chapter v benzoylation reactions -...

Chapter V

BENZOYLATION

REACTIONS

CONTENTS

CHAPTER V: BENZOYLATION REACTIONS

5.1. Introduction 165

5.2. Experimental 169

Catalyst Synthesis 169

Catalyst Characterization 169

Catalytic studies 169

5.3. Results and Discussions 171

Structural and surface properties of ZnO-TiO2

composites

171

N2 adsorption-desorption isotherms and porosity

analysis

173

Transmission Electron Microscopy (TEM) 177

Characterization of Acid Sites 178

TPD Ammonia 178

Pyridine adsorption studies on ZnO-TiO2

nanocomposites

180

5.4. Catalytic activity and selectivity 182

5.4. a) Benzoylation of toluene 182

General Discussion 187

5.4. b) Benzoylation of anisole 190

5.5. Conclusions 194

References 195

165

5.1. Introduction

Benzoylation reaction is an example of Friedel Crafts acylation reaction. Friedel

Crafts acylation of aromatic compounds is one of the most important routes for the

synthesis of aromatic ketones that are in demand for several industrial applications

from petrochemical to pharmaceutical.

Benzoylation constitutes an important class among acylation reactions [1]. Selective

benzoylation of toluene and anisole is of considerable interest due to the commercial

importance of resulting benzophenones as additives (fixative) in the perfumery

industry, photosensitizers, UV light stabilizers in plastics, cosmetics, films etc.

166

The general mechanism for acylation reaction is given as follows

Step 1:

The acyl halide reacts with Lewis acid to form a more electrophilic C, an acylium ion

Step 2:

The p electrons of the aromatic C=C bond act as a nucleophile, attacking the

electrophilic C+. This step destroys the aromaticity giving the cyclohexadienyl cation

intermediate.

Step 3:

Removal of the proton from the sp3 C bearing the acyl- group re-forms the C=C bond

and the aromatic system, generating HCl and regenerating the active catalyst.

Scheme 5.1: Mechanism for the Friedel Crafts acylation of benzene

167

Conventionally aromatic ketones are produced by electrophilic acylation of aromatic

compounds with various acylating agents like acyl halide, acid anhydride by

homogeneous acid catalysts AlCl3, BF3, FeCl3, ZnCl2, SnCl4, TiCl4, CF3SO3H,

FSO3H, HCl3, H2SO4 and HF [2-7]. However the drawbacks of this reaction are

• The acylation reactions require molar amounts of the Lewis acid catalyst

• The Lewis acid catalyst form complexes with both the acylating reagent and

the carbonyl product

• Work up is needed to decompose the complexes

• The catalyst is not reusable

• Difficulty in separation and recovery

• Disposal of spent catalyst, corrosion and high toxicity.

• Catalysts are highly moisture sensitive and hence demand moisture free

solvent, reactants, anhydrous catalyst and also dry atmosphere for their

handling

• Some catalysts are toxic

• In addition, halides of Al being strong Lewis acid also catalyze other

undesirable reactions such as isomerization and trans-alkylation reactions.

168

To overcome these difficulties, solid acid catalyst such as Nafion-H, clay,

heteropolyacids, metal oxides promoted by sulphate ions such as (SO42-/Al2O3,

SO42-/ZrO2, SO4

2-/TiO2, SO4/HfO2, SO4/Fe2O3, SO42-/SnO2), Fe or Ga substituted

H-ZSM 5, HY, H-beta basic In2O3, hydrocalcite anionic clays, Cobalt (II) bromide

have been used for the benzoylation reactions [8-13].

But these catalysts have non shape selective nature with insufficient acidity in some

cases [13]. Sulphate doped metal oxides have received much attention due to its super

acidity, however the sulphur reacting during the reaction, coke deposition at high

temperature, changes in sulphur oxidation state and phase changes from tetragonal to

monoclinic limit the industrial use of these catalysts [9].

We investigated the benzoylation of different substrates toluene, anisole and o-xylene

using non hazardous ZnO-TiO2 nanocomposite catalysts synthesized by a simple

method which resulted in high yield and selectivity towards the desired

benzophenone. The present investigation showed remarkable merits over the reported

benzoylation of toluene using triflic acid functionalized mesoporous Zr-TMS catalyst

[13] and also over benzoylation of anisole using silicotungstic acid modified

mesoporous alumina [14].

169

5.2. Experimental

5.2.1. Catalyst synthesis

The catalysts were synthesized by a procedure similar to that described earlier

(section 2.1) for pure phase TiO2. To obtain ZnO-TiO2 nanocomposites, TiO2 is

obtained insitu in presence of ZnO using calculated quantities of TiCl3, HNO3 and

ZnO such that the final samples would contain 10 %, 20 %, 40 % and 70 % ZnO. The

catalysts were designated as Z1, Z2, Z3 and Z4 respectively.

5.2.2. Catalyst Characterization

The catalysts were characterized by XRD, TEM, surface area, porosity, TPD

ammonia, N2 adsorption-desorption isotherm and by FTIR with and without pyridine

adsorption.

5.2.3. Catalytic studies

The liquid phase catalytic benzoylation reactions of toluene, anisole and o-xylene

were performed in a 20 mL round bottom flask fitted with a condenser, using 1 mL

nitrobenzene as solvent in presence of catalyst as shown in scheme 5.2.

Scheme 5.2: Reaction set up for benzoylation reactions

170

Table 5.1 shows the pre-optimized reaction conditions used for the benzoylation

reactions.

Table 5.1: Pre-optimized reaction conditions used for benzoylation of different

substrates

Proportions of acylation mixtures

Substrate toluene anisole o-xylene

Acylating agent p-toluoyl chloride p-toluoyl chloride benzoyl chloride

Molar ratio 1:1 1:1 2:1

Temperature (oC) 130 120 138

171

5.3. Results and discussions

5.3. 1. Structural and surface properties of ZnO-TiO2 composites

The powder XRD patterns of the ZnO-TiO2 nanocomposites used for the

benzoylation reaction are shown in Figure 5.1.

Figure 5.1: XRD patterns of the ZnO-TiO2 nanocomposites Z1 (10% ZnO),

Z2 (20% ZnO), Z3 (40% ZnO), Z4 (70% ZnO) used for the benzoylation reactions

Catalyst Z1 and Z2 containing 10 % and 20 % ZnO respectively showed a crystal

phase in which the major component was TiO2 of anatase type. Catalyst Z3 and Z4

with 40 % and 70 % ZnO respectively showed unusual XRD profiles which did not

conform to TiO2 or ZnO types suggesting possible formation of new phase. All the

catalysts showed crystallite size in the range 10-50 nm.

20 30 40 50 60 70 80

A

R

R

AAAA

A

Z

Z

**

**

*

**

*

*

*

*

*

*

Z ZnO

A Anatase TiO2

R Rutile TiO2

* ZnTiO3

70% ZnO

40% ZnO

20% ZnO

10% ZnO

Z4

Z3

Z2

Z1

Inte

nsit

y (

a. u

.)

2 theta

172

It can be seen from Table 5.2 that the surface areas as well as Scherrer crystallite

sizes showed an interesting trend. Thus the particle size increased from Z2 to Z4 and

the surface areas progressively decreased. The catalyst Z4, 70 % ZnO(TiO2) showed

maximum crystallite size 45 nm and minimum surface area 3 m2/g. This suggested

that the two components have possibly interacted during the synthesis stage to form a

new phase possibly ZnTiO3. This is further elaborated in section 5.4.1. As we shall

see therein, that this catalyst would exhibit highest activity in spite of large particle

size and very low surface area.

Table 5.2: Structural properties of the ZnO-TiO2 composites

Sr. No.

Code Method % ZnO Scherrer crystallite

size (nm)

BET surface area (m2/g)

1 Z1 (TiCl3 : HNO3) + ZnO 10 14 34

2 Z2 (TiCl3 : HNO3) + ZnO 20 13 45

3 Z3 (TiCl3 : HNO3) + ZnO 40 16 16

4 Z4 (TiCl3 : HNO3) + ZnO 70 45 3

5 ZC* - 100 42 42

6 TiO2 (TiCl3 : HNO3) 9 36

* ZC commercial ZnO included in the table for comparison

173

5.3.2. N2 adsorption-desorption isotherms and porosity analysis

Figure 5.2 to 5.5 shows the N2 adsorption-desorption isotherms and BJH pore

analysis of the synthesized ZnO-TiO2 composites.

Figure 5.2a: N2 adsorption desorption isotherm Figure 5.2b: Pore size distribution of Z1 of Z1


0 10 20 30 40 50 60 700.000

0.003

0.006

0.009

0.012

dV

/dD

(c

m3 /g

.nm

)

pore diameter (nm)

0.0 0.2 0.4 0.6 0.8 1.0

0

30

60

90

120Z1 [10% ZnO(TiO

2)]

qu

an

tity

ad

so

rbed

(c

m3/g

)

relative pressure (P/Po)

0 10 20 30 40 50 60 700.000

0.003

0.006

0.009

0.012

dV

/dD

(c

m3 /g

.nm

)

pore diameter (nm)

0.0 0.2 0.4 0.6 0.8 1.00

30

60

90

120

150 Z2 [20% ZnO(TiO2)]

qu

an

tity

ad

so

rbe

d (

cm

3 /g)


174



0.0 0.2 0.4 0.6 0.8 1.00

20

40

60

Z3 [40% ZnO(TiO2)]

qu

an

tity

ad

so

rbe

d (

cm

3/g

)


0 10 20 30 40 50 60 700.000

0.001

0.002

dV

/dD

(c

m3/g

.nm

)

pore diameter (nm)

0.0 0.2 0.4 0.6 0.8 1.00

2

4

6

8

10

Z4 [70% ZnO(TiO2)]

qu

an

tity

ad

so

rbe

d (

cm

3 /g)


0 10 20 30 40 50 60 700.0000

0.0001

0.0002

0.0003

dV

/dD

(c

m3/g

.nm

)

pore diameter (nm)

175

The N2 adsorption desorption isotherms generally showed H3 type hysteresis with

average pore diameter ~ 15 nm. No clear hysteresis was observed in Z4 as compared

to other samples (Figure 5.5a).

Figure 5.6 shows overlays of porosity profiles. It is clear that Z4 in comparison to

other samples is practically non porous.

0 10 20 30 40 50 60 70

0.000

0.004

0.008

0.012

dV

/dD

(cm

3 /g.n

m)

pore diameter (nm)

Z1 10% ZnO(TiO2)

Z2 20% ZnO(TiO2)

Z3 40% ZnO(TiO2)

Z4 70% ZnO(TiO2)

Figure 5.6: Overlays of porosity profiles of various ZnO-TiO2 composites

The results of BJH pore analysis of the above composites is summarized in Table 5.3.

176

Table 5.3: Results of pore analysis of the synthesized ZnO-TiO2 composites

Sr.

No.

Code Method % ZnO Scherrer

crystallite

size

(nm)

BET

surface area

(m2/g)

Pore

volume

(cm3/g)

Pore

diameter

(nm)

1 Z1 (TiCl3 : HNO3) + ZnO 10 14 34 0.19 18

2 Z2 (TiCl3 : HNO3) + ZnO 20 13 45 0.24 16

3 Z3 (TiCl3 : HNO3) + ZnO 40 16 16 0.10 21

4 Z4 (TiCl3 : HNO3) + ZnO 70 45 3 0.01 25

177

5.3.3. Transmission Electron Microscopy (TEM)

The morphology of ZnO-TiO2 samples studied for benzoylation was investigated by

TEM. Figure 5.7 shows the TEM image of sample Z4. It shows that the sample is

composed of several nanoparticles with average diameter of 50 nm. The formation of

nanosize components was thus confirmed by TEM measurements.

Figure 5.7: TEM image of sample Z4 70 % ZnO(TiO2)

178

5.3.4. Characterization of Acid Sites

(a) TPD Ammonia: In order to gain an understanding of the acidity of the catalyst, the

desorption of NH3 was carried out in several stages in the range 120 oC to 450 oC

after allowing the catalyst to adsorb ammonia at room temperature and flushed with

nitrogen to remove physisorbed ammonia. The resulting TPD profiles are shown in

Figure 5.8.

Figure 5.8: TPD profiles of various ZnO-TiO2 catalysts (A - region of weak acid

sites, B - region of medium strength acid sites, C – region of strong acid sites).

*The inset table shows the concentration of acid sites in the temperatue range 225-

325 oC (µmol/g)

179

As mentioned earlier the ammonia desorption peaks which occur at relatively low

temperature usually before 200 oC are attributed to weak acid sites, after 350 oC to

strong acid sites and the peaks in between are due to acid sites of moderate strength.

Thus the catalyst Z3 showed very low acidity. The acidity in case of Z1 and Z2 is

mainly due to the presence of weak acid sites, whereas the acidity in case of Z4 is

mainly due to the acid sites of moderate strength.

180

(b) Pyridine adsorption studies on ZnO-TiO2 nanocomposites

Pyridine was used as probe molecules to characterize the type of acid sites, Lewis or

Bronsted by infra red spectroscopy. Adsorption of pyridine is known to give

characteristic ir absorption peaks corresponding to the acid sites as discussed in

section 4.3.1. Figure 5.9, gives the FTIR spectra of Z4 catalyst with and without

pyridine adsorption. Table 5.4 shows the characteristic ir absorption frequencies

following adsorption of pyridine on catalyst Z4.

1000 1200 1400 1600 1800 2000

Z4 350

Z4 250

Z4 120

Z4 RT

171

1

Z4(without pyridine)

16

57

14

00

137

913

58

1605

12

43

10

151

044

106

9

1161

121

6 157

3

148

61

45

0

% T

(a. u

.)

wavenumber (cm-1)

Figure 5.9: IR spectra of Z4, 70 % ZnO(TiO2) with adsorbed pyridine

181

The intense band at 1450 cm-1 and 1605 cm-1 as well as the weak band at 1217 cm-1

are attributed to the presence of strong Lewis acid sites (Ls) [15-17]. The weak band

at 1570 cm-1 indicates the presence of Bronsted acid site due to the presence of

pyridinium ion PyH+ [15,18]. The band observed at 1485 cm-1 [15,17,18], is the

combination band and has contributions from both Bronsted and Lewis acid sites.

Tabe 5.4: IR absorption frequencies following adsorption of pyridine on catalyst Z4

ir frequencies (cm-1) Assignments

1217 w Ls (strong Lewis acid sites)

1450 vs Ls

1485 w L + B combination peak

1570 vw B

1605 vs Ls

Where w-weak, vw-very weak, vs-very strong

Further the pyridine adsorbed samples were heat treated from ambient to 450 oC. The

spectra showed absorption at 1450 cm-1 due to Lewis acid sites, at 1573 cm-1 due to

Bronsted acid sites and 1486 cm-1 due to combination peak. All these peaks were

present upto 250 oC. This suggested that both Lewis and Bronsted acid sites in the

catalyst bound to pyridine were of moderate strength.

182

5.4. Catalytic acivity and selectivity

a) Benzoylation of toluene

The following reaction has been investigated

O

CH3CH

3

OCH3

CH3

2,4'-DMBP

4,4'-DMBP

CH3 + CH

3

O

Cl

catalysts, 130 oC

Liquid phase

toluene p-toluoyl chloride

Others

Scheme 5.3: Reaction of toluene with p-toluoyl chloride

The product 4,4′-dimethyl benzophenone (DMBP) is of considerable importance in

industry owing to its use as additive (fixative) in the perfumery industry,

photosensitizer, UV light stabilizers in plastics, cosmetics, films etc. [13].

The reaction was carried out at 130 oC in a round bottom flask, with

toluene: p-toluoyl chloride ratio (1:1) in presence of catalysts Z1, Z2, Z3, Z4 and Z4a.

The conversion of toluene as a function of reaction time over various catalysts is

given in Figure 5.10.

183

Increasing reaction time increased the conversion of toluene over all the catalysts. Z4

catalyst showed considerably superior performance throughout the reaction and its

activity is found to be higher compared to those of other catalysts.

0 1 2 3 4 5 6

20

40

60

% c

on

vers

ion

of

tolu

en

e

time (h)

Z1

Z2

Z4

ZC

Figure 5.10: Conversion of toluene (wt %) vs reaction time over various catalysts

184

In the preliminary investigation pure ZnO and TiO2 were chosen for the benzoylation

reaction. TiO2 was surprisingly inactive for the benzoylation reaction, although earlier

investigation with TiO2 for Friedel Crafts acylation was highly encouraging [19].

On the other hand ZnO showed high activity ~ 47 %. Subsequent investigation was

carried out with ZnO-TiO2 composites and the results were quite interesting as

depicted in Table 5.5.

The main product of the reaction is 4,4′-DMBP is separated by column

chromatography and confirmed by 1H and 13C NMR.

13C NMR(CDCl3,50 MHz) � 21.3(-CH3), 194.31(-CO), 128.7(3,3′,5,5′),

130.2(2,2′,6,6′), 135.4(1,1′), 142.1(4,4′).

1H NMR(CDCl3,200 MHz) � 2.44(s, 6H), 7.30(d, 4H), 7.68(d, 4H).

185

The activities of various catalysts are compared under identical reaction conditions

using the data after 6 h run in Table 5.5. The conversion of toluene, rate of toluene

conversion and product distribution depended on the type of catalysts used.

Table 5.5: Activity selectivity profiles of ZnO – TiO2 composites for benzoylation

of toluene

Sr.

No.

Catalyst

Code

Composition Toluene

Conversion

(wt %) a

Product distribution (wt%) b

2,4′- 4,4′- Others

DMBP DMBP

1 Z1 10 % ZnO(TiO2) 20 22 78 -

2 Z2 20 % ZnO(TiO2) 45 20 77 3

3 Z3 40 % ZnO(TiO2) 23 23 77 -

4 Z4

Z4 (a)

70 % ZnO(TiO2) 60

59

22

22

75

72

3

6

5 ZC ZnO 47 22 74 4

6 TiO2 TiCl3 : HNO3 0 - - -

7 without

catalyst

- 0 - - -

a Reaction Conditions : Catalyst (g) = 0.05; Toluene (mmol) = 1; p-toluoyl chloride (mmol) = 1; Nitrobenzene (mL) = 1; Reaction temperature (oC) = 130; time (h) = 6 b 2,4′-DMBP = 2,4′-dimethyl benzophenone 4,4′-DMBP = 4,4′-dimethyl benzophenone

186

As can be seen from the Table 5.5, Z4 catalyst is found to be more active than any

other catalysts giving 60 % conversion among the catalysts investigated. The product

distribution pattern indicate that all the ZnO-TiO2 catalysts show selectivity to mainly

ortho- and para- substituted products 2,4′ and 4,4′- dimethyl benzophenones.

The selectivity for 4,4′-DMBP (para) is high for all the catalysts, followed by 2,4′-

DMBP (ortho) as seen in Figure 5.11.

Z1 Z2 Z3 Z4 Z4a ZC0

10

20

30

40

50

60

70

80

pro

du

ct

dis

trib

uti

on

catalysts

ortho

para

others

Figure 5.11: Selectivity patterns of the various catalysts toward the yield

187

5.4.1. General Discussion

As mentioned in the preceding section, TiO2 was inactive for benzoylation while ZnO

by itself showed significant activity (~ 47 %) for the reaction. Thus the ZnO-TiO2

catalyst with only 10 % ZnO was active catalyst with a maximum of 20 % yield. This

at first appears to imply that the presence of TiO2 has probably masked the activity of

ZnO. However considering the fact that a mere 10 % ZnO in the composite catalyst

could produce 20 % yield as compared to 47 % yield with a 100 % ZnO catalysts, one

could foresee a possible synergy between the components of the composite catalysts.

It was thus concluded that TiO2 acts as a good active support of the composite

catalyst.

This was confirmed when the reaction was carried out with 20 % ZnO(TiO2). Thus

pure ZnO (i.e. 100 % ZnO) and 20 % ZnO on TiO2 had the same effect on the

reaction yield ~ 45 % for both the catalysts. However selectivity to the desired

product 4,4′-DMBP was almost same in all the composite catalysts investigated, and

did not depend on individual activity of the catalysts.

While the synergy between the two components was thus confirmed, the nature of

this effect could not be understood. Thus another catalyst synthesized Z3,

40 % ZnO(TiO2) in fact showed significantly reduced activity which appears to be

due to its greatly reduced porosity (Figure 5.4b). On the other hand catalyst Z4,

70 % ZnO(TiO2) showed enhanced activity and selectivity. The activity of this

catalyst was much more than when pure or 100 % ZnO was used.

It was earlier shown that the Z4 catalyst had large crystallite size, very low surface

area and highest activity (60 % conversion) among the catalysts investigated.

188

It was thus believed that during synthesis of Z4, the components ZnO and TiO2 might

have interacted to form a new phase ZnTiO3. Hence benzoylation was investigated by

using a separately synthesized catalyst Z4a. This catalyst was prepared to have the

same composition as that of Z4 i.e. ZnO-TiO2 (7:3), but commercial ZnO was not

used for its synthesis. Instead ZnTiO3 was generated insitu by use of Zn(NO3)2, TiCl3

and HNO3 followed by evaporation to dryness and calcining the residue at 450 oC.

The resulting catalyst was ZnTiO3 as confirmed by XRD analysis (Figure 5.12) and in

agreement with the JCPDS No. (14-0033).

This catalyst Z4a interestingly gave almost the same conversion (59 %) as Z4

composite catalyst (Table 5.5).

20 30 40 50 60 70 80

* ****

**

*

**

*

*

*

* ZnTiO3

*

Z4a

Inte

nsit

y(a

. u

.)

2 theta

Figure 5.12: X-ray diffraction pattern of Z4a (a few peaks due to ZnO and TiO2 were also detected)

189

The high activity of this catalyst in spite of low surface area is believed to be due to

availability of appropriate active acid sites.

Inspection of TPD data (Figure 5.8) and pyridine adsorption (Figure 5.9) suggest that

the high activity of Z4 could be attributed to the acid sites of moderate strength.

Further comparison of pyridine adsorbed ir spectra of Z4 and Z4a (Figure 5.9 and

Figure 5.13) indicated that Z4a did not show presence of Bronsted acid sites. Z4a

showed peaks only due to Lewis acid sites and yet equally active as Z4.

Thus it was concluded that the high activity of Z4 and Z4a catalysts was due to Lewis

acid sites of moderate strength.

1000 1200 1400 1600 1800 2000

104

41

069

121

8

1450 16

09

Z4a 350

Z4a 250

Z4a 120

Z4a RT

Z4a (without pyridine)

% T

(a. u

.)

wavenumber (cm-1)

Figure 5.13: IR spectra of Z4a with pyridine adsorbed and heated at different

temperatures and also without adsorbed pyridine

190

b) Benzoylation of anisole

OMe + CH3

O

Cl

catalysts, 120oC

Liquid phase

O

CH3MeO

anisole p-toluoyl chloride 4,4′-MMBP

Scheme 5.4: Reaction of anisole with p-toluoyl chloride

Reaction of anisole with p-toluoyl chloride yields 4-methyl-4′-methoxy

benzophenone [4,4′-MMBP] as the main product along with ortho and meta side

products.

The reaction was carried out at 120 oC in a round bottom flask, with anisole and

p-toluoyl chloride ratio (1:1) in presence of catalysts discussed in the preceding

section. The results of various catalysts are compared under identical reaction

conditions after 3 h run. The main product of the reaction 4-methyl-4′-methoxy

benzophenone (4,4′-MMBP) is separated by column chromatography and confirmed

by 13C and 1H NMR.

13CNMR(CDCl3,50MHz) �21.6(-CH3), 55.5(-OCH3), 113.37(3′,5′), 128.89(3,5),

130.03(2,6), 132.32(2′,6′), 135.53(1,1′), 142.64(4), 163.06(4′), 195.41(-CO),

1H NMR(CDCl3,200 MHz) � 2.43(s,3H), 3.87(s,3H), 6.95(d,2H), 7.26(d,2H),

7.67(d,2H), 7.81(d,2H)

Figure 5.14 shows the 13C NMR spectra of the para product 4-methyl-4′-methoxy

benzophenone (4,4′-MMBP)

191

200 150 100 50 0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

195.4

1

163.0

6

142.6

4

135.5

3132.4

5130.0

3128.8

9

113.5

0

55.5

0

37.3

8

21.6

3

Figure 5.14:13C NMR spectra of 4-methyl-4′-methoxy benzophenone (4,4′-MMBP)

The results of the catalytic activities and selectivities are depicted in Table 5.6. The

main product of the reaction is 4-methyl-4′-methoxy benzophenone (4,4′-MMBP).

192

Table 5.6: Activity selectivity profiles of ZnO – TiO2 composites for benzoylation

of anisole

Sr.

No.

Catalyst

Code

Composition Anisole

Conversion

(wt %) a

Product distribution (wt%) b

ortho meta para

1 Z1 10 % ZnO(TiO2) 68 3.36 0.64 96

2 Z2 20 % ZnO(TiO2) 74 3.15 0.85 96

3 Z3 40 % ZnO(TiO2) 71 3.20 0.80 96

4 Z4

Z4a

70 % ZnO(TiO2) 87

89

2

2

2

2

96

96

5 ZC ZnO 83 5 9 88

6 TiO2 TiCl3 : HNO3 47 3 - 97

7 Without

catalyst

- 30 4 - 96

a Reaction Conditions : Catalyst (g) = 0.05; Anisole (mmol) = 2.5; p-toluoyl chloride (mmol) = 2.5; Nitrobenzene (mL) = 1; Reaction temperature (oC) = 120; time (h) = 3

The table shows that all ZnO-TiO2 composite catalysts show high benzoylation

activity for anisole conversion. Also all these catalysts showed excellent selectivity

towards the para product after 3 h run (~ 96 %). Thus anisole is a more reactive

substrate for benzoylation than toluene (Table 5.5).

193

Further, ZnO commercial showed 83 % activity, TiO2 showed only 47 % activity.

Also the catalyst Z1 which has merely 10 % ZnO in the composite catalyst gave 68 %

conversion, thus causing a clear 21 % increase in activity over that of pure TiO2

catalyst.

Keeping with this trend, 20 % ZnO(TiO2) showed further enhancement in the activity

to 74 %. However catalyst Z3 with 40 % ZnO(TiO2) showed decrease in activity

while catalyst Z4 showed maximum conversion of anisole (average conversion 88 %)

with 96 % selectivity for para product which is higher than when 100 % or pure ZnO

catalyst was used. Also the pure ZnO catalyst showed significantly less selectivity to

the desired para product (~ 88 %). When catalyst Z4a (discussed in section 5.4.1)

was used for the investigation, activity as well as selectivity to product was same or

slightly better. The para isomer is favoured because of its lower energy and the

lowest steric hindrance between the methyl and the entering acyl group.

Similarly these catalysts were also investigated for benzoylation of o-xylene. The

catalyst Z4 continued to be the most active although the overall conversions were

lower than that for anisole and toluene benzoylation.

194

5.5. Conclusions

(i) ZnO-TiO2 nanocomposite catalysts in various proportions of active components

have been synthesized.

(ii) The catalysts were investigated for benzoylation of toluene, anisole and o-xylene

at preoptimised reaction conditions.

(iii) A catalyst of composition 70 % ZnO(TiO2) showed highest benzoylation activity

for all three substrates as well as best selectivity to the desired para benzophenones.

(iv) Z4 catalyst 70 % ZnO(TiO2) nanocomposite showed good activity for the

benzoylation of toluene, anisole and o-xylene as compared to pure ZnO or pure TiO2.

(v) The high conversion and para selectivity using the above catalysts is attributed to

the synergistic effect of ZnO and TiO2 and the formation of new phase ZnTiO3.

(vi) The high para - selectivity could also be attributed to the presence of Lewis acid

sites of moderate strength.

195

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196

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197

Achievements

(i) A nanocrystalline S-doped TiO2 of high photocatalytic activity has been

synthesized by a simple process involving TiCl3, HNO3, thiourea and oxalic acid.

(ii) Developed a rapid synthesis of TiO2 from aqueous solutions of TiCl3 in

presence of small amount of “seed” for the first time.

(iii) A simple synthesis method is also developed for obtaining pure rutile phase

titania by hydrolysis of TiCl3.

(iv) A 70 % ZnO(TiO2) nanocomposite catalyst developed in this work is highly

active and selective for benzoylation of toluene and anisole.

198

Publication:

“A simple method for synthesis of S-doped TiO2 of high photocatalytic activity”,

P.P. Bidaye, D. Khushalani, J.B. Fernandes (Catalysis Letters, 134, 169-174, 2010)

Communicated/ Under Preparation

1. Investigation of ZnO-TiO2 nano composite catalysts for benzoylation of toluene

2. A rapid and facile synthesis method for nanosize rutile phase TiO2

3. A template free synthesis of mesoporous TiO2

Papers presented at National / International Symposia:

1. “Investigation of ZnO-TiO2 nanocomposite catalysts for benzoylation of

anisole” International Conference on Composites and Nanocomposites, 2011,

Mahatma Gandhi University, Kottayam, Kerala.

2. “A new approach for rapid synthesis of TiO2 from aqueous solution of TiCl3”

20th National Symposium on Catalysis 2010, IIT Madras.

3. “Selective synthesis of 4,4′-dimethyl benzophenone on ZnO-TiO2

nanocomposite” RSC West India PhD students symposium 2010, Goa-

University.

4. “Synthesis of S-doped TiO2 of high photocatalytic activity” International

Symposium on Materials Chemistry, 2008, BARC, Mumbai.

chapter v benzoylation reactions -...

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