135 CHAPTER 6 SOLVENT-FREE SELECTIVE OXIDATION OF D -PINENE OVER Co-SBA-15 CATALYST 6.1 INTRODUCTION Į-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 D-pinene such as verbenol, verbenone and Į-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.
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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),