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Template assisted synthesis of nanocrystalline sulfated titania: Active and robust catalyst for regioselective ring opening of epoxide with aniline and kinetic modeling Akhil V. Nakhate, Suresh M Doke, and Ganapati D. Yadav Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02619 • Publication Date (Web): 13 Sep 2016 Downloaded from http://pubs.acs.org on September 17, 2016

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Industrial & Engineering Chemistry Research

1

Template assisted synthesis of nanocrystalline

2

sulfated titania: Active and robust catalyst for

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regioselective ring opening of epoxide with aniline

4

and kinetic modeling

5

Akhil V. Nakhate; Suresh M. Doke; Ganapati D. Yadav*

6

Department of Chemical Engineering

7

Institute of Chemical Technology

8

Nathalal Parekh Marg, Matunga

9

Mumbai 400019

10

India

11

* Corresponding author Tel.: +91-22-3361-1001; Fax: +91-22-3361-1002,

12

+91-22-3361-1020

13

E-mail address: [email protected]

14 15 16 17 18

ACS Paragon Plus Environment

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ABSTRACT. The regioselective ring opening of epoxides with aniline affording β-amino

20

alcohols is commercially important. A variety of heterogeneous catalysts such as fuel lean

21

sulfated zirconia (FLSZ), UDCaT-5, 20%Cs2.5H0.5PW12/K-10 clay, Hβ-zeolite, nanocrystalline

22

sulfated titania (NCST) and titania were evaluated in the reaction of epichlorohydrin with

23

aniline. Nanocrystalline sulfated titania (NCST) catalyst, the most active and selective, was

24

prepared by the sol-gel method using polyvinyl alcohol (PVA) as a green template and

25

functionalized using sulfuric acid. The sulfate content was in the range 4.1 % (w/w). It was well

26

characterized using TEM, FT-IR, XRD, NH3-TPD, N2-BET surface area and TGA. The effects

27

of various parameters such as speed of agitation, molar ratio, catalyst loading and temperature

28

using NCST catalyst were investigated in the reaction of epichlorohydrin with aniline to achieve

29

good yields and excellent regioselectivity at 60°C and 0.01 g/cm3 catalyst loading. The catalyst

30

was effectively recycled for five consecutive cycles without any significant loss in its activity.

31

Furthermore, a reaction mechanism was proposed to deduce a kinetic model which was validated

32

against experimental data. The entire process is green. The protocol was extended to the reaction

33

of several epoxides and amines over NCST to make corresponding β-amino alcohols.

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KEYWORDS: β-Amino alcohols; Epoxide; Amine; Nanocrystalline sulfated titania; Kinetics;

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Green chemistry

36 37

INTRODUCTION

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Ring-opening of epoxides with amines is an important reaction for the synthesis of β-amino

39

alcohols which are versatile intermediates in pharmaceutical and agrochemical industries and

40

they are widely used in synthesis of biologically active products, such as β-blockers and chiral

41

ligands in asymmetric synthesis.

1–4

They are also used as building blocks for the production of


2

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active pharmaceutical intermediates like Oxycontin, Zyvox, Skelaxin, Coregand Toprol-XL.

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Several reports have appeared on the use of homogeneous catalysts, heterogeneous catalysts and

44

catalysts free synthesis of β-amino alcohols from the reaction of epoxide and amine.

45

Homogeneous catalysts such as porphyrin complex,5 metal halide,6 metal perchlorate,7 metal

46

triflate,8,9 sulfamic acid and ionic liquid.10,11 However, homogeneous catalyst suffer from many

47

disadvantages such as poor regioselectivity, long reaction time, high temperature, use of

48

stoichiometric amount of catalyst, and use of expensive reagents. Hence, there is need to

49

developed more efficient and reusable solid acid catalysts. Recently some heterogeneous

50

catalysts were developed for the synthesis of β-amino alcohols from the reaction of epoxide and

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amine, which included modified alumina,12,13 nano-alumino silicates,14,15 montmorillonite-K10

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clay,16 Amberlyst resin,17 zeolites,14 mesoporous titano silicates (Ti-SBA-12 and Ti-SBA-16),18

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CeO2–ZrO2,19 mesoporous Zr-Beta zeolites,20 zirconium metal organic framework,21 Ni2+-G2

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metallodendrimer sepabeads EB-EP-400,22 Nano Fe3O4,23 nanocrystalline zirconosilicates

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sulfated tungustate,25 sulfated zirconia.

56

synthesis of β-amino alcohols under microwave irradiation

57

reaction time. In other cases, catalyst preparation methods are very tedious which require harsh

58

reaction conditions and hazardous sulfonation source such as chlorosulfonic acid along with

59

chlorinated organic solvent for reaction which results in poor regioselectivity.29 Thus, there is

60

still scope for the synthesis and application of newer solid acid catalysts for the aminolysis of

61

epoxides.

26

24

A few reports are also available on catalyst free 27,28

but it requires a very long

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Metal oxides have been used either as such or as supports in combination with other

63

active components due to their well-known advantages.30–32 Titania has gained much attention

64

owing to its superior properties such as high acidity, excellent thermal stability, high specific


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surface area and activity at milder conditions.18,33 It is revealed that sulfated titania is really

66

excellent solid acid catalyst for organic reactions which have varied industrial applications.34,35

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As part of our ongoing research work on nanocrystalline metal oxides catalysts,36–40 and 40–43

68

application of heterogeneous catalysts in various organic transformations,

we herein report

69

the preparation of nanocrystalline sulfated titania (NCST) by sol-gel method using polyvinyl

70

alcohol (PVA) as a green template. Furthermore, NCST catalyst was investigated in comparison

71

with fuel lean sulfated zirconia (FLSZ), UDCaT-5, 20%Cs2.5H0.5PW12/K-10 clay, Hβ-zeolite and

72

titania as an efficient, reusable, solid acid catalyst for the synthesis of β-amino alcohols from

73

epoxides and aromatic amines providing high yields and selectivity under mild conditions

74

(Scheme 1).

75 76

Scheme 1. Regioselective ring opening of epoxide with aniline.

77 78

RESULTS AND DISCUSSION

79

Catalyst characterization

80

Fourier transform infrared spectroscopy (FTIR) analyses were carried out on FTIR,

81

spectrophotometer (Bruker) Vertex 70 in the range of 400–4000cm−1. Thermo-gravimetric

82

analysis (TGA) was carried out using Shimadzu DTG-60 H, Japan in aluminum pan with

83

temperature ramp of 10 °C/min and nitrogen flow rate of 20 ml/min. Surface area and pore size


4

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distribution were measured by using liquid nitrogen adsorption-desorption technique at -78 K in

85

a Micromeritics ASAP 2020 instrument. The crystallinity of the catalyst was analysed by XRD

86

using Cu-Kα (1.54 Å) radiation in a Bruker AXS diffractometer. Samples were step-scanned

87

from 2θ = 5–90o in 0.010o steps with a stepping time of 17.7 s. Temperature programmed

88

desorption (TPD) was carried out to determine the acidic sites possessed by the catalyst using

89

Auto Chem. II 2910 (Micrometrics, USA) with ammonia as the probe molecule. TEM images

90

were obtained by JEOL-JSM 2100 electron microscope equipped with a thermal electron

91

emission type gun. All images were collected at 200 kV using a multi scan camera.

92 93

Characterization of nanocrystalline sulfated titania (NCST)

94

Since NCST was the most active, selective and reusable catalyst, its characterization is

95

presented here. The FTIR spectra of TiO2, NCST and reused NCST are shown in Figure S1 (a-c)

96

(Supplementary Information). There are strong intense bands at 3396 and 1632 cm-1 due to

97

stretching vibration of hydroxyl group and bending vibration of adsorbed water molecule,

98

respectively, in all samples. S=O symmetric stretching frequency of NCST catalysts at 1146.38

99

and 1220.45 cm-1 and S-O symmetric stretching of sulfate bond with TiO2 at 1049.51 and 980.37

100

cm-1 represents successful incorporation of sulfonic acid moieties on NCST catalysts (b and c).

101

Thus, the 5th recycled NCST maintains its fidelity. The presence of broad absorbance band at

102

672.55 cm-1 in all samples reveals the formation of anatase TiO2.

103

TGA was performed to understand the thermal stability and the sulfur loading on catalyst. The

104

TGA spectra of NCST and pure TiO2 showed one weight loss below 100°C, which corresponds

105

to the loss of physically absorbed water molecule (Figure S2). Another weight loss in TGA

106

spectra of NCST was in the range of 300 °C which corresponds to sulfuric acid decomposition


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107

and sulfur dioxide formation. The amount of sulfuric acid functionalized on TiO2 was calculated

108

as 4.1% (w/w). The sulfur percentage was also confirmed by XPS analysis which was 3.8 %

109

(Table 1). Survey spectra of NCST showed the presence of Ti, with binding energy at 458.9 and

110

464.6 and S with binding energy at 169.4 (Figure S3).

111 112

Table 1. XPS of NCST Element

Mass (%)

Titanium (Ti)

24.12

Oxygen (O)

72.68

Sulfur (S)

3.8

113 114

TiO2 support was prepared by taking different concentrations of PVA ranging from zero

115

to 13.81 wt. % with respect to TiO2. It was observed that with increase in concentration of PVA,

116

the surface area and pore volume of TiO2 support increased. The maximum surface area and pore

117

volume of TiO2 support was found to be at 11.51 wt. % of PVA concentration in TiO2 at 118

118

m2/g and 0.38 cm2/g, respectively (Figure S4), whereas upon sulfation, these values somewhat

119

reduced to 113 m2/g and 0.28 cm3/g, respectively. The BET surface area of NCST is less than the

120

pure TiO2 due to sulfur deposition on the surface of catalyst. The same BET surface area profile

121

was noticed for the 5th recycled NCST demonstrating excellent reusability. Adsorption

122

desorption isotherms for TiO2, NCST and 5th recycled NCST show characteristic type IV

123

isotherm with an H3 is in hysteresis loop of mesoporous materials (Figure 1 a-c).

124


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125 126

Figure 1. Surface area analysis (a) TiO2 (b) NCST (c) 5th recycled NCST

127

The powder X-ray diffractogram revealed that the pure TiO2 was anatase phase (major peaks at

128

2θ values of around 25.2°, 37.8°, 47.9°, and 54.5°) (Figure 2c). The sharp and intense peak at 2θ

129

values of 25.24° corresponds to (101) plane of anatase crystal. NCST also showed the same

130

nature like pure TiO2. It suggested that sulfate ion is highly dispersed on surface of catalyst

131

(Figure 2 b). After sulfonation, no change was observed in anatase form of pure TiO2. XRD

132

pattern for NCST, 5th recycled NCST and pure TiO2 are the same (Figure 2 a).

133 134

Figure 2. XRD of (a) 5th recycled NCST (b) NCST (c) Pure TiO2


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The NH3-TPD profile of NCST and reused NCST was carried out (Figure S5 a-b). The TPD of

136

NCST exhibited the presence of maxima at 132 °C and 250–350 °C which is due to the presence

137

of a large number of acidic sites with strong and intermediate acidic strength. The total acidity of

138

catalyst measured in absorbed ammonia was 0.77 mmol/g. We observed the same NH3-TPD

139

profile for reused NCST after five recycles which illustrates the same acid strength and thus the

140

catalyst exhibits excellent catalytic activity (Figure S5 (a-b)).

141

According to TEM, it was observed that the prepared NCST catalyst is a nano material (∼10 to

142

20 nm) (Figure 3). It was found that the morphology of TiO2 is aggregated with each other

143

during the sulfonation, leading to the formation of large clusters of TiO2 nanoparticles.

144 145

Figure 3. TEM of NCST catalyst (a) Pure TiO2 (100 nm) (b) NCST (100 nm)

146 147

Catalytic activity for regioselective ring opening of epoxides with aniline

148

A variety of catalysts such as TiO2, FLSZ, 20%Cs2.5H0.5PW12/K-10 clay, Hβ- zeolites and

149

UDCaT-5 were evaluated (Table 2, entry 1-6; Scheme 2). Epoxide ring opening reaction requires

150

both Lewis and Brønsted acidity. 20% Cs2.5H0.5PW12/K-10 showed good conversion with


8

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respective limiting reactant but less selectivity because of it’s having all Brønsted acidity. Also

152

Hβ- zeolites has Brønsted acidity but showed less conversion because its acidity is lower than

153

that of 20 % Cs2.5H0.5PW12/K-10. FLSZ and UDCaT-5 have both Lewis and Brønsted sites but

154

FLSZ has higher total acidity than UDCaT-5 and hence FLSZ showed good conversion but less

155

selectivity. NCST having highest total acidity (Lewis and Brønsted) sites among all the catalysts

156

showed the best conversion and selectivity, and also reusability. NCST proves to be the best

157

catalyst providing 93 % conversion with excellent desired product selectivity (Table 2, entry 1).

158 159

Scheme 2. Epoxide ring opening of epichlorohydrin with aniline

160

Table 2. Catalyst activity for Scheme 2a

#

161 162 163

Catalyst

Time (h)

Conversion (%)

Selectivity (%) (E1:E2)

1

NCST

2

93

95:05

2

20% CsDTP-K10

2

87

65:35

3

FLSZ

2

74

70:30

4

UDCaT-5

2

62

73:27

5

Hβ-Zeolite

2

46

94:06

6

TiO2

2

18

93:07

a

Reaction condition: epichlorohydrin (0.024 mol), aniline (0.02 mol), catalyst loading (0.01 g/cm3), speed of agitation (1000 rpm), temperature (60 °C), total volume 20 mL (solvent), internal standard n-decane (1 mL).


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164

We have compared the activity of NCST catalyst with the other reported heterogeneous acid

165

catalysts for epoxide ring opening reaction of styrene oxide with aniline (Table S1;

166

Supplementary Information). NCST is the most active and selective catalyst in comparison with

167

others reported in literature where the quantity of the catalyst used was very high because those

168

catalysts were less active. NCST possesses the highest acidity (Lewis and Bronsted) among all

169

reported so far (Table S1).

170 171 172

SOLVENT STUDY The effect of solvent on the catalytic activity of the ring opening of epichlorohydrin with

173

aniline was also investigated (Table 3 entry 1-6).

Polar solvent such as tetrahydrofuran,

174

acetonitrile and 1,4 dioxane showed good conversion towards the formation of desired product

175

as compared to non-polar solvents such as toluene and benzene. In solvent free reaction we got

176

good conversion and selectivity towards the desired product but the viscosity of reaction mass

177

was noticed to increase and hence periodic sampling became very tedious. The best solvent THF

178

showed that highest rate constant and the reaction was kinetically controlled and hence for all

179

other solvents also the reaction was kinetically controlled. So, further kinetic study was carried

180

out in tetrahydrofuran as solvent.

181

Table 3. Solvent screening for regioselective ring opening of epichlorohydrin with anilinea Solvent

Conversion (%)b

1

Acetonitrile

73

2

1,4 dioxane

56

3

Toluene

32

4

Benzene

22

Entry


10

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182 183 184


5

Tetrahydrofuran

93

6

Solvent free

96

a

Reaction condition: epichlorohydrin (0.024 mol), aniline (0.02 mol), catalyst loading (0.01 g/cm3), speed of agitation (1000 rpm), temperature (60 °C), total volume 20 cm3(solvent), internal standard n-decane (1 mL) time 2h, b GC conversion.

185 186

The effect of the speed of agitation on the reaction rate was studied from 800 to 1200 rpm

187

(Figure S6). The conversion of aniline remains the same after 1000 rpm without any change in

188

regioselectivity. There was no external mass transfer resistance. Hence, all further reactions were

189

carried out at 1000 rpm. A theoretical calculation was also done as delineated in some of our

190

earlier work to ascertain that the system was free from external mass transfer resistance.46,47,50 In

191

the absence of mass transfer resistance, the rate of reaction should be directly proportional to

192

catalyst loading per unit volume which was studied from 0.005 to 0.02 g/cm3 (Figure

193

S7).Conversion of aniline increased with catalyst loading due to proportional increase in the

194

number of catalytic sites. The observed initial rate against catalyst loading was a straight line

195

(Figure S8). Aniline to epichlorohydrin mole ratio was varied at 1:1, 1:1.2, 1:1.4, 1.2:1, 1:2 by

196

keeping the total reaction volume constant. The conversion of the limiting reagent increased with

197

increasing mole ratio (Figure S9). We have used standard second order rate equation with first

198

order in aniline and first order in epichlorohydrin and the integrated form of equation ln((M-

199

XA)/(M(1-XA))) vs. time was plotted for different mole ratios (M) and fractional conversion

200

(XA) to suggest that the data are fitted adequately well (Figure S10). The effect of temperature

201

was studied ranging from 40–70°C for the aminolysis of epichlorohydrin with aniline by keeping

202

the other parameters constant. The conversion of aniline increased substantially with increase in

203

temperature (Figure S11).

204

discussed later.

This suggested a kinetically controlled regime which will be


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205 206 207 208 209

Reaction mechanism and kinetic model A possible reaction mechanism for the aminolysis of epichlorohydrin with aniline is shown (Scheme 3). Two reaction pathways are possible. On the basis of Langmuir-Hinshelwood-Hougen-Watson (LHHWS) type of mechanism.

210 211

Scheme 3. Reaction mechanism

212 213

It is assumed that all sites are of the same strength (S).

214

1. Adsorption of epichlorohydrin (A) on the vacant site ‘S’ is given by:

215 216 217

KA  → A+S ←  AS

(1)

Similarly adsorption of aniline (B) on the vacant site is presented by: KB  → B+S ←  BS

(2)


12

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218

2. Surface reaction of AS with BS, in the vicinity of the site, leading to formation of 1-chloro-3-

219

(phenylamino)propan-2-ol (E1S) and 3-chloro-2-(phenylamino)propan-1-ol (E2S) on the site.

220 221

k1   → E 1S + S AS+BS ← 

(3)

k2   → E 2 S + S AS+BS ←

(4)

222

Desorption of 1-chloro-3-(phenylamino)propan-2-ol (E1S) and 3-chloro-2-(phenylamino)propan-

223

1-ol (E2S)is shown as: 1/K

224

E1  → E1 + S E1S ← 

(5)

1/K

225 226 227 228

2  E  → E2 + S E 2S ← 

(6)

The total concentration of the sites, Ct expressed as, Ct =CS +CAS +CBS +CE1S + CE2S

or,

229

Ct =CS +KACACS +KBCBCS +KE1 CE1 CS +KE2 CE2 CS

230

or, the concentration of vacant sites, CS =

231

(7)

Ct 1+K A CA +K BCB +K E1 CE1 + K E2 CE2

(8)

232

From equation (3), the rate of formation of product E1, assuming reaction to be irreversible, is

233

given by, dC E1

234 235

dt

237

(9)

By substituting value of Cs from equation 8 dCE1

236

= k 1C A S C B S

dt

=

Ct2 [k1K A K BCA CB ]

(1+K C A

A

+K BCB +K E1 CE1 + K E2 CE2

)

2

(10)

When the reaction far from equilibrium the above equation become


13


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dCE1

238

dt

=

Page 14 of 30

wk1KAKBCACB

(1+K C +K C +K A

A

B

B

E1

CE1 + KE2 CE2

)

2

(11)

239

Where, w is catalyst loading.

240

The value of reaction rate constant, k1, can thus be evaluated by solving equation (11). We can

241

also obtain values for equilibrium adsorption coefficients for the species A, B, E1 and E2 by

242

solving this equation.

243

Similarly, we can write the rate equation for reaction (4) as dC E2

244 245

dt

=

wk 2 K A K BC A C B

(1+K

A

C A +K BC B +K E1 C E1 + K E 2 C E 2

)

2

(12)

Also the selectivity of E1 over E2 is dependent on temperature alone. dC E1 S E1 / E2 =

246

dC E 2

dt = k1 k2 dt

(13)

247

The apparent orders in aniline and epoxide are each unity as was stated earlier but the

248

adsorption constants could also be used to refine the rate equations. Thus using equation (13) and

249

experimental values of selectivity, the value for kinetic rate constant, k2, can be evaluated by

250

solver (Table 4).

251

The values of the reaction rate constants and the equilibrium adsorption coefficients are

252

presented in Table 4. Using the values of k1 and k2, Arrhenius plots were made for both the

253

reactions under consideration (Figure 4). The values for the activation energy for the two

254

reactions are 18.49 kcal/mol and 9.55 kcal/mol, respectively. It indicates that the reactions are

255

intrinsically kinetically controlled.

256 257 258

From the values of reaction rate constants, k1 and k2, and the subsequent activation energies associated with those reactions, E1 and E2, we observe the following trend (Figure 5): k1> k2 and E1> E2


14

Page 15 of 30

259 260

This implies that to obtain higher yield of product E1, high temperature should be favored, which is in concurrence to the experimental observation. This proves the correctness of the fit.

261 262

Table 4. Rate constant and equilibrium constant for reaction of aniline and epichlorohydrin

k1

k2

T 2

[L /(mol/g-

2

cat/min)]

KA

KB

K E1

K E2

(L/mol)

(L/mol)

(L/mol)

(L/mol)

[L /(mol/g-

(°C) cat/min)]

40

1.6×10-3

2×10-4

0.15

1.6×10-2

9.9×10-3

1.1×10-2

50

4.2×10-3

3×10-4

0.17

1.9×10-2

9.8×10-3

1×10-2

60

1.0×10-2

5×10-4

0.18

2.2×10-2

9.6×10-3

9.9×10-3

70

2.4×10-2

7×10-4

0.20

2.4×10-2

9.3×10-3

9.7×10-3

263 -2

R² = 1

-4 lnk1, lnk2

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


-6 R² = 0.9865 -8

-10 0.0029

0.003

0.0031

0.0032

0.0033

1/T (K-1) lnk1 vs. 1/T

lnk2 vs. 1/T

264 265

Figure 4. Arrhenius plots for both reactions


15


35

Selectivity (SE1/SE2)

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 16 of 30

28

21

14

7

0 0

30

60

90

120

Time (min.) 70° C

266 267

Figure 5. Selectivity

60 °C

S E1 S E2

50 °C

40° C

at different temperatures

268

The analysis shows that the adsorption constants for E1 and E2 (Table 4) are in the following

269

order: K E1

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