135

CHAPTER 6

SOLVENT-FREE SELECTIVE OXIDATION OF

-PINENE OVER Co-SBA-15 CATALYST

6.1 INTRODUCTION

-Pinene is a terpenoid family of organic compound which is

inexpensive, readily available and renewable starting material for the

production of a variety of valuable products such as flavors, fragrances,

medicines and agrochemicals (Erman et al 1985, Bauer et al 1997). The

oxidation products of -pinene such as verbenol, verbenone and -pinene

oxide are important intermediates for the production of fine and specialty

chemicals (Lewis and Hedrick 1965, Wender and Mucciaro 1992). The

oxidative functionalisation of olefins is an important unit operation in the fine

chemical synthesis. However, olefins can be oxidized by different ways such

as allylic C-H bond, epoxidation and oxidative cleavage of carbon-carbon

double bond. The metalperoxo species favor epoxidation of olefins and free

radical species favor allylic oxidation of olefins. It is found that epoxidation

and allylic oxidation of olefins are often competitive reactions which

normally yield mixture of products (Murphy et al 2000). However, allylic

oxidation of olefins through hydrogen abstraction is the dominant reaction.

Traditionally allylic oxidation of olefins is carried out using toxic and

expensive metallic oxidants. The development of a reaction process using

clean oxidant such as hydrogen peroxide is an environmentally acceptable

green process.

136

Allylic oxidation of -pinene was carried out using both

homogeneous and heterogeneous catalyst such as cobalt based compounds

(Lajunen and Koskinen 1994, Lajunen et al 2000, Lajunen et al 2001, Joseph

et al 2002, Lajunen et al 2003, Chakrabarty and Das 2004, Guo et al 2005,

Maksimchuk et al 2007), copper salts (Allal et al 2003), titano-silicates (Morn

et al 2000), Cr-AlPO-5 (Lempers et al1996), Cr-pillered clay (Maksimchuka

et al 2005), Fe and Cr-MIL-101 (Timofeeva et al 2012), and Uranyl-MCM-41

(Selvam et al 2011). Although these catalytic systems used drastic reaction

condition and toxic oxidants, the conversion and selectivity of the product

were low. The development of a heterogeneous catalyst for the selective

oxidation of -pinene using a green oxidant is the major demand in chemical

industry. Heterogeneous catalyst with eco-friendly oxidant was used in

chemical reactions which led to efficient process. Hence, heterogeneous

catalyst with high surface area, ordered pore arrangement and tunable pore

size are good choice in the field of catalysis.

Mesoporous SBA-15 possesses large pore diameter and thicker

wall compared to MCM-41 and MCM-48. However, siliceous SBA-15 do not

find application as catalyst due to lack of active sites. Hence incorporation of

transition metal ions into SBA-15 is a challenge under strongly acidic

condition due to hydrolysis of M-O-Si network. There are many reports on

the direct incorporation of heteroatoms such as Al, Ti, V, Co, Cr, Mn and Fe

into SBA-15 framework by direct method under suitable pH condition

(Vinu et al 2005, Selvaraj and Lee 2006, Chandrasekar et al 2007, Sathish

Kumar et al 2007, Berube et al 2010, Selvaraj et al 2010). Similarly, a few

reports are available on the allylic oxidation of -pinene using mesoporous

supported catalyst (Margolese et al 2000, Selvam et al 2011). However, cobalt

incorporated SBA-15 material has not been attempted in the allylic oxidation

of -pinene.

137

Hydrothermal synthesis of Co-SBA-15 with appropriate pH

adjustment and its catalytic performance in the liquid phase oxidation of

-pinene using H2O2 as the oxidant under solvent-free condition are presented

in this chapter. The reaction parameters such as molar ratio of -pinene/H2O2

and effect of reaction time are also studied in order to improve the conversion

and selectivity of the product. The plausible reaction mechanism is proposed

for the selective oxidation of -pinene. The stability, recyclability and

heterogeneity of the catalyst are also established in this study.

6.2 CHARACTERIZATION OF Co-SBA-15

6.2.1 X-ray Diffraction (XRD)

The small-angle X-ray diffraction patterns of SBA-15 and

Co-SBA-15 (Figure 6.1) exhibited three well resolved peaks corresponding to

(100), (110) and (200) reflections of ordered hexagonal mesopores with space

group of p6mm (Lou et al 2008). The diffraction patterns of Co-SBA-15

samples are similar to that of pure SBA-15. It is interesting to note that the

intensity of peaks increased with increase of metal content due to increased

ordering of mesoporous nature. However, the diffraction patterns of

Co-SBA-15 materials are slightly shifted to lower angle, which is attributed to

expansion of mesopores while increasing the cobalt content. The unit cell

parameters (calculated using the equation ao= 2d100 3) increased with

increase of cobalt content in SBA-15 framework and the results are presented

in Table 6.1. The increase of unit cell parameter (ao) from 10.94 to 11.47 nm

is due to dilation of mesoporous SBA-15 framework. The wall thickness of

Co-SBA-15 materials increased slightly more than that of parent SBA-15.

These results suggest the incorporation of cobalt species in the silica

framework.

138

Figure 6.1 Small angle XRD patterns of (a) SBA-15,

(b) Co-SBA-15(25), (c) Co-SBA-15(50) and

(d) Co-SBA-15(100)

Table 6.1 Textural properties of Co-SBA-15

Sample

Unit cell

parameter

(ao)

Surface

area

(m2/g)

Pore

volume

(cm3/g)

Pore

diameter

(nm)

Pore wall

thickness

(nm)

SBA-15 10.94 648 0.873 7.12 3.82

Co-SBA-15 (25) 11.47 562 0.677 7.50 3.97

Co-SBA-15 (50) 11.43 597 0.725 7.46 3.97

Co-SBA-15 (100) 11.08 628 0.832 7.20 3.88

ao calculated from =2d100 3, Pore wall thickness= ao- Dp

6.2.2 Nitrogen Sorption Studies

The nitrogen adsorption-desorption isotherms of SBA-15 and

Co-SBA-15 are shown in Figure 6.2. The N2 sorption isotherm of SBA-15

exhibited type IV isotherm with H1 hysteresis loop, which is characteristic of

0 1 2 3 4 5 6

Inte

nsi

ty (

a.u

)

2 (degree)

(100)

(200)(110)

(d)

(c)

(a)

(b)

139

0.0 0.2 0.4 0.6 0.8 1.0

(b)

(d)

(c)

Vo

lum

e o

f N

2 a

dso

rp

ed(c

m3

/g, S

TP

)

Relative pressure (p/po)

(a)

well ordered hexagonal mesoporous material. The capillary condensation

showed a sharp inflection in the range of 0.6 0.78 relative pressure, which

revealed uniform mesopores in SBA-15. The cobalt incorporated SBA-15

materials showed isotherms similar to that of SBA-15. The capillary

condensation step slightly shifted to lower range with increase of cobalt

content which clearly indicated the expansion of pores. The textural

parameters viz., surface area decreased from 649 to 562 m2/g and pore

volume decreased from 0.846 to 0.677 cm3/g while pore diameter increased

slightly with increase of cobalt content. These results clearly evidenced the

incorporation of cobalt into SBA-15 framework with a slight effect on the

micropores. The pore size distribution of SBA-15 and Co-SBA-15 are

depicted in Figure 6.3. SBA-15 exhibits a very narrow pore size distribution

while cobalt incorporated SBA-15 materials showed a broad distribution,

which clearly indicated well dispersion of cobalt species into

SBA-15 framework.

Figure 6.2 N2 sorption isotherms of (a) SBA-15, (b) Co-SBA-15(100),

(c) Co-SBA-15(50) and (d) Co-SBA-15(25)

140

Figure 6.3 Pore size distribution of (a) SBA-15, (b) Co-SBA-15(25),

(c) Co-SBA-15(50) and (d) Co-SBA-15(100)

6.2.3 Diffuse Reflectance Ultraviolet-Visible (DRSUV-Vis)

Spectroscopy

The chemical state and coordination environment of cobalt

incorporated SBA-15 were investigated by DRSUV-Vis spectra as shown in

Figure 6.4. The DRSUV-Vis spectra of all Co-SBA-15 materials showed four

distinct absorption peaks at 250, 526, 581 and 665 nm. The peak centered at

250 nm is assigned to O-Co2+

charge transfer transition. The other three peaks

at 526, 581 and 665 nm are the characteristic absorption for4A2

4T1 (P)

transition of Co2+

ion incorporated in tetrahedral coordination into SBA-15

framework (Sexton et al 1986). The intensity of absorption band increased

with increase of metal content thus confirming the increased cobalt content in

SBA-15 framework. This result is yet another strong evidence for the

incorporation of cobalt in tetrahedral coordination into SBA-15.

141

Figure 6.4 UV-DRS spectra of (a) Co-SBA-15(25), (b) Co-SBA-15(50)

and (c) Co-SBA-15(100)

6.2.4 Fourier Transform- Infra Red (FT-IR) Spectroscopy

Figure 6.5 exhibits the FT-IR spectra of SBA-15 and Co-SBA-15

with different Si/Co ratios. The pure SBA-15 exhibited a band around

1057 cm1 which is assigned to asymmetric stretching vibration of Si Si,

and the bands close to 813 cm1 is assigned to symmetric stretching and

deformation modes of Si Si framework. Co-SBA-15 materials showed

additional bands at 970 cm-1

which is attributed to stretching vibration of

Si Co bond. In addition, the intensity of Si-O-Si asymmetric stretching

vibration and deformation modes decreased with increase of cobalt content.

This result also confirmed the incorporation of cobalt into silica framework.

200 300 400 500 600 700 800

0.0

0.1

0.2

0.3

0.4

0.5

0.6

(c)

(b)400 500 600 700 800

Ab

sorb

an

ce (

a.u

.)

Wave length(nm)

Ab

sorb

an

ce (

a.u

.)

Wave length (nm)

(a)

142

Figure 6.5 FT-IR spectra of (a) SBA-50, (b) Co-SBA-15(25),

(c) Co-SBA-15(50) and (d) Co-SBA-15(100)

6.2.5 Temperature Programmed Reduction (TPR) Profile

The H2-TPR profile of Co-SBA-15 (Figure 6.6) was recorded to

understand the reducibile property and the nature of cobalt species in SBA-15

framework. The H2-TPR profile of Co-SBA-15 exhibited only one broad

reduction peak centered at 825 °C. This reduction peak is ascribed to Co2+

species that are effectively incorporated in the silica framework, which is

reduced to Co0. These results clearly revealed that cobalt species are present

in the silica framework without the presence of cobalt oxide. Furthermore, the

intensity of reduction peak area increased with increase of cobalt content due

to increased number of cobalt species in SBA-15. These results confirmed the

strong interaction of cobalt species with SBA-15 framework (Martinez et al

2003).

2000 1800 1600 1400 1200 1000 800 600

Si-O-Co

(a)

(b)

(c)

Tra

nsm

itta

nce (

a.u

.)

Wave number (cm-1

)

(d)

Si-O-Si

143

Figure 6.6 TPR profile of ( ) Co-SBA-15(25), ( ) Co-SBA-15(50) and

) Co-SBA-15(100)

6.2.6 SEM and TEM

The morphology of SBA-15 and Co-SBA-15 with Si/Co ratios of

25, 50 and 100 was studied by SEM images as shown in Figure 6.7. The SEM

image of SBA-15 showed a rod like morphology which are bundled together,

and the SEM images of Co-SBA-15 with different Si/Co ratios exhibited

similar morphology (Zhao et al 1998). The hexagonal array of mesoporosity

and homogeneous distribution of cobalt species are confirmed by TEM

images as shown in Figure 6.8. The TEM image of Co-SBA-15 (25) exhibited

well-organized hexagonal mesoporous structure with uniform channels but

did not show any distinct metal oxide particles.

200 400 600 800 1000

Co-SBA-15 (25)

Co-SBA-15 (50)

Co-SBA-15 (100)

TC

D S

ign

al

(a.u

.)

Temperature (o

C)

144

Figure 6.7 SEM images of (a) SBA-15, (b) Co-SBA-15(25),

(c) Co-SBA-15(50) and (d) Co-SBA-15(100)

Figure 6.8 TEM images of (a) SBA-15 and (b) Co-SBA-15 (25)

145

6.3 CATALYTIC ACTIVITY

The catalytic activity of Co-SBA-15 was evaluated in the oxidation

of -pinene using H2O2 as the oxidant under solvent-free condition

(Scheme 6.1). The influence of reaction parameters such as Si/Co ratios,

different molar ratio of pinene/oxidant and reaction time was also

attempted to optimize the reaction condition so as to obtain high conversion

and high selectivity of the product. The influence of cobalt content in

SBA-15, absence of catalyst, absence of H2O2 and different amount of

catalyst on the oxidation of -pinene is presented in Table 6.2. The entries

1 to 3 in Table 6.2 results revealed the influence of Si/Co ratios, and the

results cocluded that the conversion and selectivity of the product increased

with increase of cobalt content. This may be attributed to high density of

redox sites that led to high conversion and high selectivity of the product.

Additionally, the same reaction carried out in the absence of catalyst showed

not only low conversion but also resulted isomerization of -pinene (others)

which is due to insufficient reactive oxygen species. The same reaction

conducted in the absence of oxidant (H2O2) resulted only 23% conversion

with low selectivity of allylic oxidized product. The entries 6 to 8 in Table 6.2

showed the influence of catalyst amount on the conversion of -pinene, which

increased with increase of catalyst amount from 50 to 100 mg and further

increase of catalyst amount did not change the conversion. This result

revealed that 100 mg of catalyst is found to be sufficient for the oxidation of

-pinene.

Scheme 6.1 Allylic oxidation of -pinene over Co-SBA-15

CH3

CH3

CH3OH

CH3

CH3

CH3O

CH3

CH3

CH3

OCH3

CH3

CH3

+ + + OthersCo-SBA-15

H2O2

14

6

Table 6.2 Allylic oxidation of -pinene under different reaction conditionsa

S.

NoCatalyst

Conversion

(%)b

Selectivity (%)b

Verbenol Verbenone-Pinene

epoxide

Campholenic

aldehydeOthers

1 Co-SBA-15(100) 61 52.8 17.4 12.6 11.5 7.5

2 Co-SBA-15(50) 75 59.1 15.9 8.4 7.3 7.8

3 Co-SBA-15(25) 92 73.2 15.4 4.9 3.2 3.3

4 - 11 12.3 5.4 22.2 16.8 43.3

5 Co-SBA-15(25)c

23 10.3 8.4 7.9 3.8 69.6

6 Co-SBA-15(25)d

56 52.5 19.6 13.2 8.8 5.9

7 Co-SBA-15(25)e

71 54.8 21.7 12.2 8.4 2.9

8 Co-SBA-15(25)f

92 73.2 15.4 4.9 3.2 3.3

a Reaction conditions: Catalyst (100 mg), -pinene (10 mmol), H2O2 (30 mmol), Time 14 h, Room temperature,

b Determined by gas chromatograph,

cWithout H2O2,

dCatalyst (50 mg),

e Catalyst (75 mg),

f Catalyst (125 mg).

147

The plausible mechanistic pathway for the selective oxidation of

-pinene is shown in Scheme 6.2. The first step is the decomposition of H2O2

over Co2+

-SBA-15 generating active oxidant species of hydroxyl and

hydroperoxyl radicals. The hydroxyl radical abstracts a proton from the allylic

position of -pinene to form a radical intermediate (I). This radical

intermediate (I) reacts with (i) hydroxyl radical to form verbenol

(major product 1) and (ii) hydroperxyl radical to form verbenyl hydroperoxide

intermediate(II). The verbenyl hydroperoxide further reacts with Co2+

active

sites in three different pathways such as (a) distant oxygen coordinates with

Co2+

sites forming a complex, which reacts with another molecule of

-pinene to form -pinene oxide (minor product 2), (b) homogeneous

decomposition of verbenyl hydroperoxide gives hydroxyl radical and

intermediate (III). This intermediate then reacts with another molecule of

-pinene by abstracting a proton from the allylic position to form verbenol

(major) and (c) subsequent oxidation of verbenyl hydroperoxide into

verbenone (minor product 3).

148

Decomposition of H2O2

Co2+

H2O2 Co3+

OH

O OH

OH-

H+

Co3+

H2O2 +

+ + +

+ Co2++

Mechanism

Scheme 6.2 Plausible mechanism for the allylic oxidation of -pinene

CH3CH3

CH3

OH+

CH

CH3CH3

CH3

OH

CH3CH3

CH3

OH

O OH

CH3CH3

CH3

O

OH

CH3CH3

CH3

O

CH3CH

3

CH3

Co2+ +

CH3CH

3

CH3

Co2+

CH3CH3

CH3

O

OH CH3

CH3

CH3

O

Co2+Co

3+

OH+

CH3CH3

CH3

CH3CH3

CH3

O

CH3CH3

CH3

H

CH3CH3

CH3

OH

CH3

CH3

CH3

O

(Intermediate I)

(Intermediate II)(Intermediate III)

(a)

(c)

(b)

(1)

(2)(3)

149

6.3.1 Effect of Molar Ratio

Figure 6.9 depicts the influence of molar ratio of -pinene/H2O2 in

the oxidation over Co-SBA-15(25) catalyst. The conversion of -pinene

increased from 34 to 92 % with increase of molar ratio of -pinene/ H2O2

from 2:1 to 1:3, and further increase of molar ratio to 1:4 did not enhance the

conversion of -pinene. The selectivity of the product also increased with

increase of -pinene/ H2O2 ratio from 2:1 to 1:3. This is attributed to the

increased decomposition of H2O2 with increase of H2O2 concentration. The

results revealed that high concentration of H2O2 showed rapid decomposition.

It is concluded that -pinene/H2O2 ratio of 1:3 is found to be the optimal

reaction condition for selective oxidation of -pinene with high conversion

and selectivity of the product.

Figure 6.9 Effect of Si/Co ratios on the oxidation of -pinene

2:1 1:1 1:2 1:3 1:4

0

20

40

60

80

100

Co

nv

ersi

on

/ S

elec

tiv

ity

(%

)

olar ratio of -pinene/H2O2

Conversion

Verbenol

Verbenone

150

6.3.2 Effect of Reaction Time

The influence of reaction time was studied over Co-SBA-15 (25) in

the oxidation of -pinene, keeping other reaction parameters the same and the

results are depicted in Figure 6.10. The conversion of -pinene increased with

increase of reaction time up to 14 h. The selectivity of the product (verbenol)

remained the same whereas other products such as verbenone increased

slightly with increase of reaction time. This is attributed to increased

decomposition of verbenyl hydroperoxide with increase of reaction time. The

epoxide selectivity decreased with increase of reaction time. This is attributed

to the acidic property of Co-SBA-15 which may involve in the isomerisation

of epoxide yielding campholenic aldehyde. It is concluded that the efficiency

of H2O2 increased with increase of reaction time up to 14 h.

Figure 6.10 Effect of reaction time on the oxidation of -pinene

2 4 6 8 10 12 14

0

20

40

60

80

Co

nv

ersi

on

/ S

ele

cti

vit

y (

%)

Time (h)

Conversion

Verbenol

Verbenone

Pineneoxide

Campholenic aldehyde

Others

151

6.3.3 Reusability and Heterogeneity Studies

The recyclability of Co-SBA-15(25) catalyst was examined in the

oxidation of -pinene using H2O2 as the oxidant at room temperature for 14 h

under solvent-free condition. The Co-SBA-15(25) catalyst was recovered

after the reaction from the reaction mixture by filtration. The recovered

catalyst was thoroughly washed with acetonitrile to remove of the organic

substrate, dried at 100 °C for 5 h and activated at 300 °C. The activated

catalyst was used for the next catalytic cycle and the same procedure was

repeated for 5 cycles. The results revealed that Co-SBA-15 (25) catalyst

showed almost the same efficiency without significant loss of its activity up to

5 cycles (Figure 6.11). The heterogeneous nature of the catalyst was also

studied and results are presented in Figure 6.12. In order to verify the

heterogeneous nature of Co-SBA-15 (25) catalyst in the oxidation of -pinene

the following study was performed. The catalyst was removed after 6 h of

reaction and the -pinene conversion was found to be only 33%. The reaction

was further continued with filtrate for another 8 h. The reaction product was

analyzed and the results indicated that there was no conversion of -pinene

after the removal of catalyst. This suggested that the reaction is heterogeneous

in nature. It is also concluded that there is no leaching of cobalt from SBA-15

framework.

152

Figure 6.11 Recyclability studies

Figure 6.12 Heterogeneity studies

1 2 3 4 5

0

20

40

60

80

100

Con

ver

sion

(%

)

No. of cycles

2 4 6 8 10 12 14

0

20

40

60

80

100 With catalyst

Catalyst removed after 6 h

Co

nv

ersi

on

(%

)

Time (h)

Catalyst filtration

153

6.4 CONCLUSION

The cobalt incorporated hexagonal ordered SBA-15 was prepared

by hydrothermal method with lower acidic condition (pH=3). The ordering of

mesoporous nature increased with increase of cobalt content as confirmed by

XRD. The presence of isolated Co2+

site was confirmed by DRSUV-Vis and

TPR profile. The catalytic activity of Co-SBA-15 in the oxidation of -pinene

with H2O2 as the oxidant under solvent-free condition at room temperature

achieved high selectivity of the product (verbenol). The Co2+

species played

crucial role in the selective allylic oxidation of -pinene due to homogeneous

decomposition of H2O2. The selective product formation of verbenol indicated

the involvement of hydroxyl radical in this reaction. The recyclability study

revealed that cobalt species are highly dispersed in the silica framework

without leaching. The heterogeneity test performed by hot filtration method

showed that Co-SBA-15 present truly in heterogeneous nature in the

oxidation of -pinene.