Synthesis, characterizations and catalytic allylic oxidation of limonene to carvone of cobalt doped mesoporous silica templated by reed leaves

Article abstract of DOI:10.1016/j.catcom.2014.10.022

Reeds (Phragmites communis) leaves were successfully used as template in synthesis of cobalt doped mesoporous silica (Co/SiO₂/PC). The catalyst exhibited very high substrate conversion (100%) and relatively good product (carvone) selectivity (40.2%) for allylic oxidation of limonene to carvone using air as oxidant and acetic anhydride as solvent without adding any initiator. Fast hot catalyst filtration experiment proved that the catalyst acted as a heterogeneous one and it can be recycled easily and reused two times without significant loss of activity and selectivity. Graphical abstract Reeds (Phragmites communis) leaves were successfully used as template in synthesis of cobalt doped mesoporous silica which was employed as efficient heterogeneous catalyst for the allylic oxidation of limonene using air as oxidant. The catalyst exhibited high conversion and good product (carvone) selectivity and can be recycled.

Full text of DOI:10.1016/j.catcom.2014.10.022

Catalysis Communications 59 (2015) 233–237
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Synthesis, characterizations and catalytic allylic oxidation of limonene to
carvone of cobalt doped mesoporous silica templated by reed leaves
Junjie Li, Zhiqiang Li, Guoli Zi, Zhiang Yao, Zhongrui Luo, Yanmei Wang, Dong Xue, Baoqi Wang, Jiaqiang Wang ⁎
Yunnan Engineering Research Center of Photocatalytic Treatment of Industrial Wastewater, The Universities' Center for Photocatalytic Treatment of Pollutants in Yunnan Province, School of Chemical
Sciences & Technology, Yunnan University, Kunming 650091, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 August 2014
Received in revised form 18 October 2014
Accepted 21 October 2014
Available online 29 October 2014
Reeds (Phragmites communis) leaves were successfully used as template in synthesis of cobalt doped mesoporous
2
silica (Co/SiO /PC). The catalyst exhibited very high substrate conversion (100%) and relatively good product
(carvone) selectivity (40.2%) for allylic oxidation of limonene to carvone using air as oxidant and acetic anhydride
as solvent without adding any initiator. Fast hot catalyst filtration experiment proved that the catalyst acted as a
heterogeneous one and it can be recycled easily and reused two times without significant loss of activity and
selectivity.
Keywords:
©
2014 Elsevier B.V. All rights reserved.
Cobalt doped mesoporous silica
Allylic oxidation of limonene
Reed leaves
Template
Carvone
1
. Introduction
and MOFs (metal organic frameworks) as catalysts but associated with
problems of their hydrothermal and chemical stability compared to that
of oxides [11]. Biotransformation was also introduced to synthesize
carvone from limonene, which are promising for their green credentials
and their high level of sustainability but the yield of carvone is still low
[12]. Therefore, from both an economic and environmental point of
view, it continues to be a challenge to develop “green” catalysts for the
production of carvone, which are active under mild conditions, and
which can be easily recovered and reused.
Mesoporous silica has a potential for applications in field of catalysis,
sorption and separation due to their ordered pore arrangement, high
surface area and relatively high thermal stability [13,14]. And the incor-
poration of cobalt into mesoporous silica framework and the resulting
excellent catalytic activity had been widely investigated [15,16].
Biotemplating techniques have attracted considerable attention for
the syntheses of porous inorganic materials in the recent several years,
because biological templates are generally energy-conserving, green,
and can be harvested in large amounts at low costs. In addition, the meth-
od is generally performed under mild conditions, energy-conserving and
has little requirement for instrumentation [17–20]. The advantage of
biotemplating processes is that they not only yield advanced synthetic
materials in an environment benign system but also enable control over
the phase, size, morphology achieved by replication or morphosynthesis
of biological tissues and materials [21,22]. However, the application of
these biotemplated materials in selective oxidation was still very limited.
For example, a biological template (Luffa sponge) was used as macroscale
sacrificial structure builder to synthesize MFI-type zeolite frameworks
with hierarchical porosity and complex architecture [23]. Interestingly,
It is well known that allylic oxidation of cyclic alkenes is of consider-
able value for the production of unsaturated alcohols and ketones which
play a significant role in the development of value-added chemicals
from biomass [1,2]. Carvone, one of the most valuable compounds derived
from the allylic oxidation of limonene, has wide applications in food
additives, fragrance and pharmaceutical industries [3]. The classical
methods for the synthesis of carvone from limonene, such as the epoxida-
tion and the nitrosochlorination, are often performed in environmentally
unfriendly solvents, producing toxic end-products and difficult to sepa-
rate products [4]. Compared with these systems, heterogeneous catalysts
with more significant advantages for recovery and stabilities are more
desirable to liquid-phase oxidation. Recently, a number of solid catalysts
were reported for the synthesis of carvone in heterogeneous processes,
such as modified silica and titania [5–7]. Chromium containing mesopo-
rous molecular sieves MCM-41 were used to catalyze the allylic oxidation
of limonene under mild solvent-free conditions resulting in 36% substrate
conversion and 25% selectivity for carvone [5]. A high conversion of 97% of
2 5 2
limonene was attained in the presence of V O /TiO as catalyst but with
the corresponding selectivity of only 5.1% towards carvone [6]. With
limonene over Mn(Salen) complexes grafted on functionalized SBA-15,
it gave the maximum conversion 75.9% whereas the maximum selectivity
of carvone is only 7.2% [7]. Moreover, this reaction has been carried out
employing metal complexes (manganese [8], cobalt [9] and iron [10])
⁎
Corresponding author. Tel./fax: +86 871 65031567.
E-mail address: jqwang@ynu.edu.cn (J. Wang).
http://dx.doi.org/10.1016/j.catcom.2014.10.022
566-7367/© 2014 Elsevier B.V. All rights reserved.
1

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J. Li et al. / Catalysis Communications 59 (2015) 233–237
the as-synthesized biomimetic ZSM-5 replica showed catalytic activity
for cracking of n-hexane with no need for ion-exchange [24]. In our
group cobalt doped porous titania–silica was synthesized by using rice
husks as both silicon source and template. It presented good product
2
microstructure of Co/SiO /PC with pore system is well-distributed and
compact, which elaborately duplicated the original hierarchical struc-
ture such as the sophisticated vascular bundle and stomata of the reed
leaves template. The morphology of the honeycomb-like pores and
vessels, including cylindrical and also some irregular shapes with
random size at 10–20 μm, was obtained.
(
4-pyridinecarboxylic acid) selectivity (91%) and very high substrate
conversion rate (96%) for the catalytic oxidation of 4-methyl pyridine
25]. Recently, we used natural rubber latex as template in synthesis
[
of cobalt doped mesoporous alumina which was an efficient catalyst
for the oxidation of the tetralin under relatively mild reaction conditions.
It even offered significantly higher activity than same alumina prepared
by conventional templates such as poly (ethylene oxide)-block-poly
3.2. Catalytic performance
The catalytic activities of different catalysts for the oxidation of
limonene were summarized in Table 1. It is apparent that Co/SiO
shows the highest conversion and selectivity of carvone. As expected,
pristine SiO /PC and MCM-41 showed much lower conversion and
selectivity than Co/SiO /PC. This indicates that Co ions in Co/SiO /PC
2
/PC
(propylene oxide)-block-poly (ethylene oxide) (P123) [20].
Reed (Phragmites communis) is a widespread perennial plant that
2
grows in wetlands or near inland waterways. Due to the presence of
nanocavities and channels, the reed leaves possess hierarchical pore
network and complex functional patterns, an excellent gas and water
permeability for substrates and products to be diffused in and out, as
well as biogenic doped chemical elements, which might be particularly
suitable for catalysis and separation [26,27]. Herein, in the continuation
2
2
are active species for the allylic oxidation of limonene. Furthermore,
even incorporating Co into other mesoporous materials, such as
Co/MCM-41 and Co/SBA-15 with similar concentrations of cobalt,
we found that they exhibited much lower activity than Co/SiO
2
/PC
though Co/MCM-41 and Co/SBA-15 have much bigger surface area (BET
2
2
of our work, cobalt doped hierarchical mesoporous SiO
sized by using reed leaves as the template (denoted as Co/SiO
2
was synthe-
/PC)
surface areas: 945 m /g and 690 m /g, respectively) than Co/SiO
2
/PC.
2
This could be attributed to the pore diameter of Co/SiO /PC (4.6 nm)
2
and displayed good catalytic properties for the allylic oxidation of limo-
nene to carvone. Additionally, molecular oxygen to oxidation reactions
offers a green alternative to traditional toxic chemical oxidants [13]. In
our work, atmospheric air was employed as oxidant which is more
readily available and could further reduce the cost and hazards of the
oxidation process.
which is larger than Co/MCM-41(2.9 nm) and Co/SBA-15 (3.2 nm) since
the catalytic transformation of limonene to products requires
suitable catalysts with pores large enough to avoid steric hindrance [31].
To study the catalytic activity of materials with large pore diameter, we
chose the SBA-3 as the catalyst support. The pore diameter of cobalt
doped SBA-3 (4.5 nm) was smaller in comparison with pure SBA-3
(
5.4 nm), but close to that of Co/SiO
2
/PC (4.6 nm). However, we
/PC
although Co/SBA-3 has much larger surface area (780 m /g) and
similar pore diameter to the Co/SiO /PC. Therefore, it is suggested
2
. Experimental
found that Co/SBA-3 showed much lower activity than Co/SiO
2
2
Experimental details for the synthesis of catalysts, oxidation of
2
limonene and characterizations were described in detail in Supple-
mentary data.
that not only pore diameter but also pore structure and morphology
of catalyst would play an important role in catalytic oxidation of
limonene.
3
. Results and discussion
The diffusion of molecules through the pore structure, called "mo-
2
lecular traffic control", is vital for catalytic performance. Co/SiO /PC
3
.1. The characterization of catalyst
possess heterogeneous pores and complex architecture including
honeycomb-like, cylindrical and some irregular shapes, which could
help guest species to overcome the intra-diffusional resistance in host
materials and allow a rapid diffusion of bulky products through the in-
organic networks of pores and channels to ensure high selectivity of cat-
alytically active sites and to avoid over-oxidation [17,18,24]. Thus, due
The N
shown in Fig. S1. The sample showed typical isotherm of type IV having
inflection around P/P = 0.4–0.9. This is typical of mesoporous structure
of prepared material and also supported by the big average pore diameter
of 4.6 nm, a BET surface area of 435 m /g and a pore volume of 0.52 cm /g.
Moreover, the BJH pore size distribution of Co/SiO /PC (inset of Fig. S1)
2 2
adsorption/desorption isotherm of Co/SiO /PC at 77 K is
0
2
3
2
to the good performance of Co/SiO /PC, in the following we concentrate
2
on the study of the influence of various parameters on the limonene
shows one primary pore size distribution in the mesoporous region
between 2.1 and 5.6 nm which reveals that the prepared catalyst has
irregular pore channels [24,28].
2
conversion and selectivity of carvone over Co/SiO /PC.
3.3. Effect of solvents
Fig. S2 presents the wide and low (inset) angle X-ray diffraction
(
XRD) pattern of Co/SiO
2
/PC measured at 2θ of 10–80°. Obviously, the
The nature of solvents was known to have a major influence on
reaction kinetics and product conversion in the oxidation of limonene.
Therefore, the effects of various solvents on the reaction are investigated
and summarized in Table S1. Apparently, using acetic anhydride as the
solvent indicated the best catalytic performance. This is probably
because polar solvents acetic anhydride may facilitate formation of
active oxygen species and thereby enhance allylic oxidation involved
free radicals [32].
structure of silica present in our sample remained essentially amorphous
and no distinct diffractions corresponding to any crystalline cobalt are
observed. This implies that cobalt species are well incorporated into
the silica framework [29]. Similar observations have been reported for
Co/MCM-41 and Co/SBA-3 [24]. In addition, it is seen that the low
angle XRD pattern of the sample at 2θ of 0–6° shows several diffraction
lines in distinct peak in the low 2θ of region. It is clearly indicated
that the sample possesses a mesoporous structure [30], which is in
agreement with the result of N
2
adsorption/desorption measurement.
3.4. Effect of reaction temperature
To further confirm the valence state of Co species in the resulting
sample, an XPS experiment was used for the surface analysis of
The effect of reaction temperature on limonene reactions over
Co/SiO
2
/PC. As shown in Fig. S3, the binding energies of doublet for
2
Co/SiO /PC is shown in Fig. 2. It is clear that both conversion of limonene
2
+
2
p3/2 and 2p1/2 of Co
are 781.4 and 796.9 eV, respectively. It is
and selectivity of carvone increased with the increase in reaction
temperature and both values passed through a maximum at 348 K. A
further increase in the reaction temperature resulted in little change
of conversion of limonene but a fast decrease in the selectivity. This
would be due to the deep/over oxidation and ring-opening reaction
also shown that the state of Co species in Co/SiO
same as that in Co/MCM-41.
Scanning electron microscopy (SEM) images given in Fig. 1 show the
morphologies of Co/SiO /PC and their template. It is obvious that the
2
/PC catalyst is the
2

J. Li et al. / Catalysis Communications 59 (2015) 233–237
235
2 2
Fig. 1. SEM micrographs of Co/SiO /PC and their template. (a, c) Co/SiO synthesized with reed leaf as template and the template tissues after calcination (b) leaf vascular, (d) leaf stomata.
that occurred with the rising temperature when limonene was al-
most completely converted after 348 K. Because the oxidation of lim-
onene involves radical intermediates and a competition between
allylic oxidation and epoxidation. The co-products was also detected
to be limonene glycol, carveyl acetate, diepoxide, perillaldehyde
and 1,2-diacetoxy-p-mentha-8-ene. Considering the effect of tem-
perature on both conversion of limonene and selectivity of carvone,
we chose 348 K as the suitable temperature for the oxidation of
limonene.
in reaction time. Considering the effect of time on both conversion
of limonene and selectivity of carvone, the optimum yield of carvone
could be achieved at about 18 h.
3.6. Effect of catalyst concentration
Fig. 4 presents the effect of catalyst concentration on the reaction
2
over Co/SiO /PC. It is seen that an initial steep increase in the conversion
3
.5. Effect of reaction time
100
7
0
Conversion of limonene
The effect of reaction time on limonene reaction over Co/SiO
2
/PC was
60
50
80
investigated in Fig. 3. A steep increase in the conversion of limonene and
selectivity of carvone were observed when the time was increased up to
1
8 h. Beyond this time, the selectivity of carvone decreased with increase
Selectivity of carvone
60
40
3
0
40
Table 1
a
Comparison of catalytic activities of different catalysts for the oxidation of limonene.
20
Catalyst
Co/SiO /PC
Conversion of limonene (wt.%) Selectivity of carvone (wt.%) TON
20
0
1
0
2
100
35.7
40.2
22.5
34.6
12.8
21.7
18.5
632
–
SiO /PC
2
Co/MCM-41
MCM-41
Co/SBA-15
Co/SBA-3
47.5
16.7
50.2
65.1
300
–
0
320
340
360
380
317
411
Reaction temperature / K
a
Note: Reaction condition: substrate, 2 mL limonene; reaction time, 18 h; reaction
temperature, 348 K; catalyst, 100 mg; solvent, acetic anhydride 12 mL; oxidant, air. TON,
turn over number (millimole of oxidized products per millimole of metal in the catalyst).
Fig. 2. Effect of temperature on the oxidation of limonene over Co/SiO
condition: substrate, 2 mL limonene; reaction time, 18 h; catalyst, 100 mg; solvent, acetic
anhydride 12 mL; oxidant, air.)
2
/PC. (Reaction

236
J. Li et al. / Catalysis Communications 59 (2015) 233–237
1
00
2
were also under the optimum conditions with those used for Co/SiO /PC.
6
0
The typical recycling procedure was as follows: after the initial reaction,
the catalyst was separated from the reaction mixture and washed with al-
cohol and acetone, dried at 363 K, followed by the activation at 773 K for
Conversion of limonene
8
6
4
2
0
0
0
0
0
50
Selectivity of carvone
2 h. The results are also summarized in Table 2; the catalyst Co/SiO
showed excellent reusability in the oxidation reactions. Moreover, the
Co 2p XPS spectra of the fresh Co/SiO /PC and the recycled Co/SiO /PC
reacted at 348 K for 18 h) were collected to determine the state of the
2
/PC
40
30
20
10
0
2
2
(
Co active sites (Fig. S3). It was found that their spectra were similar,
which indicated that the state of the Co active site in the recycled
Co/SiO
2
/PC catalyst changed little in the first catalytic activity test
/PC.
when compared with those of fresh Co/SiO
2
In order to prove whether the catalyst is a heterogeneous one,
experiments with fast hot catalyst filtration and studying the activity
of the filtrate had been done by a modified process as Ref. [34]. We
found that after hot filtration the filtrate reacted little further comparing
that observed when the catalyst was not filtered. This observation was
also well supported by ICP-AES analysis of filtrates where negligible
amount of leaching of active cobalt ≤0.001%. These indicated that the
doped active component (cobalt) did not leach to the solution and the
6
9
12
15
18
21
24
Reaction time / h
2
Fig. 3. Effect of reaction time on the oxidation of limonene over Co/SiO /PC. (Reaction
condition: substrate, 2 mL limonene; reaction temperature, 348 K; catalyst, 100 mg;
solvent, acetic anhydride 12 mL; oxidant, air.)
2
Co/SiO /PC did act as a heterogeneous catalyst.
of limonene and a slight increase in selectivity of carvone were observed
when the amount of the catalyst was increased up to 100 mg. Beyond
this amount, both of the conversion and the selectivity decreased.
Thus only small amount of catalyst was active in the oxidation of limo-
nene. This could be attributed to the competent interaction of metal
Allylic oxidation and epoxidation are often competitive processes in
the oxidation of cyclic olefins and frequently both processes occur
simultaneously. Typically, the abstraction of the allylic hydrogen gives
allylic oxidation products and the electrophilic attack at to the double
bond results in epoxidation or ring cleavage [1]. The limonene liquid
phase oxidation may proceed by the pathway/mechanism as illustrated
in Scheme 1: (a) epoxidation, leading to limonene-1,2-epoxide or
limonene glycol (transformed by ring-opening) as the main product, or
2
oxo-species in Co/SiO /PC matrix with limonene [33], thus inhibiting
the catalytic reaction. And it is also probable that large catalyst dosages
provide excess active sites in the system to accelerate side reactions.
Therefore, 100 mg was selected as the suitable amount of the catalyst
for oxidizing limonene.
(
b) allylic oxidation, carvone, and carveol would be the main products
though trace amount of some by-products, such as perillaldehyde,
p-mentha-1,8-dien-2-ol, p-mentha-2,8-dien-1-ol as well as carveyl
acetate were also formed due to the free radical chain reaction mecha-
nism [6,35].
3
.7. Oxidation of limonene with different molar ratio of Si/Co
Meanwhile, the influence of the Si/Co on the catalytic activity is
shown in Table S2. It indicates that the suitable ratio of Si/Co is 60.
However, the decrease in limonene conversion and carvone selectivity
at higher cobalt content (Si/Co of 60) could be due to the presence of
excess amount of cobalt, which leads to competent interaction of
metal oxo-species with both alkylperoxy species and substrate [24,34].
4
. Conclusions
In conclusion, reeds (P. communis) leaves were successfully used as
template in synthesis of cobalt doped mesoporous silica (Co/SiO
2
/PC).
Co/SiO /PC was an efficient and high substrate conversion catalyst for
2
3
.8. The reusability and fast hot catalyst filtration experiment
the allylic oxidation of limonene under relatively mild reaction condi-
tions without adding any initiator. It even exhibited significantly higher
activity than Co/MCM-41. Fast hot catalyst filtration experiments
proved that the catalyst acted as a heterogeneous one; it can be reused
three times without losing its activity to any great extent.
2
To study the stability and recycling ability of Co/SiO /PC under
reaction conditions, recycling experiments were carried out. The reactions
100
60
Conversion of limonene
Acknowledgments
80
60
40
20
0
50
The authors thank the National Natural Science Foundation of China
Project NSFC-YN U1033603, 21263027, 21367024), and Program for
Innovative Research Team (in Science and Technology) in University
of Yunnan Province (IRTSTYN) for financial support.
Selectivity of carvone
(
40
30
20
10
0
Table 2
a
Recycled experiment of catalytic efficiency under optimum condition.
Catalyst
Conversion of
Selectivity of
TON
limonene (wt.%)
carvone (wt.%)
30
60
90
120
150
180
Fresh
First recycled
Second recycled
100
92.0
90.0
40.2
43.8
43.0
632
581
568
Catalyst / mg
a
Fig. 4. Effect of catalyst concentration on the oxidation of limonene over Co/SiO
Reaction condition: substrate, 2 mL limonene; reaction temperature, 348 K; reaction
time, 18 h; solvent, acetic anhydride 12 mL; oxidant, air.)
2
/PC.
Note: Reaction condition: substrate, 2 mL limonene; reaction time, 18 h; reaction
temperature, 348 K; catalyst, 100 mg; solvent, acetic anhydride 12 mL; oxidant, air. TON,
turn over number (millimole of oxidized products per millimole of metal in the catalyst).
(

J. Li et al. / Catalysis Communications 59 (2015) 233–237
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