

catalyst has a high H2 uptake value and Mo surface area (Table 4).

693

4.4

694

NiMo-HNC/LapAl catalyst

The Reaction Pathway of Eugenol HDO & Enhanced HDO performance of

695

Based on the previous literature [11, 34, 44], it is clear that the hydrogenation route is

696

less favoured by HYD than DDO over supported NiO-MoO3 and MoO3 catalyst for phenol

697

and substituted phenol conversion. The mechanistic pathway of eugenol follows the

698

following steps over the HNC-Mo supported catalysts consisting of hydrodeoxygenation

699

proceeds predominantly by demethylation of the aromatic ring followed by dehydroxylation

700

to a monohydroxyl-substituted intermediate. It is obvious that HDO of eugenol proceeds by

701

DDO and HYD over NiHNC-Mo supported catalysts. Hydrodeoxygenation proceeds

702

predominantly by demethylation of the aromatic ring followed by dehydroxylation to a

703

monohydroxyl-substituted intermediate that undergoes final dehydroxylation to benzene and

704

hydrogenation to propyl cyclohexene. It is also observed that HDO of eugenol can also take

705

place via cracking of propyl group by depropylation of eugenol. Owing to the removal of

706

alkenyl side chain of eugenol, molecules can potentially form coke on the catalyst surface. 28

707

[45, 46] Over HNC-Mo catalysts, eugenol is converted to propyl cyclohexene and benzene

708

(hydrocarbon) through DME followed by HYD and depropylation followed by DDO. The

709

propyl cyclohexene formation occurred via HYD of 4-propyl phenol i.e hydrogenation of 4-

710

propyl phenol leads to the formation of 4-propyl cyclohexanol then, finally dehydrated to 4-

711

propyl-cyclohex-1-ene. Benzene formation occurs through guaiacol and catechol via

712

depropylation followed by DDO (Figure 10, pathway 1 & 2). However, In HNC-Mo

713

catalysts, the hydrocarbon formation is due to DME followed by DDO of eugenol (Figure 10,

714

pathway 1). The HDO pathway of eugenol on the NiHNC-Mo catalyst is similar to HDO

715

pathway of eugenol on CoMoS2/γ-Al2O3 catalyst [47], while using Pt/γ-Al2O3 as a

716

catalyst [48] selectivity of 4-propylguaiacol increased due to hydrogenation ability of Pt

717

catalyst. Compared to monometallic sulfided Mo catalyst for HDO of guaiacol, bimetallic

718

CoMo catalyst showed a significant improvement in HDO rate. Hence, it has been proved

719

that the addition of cobalt enhanced the demethoxylation and deoxygenation pathways

720

leading to a higher fraction of HDO compounds [11, 49].

721

The introduction of NiMo-HNC active phase on the LapAl composite facilitated to

722

perform better HDO, compared to other supported (SiAl and Al) catalysts. The catalyst also

723

possess comparatively high amount of propyl cyclohexene and benzene as complete HDO

724

product than others which implies that the added Ni is highly favorable for the HYD and

725

DDO pathways on LapAl support. The pathway is similar to Guaiacol (GUA) HDO on

726

CoMoS/γ-alumina [11] and dibenzothiophene (DBT) HDS on NiHNC-Mo/γ-alumina [10].

727

The enhanced HDO activity of NiMo-HNC/LapAl can be correlated as support effect i.e. the

728

exposed active sites of support and metal (NiMo active species). The surface hydroxyl group

729

of support can act as adsorption sites whereas NiMo metal sites can promote the

730

hydrogenlysis (DDO pathway) and HYD of the reactant on the NiMo-HNC catalyst [11]. The

731

composite LapAl possess distinguished surface hydroxyl groups (Clay layered silica, coated

732

porous silica & alumina surfaces) as evidenced by FT-IR (Figure 3 (A) and XPS (Figure 7)

733

which are the reasons for effective adsorption of the reactant molecule on surface of LapAl

734

composite to interact with HNC-Mo and NiMo-HNC. Moreover, it is also observed that the

735

surface availability of Ni, Mo and its active species (Mo4+ and Moδ+), Mo/Ni and Ni/Al are

736

also high on LapAl composite (Table 3). Hence, the support would offer high synergism

737

between Ni and Mo, thereby leading to the formation of large number of CUS and optimum

738

number of weak acid sites which makes this catalyst to perform better HDO and

739

hydrogenation than any other catalyst. Further, It is also reported that Laponite support with 29

740

layered structure for Mn-Al hydrotalcite catalyst has been shown to exhibit enhanced activity

741

for the combustion of volatile organic compounds [50]. It can be ascertained that the support

742

composite (LapAl) in the NiMo-HNC/LapAl catalyst is more favourable for the formation of

743

monooxygenated hydrocarbons than other support composite (SiAl) and γ-alumina. Table S1

744

shows comparison of eugenol HDO of NiMo-HNC/LapAl catalyst with previously reported

745

catalysts. This result suggests that the catalyst expressed excellent HDO yield (%) under

746

atmospheric pressure at 673 K.

747 748 749 750 751 752 753 754 755 756 757 758 759 760

Figure 10 Reaction network of Eugenol HDO on HNC-Mo and Ni-HNC-Mo catalysts

761

4.5 Reusability and Physicochemical Properties of Spent NiMo-HNC/LapAl and NiMo-

762

HNC/SiAl Catalysts

763

The test for the reusability of catalysts is very important for its practical applicability

764

in industrial applications. The reusability and stability of NiMo-HNC/SiAl and NiMo-

765

HNC/LapAl catalysts were studied for five cycles towards conversion of eugenol at 673 K 30

766

using 50 cm3/min of H2 flow at WHSV: 4.27 h-1. The recyclability test was conducted after

767

pretreating with N2. The catalyst was activated every time before each run using 10% H2/He

768

mixture for 3 h at 400 °C. Figure 11 (A) shows that both the catalysts retained same HDO

769

activity up to five cycles under steady state condition indicating that the catalysts are stable

770

under the present experimental conditions. The high thermal stability of LapAl support may

771

be responsible for the high HDO activity of NiMo-HNC/LapAl catalyst.

772 773

Figure 11 Reusability and stability of NiMo-HNC/SiAl and NiMo-HNC/LapAl catalysts

774

(Reaction Conditions: Reactant: 3.5 wt% of eugenol in decalin; WHSV: 4.27 h-1;

775

Reaction time: 6-8 h; Temperature: 673 K; H2 flow rate = 50 cm3/min.)

31

776

Figure 11 (B) shows N2 adsorption and desorption isotherms of fresh and spent

777

NiMo-HNC/SiAl and NiMo-HNC/LapAl catalysts. The slight reduction in the surface area of

778

the spent catalyst may be due to coke formation on the surface of the catalyst and the surface

779

area values are indicated in Table 1. However, the pore volume and pore diameter values did

780

not change significantly.

781

5. Conclusion

782

In summary, γ-alumina mixed SBA-15 and modified Laponite support composites

783

have been prepared successfully by wet impregnation method. The interaction of γ-alumina

784

with the SBA-15 and modified Laponite framework was supported by high angle XRD and

785

N2 sorption study. From the TPD/TPR results, the H2 reduction peaks and generation of weak

786

and strong acid sites of NiMo-HNC/LapAl catalyst was found to be optimum. H2 pulse

787

chemisorption results confirm that the % dispersion and H2 consumption of HNC-Mo over

788

LapAl is higher than SiAl and γ-alumina support. FT-Raman and FT-IR results together

789

validate the successful deposition of HNC-Mo over the supports. Laponite+γ-Al2O3 support

790

was responsible for generation of a large number of active Moδ+ species with a high value of

791

Mo/Ni ratio than other supports as evidenced by XPS studies. Among the catalysts, modified

792

Laponite supported catalyst shows complete conversion of eugenol and high HDO activity at

793

400 °C under atmospheric pressure; this is due to modified framework, enhanced textural

794

properties and morphology of Laponite supported HNC-Mo. The study has revealed that

795

Laponite+γ-Al2O3 composite is a suitable choice as support for NiMo-HNC catalysts

796

expressing high intrinsic rate and synergetic factor as compared to any other composites.

797

NiMo-HNC supported Laponite+γ-Al2O3 catalyst can be practically applicable for the

798

transformation of lignin derived aromatic eugenol and upgradation of it into hydrocarbon

799

fuel.

800 801

6. Acknowledgements

802

The authors are grateful for the financial support provided by UGC - Basic Scientific

803

Research Fellowship (Award Lr.No. F.4-1/2006 (BSR)/7-7/2007 (BSR), dt. 13.03.2012). We

804

are also thankful to DRDO, UGC-DRS and DST-FIST for providing the instrumentation

805

facility in the Department of Chemistry, Anna University, Chennai, India.

32

806

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Highlights

High surface area modified Laponite synthesized by facile method at RT. Laponite & SBA-15 + alumina composites prepared by simple wet impregnation method. HNC-Mo synthesized on alumina & composites by hydrothermal method at 120 °C. Vapour phase HDO of clove oil on the catalysts were investigated at 573 − 673 K. High HDO rate and synergetic effect was observed on Laponite supported catalyst.