CHAPTER 6 SOLVENT-FREE SELECTIVE OXIDATION OF...
Transcript of CHAPTER 6 SOLVENT-FREE SELECTIVE OXIDATION OF...
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.
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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.
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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.
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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)
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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)
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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.
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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)
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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
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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)
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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)
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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).
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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).
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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)
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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
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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.
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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
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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.