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Novel silica encapsulated Cu-Al hydrotalcite catalyst: Oxidative decarboxylation of vanillyl mandelic acid to vanillin in water at atmospheric pressure Shivaji L. Bhanawase, and Ganapati D. Yadav Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04982 • Publication Date (Web): 28 Feb 2017 Downloaded from http://pubs.acs.org on March 2, 2017

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Industrial & Engineering Chemistry Research

1 2

Novel silica encapsulated Cu-Al hydrotalcite catalyst: Oxidative decarboxylation of

3

vanillyl mandelic acid to vanillin in water at atmospheric pressure

4 5

Shivaji L. Bhanawase and Ganapati D. Yadav*

6

Department of Chemical Engineering

7

Institute of Chemical Technology

8

Nathalal Parekh Marg

9

Matunga

10

Mumbai – 400 019

11

India

12 13 14 15 16 17

*Author to whom correspondence should be addressed

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E-mail: [email protected]

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Phone: +91-22-3361-1001

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Fax: +91-22-3361-1020; +91-22-3361-1002

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Abstract

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Vanillin is a versatile chemical having characteristics such as antimicrobial, antioxidant,

26

anticarcinogenic, antimutagenic, hypolipidemic and antisickling activities and is an intermediate

27

for synthesis of fine chemicals like vanilla flavour. Vanilla is the second most expensive spice

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which is extracted from vanilla beans and is in short supply. Therefore to meet demand of vanilla

29

flavour, synthesis of vanillin by chemical route is necessary. In this regard, a novel silica

30

encapsulated copper aluminium hydrotalcite (SECuAlHT) was synthesized. Different techniques

31

such as XRD, FTIR, N2 adsoption-desorption, TGA-DSC, NH3 TPD, CO2 TPD, H2

32

chemisorption, TEM, and SEM-EDXS were used to characterize SECuAlHT. Its catalytic

33

activity for oxidative decarboxylation of vanillyl mandelic acid (VMA) was investigated.

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Vanillin was obtained from VMA by oxidative decarboxylation using SECuAlHT in water as

35

solvent and atmospheric air under base-free condition. Presence of basic sites in the catalyst was

36

confirmed with CO2-TPD. Effect of different reaction conditions on catalyst activity and

37

selectivity for oxidative decarboxylation of VMA was investigated. Vanillin was efficiently

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obtained with 89 % selectivity at VMA conversion of 81 % over SECuAlHT catalyst at 100 ºC in

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8 h. The catalyst was active, selective, stable and reusable. Catalytic cycle based on Mars van

40

Krevelin mechanism was proposed and reaction kinetics studied. It is a zero order reaction. The

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apparent energy of activation was 8.7 kcal/ mol. Thus, vanillin was efficiently synthesised from

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VMA by a green catalytic route in water as a solvent.

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Keywords: Silica encapsulation; copper aluminium hydrotalcite; vanillyl mandelic acid;

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vanillin; oxidative decarboxylation; green chemistry

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1. INTRODUCTION

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Different flavorings are used as food additives to enhance the taste and aroma of food.

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Flavorings are of natural origin obtained from plants or animal by different physical processes or

51

by chemical processes are employed to clone the natural flavours. Among different flavoring

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agents, vanilla is the second most important flavor widely used in baking and perfumery.1

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Vanilla is the second expensive spice and extracted from vanilla beans. Vanilla extract is made

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up of more than 200 components in which vanillin is responsible for the characteristic flavor.

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Vanillin

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hypolipidemic, antisickling activity and is an intermediate for synthesis of a few fine chemicals.1-

57

2

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the increasing demand of vanilla flavor, synthesis of vanillin by chemical routes is necessary.

tends

to

show

antimicrobial,

antioxidant,

anticarcinogenic,

antimutagenic,

Vanillin has been used as an intermediate in synthesis of biobased polymers.3 In order to meet

59

Vanillin has been synthesized by various chemical routes as follows. From reaction of

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guaiacol with chloroform in alkaline medium,4 from cow dung,5 from reaction of 4-hydroxy

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benzaldehyde with bromine, copper bromide in presence of sodium methoxide,6 from reaction of

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3-bromo-4-hydroxybenzaldehyde with copper bromide as catalyst in the presence of sodium

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methoxide,7 from eugenol,8 from vanillic acid reduction catalyzed by enzyme,9 from

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glucovanillin,10 from O-benzylvanillic acid, from TiO2 photocatalysis of ferulic acid; trans-ferulic

65

acid; isoeugenol; eugenol; vanillyl alcohol,11 from o-nitro-chloro-benzene via guaiacol, from

66

systematic lignin oxidation,12-13 2-methoxy-p-cresol catalyzed by CoCl2,14 from vanillyl alcohol

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catalyzed by copper acetate and TEMPO,15 from substituted 4-cresols catalyzed by cobalt

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acetate,16 from guaiacol derivatives and acetic acid catalyzed by triflic acid,17 from substituted

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anisole and N-methyl-N-phenyl formamide and N-N-dimethyl formamide catalyzed by POCl3,18

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from oxidation followed by decarboxylation of vanillyl mandelic acid (VMA) obtained by

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reaction of guaiacol and glyoxylic acid in alkaline medium.19 Oxidative decarboxylation of

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VMA is catalysed by antibody,20 CoCl2,21 Bi(0,3+),22 reducible metal salts,23 copper hydroxide,19

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and copper sulphate.24 Except copper hydroxide in alkaline medium, all reactions have been

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catalyzed by homogeneous catalysts which need to be replaced by heterogeneous recyclable

75

catalyst to overcome issues of pollution and materials of construction. Heterogeneous catalysts

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are favored over homogeneous catalyst for many well-known reasons such as ease of separation

77

from reaction mass, recycle and reuse of catalysts, cheap materials of construction, better

78

economics, and above all environment friendly attributes as per principles of Green Chemistry.

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Thus heterogeneous catalysis will be helpful for sustainable development.

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Active phase of heterogeneous catalyst may be dispersed uniformly on support having

81

high surface area such as silica, alumina, titania and activated carbon to get highly active catalyst

82

compared to the parent catalyst. In order to overcome the leaching of the active phase from solid

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support, suitable encapsulation by some porous matrics should be done to maintain the fidelity

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and reusability of the heterogeneous catalyst. Only a few reports are available in literature on use

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of silica and polymer for encapsulation.25-28

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Oxidative decarboxylation of VMA has been reported by copper hydroxide in alkaline

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medium.19 To separate vanillin from reaction mass, it needs further treatment like neutralization

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by aqueous hydrochloric acid and solvent extraction. This process generates waste which need to

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be treated before its release into environment.

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In this work, oxidative decarboxylation of VMA is presented by using a novel silica

91

encapsulated Cu-Al hydrotalcite ((SECuAlHT) in water without addition of any base at ambient

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air pressure and 100 °C. Due to base-free condition, reaction mass need not require neutralization

93

by aqueous hydrochloric acid which is another novelty of the work. Thus, this method is


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environment friendly, clean and economical. Effect of various reaction conditions on catalyst

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activity and selectivity for oxidative decarboxylation of VMA was investigated. Studies were

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conducted to establish the reaction mechanism and reaction kinetics which would help in scale-

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up and reactor design.

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2. EXPERIMENTAL

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2.1 Catalyst preparation

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2.1.1 Preparation of Cu-Al hydrotalcite (CuAlHT)

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Cu-Al hydrotalcite of 3:1 mole ratio was prepared by co-precipitation method. 500 mL

102

round bottom flask (RBF) with overhead stirrer and two addition funnels was used. RBF was

103

dipped in the oil bath at 30 ºC. A solution of 0.006 mol copper nitrate trihydrate and 0.002 mol

104

aluminium nitrate nonahydrate was made in 25 mL deionised water. A solution of 0.017 mol

105

sodium hydroxide and 0.005 mol anhydrous sodium carbonate was made in 25 mL deionised

106

water and both solutions were added dropwise in RBF with stirring at 400 rpm. Brown

107

precipitate so obtained was kept at 60 ºC for 12 h and washed with distilled water until it

108

produced neutral pH supernatant water. Then supernatant water was decanted and the brown

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precipitate was centrifuged to get wet material.

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2.1.2 Preparation of silica encapsulated Cu-Al hydrotalcite (SECuAlHT)

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Wet CuAlHT material was transferred to 500 mL RBF with overhead stirrer and addition

112

funnel immersed in the oil bath at 30 ºC. Ethanol (58.3 g) and 10 ml deionised water were added

113

to RBF with stirring at 400 rpm. This colloidal solution was sonicated for 30 min with occasional

114

stirring. Cetyl trimethyl ammonium bromide (2.5 g) was added to this solution, stirred for 15

115

min, and 10.3 g tetraethyl orthosilicate was added under stirring for 15 min. Aqueous ammonia

116

solution (6 g) was added to this solution dropwise with constant stirring. Further this solution



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was stirred for 12 h. Material so obtained was washed with distilled water until neutral pH of

118

supernatant water. It was centrifuged and dried in oven at 100 ºC for 24 h which was calcined at

119

500 ºC for 6 h. This is called SECuAlHT catalyst in this work.

120

2.2 Reaction procedure

121

VMA (50 µmol), distilled water (15 mL) and 0.09 g of SECuAlHT (0.006 g mL-1) were

122

added to the glass reactor. At desired temperature, first sample (0 min) was collected. Reaction

123

mixture was stirred at specific speed and at regular interval samples were collected. In typical

124

experiment, reaction mass was stirred at 1000 rpm and at 100 ºC. Scheme 1 represents the

125

oxidative decarboxylation of VMA.

126

After reaction, reaction mass was filtered to separate the catalyst. Aqueous sodium

127

bicarbonate was added to remove VMA and vanillic acid. Reaction mass was extracted with

128

ethyl acetate to separate vanillin.

129

selectivity of the desired product vanillin was based on the moles of formation of vanillin to

130

moles of VMA reacted.

The conversion is defined with reference to VMA, and

OH

OH O

OH O

SECuAlHT, Air, Water, 100 ºC - CO 2

OH

HO

O SECuAlHT, Air, Water, 100 ºC

-H 2 O O

H

O

OH

O

131 132

Vanillyl mandelic acid

Vanillin (89%)

Vanillic acid (11%)

Scheme 1 Oxidative decarboxylation of VMA to vanillin

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3. RESULTS AND DISCUSSION

134

3.1 Catalyst characterization

135

XRD analysis of silica, CuAlHT, and SECuAlHT is shown in Figure 1. Diffractogram of

136

CuAlHT shows sharp peaks at 32.5º, 35.5º, 38.8º, 48.7º, 53.4º, 58.4º, 61.5º, 66.2º, 68.1º, 72.4º,


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and 75.1º (Figure 1a). XRD pattern of copper matches with diffractogram of copper oxide

138

(JCPDS 72-0629). Diffractogram of SECuAlHT is shown in Figure 1b; a broad peak at 15-30º

139

indicates amorphous nature of material. Copper oxide phase is well dispersed in silica and no

140

distinct peaks were obtained which indicates copper oxide phase is encapsulated in silica phase.

141 142

Figure 1 XRD of catalysts: (a) CuAlHT (b) SECuAlHT

143 144

Figure 2 shows FTIR spectra of silica, CuAlHT, fresh SECuAlHT, and three-time reused

145

SECuAlHT. Silica showed transmittance band at 3437, 1636, 3246, 1088, 800 and 470 cm-1 due

146

to stretching vibration of water, bending vibration of water, Si-OH stretching vibration, Si-O-Si

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asymmetric stretching vibration, Si-O-Si symmetric vibration and bending vibrations of O-Si-O,

148

respectively (Figure 2a). CuAlHT showed transmittance bands at 3490, 16361, 1462 and 450 cm-

149

1

150

vibration, and metal oxide stretching vibration, respectively (Figure 2b). SECuAlHT showed

due to H2O stretching vibration, H2O bending vibration, hydroxide functional group bending



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transmittance bands of H2O molecules stretching and bending vibration at 3437 and 1636 cm-1,

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respectively. The adsorption band at 3246 cm-1 is due to the stretching vibrations of Si-OH. Si-

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O-Si asymmetric stretching vibrations at 1088 cm-1, silanol at 955 cm-1, Si-O-Si symmetric

154

vibrations and bending vibrations at 800 and 470 cm-1, respectively (Figure 2c). FTIR of

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SECuAlHT has all the IR bands of silica except the IR bands of CuAlHT which confirms the

156

silica encapsulation. Silica is well dispersed on the surface of CuAlHT in SECuAlHT.

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Figure 2 FTIR of catalysts: (a) Silica (b) CuAlHT (c) Fresh SECuAlHT and (d) Reused

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SECuAlHT

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Nitrogen adsoption-desoption analysis of silica, SECuAlHT, and CuAlHT has shown IV

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type isotherm and hysteresis of type H1 (Figure 3). Mesoporous material having uniform tubular

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capillary pores open at both the ends display such type of isotherm. Table 1 shows the physical

164

properties of silica, SECuAlHT, and CuAlHT. BET surface area, pore size, and pore volume of 8 ACS Paragon Plus Environment

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SECuAlHT were 55.5 m2g-1, 5.16 nm, and 0.072 mL g-1, respectively. Due to dispersion of silica

166

on the surface of CuAlHT; surface area of SECuAlHT was increased. Pores are distributed from

167

2 nm to 5 nm (Figure 4). The catalyst SECuAlHT is mesoporous material with uniform capillary

168

pores.

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Table 1 Physiorption analysis of Silica, CuAlHT, fresh SECuAlHT, reused SECuAlHT BET surface

Langmuir surface

BJH pore volume ( BJH average pore

area (m² g-1)

area (m² g-1)

mL g-1)

size (4 V/A) Å

Silica

5.4

8.0

0.022

113.7

CuAlHT

23.4

34.5

0.11

134.4

55.5

83.0

0.072

51.6

54.9

81.5

0.070

51.5

Catalyst

Fresh SECuAlHT Three-time reused SECuAlHT 170



100

Quantity Adsorbed (A. U.)

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80

60

(c) 40

(b) 20

(a) 0 0

0.2

0.4

0.6

0.8

1

Relative Pressure (P/P0) 171 172

Figure 3 N2 adsorption-desorption isotherm: (a) Silica (b) SECuAlHT and (c) CuAlHT

173


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0.0018 0.0016 0.0014

Pore Volume (cm3/g.A)

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0.0012 0.001 0.0008 0.0006 0.0004 0.0002 0 0

50

100

150

200

250

300

Pore Diameter (Å)

174 175 176

Figure 4 Pore size analysis of catalyst SECuAlHT

177 178

TGA-DSC of catalyst is shown in Figure 5. Weight loss obtained from 30 to 170 °C is

179

due to the loss of moisture content of catalyst, and then from 170 to 360 °C is due to the change

180

of hydroxide phase into oxide phase. Above observations were confirmed by endothermic and

181

exothermic peaks at first and second weight loss, respectively. Above 400 °C, significant weight

182

loss was not observed which indicates that the catalyst is thermally stable.



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Figure 5 TGA of catalyst SECuAlHT

186 187

NH3-TPD showed a peak at 220 °C (Figure 6) of concentration 1.01 mmol g-1 which

188

indicates that SECuAlHT catalyst consists of weak acidic sites due to alumina. CO2-TPD showed

189

a peak at 150 °C (Figure 7) of concentration 0.38 mmol g-1 which indicates that the catalyst

190

consist of weak basic sites due to copper oxide. In TPR, a peak was obtained at 250 °C (Figure

191

8) which means that the catalyst was reduced at that temperature with H2 intake capacity 0.39

192

mmol g-1. This peak is due to the reduction of copper oxide into copper.

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0.012

TCD signal (a.u)

0.01 0.008 0.006 0.004 0.002 0 0

100

200

300

400

500

Temperature (°C)

194

Figure 6 NH3 -TPD of catalyst SECuAlHT

195 196 197

0.004

0.003 TCD signal (a.u)

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0.002

0.001

0 0

100

200

300

400

500

Temperature (°C)

198 199

Figure 7 CO2-TPD of catalyst SECuAlHT

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203

0.02

0.01 TCD signal (a.u)

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0

-0.01

-0.02 0

100

200

300

400

500

Temperature (°C)

204

Figure 8 TPR of SECuAlHT catalyst

205 206 207

TEM analysis of SECuAlHT is shown in Figure 9. It shows spherical silica particles and

208

encapsulated CuAlHT particles. It might be that dark spheres are of CuAlHT while faint spheres

209

are of silica.

210

500 nm

200 nm

211 212 213

Figure 9 TEM of catalyst SECuAlHT


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214

SEM analysis of SECuAlHT is shown in Figure 10. It shows spheres as well as

215

aggregates of silica. Elemental composition of the catalyst by EDXS (energy dispersive X-ray

216

spectroscopy) analysis is represented in Table 2 which shows the presence of copper, aluminum,

217

silica and oxygen as expected.

218

Figure 10 SEM of catalysts: SECuAlHT

219 220 221

Table 2 Composition of elements in SECuAlHT; fresh, reused by EDXS Catalyst

Mass % O 47.07 47.17

Fresh Reused

Al 0.58 0.68

Si 37.61 37.59

Cu 14.75 14.55

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3.2 Effect of various parameters

225

3.2.1 Agitation Speed

226

To observe the reaction conversions at different agitation speed; the reaction mixture was

227

stirred at 400, 600 and 800 rpm. At 600 rpm and 800 rpm, the same conversions were observed

228

indicating absence of mass transfer resistance at which the mass transfer of reactant molecules

229

from solvent phase to the surface of catalyst and product molecules from the surface of catalyst



230

to solvent phase take place effectively (Figure 11). So in all further reactions, speed of agitation

231

was maintained at 600 rpm. 90 80 70 60 Conversion (%)

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50 40 30 20 10 0 0

232

120

240 Time (min)

360

480

233

Figure 11 Effect of speed of agitation. Catalyst- SECuAlHT, VMA-50 µmol, Catalyst loading-

234

0.006 g mL-1, Temperature-100 °C, Duration- 8 h, Solvent- water, Total volume-15 mL, 800 rpm

235

( ), 600 rpm ( ), 400 rpm ( )

236 237

3.2.2 Catalyst loading

238

Catalyst loading was studied at 0.002, 0.004 and 0.006 g mL-1. At 0.006 g mL-1, the

239

highest conversion (81 %) was observed while at 0.002 g mL-1, the lowest conversion (55 %)

240

was observed (Figure 12). So 0.006 g mL-1 was the catalyst loading for all further reactions.

241

Initial rate of reaction was directly proportional to the loading of catalyst (Figure 13). So there

242

was a proportional increase in catalyst sites with catalyst loading. Thus, the result indicates the

243

absence of intra particle diffusion resistance and external mass transfer resistance. The Wiesz-


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244

Prater modulus was calculated, as discussed earlier29-31 from the observed rate to find that it was

245

far less than one, thereby confirming the absence of intra-particle diffusion limitation. 90 80

Conversion (%)

70 60 50 40 30 20 10 0 0

120

246

240 Time (min)

360

480

247

Figure 12 Effect of catalyst loading. Catalyst- SECuAlHT, VMA-50 µmol, Speed of agitation-

248

600 rpm, Temperature-100 °C, Duration- 8 h, Solvent- water, Total volume-15 mL, 0.002 g mL-1

249

( ), 0.004 g mL-1 ( ), 0.006 g mL-1 ( ) 8 Initial rate X 109 (mol ml-1 min-1)

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y = 1182.1x R² = 0.98

7 6 5 4 3 2 1 0 0

0.002 0.004 Catalyst loading (g mL-1)

0.006

250



251

Figure 13 Plot of initial rate versus catalyst loading, Catalyst- SECuAlHT, VMA-50 µmol,

252

Speed of agitation-600 rpm, Temperature-100 °C, Solvent- Water, Total volume-15 mL

253 254

3.2.3 Concentration of VMA

255

Concentration of VMA was varied from 3.3 µmol mL-1 to 13 µmol mL-1. At a

256

concentration 3.3 µmol mL-1; the highest conversion (81 %) was observed while at concentration

257

13 µmol mL-1; the lowest conversion (40 %) was observed (Figure 14). % Conversion was

258

decreased with increased in concentration of VMA in the reaction mixture. At the same reaction

259

conditions, number of catalyst sites remain constant which can convert same quantity of

260

molecules per unit time. As concentration of VMA increases, number of reacting molecules

261

increases which results in decrease in conversion. The conversion decreases with concentration

262

due to zero order behavior as will be explained after development of kinetic model. In all further

263

reactions, 3.3 µmol mL-1 was chosen concentration of VMA. 90 80 70 Conversion (%)

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60 50 40 30 20 10 0 0

264

120

240 Time (min)

360

480

265


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Figure 14 Effect of concentration of VMA. Catalyst- SECuAlHT, Speed of agitation-600 rpm,

267

Catalyst loading-0.006 g mL-1, Temperature-100 °C, Duration- 8 h, Solvent- water, Total

268

volume-15 mL, 3.3 µmol mL-1 ( ), 6.7 µmol mL-1 ( ), 13 µmol mL-1 ( ).

269 270 271

3.2.4 Temperature

272

Reaction mixture was heated at different temperatures from 80 °C to 110 °C. The highest

273

conversion (94 %) was observed at 110 °C while the lowest conversion (42 %) was observed at 8

274

0°C (Figure 15). Conversion was found to increase with increase in temperature. Inspection of

275

the conversion against time data in Figure 15 show that the reaction appears to be a zero order

276

reaction since conversion is linear with time. This will be discussed later. All further reactions

277

were carried out at 100°C.

278 100 90 80 70 Conversion (%)

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60 50 40 30 20 10 0 0

120

240 Time (min)

360

480

279



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280

Figure 15 Effect of temperature. Catalyst- SECuAlHT, VMA-50 µmol, Speed of agitation-600

281

rpm, Catalyst loading-0.006 g mL-1, Duration- 8 h, Solvent- water, Total volume-15 mL, 80°C (

282

), 90°C ( ), 100°C ( ), 110°C ( ).

283 284

3.2.5 Reusability study of the catalyst SECuAlHT

285

Separated catalyst from reaction mixture was washed with 10 mL water for three times

286

and then with 10 mL methanol to remove any adsorbed material. The catalyst was dried at 100

287

°C for 12 h and used for reusability study. Fresh catalyst and regenerated catalyst gave almost

288

similar conversion (81 %) and selectivity (89 %). The same procedure was repeated for three

289

times to observe the same results (Figure 16). Three-time used and regenerated catalyst was

290

characterized by FTIR (Figure 2), physiorption analysis (Table 1, Figure S3), chemisorption

291

analysis (Table 3, Figure S4-S6) and XRD analysis (Figure S7) techniques to verify the

292

properties of the reused catalyst in comparison with the fresh catalyst. XRD pattern of

293

regenerated catalyst was identical with the fresh catalyst. All the properties of fresh catalyst

294

remain the same in the regenerated catalyst and so catalyst is reusable.


Page 21 of 33

100 90 80 % conversion/ % selectivity

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


70 60 50 40 30 20 10 0 Fresh

295

1st reuse

2nd reuse

3rd reuse

296

Figure 16 SECuAlHT catalyst reusability studies. VMA-50 µmol, Speed of agitation-600 rpm,

297

Catalyst loading-0.006 g mL-1, Temperature-100 °C, Duration- 8 h, Solvent- water, Total

298

volume-15 mL, % conversion ( ), % selectivity ( ).

299 300

Table 3 Chemisorption analysis of fresh and reused SECuAlHT catalyst Catalyst Fresh Reused

Acidity in mmol g-1 1.01 1.11

Basicity in mmol g-1 0.38 0.37

H2 intake capacity in mmol g-1 0.39 0.37

301 302

3.2.6 Effect of N2, effect of O2 and effect of time

303

Reaction was carried out in air, N2 and O2 atmosphere to study the possible reaction

304

mechanism (Figure 17). Reaction progress in air was compared with N2 and O2 atmosphere.

305

When reaction was done in N2 atmosphere, the same conversions were observed up to 240 min

306

in comparison with the reaction in air. After 240 min, the conversion started decreasing which

307

indicate that oxygen was required for oxidation. And also it indicated that oxygen atoms on



308

catalyst were used and oxygen in air helps in oxidation of the reduced catalyst. However,

309

reaction conversions were similar to those in O2 atmosphere in comparison with reaction in air

310

atmosphere which supports the former conclusion. When reaction was carried out in air at

311

similar conditions for 12 h then 94 % conversion and 88 % selectivity was observed.

312

Concentration profile for conversion of VMA to vanillin and vanillic acid is shown in Figure 18.

313

This is a series reaction and thus the reaction time could be optimized to get maximum yield of

314

vanillin which is the intermediate. 100 90 80 70 Conversion (%)

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

Page 22 of 33

60 50 40 30 20 10 0 0

315

120

240

360 480 Time (min)

600

720

316

Figure 17 Effect of N2, O2, air and time. SECuAlHT, VMA-50 µmol, Speed of agitation-600

317

rpm, Catalyst loading-0.006 g mL-1, Temperature-100 °C, Duration- 8 h, Solvent- water, Total

318

volume-15 mL, In 12 h ( ), In O2 ( ), In air ( ), In N2 ( ).


Page 23 of 33

3.5

3

Concentration, mol ml-1 x 106

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


2.5

2

1.5

1

0.5

0 0

60

120

180

240

300

360

420

480

Time (min)

319 320

Figure 18 Concentration profile for conversion of VMA to vanillin and vanillic acid. Catalyst-

321

SECuAlHT, VMA-50 µmol, Speed of agitation-600 rpm, Catalyst loading-0.006 g mL-1,

322

Temperature-100 °C, Duration- 8 h, Solvent- Water, Total volume-15 mL, VMA ( ), vanillic

323

acid ( ), vanillin ( )

324 325

3.3 Reaction Mechanism

326

SECuAlHT is a redox catalyst in which CuAlHT is at the core surrounded by silica shell.

327

After calcination of catalyst at 500 ºC for 6 h, mixed copper and aluminium oxide get formed.

328

Redox sites are formed due to copper oxide. A possible mechanism based on redox sites of

329

catalyst is proposed (Scheme 2). Firstly, VMA get adsorbed on the catalyst sites where –H is

330

used by the catalyst from substrate to form carbocation. This intermediate loses –H on the

331

catalyst site to give another an intermediate α-keto acid. This intermediate undergoes

332

decarboxylation on catalyst sites which acts as basic sites to give product. Product gets desorbed



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

Page 24 of 33

333

from the catalytic sites into reaction mixture. Reduced catalytic sites are oxidized by molecular

334

oxygen as shown in Scheme 3.

335 336

Scheme 2 Reaction mechanism for oxidative decarboxylation of VMA over SECuAlHT

337

VMA acid is oxidized to α-keto acid on catalytic sites, cupric oxide to cuprous

338

hydroxide. Cuprous hydroxide is converted to cuprous oxide by losing water. Then cuprous

339

oxide is oxidized by molecular oxygen to cupric oxide. Thus regenerated catalyst is used in next

340

catalytic cycle. The above mechanism is based on Mars van Krevelin mechanism.

341 342 343 344 345 24 ACS Paragon Plus Environment

Page 25 of 33

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346


Step: 1 oxidation of VMA OH

OH O

O

+

+

2CuO

2CuOH

H O

O

HO

O OH

347 348

OH

Step: 2 decarboxylation of α-keto acid OH OH

O

O

+

CO2

O O

350

O

OH

349

H

Step: 3 oxidation of reduced catalyst 2CuOH

Cu 2 O

+

H2 O

351 Cu 2 O

+

2CuO

1/2 O 2

352 353

Overall reaction: OH

OH O

O

+

+

1/2 O 2

CO 2

+

H2 O

O HO

H

O

354

OH

355

Scheme 3 Possible mechanism for regeneration of catalyst by molecular oxygen

356



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

357

Page 26 of 33

3.4 Kinetic model

358

Both molecule of VMA and oxygen are assumed to be adsorbed on actives sites of

359

catalyst. Oxygen molecule dissociate into O atom. After surface reaction between reactants,

360

products get formed which are vanillin and carbon dioxide. It is considered that all the reacting species were weakly adsorbed and rate of reaction is

361 362

controlled by surface reaction.

363

rO2 = −

364

Where rO2 is the rate of oxidation reaction

365

C A0

366

Integration of above equation at constant oxygen partial pressure is,

367

X ACA0 = k1PO2 wt

368

This is the equation for zero order reaction. For a given partial pressure of oxygen and catalyst

369

loading X ACA0 will be linear in time which was stated earlier.

370

XA =

dC A = k1PO2 w dt

(1)

dX A = k1 PO2 w dt

k1PO2 w CA

(2)

(3)

t

(4)

0

371

Rearrangement of eq (3) also shows that fractional conversion will be inversely proportional to

372

initial concentration, which was observed.

373 374

3.5 Validation of kinetic model and evaluation of activation energy

375

Data obtained at different temperatures were used to plot XACA0 verses t which gave an

376

excellent fit thereby supporting the model (Figure 19). Plot of initial rate of reaction at different

377

initial concentration of VMA was made (Figure 20). It shows that the rate of reaction was 26 ACS Paragon Plus Environment

Page 27 of 33

378

independent of initial concentration of VMA and thereby supporting zero order reaction.

379

Activation energy calculated from Arrhenius plot (Figure 21) was 8.7 kcal mol-1 which suggests

380

that the reaction is kinetically controlled. 2.5

y = 0.0077x R² = 0.9968

2

CA0*XA*106

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


y = 0.0063x R² = 0.997

1.5

1

y = 0.0045x R² = 0.9997

0.5

y = 0.0029x R² = 0.9986

0 0

50

100

150 200 Time (min)

250

300

350

381 382

Figure 19 Kinetic plots for various temperatures. Catalyst- SECuAlHT, VMA-50 µmol, Speed

383

of agitation-600 rpm, Catalyst loading-0.006 g mL-1, Duration- 8 h, Solvent- water, Total

384

volume-15 mL, 80 °C ( ), 90 °C ( ), 100 °C ( ), 110 °C ( ).



Initial rate X 109 (mol mL-1 min-1)

8 7 6 5 4 3 2 1 0 0

0.2

0.4 0.6 0.8 1 1.2 5 Initial concentration X 10 (mol mL-1)

1.4

1.6

385 386

Figure 20 Effect of initial concentrations of VMA on initial rate of reaction. Catalyst-

387

SECuAlHT, Speed of agitation-600 rpm, Catalyst loading-0.006 g mL-1, Temperature-100 °C,

388

Duration- 8 h, Solvent- Water, Total volume-15 mL -18.4 -18.6 -18.8 y = -4373.9x - 7.2081 R² = 0.99

-19 lnK

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

Page 28 of 33

-19.2 -19.4 -19.6 -19.8 0.00255

389 390

0.0026

0.00265

0.0027 1/T (K)

0.00275

0.0028

0.00285

Figure 21 Arrhenius plot

391 28 ACS Paragon Plus Environment

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392 393


4. CONCLUSION A novel silica encapsulated Cu-Al hydrotalcite

was prepared. The catalyst was

394

characterized by different techniques to explore textural properties, surface area, information

395

about active sites, thermal stability, etc. It was used for oxidative decarboxylation of vanillyl

396

mandelic acid (VMA) to vanillin which uses atmospheric oxygen in base-free condition. Speed

397

of agitation, catalyst loading, concentration of VMA and temperature were found to influence the

398

rate of reaction and theses parameters were tuned to obtain maximum conversion with highest

399

selectivity. Presence of O2 atmosphere in reaction increases the initial rate of reaction but later

400

gives the same rate of reaction as in atmospheric air. Reaction should be carried out for longer

401

periods (12 h) in an atmospheric air at VMA concentration of 50 µmol, speed of agitation of 600

402

rpm, catalyst loading of 0.006 g mL-1 and water as solvent at 100 °C. The catalyst was reused

403

for 4 cycles and gave the same conversion and selectivity. Based on reaction mechanism, kinetic

404

model was developed. Calculated activation energy of reaction was 8.7 kcal mol-1. Vanillin is

405

efficiently synthesised from VMA by green route. It is possible to commercialise this process to

406

produce vanillin economically.

407 408

ACKNOWLEDGEMENTS

409

S.L.B. thankful to the University Grants Commission (UGC) for BSR fellowship. G.D.Y.

410

received support from the R.T. Mody Distinguished Professor Endowment and DST-GOI as J.C.

411

Bose National Fellowship.

412

AUTHOR INFORMATION

413

Corresponding Author

414

Tel: +91-22-3361-1001, Fax: +91-22-3361-1020; +91-22-3361-1002,



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

415

E-mail: [email protected]

416

SUPPORTING INFORMATION STATEMENT

417

Chemicals, techniques for catalyst characterization, assembly for experiment, analysis method,

418

GCMS and 1H NMR of vanillin, and N2 adsorption-desorption isotherm, NH3 TPD, CO2 TPD,

419

TPR, XRD of reused SECuAlHT .

Page 30 of 33

420 421

REFERENCES

422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452

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509


Graphical abstract

510

511


Novel Silica-Encapsulated CuâAl Hydrotalcite ... - ACS Publications

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